EE6009 unit 4

SRI KRISHNA KUMAR S
HTS953
AP/EEE

UNIT IV - ANALYSIS OF WIND AND PV SYSTEMS
Stand alone operation of fixed and variable speed wind energy conversion
systems and solar system- Grid connection Issues –Grid integrated PMSG,
SCIG Based WECS, Grid Integrated solar system
CO No. Course Outcomes
Knowledge
Level
C410.1 Discuss and analyze the various types of renewable energy sources K2
C410.2 Analyze the performance of IG,PMSG,SCIG AND DFIG K4
C410.3
Design different power converters namely AC to DC,DC to DC and Ac to
AC converters for renewable energy sources
K3
C410.4
Analyze various operating modes of wind electrical generators and solar
energy systems
K4
C410.5 Design various types of hybrid systems. K3
Course Outcomes: At the end of the course, the student will be able to:

Fixed speed systems are the simplest and most widely used arrangement.
They operate at constant (or nearly constant) speed [also called constant speed constant
frequency (CSCF) mode of operation].
This arrangement, in general, has simple and reliable construction of the electrical part while
the mechanical parts are subject to higher stresses and additional safety factors need to be
incorporated in the mechanical design.
This arrangement can use induction generator (IG) and the wound rotor synchronous
generator (SG) as the electric machine.
o The reasons for this popularity are mainly due to its simplicity,
high efficiency
low maintenance requirements.

The wind turbine is therefore effectively fixed speed. FSIGs do not have the capability of
independent control of active and reactive power, which is their main disadvantage
Their great advantage is their simple and robust construction, which leads to lower capital
cost. In contrast to other generator topologies, FSIGs offer no inherent means of torque
 This wind power system operates at a constant rotor speed regardless of the wind speed
variations.
 The rotor speed is controlled according to the grid frequency.
The electrical machine equipped with such wind turbines is SCIG.
 Sometimes a PMSG can be used.
 Fixed-speed WECSs have advantage of being simple, robust, and reliable with a low-cost
generator and easy control.
However, such wind power systems also have drawbacks due to limited control when wind
speed changes continuously.

With the increase in the size of turbine, the inherent problems of the constant speed systems
become more and more pronounced, especially in areas with relatively weak grids. To overcome
these problems, the trend in modern generator technology is toward variable-speed concepts.
o A variable-speed system keeps the generator torque constant and it is the generator speed
which changes. Variations in the incoming power are absorbed by rotor speed changes.

The variable-speed system therefore incorporates a generator control system that can operate
with variable speed. In this arrangement the variable-voltage variable frequency (VVVF) power
generated by the machine is converter to fixed-frequency fixed voltage power by the use of back
to back power converters.
The machine side converter supplies the lagging excitation to the machine while the line side
converter maintains unity power factor at grid interface and also regulates the dc link voltage
constant.
It can operate without gear box, with a good multi-pole design. This is an important objective
since gear box is a component that has a tendency to fail.
o The advantages of this scheme are that mechanical oscillations in the drive
train are absent as it is in fixed speed systems.

Compared to the squirrel-cage induction generator, the main difference that the
doubly- fed induction generator configuration provides is the access to the rotor
windings, thereby giving the possibility of impressing the rotor voltage.
With this arrangement, power can be extracted from or fed to the rotor circuit and the
generator can be magnetized from either the stator circuit or the rotor circuit
Basically two methods of speed control
rotor resistance control
back to back converter control

The main features of this configuration are listed below:
1) Reduced converter cost, as they have to be rated for slip power only (typically about 0.25
pu).
2) Converter on the rotor side enables both positive and negative slip power control through
control of rotor current in phase magnitude and frequency. This allows both sub
synchronous and super-synchronous operation.
3) DC link capacitor acts as a source of reactive power, which in a way can control power
factor on the stator side.
4) Line side converter has ability to work as active filter, apart from maintaining unity
power factor operation and regulating dc bus voltage.
5) Reduced cost and weight of inverter filter and EMI filters (to about 0.25pu of total
system power). Inverter harmonics represent a fraction of total system harmonics.
6) System efficiency is better, due to reduced losses in the converters.

Block Diagram of Standalone PV System

Standalone PV System
• These systems which use photovoltaics technology connected, and are
not connected to a utility grid.
• The standalone system is also known as offgrid system.
• These systems are suitable for small devices and equipments not close
to an electricity supply.

Electrical schematic of the grid-connected photovoltaic system.

Electrical component layout of the grid-connected wind power system.

Electrical schematic of the grid-connected variable speed wind power system.

Four conditions which must be satisﬁed before the synchronizing switch will permit the closure
are as follows
Synchronizing with Grid
• the frequency must be as close as possible with the grid frequency, preferably about one-third
of a hertz higher.
• the terminal voltage magnitude must match with that of the grid, preferably a few percent
higher.
• the phase sequence of the two three-phase voltages must be the same.
• the phase angle between the two voltages must be within 5 degrees.

Taking the wind power system as an example, the synchronizing process speciﬁcally runs as
follows:
1. With the synchronizing breaker open, the wind power generator is brought up to speed
using the machine in the motoring mode.
2. Change the machine into the generating mode, and adjust the controls such that the site
and grid voltages match to meet the,above requirements as close as possible.
3. The match is monitored by the synchroscope or three synchronizing lamps, one in each
phase. The voltage across the lamp in each phase is the difference between the renewable
site
voltage and the grid voltage at any instant. When the site and the grid voltages are exactly
equal in all three phases, all three lamps will be dark. However, it is not enough for the lamps
to be dark at any one instant. They must remain dark for a long time. This condition which
will be met only if the generator and the grid voltages have nearly the same frequency. If
not, one set of the two three-phase voltages will rotate faster relative to the other, and the
phase difference between the two voltages will light the lamps.
4. The synchronizing breaker is closed if the lamps remain dark for ¼ to ½
second.

Operating Limit
The link line connecting the renewable power site with the utility grid introduces the operating
limit in two ways, the voltage regulation and the stability limit.
In most cases, the line can be considered as an electrically short transmission line.
The ground capacitance and the ground leakage resistance are generally negligible and are
ignored.
The equivalent circuit of such a line, therefore, reduces to a series resistance R and reactance L.
Such an approximation is valid in lines up to 50 miles long.
The line carries power from the renewable site to the utility grid, or from the grid to the
renewable site to meet local peak demand.
There are two major effects of the transmission line impedance, one on the voltage regulation
and the other on the maximum power transfer capability of the link line.
Voltage Regulation

Block diagram of grid connected PMSG based wind turbine system

Plant Economy
Energy Delivery Factor

Plant Economy
Initial Capital Cost

Plant Economy
Availability and Maintenance

Plant Economy
Energy Cost Estimates
The principal decision-making parameter of an electrical power plant is the unit cost of
energy (UCE) per kWh delivered to the paying consumers. It takes into account all
economy factors discussed above, and is given by the following:

SRI KRISHNA KUMAR S
HTS953
AP/EEE
krishnakumar.rvs@gmail.com

EE6009 unit 4

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EE6009 unit 4