Improvement of directional relays based on constant impedance angle control of inverter interfaced distributed generations

Abstract

With a large number of Inverter Interfaced Distributed Generations (IIDGs) being connected to active distribution network, there is a serious problem that the sensitivity of directional relay declines. The basic reason has been found that the phase angle of IIDG equivalent impedance is no longer constant and fluctuates in a large range. Researchers have proposed some methods to improve the directional relay performance by means of controlling the characteristic of IIDG equivalent impedance after a fault. However, all existing methods ignore the mandatory requirement of low voltage ride through, resulting in less practical value. This paper proposes a novel IIDG control strategy of constant impedance angle considering the requirement of low voltage ride through and presents a coordination scheme of control and protection to improve the performance of directional relay. Firstly, this paper divides voltage drop degree represented by k into three intervals, which are k>0.9, 0.2⩽k⩽0.9 and k<0.2. In each interval, the calculation methods of inner loop reference currents for d-axis and q-axis are derived respectively to achieve the proposed control strategy. Then, the influence of fault current limitation on the adaptability of the proposed control strategy is analyzed in the interval of 0.2⩽k⩽0.9. In addition, the switching scheme of control strategies and the coordination scheme are also designed in detail. Finally, digital simulations verify that the proposed control strategy can achieve the control objective of constant impedance angle while satisfying the requirement of low voltage ride through, regardless of how deep the voltage drop degrees are. Afterwards, the proposed coordination scheme is compared with the scheme in which the pre-fault constant PQ control strategy continues to be used after a fault. The comparison results demonstrate that the coordination scheme can improve the performance of directional relays effectively.