Yellaiah Ponnam et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 1( Version 1), January 2014, pp.248-255

RESEARCH ARTICLE

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Power Quality Improvement Using Hybrid Power Flow Controller in Power System Manidhar Thula1, Voraganti David2 ,Yellaiah Ponnam3 (Assistant Professors in Dept.of EEE, GNIT, Ibrahimapatnam Affiliated to JNTU Hyderabad, A.P) 1,2,3

Abstract This paper discusses the applicability of Hybrid Power Flow Controller (HPFC) as an alternative to Unified Power Flow Controller (UPFC) for improvement of power system performance. UPFC is a flexible AC transmission system (FACTS) device containing two switching converters, one in series and one in shunt. To configure the HPFC, one of the switching converters of the UPFC is replaced by thyristor controlled variable impedances, thus reducing the cost. In this paper, the HPFC has been configured by multilevel Voltage Source Converter (VSC) used for the shunt compensation branches and a thyristor controlled variable impedance used for series compensation. It is shown that with suitable c o n t r o l the HPFC can inject a voltage of required magnitude in series with the line at any desired angle, just like UPFC. This helps in providing compensation equivalent to UPFC and improving the steady state stability limits of the power system. Keywords — Flexible AC Transmission Systems, Unified Power Flow Controller, Hybrid Power Flow Controller. comparatively cheaper. In case it is imperative to install a UPFC in I. INTRODUCTION a particular line in a given system, the idea of the The demand for electrical power is rising Hybrid Power Flow Controller (HPFC) proposed in across the world. Setting up of new generating [5] can possibly be an alternative solution without facilities and building or upgrading the significant reduction in versatility. The HPFC is a transmission system is constrained by economic blend of switching converter based FACTS devices and environmental factors. Flexible AC along with variable impedance type FACTS Transmission System (FACTS) provides an avenue devices. The motivation behind the proposal of the to utilize the existing system to its limits without HPFC is to provide possible alternative solutions to endangering the stability of the system, thus the UPFC as far as economy is concerned, and to providing efficient utilization of the existing system. improve the dynamic performance of the Variable FACTS devices can be broadly classified Impedance type FACTS devices via coordination into two types, namely (a) Variable Impedance with VSC based FACTS devices. In order to type devices, e.g. Static Var Compensator (SVC) conserve the properties of the UPFC, and to or Thyristor Controlled Series Capacitor (TCSC) configure the HPFC, the shunt converter in the and (b) Switching Converter type devices which UPFC is replaced by two half sized shunt converters generally use Voltage Source Converters with their DC links connected back to back, so that (VSC‟s), e.g. Static Synchronous Compensator the effective cost of the shunt converter remains (STATCOM) or Unified Power Flow Controller comparable. On the other hand, the series converter (UPFC). The dynamic performance of VSC based has been replaced by a thyristor controlled variable FACTS devices have been observed to be better impedance type FACTS device which reduces the than that of the variable impedance type FACTS cost of the series compensator considerably. devices [1]. Among the VSC based FACTS devices, The steady state analysis of the HPFC the UPFC [2, 3] is capable of controlling all the has been presented in [5] with simplified models. parameters that effect power flow in a transmission This paper focuses on the control structure and the line either simultaneously or selectively. But the comparison of the steady state performance of the main constraint in the use of the UPFC is its cost. HPFC with a model of the UPFC of equivalent The VSC especially for the transmission voltage rating. In the configuration of the HPFC, the two level comes at a very high cost. There are shunt VSC‟s are multilevel converters to suit the reportedly very few installations of UPFC around higher voltage level. A fixed capacitor with the world [4], as compared to the number of Thyristor Controlled Reactor (TCR) in parallel has installations of SVC and TCSC which are been used as the series compensator. A metal oxide www.ijera.com

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Yellaiah Ponnam et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 1( Version 1), January 2014, pp.248-255 varistor (MOV) is also connected in parallel to provide protection against over voltages. The models of the HPFC and a UPFC of equivalent rating have been connected in a single machine infinite bus (SMIB) system one at a time and the steady state performance have been compared. The complete system has been simulated using PSCAD/EMTDC.

II.

THE CONCEPT OF THE UPFC & THE HPFC

A. Unified Power Flow Controller The UPFC is configured as shown in Fig. 1. It comprises two VSC‟s coupled through a common dc terminal. VSC–1 is connected in shunt with the line through a coupling transformer and VSC–2 is inserted in series with the transmission line through an interface transformer. The DC voltage for both converters is provided by a common capacitor bank (CDC). The series converter is controlled to inject a voltage Vpq in series with the line, which can be varied between 0 and Vpqmax. Moreover, the phase angle of the phasor Vpq can be varied independently

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B. Hybrid Power Flow Controller (HPFC) The configuration of the HPFC followed in this paper is shown in Fig. 2. It comprises of two VSC‟s coupled through a common DC circuit. The VSC‟s are connected in shunt with the transmission line through coupling transformers, each on either side of the TCSC. Each VSC is half the rated capacity of the shunt VSC in the UPFC. The proposed version of HPFC in [3] used Current Sourced Converters (CSC) in shunt. However, VSC has been chosen in this paper due to the fact that VSC‟s offer better dynamic performance when compared to CSC‟s and also VSC‟s use self commutated converters which offer better versatility when compared to the line commutated converters used in CSC‟s. Also line commutated converters have the risk of having a commutation failure which does not occur in self commutated converters. Just like the UPFC, the HPFC injects a voltage in series with the transmission line voltage and by varying the phase angle of this voltage vector, offers control of the real and reactive power flow through the line. The magnitude of the injected series voltage can be varied by varying the impedance of the series compensator through the firing angle of the thyristors. The phase angle of the injected series voltage can be controlled by controlling the VAR outputs of the shunt compensators. Actually the injected voltage is the vector difference between the voltages V1 and V2. Therefore the angle of the injected voltage can be

Figure 1. Basic Configuration of the UPFC. between 0o and 360o. In this process the series converter exchanges both real and reactive power with the transmission line. While the reactive power is internally enerated/absorbed by the series converter, the real power generation/absorption is made feasible by the DC capacitor. VSC–1 is mainly used to supply the real power demand of VSC–2, which it derives from the transmission line itself. The shunt converter maintains the dc bus voltage constant. Thus the net real power drawn from the ac system is equal to the losses of the two converters and their coupling transformers. In addition, the shunt converter functions like a STATCOM and to regulate the terminal voltage of the interconnected bus independently, by generating/absorbing requisite amount of reactive power.

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Figure 2. Basic Configuration of the HPFC. varied by varying the magnitudes of V1 and V2. These magnitudes depend on the reactive power output of the shunt connected converters and hence can be controlled. This can be explained using Fig. 3. Considering a constant bus voltage V2, and a particular value of the magnitude of the injected voltage Vc, angle of Vc will vary along a circular locus depending on the magnitude of bus voltage V1.

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Yellaiah Ponnam et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 1( Version 1), January 2014, pp.248-255

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UPFC & HPFC

Figure 3. Injection of Series Voltage by the HPFC.

Figure 4. Multilevel Inverter (3-level) Here VCmax and VCmin are determined by the limits of the variable impedance of the series compensator. The shunt compensators draw a small amount of active power from the line in order to maintain the DC bus voltage constant. C. Voltage Source Converter (VSC) A VSC is essentially a self commutated DC to AC converter, generating balanced three phase voltages. The configuration shown in Fig. 4 is a basic diode clamped multilevel inverter. The switching device is Insulated Gate Bipolar-junction Transistor (IGBT). Pulse Width Modulation (PWM) switching technique is used to get an output voltage closer to sinusoid. In this paper, multilevel inverter [6, 7] has used so that the voltage stress on each switch is reduced. Also the use of multilevel inverter reduces the harmonic content of the voltage generated by the VSC.

III.

CONTROL STRUCTURE OF THE

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A. The Shunt Compensator Control Strategy Fig 5 shows the control structure of the shunt converter [8 11]. The main objective of this control is to maintain required voltage at the point of common coupling (PCC) and to control of the DC link capacitor voltage simultaneously. These two control actions take place in a decoupled manner by the use of Parks transformation. A phase locked loop (PLL) synchronizes the positive sequence component of the three-phase terminal voltage at PCC. The outer loop of the PCC voltage regulator compares the voltage reference (Etref) with the measured PCC voltage and the error is fed to a PI controller which provides the reference current for the quadrature axis, Iqref. In the inner loop, this Iqref is compared with the measured value of quadrature axis current (Iq) and the error is fed to a second PI controller. As Iq is in quadrature with the terminal voltage, the reactive power output of the converter (and in turn the PCC voltage) is controlled through this part of the controller. The outer loop of the dc link voltage regulator compares the preset dc link voltage reference (VDCref) with the measured dc link voltage and the error is fed to a PI controller which provides the reference current for the direct axis, Idref. In the inner loop, this Idref is compared with the measured value of direct axis current (Id) and the error is fed to a second PI controller. The direct axis current (Id) being in phase with the terminal voltage helps to control the active power so as to either increase or decrease the DC link voltage (and to supply the active power requirements of the series converter in the case of the UPFC). The current regulators (inner loop) generates signals Esd and Esq. These are then transformed to a-b-c frame to get the reference waves for the PWM. These signals are compared with the carrier waves (which are triangular waves whose peak to peak value is either equal to or greater than the amplitude of the reference waves) in order to generate the PWM switching pulses for the inverter. B. The Series Compensator Control Strategy As mentioned in section I, the series compensator of the HPFC consists of a fixed capacitor shunted by a TCR. The control structure for this compensator [12] is shown in Fig. 6(b). The active power flow (P) through the line containing the series compensator is taken as the control variable. The measured value of P is compared with the reference value of active power 250 | P a g e

Yellaiah Ponnam et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 1( Version 1), January 2014, pp.248-255 flow (Pref) and the error is fed to a P-I controller. The output of the P-I controller is the firing angle (α) of the thyristors of the TCR. This value of firing angle (α) is limited between 1450 and 1800 to keep the net impedance of the compensator within the capacitive operation zone (α). The output of the limiter is supplied to the firing circuit of the series compensator. In case of

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UPFC, the series converter provides simultaneous control of real and reactive power flow in the transmission line. To do so, the series converter injected voltage is decomposed into two components. One component of the series injected voltage is in quadrature and the other in-phase with the line current „i‟.

Figure 5. Control structure for the shunt converter for the UPFC as well as the HPFC. Transmission line reactive power (Q2) is controlled by modulating the bus voltage reference „V2‟. The voltage „V2‟ is controlled by injecting a component of the series voltage in- phase with the line current „i‟.

IV. Fig. 6. (a) Basic module of the series compensator. (b) Control structure.

Fig. 7 Control structure for the series converter for the UPFC. The quadrature injected component controls the transmission line real power flow. The in-phase component controls the transmission line reactive power flow. Fig. 7 shows the series converter control system [8]. The transmission line real power flow (Pline) is controlled by injecting a component of the series voltage (Vseq) in quadrature with the line current „i‟. The www.ijera.com

COMPARISON OF RESULTS OF COMPENSATION WITH HPFC AND UPFC

The HPFC and the UPFC have been tested in a Single Machine Infinite Bus (SMIB) system shown in Fig. 8. The generator has been modeled as a voltage source behind the transient reactance (Classical model). Detailed data of the SMIB system, the HPFC and the UPFC are given in the Appendix (Table A1 and Table A2). At first, with no compensator connected in the system, 73 MW power flows through the transmission line from the alternator to the infinite bus when the angle between the generator voltage and infinite bus voltage (δ) is kept equal to 22°. Now the HPFC is connected as shown in Fig.2. The PCC voltages for both the converters (V1 and V2) are

maintained at 230 kV and the angle δ is maintained at 22°. This results in an increase in the power flow through the line to 100 MW. A plot of the steady state power in the uncompensated and the compensated system is shown in Fig. 9. This increase in power flow takes place because of the voltage injection by the HPFC. 251 | P a g e

Yellaiah Ponnam et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 1( Version 1), January 2014, pp.248-255

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Fig. 8 Schematic diagram of the SMIB system The corresponding phasor diagram of V1, V2 and injected voltage Vc is shown in Fig. 10. Next, the power flow through the line is maintained at 100 MW and the angle δ is allowed to vary. The steady state values of δ for the uncompensated system and the system compensated by HPFC are plotted in Fig. 11. It can be seen that the value of δ for the uncompensated system is 0.54 radian which comes down to 0.39 radian when HPFC is connected to the system. A decreased value of δ means more power can be transmitted through the line. Now if the UPFC is connected to the system, the value of δ becomes 0.35 radian which is also shown in Fig. 11. Thus all these results indicate that compensation with HPFC increases the power carrying capacity of a transmission line and the effect of HPFC and UPFC in this regard are comparable.

Fig. 11 Value of δfor uncompensated and compensated system

Fig. 12. Comparison of the steady state reactive power generation: Case5.

Fig. 9 Power flow for uncompensated and HPFC compensated system

Fig. 10 PCC voltages of the shunt converters of the HPFC and the series voltage injected by the HPFC www.ijera.com

Simulations have been performed for the UPFC and the HPFC in order to prove that, just like the UPFC, the HPFC injects a voltage source of controllable magnitude and phase angle, in series with the transmission line. In order to fulfill this particular objective, the following cases have been considered where the reference variables for the UPFC and the HPFC have been adjusted in such a manner that the bus voltages „V1‟ and „V2‟ are maintained at: Case 1: V1 = V2 = 230 KV (Line to line). Case 2: V1 = 237 KV, V2 = 230 KV. Case 3: V1 = 222 KV, V2 = 230 KV. Case 4: V1 = 235 KV, V2 = 225 KV Case 5: V1 = V2 = 237 KV. Case 6: V1 = 225 KV, V2 = 235 KV

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Fig. 13. Phasor diagrams showing the injected series voltage - cases 1, 2 and 3.

Fig. 14. Phasor diagrams showing the injected series voltage - case 5.

Case 1

TABLE I COMPARISON OF INJECTED SERIES VOLTAGES HPFC UPFC Voltage Phase angle of Voltage Phase angle of across the the injected across the the injected series series voltage series series voltage branch with respect to branch with respect to bus „V bus „V02‟ 18.05 KV -94.0474 0 2‟ 19.12 KV -94.5880

Case 2 Case 3

17.35 KV -80.31950 19.55 KV -108.03560

18.60 KV 21.05 KV

-81.85860 -107.72250

Case 4

18.36 KV -75.65630 16.32 KV -93.65220 18.95 KV -111.76280

19.64 KV

-77.59800 -94.17210

Case 5 Case 6

17.64 KV 20.35 KV

-111.46310

TABLE II OPERATING CONDITIONS OF HPFC AND UPFC: CASE 5 Active power of the shunt branch

VSC – 1

HPFC -1.1891 MW

VSC – 2

-0.9768 MW

Active power of the series branch Reactive power of the shunt branch Reactive power of the series branch Voltage across the series branch Power flow through the line

0.0315 MW 17.3178 MVAR 17.3190 MVAR

VSC – 1 VSC – 2

UPFC 0.5990 MW -2.1685 MW 31.6142 MVAR

In all the cases, the synchronous machine has been treated as a constant voltage source with the sending end voltage at 230 KV, both the UPFC and the HPFC try to maintain the power flow through the line constant at 100 MW. Fig. 12 compares the steady state operating condition of the HPFC and the UPFC for case 5. Fig 13 and 14 show the phasor diagram of the injected series voltage of the UPFC and the HPFC for cases 1 to 4 as above. A comparison of the magnitude and phase angle of HPFC with those for the UPFC is given in Table I. It can be seen that the magnitude and angle of the voltage injected by the HPFC for all the five case are pretty close to those in case of compensation by UPFC. Similarly, Table II shows a comparison of active and reactive power of the series and shunt branch and line power flow for compensation with HPFC and UPFC. It is clearly understood from Figures 11, 12, and table I, that the HPFC behaves just like the UPFC in its principle, in other words, the HPFC injects a voltage source of controllable magnitude and phase angle, in series with the transmission line, thus controlling the real and reactive power flow through the line. Also Fig 10 shows that the reactive power generated by the VSC‟s is found to be almost the same. Hence the fact that two half sized VSC‟s are used for the HPFC is justified.

V.

CONCLUSION

In this paper, the steady state performance of the HPFC has been studied. The HPFC configuration used here has two shunt connected VSC‟s around a series connected variable impedance type reactive compensator. The control structure for the HPFC and the UPFC has been presented. The HPFC and the UPFC have been connected to an SMIB system. It has been shown that the HPFC, similar to UPFC, can inject a voltage source of controllable magnitude and phase angle in series with the line. Also HPFC, with proper control, is found to increase the power flow through a line and reduce the value of the angle between the voltages at the two ends of the line. Thus, the performance characteristics of the HPFC are similar to that of the UPFC without significant reduction in versatility. Thus the HPFC can be regarded as a cost effective alternative to the UPFC. APPENDIX TABLE A1 PARAMETERS OF THE HPFC

11.8244 MVAR

13.1143 MVAR

16.32 KV

17.64 KV

100 MW

100 MW

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Yellaiah Ponnam et al Int. Journal of Engineering Research and Applications ISSN : 2248-9622, Vol. 4, Issue 1( Version 1), January 2014, pp.248-255 Capacitance 41.1 μF. Inductance 0.05 H. Shunt Compensator Parameters VSC-1 VSC-2 11/230 KV, Y/Δ, 30 11/230 KV, Y/Δ, 30 MVA. Reactance = MVA. Reactance = Transformer details 0.1 pu (With 0.1 pu (With respect to respect to Transformer rating). Transformer rating).

Series Compensator

Lf = 0.0001 H Rf = 0.003 400ΩμF

Filter Inductance Filter Capacitor DC link Capacitance Rated DC bus voltage

Lf = 0.0001 H Rf = 0.003 400ΩμF 3 mF 22 KV

TABLE A2 PARAMETERS OF THE UPFC Shunt Compensator Parameters 11/230 KV, Y/Δ, 60 MVA. Transformer Reactance = 0.1 pu (With respect to transformer rating). Filter Inductance Lf = 0.0001 H, Rf = 0.003 Ω Filter Capacitor 400 μF DC link Capacitance 3 mF Rated DC bus 22 KV voltage Series Compensator Parameters Number of 1 Phase Units = 3 Primary side rated voltage = 11 KV Secondary side rated voltage = 33 KV Transformer Primary side connection = Δ Rated capacity of each unit = 8 MVA Reactance = 0.1 pu (With respect to the rating of the individual unit) Filter Inductance

Lf = 0.0001 H, Rf

Filter Capacitor DC link Capacitance DC bus voltage

= 0.003 Ω

400 μF 3 mF 22 KV

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ABOUT AUTHORS Manidhar. Thula, Asst.Professor Yellaiah. Ponnam, Asst.Professor Received B.Tech degree in Electrical and Electronics Engineering from the University of JNTUH, M.E in Industrial Drives & Control from College of Engineering, Osmania University, Hyderabad. He is currently working as Asst. Professor in EEE Department of Gurunanak Institutions, Hyderabad, His currently research interests Power electronics & Drives, Application of Power electronics in Power systems and Power quality.

Received M.Tech degree in Control Systems in Dept. of Electrical and Electronics Engineering, JNTU Hyderabad. He is currently working as Asst. Professor in EEE Department of Guru Nanak Institute of Technology ,Hyderabad, His is doing currently research in Real time application in control systems, Fuzzy logic controller, Power electronic drives and FACTS

Voraganti David Asst.Professor Received B.Tech degree in Electrical and Electronics Engineering from the University of JNTUH, M.Tech in Power Electronics from the University of JNTU-Hyderabad. He is currently Asst. Professor in EEE Department of Guru Nanak Institute of Technology, Hyderabad. His currently research interests include, Power electronics & Drives, Application of Power electronics in Power systems.

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