Junhao Li

Partial discharge characteristics over differently aged oil/pressboard interfaces

The choice of the materials used as the dielectric media in large transformers continues to be a combination of pressboard and mineral oil. However, a number of failures in the field have been attributed to the surface discharge or creeping discharge over the oil/pressboard interface. This paper reports on an experiment to study surface discharge over oil/pressboard. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD) from inception to flashover. The max PD charge, pattern, frequency and current pulse were measured during the flashover process. Four oil/pressboard specimens with different aging degrees were used and their influence on the flashover process over oil/paper interface was studied in this experiment. The results indicate that the max PD charge and PD frequency increase with the increase of applied voltage. The oil aging has great influence while the pressboard aging has little influence on PD characteristic over oil/pressboard. The PD pattern and current pulse shape show great difference for various applied voltages. The results therefore suggest that the PD detection on the oil/pressboard can be used effectively to implement the early monitor of surface discharge thereby to avoid the formation of surface flashover.

Measurement and simulation of partial discharge in oil impregnated pressboard with an electrical aging process

The continuous test on oil impregnated pressboard insulation with internal void defect was developed and the phase resolved partial discharge (PRPD) pattern of partial discharge (PD) signals during the electrical aging process was measured. Two different void structures which have different void volume were used in this experiment. It shows that the PD pattern could be classified into five stages and a great diversity in the first four stages is observed. The larger void volume leads to larger PD magnitude. The computer numerical simulation model which is based on a physical discharge process was used and the causes of PD pattern change were interpreted by comparison with computer numerical simulation results. The initial values and change tendency of gas pressure and surface conductivity were determined through experiment. The model parameters in different stages have been studied as well as the insight into the physical changes in the void during electrical aging. The results provide rules for the identification of the electrical aging stage through the partial discharge measurements.

Characteristics and Development Mechanisms of Partial Discharge in SF 6 Gas Under Impulse Voltages

It is well known that impulse voltages strongly affect the reliability of gas-insulated switchgear (GIS) under actual operational conditions. The alternating-current (ac) withstand voltage test and its corresponding partial discharge (PD) measurement are inadequate for the detection of all the hidden flaws in GIS. Such inadequacy can be compensated by additional impulse withstand voltage test and its PD detection measurement. However, the bulkiness and inefficiency of aperiodic impulse voltage generator make it unsuitable for on-site application. Therefore, in practical tests, oscillating impulse voltage (OIV) is generally adopted instead of aperiodic impulse voltage. In this paper, we measure the PD inception voltage (PDIV) and the breakdown characteristics of a needle-plate gap in SF6 gas under four types of impulse voltages by electrical and optical methods. The 50% probability PDIV is analyzed as a function of gas pressure. The physical mechanisms of the PD under the four impulse voltages are discussed. The PD characteristics under ac voltage are investigated for comparison. Experimental results reveal that the PD development process under the two aperiodic impulse voltages was mainly dominated by the precursor mechanism which consists of streamer and leader stages, while the PD propagation under the two OIVs was affected by the displacement current produced by oscillation. The difference of the PD characteristics under the aperiodic impulse voltage and OIV, and ac voltage are also presented.

Coupled Electromagnetic-Thermal Modeling the Temperature Distribution of XLPE Cable

Current carrying capacity of power cable should be determined by the maximum operating temperature which the insulation can withstand. The temperature rise in cable is mainly due to Joules loss in conductor and induced eddy loss in sheath. In this paper, based on the coupled electromagnetic-thermal theory, the power losses calculated in electromagnetic field analysis are used as the input data of thermal analysis to predict the temperature rise in different layers of cable. Using the simplified cable model, the temperature distributions in single phase and three phases cable with different arrangements are calculated and compared. The results show that three phases cable get the higher temperature distribution according to the proximity effects between the different phases compared to the single phase cable. As a result, in application, the optimized arrangement of cable can be applied to make the temperature rise of cable as low as possible.

Partial discharge characteristics over SF 6 /epoxy interfaces under impulse voltage

Gas insulated switchgears (GIS) have been widely used due to their high reliability, low maintenance and compact size. The field experiment is an important step to ensure the safety of GIS. The AC withstand voltage test and its corresponding partial discharge (PD) measurement are inadequate for the detection of all the hidden flaws in GIS. Such inadequacy can be compensated by additional impulse withstand voltage test and its PD detection measurement. This paper reports on experiments to study surface discharge over SF6/epoxy under impulse voltage. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD). The PD signals under four types of impulse voltages were detected by electrical methods. The 50% probability PD inception voltage (PDIV50), time lags and maximum discharge magnitude were analyzed as a function of gas pressure. The PD characteristics under AC voltage were investigated for comparison. The results indicate that the PD signals under impulse voltage can be detected using electrical method, the number of discharges under oscillating impulse voltage was significantly larger, which was due to the influence of source oscillation. The oscillating lightning impulse has the lowest impulse ratio and it is recommended as the best test waveform choice for the field impulse withstand test with PD detection.

PD characteristics of oil-pressboard insulation under AC and DC mixed voltage

The insulation problem of converter transformers has become one of the core issues in HVDC system, which is receiving more and more attention. A converter transformer has complex structures and requires high insulation level. The voltage waveform which the converter transformer insulation withstands is different from AC transformers. In the valve side of the converter transformer, the insulation endures a mixed voltage of AC voltage, DC voltage, harmonics and the pulsating component etc. Partial discharge (PD) occurring in oil-pressboard insulation is the one of the main causes of insulation breakdown, and it is the main factor of electrical aging of materials. Even though PD characteristics under pure AC or DC voltage waveform have been studied by many researchers, little research has been done with the mixed voltage. In this paper, the AC + DC mixed voltage experiment system is set up to study the PD characteristics under mixed voltage waveforms. The study results indicate that the pressboard withstands most of the DC component and void withstands most of the AC component when applied mixed voltage. For PD inception voltage, it is decided by both the AC component and DC component. For PD development, AC component has more significant influence than DC component. However, the DC component is found to promote PD activity under AC half cycle with the same DC polarity and inhibit PD activity under AC half cycle with the opposite DC polarity. The results in this paper provide an experimental foundation for the converter transformer insulation breakdown mechanism and analysis.

Feature extraction methods for time frequency energy distribution of PD pulse

Feature extraction methods for time-frequency energy distribution (TFED) using moment, marginal and Renyi information are studied in this paper. They are developed by combination of TFEDpsilas own characteristics with mathematic statistic and information entropy. The Wigner-Ville distribution (WVD) of a simulated PD pulse is chosen as the subject investigated to illustrate those methods, which gives the result of time and frequency moments, time and frequency marginals and Renyi information for the TFED. The results show that those methods can exactly and easily make feature extraction for the TFED of PD pulse, which can be used to PD pattern recognition and in-depth study of PD mechanism in the electrical insulation system.

Simulating Propagation Characteristic of UHF Signal for PD Monitoring in Transformers by FDTD Technique

Partial discharge (PD) in transformer can be detected on-line by UHF technique. However, the relationship between the UHF signal received by UHF sensors and the PD sources is not very clear. It is very important studying the propagation properties of the PD signal. In this paper, the electromagnetic waves emitted by PD sources are investigated. The finite difference time domain (FDTD) technique is used to model wave transients. The model consists of a tank and a PD source. The wave transients at different time and position are simulated. The results show the propagation transients of the PD signal in the tank and wave propagation attenuates with distance between the source and the sensors.

Study on Oscillating Switching Impulse Voltage Generation for Power Transformer Onsite Test

In this paper, the authors describe an oscillating switching impulse voltage generation method using a power transformer induction principle through which the power transformer itself can produce the oscillating switching impulse voltage in accordance with the requirements of the IEC60060-3 standard. The circuit of the oscillating switching impulse voltage generator is proposed. Estimation for the circuit parameters is also pointed out in this paper. An oscillating switching impulse voltage generator is set up to verify the proposed method. Two power transformers are utilized to generate the oscillating switching impulse voltage. The simulating and experimental results verify that the proposed oscillating switching impulse voltage generation method can generate the oscillating switching impulse voltage accorded with the requirements of the IEC60060-3 standard. The steps and consideration for the oscillating switching impulse generation are suggested in this paper. The power transformer onsite oscillating switching impulse test can be easily carried out by the steps and consideration.

UHF characteristic of PD in oil-paper insulation under DC Voltage

A test system with UHF and UWB detection technologies developed to measure PD under DC voltage is introduced in this paper. According to the typical defects in oil-paper insulation of converter transformer, six corresponding PD models are designed. The test results of PD UWB detection under DC voltage show that the PD current pulses have a very short rise-time, which are capable of radiating UHF signals at frequencies up to at least 300 MHz, and these electrical magnetic waves can be picked up by the UHF sensor. The DBM spectrum analysis is made for all UHF signals of the models, which shows that they differ with various defects, and the maximum amplitudes of the energy distribution against the UHF are also different. These indicate the on-line monitoring for the UHF PD signal under DC voltage should be realized utilizing the narrowband filter with adaptive center frequency.