Analysis And Suppression For Tie-line Power Fluctuation Of Large-scale Interconnected Power Systems

Since the ultra-high voltage (UHV)transmission line was put into operation, the two large-scale regional power systems in Central and North China were interconnected and constituted a typically long-chain weak interconnected systems. There are irregular power fluctuations on the UHV tie-line. It has become one of the important challenges that limit the capability of power transmission on the UHV tie line. Further the irregular power fluctuations increase the risk of operation security to the large-scale interconnected power system.In the literature, the previous studies for the power fluctuation on the tie-line have been concentrated on the mechanism and mitigation approaches of low-frequency oscillations, which have achieved remarkable conclusions. However, these studies ignored the characteristics of the random power fluctuations. Currently, there is not a comprehensive and systematic theory to explain the power fluctuations on the UHV tie line both at home and abroad. The mechanism and influence factors of the power fluctuations on tie-line cannot be effectively modeled and quantitative calculated. Moreover, the mitigation approach of the random power fluctuations on the tie-line is not mature. With the foundation of the large-scale synchronous interconnected power systems, the mechanism and mitigation technology of the power fluctuation issue on UHV tie-line has becomes a key issue.Based on the above factors, this paper takes fluctuation power on the UHV tie-line as the object of study. The mechanism of power fluctuations is analyzed. This paper proposes an amplitude estimation method for the power fluctuation on the tie lines based on the load fluctuation. The methods of risk assessment for the power transmission on the tie-line are presented based on the characteristics of the wind power. Then the method to inhibit the stochastic power fluctuations on the tie-line is designed based on multi-regional coordinated generation control.Through the study of full differential formulation for the transmitted power on the tie-lines, the paper discusses the relationship between the change of two regional power grids' frequency deviation and the stochastic power fluctuations on the tie-lines. The results reveal that the power fluctuations on the tie-lines are mainly composed of the power fluctuation caused by voltage deviation and frequency deviation. Hence, the power fluctuations can be approximately described by the frequency deviation throughout the tie-line. Then according to the characteristic of system frequency response, the dynamic changing processes from "disturbance response phase" to "new steady state phase" of power fluctuations have been analyzed based on power perturbation.Due to the stochastic of tie-lines power fluctuations, it is difficult to accurately obtain the magnitude's fluctuations. However, the distribution, mean and standard deviation of power fluctuations'amplitude can be analyzed by probabilistic methods, which can provid the decision basis for the control and operation of the transferred power on the tie-line. Based on the load distribution characteristics and stochastic volatility model, the estimation methods of tie-lines power fluctuations'amplitude have been proposed by introducing the Monte Carlo method. As the latter takes into account the time schedule of load sample during sampling process, it fits the operating characteristics more accurately.The over load problem caused by tie-line power fluctuation is researched based on risk theory. An appropriate severity function is adopted. The traditional calculation method of risk cannot distinguish high risk-low probability and low risk-high probability event. Based on this, an evaluation risk framework of transmission capacity is established based on the Partitioned Multi-objective Risk Method.This paper studies and proposes a method to suppress the tie-line power fluctuations by coordinating the regional automatic generation control based on the distributed model predictive control theory. Based on the distributed model predictive control theory of regional Interconnection system' active power, this method achieves the online optimization control of regional automatic generation control, by solving the Nash equilibrium control solution which suppress the tie line power fluctuations'during predictive control cycle. Finally, the rapid mitigation of the tie-line power fluctuations is achieved by using rolling optimization of model predictive control.The power fluctuation on the UHV tie-line of interconnected power systems is a slow dynamic process. It is also the power fluctuation's reflection on tie-line after redistributing in the whole interconnected system. For a weak-linked interconnected system, the large tie-line stochastic power fluctuation is inevitable. To ensure the power exchange capacity of the interconnected power systems and the safe and stable operation of the systems, the most fundamental way is to build a strong structure of power systems.