Device Protection And Adverse Effects Countermeasures Of Medium Voltage High Power NPC Three Level Converters Based On IGCTs

With technology advancements in semiconductor devices such as gate commutated thyristors (GCTs) and insulated gate bipolar transistors (IGBTs), modern high power medium voltage (MV) drives are increasingly used in petrochemical, mining, steel and metals, transportation and other industries to conserve electric energy, increase productivity and improve product quality. This thesis focused research on the three-level Neutral Point Clamped (NPC) inverter with IGCT devices, and investigated its absorption circuits design, the short circuit breakdown protection and inhibition of converter adverse effects.Firstly, a novel design method of the absorbing circuit in IGCT converters is proposed. After a detailed analysis of the absorbing circuit transient states, an equivalent circuit is presented and its time-domain analysis is made. The optimization of the absorbing circuit parameter matching, with the constraints of device safe operation area, circuit dynamic time and device heat consumption, is a nonlinear optimization problem, and can be solved with the MATLAB optimization tools. Based on the simulation and experiment contract, the stray inductance, which has a great influence to the shut off characteristics of the device, can be extracted to make the absorbing circuit parameter more optimization. Simulation and experiments are made to verify the goodness of the result.Secondly, this thesis presents the fault detection and protection methods associated with IGCT three-level neutral point clamped voltage source converter. The inner device short circuit in the three-level phase leg can lead to DC-Link voltage unbalance and doubling the stress on capacitors and devices, which must be suppressed for the safe operation of the converter. The reason of device malfunction and short circuit protection strategy are analyzed, based on which a novel method is proposed, and it can guarantee the short circuit current exhaustive releasing as well as realize thermal stress equilibrium in the bridge arm. Compared with the traditional all-off protection strategy, this method can better protect the security of the device and saves the cost, the scheme is verified with simulation.Thirdly, Medium voltage converters use long cable to connect the inverter to the motor side in long distance pulse width modulation driving system, which brings some notable adverse effects simultaneously such as PWM pulse over-voltage and common-mode voltage oscillation. In this thesis, based on transmission theory and common-mode voltage analysis, the model of the long distance PWM driving system was founded and a filter design method was proposed which the parameter such as cable length, cable inductance, cable capacitance, reflection coefficient were considered synthetically. The validity of the proposed design method was supported by simulation and experiment results.