Practical aspects of partial discharge measurement for HVDC cables

High-voltage direct current (HVDC) cables are increasingly being installed to connect new offshore wind farms. Unplanned outages of these connectors can cause high economic impacts. Hence, there are requirements for condition-based maintenance that can improve operational reliability. Partial discharge (PD) is indicative of insulation defects. PD monitoring for AC cables is well established, but before applying the technique to HVDC cable connections, it is important to characterise PD behaviour under DC conditions and the attenuation in HVDC cables. This paper investigates PD activity under non-ideal DC stress, PD signal attenuation in HVDC cables, and electromagnetic noise in converter stations. Under the voltage of superimposed DC and harmonics, PD pulses tend to synchronise with the phase of harmonics. Therefore, synchronised recording of PD pulses can produce phase resolved patterns as an additional tool for insulation diagnostics. Modelling of attenuation in a HVDC transmission cable indicates that a detection bandwidth of tens of kHz to a few MHz may improve detection sensitivity when measuring PD current pulses over very long cable runs is carried out through sensors such as high frequency current transformers (HFCTs) installed at cable ends. Additionally, the RF spectrum measured in a converter station cable hall did not include any switching-related signals, demonstrating the viability of RF sensors based PD monitoring for the HV components associated with the cable connections

The use of GIS and water quality index to assess groundwater quality of krimat aquifer (Essaouira ; Morocco)

The aim of this present study was to evaluate groundwater quality in the upstream part of the Essaouira basin. A detailed geochemical study of groundwater region is described, and the origin of the chemical composition of groundwater has been qualitatively evaluated, using multivariate statistical methods (PCA, HCA), and Water Quality Index (WQI) was used to determine the suitability of water for drinking. To attempt this investigation, 38 samples were analysed for various physicochemical parameters such as temperature, pH, TDS, Na, NO3, K, Ca, HCO3, Cl, Mg, and SO4. The results obtained showed that the facies characterizing the study area was a combination of Ca-SO4 and mixed Ca-Mg-Cl. Hydrochemical approach based on the bivariate diagrams of major ions indicates that the origins of groundwater mineralization are the result of (I) evaporite dissolution; (II) cation-exchange reactions; and (III) evaporation processes. The WQI values range from 82.3 to 390.9, and therefore the water samples can be categorized into five groups: excellent water to water unsuitable for drinking. In global, 61% of the groundwater sampled had poor water quality, 18% were very poor water quality, 16% are unsuitable for drinking, and just 6% represent a good quality. However, the results of this paper indicate that most water is not safe for drinking and needs further treatment

Investigating the effects of VSC harmonic content on PD diagnostics for HVDC insulation systems

This paper reports on experimental investigations in which both HVDC and HVAC voltages were applied to solid dielectric insulation samples containing known defects. The insulation samples were subjected to HVAC and HVDC containing superimposed harmonic ‘ripple’, similar to that produced by the converter switching operation. Of particular interest were the effects of the applied voltage on partial discharge (PD) amplitude, repetition rate and pulse-spacing, which showed a correlation with harmonic amplitude and phase position. Under HVAC, the phase of the power frequency cycle is the dominant driver of PD activity and forms the basis of most diagnostic techniques for PD. However, under HVDC, PD activity can be influenced by comparatively minor features of the applied voltage. Established knowledge rules for defect identification based on repetitive PD characteristics at HVAC are therefore unlikely to be applicable directly to HVDC systems. Given the rapidly expanding HVDC infrastructure, accruing new knowledge rules through laboratory tests and online measurement trials will be essential in assisting network operators to identify incipient defects with a greater degree of confidence on these systems in the futur

Partial discharges under HVDC conditions

This paper describes partial discharge (PD) phenomena in HVDC insulation, including diagnostic methods that are being developed. PD detection and analysis is used to assess the condition of electrical insulation in many types of HV equipment during manufacture, acceptance testing and condition assessment of equipment in the field. In HVAC systems, the phase of the power frequency cycle has a dominant influence on PD activity. Consequently, phase-resolved PD patterns form the cornerstone of many diagnostic techniques for HVAC insulation. However, this approach cannot be used for HVDC systems. The increasing use of HVDC in recent years has led to a growing interest in how PD diagnosis techniques developed for AC systems can be adapted for use with HVDC equipment. Under the influence of an electrostatic (DC) field, the flow of PD current is unipolar, which means that mobile charges in the insulation are subject to forces that cause them to migrate from one conductor to the other, with conductivity of the insulation playing a significant role. This can lead to the accumulation of localised regions of trapped charge. While PD pulses tend to occur much less frequently under HVDC conditions than under comparable HVAC excitation, this trapped charge can cause bursts of discharge activity when the steady-state equilibrium is disturbed, most notably during energising and de-energising. Measurements are presented to illustrate PD behaviour in various test objects subjected to HVAC, HVDC and composite (HVDC plus modulation) voltages. Additional tests on defective 33 kV cables are described, which include DC ramp tests, polarity reversal and soak testing at DC overpotentials. Results results demonstrate how PD activity can be influenced by comparatively minor features of the applied HVDC voltage, such as those associated with ripple from converter switching. Finally, PD data acquisition and analysis techniques are discussed with a view to optimising the application of PD detection under HVDC conditions

Partial discharges under HVDC conditions

This paper describes partial discharge (PD) phenomena in HVDC insulation, including diagnostic methods that are being developed. PD detection and analysis is used to assess the condition of electrical insulation in many types of HV equipment during manufacture, acceptance testing and condition assessment of equipment in the field. In HVAC systems, the phase of the power frequency cycle has a dominant influence on PD activity. Consequently, phase-resolved PD patterns form the cornerstone of many diagnostic techniques for HVAC insulation. However, this approach cannot be used for HVDC systems. The increasing use of HVDC in recent years has led to a growing interest in how PD diagnosis techniques developed for AC systems can be adapted for use with HVDC equipment. Under the influence of an electrostatic (DC) field, the flow of PD current is unipolar, which means that mobile charges in the insulation are subject to forces that cause them to migrate from one conductor to the other, with conductivity of the insulation playing a significant role. This can lead to the accumulation of localised regions of trapped charge. While PD pulses tend to occur much less frequently under HVDC conditions than under comparable HVAC excitation, this trapped charge can cause bursts of discharge activity when the steady-state equilibrium is disturbed, most notably during energising and de-energising. Measurements are presented to illustrate PD behaviour in various test objects subjected to HVAC, HVDC and composite (HVDC plus modulation) voltages. Additional tests on defective 33 kV cables are described, which include DC ramp tests, polarity reversal and soak testing at DC overpotentials. Results results demonstrate how PD activity can be influenced by comparatively minor features of the applied HVDC voltage, such as those associated with ripple from converter switching. Finally, PD data acquisition and analysis techniques are discussed with a view to optimising the application of PD detection under HVDC conditions