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As a new control strategy, Nonlinear PID(NLPID) controller has been introduced in the power system successfully. The controller is free of planting model foundation in the design procedure and realized simply. In this paper, a nonlinear PID controller used for superconducting magnetic energy storage (SMES) unit connected to a power system is proposed. Purpose of designing such controller is to improve the stability of the power system in a relatively wide operation range. The design procedure takes into account the active and reactive power cooperative control scheme and the simple structure so as to be apt to the practical use. Circuit design and implementation of the proposed controller is presented subsequently. At the same time, simulation is carried out to investigate the performance of the proposed controller in a high order nonlinear power system model under the MATLAB environment. The results show satisfactory performance and good robustness of the controller. The feasibility of the controller is testified as well.