Measurement of impedance angle for anti-islanding protection in multi-source distribution network

With the increase of the proportion of distributed generators (DGs) access in the distribution network, it transforms from a traditional radial network to a multi-terminal active network, which brings many challenges to relay protection. If part of the distribution network is disconnected from all upstream networks, it can create unplanned islands that can compromise personal safety, power quality, and out-of-phase reclosing. As a result, it is essential to equip DGs with anti-islanding protection. Currently, anti-islanding protection based on impedance measurement is of great interest because it has the lowest risk of rejection and misoperation. However, in multi- source distribution networks, signal interference can lead to protection failure. In light of this, we proposed an impedance angle measurement anti-islanding protection based on voltage synchronous injection, where each DG uses the bus voltage as a reference to achieve synchronous injection of the islanding detection signal. The measured impedance angle was then used to identify the state of islanding operation. Combining the analyses of line impedance, injected signal frequency and other relevant factors, an impedance angle based islanding identification criterion is proposed. Furthermore, the realization method of signal injection and impedance angle detection in combination with the DG control strategy was presented. Finally, real-time digital simulation (RTDS) experiments show that the proposed method can accurately identify islands under zero power mismatch con-ditions and independent of grid disturbances.