Research On Wide-Area Backup Protection Scheme Capable To Identify Power Flow Transfer

The interconnection of power networks has promoted the development of both economy and society. However, it has brought about some negative aspects that the network structure has become more complicated, the difficulty to control the network has been increased and the range of disturbance has expanded. When power system is under heavy-loaded condition, after a faulted transmission line is removed, the power flow on it will be transferred to other lines, resulting in the overload on them. Traditional backup protection will operate to remove these non-faulted lines because it cannot distinguish non-faulted overload from internal short-circuit faults, which will aggravate the transfer of power flow and will promote the process of cascading trips. This is considered as the main factor of several blackouts worldwide in recent years. In order to prevent backup protection from incorrect operation caused by transfer power flow to deteriorate power grid, based on wide-area synchronized measurement technology, research on wide-area backup protection scheme capable to identify power flow transfer is carried out in this paper.First, the study of the proposed method based on DC sensitivity factors to identify power flow transfer is conducted. After the features of DC sensitivity factors matrix are concluded, the disadvantage of this method that backup protection may be blocked inappropriately because the identification only depends on the criterion of active power, and its reasons are analyzed in detail. In order to improve the reliability of identification, the additional criterion is put forward, as well as the improved power flow transfer identification scheme.Second, on the basis of wide-area information, a novel algorithm for power flow transfer identification concerning power swing is proposed. A new concept of power flow transfer characteristic(PFTC) that can characterize the significant differences between power flow transfer and kinds of short-circuit faults is introduced, and its theoretical foundation is listed. The different ranges of PFTC when power flow transfer or each kind of short-circuit fault occurs with power angle changing from 0 degree to 180 degree are derived respectively. Then the identification criteria and delay time are put forward in accordance with the derived conclusion. The logical relationship among the criteria is also stated.At last, a wide-area backup protection scheme capable to identify power flow transfer is designed. The implementation process of the scheme whose realization needs the cooperation of dispatching center and substations is elaborated. In this paper, the feasibility and validity of not only the improved scheme but also the novel one are demonstrated by a large number of simulation cases. The achievements of the paper can provide some reference for the effective prevention of cascading trips.