Three-Phase Multi-Pulse Converter with Phase-Shifting Transformer

Transcription

1 89 International Journal of Electronics, Electrical and Computational System Three-Phase Multi-Pulse Converter with Phase-Shifting Transformer Department of Electrical Engienring MITS Gwalior Abstract-In the recent year various technique of rectification is developed one of them is multi-pulse rectifier by using zigzag transformer to meet the power quality requirement for example power factor, THD, form factor etc. The three-phase multi-pulse AC to DC conversion system consist a phase-shifting transformer and a three-phase converter. With the help of every such converter can get a 6-pulse AC to DC conversion, for producing more sets of 6-pulse systems, in this case a uniform phase-shift is required and hence by using a proper phase-shifting angle, 12, 18, 24, 30, 36 and higher pulse systems have been produced for feeding a non-sinusoidal load. With this technique as the number of pulse increases the power quality is also improving gradually. These types of converters are also known as isolated converter because of isolated transformer is used in this rectifier. This type of converter is basically use for high voltage application. This paper proposed comparative study of three-phase multi-pulse AC-DC converters. KEYWORDS- Multi-pulse converters, harmonics reduction technique, form factor, ripple factor INTRODUCTION- In the last few years a lot of work has been done for improvement of power quality in AC-DC converter by using different concepts and applications. In this paper the proposed AC-DC converter system consists a phase-shifting transformer and a three-phase converter between the supply and load side of the system. This type of converter provides 6-pulse AC to DC conversion, so for producing more number of sets of 6-pulse systems, a phase-shift is required and hence with the help of proper phase-shifting angle, higher pulse like as 12, 18, 24, 30, 36 and 48 pulse systems can be produced. Basically a great part of consumed electric power is processed by front end connected six-pulse diode or thyristor AC-DC converters [1]. These 6-pulse AC-DC converters is still considered the most important converters in the power electronics universe because they have low cost structure even these six-pulse converter have low power factor and introduce excessive harmonic current in the AC source. The six-pulse converter does not meet the power quality requirement as per International Standards, such as IEEE-519 [2], until a heavy and expensive AC line filters are used in circuit to reduce these harmonics contents and to get a good power factor for varying load, there are some techniques have been proposed in recent years for this problem shown in fig.2. There are three method for reduction of harmonics in fig.2 one of them is the multi-pulse converter technique in this technique phase-shifting transformers or autotransformers can be used according to the application to provide cancellation of certain harmonics [3,4]. This is a good solution for uncontrolled multi-pulse AC-DC converters. However, in case of output voltage is needed controlled outputs then the thyristors are used in place of diode and for that the rectifier presents a low displacement factor and a lower power factor but this technique is expensive and bulky, because of input power transformer. In fig.2 another technique is given that is pulse width modulated controlled rectifier technique by using high frequency modulation, which operates at a unity power factor and sinusoidal input currents [5,6]. However, rectifiers of this type are not use at high power applications, because of its high switching losses. In case of full wave converter the main problem is due to isolated configuration that is transformer size which is nearly %and the power transferred as only 50% of the winding[7] but isolation is required in case of high voltage application for stepping down the voltage value. Thus these type of transformer frequently used.

2 Many methods based on the multi pulse switching have been proposed which are based on autotransformer or multi winding isolation transformer although both of them provides small size of transformer but need a complex structure also the autotransformer lacks the isolation between input and output & other also can be completely damaged by accidental overloading of one of the any phase because all primary windings are common. Fig 1 shoes the block diagram of multi-pulse rectification technique. Fig.1 Block Diagram The above block diagram has five blocks according to the working procedure. Three phase line reactors are used for reduction for harmonics contents in input. 2. PROPOSED MULTI-PULSE AC-DC CONVERTERS- In this paper the proposed multi-pulse AC-DC converter have a phase shifting transformer (Zigzag transformer). With the help of this transformer we get a uniform phase shift. Thus we produce 12, 18, 24 and more pulse rectifier. Nowadays because of non-sinusoidal load harmonics is main problem to solve this problem and meet the power quality requirement we have various technique which is shown in fig.2[8]. one of them is multi-pulse converters which have various classification according to number of pulse. HARMONICS REDUCTION TECHNIQUES FILTER PWM MULTIPLE CONVERTERR Active Passive Fig.2 Various Harmonics Reduction Techniques 2.1. ZIGZAG TRANSFORMER- A zigzag or phase shifting transformer are special purpose transformer. Its winding are basically connected in zigzag or interconnected star connection such that its output is the vector sum of two phases offset by 120[9]. The primary winding of this transformer connected in zigzag configuration for this each winding on each phase has two halves. With the help of this connection we can suppress all triplen harmonics e.g. 3 rd,9 th, 15 th etc[10] MULTI-PULSE METHOD- With the help of multi pulse method the harmonics contents in ac-dc converter is reduced because of multiple converter used in circuit that s why harmonics produced by one converter is cancelled by harmonics produced by another converter. Thus certain harmonics related to number of converters are eliminated from the power source. 90

3 There are two type of pulse converter according to controlling techniques first one uncontrolled and second one is controlled multi-pulse converter. Uncontrolled multi-pulse has fixed output and controlled multi-pulse converter has controlled output which can control by the firing angle α. As we know that in case multi-pulse converter as the number of pulse increase the harmonics contents will reduced and the THD is depend upon phase shift of the multi-pulse converter and it is given by following equation [11] Phase shift=60 /number of six-pulse converter (1) 3. MATLAB BASED SIMULATION- Fig 7 to fig 8 shows the implemented uncontrolled AC-DC converter with a load in MATLAB using SIM POWER SYSTEM toolbox. In this model multi-pulse converter are designed by using a zigzag transformer. These models are given following SIX-PULSE CONVERTER- Simulink model of six-pulse AC-DC converter is shown in fig.3. This AC-DC converter is used in various AC-drives because it has low initial cost and simple structure but in case of full load condition its input current harmonics exceed 100% when no harmonics filter is connected. Six-pulse converter has 5 th, 7 th and 11 th harmonics are dominant harmonics contents. Fig.3 Six-Pulse Converter 3.2. TWELVE-PULSE CONVERTER- Fig.4 shows simulink model of twelve-pulse converter which has two six-pulse converters connected in series or parallel according to the application. The three phase input of these two six-pulse converters has 30 phase difference to each other. Due to this 5 th and 7 th harmonics content are canceling to each other and 11 th and 13 th are dominant harmonics content. Fig.4 Twelve-Pulse Converter 3.3. EIGHTEEN-PULSE CONVERTER- The simulink model of eighteen pulse converters is shown in fig.5 it has three six-pulse converters. The phase difference in these converters are 20 to each other due to this lower harmonics content are canceled and dominant harmonics content are 17 th and 19 th. 91

4 Fig.5 Eighteen-Pulse Converter 3.4. TWENTY-FOUR-PULSE CONVERTER- The model is shown in fig.6 it has 15 phase difference between all four six-pulse converters. It has much lower harmonics contents on both AC and DC side. Fig.6 Twenty-four-Pulse Converter 92

5 3.5. THIRTY-PULSE CONVERTER- Thirty-pulse converter has five set of six-pulse converter all the converters are at a phase difference of 12 to each other. The simulink model of thirty-pulse converter is shown in fig.7 Fig.7 Thirty-Pulse Converter 3.7. THIRTY-SIX-PULSE CONVERTER- The corresponding Simulink model is shown in fig.8. it has six set of six-pulse converter at a phase difference of 10 to each other because of this all the lower harmonics contents in input current acr canceled to each other only higher order harmonics are remaining 93

6 Fig.8 Thirty six-pulse Converter 3.8. FOURTY-EIGHT-PULSE CONVERTER- This type of converter are basically use for high power application. It has eight set of six-pulse converters at a phase difference of 7.5 to each other. The simulink model is shown in fig.9 94

7 Fig.9 Forty-eight-Pulse Converter 4. RESULT AND DISSCUSSION- The power quality indices have been listed in table 1 obtained from simulink model shown in fig.3 to fig.8. The simulink model have a three phase 230 volt 50Hz supply and varying load. Table.1 has been obtained by using R, RL and RC load. From the table we can say that as the no of pulse increases in AC-DC converter the power quality improves. In all simulink model source impedance is connected to source side for meet IEEE 519 power quality requirements. Table1 has % THD of input current and input voltage, output voltage and current in volt and ampere respectively, power factor, ripple factor and input current in ampere for R, RL and RC load for 6, 12, 18, 24, 30, 36 and 48-pulse converters. Table.1 is shown bellow 95

8 96 SrNo. Topology Type of Load %THDIa FF Ripple Factor 1. R PULSE RL RC PULSE R RL RC PULSE R RL RC PULSE R RL RC PULSE R RL RC PULSE R RL RC R PULSE RL RC Table 1 Result of Proposed Multi-Pulse AC-DC Converters 5. CONCLUSION- In this paper proposed multi-pulse AC-DC converter by using zigzag transformer is a method to reduce harmonics in input because of sinusoidal load. These types of converter are basically used for high voltage application. A comparative analysis of three-phase multi-pulse converter has been done in this paper. The discussed IPQCs can be considered to be the better alternatives for power quality improvement because of the reduced size of the converter, lower cost and higher efficiency as compared to other methods of power quality improvement. REFRENCES [1]. B.R.Pelly, Thyristor Phase-Controlled Converters and Cycloconverters New York: Wiley- Interscience, [2]. IEEE Std , IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems. [3]. J.Schaeffer, Rectifier Circuits: Theory and Design NewYork: Wiley-Interscience, [4]. D.A.Paice, Power Electronic Converter Harmonics IEEE Press, [5]. T.Kataoka, K.Kawakami, J.Kitano, A Pulsewidth Modulated AC to DC Converter Using Gate Turn-off Thyristors, IEEE IAS Annual Meeting, pp , [6]. Y.Sato et al., A New Control Strategy to Improve AC Input Current Waveform of High-Power Parallel Connected PWM Rectifiers, IEEE PCC - Yokohama, 1993, pp [7]. R. W. Lye (Editor), Power Converter Hand Book-Theory, Design, Applications, Power Delivery Department, GE Canada, Ontario, March [8]. G.K.Dubey, S.R.Doradla, A.Joshi, R.M.K.Sinha, Thyristorised Power Controllers, New Age International publishers, July [9]. Lawhead, Larry et al (2006). "Three phase transformer winding configurations and Differential relay compensation". [10]. Khera, P.P. (1990). "Application of zigzag transformers for reducing harmonics in the neutral conductor of low voltage distribution system". [11]. Bhim Singh, G.Bhuvaneswari and Vipin Garg, A novel harmonic mitigator based 12-pulse rectification for vector controlled induction motor drives, Int. J. Energy Technology and Policy, Vol.4, Nos.1/2, 2006, pp