IRJET- Series and Shunt Compensation in UPFC using Cascaded Multilevel Inverter- A Transformerless Approach

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 12 | Dec 2018 www.irjet.net p-ISSN: 2395-0072
© 2018, IRJET | Impact Factor value: 7.211 | ISO 9001:2008 Certified Journal | Page 1283
SERIES AND SHUNT COMPENSATION IN UPFC USING CASCADED
MULTILEVEL INVERTER- A TRANSFORMERLESS APPROACH
R. Nagananthini
Assistant Professor, Department of Electrical and Electronics Engineering, Bannari Amman Institute of Technology,
Sathyamangalam, India-638401
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ABSTRACT:- In this paper, reactive power compensation
is done by using two level Cascaded Multilevel Inverter
arrangement with a transformer-less connection. H-Bridge
based FACTS controller is used to compensate the shunt
voltage and series current during fault condition.
Transformer-less approach is to eliminate the certain power
related issues in using transformer. Control mechanism for
shunt voltage and series current compensation is done
separately with PI controller. Capacitor sources for the
multilevel inverter are control for the independent control of
real and reactive power. For different operating conditions
the DC link voltages of the capacitor sourced inverter is
maintain constant. MATLAB Simulink results were presented
for various load conditions and the THD was analyzed and
compared for the system with and without transformer.
Throughout this operation the DC link voltages were
maintained constant to ensure a smoother operation
Keywords:Transformer-less operation, series and shunt
compensation, Cascaded Multilevel Inverter, Unified
Power Flow Controller, SPWM Technique, DC link voltage.
1. INRODUCTION
In present day electrical technology the usage of FACTS
devise in transmission and distribution side is uplifted.
The vital and proficient role of the Flexible AC
transmission system is the reactive power compensation.
FACTS devices such as STATCOM, SSSC, UPQC, UPFC, SVC
and TCSC have been in use for voltage and current
compensation. Apart from Q-compensation improving the
power quality and voltage regulation are the notable
advantages of facts devices[1]. Multilevel inverters are the
desirable solution for reactive power compensation
dealing in high power applications. This is because
thistype of inverters deal with plethora of DC sources and
it is done through capacitors [2] &[3].A small amount of
active power is being drawn from the source inorder to
compensate the converter losses. AnyhowCapacitor
voltage unbalancingoccurs due to the incongruous nature
of switching and conduction loss. Balancing is the
paramount test in multilevel inverters. In [4] to [7]
number of control schemesfor the voltage and current
compensation has been discussed with different
topologies. Analyzing all the three types of multilevel
inverter the cascaded h-bridge type of inverter is
dominance in certain factors like clamped diode vacancy
and downsized capacitors. Large number of capacitors is
main inconvenience in other types of multilevel inverter.
Since controlling the DC link voltage is the predominant
task, which become a unattainable task when the inverter
has large number of capacitors[8]. From this cascaded h-
bridge inverter is the best explication for static var
compensation techniques. Using h-bridge inverter is also
helpful in another way, since the power supply to the
inverters will be the capacitors therefore separate power
source is eliminated which in turn will stamp out the use of
transformer[9]. Thus H-bridge inverter is used for the
transformer-less operation and eliminates the
disadvantages caused due to the pole- mounted
transformer.
Gyugi was the first one to propose the idea of unified
powerflow controller in the year 1992. In last two decades
because of the combined advantage of all the FACTS
devices and separate control of active and reactive power,
a wide research is going in this area on the domain of
modeling, analysis and control [10]. Other notable
advantages of this sophisticated type of FACTS devices
arevoltage and phase angle regulation. Due to this ability
of controlling in series as well as in shunt it has many
practical applications [11]. These applications requires
isolation transformer to separate each of its converter
from the transmission line as shown in fig.1. Apart from
this, pole mounted transformers are used in high voltage
high power converters in order to meet the best quality
outcome based on the required volt-ampere rating.
Fig-1:Series and Shunt compensation with transformer
arrangement

The disadvantages that are associated in this pole mounted
transformer are its high cost which accounts 40% of the
total system cost, huge size which is 90% of the system
weight and high loss which is 50% of the total power loss.
The occurrence of failure is also high, is a serious term to
be considered. Response of the system is the important
outcome to calibrate the performance of the system,
because of the larger time constant the system with pole
mounted transformer has poor dynamic response which
will drag down the control performance of the system.
With poor dynamic and control performance this will not a
perfect solution for a power transmission system by solar
and wind energy, which needs fast track active power
control. Summing up all this transformer-less operation
will wipe out the loss, cost and size demerits of the system
when it is under the transformer operation [12].The
configuration is changed as shown in fig.2. to achieved the
similarity in transformer operation and to achieve
reliability.
Fig-2:Shunt and Series connection configuration.
2. TRANSFORMER-LESS COMPENSATION TECHNIQUE:
Transformer is eliminated in this scheme, like it is used in
conventional back-back DC coupling. Such that this
arrangement will be low in cost, less in size, compact in
size, fast in response and paramount in efficiency. The
alteration is differed in the way of placing the series and
shunt inverter. Series compensation comes first to the
shunt compensation. This method of arrangement is to
ensure thatthere is no swap of active power between the
converters in the circuit. Without active power exchange
the need of transformer is eliminated. This ensures a
higher reliability and wider flexibility of the system.
Fig-3:Operation of transformer-lesscompensation-phasor
diagram
The basic operation of the transformer-less reactive power
compensation system is differed from the conventional
type because of its unique configuration. From fig.3 the
idea behind the transformer-less operation is defined by
the sending end voltage and current vectors and receiving
end voltage and current vectors.
- First stage of Sending end voltage
-Receiving end voltage
-Second stage of sending end voltage
- Current of series connected H-Bridge Cascaded
Multilevel Inverter
-Current of shunt connected H-bridge cascaded
Multilevel Inverter
-Active power flow into the series connected H-Bridge
Cascaded Multilevel Inverter
-Active power flow into the shunt connected H-Bridge
Cascaded Multilevel Inverter
δ-the angle between the first stages of sending end voltage
to the desired voltage level
Initial stage of sending and receiving end voltage will be
and respectively. In order to attain the desired voltage
the series connected H-Bridge cascaded type Multilevel
Inverter is controlled. Because of this voltage the sending
end voltage is shifted to a new vector is the second stage
of sending end voltage.This shift in voltage will control the
active and reactive power. The shunt connected H-bridge
cascaded type Multilevel Inverter is controlled in such a
way to inject the current to the second stage of sending
end voltage to wipe out the active power.
From this we can recognize the controlled voltage source
is the series connected cascaded type multilevel inverter
and controlled current source is shunt connected cascaded
type multilevel inverter. Since the currents are
parallel to their voltages this ensures that there is

no swap of active power between the converters. To prove
this the active power flow in the series connected H-Bridge
CMI can be expressed as
(1)
active power flow in the series connected H-Bridge CMI
can be expressed as
- )
= )cos (90◦ − δ) cos ρ +
cos (δ − ρ) (2)
Where is the First stage of Sending end voltage, is the
Current of series connected H-Bridge Cascaded Multilevel
Inverter. Thus the active power flow in both shunt and
series will be null. From this the power flow control and
command over the line for the series converter is the
desired value of voltage phasor and the power flow
control and command for the shunt connected inverter is
the desired current phasor value is , since is
calculated from(1) and (2). From this the operation and it
is affirm that the functionality of transformer-less system
is similar to the pole-mounted transformer operation.
2.1 CURRENT COMPENSATION CONTROL
METHODOLOGY:
The control methodology used in the control of current
compensation is by generating the sine and cosine signals
and applying them to the three phase supply voltage
( ). Converter currents ( ) in the
synchronously rotated reference frame medium are
transformed by the generated sine and cosine signals. Low
pass filter is used to eliminate the frequency ripples
caused due to switching the circuitry. References voltages
are generated by the controller. Total DC link Voltage
which is the summation of the DC link voltages across each
converter. For the regulation of the total DC link voltagethe
converter lends a small amount of active current and give
the desired reactive current. Apart from this control, one
more control methodology is needed for the control of
individual inverter DC link voltages. The active power
transfer between the supply source and the inverter is
depends on the value and it is comparatively less, when
it is supplied to the grid from the inverter. The value of
depends on the direct axis and quadrature axis voltage
√ so the q-axis voltage reference component
of the shunt connected inverter is analyzed from (3) to
control the DC link voltage of the shunt connected inverter.
= ) ( ) (3)
For the control of the series connected inverter the q-axis
voltage reference component is analyzed by (4)
(4)
The DC link voltage of the series CMI is 0.36 times
lesser than the shunt connected CMI. Inorder to express
the DC link voltage of the inverter as a total DC link voltage
and from this the DC link voltage of
inverter 1 & 2 can be calculated from (5) and (6) in terms
of total DC link voltage
(5)
= 0.27 (6)
Where
= DC link voltage of series/shunt connected H-bridge
Cascaded Multilevel Inverter1
=DC link voltage of series/shunt connected H-bridge
Cascaded Multilevel Inverter1
Since two inverters are used in the H-Bridge CMI, so the
direct and quadrature axis component are split into two on
its total DC link voltage values as in equation (6) and (7).So
the direct axis voltage component is split into
(7)
= 0.27 (8)
Equation (7) and (8) refers the positive reference voltage
components whereas the negative voltage sequence
components and are controlled using the below
formulas, but the rotating reference frame will be
negatively synchronous
+ (9)
+ (10)
Inorder to block the negative sequence current to flow
through the inverter the negative sequence current
referencevalues are assigned to zero.
2.2 VOLTAGE COMPENSATION CONTROL
METHODOLOGY:
The control methodology used for compensation in series
inverter is as follows.The source and load voltages are
compared to obtain the reference voltage signals (11), (12)
& (13) which is phase shifted by 120 degree. With the help
of relay sub-system the switching signals are produced as
shown in fig.4.These signals are applied to the upper arm
of the series and shunt connected H-bridge cascaded
multilevel inverter circuitry.
The NOT gate output of the signals which are
synchronously rotated reference frame signals are applied

to the lower arm signals of the series/shunt connected
inverter.
= sin (ωt)(11)
= sin (ωt- (120 ))(12)
= sin (ωt +(120 ))(13)
Fig-4.Switching Methodology in voltage compensation
Obtained level of voltages aregiven through the NOT
process to provide the switching signals to the remaining
switches.
This control methodology ensures the way of zero active
power exchange that is the time of triggering will make the
series and shunt connected inverter to produce the voltage
phasor which is perpendicular to the reference voltage
signals.
3. RESULTS AND DISCUSSION:
The Q-compensation is done in both series and shunt level
using cascaded H-bridge inverter. MATLAB is used for
observing the performance of the system.
Operating load conditions are changed accordingly to
predict its performance by the output waveform.fig.5 is the
complete MATLAB simulation circuit which comprises the
series and shunt control block and its connection to the
transmission line without transformer
Fig-5:Complete circuitry of Shunt and Series connection
without Transformer
For current regulation the control is taken by the H-bridge
cascaded Multilevel Inverter circuit which is connected in
parallel as shown in fig.5. Load analysis for reactive and
harmonic basis is analyzed. Similarly for the voltage
regulation the control is taken by the H-bridge cascaded
Multilevel Inverter which is connected in series. And for
reactive and harmonic load based conditions are taken
account as a whole i.e., with shunt and series and the
analysis is done accordingly. In both the control
methodology the sinusoidal pulse width modulation
technique was used for switching pulse generation.
Table-1:.Specifcation of parameters in MATLAB
S.NO PARAMETERS SPECIFICATION
1. Rated power of the system 2 MVA
2. Carrier frequency 30KHz
3. DC link voltage of
series/shunt connected
Inverter 1
241V
4. DC link voltage of
series/shunt connected
Inverter 2
659V
5. Equivalent DC capacitance
of each H-bridge CMI
13000 μF
6. line
inductance equivalent
0.22 p.u
7. line inductance
equivalent
0.46.p.u

Fig-6: Current compensation Transformer-less connection
Simulation diagram
Another problem that was faced is the presence of current
rippleeven after the compensation. This was overcome by
the proper and values of the system. Fine tuning of
this PI controller value helps to reduce it. In order to fully
overcome this ripples when it comes for a prototype
optimization techniques like meta-heuristic algorithms are
suggested.
Fig-7: Control methodology for current compensation
For the sine and cosine signal generation the voltages from
the grid is used as shown in fig.6. Transformation
techniques like Inverse Park and park transformation
technique is used to convert the voltage and current of
three phase quantities to two phase quantities.
The direct and quadrature axis are the two phase
components retrieved from the three phase. The direct
axis lags 90 degree to the quadrature axis component as
shown in fig.10. This is then used to produce the positive
and negative sequence voltage and current components.
But the output of the control methodology again uses the
transformation technique to produce the switching signals.
Fig-8: Carrier signal and Reference signal
Fig-9:Switching signal output by SPWM technique
Fig-10: Sine and Cosine wave generation
For a system using transformer in supplying voltage to the
inverter circuit, the low voltage side is connected to the
Cascaded H-bridge inverter in series as well as in parallel
connection and the high voltage side is associated with the
transmission line system. Since for the voltage
compensation circuit the 1:1 transformer arrangement is
vital as mentioned before, thus the output of the cascaded
H-bridge inverter output is networked to one side of the
linear type transformer and then it is associated to the
transmission line system as shown in fig.13.
Fig-11: Output of Balanced DC link voltage

In order to compensate the losses that are associated with
the transmission line parameters the RL series branch is
connected in every output terminal before it is in contact
with the network.
Fig-12: Transformed two phase quantity (direct and
quadrature) output
The challenge that is mentioned before is the DC link
voltage maintenance, here the Cascaded H-Bridge
Multilevel Inverter 1 in both shunt and series level is
maintained near 241V and the Cascaded H-Bridge
Multilevel Inverter 1 in both shunt and series level is
maintained near 649V and total of around 1000V is
maintained as shown in fig.12. Thus the total DC link
voltage will be the summation of individual DC link
voltages across the circuitry to ensure the proper and
smooth operation of the system as shown in fig.11.
Fig-13:H-bridge inverter output a)voltage b)current
Fig-14: Voltage compensation with linear transformer
connection
Fig-15: Transformer-less uncompensated output
waveforms of grid voltage and current
Fig-16: Transformer-less compensated output waveforms
of grid voltage and current
4. CONCLUSION:
Current and voltage is compensated by the shunt and
series connected H-bridge cascaded multilevel inverter
circuitry, with-out using transformer. The connection done
is the face to face connection which is different in
arrangement of the series and shunt converters when

compared to the back to back connection. A three level
inverter output voltage is achieved by the specific
switching control strategy for series and shunt connected
inverter. Reactive and harmonic load analysis are applied
and compensation in voltage and current is achieved
during this conditions. The paramount task of balancing
the DC link voltage is achieved till the end of the operation.
Eliminating the disadvantages of the transformer
dependentmodule is the main idea behind, but it should be
realized only in a prototype model by its less size, reduced
weight and reduced cost. From the MATrix LABoratory
Simulink point the advantages is compared with the circuit
consist of transformer by its reduced total harmonic
distortion.
REFERENCES:
1) ’Cascaded Two-Level Inverter-Based Multilevel
STATCOM for High-Power Applications’ N. N. V.
Surendra Babu and B. G. Fernandes, IEEE transactions
on power delivery, 2014.
2) ‘Transformer-Less Unified Power-Flow Controller
Using the Cascade Multilevel Inverter’, Fang Zheng
Peng, Yang Liu, Shuitao Yang, Shao Zhang, Deepak
Gunasekaran, and Ujjwal Karki, IEEE transactions on
power electronics, 2016.
3) ‘Modulation and Control of Transformer-less UPFC’
Shuitao Yang, Yang Liu, Xiaorui Wang, Deepak
Gunasekaran, Ujjwal Karki, and Fang Z. Peng,IEEE
transactions on power electronics, 2016
4) ‘Transformer-less UPFC Using Multilevel
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International Journal Of Engineering And Computer
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IRJET- Series and Shunt Compensation in UPFC using Cascaded Multilevel Inverter- A Transformerless Approach

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