Impedance Differential Protection for Active Distribution Network

The current differential protection is introduced into the active distribution network to improve the selectivity and reliability of protection. However, it is difficult to meet the synchronous requirement of differential protection for distribution networks. Therefore, this paper proposes an impedance differential protection for the active distribution network. The principle and criterion of the impedance differential protection are established according to the significant differences between differential impedance and restraint impedance during normal operation, external fault, and internal fault. Furthermore, an auxiliary criterion is constructed by the amplitude characteristic of fault current to cope with the existing dead-zone fault. A complete protection scheme based on the proposed principle is designed. Several major factors are taken full consideration in the scheme including transition resistances, current transformer saturation, unsynchronized data, various penetrations, and underground cables. Extensive digital simulations are conducted to verify the principle. Theoretical analyses and simulations demonstrate the effectiveness and advantage of this principle. Compared with current differential protection, the impedance differential protection significantly reduces the requirement of data synchronization and transmission bandwidth, which plays an important role in adapting to the communication infrastructure of distribution network.