Test verification of different types of lightning impulse voltage distribution

Author： GOZ Electric Time：2024-10-15 09:51:47 Read：19

1. Test platform

In order to further study the distribution characteristics of different types of impulse voltage waveforms in actual transformer windings, a test platform for measuring voltage waveforms at different positions inside transformer windings was built, as shown in Figure 4.

Figure 1

The lightning impulse voltage is generated by charging the main capacitor C0 to a certain voltage value U through the half-wave rectifier circuit of the industrial frequency AC power supply, and then through the rapid conduction and disconnection of the ICBT control circuit, the voltage on the main capacitor C0 passes through the wave head resistor Rf, the wave tail resistor Rt, and the wave modulation inductor La. And the wave modulation capacitor C1 discharges to the head end of the winding model to generate a lightning impulse voltage waveform that meets the standard requirements. Adjusting the value of resistance and inductance can generate oscillating lightning impulse voltages with different waveform parameters, and removing the wave modulation inductor can generate a standard lightning impulse voltage waveform.

The winding model winding simulates the actual structure of the high-voltage winding of a 220kV transformer. The design adopts an inner screen continuous winding, with a total of 72 coils. Among them, the shielded winding has 18 sections, the first 16 sections span four screens, 2 sections span two screens, and the rest are continuous windings. In order to measure the oscillating lightning impulse distribution between each cake, each double cake unit of the wire cake is led out to the outside of the oil tank through the epoxy resin plate, numbered 1~36.

Since the winding model uses air insulation and the equivalent population capacitance is small, the method of parallel compensation capacitor is adopted. The parallel capacitor (0.5nF) is used as the modulation capacitor at the population of the winding model to increase the inlet capacitance of the winding model and obtain the oscillating lightning impulse voltage that meets the standard requirements. The lightning impulse voltage is applied to the head end of the winding, and the end of the winding is grounded through a 1Ω damage resistor to simulate the actual situation of the winding lightning impulse test. The voltage waveform of each terminal to the ground is measured in turn by an oscilloscope (TEK4104, bandwidth 1GHz, sampling rate up to 5Gsamples/s), and the potential distribution of different types of impulse voltages inside the winding model can be obtained.

2. Time domain and frequency domain waveforms of impulse voltage inside the winding

By changing the wave head, wave tail resistance and wave modulation inductance values in the modulation circuit, the standard lightning impulse and oscillating lightning impulse voltage waveforms are obtained. In order to facilitate the comparison of the impact of waveform oscillation on the voltage distribution in the winding, the wave front time and wave tail time of the two waveforms are basically the same.

The time domain and frequency domain waveforms of the voltage inside the transformer winding of the standard lightning impulse are shown in the figure (omitted). When the standard lightning impulse voltage is applied, the voltage inside the winding is distorted, showing continuous oscillation. The oscillation frequency is mainly 87kHz. The middle winding oscillates more violently than the windings at both ends, and the overall waveform shows an oscillation attenuation trend. The oscillation of the voltage inside the winding indicates that the natural frequency of the transformer winding is 87kHz. The transformer winding is resistive near this frequency, inductive below this frequency, and capacitive above this frequency, and a series of resonant peaks will appear in the high frequency band. The spectrum of the voltage of each pie of the winding is a single peak, with a peak near the natural frequency, and the frequency component of the peak in the middle of the winding is greater than that at both ends of the winding.

The time domain and frequency domain of the voltage inside the transformer winding of the oscillating lightning impulse are shown in the figure (omitted). When a 280kHz oscillating lightning impulse voltage is applied, the voltage waveform in the winding also oscillates to varying degrees. The oscillation frequency of the winding near the head end is mainly 280kHz at the beginning, which is consistent with the oscillation frequency of the externally applied voltage. Then, when the externally applied voltage enters the non-oscillation stage, the oscillation frequency of the middle winding is close to the natural frequency (87kHz); except for the first two peaks, which are basically the same as the frequency of the externally applied voltage, the rest of the oscillation frequency of the middle winding is basically the same as that under the action of the standard lightning impulse, mainly the natural frequency of the winding. There are two peaks in the winding spectrum, one is the oscillation frequency of the externally applied voltage of 280kHz, and the other is the natural frequency of the transformer of 87kHz. The 280kHz component of the voltage waveform at the head end of the winding is larger, while the 87kHz component in the middle is larger.

3. Distribution characteristics of impulse voltage inside the winding

The maximum values of the voltage waveform at different taps can be statistically analyzed to obtain the distribution characteristics of the voltage peaks at different positions of the winding. The voltage amplitude distribution of different types of lightning impulse voltages in the winding is shown in the figure (omitted), and all voltage peaks are normalized.

As the winding position moves downward, the oscillation voltage peak of the standard lightning impulse decreases slowly due to the natural frequency of the winding, while the voltage peak of the oscillating lightning impulse decreases rapidly along the winding. The voltage peak of the standard lightning impulse is generally greater than the voltage peak of the oscillating lightning impulse. Therefore, from the perspective of the main insulation, the external voltage amplitude should be appropriately increased when conducting the oscillating lightning impulse voltage test.

For the inter-turn insulation of the winding, the voltage difference between adjacent turns determines the voltage that the inter-turn insulation bears. Under the oscillating lightning impulse voltage, the harm to the longitudinal insulation of the winding head end is more serious, and the waveform distortion caused by it is greater than that of the standard lightning impulse voltage. For the voltage gradient at the end of the winding, the standard wave is more stringent. This is because when the oscillation frequency is high, the voltage peak and gradient at the end of the winding are both smaller than the measurement results under the standard wave, so the oscillating wave will be relatively loose in the assessment of the insulation between the turns and cakes at the end of the winding.