Calculation Method Of Probabilistic Load Flow For Wind Power Integrated Power Systems And Its Application In Safety Assessment

With the change of operating conditions especially integration of the large-scale wind power, power system will be more random and the state will be even more uncertain. Also, higher requirements will be put forward to the grid analysis. Probabilistic Load Flow (PLF) is a powerful to solve the problem because it can give probability distributions of state variables considering various disturbances. Therefore, calculation method of PLF and its applications in safety evaluation against a background of wind power integration is analyzed.PLF calculation for wind power integrated power system is divided into two categories:PLF for Long and Medium Term (LMT-PLF) and Short Term (ST-PLF). The randomness of wind power is treated as random variable between zeros and full output for LMT-PLF while prediction errors of wind power output for ST-PLF.With regard to greater fluctuation of wind power output in long and medium term, a multi-linearization LMT-PLF model for wind power integrated system is formulated to solving the accuracy problem of the current analytic method for LMT-PLF due to linearization power flow equation. The model was solved by semi-invariant method and the semi-invariant calculation method for multi-linearization model is deduced. C-type Gram-Charlier series is introduced to avoid getting the negative value of probability density function of node voltage or branch power when the A-type Gram-Charlier series expansion is used to estimate distribution of random variables for wind power integrated system. Case study shows that the proposed method has much higher accuracy than the current analytic method, and is more adaptive to scale of wind power.In view of the fact that the current method is difficult to cope with greater fluctuation of wind power output in long and medium term due to sole slack bus, idea of dynamic power flow is introduced into PLF to research the model and algorithm of LMT-PLF considering frequency modulation. Based on processes of frequency response in real system, the analytical model of dynamic unbalance power distribution was established. Then, the model was combined with traditional PLF model to establish the model of LMT-PLF considering frequency modulation. The impact of insufficient frequency modulation is considered in proposed model. A hybrid method is proposed to solving the proposed model. Test results demonstrate that the proposed method can ensure reasonable fluctuations for all generators and results of LMT-PLF according with real system.To solve the practicality of the current method, a practical ST-PLF algorithm for wind power integrated system was proposed based on sequence operation theory. Because it is assumed that the distribution models of system random variables are known in all the current PLF methods while the distribution models are difficult to obtain in real system. Considering the characteristics of PLF calculation, feasibility of applying the current sequence operation theory to PLF was discussed firstly and extended sequence operation was proposed. Extended probabilistic sequence was used to express the stochastic behavior of system and sequence transformation of random variables was realized based on samples of operating data. Probabilistic distribution of power flow was obtained quickly by extended sequence operation between system random variables. Example analysis indicates that the proposed method can achieve ST-PLF calculation accurately without probability distributions of system random variable and improve practicality of the current method.In order to evaluate voltage random fluctuation for wind power integrated system, three evaluation indexes including voltage distribution index, skewness index and retention index is defined based on PLF results to describe characteristics of voltage random distribution. The LMT-PLF is taken as example to verify the effectiveness of these proposed indexes and results illustrate that the proposed indexes can evaluate voltage fluctuation effectively in overall and the partial two aspects. At last, the impacts of some factors on voltage fluctuation are analyzed.For the purpose of evaluating static safety effectively and efficiently for power system with large-scale wind power integration, PLF is introduced into evaluation. PLF was solved by analytic method to improve method efficiency. Safety indexes were defined based on PLF to obtain quantitative assessment of system safety rapidly. Safety indexes involve two aspects:safety probability and safety risk. Vulnerability analysis was achieved based on safety indexes of components. Test results of ST-PLF demonstrate that the proposed method has higher efficiency than the traditional method for uncertain security assessment, and provide beneficial references for safety assessment of power system with intermittent energy integration.