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

  Currently, there are problems in the handover tests 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 both equipment and standards perspectives.

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 IEC840 or CIGRE WG21.0 3 recommended procedures, 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 factory 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 any accidental damage during transportation, handling, storage, installation, and backfilling. For a long time, due to the large test capacity and test equipment limitations, the DC withstand voltage test method has been used for cross-linked cables before commissioning, but it has many defects. Therefore, non-DC methods for withstand voltage testing of cross-linked cables have gained increasing attention.

2. Standard Issues

    Due to equipment capacity and size issues, there is currently no national standard for on-site AC withstand voltage tests of high-voltage power cables after installation. The CIGRE Working Group 21's "Recommended Guidelines for Acceptance Tests of High-Voltage Extruded Insulation Cables" raises doubts about the currently used DC withstand voltage test method and recommends the use of power frequency and near-power frequency (30-300Hz) AC test methods. In the IEC60840 standard, for cable tests after installation at 45-150kV, in addition to the original DC test standard, an AC test standard of 1.7U0 for 5 minutes or 1 U0 for 24 hours has been added. For the 220kV level, the IEC 62067/CD draft cancels the DC test standard for tests after cable installation, leaving only AC test requirements, i.e., 20-300Hz 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 20-300Hz. For cables of 35kV and below, the test voltage is 1.6 times the phase voltage; for 110kV cables, it is 1.36 times the phase voltage; and for 220kV and above, it is 1.12 times the phase voltage.

II. Selection of AC Withstand Voltage Test Methods

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

    Due to the large capacitance of the tested XLPE cables, the capacity of the test transformer required for power frequency tests is also large, making the test equipment bulky and unsuitable for on-site use. Using 0.1Hz as the test power supply theoretically reduces the test transformer capacity to 1/500, significantly reducing the weight of the transformer, making it easier to move to the site for testing. Currently, this method is mainly used for medium and low voltage cable tests. Since 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 of 110kV 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 another method for acceptance tests of extruded insulation cable lines. This method is more effective than DC withstand voltage tests, but compared to power frequency voltage tests, its effectiveness in checking the main insulation and accessory defects of cables is still not ideal. First, the wave attenuation is severe, 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 a resonant state. In this way, most of the capacitive current on the test object in the test circuit cancels out with the inductive current on 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 multiple). Therefore, the capacity of the test power supply is reduced, and the weight is greatly reduced. Resonant withstand voltage test systems are divided into two types based on adjustment methods: adjustable inductance and adjustable frequency.

    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 on the test object. When operating in a resonant state, if the insulation point of the test object breaks down, the current immediately detunes, and the circuit current rapidly drops to a fraction (one-tenth) of the normal test current. In the event of flashover breakdown, due to the loss of resonance conditions, not only does the short-circuit current immediately drop, but the high voltage also disappears immediately, and the arc can be extinguished. The re-establishment process of recovery voltage is very long, making it easy to disconnect the power 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 30-300Hz, which complies with the recommended frequency of 20-300Hz 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. The number of operators should be no less than two. During use, strictly comply with the unit's safety regulations for high-voltage tests.