Authors: Yuanhang Yao
Source: Institutional Repositories DataBase (IRDB) (article, 2025-03-25)
Field: Materials Science — Materials Chemistry

Elucidation of Impulse Voltage Degradation Mechanism in Cast Resin Transformers Cast resin transformers (CRTrs), characterized by windings insulated with epoxy resin, are widely used in modern power distribution systems due to their superior performance. However, an increasing number of partial discharge (PD)-related electrical failures have been reported involving CRTrs operating beyond 20 years, underscoring the need for an in-depth investigation into their long-term degradation mechanisms. In particular, the aging mechanisms under high impulse voltages, such as those caused by lightning or switching overvoltages, remain insufficiently understood, and the lack of reliable diagnostic tools complicates the prediction of remaining service life and maintenance planning. This dissertation addresses these challenges through experimental studies aimed at elucidating the insulation degradation mechanisms induced by impulses in CRTrs. Chapter 2 investigated PD behavior in an aged 22 kV CRTr subjected to multiple impulse voltages of varying magnitudes, polarities, and cycles. PD inception voltage, PD maximum charge, and phase-resolved partial discharge patterns were analyzed. Results show that insufficient impulse magnitudes did not significantly accelerate insulation degradation, prompting further investigation. Chapter 3 studied systematically investigates the PD behavior and aging characteristics of CRTr windings under multiple bipolar impulse. A higher-magnitude impulse voltage application and impulse PD detection systems were developed to study impulse PD generation, and development, using a high-frequency current transformer. Impulse PD magnitude was precisely calculated by the current integration method. Results indicate that higher-magnitude impulse voltages cause impulse PD activity in the windings. A three-stage mechanism, space charge accumulation, residual charge deposition, and breakdown, is proposed to describe the degradation process. Impulse PD-induced space charge accumulation and residual charge deposition significantly accelerate insulation aging and cause irreversible degradation. Chapter 4 extended the investigation to twisted-pair samples coated with epoxy resin, simulating turn-to-turn insulation. AC PDIV and capacitance of the twisted pair samples were measured before and after bipolar impulse applications. Experiments showed that changes in AC PDIV and capacitance indicate aging trends. Microscopic analysis revealed the existence of air voids, a key indicator of insulation degradation, further supporting insulation degradation mechanisms induced by an air cavity. In Chapter 5, a polyethylene terephthalate film model with an artificial void was designed. The role of residual charges in the subsequent discharge was elucidated by using surface charge mapping method. Observations of surface morphology changes, and AC PD characteristics were analyzed to provide insights into the mechanisms of insulation degradation. Experiments demonstrated that residual charges influence PD characteristic, with bipolar impulses accelerating aging more effectively than unipolar impulses. Surface charge mapping quantitatively assessed residual charge accumulation and their relationship with impulse PDs, revealing its influence mechanism on the development of and the subsequent PD behaviors. A decrease in the steady PDIV was observed due to impulse PD exposures. This research concludes that impulse PD induces irreversible insulation degradation through polymer chain scission and surface erosion, decreasing PDIV and increasing vulnerability to breakdown. The main contributions and innovations of this work are: 1. Development of a PD detection method under high-magnitude impulses. 2. Proposal of a three-stage degradation mechanism driven by space charge and residual charge dynamics. 3. Introduction of surface charge mapping to quantify residual charges within voids and their effects on the subsequent PD behavior. 4. Elucidation of degradation mechanism caused by impulse PDs through molecular chain scission and surface erosion, leading to irreversible insulation degradation. The outcomes contribute to paving the way for the accelerated aging tests in the windings of the CRTr by bipolar impulse voltages, emphasizing the dangers of bipolar switching overvoltages or lightning overvoltages on the insulation status of the transformers in the power systems, clarifying the hazards of the presence of air cavities on solid insulation transformers, proposing residual charge and space charge as an indicator for insulation monitoring, guiding the development of advanced diagnostic methods based on the measurement of the steady PDIV to predict the operational lifetime of CRTr and enhance power system reliability. Impulse (physics) ; Transformer ; Voltage ; Degradation (telecommunications) ; Mechanism (biology) High voltage insulation and dielectric phenomena ; Electrostatic Discharge in Electronics ; Magnetic Properties and Applications