Features of Variable Frequency Series Resonance Test Device

I. Product Overview:

    Currently, more and more XLPE cross-linked polyethylene insulated power cables are replacing the traditional oil-impregnated paper insulated power cables both internationally and domestically. However, in the pre-commissioning test methods for cross-linked cables, due to the large capacitance of the test object and the limitations of test equipment, the DC withstand voltage test method is still used. In recent years, research results from many international and domestic research institutions have shown that DC testing causes varying degrees of damage to XLPE cross-linked polyethylene cables. Some research viewpoints suggest that the XLPE structure has the ability to store and accumulate unipolar residual charges. If the DC residual charges cannot be effectively released after the DC test, the combination of DC residual charges and the AC voltage peak after commissioning may cause cable breakdown. Some domestic research institutions believe that during the DC withstand voltage test of cross-linked polyethylene cables, due to the space charge effect, the actual electric field strength in the insulation can be up to 11 times higher than the working electric field strength of the cable insulation. Even if the cross-linked polyethylene insulated cable passes the DC test without breakdown, it can cause serious damage to the insulation. Furthermore, since the electric field distribution under applied DC voltage differs from that under operating AC voltage, the DC test cannot truly simulate the overvoltage that the cable withstands under operating conditions, nor can it effectively detect defects in the cable, cable joints, and construction processes. Therefore, the use of non-DC methods for withstand voltage testing of cross-linked cables has attracted increasing attention.

    At the same time, AC withstand voltage tests for various large transformers, as well as AC withstand voltage tests for thermal and hydroelectric generators, are also conducted periodically. The test equipment required for these tests demands large capacity, and resonance methods are typically used for testing, but they must be performed under power frequency conditions or equivalent power frequency conditions. The equivalent power frequency condition generally adopts a frequency range of 45-65 Hz, but many testing organizations require a 50Hz test power supply for AC withstand voltage tests on such equipment.

    Our company's series of variable frequency automatic resonance test devices are mainly used for variable frequency AC withstand voltage tests of 10kV, 35kV, and 110kV cross-linked rubber-plastic power cables, 66kV, 110kV, and 220kV gas-insulated switchgear (GIS), and power frequency AC withstand voltage tests for hydroelectric and thermal generators or power transformers. The basic principle is to use an adjustable (30-300Hz) series resonance test device to resonate with the capacitance of the test object to generate an AC test voltage. Due to the large capacitance of cables, traditional power frequency test transformers are heavy, bulky, and the high-current working power supply is not easily available on site, so series resonance AC withstand voltage test equipment is generally adopted. The input power capacity can be significantly reduced, the weight is reduced, and it is easy to use and transport. Initially, variable-inductance series resonance equipment (50Hz) was mostly used, but it had disadvantages such as poor automation and high noise. Therefore, variable frequency resonance is now mostly used, which can achieve a higher quality factor (Q value) and has advantages such as automatic tuning, multiple protections, noise reduction, flexible combination methods, and light single-component weight.

II. Advantages of Series Resonance Power Supply in Power System Applications:

    1. The required power supply capacity is greatly reduced. The series resonance power supply uses the resonance between the resonant reactor and the capacitance of the test object to generate high voltage and high current. In the entire system, the power supply only needs to provide the active power consumption part of the system, so the power required for the test is only 1/Q of the test capacity.

    2. The weight and volume of the equipment are greatly reduced. In the series resonance power supply, not only are the bulky high-power voltage regulation devices and ordinary high-power power frequency test transformers eliminated, but the resonant excitation power supply only needs 1/Q of the test capacity, greatly reducing the system weight and volume, generally to 1/5-1/10 of ordinary test devices.

    3. Improve the waveform of the output voltage. The resonance power supply is a resonant filter circuit that can improve the waveform distortion of the output voltage, obtain a good sine waveform, and effectively prevent false breakdown of the test object caused by harmonic peaks.

    4. Prevent large short-circuit currents from burning the fault point. In the series resonance state, when the insulation weakness of the test object is broken down, the circuit immediately detunes, and the loop current rapidly drops to 1/Q of the normal test current. In contrast, when using parallel resonance or test transformer methods for withstand voltage tests, the breakdown current immediately rises by dozens of times. Comparing the two, the short-circuit current and breakdown current differ by hundreds of times. Therefore, series resonance can effectively find insulation weaknesses without the concern of large short-circuit currents burning the fault point.

    5. No recovery overvoltage will occur. When the test object breaks down, due to the loss of resonance conditions, the high voltage immediately disappears, the arc is extinguished immediately, and the re-establishment process of the recovery voltage is very long, making it easy to disconnect the power supply before the flashover voltage is reached again. This voltage recovery process is an intermittent oscillation process of energy accumulation, with a long duration, and no recovery overvoltage will occur.

III. Main Features of the Product:

    1. The frequency modulation and power components use advanced high-quality components imported from Japan;

    2. Fully utilize the company's existing resources, and independently develop, design, and produce all components of the equipment: variable frequency power supply

, excitation transformer, high-voltage reactor, capacitor compensator, and high-precision capacitive voltage divider;

    3. Equipped with fully automatic (automatic tuning, automatic voltage boosting), fully manual (manual tuning, manual voltage boosting), and semi-automatic (automatic tuning, manual voltage boosting, and manual tuning, automatic voltage boosting) multiple functions, which can be switched arbitrarily;

    4. Equipped with settings for test voltage, voltage application time, alarm current setting, alarm voltage setting, frequency range, and starting voltage;

    5. Equipped with discharge protection function. When a flashover occurs on the test object, or when the resonance circuit suddenly detunes due to other reasons, the variable frequency control power supply immediately and automatically cuts off the output quickly, and displays the protection type and flashover voltage value;
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