Computation and analysis of dielectric losses in MV power electronic converter insulation

Abstract

The newly available Medium Voltage (MV) Silicon-Carbide (SiC) devices enable a great extension of the design space of MV inverters. This includes the utilization of unprecedented blocking voltages, higher switching frequencies, higher commutation speeds, and high temperature operation. However, all these factors considerably increase the insulation stress. This paper details the computation of dielectric losses, which are directly related to the insulation stress and can be used for the insulation design and diagnostic. After a review of the method used to compute dielectric losses, scalable analytical expressions are derived for the losses produced by PWM waveforms of DC-DC, DC-AC, and multilevel DC-AC inverters. Finally, a Medium-Frequency (MF) transformer is analyzed and the impacts of the insulation material and the operating temperature on the dielectric losses are discussed. It is found that the insulation losses can represent a significant share (17 %) of the total transformer losses.

Electrical shielding of MV/MF transformers subjected to high dv/dt PWM voltages

“…single-cell or multi-cell structures. In [11], [12], the transformer of the single-cell structure is identified as particularly critical with respect to the electric fields since one transformer has to deal with the complete CM and Differential-Mode (DM) voltages. Therefore, a single-cell series-resonant DC-DC converter, depicted in Figs.…”

“…A shell-type winding arrangement reduces the stray field in the winding window and, therefore, the high-frequency winding losses. The LV winding is placed near the wound limb for several reasons: this winding arrangement improves the cooling of the MV winding, which is critical [11], the capacitance between the MV winding and the core is reduced, and the realization of the MV cable terminations is facilitated. The MV winding is divided in two chambers and three layers which represents a good trade-off between the insulation stresses, the winding capacitance, and the complexity of the winding scheme [22].…”

“…Additionally, the increased voltages and switching speeds lead to large CM currents [8], more intense excitations of parasitic resonances of the transformer [4], [9], and issues related to insulation coordination, such as dielectric losses or partial discharges [10], [11]. For all these effects, it appears that the magnetic and electric fields are of key importance for MV/MF transformers [12], [13].…”

“…To the knowledge of the authors, the electric field of MV/MF transformers has been studied inside the insulation [11], [12] but not at the surface of the insulation and in the surrounding air. Therefore, this paper analyzes the electric near-field and methods to mitigate excessive field strengths.…”

Dielectric Losses in Dry-Type Insulation of Medium-Voltage Power Electronic Converters

“…Medium-Frequency (MF) energy conversion [2], [5], [9]. The design of such MV/MF converters is typically examined at system level [3], [4], [6], [7], [10] and component level [9], [11]- [13] while the electrical insulation is often neglected [14]- [17].…”