Strategy Design and Transient Analysis of ITER PPEN Transformer On-Load Tap Changer During Converter Operation

Three identical three-winding main step down transformers rated at 300/250/150 MVA are installed in ITER Pulsed Power Electrical Network (PPEN) to distribute power to three independent 66 kV and 22 kV busbars for feeding Coil Power Supply System (CPSS). The On-load Tap Changer (OLTC) is installed in primary side of PPEN main step down transformer to limit the voltage in required range under the control of Reactive Power Compensation and Harmonic Filter (RPC&HF) system. A comprehensive OLTC tap changer strategy has been proposed to maintain the 66 kV busbar voltage with consideration of hysteresis band to avoid strategy instability. The circuit models of OLTC tap changer transient processes have been built to analyze the terminal voltage transients quantitatively in both amplitude and phase. And the dynamic performance during OLTC tap transition has been verified to be in accordance with the analysis by simulation. And during future campaign, the most critical situation for CPSS is that the plasma operation system would give step voltage reference from minimum to maximum or vice versa to CPSS according to the operation scenarios. Hence, the converter system operation robustness has been verified by simulation on MATLAB/SIMULINK during PPEN OLTC tap changer transition in the case of this most critical situation during plasma operation.