A set of fault location algorithms for underground medium voltage cables, two incipient fault detection schemes for distribution cables and a state estimation method for underground distribution networks are developed in this thesis.
Two schemes are designed to detect and classify incipient faults in underground distribution cables. Based on the methodology of wavelet analysis, one scheme is to detect the fault-induced transients, and therefore identify the incipient faults. Based on the analysis of the superimposed fault current and negative sequence current in time domain, the other scheme is particularly suitable to detect the single-line-to-ground incipient faults, which are mostly occurring in underground cables. To verify the effectiveness and functionalities of the proposed detection algorithms, different fault conditions, various system configurations, real field cases and normal operating transients are examined. The simulation results have demonstrated a technical feasibility for practical implementations of both schemes.
Based on the methodology of the direct circuit analysis, a set of location algorithms is proposed to locate the single phase related faults in the typical underground medium voltage cables. A large number of complex nonlinear equations are effectively solved to find the fault distance and fault resistance. The algorithms only utilize the fundamental phasors of three-phase voltages and currents recorded at single end, normally at substation. The various system and fault conditions are taken into account in the development of algorithms, such as effects of shunt capacitance, mutual effects of metallic sheaths, common sheath bonding methods and different fault scenarios. The extensive simulations have validated the accuracy and effectiveness of the proposed algorithms.
In order to extend the proposed fault location algorithms to underground distribution networks, a state estimation algorithm is developed to provide the necessary information for the location algorithms. Taking account of the complexity and particularity of cable circuits, the problem of the state estimation is formulated as a nonlinear optimization problem that is solved by the sequential quadratic programming technique. The simulation studies have indicated that the proposed fault location scheme incorporating with the state estimation algorithm can achieve good performance under different load and fault conditions
