Analysis of voltage distribution characteristics of air-core reactor under impulse voltage

Author： GOZ Electric Time：2024-10-01 09:32:41 Read：34

Aiming at the problem that the voltage distribution of the internal winding of the air-core reactor under the lightning overvoltage cannot be quantitatively calculated, a high-frequency impedance network equivalent analysis model suitable for the air-core reactor structure is established. According to Kirchhoff's basic law, a calculation method for transient and steady-state voltage distribution under lightning voltage is proposed, which can calculate the voltage distribution under lightning voltage in the reactor. Taking the actual parallel air-core reactor as an example, the voltage distribution under the lightning voltage calculated and the voltage distribution under the actual test are analyzed. According to the calculation results, it is found that the voltage and unevenness coefficient of the 1-2 turns of the coil in the inner and outer envelopes are both large, which is mainly related to the voltage clamping caused by the inner and outer voltages to the ground.

Keywords: air-core reactor; lightning overvoltage; transient voltage; impedance distribution network

As an important power equipment, the reactor plays a role in compensating stray capacitive current, limiting the closing inrush current, limiting the short-circuit current, and filtering, smoothing, starting, lightning protection, and wave blocking for circuits and equipment in the power system. In the actual operation of air-core reactors, they are not only subjected to the impact of power frequency overvoltage, but also to the impact of lightning overvoltage. Lightning impulse overvoltage can quickly rise to its voltage peak in a short time. From the voltage waveform of lightning voltage, it can be seen that the voltage waveform is very steep and almost rises in a straight line during the wavefront time. When lightning overvoltage acts on the reactor, the reactor needs to withstand extremely high-amplitude voltage in a short time, which will not only damage the insulation structure of the power reactor, but also seriously endanger the stable operation of the power grid.

Under the action of impulse overvoltage, electromagnetic induction will be generated between the coils of the air-core reactor, and an oscillation process will occur inside the coil. The analysis of this process can not only obtain the initial distribution of voltage on the reactor coil winding under lightning voltage impact, but also calculate the voltage difference between each winding turn, which can provide reference data for selecting parameters such as insulation thickness and airway thickness during the design and manufacturing process, so as to design the optimal reactor insulation structure that meets the insulation margin requirements.

Since the number of air-core reactors of different voltage levels is different, and the number of parallel turns of the coils in each package is large, and there are parallel windings, the calculation results after modeling using simulation software are very different from the actual empirical values, and the voltage distribution parameters cannot be effectively obtained. Therefore, it is of great engineering significance to accurately and quickly obtain the voltage distribution characteristics of the reactor under lightning voltage impulse through effective numerical calculation methods.

By analyzing the equivalent impedance model of the air-core reactor under high-frequency voltage, the initial transient capacitance distribution model and the steady-state impedance distribution model are established respectively, and the corresponding node voltage iterative calculation method is proposed to realize the voltage distribution calculation of each layer of coils inside the reactor under lightning voltage. The main conclusions are as follows.

1) When calculating the inter-turn capacitance of the reactor coil, the influence of the insulation inside the adjacent corners between the turns on the inter-turn capacitance needs to be considered; the stray capacitance to the ground can be expressed by calculating the stray capacitance to the ground of the air insulation part outside the reactor.

2) Affected by the skin effect, the AC resistance value increases, and the increase phenomenon is more obvious under high frequency conditions, and the AC resistance value is much larger than the DC resistance value.

3) The initial voltage distribution unevenness coefficient is greater than the voltage distribution during stability, and the voltage distribution unevenness phenomenon becomes more serious with the increase of the height above the ground. The maximum voltage difference occurs between the turns at the first end of the reactor envelope.

4) The turn-to-turn voltage difference and unevenness coefficient of the innermost and outermost layers of the air-core reactor are large, and the rest are relatively small.

The proposed calculation method for the internal voltage distribution of the reactor under lightning voltage can effectively calculate the maximum voltage position and the impulse voltage unevenness coefficient of air-core reactors of different capacities and types under lightning voltage, and can evaluate the reliability of the turn-to-turn insulation based on its unevenness coefficient and the maximum amplitude of the lightning voltage that may occur, which provides a certain theoretical basis for manufacturers to optimize the design of the reactor structure and consider the insulation margin.