Characteristics of Variable Frequency Series Resonance Test Device

I. Product Overview:

    Currently, more and more XLPE cross-linked polyethylene insulated power cables are replacing traditional oil-impregnated paper insulated power cables both internationally and domestically. However, due to the large test capacity and test equipment limitations, the DC withstand voltage test method is still used for pre-commissioning tests of cross-linked cables. 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 perspectives suggest that the XLPE structure has the ability to store and accumulate unipolar residual charges. If the DC residual charges cannot be effectively discharged after a DC test, the combination of DC residual charges and the peak AC voltage after commissioning may cause cable breakdown. Domestic research institutions believe that during DC withstand voltage tests on cross-linked polyethylene cables, due to space charge effects, 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 cross-linked polyethylene insulated cables pass the DC test without breakdown, it can cause severe damage to the insulation. Furthermore, since the electric field distribution under applied DC voltage differs from that under operating AC voltage, DC testing cannot truly simulate the overvoltage that cables experience under operating conditions, nor can it effectively detect defects in the cable, cable joints, or construction processes. Therefore, non-DC methods for withstand voltage testing of cross-linked cables are gaining increasing attention.

    At the same time, AC withstand voltage tests for various large transformers, as well as AC withstand voltage tests for thermal and hydro generators, are also conducted periodically. These equipment tests require large-capacity test equipment, typically using resonance methods, but must be performed under power frequency conditions or equivalent power frequency conditions. The equivalent power frequency condition generally uses a frequency range of 45-65 Hz, but many testing organizations require a 50 Hz 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 on 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 on hydro 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 require large current power supplies that are not easily available on site. Therefore, series resonance AC withstand voltage test equipment is generally used. The input power capacity can be significantly reduced, weight is reduced, and it is easy to use and transport. Initially, variable inductance series resonance devices (50Hz) were used, but they had disadvantages such as poor automation and high noise. Therefore, variable frequency resonance is now mostly used, which can achieve higher quality factors (Q values) and has advantages such as automatic tuning, multiple protections, reduced noise, 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 test object's capacitance to generate high voltage and large current. In the entire system, the power supply only needs to provide the active power consumption part of the system. Therefore, 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 regulating devices and ordinary high-power power frequency test transformers omitted, but the resonant excitation power supply only needs 1/Q of the test capacity, greatly reducing the system weight and volume, typically to 1/5-1/10 of ordinary test devices.

    3. Improve the output voltage waveform. 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 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, and the arc is extinguished immediately. Moreover, 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, which is long and does not produce any recovery overvoltage.

III. Main Product Features:

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

    2. Fully utilize the company's existing resources, completely 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. It has multiple functions including 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), which can be switched at will;

    4. It has settings for test voltage, pressurization time, alarm current setting, alarm voltage setting, frequency range, and starting voltage;

    5. It has discharge protection function. When the test object flashes over, or when the resonant 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;

    6. It measures and displays output voltage, output frequency, pressurization time, protection action type, and other related information. When the test is completed, the voltage automatically drops to zero;

    7. Large LCD full Chinese interface platform technology, full touch screen operation, data saving.

    8. Summary of technical features:

    (1) Advanced digital and power technology: The power components use intelligent power modules (IPM) newly introduced abroad that integrate driving, protection, and power conversion, greatly improving the reliability of the entire machine and reducing volume and weight.