Sufficient Overcurrent Protection For Distribution Network With DG Considering LVRT Characteristics

Distributed generation greatly improves power system reliability and flexibility. However, the traditional overcurrent protection will be affected as the radial structure of distribution network has been changed into structure of multiple power. With the increase of DG capacity, DG may be cut off immediately when fault occurs in distribution network, which may cause the instability of system. In order to ensure the safety and stability, DG must have the capability of low voltage ride through (LVRT) and cannot be cut off. However, characteristics of fault and influence on traditional current protection will change due to LVRT. Considering this, researches of the access of DG which consider LVRT in-depth are mainly as follows:Firstly, requirements of LVRT and impact on traditional overcurrent protection due to access of DG have been researched. The analysis shows that fault characteristics of DG are affected by control scheme of LVRT. In view of the incomplete and disperse of information, sufficient protection strategy has been proposed based on finite quantity of state information of distribution network. According to fault characteristics which can be identified, the overcurrent protection is changed into sufficient current protection with the consideration of LVRT characteristics.Secondly, LVRT characteristics of DFIG and IIDG have been researched. Basic principle of Crowbar protection for LVRT is analyzed. The conclusion shows that when fault is close to DFIG, DFIG can only provide instantaneous short circuit current with Crowbar protection access. If output voltage of IIDG sag severily, short circuit current will be limited. Steady state current will be 2 times the rated value which operates in LVRT mode, and steady state current will be less than 2 times the rated value which operates in grid-connected mode.And then the impact of LVRT of DG on current protection of distribution network is analyzed. The influence of LVRT on overcurrent protection of DFIG output tie line and feeder has also been analyzed. Overcurrent instantaneous protection and it backup protection of tie line will be influenced due to fault current of DFIG with Crowbar protection, which decay rapidly. Current instantaneous protection of downstream feeders will be influenced seriously, but time delay instantaneous current protection is influenced weakly.Finally, sufficient current protection scheme considered LVRT of DG has been researched. For output tie line of DFIG, fast algorithm is applied to instantaneous overcurrent protection, self-hold circuit and low voltage criterion are applied to time delay instantaneous protection, LVRT standard curve is coordinated with definite time overcurrent protection. For feeder of distribution network, downstream instantaneous current protection open selectively by detecting signal of Crowbar. A setting value calculating method of sufficient current protection for LVRT of IIDG is put forward. Finally, the feasibility and effectiveness of proposed scheme is verified by data and simulation.