Active Current Balance between Parallel Thyristors in Multibridge AC-DC Rectifiers

New technology in digital excitation control provides a method to balance thyristor currents in parallel power rectifier bridges that provide current to a synchronous generator field. Balancing current between parallel bridges feeding a common load has traditionally been accomplished by using current balancing transformers in each bridge or adding inductance in series with the thyristors or silicon controlled rectifiers (SCRs). This paper introduces active current balance, a new technique that modifies the average current in parallel leg thyristors to facilitate current balance between the bridges. This is accomplished by periodically inhibiting the firing of thyristors that carry more than the average current. The thyristors that are fired carry more current during this interval, increasing their average current. The thyristors that are not fired do not carry current during this interval and thus their average current is reduced. Using this technique (referred to in this article as "skip firing"), the average current in parallel legs can be adjusted through feedback and control to equalize the average thyristor current loading. Additional feedback and control parameters are proposed, such as heatsink temperatures, which can be measured and utilized to program the "skip firing," in order to balance thyristor heat-sink temperatures.