Testing of active current balance in parallel thyristor bridges

Test results of active current balance (ACB) at Georgia Power´s McDonough Plant show the effect of ACB applied to parallel thyristor bridges in generator excitation applications. This paper presents test results of ACB, a new technique that adjusts the average current in parallel leg thyristors to facilitate current balance between the parallel operating bridges. The unbalance between parallel thyristors is typically caused by differences in inductance feeding the thyristors and by the conducting characteristics of the thyristors. Balance is accomplished by periodically inhibiting the firing of thyristors that carry more than the average current. The thyristors that are conducting carry more current during this interval and their average current is increased. Test results show that the current balance between the parallel operating thyristors in the bridges is significantly improved when ACB is enabled. The results of rapid changes in load while ACB is enabled are also shown.