Technical developments and practical experience in large scale introduction of on-line PD diagnosis

On-line Partial Discharge (PD) detection and location systems for medium-voltage cables are at present being introduced in Dutch utilities and worldwide. The technical challenges now move from the development of the diagnostic technique itself to efficient implementation on a large scale. In this paper we discuss several implementation related challenges and will propose adequate solutions. These challenges include robust algorithms to determine time of arrivals of distorted PD waveforms, signal propagation along cable types and configurations as three-core and cross bonded cables, and effect of ring main units or substations on signal propagation. Algorithms based on signal energy and on phase angle in frequency domain are preferred above e.g. threshold detection to determine PD arrival times. By introducing effective dielectric properties, cable parameters for accurate fault location as characteristic impedance and propagation velocity can be estimated also if data on semiconducting layers are unavailable. Models are proposed for cross-bonded connections and for three-core cables with common earth screen. A pulse injection circuit, already included in the PD equipment for time synchronisation, can be employed to extract a model for PDs passing ring main units or even entire substations.