Analytical and Simulation Fair Comparison of Three Level Si IGBT Based NPC Topologies and Two Level SiC MOSFET Based Topology for High Speed Drives

Abstract

Wide bandgap (WBG) power devices such as silicon carbide (SiC) can viably supply high speed electrical drives, due to their capability to increase efficiency and reduce the size of the power converters. On the other hand, high frequency operation of the SiC devices emphasizes the effect of parasitics, which generates reflected wave transient overvoltage on motor terminals, reducing the life time and the reliability of electric drives. In this paper, a SiC metal-oxide-semiconductor field-effect transistor (MOSFET) based two level (2L) inverter is systematically studied and compared to the performance of Si insulated-gate bipolar transistor (IGBT) based three level (3L) neutral point clamped (NPC) inverter topologies, for high speed AC motor loads, in terms of efficiency, overvoltages, heat sink design, and cost. A fair comparison was introduced for the first time, having the same output voltage capabilities, output current total harmonic distortion (THD), and overvoltages for the three systems. The analysis indicated the convenience of using the SiC MOSFET based 2L inverter for lower output power. In the case of the maximum output power, the heat sink volume was found to be 20% higher for the 2L SiC based inverter when compared to 3L NPC topologies. Simulations were carried out by realistic dynamic models of power switch modules obtained from the manufacturer's experimental tests and verified both in the LTspice and PLECS simulation packages.Even though the modern switching devices offer many benefits, they still experience many problems mainly connected to the fast switching such as high dv/dt-rates, also in combination with impedance mismatch (machine against cabling and surge), high di/dt, crosstalk [5], etc. Moreover, the fast switching leads to transient overvoltage which increases the strain for the machine's insulation and accelerated aging [4,6]. This, together with the fact that often, the power cable between the inverter and the motor is long, leads to significant voltage overshoot due to reflected wave phenomenon. All of these problems resulting from the high dv/dt spikes could degrade the reliability and efficiency of motor drive systems [7][8][9]. In some cases, motor transient peak voltages can be even up to 3-4-times the DC bus voltage [10]. Moreover, besides the effects of the overvoltages on a whole winding, also the stress on the inter-turn insulation of the motor windings must be considered, especially as the rise time becomes shorter. This can result in relevant voltage drops across one turn, going beyond the inter-turn insulation design limit considered under sinusoidal operation [6,7].Currently, insulated-gate bipolar transistor (IGBT) based two level (2L) VSI, and three level (3L) neutral point clamped (NPC) and T-type neutral point clamped (T-NPC) inverters are commercially available and widely used by industry. However, in some cases, the application of 2L inverters is limited due to several drawbacks. They are usually connected with increased losses (in the case of higher fundamental freq...

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“…The losses are measured at V dc = 100 V and the inductive load of R = 10 Ω, L = 10 mH. (38) The heat sink volume can be determined by Equation (39). It was found from the quantity of separate extruded, naturally cooled heat sinks with the thermal resistance of heat sinks, obtained from the curve fitting [39].…”

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