| With the fast development of UHVDC transmission project in China, several hybrid AC-DC systems contained UHVDC will be built. UHVDC can not only improve the flexibility and transmission capacity, but also induce the mutual fault problems, influencing the reliability of AD and DC line protections. On the one hand, AC line fault can easily introduces UHVDC commutation fault, which will also distort the AC signals and make the AC line protection act incorrectly. What’s more, the current UHVDC line protections have much problems, such as low reliability and threshold determination. At this time, the UHVDC line protection can hardly recognize AC fault and inside-line fault, which can seriously influence the operational stability. Therefore, this paper researched on the key issues of transmission line protections of hybrid AC-DC system contained UHVDC, one is that the AC line protections cannot act correctly under commutation failure, the other is that the UHVDC line protections have low reliability under high resistance fault. The main work in this paper are as follows:Firstly, this paper establishes the hybrid AC-DC system model, which contains UHVDC. Based on the electromagnetic transient simulation software, which is called PSCAD/EMTDC, this paper establishes the detailed hybrid AC-DC system model, with UHVDC connected to the AC system through double circuit transmission lines. The converters are established by detailed models, and the generators are established with excitation and speed regulating systems. Dynamic properties of hybrid AC-DC system under different operation conditions are analyzed in this paper. The research results show that, the established hybrid AC-DC system model in this paper can reflect electromagnetic properties of different operation conditions, laying solid foundations for subsequent protection related issues in hybrid AC-DC system.Secondly, the features of fault AC signals under commutation failure are analyzed in this paper. The AC line protection reliability is seriously influenced by fault signal induced from HVDC system under commutation failure situation, and the key to AC protection research is the precise features of AC fault signal. A modified two-stage FFT algorithm is used to precisely analyze the features and distortion rules of fault AC signals. The analyzing results show that, large amounts of harmonics and inter-harmonics are contained in the fault AC signals under commutation failure, and the harmonic distortion rates of parallel lines are higher, which are influenced by fault length and fault type.Thirdly, performance of directional protection based on variation of power-frequency components under commutation fault is analyzed in this paper. Through the performance analysis of directional protection based on variation of power-frequency components in AC-DC system, the influence reason of non-characteristic inter-harmonics introduced by commutation failure and solutions to improve the protection reliability are also analyzed in this paper. The results show that, the non-characteristic inter-harmonics will seriously reduce the accuracy of power frequency phasor, extracted by FFT algorithm, which is widely used in current protection schemes. As a result, it will further affect the AC protection performance, and the influence is aggravated as the increasing content of non-characteristic inter-harmonics. They are probably effective way to improve the AC protection reliability in AC-DC system, such as precise power-frequency phasor extraction method, and so on.Lastly, a pilot protection scheme for UHVDC with fast reaction to high resistance faults is researched in this paper. To solve the problems in the current UHVDC transmission line protections, such as low reliability under high resistance fault and threshold determination problem, least-squares approximation is used to characterize the polarity features of fault voltages and currents of low frequency band, which are acquired by wavelet analysis. The pilot protection scheme can accustom to high resistance and noising, and overcome the problem of threshold determination through various simulations. In addition, the algorithm has low calculated amount and low requirement for communication channel and sampling rate, which can be realized under the current hardware conditions of UHVDC system.Above all, the influence principle and its solutions for transmission line protection relays under commutation failure are discussed, and the problem of low reliability and threshold determination for UHVDC line protection under high fault resistance is solved, which can provide supports for improving the action performance of line protections in AC-DC system... |
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