Studies On Out-of-Step Splitting Criterion And Control Scheme Based On Wide Area Measurements

The advancement of interconnected power grids provides strong supports for improvement of socioeconomic situation. Meanwhile, it also brings new challenges to security and stability of power systems. Huge efforts have been made to avoid blackouts worldwide but cascading outages which evolved into catastrophic blackouts still happened. As a result, more and more research on how to deal with unstable conditions of interconnected power system is necessary and resonable. Out-of-Step splitting control is one of important means to defend power systems blackouts, ensure stability and reliability of power systems. When oscillation occurs in interconnected power systems, if proper automatic control actions or operator intervention are not taken decisively, the system may be susceptible to further failures and subsequent cascading. It is imperative to develop new strategies of splitting control schemes to detect instability and take remedial actions to prevent power system from worldwide blackout emergency.The long time experiences for power system operation have shown that no matter how strict the requirements for power system stability, and how perfect the measures are, there would be some unpredictable casual faults working together to cause instability. However, the expense for too strict security requirements is very high. When the stability of power system is destroyed, the key question is how to detect and prevent oscillations rationally and quickly, and how to restoration rapidly. In China, when an interconnected grid is subjected to large disturbances and becomes out-of-step, out-of-step splitting control has been used widely as the last resort for preventing widespread blackout. But there are some problems of out-of-step splitting controls: 1) most of the criterions are designed based on equivalent two-machine system model, splitting controls will not be suitable to the interconnected power grids; 2) a lot of conventional out-of-step criterions are based on local signals, which can not follow drift of the dynamics oscillation center and can not be adaptive to the change of system network topology; 3) out-of-step splitting schemes are not able to adjust to complex oscillation conditions. In this paper, though analysising the charateristics of electrical parameters of out-of-step interface and current splitting schemes in the actual field, new out-of-step splitting criterions and splitting control schemes based on WAMS information are proposed. The splitting criterion and splitting control scheme of power system have correct functions of initiating the separation equipment, capturing the oscillation center and out-of-step interface, coordinating splitting controls and splitting the out-of-step area from the main system timely and accurately when the out-of-step happens in the interconnected system. The main research work and innovative fruits of the dissertation are as follwing.1. Based on the Phasor Measurements Units (PMUs), an adaptive ucosφimproving criterion for out-of-step splitting to obtain quick and selectable disconnecting actions is proposed in the paper. The ucosφout-of-step splitting criterion can identify out-of-step oscillation, short-circuit fault and synchronous oscillation which based on the changing track of the voltage of oscillation center. However, in practical system ucosφmay not able to represent voltage at any location of the out-of-step center and has no specific physical meaning. On the other hand, the ucosφcriterion based on local information may be misjudged because amplitudes of equivalent electric potential of both sides are different in practical system, so the power system might be in a dangerous condition. The change of angle difference of voltage phasor based on PMUs can reflect the out-of-step condition directly. It can help to confirm the location of the oscillation center. The reliability of ucosφsplitting criterion and the accuracy of splitting action are ensured with the assistant of angle difference of voltage phasor.In the simulations of the 10 machines 39 buses New England system and a practical Shandong power system, the splitting spots are set by the identification and aggregation of coherent generators and according to the active power balance. The improving out-of-step splitting criterion identifies out-of-step condition of system. Simulation results demonstrate the effectiveness of the proposed approach.2 . An adaptive out-of-step detecting composite criterion for out-of-step splitting interface is proposed. When the synchronization loss takes place, disconnecting the interface of out-of-step is considered the main solution to prevent power system from collapse. Through the analysis of the process of asynchronous oscillation and systematic researches on existing out-of-step criterions of power system, making several criterions work together and reusable and taking fully advantage of every criterion are possible. An adaptive out-of-step composite criterion for splitting interface based on WAMS includes the main criterion and the auxiliary criterion. The main ucosφcriterion which reflects the changing track of oscillation center voltage detects the synchronism loss. The auxiliary criterion reflecting changes of three-phase currents ensures the reliability of splitting action which is independent of the oscillation center. These two criterions are used together for determining out-of-step accurately and duly.In the simulations of Shandong power system in China, the islanding interface is determined based on power system dispatching area and generator coherency. And the composite criterion can be operated based on the confirmed interface. Simulation results have demonstrated that the proposed criterion can adapt to the changes of operating conditions, capture the position of out-of-step center dynamically and cope with the shortcoming of a single criterion.3. An out-of-step splitting control system scheme based on wide area measurement information for power system is proposed in this paper. This system scheme based on WAMS and multi-agent is designed to prevent worldwide collapse of interconnected power system. This system which has distributed hierarchical control structure includes control center level, region control level and local execute level. Three levels are coordinated and cooperated, so the splitting control system can observe power system more comprehensively, identify out-of-step condition correctly, split the out-of-step system duly and ensure the stability of the whole system. The control center level collects and processes the overall informations, searches splitting interfaces online, identifies the out-of-step oscillation between region power systems and coordinates with emergency control. The region control level which includes region splitting stations, identifies the out-of-step oscillation in the region power system and coordinates with out-of-step interfaces. The local execute level which includes local splitting stations, splitting control measures are carried out. Three control levels can work both independently and coordinately to maintain stability of the whole system.Simulation results for the 10 machines 39 buses New England system and the practical Huabei-Huazhong-Huadong interconnected power system have demonstrated that the splitting scheme which is easy to be applied in the engineering field can identify the out-of-step condition hierarchically, split the instability area cutting off from the whole system timely and correctly. So the asynchronous oscillation between interconnected power systems will be eliminated and the widespread outages will be averted.