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

•A novel control strategy is proposed to achieve the goal of constant impedance angle.•A coordination scheme of control and protection is designed to improve directional relay.•The influence of current limitation on the proposed control strategy is analysed. 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