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This paper presents an investigation of effects of high voltage direct current (HVDC) superimposed voltage harmonics on partial discharge (PD) behavior. Firstly, a single air filled void embedded in a slab of polymeric insulation was modeled using finite element methods (FEM). PD phenomena activated under the influence of rippled DC voltages were simulated using an interlinked environment of MATLAB and multiphysics simulation package COMSOL. Secondly, the results of the modeling were backed up by experimental tests, where HVDC superimposed harmonics were applied to artificial void samples in the lab and trends in the resultant PD characterized with varying harmonic orders and amplitudes. The results of both modeling and experiment are generally in agreement showing that as the harmonic orders and levels increase, the repetition rate of PD pulses rises in comparison to a purely DC applied voltage having the same peak amplitude. The findings of this study will assist in understanding of PD behavior under HVDC conditions where harmonic rippling occurs on the voltage waveform.

Transient overvoltages are inevitable phenomena in power systems. They occur in high voltage direct current systems for much the same as the reasons they happen in high voltage alternating current systems and could be of either internal or external origin. Moreover, the switching operation of the power electronic valves in HVDC converters add to these transient sources. The effects of such transient overvoltages become more critical in the case of non-restoring insulation media like solid dielectrics that form the main insulation media of extruded HVDC power cables. In addition, the intrinsic behavior of solid dielectric under DC fields results in space charge accumulation due to inhomogeneity of the insulation, increasing the likelihood of a fault during overvoltage incidents or even during normal operations like polarity reversal. This paper presents the results of PD measurements that have been conducted on artificial test samples at HVDC superimposed with standard switching overvoltages and operational polarity reversal, which are common in modern converter schemes. The results show that under such transient conditions, PD occurs with higher repetition than the normal voltage level in the test sample. The results of this investigation should be of interest to network operators for asset management on fleets of HVDC cables that may typically be subject to the reported transients and polarity reversal effects.

This paper presents an investigation into the influence of different operating conditions of HVDC converters on partial discharge behavior occurring within solid dielectrics. Cables at the DC side undergo different stresses due to transients and harmonics that may result in an intensified level of partial discharge (PD) activity and could affect their insulation integrity. Most DC converter schemes, nowadays, are bidirectional with the valve firing angles at the heart of power conversion and power flow control. Depending on the mode of converter operation and the properties of the interconnected AC systems, varying levels of harmonics, either characteristic or non-characteristic, appear on the DC side. By reproducing down-scaled equivalent DC outputs of conventional converters under controlled laboratory conditions, PD pulses occurring under characteristic HVDC voltages have be measured. PD was produced using a range of insulation samples containing known defects and measured using an HFCT sensor. Characteristics of the acquired PD pulses have been analyzed with the aim of finding a promising condition monitoring tool to identify the influence of the superimposed harmonics on PD activity. The results will aid HVDC network operators in identifying incipient cable faults by contributing to diagnostic knowledge rules for interpretation of PD parameters under known operating conditions.

Insulation is a critical component of any electrical power system, which, when subjected to excessive stress, may lead to premature ageing and eventual failure. Partial discharge (PD) measurement is a well-developed insulation diagnostic method, particularly under alternating voltage (AC), for quality assessment and condition monitoring of power apparatus. However, this technique is still immature when applied to DC operating equipment, especially high voltage direct current (HVDC) systems where rippled and distorted voltage waveforms appear at the output terminal of the converters. This paper presents the results of experimental PD measurements and analysis for commonly encountered PD sources: corona, surface and void discharging defects under the application of DC and DC superimposed with harmonics. The results show that PD activity under rippled voltage is different than that under pure DC. PD occurs with higher repetition rate, and is - in varying degrees - influenced by the relative magnitudes of the harmonics, depending on the PD source. Since it appears that PD is more severe under the influence of harmonic voltage ripple, the results should be of interest to network operators and will contribute to understanding of the phenomena in HVDC insulation diagnostics.

Partial discharge (PD) measurement as a non-destructive diagnostic tool is a proven technique that has served in both design improvement and condition monitoring of insulation systems. An understanding and interpretation of the physical phenomena and influencing parameters of PD in AC systems have been established through experience over the past decades, however, flexible alternating current transmission system (FACTS) and high-voltage direct current (HVDC) systems present new challenges. These are due to the differences in physical space charge phenomena and the complexities introduced by power electronic switching. More meaningful information on the physical condition of the dielectric insulation could be gained given a better understanding of the physical discharge phenomena occurring in the region of a defect. Numerical modelling of these complex phenomena can go some way to achieving this. In this paper, PD behavior in HVDC systems has been investigated with the employment of Finite Element Methods (FEM). An insulation defect, consisting of a single void within a polymeric insulation layer, has been modeled to investigate the idealized behavior of such a defect in, for example, a sub-sea HVDC shielded cable. The multi-physics simulation package COMSOL interlinked with MATLAB were used to study the electric field distribution imposed by the HVDC waveform and the PD process. It was found that additional harmonic content results in an increased PD repetition rate to be proportional to the dominant harmonic amplitude.

Partial discharge measurement is among the most important diagnostics methods of insulation systems in high voltage equipments, which makes it convenient to assess the insulation status and its prospective condition. Partial discharge activities may stem from various defects, and correspondingly behave differently. Since the origins of the PD activities are of major concern in insulation diagnostics, a large number of recognition methodologies have been proposed and used for this purpose. Among them, Phase Resolve Partial Discharge (PRPD) analysis has gained more attractions. However, sometimes the complexity of the patterns is of much more sophistication where utilization of intelligent based or expert system is inevitable. In this paper an Adaptive Neuro-Fuzzy Inference System (ANFIS) based classification method has been considered, where statistical features are extracted from the measured PRPD data first, then Fuzzy IF-THEN classification rules which are obtained from experts are employed in training procedures of the ANFIS model, and finally the classification of the PD defects is done automatically by the trained system, which makes it ready to be interpreted for decision making purposes.

Condition monitoring of high voltage equipments has always been a central part of condition based maintenance programs which is highly preferred by generation, transmission and distribution companies. Through their development there has been a great deal of changes in condition monitoring systems aimed at providing systems of high reliability, modularity and adaptivity as well. Multi agent systems with the aforementioned features can be appropriate means of implementing distributed, modular and intelligent monitoring systems. In this paper designing of a high voltage monitoring system based on multi agent system will be considered in which tasks are distributed among different levels where the tasks and purposes of each level are defined according to the whole goals of the system. It finally results in an intelligent multi agent system combined of different levels.