| More extra-high voltage (EHV) and ultra-high voltage (UHV) transmission lineswill appear with development of power system in China. Study of new protectionprinciple is an important approach to ensure quick and correct switching off faultsand to maintain power system stability. On account of influence of currenttransformer saturation and distributed capacitive currents of the protected line, greatunbalance current will be produced during external faults and make protection tomal-operation, so that some restraining measures must be adopted. There are manyrestraining principles proposed, but at present, they can't assure strong brakingeffect during external faults as well as high sensitivity during internal faults.Therefore some protections adopt complicated restraining characteristic of polygonalform,it makes protection complicated and difficult to select proper settings for it.Restraining characteristic of microprocessor-based adaptive segregateddirectional current differential pilot protection principle proposed in this paper putsforward a nonlinear restraining characteristic, the restraining quantity increases withincrease of the current of other side nonlinearly and will change into operatingquantity during internal faults. This assures sufficient security during external faultsand sensitivity during internal faults and resolves the aforementioned problem. Thisprinciple is original and firstly proposed in this paper.Values of restraining factorin this principle can be selected as any big values without any restricts.At the same time, increase of restraining quantity does not impact on sensitivityduring internal faults , just the contrary, it increases protection sensitivity. In thispaper the criterion and operating characteristics of it are analyzed and compared withother principles in detail.The comparison shows the advantage of the new principle.Many simulation studies of the protection on EHV and UHV transmission lines500kV and 750kV show the correctness and advantage of the new principle.K ′′ /K′A microprocessor-based segregated current differential pilot protection programhave been worked out based on the new principle. This protection will not mal --operate during CT circuit breaking at one side, at the same time the protection at theother side will not start , so the protection will not mal-operate as a whole. Theprotection can be blocked with a time delay by using the communication channelfor the purpose to prevent the protection from mal-operation owing to external faults.In the protection program three working modes are considered such as output trippingcommand through AND gate, OR gate or transfer trip by communication channel inorder to increase reliability and the flexibility.The program is developed based both on sampled values and on phasors usingthis new principle. Directional overcurrent protection of zero-sequence was used asback up protection, it ensures sufficient sensitivity of protection during ground faultthrough high intermediate resistances. |
