This paper presents experimental research on creeping discharge by using cylinder-plate electrode configurations under AC voltages. The process of creeping discharge-caused damage on the oil/pressboard insulation was studied. First, the electric field distribution was achieved by Multiphysics software simulation. Afterwards, the phenomena that occurred in the entire damage process, such as “white smoke”, “white mark”, and “black mark”, were recorded and analyzed. Furthermore, the micromorphology of the oil-impregnated pressboard was observed via scanning electron microscope (SEM). Finally, the inner mechanism of the damage to the oil/pressboard insulation was explored according to the phenomena and the SEM morphologies. Results showed that during damage processes, the high electrical field strength (nearly 25.853 MV/m) at the weak-link point between the cylinder electrode and the pressboard directly caused the incipient discharge. The cavity, moisture, impurity, solid particle, and formation of a gaseous channel all contributed to the development of the damage. The “white smoke” consisted of gases that stemmed from the ionization of oil and evaporation of moisture. The “white mark” was the gas channel and pressboard carbonization was caused by discharge and high temperature, both of which were also the main causes of the emergence of “black mark”. SEM images revealed that the pressboard successively experienced “white solid”, “crack”, and “pitting”, which changed its surface roughness. The distorted electric field caused by gases, solid particles, and pitting further damaged the oil/pressboard. The pitting evolved into the starting point of the electrical trees and gradually led to the final breakdown.

https://journals.tubitak.gov.tr/elektrik/issues/elk-16-24-3/elk-24-3-53-1310-158.pdf

The process of creeping discharge-caused damage on oil/pressboard insulation

The present paper investigates progressive creeping discharge at oil/pressboard interface in power transformer. Using a needle-to-plate electrode configuration, AC constant stress tests were carried out with differently aged pressboard samples. Two distinctly different discharge patterns have been observed, namely chronic creeping discharge and acute creeping discharge. Chronic creeping discharge happens with all pressboard samples, whereas acute creeping discharge happens only with massively-aged pressboard. The results of partial discharge detection, on-line temperature monitoring and gas analysis indicate that the chronic creeping discharge is closely correlated o oil phase, while the acute creeping discharge involves multiple discharge mechanisms. It is also found carbonization crack in pressboard interior is the principal cause of white mark and bubble generation during acute creeping discharge.

Evolution and discharge pattern of creeping discharge at aged oil/pressboard interface

Creepage discharge faults in air on solid insulating material play a vital role in degradation and ageing of material which ultimately leads to breakdown of power system equipment. And electric discharge decompose air in to its byproducts such as Ozone and NOx gases. By analyzing air decomposition gases is a potential method for fault diagnostic in air. In this paper, experimental research has been conducted to study the effect of creepage discharge on rate of generation of air decomposition by-products using different insulating materials such as RTV, epoxy and fiberglass laminated sheet. Moreover XRF analysis has been done to analyze creepage discharge effect on these insulating materials. All experiments have been done in an open air test cell under constant temperature and pressure conditions. While analysis has been made for low and high humidity conditions. The results show that the overall concentration of air decomposition byproducts under creepage discharge in low humidity is 4% higher than concentration measured in high humidity. Based on this study a mathematical relationship is also proposed for the rate of generation of air decomposition by-products under creepage discharge fault. This study leads to indirect way for diagnostic of creepage discharge propagation in air.

Experimental Study on Air Decomposition By-Product Under Creepage Discharge Fault and Their Impact on Insulating Materials

Partial discharge (PD) measurement and recognition is of great importance to assess the health condition of power transformers. Surface discharge is a typical PD event often happened within oil-paper insulation system. In this work, surface discharge over oil/pressboard interface is observed in laboratory using a digital PD measurement system. Accelerated aging experiment is performed to obtain aged oil and pressboards with different aging degrees. Then, the influences of insulation aging, electrode size as well as applied voltage on surface discharge characteristic are analyzed. Experimental results relevant to surface discharge measurements performed on lab-scale rod-to-plane electrode under different circumstances are presented with the purpose to spotlight surface discharge distributions aimed at identifying the basic nature of surface discharge over oil/pressboard interface. The obtained results would provide useful information to condition diagnosis of transformers.

Experimental investigations on surface discharge characteristics over oil/pressboard interface based on a rod-to-plane electrode

A comparative study of the effect of surface discharge on the impulse breakdown voltage of oil-impregnated pressboard insulation

To research the creepage performance of high moisture pressboard, possible high moisture of pressboard in service power transformer has been analyzed, a creepage discharge simulation and test equipment and measurement system have been set up in laboratory, which were consisted of creepage discharge model, test tank, UHF sensor, gas-phase chromatographic instrument and partial discharge detector. Creepage discharge growth from inception to stop has been simulated by raised voltage step by step up to 50kV, ultra high frequency signal, apparent discharge magnitude and dissolved gases in oil have been measured in the growth. The results show that moisture of pressboard in part may reach 7% in service power transformer; creepage discharge inception voltage has a remarkable decrease under this high moisture; creepage discharge appeared at small oil gap firstly and bright spark can been observed; creepage discharge was intermittent and even stop at some time; lots of bubbles were generated in the creepage growth; C2H2 and H2 generated by creepage discharge was very high.

Creepage discharge performance for high moisture pressboard

To research the diagnosis method based ultra high frequency (UHF) signal and dissolved gases analysis (DGA) in oil of severity degree for oil/pressboard insulation surface discharge, a surface discharge simulation and test equipment and measurement system have been set up in laboratory, which were consisted of surface discharge model, test tank, UHF sensor, gas-phase chromatographic instrument and partial discharge detector. Surface discharge growth from inception to breakdown has been simulated, test voltage was raised step by step, ultra high frequency signal and dissolved gases in oil have been measured in the growth. The results show that severity degree of surface discharge can be divided tree stages: inception stage, growth stage and critical stage according to statistics spectrum of the ultra high frequency signal; ultra high frequency signal and dissolved gases in oil both can indicate the oil/pressboard surface discharge severity degree, however, dissolved gases analysis method was not as sensitive as ultra high frequency method; C2H2/total hydrocarbon generated by oil/pressboard surface discharge was more than 10%; Surface discharge was in critical stage when C2H2/total hydrocarbon value more than 50% and raised rapidly.

Jianfeng Xu

Papers

Creepage discharge performance for high moisture pressboard

Diagnosis of severity degree for oil/pressboard insulation surface discharge

Experimental research on the evolution of creepage discharge in aged pressboard