Seamless transfer strategy considering power balance in parallel operation

This paper presents a power reference modifier for seamless transfer considering the power balance in the parallel operation. When the PCS (Power Conditioning System) is disconnected from the grid, the PCS should operate in the stand-alone mode to supply energy to the critical loads. If multiple PCSs work as one PCS, slave PCSs maintain the current control mode even if master PCS change its control mode from the current control to the voltage control. If the power references of slave PCSs are not changed properly based on the load condition, the power balance condition may not be met. In that case, the voltage applied to the critical loads might be beyond or below the rated voltage. To avoid this phenomenon, the power values absorbed into the loads should be monitored consistently and the power references of the slave PCSs should be modified properly based on them. But the master PCS which transfers the power references to the slave PCSs and works as voltage source in stand-alone mode can obtain it indirectly by calculating its own power values without monitoring the power consumed by critical loads. Using those, simple controller for modifying the power references of the slave PCSs can be configured. The effectiveness of the proposed power reference modifier has been verified through the experimental results. By applying the proposed method, the transition from the grid-connected mode to the stand-alone mode works satisfying Computer Business Equipment Manufacturers Association (CBEMA) curve.