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Novel Soft-switching Isolated Three-phase Bidirectional AC/DC Converter Staffan Norrga Royal Inst. of Technology Electr. Machines and Power Electronics SE-100 44 Stockholm, Sweden Abstract-- A novel ac/dc converter topology comprises a Phase Phase Phase Phase Phase leg 1 leg 2 leg 3 leg 1 leg 2 voltage source converter with capacitive snubbers and a 3-by-2 cycloconverter, connected by an MF transformer. By alternately commutating the two converters it is possible to achieve soft itr Cs Cs switching conditions for all of the semiconductor elements. DC utr Ud Furthermore, it is shown that resonant commutation of the side voltage source converter is possible by utilizing the cycloconverter Ntr: 1 for short-circuiting the transformer terminals. Cs Cs uac,1 I. INTRODUCTION uac,2 Isolated ac/dc converters incorporating a voltage source uac,3 converter (VSC) and a direct converter connected by an MF transformer, as shown in Figure 1, have been treated extensively in the literature [1]-[4]. Both circuits with single iac,1 iac,2 iac,3 or phase terminal and three phase terminal on the ac side have AC side been studied. A wide range of applications including UPS systems and active filters has been envisaged. The basic Figure 2. Topology of the proposed converter. function of the converter concept is that the VSC produces a of the transformer and thereby reduce the benefits of using an medium frequency ac voltage that magnetises the transformer MF transformer. In [5] and [6] a concept is described which and that this voltage is converted to a desired PWM voltage by utilises snubber capacitors in the VSC and a specialised the direct converter. The PWM voltage is applied to an commutation algorithm in order to achieve soft commutation inductive filter on the output of the direct converter whereby for all semiconductor elements in all points of operation. the power flow may be controlled. Several options exist Similar concepts have also been proposed for multilevel concerning the commutation mode of the two converters. The converters [7], [8]. In the present article the prospects for direct converter can either operate by source commutation [3] adapting the ideas from [5] to a system with a three-phase in which case the VSC acts as source, or by forced terminal on the ac side will be analysed. Furthermore, a brief commutation. In case forced commutation is employed the discussion of the prospects for applying the concept in HVDC switching losses can be reduced by having the VSC output converter stations will be made. zero voltage during the commutations of the direct converter [4]. In order to reduce the switching losses of the VSC it is II. TOPOLOGY possible to have the ac side current freewheeling through the direct converter during commutations of the VSC whereby The topology of the proposed converter system is displayed zero-current switching can be achieved [1]. in Figure 2. A voltage source converter with two phase legs is The above mentioned methods of reducing the switching connected to one of the windings of a single phase losses can generally not be applied in such a way that the loss transformer. This converter is equipped with snubber reduction is obtained in both the VSC and the direct converter capacitors Cs connected in parallel to each of the simultaneously. The semiconductor switching losses in either semiconductor valves. The capacitors should be sufficiently converter may limit the maximal possible operation frequency large to allow for zero-voltage turn-off of the switches of the converter. The other winding of the transformer is connected Direct MF to a 3-by-2 cycloconverter, which in turn is connected to a VSC converter transformer passive line filter. The valves of the cycloconverter do not need any turn-off capability, they may well be realised by LF AC DC thyristors connected in anti-parallel. However, for practical side side reasons it may be more beneficial to use a configuration with diodes and IGBT transistors as indicated in the figure. Figure 1. Isolated ac/dc converter topology with In order to simplify the analysis of the operation of the MF transformer and direct converter. system coupling functions that relate the relevant voltages and This expression is positive which implies that utr and itr are currents are defined. For each phase leg of the cycloconverter of the same sign, i. e. the instantaneous power flow is directed a coupling function kac,i may be defined such that kac,i=+½ from the dc side to the ac side. Thereby the conditions are set when the phase leg i. connects the corresponding terminal of for a snubbered commutation of the VSC. The stages of such a the filter to the upper transformer terminal and kac,i=-½ for the commutation are outlined in Figure 4 for the case where itr is lower terminal. Thus follows positive. The process is initiated by turning off the switches uac ,i = kac ,i utr N tr (1) that conduct. Thereby the current is diverted to the snubber capacitors which thus are recharged. When the potential of and both phase terminals have fully swung to the opposite dc rail itr = N tr ∑ kac ,i iac ,i . (2) the diodes in the incoming valves take over the current. i Finally, the switches that are anti-parallel to these diodes are turned on at zero-voltage and zero-current conditions. Thereby III. PRINCIPLE OF OPERATION the VSC is prepared for a subsequent current direction reversal. A. Commutation The reversal of the transformer voltage utr by the VSC By alternately commutating the two converters it is possible commutation causes the condition in expression. (3) to become to allow the direct converter to solely operate by source fulfilled for all three cycloconverter phase legs. Thus the cycle commutation (natural commutation) whereas snubbered or may be repeated by again commutating the cycloconverter etc. zero-voltage commutation is always enabled for the VSC. In The principal voltage and current waveforms during a this section such a commutation scheme will be analysed. sequence of commutations as devised above can be found in In order for a natural commutation of one of the phase legs Figure 7. It should be pointed out that the durations of the of the cycloconverter to be possible the condition commutation processes have been greatly exaggerated in this figure, for clarity. In practice they occupy only a very minor utr kac,i iac ,i < 0 (3) fraction of the commutation cycle. has to be fulfilled. Figure 3 schematically shows an example of B. Resonant VSC commutation at low load such a commutation. For simplicity the assumption is made At low currents the commutation of the VSC may become that the transformer can be represented by its leakage unduly lengthy as the recharging of the snubber capacitors inductance whereas the voltage source converter can be becomes slower. In the extreme case of zero load it is not represented by a constant voltage throughout the process. The possible at all to commutate the VSC in the fashion described commutation is started by turning on the non-conducting valve above. However, a quasi-resonant mode of commutation is in the direction of the current through the phase terminal. The proposed that solves this problem. By short-circuiting the voltage supplied by the VSC appears across the leakage primary winding of the transformer using the cycloconverter inductance and the incoming valve gradually takes over the during the commutation of the VSC it is possible to initiate a current. Finally the initially conducting valve turns off as the resonant process, governed by the snubber capacitors and the current through it goes to zero and the switches in this valve leakage inductance of the transformer. This resonant process can be gated off. The current derivatives during both turn-on can be utilised for recharging the snubber capacitors. In and turn-off are determined by the transformer leakage Figure 5 the stages of such a resonant commutation are shown. inductance and are thus relatively low. As a result of the commutation the coupling function changes sign, i. e. 1. 2. utr kac,i iac,i > 0 (4) itr itr becomes valid. When all of the cycloconverter phase legs have been commutated it follows from eq. (2) and (4) that 1 utr itr = N tr ∑ utr kac ,i iac ,i = N tr ∑ utr iac ,i . (5) i i 2 3. 4. 1. 2. 3. Lλ Lλ Lλ itr itr Lfilt Lfilt Lfilt -NUd -NUd -NUd Figure 3. Commutation of a cycloconverter phase leg. Figure 4. VSC Commutation. 1. Power flow from AC side to DC side 2. Enhancement stage 3. Resonant stage 4. Ramp-down stage 5. Cycloconverter natural turn off 6. Power flow from AC side to DC side Figure 5. Stages of a resonant commutation of the VSC. The corresponding voltage and current waveforms are side current. At this level the cycloconverter valves that were displayed in Figure 6. Initially, stage 1, the power flow is turned on in stage 2 turn off as the current through them goes directed from the dc side to the ac side just as in the case of the to zero, stage 5. Finally the switches in these valves are gated conventional VSC commutation. The process is started, off at zero-current conditions, stage 6. stage 2, by turning on valves in the cycloconverter so as to The enhancement current allows for a complete provide a path in the direction of the voltage applied by the commutation despite losses in the resonant circuit and VSC. This causes the transformer current to start increasing variations the ac side currents during the resonant process. linearly as the voltage from the VSC appears across the Although only one of the phase legs of the cycloconverter is transformer leakage inductance. This is allowed to continue shown in the figure it is obvious that all of the phase legs may until the current has increased by a certain predefined amount, be switched in the described fashion. This provides additional hereafter named enhancement current, ienh. Thereafter the two paths for the resonant current and thereby reduces the stress on switches in the VSC that carry current are turned off whereby the semiconductors. the resonance process takes place, stage 3. The duration can be In summary, the described method offers an alternative way shown to be, of commutating the VSC that is not dependent on the ac side currents. This is of vital importance for the feasibility of the i + i tr = Lλ Cs π − 2 arctan Z r tr ,1 enh (6) system as otherwise hard switching would have to be used to U d commutate the current from switch to diode with sufficient speed at low load. Hard switching in this case amounts to rapid where discharge of the snubber capacitors into the IGBT that turns N tr Z r = Lλ Cs and itr ,1 = 2 ∑i i ac ,i on. This would lead to very high switching losses and possibly neglecting losses, where Lλ is the leakage inductance in tr secondary side quantities and Ntr is the turns ratio of the utr transformer. From the expression it is obvious that both the ac side currents as well as the enhancement current act to shorten the resonant interval as they contribute to the recharging of the uAC,i capacitors. The peak transformer current during the resonant process amounts to itr ienh 2 U = d + ( itr ,1 + ienh ) 2 t itr , pk (7) Zr When the transformer voltage has been fully reversed the diodes in the VSC valves that initially blocked take over the current and the switches anti-parallel to these diodes can be turned on at zero-voltage and zero-current conditions. 1. 2. 3. 4. 5. 6. Subsequently, stage 4, the current through the transformer is forced down linearly to the value that corresponds to the ac Figure 6. Current and voltage curve shapes during a resonant VSC commutation. utr uac,2* uac,1* itr uac,1 uac,3* uac,2 uac,1 uac,3 uac,2 D A1 A3 A2 D A1 A3 A2 D D: VSC commutation Ai: Commutation of cycloconverter phase leg i uac,3 Figure 7. Principal curve shapes during commutation sequence enabling constant soft commutation. Sawtooth carrier for Sawtooth carrier for also destruction of the IGBT. iac,i<0 iac,i>0 C. Modulation Figure 8. Carrier-based modulation method The control system for the proposed converter should fulfil enabling constant soft commutation. two main demands apart from maintaining soft commutation as outlined in the previous two sections. Firstly, proper operation direction, are used as shown in Figure 8. For simplicity, the of the transformer should be ensured by avoiding low impact on the curveforms caused by the commutation frequency or dc components in the voltage applied to the processes is not shown in the figure. In practice it may be transformer utr. This is achieved by always making the VSC necessary to adjust the timing of the switchings for the voltage- commutations at constant intervals, whereby essentially a time area lost or gained during the commutations. square voltage will be applied to the transformer. Secondly it should be possible to obtain desired PWM patterns in the ac- IV. SIMULATION STUDY side phase potentials uac,i. By making the commutations of the In order to verify the control algorithm a simulation model cycloconverter phase legs at appropriate instants in the interval of a 40 kVA converter has been created in the simulation between two VSC commutations the width of the pulses can be software package Simplorer. The parameters of the chosen freely. This may be effected in a multitude of ways. To simulated system can be found in TABLE 1. The parameters show the potential of the concept a rather simple and have been chosen such that it will be possible to build a straightforward modulation derived from carrier-based prototype using IGBT modules with a blocking voltage of methods will be described. 1200 V. The control logic needed to achieve the desired Following each VSC commutation the sign of each phase potential uac,i is opposite to the sign of the current through the corresponding phase outlet iac,i which follows from expressions TABLE 1. - Parameters of the simulated converter. (1) and (3) (expression (3) is fulfilled in this case). This Rated power 40 kVA dependency implies that the instants for commutating the Dc link voltage 700 V cycloconverter phase legs have to be determined in different Transformer operating frequency 4 kHz ways depending on the direction of iac,i. This may be Transformer turns ratio 1.0 Transformer leakage inductance 5 µH interpreted as a carrier based method where sawtooth carriers Snubber capacitance, per valve 0.3 µF with either positive or negative slope, depending on the current Filter inductor, (per phase) 1.5 mH (0.35 pu) 800.0 400.0 utr 500.0 250.0 2itr voltage (V), current (A) 250.0 125.0 voltage (V) 0 0 -250.0 -125.0 -500.0 -250.0 -800.0 -400.0 4.200m 4.300m 4.400m 4.500m 4.600m 4.700m 4.200m 4.300m 4.400m 4.500m 4.600m 4.700m time(s) time(s) Figure 9. Simulated curve shapes of transformer voltage and current. Figure 10. Simulated curve shape of phase voltage uac,1. Operation at rated load. Operation at rated load. commutation sequence was implemented using a number of the phase voltages is shown in Figure 10 for the same time coupled state machines. All semiconductor elements were interval. The curveshape follows a two-level PWM pattern modeled as essentially ideal. Small RC-snubbers were except during commutations of the cycloconverter when the connected in parallel to the cycloconverter valves as such voltage momentarily goes to zero. The three phase currents snubbers most likely would be used in a real system to handle can be seen in Figure 11, showing the curve shape during an diode reverse recovery. In the simulations the converter system entire fundamental cycle. The curves are essentially sinusoidal is connected to a 230 V/50 Hz three-phase ac voltage source with minor notches occurring at zero crossings. These result and it is controlled to operate at rated power in such a fashion from the previously mentioned changes between positive and that either cosφ = 1 or cosφ = -1 at the connection point. This negative carrier slopes in the modulation algorithm. Figure 12, implies that the PWM voltages, uac,i, are slightly out of phase finally, displays a frequency spectrum of one of the ac side with the ac side currents, iac,i, in order to compensate for currents, iac,1. The dominant harmonics are grouped around reactive power in the filter inductors. even multiples of the transformer operating frequency. Figure 9 through Figure 12 display results from a simulation In Figure 13 the VSC output voltage and current are in which the system operates at rated load with power flowing displayed during a couple of commutation cycles when the from the dc side to the ac side. Figure 9, firstly, shows VSC system operates at low load and resonant commutation has to output voltage and current during a few commutation cycles be used for the VSC. The curveshapes are principally the same (cf. Figure 7). The voltage has essentially a square waveform as during full load (cf. Figure 9) but during the resonant and the frequency equals the switching frequency. Stepwise commutations the current momentarily increases. changes in the current occurs at commutations of the cycloconverter. The transients that appear in the current are V. APPLICATION IN HVDC CONVERTER STATIONS likely to be caused by the cycloconverter RC snubbers. One of High voltage direct current (HVDC) systems based on 200.0 iac,3 iac,2 iac,1 200 100 150.0 70 50 100.0 30 20 current (A) 50.0 10 current (A) 7 5 0 3 2 -50.0 1 700 m 500 m -100.0 300 m 200 m -150.0 80 m -200.0 50 m 2.00m 5.00m 10.00m 15.00m 20.00m 25.00m 30.00m -1.00k 0.0k 10.00k 20.00k 30.00k 40.00k 50.00k frequency(Hz) time(s) Figure 11. Simulated curve shapes of ac side currents. Operation at rated load. Figure 12. Frequency spectrum of ac side current iac,1. Operation at rated load. 800.0 Also for interfaces between HVDC transmission systems and renewable energy sources, such as wind generators, the 500.0 utr studied converter topology may represent a promising solution. voltage (V), current (A) 250.0 2itr VI. CONCLUSIONS 0 A three-phase isolated bi-directional ac/dc converter topology is described together with a control algorithm. It is -250.0 shown that soft commutation is possible for all semiconductor resonant current elements in all points of operation without the need for any during VSC -500.0 commutations auxiliary semiconductors. Simulations verify the operation of the concept. -800.0 9.600m 9.650m 9.700m 9.750m 9.800m 9.850m REFERENCES time(s) [1] T. Kawabata, K. Honjo, N. Sashida, K. Sanada, M. Koyama, "High Figure 13. Simulated curve shapes of transformer voltage and current. frequency link DC/AC converter with PWM cycloconverter", Operation at low load. Resonant commutation employed for the VSC. Conference Record of the 1990 Industry Applications Society Annual Meeting, pp. 1119 -1124, vol.2, 1990. voltage source converters have lately emerged as a competitive [2] I. Yamato and N. Tokunaga, “Power loss reduction techniques for three alternative to classic HVDC technology based on current phase high frequency link DC-AC converter”, 24th Annual IEEE Power Electronics Specialists Conference, PESC '93 Record., pp. 663 -668, source converters [9]. The converter stations in such a system 1993. are equipped with three-phase voltage source converters, in [3] M. Matsui, M. Nagai, M. Mochizuki and A. Nabae, “High-frequency which each valve is realised by several series-connected IGBT link DC/AC converter with suppressed voltage clamp circuits-naturally modules, see Figure 14a. On the ac side generally transformers commutated phase angle control with self turn-off devices”, IEEE Transactions on Industry Applications, vol. 32, no. 2, pp. 293 -300, are attached in order to adapt the output voltage to March-April 1996. transmission networks operating at medium or low voltage. [4] M. A. Rodrigues, E.R. da Silva, C.B. Jacobina and A.M.N. Lima, With the studied converter topology the dc/ac conversion can “PWM strategy for switching loss reduction in a high frequency link DC to AC converter”, 30th Annual IEEE Power Electronics Specialists be integrated with transformation by a medium frequency Conference, PESC ‘99 Record , vol. 2 , pp. 789 -794, 1999. transformer thus enabling lower transformer losses. In addition [5] S. Norrga, “A Soft-switched Bi-directional Isolated AC/DC Converter soft commutation for all semiconductor valves is achieved. An for AC-fed Railway Propulsion Applications”, IEE PEMD 2002 conference proceedings pp. 433-438, Bath UK, 16-18 April 2002. interesting option is furthermore to utilise a modification of the [6] S. Norrga, “A Novel Soft-switched Bi-directional Isolated AC/DC studied topology employing only one phase leg in the VSC, Converter”, PCIM 2002 conference proceedings, Nuremberg Germany, see Figure 14b. In this case one of the transformer terminals is 2002. [7] F. Iturriz, P. Ladoux, “Phase-Controlled Multilevel Converters Based on connected to a midpoint in the dc link created by bus-splitting Dual Structure Associations”, IEEE Trans Power Electronics, vol. 15, capacitors. This solution implies a very significant reduction in no. 1,pp 92-102, Jan 2000. the number of series-connected IGBT sub-valves on the high [8] C. Chabert, A. Rufer, “Multilevel Converter with Two-stage voltage side compared to the conventional case. As these sub- Conversion”, EPE 2001 Conf. Proceedings, Graz, Austria, 2001. [9] L. Stendius, K. Svensson, “HVDC Light - an excellent tool for city valves require complex gate drives and voltage-sharing center infeed”, PowerGen Conference Proc., Singapore, 1999. circuitry they tend to be expensive and a reduction in their number is therefore highly desirable. a.) Conventional VSC-based solution b.) Studied topology with MF transformer MF transformer LF transformer Three HV main phase legs One HV main phase leg Figure 14a,b. HVDC application. Comparison between a conventional system and the studied solution

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