| This thesis was supported by Natural Science Foundation of Chongqing(CSTC 2009BB6190) and independent research project of State Key Laboratory of Power Transmission Equipment & System Security and New Technology(2007DA1051270 9208).Distributed generation (DG) develops rapidly due to many advantages such as less pollution, high energy efficiency and flexible installation locations. As distributed generation has many advantages,the distributed generators start to be embed in distributed network, these small or micro DGs include synchronous generator (SG), induction generator (IG) and the power source through the power electronic interface. After embing DG, the way of distribution network receives and consumes energy from the main network only has been changed, but it may affect the transient stability of the distribution network. Local dynamic compensation or the use of energy storage devices can improve the transient stability effectively of distribution network with DG. Flywheel energy storage system (FESS) has become an energy storage technology with practical promise at present due to some advantages, such as longer life, higher environmental friendliness, lower unit energy storage cost, etc. Based on the study of the FESS, converter and control model, the function of FESS for enhancing transient stability of distribution networks containing IG and SG is studied, control strategy of FESS is proposed. The main contents are as follows:â‘ The status and advantages of FESS are discussed,the influence of DG after connected to distribution networks and the existing transient stability problem of the distribution network are analyzed, the measures to improve transient stability of distribution network and its shortage at present are researched, the feasibility of improving transient stability of distribution network by FESS is demonstrated.â‘¡The characteristics of FESS are researched, the flywheel model and the fifth-order mathematical models of energy storage motor are established. Induction motor is selected as the energy storage motor of flywheel, the flywheel is equivalent to an additional moment of inertia of induction motor. By coordinate transformation, the model of energy storage motor of multivariate, high order, nonlinear and strong coupling in three-phase stationary coordinate is simplified in dq synchronous rotating coordinate, and the fifth-order mathematical model is obtained. â‘¢The transient stability characteristics of IG and SG are analyzed, from point of enhancing transient stability of distribution networks, control strategy of bidirectional PWM converter of FESS is proposed. For the motor-side converter, field oriented control strategy is proposed, which has better robustness for voltage fluctuation of the power grid and easy to implement. For the grid-side converter, the cascade control scheme is used, which can achieve independent control of the active and reactive.â‘£FESS model and distribution system model, which includes a synchronous generator and a induction generator are established in PSCAD/EMTDC simulation platform, showing that the proposed theoretical analysis and realization method of improving the transient stability of distribution networks using FESS in the paper are right. |
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