By hvhipot 
Why conduct series resonant withstand voltage test on transformers
Firstly, the capacity of the testing equipment required for series resonance testing of high voltage and large capacity equipment is increasing. Conventional power frequency withstand voltage methods often cannot meet the needs of on-site testing, so the series resonance withstand voltage testing method is widely used in transformer on-site testing.
The series resonance withstand voltage test is an important test item for assessing the electrical strength of equipment such as transformers, reactors, and voltage transformers. Taking transformers as an example, the power frequency AC withstand voltage test only checks the electrical strength of the winding insulation, namely the insulation between high voltage, medium voltage, low voltage windings and the grounding parts such as the oil tank and iron core. However, longitudinal insulation, which refers to the insulation between turns, layers, and segments of the winding, has not been inspected. The series resonance withstand voltage test is to apply a voltage on the low voltage side of the transformer that is a certain multiple higher than the rated voltage, and to obtain the required test voltage on the high voltage winding by the electromagnetic induction of the transformer itself to test the main insulation and longitudinal insulation of the transformer. Especially for transformers with neutral point graded insulation, due to the inability to use external high voltage for power frequency AC withstand voltage testing, their main insulation and longitudinal insulation are evaluated by induction withstand voltage testing. In order to increase the test voltage without saturating the iron core, the method of increasing the power frequency is often used, which can be understood from the potential equation of the transformer.
E=KfB
In the formula, E represents the induced potential;
K – constant;
F – frequency;
B – Magnetic flux density.
From this, it can be seen that in the series resonance withstand voltage test, if the magnetic flux density is to remain constant, the frequency needs to be doubled correspondingly when the voltage is doubled. Therefore, the frequency of the induction withstand voltage test power supply should be greater than twice the rated frequency, generally using power frequencies of 100Hz, 150Hz, and 200Hz.
There are several methods to obtain such a high-frequency power supply:
1. High frequency generator set. It is composed of an electric motor driven to a high-frequency generator. The voltage regulation of the generator set is achieved by changing the excitation variable resistor of the exciter to adjust the excitation of the generator rotor, thereby achieving the goal of smooth and adjustable stator output voltage of the generator. This method is commonly used in manufacturing plants.
2. The test power supply for the winding asynchronous motor with double frequency is obtained by reverse traction. This method is called the anti drag method. It is actually an example of using wound asynchronous motors as asynchronous frequency converters.
3. Connect the three windings into an open triangle to obtain a triple frequency test power supply. This is a relatively easy method to implement on-site induction withstand voltage testing. They can be composed of three single transformers, or some use five column transformers as dedicated triple frequency power supplies.
4. Controllable silicon variable frequency voltage regulation inverter power supply. The application of thyristor inverter technology to generate high frequency and use it as an induction voltage withstand test power supply has significant advantages. If it is lightweight and can use a 380V low-voltage AC power supply, the device also has voltage regulation function, saving a lot of equipment, etc. Therefore, it is a promising power supply device for frequency doubling induction withstand voltage testing.