Large-scale Power System Dynamic Equivalent In The Aggregate Problem

Power system dynamic equivalence consists of reducing the number of equations to be solved while retaining the most important characteristics of the full system. It is achieved by representing the study area of interest with its full model and the remainder of the system external to the study area with reduced-order equivalents. The large-scale power system dynamic equivalence is very important for physical simulation system and real-time online dynamic security analysis.Coherent equivalent algorithm is a classical method applied in the large-scale power system off-line transient stability analysis. It is typically done in three steps:1. Identification of the groups of coherent generators.2. Reduction of the generator buses and network.3. Aggregation of the generator models and their associated control devices.In the first step, coherent machines are identified using linear simulations or manual identifications; In the second step, the coherent generator buses are reduced by constant MVA technology, the reduction of the load is done by current sink reduction (CSR) algorithm. The above methods are classical and effective.The third step, namely, the aggregation of the generator models and their associated control devices, is one of the important procedures in dynamic equivalence. Electric Power Research Institute (EPRI, China) has developed the Power system dynamic equivalence Program (PSDEP) to systematically obtain dynamic equivalents. PSDEP contains weighted-average and Powell frequency-domain algorithms (Podmore method) to compute the parameters of the aggregate generator, exciter, governor, and PSS. However, the frequency-domain algorithm assumes that each part of the generator unit is independent and separate, so each part can be aggregated separately. But in practice, these parts of the unit are interactive and interdependent.In this paper the focus point is the procedure of the aggregation, and a time-domain algorithm is developed. Based on this method, each part of the unit such as the generator, exciter, governor and turbine has been related as the practice. BFGS nonlinear optimal algorithm is adopted to determine optimal parameters for the aggregated units. This method has been used in the equivalence of the Yangcheng Power Plant Transmission System and the results are of satisfying.