The generator AC withstand voltage test requires the use of power frequency voltage. The national standard defines the frequency of power frequency voltage as 45Hz to 65Hz. However, in many test scenarios, the generator is required to undergo an AC test of 50Hz ± 2Hz. Moreover, the generator stator winding has a relatively large capacitance to ground. Most domestic test units use resonant transformers to conduct the test on it. However, the resonant transformers themselves are very heavy, often weighing several tons, making on-site tests very inconvenient. In response to this situation, a frequency and inductance adjustable AC withstand voltage resonant boost device for generators has been designed and manufactured.

The device can not only achieve the resonant state by adjusting the inductance of the reactor, but also by adjusting the working frequency of the system, and it can also achieve the resonant state by using both of the above two methods simultaneously. When the test requires a certain frequency, the frequency of the excitation source is adjusted to the required frequency, and then the inductive reactance of the reactor is adjusted according to the capacitance of the test sample to make the system resonate and generate high voltage. During the voltage rise process, if the resonant point deviates, the resonant point can be accurately found by fine-tuning the frequency to ensure a high Q value, thereby significantly reducing the power capacity and the volume and weight of the test equipment.
Compared with similar domestic generator AC withstand voltage devices, it has several revolutionary technical advantages as follows:

1.When conducting AC withstand voltage tests on generators of the same voltage level and capacity, the overall weight of the equipment is significantly reduced, and the weight of each individual piece of equipment is less than 60 kilograms, completely solving the problem of difficult on-site transportation.

2. The entire set of equipment features an advanced principle and simultaneously adopts the new frequency modulation and inductance adjustment technology, strictly ensuring the requirements for the power frequency withstand voltage test of the generator. It has a wide range of applications, significantly enhancing the value of the equipment.

3. It is equipped with a 20Hz-300Hz variable frequency control source, which features strong protection, good output waveform, excellent stability, multiple working modes, and convenient operation. Moreover, it can be powered by either 220V or 380V, making it easy to obtain power on site.

4. The method of adjusting the inductance first and then the frequency can accurately locate a larger Q point and avoid the resonance shift caused by the voltage rise.

5. Flexible configuration and strong expandability. Different types of reactors can be selected or multiple sections can be connected in series or parallel to meet the test requirements of all capacitive test samples in the power system. At the same time, it can be used in both power frequency test scenarios and variable frequency test scenarios. The device can be used for AC withstand voltage tests on generators as well as on cables, achieving multiple uses with one machine and offering high cost performance.

The test transformer is bulky and large, and the working power supply with a large current is not easy to obtain on site. By using series and parallel resonant AC withstand voltage test equipment, the capacity of the input power supply can be significantly reduced, the weight can be lightened, and it is convenient for use and transportation.

There are four viewpoints on the test frequency range both at home and abroad: 1. Wide frequency range (30-300Hz); 2. Power frequency range; 3. Close to power frequency; 4. Ultra-low frequency.

According to the research of the International Council on Large Electric Systems (CIGRE), it is reliable that there is no significant difference in the breakdown characteristics of several typical insulation defects within the frequency range of 30-300 Hz for cross-linked polyethylene cables. This has been widely adopted. This AC voltage can reproduce the same electric field as in the operating conditions and has been proven to be the most effective method. For instance, the preventive test regulations for power equipment in Jiangxi Power Grid recommend the resonant withstand voltage test within the frequency range of 30-300 Hz.

The series resonance withstand voltage test laboratory achieves series resonance by using the inductance of the reactor and the capacitance of the test subject. This has become a new direction and trend in high-voltage testing and has been widely applied both at home and abroad.

There are two types of series resonance: adjustable inductance type and adjustable frequency type. The adjustable inductance type uses an inductor with a core whose air gap can be adjusted. By adjusting the inductance, the circuit can be made to be in a power frequency resonance state. Its disadvantages are high noise, complex mechanical structure, heavy equipment, and difficult transportation.

The frequency modulation type uses a fixed inductance reactor. By adjusting the frequency of the excitation power supply to be the same as the natural frequency of the test circuit, the series and parallel circuits reach a resonant state, thereby generating high voltage or relatively large current on the test sample to achieve the purpose of withstanding voltage test on the test sample. Its characteristics are small volume, light weight, high quality factor, and convenient use of the test equipment.

All kinds of large power transformers, power cables, steam turbines and hydro turbines, and other capacitive equipment must undergo AC withstand voltage tests in strict accordance with the test regulations on a regular basis. Under power frequency conditions, due to the large capacitance of the test subject or the high test voltage requirement, the power capacity of the test device also has relatively high demands. Traditional power frequency withstand voltage devices are often large in size and heavy in weight, which are not convenient for on-site transportation and difficult to be combined arbitrarily, with poor flexibility. Therefore, the current better method is to use the variable frequency resonance method for withstand voltage tests.