Navpreet Singh Tung et al. / International Journal of Engineering Science and Technology (IJEST)
Dynamics of IGBT based PWM Converter A Case Study Navpreet Singh Tung M-Tech Scholar, Electrical Engg. , Lovely Professional University ,India
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Amit Bhardwaj M-Tech Scholar, Electrical Engg. , Lovely Professional University ,India
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Tarun Mittal M-Tech Scholar, Electrical Engg. , Lovely Professional University ,India
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Vijay Shukla Electrical Engg. , Lovely Professional University
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Abstract— Optimizing the efficiency and dynamics of power converters is a critical tradeoff in power electronics. The increase of switching frequency can improve the dynamics of power converters, but the efficiency may be degraded as well as the switching losses. As power semiconductor devices like diodes, MOSFETS, IGBTs, Thyristors, BJTs have their own characterstics and dynamic responses. It is desired to analyze and observe the dynamics of different semiconductor devices before they actually employed in the model. Inclusion of different PWM techniques help in the removal of power line interferences like harmonic losses, unwanted ripples, chopped frequencies, spikes. In this paper, we have studied and analyzed the dynamics of IGBT based PWM converter with subjected to different conditions like transient state, steady state feeding the RLC load. Snubber circuits are used to reduce the switching losses. The IGBT based PWM converter reflects the better dynamics with improved efficiency and reduced harmonics as compared to some other power semiconductor devices when FFT is performed and subjected to standard parameterized RLC load under steady state and transient analysis. Index Terms – Modulation Index(MI),Pulse Width Modulation(PWM),FFT(Fast Fourier Transform) 1.
Introduction
THE DEMAND of high performance power converter is increased exponentially with the increasing spectrum of power converter’s application fields [1]. In order to optimize the transient and steady state performance of power converters and to enhance power density, high switching frequency is an effective method. However, switching frequency rise causes higher switching losses and greater EMI losses [2]. This, in turn, lim its the increase of switching frequency and hinders the improvement of system performance. Various active and passive soft-switching techniques have been introduced to reduce switching losses [3]–[6]. While these can create more favorable switching projections for various active power devices, they will generally increase the complexity of control and sometimes are affected by the variable input and output condition. In the trends of using power modules, space is limited for placing the added elements. The complexity of power stage and control circuit also reduces the reliability of soft-switched converters. Multi-converter paralleling method, which employs low-power converters in parallel to enhance the power rating, has been proposed to enhance the power processing capability [7]–[12]. However, parallel operation has interaction problem that causes circulating current [11], [12]. To avoid the circulating current, approaches such as isolation, high impedance, and one-
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Navpreet Singh Tung et al. / International Journal of Engineering Science and Technology (IJEST)
converter approach are utilized. These efforts increase the control complexity. The interleaving operation employs N converters to operate in parallel with interleaved clocks, so the total dynamics can reach higher performance due to the fact that the equivalent frequency is N times the single converter frequency. Nevertheless, the circulating current phenomenon also exists. A single-phase boost-type zero-voltage-transition (ZVT) PWM converter proposed in [13] adopts an additional shunt resonant network to form an additional Boost cell to realize soft switching of the main switches. However, the auxiliary switches operate in hard switch and high frequency. A similar topology of single-phase rectifier is given in [14], where total harmonic distortion of the input line current is reduced and the efficiency improved. Its operation is different from the ZVT circuit. The boost-type topology, however, is not very effective to enhance the output voltage performance in that the capacitor ripple voltage is determined by the low frequency. Hence, this topology is not suitable for improvement of dc output transient and steady state performances. Moreover, the main Boost circuit and the added cell are coupled, and the added Boost cell has an effect on the inductor current input [15]. Splitting the filter inductor of buck converter into two parts with added auxiliary active switch and diode has been proposed to improve the output voltage response at load current step-down transient situation, but not at load current stepup transient situation [16]. Additional transformer and switches are needed to realize the improvement at step-up transient [17]. To make the circuits in [16] and [17] function as designed, it is required to detect the load transient event, then to trigger or shut down the auxiliary switch. This increases the complexity of the control circuit. Moreover, oscillations at the output voltage occur due to the frequent on and off operations at each transient event. On the other hand, high-frequency switching converter or linear power supply in parallel with low-frequency converter proposed in [18] and [19] enhances the output voltage response. Paralleling highfrequency converter approach also requires the load transient information, while linear power supply method suffers from low efficiency. Moreover, the parallel structure brings about the circulating current problem. Additional current sharing control is needed to overcome this problem. 2.
Model Layout
A 60 Hz, voltage source feeds a 50 Hz, 50 kW load through a double conversion converter. The 600V, 60 Hz voltage obtained at secondary of the Y connected transformer is first rectified by a six pulse diode bridge. The filtered DC voltage is applied to an IGBT two-level inverter generating 50 Hz. The IGBT inverter uses PWM at a 2 kHz carrier frequency. The circuit is discretized at a sample time of 2 us The load voltage is regulated at 1 pu (380 V rms) by a PI voltage regulator using abc_to_dq and dq_to_abc transfomations. The first output of the voltage regulator is a vector containing the three modulating signals used by the PMW Generator to generate the 6 IGBT pulses. The second output returns the modulation index. The Discrete 3-Phase PWM Pulse Generator is used.
Fig 2. IGBT - Diode Bridge.
Fig 1. Layout of PWM Converter
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Navpreet Singh Tung et al. / International Journal of Engineering Science and Technology (IJEST)
Fig 3. Schematic of PWM Converter
3.
Performance Analysis 3.1 Steady State Analysis
After a transient period of approx. 50 ms, the system reaches a steady state as observed in Fig.4. Observe voltage waveforms at DC bus, inverter output and load. Harmonics generated by the inverter around multiples of 2 kHz are filtered by the LC filter. As expected the peak value of the load voltage is 537 V (380 Vrms). In steady state, the mean value of the modulation index is m = 0.80 and the mean value of the DC voltage is 778 V. The fundamental component of 50 Hz voltage in the chopped inverter voltage is therefore: Vab = 778 V * 0.612 * 0.80 = 381 V rms 3.2 FFT Analysis 'FFT Analysis' displays the 0 - 7000 Hz frequency spectrum of signals as shown in Fig.6(a,b,c,d). The FFT is performed on a 2-cycle window starting at t=0.1-2/50 (last 2 cycles of recording). For load voltage- 'Vab Load' observe the frequency spectrum of last 2 cycles. Notice harmonics around multiples of the 2 kHz carrier frequency. Maximum harmonic is 1.4 % of fundamental and THD is 2%. Table 1. FFT Analysis
Observe diode currents showing commutation from diode 1 to diode 3 and currents in switches 1 and 2 of the IGBT/Diode bridge (upper and lower switches connected to phase A) in Fig.5. These two currents are complementary. A positive current indicates a current flowing in the IGBT, whereas a negative current indicates a current flowing in the anti-parallel diode.
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Navpreet Singh Tung et al. / International Journal of Engineering Science and Technology (IJEST)
4.
Transient State Analysis
As observed in Fig. 4-5, transient state remains till approximately. 50 ms . Output of rectifier increases from 0 to 1000.Some peak reverse recovery happens as shown then reaching towards its steady state value. MI – m initially follow linear region with initial value of 0.5 then follows constant region with some over-modulation at m=1.It decays and going towards steady state region. We can observe some amplitude variation in Vab_load and Vab_inverter with harmonics.
Fig 4. Variation of Vdc, Vab_inverter, Vab_load, m
Fig 5 . Variation of Diode currents and Switches (IGBT)
Fig 6.a – Vdc FFT Analysis
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Fig 6.b Vab inverter FFT Analysis
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Navpreet Singh Tung et al. / International Journal of Engineering Science and Technology (IJEST)
Fig 6.c Vab load FFT Analysis
Fig 6.d – MI(m) FFT Analysis
Figure 7. Diode bridge
5.
Figure. 8– Initial values of parameters
Circuit Analysis 5.1 3 phase IGBT PWM Inverter(Fig.2)
It implements a universal three-phase power converter that consists of up to six power switches connected in a bridge configuration with forced-commutated device- IGBT. Series RC snubber ckt is connected parallel to lower down the switching losses on IGBT. The bridge operates satisfactorily with purely resistive snubbers as long as firing pulses are sent to switching devices. If firing pulses to forced-commutated devices are blocked, only anti-parallel diodes operate, and the bridge operates as a diode rectifier. In this condition appropriate values of Rs and Cs must also be used. When the system is discretized, use the following formulas to compute approximate values of Rs and Cs: a)Rs >2 Ts/Cs b)Cs
