Condition monitoring of HVDC transmission systems for offshore wind

The work presented in this paper reviews the methods of detecting PD in a HVDC system. The behavior of PD under DC conditions for common defects types is discussed. The partial discharges studied in this paper are limited to three dielectric samples with discharges from well-defined discharge sites. These include corona, surface discharges and internal voids. The samples were subjected to AC and DC excitation, AC excitation as a sense check to ensure the samples yield the expected PD activity. A further area of investigation was the application of analysis techniques to enable the identification of the type of PD event. The ability to determine the type of PD apparent in the HVDC system feeds directly into the condition monitoring of the HVDC system and identification of the insulation fault

Partial discharge activity in polymeric cable insulation under high voltage AC and DC

The focus of this paper is to report on analysis of partial discharge characteristics in insulation samples containing artificially created voids under AC and DC excitation. Samples were initially tested under AC conditions to determine that the artificial void was the dominant source of PD. Once this was proven the samples were tested under DC to generate knowledge on PD under these conditions. The results were analyzed, with differences between AC and DC PD in artificial voids determined. The impact of these differences on potential methods of PD detection are also briefly discussed

Partial discharge detection and location for HVDC cables

This poster is concerned with the initial review of literature in the area of partial discharge and location for HVDC cables, as part of an on-going PhD research project. HVDC cable systems have been utilised in the transmission of energy for some time, and look to be increasingly employed in the future. However, whilst the technology exists to allow such systems to be installed, the monitoring of such systems is still a novelty and continues to present many challenges. Similarly, partial discharge detection and analysis has become a key part of condition monitoring of high voltage electrical equipment insulation, with transformers, generators, cables and transmission lines all often tested for PD. However the characteristics of PD at DC voltages are less well understood. Additional challenge comes from the length of HVDC cables, as partial discharges are measured at cables ends which can be hundreds of kilometres apart. The project aims to understand the characteristics of partial discharge inception from HVDC cables, including consideration of different insulation types, and to investigate and consider technologies for the detection and location of partial discharges up to 300 km. This poster reviews the current literature in this area, and identifies gaps in current knowledge, that the project seeks to fill

Fault location and diagnosis in a medium voltage EPR power cable

This paper presents a case study on fault location, characterization and diagnosis in a length of shielded 11 kV medium voltage ethylene-propylene rubber (EPR) power cable. The defect was identified on-site as a low resistance fault occurring between the sheath and the core. A 43 m section was removed for further analysis. The fault resistance was characterized and the location of the defect pinpointed to within a few cm using a combination of time-difference-of-arrival location and infra-red imaging. A combination of X-ray computed tomography, scanning electron microscopy and energy dispersive X-ray spectroscopy were then applied to characterize any abnormalities in the dielectric surrounding the breakdown region. A significant number of high density contaminants were found to be embedded in the dielectric layer, having an average diameter of the order of 100 um, a maximum diameter of 310 um and an average density of 1 particle per 2.28 mm3 . Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to determine the geometry and elemental composition of some initial contaminant samples. It was concluded that contamination of the EPR layer, combined with an observed eccentricity of the cable’s core and sheath resulting in a reduced insulation gap, may have led to an electric field concentration in the region of the defect sufficient to initiate breakdown. Preventative strategies are discussed for similar families of cables, including more stringent dielectric testing requirements at the manufacturing stage and PD monitoring to detect incipient failure

Voltage distribution in group-grounded 8 x 2 solar PV panel assembly during lightning strike

This paper is focused on the effect of group grounding of solar PV assemblies using both end-point and mid-point grounding on the potential rise across the solar PV panels during a lightning strike. This system consists of 16 assemblies forming an 8x2 array. Simulation has been carried out for various lightning attachment points in PSPICE using the lossy transmission line model. Voltage drop at various points in the assembly is determined for various soil resistivities. Based on the simulation results, group grounding of solar PV panels with middle grounding shows a lower voltage transient potential rise compared to end grounding

Application of k-means method to pattern recognition in on-line cable partial discharge monitoring

On-line Partial Discharge (PD) monitoring is being increasingly adopted in an effort to improve asset management of the vast network of MV and HV power cables. This paper presents a novel method for autonomous recognition of PD patterns recorded under conditions in which a phase-reference voltage waveform from the HV conductors is not available, as is often the case in on-line PD based insulation condition monitoring. The paper begins with an analysis of two significant challenges for automatic PD pattern recognition. A methodology is then proposed for applying the K-Means method to the task of recognizing PD patterns without phase reference information. Results are presented to show that the proposed methodology is capable of recognising patterns of PD activity in on-line monitoring applications for both single-phase and three-phase cables and is also effective technique for rejecting interference signals

MV cable lifetime improvement analysis through transformer tap changes

Cable life depends mainly on the thermal stress, which relates to the current applied on the cable. Voltage changes in medium voltage (MV) cables due to transformer tap changes will also change the current flowing through the cable, which will change the cable temperature. In order to extend the cable life, this paper aims to simulate and analyse the potential thermal lifetime improvement of cables through long-term tap changes within the statutory levels. Firstly, the IEC standard (60287) method for rating and modelling cables is applied to evaluate the cable temperature under different voltages and relative currents. Different cable configurations will also be considered in simulations as temperature is dependent on the cable dimensions. Then, typical thermal lifetime analytical expressions will be used to evaluate the long-term influence of voltage changes. Lastly, the obtained thermal lifetime assessments under different transformer tap changes and different cable configurations will provide a potential understanding of cable lifetime changes through implementation of permitted regulatory voltage changes

Nanocomposites based on magnesium-oxide/aluminum-nitride/polypropylene for HVDC cable insulation

Abstract—Polypropylene (PP) with high thermal stability and good electrical properties, has attracted much attention for its potential to take the place of cross-link polyethylene (XLPE) as HVDC insulation because PP is more easily recycled than XLPE due to its thermoplasticity. Due to the adverse effect of electric field reversal under HVDC application, there is a need to find the new polymer insulation material with higher thermal conductivity and good electrical performance. This paper investigates the effect of introducing aluminum nitride (AlN) and magnesium oxide (MgO) into PP on the electrical properties of the resulting the new nanocomposites. In the sample preparation, AlN and MgO were surface-modified by KH570 (γ- methacryloxypropyltrimethoxy silane) and then introduced into PP by the solution method to manufacture the nanocomposite materials. The measurements made were the voltage breakdown characteristics and the DC conductivity. The results obtained show that the combination of AlN and MgO can slightly decrease the DC conductivity of PP/AlN/MgO nanocomposites compared with pure PP. The breakdown strength was slightly decreased. which shows that the adverse effect of AlN on the electrical performance of PP can be compensated by introducing MgO nanoparticles. Hence, the new polymer with high thermal conductivity and good electrical properties could be manufactured by combining two kinds of nanoparticles. Keywords — nanocomposites, magnesium-oxide, aluminum-nitride, polypropylene, electrical performance

Effect of group grounding on the potential rise across solar PV panels during lightning strike

Grounding systems play a vital role for dissipating the energy originating from a lightning stroke to the earth, functioning as an important component for protection and safety. In a solar photovoltaic (PV) farm, solar PV panels are fixed on a grounded structure with bolts and nuts. The structure, the frame of the PV panels, and the bolts and nuts are metallic (together called the assembly) and the layout of all assemblies of the entire solar farm depends on the terrain where they are installed. Lightning protection systems which are installed on a solar PV farm are mostly based on a Franklin rod (connected to a down-conductor) as the preferred point of attachment. These lightning rods can be installed either as isolated systems or as non-isolated systems from the solar panel assemblies. This paper is focused on the effect of group grounding on the potential rise across the solar PV panels during lightning strike. This whole assembly is simulated for various lightning attachment points in PSPICE using the lossy transmission line model. Voltage drop at various points in the assembly is measured for various soil resistivity. Based on the simulation results, group grounding of solar PV panels with middle grounding shows relatively low voltage drops compared to end grounding

Thermoplastic materials aging under various stresses

The most popular cable insulation material used is XLPE due to its excellent electrical and thermal properties. However, it does not lend itself to ease of recycling. As a result of an increase in concern worldwide regarding environmental protection, it is the objective of this work to investigate whether a thermoplastic material could be used to replace XLPE for cable insulation. Among thermoplastic materials, HDPE is regarded as one with the most similar properties as XLPE. Although it is clear that the performance of polymeric material changes with different stresses, especially polymer nanocomposites aging process under AC electric field stresses, there are also not many publications on how a superimposed AC voltage would affect the insulation’s performance in HVDC power systems. This paper reports the dielectric properties of HDPE under thermo-electrical stresses. DC stress with and without a superimposed AC stress were applied in the experiments undertaken. The degradation of materials with change in frequencies are summarized and discussed