 Open Access
 Total Downloads : 449
 Authors : Sundaraiah Nayini, Ramakrishna Raghutu
 Paper ID : IJERTV3IS10203
 Volume & Issue : Volume 03, Issue 01 (January 2014)
 Published (First Online): 11012014
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Coordinated PSS and STATCOM Controller for Damping Low Frequency Oscillations in Power Systems
Sundaraiah Nayini1 Ramakrishna Raghutu2
Department of Electrical Engineering Department of Electrical Engineering Mallreddy en gineering college for women GMR inistitute of technology
Abstract: The low frequency oscillations have become the main problem for power system small signal stability. They restrict the steadystate power transfer limits, which therefore affects operational system economics and security. In this work, using Parameters of the classic PSS and STATCOM internal AC and DC voltage controllers and AC damping stabilizer is designed in order to damp the Low Frequency Oscillations (LFO). The STATCOM is used for adjusting the voltage in line and the dynamic effect of STATCOM is proposed. The study is performed on the linearized power system model with presence of STATCOM and PSS. The designing results are performed using nonlinear simulation. The results show that the design according to simultaneous optimization is an effective procedure for power system stability improvement.
Keywords: STATCOM, PSS, FACTS devices, Power System Stability.
I.INTRODUCTION
The low frequency oscillations have become the main problem for power system small signal stability. They restrict the steadystate power transfer limits, which therefore affects operational system economics and security. Using PSS create change in oscillation stability. To increase power system oscillation stability, the installation of supplementary excitation control, power system stabilizer (PSS), is a simple, effective and economical method [1].
These oscillations may sustain and grow to cause system separation if no adequate damping is available. Although PSSs provide supplementary feedback stabilizing signals, they suffer a drawback of being liable to cause great variations in the voltage profile and they may even result in leading power factor operation under severe disturbances.
The recent advances in power electronics have led to the development of the flexible alternating current transmission systems (FACTS) [2]. Generally, a potential motivation for the accelerated use of FACTS devices is the deregulation environment in contemporary utility business. Along with primary function of the FACTS devices, the real power flow can be regulated to mitigate the low frequency oscillations and enhance power system stability. This suggests that FACTS will find new applications as electric utilities merge and as the sale of bulk power between distant and ill interconnected partners become more wide spread.
Recently, several FACTS devices have been implemented and installed in practical power systems such as static VAR compensator (SVC), thyristor controlled series capacitor (TCSC), and thyristor controlled phase shifter (TCPS) [3].
The emergence of FACTS devices and in particular gate turnoff (GTO) thyristorbased STATCOM has enabled such
technology to be proposed as serious competitive alternatives to conventional SVC. From the power system dynamic stability viewpoint, the STATCOM provides better damping characteristics than the SVC as it is able to transiently exchange active power with the system [4].
The method of phase compensation [5] and damping torque analysis method [6] are conventional methods for design and control of power system stabilizers [7]. Also multivariable design method has been performed on coordination between internal AC and DC voltage controllers [8]. The coordination between the AC and DC voltage PI controllers was taken into consideration. However, the structural complexity of the presented multivariable PI controllers with different channels reduces their applicability. Moreover, the utilization of damping capability of the STATCOM has not been addressed. The STATCOM damping characteristics have been addressed in [9]. However, the coordination among the STATCOM damping controllers and AC and DC voltage PI controllers has not been investigated. The coordination among different Controllers has been taken into consideration

where different damping channels of the STATCOM have been assessed in terms of their respective effectiveness on electromechanical modes controllability. In this study, to improve power system dynamic stability and voltage regulation, coordination among internal AC and DC voltage controller of STATCOM and ACdamping stabilizer and also PSS are performed. The studies are performed on a single machine infinitebus power system. The linearized power system model is used for the studies [11].The parameters design is considered as an optimization problem and the genetic algorithm is used for searching optimized parameters [12].

POWER SYSTEM MODEL WITH STATCOM
Fig.1 shows a SMIB system equipped with a STATCOM. A single machine infinite bus (SMIB) system installed with STATCOM is considered for the analysis of stability.A simple STATCOM is incorporated which consists of a step down transformer (SDT) with a leakage reactance, three phase GTO based voltage source converters (VSCs), and a DC capacitor. The VSC generates a controllable ACvolt age source behind the leakage reactance. The voltage difference between the STATCOMbus AC voltage and produces active and reactive power exchange between the STATCOM and the power system, which can be controlled by adjusting the phase and the magnitude Vo.
v sin xLB cv
B x dc
cos
I sdt
tlq x x
x
LB tL
x
q
xL x x LB (1 LB )x
SDT SDT
Power System Linearized Dynamic Model
The nonlinear dynamic equations are linearized around a given operating point to have the linear model as given below
Pe K1 K2Eq 'KpdcVdc KpcC Kp KpcuE KpmTm
Fig.1. SMIB power system equipped with STATCOM
Eq ' K4 K3Eq 'KqdcVdc Kqcc

Kq KqeUE KqmTm


POWER SYSTEM NONLINEAR DYNAMIC MODEL WITH STATCOM
Vt K5 K6Eq 'KvdcVdc Kvc c
Kv

Kve UE KvmTm
V .dc K K E 'K V K
C K
K U

K T
During lowfrequency oscillations, the current induced in a damper winding is negligibly small hence the damper
7
Where
8 q 9 dc DC D
ce E
cm m
windings are ignored in system modelling. The natural oscillating frequency of the d and q armature windings of a
synchronous machine is extremely high; their Eigen modes
K1 to
K9 , K pDC , K pc , K p KqDC Kqc Kq KvDC Kvc Kv Kdc Kd
will not affect the low frequency oscillations hence they can
K Eq 'Vb cos X q X d ' I
V COS X q X d ' I
V sin
be described simply by algebraic equations. The field winding 1
circuit of the machine must be described by a differential equation, not only because of its low Eigen mode frequency,
X E
X E
tld b
X d
tlq b
but also because it is connected directly to the excitation system to which the supplementary excitation controller is applied. The torque differential equation of the synchronous machine also must be included in the model.
K2 I
tlq

( X q X d ' ) I
X d
( X d X d ')
tlq
m
. (P
Pe D) / M
K3 1
X d
o
. ( 1)
( X X ' )
The real power output of the generator is described as
K4 d d Vb sin
Pe Vtd
<>Itd
Vtq
Itq
X d
Eq E'q
( X d

X d
')I
tld
K Vtd X q
5 V X
Vb
cos Vtq
V
X d '
X
Vb
sin
Vt Vtd jVtq
The real power output of the generator is described as
t E
K Vtq Vtq
t d
X d '
6
Vt Vt
x
X d
Eq 'VB
cos LB cv
x dc
sin
K7
Vb C cos sin


Vb C cos sin
I sdt
X d CDC
X E CDC
tld x x
x x x LB (1 LB )x '
C cos
d
tL LB
tL x
SDT
xsdt
K8
X d
CDC
K C cos
X LB
C sin
C sin
X LB
C cos
d
9 X
CDC
X SDT
X E
CDC
X SDT
KqDC
( X d X d ')
X d
X LB
X SDT
C sin
0 0
0 1
0

K pc
0

k p
( X X ') X
M M M
K d d LB V
sin
K K
d
X
qc X
DC
SDT
U 0 0

qc
T '
q
T '

( X d X d ')

dpo
dpo
Kq X
CVDC cos
K A 0 K A Kvc
K A Kv
d
V X X
V X ' X
TA
TA TA
K td q LB C cos tq d LB C sin
0 0
Kdc
Kd
VDC
Vt X E
XSDT
Vt X d
XSDT
DC
DC


POWER SYSTEM STABILIZER
KVC
Vtd Vt
X q
X E
X LB V X SDT
cos Vtq
Vt
X d '
X d
X LB V X SDT
sin
One problem that faces power systems nowadays is the low frequency oscillations arising from interconnected systems. Sometimes, these oscillations sustain for minutes and grow to cause system separation. The separation occurs if no adequate

C cos X
C sin X
damping is available to compensate for the insufficiency of
Kdc LB Vdc sin LB C cos
the damping torque in the synchronous generator unit . This
X d CDC X SDT
K C cos XLB C V
X E CDC X SDT
X

cos C sin XLB C V

sin
insufficiency of damping is mainly due to the AVR exciters
high speed and gain and the systems loading. In order to overcome the problem, PSSs have been successfully tested
d
X
d X

CDC
dc
SDT
XE

CDC
dc
SDT
and implemented to damp low frequency oscillations. The PSS provides supplementary feedback stabilizing signal in
Are linearization constants The 20 constants of the model depend on the system parameters and the operating condition. The expressions for the constants are derived and are as follows.
SMIB POWER SYSTEM STATESPACE MODEL
WITH STATCOM
The SMIB power system statespace model is obtained from the linearized dynamic equations as
X AX BU
the excitation system. The basic function of PSS is to damp electromechanical oscillations. To achieve the damping, the CPSS proceeds by controlling the AVR excitation using auxiliary stabilizing signal. The CPSSs structure is illustrated in Fig 2 shows
Fig.2 PSS structure
The block diagram of a linearized model of the SMIB power system with STATCOM is shown in fig 3
q
x [ E '
E fd
VDC ]
U [U E Tm C ]
0
K1
M
b
0

D M
0 0
K 2 0
M
0

k pDC

M
A
K 4 '
T
dpo
K 3 '
T
dpo
1
T
'
dpo
K qDC
M
K A K 5
0 K A K 6 1

K A K vDC
TA
K 7 0
TA TA
K8 0
T
A
K 9
Fig.3 Modified HeffronPhillips transfer function model


SIMULATION RESULTS
The disturbance is given as step input and the output response is taken from omega, delta, change in power and DC voltage. The system shows with STATCOM and PSS in a time span of 10 seconds for light, nominal and heavy condition. From the Fig.4 to Fig.11 we concluded that STATCOM and PSS are more effective than the STATCOM.
4
x 10
3
x 10
20
15
delta Pe (pu)
10
STATCOM and PSS STATCOM
5
STATCOM and PSS
4 STATCOM 5
3
delta omega (pu)
0
2
1
5
0 0 1 2 3 4 5 6 7 8 9 10
Time in S
1 Fig.6Time response of Pe
2 and PSS at light load.
4
x 10
with STATCOM & STATCOM
1
3
0 1 2 3 4 5 6 7 8 9 10
Time in S
STATCOM and PSS
Fig.4 Time response of with
and PSS at light load
STATCOM & STATCOM
0.5
STATCOM
0.06
SATCOM and PSS 0
delta Vdc (pu)
STATCOM
0.05
0.04
0.5
delta (rad)
0.03 1
0.02
0.01
0
0 1 2 3 4 5 6 7 8 9 10
Time in S
1.5
2
0 1 2 3 4 5 6 7 8 9 10
Time in S
Fig.5 Time response of
and PSS at light load.
with STATCOM & STATCOM
Fig.7 Time response of Vdc
with STATCOM &
STATCOM and PSS at light load
4
x 10
6
5
STATCOM and PSS STATCOM
0.04
0.03
STATCOM and PSS STATCOM
4 0.02
delta omega (pu)
delta Pe (pu)
3 0.01
2 0
1 0.01
0 0.02
1 0.03
9
10
0.04
0
1
2
3
4
5 6
7
8
9
10
Time in S
Time in S
2
0 1 2 3 4 5 6 7 8
Fig.8 Time response of with and PSS at high load.
0.09
STATCOM & STATCOM
SATCOM and PSS
Fig.10Time response of with Pe
and PSS at high load
4
x 10
2
STATCOM & STATCOM
STATCOM and PSS
0.08
0.07
STATCOM STATCOM
0
0.06 2
delta (rad)
delta Vdc (pu)
0.05
4
0.04
0.03 6
0.02
8
0.01
0
0
1
2
3
4
5
6
7
8
9
10
10
0
1
2
3
4
5
6
7 8
9
10
Time in S
Time in S
Fig.9 Time response of and PSS at high load.
with STATCOM & STATCOM
Fig.11 Time response of with Vdc
STATCOM and PSS at high load.
STATCOM &

CONCLUSIONS

This work the coordination among PSS and STATCOM was resented and discussed for power system dynamic stability improvement. The design results are performed for three loading conditions and also the nonlinear simulations are performed loading conditions. A MATLAB/SIMULINK has been developed for a single machine infinite bus power system with STATCOM and PSS. The design results are confirmed with nonlinear simulations. The results of coordinated design show dynamic stability improvement. With nonlinear simulation in coordinated design case has been shown that the oscillations are damped properly.
Appendix
,
System data: M=8 MJ/MVA, D=0, Tdo =5.044s,
,
X d =1pu, X q =0.6pu, X d =0.3pu, K A =10, TA =0.05s,
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