Integration of online partial discharge monitoring and defect location in medium-voltage cable networks

Partial discharge (PD) diagnostics is a proven method to assess the condition of underground power cables. PDs are symptomatic for a defect (weak spot) that may evolve into a complete breakdown. A PD induces a small pulse in the conductor(s) and earth screen that propagates through the cable in both directions. In previous research an online PD detection and location system was developed for medium-voltage cables. This system is currently introduced by utilities on an increasingly large scale. This thesis deals with several new challenges that are related to the integration of online PD monitoring in different medium-voltage power cable network configurations. A transmission line model of the power cable is required to enable optimal PD detection. A three-core power cable with common earth screen has multiple coupled propagation modes. The propagation modes are decoupled into a modal solution. A practical method to measure and analyze the cable parameters is developed and validated by measurements on a cable sample. Detailed prediction of multiple reflections was achieved, including the mixing of propagation modes having distinct propagation velocities, validating both the model and the measurement method for three-core cables with common earth screen. The semiconducting layers in a cable with cross-linked polyethylene (XLPE) insulation have a significant influence on the transmission line parameters. Unfortunately, the dielectric properties of these layers are usually unknown and can differ between similar cable types. It is shown that nonetheless the characteristic impedance and propagation velocity of single-core and three-core XLPE cables can be calculated using information available from the cable specifications. The calculated values are validated using pulse response measurements on several cable samples. The accuracy of the calculated characteristic impedance and propagation velocity is 5–10%, which is sufficient for estimating PD pulse shape and amplitude in a cable circuit. The online PD monitoring system was initially developed to be installed on a single cable connection between two consecutive ring-main-units (RMUs). It is more efficient to monitor two or more consecutive cables, with one or more RMUs or substations in between, using a single monitoring system. Models for RMUs and substations are proposed and verified by measurements. The influence of RMUs and substations along the cable under test on the PD detection sensitivity and location accuracy is studied using these models and measurements. The influence of a compact RMU with two connected cables is neglectable if the total cable length is longer than approximately 1km. The longer the total cable length, the smaller the influence of an RMU along the cable under test. An RMU or substation along the cable under test with more than two connected cables introduces a significant signal loss, decreasing the detection sensitivity significantly. The higher the number of connected cables, the higher the signal loss. In the equipment found in some substations and RMUs it is not possible to install a PD measurement unit at a desired location. The models and measurements are used to study the feasibility of a single-sided PD measurement, including PD location, with the problematic RMU/substation at the far end. The study shows that a major part of an incoming pulse reflects on a large RMU/substation with many connected cables. This reflection enables a single-sided PD measurement from the other cable end. A single-sided measurement has the disadvantage that the maximum cable length that can be monitored is halved and that it is sensitive to reflections on joints in the cable under test and other connected cables. Accurate location of PDs in cables, based on arrival times, is imperative for the identification and assessment of defects. Five algorithms that determine the time-of-arrival of pulses are evaluated to investigate which method yields the most accurate location under different circumstances. The methods are evaluated analytically, by simulations, and by measurements on a cable system. From the results the energy criterion method and the phase method show the best performance. The energy criterion is a robust method that achieves location accuracy of 0.5% of the cable length or better. The phase method has a good performance only if the phase shift introduced at the transmission from the cable under test to the RMU is known accurately. For maximum detection sensitivity the PD monitoring system uses matched filters. These filters are constructed using predicted PD waveforms. These predictions are based on a series of online system identification measurements and a standard cable model. Due to signal distortions that are not taken in account in the model, e.g. an RMU along the cable under test, the predictions can be inaccurate, resulting in a sub-optimal PD detection. An automated procedure that creates new signal templates, based on measured PD signals, is proposed and tested on signals measured during online PD measurements on multiple cable systems. The algorithm generated new templates for PD signals and for disturbing pulses. New PD pulse templates for may be used to improve the cable system model. Disturbing pulse templates may be used to improve disturbance rejection by the measurement unit.