International Journal of Advances in Electrical and Electronics Engineering 158 Available online at www.ijaeee.com & www.sestindia.org ISSN: 2319-1112 An Improved CSI with the Use of Hybrid PWM and Passive Resonant Snubber Latha. R 1,Walter raja rajan.b 2 rlatharam@yahoo.co.in walter.engg@gmail.com ABSTRACT A hybrid pulse width modulation(pwm) scheme that combines the advantages of unipolar switching scheme (USS) and bipolar switching scheme (BSS) for a grid-connected current source inverter is presented. The former switching scheme has the advantage of low switching loss while the latter one has the advantage of less total harmonic distortion(thd). The inverter bridge is predominantly operated under USS over a line cycle, within which only one switch is switched at high frequency. It is momentarily operated in BSS around the zero-crossing region of the line voltage, within which two switches are switched at high frequency. To further reduce the switching loss, the concept of passive resonant snubber is applied to the high-frequency switches in the inverter. The circuit is simulated using MATLAB simu-link. The proposed control scheme is also to be implemented for a 60V, 50Hz, 360W inverter. Index Terms Bipolar switching schemes (BSSs), current-source inverters (CSIs), pulse-dropping, pulse width modulation (PWM), resonant snubbers, unipolar switching scheme (USS). I. INTRODUCTION In today s environmentally conscious world, alternative energy sources are becoming a vital issue. Renewable energy technologies generate energy from sources that release much less, or no, carbon dioxide - one of the major green house gases causing global warming. However, their output power fluctuates with the ambient environment, such as the sunshine variation and weather condition, resulting in an unstable supply of power. As most renewable energy sources, such as the solar cells, deliver dc power, the above issue is typically tackled by feeding the generated power into the grid through a grid connected inverter[2]. There are two main categories of inverters, namely voltage source inverters (VSIs) and current source inverters (CSIs). Much research has been done on new modulation schemes and control strategies for the VSI. However, for grid connected applications, control of the CSI is comparatively simpler, because CSI can buffer the output from the grid voltage fluctuations and generate a predetermined magnitude of current to the grid. The CSI has inherent short-circuit protection and the advantage of rapidness in system control. There are two switching schemes namely unipolar switching scheme (USS) and bipolar switching scheme (BSS) for inverter applications [1]. Their waveforms are shown in Fig.2. In order to improve the output spectra sinusoidal pulse width modulation (SPWM) technique is used[6]. It is well known that the power switching devices require a finite time to turn-on or turn-off completely. Their switching characteristics cannot respond to narrow gate pulses, and thus pulse dropping occurs at the inverter output. pulse dropping means sine and triangular wave modulation required for the operation is less effective and some pulse of the carrier signal are not effective to modulate the voltage. This pulse dropping phenomenon introduces undesirable harmonics. In unipolar switching scheme the pulses near the zero-crossing region are practically too narrow and they will disappear at the output. Which is shown in Fig. 2(a). Although pulse dropping also occurs around the peak of the modulating signal. Which is equivalent to over modulation. Some compensated modulation techniques are proposed to deal with this issue. However, investigations into the effect of pulse-dropping around the zero-crossing region on the output spectrum and its remedial measures are less impressive. An advantage of BSS is that the duty cycle of the pulses around the zero crossing regions about 0.5. Which is shown in Fig. 2(b). The output waveform
IJAEEE,Volume2, Number 1 Latha. R and Walter raja rajan.b. Figure 1: structure of CSI (a) USS (b) BSS (c) HSS Figure 2: Waveform of different Switching scheme distortion is low within this region, but the switching loss in this scheme is higher when compared to USS. The hybrid switching scheme (HSS) combines the advantages of USS and BSS, which is proposed in this paper for a grid connected CSI. Its output
160 An Improved CSI with the Use of Hybrid PWM and Passive Resonant Snubber waveform is shown in Fig. 2(c). The switched current is supplied predominantly in USS and momentarily in BSS around the zero-crossing regions. In USS, only one of the four switches is switched at high frequency over one half cycle while the other switches are switched at the line frequency. Whereas in BSS two switches are switched at high frequency. The averaged switching frequency and thus the switching loss is reduced. To further reduce switching losses the snubber circuit is applied to the high frequency switches in the inverter bridge [4]. The switched current i 0 is supplied to the ac mains through a low pass filter. The output low pass filter reduces the high frequency harmonic content of the line current caused by PWM switching [3]. Generally the line filter consists of only a filter inductor (L filter) for VSIs but other configurations inductors and capacitors such as CL filters used for CSIs. II. IMPLEMENTATION OF THE HSS AND PASSIVE RESONANT SNUBBER A. Circuit Description: The fig.1(b) shows current source inverter with passive resonant snubber. The inverter bridge consists of four switches S 1, S 2, S 3, S 4. The proposed snubber is passive one. This snubber circuit consists of Diodes, Capacitors, and (a) Switching pattern S 1-S 4 (b) Voltage and current waveform of S 1 Figure 3: Timing Diagram Inductors. Which is used to reduce the switching losses, and to reduce the high frequency harmonic content, the capacitive inductive filter is used. B. Modes of operation: The switching pattern of the four switches is shown in Fig. 3(a). The switches S 1 and S 2 are switched at high frequency in positive half and negative half line cycle. Their duty cycles are pulse width modulated. The switches S 3 and S 4 are switched at the line frequency, except around the zero crossing region. To reduce the switching losses of high frequency switches S 1 and
IJAEEE,Volume2, Number 1 Latha. R and Walter raja rajan.b. S 2, the concept of passive resonant snubber is applied [7]. The key voltage and current waveforms associated with the operations of S 1, L 11, C 11, C 12, D 11, D 12, and D 13 in one switching cycle is shown in Fig. 3(b). There are nine topologies, as shown in Fig. 4. Mode 1 [t 1, t 2 ] [fig. 4(a)] --- switch S 1 is switched on with zero current, due to the presence of inductor L 11. The input current I dc will be taken up gradually by S 1. The current through S 1, i s1, which is equal to the current through the inductor L 11, i L11 will increase linearly while the current through the switch S 3, i s3 will decrease linearly. The current through the inductor L 21 is equal to I dc. The voltage on the snubber capacitor C 11, vc 11 is v s and the voltage on the capacitor C 12, vc 12 is equal to zero. Mode 2 [t 2, t 3 ] [fig. 4(b)] --- A resonant path formed by L 11, C 11, C 12, and D 12 is created. The voltage on the capacitor C 11 will reduce from the original value of v s to zero and C 12 undergoes charging. (a) Mode 1 [t 1, t 2 ] (b) Mode 2 [t 2, t 3 ]
162 An Improved CSI with the Use of Hybrid PWM and Passive Resonant Snubber (c) Mode 3 [t 3, t 4 ] (d) Mode 4 [t 4, t 5 ] (e) Mode 5 [t 5, t 6 ]
IJAEEE,Volume2, Number 1 Latha. R and Walter raja rajan.b. (f) Mode 6 [t 6, t 7 ] Fig. 4 Modes of operation (a) (f) (g) Mode 7 [t 7, t 8 ]
164 An Improved CSI with the Use of Hybrid PWM and Passive Resonant Snubber (h) Mode 8 [t 8, t 9 ] (i) Mode 9 [t 9, t 10 ] Figure 4: Modes of operation (g) (i) Mode 3 [t 3, t 4 ] [Fig. 4(c)] --- The second resonant path formed by L 11, C 12, D 11, and D 12 is created. The current through L 11 will reduce to I dc, and capacitor C 12 undergoes charging. Mode 4 [t 4, t 5 ] [Fig. 4(d)] --- The switch current is equal to I dc. At the end of this stage the switch S 1 is turned off. Mode 5 [t 5, t 6 ] [Fig. 4(e)] ---The switch is turned off with zero voltage. Capacitor C 11 will be charged up linearly. Mode 6 [t 6,t 7 ] [Fig. 4(f)]---When the voltage on the capacitor C 11 is increased to a level equal to the difference between v s and vc 12,diode D 13 will be ON, and L 11 and C 12 are discharged to the load. Mode 7 [t 7,t 8 ] [Fig. 4(g)]---The diode D 12 will be ON. L 11 and C 12 continue to be discharged to the load. This stage ends at t 8 when the current through the inductor L 11, i L11 is equal to zero. Mode 8[t 8,t 9 ][Fig.4(h)]---The capacitor C 12 will be discharged by I dc through the diode D 13 to the load. This stage ends at t9 when voltage on the capacitor C 12, vc 12 is equal to zero. Mode 9 [t 9,t 10 ] [Fig. 4(i)]---The capacitor C 12 is completely discharged and I dc will supply to the load. III.SOFT SWITCHING CRITERIA AND SIMPLIFIED DESIGN PROCEDURES A. Soft-Switching Criteria The soft switching criteria of the proposed CSI are listed as follows. 1) Criteria 1---The zero current switching is achieved when the snubber capacitor C 11 has to be discharged in mode 2,that is, C 12 >C 11 2) Criteria 2 --- In order to achieve the zero current switching in mode 1,L 11 has to be discharged before C 12 in mode 7 B. Simplified Design Procedures i. Design of snubber circuit Step 1--- By considering the maximum duration of mode 1,Δt 1, max, the snubber inductor L 11 is, L 11 = Δt 1, max. v s, min / I dc Step 2---By considering the maximum current stress i max on the switch in mode 2, the snubber capacitor C 11 = I dc.δt 1 / v s, min ii. Design of Low pass filter The filter cut off frequency is referred as resonant frequency fr, Fr=1/2π LC Hertz The Resonant frequency is selected to be small compared to switching frequency,
IJAEEE,Volume2, Number 1 Latha. R and Walter raja rajan.b. Fsw = 20 khz The cutoff frequency is chosen to have a reasonable ratio,fc/fsw =0.125 IV.SIMULATION MODEL OF CSI A 360W,60V,50Hz grid connected current source inverter bridge is built using 4 MOSFET switches S 1,S 2,S 3,S 4 in simulink. The snubber circuit is added to reduce switching loss, the snubber capacitors C 11 =C 12 =53.8nF, and Figure 5: Simulation circuit of Proposed current source inverter Figure 6: Simulated output voltage And current C 21 =C 22 =330nF. The snubber inductors L 11 =L 12 =2µH. The CL low pass filter blocks harmonics content caused by PWM switching. The capacitance and inductance of filter is 4µF and 600µH respectively. In the model, sinusoidal wave at 50Hz and carrier wave are compared to generate PWM signals. The switching frequency of the inverter is 20kHz.The simulation diagram of CSI is shown in Fig.5. V.RESULTS AND DISCUSSION Fig. 6. shows the output power of CSI operating in three switching schemes, including USS,BSS, AND HSS with the proposed snubber. Among them, the USS gives the highest efficiency in one half line cycle. However, the THD of the USS is highest around the zero crossing region. The BSS gives. The lowest efficiency as all switches are switched at high frequency in one half line cycle. However, the THD of BSS is lower when compared to USS. The proposed HSS combines the advantage of USS, which has good efficiency, and BSS, which has good THD. The efficiency of HSS is about 0.2% is less than that of USS, because the CSI is predominantly switched under USS. The THD of HSS is the lowest among the three switching schemes because HSS utilizes the advantage of BSS that has good current waveform around the zero crossing region.
166 An Improved CSI with the Use of Hybrid PWM and Passive Resonant Snubber VI.CONCLUSION This paper presents the use of HSS and resonant passive snubber for a grid connected CSI. The HSS can effectively tackle the pulse dropping phenomenon found in USS by introducing BSS in the pulse dropping region. This switching scheme and passive snubber is also to be applied for 360-W CSI. Thus the proposed CSI is simulated using MATLAB simu-link and desired outputs are obtained. Further, the proposed CSI will be implemented and compared with the theoretical values. REFERENCES [1] A. Ahfock and L.Bowtell, Comparision between unipolar and bipolar single phase grid connected inverters for PV Applications, in proc. Aust. univ. power Eng.conf.(AUPEC 2007,Dec,9-12,pp.1-5. [2] F. Blaabjerg, S.B.Kjaer, and J.Pederson, A review of single phase grid connected inverters for photovoltaic modules, IEEE Trans.Ind.Appl.,vol.41, no. 5, pp. 1292-1306,Sep./Oct. 2005. [3] Muhammed H.Rashid, Power Electronics,Circuits, Devices, and Applications. [4] K.Fujiwara and H.Nomura, A novel lossless passive snubber for soft switching boost type converters, IEEE Trans.Power Electron.,vol. 14, no. 6, pp. 1065-1069, Nov. 1999. [5] River T.H.Li and Henry S.H.Chung, Application of Hybrid PWM and passive resonant snubber for a grid connected CSI IEEE Trans 2008. [6] L.Salazar,G.Joos, and P.Ziogas, A low loss soft switching PWM CSI, in Proc.IEEE Power Electron.Spec.conf., 1992, pp. 1098-1104. [7] X.Wu,X.Jin,L.Huang, and G.Feng, A lossless snubber for dc/dc converters and its application in PFC, in proc. IPEMC,2000, pp. 1144-1149.