Comprehensive Analysis and Experiment of Extreme Faults in MMC Based on IGCT

High-voltage direct current voltage source converter (HVdc-VSC) technology is an important solutions for large-scale renewable sources. The MMC is one of the most popular topologies for the HVdc-VSC system. The extreme faults affect the safety and reliability of the MMC greatly. This article gives comprehensive analysis and experiment of extreme faults in the MMC based on an IGCT. Causes and characterization of the extreme faults are analyzed in detail. A new protection solution based on the CP-IGCT and fast recovery diode with HS-FRD is proposed. The surge current capability of HS-FRD is validated via experiment successfully. Theoretical calculation shows that the protection solution based on CP-IGCT, and HS-FRD can help reduce the maximum fault current below 50% when the anode inductor is 0.8 μH in the extreme shoot-through fault. Test results show that the protection solution can realize the discharge of the dc capacitor below 4.5 kV in the extreme shoot-through fault. The maximum fault current in the MMC based on the IGCT is only about 600 kA, while the maximum fault current in the MMC based on the IGBT is over 1100 kA. Explosion proof of CP-IGCT is validated during the tests, which is similar to that of the protection thyristor. However, the IGBT cannot guarantee the stability of the package and may cause severe safety problem. The failed CP-IGCT has the short-circuit failure mode and can be used for bypass of the faulty MMC submodule. Via comparison, the proposed protection solution has lowest cost and complexity, as well as the highest safety.