Application of Portable AC Withstand Voltage Test Device for Cross-linked Cable in High-voltage Cross-linked Cable Withstand Voltage Test

  Currently, there are problems in the handover test of XLPE cross-linked cables in China. This paper discusses the feasibility of using variable frequency resonant devices for on-site AC withstand voltage tests of high-voltage power cables from the perspectives of equipment and standards.

I. Problem Statement

1. Problems and Current Status

    In recent years, with the implementation of urban network renovation projects, high-voltage cross-linked cables have been widely used. According to the recommended procedures of IEC840 or CIGRE WG21.03, the purpose of on-site tests is not to verify the manufacturing quality of cables or cable accessories, as that has already been confirmed through type tests and routine tests. The purpose of on-site acceptance tests is to check whether the cable installation and accessory installation are correct, and whether the cable has suffered accidental damage during transportation, handling, storage, laying, and backfilling. For a long time, due to the large capacitance of the test object and limitations of test equipment, the DC withstand voltage test method has been used for cross-linked cables before commissioning, but it has many drawbacks. Therefore, non-DC methods for withstand voltage testing of cross-linked cables have gained increasing attention.

2. Standard Issues

    Due to issues such as equipment capacity and size, there is currently no national standard for on-site AC withstand voltage testing of high-voltage power cables after laying. The CIGRE Working Group 21's "Recommendations for Acceptance Tests of High-Voltage Extruded Insulation Cables" raises doubts about the DC withstand voltage test method currently used and recommends the use of power frequency and near-power frequency (30-300 Hz) AC test methods. In the IEC60840 standard, for cables rated 45-150 kV, in addition to the original DC test standard, an AC test standard of 1.7 U0 for 5 minutes or 1 U0 for 24 hours has been added. For the 220 kV level, the IEC 62067/CD draft cancels the DC test standard after cable laying, leaving only the AC test requirement, i.e., 20-300 Hz, 1.4 U0 for 60 minutes. In the enterprise standard "Preventive Test Procedures for Power Equipment" issued by China Southern Power Grid in 2004, the AC withstand voltage test standard for XLPE cable main insulation is specified, with a recommended test frequency of 20-300 Hz. For cables rated 35 kV and below, the test voltage is 1.6 times the phase voltage; for 110 kV cables, it is 1.36 times the phase voltage; and for 220 kV and above, it is 1.12 times the phase voltage.

II. Selection of AC Withstand Voltage Test Methods

1. Very Low Frequency (0.1 Hz) Withstand Voltage Test

    Since the capacitance of the XLPE cable under test is large, the capacity of the test transformer required for power frequency testing is also large, making the test equipment bulky and unsuitable for on-site use. Using 0.1 Hz as the test power supply can theoretically reduce the capacity of the test transformer to 1/500, significantly reducing its weight, making it easier to move to the site for testing. Currently, this method is mainly used for testing medium and low voltage cables. However, because the test conditions are not as realistic as near-power-frequency AC voltage and the voltage level is relatively low, it cannot be used for high-voltage cables rated 110 kV and above.

2. Oscillating Voltage Test

    The oscillating voltage test uses a DC power supply to charge the cable. When the test voltage is reached, the discharge gap breaks down and discharges through an inductor coil, applying a damped oscillating wave voltage of a certain amplitude and frequency in the kHz range to the cable as an alternative method for acceptance tests of extruded insulation cable lines. This method is more effective than the DC withstand voltage test, but compared to power frequency voltage testing, its effectiveness in checking the main insulation and accessory defects of cables is still not ideal. First, the wave attenuates severely, making it difficult to meet the needs of long cables; second, it increases partial discharge, causing significant damage to the cable.

3. Resonant Withstand Voltage Test

    The resonant withstand voltage test method adjusts the inductance and test frequency of the test system to bring the circuit into resonance. In this state, most of the capacitive current in the test object cancels with the inductive current in the reactor, and the energy supplied by the power source is only the active power consumed in the circuit, which is 1/Q of the test object's capacity (Q is the system's resonance multiplication factor). Therefore, the capacity of the test power supply is reduced, and its weight is greatly decreased. Resonant withstand voltage test systems are divided into two types based on adjustment methods: adjustable inductance type and adjustable frequency type.

    The adjustable inductance resonant test system can meet withstand voltage requirements, but due to its heavy weight and poor mobility, it is mainly used in laboratories. The variable frequency series resonant withstand voltage test uses the inductance of the reactor and the capacitance of the test object to achieve capacitive resonance, obtaining high voltage and large current on the test object. It is a new method and trend in high-voltage testing and has been widely applied both domestically and internationally. Variable frequency series resonance is a resonant current filtering circuit that can improve power supply waveform distortion, obtain a better sinusoidal voltage waveform, and effectively prevent harmonic peaks from causing false breakdowns of the test object. When operating in resonance, if the insulation point of the test object breaks down, the current immediately detunes, and the loop current rapidly drops to one-tenth or less of the normal test current. In the event of flashover breakdown, due to the loss of resonance conditions, both the short-circuit current and the high voltage immediately disappear, and the arc can be extinguished. The re-establishment process of recovery voltage is very long, making it easy to disconnect the power supply before reaching the flashover voltage again. Therefore, it is suitable for insulation withstand voltage tests of high-voltage, large-capacity power equipment.

    The variable frequency resonant test system not only meets the withstand voltage requirements of high-voltage XLPE cables but also has the advantages of light weight and good mobility, making it suitable for on-site testing. After analysis and comparison, the variable frequency series resonant test device uses a fixed reactor as the resonant reactor and achieves resonance by adjusting the frequency, with a frequency adjustment range of 30-300 Hz, which complies with the recommended frequency of 20-300 Hz for resonant withstand voltage tests in the enterprise standard "Preventive Test Procedures for Power Equipment" issued by China Southern Power Grid in 2004.

III. Precautions for Using Resonant Test Devices

    1. Most resonant power supply products are high-voltage test equipment and should be used by professional high-voltage test personnel. Before use, carefully read the instruction manual and undergo repeated operational training.

    2. At least two operators are required. During use, strictly comply with the safety operation procedures for high-voltage testing in the unit.