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an essential role in the analysis of electrical power systems. They ... extended to custom power equipment, namely Distribution Static. Compensator ...
2009 Third International Conference on Power Systems, Kharagpur, INDIA December 27-29 Paper no.:229

Modeling of Facts and Custom Power Devices in Distribution Network to Improve Power Quality Ashwin Kumar Sahoo

T. Thyagarajan

Department of Electrical Engineering SSN College of Engineering Chennai, India Email: [email protected]

Department of Instrumentation Engineering MIT, Anna University Chennai, India Email: [email protected]

Abstract— Electromagnetic transient studies have always played an essential role in the analysis of electrical power systems. They provide priceless information relating to the behaviour of the system in the event of different forms of transient phenomena, which can hardly be achieved by other means. In this paper, an electromagnetic transient model of Fixed Capacitor/ Thyristor Controlled Reactor (FC-TCR) is developed and applied to the study of transients due to load variations. The work is then extended to custom power equipment, namely Distribution Static Compensator (D-STATCOM) and Dynamic Voltage Restorer (DVR) aimed at enhancing the reliability and quality of power flows in low voltage distribution networks. A new PWM based control scheme that requires only voltage measurements and no reactive power measurements has been proposed for both the custom controllers. PSCAD/ EMTDC simulation tool is used for model implementation and to carry out extensive simulation studies. Comprehensive results are presented to assess the performance of each device as a potential custom power solution. Keyword-Custom power, D-STATCOM, DVR, PSCAD/EMTDC, VSC

I.

The transient analysis and modeling is performed with the state-of-the-art digital simulator PSCAD/EMTDC V4.2 (Manitoba, 2005). PSCAD/ EMTDC is an industry standard simulation tool for studying the transient behavior of electrical networks. It provides a flexible user interface to make use of EMTDC, enabling an integrated visual environment that supports all aspects associated with the simulation, including circuit assembly, run-time control, analysis and reporting [2]. It’s great many modeling capabilities and highly complex algorithms are transparent to the user, leaving him free to concentrate his efforts on the analysis of results rather than on mathematical modeling. The paper is organized as follows: in Section II, the FCTCR arrangement of SVC is discussed. Section III presents the theory behind Voltage Source Converter (VSC) - Based Controllers namely, D-STATCOM and DVR. In Section IV, the PWM scheme adopted in this paper for D-STATCOM and DVR is described. Then in Section V, the test cases are presented and the simulation results are discussed and, finally, in Section VI, some conclusive remarks are drawn.

INTRODUCTION

The transient response of any natural system is the way in which the response of the system behaves as a function of time. Electromagnetic transient studies have always played an essential role in the analysis of electrical power systems. A considerable percentage of power system studies rely on electromagnetic transient simulations. They provide substantial information relating to the behavior of the system in the event of different forms of transient phenomena, which can hardly be achieved by other means. This paper addresses the transient studies of electrical networks with embedded, power electronics-based, FACTS and Custom Power (CP) controllers. The FACTS controller considered here is the basic Static Var Compensator (SVC) with FC-TCR arrangement. The CP controllers include Distribution Static Compensator (DSTATCOM) and Dynamic Voltage Restorer (DVR) .The modeling approach adopted in this paper is graphical in nature, as opposed to mathematical models embedded in code using a high- level computer language [3].

II.

STATIC VAR COMPENSATOR

The Static Var Compensators are the most widely installed FACTS equipment at this point in time. They mimic the working principles of a variable shunt susceptance and use fast thyristor controllers with settling times of only a few fundamental frequency periods. From the operational point of view, the SVC adjusts its value automatically in response to changes in the operating conditions of the network. By suitable control of its equivalent reactance, it is possible to regulate the voltage magnitude at the SVC point of connection, thus enhancing significantly the performance of the power system [6],[7],[8]. In its simplest form, SVC consists of a TCR in parallel with a bank of capacitors. Fig. 1(a) shows the schematic diagram of the most basic FC/TCR arrangement of the SVC. Its equivalent variable susceptance representation is shown in Fig. 1(b). An ideal variable shunt compensator is assumed to contain no resistive components, i.e. GSVC =0.

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2009 Third International Conference on Power Systems, Kharagpur, INDIA December 27-29 Paper no.:229 STATCOM). In its simplest form, it consists of a two-level VSC, a dc energy storage device; a coupling transformer connected in shunt with the ac system, and associated control circuits. Fig. 2 shows the schematic representation of the DSTATCOM. The active power flow is controlled by the angle between the ac system and VSC voltages and the reactive power flow is controlled by the difference between the magnitudes of these voltages [4]. The D-STATCOM controller continuously monitors the load voltages and currents and determines the amount of compensation required by the ac system for a variety of disturbances.

Figure 1(a). Schematic representation of an FC-TCR arrangement of SVC

Figure 2. Basic Configuration of D-STATCOM

The VSC connected in shunt with the ac system serves three distinct purposes: 1) Voltage regulation and compensation of reactive power; Figure 1(b). Variable Susceptance representation

Accordingly, it draws no active power from the network. On the other hand, its reactive power is a function of nodal voltage magnitude at the connection point, say node j , and the SVC equivalent susceptance, BSVC . Pj = 0 Qj = -│Vj │2 BSVC The graphical modeling and implementation of FC-TCR arrangement are explained in Section V of this paper. III.

VSC-BASED CONTROLLERS

In this section, an overview of the VSC-based Custom Power controllers is presented. A. D-STATCOM The STATCOM, when used in low-voltage distribution systems is normally identified as Distribution STATCOM (D-

2) Correction of power factor; 3) Elimination of current harmonics. In this paper, the D-STATCOM is used to regulate voltage at the point of connection. The control scheme is explained in Section IV of this paper. B. DVR The DVR consists of a VSC, a switching control scheme, a DC energy storage device and a coupling transformer similar to D-STATCOM, but here the coupling transformer is connected in series with the ac system. It is commonly used for voltage sag mitigation at the point of connection. Fig. 3 shows the schematic representation of DVR. The VSC generates a three-phase ac output voltage which is controllable in phase and magnitude. These voltages are injected into the ac distribution system in order to maintain the load voltage at the desired voltage reference. The control scheme implemented for DVR is explained in Section IV of this paper. 2

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2009 Third International Conference on Power Systems, Kharagpur, INDIA December 27-29 Paper no.:229

The D-STATCOM control system exerts voltage angle control as follows: an error signal is obtained by comparing the reference voltage with the rms voltage measured at the load point. The PI controller processes the error signal and generates the required delay angle to drive the error to zero, i.e., the load rms voltage is brought back to the reference voltage. In the PWM generators, the sinusoidal vcontrol signal is phasemodulated by means of the delay angle. The modulated signal vcontrol is compared against a triangular signal (carrier) in order to generate the switching signals for the VSC valves. V. Figure 3. Basic Configuration of DVR

IV.

PWM CONTROL SCHEME

This section describes the PWM-based control scheme with reference to the D-STATCOM and DVR. The aim of the control scheme is to maintain constant voltage magnitude at the point where a sensitive load is connected, under system disturbances. The control system only measures the rms voltage at the load point i.e., no reactive power measurements are required [1]. The VSC switching strategy is based on a sinusoidal PWM technique which offers simplicity and good response. Because distribution network is a relatively lowpower application, PWM methods offer a more flexible option than the fundamental frequency switching methods favored in FACTS applications. Besides, high switching frequencies can be used to improve on the efficiency of the converter, without incurring significant switching losses.

TEST CASES AND SIMULATION RESULT

This section is divided into three parts. Simulations relating to FC-TCR are presented first followed by the simulations carried out for D-STATCOM and DVR. A. FC-TCR Simulation and Results Fig.4 shows the test system implemented in PSCAD/EMTDC to carry out simulations for the FC-TCR topology. The aim of the SVC in this application is to provide voltage regulation following load variations. Initially, the SVC is operated in open-loop mode and for this condition, the power exchange between the SVC and the ac system is zero. When the breaker BRK1 is closed, the load is increased and the voltage at the load point experiences voltage sag of nearly 16%. When the load is increased, the SVC controller operation changes to closed-loop mode in order to adjust the SVC effective impedance Xsvc so that it injects capacitive current into the system to restore the voltage to the original value. The SVC parameters have been determined according to the compensation requirements for the case when the second load is connected.

Figure 4. Control Scheme and test system implemented in PSCAD/EMTDC to carry out FC-TCR simulation

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2009 Third International Conference on Power Systems, Kharagpur, INDIA December 27-29 Paper no.:229 The control scheme implemented for FC-TCR topology works as follows: The amplitude of the bus voltage Vbus is measured and filtered. Then it is compared against the voltage reference Vref. The voltage difference between the two signals is processed by a PI controller which causes a corresponding change in the firing angle α. The value provided by the PI controller is used as the input to the TCR firing control unit. The zero crossing of the Vbus voltage is taken as the reference for the firing angle. Two simulations are carried out as follows: 1) In the first case, SVC is operated in open-loop mode with α=115°. The voltage Vrms at the load point is close to 0.96p.u. At time t=0.6s, Load 2 is switched on by closing BRK1. Under this condition, the voltage at the load point drops by as much as 16%, giving a Vrms value equal to 0.8p.u., as shown in Fig.5. 2) In the second case, the closed-loop mode of SVC is enabled. At t=0.6s, the SVC begins to control the firing angle of the thyristors to α=162° so that the voltage is regulated and driven back to the original value as shown in the Fig.6.

Figure 6. Voltage Vrms and Vload with SVC in Closed loop mode

It is clear from these results that the SVC is an effective system controller which may be used to provide voltage regulation at the point of connection and to improve substantially the voltage quality in power systems. B. D-STATCOM Simulations and Results Fig.7 shows the test system implemented in PSCAD/EMTDC to carry out simulations for the DSTATCOM. The test system comprises a 230 kV, 100MVA transmission system, represented by a thevenin equivalent feeding into the primary side of a 3- winding transformer. A varying load is connected to the secondary side of the transformer. A basic two-level, six-pulse D-STATCOM is connected to the tertiary winding to provide instantaneous voltage support at the load point. The capacitor on the dc side provides the D-STATCOM energy storage capabilities.

Figure 5. Voltage Vrms and Vload with SVC in Open loop mode

A set of simulations was carried out for the test system shown in Fig. 7. The simulations relate to three main operating conditions:

Figure 7. Control Scheme and test system implemented in PSCAD/EMTDC to carry out D-STATCOM simulation

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2009 Third International Conference on Power Systems, Kharagpur, INDIA December 27-29 Paper no.:229 1) In the simulation period 0.3-0.6s, the load is increased by closing Switch D. In this case, the voltage drops by almost 20% with respect to the reference value.

Fig. 8(a) shows the rms voltage at the load point for the case when the system operates without D-STATCOM. Similarly, a new set of simulations was carried out but now with the DSTATCOM connected to the system.

Vrms(pu) Vab (kV)

15.0 10.0 5.0 0.0 -5.0 -10.0 -15.0

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Figure 8(b). RMS and Line Voltage with D-STATCOM

rms, line voltage 1.20 1.00 0.80 0.60 0.40 0.20 0.00

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3) In the simulation period 0.9–1.2s, Switch B is closed, connecting a capacitor bank to the high voltage side of the network. The rms voltage increases 20% with respect to the reference voltage.

1.20 1.00 0.80 0.60 0.40 0.20 0.00

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2) At 0.6s, the switch D is opened and remains so throughout the rest of the simulation. The load voltage is very close to the reference value

rms, line voltage rms voltage

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The results are shown in Fig. 8(b) , where the very effective voltage regulation provided by the D-STATCOM can be clearly appreciated. When the load Switch D closes, the DSTATCOM supplies reactive power to the system, and when Switch D opens and Switch B closes, the DSTATCOM absorbs reactive power in order to get the voltage back to reference. In spite of sudden load variations, the regulated rms voltage shows a reasonably smooth profile, where the transient overshoots are almost nonexistent. The magnitude of these transients is kept very small with respect to the reference voltage.

Figure 8(a). RMS and Line Voltage without D-STATCOM

Figure 9. Control Scheme and test system implemented in PSCAD/EMTDC to carry out DVR simulation

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2009 Third International Conference on Power Systems, Kharagpur, INDIA December 27-29 Paper no.:229 C. DVR Simulations and Results Fig. 9 shows the test system used to carry out the various DVR simulations. The DVR coupling transformer is connected in delta in the DVR side, with a leakage reactance of 10%. A unity transformer turns ratio was used i.e. no booster capabilities exist. The capacity of the dc storage device is 5 kV. Two simulations are carried out as follows: 1) The first simulation contains no DVR and a three phase fault is applied near the source point, via a fault resistance of 0.66 Ω, during the period 0.3–0.6s. The voltage sag at the load point and the variation of load rms voltages for a three phase short circuit fault is presented in Fig.10. 2) The second simulation is carried out using the same condition as above but now with the DVR in operation.

point is maintained close to the original value, as shown in Fig. 11. The PWM control scheme controls the magnitude and the phase of the injected voltages, restoring the rms voltage very effectively. The sag mitigation is performed with a smooth, stable, and rapid DVR response; no transient overshoots are observed when the DVR comes in and out of operation .It should be noted that in the DVR, the dc voltage is supplied by a dc source as opposed to the dc capacitor used in the DSTATCOM. Several simulations were carried out to assess the performance of the DVR. As expected, the DVR required a higher rating of dc storage device to provide appropriate levels of sag mitigation when the fault was applied near the source point. This is due to the short electrical distance between the point in fault and the DVR coupling transformer. Clearly, the Controller must be designed to satisfy the most sever case, where the voltage sag is due to a fault quite close to the sensitive load. VI.

Figure 10. Variation of load RMS voltage and line voltage during three phase fault without DVR

CONCLUSION

This paper has discussed the electromagnetic transient models of FC-TCR arrangement of Static Var Compensator (SVC), Distribution STATCOM (D-STATCOM) and Dynamic Voltage restorer (DVR). These models were then applied to the study of power quality and their voltage regulation capabilities were studied. A new PWM-based control scheme has been implemented to control the power switches in the VSC used in the D-STATCOM and DVR. This PWM control scheme only requires voltage measurements as against fundamental frequency switching schemes already available in the PSCAD/EMTDC. This characteristic makes it ideally suitable for low-voltage custom power applications. The control scheme was tested under a wide range of operating conditions, and it was observed to be very robust in every case. The simulations carried out showed that the FC-TCR arrangement of SVC, the DVR and the D-STATCOM provide excellent voltage regulation capabilities. REFERENCES [1] [2] [3]

[4]

Figure 11. Variation of load RMS voltage and line voltage during three phase fault with DVR

Using the facilities available in PSCAD/EMTDC, the DVR is simulated to be in operation only for the duration of the fault, as it is expected to be the case in a practical situation. When the DVR is in operation the voltage sag is mitigated almost completely, and the rms voltage at the sensitive load

[5] [6] [7] [8]

O. Anaya-Lara, E. Acha, “Modeling and analysis of custom power systems by PSCAD/EMTDC”, IEEE Trans. on Power Delivery, Vol. 17, No. 1, pp.266-272, January 2002. Introduction to PSCAD / EMTDC, Manitoba HVDC Research Centre, March 2000. A. M. Gole , O. B. Nayak, T. S. Sidhu, and M. S. Sachdev, “A graphical electromagnetic simulation laboratory for power systems engineering programs,” IEEE Trans. Power Syst., vol. 11, pp. 599–606, May 1996 E Acha, V G Agelidis, O Anaya-Lara, T J E Miller, “ Power Electronic Control in Electrical Systems”, Newnes Power Engineering series,2002. “Introducing custom power,” IEEE Spectrum, vol. 32, pp. 41–48, June 1995. N. Hingorani, “FACTS — Flexible ac transmission systems,” in Proc.IEE5th Int. Conf. AC DC Transmission, London, U.K., 1991, Conf. Pub. 345, pp. 1–7. N. G. Hingorani and L. Gyugyi, Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems. New York: Wiley, 2000, vol. I Mathur, R. Mohan and Varma, Rajiv K., “ Thyristor-Based Facts Controllers for Electrical Transmission Systems”, John Wiley & Sons.

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2009 Third International Conference on Power Systems, Kharagpur, INDIA December 27-29 Paper no.:229

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