The AC withstand voltage test is the most rigorous, effective, and direct test method for evaluating the insulation strength of electrical equipment, and it is also a test method required by current electrical regulations. It is currently a guiding judgment for determining whether electrical equipment can be put into operation in China, and it is also an important detection method to ensure the insulation level of equipment and avoid insulation breakdown accidents. With the continuous development of China's power system, more and more high-voltage and large capacity electrical equipment and cross-linked polyethylene cables are being widely used. In daily maintenance and status check work, AC voltage withstand test is an essential test item. Due to the bulky and inconvenient transportation of traditional power frequency test transformers, and the need for a large capacity test power supply when conducting AC voltage withstand tests on large capacity electrical equipment, it is sometimes impossible to conduct tests on site. Therefore, a new AC voltage withstand test device is needed for testing. With the development of power technology, new testing methods have gradually been adopted. Among them, the series resonant voltage withstand test is one of them that is suitable for China's national conditions. It applies the basic principles of resonant circuits and can be divided into three types according to different adjustment methods: inductive, capacitive, and variable frequency. Through the comparison of the three types of series resonant devices in practical applications, it is found that the variable frequency series resonant device is more suitable for the actual needs on site. Therefore, in the field equipment AC test, the most commonly used is the variable frequency series resonant device, which can meet various AC voltage withstand tests.
By referring to various graphic and textual materials, we can understand the commonly used on-site adjustment methods for series resonance withstand voltage testing, including inductive and variable frequency methods. However, regardless of the method, it is hoped that the high-voltage circuit can be adjusted to an inductive impedance equal to the capacity, that is, o L=1/C. This way, only a small voltage needs to be applied to the intermediate transformer, and a high voltage QU (Q=L/R=1/o CR, quality factor) will be generated on the test sample
Whether sufficient high voltage can be generated on the test sample depends on the following three conditions:
① Is the total inductance of the high-voltage circuit equal to the total capacity;
② Is the quality factor value of the high-voltage circuit large enough;
③ Is the voltage U applied to the intermediate transformer large enough.
The most important component of the inductive device is the iron core reactor with adjustable inductance, which adjusts the iron core air gap of the reactor to change its inductance. The advantage of this device is that the inductance of the reactor can be made very large, which can be used for withstand voltage testing of large capacitance samples. The size of the inductance can be linearly adjusted, and the series circuit can be made to reach the resonance point exactly according to the capacitance of the sample. By first applying a low voltage to adjust the inductance of the reactor to produce resonance in the series circuit, and then increasing the voltage, the inductance of the reactor can be adjusted until resonance occurs under the test voltage. This is both safe and can achieve complete resonance of the circuit. The disadvantage is that the manufacturing of the reactor is complex, and the weight of the device increases.
The variable frequency device relies on a high-power variable frequency power supply to adjust the power frequency, so that the circuit reaches the resonance point. The inductance of the reactor used in the device is fixed, and the frequency of the test power supply varies with the capacitance. The variable frequency series resonance test device uses the principle of series resonance, uses an excitation transformer to excite the series resonance circuit, adjusts the output frequency of the variable frequency controller, and makes the circuit inductance L and the test sample C.
Series resonance, where the resonant voltage is the voltage applied to the test sample, has the advantages of reaching the resonance point exactly, making the reactor simple, lightweight, and having a high quality factor of the circuit. However, the disadvantage is that it requires a high-power power supply, which has high requirements for the stability of the power supply's voltage and frequency. The adjustment range of variable frequency type is larger than that of inductive type. With the advancement of electronic equipment, variable frequency type is generally used now.
Issues with series resonance withstand voltage test in actual on-site situations
In the actual use of variable frequency series resonant devices on site, we often encounter a series of problems such as low quality factor, low resonant frequency, and high current in the test circuit, which should be noted by the testing personnel. Corresponding measures should be taken to address these issues to ensure the smooth progress of the AC withstand voltage test.