Improvement of Power Factor and Harmonic Reduction with VSC for HVDC System

Transcription

1 International Journal of Engineering an Management Research, Volume-3, Issue-2, April 2013 ISSN No.: Pages: Improvement of Power Factor an Harmonic Reuction with VSC for HVDC System S. Raha Krishna Rey 1, Y.Rambabu 2, V.K.R.Mohan Rao 3, G.Venugopal 4 1,3 Associate Professor, Department of EEE, Holy Mary Institute of Tech., & Sc., Keesara, Hyeraba, INDIA. 2,4 Assistant Professor, Department of EEE, Holy Mary Institute of Tech., & Sc., Keesara, Hyeraba, INDIA. ABSTRACT This paper eals with analysis, simulation an control of a two level 48-pulse voltage source converter for High Voltage DC (HVDC) system. A set of two-level 6-pulse voltage source converters (VSCs) is use to form a 48-pulse converter operate at Funamental Component of Switching Frequency (FCSF). The performance of the VSC system is improve in terms of reuce harmonic level at FCSF an THD (Total Harmonic Distortion). The performance of the VSC is stuie to improve power factor an reuce harmonic istortion. Simulation results are presente for the propose two level multi pulse converter. Inex Terms Two-Level Voltage Source Converter, HVDC Systems, Multi pulse, Funamental Frequency Switching, Harmonics, I. INTRODUCTION VDC schemes employing line-commutate, current H source converter using thyristors have been wiely use in power transmission. The control schemes for such systems are well establishe an operating successfully all over the worl [1]. Voltage Source Converter (VSC) base HVDC systems using self commutate evice technology has attracte increasing attention an a number of installations are now in operation all over the worl transmitting more than 70,000 MW power [1]. One of the principle avantages of VSC HVDC system is that no external voltage source require for commutation. It can inepenently control the reactive power flow at each AC network, an reactive power control is inepenent of the active power control. These features make the VSC base HVDC system attractive for connection of weak AC systems, islan networks an renewable sources to a main gri [2]. A VSC base HVDC system uses a basic three phase 6-pulse voltage source converter brige as its main unit. This VSC brige is force to operate at very high switching frequency in orer to minimize the effect of harmonics in the system. Therefore, VSC in HVDC system has high power loss an high cost compare to conventional HVDC system [3]- [4]. HVDC conversion is implemente mostly by mono polar or bipolar configurations of 12-pulse series connecte thyristor converters. In such a case the resulting high contents of 12- pulse converter relate harmonics can couple into nearby telephone lines an cause noise in the communication network. This may also cause mal-function of protective relaying an circuit breakers [5]. To avoi such unesirable harmonic effects, tune passive filters have been employe on the AC sie of the converter. To reuce the harmonic istortion in VSC base HVDC systems without using conventional filters, there are three feasible solutions. These are through the use of multi-pulse converter, the multilevel converter, an the pulse with moulation (PWM) technique. The PWM converter shoul switch many times within one power cycle to synthesize its output waveform, therefore its switching loss is reasonably high, an this greatly limits its evelopment in high power applications. Magnetic couple multi pulse converter has two or more briges an evelops the staircase voltage waveform by varying transformer turns ratio with zigzag connections [5]. In these multi pulse converter circuits the converter briges are operate at funamental frequency switching (FFS) thus reuce the switching losses substantially. Pulse number can be increase in multiples of six, an an increase in every six pulse VSC reuces the harmonics in the system proportionally. The THD of steppe voltage of two level multi pulse VSC converters are given in Table I. From this table, it is clear that VSC only with pulse number 48 qualifie the IEEE stanar 519, where the THD is less than 5%. A 48- pulse voltage source converter is alreay reporte for STATCOM applications [7]. This paper proposes a 48-pulse voltage source converter (VSC) for HVDC system. The reason for choosing 48- pulse operation in this work is that a 48-pulse voltage source converter will give THD within IEEE 519 stanar compare to other pulse number such as 30 an 36 as shown 6

2 in Table I. Pulse number % VSC voltage THD TABLE I STANDARD VOLTAGE THD OF TWO-LEVEL MULTIPULSE CONVERTERS The control of propose 48-pulse voltage source converter is emonstrate an valiate for HVDC system. The results show a substantial reuction in voltage an current harmonics an THD is well controlle within the limit of IEEE 519 stanar. This 48-pulse voltage source converter is emonstrate a potential caniate for high power an high voltage AC-DC conversion with minimum switching losses an reuce voltage an current THDs. II. CONVERTER SYSTEM CONFIGURATION Fig. 1 shows the circuit configuration of a 48-pulse voltage source converter base back-to-back HVDC system. Eight two-level converters are use in this configuration. In this work the rectifier operation is simulate by consiering resistance as an equivalent of an inverter. These converters are connecte in parallel at the c sie. The HVDC system is rate for 100 MW with an each unit of 12.5 MW. It uses common c link capacitors. Total of (8X6) 48 soli state switching evices are use on each sie converter system. The DC link voltage can be selecte accoring the system configuration by appropriate turns ratio of the converter transformer. For back-to-back HVDC system it can be esigne for low DC link voltage. The HVDC system is moele as eight units of 6- pulse converters that are connecte in parallel with appropriate phase shift to achieve the 48 pulse converter operation. Each 6-pulse converter uses a set of Y/ZZ transformers connection for phase shift. The transformers are esigne to give a phase shift of 7.5q between two ajacent 6-pulse converters. The phase shift value is chosen in such a way to have an ientical transformer esign. This reuces the magnetic losses in transformers. Transformers seconary winings are connecte in Y configuration. The primary winings of these transformers are connecte in series an these consist of zigzag connections. The zigzag connection is use as a phase shifting wining an gives the phase shift of 7.5q between the two ajacent 6- pulse converters. Appropriate phase shift is also introuce in the gate pulses of an iniviual converter signal [8]. The net 48- pulse converter AC output voltage is given by This converter behaves as a 48-pulse converter where the minimum harmonics are of the orer 47 th an 49 th. This converter generates an almost equivalent to sinusoial waveform consisting of steppe waveform equivalent to a 48- pulse converter an THD of voltage an current is well maintaine within IEEE 519 stanar an qualify for the application in HVDC system without using PWM technique where the switching loss is quite high. The synthesis of sinusoial waveform using the steppe waveform is shown in Fig. 2. The system parameters use for the simulation are given in Appenix. III. CONTROL ALGORITHM The objective of the control algorithm of VSC is to maintain the DC voltage at the given reference value an to control the active power flow from AC gri to DC sie, along with supplying require reactive power to the AC mains. A set of capacitors is use at the DC bus to support the DC bus voltage at the require value to make the real power balance between the two sies of the converter, which is most important for the successful operation of the VSC base HVDC system. The store energy in the capacitors reuces or increases if the active power is not balance between two sies of converter stations. It consists of two controllers, one is the DC voltage controller an other one is the current controller [9]. 7

3 where P * is the reference real power to be transmitte from one sie to another sie, K V is proportional gain constant, V s is rms supply voltage, an V * c is reference DC voltage. The reference value of the reactive current (I q ) is supplie irectly to the inner current loops an is regulate equal to zero in this stuy. The first term in (2) ecies the power flow in the system an secon term achieves DC voltage regulation by means of controlling the aitional amount of active power flowing from AC sie to DC sie. When V c is lower than the V * c, then i * is increase as shown in (2), so that a small amount of aitional active power flows into the DC link capacitor through rectifier, thus V c rises up to V * c. When V c is higher than the V * c, then i * is ecrease so that the amount of active power flowing into the DC link capacitor is reuce, thus V c is lowere to V * c. A. DC Voltage Controller The DC voltage controller is shown in shown in Fig. 3a in which reference currents (i *, i q *) are achieve by the DC voltage controller from the reference real power an reference DC voltage as given below [10] B. Decouple Current controller The ecouple current controller shown in shown in Fig. 3b. The output of the DC voltage controller is fe to the current ontroller. The voltage an current relation of the converter is given by va v1a ia vb v1 b = ( R+ L1 ) ib t (4) v c v 1c i c Three phase to two phase transformation can be applie to (4) as 8

4 Here v 1, v 1q are the -axis an q-axis components of v 1, while i, an i q are the -axis an q-axis components of i s. v is the -axis component of v s whereas v q is always zero because the supply voltage vector is aligne with the -axis. The instantaneous active power P, an the reactive power Q are rawn from the utility gri as P= V. i + V. i (6) q q Q= V. i + V. i (7) q q The control of i an i q ecies P an Q inepenently. This ecouple current control is applie to the system in orer to achieve an inepenent control of i an i q. The AC voltage commans in the an q axes, are as v *, an v q *. The inner current controller inclues a feeback PIcontroller. The reference currents (i *, i q *) from c voltage controller are given as inputs to the current controller, an these provie reference voltages (v *, v q *). The operation of the current controller can be explaine by using (8) an (9) as * = (. + ω. q) { p1( ) + 11 ( ) }(8) q* = q (. q + ω. ) { p2( ) + 12 ( ) }(9) V V Ri Li K i i K i i V V Ri Li K i i K i i where K p1 an K p2 are proportional gain, K I1 an K I2 are integral gain, i, v an v q are -q values of supply voltage v s, an i, i q are -q values of the supply current (i s ). Here i * an i q * are the current commans in the an q axes. The first an secon terms of the right han sie cancel the steay state voltage appearing across the AC-link inuctor L 1. The thir term constitutes feeback control loops of the currents i an i q. The phase shift is calculate by using the (10) as v * 1 q δ* = tan v * (10) where δ * is the angle at which the converter evices are gate. It is the phase shift angle from the funamental supply voltage. IV. MATLAB BASED SIMULATION The propose two-level 48-pulse converter is simulate in the MATLAB environment with Simulink an Power System Block set (PSB) toolboxes. Fig. 4a shows the MATLAB moel of two-level 48-pulse converter an Fig. 4b shows the transformer an converter connection to realize 48-pulse converter moel. In this moel, eight two-level GTO VSC briges are use an connecte in parallel in the DC sie. The control algorithm is implemente using Simulink blocks. Three phase supply of 33kV, 50 Hz is connecte to the converter through an interfacing reactance with a value of 0.2 pu. 9

5 an transformer The primary winings of the transformer are connecte in series therefore, the total supply voltage is share equally by eight series connecte primary winings of the transformer. Three phase AC input is fe to the brige through an interface reactance. The voltages at the two sie of the reactance an the voltages at the two sies of the transformer are measure as supply an converter voltages. A DC capacitance is use to store energy at the DC bus. Fig. 4a MATLAB moel of two-level, 48-pulse VSC system Fig. 4b MATLAB moel of two-level, 48-pulse voltage source converter V. RESULTS AND DISCUSSION The steay state performance an ynamic behavior of the propose two level, 48-pulse voltage source converter are simulate to emonstrate its capability. Fig.5 shows the steay state behavior of moele converter system. In this figure, it shows the nature of the supply voltage (v abc ), ac mains current (I abc ), voltage (v pri ) an current (I pri ) at the primary sie of the converter transformer an voltage (v sec ) at the input of the converter, real power (P) rawn from supply, reactive power eman (Q), DC voltage (V c ), an angle elta (G) for require power flow an DC power (P c ) uring steay state is shown to emonstrate its behavior. The reference power comman is set at 100 MW throughout the steay state operation. The phase voltage an phase current are in phase with each other as the reactive power is maintaine to zero. Fig. 5 also shows the voltage an current at supply sie, at the primary sie of the converter transformer an at the input of the converter. Voltages reflecte at the input of the converter ue to switching operation are square wave in nature accoring to the switching pattern of the converter. All the seconary voltages are ae up an reflecte voltage on the primary sie of the transformer is a 48-pulse converter steppe voltage waveform almost equivalent to sinusoial waveform. This voltage becomes smooth at the supply sie ue to the aition of phase shifte voltages. Harmonics spectra of a 48-pulse converter voltage waveform at the primary wining of the transformer are shown in Fig. 7, which is very close to sinusoial waveform mae up of steppe waveform with a THD of 2.9% only. The THD of converter voltage observe here for 48-pulse operation is much less than the value shown in the Table I, this is because of the value of interface reactance. The AC current is equally share by the number of converters as they are ientical in nature. Fig. 6 shows the ynamic behavior of the converter system, where the ynamic conition is introuce in the system by changing the power flow. Initial reference power is set at 75 MW an at 1s, the reference active power is increase to 100 MW. The controller respons immeiately for the change in the system conition to bring back the system DC voltage to its reference value. The 10

6 change in power flow is accompanie by changing the angle elta (G) an keeping all other parameters of the system remain unchange. In Fig. 7 it may be observe that the THD of voltage is as 0.01% an THD of current is foun as low as 0.43%, at 100 % active power. This shows that the converter results in low THD in current below the IEEE stanar 519 [6]. This makes the supply voltage an current almost free from harmonics. Various harmonics spectrum shown in Fig. 7 is observe uring steay state conition an at full loa power. (a) Fig. 5 Steay state performance of propose 48-pulse voltage source converter Fig. 6 Dynamic performance of propose 48-pulse voltage source converter 11

7 ( b ) VI. CONCLUSION A 48-pulse two-level voltage source converter has been evelope an controlle for HVDC system. The transformer connections with appropriate phase shift have been use to form a 48-pulse converter. The operation of the propose converter configuration has been simulate. The power factor is improve an harmonic istortion is reuce for the propose converter configuration. REFERENCES [1] J. Arrillaga, Y. H. Liu an N. R. Waston, Flexible Power Transmission, The HVDC Options, John Wiley & Sons, Lt, Chichester, UK, [2] Gunnar Asplun Kjell Eriksson an kjell Svensson, DC Transmission base on Voltage Source Converter, in Proc. of CIGRE SC14Colloquium in South Africa 1997, pp.1-8. [3] Y. H. Liu R. H. Zhang, J. Arrillaga an N. R. Watson, An Overview of Self-Commutating Converters an their Application in Transmission an Distribution, in Conf. IEEE/PES Trans. an Distr.Conf. & Exhibition, Asia an Pacific Dalian, China [4] B. R. Anerson, L. Xu, P. Horton an P. Cartwright, Topology for VSC Transmission, IEE Power Engineering Journal, vol.16, no.3, pp , June [5] G. D. Breuer an R. L. Hauth, HVDC s Increasing Poppularity, IEEE Potentials, pp.18-21, May [6] IEEE Stanar , IEEE Recommene Practices an Requirements for Harmonic Control in Electrical Power Systems, IEEE Inc., New York, [7] M.S. EL-Moursi an A. M. Sharaf, Novel controllers for the 48-pulse VSC STATCOM an SSSC for voltage regulation an reactive power compensation, IEEE Trans. on Power Systems, vol.20, no.4, pp , Nov [8] Zhengping Xi an S. Bhattacharya, Magnetic Saturation in Transformers use for a 48-pulse Voltage-Source Converter base STATCOM uner Line to Line System Faults, in. Prof of IEEE Power Electronics Specialists Conference, 2007, PESC 2007, IEEE, June 2007, pp [9] Makoto Hagiwara, Hieaki Fujita an H. Akagi, Performance of a Self- Commutate BTB HVDC Link System uner a Single-Line to-groun Fault Conition, IEEE Trans. on Power Electronics, vol.18. no.1, pp Jan [10] Makoto Hagiwara an Hirofumi Akagi, An Approach to Regulating the DC-Link Voltage of a Voltage Source BTB system uring power flow line faults, IEEE Trans. on Inustry Applications, vol. 41, no. 5, Sep/Oct pp [11] A Two-Level, 48-Pulse Voltage Source Converter for HVDC Systems Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008, D. Mahan Mohan, Bhim Singh an B. K. Panigrahi. 12