Study Of Dynamic Equivalent Of Regional Power Grid And Control Measurements Of Low Frequency Oscillation

With the development of the interconnected power system, the scale and complexity ofthe system becomes more and more wider. It brings many advantages, meanwhile, whichtakes the simulation of the power system more complex and time-consuming, difficult torealization. For dispatch department of each net and province, the most important parts theycare are that closely connected to them, yet pay little attention to those not closelyconnected. With proper equivalents to those parts, not only will the network structure besimplified, the system order be lowered, but also the simulation time will be saved withoutcausing large influence to the problems of interest.On the other hand, under the trend of constructing nation-wide network, the problemof low frequency oscillation and ultra-low oscillation within regional interconnected powergrid become more serious, which decreases dynamic stability of the system and makes itunable to achieve the largest utilization factor of the system transmission capacity. Theapplication of wide-area measuring system now provides power system with condition ofusing global information. The supplementary damping controller (SDC) designed on thisbasis will play a huge role.Therefore, this paper analyses and investigates the problems stated above. It beginswith the current research and application situation of dynamic equivalence and themeasures to control low frequency oscillation. It introduces the research method and theapplication situation of this problem in detail, and points out relevant difficulties and keytechnologies. The whole paper is consisted of the following three parts:Firstly, this paper summarizes the basic theory, algorithms and specific steps ofrealization of coherency equivalence, in which gives three key steps of this method, namely,coherency division, parameter aggregation and network reduction, the features ofcommonly used methods and applicability conditions of the steps are discussed, meanwhile,the equations in each algorithm are deduced.Secondly, simulations are conducted on two standard cases applying the above methodin order to analyze and compare the system before and after equivalence on three main aspects: steady state, small signal disturbance and large disturbance. Results have shownthat with the above method, the dynamic response of the system in interest can be wellreserved. Meanwhile, the parameters are well adaptive to different types of faults. Then thismethod is applied to an actual power grid. Simulation results have shown again the validityand applicability of the method.Finally, on a typical four-machine-two-area example, with residue method, theparameters of conventional PSS and the SDC of FPC (Flexible Power Conditioner) aredesigned respectively using wide-area feedback signals. Simulation results have shown thatthe inter-area oscillation mode which is weakly damped can be effectively controlled withthe adoption of SDC based on wide-area signals, thus improving the damping and dynamicstability level of the whole system.