Study On Controlled Islanding Strategy Based On Power-frequency Sensitivity

With the rapid expansion of the scale of the power system, the network structure becomes more and more complex. At the same time, affected by the reform of electric power market, the operation point of the large-scale interconnected power grid is gradually approaching its operating limits. The grid security and stability margin is reduced, it is likely to trigger the cascading events and result in system crash without efficient measure.When a big disturbance occurs, as the first and second defenses, the relays and automatic devices should work to eliminate faults, inhibit the fluctuations of frequency and voltage. In this case, the grid can maintain stable operation and provide normal power supply. System splitting is the last defense to prevent system from crash and avoid widespread blackouts. When long-term failure or cascading events cause the unnormal operation of system, system splitting would split the entire system into several independent power islands to keep power supply to important loads. A successful system splitting can limit the accident range and avoid collapse of the whole network.System splitting is typically divided into passive islanding and controlled islanding. Passive splitting is to split the system at the set point of system after detecting that the system can’t guarantee stable operation. It exists unreasonable point, lack of global cooperation and other issues. While the controlled islanding is to actively separate system to two or more independently stable islands to stop the expansion of cascading failures, in order to guarantee as large range of power supply as possible.Controlled islanding determines and performs the optimal solution online based on the WAMS. Controlled islanding can improve the reliability and robustness of the smart grid, enhance the grid economic benefit, and much more adapt to the development of new energy. It is an important part of self-healing function of the smart grid. This paper has done some work in the following areas:First, this paper makes a literature review of the research. The application platform and data support system of controlled islanding study-WAMS has been introduced. This paper introduces the controlled islanding strategy decision-making process, model and algorithm of searching for the optimal strategy.Second, considering that obvious space distribution characteristic of frequency exists in large-scale interconnected system, this paper introduces the power-frequency sensitivity, based on which this paper puts forward the evaluation model of the controlled islanding strategy. The model can evaluate dynamic response of islands after controlled islanding from two aspects, the frequency offset and the unbalanced power.Finally, this paper presents a complete mathematic model of searching for the controlled islanding strategy based on power-frequency sensitivity. The objective function of the model is the minimum of the frequency deviation. The constraints are power balance constraints, system splitting constraints, islands connectivity constraints and so on. The main variable of the model is on or off information of the circuits. The model is a typical mixed integer linear program (MILP) problem. In order to verify the effectiveness of the MILP model, this paper modeled and searched the optimal solution based on IEEE-9 node system and IEEE-118 node system respectively. Besides, detailed comparison and analysis have been done between different models based on different objectives. Simulation results show that the proposed model can get optimal solution of minimum frequency offset as well as smaller power unbalance. The strategy of the proposed model can improve the frequency stability of islands formed and is conductive to the safe and stable operation of island restoration.