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1 A ZVS Grid-Connected Three-Phase Inverter Driven Renewable Energy Sources 1 CHENNAMSETTI PAVANI, 2BEELA RAJESH M.Tech [Scholar], EEE Dept, VITAM College of Engineering, Visakhapatnam, AP-India 2 Assisstant Professor, EEE Dept, VITAM College of Engineering, Visakhapatnam, AP-India 1 few years, there have been many studies on soft switching Abstract-- A six-switch three-phase inverter is widely used in a techniques for a three-phase converter. And they can high-power grid-connected system. However, the anti parallel generally be divided into two configurations according to diodes in the topology operate in the hard-switching state under the traditional control method causing severe switch loss the position where the soft-switching function is realized  as dc-side and ac-side soft-switching circuits. and high electromagnetic interference problems. In order to solve the problem, this paper proposes a topology of the traditional six-switch three-phase inverter but with an The active-clamping ZVS-PWM half-bridge inverter [6-8] also has lower voltage stress (1.01–1.1 times as high as the additional switch and gave a new space vector modulation dc-bus voltage). According to , in this active-clamping (SVM) scheme. In this way, the inverter can realize zero- ZVS-PWM half-bridge inverter, to achieve better soft- voltage switching (ZVS) operation in all switching devices and switching performance, the slow reverse recovery switch suppress the reverse recovery current in all anti parallel diodes antiparallel diode is the primary choice because the diode very well. And all the switches can operate at a fixed frequency reverse recovery energy is used to obtain the with the new SVM scheme and have the same voltage stress as the dc-link voltage. In grid-connected application, the inverter can achieve ZVS in all the switches under the load with unity power factor or less. The aforementioned theory is verified in a 30-kW inverter prototype. soft commutation condition. In the ZVS dc-link single-phase full-bridge inverter , the switch voltage is clamped to the dc-link voltage. The PWM modulation scheme is modified to achieve ZVS under different power factor loads. Besides the dc-side soft-switching technique, there are also some ac- Index Terms-- ZVS; ZCS; hard switching. side soft-switching techniques suitable for higher power application. The auxiliary resonant commutated pole I. INTRODUCTION (ARCP) converter achieves zero-voltage turn-on for main IN A high-power grid-connected inverter application, the switches and zero-current turn-off for an auxiliary switch six-switch three-phase inverter is a preferred topology with . several advantages such as lower current stress and higher The ARCP converter has excellent performance, but two efficiency. To improve the line current quality, the switching lowfrequency capacitors are necessary in the resonant cell frequency of the grid-connected inverter is expected to and it is difficult to control the capacitors’ midpoint voltage increase. Higher switching frequency is also helpful for without an additional control circuit. A new ZVS-PWM decreasing the size and the cost of the filter. However, single-phase fullbridge inverter using a simple ZVS-PWM higher switching frequency leads to higher switching loss commutation cell is proposed in . No auxiliary voltage . The soft-switching technique is a choice for a high- source or low-frequency center-tap capacitor is needed in power converter to work under higher switching frequency the cell. The main switches operate at ZVS and the auxiliary with lower switching loss and lower EMI noise. In the past switches operate at ZCS. The inductor-coupled ZVT inverter achieves the zero-voltage turnon condition for main switches and the near-zero current turn-off condition for 2 auxiliary switches –. This topology offers several which is similar to the rectifier topology proposed in . All advantages over the ARCP. The problems associated with the soft-switching advantages under the rectifier condition the split dc capacitor bank are avoided, and the ZVT can be achieved in a grid-connected inverter application, and operation requires no modification compared to normal the voltage stress in both main switches and the auxiliary space vector modulation (SVM) schemes. The peak current switch is the same as the dc-bus voltage. The operation stress of the auxiliary switches is half of that of the main principle of this SVM scheme is described in detail. The switches. experimental results of a 30-kW hardware prototype are The major problem of this topology is to use coupled presented to verify the theory. inductors, which are normally bulky in high-power applications. An improved ZVS inverter used two coupled magnetic components in one resonant pole to ensure the II. SWITCHING TECHNIQUES A. HARD AND SOFT SWITCHING main switches operating under the ZVS condition and the In the 1970’s, conventional PWM power converters were auxiliary switches operating under the ZCS condition when operated in a switched mode operation. Power switches have the load varies from zero to full. Since an independent to cut off the load current within the turn-on and turn-off coupled magnetic component structure avoids the unwanted times under the hard switching conditions. Hard switching magnetizing current antiparallel loop, the size of the coupled refers to the stressful switching behavior of the power inductors can be minimized with lower magnetizing electronic devices. The switching trajectory of a hard- inductance, and its saturation can be eliminated. The ZVS switched power device is shown in Fig.1. timing requirement is also satisfied over the full load range I Safe Operating Area by using the variable timing control with simple and reliable ZV detection. The zero-current transition (ZCT) inverter On Hard-switching achieves ZCS in all of the main and auxiliary switches and their antiparallel diodes. This topology needs six auxiliary snubbered switches and three LC resonant tanks. The simplified threeswitch ZCT inverter  needs only three auxiliary switches to achieve zero-current turn-off in all of the main switches and auxiliary switches. Compared with the six-switch ZCT Soft-switching Off V inverter, the resonant tank current stress of the three-switch ZCT inverter is higher. Fig.1 Typical switching trajectories of power switches. The structure of the ZVS-SVM controlled three-phase PWM rectifier is similar to the ACRDCL converter. With the special SVM scheme proposed by the authors, both the main switches and the auxiliary switch have the same and fixed switching frequency. The reverse recovery current of the switch be turned ON under the zero-voltage condition. Moreover, the voltage stress in both main switches and the auxiliary switch is only 1.01–1.1 times of the dc-bus voltage. In this paper, a ZVS three-phase grid-connected inverter is proposed. The topology of the inverter is shown in Fig. 2, During the turn-on and turn-off processes, the power device has to withstand high voltage and current simultaneously, resulting in high switching losses and stress. Dissipative passive snubbers are usually added to the power circuits so that the dv/dt and di/dt of the power devices could be reduced, and the switching loss and stress be diverted to the passive snubber circuits. However, the switching loss is proportional to the switching frequency, thus limiting the maximum switching frequency of the power converters. 3 Typical converter switching frequency was limited to a few frequency modulation (FM) for output regulation. Variable tens of kilo-Hertz (typically 20kHz to 50kHz) in early switching frequency operation makes the filter design and 1980’s. The stray inductive and capacitive components in control more complicated. the power circuits and power devices still cause considerable In late 1980’s and throughout 1990’s, further improvements transient effects, which in turn give rise to electromagnetic have been made in converter technology. New generations interference (EMI) problems. Fig.2 shows ideal switching of soft-switched converters that combine the advantages of waveforms and typical practical waveforms of the switch conventional PWM converters and resonant converters have voltage. The transient ringing effects are major causes of been developed. These soft-switched converters have EMI. switching waveforms similar to those of conventional PWM converters except that the rising and falling edges of the waveforms are ‘smoothed’ with no transient spikes. Unlike the resonant converters, new soft-switched converters usually utilize the resonance in a controlled manner. Resonance is allowed to occur just before and during the turn-on and turn-off processes so as to create ZVS and ZCS conditions. Other than that, they behave just like conventional PWM converters. With simple modifications, Fig.2. Typical switching waveforms of (a) hard-switched many customized control integrated control (IC) circuits and (b) soft-switched devices designed for conventional converters can be employed for soft-switched converters. Because the switching loss and In the 1980’s, lots of research efforts were diverted towards stress have been reduced, soft-switched converter can be the use of resonant converters. The concept was to operated at the very high frequency (typically 500kHz to a incorporate resonant tanks in the converters to create few Mega-Hertz). Soft-switching converters also provide an oscillatory (usually sinusoidal) voltage and/or current effective solution to suppress EMI and have been applied to waveforms so that zero voltage switching (ZVS) or zero DC-DC, AC-DC and DC-AC converters. This chapter current switching (ZCS) conditions can be created for the covers the basic technology of resonant and soft-switching power switches. The reduction of switching loss and the converters. Various forms of soft-switching techniques such continual improvement of power switches allow the as ZVS, ZCS, voltage clamping, zero transition methods etc. switching frequency of the resonant converters to reach are addressed. The emphasis is placed on the basic operating hundreds of kilo-Hertz (typically 100kHz to 500kHz). principle and practicality of the converters without using Consequently, magnetic sizes can be reduced and the power much mathematical analysis. density of the converters increased. Various forms of resonant converters have been proposed and developed. B. Resonant Switch However, most of the resonant converters suffer several Prior to the availability of fully controllable power problems. When compared with the conventional PWM switches, thyristors were the major power devices used in converters, the resonant current and voltage of resonant power electronic circuits. Each thyristor requires a converters have high peak values, leading to higher commutation circuit, which usually consists of a LC resonant conduction loss and higher V and I ratings requirements for circuit, for forcing the current to zero in the turn-off process. the power devices. Also, many resonant converters require This mechanism is in fact a type of zero-current turn-off 4 process. With the recent advancement in semiconductor Resonant-type DC-DC Converters technology, the voltage and current handling capability, and the switching speed of fully controllable switches have significantly been improved. In many high Conventional Resonant Converters power Quasi-Resonant Converters Multi-Resonant Converters applications, controllable switches such as GTOs and IGBTs Constant Frequency Operation Phase Shift-modulated have replaced thyristors. However, the use of resonant Variable Frequency Operation Load-Resonant Converters circuit for achieving zero-current-switching (ZCS) and/or zero-voltage-switching (ZVS) has also emerged as a new Series Resonant Converters technology for power converters. The concept of resonant Parallel Resonant Converters Constant Frequency Operation Variable Frequency Operation Series-Parallel Resonant Converters Fig.5: Classification switch that replaces conventional power switch is introduced in this section. A resonant switch is a sub-circuit comprising III. ZVS-QRC a semiconductor switch S and resonant elements, Lr and Cr. In these converters, the resonant capacitor provides a The switch S can be implemented by a unidirectional or zero-voltage condition for the switch to turn on and off. A bidirectional switch, which determines the operation mode quasi-resonant buck converter designed for half-wave of the resonant switch. Two types of resonant switches, operation is shown in Fig.6 - using a ZV resonant switch. including zero-current (ZC) resonant switch and zero- Basic relations of ZVS-QRCs are given in Equations (1a- voltage (ZV) resonant switches, are shown in Fig.3 and 1e). Fig.4, respectively. ILr Lr Dr Lr Lr Lf + v oi - Cr Df Cf + vc - Cr S Vi Io S Cr (a) Fig.6: Schematic diagram. (b) Fig.3 Zero-current (ZC) resonant switch. M Lr Lr S (a) Lr Cr Zr Cr S Vo Vi Cr (b) fr Fig.4 Zero-voltage (ZV) resonant switch. C. CLASSIFICATION 1 2 Lr Cr r The classification of the switching techniques are presented here in this fig.5. (1a) (1b) (1c) RL Zr (1d) fs fr (1e) + Vo - 5 IV. RESULTS The results pertaining to the schematic discussed in the previous section are presented here in this section. When the switch S is turned on, it carries the output current Io. The supply voltage Vi reverse-biases the diode Df. When the switch is zero-voltage (ZV) turned off, the output current starts to flow through the resonant capacitor Cr. When the resonant capacitor voltage VCr is equal to Vi, Df turns on. This starts the resonant stage. When VCr equals zero, the anti-parallel diode turns on. The resonant capacitor is shorted and the source voltage is applied to the resonant (b) Relationship between M and . inductor Lr. The resonant inductor current ILr increases linearly until it reaches Io. Then Df turns off. In order to Fig.7 Half-wave, quasi-resonant buck converter with ZVS. achieve ZVS, S should be triggered during the time when the anti-parallel diode conducts. It can be seen from the waveforms that the peak amplitude of the resonant capacitor voltage should be greater or equal to the input voltage (i.e., Io Zr > Vin). From Fig.7(a), it can be seen that the voltage conversion ratio is load-sensitive. In order to regulate the output voltage for different loads r, the switching frequency ZVS converters can be operated in full-wave mode. The circuit schematic is shown in Fig.8(a). The circuit waveforms in steady state are shown in Fig.8(b). The operation is similar to half-wave mode of operation, except that VCr can swing between positive and negative voltages. The relationships between M and at different r are shown in Fig.8(c). should also be changed accordingly. ILr 1 cycle ILr IO t0 t1 t1 ' t1 " Lr t t2 ' t3 Lf + v oi - + vc - t2 0 Dr Cr Io Df + Vo - Cf t4 (a) Schematic diagram. vc 1 cycle ZrIO ILr IO t2 0 t0 t1 t1' t1" vi 0 t t0 t1 t1 ' t1 " t2 t2 ' t3 t t2 ' t3 t4 t3 t4 vc t4 (a) Circuit waveforms. ZrIO vi 0 t t0 t1 t1 ' t1" t2 t2 ' (b) Circuit waveforms. 6  D. M. Divan, “Static power conversion method and apparatus having essentially zero switching losses and clamped voltage levels,” U.S. Patent 48 64 483, Sep. 5, 1989.  M. Nakaok, H. Yonemori, and K. Yurugi, “Zero-voltage soft-switched PDMthreephaseAC–DC active power converter operating at unity power factor and sinewave line current,” in Proc. IEEE Power Electronics Spec. Conf., 1993, pp. 787–794.  H. Yonemori, H. Fukuda, and M. Nakaoka, “Advanced (c) Relationship between M and . three-phase ZVS- PWM active power rectifier with new Fig.8 Full-wave, quasi-resonant buck converter with ZVS. resonant DC link and its digital control scheme,” in Proc. Comparing Fig.7(b) with Fig.8(c), it can be seen that M is IEE Power Electron. Variable Speed Drives, 1994, pp. 608– load-insensitive in full-wave mode. This is a desirable 613. feature. However, as the series diode limits the direction of  G. Venkataramanan, D. M. Divan, and T. Jahns, the switch current, energy will be stored in the output “Discrete pulse modulation strategies for high frequency capacitance of the switch and will dissipate in the switch inverter system,” IEEE Trans. Power Electron., vol. 8, no. 3, during turn-on. Hence, the full-wave mode has the problem pp. 279–287, Jul. 1993. of capacitive turn-on loss, and is less practical in high  G. Venkataramanan and D. M. Divan, “Pulse width frequency operation. modulation VII. CONCLUSION with resonant dc link converters,” inProc.Conf.RecordIEEEInd.Appl.Soc.Annu. Meeting, 1990, pp. 984–990. Through study and analysis, of the ZVS schematic and its  Y. Chen, “A new quasi-parallel resonant dc link for soft- results are made with a motive to understand its switching PWM inverters,” IEEE Trans. Power Electron., characteristics. In practice, ZVS-QRCs are usually operated vol.13,no.3,pp.427–435,May 1998. in half-wave mode rather than full-wave mode. By replacing the ZV resonant switch in the conventional converters, various ZVS-QRCs can be derived. A comparative analysis in the previous section paved a path towards vital conclusions in its applications. V. REFERENCES  N. Mohan, T. Undeland, and W. Robbins, Power Electronics: Converters, Applications and Design. New York: Wiley, 2003, pp. 524–545.  M. D. Bellar, T. S. Wu, A. Tchamdjou, J. Mahdavi, and M. Ehsani, “A review of soft-switched DC–AC converters,” IEEE Trans. Ind. Appl., vol. 34, no. 4, pp. 847–860, Jul./Aug. 1998.