Application Cases

Experiment Title: Space Charge Measurement System

Testing Equipment:Voltage amplifier, electrode system, function generator, high-voltage high-frequency pulse source, oscilloscope, computer, etc.

Experimental Process:

Figure 1: Structural Diagram of Space Charge Measurement System

In this study, the space charge in oil-paper insulation was measured at room temperature. The detection mechanism of space charge inside the sample used the PEA (Pulsed Electroacoustic) method. The detection of space charge at different phases under an alternating electric field was based on the AEPS (Automatic Equal Phase Shifting) principle. The simplified diagram of the measurement system is shown in Figure 1.

The voltage amplifier can provide an alternating or direct current electric field, while the pulse source provides the excitation pulses. Both the alternating/direct current electric field and the pulse electric field are coupled onto the sample under test. In the electrode system, the upper electrode is a semiconductor electrode made of a blend of ethylene-vinyl acetate copolymer and conductive carbon black, with a thickness of 1.8 mm and a volume resistivity of less than 100 Ω•m. The lower electrode is an aluminum electrode.

Figure 2: Space Charge Response to Different Pulse Excitations Without Polarization Field

In space charge measurements, the space charge response of the sample under different external polarization fields is of interest. However, if the pulse electric field used as the excitation source is too high, it can significantly affect the space charge response and introduce unnecessary errors. For example, with a 270-μm-thick oil-impregnated insulating paper sample and no external polarization field applied, the space charge responses obtained when applying 180 V and 500 V pulse voltages to the sample are shown in Figure 2.

Figure 3: Space Charge Response at the Instant of Applying Voltage Under a Direct Current Electric Field of 2 kV/mm and a Pulse Voltage of 180 V

At a position 235 μm from the lower electrode, almost no charge peak was observed when the pulse voltage was 180 V, while a charge peak with a peak value of -0.002 V was observed when the pulse voltage was 500 V. At a position 505 μm from the upper electrode, due to the severe dispersion and attenuation of acoustic wave signals in oil-impregnated insulating paper, no distinct charge peaks were observed under both pulse voltage values. Although the charge peak caused by the pulse electric field is not large, it can have a non-negligible effect under low field strength. As shown in Figure 3, the space charge response at the instant of applying voltage to oil-impregnated insulating paper under a direct current electric field of 2 kV/mm and a pulse voltage of 180 V has a charge peak with a peak value of -0.01 V at the lower electrode position. If a pulse excitation with a voltage value of 500 V were used, the charge peak caused by the pulse electric field would increase this peak value by 20%, seriously affecting the analysis of the space charge response under low field strength.

In this study, the oil-impregnated insulating paper samples used for space charge measurement experiments included samples that had not been aged, samples aged for 48 hours and 96 hours at a thermal aging temperature of 120°C (hereinafter referred to as thermally aged 48 h and thermally aged 96 h, respectively). During the experiment, the samples to be tested were soaked in insulating oil corresponding to the aging conditions for space charge measurement, with a pulse frequency of 2010 Hz and a pulse voltage value of 180 V selected.

1. Direct Current Electric Field

During measurement, the polarization field strength was increased stepwise to 0.5 kV/mm, 1 kV/mm, 3 kV/mm, 5 kV/mm, 7 kV/mm, 10 kV/mm, 15 kV/mm, 20 kV/mm, 30 kV/mm, and 40 kV/mm. At each field strength, a corresponding 1800-s short-circuit measurement was performed after polarization for 3600 s. During the polarization and short-circuit processes, the space charge response waveform was measured every 3 s, and the oscilloscope averaging count was set to 4000.

2. Power Frequency Alternating Current Electric Field

The settings for the polarization field strength during measurement were similar to those for the direct current electric field, increasing stepwise to 0.5 kV/mm, 1 kV/mm, 3 kV/mm, 5 kV/mm, 7 kV/mm, 10 kV/mm, 15 kV/mm, 20 kV/mm, 30 kV/mm, and 40 kV/mm. At each field strength, a corresponding 1800-s short-circuit measurement was performed after polarization for 3600 s. During the polarization and short-circuit processes, the data were saved using the fast acquisition function of a high-performance oscilloscope, with the acquisition count set to 4020. Since it takes some time for the oscilloscope to save the data, the space charge response waveform and the corresponding instantaneous voltage signal were measured every 10 minutes. After the data were saved, they were transferred to a computer offline and phase identification was performed based on the AEPS principle to obtain the space charge response waveforms at different phases.

Experimental Results:

Similar to the case of power frequency alternating current polarization field, a corresponding 1800-s short-circuit measurement was performed after polarization for 3600 s at each field value. During the polarization and short-circuit processes, the data were saved using the fast acquisition function of a high-performance oscilloscope, with the acquisition count set to 4020. The space charge response waveform and the corresponding instantaneous voltage signal were measured every 10 minutes. After the data were saved, they were transferred to a computer offline and phase identification was performed based on the AEPS principle to obtain the space charge response waveforms at different phases.

Recommended Voltage Amplifier: ATA-2161

Figure: ATA-2161 High-Voltage Amplifier Specifications

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