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Wu T.,Hubei Engineering University | Ruan J.,Hubei Engineering University | Hu Y.,State Grid Electric Power Research Institute of China | Liu B.,Central China Grid Company Ltd | Chen C.,Hubei Engineering University
Zhongguo Dianji Gongcheng Xuebao/Proceedings of the Chinese Society of Electrical Engineering | Year: 2011

Forest fires near power transmission lines always threaten the reliability of transmission lines and safety of power grids. Combining video record of the 500 kV transmission line fire induced flashover and breakdown characteristics of rod-plane gap under wood crib fires, the flashover mechanism of 500 kV line under forest fires was investigated. Results are as follows: the dielectric resistance increases with the height of flame due to temperature that affects the thermal ionization of alkali mental salts; the breakdown strength of rod-plane gap under wood crib fire is reduced to 23% of that air gap under standard atmosphere; flame temperature, conductivity and particles such as smokes and flying ashes promote the electronic avalanches and streamers development and lead to the dielectric strength reduction of the transmission gap acutely; pre-discharges initiated by flying ashes and smoke are connected to flame tip and promote the burning of fire. Electrical arc enhances the fire intensity and lifts the height of flame. Based on field records and experimental results, it is concluded that flame temperature, flame conductivity and flying ashes are ascribed to factors leading to transmission line flashover due to fire. Measures to reduce transmission line breakdown under forest fire such as increasing height of transmission line conductors above ground, restricting the vegetation species, height and density near transmission line corridors are proposed. © 2011 Chinese Society for Electrical Engineering.

Liu H.,Central China Grid Company Ltd | Jia X.,State Grid Electric Power Research Institute of China
Dianli Xitong Zidonghua/Automation of Electric Power Systems | Year: 2011

By proceeding from the implementation of the digital algorithm of relays, a discussion is made of the contradiction between the action speed of rapid relays and the length of data window. In order to solve the contradiction, various algorithms are adopted for relays. Short data window algorithms are fast and long ones are accurate. The long data window full wave Fourier algorithm is selected to ensure the accuracy of the calculation and simplify the Fourier algorithm used in variation relays. The operation equation of amplitude comparison relays of rapid relays is expressed as the operating value being greater than the braking value. To meet the requirement on the variation relay to serve as the rapid relay, the braking coefficient is introduced into the braking value according to the variation features of the operating value to correct the operation characteristics in real-time so as to meet the requirement of action speed. © 2011 State Grid Electric Power Research Institute Press.

Liu H.,Central China Grid Company Ltd | Li Y.,Huazhong University of Science and Technology
Zhongguo Dianji Gongcheng Xuebao/Proceedings of the Chinese Society of Electrical Engineering | Year: 2010

The three elements of distance relay in the voltage plane are the polarization voltage, operation voltage and operation characteristic, among which the polarization voltage is the most important. The relay could ideally select pre-fault voltage as polarization voltage, while it practically employs the positive sequence voltage. The difference between the two kinds of polarization voltage is negligible in the case of low impedance ground fault, while significant in the case of high impedance ground fault. Thus, this paper uses the memory and the non-fault phase parameters to gain the pre-fault voltage, and use it as the new polarization voltage. Thus it puts forward a new self-adaptive high impedance ground distance relay. The theoretical analysis and simulation result show that the performance of the distance relay based on the voltage plane is better than that based on the impedance plane. © 2010 Chinese Society for Electrical Engineering.

Liu H.,Central China Grid Company Ltd | Zhou Z.,China Electric Power Research Institute
Zhongguo Dianji Gongcheng Xuebao/Proceedings of the Chinese Society of Electrical Engineering | Year: 2011

Existing distance protection of transmission lines depends on setting calculation to avoid operation under over-load conditions. After operation of distance protection, the line load transferring to other transmission lines may result in fault expansion, so it's necessary to find some new countermeasures to avoid the mal-operation of distance protection in post-fault over-load condition. This paper proposed a new criterion to identify between over-load and fault in voltage co-ordinate instead of that in impedance co-ordinate. In the criterion, an operation area was constructed by considering compensation voltage as operation voltage and the polarization voltage as reference voltage. The protection will operate when the operation voltage is in the operation area. In the voltage co-ordinate, the load voltage is on the line between the two power source voltages, so it's easier for the protection to decide whether operate or not. The criterion can identify between fault and over-load radically and make distance protection avoid mal-operating after the load transferring. Its application will be benefit for the stability of power system. Simulation results show that the method is effective. © 2011 Chinese Society for Electrical Engineering.

Xu D.,China Electric Power Research Institute | Xia S.-L.,Central China Grid Company Ltd | Ding Q.,China Electric Power Research Institute | Ma Z.-M.,Northeast Dispatching Branch
Dianli Xitong Baohu yu Kongzhi/Power System Protection and Control | Year: 2012

Single mixed-integer programming (MIP) method is hard to obtain satisfactory solution in reasonable time when it solves large-scale unit commitment problems (UCP). In order to solve the problem, we propose a heuristic MIP method. Based on the unit on/off inherent mechanism, this algorithm determines the part of integer variables based on priority list method by comprehensive analysis of load curve and unit on-off characteristics. MIP method is combined to solve large-scale UCP, which can expand calculation scale and accelerate the calculation speed. The improved MIP is tested on standard and actual system and compared with existing reference. The comparison results prove the correctness, validity, and practicability of the proposed algorithm.

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