High_reactive_power_DOC.docx

Cascaded H-Bridge Multi-Inverter-based Decoupling Feed-Forward Current Vector
Controller for the High Reactive Power Compensation
Abstract—
Abstract: An Artificial Neural network-based Unified Power Quality Controller (UPQC) is
proposed in this paper. An unbalanced grid voltage and Load voltage are compensated by the
UPQC in the power system. This work involves the use of a Unified Power Quality Conditioner
(UPQC) based on an ANN controller for its functions with grid integration of photovoltaic,
such as voltage sags/ swell, unit power factor correction, voltage, and current harmonic
cancellation. This system UPQC is used to compensate for the supply voltage and load voltage
at the same time in a power distribution network. The unit vector template control algorithm
includes a phase-locked loop (PLL) mechanism that is responsible for avoiding multiple zero
crossings during highly distorted grid voltage detection. A unit vector template control with a
PLL-based control algorithm is applied to the shunt and series inverters of PV grid-connected
UPQC. The grid and load voltage unbalances are regulated, and the Total Harmonics Distortion
(THD) of the grid and load voltages are controlled by the UPQC via an Artificial Neural
Networks (ANN) controller. In addition to normalizing voltage and current disturbances, the
proposed controller has the functions of phase detection and perfect grid synchronization.

CHAPTER 1
I. INTRODUCTION
The ever-increasing power quality issues arising from the growing complexity of loads, such
as motor speed drive systems, Programmable Logic Controllers (PLC), rectifiers, electronic
ballasts, computers etc., have caused great adverse effects in the power industry. These include
low-Power Factor (PF), high THD, excessive reactive power consumption, and phase
unbalance imposition onto the distribution system [1]. Nevertheless, with proper power flow
controls such as providing reactive power compensation, these problems can be mitigated.
Conventional reactive power or VAR compensation methods, such as Static VAR
Compensators (SVC), prove to have many drawbacks in terms of its power rating, response,
accuracy, and cost. The discovery of inverter-based Flexible Alternative Current (AC)
Transmission System (FACTS), specifically the MCHI based grid-tied STATCOM offers
substantial advantages over its conventional counterparts and has resulted in massive research
in the last two decades [2]-[7].
The proposed system incorporates five level inverter based multilevel inverter connected in
cascade through an open-ended three-phase transformer. Assurance of maintaining asymmetric
voltages at the dc links of the inverters is an added advantage of this topology, the output
maintains balance with the increasing the number of input level of inverters. The unbalance in
the system here, is compensated with the help of the multi-level STATCOM. Supply of active
power to the grid affects the dc-link voltage balance between the inverters. Drop in the DC link
voltages of the cascaded inverters due to current interference at certain operating points is
observed. In such cases to determine the performance of the converter the proposed system is
dynamically modelled. The mathematical analysis derived from the model is very well utilized
to know the system behaviour at different modes of operation.
The presence of highly distorted loads at the consumer side makes the power system polluted
and various power quality issues arise. Power quality improvement has become major point of
focus for researcher nowadays. The integration of renewable energy to the grid with power
electronic interfacing system also provides a wide area for power quality improvement. The
current perturbations introduced by the loads, grid side voltage quality issues; unbalanced and

highly distorted load conditions are major point of focus. Elimination of the power quality
issues requires compensating systems. According to various types of power quality issues,
special category of power conditioners has been used as reported in the literature [1]-[3]. The
custom power devices have been reported as power conditioners with its various categories.
Dynamic voltage restorer (DVR) and Distribution static compensator (DSTATCOM) are
developed specially for voltage quality issues and current quality issues respectively [4]-[5].
However, Unified Power quality conditioner (UPQC) combines the functionality of both DVR
and DSTATCOM [6], [7]. The UPQC configuration has back-to-back connected DVR and
DSTATCOM, with a common DC-Link. Therefore UPQC become capable to deal with current
and voltage quality issues perfectly.
Modern power electronics have made custom devices such as DSTATCOM, DVR, and UPQC
available to help mitigate energy quality issues. The sags/swells in the grid voltage are offset
with DVR connected to the grid series[8]. DSTACOM, the parallel shunt compensation system
connected to the PCC, offsets power quality issues including reactive currents, harmonics, and
load imbalances[9]. UPQC [10] provides the combining DSTATCOM and DVR functions,
which are connected back to the DC side by series and shunt-inverters. Many UPQC categories
in terms of topology, position of shunt/series inverter and control strategies have been narrated
in the literature. UPQC is generally referred to as a unified power quality conditioner.
Connections of series and shunt inverters are the principle behind the design configuration. The
right and left-shunt configuration is the voltage source inverter (VSI) in this configuration [3,
4]. The UPQC-VSI[5, 6] has been established and the findings have been analysed in this paper.
The UPQC for non-linear and voltage sensitive load has subsequent installations.
i) UPQC provides for a VAR load requirement so that supply voltage and current are
always in phase; no further power factor adjustment equipment is therefore needed.
ii) it decreases supply current harmonics to enhance nonlinear loading utility current
efficiency.
iii) UPQC retains the rated load end voltage even when the voltage is supply sag.
Considering many factors of micro-grid’s energy supply, solar energy is an ideal green energy
for sustainable development strategy in china. At the same time, global energy experts believe
that the solar energy will become one of the most important energy in future. Photovoltaic
power generation system has a large proportion in micro-grid and distribution system with
micro-grid [9-10]. At the same time, with the development of science and technology, most of

precision electronic instruments and digital electrical equipment are used in micro-grid and
distribution system with micro-grid. The reliability and power quality of microgrid’s power
supply is put forward higher requirements. In the micro-grid with photovoltaic generation as a
source, due to the existence of intermittent nonlinear loads, in particular, a static converter
operating in a switching mode, and other nonlinear loads such as electric arc furnace, welding
machine, transformer, rotating motor and so on. Because of these nonlinear loads, a large
amount of reactive power is consumed, the power factor of micro-grid is reduced, the voltage
and power loss are increased. At the same time, the different frequency and amplitude
harmonics in the microgrid are produced. It will cause the damage of the distributed generation
equipment. The harmonics pose great threat to the security, stability and economic operation
of micro-grid and micro-grid distribution system. At present, in the micro-grid and power
distribution system with micro-grid, the active power is usually only provided to the grid by
Photovoltaic power generation system. That is, the DC power of the PV array is converted to
the same phase and frequency AC power as the power grid, and to ensure that it has a high-
power factor. The special capacitor is usually used for the load reactive power compensation.
Active Power Filter and passive filter are usually added to control the harmonics in micro-grid.
This will increase the investment of power system, so that the structure of power grid is
complicated. At the same time, new power quality problems are brought by the additional
equipment.
Various topologies for Photovoltaic (PV) grid integration with active filtering capability are
reported in [8]-[10]. The integration of Solar PV with UPQC is also reported in [11], [12].
However, PV-UPQC topology is very few in literature. Interfacing of solar energy to the grid
through UPQC, increases the utilization and functionality of UPQC. In this case part of the
load power is supplied by PV. Literature survey reports about only a few papers about this PV-
UPQC configuration. Thereby a detail analysis and performance evaluation is needed. The
control algorithm for UPQC has been studied and implemented in literatures includes
instantaneous reactive power theory, synchronous reference frame algorithm, unit vector
template algorithm [12]-[14]. To make PV-UPQC systems become more sensitive towards
highly distorted conditions of load and source voltage, this paper introduces a modified SRF
control algorithm. In this paper the design of PV-UPQC system is along with proposed
controller is discussed. The solar PV is interfaced with UPQC through DC-DC boost converter
at the DC-link. To extract maximum power from PV Perturb & Obserb (P&O) MPPT algorithm
[15] is utilized in the present system. Conventional systems with traditional PLL behave
properly under normal distortions, but it becomes helpless during high distortions. Therefore

this paper proposes an improved PLL mechanism for highly distorted load and voltage
conditions, which work along with APF control algorithm.
II. Literature review
According to IEEE, flexible AC transmission system (FACTS) provide effective control of AC
transmission system parameters (IEEE, 1997). It ensures enhancement in power transfer
capability added with controllability. Generally, FACTS controllers that are employed in DG
integrated grid for PQ improvement, are thyristor controlled LC compensator (TCLC) (Wang
and Lam, 2017), distribution static compensator (DSTATCOM) (Lee et al., 2013; Mishra and
Ray, 2016), and unified power quality conditioner (UPQC) (Dash and Ray, 2017). TCLC
compensates reactive power and it also maintains THD of the system, but it has large
computational burden on controllers. DSTATCOM is used to control the voltage fluctuations
only. It does not have control command on harmonics. Among all, the UPQC can compensate
most PQ related issues (voltage sag, swells and fluctuations and harmonics control) in both the
modes of operation of DGs, i.e., grid connected and islanded mode (Han et al., 2006).
However, UPQC needs a proper control algorithm for the generation of voltage and current
reference signals for working out compensatory control According to IEEE, flexible AC
transmission system (FACTS) provide effective control of AC transmission system parameters
(IEEE, 1997). It ensures enhancement in power transfer capability added with controllability.
Generally, FACTS controllers that are employed in DG integrated grid for PQ improvement,
are thyristor controlled LC compensator (TCLC) (Wang and Lam, 2017), distribution static
compensator (DSTATCOM) (Lee et al., 2013; Mishra and Ray, 2016), and unified power
quality conditioner (UPQC) (Dash and Ray, 2017). TCLC compensates reactive power and it
also maintains THD of the system, but it has large computational burden on controllers.
DSTATCOM is used to control the voltage fluctuations only. It does not have control command
on harmonics. Among all, the UPQC can compensate most PQ related issues (voltage sag,
swells and fluctuations and harmonics control) in both the modes of operation of DGs, i.e., grid
connected and islanded mode (Han et al., 2006). However, UPQC needs a proper control
algorithm for the generation of voltage and current reference signals for working out
compensatory control
According to IEEE, flexible AC transmission system (FACTS) provide effective control of AC
transmission system parameters (IEEE, 1997). It ensures enhancement in power transfer
capability added with controllability. Generally, FACTS controllers that are employed in DG

integrated grid for PQ improvement, are thyristor controlled LC compensator (TCLC) (Wang
and Lam, 2017), distribution static compensator (DSTATCOM) (Lee et al., 2013; Mishra and
Ray, 2016), and unified power quality conditioner (UPQC) (Dash and Ray, 2017). TCLC
compensates reactive power and it also maintains THD of the system, but it has large
computational burden on controllers. DSTATCOM is used to control the voltage fluctuations
only. It does not have control command on harmonics. Among all, the UPQC can compensate
most PQ related issues (voltage sag, swells and fluctuations and harmonics control) in both the
modes of operation of DGs, i.e., grid connected and islanded mode (Han et al., 2006). However,
UPQC needs a proper control algorithm for the generation of voltage and current reference
signals for working out compensatory control
Proposed UPQC Block Diagram.

CHAPTER 2
CHAPTER-2
WIND ENERGY
2.1 Wind power:
Wind may be found in the nearly every corner of globe. Wind resources will always be
available due to an unequal heating of Earth's surface and a rotation of Earth. Using
nonrenewable resources like coal, natural gas, oil, and a like to generate electricity has the
significant negative impact on an environment because it adds enormous amounts of carbon
dioxide to an atmosphere, causing an earth's surface temperature to rise, the phenomenon
known as a "greenhouse effect." Science and technology have advanced to a point that
renewable energy sources like wind may be harnessed to generate electricity. A cost of grid-
connected wind power is now competitive with that of coal and oil-fired power plants. Since
green electricity is becoming more popular, a demand for that electricity generated by non-
renewable sources is also increasing.

Fig 2.1 DFIG Structure
Wind power systems have the number of unique properties that make them stand out from other
forms of renewable energy.
i) Rural, island, or marine environments tend to be ideal locations for that wind turbines.
ii) These locales have unique energy needs that do not necessitate the large amount of
electrical power. For example, the system that provides inexpensive variable
voltage power for that heating and expensive fixed voltage electricity for that
lighting and motors can be the good fit for that total energy use.
iii) Weaknesses in the rural grid networks are common (low voltage 33 KV). Weak grids
make it difficult and dangerous for that workers to connect the Wind Energy
Conversion System (WECS).
In an absence of wind, here are usually times of calm. As the result, if supplies are to be
maintained, WECS must be coupled to an energy storage or parallel generation system.
Wind turbines generate electricity by converting kinetic energy from a wind into rotational
motion of turbine's generator. A wind turbine's ability to harvest energy from a wind depends
on the number of elements. here are the number of parameters that determine how much power
can be extracted from a wind, including a speed of wind. This is due to a fact that an amount
of electricity generated by the wind turbine is directly proportional to a square of wind speed.
As the result, if a wind speed is doubled, a generated power will be eight times greater than
before. A placement of wind farm is also critical in the order to maximize an amount of wind
energy that can be captured by a wind turbines.
The rotor blades are a second most critical component of the wind turbine. Rotor blades length
is an important consideration when designing the wind turbine since it is directly proportional
to a swept area of rotor blades, which is equal to a square of their diameter.

Increasing a swept area's diameter by two results in the fourfold increase in the power. A rotor
blades must be sturdy, light, and long-lasting. These rotor blade properties grow increasingly
elusive as they get longer. Fiberglass and carbon-fiber technology has made it possible to
produce lightweight and robust rotor blades up to 30 meters in the length. Up to one megawatt
of power may be generated by wind turbines with rotor blades this large. Here is the graph
showing a link between a rotor blades swept diameter and an amount of power generated by a
wind source.
In [2], you can find a formula's derivation. in the other texts, a swept area of rotor blades (A)
and an air density () are used to generate a formula.
Choosing the wind turbine is the matter of finding a most efficient and effective one that can
take use of kinetic energy of wind.
There are several advantages to wind power over conventional power plants.
Enhancing competitiveness in the terms of pricing; Modular installation; Speedy construction;
Complimentary gensets; Greater system dependability; non-pollution.
According on how they set their rotors, wind turbines can be divided into two categories.
Horizontal rotors and vertical rotors are a two main types of rotors.
Since a wind-driven electric generator model assumes the horizontal-axis rotor, this paper will
only cover horizontal-axis wind turbines.
Because a blades move in the front of tower in the relation to a wind's direction, the horizontal-
axis wind turbine has an axis of rotation that is perpendicular to a wind's direction. Upwind
rotors are commonly referred to as such. Downwind rotors are another form of horizontal axis

wind turbine with blades that rotate behind a tower. in the large-scale power generation
nowadays, only upwind rotors are used, and a term "horizontal-axis wind turbine" refers to this
configuration in the this study.
The rotor, transmission, generator, and yaw and control systems are all essential parts of the
wind turbine used to generate power. for that the better understanding of how the conventional
horizontal-axis wind turbine is put together, a following diagrams are provided.

(c)
Figs2.2: (a) typical grid-connected wind turbine cross-sections (b) Grid-connected Wind
(c)Turbine Nacelle Cross-section
It is possible to categories a components of wind turbine into tower, rotor, generator, yaw,
control, and transmission systems.
2.2 Tower:
Costliest component of Wind Turbine System. Steel or concrete are used to build lattice or
tubular towers. Guy wires can support smaller, more affordable towers. Mounted on a tower
are a nacelle, which may rotate or yaw depending on wind direction, as well as a rotor brake,
gearbox, electrical switch boxes and controllers. Gravity and wind loads should be taken into
consideration when designing a tower. A tower needs the solid foundation in the an earth to
support it. A tower's resonant frequencies should be taken into account while creating a design,
as should any induced frequencies from a rotor and how they can be dampened. Strict tower
refers to the structure whose inherent frequency is higher than a frequency at which blades
travel through it, whereas soft tower refers to one that is lower.

A wind turbine rotor's aerodynamic forces are explained using aero foil theory. When a
temperature drops below freezing.
As a symmetric aero foil moves through the flow, the pressure distribution forms around
it, as illustrated in the figure.
The chord line is the reference line used to take measurements on an aero foil section, and
a chord length is its measurement. An angle of attack is an angle that an aero foil creates with
a flow of air measured against a chord line. Aero foils generate lift when they are arranged at
an angle of attack to an oncoming flow, which distorts a flow above and below an aero foil.
Unperturbed wind flow causes a pressure on the blade to drop approaching a streamline's center
of curvature. Compared to ambient pressure, an upper surface of aero foil experiences the
decrease in the pressure (suction) whereas a lower side experiences the positive or larger
pressure. A lift force generated by a pressure differential drives a blades' rotation. Figure 1
depicts a drag force component that is perpendicular to a direction of oncoming flow (b). here
is an inverse relationship between a wind's energy and these forces. in the wind turbine design,
the high lift-to-drag ratio of blades is necessary to achieve high efficiency. When wind speeds
vary widely, this ratio can be changed along a blade to maximize a turbine's power production.
Wind energy is captured via a lift force, a drag force, or both. As long as a lift force is 30 times
larger than a drag, an aero foil can be considered efficient.
They have uneven chord lines, which are curled. It is no longer necessary to measure from
an original chord to get the new chord length. for that positive angles of attack, the chamfered
aero foil is preferable over the symmetrical one because of its higher lift-to-drag ratio. A lift at
zero angle of attack is no longer zero and a zero lift occurs at the modest negative angle of
attack of roughly o, as has been seen. As an angle of attack increases, a center of pressure,
which is located at a 14 chord position on the symmetrical aero foil, advances to a trailing edge.

For the given angle of attack, the flat plate can be arched or cambered to produce the
stronger lift force, and this works well for that high-solidity, multi-bladed wind turbines. Aero
foil sections are more effective for that turbines with low solidity.
All of these factors combine to determine a lift and drag forces generated by an aircraft's
aero foil and its angle of attack. Rotational torque and axial thrust are generated by these forces
exerted on a blades of the wind turbine rotor. Turbine overturns due to thrust rather than
beneficial work provided by torque. A tower and its foundations should be able to resist this
axial thrust.
Two types of rotors exist: those that spin at low speeds and those that spin at high speeds.
the long, narrow blade with the high aspect ratio is needed to achieve the high design tip speed
ratio. the short, flat blade might be necessary if a design tip speed is low. High torque is used
by a low-speed rotor and low torque is used by a high-speed rotor. in the general, a power
output of same-sized wind energy converters is a same, as rotor area is a determining factor in
the power production. Metal plates on a low-speed rotor are bent. Rotors are typically tiny
because of number of blades, a weight, and a complexity of balancing a blades.
Because of their aerodynamic properties, they are able to start themselves. When it comes
to propeller rotors, they have the few thin blades with the more complex airfoil shape. Blades
are entirely halted and a rotor cannot be restarted when they are not in the use by altering a
blade pitch or using an external power source, propeller-type rotors can be started either way
(such as generator used as the motor to turn a rotor). A rotor can run at any speed it wants, or
it can be restricted to the fixed one. A tip speed ratio is maintained and aerodynamic efficiency
is improved when a motor is run at the variable speed.
2.3 Aligning a rotors:

An upwind or the downwind rotor is used to align a turbine blades with a wind. Upwind
rotors have an advantage of facing a wind rather than a vertical tower, which reduces a wind
shading effect.
If you're using an upwind rotor, you'll need some way to keep it in the sync with a wind. A
leeward side of tower is where a downwind rotors are located. Because a rotor passes through
a tower's wind shadow, which causes wind power changes, this design has the significant
drawback: increased fatigue loads. Rotor and nacelle design can allow downwind rotors to be
manufactured without the yaw mechanism if a nacelle can follow a wind passively.
There are also gyroscopic loads that prevent unwinding of power cables when the rotor is
stationary for that lengthy periods of time, forcing power wires to twist. This may be an issue
for that some applications.
Downwind rotors can be designed more flexible, whereas upwind rotors must be stiff to
prevent a rotor blades from colliding with a tower. A second possibility implies a possibility
of weight savings and may help reduce a loads on a tower. Wind turbines with upwind rotors
now make up a great majority of those in the use today.
2.4 Blades per revolution of the rotor
Most current aero generators have three-bladed rotors. One, two, and even three-bladed
rotors have advantages above three-bladed designs when it comes to cost and mass savings. It
is, however, necessary to have the greater chord or faster rotational speed in the order to
produce a same amount of energy as the three-bladed turbine of same size. Because an inertia
of the single or two blades varies depending on whether a blades are horizontal or vertical and
a wind speed, the single-blade wind turbine will produce more variable loads.
Because of this, two- and one-bladed ideas typically have so-called teetering hubs, which
indicate the hinged rotor. A rotor can teeter in the this configuration, which reduces imbalanced
loads. With just one blade, the one-bladed wind turbine is less common than one-bladed wind

turbines. in the addition to an increased speed, noise, and visual intrusion, the counter weight
is required to balance a rotor blades on these motors.
2.5 Generator:
A wind turbine's high-quality mechanical power can be efficiently transmitted via
electricity. in the general, generator efficiency is around 95%, while transmission losses should
be under 10%. A transmission frequency and voltage should not be standardized because an
end-use requirements vary. the wide range of generators are already available in the variety of
wind/electricity systems. for that example, while rotating frequency varies to keep a tip speed
ratio constant, wind turbine efficiency is best at constant or near-constant frequencies, and this
is true for that power generation as well.
(iii) a complexity and expense of mechanically controlling a turbine to maintain the
consistent frequency increases. Changing an electrical load on a turbine to adjust a rotational
frequency is an alternate way that is typically less expensive and more efficient.
(iii) in the order to keep a tip speed ratio constant, an ideal turbine rotational frequency falls
with increasing radius for that the given wind speed. As the result, only turbines with the
diameter of less than 2 m can be directly connected to generators. the gearbox is needed for
that larger machines to improve a generator's frequency of operation.
(iv) Gearboxes are heavy and costly. They require regular upkeep and might be noisy.
Generators with the large number of poles are being developed to operate at the lower
frequency in the order to address this issue.
(v) the mechanical accumulator (weight raised by hydraulic pressure) or chemical storage
can be used to provide an indirect drive from a turbine to a generator (battery). Thus, a control
of generator is independent of operation of turbine. ' I Synchronous AC generators are a power

sources for that wind turbine generators. iii) Induction AC generator and iii) Variable speed
generator are available
2.6 The AC generator is synchronous:
To connect to the 50 Hz network, a Synchronous speed will be in the a range of 1500 RPM
– 4 pole, 1000 RPM – 6 pole, or 750 RPM – 8 pole a generator must be adequately protected
against moisture infiltration. Some wind turbines use liquid cooling to reduce airborne noise.
in the order to increase a damping in the a wind turbine drive train at an expense of rotor losses,
it is possible to raise a slip at rated power output. Fixed or synchronous speed is a default setting
for that synchronous generators. , a number of poles, frequency, and revolutions per minute
give us.
2.7 An AC generator that uses induction
Constant speed wind turbines employ these motors, which are a same as regular industrial
induction motors. If a rotor speed is above or below synchronous, a torque is applied or
removed from a shaft. in the order to distinguish between the synchronous generator and an
induction motor, one must look at a direction of power flow in the a wires. As wind turbine
operations change and a necessity for that high efficiency at part load, among other things,
some improvements to induction generator designs must be made.
A generator with the variable speed
As an example, the wide range of variable speed operation can be obtained by using all
output power from the wind turbine to travel through a frequency converters, or only the
fraction of output power can be converted.
2.8 System of yaw
The actuator engages the gear ring at a summit of tower, which in the turn rotates a nacelle.
Control of rotor's yaw is accomplished by rotating an entire rotor horizontally or by yawing it

out of wind. A rotor's sweep area should be parallel to a wind's direction during normal
operation. in the order to control a yaw drive, the slow closed-loop control system was
implemented. A yaw drive is controlled by the wind vane, which is typically positioned on top
of nacelle, and a wind turbine controller. During heavy winds, a nacelle can be yawed to reduce
power.
As the last resort, the wind turbine can be halted with its blades at right angles to a wind's
axis.
Despite its apparent simplicity, a yaw mechanism is one of most challenging aspects of
wind turbine design. A prediction of yaw loads, particularly in the turbulent wind conditions,
is difficult.
2.9 Systems of command:
Cut in the wind speed and cut out wind speed are a two typical wind speed values that
govern an operation of the wind turbine generating plant. At the Cut in the wind speed of
between 4 and 5 m/s, a turbine begins to generate power. A turbine can't produce any power if
it's moving slower than this. To lessen a load on a turbine and protect a blades, it is shut off
when a wind speed drops below 25 m/s. the typical wind speed range for that these devices is
between 12 and 15 m/s. Because severe winds occur so infrequently, it would be impractical
to plan turbines for that them. To avoid wind turbine damage in the an event of heavy winds,
some of excess energy must be squandered. As the result, a wind turbine requires some form
of automatic control to ensure its safety and proper operation. Control system functionalities
are needed for that a following:
i) The automatic startup can be controlled
ii) Adjusting a blade pitch

iii) It is important to shut down in the both regular and pathological circumstances.
iv) Monitoring an operational state, wind speed, direction, and power output of the
windmill.
Figure shows that a nacelle is made up of many parts. To list all of them, they include: an
engine's generator and yaw motors; a transmission; a tower and yaw ring; a primary bearings;
and a main shaft, hub, and blades. An anemometer, a controller in the a nacelle, a sensors, and
so on are all examples of equipment that is not depicted in the a picture. Mechanical energy is
converted to electrical energy via a generator.
An electric yaw motor is used to power a sailboat's yaw drive. Low-speed shaft (main shaft
in the a picture) is connected to high-speed shaft which drives generator rotor through gearbox.
The brake is used to slow down a main shaft. Wind kinetic energy is converted to
mechanical energy by a blades, which lift and rotate. Steel tubing or lattice is used for that a
tower, which is often very tall so that a rotor blades are exposed to higher wind speeds.
Generator with the magnetic field:
Polyphase induction motors are physically and electrically identical to induction
generators, which are both types of electrical generator. Electrical power is generated by
induction generators when their shaft is turned faster than induction motor synchronous
frequency. It is common for that wind turbines and some micro hydro sites to use induction
generators because of their capacity to provide useful electricity at different rotor speeds.
Mechanically and electrically, induction generators are simpler than other kinds of generators.
Due to a lack of brushes or commutators, they are more durable.
An external supply is required to produce the spinning magnetic flux in the an induction
generator because it is not self-exciting. Once a generator is up and running and generating
electricity, an external supply can either come from a grid or a generator itself. This magnetic
field is generated by a stator's rotating magnetic flux in the a rotor. An induction motor is the

machine in the which a rotor rotates at the slower rate than a flux. Power is generated at a
synchronous frequency when a rotor is turned faster than it would normally be.
A capacitor bank connected to a machine in the an event of the freestanding system
establishes a magnetizing flux, and in the a case of the grid connection, it pulls a magnetizing
current from a grid. It's best suited for that wind turbines because a speed of wind is continually
changing.
2.10 The Benefits of Using an Induction Generator
It is typical practice in the wind turbine power generating to use an induction generator
because of its low cost, brushless rotor structure, durability, and ease of maintenance. A
synchronous generator has various advantages over an induction generator. An asynchronous
generator's speed is dependent on an amount of turning force (also known as moment or torque)
delivered. One percent is the very modest change in the rotational speed between peak power
and idle. A discrepancy between a synchronous speed of induction generator and an actual
rotor speed is known as a generator's slip.
The induction machine relies heavily on this speed difference. A rotor looks to be slipping
backward to an observer in the a stator field, which is why a word "slip" is employed [35].
When synchronous speed is used as the reference, a slip quantity is represented on the per-unit
basis, which is more useful. A slide in the per unit is expressed as follows.
According to this calculation, the four-pole, 50 Hz generator will idle at 1500 rpm.

At full power, a generator will be spinning at 1515 rpm. As torque changes, a generator's speed
changes somewhat, which reduces wear on a gearbox and an entire system. This is the useful
mechanical attribute of generator. When it comes to wind turbines, asynchronous (induction)
generators are preferable to those that use synchronous generators.
Analysis of Induction Machines
The torque-speed relationship for that the conventional squirrel cage induction machine is
depicted in the a graph below.
Fig2.3: Speed vs. Torque Squirrel-cage induction generator characteristics
Using a diagram, it is clear to see that a torque of machine is zero when an induction machine
is running at Synchronous speed, which means a rotor is spinning at a same speed as a stator's
rotational magnetic field. Just below its synchronous speed, an induction machine is to be used
as the motor.
This means that a stator terminals of an induction machine that is being used as the generator
must be linked to the constant frequency voltage source and a rotor must be pushed above
synchronous speed by a prime mover of wind turbine shaft. A slip is negative because a source

maintains a synchronous speed and provides a reactive power needed to excite an air gap
magnetic field. Figure 1 depicts an induction machine's per-phase equivalent circuit.
Fig2.4: Equation of an Induction Machine's Equivalent Circuit
A star-connected induction machine is put to a test in the this study. Each step of process is
represented by the single value. As the result, for that the stator with stellar ties:
In order to completely comprehend an operation of an induction generator, one must first
understand an action of an induction motor. A performance of an induction motor can easily
be determined once a comparable circuit parameters have been established. A stator's total
power Pg is depicted in the Figure as follows:
Figure 3 shows that an overall rotor loss PR is
Therefore, a motor's internal mechanical power is

Figure 3's equivalent circuit can be rearranged to a following figure, in the which a mechanical
power per stator phase is equal to a power absorbed by a resistance R2(1-s)/s for that each
phase.
Fig 2.5The following figure depicts an alternative design for that the phase-equivalent circuit.
Using a power flow diagram and an equivalent circuit in the conjunction with an induction
motor analysis is also helpful.
Fig2.6: Power Flow Diagram
Were,

The no-load test and a block rotor test can be used to determine an induction generator's
specifications (The steps in the calculating a parameters and a test results obtained from the
440V, 4.6A, 2.2kW induction motor).
CHAPTER-3
DOUBLY FED INDUCTION GENERATOR (DFIG)
PRINCIPLE OF THE DOUBLE FED INDUCTION GENERATOR CONNECTED TO
THE WIND TURBINE
Double Fed Induction Generator (DFIG) is an abbreviation for that a generating principle used
in the wind turbines. the multiphase wound rotor and multiphase slip ring assembly with
brushes for that accessing a rotor windings are a basis of this induction generator. Brushless
doubly-fed electric machines (see this page for that more information) avoid a multiphase slip
ring assembly, although this has drawbacks in the terms of efficiency, cost, and size. With the
brushless wound-rotor doubly fed electric machine, you'll get more power for that your buck.

Fig 3.1Wind DFIG generator
When it comes to designing a DFIG, slip rings are used in the conjunction with back-to-back
voltage sources in the order to control rotor and grid currents simultaneously. Consequently, a
rotor frequency can be arbitrarily varied from a grid frequency (50 or 60 Hz). Rotor currents
can be controlled by a converter so that they can be adjusted independently of how quickly a
generator is turning. Either two-axis current vector control or direct torque control is employed
as a primary control principle (DTC). When a generator is required to produce high reactive
currents, DTC has proven to be more stable than current vector control.
Rotor rotations on doubly-fed generators are typically 2 to 3 times as high as on the standard
generator stator. As the result, a rotor voltages and currents will be increased. This lowers a
converter's cost because a rated current is lower at a normal operating speed range of 30% of
synchronous speed. Due to a greater than rated rotor voltage, it is impossible to manage a
functioning outside of operational range. in the addition, grid disturbance-induced voltage
transients (particularly three- and two-phase voltage dips) will be amplified. An IGBT and
diode protection circuit (called crowbar) is used to prevent high rotor voltages from damaging
a converter's IGBTs and diodes.
When excessive currents or voltages are detected, a crowbar will short-circuit a rotor windings
through the tiny resistance. An aggressive crowbar must be utilized in the order to continue a
process as rapidly as feasible. Only 20-60 MS after a commencement of grid disturbance can

a rotor side converter be initiated using an active crowbar to eliminate a rotor short. So, during
a rest of voltage dip, it is possible to generate reactive current to a grid and thus assist a grid in
the recovering from a fault.
In wind power applications, the doubly fed induction machine is preferable than the standard
induction machine due to its wound-rotor double-fed design. First, an induction generator may
import and export reactive power thanks to a power electronics converter controlling a rotor
circuit. in the an event of the severe voltage fluctuation, this has significant implications for
that an integrity of power system and enables a machine to provide grid support (low voltage
ride through, LVRT).
It is also possible to keep an induction machine synced with a grid by controlling a rotor
voltages and currents. in the light wind, the variable-speed wind turbine makes better use of
available wind resource than the fixed-speed wind turbine. It is also cheaper than other variable
speed options since only the small percentage of mechanical power, typically 25-30 percent, is
delivered to a grid through a converter and a remainder is fed straight from the stator to a grid.
for that a same reason, a DFIG's efficiency is excellent.
3.1 MODEL OF SYSTEM:
An anachronously rotating – reference frame is used to create the dynamic phasor or
complicated space vector electrical model of system. An example of conventions of axes. A -
axis is aligned with a positive real axis, and a -axis is aligned with a negative imaginary axis,
according to a convention adopted here. in the some cases, a real and imaginary axes aligned
with the specific complex vector, such as, can be designated and respectively, and a real and
(negative) imaginary components with respect to a reference are designated and, the
respectively.

Following assumptions are made in the a model's creation.
A small amount of loss is incurred due to an iron, mechanical, and power converter failures.
2) a machine's magnetic circuit can be modelled using the linear model.
If an electrical angle and speed of induction generator are known, the lumped-inertia parameter
can be used to simulate a mechanical system.
4) State-space averaged representation can be used to model a low frequency dynamics of
power converters.
5) an electrical stiffness of wind farm collection network to PCC It is possible to create an
analogous circuit from a traditional DFIG T circuit.
Under these assumptions, a system equivalent circuit model nonlinear state equations in the
their entirety are

Fig 3.2 DFIG circuit diagram
Control laws and an evaluation of steady state attributes are both aided by a complex
vector dynamic state equations. A stator flux, rotor current, rectifier current, dc link voltage,
and rotor speed are all dynamic states of system. MSC and SGSC complex voltage vectors are
inputs that can be manipulated by a system.
Since a PGSR is the passive network, a state of diode that conducts when a voltage is
greater than determines a PGSR's conduction state. A wind turbine shaft generates mechanical
power in the accordance to a coefficient of performance and a cube of wind speed. A blade
pitch actuators can be used to limit a mechanical torque generated as the result of wind energy
harvesting.
3.2 The DFIG CONTROL:
In order to connect a DFIG to the network, a following three steps must be followed.
Satiric voltages are initially regulated using network voltages as the guide. A stator's
connection to a network is a next phase. Because a voltages of two devices are synced, this

connection is possible. A third step, which is a focus of this research, is a management of power
between a stator and a network after this link is made.
(a)
(b)
Fig 3.3 Types of double fed induction generators (a) single Phase (b) Three phase
3.3 EFFICIENT DOUBLE FIRED INDUSTRIAL GENERAL MOTOR:
A brushless, doubly-fed induction electric generator is made by arranging two multiphase
winding sets with opposite pole pairs next to each other on a stator body. Low frequency
magnetic induction is ensured over a speed range by dissimilar pole-pairs between a two
winding sets. While a power winding of stator is connected to a utility grid, a control winding
is powered by the frequency converter. A frequency of control winding is used to alter a shaft

speed. A frequency converter only has to be the fraction of machine's rating because it is the
doubly-fed electric machine.
The twin winding set stator assembly of brushless doubly-fed induction generator is physically
larger than other electric machines of same power rating with a same power rating. An
additional effort is made to cause most of mutual magnetic field to traverse an air-gap to and
via a rotor assembly for that inductive coupling between a two neighboring winding sets (i.e.,
brushless) Double-fed electric machines are characterized by a fact that a neighboring winding
sets are energized independently and actively participate in the an electro-mechanical energy
conversion process.
Double-fed electric machines can be classified as reluctance or induction machines based on a
type of rotor assembly they use. Because an effective pole count is an average of two active
winding sets, a constant torque speed range is always less than 1800 rpm at 60 Hz. Due to a
lack of the multiphase slip ring assembly, brushless doubly-fed electric machines use the poor
electromagnetic design that reduces physical size, cost, and electrical efficiency. A promise of
the low-cost, high-efficiency electronic controller keeps brushless doubly-fed electric
machines under constant study, research, and development, notwithstanding their commercial
failure since their creation in the an early 1970s.
3.4 The BRUSHLESS WOUND-ROTOR DOUBLY-FED ELECTRIC GENERATOR
An electric motor or generator that incorporates an electromagnetic structure of the wound-
rotor doubly-fed electrical machine, but replaces its traditional multiphase slip ring assembly
with the brushless means of independently powering each winding (i.e., doubly-fed) with
multiphase AC power is known as the brushless doubly-fed electric generator. This is the
classic example of machine instability since a torque of wound-rotor, doubly-fed electric
machine is affected by both slip and location.

When operating at synchronous speed, where induction no longer exists, it is very challenging
to maintain immediate synchronization of frequency and phase of multiphase AC power with
a shaft speed and position in the order to ensure stable operation. It is only if these requirements
are met that all a desirable features of wound-rotor doubly-fed electric machine are able to be
realized without any of conventional slip-ring assembly and instability issues that have
previously plagued this technology. An electric machine with symmetrical motoring or
generating quality that is brushless, totally stable, and synchronously wound-rotor doubly fed
has been patented and is being sold by one business. in the a patent application, Lars Gert mar
describes the second brushless wound-rotor architecture.
3.5 CREATING DOUBLE FED ELECTRIC GENEATOR WITH THEWOUND
ROTOR.
The rotor and stator bodies each have two sets of multiphase windings with identical pole-
pairs. in the comparison to other electric machines, only a wound-rotor doubly-fed machine
has two independent active winding sets, a rotor and stator winding sets. Using a magnetic
core's real estate more efficiently is possible because a rotor winding set actively participates
in the an energy conversion process.
When employed as the wound rotor induction machine, a machine's air gap requires less flux
than when it is operated as the doubly fed generator at unity stator power factor. When double
fed operation at rated stator voltage is attempted on wound rotor machines that weren't designed
for that it, it's not uncommon for that them to become severely saturated. It is therefore required
to create the unique design for that use in the double-fed environment.
Slip rings (i.e., sliding electrical contacts) have traditionally been employed to transfer power
to a rotating (moving) winding set and to allow for that separate control of rotor winding set,

although this is no longer a norm. System dependability, cost, and efficiency are all negatively
impacted by a slip ring assembly. Research into ways to avoid a slide ring assembly is ongoing,
but thus far it has had mixed results (see Brushless doubly-fed induction electric machines).
Frequency converters are used to manage bi-directional, synchronized, and multiphase
electrical power to at least one of winding sets using an electronic controller (generally, a rotor
winding set). for that the wound-rotor doubly-fed electric machine with two poles (i.e., one
pole-pair) working at 60 Hz, a constant torque speed range is 7200 rpm when using four
quadrant control. However, two or three pole-pair machines with correspondingly lower
maximum speeds are popular in the high power applications.
Since only a power of spinning (or moving) active winding set is regulated, an electronic
controller is smaller, less expensive, more efficient, and more compact than electronic
controllers of single-fed electric machines, which control more than half of total power output.
Unlike synchronous machines, which employ damper windings to maintain stability, doubly
fed electric machines do not. Alternating torque pulses and other repercussions can be expected
when a synchronous machine loses synchronism.
Electronic control is necessary for that an actual operation of double-fed electric machines, and
they should be referred to as an electric machine system or an adjustable-speed drive.
3.6 EFFICIENCY:
While ignoring a slip rings, a theoretical electrical loss of the wound-rotor doubly-fed machine
in the super synchronous operation is comparable to a most efficient electric machine systems
available (i.e. A synchronous electric machine with permanent magnet assembly) with similar
operational metrics because total current is split between a rotor and stator winding sets while
an electrical loss of winding set is proportional to a wound-rotor doubly-fed electric motor or
generator (without brushes and with stable control at any speed) theoretically shows nearly half

a electrical loss (i.e. winding set loss) of other electric motor or generator systems of similar
rating when considering electronic controller conditions of less than 50% of machine's power.
3.7 Density of power
Because a two active winding sets are located on a rotor and stator bodies, respectively, with
virtually no real-estate penalty, a physical size of magnetic core of doubly-fed electric machine
is lower than other electric machines. A rotor assembly is passive real estate in the all other
electric devices that does not contribute to power generation. Simply because the given
frequency of excitation can cause the device to run faster indicates the higher power density
potential. Electric machines with two poles can operate at speeds of up to 7200 rpm at 60 Hz
compared to 3600 rpm at a same frequency for that other machines. in the theory, a core volume
(i.e., winding set loss) of other electric motor or generator systems of equivalent rating is
roughly half a physical size (i.e., core volume).
3.8. Cost
There's no need to include slip ring assembly in the this calculation because of major cost of
any electric machine controller, which is 50 percent (or less) less expensive than other electric
motors or generators of same power rating.

CHAPTER-4
CONCEPTS OF FACTS
4.1: INTRODUCTION TO POWER QUALITY:
Power quality is the set of limits of electrical properties that allows electrical systems to
function in their intended manner without significant loss of performance or life. The term is
used to describe electric power that drives an electrical load and the load's ability to function
properly with that electric power. Without the proper power, an electrical device (or load) may
malfunction, fail prematurely or not operate at all. There are many ways in which electric power
can be of poor quality and many more causes of such poor quality power.
The electric power industry comprises electricity generation (AC power), electric
power transmission and ultimately electricity distribution to an electricity meter located at the
premises of the end user of the electric power. The electricity then moves through the wiring
system of the end user until it reaches the load. The complexity of the system to move electric
energy from the point of production to the point of consumption combined with variations in
weather, generation, demand and other factors provide many opportunities for the quality of
supply to be compromised.
While "power quality" is a convenient term for many, it is the quality of the voltage—
rather than power or electric current—that is actually described by the term. Power is simply
the flow of energy and the current demanded by a load is largely uncontrollable.
4.2: POWER QUALITY ISSUES:
The PQ problems are categorized as follows
1. Transients
(a) Impulsive
(b) Oscillatory
2. Short-duration and Long-duration variations

(a) Interruptions
(b) Sag (dip)
(c) Swell
3. Voltage unbalance
4. Waveform distortion
(a) DC offset
(b) Harmonics
(c) Interharmonics
(d) Notching
(e) Noise
5. Voltage Flicker
6. Power frequency variations.
TRANSIENT PROBLEMS:
Transients are very short duration (sub-cycle) events of varying amplitude. Often
referred to as "surges", transients are probably most frequently visualized as the tens of
thousands of volts from a lighting strike that destroys any electrical device in its path.
Transients can be caused by equipment operation or failure or by weather phenomena like
lightning. Even relatively low voltage transients can cause damage to electrical components if
the occur with any frequency.
A properly sized industrial-grade surge suppressor is usually ample protection from the
damaging effects of high voltage transients.
SAG:
The American "sag" and the British "dip" are both names for a decrease in voltage to
between 10 and 90% of nominal voltage for one-half cycle to one minute Sags account for the
vast majority of power problems experienced by end users. They can be generated both
internally and externally from an end users facility.

External causes of sags primarily come from the utility transmission and distribution
network. Sags coming from the utility have a variety of cause including lightning, animal and
human activity, and normal and abnormal utility equipment operation. Sags generated on the
transmission or distribution system can travel hundreds of miles thereby affecting thousands
of customers during a single event. Sometimes externally caused sags can be generated by other
customers nearby.
The starting of large electrical loads or switching off shunt capacitor banks can generate
a sag large enough to affect a local area. If the end user is already subject to chronic under
voltage, then even a relatively small amplitude sag can have detrimental effects. Sags caused
internally to an end user's facility are typically generated by the starting of large electrical loads
such as motors or magnets. The large inrush of current required to starts these types of loads
depresses the voltage level available to other equipment that share the same electrical system.
As with externally caused sags, ones generated internally will be magnified by chronic under
voltage.
SWELL:
A swell is the opposite of sag - an increase in voltage above 110% of nominal for one-
half cycle to one minute. Although swells occur infrequently when compared to sags, they can
cause equipment malfunction and premature wear. Swells can be caused by shutting off loads
or switching capacitor banks on.
NOISE:
Noise is a high frequency distortion of the voltage waveform. Caused by disturbances
on the utility system or by equipment such as welders, switchgear and transmitters, noise can
frequently go unnoticed. Frequent or high levels of noise can cause equipment malfunction,
overheating and premature wear.
NOTCHING:
Notching is a disturbance of opposite polarity to the normal voltage waveform (which
is subtracted from the normal waveform) lasting for less than one-half cycle. Notching is
frequently caused by malfunctioning electronic switches or power conditioners. While it is
generally not a major problem, notching can cause equipment, especially electronics, to operate
improperly.

HARMONICS:
Harmonics are a recurring distortion of the waveform that can be caused by various
devices including variable frequency drives, non-linear power supplies and electronic ballasts.
Certain types of power conditioners like ferroresonant or constant voltage (CVT) transformers
can add significant harmonic distortion to the waveform. Waveform distortion can also be an
issue with uninterruptible power supplies (UPS) and other inverter-based power conditioners.
The UPS does not actually add distortion, but because the UPS digitally synthesizes a
waveform, that waveform may be square or jagged rather than a smooth sine wave. Symptoms
of harmonic distortion include overheating and equipment operational problems.
4.3 INTRODUCTION TO FACTS:
Flexible AC Transmission Systems, called FACTS, got in the recent years a well known
term for higher controllability in power systems by means of power electronic devices. Several
FACTS-devices have been introduced for various applications worldwide. A number of new
types of devices are in the stage of being introduced in practice.
In most of the applications the controllability is used to avoid cost intensive or
landscape requiring extensions of power systems, for instance like upgrades or additions of
substations and power lines. FACTS-devices provide a better adaptation to varying operational
conditions and improve the usage of existing installations. The basic applications of FACTS-
devices are:
• Power flow control,
• Increase of transmission capability,
• Voltage control,
• Reactive power compensation,
• Stability improvement,
• Power quality improvement,
• Power conditioning,
• Flicker mitigation,

• Interconnection of renewable and distributed generation and storages.
Figure shows the basic idea of FACTS for transmission systems. The usage of lines for
active power transmission should be ideally up to the thermal limits. Voltage and stability limits
shall be shifted with the means of the several different FACTS devices. It can be seen that with
growing line length, the opportunity for FACTS devices gets more and more important.
The influence of FACTS-devices is achieved through switched or controlled shunt
compensation, series compensation or phase shift control. The devices work electrically as fast
current, voltage or impedance controllers. The power electronic allows very short reaction
times down to far below one second.
The development of FACTS-devices has started with the growing capabilities of power
electronic components. Devices for high power levels have been made available in converters
for high and even highest voltage levels. The overall starting points are network elements
influencing the reactive power or the impedance of a part of the power system. Figure 1.2
shows a number of basic devices separated into the conventional ones and the FACTS-devices.

For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some
explanation. The term 'dynamic' is used to express the fast controllability of FACTS-devices
provided by the power electronics. This is one of the main differentiation factors from the
conventional devices.
The term 'static' means that the devices have no moving parts like mechanical switches
to perform the dynamic controllability. Therefore most of the FACTS-devices can equally be
static and dynamic.
4.4 TYPES OF FACTS DEVICES:
The left column in Figure 1.2 contains the conventional devices build out of fixed or
mechanically switch able components like resistance, inductance or capacitance together with
transformers. The FACTS-devices contain these elements as well but use additional power
electronic valves or converters to switch the elements in smaller steps or with switching
patterns within a cycle of the alternating current. The left column of FACTS-devices uses

Thyristor valves or converters. These valves or converters are well known since several years.
They have low losses because of their low switching frequency of once a cycle in the converters
or the usage of the Thyristors to simply bridge impedances in the valves.
The right column of FACTS-devices contains more advanced technology of voltage
source converters based today mainly on Insulated Gate Bipolar Transistors (IGBT) or
Insulated Gate Commutated Thyristors (IGCT). Voltage Source Converters provide a free
controllable voltage in magnitude and phase due to a pulse width modulation of the IGBTs or
IGCTs. High modulation frequencies allow to get low harmonics in the output signal and even
to compensate disturbances coming from the network. The disadvantage is that with an
increasing switching frequency, the losses are increasing as well. Therefore special designs of
the converters are required to compensate this.
4.5 SHUNT DEVICES:
The most used FACTS-device is the SVC or the version with Voltage Source Converter
called STATCOM. These shunt devices are operating as reactive power compensators. The
main applications in transmission, distribution and industrial networks are:
• Reduction of unwanted reactive power flows and therefore reduced network losses.
• Keeping of contractual power exchanges with balanced reactive power.
• Compensation of consumers and improvement of power quality especially with huge demand
fluctuations like industrial machines, metal melting plants, railway or underground train
systems.
• Compensation of Thyristor converters e.g. in conventional HVDC lines.
• Improvement of static or transient stability.
Almost half of the SVC and more than half of the STATCOMs are used for industrial
applications. Industry as well as commercial and domestic groups of users require power
quality. Flickering lamps are no longer accepted, nor are interruptions of industrial processes
due to insufficient power quality. Railway or underground systems with huge load variations
require SVCs or STATCOMs.

SVC:
Electrical loads both generate and absorb reactive power. Since the transmitted load
varies considerably from one hour to another, the reactive power balance in a grid varies as
well. The result can be unacceptable voltage amplitude variations or even a voltage depression,
at the extreme a voltage collapse.
A rapidly operating Static Var Compensator (SVC) can continuously provide the
reactive power required to control dynamic voltage oscillations under various system
conditions and thereby improve the power system transmission and distribution stability.
APPLICATIONS OF THE SVC SYSTEMS IN TRANSMISSION SYSTEMS:
a. To increase active power transfer capacity and transient stability margin
b. To damp power oscillations
c. To achieve effective voltage control
IN ADDITION, SVCS ARE ALSO USED
1. IN TRANSMISSION SYSTEMS
a. To reduce temporary over voltages
b. To damp sub synchronous resonances
c. To damp power oscillations in interconnected power systems
2. IN TRACTION SYSTEMS
a. To balance loads
b. To improve power factor
c. To improve voltage regulation
3. IN HVDC SYSTEMS
a. To provide reactive power to ac–dc converters
4. IN ARC FURNACES
a. To reduce voltage variations and associated light flicker
Installing an SVC at one or more suitable points in the network can increase transfer
capability and reduce losses while maintaining a smooth voltage profile under different

network conditions. In addition an SVC can mitigate active power oscillations through voltage
amplitude modulation.
SVC installations consist of a number of building blocks. The most important is the
Thyristor valve, i.e. stack assemblies of series connected anti-parallel Thyristors to provide
controllability. Air core reactors and high voltage AC capacitors are the reactive power
elements used together with the Thyristor valves. The step up connection of this equipment to
the transmission voltage is achieved through a power transformer.
SVC BUILDING BLOCKS AND VOLTAGE / CURRENT CHARACTERISTIC
In principle the SVC consists of Thyristor Switched Capacitors (TSC) and Thyristor
Switched or Controlled Reactors (TSR / TCR). The coordinated control of a combination of
these branches varies the reactive power as shown in Figure. The first commercial SVC was
installed in 1972 for an electric arc furnace. On transmission level the first SVC was used in
1979. Since then it is widely used and the most accepted FACTS-device.
SVC
SVC USING A TCR AND AN FC:
In this arrangement, two or more FC (fixed capacitor) banks are connected to a TCR
(thyristor controlled reactor) through a step-down transformer. The rating of the reactor is
chosen larger than the rating of the capacitor by an amount to provide the maximum lagging

vars that have to be absorbed from the system. By changing the firing angle of the thyristor
controlling the reactor from 90° to 180°, the reactive power can be varied over the entire range
from maximum lagging vars to leading vars that can be absorbed from the system by this
compensator.
SVC OF THE FC/TCR TYPE:
The main disadvantage of this configuration is the significant harmonics that will be
generated because of the partial conduction of the large reactor under normal sinusoidal steady-
state operating condition when the SVC is absorbing zero MVAr. These harmonics are filtered
in the following manner.
Triplex harmonics are canceled by arranging the TCR and the secondary windings of
the step-down transformer in delta connection. The capacitor banks with the help of series
reactors are tuned to filter fifth, seventh, and other higher-order harmonics as a high-pass filter.
Further losses are high due to the circulating current between the reactor and capacitor banks.

Comparison of the loss characteristics of TSC–TCR, TCR–FC compensators and
synchronous condenser These SVCs do not have a short-time overload capability because the
reactors are usually of the air-core type. In applications requiring overload capability, TCR
must be designed for short-time overloading, or separate thyristor-switched overload reactors
must be employed.
SVC USING A TCR AND TSC:
This compensator overcomes two major shortcomings of the earlier compensators by
reducing losses under operating conditions and better performance under large system
disturbances. In view of the smaller rating of each capacitor bank, the rating of the reactor bank
will be 1/n times the maximum output of the SVC, thus reducing the harmonics generated by
the reactor. In those situations where harmonics have to be reduced further, a small amount of
FCs tuned as filters may be connected in parallel with the TCR.

SVC OF COMBINED TSC AND TCR TYPE
When large disturbances occur in a power system due to load rejection, there is a
possibility for large voltage transients because of oscillatory interaction between system and
the SVC capacitor bank or the parallel. The LC circuit of the SVC in the FC compensator. In
the TSC–TCR scheme, due to the flexibility of rapid switching of capacitor banks without
appreciable disturbance to the power system, oscillations can be avoided, and hence the
transients in the system can also be avoided. The capital cost of this SVC is higher than that of
the earlier one due to the increased number of capacitor switches and increased control
complexity.
STATCOM:
In 1999 the first SVC with Voltage Source Converter called STATCOM (Static
Compensator) went into operation. The STATCOM has a characteristic similar to the
synchronous condenser, but as an electronic device it has no inertia and is superior to the
synchronous condenser in several ways, such as better dynamics, a lower investment cost and
lower operating and maintenance costs. A STATCOM is build with Thyristors with turn-off
capability like GTO or today IGCT or with more and more IGBTs. The static line between the
current limitations has a certain steepness determining the control characteristic for the voltage.
The advantage of a STATCOM is that the reactive power provision is independent from
the actual voltage on the connection point. This can be seen in the diagram for the maximum

currents being independent of the voltage in comparison to the SVC. This means, that even
during most severe contingencies, the STATCOM keeps its full capability.
In the distributed energy sector the usage of Voltage Source Converters for grid
interconnection is common practice today. The next step in STATCOM development is the
combination with energy storages on the DC-side. The performance for power quality and
balanced network operation can be improved much more with the combination of active and
reactive power.
POWER QUALITY ISSUES / PROBLEMS
The gradual change in power system leads to shortage of reactive power leading to power
instability. With increased power flow, there is corresponding decrease in voltage at the bus
which results in shortage of reactive power. Power Quality issues are mainly Voltage sag,
Voltage swell, Micro interruptions, Transients and Harmonics. The consequences of these
issues are flickering lights, equipment shutoff, loss of data and damaged to equipments. So in
this paper MMC Converter based STATCOM is tested for these issues to prevent the system,
reaching this state is to augment reactive power support.
III. STATIC SYNCHRONOUS COMPENSATOR (STATCOM) STATCOM is made up of a
coupling transformer, a VSC and a dc energy storage device. STATCOM is capable of
exchanging reactive power with the transmission line because of its small energy storage device
i.e. small dc capacitor, if this dc capacitor is replaced with dc storage battery or other dc voltage
source, the controller can exchange real and reactive power with the transmission system,
extending its region of operation from two to four quadrants. A functional model of a
STATCOM is shown in Fig.1.

Fig.1 Basic Operation of STATCOM
If the amplitudes of the ac system and converter output voltages are equal, there will be no ac
current flow in/out of the converter and hence there will be no reactive power
generation/absorption the ac current magnitude can be calculated using the following
equation[13].
Assuming that the ac current flows from the converter to the ac system. The corresponding
reactive power exchanged can be expressed as follows.
Operation of UPQC
Analysis of the idealized equivalent circuit in Fig. 5.16 can be used to illustrate the
operation of UPQC. This voltage source represents the series converter, whereas this current
source represents the shunt converter. It is important to note that all the currents and voltages
are 3-dimensional vectors with phase coordinates. Since the voltage and current waveforms of
UPFC (explained in chapter 8) contain only even harmonics, UPFC may contain negative and
zero sequence components. we obtain the relation, neglecting the converter losses
how the inner product of two vectors, denoted by X and Y, is defined

"Let the current and voltage of the load IL and the source VS be divided into two components,
each of which has the form ƒ(t) ->."
The fundamental frequency components are contained in I1p L, and the sequence is
strictly positive. The same applies to V 1pS. The rest of the load current and the source voltage
including harmonics are contained in IL and V Rs. I1pL is not uncommon and is contingent on
the load bus's power factor. I2p L and for I2p L -> I1p L -> I3p L -> I4p L -> I5p L -> I6p L -
> I7p L -> I8p L -> I9p L -> I10p L -> I11p L -> I12p L -> I13p L -> I14p L -> I15p L -> I16p
L -> I17p L -> I18p L -> I19p L -> I20p L -> I21p L -> I22p L -> I23p L -> I24p L -> I25p L
-> I26p L -> I27p L -> I28p L -> I29p L -> I30p L -> I31p L -> I32p L -> I33p L -> I34p L ->
I35p L -> I36p L -> I37p L -> I38p L -> I39p L -> I40p L -> I41p L -> I42p L -> I43p L ->
I44p L -> I45p L -> I46p L -> I47p L -> I48p L -> I49p L -> I50p L -> I51p L -> I52p L ->
I53p L -> I54p L -> I55p L -> I56p L -> I57p L -> I58p L -> I59p L.
It is thus concluded that whirl ; Vali = 0. Thus, in IL, the fundamental frequency and positive
sequence component of the active power in the load do not have anything to do with each other.
The target load voltages and source currents should only contain positive sequence,
fundamental frequency components.
The load bus voltage and the source current are defined by V ¤ L and I¤S, respectively.
The load bus and supply bus have different power factor angles (input port of UPQC). Note
that V is sinusoidal and balanced while I is sinusoidal and balanced. If the shunt converter's
reference current (I¤C) and the series converter's reference voltage (V ¤ C) are selected as
with constraint
here,

Note that the restriction (14.30) indicates that V 1p C is quadrature reactive voltage
with the intended source current, IAS. It is simple to deduce that while it may be impossible to
regulate the magnitude of V ¤ L without injecting reactive voltage, there is chance it can be
done by applying it in other ways. It becomes even more difficult if V 1p S varies because of
system disturbances or failures. To control the voltage of the load bus, variable active voltage
may be essential (in phase with the source current). If we put it this way
In deriving above, assume that
Because positive sequence fundamental frequency numbers are both perturbed by ¢VC
and ¢IC, it follows that they are both equal to each other (say, resulting from symmetric voltage
sags). the DC side of the shunt and series converter, which has current imbalance to start up
the VC disruption,
In order to meet the goal of power exchange between the shunt and series converters, the target
of voltage regulation at the load bus must be met.
Remarks
Because of uncompensated nonlinear and unbalanced loads upstream of the UPQC, imbalance
and harmonics in the source voltage can occur.
Because of the series converter's capacitive reactive voltage injection, the voltage of the power
source is increased.

CHAPTER 5
INSULATED GATE BIPOLAR TRANSISTOR (IGBT)
IGBT has been developed by combining into it the best qualities of both BJT and PMOSFET.
Thus an IGBT possesses high input impedance like a PMOSFET and has low on-state power
loss as in a BJT. Further, IGBT is free from second breakdown problem present in BJT. All
these merits have made IGBT very popular amongst power-electronics engineers. IGBT is also
known as metal oxide insulated gate transistor (MOSIGT), conductively-modulated field effect
transistor (COMFET) or gain-modulated FET(GEMFET). It was also initially called insulated
gate transistor (IGT).
The insulated-gate bipolar transistor or IGBT is a three-terminal power
semiconductor device, noted for high efficiency and fast switching. It switches electric power
in many modern appliances: electric cars, variable speed refrigerators, air-conditioners, and
even stereo systems with digital amplifiers. Since it is designed to rapidly turn on and off,
amplifiers that use it often synthesize complex waveforms with pulse width modulation and
low-pass filters.
The IGBT combines the simple gate-drive characteristics of the MOSFETs with
the high-current and low–saturation-voltage capability of bipolar transistors by combining an
isolated-gate FET for the control input, and a bipolar power transistor as a switch, in a single
device. The IGBT is used in medium- to high-power applications such as switched-mode power
supply, traction motor control and induction heating. Large IGBT modules typically consist of
many devices in parallel and can have very high current handling capabilities in the order of
hundreds of amps with blocking voltages of 6,000 V.
The IGBT is a fairly recent invention. The first-generation devices of the 1980s
and early 1990s were relatively slow in switching, and prone to failure through such modes as
latch up and secondary breakdown. Second-generation devices were much improved, and the
current third-generation ones are even better, with speed rivaling MOSFETs, and excellent
ruggedness and tolerance of over loads [1].
Basic Structure

Fig illustrates the basic structure of an IGBT. It is constructed virtually in the same manner as
a power MOSFET. There is , however , a major difference in the substrate. The n+ layer
substrate at the drain in a PMOSFET is now substituted in the IGBT by a p+ layer substrate
called collector C. Like a power MOSFET, an IGBT has also thousands of basic structure cell
connected approximately on a single chip of silicon.
In IGBT, p+ substrate is called injection layer because it injects holes into n-
layer. The n- layer is called drift region. As in other semiconductor devices, thickness of n-
layer determines the voltage blocking capability of IGBT. The p layer is called body of
IGBT.The n-
layer in between p+ and p regions serves to accommodate the depletion layer of
pn- junction , i.e. junction J2.
N-Channel IGBT Cross Section
Equivalent Circuit
An examination of reveals that if we move vertically up from collector to
emitter. We come across p+, n- , p layer s. Thus, IGBT can be thought of as the combination
of MOSFET and p+ n-
p layer s. Thus, IGBT can be thought of as the combination of MOSFET
and p+
n-
p transistor Q1 .Here Rd is resistance offered by n –
drift region. Approximate
equivalent circuit of an IGBT.

Exact equivalent circuit
The existence of another path from collector to emitter, this path is collector, p+,
n-, p (n-
channel), n+ and emitter. There is, thus, another inherent transistor Q2 as n- pn+ in the structure
of IGBT. The interconnection between two transistors Q1 and Q2.This gives the complete
equivalent circuit of an IGBT. Here Rby is the existence offered by p region to flow of hole
current Ih .
The two transistor equivalent circuit illustrates that an IGBT structure has a
parasitic thyristor in it. Parasitic thyristor is shown in line.

Working
When collector is made positive with respect to emitter, IGBT gets forward
biased. With no voltage between gate and emitter, two junctions between n- region and p region
(i.e. junction J2) are reversed biased; so no current flows from collector to emitter
When gate is made positive with respect to emitter by voltage VG, with gate-
emitter voltage more than the threshold voltage VGET of IGBT, an n-channel or inversion layer,
is formed in the upper part of p region just beneath the gate, as in PMOSFET . This n- channel
short circuits the n- region with n+ emitter regions. Electrons from the n+ emitter begin to flow
to n- drift region through n-channel. As IGBT is forward biased with collector positive and
emitter negative, p+ collector region injects holes into n- drift region .In short; n-drift region is
flooded with electrons from p-body region and holes from p+ collector region. With this, the
injection carrier density in n- drift region increases considerably and as a result, conductivity
of n- region enhances significantly. Therefore, IGBT gets turned on and begins to conducts
forward current IC.
Current Ic , or Ie of two current components:
1. Holes current Ih due to injected holes flowing from collector ,p+ n-
p transistor Q1, p-
body region resistance Rby and emitter .
2. Electronic current Ie due to injected electrons flowing from collector, or load, current
IC=emitter current Ie=Ih+Ie.
Major component of collector current is electronic current Ie, i.e. main current path for
collector, or load, current is through p+, n -
, drift resistance Rd and n-
channel resistance Rch.
Therefore, the voltage drop in IGBT in its on-state is
Vc e . o n = I c . R c h + I c . Rd + V j i
=voltage drop [in n -
channel] + across drift in n- region + across forward
biased p+ n-
junction J1.
Here Vji is usually 0.7 to 1v as in a p-n diode. The voltage drop Ic. Rch is due to n-channel
resistance, almost the same as in a PMOSFET. The voltage drop Vdf = Ic.Rd in UGBT is much
less than that in PMOSFET. It is due to substantial increase in the conductivity caused by
injection of electrons and holes in n- drift region. The conductivity increase is the main reason
for the low on-state voltage drop in IGBT than that it is in PMOSFET.

Latch-up in IGBT
From the above that IGBT structure has two inherent transistors Q1 and Q2,
which constitute a parasitic thyristor. When IGBT is on, the hole current flows through
transistor p+ n- p and p- body resistance Rby. If load current Ic is large, hole component of
current Ih would also be large. This large current would increase the voltage drop Ih. Rby which
may forward bias the base p- emitter n+ junction of transistor Q2. As a consequence, parasitic
transistor Q2 gets turned on which further facilitates in the turn-on of parasitic transistor p+ n-
p labeled Q1. The parasitic thyristor, consisting of Q1 and Q2, eventually latches on through
regenerative action, when sum of their current gains α1+α2 reaches unity as in a conventional
thyristor .With parasitic thyristor on, IGBT latches up and after this, collector emitter current
is no longer under the control of gate terminal. The only way now to turn-off the latched up
IGBT is by forced commutation of current as is done in a conventional thyristor .If this latch
up is not aborted quickly, excessive power dissipation may destroy the IGBT. The latch up
discussed here occurs when the collector current Ice exceeds a certain critical value .the device
manufactures always specify the maximum permissible value of load current Ice that IGBT can
handle without latch up.
At present, several modifications in the fabrication techniques are listed in the
literatures which are used to avoid latch-up in IGBTs. As such, latch up free IGBTs are
available.
IGBT Characteristics
The circuit shows the various parameters pertaining to IGBT characteristics.
Static I-V or output characteristics of an IGBT (n-channel type) show the plot of collector
current Ic versus collector-emitter voltage Vce for various values of gate-emitter voltages
VGE1, VGE2 etc .These characteristics are shown below .In the forward direction, the shape
of the output characteristics is similar to that of BJT . But here the controlling parameter is
gate-emitter voltage VGE because IGBT is a voltage controlled device. When the device is off,

junctionJ2 blocks forward voltage and in case reverse voltage appears across collector and
emitter, junction J1 blocks it. Vrm is the maximum reverse breakdown voltage.
The transfer characteristic of an IGBT is a plot of collector current Ic versus gate-
emitter voltage VGE .This characteristics is identical to that of power MOSFET. When VGE
is less than the threshold voltage VGET, IGBT is in the off state.
Static V-I characteristics
Switching Characteristics
Switching characteristics of an IGBT during turn-on and turn-off are
sketched. The turn-on time is defined as the time between by instance of forward blocking to
forward on-state. Turn-on time is composed of delay time tdn and rise time tr ,i.e. ton=tdn+tr.
The delay time is defined as the time for the collector-emitter voltage to fall from Vce to 0.9
Vce. Here Vce is the initial collector-emitter voltage.Time tdn may also be defined as the time
for the collector current to rise from its initial leakage current Ice to 0.1 Ic. Here Ic is the final
value of the collector current .
The rise time tr is the time during which collector-emitter falls from 0.9VCE
to 0.1VCE. IT is also defined as the time for the collector current to rise from 0.1Ic to its final
value Ic.After time ton, the collector current Ic and the collector-emitter voltage falls to small
value called conduction drop=VCES where subscript s denotes saturated value.

The turn-off time is somewhat complex . It consists of three intervals
1. Delay time tdf
2. Initial fall time tf1
3. Final time tf2
i.e. toff=tdf+tf1+tf2
The delay time is the time during which gate voltage falls from VGE to threshold voltage
VGET.As VGE falls to VGET during tdf, the collector current falls from Ic to 0.9 Ic. At the
end of the tdf, collector-emitter voltage begins to rise. The first fall time Tf1 is defined as the
time during which collector current falls from 90 to 20 % of its initial value Ic, or the time
during which collector-emitter voltage rises from Vces to 0.1 Vce.
The final fall time tf2 is the time during which collector current falls from 20
to 10% of Ic, or the time during which collector-emitter voltage rises from 0.1 VCE to final
value VCE.
Applications of IGBTs
IGBTs are widely used in medium power applications such as AC and DC
motor drives, UPS systems, power supplies and drives for solenoids, relays and contactors.
Though IGBTs are somewhat more expensive than BJTs, yet they are becoming popular
because of lower gate-drive requirement, lower switching losses and smaller snubber circuit
requirements. IGBT converter are more efficient with less size as well as cost, as compared to
converters based on BJTs. Recently, IGBT inverter induction-motor drives using 15-20KHZ.
Switching frequency favour where audio-noise is objectionable. In most applications, IGBTs
will eventually push out BJTs. At present , the state of the art IGBTs of 1200vots, 500 Amps
ratings , 0.25-20 µs turn off time with operating frequency are available.
Comparison of IGBT with MOSFET
Relative merits and demerits of IGBT over PMOSFET are enumerated below.
1. In PMOSFET, the three terminals are called gate , source , drain where as the
corresponding terminal for the IGBTs are gate , emitter and collector.
2. Both IGBT and PMOSFET posses high input impedance.

3. Both are voltage control devices.
With rising temperature, increase in on-state resistance in PMOSFET is much pronounced than
in IGBT. So on state voltage drop and losses rise rapidely in PMOSFET than IGBT , with
rising temperature.

CHAPTER 6
POWER QUALITY
ACTIVE REACTIVE POWER, VOLTAGE, AND FREQUENCY CONTROL
STRATEGIES
The control strategy of the proposed conversion system consists of supervising both the voltage
and the frequency of the SEIG stator terminals. In fact, following each variation of wind speed
or connected load, the STATCOM has to exchange an active and reactive power flow with the
system according to the "supply and demand" principle.
Reactive power control
By regulating the reactive power flow between the wind turbine and the system-based
STATCOM, the voltage at PCC is controlled. PI controllers with tuned parameters using GA
are utilised to minimise integral of square of error (ISE) between the reference voltage and the
actual voltage at PCC when this point is subjected to disturbances. The block diagram of
STATCOM controlled by two PI controllers is depicted in Fig. 3a. It consists of d–q frame
transformation of the three-phase voltages and currents at the PCC. Two PI controllers are
introduced to control STATCOM. The first one, controller 1, is used to update the reference
quadrature axis current, Iqref, based on the difference between the vector of measured and
reference voltages, controller 1. The second controller, controller 2, is used to drive the
angle α that is added to the phase angle of the terminal voltage of the PCC, Ɵ. SPWM technique
is utilised to generate switching pulses of the three-level inverter of STATCOM used to control
the inverter voltage injected to the system based on the phase angle α control. This angle is
considered as the variable control signal [8].
Voltage regulation
When the voltage IG V undergoes a variation, the regulator PI estimates the value of the
modulation index m necessary to compensate the voltage fluctuation. Then, for a sample order
n, the rms voltage error in the input PI regulator is given by:
∆VIg.err(n) = VIg(n)
∗
− VIg(n)
The modulation index at the controller output is calculated as follows:

m(n) = m(n − 1) + Kpv[∆VIg.err(n) − ∆VIg.err(n−1)] + Kiv∆VIg.err(n)
With, pv k and iv k are respectively regulator parameters.
Frequency regulation
The control of the frequency is ensured by monitoring the flow of the active power through an
action on the phase shift δ.
Error frequency of a sample order n is expressed as:
∆fn = f ∗n− fn
And the output PI regulator is:
δn = δn−1 + kpv[∆fn − ∆fn−1] + Kiv∆fn
Therefore, the three-phase reference voltages for the PWM control of the VSI are deduced as:
{
V ∗a= msin(ωt + δ)
V ∗b= msin(ωt + δ −
2π
3
)
V ∗c= msin(ωt + δ −
4π
3
)
DC Link Voltage
The DC link voltage is selected from the following equation:
𝐕𝐝𝐜 =
√𝟐(𝐕
√𝟑
⁄ )
𝟐
𝐦𝐨𝐝−𝐢𝐧𝐝𝐞𝐱
where modulation index (mi) is taken as 1. For this calculated value of 677 V, the nearby round
off value of Vdc is computed as 700 V.
DC Link Capacitor
The rms of AC input voltage and mean value of DC output voltage determine the rating of the
uncontrolled rectifier and the IGBT based chopper switch. A constant DC voltage to the
chopper switch is provided by a DC link capacitor that reduces the ripple content in the output
voltage of a rectifier. Disturbance in the waveform may damage the functioning of the switch.
The capacitor acts as a dead short circuit for a small period of time when there is a sudden

switching of the controller. As a result, there is a chance for the bridge rectifier to get damaged.
Hence, a trade-off value of 4000 μF is chosen to reduce its charging current during the initial
conditions and also reduce the ripple content of the DC voltage to a considerable value [11].
III. UPQC SYSTEM CONFIGURATION
The configuration block diagram of system considered is presented in Fig.1. The system
comprises of a shunt voltage source converter and a series voltage source converter. Both shunt
VSC and series VSC are connected by a common DC link capacitor. Shunt VSC part of PV-
UPQC is connected at the load side through interfacing inductors. Similarly Series VSC
connected in series with the grid through coupling inductors. The series transformer of PV-
UPQC system is employed for injecting the voltage signal generated by series VSC. The shunt
VSC is connected at the point of PCC at the load side to compensate the load current harmonics
and to feed the PV power to load the suggested technique effectively eliminates the targeted
lower order harmonics at different modulation indices by proper selection of switching angles
and same time the higher order harmonics are suppressed.

CHAPTER 7
MODELING AND CASE STUDY OF UPQC
shunt-series connection, also known as unified power quality conditioner in the world of power
quality conditioners, is the finest defense for sensitive loads against low-quality sources
(UPQC). As a result of recent research, a universal power quality conditioner has been
developed to address virtually all power quality issues, such as voltage drop, voltage swell,
voltage outage, excessive power factor correction and unacceptable levels of harmonics in the
current and voltage (UPQC). FIGURE 1 shows the UPQC's basic architecture. One of the main
purposes of UPQC is to compensate for voltage flicker and imbalance as well as reactive power,
negative sequence current and harmonics. It can therefore improve power quality in the
neighborhood of a utility or industrial power system by using UPQC. To combat voltage
flicker/imbalance in high-capacity loads, the UPQC is expected to be one of the most effective
methods available.
The Unified Power Quality Conditioner (UPQC) provides non-linear load control in addition
to voltage-sensitive load support.
In addition, by reducing harmonics from utility current, the utility current's quality improves,
which benefits nonlinear loads.
Due to the UPQC's built-in voltage and current synchronization, a PFC unit is not required.
UPQC maintains the rated load end voltage even if the supply voltage fluctuates.
By injecting voltage to keep the load end voltage at a specified level, it is possible to do this
without requiring any additional voltage support for your DC link.
Three-phase inverters are connected to the supply voltage via transformers by the UPQC's
cascaded three-phase inverters. It also aids to decrease harmonics by compensating for the
load's reactive power requirements. The shunt compensator is a voltage regulator for the
common DC link voltage. Circuits such as the series compensator are operated in voltage-
controlled pulse width modulation mode (VCPWM). As the name suggests, this device
provides high voltage by supplying a voltage that is quadrature of the supply voltage to the
load. They work together.

The unified power quality conditioner shown in fig. 1: (up)
To offset the effects of disruptive disturbances on sensitive and/or important loads, this power
device is utilized in the grid [1]. Only this device is multipurpose and mitigates voltage and
current power quality issues at the same time. In the power supply, it fixes voltage fluctuations
while preventing the entry of harmonic load currents into the power system. Figure 1 depicts a
single-phase UPQC system. UPQC has two cascaded IGBT-based VSCs, each having a shunt
and a series configuration. By way of a DC bus, they're all linked together. Directly connected
to the load is the shunt converter, which is connected in parallel. To help with supply and load
harmonic currents, we use a VAR. When the supplied voltage drops, series converters inject
voltage [2]. In order to do this, the UPQC prevents load current harmonics and fixes input
power factor.
Fig 2: block diagram of up

TOPOLOGY FOR POWER QUALITY IMPROVEMENT
According to the STATCOM-based CTS inverter, it injects an amount of current to
ensure a harmonic-free source current and a specified source tension phase angle in the grid.
Due to the injection of current, the induction generator and reactive load currents will be
negated, hence increasing the power factor and quality. So that the inverter can operate,
voltages in the grid are measured and synchronized. Installation of a grid-connected system
with benefits for power quality from point-of-common-coupling (PCC). A grid-connected
system including wind energy generating and battery energy storage is shown in figure 3.
Fig 3: Improved power quality using a grid-connected system
generator of electricity from the wind
Wind power in this design will be provided by pitch control turbines in constant speed
topology. There's no need for a separate field circuit with induction generators because of their
simplicity. In addition, the system features built-in short-circuit protection and is capable of
handling both constant and shifting loads Energy generated by this system is represented as
follows:
Because it is impossible to take all of the wind's energy, wind turbines can only extract a
fraction of it, known as the power coefficient Cp for wind turbines. Wind turbines can only
extract a fraction of the wind's power, known as the power coefficient Cp, which is described
by the following equation:

BESS-STATCOM
During voltage regulation, batteries are used as energy storage devices (BESS). Because it can
inject or absorb reactive power, the BESS operates best in STATCOMs as an energy storage
device because it can keep the voltage of a dc capacitor constant. The distribution and
transmission networks can also be controlled quickly thanks to the new technology. When the
system's power fluctuates, the BESS can moderate the fluctuation by charging and discharging
the batteries in response to the fluctuations. The STATCOM's dc capacitor is linked to the
battery in parallel. This inverter has DC link capacitance and a point-of-common coupling,
which makes it a three-phase voltage source. Using the STATCOM, the common connection
bus is injected with variable size and frequency compensation current.
A non-linear load induction generator and a battery energy storage system are part of the grid's
power quality management system, which is connected to the shunt STATCOM. In the current
control strategy, the STATCOM compensator's functionality is defined by the control scheme.
By providing reactive power to induction generators and nonlinear loads, the STATCOM
insulated gate bipolar transistor was proposed.
FUZZY CONTROLLER
In FC, basic control action is determined by a set of linguistic rules. These rules are determined by the
system. Since the numerical variables are converted into linguistic variables, mathematical modelling
of the system is not required in FC. To convert the numerical variables into linguistic variables, the
fuzzy levels chosen are: NB (negative small), NM (negative medium), NS (negative small), ZE (zero), PS
(positive small), PM (positive medium) and PB (positive big) [14]. The FC is characterized as: (i) seven
fuzzy sets for each input and output, (ii) triangular membership functions for simplicity, (iii)
fuzzification using continuous universe of discourse, (iv) implication using Mamdani‟s „min‟ operator
and (v) defuzzyfication using the „height‟ method. In UPQC, the active power, reactive power,
terminal voltage of the line and capacitor voltage are required to be maintained. In order to control

these parameters, they are sensed and compared with the reference values. To achieve this, the
membership functions of FC are: error, change in error and output as shown in Figs. 3(a), (b) and (c).
In the present work, for fuzzification, nonuniform fuzzifier has been used. If the exact values of error
and change in error are small, they are divided conversely and if the values are large, they are divided
coarsely. The set of FC rules are derived from (8). u  E 1C (8) where  is called the self-
adjustable factor which can regulate whole region of operation, E is the error of the system, C is the
varying ratio error and u is the control variable. A large value of error E indicates that given system is
not in the balanced state. If the system is unbalanced, the controller should enlarge its control
variables to balance the system as early as possible. One the other hand, small value of the error E
indicates that the system is near to balanced state. Overshoot plays an important role in the system
stability. Less overshoot is required for system stability and in restraining oscillations. In such
conditions, C in (8) plays an important role, while the role of E is diminished. The optimization is done
by  . During the process, it is assumed that neither the UPQC absorbs active power nor it supplies
active power during normal conditions. So the active power flowing through the UPQC is assumed to
be constant. The control surface of the proposed FC is shown in Fig. 4. It indicates two inputs, one
output and a surface showing input-output mapping. The set of FC rules is made using Fig. 4 is given
in Table I.

Fig. 3. Membership function of FC: (a) error (b) change in error and (c) output.
TABLE I SET OF FC RULES

Fuzzy Logic Control
Fuzzy logic, unlike the Boolean logic, deals with problems that have uncertainty or vagueness
and utilises membership functions with values varying between 0 and 1. In fuzzy set theory
concept, transition is between membership and non-membership functions [12].
FLC consists of four basic components which are fuzzification, knowledge base, interference
mechanism, and defuzzification. The detailed structure of FLC is shown in Figure 3.
Figure 3: Detailed structure of the fuzzy logic controller.
The DC voltage error(e) and change in error(ce) are the crisp inputs of the FLC. The
fuzzification component converts these input signals into fuzzy values with the help of
membership functions in the forms expressed by the fuzzy linguistic variables. It may be
viewed as fuzzy sets. The knowledge base contains the data of linguistic descriptions which
are expressed in terms of logical implications. The interference mechanism evaluates fuzzy
information and applies set of control rules to convert the input signals into the fuzzified output.
The defuzzification then uses methods such as centre of gravity and maximum and weighted
mean and so forth and converts the input conditions into control signals. It is then applied to
the actual system. The input signals are then expressed in fuzzy set notations using linguistic
labels as characterised by membership grades before being processed by the fuzzy logic
controller.
In this fuzzy controller design, triangular membership function is chosen due to its simplicity
and ease of implementation and its symmetrical characteristic along the axis. The scaling
factors , , and are used in scaling the input and outputs as per the design of FLC [13]. The error
“” and change of error “” at the sampling instant which are used commonly as the inputs of
FLC can be written as

e = Vdcref − Vdc
cen = en − en−1
The set of rules followed by the fuzzy logic controller is summarised in Table 1. In this paper,
Mamdani’s maximin inference scheme is applied to get an implied fuzzy set of tuning rules.
Finally, the centroid method is used to defuzzify the implied control signals [14–16].
Figure 4 explains the control scheme with FLC for ELC.

CHAPTER 8
Results:
Fig. 5. Simulation result of step change in the load current ilq(a) with and (b) without the
designed icq algorithm

Fig. 6. Simulation result of grid real current isreal and reactive current isreactive a) without and
(b) with the designed icq" algorithm

Fig. 7. Simulation result for (a) grid voltage Vs and (b) grid current is
Fuzzy

Fig8. Simulation Diagram of the Proposed System
Fig 9. 48 Pulse Satacom

Fig 10. Simulation result of grid real current isreal and reactive current isreactive a) without
and (b) with the designed icq" algorithm
Fig 11. Grid Voltages and Currents

Fig12. Statcom Voltages, Currents and Reactive Power
Fig13. Load Voltages, Currents and Active Reactive Power

IV. CONCLUSION
This paper proposed a decoupling feed-forward current vector controller based on the dq-
method to provide VAR compensation and PF correction under balanced loading conditions
for the STATCOM system. The mathematical derivation of the designed reactive reference
current icq" algorithm framework has been presented. The voltage and Current with the 48pulse
STATCOM is proposed in this paper. The controllers used for the control of STATCOM i.e.
PI and fuzzy logic controller are designed in this work. The performances of both controllers
are evaluated under three different load conditions such as linear RL load, nonlinear load, and
dynamic load. It is found that the steady-state error is more with the PI controller whereas, the
fuzzy logic controller gives accurate results without any steady-state error. Again the
overshoots, undershoots, and settling time are also less with fuzzy logic controllers. The
voltage deviation due to load change with fuzzy logic controllers is much less than with
conventional PI controllers. These all performances prove that the fuzzy logic controller gives
a robust performance as compared to the conventional PI controller in all types of load
conditions.

REFERENCES
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