Research On Grounding And Protection For Distribution Network With Distributed Generation

With the rapid development and application of distributed energy, it will greatly improve the efficiency of energy use, but also can change the structure of the original distribution network,that causes a series of problems in terms of relay protection. Combined with current situation of the research on the relay protection of distribution network containing DG, this paper has designed schemes of grounding and relay protection, and that is consistent with the distribution network attitude of acceptance to DG. Specifically, mainly include the following:(1)Combined with the principle and fault characteristics, the establishment of grid connected model of different DG was completed. the synchronous motor and induction motor type of DG are identical in fault output characteristics, so they could be equivalent to a voltage source in series with impedance, the inverter type DG could be equivalent to a controlled current source constraint by current because of the protection module of inverter, the equivalent models could meet the computational requirements and also simplify the process of calculation.(2)Based on the features and specifications of distribution network grounding system, a mixed grounding scheme is designed to adapt to the DG. The scheme includes the grounding type of DG, and the distribution network connection modes, a neutral point of DG can turn on and off is designed to adapt to grid connected and islanded operation, the connection modes is that TN-C-S(TN-S) grounding system on the occasion of equipotential connection, while TT grounding system with the protection of RCD in the other case could satisfy safety requirements.(3)According to the relay protection model of medium voltage distribution network, the influence of DG is analyzed and the corresponding countermeasures are given. This paper has made a quantitative analysis of the effect of DG to current protection, and also introduced a fault current limiter( FCL) to weaken this effect. To prevent asynchronous reclosing and arc-reignition in distribution network with DG, a disconnect reclosing device should be added on the side of the distributed generation.(4)Considering the effects of protection at different regions, a relay protection scheme is proposed to adapt to DG. Based on the requirements of distributed generation grid connected and current protection, the penetration capacity at different grid bus was calculated. While DG at higher capacity connect to the network, the downstream protection should deploy current and voltage interlock protection and the upstream should deploy new another protection, so that distribution network with DG could operate in a safe state.