Proc. Int. Conf. on Control System and Power Electronics, CSPE
Improving Critical Clearing Time of Grid
Connected Squirrel Cage Induction Generator based
Wind Generation System using D-STATCOM
Aarthi Shanmugam and Kalyan Kumar Boddeti, Member, IEEE
Department of Electrical Engineering,Indian Institute of Technology Madras, Chennai,
India.
aarthishanmukam@gmail.com, bkalyan@ee.iitm.ac.in
Abstract—The impact of the wind generation on the power systems is no
longer negligible if high penetration levels are going tobe reached. The
regulatory standards require the wind generationsystem to ride-through
disturbances such as faults and support the grid during such events. This fault
ride-through capability is determined by its corresponding critical clearing time
(CCT).In this work a method is proposed to enhance the fault- ridethrough
capability of squirrel cage induction generator basedwind generation system
using Distributed Static Compensator (D-STATCOM). The simulation results
are presented.
Index Terms—Squirrel Cage Induction Generator (SCIG), Fault-Ride Through
(FRT) Capability, Critical Clearing Time (CCT), Critical Speed, Distributed
Static Compensator (DSTATCOM).
Nomenclature
Induction Generator
vds , vqs , ids , iqs Stator voltages and currents in dq0 reference frame
 ds , qs ,  dr , qr Stator and rotor flux linkages in dq0 reference frame
rs rr , xs xr Stator and Rotor winding resistance and reactance
,
,
xm Mutual reactance between stator and rotor winding , xs Transient reactances
Eds , Eqs Voltage behind transient reactance in dq0reference frame , s Slip
Te Electromagnetic torque developed , Tm Mechanical torque System
xtf Transformer reactance , xtr Transmission line reactance
POC Point of connection of SCIG with the grid
1 Introduction
Wind power, as one of the most popular renewable energy due to its environmental
impact and availability, has experienced a rapid growth in the recent years. Newer
© Elsevier, 2012
545
connection conditions usually require wind generators to participate in real power
control, to ride-through network faults and contribute to system stability after fault
clearance [1]. Although there is a growing interest in usage of doubly fed induction
generators, squirrel cage induction generators are still in use due to their simplicity,
robustness, low cost, and low maintenance.
During the occurrence of short circuits in the network, induction generator rotor
tends to accelerate due to the abrupt reduction in the electrical torque. Whether the
generator will become unstable or not will depend on the fault clearance time [1].
More specifically, it will depend on whether the fault will be eliminated before the
generator rotor speed reaches a maximum critical speed [2]. The grid with large wind
farms connected to it will have adverse effect when a wind generator is disconnected
immediately after a fault. Therefore, it is recommended that the wind operators
remain connected to the grid during thefault for a specified amount of time which is
termed as ”ride through” time. To achieve this, the wind operators try to make their
generators critical clearing time equal to or lesser than the ride through time.
Distributed Static Compensator (D-STATCOM) has been used in this paper to
improve the CCT of the SCIG based wind generation system.
2 Calculation of Critical Speed and Critical Clearing Time (CCT)
In order to calculate the critical speed and CCT, a three phase fault at the mid-point of
transmission line 2, of the system shown in Fig. 1, is considered. The torque-slip
characteristics of induction generator for pre-fault and during fault are shown in Fig.
2. It is assumed that after the fault is cleared, system returns to its pre-fault condition.
Fig. 1 & 2 : Single line diagram of SCIG based wind generation system connected to grid &
Pre-fault and during fault Torque-Slip characteristics of SCIG connected to a grid
In steady state, the electromagnetic torque T e is equal to the mechanical torque Tm at
two different slips s0 and scrit. The operating point a, corresponding to s0 is the stable
operating point whereas the operating point a0, corresponding to slip scrit is the
unstable operating point. The machine is operating initially at the stable operating
point a corresponding to slip s0.[3] Immediately after the occurrence of the fault, T e
decreases due to change in system topology. Now, the machine is operating at point c
as shown in Fig. 2. As Tm>Te, from machine motion equation, there is a net
accelerating torque and the slip gradually increases. [4] The system will be stable only
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if the fault is cleared before the slip reaches scrit, that is at point d which corresponds
to fault clearing slip sfl. Once the fault is cleared, the system shifts from point d on the
Torque-Slip characteristic curve during fault to point b in the pre-fault Torque-Slip
characteristic curve. It oscillates at point b and the peak of the oscillation should not
cross a0, then the machine will settle down at its stable equilibrium point
corresponding to point a. Any increase in slip beyond scrit, corresponding to a0 will
make the system unstable. The maximum speed  crit the generator can reach can be
obtained from the corresponding slip (2).
(1)
0  (1  s0 ) s
crit  (1  scrit )s
(2)
The fault should be cleared before the slip of the Induction Generator reaches the
critical slip, scrit. The exact slip at which the fault should be cleared, sfc, can be found
out by trail and error through repeated time domain simulations of IG dynamic
equations .
3 Modelling of D-STATCOM
Distributed Static Compensator (D-STATCOM) is used in this paper to improve the
CCT of the system [5] & [6]. The modelling of D-STATCOM is done by converting
single machine infinite bus (SMIB) into single machine finite bus(SMFB) [7], as
shown in Fig. 6.
Fig.3: Steady state equivalent of SMIB system with D-STATCOM
The shunt injected current Ish can be resolved into two components: one is in phase
(Ip) with Vm and another in quadrature (Iq) with Vm. Thus Ish can be expressed as
I sh  ( I p  jI q )e j m
(3)
Where,  m is the angle of the dependent voltage source of the SMFB system as
shown in Fig. 6. With the injected current of D-STATCOM defined in (3), the voltage
Vm and angle  m, of the SMFB system are given as
( mEs cos   nVb ) sin  m  ( mEs sin  ) cos  m  I p  0
(4)
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Vm 
mEs cos(   m )  nVb cos  m  I q
m n p
(5)
Where,
1
1
1
;n 
;p
X1
X2
X3
X 1  xs  xtf
m
xtr
during pre- fault and post- fault condition
2
 xtr during fault
X2 
X 3   during pre – fault and post- fault condition
x
 tr during fault
2
4 Control Strategy for D-STATCOM
The control scheme of D-STATCOM consists of two control loops, one for the real
power flow tracking and another for the reactive power flow tracking at the Point of
Coupling (POC) [8]. This is done by the appropriate selection of injecting current
(Ish ) , which can be decomposed into real and imaginary quantities as,
Iˆsh  ( I p  jI q )e j m
Ssh  Vˆm ( I sh )*
(6)
Fig. 4 & 5 :Phasor diagram of SMIB system with D-STATCOM using Q-control & PQ-control
There are two control strategies for D-STATCOM according to the values of DSTATCOM parameters Ip and Iq as shown in Fig 4 & 5.
Q Control: In this control strategy, only the reactive power is being controlled using
the quadrature component of the injected current, Iq.
PQ Control : In this control, both the real and the reactive power are being
controlled. Ip and Iq are varied to track the real and reactive power flow references at
POC.
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5 Simulation Results
A 3-phase fault is applied at the mid-point of transmission line 2. The sfc and tfc
obtained for different D-STATCOM control strategies are shown in Table I.
Table 1. Comparison of CCT for Different D-STATOM Control Strategies
Without D-STATOM
With D-STATOM
a. Q control
b. PQ control
t fc(ms)
255
sfc(pu)
0.2550
375
435
0.2569
0.2578
Fig. 6, 7, 8 & 9: Variation of Slip, Real power at POC, Reactive power at POC & voltage at
POC for different control modes
Without D-STATCOM: The critical slip of the induction generator as calculated
from(2), is 0.2763. The fault clearing time as obtained by trial and error through
repeated time domain simulations, corresponding to the fault clearing slip 0.2550, is
225ms. It is observed that the fault clearing slip sfc is less compared to the critical slip
scrit. At the instant of fault, Te decreases therefore real power generation decreases,
speed of the induction generator increases drastically in super-synchronous region as
can be observed from Fig.6-7. It can be observed from Fig. 8-9 that the reactive
power supply from the grid is reduced because of the fault on one transmission line
leading to the reduction of terminal voltage of the induction generator.
case a: The value of Ip and Iqwill vary according to the PI controller. In this control,
only Iq is allowed to vary and Ip is kept as zero. The critical clearing time with DSTATCOM in the system, and operating in Q control mode, is obtained as 375ms,
which is 150ms above the CCT without D-STATCOM.
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case b: In the PQ control strategy the value of Ip and Iqare varied in accordance with
the reference real and reactive power. The tcrit obtained as in case b is 210ms higher,
as compared to without D-STATCOM case.
From Figs. 8-9, it can be seen that for case a, the CCT is less compared to case b.
For case b, the peak over shoot and under shoot in power oscillations is more
compared to the Q control mode of D-STATCOM operation, that is case a. Hence, DSTATCOM in PQ control mode is more suitable for improving CCT as compared to
Q control mode of operation.
6 Conclusion
In this paper, DSTATCOM is operated with different control a strategy that is, Q
control and PQ control to improve the CCT of the system. Even though the settling
time is more for PQ control mode as compared to Q control mode, the CCT and the
transient performance of the system is improved in former case. It can be concluded
that PQ control strategy of D-STATCOM produces a better and optimum
improvement in CCT.
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