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Abstract A power line carrier (PLC) communication characteristics‐based method is proposed for single‐phase‐to‐ground fault location in neutral isolated medium voltage meshed distribution networks in this paper. The carrier signals with a time‐varying frequency and constant amplitude are processed by a set of PLC transmitters and receivers, whose placement is optimized by regarding the power network as an undirected graph. Two signal encoding and decoding algorithms for the PLC terminals are proposed to avoid using expensive timing systems between the terminals. The fault location technique is implemented by comparing the cosine similarity of amplitude attenuation and phase offset between the fault and a feature library. The node corresponding to the maximum cosine similarity of the characteristics between the present fault and the library is selected as the location of the current fault. Only one set of low‐cost PLC communication terminals and the widely available power lines are needed in the fault location system, making this approach highly practical. Numerical simulations using MATLAB/Simulink have been performed to verify the technique's feasibility. The results show that the method can accurately locate faults in neutral isolated medium voltage meshed distribution networks. Besides, the presented approach achieves a high level of accuracy in estimating transition resistance values.

In order to evaluate the insulation condition of oil-immersed transformers effectively, a weight allocation method considering expert experience and degradation state is proposed in this paper. Firstly, a hierarchical structure for insulation condition evaluation is built based on dissolved gas in oil, ambient temperature and defect information, and subjective weights is assigned to each index based on expert experience. Then, the method of combined weight is adopted to introduce entropy weight into the weight allocation of index layer, thus weights of the index layer are dynamically adjusted to reflect the differences in the individual operation process while retaining expert experience for the criterion layer. Furthermore, the transformer insulation state is subdivided into five states from common four to realize more accurate perception of insulation state. Finally, case study of four 500 kV transformers shows that the method proposed can effectively identify normal transformers and discriminate abnormal transformers compared with the method based only on objective or subjective weight, which verifies the rationality of the method.

The gas pressure of gas insulated switchgear (GIS) is the key factor affecting the sensitivity of partial discharge (PD) detection. In this paper, four defect models, tip, suspension, void, surface, are constructed on the PD experimental platform. PD experiments are carried out at different pressures, 0.2 MPa-0.5 MPa. Ultra-high frequency (UHF), high-frequency current transform (HFCT) and acoustic emission (AE) methods are used, and the sensitivity of HFCT and AE methods for PD signal detection is compared. The results show that SF6 pressure is a key parameter affecting tip discharge, suspension discharge and surface defect discharge. Under these defects, the partial discharge initial voltage (PDIV) is proportional to SF6 pressure, and the discharge amplitude and pulse number are inversely proportional to SF6 pressure. For the void defect, the change of external SF6 pressure does not affect the void gas pressure, and thus does not significantly affect the cavity discharge intensity. The HFCT method can effectively detect suspension defects, however, it cannot detect the PD signals of void and tip defects. As the SF6 pressure decreases, the sensitivity of HFCT method to detect surface defects increases, although the sensitivity is not comparable to that of the UHF method. The AE method can detect suspension and tip defects effectively, but cannot detect PD signals of void and surface defects. The detection sensitivity of the AE method for tip defects gradually increases with the decrease of SF6 pressure. When the pressure is 0.2 MPa, the detection sensitivity is equivalent to that of the UHF method.

In cable fire test,a fire source is essential for the fire performance evaluation. Most of the existing fire sources for power cable are based on chemical fuel,which is unable to represent the cable fire caused by fault arc. Therefore,to simulate the ignition of power cable by fault arc,a fire source based on electrical arc is developed in this work. Firstly,the induced voltage on the metal armor layer is theoretically analyzed when the 110 kV cable has the ground wire stolen or the cross interconnection wires on both sides are stolen. Voltages can reach thousands of volts. Secondly,the pre-breakdown voltage and sustained voltage of the metal-to-ground arc in power cable and the Jacob's Ladder arc are compared. It is found that the breakdown characteristics of both arcs are similar for applied ac voltage with frequencies of 50 Hz and 20 kHz. Thirdly,a fire source based on Jacob's Ladder arc is proposed for the ignition of power cables. Experimental results show that the 110 kV cable can be successfully ignited by this fire source,and the flame is able to spread both vertically and horizontally. Finally,the conclusion is that the high frequency Jacob's Ladder arc can simulate the characteristics of the fault arc and effectively ignite the 110 kV cable.