Abstract：The remarkable evolution in the urban distribution networks highlights its active-distribution-characteristic. And the number of distribution feeders with unmeasurable branch structures increases greatly. To adapt to this feature, the performance of non-unit protection degrades to a great extent. For particularly important feeders, pilot protection is urgently needed to ensure the reliability and sensitivity of fault removal. Due to the differential characteristics of branch current, phasor differential protection is not applicable in principle. The pilot distance protection and directional comparison pilot protection both require voltage information while Potential Transformers (PTs) are not available for the protection of distribution network. In addition, both factors mentioned above make it difficult to analyze the action behavior of protection. Therefore, it is of great significance to design a new pilot protection criterion. Firstly, the influence of unmeasurable branches is fully considered, and the idea of using only the phase difference of negative sequence current, the fault component that always exists during the asymmetric fault, to reflect the fault is proposed. Next, considering the passive property equivalence of motor type distributed generation (DG) and inverter type DG in negative sequence network, the phase comparison range of negative sequence current on both sides of the line in case of internal fault and external fault is analyzed respectively, and the difference between them is plotted in the phase comparison plane. Finally, based on this difference, the negative sequence phase comparison criterion of each level of line is set, and combined with the auxiliary criterion of negative sequence current check, a negative sequence current phase comparison protection suitable for branched distribution network line is constructed. Simulation results of a 10 kV distribution network model in PSCAD/EMTDC show that the data processing time required for protection is about 20 ms. This meets the requirements of distribution network differential protection. For six typical locations f1~f6, and different types of asymmetric faults, phase to phase fault and two-phase grounding fault, the proposed negative sequence phase comparison protection does not mis-operate when external fault (f1, f5, f6). It acts correctly when internal fault (f2, f3, f4), which is not affected by transition resistance and has high sensitivity. When the branch property (inductive, resistive) and capacity (0.8~1.5 MV·A) change, the protection can act correctly. Compared with the existing amplitude differential protection with branches, the proposed criterion does not rely on a priori condition and can be applied to the active distribution network with flexible operation modes, such as downstream load is lost and the downstream load proportion is small, without mal-operation. The following conclusions can be drawn through simulation analysis: (1) The proposed protection has good rapidity, which can provide rapid protection for the branched distribution network. (2) By simulating the resistive and inductive properties of unmeasurable branch loads and possible fluctuations in real systems, it is verified that the proposed criterion can effectively deal with the distribution network with unmeasurable branches. The protection has clear action boundary, good selectivity and high sensitivity. (3) Compared with the existing amplitude differential protection considering unmeasurable branches, the proposed criterion is not subject to the prior conditions such as the proportion of new energy not exceeding 25%. It has good adaptability to the flexible operation mode of active distribution network and has advantages in security. In the appendix, the design of criterion margin angle considering frequency offset and inverter harmonic characteristics is given. Furthermore, the design related simulation verifies that the above two factors have no influence on the judgment results of the proposed protection. The protection is robust.
肖澍昱, 林湘宁, 魏繁荣, 吴宇奇, 李正天. 面向含不可测分支配电线路不对称故障可靠辨识的负序电流比相保护判据[J]. 电工技术学报, 2023, 38(9): 2435-2447.
Xiao Shuyu, Lin Xiangning, Wei Fanrong, Wu Yuqi, Li Zhengtian. Phase Comparison Protection of Negative Sequence Current for Distribution Lines with Unmeasurable Branches. Transactions of China Electrotechnical Society, 2023, 38(9): 2435-2447.
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2023, Vol. 38 
