1. Problem Statement
1.1 Problem 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 WG21.03 recommended procedures, the purpose of on-site testing is not to verify the manufacturing quality of cables or cable accessories, as this 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 been accidentally damaged during transportation, handling, storage, laying, and backfilling. For the test methods before the commissioning of cross-linked cables, due to the large test capacity and test equipment limitations, the DC withstand voltage test method has been used for a long time, but it has many defects. Therefore, the use of non-DC methods for withstand voltage testing of cross-linked cables has gained increasing attention. Infrared thermometer is a non-contact temperature measuring instrument that modulates and linearizes information for indication, display, and control purposes. It is mainly used for railway rails, ultra-high voltage cable joints buried deep underground, gas analysis, and control applications.
1.2 Standard Issues Due to equipment capacity and size issues, there is currently no national standard for on-site AC withstand voltage testing of high-voltage power cables after installation. The CIGRE Working Group 21's "Recommendations for Acceptance Tests of High Voltage Extruded Insulation Cables after Installation" 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 cables rated 45-150kV, in addition to the original DC test standard, AC test standards of 1.7U0 for 5 minutes or 1U0 for 24 hours have been added. For the 220kV level, the IEC62067/CD draft cancels the DC test standard after cable installation, leaving only AC test requirements, i.e., 20-300Hz, 1.4U0 for 60 minutes. In the enterprise standard "Preventive Test Regulations 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. The test voltage is 1.6 times the phase voltage for cables of 35kV and below, 1.36 times the phase voltage for 110kV cables, and 1.12 times the phase voltage for 220kV and above cables.
2. Selection of AC Withstand Voltage Test Methods
2.1 Very Low Frequency (0.1Hz) Withstand Voltage Test Due to the large capacitance of XLPE cables under test, the capacity of the test transformer required for power frequency testing is also large, making the test equipment heavy and unsuitable for on-site use. Using 0.1Hz as the test power source theoretically reduces the test transformer capacity to 1/500, significantly reducing the weight of the test transformer, making it easier to move to the site for testing. Currently, this method is mainly used for testing medium and low voltage cables. Since the authenticity of test conditions is not as good as near-power frequency AC voltage, and the voltage level is relatively low, it cannot be used for high-voltage cable tests of 110kV and above.
2.2 Oscillating Voltage Test The oscillating voltage test charges the cable with a DC power source, and 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 testing of extruded insulation cable lines. This method is more effective than the DC withstand voltage test, but compared to power frequency voltage testing, its effect on detecting defects in the cable's main insulation and accessories 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.
2.3 Resonant Withstand Voltage Test Method This method adjusts the inductance and test frequency of the test system to achieve resonance in the circuit. In this state, most of the capacitive current in the test circuit is offset by 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 multiple). Therefore, the capacity of the test power source is reduced, and the weight is greatly decreased. Resonant withstand voltage test systems are divided into two types based on adjustment methods: adjustable inductance type (VISR) and adjustable frequency type (VFSR). 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 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 a fraction (one-tenth to one-hundredth) of the normal test current. In the event of flashover breakdown, due to the loss of resonance conditions, in addition to the immediate drop in short-circuit current, the high voltage also disappears immediately, and the arc can be extinguished. The re-establishment process of recovery voltage is very slow, making it easy to disconnect the power supply before reaching flashover voltage again. Therefore, it is suitable for insulation withstand voltage testing 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, we adopted the VFSR variable frequency series resonant test device produced by Shanghai Siyuan Electric Co., Ltd. This 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 testing in the enterprise standard "Preventive Test Regulations for Power Equipment" issued by China Southern Power Grid in 2004.
3. 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) There should be at least two operators. During use, strictly comply with the unit's safety operating procedures for high-voltage tests.
(3) To ensure the safety and correctness of the test, in addition to being familiar with this product manual, strictly follow national standards and regulations for test operations.
(4) All connection wires must not be connected incorrectly, especially the grounding wire, otherwise the test device may be damaged.
(5) When this device is in use, it outputs high voltage or ultra-high voltage, so it must be reliably grounded and attention should be paid to safe operating distances.
(6) The series resonant test system generates high voltage by resonating the resonant reactor with the test object. That is, whether high voltage can be generated mainly depends on whether the test object and the resonant reactor resonate. Therefore, when test personnel analyze that the required high voltage cannot be generated on site, they should analyze what disrupts the resonance conditions, whether the circuit is connected, etc.
(7) The excitation transformer of the series resonant test system has specific voltage and current requirements. When selecting a replacement, voltage and current must be considered, and ordinary test transformers with only the same capacity cannot be used.
4. Conclusion
DC withstand voltage testing cannot simulate the operating conditions of high-voltage cross-linked cables, has poor test effectiveness, and poses certain hazards. Therefore, it is not recommended to use DC withstand voltage methods during on-site acceptance tests. AC withstand voltage testing is an effective means for on-site verification of the installation quality of cross-linked cables and accessories. The variable frequency resonant device we use complies with the enterprise standards of China Southern Power Grid, IEC, and national standards. By connecting reactors in series and parallel, it can meet the on-site AC withstand voltage requirements for high-voltage cross-linked cables.
