Figures-of-Merit and current metric for the comparison of IGCTs and IGBTs in Modular Multilevel Converters
IGCTs and IGBTs are compared in the case of a HVDC MMC. Because of the different principles IGBTs and IGCTs operate on, they cannot be compared directly from the figures quoted in their datasheets. Therefore, we introduce new figures of merit (FOMs) and a current metric, as a means for easy and accurate selection of semiconductor switches for an MMC application. Simulation is used to validate the FOMs. This analysis supports the growing interest in IGCTs for MMCs that can be seen in some papers.
Using an analytical model of the MMC, an equivalent current rating to the IGBT dc-current has been built for the IGCT based on the IGCT average current. The ratings of the different semiconductors are displayed Fig. 1.
Two figures of merit have been built to reflect the losses of the semiconductors: the conductive losses for one FOM, and the switching losses for the other one. The conductive losses FOM is the ratio between the average on-state voltage and the blocking voltage. The switching losses FOM is the ratio of the switching energy (written on the datasheet) over the product of the current and the voltage used for the measure of the datasheet switching energy.
To verify the relevance of those FOMs, the losses of the semiconductors have been calculated with an MMC dynamic model, imitating a real-time MMC operation, with a balancing control algorithm and a nearest level modulation MMC control. For the particular case of one MMC, Fig. 2. illustrates the coherence betweenthe FOMs (left) and the simulated losses (right).