106 CHAPTER 5 FAULT AND HARMONIC ANALYSIS USING PV ARRAY BASED STATCOM 5.1 INTRODUCTION Inherent characteristics of renewable energy resources cause technical issues not encountered with conventional thermal, hydro or nuclear power. These issues make operation of the renewable energy resources and their integration with the grid system a technical challenge. The rapid development of the renewable energy power industry, together with the rising challenges, has drawn many of the world’s leading professional associations and organizations into this fast growing field. Among all the rising challenges, one important issue is how to integrate renewable energy sources with the grid through power electronic converters as well as associated control system design. Although traditional approaches have been developed for power converter control of renewable energy systems, there is a critical need to develop new and improved power converter control technologies for many reasons such as 1) The existing power converter control technologies in grid integrated renewable energy generation systems do not perform well in some cases. 2) Unbalance and high harmonic distortion have been found in renewable energy conversion systems, which not only affect the grid system but also affect the renewable energy sources. 3) The existing power converter control mechanism has an inherent deficiency, which can cause malfunctions of the system, such as
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CHAPTER 5
FAULT AND HARMONIC ANALYSIS USING PV ARRAY
BASED STATCOM
5.1 INTRODUCTION
Inherent characteristics of renewable energy resources cause technical
issues not encountered with conventional thermal, hydro or nuclear power.
These issues make operation of the renewable energy resources and their
integration with the grid system a technical challenge. The rapid development
of the renewable energy power industry, together with the rising challenges,
has drawn many of the world’s leading professional associations and
organizations into this fast growing field.
Among all the rising challenges, one important issue is how to
integrate renewable energy sources with the grid through power electronic
converters as well as associated control system design. Although traditional
approaches have been developed for power converter control of renewable
energy systems, there is a critical need to develop new and improved power
converter control technologies for many reasons such as
1) The existing power converter control technologies in grid integrated
renewable energy generation systems do not perform well in some
cases.
2) Unbalance and high harmonic distortion have been found in renewable
energy conversion systems, which not only affect the grid system but
also affect the renewable energy sources.
3) The existing power converter control mechanism has an inherent
deficiency, which can cause malfunctions of the system, such as
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abnormal DC capacitor voltage, active and reactive power, or output
currents. These malfunctions may make the gird integration of the
renewable energy sources unstable and may even cause power system
trips [69], [72], [85].
This chapter investigates the effectiveness of PV based STATCOM,
explained in the previous chapter, in increasing the system stability during
fault condition and reducing Total Harmonic Distortion (THD). The proposed
PV based STATCOM is tested in a Distributed Generation system consisting
of a grid interconnected wind farm and solar farm.
5.2 DOUBLY FED INDUCTION GENERATOR (DFIG)
DFIG is an abbreviation for Doubly Fed Induction Generator, a
generating principle widely used in wind turbines. It is based on an induction
generator with a multiphase wound rotor and a multiphase slip ring assembly
with brushes for access to the rotor windings.
A doubly fed induction machine is a wound-rotor doubly-fed electric
machine and has several advantages over a conventional induction machine in
wind power applications:
• First, as the rotor circuit is controlled by a power electronics converter,
the induction generator is able to both import and export reactive
power. This has important consequences for power system stability and
allows the machine to support the grid during severe voltage
disturbances (low voltage ride through, LVRT).
• Second, the control of the rotor voltages and currents enables the
induction machine to remain synchronized with the grid while the wind
turbine speed varies. A variable speed wind turbine utilizes the
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available wind resource more efficiently than a fixed speed wind
turbine, especially during light wind conditions.
• Third, the cost of the converter is low when compared with other
variable speed solutions because only a fraction of the mechanical
power, typically 25-30 %, is fed to the grid through the converter, the
rest being fed to grid directly from the stator. The efficiency of the
DFIG is very good for the same reason.
Figure 5.1 The wind turbine and the doubly-fed induction generator
5.2.1 DFIG Construction and Working Principle
The principle of the DFIG in figure 5.1 is that rotor windings are
connected to the grid via slip rings and back-to-back voltage source converter
that controls both the rotor and the grid currents. Thus rotor frequency can
freely differ from the grid frequency (50 or 60 Hz). By using the converter to
control the rotor currents, it is possible to adjust the active and reactive power
fed to the grid from the stator independently of the generator's turning speed.
The control principle used is either the two-axis current vector control or
direct torque control (DTC). DTC has turned out to have better stability than
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current vector control especially when high reactive currents are required
from the generator.
The doubly-fed generator rotors are typically wound with 2 to 3 times
the number of turns of the stator. This means that the rotor voltages will be
higher and currents respectively lower. Thus in the typical ± 30 % operational
speed range around the synchronous speed, the rated current of the converter
is accordingly lower which leads to a lower cost of the converter. The
drawback is that controlled operation outside the operational speed range is
impossible because of the higher than rated rotor voltage. Further, the voltage
transients due to the grid disturbances (three- and two-phase voltage dips,
especially) will also be magnified. In order to prevent high rotor voltages and
high currents resulting from these voltages from destroying the IGBTs and
diodes of the converter, a protection circuit (called crowbar) is used.
The crowbar in figure 5.1 will short-circuit the rotor windings through
a small resistance when excessive currents or voltages are detected. In order
to be able to continue the operation as quickly as possible an active crowbar
has to be used. The active crowbar can remove the rotor short in a controlled
way and thus the rotor side converter can be started only after 20-60 ms from
the start of the grid disturbance. Thus, it is possible to generate reactive
current to the grid during the rest of the voltage dip and in this way, it helps
the grid to recover from the fault.
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5.2.2 POWER FLOW IN DFIG
Figure 5.2 Power flow in DFIG
Pm Mechanical power captured by the wind turbine and transmitted to the
rotor
Ps Stator electrical power output
Pr Rotor electrical power output
Pgc Cgrid electrical power output
Qs Stator reactive power output
Qr Rotor reactive power output
Qgc Cgrid reactive power output
Tm Mechanical torque applied to rotor
Tem Electromagnetic torque applied to the rotor by the generator
ωr Rotational speed of rotor
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ωs Rotational speed of the magnetic flux in the air-gap of the generator,
this speed is named synchronous speed. It is proportional to the
frequency of the grid voltage and to the number of generator poles.
J Combined rotor and wind turbine inertia coefficient
The mechanical power and the stator electric power output are
computed as follows: [87]
m m rP T ω= (5.1)
s em sP T ω= (5.2)
For a loss less generator the mechanical equation is:
rm em
dJ T Tdtω
= − (5.3)
Generally the absolute value of slip is much lower than 1 and
consequently, Pr is only a fraction of Ps. Since Tm is positive for power
generation and since ωs is positive and constant for a constant frequency grid
voltage, the sign of Pr is a function of the slip sign. Pr is positive for negative
slip (speed greater than synchronous speed) and it is negative for positive slip
(speed lower than synchronous speed). For super-synchronous speed
operation, Pr is transmitted to DC bus capacitor and tends to raise the DC
voltage. For sub-synchronous speed operation, Pr is taken out of DC bus
capacitor and tends to decrease the DC voltage. Cgrid is used to generate or
absorb the power Pgc in order to keep the DC voltage constant. In steady-state
for a loss less AC/DC/AC converter Pgc is equal to Pr and the speed of the
wind turbine is determined by the power Pr absorbed or generated by Crotor.
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The phase-sequence of the AC voltage generated by Crotor is positive
for sub-synchronous speed and negative for super-synchronous speed. The
frequency of this voltage is equal to the product of the grid frequency and the
absolute value of the slip.
Crotor and Cgrid have the capability of generating or absorbing reactive
power and could be used to control the reactive power or the voltage at the
grid terminals.
5.3 SIMULATION OF WIND ENERGY SYSTEM
Wind turbines using a doubly-fed induction generator (DFIG) consist
of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM
converter modeled by voltage sources. The stator winding is connected
directly to the 50 Hz grid while the rotor is fed at variable frequency through
the AC/DC/AC converter. The DFIG technology allows extracting maximum
energy from the wind for low wind speeds by optimizing the turbine speed,
while minimizing mechanical stresses on the turbine during gusts of wind.
Figure 5.3 grid voltage
Figure 5.3 shows the grid voltage during normal conditions. The value
of the grid voltage is maintained at 1 p.u. The initial transients in the
waveform are due to the synchronization of DFIG with the Grid.
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Figure 5.4 grid current
The figure 5.4 shows the grid current during the normal condition. A
constant load is connected and hence the grid current is maintained a constant
value.
Figure 5.5 rotor voltages
Figure 5.6 rotor currents
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Figure 5.7 stator voltages
The rotor voltage, rotor current and the stator voltages of the DFIG is
shown in the figure 5.5, 5.6 and 5.7 respectively.
The wind speed is set to 5m/s initially and then increased to 14m/s at
time t=1 second. The reactive power produced by the wind turbine is
regulated at 0 Mvar as in figure 5.8.
Figure 5.8 P, Q in pu
5.4 PV ARRAY BASED STATCOM SIMULATION
The PV based STATCOM modelled in chapter 3 as shown in figure
5.9, is used here for validating its ability in increasing the system stability
during fault condition.
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Figure 5.9 PV array based STATCOM
5.5 SYSTEM DESCRIPTION
The simulation results carried out using MATLAB SIMULINK
software. The actual system is created using the model available in the
MATLAB Simulink model library and the system performance has been
anlaysed and verified.
The following system is considered as the test system. A 9 MW wind
farm consisting of six 1.5 MW wind turbines connected to a 25 kV
distribution system exports power to a 120 kV grid through a 30 km, 25 kV
feeder as in figure 5.10.
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Figure 5.10 Test System
The ratings of the system considered are given below
Grid voltage: 120KV
Inductance MVA: 2500MVA
Transformer 1 Rating
Primary voltage: 120KV
Secondary Voltage: 25KV
MVA Rating: 47MVA
Transformer 2 Rating
Primary Voltage: 25KV
Secondary Voltage: 575KV
MVA Rating: 6*1.75MVA
Wind Turbine
No of Turbines: 6
Stator RMS Voltage: 575KV
Rotor RMS Voltage: 1975V
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Double tuned filter
Harmonics Tuned: 11th and 13th.
5.6 FAULT ANALYSIS USING PV BASED STATCOM
The integrated system is tested by implementing a single phase to
ground fault in phase “A” at 25kV bus. It is then tested whether the PV based
STATCOM is able to provide compensation for the system at that bus to
which it is connected.
This analysis is carried out with the help of the following three cases.
Case 1: Voltage and current values at PCC during normal condition
Case 2: Voltage and current values at PCC after the occurrence of fault.
Case 3: Voltage and current values at PCC after the action of PV based
STATCOM
Case 4: Fault ride through capability of PV based STATCOM
5.6.1 Case1:Under normal condition the voltage and current profile in the
25kv bus is as shown in figure 5.11. Here after an initial fluctuation for
about 0.2 sec the voltages and currents profile is well within the ±5%
pu value criteria. The reason for the fluctuation is due to change over
of speed from 5m/s to 14m/s. The simulation is conducted for duration
of 2 sec. Also the zoomed view of the voltages and currents is shown
in figure 5.12.
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Figure 5.11 voltage and current at 25kv bus under normal condition
Figure 5.12 Zoomed view of voltages and currents profile at 25 kV bus
under normal condition
5.6.2 Case2:When the system is under normal condition single phase short
circuit fault is created on the “phase A” line at the 25kv bus at the
instant of 0.8S. The voltage decreases and current increases after the
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instant at 0.8S as shown in the figure 5.13 and propagates till the
complete cycle of the simulation.
Figure 5.13 voltages and current profile at the 25kv bus after fault
5.6.3 Case3: In this case the fault is implemented at 25kv bus at about 0.8
sec and is allowed to propagate. At 1 sec PV array STATCOM is
brought into action and after 1.2 sec system is compensated and system
is restored to normal condition after 1.2 sec as shown in figure 5.14.
Figure 5.14 Voltages and Current profile at 25kv bus after STATCOM action
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5.6.4 Fault Ride Through (FRT) Capability: In order to evaluate the Fault
ride through capability of PV based STATCOM following unbalanced
faults, a single-phase short-circuit (phase a) is simulated in the test
system shown in figure 5.10 at t=1s and with a clearing time of 500
ms.
Figure 5.14 a Voltage at PCC without PV based STATCOM
The voltage at the PCC is shown in the figure without PV based
STATCOM. As it can be observed from figure 5.14 a, the voltage at phase a,
becomes zero during the fault condition.
Figure 5.14 b Voltage at PCC with PV based STATCOM
The voltage at the PCC after connecting PV based STATCOM in
figure 5.14 b, shows the continuous supply of phase a even during the fault
duration of 1s to 1.5s.This is due to the reactive power injection by the PV
based STATCOM thereby incorporating the FRT capability for the system.
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The results of the three cases are summarized below.
Table 5.1 Tabulation of System Results
Condition Voltage profile Current profile
Case 1:
Under normal condition
Voltage is maintained
at 1 PU.
Current is maintained as
per system requirement
Case 2:
Fault occurs at 0.8sec
and sustains till the
complete simulation
Voltage profile finds a
dip below 1 PU.
Due to short circuit,
current increases
enormously and exceeds
system limit
Case 3:
Fault occurs at 0.8sec,
STATCOM acts at 1sec
Voltage falls below
1PU and after
inclusion of
STATCOM the profile
is restored at 1.2 sec
within limits
Current increases
drastically and exceeds
the system limit. And
then restored to normal
condition after inclusion
of STATCOM at 1.2 sec
5.7 HARMONIC ANALYSIS USING PV BASED STATCOM
The nonlinear loads draw non-sinusoidal currents from the utility and
contribute to numerous power systems problems. The currents drawn from the
grid are rich in harmonics with the order of 6k ±1, that is, 5, 7, 11, 13, etc.
These harmonics currents result in lower power factor, overheating and
electromagnetic interference (EMI). In recent past, there has been
considerable interest in the development and applications of active filters due
to the increasing concern over power quality at both distribution and
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consumer levels. In order to address this matter, the power electronics circuits
incorporating power switching devices and passive energy storage circuit
elements, such as inductors and capacitors which are known as active filter
are used [89].
Effects of Harmonics
The reason for which the harmonics have to be eliminated from the
power system is due to the following effects
• Failure of electrical / electronic components
• Overheating of neutral wires
• Transformer heating
• Failure of power factor correction capacitors
• Losses in power generation and transmission
• Noise coupling on telephone lines etc
5.7.1 Harmonic Analysis of DFIG for a Wind Energy Conversion
System
Doubly Fed Induction Generators (DFIGs) are widely used in wind
generation. The possibility of getting a constant frequency ac output from a
DFIG while driven by a variable speed prime mover improves the efficacy of
energy harvest from wind. Unlike a squirrel-cage induction generator, which
has its rotor short-circuited, a DFIG has its rotor terminals,which are
accessible.
The rotor of a DFIG is fed with a variable-frequency (ωr) and variable-
magnitude three-phase voltage. This ac voltage injected into the rotor circuit
will generate a flux and a stator voltage/current with a frequency ωr if the
rotor is standing still. When the rotor is rotating at a speed ωm, the net flux
linkage and the stator voltage/current will have a frequency ωs= ωr+ ωm.
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When the wind speed changes, the rotor speed ωm will change, and in order to
have the net flux linkage at a frequency 60 Hz, the rotor injection frequency