Nonlinear Control of Static Synchronous Compensator STATCOM for Transmission System

International Journal of Trend in Scientific Research and Development (IJTSRD)
Volume 3 Issue 5, August 2019 Available Online: www.ijtsrd.com e-ISSN: 2456 – 6470
@ IJTSRD | Unique Paper ID – IJTSRD28002 | Volume – 3 | Issue – 5 | July - August 2019 Page 2231
Nonlinear Control of Static Synchronous Compensator
(STATCOM) for Transmission System
Yadana Aung, Phyu Phyu Win, Moe Phyu Thel
Department of Electrical Power Engineering, Technological University, Mandalay, Myanmar
How to cite this paper: Yadana Aung |
Phyu Phyu Win | Moe Phyu Thel
"Nonlinear Control of Static Synchronous
Compensator (STATCOM) for
Transmission
System" Published
in International
Journal of Trend in
Scientific Research
and Development
(ijtsrd), ISSN: 2456-
6470, Volume-3 |
Issue-5, August 2019, pp.2231-2234,
https://doi.org/10.31142/ijtsrd28002
Copyright © 2019 by author(s) and
International Journal ofTrend inScientific
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ABSTRACT
The Static Synchronous Compensator (STATCOM) is a shunt controller, which
is a member of FACTS devices. In this paper, an effective and robust controller
for STATCOM device on transmission lines, a Single Machine Infinite Bus
(SMIB) system is modeled. A state space mathematical model is constructed
which considers both electromechanical oscillations and reactive current of
the STATCOM at the installation site. Based the obtained third-order model,
state feedback linearization and linear quadratic regulation (LQR) approach
are applied to obtain a nonlinear control law. The controllers are simulated
and tested under different operating conditions comparing with the
conventional PI controller.
KEYWORDS: FACTS; STATCOM; feedback linearization; nonlinear control; PI
controller
I. INTRODUCTION
The power flows in some of the transmission lines areoverloaded, which has as
an overall situation requires the review of traditional transmission methods
and practices, and the creation of new concepts, which would allow the use of
existing generation and transmission lines up to their full capabilities without
reduction in system stability and security. Series capacitor,shuntcapacitorand
phase shift are different approaches to increasethepower systemtransmission
lines load ability. They are very useful in a steady state operation of power
systems but from a dynamical point of view, their time response is too slow to
effectively damp transient oscillations.
In the control of electric power systems, reactive power
compensation is an important issue. Reactive power
compensation is traditionally realized by connecting or
disconnecting capacitororinductorbankstothebus through
mechanical switches that are slow and imprecise. With the
developments in power electronics, a new kind of
compensator was introduced. This compensator is static
synchronous compensator(STATCOM), whichisthemember
of FACTS devices [1]. STATCOMis basedonself-commutated
solid state power electronic devices to achieve advanced
reactive power control.
Much work about STATCOM steady state performance
control has been done, which is based on steady state vector
(phasor) diagram analysis to power system quantities. This
kind of control approach, usually a proportional integrated
control (PI control) is convenient to the traditional power
system analysis method and not necessary to build a special
mathematic model for controller design. However, the
system response is slow due to the calculation of active and
reactive power, that need several periods (T) of the power
system, and not effective when the change of power system
is rapid[3][4].
As power system becomes more complex and more
nonlinear loads are connected, the control of power system
transient response is becoming a very critical issue.
Therefore, it is necessary to study nonlinearcontrolstrategy
to control the STATCOM dynamic characteristics and
capabilities to improve transient stability [5].
STEADY STATE MODEL
A STATCOM is always connected in shunt with the ac system
through some magnetic coupling, namely, the coupling
transformer or interface reactor. A typical STATCOM
connection is shown in Fig.1; it consists of a voltage source
converter (VSC) using either a GTO or IGBT as a switching
device, and a capacitor, Cs, on the DC side as an energy-
storage device. The resistance Rp in parallel with Cs
represents both the capacitor losses and switching losses.
The STATCOM is connected to the ac system through
magnetic coupling, represented by leakage inductance Ls
and resistance Rs. The STATCOM improves the desired
power system performance, including dynamic
compensation, mitigatingthesynchronous resonances (SSR)
bu modeling the reactive power at the common coupling
point, and so forth.
Fig.1 A Typical STATCOM Connection to AC System
IJTSRD28002

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MATHEMATICAL EQUATION
A single machine infinite bus system (SMIB) is shown in
Fig.2. The STATCOM is placed at the middle of the
transmission line which is generally considered to be the
ideal site.
Fig.2 A Single Machine Infinite Bus (SMIB) System with
STATCOM
The synchronous generator is represented by the classical
second order model. The system dynamics is described by
the following equations.
δ̇ = ω − ω (1)
ω̇ =
ω
[P − P −
ω
(ω − ω )] (2)
where P =
′
sin(δ − δ ) (3)
The voltage magnitude and angle at bus m can be written as
V =
′ (δ δ ) δ
+ I (4)
= V + X I (5)
δ = tan
′ δ
′ δ
(6)
When the STATCOM is installed at the transmission line, the
STATCOM reacrive output current is given by the STATCOM
control theory as
I ̇ = (−I + Ku) (7)
The target of the nonlinear control strategy here is to keep
the whole system stable by regulating the reactive power
exchange between the STATCOM and ac system. For the
system shown in Fig.2, the system stability can be evaluated
by checking the absolute value of the relative angle of the
generator within 180°
. This relative angle is used as a
stability index of the connected power systems. After a large
disturbance, the smaller the relativeangleis,themorestable
will be. Therefore, the relative rotor angle of thegenerator is
taken as the output equation for the nonlinear control
design.
y(t) = δ (8)
Combining equations (1), (2) and (8), obtain the state
equation and the output equation of the power system
installed with the STATCOM
δ̇ = ω − ω (9)
ω̇ =
ω
P − P −
ω
(ω − ω ) (10)
Iq
̇ =
1
T
(-Iq+Ku) (11)
The output equation is
y(t) = h(X(t) = δ (12)
The dynamic equation of a synchronous generator can be
written in a state space as
Ẋ (t) = f(X) + g(X)u (13)
y(t) = h(X) (14)
where X = [δ ω I ] ; u is the control variable of the
STATCOM
f(x)=
⎣
⎢
⎢
⎡ ω-ω0
ω0
H
Pm-
ω0
H
C1 Vm0+XeqIq -
D
H
(ω-ω0)
-
1
T
Iq ⎦
⎥
⎥
⎤
(15)
g(x) = 0 0 (16)
h(x)=δ (17)
and
C1=
Eq
'
X1
sin⁡(δ-δm) (18)
NONLINEAR CONTROL DESIGN OF THE STATCOM
The development of the feedback linearization techniques
provides a powerful tool for the design of controllers for
nonlinear systems. The first step in the design procedure is
the establishment of the linearization conditionfortheSMIB
system. Hence the SMIB system has relative degree r=3,
which is equal to the system order. A nonlinear system in X
coordinate can be transferred into the following Z
coordination
𝑧 =δ (19)
z = ω − ω = ∆ω (20)
z = ω̇ =
ω
P −
ω
C V + X I − ∆ω (21)
Then the nonlinear system can be transferred into a linear
system of the form
Ż = AZ + Bv (22)
y = CZ (23)
where ‘v’ is the control input of the linear system and
Z =
z
z
z
, A =
0 1 0
0 0 1
0 0 0
, B =
0
0
1
(24)
C = [1 0 0], D = [0] (25)
The optimal control vector ‘v’ is
v = −K∗
Z = −k ∗
z − k ∗
z − k ∗
z (26)
v = −k ∗
δ − k ∗
∆ω − k ∗
ω̇ (27)
where k ∗
= 1.0, k ∗
= 7.6, k ∗
= 3.9
The nonlinear control design for the system using
STATCOM is obtained as

u = −
α( )
β( )
+
β( )
v (28)
α(x) =
ω
C X I (29)
β(x) = −
ω
C X (30)
u =
ω
(k ∗
δ + k ∗
∆ω + k ∗
ω̇ ) + I (31)
PI CONTROL SCHEME OF STATCOM
The effectiveness of the proposed nonlinear controller in
damping power system oscillations is evaluated through a
comparison with a conventional PIcontrollershowninFig.3.
The gains of controller were selected based on a pole
placement technique.
Fig.3 Block Diagram of Conventional PI Control
The method produces anoptimumcontrolfunctionforlinear
system designed for a specific operating point. The transfer
function of PI controller is given as
GPID(s)=KP+
KI
s
(32)
The controller transfer function can be written as
Gc(s)=
sTw
1+sTw
KP+
KI
s
(33)
Here, KP and KI are the gains in proportional and integral
loops, Tw is the washout time constant.
SIMULATION RESULTS
The performance of the STATCOM for the stabilization of
synchronous generator is evaluated bycomputersimulation
studies. The STATCOM is installed at the middle of the
transmission line. The time constant of the STATCOM
reactive output current is set to 0.01 sec. The transient
performances of the rotor angle are shown in Fig.4 whenthe
generator is loaded at power 0.8 p.u. Fig.5 shows the
transient response of the mid-bus voltage.Itcanbeseenthat
from the simulation results, the nonlinearcontrolissuperior
over the conventional PI control not only in suppressing the
STATCOM bus voltage fluctuation, but also in restrainingthe
power oscillation.
The transient performances of the rotor angle are shown in
Fig.6 when the generator is loaded at power 1.0 p.u. Fig.7
shows the transient response of the mid-bus voltage. When
the generator loadwas increased(P=1.0p.u),STATCOM with
conventional PI controlaremoreunstable,but theSTATCOM
using nonlinear control is effectively damp the system
oscillation and completely stable at 7 second. This proved
that nonlinear control technology provides new promising
way to further improve the operation security and dynamic
performance of the system.
Fig.4 Transient Response of Rotor Angle (Generator
was Loaded to P=0.8 p.u )
Fig.5 Transient Response of Mid-bus Voltage
(Generator was Loaded to P=0.8 p.u )
Fig.6 Transient Response of Rotor Angle (Generator
was Loaded to P=1.0 p.u )

Fig.7 Transient Response of Mid-bus Voltage
(Generator was Loaded to P=1.0 p.u )
CONCLUSION
The control strategy of a STATCOM in a single machine
infinite bus system was derived to improve system stability
and damping power system oscillation. The control of the
STATCOM is selected based on the linearized mathematic
model of the system by applying nonlinear control. The
effectiveness of the proposed control strategy in improving
the power system dynamic stability has been verified
through nonlinear time-domain simulations under different
load conditions. This control approach for STATCOM shows
better performance compare with the conventional PI
controller.
REFERENCES
[1] N. G. Hingorani, L. Gyugyi, “UnderstandingFACTS”IEEE
Press, 2000
[2] A. Hammad, B. Roesle, “New Roles for Static Var
Compensators in Transmission Systems”, Brown Boveri
Review, June, 1986
[3] Dong Shen and P. W. Lehn, “Modeling, Analysis and
Control of a Current SourceInverter-BasedSTATCOM”,
IEEE Transctions on Power Delivery, Vol.17, No.1,
January, 2002
[4] M. A. Abido, Ch. Weindl, G. Herold, “STATCOM-Based
Damping Stabilizers for Power System Stability
Enhancement”, 11th International Power Electronics
and Motion Control Conference EPE-PEMC,September
2004.
[5] W. Mielczarski and A. M. Zajaczkowski, “Multivariable
Nonlinear Controller for a Synchronous Generator”
Optimal Control Applications and Methods, Vol.
15,1994
[6] D. I. Kim, I. J. Ha and M. S. Ko “Control of Induction
Motor via Feedback Linearization with Input-output
Decoupling” International Journal of Control,Vol.51,
No.4,1990
[7] P. W. Lehn and M. R. Irvani, “ExperimentalEvaluationof
STATCOM Closed Loop Dynamics”, IEEE Transctions on
Power Deliver, Vol.13, No.4, pp.13781384,1998
[8] Alberto Isidori, “ Nonlinear Control Systems: an
Introduction”, Berlin, Springer-Verlag,1989
[9] S. Mori, K. Matsuno, M. Takeda, et al, “ Development of
a Large Static Var Generator using Self-commutated
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[10] R. Marino, “An Example of a NonlinearRegulator”,IEEE
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[11] O. Akhrif, F. A. Okou, L. A. Dessaint and R. Champagne,“
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