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Measuring the transient voltages, including lightning or switching impulse overvoltage, is helpful for accident analysis of power system and insulation design. In this paper, a contactless transient voltage measuring system based on a metal-free integrated electro-optic electric field sensor was proposed, implemented, and tested. The lightning and switching impulse transients are proportional to the electric field they generate, and vice versa, enabling measurement of the transient voltage by measuring the associated electric field. The proposed method uses an electric field sensor to directly measure the field generated by the voltage without any capacitive coupling. In addition, a method to decouple the multiphase coupling was proposed. The performances of the measuring system were tested by simulation experiments and compared with that of a standard divider. The field data of the transient overvoltage on a 220 kV bus during the process of putting a capacitor voltage transformer into operation were given. The measurement results were found to coincide with the related physical process.

Electrode have an important influence on the breakdown of liquid dielectrics. The interface of electrodes and liquid dielectrics may affect the injection and distribution of space charge in liquid dielectrics, and thereby affecting the breakdown of liquid dielectrics. To find experimental evidence, the Kerr electro-optic method was used to observe the space charge in transformer oil under different electrode materials (Brass, Aluminum). With the use of array photodetector, the space-time evolution of space charge in liquid dielectrics can be directly measured under a single impulse voltage. The results show that when impulse voltage was applied to the electrodes, the Br (+)-Br (-) electrodes injected unipolar positive space charges, the Al (+)-Al (-) electrodes injected unipolar negative space charges, the Br (+)-Al (-) electrodes injected bipolar homo-charges, and the Br (-)-Al (+) didn’t inject significant space charges. The reason of this phenomenon may be the different properties of electric double layer formed by different electrodes and transformer oil. Considering that the homo-charge injection can weaken the electric field near the electrode, the breakdown voltage under Br (+)-Al (-) should be highest, which is consistent with other researchers’ conclusions. This study provides an experimental basis for selecting a suitable electrode combination to strength the breakdown voltage of transformer oil.

The proper work of main transformers plays an important role in the stability of the power grid. The development of power data allows us to evaluate the instant status of the main transformer. If we use appropriate data classification algorithm, the laws and values hidden behind the mass power data can be discovered. This paper took basic power data, main transformers’ status data and oil chromatography data into consideration, and used Logistic regression and Naive Bayes algorithm to evaluate 110kV main transformers’ defects and faults from regression analysis and probability statistics seperately. The results of these two algorithms were obtained and compared. Finally, a more accurate algorithm of evaluating main transformer fault was selected, which provided a new reference for the application of power big data.

The grounding pole line is an important equipment of the UHVDC transmission system. Due to the low steady state voltage of the grounding pole line during the normal operation, the grounding pole line adopts the external insulation of the 35kV AC line. However, 110kV voltage grade is used mostly for tower of the grounding pole. So the external insulation is lower. When the DC system runs out of balance, there will be overvoltage on the grounding pole line, which may cause the external insulation to be broken down. The “7.13” grounding line fault occurred in 2015 in ± 800 kV Bin-jin HVDC project. Due to the high voltage DC casing breaking down in Jinhua, the system turned directly into the unipolar operation. The overvoltage of the Yibin to Jinhua grounding line at the moment of the casing breakdown caused the insulation breakdown of the grounding electrode without causing action of the line arrester. Under the condition that the DC continuous flow can not be extinguished, the accident of grounding insulator, ground wire and conductor burning has occurred. The occurrence of “7.13” accident has exposed the overvoltage and insulation coordination design defects of the grounding electrode circuit of the HVDC system. In this paper, the “7.13” fault of ± 800 kV Yibin to Jinhua DC project was taken as the analysis object. The simulation model was established by using the power system analysis software, MATLAB Sim Power System. The internal overvoltage characteristics and distribution of the grounding circuit of the UHVDC system were analyzed. The grounding line internal overvoltage level was calculated. Analysis showed that the overvoltage inside the UHV grounding circuit was depended on grounding line length, system operation mode, outgoing arrester parameters. To inspect the grounding electrode line of the converter station, the closer the grounding position of the high voltage pole was to the inspected converter station, the higher the overvoltage of the line of the grounding electrode. For the ground electrode line overvoltage caused by the high voltage pole line grounding, it showed a gradual downward trend from the converter station to the grounding pole on the grounding electrode line. The maximum overvoltage on the grounding electrode line is often caused by grounding fault on the high voltage side when the DC system was full power. Such overvoltage is the control value for the external insulation operation of the grounding electrode line. Based on calculation results of overvoltage, the appropriate insulation configuration adjustment scheme for grounding electrode line was given. After the “7.13” fault, the SGCC made a unified adjustment for external insulation configuration of the UHVDC grounding electrode lines. Number of insulator strings on the grounding electrode line has been increased to 5 or 6 pieces from 3 or 4 pieces. Distance of the insulator parallel gap was increased to 68cm from 20cm. Thus the problem was avoided that the MOA on the grounding conductor outlet could not protect the external insulation. The conclusion is of great significance to guide the external insulation design, fault analysis and operation and maintenance of the DC grounding line.

Transient voltages in the power grid are the key for the fault analysis of a power grid, optimized insulation design, and the standardization of the high-voltage testing method. The traditional measuring equipment, based on electrical engineering, normally has a limited bandwidth and response speed, which are also featured by a huge size and heavy weight. In this paper, an integrated optical electric-field sensor based on the Pockels effect was developed and applied to measure the transient voltages on the high-voltage conductors in a non-contact measuring mode. The measuring system has a response speed faster than 6 ns and a wide bandwidth ranging from 5 Hz to 100 MHz. Moreover, the sensors have the dimensions of 18 mm by 18 mm by 48 mm and a light weight of dozens of grams. The measuring systems were employed to monitor the lightning transient voltages on a 220 kV overhead transmission line. The switching transient voltages were also measured by the measuring system during the commissioning of the 500 kV middle Tibet power grid. In 2017, 307 lightning transient voltages caused by induction stroke were recorded. The characteristics of these voltage waveforms are different from the standard lightning impulse voltage proposed by IEC standards. Three types of typical switching transient voltage in 500 kV AC power grid were measured, and the peak values of these overvoltages can reach 1.73 times rated voltage.

The internal insulation defects of high voltage power equipment usually cause internal short circuit faults. The partial discharge (PD) test is one of the most effective technical methods to detect these defects. The electric charge of impulse current carefully measured in the PD test can be used to estimate whether there is a significant insulation defect in the power equipment. Moreover, because discharge processes vary with dielectric insulation, the characteristics of PD impulse currents could be used to distinguish the types of dielectric insulation. In this paper, the characteristics of PD impulse current in three typical dielectric insulation have been experimentally studied. First, a special discharge device was developed that can generate PDs in three typical dielectric insulation, including air, inert gas, epoxy, and insulating oil. The measuring circuit for PD impulse current was also established. For each type of dielectric insulation, three kinds of measuring impedance were used, and 20 tests were carried out for each measuring impedance. The characteristics of PD impulse currents, such as the rise time, the equivalent half time, the oscillation period, the frequency distribution and so on, were analyzed. By comparing these characteristics, a series of criteria used to identify the types of PDs were proposed.

Natural lightning flashes are stochastic and uncontrollable, and thus it is difficult to observe the formation process of a downward negative stepped leader (NSL) directly and in detail. In this paper, the formation processes of NSLs in controllable laboratorial long-air-gap discharge experiments were studies. First, a series of negative long-air-gap discharge experiments with scales of 4∼10 m were designed and carried out. According to the observation results of these experiments, two kinds of formation processes for NSLs with different scales were presented, and several of the characteristic parameters, including the scale, the propagation velocity and the dark period, were obtained. By comparing these characteristics with that in natural flashes, the similarity between the NSLs in the simulation experiments and those in natural flashes was proved, and the actual formation of NSLs in natural flashes was deduced.

High speed CCD cameras, current measured devices and impulse voltage divider were utilized to investigate the characteristics of rod-rod discharges. The discharge experiments in a 3m rod-rod gap were carried out under positive and negative switching impulse voltage. The characteristics of leaders propagation stimulated by positive voltage were compared with that stimulated by negative voltage. Although with certain similarities, the leaders which energized by different polarity voltage share differences such as the varied process of leader with the time, the length of upward leader, the proportion of upward leader, and the velocity of upward leader. These works have revealed the discharge process of rod-rod air gap more clearly, and are benefit for the study on the discharge mechanisms of the rod-rod discharges.

Natural lightning flashes are stochastic and uncontrollable, and thus, it is difficult to observe the formation process of a downward negative stepped leader (NSL) directly and in detail. This situation has led to some dispute over the actual NSL formation mechanism, and thus has hindered improvements in the lightning shielding analysis model. In this paper, on the basis of controllable long air gap discharge experiments, the formation conditions required for NSLs in negative flashes have been studied. First, a series of simulation experiments on varying scales were designed and carried out. The NSL formation processes were observed, and several of the characteristic process parameters, including the scale, the propagation velocity, and the dark period, were obtained. By comparing the acquired formation processes and the characteristic parameters with those in natural lightning flashes, the similarity between the NSLs in the simulation experiments and those in natural flashes was proved. Then, based on the local thermodynamic equation and the space charge estimation method, the required NSL formation conditions were deduced, and the space background electric field (E b) was proposed as the primary parameter for NSL formation. Finally, the critical value of E b required for the formation of NSLs in natural flashes was determined to be approximately 75 kV/m by extrapolation of the results of the simulation experiments.