Protection Circuitry Design to Mitigate Failure Propagation for 10 kV SiC MOSFETs in a 22 kV DC 13.8 kV AC Flying Capacitor Multilevel Converter

The operation and failure modes of the flying capacitor multilevel converter topology are well-known and the protections for Si IGBT-based converters are well established. However, when it comes to high voltage SiC MOSFET-based converters there are some particularities that must be evaluated: shorter energy withstanding time, higher dv/dt and higher insulation stress. This paper provides an assessment of the failure modes for a 7-level 13.8 kV AC 22 kV DC 1.1 MVA three-phase flying capacitor converter using 10 kVSiC MOSFETs, an analysis of the fault propagation mechanism between cells and the design of a transient voltage suppressor (TVS) diode-based protection module to prevent it.

I. Introduction

Wide bandgap devices such as 10 kVSiC MOSFET modules enable the design of medium voltage converters with the benefit of elevated density and efficiency [1]–[6]. This device is the candidate to replace the 6.5 kV Si IGBT in medium-voltage applications. Hence, to further demonstrate the density and efficiency benefits brought by the 10 kVSiC MOSFETs, a phase leg of a 1.1 MVA 7-level l3.8 kV ac 22 kV dc flying capacitor multilevel using 10 kVSiC MOSFET half-bridge modules was designed and constructed as shown in Fig. 1(a) and detailed on [7]. Although the failure modes of this topology were comprehensively explored in [8] and [9], an re-assessment of the fault modes and protection schemes is necessary considering the particularities of the 10 kVSiC MOSFET. In general, the SiC devices have lower short-circuit withstand time due to the smaller chip area and high current density compared to their Si counterpart [10]. Si IGBT must withstand a short-circuit for 10 s by IEC60747-9 standard, while SiC devices is around 1.5 s[11–12]. This requires not only a faster short-circuit protection in comparison to the IGBT-based converter, but a different protection scheme especially in case of a severe failure where both devices failed in one cell. Moreover, SiC devices are subjected to higher dv/dt due to its high blocking voltage and fast switching speed. The abrupt change in voltage imposes higher stress on the insulation system of the 10 kVSiC modules and increases the possibility of an insulation breakdown [13]. In this work, the flying capacitor converter fault modes are assessed in Section II and Section III. Section IV proposes a protection scheme for the defined Cell Short-Circuit Fault (CSCF) and a selection of the clamping device is showed. Section V presents the protection module performance, the tests were conducted with rated voltage. Moreover, the impact of the paralleled protection module on the switching transient performance was validated through Double Pulse Tests (DPT) performed at 4 kV, 40 A peak.