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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 Abstract-- A six-switch three-phase inverter is widely used in a high-power grid-connected system. However, the anti parallel diodes in the topology operate in the hard-switching state under the traditional control method causing severe switch loss 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 additional switch and gave a new space vector modulation (SVM) scheme. In this paper the simulation of ZVS three phase inverter, ZVS three phase inverter with PV cell are produced and a comparative analysis basing on various reports generated in the matlab tool is performed. Index Terms-- ZVS; ZCS; hard switching. I. INTRODUCTION IN A high-power grid-connected inverter application, the six-switch three-phase inverter is a preferred topology with several advantages such as lower current stress and higher efficiency. To improve the line current quality, the switching frequency of the grid-connected inverter is expected to increase. Higher switching frequency is also helpful for decreasing the size and the cost of the filter. However, higher switching frequency leads to higher switching loss [1]. The soft-switching technique is a choice for a highpower converter to work under higher switching frequency with lower switching loss and lower EMI noise. In the past few years, there have been many studies on soft switching techniques for a three-phase converter. And they can generally be divided into two configurations according to the position where the soft-switching function is realized [26] as dc-side and ac-side soft-switching circuits. The active-clamping ZVS-PWM half-bridge inverter [6-8] also has lower voltage stress (1.01–1.1 times as high as the dc-bus voltage). According to [6], in this active-clamping ZVS-PWM half-bridge inverter, to achieve better softswitching performance, the slow reverse recovery switch antiparallel diode is the primary choice because the diode reverse recovery energy is used to obtain the soft commutation condition. In the ZVS dc-link single-phase full-bridge inverter [7], 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 acside soft-switching techniques suitable for higher power application. The auxiliary resonant commutated pole (ARCP) converter achieves zero-voltage turn-on for main switches and zero-current turn-off for an auxiliary switch [4]. The ARCP converter has excellent performance, but two lowfrequency capacitors are necessary in the resonant cell and it is difficult to control the capacitors’ midpoint voltage without an additional control circuit. A new ZVS-PWM single-phase fullbridge inverter using a simple ZVS-PWM commutation cell is proposed in [5]. No auxiliary voltage source or low-frequency center-tap capacitor is needed in the cell. The main switches operate at ZVS and the auxiliary 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 auxiliary switches [6]–[8]. This topology offers several advantages over the ARCP. The problems associated with the split dc capacitor bank are avoided, and the ZVT operation requires no modification compared to normal space vector modulation (SVM) schemes. The peak current stress of the auxiliary switches is half of that of the main switches. The major problem of this topology is to use coupled 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 main switches operating under the ZVS condition and the auxiliary switches operating under the ZCS condition when the load varies from zero to full. Since an independent coupled magnetic component structure avoids the unwanted magnetizing current antiparallel loop, the size of the coupled inductors can be minimized with lower magnetizing inductance, and its saturation can be eliminated. The ZVS timing requirement is also satisfied over the full load range by using the variable timing control with simple and reliable ZV detection. The zero-current transition (ZCT) inverter achieves ZCS in all of the main and auxiliary switches and their antiparallel diodes. This topology needs six auxiliary switches and three LC resonant tanks. The simplified threeswitch ZCT inverter [23] 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 inverter, the resonant tank current stress of the three-switch ZCT inverter is higher. 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 2 proposed. The topology of the inverter is shown in Fig. 2, which is similar to the rectifier topology proposed in [8]. All the soft-switching advantages under the rectifier condition can be achieved in a grid-connected inverter application, and the voltage stress in both main switches and the auxiliary switch is the same as the dc-bus voltage. The operation principle of this SVM scheme is described in detail. The experimental results of a 30-kW hardware prototype are presented to verify the theory. II. SWITCHING TECHNIQUES A. HARD AND SOFT SWITCHING In the 1970’s, conventional PWM power converters were operated in a switched mode operation. Power switches have to cut off the load current within the turn-on and turn-off times under the hard switching conditions. Hard switching refers to the stressful switching behavior of the power electronic devices. The switching trajectory of a hardswitched power device is shown in Fig.1. I Safe Operating Area On Hard-switching snubbered Soft-switching Off V Fig.1 Typical switching trajectories of power switches. 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. Typical converter switching frequency was limited to a few tens of kilo-Hertz (typically 20kHz to 50kHz) in early 1980’s. The stray inductive and capacitive components in the power circuits and power devices still cause considerable transient effects, which in turn give rise to electromagnetic interference (EMI) problems. Fig.2 shows ideal switching waveforms and typical practical waveforms of the switch voltage. The transient ringing effects are major causes of EMI. Fig.2. Typical switching waveforms of (a) hard-switched and (b) soft-switched devices In the 1980’s, lots of research efforts were diverted towards the use of resonant converters. The concept was to incorporate resonant tanks in the converters to create oscillatory (usually sinusoidal) voltage and/or current waveforms so that zero voltage switching (ZVS) or zero current switching (ZCS) conditions can be created for the power switches. The reduction of switching loss and the continual improvement of power switches allow the switching frequency of the resonant converters to reach hundreds of kilo-Hertz (typically 100kHz to 500kHz). Consequently, magnetic sizes can be reduced and the power density of the converters increased. Various forms of resonant converters have been proposed and developed. However, most of the resonant converters suffer several problems. When compared with the conventional PWM converters, the resonant current and voltage of resonant converters have high peak values, leading to higher conduction loss and higher V and I ratings requirements for the power devices. Also, many resonant converters require frequency modulation (FM) for output regulation. Variable switching frequency operation makes the filter design and control more complicated. In late 1980’s and throughout 1990’s, further improvements have been made in converter technology. New generations of soft-switched converters that combine the advantages of conventional PWM converters and resonant converters have been developed. These soft-switched converters have 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, many customized control integrated control (IC) circuits designed for conventional converters can be employed for soft-switched converters. Because the switching loss and stress have been reduced, soft-switched converter can be operated at the very high frequency (typically 500kHz to a few Mega-Hertz). Soft-switching converters also provide an effective solution to suppress EMI and have been applied to 3 DC-DC, AC-DC and DC-AC converters. This chapter covers the basic technology of resonant and soft-switching converters. Various forms of soft-switching techniques such as ZVS, ZCS, voltage clamping, zero transition methods etc. are addressed. The emphasis is placed on the basic operating principle and practicality of the converters without using much mathematical analysis. B. Resonant Switch Prior to the availability of fully controllable power switches, thyristors were the major power devices used in power electronic circuits. Each thyristor requires a commutation circuit, which usually consists of a LC resonant circuit, for forcing the current to zero in the turn-off process. This mechanism is in fact a type of zero-current turn-off process. With the recent advancement in semiconductor technology, the voltage and current handling capability, and the switching speed of fully controllable switches have significantly been improved. In many high power applications, controllable switches such as GTOs and IGBTs have replaced thyristors. However, the use of resonant circuit for achieving zero-current-switching (ZCS) and/or zero-voltage-switching (ZVS) has also emerged as a new technology for power converters. The concept of resonant switch that replaces conventional power switch is introduced in this section. A resonant switch is a sub-circuit comprising a semiconductor switch S and resonant elements, Lr and Cr. The switch S can be implemented by a unidirectional or bidirectional switch, which determines the operation mode of the resonant switch. Two types of resonant switches, including zero-current (ZC) resonant switch and zerovoltage (ZV) resonant switches, are shown in Fig.3 and Fig.4, respectively. 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 inductor Lr. The resonant inductor current ILr increases linearly until it reaches Io. Then Df turns off. In order to 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.5(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 should also be changed accordingly. 1 cycle ILr IO t2 0 t0 t1 t1' t1" t t 2' t3 t4 t3 t4 vc ZrIO vi 0 t t0 t1 t1 ' t1" t2 t 2' (a) Circuit waveforms. 1 0.9 0.9 0.8 0.8 0.7 M 0.6 Lr Lr 0.5 0.5 0.4 0.3 0.2 0.2 Cr S 0.1 0.1 S Cr 0 0 (a) (b) Fig.3 Zero-current (ZC) resonant switch. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 g (b) Relationship between M and g. Fig.5 Half-wave, quasi-resonant buck converter with ZVS. Lr Lr Cr S S (a) Cr (b) Fig.4 Zero-voltage (ZV) resonant switch. IV. RESULTS The results pertaining to the schematic discussed in the previous section are presented here in this section. When the ZVS converters can be operated in full-wave mode. The circuit schematic is shown in Fig.6(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 g at different r are shown in Fig.8(c). 4 ILr Cr Dr Io Lr Lf + v oi - + vc - Df + Vo - Cf (a) Schematic diagram. Fig.8: Output Voltage & Current of Proposed Three Phase ZVS based Grid Connected Inverter 1 cycle ILr IO t2 0 t0 t1 t1' t1" t t 2' t3 t4 t3 t4 vc ZrIO The corresponding output waveforms of the proposed model presented in the circuit given as ZVS three phase inverter is given in the Fig.8. Similarly Fig.10 depicts the simulation results of the Fig.9 circuit. ZVS using PV cell vi 0 t t0 t1 t1 ' t1" t2 t 2' (b) Circuit waveforms. 1 0.9 0.9 0.8 0.8 0.7 M 0.6 0.5 0.5 0.4 Fig.9: simulation diagram of a ZVS using PV cell 0.3 0.2 0.2 1 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 g (c) Relationship between M and g. Fig.6 Full-wave, quasi-resonant buck converter with ZVS. Comparing Fig.5(b) with Fig.6(c), it can be seen that M is load-insensitive in full-wave mode. This is a desirable feature. A thorough description of the proposed ZVS three phase inverter model with and without PV cell are given in this section. There are presented in the Fig.7 through Fig.10. ZVS 3 phase inverter Fig 7 Simulation diagram of ZVS three phase inverter Fig.10: Output waveforms of the proposed ZVS 3 phase inverter VII. CONCLUSION Through study and analysis, of the ZVS schematic and its results are made with a motive to understand its characteristics. In practice, ZVS-QRCs are usually operated 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 [1] N. Mohan, T. Undeland, and W. Robbins, Power Electronics: Converters, Applications and Design. New York: Wiley, 2003, pp. 524–545. 5 [2] 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. [3] 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. [4] 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. [5] H. Yonemori, H. Fukuda, and M. Nakaoka, “Advanced three-phase ZVS- PWM active power rectifier with new resonant DC link and its digital control scheme,” in Proc. IEE Power Electron. Variable Speed Drives, 1994, pp. 608– 613. [6] G. Venkataramanan, D. M. Divan, and T. Jahns, “Discrete pulse modulation strategies for high frequency inverter system,” IEEE Trans. Power Electron., vol. 8, no. 3, pp. 279–287, Jul. 1993. [7] G. Venkataramanan and D. M. Divan, “Pulse width modulation with resonant dc link converters,” inProc.Conf.RecordIEEEInd.Appl.Soc.Annu. Meeting, 1990, pp. 984–990. [8] Y. Chen, “A new quasi-parallel resonant dc link for softswitching PWM inverters,” IEEE Trans. Power Electron., vol.13,no.3,pp.427–435,May 1998.