Analyses of partial discharges in dielectric samples under DC excitation

The main focus of the paper is to develop a better understanding of partial discharges under DC excitation. Partial discharges studied will initially be limited to discharges from well-defined discharge sites. These include corona, surface discharges and internal voids. The samples are first tested under AC excitation as a sense check to ensure the samples yield the expected PD events. The samples were then subjected to DC excitations where the PD events were recorded and subsequently analysed. A number of analysis techniques will be applied to potentially enable the identification and classification of the type of PD event occurring in the DC system under investigation

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

PD activity in void type dielectric samples for varied DC polarity

This paper discusses the DC testing of a dielectric sample (with voids) under DC conditions. The ramp test method was employed to assess whether the tests were repeatable. Three DC ramp tests were performed in quick succession. The polarity of the first/third tests was positive and the polarity of the second test was negative. The resultant partial discharge activity was analyzed for each series of tests and compared for positive and negative conditions. The results show that the inclusion of a negative ramp between two positive ramps enabled similar PD activity to be recorded in the first and second positive ramp tests. The negative cycle was used to re-initialise the test sample and allows trapped charge to migrate enabling similar PD activity in the subsequent positive ramp test. Changes in PD behavior were observed in the distribution of PD data between the tests

Long term testing and analysis of dielectric samples under DC excitation

This paper details testing conducted under DC conditions on a dielectric sample containing internal voids. The DC testing was conducted using a ramp method to vary the voltage applied to the dielectric sample. The dielectric sample was de-energised for a week prior to two separate identical ramp tests and the results are presented showing the variability of PD activity. After the second ramp test an additional ramp test was performed in quick succession and PD activity was reduced, emphasizing the importance of de-energising the sample between tests. A major challenge associated with void type dielectric samples is ensuring that repeatable results are generated and possible approaches are discussed

Laboratory and field partial discharge measurement in HVDC power cables

A range of experimental and field measurements of partial discharge (PD) activity under high voltage direct current (HVDC) conditions have been conducted with the goal of developing effective monitoring techniques for PD in HVDC cables and ancillary equipment, particularly in offshore renewable energy HVDC grid installations. Laboratory measurements on insulation test objects and cross linked polyethylene (XLPE) cable samples have been conducted to better understand the characteristics of PD activity under direct current (DC) stress in comparison with AC. In addition, long-term PD measurements carried out at both an HVDC cable aging laboratory and an in-service HVDC interconnector circuit are presented together with a description of the monitoring system architecture

Aging behaviour of polypropylene under various voltage stresses

Practical HVDC systems have superimposed AC or transients and the synergistic effect of these factors on polymer aging would be of interest. Although it is clear that partial discharges under AC stress will gradually degrade the insulation behavior of a polymeric material, there are not many publications detailing the effect of superimposed AC voltages on polymer performance in a HVDC power system. Assuming polypropylene (PP) is suitable for use as electrical insulation, this paper considers the behavior of PP under various voltage ratios and temperatures. Factors which cause the degradation of PP will be summarized and explained. To simulate the working condition, electro-thermal aging equipment will be used. Fourier transform infrared spectroscopy - attenuated total reflection (FTIR-ATR) measurement and dielectric spectroscopy measurement will be carried out before and after aging

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

Assessment of HDPE aged under DC voltage combined with AC harmonic stresses of various frequencies

One of the challenges on the increasing reliance on isolated renewable generation sources is the transmission of power from these sources to centers of power demand. One possible approach is the use of high voltage direct current (HVDC) transmission. The power electronic converters are key components in HVDC transmission system. The converters produce the intended DC voltage for transmission but there may also be AC harmonics superimposed. The superimposed harmonics on the HVDC may have synergistic effects and may lead to further degradation in the cable insulation. Previous research has shown that partial discharge was the main cause of degradation in polymeric insulation under AC stress. However, few publications have demonstrated the effect of combined stress on cable insulation degradation. Additionally, the most popular cable insulation material, cross-linked polyethylene (XLPE), cannot be recycled. Alternative materials which can be recycled have been proposed and one such solution could be thermoplastic materials. In this study, HDPE was investigated as a reference material for thermoplastics and their potential use as insulation in HVDC cables. In this paper the effect of frequency on HDPE degradation under superimposed stresses was studied using the following approaches; equivalent phase resolved partial discharge (PRPD) plots, fourier transform infrared spectroscopy - attenuated total reflection (FTIR-ATR) and dielectric spectroscopy (DS) measurements were carried out. The results show that during aging and with a frequency increase, the voltage of PD events increased which in turn created more polar molecule groups on the surface. The amount of polar molecule groups was found to affect ε' and tanδ, with both increasing when more polar molecules were created. The results show that applying a higher AC frequency enhances polymer degradatio

Impact of increased penetration of low-carbon technologies on cable lifetime estimations

Cables are the largest assets by volume on power distribution networks and the assets with the least health information routinely gathered. Projections over the next 8 years suggest increased penetration of low-carbon technology (LCT) at the distribution level with higher loads resulting from electric vehicle (EV) and heat pump uptake. Over this period, increased cable loading will directly influence their lifetimes and may mean that existing asset management practices need to be revised to understand the specific impact on end-of-life assessment. Accordingly, this paper uses a physics-based thermal lifetime model based on projected uptake trends for LCTs to evaluate the impact on distribution cable lifetime. Two case studies are presented considering portions of network and the ultimate impact on asset life over the next decade. Two commonly used cables are considered to quantify the lifetime reduction caused by LCT for asset fleets. The paper shows that the projected uptake of EVs and heat pumps will have a detrimental effect on cable life with a 30% reduction in cable lifetime possible

Full-depth englacial vertical ice-sheet velocities measured using phase-sensitive radar

We describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling. Using a ground-based phase-sensitive radio-echo sounder (pRES) during seven Antarctic field seasons, we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on Berkner Island, Roosevelt Island, Fletcher Promontory and Adelaide Island. Recorded changes in reflector positions yield 'full-depth' profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides. We interpret these variations by comparing them to the results of a full-Stokes simulation of ice-divide flow, qualitatively validating the model and demonstrating that we are directly detecting an ice-dynamical phenomenon called the Raymond Effect. Using pRES, englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice. We discuss how these measurements could be used with inverse methods to measure ice rheology, and to improve ice-core dating by incorporating pRES-measured vertical velocities into age modelling