| Abstract |
This work presents three methods of resistive balancing for a high voltage switch, that is formed by the series connection of press-pack IGBTs, namely the Director Switch (DS). The switch is a key component that enables two novel HVDC Voltage Sourced Converter (VSC) topologies: the Alternate-Arm Converter (AAC) and the Series Bridge Converter (SBC), which offer fault-blocking capability, a better efficiency, cost and size than the established Modular Multilevel Converter (MMC) solution. An important DS requirement is that the gate electronics for each IGBT shall be self-powered, as breaching the high voltage insulation barrier to ground to provide auxiliary power is unfeasible. This is achieved by storing energy in a capacitive clamp snubber on each IGBT level, which then powers a DC-DC converter. However, due to the constant power load, or negative impedance of the electronics, the capacitor and collector-to-emitter voltage of the IGBTs soon becomes highly imbalanced during operation, causing some of the levels to turn-off, others to take a very high voltage share. A way of maintaining voltage balance is to introduce or emulate a resistive load that can overcome the negative impedance of the electronic load. This is achieved in three different ways. At start up, when a higher current than normal is required, a resistive current is emulated by enabling that higher demand only when the switch voltage is above a given threshold. During operation, a protective crow-bar resistor of a low-ohmic value is modulated with a fixed duty ratio, to emulate a higher resistive value than that present. Finally, a resistive load can be directly connected across the clamp snubber capacitor. All these techniques are complementary and can be used simultaneously. The successful operation of these three techniques is demonstrated by showing experimental results on a 7-level switch, using 2.5 kV press-pack IGBTs. |
