Brief Introduction to Transformer Winding Voltage Distribution under Different Types of Lightning Impulse Voltage

Author： GOZ Electric Time：2024-10-13 09:31:33 Read：15

As one of the important components of the power system, the safe and reliable operation of power transformers ensures the normal operation of the power grid. In addition to the long-term power frequency voltage, power transformers are also subject to the operation overvoltage caused by the action of other equipment on the line and the lightning impulse overvoltage caused by lightning strikes. Since the amplitude of the lightning impulse overvoltage is high, the equivalent frequency is high, and the wave front is particularly steep, when it acts on the transformer winding, the end of the transformer winding will experience an abnormally high voltage in a very short time. Unlike the power frequency and operation impulse voltage, the distribution of the lightning impulse voltage on the transformer winding is nonlinear, which will lead to a high concentration of the local electric field of the transformer, which will easily lead to damage to the transformer insulation. Therefore, lightning overvoltage protection plays a vital role in the safety of the transformer, and the on-site impulse withstand voltage test has become an important part of electrical engineering.

At present, the 1.2/50μs standard lightning impulse voltage (Standard lightning impulse, SLI) is commonly used as the lightning impulse withstand voltage test voltage waveform of the transformer. However, the application of lightning impulse test on site is limited because the equipment that generates the double exponential impulse voltage is large, difficult to move, and complex to install. Studies have shown that the lightning impulse voltage to which the transformer is subjected during actual operation usually manifests itself in the form of oscillation, with an oscillation frequency of about tens of kilohertz to several kilohertz. Therefore, the standard introduced by IEC in 2005 recommends the use of oscillating lightning impulse voltage (OLI) as the on-site lightning impulse withstand voltage test waveform, which has the advantages of high generation efficiency, suitability for on-site use, and close to the actual action waveform of the equipment.

Due to the nonlinear distribution of impulse voltage in transformer windings, the voltage distribution characteristics of different types of lightning impulse voltage in transformer windings are significantly different. Therefore, mastering the distribution characteristics and changing laws of different types of lightning impulse voltage in transformer windings is of great significance for the operation and design of transformers and the implementation of on-site lightning impulse tests.

1. Simulation study of transformer winding voltage distribution

The transformer used in the simulation has a rated voltage of 220kV, which is an end-out line and an inner shield-continuous winding. The winding consists of 72 coils, and the first 18 coils are wound using the inner shield method to increase the longitudinal capacitance of this part to adjust the impulse voltage distribution of the winding; the remaining coils are wound using the uniform continuous method. A tap is drawn from every two coils of the winding to facilitate the experimental study of the voltage distribution.

(1) Calculation of winding equivalent RLC network parameters

Based on the geometric design parameters of the winding, the author uses the theoretical formula calculation method to calculate the circuit network parameters of the winding, and then simulates the winding voltage distribution under different types of lightning impulse voltages. The winding equivalent circuit network is composed of several identical RLC units in cascade. For the simulation of the winding, every two coils are selected as one unit, totaling 36 units.

(2) Winding voltage distribution under standard lightning impulse

The standard lightning impulse used in the simulation is an ideal waveform with a wave front time of T = 1.2μs and a wave tail time of T = 50.0μs. Under the action of the standard lightning impulse voltage, the voltage distribution on the transformer winding is significantly distorted. Its waveform no longer follows the shape of the input voltage, but oscillates. The closer to the end of the winding, the lower the voltage amplitude is. At the same time, the natural oscillation of the winding is more obvious.

2. Conclusion

The authors of this paper studied the distribution characteristics of standard lightning impulse voltage and oscillating lightning impulse voltage in transformer windings. The research results show that for standard lightning impulse voltage and oscillating lightning impulse voltage with the same wave front time and wave tail time, there are obvious differences in the distribution inside the winding. Compared with the standard lightning impulse, the oscillating lightning impulse has a stricter assessment on the insulation of the winding end, but a relatively loose assessment on the main insulation of the winding.