Current sharing design of ITER poloidal field converter bridges

Due to high rated current and special Fault Suppression Capacity (FSC) criterion, 12 thyristors connected in parallel are applied in one bridge arm of ITER Poloidal Field (PF) converter. A large amount of parallel thyristors will give rise to significant difficulties in the current sharing design due to the electromagnetic coupling between different parts of the bridge arm. This paper presents a new method to analyze the influence of the electromagnetic coupling on current sharing performance and guide the bridge arm structure design. At first, the bridge arm is decomposed to many segments and the stray inductances are extracted. Then, an equivalent circuit of the decomposed model is built and a mathematical model is established to analyze the circuit. It will result in the current waveforms of parallel thyristors and thus provide the current sharing information. Based on the mathematical model, the optimization design of PF converter is conducted along with conventional methods. Experimental tests are also performed, and the results fit well with the analyzed ones. The current sharing coefficient of the 12-parallel thyristors is over 0.8, which is far beyond the value required by ITER Organization. In a word, the method presented in this paper provides an effective tool for the current sharing structure design for high-current fusion power supplies.