Research On Island Detection And Island Protection Based On Wide - Area Phase Comparison

According to the structural characteristics and development needs for the current active distribution network with DER, the stability, flexibility and quality of the power system has been improved while DER operating connected with network. But it also brought some other new problems such as voltage sag, protection misoperation. Especially when power system is operating for the non intentional islanding, the stability of power system may be affected seriously, and threaten the safety of power equipments and person. So it is necessary to make effective islanding protection for the non intentional islanding, which can’t maintain its stability. Cut DER from large power grid, as far as possible to shorten the islanding operation time, to avoid a series of safety problems. But effective islanding protection depends on the rapid and reliable islanding detection. The common islanding detection methods have certain shortcoming and limitations. The remote detection methods are based on communication with higher cost. The passive detection methods based on local electrical variables, have certain non-detection-zone when the islanded system power matching degree is high. The active methods based on disturbance signal injected into power grid, have certain influence to power quality.In this paper, grid-connected inverter based on PQ control mode with Constant power output has been researched, RLC parallel load model is established, The system voltage, Frequency characteristics and the influence of unbalanced power have been theoretical deduced when DER is on grid-connected operation and islanding, The results found that the voltage and frequency of islanding system is major related to local load characteristics, this is very different with power grid which have a constant external characteristic, The voltage amplitude change after islanding was mainly decided by the unbalanced voltage (△P),The bigger△P and the voltage amplitude changes. For a higher level of active match (△P is relatively small) case, the variation of voltage amplitude is small and it’s difficult to reflect island characteristics, while a decisive influence on the island system frequency due to△Q is still significant now. At the same time, the voltage phase change brought about by frequency change is more prominent. So the islanding detection method based on WAN phase comparison is proposed, the island detection criterion is the voltage phase difference between large power bus side and DER side. The synchronous measurement and calculation of voltage phase on both sides are completed by FTU, and realize information exchange through the rapid communication system.By configurating FTU in large power bus and DER, the voltage phase of large power bus and DER (θs(t),θDER(t)) are calculated in real-time. The FTU of large power bus side will transport time-stamped θs(t) via fast fiber optic communications system to the FTU of DER side; the FTU of DER side execute the voltage phase difference comparison between both sides with the same time scale, considering certain measurement and communication error, compared with island determination threshold setting θz, completing island detection. According to the test results obtained by the FTU of DER side, islanding protection starts and DER interconnection switch is disconnected and stop outputting power transportation. Based on Matlab, established the islanding detection simulation model of active distribution network with DER. System voltage, frequency and phase change characteristics of grid-connected and islanding operation are analyzed by the simulation model. The local load mutation and distribution network are on to islanding detection’s influence is ruled out. Based on the local load characteristic, moreover fully considering the impact of the imbalance of power on islanding detection. Experimental results and theoretical analysis are matched, verified the validity and applicability of the method.