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 Total Downloads : 878
 Authors : P. C. Pradhan, P. K. Ray, R. K. Sahu, J. K. Moharana
 Paper ID : IJERTV2IS121198
 Volume & Issue : Volume 02, Issue 12 (December 2013)
 Published (First Online): 30122013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
A STATCOMControl Scheme used for Power Factor Improvement of Grid connected weak bus System
P. C. Pradhan*, P. K. Ray **, R. K. Sahu *** and J. K. Moharana****
* Faculty and Research Scholar, Dept. of EE, DRIEMS, Cuttack, ** Dept.of EEE, IIIT, BBSR, *** Dept.of EE, VSSUT, Burla and **** Dept.of EEE,GITA,BBSR
Abstract– Improvement of power factor of long weak distribution lines connected to grid is a challenging problem, particularly when it may not be economic to simply upgrade the network. The STATCOM (STATics synchronous COMpensator) offers an attractive alternative, with their potential to provide both steady state and transient voltage compensation for a limited capital investment. However, operation of these systems with weak networks needs careful attention to achieve a stable and fast response under all supply conditions. This paper proposes a new control strategy in the STATCOM to improve power factor at a distribution weak bus which is more robust and works well under all system conditions. Simulation results are presented to verify the stability of this control strategy across a range of operating conditions. The operation of the STATCOM with supply system makes the rural consumers healthy and wealthy.
Index Terms: Controller design, PI Controller, STATCOM

INTRODUCTION
Inductive loads and diode rectifiers are widely employed in industrial fields and consumer products thanks to advantages of low cost, simple structure, robustness and absence of control. However, this type of converters results in only unidirectional
voltage regulation, voltage unbalance, low power factor and harmonics often become a significant problem. As is well known, the current is proportional to the voltage in case of a linear load as shown in Fig.1(a) where as the current is not proportional to the voltage in case of nonlinear load (as shown in Fig.1(b)). A linear load draws active power from the grid with only fundamental component being present in the current and absorbs/injects reactive power from/to the grid. However, a nonlinear load draws active power from the grid, where the current has fundamental and harmonics. These harmonics do not provide extra power but unnecessarily, yet unavoidably, increase the system voltampere (VA). This shows up as an increase in the rms current in the lines and leads to an extra heating of the transmission conductors and system elements. Hence, the compensation of reactive power is necessary for both linear and nonlinear loads. Harmonic compensation upto standard values and power factor improvement are the main issues for such loads. The power factor PF is defined as the ratio of the active power P to the apparent power S . Thus
power flow, low input power factor, high level of harmonic input currents, malfunction of sensitive electronic equipment,
PF P
S
(1)
increased losses and also contributing to inefficient use of electric energy. And also the inductive loads especially
For purely sinusoidal voltage and current, the standard expression is obtained as
inductive motors create low power factor at the electric supply
system. Recently, many promising power factor correction
PF cos
(2)
(PFC) techniques have been proposed for rectifiers and
where
cos
is popularly known as the displacement factor.
inductive loads. Apart from application of active and passive filters, the best solution is in using pulse width modulated (PWM) rectifiers. Research interest in threephase PWM rectifiers has grown rapidly over the last few years due to some of their important advantages, such as power regeneration capabilities, control of dcbus voltage over a wide range, and low harmonic distortion of input currents. In recent years power systems have become very complex with interconnected long distance transmission lines. The interconnected grids tend to become unstable as the heavy loads vary dynamically in their magnitude and phase angle and hence power factor. Commissioning new transmission systems are extremely expensive and take considerable amount of time to build up. Therefore, in order to meet increasing power demands, utilities must rely on power export/import arrangements through the existing transmission systems. In specially, the distribution networks supplying rural consumers are often quite weak because of the long distances involved and the relatively high R/X ratio of the cables that are used. Hence, as demand increases on these networks, power quality issues such as poor
The survey of the power factor for a year of the rural consumers is given in Table.I
Month of the Year
Demand (KW)
Demand (KVA)
Actual Power Factor (%)
January February
200
150
245
224
81.63%
66.69%
Month of the Year
Demand (KW)
Demand (KVA)
Actual Power Factor (%)
January February
200
150
245
224
81.63%
66.69%
Fig.1: Linear and nonlinear loads of consumers connected to supply system
March
125
175
71.43%
April
224
256
87.50%
May
208
289
71.97%
June
210
299
70.23%
July
223
289
77.16%
August
211
278
75.90%
September
204
265
76.98%
October
198
245
80.82%
November
156
198
78.79%
December
201
265
75.85%
Table.I: It shows the power factor of the rural consumers
Power electronic devices are gaining popularity for applications in the field of power transmission and distribution systems. The reactive power (VAR) compensation and control have been recognized [1] as an efficient & economic means of increasing power system transmission capability and stability. The use FACTS (Flexible AC Transmission System) devices in a power system can potentially overcome limitations of the present mechanically controlled transmission systems. By facilitating bulk power transfers, these interconnected networks help minimize the need to enlarge power plants and enable neighboring utilities and regions to exchange power. The stature of FACTS devices within the bulk power system will continually increase as the industry moves toward a more competitive posture in which power is bought and sold as a commodity. As power wheeling becomes increasingly prevalent, power electronic devices will be utilized more frequently to insure system reliability and stability and to increase maximum power transmission along various transmission corridors. The FACTS device, such as STATCOM has been introuced more recently which employs a VSI with a fixed DC link capacitor as a static replacement of the synchronous condenser. In a traditional synchronous condenser, the field current of the synchronous motor controls the amount of VAR absorbed/injected and hence in a similar way, the firing instant of the 3phase inverter controls the VAR flow into or out of the STATCOM. Large numbers of capacitor banks or inductor banks are no more required. Only a fixed set of capacitor provides the required VAR control, with a rapid control of bus voltage and improvement of utility power factor. It offers several advantages over conventional thyristorised converters [2] in terms of speed of response. The STATCOM is a voltage source inverter (VSI) based device, which regulates distribution bus voltage using reactive power compensation. The potential of STATCOM to improve supply quality and increase line utilization in weak distribution networks is well documented [3, 4]. However, many of the proposed control strategies assume a stiff, balanced grid source, and this is often not the case in practice. Recently, there has been some research focus on the performance of STATCOM devices operating under unbalanced supply conditions. Direct voltage control algorithms used to compensate for supply unbalance in distribution networks were proposed in [5] and [6]. However, the results in [6] show a relatively slow dynamic response because of the filters employed. Also, both algorithms have been developed for a VSI device interfaced to the distribution
network through a simple inductive filter, and have not been tested for the more complex LCL filter considered in this work. A multivariable control strategy was proposed in [7] for a STATCOM with a LCL filter interface. Although this strategy is shown to achieve good steady state and dynamic responses under balanced and unbalanced supply conditions, it is complex and sensitive to variations in system parameters. The penalty paid for this improvement is in terms of introduction of some harmonics, which requires separate handling using active filtration techniques. Moran et al [8] have shown in details how the utilization of Sinusoidal Pulse Width Modulation (SPWM) techniques reduces harmonic distortion. It has also been shown that an increase of modulation index reduces the size of the link reactor and stress on switches which are significant issues in practical implementation. The modeling and analysis of STATCOM steady state and dynamic performance with conventional control method have been studied by Schauder and Mehta [9] using nonlinear controller. In [10, 11] the dynamic responses and steady state behavior of STATCOM with Space Vector Pulse Width Modulation (SVPWM) has been studied and the advantages of introducing SVPWM inverter with higher values of modulation index are highlighted.
The controllable reactive power allows for a rapid control of bus voltage and power factor at the system or at the load end. To compensate for the distorted current drawn by the rectifiers from the utility grid, the STATCOM and its current controller must have the capability to track source PWM (Pulse Width Modulation) converters. The linear control is more suitable for STATCOM application reported in [13, 14]. The present paper suggests the design of a linear current controller and voltage controller on the basis of gain and time constant adjustment along with the parameter of the coupling inductor and storage capacitor.
The present paper goes on to develop closed loop model for investigating transient performance of the STATCOM by using controller parameter. First, in Section 2 focuses on modeling of the power system and Section 3 gives state space model of the STATCOM with the system. Secondly, in Section 4, a current and voltage controllers are designed. The simulated responses with the designed controller parameters are presented in Section 5. This scheme is both an extension and a significant improvement of the scheme suggested by Shauder et al [9] and Sensarma et al [4].The results obtained have been compared and appropriate conclusions have been drawn.

MODELING OF THE POWER SYSTEM
Fig.2: Interconnected Six Bus system
The 6bus system is a simple power system network given in Fig.2 and scaled model of a bus is shown in Fig.3.The 6bus system is intended to illustrate in a simple context notions
current Fig.5.
Ica to be injected into the system at PCC as shown in
of transfer capability and the impact that various actions have on the given transfer capability. Buses 1, 2, 3 of the system diagram are generators and buses 4, 5, 6 of the diagram are loads. The primary flow of power is from the top of the diagram to the bottom of the diagram and also from left to right. Reactive demand by the loads (signified by the empty portion of the load arrows) is large. The network has 10 branches and each branch represents a transmission line. The model is an AC power flow model; it represents real and reactive power flows and power system nonlinearity. Operational limits relating to transmission line flow, voltage magnitude, and voltage collapse are represented. The weak bus no 5 is connected to industrial area and also local consumers. The power factor of this grid (Bus no.5) is poor and hence its power factor can be improved as the model given in Fig.4.
Fig.3: Scaled distribution network model
Fig.4: A single Bus connected with STATCOM

MODELING OF THE STATCOM AND ANALYSIS

Modeling
The modeling of the STATCOM, though well known, is reviewed in the lines below, for the sake of convenience. The modeling is carried out with the following assumptions:

All switches are ideal

The source voltages are balanced

Rs represents the converter losses and the losses of the coupling inductor

The harmonic contents caused by switching action are

negligible
The 3phase stationary abc coordinate vectors with 1200 apart from each other are converted into 2phase stationary coordinates (which are in quadrature). The axis is aligned with a axis and leading axis and both
converted into dq twophase rotating coordinates. The Parks abc to dq transformation matrix is
Fig.5: Schematic diagram of STATCOM
Fig.6: Phasor diagram for inductive load operation

Operating principle
As is well known, the STATCOM is, in principle, a static (power electronic) replacement of the ageold synchronous condenser. Fig.5 shows the schematic diagram of the
Cos(t)
2
2
K Sin(t) 3
1/ 2
Cos(t 2 / 3) Sin(t 2 / 3) 1/ 2
Cos(t 2 / 3)
Sin(t 2 / 3) (3)
1/ 2
STATCOM at PCC through coupling inductors. The
fundamental phasor diagram of the STATCOM terminal voltage with the voltage at PCC for an inductive load in operation, neglecting the harmonic content in the STATCOM
The actual proposed circuit is too complex to analyze as a whole, so that it is partitioned into several basic subcircuits, as shown in Fig.5. The 3phase system voltage vs,abc lagging with
terminal voltage, is shown in Fig.6. Ideally, increasing the
the phase angle to the STATCOM output voltage v
o,abc
and
amplitude of the STATCOM terminal voltage Voa above the
amplitude of the utility voltage Vsa causes leading (capacitive)
differential form of the STATCOM currents are defined in (4) and (5).
vsa
Sin(t )
(4)
the transient responses take about one and half power cycle to reach at their steady state values.
v v 2V
2
Sl
Parameters
Symbol
Values
1
Frequency
f
50 Hz
2
Angular Frequency
w
314 rad/sec
3
RMS linetoline Voltage
Vs
230V
4
Coupling Resistance
Rs
1.0
5
Coupling Inductance
Ls
5.0mH
6
DClink capacitor
C
500 F
7
Modulation Index
M
0.979
8
Phase angle
50
9
Load Resistance
RL
52
10
Load Inductance
LL
126mH
11
Load Power factor
0.79
Sl
Parameters
Symbol
Values
1
Frequency
f
50 Hz
2
Angular Frequency
w
314 rad/sec
3
RMS linetoline Voltage
Vs
230V
4
Coupling Resistance
Rs
1.0
5
Coupling Inductance
Ls
5.0mH
6
DClink capacitor
C
500 F
7
Modulation Index
M
0.979
8
Phase angle
50
9
Load Resistance
RL
52
10
Load Inductance
LL
126mH
11
Load Power factor
0.79
Sin(t )
s, abc
sb
3 s 3
vsc
Sin(t
2
L d i
R i v
3

v

(5)
s dt
c,abc
s c,abc
s,abc
o,.abc
where , V , , R and L have their usual connotations. The
s s s
above voltages and currents are transformed into dq frame
L d i
R i

wL i

v v
s dt cq
s cq
s cd
sq oq
(6a)
d
Ls dt icd
wLsicq Rsicd vsd vod
(6b)
The switching function S of the STATCOM can be defined as follows
Sin( wt)
Table.II: It shows the system parameters
Sa
S S
b
Sc
2
m Sin( wt
3
2
)
3
(7)
Sin( wt
2 )
3
The modulation index, being constant for a programmed PWM, is given by,
MI
vo, peak 2
m
Vdc 3
(8)
Fig.7: Steady state responses of Icq , Icd and Vdc
The STATCOM output voltages in dq transformation are
vo,qdo m0 1
0T v
(9)
dc
dc
The dc side current in the capacitor in dq transformation
i m0 1
0i i
i T
(10)
dc cq cd co
The voltage and current related in the dc side is given by
Fig.8: Steady state responses of Pc and Qc
dvdc m i
(11)
dt C cd
The complete mathematical model of the STATCOM in dq
frame is obtained as given in (12)
Rs w 0
icq
Ls
icq
Sin
d i
w


Rs

m i
Vs Cos
(12)
Fig.9: Transient responses of icq in capacitive and inductive
dt cd
L L cd L
v
s s v s 0
dc 0
m 0 dc
C
3.3. Steady State and transient Analysis
The detailed steady state and transient responses with the Table.II are given in Fig.710 and responses suggest the static and dynamic conditions of the STATCOM. It can be seen that
Fig.10: Transient responses of vdc in capacitive and inductive


DESIGN OF CONTROLLERS:
With the assumption of the system voltage and STATCOM output voltage are in phase and hence the equation (12) can be modified as given in equation (13)
Gq (s)
I q (s)
oq
oq
V * (s) Rs
1

sLs
, Gd (s)
I d (s)
od
od
V * (s) Rs
1

sLs
(17)
Rs
d icq
Ls
icq
1 vsq
voq
dt i
Rs i
L v
v
(13)
Fig.12: Current control of inverter of equivalent decoupled SISO systems
cd
cd
Ls
s sd
od
For similar dynamic behaviour of the d and q – axis currents, both the d and q – axis controllers are identical and its transfer
So the equation (13) is a Multiple Input and Multiple Output
(MIMO) system and its input and output are given as
function is given in (21)
oq cq
oq cq
v i
I cq (s) I cd (s) 1
u ,y
(14)
G (s)
i * *
(18)
vod icd
Voq (s) Vod (s) Rs sLs
The block diagram of the STATCOM in dq transformation as
The transfer function of a PI controller is
per (13) is shown in Fig.11.The instantaneous voltage of the system and the STATCOM are independent, but the active and the reactive currents are coupled with each other through the reactance of the coupled inductor. So it is very essential to decouple the active and reactive current from each other and
G pi (s) K 1
K
1
1
K p i s i s
K
(19)
design the controller for tracking the required value.
With
K p K , Ki
i
. The transfer function in open loop of
PI controller associated with the transfer function on the a.c. system is
1 R
G (s).G (s) K 1 1 s
(20)
Fig.11: Equivalent Diagram on a.c.side of STATCOM
pi i
s
Ls
1 s Ls
i
i
Rs
4.1 Design of current controller:
The current controller design for the above system can be done
While taking i
and on simplification reduces to
Rs
using the strategy [89] attempts to decouple the d and q axes equations, so that the MIMO system reduces to two independent
G (s).G (s) K
(21)
Single Input Single Output (SISO) system. Hence, the control
pi i
sLs
inputs vod
and voq
are configured as
The closed loop transfer function is
oq
oq
voq v*
*

wLsicq vsq
(15) T 1
L
(22)
vod
vod wLsicd


vsd
1 s s
The equation (16) can be obtained by replacing (13) by (15). Hence each row of (16) is independent of each other and thus defines an independent SISO system. Conventional frequency domain design methods can now be directly applied for current
K
Thus the system behaves like a first order with an apparent time constant as
controller. Taking the Laplace transformation of both sides of
(17) and rearranging terms are given by (18) and their
Ls
i K/p>
(23)
decoupled SISO system is shown in Fig.12.
The gain of K can be adjusted such a way that if it is increased
Rs 0
*
too high then the system behaves as second order, otherwise
icq
Ls icq
1 voq
(16)
responses very slow. Hence the numerical values for
i
Rs i
L *
K p and Ki are decided from the circuit parameters Ls and Rs
cd 0
cd
Ls
s vod
from the required value of K. So the parameters of PI controller are defined as
K p K , Ki
KRs
Ls
(24)
where , v
C
K * Kdc
and taking v
1msecond and with the
where,
Ls
which is taken as 0.3mseconds and with the
parameters of Table. I, the value of Kdc 1.08
K
K
i
p
parameters given in TableII, value of
K pi 16.9
and
3
3
Kii 3.310
are calculated. These parameters are used
in d and q – axis current controller. The structure of the effective closed loop system is shown in Fig.13 and is replicated in both the d and q – axis current controllers.
Fig.14: DC link voltage control loop
Then Proportional Integral controller is considering for the voltage control. Hence, the transfer function of PI controller in
(19) is associated with the transfer function on dc side is
Fig.13: Effective closed loop current control system
Gv (s).G pi (s)
ol
ol
v
v
K 1
1 1
(31)
4.2. Design of voltage controller:
s sC
The relation between dc voltage vdc and dc current idc is
After taking v C
and on simplification
v 1 i dt
(25)
1 s
dc C dc
G (s).G (s) K
v pi
v pi
v
2 2
(32)
The transfer function can be written as
ol
s v
G (s) Vdc 1
v I sC
(26)
The transfer function in closed loop
dc
1 s
Neglecting the power loss in the source resistance and power
G (s).G
(s)
v
(33)
losses in the switches, balancing the power on both sides,
v pi
cl
1 s
s 2 2
v
vsdicd
vdcidc
(27)
v K
From the above equation, we have
So the system behaves like a second order system. As
i v V 230 2
dc sd s
0.46
(28)
v
and the initial slope in magnitude plot at break
icd
vdc
Vdc
500 v K
With Vdc as the reference, the voltage control loop is shown in Fig.14 and it consists of inner d – axis current control loop. The
active power is supplied by the d axis current which is nothing but the ripple current of the capacitor. To make the steady state
point is approximately 20db/decade and hence it reduces to first order system. The value of K can be determined form root locus with approximate settling time as given in (34) and implementation block diagram is shown in Fig.15.
K
error of the voltage loop zero Proportional control is adopted here and it produces the reference d axis current for the control of the d axis current. The design of voltage controller is as follows:
The open loop transfer function of DC bus voltage controller is
K pv K 0.15, Kvi
200
C
(34)
Gop
K * Kdc
sC
(29)
The closed loop transfer function with unity feed back gain is
Gcl
1
1 sC K * Kdc
(30)
Fig.15: Implementing scheme for linear loads


SIMULATION RESULT 5.1: Simulation Result of the load:
Fig.17: System voltage and system current
A linear load, simulated with
R L
parameters (given in
Table.II), is connected to the grid at Bus no.5. The waveforms of the grid side phasea voltage ( vsa ) and current ( isa ) at point of common connection (PCC) (without the STATCOM in operation) are shown in Fig.16. It may be mentioned that here
and elsewhere (unless otherwise mentioned)
vsa is plotted to a
reduced scale of 10:1. Under steady state it is seen that the
Fig.18: System voltage and STATCOM current
power angle is
39.640 (so that power factor is
0.77 ). The
STATCOM will now act in closedloop with this system along with the proposed controllers in order to improve this power factor.
Fig.16: Grid phase a voltage and current
5.2. Simulation Results with STATCOM:
Then after PI controller is applied to control DC link voltage and PI controllers are used to control d and q axis current of STATCOM using the same control block as above. These controllers work and STATCOM functions at initial value of DC link voltage less than 550V. The relevant outputs at initial DC link voltage of 550V are shown in Fig.17 to 23.Fig17 shows the dynamics of system voltage and system current after using PI controller at DC link voltage and it shows the over shoot of only 20A and the same dynamics is obtained in case of STATCOM current as shown in Fig.18. Figs.19 and 20 show the dynamics of DC link voltage and current. Figs.21 and 22 show the change of STATCOM current and DC link voltage due to change of reference current (reactive current of load).The system voltage and the STATCOM voltage are shown in Fig.23 and both are inphase as it signifies for linear model. These controllers work well without spike at initial voltage of 700V as shown in Fig.24 of system voltage.
Fig.19: DC link voltage
Fig.20: DC link current
Fig.21: Reference current
Fig.22: DC link voltage due to change of reference current
Fig.23: System and STATCOM output voltage
Fig.24: System voltage and system current

CONCLUSION
The investigation of performances of the STATCOM for the power factor improvement with linear loads at weak bus connected to the grid has been carried out. The proposed control strategy has been simulated. The STATCOM works as a power factor compensator. It has also been shown that the interaction between a STATCOM device and a supply system makes the rural consumers healthy and wealthy.

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