STATCOM Design for Voltage Control using Synchronous Vector PI Controller

STATCOM Design for Voltage Control using Synchronous Vector PI Controller

Raj Kamal Kakoti Electrical Department GIMT

Azara, Guwahati, Assam 781017

Abstract The objective of this paper is to improve Voltage profile of the system by controlling reactive power using a synchronous vector PI controller. Voltage in a power system is mainly affected by reactive loads and faults. This paper proposes modeling of a STATCOM (STATic+ COMpensator) with a synchronous vector PI controller under a nonlinear load. The prototype has been programmed in MATLAB/SIMULINK environment which has been tested on a standard IEEE 14 bus system. It has been observed that after installing STATCOM the voltages of the buses are found to be within the acceptable limits.

Keywords FACTS, Reactive power, PI vector controller, STATCOM, Voltage control.

1. INTRODUCTION

   After deregulation of the electricity market, the electrical utilities have been trying to improve the efficiency of the power system networks. One of the most concerned problems today is voltage swell or sag [1]. The voltage sag/swell magnitude is ranged from half cycle to one minute. System voltage is directly proportional to the reactive power of the system. Thus whenever there is a disturbance in the reactive power of the system, the system voltage profile gets disturbed. Moreover increased penetration of renewable energy sources such as wind and solar power plants is also disturbing the reactive power of the system. As for instance the solar power plant is employing a great number of semiconductor devices to convert the solar energy into electrical energy [2-3], the wind power plant is also absorbing great VARs during its running condition [3-4]. So all these problems are having a great impact on reliable and secure power supply which is very important in the world of Globalization and Privatization of electrical systems. New approaches have been coming up for power system operation and control for congestion management[5-6], reliable operation and counteracting of various dynamic disturbances[7] such as transmission lines switching, loss of generation, short-circuits and load rejection. To counteract these problems, the power industries are shifting towards large deployment of FACTS devices [8]. The FACTS

   mitigation of system oscillations [9]. Over the period of time various FACTS technologies were developed [10]. With respect to FACTS equipment, voltage sourced converter (VSC) technology, which utilizes self-commutated technique such as GTOs, GCTs and IGBTs has been successfully applied in a number of installations world-wide for static synchronous Compensator [11-12], unified power flow controllers [13] and back to back dc ties [14].

   Amongst the FACTS devices, STATCOM is the most preferred device in the power industries for voltage and reactive power compensation [15]. Analogous to a synchronous condenser, STATCOM is operated as a shunt connected static VAR compensator whose capacitive and inductive output current can be controlled independent of ac system voltage. The converters used in STATCOM can be either voltage source or current source converters but voltage source converters are more economical [16] due to the high conduction losses in switches of current source converters

   .The shunt connection of STATCOM to the Grid is shown in Fig.1

   STATCOM provides the following advantages:

   • Quick response to system disturbance.

   • Smooth voltage control over a wide range of operating conditions.

   • Damping of power oscillations.

   • Transient stability improvement.

   • Alsop control of active power possible(with a DC energy source).

   • No Sub Synchronous Resonance.

   • Less space required due to availability in modules.

   Vdc

   Coupling Transformer

   technology has the principal role of enhancing controllability and power transfer capability in power system. This technology enables the loading of transmission line closer to its thermal limit. FACTS devices can also be effectively used for power flow control, load sharing along parallel feeders, voltage regulation, and enhancement of transient stability and

   AC system

   Voltage sourced

   converter

   Fig 1: Connection of a STATCOM to ac system

   The ideal V-I characteristic of a STATCOM is shown in Fig. 2. STATCOM can be used both for leading VARs and

   		Vr
   		Vy
   		Vb

   		Vr
   		Vy
   		Vb

   Rt Lt Er

   lagging VARs compensation.

   ing			voltage		swi
   		Rated capacitive current	Rated inductive current

   ing			voltage		swi
   		Rated capacitive current	Rated inductive current

   Limit for safe

   switch

   STATCOM

   Limit for safe tching

   Rt Lt

   Rt Lt

   Ey Eb

   Voltage sourced

   Rsc

   Vdc

   AC system

   Coupling Transformer

   equivalent

   converter

   Figure 4: Equivalent Diagram of a STATCOM

   Control range

   STATCOM

   TABLE I NOMENCLATURE

   Leading VARs Lagging VARs

   current

   Figure 2: Ideal V-I characteristic of STATCOM

   By implementing various hybrid converters the ideal characteristic can be made drooping as per the requirement for VAR compensation [17]. One of such modified drooping characteristic is shown in Fig.3.

   Limit for safe switching

   Rated capacitive current

   STATCOM

   voltage

   Rated inductive current

   Limit for safe switching

   Control range

   STATCOM

   Symbol	Description
   Vr, Vy, Vb	Line voltages
   Rt	Equivalent resistance of the coupling transformer
   Lt	Equivalent inductance of the coupling transformer
   ws	System Frequency
   Er , Ey , Eb	Voltages available in the STATCOM output
   RSC	Switching loses equivalent resistance
   VDC	Voltage available across the capacitor
   C	Capacitance across the converter
   s	Phase angle
   Vs	R.M.S phase voltage
   	Firing angle
   k	Factor relating Vdc and Vs

   Symbol	Description
   Vr, Vy, Vb	Line voltages
   Rt	Equivalent resistance of the coupling transformer
   Lt	Equivalent inductance of the coupling transformer
   ws	System Frequency
   Er , Ey , Eb	Voltages available in the STATCOM output
   RSC	Switching loses equivalent resistance
   VDC	Voltage available across the capacitor
   C	Capacitance across the converter
   s	Phase angle
   Vs	R.M.S phase voltage
   	Firing angle
   k	Factor relating Vdc and Vs

   current

   Leading VARs Lagging VARs

   Figure 3: Drooping V-I characteristic of STATCOM

   The ac KVL equations for the circuit can be expressed in matrix form,

   dir

2. MATHEMATICAL MODELLING OF THE SYSTEM

   dt

   i E V

   1. Modelling of STATCOM

      di

      R r

      r r

      1

      y t i E

      V

      (1)

      The methodology proposed in this paper as with reference to [17], under the method of linearization. In fig 4. the

      dt L y

      t i

      L y y

      t E V

      equivalent model of the STATCOM being connected to the AC supply has been shown.

      dib

      dt

      b

      b b

      The AC system phase voltage and the output of the STATCOM (neglecting harmonics) is given by (2) & (3) respectively,

      Vr

      2Vs sin(wst s )

      (2)

      Vr kVDC sin(wst )

      (3)

      The above system is now transformed to a synchronous reference frame (on p.u. basis),

      did

      dt

      controllers of current vector components in a rotating synchronous frame (d-q) [21]. This controller has been used

      id

      Vs coss

      as the control scheme in this paper as this scheme eliminates

      diq

      A * i

      ws V

      sin

      (4)

      the errors completely and also operates satisfactorily in high

      dt

      s q

      L s s

      frequency system. The only disadvantage of this controller is

      V

      t 0

      that its dynamic properties are inferior to that of other non-

      dVDC

      DC

      linear controllers.

      PWM

      modulator

      PWM

      modulator

      dt isdref

      Current regulator

      Coordinate transformation

      where,

      R w kw cos( )

      PI

      isq

      isq

      –

      –

      +

      PI

      PI

      ref

      DQ

      – To RYB

      – To RYB

      to –

      Three- phase converter

      w

      w

      t s

      L

      s s +

      s L –

      – To DQ

      – To DQ

      RYB

      to –

      RYB

      to –

      t t

      isd ir

      A w

      Rt ws kws sin( s ) (5) iy

      s s

      Lt Lt

      Cw

      isq

      ib

      Non

      M cos( )

      M sin( ) s

      linear

      and

      k s k s

      Lt

      Figure 5: Synchronous vector Controller (PI) Scheme

      load

      Mk 1.5* k * ws *C

      (6)

3. IMPLEMENTATION

   The injected active power and reactive power at the bus

   connected to the STATCOM are given by,

   The design of STATCOM with Synchronous Vector PI controller, presented in Fig. 4, has been implemented on a

   P Vs *cossid sinsiq

   Q Vs *sinsid cossiq

   (7)

   (8)

   standard IEEE 14 bus system (Fig. 6) using MATLAB. Table II represents the data of IEEE 14 bus system [22].

   The characteristic equation of the linearized system of (4) is 1

   13 14

   12

   given by,

   2R w w C R w R w 2w C 3k 2w2C

   11 10 8

   6 9

   7

   7

   s3 s2 t s s s t s t s s w2 s

   L R L L R

   s 2L 5

   t SC t

   t SC t

   w3C R2 3k 2 w3CR

   (9)

   s 1 t s t 0

   2 3 4

   R L2 2L2

   SC t t

   Now with reference to [18], the per unit values of

   Rt = 0.01, Lt=0.15, C=0.88, Rsc =100/K, K=4/, ws=377

   are taken and the characteristic equation (9) has been solved for the Eigen values in MATLAB.These parameters yield the following Eigen values for the Linearized system,

   s=-3.8 & s=-25.35±j147

   These values show that the STATCOM is highly overdamped and also has a high frequency of oscillations. Now in steady state (4) has been solved.

   Figure 6: Standard IEEE 14 bus system

   TABLE II

   BUS DATA OF 14-BUS SYSTEM

   Bus	Voltage	Angle	PGi	QGi	PLi	QLi
   1	1.0600	0.00	188.8 6	-9.90	188.8 6	0.00
   2	1.0450	-4.05	40.00	34.63	21.70	12.7 0
   3	1.0100	-11.17	0.00	23.90	94.20	19.0 0
   4	1.0202	-8.237	0.00	0.00	47.80	– 3.90
   5	1.0232	-6.915	0.00	0.00	7.60	1.60
   6	1.0700	-10.32	20.00	15.83 8	11.20	7.50
   7	1.0528	-9.397	0.00	0.00	0.00	0.00
   8	1.0900	-7.638	20.00	23.31 1	0.00	0.00
   9	1.0356	-11.16	0.00	0.00	29.50	16.6 0
   10	1.0341	-11.30	0.00	0.00	9.00	5.80
   11	1.0482	-10.94	0.00	0.00	3.50	1.80
   12	1.0537	-11.20	0.00	0.00	6.10	1.60
   13	1.0473	-11.27	0.00	0.00	13.50	5.80
   14	1.0225	-12.23	0.00	0.00	14.90	5.00

   td>

   1.0900

   Bus	Voltage	Angle	PGi	QGi	PLi	QLi
   1	1.0600	0.00	188.8 6	-9.90	188.8 6	0.00
   2	1.0450	-4.05	40.00	34.63	21.70	12.7 0
   3	1.0100	-11.17	0.00	23.90	94.20	19.0 0
   4	1.0202	-8.237	0.00	0.00	47.80	– 3.90
   5	1.0232	-6.915	0.00	0.00	7.60	1.60
   6	1.0700	-10.32	20.00	15.83 8	11.20	7.50
   7	1.0528	-9.397	0.00	0.00	0.00	0.00
   8	-7.638	20.00	23.31 1	0.00	0.00
   9	1.0356	-11.16	0.00	0.00	29.50	16.6 0
   10	1.0341	-11.30	0.00	0.00	9.00	5.80
   11	1.0482	-10.94	0.00	0.00	3.50	1.80
   12	1.0537	-11.20	0.00	0.00	6.10	1.60
   13	1.0473	-11.27	0.00	0.00	13.50	5.80
   14	1.0225	-12.23	0.00	0.00	14.90	5.00

   (10)

   (11)

   1. Modeling of Control scheme

   Various current control schemes for three phase voltage source PWM converters have been explained in [20]. Out of these schemes the Synchronous vector controller (PI) been implemented in this paper. The schematic diagram of this scheme is shown in Fig 5. The synchronous vector PI controller is used when the phase or amplitude errors are needed to be completely eliminated. It employs two PI

4. RESULTS

   Load flow analysis is performed initially to identify bus voltages which are out of the tolerance level (5%). The results of load flow analysis in steady state show that the voltages in buses 9 and 13 violate the tolerance level. Hence the modeled STATCOM is connected to these buses to bring the voltage level within tolerance limit.

   Table III shows the load flow analysis of 14-bus system. It can be observed from Fig. 7 that the voltage profile is going out of the tolerance level in bus 9 and 13.

   Table IV shows the load flow analysis of 14-bus system with STATCOM been connected at buses 9and 13 respectively. It is evident from Fig. 8, that the voltage profile has been improved considerably. Thus it can be concluded that STATCOM results in improvement of voltage profile by controlling the reactive power.

   TABLE III

   RESULTS OF LOAD FLOW WITHOUT STATCOM

   Bus	Voltages	Angle
   1	1.0400	0.0000
   2	1.0430	-5.3543
   3	1.0196	-7.5308
   4	1.0104	-9.2841
   5	1.0100	-14.1738
   6	1.0392	-14.0644
   7	1.0020	-12.8649
   8	1.0100	-11.0581
   9	0.9335	-16.5031
   10	1.0145	-15.6550
   11	0.9991	-16.3007
   12	0.9944	-16.9077
   13	0.9428	-17.8067
   14	1.0132	-16.0084

   Figure 7: Graph of load flow studies without STATCOM

   TABLE IV

   RESULTS OF LOAD FLOW WITH STATCOM

   Bus	Voltages	Angle
   1	1.0400	0.0000
   2	1.0430	-5.3543
   3	1.0200	-7.5318
   4	1.0108	-9.2848
   5	1.0190	-14.1692
   6	1.0402	-14.0508
   7	1.0023	-12.8655
   8	1.0100	-11.8168
   9	1.0000	-16.7794
   10	1.0240	-15.7112
   11	1.0091	-11.7434
   12	1.0076	-17.0359
   13	1.0000	-18.0205
   14	1.0330	-15.0935

   Figure 7: Graph of load flow studies with STATCOM

5. CONCLUSION

This paper illustrates the design of a STATCOM model using a synchronous vector PI control technique to meet the voltage dip problem. The results obtained and the analyses are the justifications of the design.

The present work can be extended to incorporate the following sectors:

1. Conduction loss calculation of the switches.

2. Harmonic analysis and appropriate filter design.

3. Operation during various fault conditions.

4. Optimization of the capacitance value.

5. Dynamic performance.

New trends in the current control techniques have come up such as hysteresis controller neural networks and fuzzy-logic based controllers. These controllers can also be implemented on the model discussed above.

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