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Transcript
Power Circuit of the inverter: Fig. 1 represents the power circuit of a 5-level inverter with two cells in each phase, used for medium voltage drives. Each cell, shown in Fig. 2, has a three phase secondary winding at the input side. The output part of the cell has a single-phase inverter with Insulated Gate Bipolar Transistors (IGBTs). The output voltage of the cell has three values: +VDC, 0 and –VDC. A more detailed description of this topology can be found in [1] and [3]. A contactor is added at the output of each cell, compared to the original solution. N Another strategy to correct the unbalance is to change the phase of voltages VA, VB and VC, generated by the inverter, as observed in Fig. 3b. This is achieved by giving to the reference voltages of phases B and C the angles 135.52º and 224.47º, instead of 120º and 240º, respectively. This solution generates a higher load voltage. VA VA VCA 12 0º VCA VAB VAB 88.96º 120º VC º 52 5. 13 Introduction: Multilevel inverters have shown an important development on the last years, due to their capability of increasing the voltage and power delivered to the motor with semiconductors which are available today [1]. Due to this high power, these multilevel inverters have a big impact on production, generating important losses when they fail. For this reason, the investigation of methods to increase the reliability of these inverters is of high interest [2]. This paper presents a method to increase the reliability of a 5-level cascaded multicell inverter [3], introducing a reduced amount of additional hardware and changing the reference voltages of the modulator. A simple method to generate balanced voltages is to bypass a cell in each of the other two phases (B and C). However, this method produces a reduction of 50% in the load voltage and does not use all available cells. 13 5.5 2º This paper presents a method to operate a cascaded 5level inverter with faulty cells. The addition of a contactor and the displacement in the reference voltages allows for a balanced operation, increasing the reliability of the converter. 0º J. Rodríguez , P. Hammond, J. Pontt and R. Musalem Operation with one faulty cell: A damaged cell, for example A1 in Fig. 1, is bypassed by changing the position of the bypass contactor. The cell generates in this condition the output Vcell = 0. The faulty phase (A) will generate only 50% of the voltage generated by any of the other two phases. This will generate an important voltage unbalance in the load, as shown in Fig. 3a, which is unacceptable for a rotating electrical machine. 12 A method to increase the reliability in a 5-level inverter VB VC VBC VB VBC b) a) Fig. 3. Voltages generated with one faulty cell: a) unbalanced; b) balanced. Results: Fig. 4 represents the balanced operation of the inverter in normal conditions, with two cells in each phase working properly. 2000 [v] CELL A1 Vcell CELL A2 CELL B1 VB CELL B2 A CELL C1 a) -2000 0 CELL C2 B VAB a) 0 VC 2000 [v] 0 C iB 0.005 0.01 0.015 0.02 0.025 0.03 0.035 b) b) -2000 0 iA iC 2000 [v] 0 MEDIUM VOLTAGE INDUCTION MOTOR 0.005 0.01 0.015 0.02 0.025 0.03 0.035 c) c) 0 10 [A] 0.005 0.01 0.015 0.02 0.025 0.03 0.035 BYPASS CONTACTOR 0.04 d) d) 0 FROM DEDICATED SECONDARY OF INPUT TRANSFORMER 0.04 -2000 Fig. 1. Power circuit of the inverter -10 COIL 0 Vcell + 0.04 VDC OUTPUT OF POWER MODULE Fig. 2. Power circuit of each cell. Control of the inverter: Each cell operates with sinusoidal unipolar pulse width modulation, comparing a sinusoidal control voltage with a triangular carrier signal. The carrier has a frequency of 600 [Hz]. The carriers of the two cells in series connection are phase shifted to reduce the distortion of the load voltage, producing an additional equivalent switching frequency of 1200 [Hz]. 0.005 0.01 0.015 0.02 Time [s] 0.025 0.03 0.035 0.04 Fig. 4. Balanced operation of a 5-level cascaded multicell inverter (two cells per phase): a), b), c) line voltages in Volts [v]; d) load currents (iA, iB, iC) in Amperes [A]. Fig. 5 shows the unbalanced operation with one cell bypassed in phase A, maintaining the shifting of 120º between the reference voltages. This situation generates a high unbalance in the load voltages and currents. Finally, Fig. 6 shows that the proposed method generates balanced operation with one faulty cell. The reference voltages have the phase-shifting of Fig. 3b. This solution produces a current 50% higher than the simplest alternative of balanced operation with one cell per phase. 1 a) 0 -1 2000 [v] 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 b) -2000 2000 [v] 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 c) -2000 2000 [v] 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 d) -2000 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 10 [A] e) 0 -10 0 0.005 0.01 0.015 0.02 Time [s] 0.025 0.03 0.035 0.04 Fig. 5. Unbalanced operation with one faulty cell: a) reference voltages; b), c), d) line voltages in Volts [v]; e) load currents (iA, iB, iC) in Amperes [A]. 1 a) 0 -1 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 2000 [v] 0 0.04 b) -2000 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 2000 [v] 0 0.04 c) -2000 2000 [v] 0 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 d) -2000 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 10 [A] e) 0 -10 0 0.005 0.01 0.015 0.02 Time [s] 0.025 0.03 0.035 0.04 Fig. 6. Balanced operation with one faulty cell: a) reference voltages; b), c), d) line voltages in Volts [v]; e) load currents (iA, iB, iC) in Amperes [A]. Conclusions: The addition of a simple contactor at the output of each cell will produce a significant increase in the reliability of the inverter, permitting balanced operation with one faulty cell. The solution is not expensive and it is easy to implement. Acknowledgments: The authors acknowledge the support received from the Chilean Research Fund CONICYT under grant 1030368 and from the University Federico Santa María. References: 1 J. Rodriguez, J. S. Lai, F. Z. Peng, “Multilevel Inverters: A Survey of Topologies, Controls and Applications”, IEEE Transactions on Industrial Electronics, Vol. 49, No. 5, October 2002, pp. 724-738. 2 C. Turpin, P. Baudesson, F. Richardieu, F. Forest, T. Meynard, “Fault Management of Multicell Converters”, IEEE Transactions on Industrial Electronics, Vol. 49, No. 5, October 2002, pp. 988-997. 3 P. W. Hammond, “A new approach to enhance power quality for medium voltage drives”, IEEE Transactions on Industry Applications, Vol. 33, No. 1, January/February 1997, pp. 202208.