| Large scale interconnected network is a basic situation currently and tendency in thefuture. It is also an essential part of China’s smart grid plan. With the features of high order,nonlinearity, and difficulty to control, large scale power system is the most complicateddynamic system in the world. Elements which compose this grand system are stillincreasing and make it more and more complicated. It is well known that fast and accuratecalculation of large scale power systems is a basic requirement for on-line dynamic securityanalysis now and in the future. Therefore, it is greatly useful and necessary, both on theoryand on engineering, to study the dynamic equivalent method with good accuracy, generality,and to some extent robust, which should be convenient for practical use and be adaptive todifferent stability situations in power systems.This paper firstly summerizes the complexity and diversity of the dynamiccharacterisitcs of regional power systems and the main influence factors and elements,combining the physical reality of the development of regional power systems in China. Andthen thoroughly discusses coherency based dynamic equivalent and the real-time dynamicequivalent method based on real-time measurement data from Wide Area MeasurementSystem and parameter identification. Theories, equivalent models, and approach to find outthe equivalent parrameters are specifically stated out. Using assessment indexes such asaccuracy, speed, generality, scope of application, and instantaneity, performances of thistwo dynamic equivaent methods are compared and evaluated comprehensively.Rigorous in theory and maturely developed, coherency-based equivalence is widelyused in engineering. The equivalent models of this method are consisted of physicalelements of power system, which can be used directly in transient stability analysisprograms. The weighting aggregation method based on coherent generators’ rated capacityhas marked advantage on speed, and the accuracy of it can satisfy engineering requirements,which is simulated with CEPRI8-machine36-bus ac/dc hybrid power system. And it issuggested to apply to aggregation of inducted motors of loads, which can imrove thedisadvantage of coherent dynamic equivalent on load modeling. For the weakness of largequantities of off-line computing for differentiating coherent generators, an improvementbased on history WAMS information and online matching is put forward in this paper.In the real-time dynamic equivalent method based on parameter identification, a parallel dynamic equivalent model composed of an equivalent generator and an equivalentsynthesis load is firstly put forward. Identifiability analysis is carried out for this nonlineardynamic equivalent model. Particle Swarm Optimization algorithm is utilized for parameteridentification of the dynamic equivalent model, where the variance of output errors of theequivalent boundaries is the optimization objective.This dynamic equivalent method is verified on both CEPRI8-machine36-busexample system and central China grid. Load model is a remarkable factor that relate to thestability of the whole power system, which is fully considered in this paper. Dynamicequivalent of neteworks with HVDC system is processed. The simulation results show thatthe dynamic equivalent model can reflect the dynamic responses and the transient stabilityof the original system, no matter which is transient stable or unstable, angle unstable orvoltage stable. When we change the load model in the original system, the staticcharacteristic model with both voltage and frequency factors can throughout represents thedynamic behavior of the original system, which confirms its robustness of this dynamicequivalent model. All the simulation work was performed on PSASP. |
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