Development Of An IGBT-Based 750kVA Three-phase NPC Three-level Universal Converter Module

The IGBT-based Neutral-Point-Clamped (NPC) three-level converter is widely used in medium voltage Variable Frequency Driver nowadays. This paper covers the system design, 3-D configuration, cooling system and drive, control and protection design of an IGBT-based 750kVA NPC three-level inverter module, by considering the conceivable problems caused by heat, EMI and the stray inductance during commutationsThe voltage overshoots on IGBTs induced by stray inductance is one of the crucial problem when devices are turned off. This paper presents the bus bar design of the inverter module to minimize the stray inductance. ANSOFT Q3D is used in comparative evaluation for 3 types of multi-layer bus bar structures and a novel low-inductance laminated configuration with divided connections is proposed for the inverter modules. Combined with specially arranged snubber capacitors distribution, reduced voltage overshoots across IGBTs at turn-off and lower loss within the electrolytic capacitors are achieved without any RC snubber, along with optimized high frequency current sharing. The stray inductance of the bus bar is calculated theoretically and in simulation as a contrast to the test results. Feasibility and improved performance are confirmed by experiments.A water-cooling heat sink is designed for the placement of 15 power devices, 4 voltage sensors, 3 balance resistors and several thermal switches. It is supposed to take away the heat produced by power loss and make contribution to a compact package. The inside ducts are designed to be very slim to avoid more than 100 screws in the heat sink.Fundamentals of 2 types of device-level short circuit protection by SCALE chip are analyzed and threshold current are tested in a series of short circuit experiments to guarantee that all the devices work in the SOA. Control units and 3 types of system-level protection are also shown in this paper. Due to the well-arranged component placement and PCB layout, drive, control and protect circuit are not interfered in the bad electromagnetic environment in high power experiment. The test results under various load conditions are shown at the end of paper.