Open Switch Fault Tolerant Control Strategy for Paralleled Three-Level NPC Back-to-Back Power Converters Fed DFIM Unit

In a large-rated adjustable speed hydro generating unit, the design of fault tolerant control operation is a crucial process due to dynamic slip frequency variation and the existence of overvoltage across rotor windings during the isolation process of the faulty converter. To overcome these practical challenges, a fault tolerant control strategy is designed in this article for a parallel-connected 3L-NPC-fed 250-MW doubly fed induction machine (DFIM) unit. The basic idea is to limit reactive power support under fault tolerant operation to enhance rotor active current amplitude
$({i}_{m}^{d})$
. Meanwhile, the pulsewidth modulation (PWM) signal to the faulty converter is disabled, and healthy converters are overloaded to 115% for 10–20 s. The overloaded operation in machine side converter shall increase the thermal stress on power devices. To handle this issue, the average switching cycle of power devices is reduced with the help of a state machine controller. Once the system is reconfigured, reactive power support to the grid is provided with the help of a line side converter by adjusting the reactive control component
$({i}_{g}^{qref})$
. The effectiveness of the designed coordinated fault tolerant control strategy is investigated in MATLAB/Simulink2014A environment for the 250-MW DFIM unit under different case scenarios. Experimental validation is performed with the three-channel 3L-NPC power converter-fed 2.2 kW DFIM unit.