Investigation of Multi Input Adaptive
Control for STATCOM
Nikolay Djagarov
Zhivko Grozdev
Milen Bonev
Technical University
Electrical Equipment
and Power Supply
Department
Varna, Bulgaria
jagarov@ieee.bg
Technical University
Electrical Equipment
and Power Supply
Department
Varna, Bulgaria
grozdew@yahoo.com
Technical University
Electrical Equipment
and Power Supply
Department
Varna, Bulgaria
bonevi_km@tu-varna.bg
Abstract— In the present report is presented the advanced
adaptive control of static compensator (STATCOM) consisting of
conventional PI-regulator and multi input adaptive regulator.
The adaptive static modal regulator identifies the controlled
object by optimal observer in real time. By means of identified
parameters and variables of controlled object is calculated the
control signal. The operation of power system connected through
transmission line into infinity buses is being investigated. The
power system consists of generator units, static and dynamic
loads and STATCOM. The results from investigations show the
effectiveness of suggested advanced adaptive control.
Keywords-component; STATCOM, FACTS, adaptive control,
singular adaptive observer Introduction (Heading 1)
I.
INTRODUCTION
The development and improvement of powerful
semiconductor technology in recent years contribute to the
development and implementation of flexible alternate current
transmission systems (FACTS). The FACTS devices represent
a group of innovative devices based on power electronics
which are used primarily in high-voltage transmission lines.
These devices perform dynamic compensation of power
systems by improving natural characteristics of transmission
lines, increasing the transmission power and governing the
form of voltage lines.
The static synchronous compensator represents shunt
connected static compensator which generates or absorbs
reactive power in order to maintain or adjust the parameters of
the power system [1,2,3,4].
The main advantages of static synchronous compensators
are:
- Voltage control under different operating modes;
- Reactive power control;
- Improving the power system stability in energy
transmission at long distance;
- Improving insensitivity of the network by controlling
the voltage during major disturbances, such as. short
circuit;
- Improving the reliability of power systems;
- Reducing the costs and increasing capabilities of the
managing controllers.
For control of FACTS are used all known methods from
the theory of control: classical PI-regulators, methods of fuzzy
logic and neural network, regulators with adjusted parameters,
regulators with variable structure and others. The main trend is
to make these controllers adaptive [5].
However, this relates to the need of large computational
resources, which will worsen their performance and hence the quality of regulation.
The adaptive stabilizer suggested by us uses optimal
singular adaptive observers [6]. These observers based on
measured parameters of the controlling object identify the
parameters and variables of minimal model of Frobenius. The
main difference of this identification from the known is that
not only the current vector is estimated but also the initial
vector. This avoids iterative solution of Riccati equations and
hence is achieved very high speed of identification and
calculation of control signal. Thanks to this, the calculation
time of the control signal and feedback is negligible small in
comparison with the speed of running processes in the system.
Therefore, these regulators improve all parameters of the
transition process, damping the oscillations and improving
power system stability as whole.
II.
STUDIED POWER SYSTEM
On Fig.1 is presented studied power system including
infinity power system, transmission line, static activeinductive load, dynamic load and STATCOM connected to
load bus.
The suggested STATCOM represents GTO-based multilevel voltage source converter (VSC) connected in parallel
with power system by coupling transformers. The four sets of
three-phase voltages obtained at the output of the four threelevel inverters are applied to the secondary windings of four
phase-shifting transformers (with phase shifting ±7,5°). In
this way of connecting of power transformers are neutralized
the odd harmonics up to 45th harmonic with exception of 23rd
and 25th (which is ideal for balancing of power system). By
regulating of amplitude and phase of the generated voltage
from compensator can be regulated the active and reactive
power generated/absorbed to/from power system. Also by
adjusting the output voltage amplitude of the compensator can
978-1-4577-1829-8/12/$26.00 ©2012 IEEE
be regulated the reactive power flow and voltage of the line.
On the other hand the DC-voltage of capacitors controls the
generated voltage of compensator output. In this way for
change of reactive power flow of STATCOM the controller of
the compensator first must temporary absorb reactive power
from power system for charge the power capacitors. This
active power flow is controlled by managing of firing angle of
GTO- thyristors [7]. of pagination anywhere in the paper. Do
at the AC-terminal R, ERN1 with respect to the capacitor centre
tap, is determined by the ON and OFF states of the four
switches GR1, GR2, GR3 and GR4.
III.
CONTROL OF STATCOM
The control of compensator consist from three control
loops; adaptive voltage regulator; current regulator; DC
Dynamic load
Power
System
IM
Transmission
line
Static load
STATCOM
Controller
48 GTO
Firing pulses
STATCOM
Fig.1. Diagram of studied power system
not number text heads-the template will do that for you.
regulator. The basic idea of STATCOM control is keeping the
On the Fig.2 is presented circuit diagram of one 3-level
amplitude of voltage in connection node close to the reference
GTO-bridge converter module, with limbs R, S and T. With
value, which is given from operator.
the capacitor centre tap N connected through diodes, each ac
The structure of the STATCOM control is shown in Fig.3.
node has three voltage levels, +0,5Udc, 0, and -0,5Udc. The
The voltage of connection node UT and the current of
capacitor voltages are regulated and equalized at 0,5Udc by
compensator IS are measured. The inner current control loop
another DC-voltage feedback regulation loop and an inner
forces the voltage source converter to behave as a controlled
DC-voltage equalization feedback loop [8]. The phase voltage
current source. For connection of STATCOM to present node
S
T
is used power transformer Tr. The node voltage is controlled
R
by a two input adaptive controller and gives the reference
+
signal for the q-current controller. To regulate the DC voltage
GR1
of the outer control loop to its constant value a PI controller is
+
used. To design the PI controller parameters, the inner control
UDC1
loop is modeled as a first order delay element and the dynamics
of the DC link are taken into account. The controller is tuned
GR2
with the symmetrical optimum [9]. The inner current control is
US
UR
UT
performed
with PI-controllers in rotating dq-axes coordinates.
UDC
N
Grid synchronization is done with a PLL algorithm.
GR3
+
-
UDC2
GR4
Fig.2. Three level diode - clamped converter.
The basis of the adaptive regulator, which regulates
voltage of static synchronous compensator, is that in real time
it can continuously identify the controlled object by lineal
model from low order and after that it creates the controlling
signal. The performed numerous investigations [10] show that
for the needs of regulator can be used models from 2nd order.
This provides very high performance and accuracy. In the
Two-input
adaptive
regulator
z(k)
Iqref
UDC
Idref voltage
control
Current
regulator
UTref
US,dq d,q
P, Q
calculation
QS
PWM
STATCOM
Tr
IS,abc
d,q
a,b,c
QS
UT
UDC
US,abc
a,b,c
IS,dq
UDCref
Ȗ
PLL
UT,abc
UT,dq d,q
a,b,c
Fig.3. Block diagram of the STATCOM control
presented diagram is used two-input optimal singular adaptive
(OSA) observer (MISO – multi input single output). As it can
be seen on Fig.3 on the input of OSA observer are feeded
discrete samples from input UT(k) (voltage of connection
point) and QS(k) (reactive power of STATCOM) and also
signal z(k), representing limited input sequence used for
identification.
The observed system might be present by a following type
of a linear model in the state space described through the
following differential equations [6]:
x k 1
A.x k B.>u k z k
@
A.x k B.v k
T
> y ( 0)
y (1)@
y (3)
y ( 4)
y (5)
Y
T
Y
>y (2)
(1)
(2)
c .x k
ªY º
« » ; Ui
«¬Y »¼
x(0)=x0,; k=0,1,2,….;
where: x(k), x(k+1) are an unknown current state vector in two
neighbor moments of discretization; x(0) is an unknown initial
state vector; u(k) is an input signal; z(k) is a limited input
sequence for identification;
A
ª0
« at
¬
1º
»
¼
ª0 1 º
«
» ; bi
¬a 1 a 2 ¼
Y
ª bi1 º
« »;
¬bi 2¼
(3)
c
ª 1º
«0 » ; B
¬ ¼
>b1
H z
c . z.I - A
1
.B
as and following difference equation “input/output”,
Y z H z .U z
H z
L . z .Q z
can be presented in following way:
y (1) º
y ( 2 ) »»
y ( 3) » ; U
0i
»
y ( 4) »
y (5 ) »
»
y ( 6 ) »¼
y (7)@
ui (0) º
u i (1) »»
ui ( 2) » ;
»
u i ( 3) »
ui ( 4) »
»
u i (5) ¼»
(9)
ª 0 º
«u 0 »
¬ i ¼
The estimation of next vector is calculated:
hi T
>hi1
hi 2 @
(10)
With the help of linear system of algebraic equations:
(4)
ȍ 2 .Ĭˆ Y
>U 1
U 2 Y @ ; Ĭˆ T
(11)
>h
T
i1
hi2T
aT
@
(5)
where: ȍ 2
(6)
The lower-triangle Toeplitz matrix with dimension 2x1 is
shaped:
ǻ ª0º
A « »
(13)
¬ a2 ¼
which taking into account the ratio:
1
ª y (0)
« y (1)
«
« y (2)
«
« y (3)
« y (4)
«
«¬ y (5 )
ª u i (1)
«u ( 2 )
« i
« u i ( 3)
«
«u i ( 4 )
« u i (5)
«
¬« u i ( 6 )
y ( 6)
where: Y - Hankel matrix; U 0i , U i - Toeplitz matrices.
bi @ ; 1 d i d r
For the (1) and (2) describing the studied power system
correspond to following “input/output” difference equations
[7]:
T
(7)
Q z .U z
The input/output data are shaped in following matrices and
vectors.
Y
t
yk
L z .Y z
(12)
The estimation of the vector bi, is calculated by linear
system equations of following type:
bˆ i
where: bi
>bi1
ǻ hˆ i A .bi
disconnection from circuit breakers for time 5,03sec. Parts of
parameters of STATCOM and power system are shown
below.
(14)
QSTATCOM [MVar]
bi 2 @
100
The estimation of initial vector x̂ 0 is calculated by the
optimal estimator of following type:
(15)
xˆ 0 Y ȍ 3 .bˆ*
80
*
b1T b2T
where: ȍ 3 >U 01 U 02 @ ; bˆT
The current vector is estimated by the degenerate OSA
observer of the following type:
xˆ k 1 Aˆ .xˆ k 1 Bˆ .u k
(16)
ˆx 0 xˆ 0 ; k=0,1,2,…
40
>
@
The investigations have shown that controlling system can
be identified with the help of model from second order i.e.
n 2
I qref p aˆ t .xˆ p aˆ1.xˆ1 aˆ 2 .xˆ 2
(17)
60
20
0
-20
-40
-60
10
10.05
10.1
10.15 10.2
time [sec]
10.25
10.3
10.35
Fig.5. Reactive power of STATCOM
where: p=k, k+1, k+2
DG [deg]
IV.
SIMULATION STUDY
50
For prove of rightness and effectiveness of studied system
work a computer model of suggested system and control was
created in MATLAB space. Different disturbances causing
transient processes have been simulated. The simulated
transient processes are: three-phase earth short circuit and it’s
disconnection from circuit breaker, connection/disconnection
of
powerful
static
active-inductive
load,
connection/disconnection of powerful dynamic load. The
obtained simulations are compared with system of identical
parameters in with conventional control (PI-controller) for
STATCOM. The following figures show some typical
parameters of investigated power system. On the figures:
STATCOM with adaptive two-input controller-blue line with
PI-controller-red line.
First transient process (Fig.4yFig.7) which was is
simulated present three-phase earth fault at time 5sec and it’s
40
30
20
10
0
-10
-20
5
5.05
5.1
time [sec]
5,15
Fig.6. Firing angle for GTO-thyristors
ISTATCOM [p.u.]
UT [p.u.]
3.5
3
1.01
2.5
1.005
2
1
1.5
1
0.995
0.5
0.99
0
5
5,05
5,1
5,15
time [sec]
5,2
5,25
Fig.4. Voltage of STATCOM buses
5,3
5
5.05
5.1
5.15 5.2 5.25
time [sec]
5.3
5.35
Fig.7. Current of STATCOM
5.4
DG [deg]
Second performed transient process (Fig.8yFig.13), present
connection at 1,5sec and disconnection of powerful dynamic
load. Parts of parameters of STATCOM, power system and
dynamic load are shown below.
10
5
0
UT [p.u.]
1.02
-5
1
-10
0.98
1.5
1.6
1.7
0.96
1.8
1.9
time [sec]
2
2.1
2.2
Fig.10. Firing angle for GTO-thyristors
0.94
Iqref [p.u.]
0.92
1
0.8
1.5
1.6
1.7
1.8
time [sec]
1.9
2
2.1
0.6
Fig.8. Voltage of STATCOM buses
0.4
0.2
0
QSTATCOM [MVar]
-0.2
-0.4
100
-0.6
1.5
50
1.6
1.7
1.8
1.9
time [sec]
2
2.1
2.2
Fig.11. Control current of STATCOM
0
UDC
4
[V]
x 10
2.4
-50
2.2
-100
1.5
1.6
1.7
1.8
1.9
2
time [sec]
2.1
2.2
2.3
2.4
2
Fig.9. Reactive power of STATCOM
1.8
1.6
1.4
1.2
1.5
1.6
1.7
1.8
1.9
time [sec]
2
2.1
Fig.12. Voltage of STATCOM capacitors
2.2
V.
CONCLUSION
It is proposed new combined adaptive control of
STATCOM consisting of a conventional current regulator and
two-input adaptive regulator on the basis of adaptive observer.
Through the estimated variables and parameters of the
identification model is calculated the control signal in channel
for voltage regulation.
Conducted simulation studies show the effectiveness of the
proposed adaptive management. At disturbances in the power
system from proposed diagrams is possible to observe:
reduction the transition time; reduction the oscillation of
transition process; limitation the deviations of the parameters
of the system; facilitation the work of STATCOM.
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Authors:
Nikolay Filev Djagarov,
Technical University, 9010 Varna, “Studentska”1, Electric
Supply and Electrical Equipment Department, Bulgaria, tel:
+35952383265; e-mail: jagarov@ieee.bg
Zhivko Genchev Grozdev,
Technical University, 9010 Varna, “Studentska”1, Electric
Supply and Electrical Equipment Department, Bulgaria, tel:
+35952383345; e-mail: grozdew@yahoo.com
Milen Bonev Bonev,
Technical University, 9010 Varna, “Studentska”1, Electric
Supply and Electrical Equipment Department, Bulgaria, tel:
+35952383345; e-mail: bonevi_km@tu-varna.bg