Wu Guangning

Microscopic degradation mechanism of polyimide film caused by surface discharge under bipolar continuous square impulse voltage

Polyimide (PI) film is an important type of insulating material used in inverter-fed motors. Partial discharge (PD) under a sequence of high-frequency square impulses is one of the key factors that lead to premature failures in insulation systems of inverter-fed motors. In order to explore the damage mechanism of PI film caused by discharge, an aging system of surface discharge under bipolar continuous square impulse voltage (BCSIV) is designed based on the ASTM 2275 01 standard and the electrical aging tests of PI film samples are performed above the partial discharge inception voltage (PDIV). The chemical bonds of PI polymer chains are analyzed through Fourier transform infrared spectroscopy (FTIR) and the dielectric properties of unaged and aged PI samples are investigated by LCR testers HIOKI 3532-50. Finally, the micro-morphology and micro-structure changes of PI film samples are observed through scanning electron microscopy (SEM). The results show that the physical and chemical effects of discharge cut off the chemical bonds of PI polymer chains. The fractures of ether bond (C—O—C) and imide ring (C—N—C) on the backbone of a PI polymer chain leads to the decrease of molecular weight, which results in the degradation of PI polymers and the generation of new chemical groups and materials, like carboxylic acid, ketone, aldehydes, etc. The variation of microscopic structure of PI polymers can change the orientation ability of polarizable units when the samples are under an AC electric field, which would cause the dielectric constant e to increase and dielectric loss tan δ to decrease. The SEM images show that the degradation path of PI film is initiated from the surface and then gradually extends to the interior with continuous aging. The injection charge could result in the PI macromolecular chain degradation and increase the trap density in the PI polymer bulk.

An optimized transmission line model of grounding electrodes under lightning currents

In this paper, an optimized transmission line model (OTL) for modeling transient behavior of grounding electrodes under lightning currents is presented. The soil ionization effect is considered in OTL, and all electromagnetic couplings between different parts of grounding electrode are also considered by selecting the size of segment conductor properly and calculating the mutual coupling parameters between segment conductors accurately. Comparing with the traditional transmission line model, the optimized model can be used to accurately predict the effective length and transient potential rise (TPR) of grounding electrodes. Transient behaviors of grounding electrodes are simulated by OTL and the results are in good agreement with those of the electromagnetic model proposed by Grcev, and experiment results performed by Electricité de France and Geri. Furthermore, non-uniform discharging phenomenon of grounding electrode under lightning current is discussed, and the effective lengths of horizontal grounding electrode under lightning currents are presented.

Partial discharge characteristics of interturn insulation used for inverter-fed traction motor under bipolar impulses

Inverter-fed traction motors, which are employed for electric-multiple-units (EMUs), are subjected to Pulse Width Modulated (PWM) impulse voltage. Impulse voltage has fast rise/fall time and high repetition frequency. The failure mechanism of insulation under impulses is different from that of standard 50 Hz sinusoidal condition. Partial discharge (PD) characteristics (discharge number, mean discharge quantity and phase resolved partial discharge (PRPD) pattern) with different frequencies and different rise times of impulse voltage were investigated. The results reveal that discharge number and mean discharge quantity increase, while PRPD pattern moves to the zero of impulse voltage with decrease of rise time. Discharge number and PRPD pattern follow that trend with increase of frequency, the value of mean discharge quantity is similar. Memory effect of the deposited space charges on PD was studied. PRPD pattern moves to the zero of bipolar impulse voltage due to the role of space charges, with increase of frequency and decrease of rise time. It is revealed that available discharge time is prolonged and overvoltage across the cavity increases with augment of frequency, which results in discharge number increasing. Under the condition of shortening the rise time, with the same statistical time lag tL, ΔV would increase if dV/dt is higher, which results in enhancement of discharge quantity. Discharge number increases with reduction of rise time, because the decreased multiple of td is lower than the increased multiple of dV/dt. Study of PD characteristics and role of space charges is helpful to the exploration of the destructive mechanism of insulating materials under bipolar impulse.

Insulation life-span models for electrical and thermal aging under continuous high square impulses voltage

Stator insulation of inverter-fed motors experiences continuous impulses with significant harmonics and transients, which may result in early failure. There are many differences in insulation aging mechanism between under continuous impulses and sine waves. In order to test insulation properties under continuous impulses, a new insulation aging test system is used to simulate the aging effect of converter on the winding insulation in the inverter-fed motors. The influences of multi-stress including impulse amplitudetemperature, impulse frequency-temperature and impulse frequency-amplitude on insulation lifetime are investigated. And the test data under different conditions are analyzed by the statistic analysis method of Weibull distribution. Finally, insulation life models for multi-stress is developed based on the above test results, which is helpful for the insulation design of inverter-fed motor.

The measurement and simulation research of electrified railway harmonic

Harmonic pollution in modern power system is a major problem. In order to analyze the influence of harmonic caused by electric locomotive on power system, taking Ynd11 transformer connection of a traction station as a example, this paper presents a dynamic model of electric locomotive and traction power supply system, and conducts simulation analysis of the injection of 110 kV side of the imbalance harmonic current, voltage. Comparing the simulation results with the actual measurement data at a traction station is almost the same, which verifies the correctness of the model. Finally, the paper simulates the different locomotive traction power impact on the power system, including the current harmonics, voltage harmonics, negative sequence voltage changes. The results show that harmonic current and voltage increases as power increasing, but it changes trend complicated, negative sequence voltage changes in a nearly line, which presents a reliable basis for the further study of electric locomotive on power system.

Study of partial discharge characteristics at repetitive square voltages based on UHF method

Due to overvoltage produced by inverter output, inverter-fed motor insulation systems often experience fast electrical aging process, especially when partial discharge (PD) is incepted. Before putting into use, the PD detection should be performed on inverter-fed motors at repetitive square voltages to avoid the PD caused insulation deterioration when the motors are collected to inverters having specific characteristics. However, unlike PD tests at AC/DC voltages proposed in IEC 60270, the PD detection at repetitive square voltages is much more complex because of serious interference generated by impulse generator. To solve the problem, ultra-high frequency (UHF) method seems recommendable for its preferable signal-to-noise ratio (SNR). The chief aim of this study is to investigate PD pulse and statistical characteristics of turn-to-turn insulation for inverter-fed traction motors. A square-shaped Archimedes antenna, specially designed for the PD detection at repetitive square voltages of fast rise times, was used to perform PD tests on turn-to-turn insulation models. Time and frequency analysis results indicate that energy component of generator disturbance and PD pulses are mainly distributed in the 0–0.5 GHz and 0.6–1.5 GHz range, respectively. Based on the results, suitable filter was designed for power disturbance suppression. Additionally, resorting to the sensor unit (i.e. antenna and filter) and the PD test system, the PD statistical features at square voltages of different frequencies were obtained. Experimental results show that higher frequency will give rise, statistically, to PD of lower magnitudes distributing at smaller phases. A reasonable interpretation of this phenomenon was presented. Lastly, according to the PD statistical features, some suggestions for the PD detection system design, generator parameter optimization and the PD pulse extracting were given. The results of this work would be beneficial to the increase of the sensitivity when performing the PD detection on insulation systems for inverter-fed motors at repetitive square voltages and thus, improving the reliability of inverter-fed motors.

Research on electrical aging lifetime model of insulation material under high voltage square impulses for inverter-fed motors

High voltage square impulses from converter can make stator insulation of inverter-fed motors damaged much earlier. There are many differences in insulation aging mechanism between under continuous impulses and under sine waves. In order to test insulation properties under continuous impulses, a new insulation aging test system is used to simulate the aging effect of converter on the winding insulation in the inverter-fed motors. The influences of impulse amplitude, frequency, on insulation lifetime are investigated by using polyimide film. Finally, an insulation electrical aging lifetime model is developed based on the test results.

The Influence of Organic Acid on Thermal Aging of Power Transformer Oil-paper Insulation

To research the influence of one of the aging product (organic acid) of cellulose on oil-paper insulation, low molecular weight of the carboxylic acid (formic, acetic) and high molecular weight of the carboxylic acid (stearic) have been added to new electric insulating oil to arrive at a neutralization value of 0.3mgKOH/g, the initial moisture content in insulation paper have been controlled at 0.5% and 2.75%. The distribution characters of organic acid in oil-paper insulation and the influence of organic acid on thermal aging of oil-paper insulation at 70°C, 90°C and 110°C have been investigated. Experimental results show that formic and acetic could be easily absorbed by insulation paper while stearic could easily dissolve in electric insulating oil. There is no significant influence of temperature and initial moisture content on the distribution characters of organic acid in oil-paper insulation. Low molecular acid would work together with moisture to accelerate the aging speed of insulation paper.

Effect of repetitive square voltage frequency on partial discharge features

The inverter-fed motor is a key component of environmental-friendly hybrid cars, high-speed trains, and other industrial applications. After the widespread use of inverter-fed motors, the repetitive impulse surges from inverters were found to cause new insulation problems in inverter-fed motors that were linked to premature insulation failure. The partial discharge (PD) induced by the overvoltage inner stator windings was found to be the root cause of insulation reliability problems. To mitigate this problem, PD detection and life testing at repetitive square voltages should be performed on new types of insulation models and entire motor insulation systems. The obtained partial discharge inception voltage (PDIV) and lifetime of insulation in the tests can be used to evaluate the insulation status of inverter-fed motor insulation systems and thus to improve the insulation designs. At AC voltage conditions, the voltage frequency can significantly affect PD activity. At repetitive square voltage with rapid rise time, the influence of the square voltage frequency on PD activity should be investigated (1) to suggest values for the repetitive square voltage frequency selection for the PDIV and lifetime tests, and (2) to increase the signal-to-noise ratio when extracting PD pulses from strong disturbances generated by the operating switch of impulse generators. Therefore, the main purpose of this paper was to investigate the effects of repetitive square voltage frequency on PD events. Insulation models of both Type I (low-voltage inverter-fed motor) and Type II (high-voltage inverter-fed motor) were used to obtain single-PD and multi-PD statistical features, respectively. Experimental results indicated that higher frequencies caused less PD events with lower PD magnitudes and shorter delay times in one square voltage cycle. This phenomenon was attributed to the different surface charge decay ratios and the initial electron emission probabilities resulting from different square voltage frequencies. Finally, a number of considerations for PD measurements on inverter-fed motors were provided to improve the sensitivity of the PD test system and to obtain high objective insulation lifetime test results, which are used to assess the insulation status of inverter-fed motors.

Notice of Retraction Study on the test of oil-paper insulation by frequency domain spectroscopy

This paper used the dielectric spectrum method (Frequency Domain Spectroscopy), with laboratory equipment and facilities, built the experimental test platform for oil-paper dielectric response test. Simulated the actual operation of the transformer, and accelerated thermal aging tests by laboratory simulation of transformer oil-paper insulation aging, studied the dielectric spectra at different temperatures and different conditions of water content variation; analysis the dielectric properties of oil-paper insulation, summed up the relationship between the effect factors and the frequency dielectric spectrum, so it can supply an important basis for the correct diagnosis of transformer insulation condition, assessment of transformers remaining life and improving power supply reliability.