Analysis of capacitive voltage rise during off-line partial discharge tests on power transformers

Partial discharge test on a transformer is carried out according to the items in IEC 60. During the test, unproved measuring system is calibrated by proved system at a voltage no less than 50% the rated testing voltage. The result is then extrapolated linearly, leading to an error related to the distribution of stray capacitance, which varies with the testing frequency, especially to large transformers. In this paper a factor, named the capacitive rise fact, is introduced to assess the rise. The factor can be adjusted to some extent by changing the reactance that is connected to the LV side of the testing circuit to lower the capacity of the power source. However, the factor changes when the voltage divider on the high voltage side is removed after the voltage ratio has been calculated, and a great error is resulted under unfavorable conditios.

A Study on Measurement Error during Alternating Current Induced Voltage Tests on Large Transformers

The large transformer is pivotal equipment in an electric power supply system; Its partial discharge test and the induced voltage withstand test on large transformers are carried out at a frequency about twice the working frequency. If the magnetizing inductance cannot compensate for the stray capacitance, the test sample turns into a capacitive load and a capacitive rise exhibits in the testing circuit. For self-restoring insulation, a method has been recommended in IEC60-1 that an unapproved measuring system be calibrated by an approved system at a voltage not less than 50% of the rated testing voltage, and the result then be extrapolated linearly. It has been found that this method leads to great error due to the capacitive rise if it is not correctly used during a withstand voltage test under certain testing conditions, especiaUy for a test on high voltage transformers with large capacity. Since the withstand voltage test is the most important means to examine the operation reliability of a transformer, and it can be destructive to the insulation, a precise measurement must be guaranteed. In this paper a factor, named as the capacitive rise factor, is introduced to assess the rise. The voltage measurement error during the calibration is determined by the parameters of the test sample and the testing facilities, as well as the measuring point. Based on theoretical analysis in this paper, a novel method is suggested and demonstrated to estimate the error by using the capacitive rise factor and other known parame- ters of the testing circuit.

Influence of moisture on the interface charge of oil-pressboard composite insulation under DC voltage

The distortion of the electric field in the oil–pressboard composite insulation caused by the accumulation of the interface charge is detrimental to both the insulation design and operation of converter transformers.The influence of moisture content on the surface charge accumulation of oil–pressboard insulation under DC voltage was studied in this study.In accordance with the Kerr electro-optic effect,the electric field strengths in transformer oil and the surface charge density were acquired after applying the positive and negative DC voltages in three oil–pressboard insulation models with different moisture content,respectively.The resistivities of the oil and pressboard in three models,namely Model 1#with 3.8–4.2 ppm moisture in oil and 0.35–0.37%moisture in pressboard,Model 2#with 7.6–7.9 ppm moisture in oil and 0.79–0.82%moisture in pressboard and Model 3#with 14.9–15.4 ppm moisture in oil and 1.39–1.42%moisture in pressboard,was also measured.The results indicate that:(i)as negative charges in oil accumulated on the pressboard surface in a much greater speed than the positive ones,the electric field in transformer oil under negative DC voltage decreases more rapidly with time than that under positive DC voltage;(ii)the increase of the moisture content in both oil and pressboard,under either positive or negative DC voltage,leads to the decrease of both the electric field strength in transformer oil and the charge density with time;and(iii)the increase of moisture content could not only decrease the resistivity of both oil and pressboard,but also the ratio of the resistivity between the pressboard and the oil.On the basis of the Maxwell–Wagner theory,the decrease of the ratio between the pressboard and oil could lead to the decrease of the interfacial charge density,leading to the slow transient process of the electric field in transformer oil under DC voltage.

Simulations and experiments of the generation apparatus of the DC electric field with the space charge

To make accurate measurements,field mills must be calibrated in a known DC electric field space charge in which the quantities can be control1ed.A simulation model of the generation apparatus of the electric field with a space charge has been built.Based on the combination of the electric field and the transfer of a dilute substances model,the in-fluence of openings in the metal screen was calculated.Simulation results showed that the opening size of the high‐voltage electrode has a great influence on the distribution of the electric field and ion density at the calibration zone.Next,a generation apparatus was designed and developed and ion mobility was researched.The experimental results showed that ion mobility decreased with an increase in electric field strength.However,ion mobility tends to be saturated when the electric field reaches a certain critical value.Two fitting formulas of ion mobility of positive and negative charge are proposed that can be used to predicate ion mobility in a high electric field.

Received signal strength indication-based localisation method with unknown path-loss exponent for HVDC electric field measurement

The electric field environment under high-voltage direct current(HVDC)transmission lines is an important design consideration.In the wireless sensor networks for electric field measurement system under the HVDC transmission lines,it is necessary to obtain the electric field distribution with multiple sensors.The accurate localisation of sensing nodes is essential to the analysis of measurement results.However,most current techniques are limited to constant measurement environment with fixed and known path-loss exponent.Here,the authors report a localisation method based on received signal strength indication with unknown path-loss exponent for the localisation of one-dimensional linear topology wireless networks in the electric field measurement system.The optimisation method is utilised to obtain the optimal pass-loss parameter without involving the previous environment parameters.Afterwards,simulations are employed to demonstrate the feasibility and the effectiveness of the proposed method by comparing with other methods.

Shielding measures of power transformer to mitigate stray loss and hot spot through coupled 3D FEA

Leakage flux generated in the power transformers cause stray losses in metallic parts of the transformer.Stray losses largely affect the transformer hot spot which leads to thermal failure.It is necessary to mitigate the impact of leakage flux on metallic parts by employing magnetic shunts and eddy current shields.Preeminently clamping structures like flitch plate and end frames experience substantial losses,hence combinations of shielding can be used for better loss reduction.In this study,a case study is performed on a 315 MVA,420/27 kV single-phase generator power transformer.This work is based on coupled three-dimensional(3D)finite-element analysis(FEA)of the transformer model.A new type of magnetic shunt is explored;also different design combinations of shielding are specified in this work for efficient mitigation of hot spots as well as tank walls.

Effect of TiO_(2)coating on the surface condition and corona characteristics of positive DC conductors with particle matters

Persistent severe smog and dust weather exists in many regions of China.Particles accu-mulate on the surface of high‐voltage transmission lines,which affects the surface morphology of the transmission line and induces corona discharge.This work mainly studies the influence of electric field and photocatalysis on the corona characteristics of transmission lines.In this work,plasma spraying is used to prepare a dense,smooth and excellent photocatalytic TiO_(2)coating on the surface of transmission wires.An electron microscope and a white light interference profilometer is used to study the surface condition of the samples.The variation in the surface condition of the samples is observed under different fouling,light time and electric field intensity.The corona characteristics of power transmission lines under different surface conditions are studied.The results show that light and applied electric field promote the photocatalytic effect of TiO_(2),which helps to decompose the contamination on the surface of power transmission wires and thus reduce the corona discharge of transmission lines.The increase in corona inception voltage reduces the corona current pulse repetition rate and the ion current density during corona discharge.