Advanced islanding detection utilized in distribution systems with DFIG

The penetration of distributed generation (DG) in electrical power systems is rapidly increasing these days and more attention is drawn to maintain a healthy distribution system. Islanding operation of DGs is one of the biggest challenges to the distribution system stability. Fast and accurate islanding detection can avoid the possibility of damages to the DGs when they are un-intentionally reconnected to the grid and also provide useful information to the protection and automation design of the stand alone operated system. Rate of change of frequency (ROCOF) method is one of the most commonly employed anti-islanding protection techniques, it offers fast detection and easy implementation. However, it is often easily affected by the system disturbance and might not able to detect the islanding situation if the power imbalance between the DG and the load is small. This paper investigates an inter-lock method which can improve the performance of rate of change of frequency (ROCOF) by applying system impedance estimation. It was found that this new method can help in verifying the ROCOF relay islanding detection and avoiding false operations of ROCOF in a grid connected distribution system which has large load variations. The proposed method was verified using the experimental testing results derived from both an experimental testing model which includes an 8 kW Double Feed Induction Generator (DFIG) and a 9 MW DFIG simulation system.