Parallel Operation of Electronic Power Transformers Without Control Interconnections

A novel parallel control strategy of electronic power transformers (EPT) without control interconnections is proposed. The proposed control strategy is based on the well-known power droop control method, which consists of an active power sharing control loop and a reactive power sharing control loop. The phase synchronization control loop is added to the corresponding control loop to reduce the phase differences of output voltages among paralleled EPTs. Linear and nonlinear loads can be properly shared due to the addition of a current harmonic control loop in the control loop. The derivative control link is also included in order to improve the dynamic response of the system. A small signal model is developed to analyze the stability and the transient response of the system and to facilitate the design of control parameters. Finally, on the basis of the parallel system with two EPTs, the solution of the average-power-sharing problem of AC side is discussed in detail using the proposed control strategy and correlative simulations are carried out. The simulation results show that the excellent power sharing and voltage regulation characteristic, a proper dynamic response and good current-sharing performance under the nonlinear load condition can be achieved.