power factor improvement in three phase ac-ac ...

P.Vasuki et al. / International Journal of Engineering Science and Technology (IJEST)

POWER FACTOR IMPROVEMENT IN THREE PHASE AC-AC CONVERTER THROUGH MODIFIED SPWM P.VASUKI* P.G Scholar Electrical and Electronics Engineering Kumaraguru college of Technology Coimbatore-641049, India

R.MAHALAKSHMI Assistant Professor Electrical and Electronics Engineering Kumaraguru college of Technology Coimbatore-641049, India Abstract: Recently, a new generation of ac-ac single-phase and three-phase power converters with more commutations per half cycle has been proposed for ac power due to the increasing availability and power capability of high frequency controlled-on and off power semiconductor switching devices. This paper presents three phase ac-ac converter whose control strategy is based on modified sinusoidal pulse-width modulation switching technique. As majority of the industrial loads are being inductive, the power factor is less. To improve the power factor, the delayed current is shifted to the input voltage, through a modification of the classical sinusoidal pulse width modulation switching technique. In this way, the decrease in the phase angle between the input current and voltage is feasible, and consequently, high cost compensation capacitors can be avoided. The improvement of power factor through this switching technique on the proposed converter is investigated and verified via simulation using the software Matlab/Simulink. Keywords: Power factor; modified sinusoidal pulse width modulation; AC-AC Converter; Voltage regulator 1. Introduction The ac voltage regulator is used as one of the power electronic systems to control the output ac voltage for power ranges from a few watts up to fractions of megawatts. Phase-angle control and integral-cycle control of thyristors have been traditionally used in these types of regulators. Some techniques have advantages like simplicity and to control large amount of power economically. However, they suffer from disadvantages, such as retardation of the firing angle, causing a lagging power factor(PF) at the input side, in particular, at large firing angles, and high low-order harmonic contents in both load and supply voltages/currents [1]. The recent developments in the field of power electronics make it possible to improve the power system utility interface. Line-commutated ac controllers can be replaced by pulse width modulation (PWM) ac chopper controllers, which have better performance, and the above problems can be improved [2]–[4]. In this case, the input supply voltage is chopped into segments, and the output voltage level is decided by controlling the duty cycle of the chopper switching function. The advantages gained include better input PF, transient response and elimination of the low- order harmonics [5]–[7]. Most researchers in ac choppers have not considered the variation of the input PF [8]. For three phase ac-ac converters, which is mainly used in industrial motor drives the power factor is very less. In case of a controlled R–L load through a power electronic converter, high harmonics and a lag of the current fundamental harmonic appear, both reducing the PF. To remove the high harmonics, especially by low converter switching frequencies, the filter has large dimensions. To avoid this, the converter switching frequency has to be high. The Hysteresis Current Control (HCC) technique, which is one of the methods mostly used, it is possible to achieve high PF, but with relatively low efficiency [9–11]. Through the sinusoidal pulse width modulation (SPWM) switching technique, the lag of the input source current Iin (t) and the input voltage

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Vs cannot become zero. Contrary to this, by using modified pulse width modulation switching technique(αSPWM)[12] in the proposed three phase ac-ac converter, this lag can be made to almost zero, and consequently, the PF will be very much improved. This paper is organized in the following way. Section II describes the α-SPWM technique. The principle of operation of the proposed converter is dealt in Section III. Finally, simulation and performance of the proposed converter is investigated in Section IV. 2. Modulation Strategies: AC-AC Converter The two main controlled strategies employed with ac-ac converters are discussed in this paper • Conventional SPWM Technique • Modified SPWM Technique 2.1 Conventional SPWM Technique The switching pulses for the operation of the SPWM converter are created by comparing a triangular waveform (Vtr) with a sinusoidal waveform, according to Fig.1, in which the sinusoidal waveform signal (Vc) is absolutely similar and in phase with input voltage Vs.

Fig.1 Conventional SPWM pulses

Applying these pulses to the electronic switching elements of a single phase ac-ac converter for supplying an R–L load as shown in Fig.2 the input current Iin has the waveform shown in, Fig. 3(a) which has been calculated through simulation.

Fig. 2 Single Phase AC-AC Converter

Fig. 3(a) Input Voltage and Current Conventional SPWM converter

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Fig. 3(b) Input Voltage and Current Without a converter

In case that an R–L load is supplied through a controlled converter (Fig.2) using the conventional SPWM switching technique, the lag of the current Iin behind the voltage Vs as shown in Fig.3(a), decreases in comparison to those in case without converter according to Fig.3(b). However, the lag of the current Iin to the voltage Vs cannot become zero. 2.2 Modified Spwm Technique The comparison of a triangle waveform, i.e., triangular signal (Vtr) with a sinusoidal waveform, which is similar to input voltage but not in phase with Vs (Vcα) enables the generation of the switching pulses as shown in Fig. 4

Fig. 4 α-SPWM Pulses For α =35˚

The waveform Vcα is strongly similar to the sinusoidal waveform Vc but shifted to the left by an angle α, so that this signal leads to the voltage Vc by an angle. The corresponding switching pulses are generated and given to the converter. Fig. 5 show the input current Iin and voltage Vs for the single phase ac-ac converter calculated through Matlab/Simulink simulation.

Fig. 5Vs and Iin α=35˚

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According to Fig. 5 the current Iin is shifted to the voltage Vs by α=35˚ and so the fundamental current harmonic is in phase with this Voltage. In general, the application of such pulses (shifted to the left by an angle a) to a controlled converter consisting of metal oxide-semiconductor field-effect transistor (MOSFET) or IGBT has two advantages: • Input current is shifted left to the input voltage, that is, the trade-off in this work • The most high harmonics of the input current appear in the area of high order as by the conventional SPWM, which can be eliminated by the use of a small filter. 3.Proposed Three Phase Ac-AC Converter 3.1 Circuit Description The naturally commutated thyristor controllers introduce lower order harmonics in both the load and supply side and have low-input PF. Normally in ac choppers single pulse can be modulated to control the output or load voltage. However, the load voltage has almost square wave shapes, and therefore the load voltage and the line current has higher order harmonics. If multiple output voltage pulses are used instead of single pulse, it significantly reduces the harmonics. So, the performance of ac voltage regulators can be improved by PWM control. The Fig. 6 shows the schematic block diagram of the proposed three phase ac-ac converter employing α-SPWM technique. The instantaneous input phase voltages are (1) (2) (3) The instantaneous input line voltages are (4) (5) (6)

Fig. 6 Block Diagram of Three Phase AC-AC Converter

The circuit configuration for one phase of the three phase ac voltage regulator for PWM control is shown in Fig. 7. The AC source offers a sinusoidal voltage VS and has the internal impedance Rg–Lg. A small Lf–Cf input filter is used to absorb the high-order harmonics of the input current Iin. The switching power elements insulated

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gate bipolar transistor IGBT1 and IGBT2 with external antiparallel diodes control the load current IL, which can flow bidirectionally. By using fast switching devices, PWM techniques can be applied to the ac voltage controllers for producing variable voltage with a better input PF.

Fig. 7 One Phase of Three Phase AC voltage regulator

3.2 Principle Of Operation The two main modes of operation are active and freewheeling modes. A diode connected in anti-parallel with each parallel switch is used to complete the freewheeling current path .It also prevent reverse voltages from appearing across the switches. The switching sequence of the devices are given below in Table 1. Table 1 Switching Sequence

Vs>0 Vs