A six-switch three-level current source inverter

A Six-Switch Three-level Current Source Inverter N. Vazquez, H. Lopez, C. Hernandez, R. Osorio, E. Rodriguez, J. Arau Instituto Tecnológico de Celaya Email: [email protected] Abstract - In several applications DC/AC converters are widely used; traditionally they can be classified in voltage source inverters (VSI) and current source inverters (CSI). Their use depends on the application. Other possibility for the DC/AC conversion is the multilevel configuration, and the most analyzed is the VSI. In this paper a multilevel current source inverter (MCSI) is proposed. The operation, analysis and implementation are presented; also simulation and experimental results are shown. I. Introduction. The dc/ac converters can be classified in two types: the voltage source inverter (VSI) and the current source inverter (CSI) [1]–[15]. The VSI has been studied extensively [1]–[8], this type of converter can be classified in half bridge inverter (HBI), full bridge inverter (FBI) and multilevel inverter (MI); and the last one can be subdivided in neutral point clamped inverter (NPC), flying capacitor inverter, cascade full bridge inverter [1] and recently the hybrid and hexagram inverter [3]–[5]. Also many works have been studying the MI [8]. In Fig. 1 the topologies of different type of multilevel VSI are presented. Nowadays the CSI has been studied less than the VSI, however it is used extensively in motor drives and it is Vin Vin Basic concepts are different for each inverter. VSI requires considering a blanking time, but an overlapping time is prohibited in one inverter leg. CSI needs an overlapping time, but blanking time is strictly prohibited in the upper/lower switches of the inverter legs. Finally, the ZSI allows both in one inverter leg. Current source converters feature a simple structure, low switching dv/dt, and reliable over-current/short-circuit protection. On the other side, multilevel converters offer the advantage that a low harmonic content can be obtained with relatively low switching frequency [1]. In this paper a type of CSI is discussed; in fact a multilevel type, that combines the characteristics of CSI and multilevel converter. This converter is quite simple, it uses fewer or equal amount of semiconductors than the multilevel CSI reported in literature; and also the current is equally distributed in the inductors associated to the topology. Vin S1 S1 Sa recommended in applications when boosting capabilities are required; this type of converter can be classified in pulse with modulated (PWM) CSI [13]–[15], load-commutated inverters (LCIs) [12] and there are also two multilevel topologies of CSI [17],[18]; the last type can be subdivided in the inverters that we referred as the “embedded” configuration [13], and the “two stage” configuration [14]. In Fig. 2 the topologies of multilevel CSI are presented. vd 2Vin Vin Sa S3 S3 Sc Sc (a) S1 S3 Sa Sc S1 S3 Sa Sc vd vd Vin (b) (c) Fig. 1. Different topologies of multilevel VSI. (a) NPC converter. (b) Flying capacitor converter. (c) Cascaded converter. 978-1-4244-8067-8/10/$26.00 ©2010 IEEE 145 Sb Sa S2 S1 Vin Operation of the converter In order to explain the operation of the proposed converter, the input current is considered constant (large inductors); and when the switches S1, S2, and S3 are turned on the other switches are turned off, in fact the control signal of S1 is exactly the opposite of Sa and so on for the other switches. In practice there is a small overlapping in the control signals S1 and Sa, this is because always a current path must be provided to the inductors in order to avoid the semiconductor damage. Io S3 S4 Is Sd Sc Fig. 4 shows the current generated by the proposed MCSI. These waveforms are at low frequency and only the positive semicycle is shown in order to illustrate the behavior. Control signals for the switches S1, S2 and S3 are also included. According to Fig. 4 the converter operates as follows: (a) S2 S1 Vin Sb Sa Io S3 S4 Is (b) Fig. 2. Different topologies of multilevel CSI. (a) Embedded converter. (b) Two stage converter. II. PROPOSED CONVERTER. The proposed idea consists of a CSI, but a multilevel type that permit to produce an output current with a lower harmonic content compared with the semiconductors switching frequency, the proposed converter injects current to the output in a parallel connection. In Fig. 3 the proposed three level topology is shown; it consists of two CSI connected in parallel. During t0 – t1. S1, S2 and S3 are turned on, the equivalent subcircuit is shown in Fig. 5(a), then zero current is injected (also a zero current can be produced when S1, S2 and S3 are turned off). During t1 – t2. S1 is maintained on, S2 and S3 are turned off, the equivalent subcircuit is shown in Fig. 5(b), and then a positive current is delivered. During t2 – t3. The same switches combination that during t0-t1 is used. Then a zero current output is obtained. During t3 – t4. A positive current is injected to the load, but in this case the other combination is used. S2 is turned on, S1 and S3 are turned off. During t4 – t5. S1 and S2 are turned on, and S3 is turned off, the equivalent subcircuit is shown in Fig. 5(c), and then a double magnitude of positive current is obtained. After t5. During the rest of time the converter operation is similar to the before mentioned. S1 S2 Vin Sa S1 S2 S3 Io S3 Sb Is Sc I0 t4 t2 t0 Fig. 3 Proposed three level current source inverter t1 t3 t5 Fig. 4. Control signals and output current. Top to down: Control signal of S1, control signal of S3, control signal of S2 and output current 146 Vin Sa S1 S2 S3 Io Is Sa Vin Sb Sc S1 S2 S3 Io (a) Vin Sb Is Sc (b) Sa S1 S2 S3 Io Sb Is Sc (c) Fig. 5. Subcircuits of the proposed multilevel CSI. (a) Subcircuit for zero current. (b) Subcircuit for positive current. (c) Subcircuit for double positive current. III. EXPERIMENTAL AND SIMULATION RESULTS Numerical simulations are shown in Figs. 6 and 7. They were made at 600Hz, with an inductor of 40mH due to the response time of the simulator. Fig, 6 shows the output current with a modulation index of 0.99 and also the inductors current, and as it can be observed they are well balanced. Fig. 7 illustrates the performance of the system with a lower modulation index, particularly for this case the output current and the control signals of the switches S1, S2 and S3 are shown. As it can be observed the output has a higher switching frequency than the semiconductors. In Figs. 8 and 9 some experimental results of the system are shown. The input inductors are of 400mH; the switching devices are IRG4PC40 without anti-parallel diode, as a consequence the extra topology diodes are not required. In Fig. 8 the output current of the proposed converter and the inductor currents are shown; it can be observed that they are well balanced; the converter operates as it was expected. In Fig. 9, the output current and its respective harmonic content are shown; it can be observed that the first high frequency harmonic appears around 50 KHz, this is because the carrier frequency is 25 KHz. The THD obtained in this test is lower than 5%. IV CONCLUSIONS This paper presents a different method to built a multilevel current source inverter, it is suggested a parallel connection. The converter is different to the previous published schemes. For a single-phase three level CSI, six semiconductors and two inductors are used. The proposed idea can be extrapolated to three phase systems, but also more levels can be added. REFERENCES [1] Muhammad H. Rashid. “Power Electronics: Circuits, Devices, and Applications” Third Edition, Ed Prentice Hall [2] M. A. Pérez, P. Cortés, J. Rodríguez. “Predictive Control Algorithm Technique for Multilevel Asymmetric Cascaded H-Bridge Inverters” IEEE Transactions on Industrial Electronics. Vol 55, No 12, December 2008. pp 4354-4361. [3] A. Chen, X. He. “Research on Hybrid-Clamped Multilevel-Inverter Topologies”. IEEE Transactions on Industrial Electronics. Vol 53, No 6, December 2006. pp 1898-1907. [4] C. Rech, J. R. Pinheiro. “Hybrid Multilevel Converters: Unified Analysis and Design Considerations”. IEEE Transactions on Industrial Electronics. Vol 54, No 2, April 2007. pp 1092-1104. [5] J. Wen, K. M. Smedley. “Hexagram Inverter for Medium-Voltage Six-Phase Variable-Speed Drives” IEEE Transactions on Industrial Electronics. Vol 55, No 6, April 2008. pp 2473-2481. [6] H. du T. Mouton. “Natural Balancing of Three-Level Neutral-PointClamped PWM Inverters”. IEEE Transactions on Industrial Electronics. Vol 49, No 5, October 2002. pp 1017-1025. [7] G.P. Adam, S.J. Finney, A.M. Massoud, B.W. Williams. “Capacitor Balance Issues of the Diode-Clamped Multilevel Inverter Operated in a Quasi Two-State Mode” IEEE Transactions on Industrial Electronics. Vol 55, No 8, August 2008. pp 3088-3099 [8] J. Rodríguez, J. Pontt, P. Correa, P. Cortés, C. Silva. “A New Modulation Method to Reduce Common-Mode Voltages in Multilevel Inverters”. IEEE Transactions on Industrial Electronics. Vol 51, No 4, August 2004. pp 834-839. [9] G. Joos, G. Moschopoulos, P. D. Ziogas, “A High Performance Current Source Inverter”, IEEE Transactions on Power Electronics, Vol 8, No 4, Oct. 1993, pp 571-579 [10] J. R. Espinoza, G. Joos. “Current–Source Converter On-Line Pattern Generator Switching Frequency Minimization”. IEEE Transactions on Industrial Electronics, Vol 44, No 2, April 1997. pp 198-206. 147 Output current Output current S1 S2 Inductors current Fig. 6. Output current and inductors current. Top to down: Output current (500mA/div), inductors current (125mA/div). Time: 1ms/div S3 Fig. 7 Output current and control signals of the switches. Top to down: Output current (500mA/div), Control signal S1, S2 and S3 respectively. Time: 1ms/div. Fig. 8. Output current and inductor currents. [11] D. N. Zmood, D.G. Colmes. “A Generalised Approach to the Modulation of Current Source Inverters”. IEEE Power Electronics Specialists Conference PESC 1998. pp 739-745. [12] S. D. Sudhoff, E. L. Zivi, and T. D. Collins, “Start up performance of load-commutated inverter fed synchronous machine drives,” IEEE Transactions on Energy Conversion., vol. 10, no. 2, pp. 268–274, Jun. 1995. [13] F.L.M. Antunes, H.A.C. Braga, I. Barbi, “Application of a Generalized Current Multilevel Cell to Current-Source Inverters”. IEEE Transactions on Industrial Electronics. Vol 46, No 1, April 1999. pp 31-38. Fig. 9. Output current and harmonic content. [14] P. G. Barbosa, H. A. Carvalho, M. do C. Barbosa, E. Coelho. “Boost Current Multilevel Inverter and Its Application on Single-Phase GridConnected Photovoltaic Systems”. IEEE Transactions on Power Electronics, Vol 21, No 4, July 2006. pp 1116-1124. [15] Y. Suh, J. K. Steinke, P. K. Steimer. “Efficiency Comparison of Voltage-Source and Current-Source Drive Systems for MediumVoltage Applications”. IEEE Transactions on Industrial Electronics. Vol 54, No 5, April 2007. pp 2521-2531. 148