47 CHAPTER 4 STATCOM BASED LOAD VOLTAGE STABILITY OF SEIG 4.1 INTRODUCTION STATCOM is a voltage-source converter based device, which converts a DC input voltage into an AC output voltage in order to compensate the active and reactive needs of the system. STATCOM has better characteristics than SVC; when the system voltage drops sufficiently to force the STATCOM output to its ceiling, its maximum reactive power output will not be affected by the voltage magnitude. Therefore, it exhibits constant current characteristics when the voltage is low under the limit. Srinivas.P and Devakumar.M.L.S (2010) dealt with the Optimization of Power Factor and Energy Management in Wind Energy Station. PraneshRao, M. L. Crow, and Zhiping Yang (2000) dealt with the STATCOM Control for Power System Voltage Control Applications. P. W. Lehn, and M. R. Iravani (1998) dealt with the Experimental evaluation of STATCOM closed loop dynamics.Raimondset al (2011) dealt with the static synchronous compensator for reactive power compensation under distorted mains voltage conditions. Sidhartha Panda and PadhyN.P (2007) dealt with the power electronics based FACTS controller for stability improvement of a wind energy embedded distribution system. Kuang Li et al (2007) dealt with the strategies and operating point optimization of STATCOM control for voltage unbalance mitigation in three-phase three-wire systems. Jianye Chen and et al. (2006) dealt with the analysis and implementation of thyristor based STATCOM. Zhiping,and et al. (2000) dealt with the improved STATCOM model for power flow analysis. Shaheen H.I, RashedG.
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47
CHAPTER 4
STATCOM BASED LOAD VOLTAGE STABILITY OF SEIG
4.1 INTRODUCTION
STATCOM is a voltage-source converter based device, which converts a
DC input voltage into an AC output voltage in order to compensate the active and
reactive needs of the system. STATCOM has better characteristics than SVC; when
the system voltage drops sufficiently to force the STATCOM output to its ceiling,
its maximum reactive power output will not be affected by the voltage magnitude.
Therefore, it exhibits constant current characteristics when the voltage is low under
the limit.
Srinivas.P and Devakumar.M.L.S (2010) dealt with the Optimization of
Power Factor and Energy Management in Wind Energy Station. PraneshRao, M. L.
Crow, and Zhiping Yang (2000) dealt with the STATCOM Control for Power
System Voltage Control Applications. P. W. Lehn, and M. R. Iravani (1998) dealt
with the Experimental evaluation of STATCOM closed loop dynamics.Raimondset
al (2011) dealt with the static synchronous compensator for reactive power
compensation under distorted mains voltage conditions. Sidhartha Panda and
PadhyN.P (2007) dealt with the power electronics based FACTS controller for
stability improvement of a wind energy embedded distribution system.
Kuang Li et al (2007) dealt with the strategies and operating point
optimization of STATCOM control for voltage unbalance mitigation in three-phase
three-wire systems. Jianye Chen and et al. (2006) dealt with the analysis and
implementation of thyristor based STATCOM. Zhiping,and et al. (2000) dealt with
the improved STATCOM model for power flow analysis. Shaheen H.I, RashedG.
I.andChengS. J (2008) d
static synchronous compen
4.2 PROBLEM STA
An Approach to m
based STATCOM Contr
steady state performance
(VSC) by which the stator
proposed.
In this approach th
excitation reference frame
of q-axis current on the
controlled by d-q equivale
axis current of SEIG is co
The proposed STATCOM
supplies the required react
4.3 BASIC MODEL
Figu
dealt with the nonlinear optimal predictive
pensator (STATCOM).
TATEMENT
maintain the load voltage using d-q equivalent
ntroller and basic STATCOM Controller is
e of STATCOM based on six pulse voltage so
tor flux oriented vector control of terminal volta
the total flux is aligned to the d-axis of the sta
me. A decoupling signal is also generated to ca
e d-axis flux or total flux. The load voltage o
alent model using STATCOM controller. The tr
compared with the reference voltage of VSC o
OM eliminates the harmonics, provides load
active power to the load and the generator.
L OF A STATCOM
ure 4.1 Basic Model of the STATCOM
48
ve controller for
t model of SEIG
s proposed. The
source converter
ltage for SEIG is
stator flux in the
cancel the effect
e of the SEIG is
transformed d-q
of STATCOM.
d balancing and
4.4 BASIC STATCO
Figure 4
4.5 STATCOM BAS
A static synchrono
alternating current elect
electronics voltage-source
AC power to an electrici
provide active AC power
STATCOM is installed to
and often poor voltage reg
load voltage stability.
COM CONTROL SCHEME
4.2: Basic STATCOM Control Scheme
ASED LOAD VOLTAGE STABILITY O
nous compensator (STATCOM) is a regulating
ctricity transmission networks. It is based
ce converter and can act as either a source or s
icity network. If connected to a source of pow
er. It is a member of the FACTS family of dev
to support electricity networks that have a poo
regulation. Hence, the STATCOM is proposed
49
OF SEIG
g device used on
ed on a power
r sink of reactive
ower it can also
evices Usually a
oor power factor
ed to enhance the
50
4.6 DESIGN OF BASIC STATCOM CONTROLLER FOR THE
LOAD VOLTAGE STABILITY OF SEIG
To enhance the load voltage stability, the STATCOM is proposed as the
active VAR supporter. Figure illustrates the STATCOM compensated load voltage
stability system. Even though, the primary purpose of the STATCOM is to support
the load voltage by injecting or absorbing reactive power, it is also capable of
improving the voltage stability. This thesis investigates the performance of
STATCOM based on six pulse voltage sourced converter.
Figure 4.3: STATCOM Controller
STATCOM
SEIG
Load
51
4.7 STATCOM CONTROLLER TO ENHANCE THE TERMINAL
VOLTAGE STABILITY USING D-Q EQUIVALENT MODEL
Figure 4.4: STATCOM controller to enhance the terminal voltage stability using
D-Q equivalent model
52
In general, electrical loads are linear in nature. However, due to the
extensive use of solid-state controllers in different appliances, they draw harmonic
current from AC sources and behave as non-linear loads. Adjustable speed drives
used in pumps, compressors, air conditioner and other domestic appliances such as
TV’s, computers, SMPS and UPS consist of either three-phase or single-phase
rectifiers at the front-end. These non-linear loads draw non-sinusoidal currents from
the generating system, therefore injecting harmonics into the system. An SEIG is an
isolated system, which is small in size and the injected harmonics pollute the
generated voltage.
A dynamic model of an SEIG-STATCOM system with the ability to
simulate varying loads has been developed using a stationary d-q axes reference
frame. This enables to predict the behavior of the system under transient conditions.
The simulated results show that by using a STATCOM based voltage regulator the
SEIG terminal voltage can be maintained constant and free from harmonics under
linear and nonlinear loads.
4.8 SYSTEM CONFIGURATION AND CONTROL SCHEME
The schematic diagram of an SEIG with excitation capacitor, STATCOM,
load and control scheme is shown in Fig. Excitation capacitors are selected such that
the SEIG generates rated voltage at rated speed under no load. The additional demand
for reactive power is fulfilled using the STATCOM under varying loads. The
STATCOM acts as a source of lagging or leading current to maintain the constant
terminal voltage despite variations in load. The STATCOM consists of a three-phase
IGBT based current controlled voltage source inverter, a DC bus capacitor and AC
inductors. The output of the inverter is connected through the AC filtering inductor to
the SEIG terminals. The DC bus capacitor is used as an energy storage device and
provides the self-supporting DC bus of the STATCOM.
53
The control technique is used to regulate the terminal voltage of the SEIG is
based on the generation of source currents. (They have two components, in-phase and
quadrature, with AC voltage.) The in-phase unit vectors (ua, ub and uc) are three-phase
sinusoidal functions, computed by dividing the AC voltages va, vb and vc by their
amplitude Vt. Another set of quadrature unit vectors (wa, wb and wc) are sinusoidal
functions obtained from in-phase vectors (ua, ub and uc). To regulate the AC terminal
voltage (Vt), it is sensed and compared with the reference voltage. The voltage error is
processed in the PI controller. The output of the PI controller (Ismq*) for the AC
voltage control loop determines the amplitude of the reactive current to be generated
by the STATCOM. Multiplication of quadrature unit vectors (wa, wb and wc) with the
output of the PI based AC voltage controller (Ismq*) yields the quadrature component
of the reference source currents (isaq*, isbq* and iscq*). To provide a self-supporting DC
bus for STATCOM, its DC bus voltage is sensed and compared with the DC
reference voltage. The error voltage is processed in another PI controller. The output
of the PI controller (Ismd*) determines the amplitude of the active current.
Multiplication of in-phase unit vectors (ua, ub and uc) with the output of the PI
controller (Ismd*) yields the in-phase component of the reference source currents (isad*,
isbd*and iscd*). The instantaneous sum of quadrature and in-phase components gives
the reference source currents (isa*, isb* and isc*), which are compared with the sensed
line current (isa, isb and isc). These current error signals are amplified and compared
with the triangular carrier wave. If the amplified current error signal is equal to or
greater than the triangular carrier wave, the lower device of the inverter phase is
turned on and the upper device turned off. If the amplified current error signal is equal
to or less than the triangular carrier wave the lower device of the inverter phase is
turned off and the upper device turned on. A non-linear load draws non-sinusoidal
currents which causes harmonics to be injected into the generating system. Under
unbalanced load conditions, SEIG currents may be unbalanced which may cause the
machine to be derated. STATCOM is able to filter out the harmonics and balance the
unbalanced load resulting in balanced and sinusoidal currents and voltages in the
generator.
54
4.9 MODELING OF CONTROL SCHEME OF STATCOM
Different components of the SEIG-STATCOM system shown in Fig. 1 are
modeled as follows. From the three-phase voltages at the SEIG terminals (va, vb and
vc), their amplitude (Vt) is computed as:
Vt={(2/3)(Va2+Vb
2+Vc
2) (4.1)
It (Vt) is filtered to eliminate ripples if there are any present.
The unit vector in phase with va, vb and vc are derived as:
ua=va/Vt; ub=vb/Vt; uc=vc/Vt (4.2)
The unit vectors in quadrature with va, vb and vc may be derived using a quadrature
transformation of the in-phase unit vectors ua, ub and uc as:
Wa=-ub/ 3+uc/ 3 (4.3)
Wb= 3ua+(ub-uc)/2 3 (4.4)
Wc=- 3ua+(ub-uc) )/2 3 (4.5)
Figure 4.5: Simulation model for the system with STATCOM
55
4.10 QUADRATURE COMPONENT OF REFERENCE SOURCE
CURRENTS
The AC voltage error Ver(n) at the nth
sampling instant is:
Ver(n) = Vtref(n)– Vt(n) (4.6)
Where Vtref(n) is the amplitude of the reference AC terminal voltage and Vt(n) is the
amplitude of the sensed three-phase AC voltage at the SEIG terminals at the nth
instant. The output of the PI controller (I*smq(n)) for maintaining constant AC terminal
voltage at the nth sampling instant is expressed as: