| Power system transient signal reflect the deep-level fault information of the system. Because the amplitude of the transient component in distribution network is much larger than the steady-state component, so the fault line selection scheme based on transient signal is more reliable. The phase selector based on transient signal can achieve faster action speed, has more protection margins. Fault location principle based on transient signal is more accurate, less influenced by parameter of the lines and fault conditions. Recently, theories of protection and fault location based on wavelet and transient signal have been fully demonstrated, the research is gradually moving towards the stage of application device development.In this background, according to the theoretical features and equipment foundation of our lab, the first part of the paper developed a wavelet analysis system for power system transient signal based on the virtual instrument development software LabWindows/CVI, NI PCI-6133 high-speed synchronous data acquisition card and Appollo150 IPC. It can achieve high-speed transient signal data acquisition, fault line selection, fault phase selection and fault location function, divided into four corresponding modules. The data acquisition module is realized by NI DAQmx, multi-threading technology. Fault line selection module is realized based on travelling wave's amplitude and polarity of each line. Fault phase selection module includes two methods, respectively method based on transient signal energy and method based on travelling wave. Fault location module realizes A-type single-ended and D-type double-ended algorithm, taking advantage of the modulus maxima to accurately determine the arrival time of traveling wave. Performance of each module was tested by a three-phase standard source, simulation data generated by a 35kV model of distribution network and 500kV model of transmission network. All test results show that the function of each module is complete. Every module achieves the desired effect. The system can be used as teaching or experiment platform, and is hopeful to be used in practice, making the wavelet analysis tool practical.On account of transient signal is easily influenced by many factors, such as fault time, fault resistance, et al. It shows localized, random, and non-stationary characteristic. Protection scheme based on transient signal can't always achieve perfectly, especially in the distribution network, because of the influence of fault resistance, process of transient may be unobvious. Therefore, fault line selection method based on transient signal may fail. In reality, the difficult of fault line selection particularly in resonant grounded system is always existed, the fault line selection device have highly dropout rate. In view of this, the second part of the paper focuses on transient signal and its frequency spectrum characteristic research in order to evaluate the applicability of fault line selection methods based on transient signal. Based on the equivalent circuit of resonant grounded system, the changing law of the transient signature is analyzed theoretically when the fault resistance change, concludes that the circuit is firstly in under-damped state, then entered damping state, and finally makes the transition to under-damped state again. A simulation model of resonant grounded system with cable lines is established, the changing law of the zero-sequence transient current and its spectrum is analyzed when cable lines are added in distribution network and when fault angle, fault resistance or fault distance change. Simulation results show that the adding of cable lines makes the principal component of the spectrum moves to low-band. The main components of all lines transient current's frequency spectrum may exist in different frequency band when the fault distance is short. Transient signal and its spectrum are sensitive to various fault conditions. Fault line selection methods based on transient signal may fail when the fault resistance is large.Since fault line selection methods based on fault steady-state component and transient-state component all have advantages and disadvantages. If we confuse them, complement each other to achieve the advantages of both, it is hopeful to solve the difficult of fault line selection. Starting from this point, the third part of the paper research the fusion fault line selection method based on DS evidence theory. The author constructed three fault degrees, namely fifth harmonic degree (FHD), transient energy degree (TED) and correlation degree of the wavelet packet coefficients in the characteristic frequency band (CFCD). Based on the simulation model, the adaptability of the three degree is tested, concludes that in high impedance fault condition, the FHD and CFCD have poor performance, but the TED still have high accuracy. Subsequently, we assigned the basic credibility of the corresponding evidence. A fusion method based on dynamic absolute credibility (DAC) is proposed. The method relies on transient factor that defined based on zero-sequence voltage to dynamically assign the absolute credibility of each evidences, so we can allocate the evidence conflict reasonably. The performance of the proposed fusion method and the traditional fusion method is contrasted. Results show that the proposed fusion scheme realized higher accuracy than the traditional evidence fusion. |
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