Pulse Rectifiers Current-Type for Applications in Modern Frequency Converters Hlisnikovsky Pavel, Chlebis Petr Department of Power Electronics and Ele

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1 Pulse Rectifiers Current-Type for Applications in Modern Frequency Converters Hlisnikovsky Pavel, Chlebis Petr Department of Power Electronics and Electric Drives VSB Technical University of Ostrava Ostrava-Poruba, CZECH REPUBLC Phone Fax ; WWW: Abstract - This document occupy with the pulse rectifier current-type as a modern controlled rectifier, which is very suitable as an input unit of indirect frequency converters current-type, because its low emissions of harmonics into the network, the controlled power factor and the controlled current in link. There is a comparison between the pulse rectifier and the classical thyristor rectifier. There is a description of the zero current switch pulse rectifier as suitable input unit of indirect frequency converter currenttype with the series resonant link. The vector pulse width modulation and the delta modulation as the suitable control methods for the pulse rectifiers or for the whole frequency converter are also described in the contribution. The measured results on the realized physical models are enclosed at the end of the contribution.. PULSE RECTFER CURRENT-TYPE. The pulse rectifier current-type belongs to modern converters at the present time. This converter is suitable for many applications of different kinds (e. g. [4]), but especially as the input unit of an indirect frequency converter current-type. An output frequency is controlled by an output inverter currenttype, whose output AC current can be controlled by the input unit of a converter - the pulse rectifier (This is given by a modulation of the output current). The first-rate rectified current in the link with possibility of its quick control and the minimal backward influences on the supply network can be achieved by the pulse rectifier current-type. The main advantages of the pulse rectifier current-type in comparison with the till now used classical thyristor rectifier are these: Almost the harmonic current consumption from the supply network. t means a lower content of the harmonics emitted into the supply network and the low interference of the surrounding electrical instruments. The minimisation of sizes and also price of the commutating chokes at the input side of the converter. The lower ripple of the output rectified current. t enables to use a smaller smoothing choke at the side or do not use it. The power factor can be set in wide range, so the compensators of the reactive power are not needed. The pulse rectifier can perform the function of compensator itself. The better dynamics of whole drive is caused by elimination of the distance-velocity lag of the converter. The quick reaction on the control changes is given first of all by high sampling rate. The constant power factor in the whole regulation range during the control of the output rectified current, which is based on the vector pulse width modulation of the current. There is no not passed commutation, the effect, which is very dangerous at the rectifiers with the phase control. The converter retains its preferences even at the deformed network voltage and it is able to manage with the network voltage drop. The pulse rectifier current-type can be applied on the indirect frequency converters current-type in two versions: The pulse rectifier current-type with the hard switching The pulse rectifier current-type with the soft switching The pulse rectifiers are mainly supplied by three-phase supply network, but the single-phase pulse rectifier can be also implemented for the traction applications. A. The pulse rectifier current-type with the hard switching Figure 1 shows the pulse rectifier current-type with the hard switching. The semiconductor switches S1 to S6 must be realized by reverse blocking switching semiconductor device (such as GTO or GCT thyristor). The input part of the converter is the condenser battery, which does an accumulation of the phase current during switching, eliminates the inductance of the supply network and removes accidental overvoltages. + S1 S3 S5 3x L δ L1 L2 L3 C1 C2 C3 S4 S6 S2 Fig. 1 Three-phase pulse rectifier current-type The disadvantages of the pulse rectifier current-type with the hard switching are heavy switching losses, which are caused by the high switching frequency of the semiconductor devices. The high frequency is needed for the first-rate rectified current in the link and its control. The switching losses can be a little minimized by suitable choice of switching combinations. The switching losses are minimized much more effectively by soft switching. - U 1

2 B. The zero current switch pulse rectifier current-type The total scheme of the indirect frequency converter currenttype with the zero current switch pulse rectifier current-type is shown on the fig. 2. L switches for a vector. The desired position of the current vector inside the sector is realized by switching adjacent active vectors (the left and the right active vector) for the specific time. The size of the desired current vector is decreased by zero vector, which is specific for each sector. The size of the current vector L1 S11 S13 S15 C R S22 S26 S24 L21 L2 L3 L SAT L R L22 L23 C11 C12 C13 S14 S16 S12 S25 S23 S21 C21 C22 C23 Fig. 2 ndirect frequency converter current-type with soft switching using ZCS The ZCS pulse rectifier current-type has all advantages list above (chapter 1), but it also minimizes switching losses much more, which would be otherwise the main part of the total losses of the converter with the hard switching. The ZCS is achieved by resonance process in the link. The control unit evaluates the zero current on the switch and then the switch or the whole branch of the converter is turned on/off during zero current. The switching losses on the switch are almost zero at the zero current because they are product of current and voltage on the switch. The disadvantage of the ZCS pulse rectifier current-type is complicated control of the converter. The control unit must be able of real time control and particularly to control the resonance process in the link. The requirements on the control can be realized by modern signal processor.. METHODS OF CONTROL. The suitable modulation of the current is necessary for the harmonic current consumption from the electrical network and for the hard output rectified current without ripple. The modulation is a core of the whole control. The vector pulse width modulation (VPWM) or the delta modulation has excellent properties. These modulations can be used in control of indirect frequency converters current-type for both the input unit (the pulse rectifier) and the output unit (the current inverter). A. Vector pulse width modulation This type of vector pulse width modulation (VPWM) is the current modulation and it is similar to VPWM of current inverter. The VPWM consists of the six active current vectors and the three zero current vectors (see fig. 3). The active vectors divide the vector diagram to the six sectors at 60 (s0 to s5). The position of the desired current vector in the direction one of the six active vectors is realized by turn on switches, which are marked at active vector in fig. 3. There is a difference in comparison with voltage VPWM because there are only two is smaller by longer time of the zero vector. The end point of the desired current vector must moves after the circle for the harmonic current consumption. This circle is the inscribed circle of the hexagon. S2,S3 i3 i02 i4 S3,S4 inscribed circle s2 s2 i03 i01 s1 m S1,S2 60 i5 i01 is realized by turn on S1,S4 i02 is realized by turn on S3,S6 i03 is realized by turn on S2,S5 (according to the fig.1) i2 the left vector of the sector no α α s3 s4 i6 S5,S6 S4,S5 s0 30 i01 the zero vector of the sector no. 0 i03 s5 max s5 i1 S6,S1 the right vector of the sector no. 0 i02 Re the active vectors: i1, i2, i3, i4, i5, i6 the zero vectors: i01, i02, i03 the zero vectors have zero size and their position is in the centre of diagram Fig. 3 Diagram of the vector pulse width modulation of the current for the pulse rectifier current-type with hard switching Moving after the circle is achieved by switching the left, the right and the zero vector for the specific time according to the following equations: 2

3 The time of the left active vector: T2 = T sin max ( α ) The time of the right active vector: T1 = T sin max The time of the zero vector: T = T - 0 T1 - T2 ( 60 -α ) (2.0) (2.1) (2.2) is the desired size of current vector and it is also the output rectified current in the link. max is the maximal size of current vector, which is equal to radius of the inscribed circle in the hexagon (for condition of sinusoidal course of phase current). The angel α is the angle in the sector with values from 0 to 60. T is the total switching period, which consists of the right vector time, the left vector time and the zero vector time. The vectors are switched in sequence half zero - right - left - half zero vector. This modulation can be applied on the pulse rectifier currenttype with hard switching. The delta modulation is much more suitable for the converter with soft switching. B. Delta modulation The delta modulation is a discrete method, which comes from two-value hysteretic regulation of quantities. The hysteresis is achieved by discrete sampling. The principle of the modulation for the input unit of the indirect frequency converter currenttype is: 1. The calculation of the differences between desired and real instantaneous values of the input phase currents. 2. The determination of the switching combination of the next period for minimizing the differences. This modulation can be also applied on the output unit (the current inverter) of the indirect frequency converter currenttype, where the differences are determined between output phase voltages, so the principle is: 1. The calculation of the differences between desired and real instantaneous values of the output phase voltages. 2. The determination of the switching combination of the next period for minimizing the differences. Table 1 shows the switching principle of the switches for the input unit of the indirect frequency converter current-type according to the two previous upper conditions and according to the fig. 2. Switching combinations for the output unit are similar with some differences. Table 1. Switching combination for the input unit of the indirect frequency converter current-type. Condition Switching combination dif 1 < dif 2 < dif 3 S 12, S 15 dif 3 < dif 2 < dif 1 S 11, S 16 dif 1 < dif 3 < dif 2 S 12, S 13 dif 2 < dif 3 < dif 1 S 14, S 11 dif 2 < dif 1 < dif 3 S 14, S 15 dif 3 < dif 1 < dif 2 S 16, S 13 dif 1 = 11 - * 11 ; dif 2 = 12 * * 12 ; dif 3 = * - desired value. RESULTS. The qualities and the behaviour of the three-phase pulse rectifier current-type were verified by simulations in the PSPCE OrCAD software and next by measuring on the realized physical models. The physical model of the pulse rectifier with the nominal output rectified current 10A was realized by reverse blocking switches with GBT and fast power diode in series. Figure 4 shows phase current and line voltage of the three-phase pulse rectifier current-type with hard switching. Figure 5 shows phase current and phase voltage with leading power factor cosϕ = 0.86 of the three-phase pulse rectifier current-type with hard switching. Figure 6 shows phase current and phase voltage with unite power factor cosϕ = 1 of the three-phase pulse rectifier current-type with hard switching. Figure 7 shows phase current and phase voltage with lagging power factor cosϕ = 0.86 of the three-phase pulse rectifier current-type with hard switching. Figures number 5, 6, 7 and 8 are measured without commutating chokes and without smoothing chokes. Figure 8 shows phase currents harmonics spectrum of this pulse rectifier up to the harmonic number 13. Figure 9 refers to the indirect frequency converter current-type with soft switching using ZCS and shows resonant current pulses in link of the converter. Figures 10 and 11 shows output phase voltage and current of ZCS indirect frequency converter current-type working with the not loaded induction motor in case of figure 10 and loaded induction motor in case of figure 11. The resonant frequency was about 7.5kHz and the induction motor is P N = 1kW and U N = 127/220V. The quality of the input and the output quantities were evaluated by Norma Wide Band Power Analyser D6000 and by digital oscilloscopes. As the control unit for the pulse rectifier was used modern signal processor TMS320LF2407. The signal processor TMS320F2812 was used for the control of the whole indirect frequency converter current-type with soft switching using ZSC. 3

4 Fig. 4 Courses of phase current (up) and supply line voltage (down) of the pulse rectifier current-type Fig. 7 Courses of phase current (up) and supply phase voltage (down) with lagging power factor Percentual value of harmonic 1 0, Harmonic Phase current Fig. 5 Courses of phase current (up) and supply phase voltage (down) with leading power factor Fig. 8 Spectrum of phase currents harmonics of the threephase pulse rectifier current-type Fig. 6 Courses of phase current (up) and supply phase voltage (down) with power factor equal 1 Fig. 9 Resonant current pulses in link of indirect frequency converter (up) and switching pulses (down) 4

5 Fig. 10 Output phase voltage (up) and output phase current (down) of the ZCS indirect frequency converter current-type with not loaded nduction Motor Fig. 11 Output phase voltage (up) and output phase current (down) of the ZCS indirect frequency converter current-type with loaded nduction Motor V. CONCLUSON. The contribution summarizes the results of theoretical analyses and particularly practical measurement from an area of semiconductor converters current-type, which by using modern reverse blocking switching semiconductor devices bring the new possibilities of solutions of these converters, especially for high power. ACKNOWLEDGEMENT We would like to express thanks to VSB Technical University of Ostrava, Department of Power Electronic and Electric Drives and The Ministry of Education of The Czech Republic. n the paper there are the results of the project LN00B029, which was supported by The Ministry of Education of The Czech Republic. REFERENCES [1] F. Vondrasek, Current Pulse Rectifiers. Proceeding XXV. Nationwide Conference about Electric Drives, Pilsen, 2001, pp [2] F. Vondrasek, Power Electronics (Book),Pilsen,1998. [3] P. Chlebis, Resonant Converters for AC Electric Drives, (Habilitation Work), Ostrava, VSB Technical University of Ostrava, [4] A. Klos, Netzrückwirkungen in Theorie und Praxis, AT Verlag Arrau, Stuttgart, 1981, SBN [5] V. Kus, Backward nfluences of Semiconductor Converters on Supply Network, BEN - Technical Literature, Praque, 2002, SBN [6] A. Weber, Symmetrische GCTs für stromgeführte Umrichter, ABB Semiconductors AG, September [7] P. Chlebis and V. Damec, New Concept of Single-Phase Traction Supply, n: EDPE nternational Conference on Electrical Drives and Power Electronics, The High Tatras, Slovakia, September 2003 [8] A. Weber und kol., Rückwärts blockierence GCTs, ABB Semiconductors AG, October