Yixuan Li

Bio: Yixuan Li is an academic researcher from Southwest Jiaotong University. The author has contributed to research in topics: Partial discharge & Voltage. The author has an hindex of 1, co-authored 3 publications receiving 8 citations.

Partial discharge characteristics of oil-paper insulation for on-board traction transformers under superposed inter-harmonic AC voltages

Abstract: Oil-paper insulation of higher voltage winding and traction winding in on-board traction transformers frequently withstand large AC fluctuation voltages mainly combined with AC and inter-harmonic components. This paper presents the characteristics of partial discharge (PD) of oil-paper insulation under superimposed inter-harmonic AC voltages. The experimental results show that the addition of the inter-harmonic to the fundamental sinusoidal component results in variation of peak voltage and dv/dt along the phase. This superimposed inter-harmonic AC voltages has a significant impact on PD including partial discharge inception voltage (PDIV), breakdown voltage, PD repetition rate, and phase resolved partial discharge (PRPD). In addition, a dynamic characterization method of PD statistical parameters is proposed to determine the developing trend. The critical values and inflection points of PD are obtained. Besides, the theoretical mechanism is expounded to better interpret and understand the influence of inter-harmonic component on PD properties. These methods will provide the dynamic datasets and more information for insulation condition assessment of on-board traction transformer.

The Evolution of Trapping Parameters on Three-Layer Oil-Paper of Partial Discharge Degradation for On-Board Traction Transformers

Abstract: Trap plays an important role in the charge transportation of three-layer oil-paper and the investigation of trap parameters at different degradation degree is of great significance for further research of the partial discharge (PD) mechanism. This article considers the turn-to-turn structure of on-board traction transformer. A defect model of three-layer oil-paper insulation was designed for PD test. The PD degradation samples are selected from the inception of PD to the breakdown of PD to study the characteristics of surface potential decay on degradation samples by using isothermal surface potential decay method. Given that, the trap distribution at different degradation stages was calculated and its influence on PD characteristics was investigated. The result shows that AC fluctuation voltages caused by inter-harmonic has a great influence on PD distribution, surface charge distribution and trap distribution. With the increase of degradation, the decay rate of surface potential increases and the decay rate of AC fluctuation voltages is larger than that pure AC. And also, the number of shallow trap with AC fluctuation voltages is larger than that with pure AC. The rate of free charge increases with the number of shallow trap increasing. More free electrons involves in the process of PD, accelerating the aging rate of three-layer oil paper samples. Therefore, it can be concluded that surface charge and trap distribution are helpful to clarify PD mechanism.

Investigation of Surface Charge Accumulation Characteristics of Insulators in HSS

Abstract: As the key equipment of flexible DC transmission, the operational reliability of DC high speed switch (HSS) is of great significance. However, surface charge gradually accumulated on the insulators under DC voltage when the HSS double-break is in the opening position, which further resulting in the electric field distortion and even discharge. In this paper, a $3\mathrm{D}$ finite simulation model was established on the basis of the operating conditions of HSS. What’ $\mathbf{s}$ more, the worst operation condition of HSS that under the opening position was calculated and analyzed. Then the multi-coupled surface charge accumulated model which considered charge drift and diffusion was established based on the obtained most severe condition. Morever, the electric field distribution and charge distribution of insulators in HSS breaker chamber was computed. The gas-solid charge accumulation characteristics of insulators and its effect on electric field was analyzed. The results show that the surface charge density of nozzle is much greater than the FRP pipe inside the breaker chamber. The obtained results provide a guidance for the design of HSS double-break.

Commutation Scheme of Seven-Level Hybrid-Clamped Converters With Suppressed Deadband-Induced Voltage Spikes

Abstract: A seven-level hybrid-clamped (7L-HC) converter is a promising option for medium-voltage drives due to its low number of devices and the ability to well balance all the capacitors. However, like other hybrid-clamped multilevel converters, the freewheeling paths during deadtime can produce unexpected output voltage levels. The unexpected output voltage can cross a few consecutive output levels, introducing a large dv / dt to the system. To address this problem, in this article, we conduct a detailed study of the mechanism that produces such undesirable voltages and propose a commutation scheme to coordinate the turn- on / off sequence of switches in the 7L-HC converters. The proposed method is based on an easy-to-implement bitwise operation of switching vectors, where the number of switching actions is not increased. The proposed method is validated by both simulation and experimental results.

The Evolution of Trapping Parameters on Three-Layer Oil-Paper of Partial Discharge Degradation for On-Board Traction Transformers

Abstract: Trap plays an important role in the charge transportation of three-layer oil-paper and the investigation of trap parameters at different degradation degree is of great significance for further research of the partial discharge (PD) mechanism. This article considers the turn-to-turn structure of on-board traction transformer. A defect model of three-layer oil-paper insulation was designed for PD test. The PD degradation samples are selected from the inception of PD to the breakdown of PD to study the characteristics of surface potential decay on degradation samples by using isothermal surface potential decay method. Given that, the trap distribution at different degradation stages was calculated and its influence on PD characteristics was investigated. The result shows that AC fluctuation voltages caused by inter-harmonic has a great influence on PD distribution, surface charge distribution and trap distribution. With the increase of degradation, the decay rate of surface potential increases and the decay rate of AC fluctuation voltages is larger than that pure AC. And also, the number of shallow trap with AC fluctuation voltages is larger than that with pure AC. The rate of free charge increases with the number of shallow trap increasing. More free electrons involves in the process of PD, accelerating the aging rate of three-layer oil paper samples. Therefore, it can be concluded that surface charge and trap distribution are helpful to clarify PD mechanism.

The Physical Mechanism of Frequency-Induced Inflection Point for Oil–Paper Insulation Under High-Frequency Square-Wave Voltage

Abstract: High-frequency (HF) oil-immersed transformers with small size and light weight have been paid wide attention by researchers. Its oil–paper insulation frequently withstands HF over-voltages at frequencies of several kilohertz and above, resulting in the too early failure of oil–paper insulation. In order to investigate the effect of HF on oil–paper insulation, this article investigates the partial discharge (PD) characteristics of oil–paper insulation under HF stress. The experimental results show that a frequency-induced inflection point is existed near 10 kHz and the PD parameters of HF voltages is much higher than that ac voltages. Besides, the theoretical mechanism is expounded to interpret and understand the formation process of frequency-induced inflection points more easily. It can be found that the change in frequency causes the change in the charge decay time constant and the residual charge on the insulating surface, resulting in the formation of the frequency-induced inflection point at HF. These results might contribute to determine the characteristics of PDs between HF voltages and ac voltages. It can provide references for the research and production of HF transformers.

Effect of Harmonic Voltage On Partial Discharge Properties of LN 2 /PPLP Insulation for HTS DC Cable

Abstract: In this paper, the effect of harmonic voltage on partial discharge properties of LN2/PPLP (Liquid nitrogen/ Polypropylene-Laminated-Paper) insulation system for high-temperature superconducting (HTS) DC cable is studied. The harmonic components with a frequency from 50 to 350 Hz are superimposed on the DC voltage to analyze the mechanism of the harmonic amplitude and order on partial discharge (PD). The experimental results show that when the V rms of the composite voltage remains consistent, the increase in the proportion of the harmonic amplitude will cause the enhancement of partial discharge amplitude, discharge counts and pulse repetition rate, especially as the amplitude of harmonic component exceeds that of DC voltage. The increase of the harmonic order results in variation of the derivative of the applied voltage with time (dv/dt) along the phase, which further causes the pulse repetition rate to increase. It is concluded that the harmonic components superimposed on DC voltage promote the occurrence of PD, which poses a significant threat to the insulation of superconducting DC cable.

A crest factor-based technique for the analysis of polluted insulator leakage current under harmonically distorted supply voltage

Abstract: High-voltage insulators are one of the essential components of a power network, and regular monitoring of them ensures the reliable operation of the network. Therefore, several research groups have proposed different condition assessment indices by analyzing the measured leakage current of the polluted insulator at different environmental conditions. At laboratory conditions, those indices are developed under pure or nearly sinusoidal voltage waveforms. However, a considerable amount of harmonic distortion is prevalent in the supply voltage in real life owing to the ever-increasing trend in the use of power electronic devices in the power network; as a result, insulators in the power network are immediately subject to this distortion. Therefore, in the presence of distorted supply voltage, the efficacy of these current indicators developed for pollution severity assessment under purely sinusoidal voltage will become dubious. In this work, authors have investigated the impact of distorted supply voltage waveforms on the leakage current of an artificially polluted silicone rubber insulator. The result of this work indicates that in combination with contaminant severity and relative humidity, supply voltage distortion causes significant changes in the measured leakage current. Based on these findings, the authors further proposed a new crest factor-based index to predict the pollution level of a silicone rubber insulator.