Correlation between surface charge and creepage discharge on polymeric dielectrics under high-frequency high-voltage stress

In the present work,creepage discharge characteristics,i.e.amplitudes,phases,and repetitiveness,and surface charge dynamic behaviors under a 20 kHz high-frequency sinusoidal waveform high-voltage electrical stress were captured in a discharge chamber with temperature and humidity control.The results showed that the creepage discharges mostly occurred in the positive half phase,whose maximum amplitude increased with the development of discharge.The inception voltage of the creepage discharge is independent of the frequency of the external electrical stress.Once the discharge occurred,there were a large number of positive and negative particles ionized by a high electric field.Because of the much higher velocity of electrons than positive ions,the energetic discharge-produced electrons are likely to disperse away along the surface and be accumulated through adsorption,collision,and reactions.Moreover,the positive ions join the high-conductive discharge channel and disappear though the ground electrode.Thus,after high-frequency creepage discharge,only negative charges remained on the dielectric surface,as measured.Particularly,the creepage discharges mostly occurred in the positive half phase,owing to the reverse electric field induced by the accumulated negative charges.With the development of creepage discharge,some large-amplitude discharges began to occur in the positive-peak-phase region.The research concluded that the synergistic effect of negative surface charges and large-amplitude discharges eroded the dielectrics and excited the streamer to creep toward the ground electrode until flashover along the surface.Therefore,the correlation between high-frequency creepage discharge and surface charge is preliminarily revealed.

Statistical analysis and interpretation on lightning impulse breakdown voltages of ester liquids under varying field uniformity

Ester liquids(ELs),as the alternative transformer liquids to the conventional mineral oils(MOs),have been increasingly used in power transformers with high voltage ratings.Among various properties,the lightning impulse breakdown voltages(LIBDVs)have been studied elaborately in the past two decades under different electric field uniformities.The electric field uniformities do have an effect on the breakdown characteristics,for example,breakdown mechanisms,polarity effects,and differences of LIBDV between ELs and MOs,which is quantitatively investigated based on the LIBDVs collected from the literatures.The breakdown mechanism transition phenomenon between streamer initiation dominated breakdown and streamer propagation dominated breakdown has been reviewed firstly.Then,the transition field factor boundaries to observe the polarity effect on LIBDVs are taken at around 20 for ELs and 10 for MOs.Besides,the transition field factor boundaries to observe the LIBDV distinctions between ELs and MOs are about 109 and 14 under positive and negative lightning impulse,respectively.The systematic analysis and interpretation on the transition field factor boundaries among different phenomena can provide insights on insulation design of ELs filled power transformers.

Analysing the power transformer temperature limitation for avoidance of bubble formation

Failure of a transformer is a costly incident which can impact power system operation and result in loss of supply to customers.Transformers can fail through a number of mechanisms,one of which is from the formation of bubbles within the insulation.Currently,the loading guidance is set to avoid certain overload temperatures as high temperature is the main cause of bubbles forming.This paper analyses the limit set within the loading guides,comparing the most commonly cited temperature limit of 140℃ with values taken from across the full spectra of available studies in literature.It is concluded that there is sufficient evidence for further investigation into formation of bubbles from transformer insulation as employing a single temperature limit may be inadequate.Use of a single temperature value is unsuitable,given the wide range of potential scenarios that the transformer fleet can present.It has become the go-to rationale that the moisture content of the solid insulation is the key driver for transformer bubbling temperature.This is true,however,it is shown that other factors such as solid insulation aged condition(determined through degree of polymerisation value)and the insulation type are also important considerations.

Failure investigation and asset management of combined measuring instrument transformers

Asset management of instrument transformers(ITs)has gained momentum in recent years due to their large population in the network and the potential adverse impacts of their increasing number of failures.This study investigates the failure cause of a family of combined measuring ITs via temperature profile simulations and consideration of moisture migration behaviour in an oil‐impregnated paper insulation system.The tem-perature difference experienced by an IT,particularly throughout summer,together with a relative saturation hysteresis phenomenon could have caused a lower dielectric strength that led to failures.The study also offers insights into factors influencing the probability of failures through analysis on the lifetime data from both graphical survival function plots and statistical Cox model.In‐service age,operating voltage level and environment appear to have an influence on failure rate.

Effect of sinusoidal waveform and frequency on space charge characteristics of polyimide

The charge phenomena under sine and half‐wave sine voltages within the frequency range of 500 Hz are studied here.Based on the pulsed electro‐acoustic method,the traditional circuit design under high-frequency voltages is first analysed.It is found that the selection of a 186 pF protection capacitor and a 333 kΩprotection resistor can ensure that the actual voltage applied to the sample is consistent with the expected input.Based on this design,experimental results show that the polarity of the charge accumulated in the depth of the sample is determined by that of the upper electrode.Comparison results under special voltages with different amplitudes and frequencies indicate that the amount of accumulated charge under sine voltages are larger than those under positive and negative half-wave sine and DC conditions,and the samples under lower-frequency conditions show more charge accumulation.The maximum electric field strengths appear at 90 and 270 degrees of the sine voltage with a frequency of 10 Hz,and their values are 68.55 and 81.82 kV/mm,respectively.Therefore,the charge characteristics are easily affected by the voltage’s waveform and frequency parameters.The results obtained here can provide guidance for the application of insulating materials under special voltage environments.

Transformer winding type recognition based on FRA data and a support vector machine model

Frequency response analysis (FRA) is regarded as the most effective technique to detect mechanical faults of transformers. Over the years, FRA measurement data have been collected by utilities into transformer asset databases. The characteristic of FRA data is fundamentally determined by the transformer's equivalent electrical circuit, which consists of inductance and capacitance parameters that are windings' design and structure dependent. Different winding types tend to have different FRA characteristics, and a transformer's design information such as winding type, dimension etc. is often not known to the utility but critically important for asset management. This study reviews the state-of-the-art transformer FRA databases and application of machine learning techniques in this field, and proposes to apply a support vector machine (SVM) model onto the FRA data to identify the winding type. The SVM model is first trained by FRA traces of transformers with known winding types, and after testing, the SVM model is then applied to FRA traces with unknown winding information. A set of data from the UK's National Grid FRA database, was used to demonstrate and verify the SVM model. All transformers used in this study are 400/275/13 kV transmission transformers, which were designed using four different winding types, namely multiple layer, plain disc, interleaved disc and single helical windings. The proposed method can successfully identify the correct winding type.