1. Preface
Oil-paper insulation has been used in medium-voltage and high-voltage cables for more than 50 years. Due to the large capacitance of these cables, power frequency AC tests have not been performed on-site. Moreover, the insulation resistance of oil-paper insulated cables is much lower than that of rubber-plastic cables. DC voltage testing has been used for decades to judge the quality of paper-insulated cables, and practical experience has shown that it is effective and can provide reliable information on the hazards of defects. Therefore, DC withstand voltage testing, as a field acceptance test and periodic preventive test item for oil-paper insulated cables, has played a good role in detecting insulation defects and ensuring the safe operation of the power grid.
With the development of power technology, cross-linked polyethylene (XLPE) power cables are increasingly used. In recent years, with the implementation of urban network renovation projects in Shandong Power Grid, high-voltage cross-linked cables, especially 110kV XLPE cables, have been widely used in various municipal power supply bureaus. 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 this has already been confirmed in type tests and routine tests. The purpose of on-site acceptance tests is to check whether the cable installation and accessory installation are correct. During transportation, handling, storage, laying, and backfilling, cables may be accidentally damaged. The inspection method is, according to IEC229, for cables with an outer sheath thickness greater than or equal to 2.5mm, apply 10kV DC between the cable shield and ground for 1 minute. For the main insulation of cables, IEC recommends two methods:
DC withstand voltage: 3U0 for 15 minutes; AC withstand voltage: U0 for 5 minutes or 1 U0 for 24 hours.
Traditional DC withstand voltage has the advantages of light test equipment, good mobility, and low capacity, and is effective for oil-paper insulated cables. However, for XLPE AC cables, both theoretically and practically, it has been proven that DC withstand voltage methods are not suitable.
2. Problems with DC Withstand Voltage Test for XLPE Cables
A general principle of high-voltage testing technology: the test voltage field strength applied to the test object must simulate the operating conditions of high-voltage electrical equipment. The pass or fail conclusion from high-voltage tests should represent whether weak points in high-voltage electrical equipment pose a hazard to future operation. This means that the failure mechanism during testing should have the same physical process as the mechanism during operation. According to this principle, the problems of DC withstand voltage testing on XLPE cables are mainly manifested in the following aspects:
2.1 Under DC voltage, the electric field distribution in cable insulation depends on the volume resistivity of the material, while under AC voltage, the electric field distribution depends on the dielectric constant of each medium. Especially in cable accessories such as terminal heads and joint boxes, the DC electric field strength distribution is completely different from the AC electric field strength distribution. Moreover, the aging mechanism of insulation under DC voltage is different from that under AC voltage. Therefore, DC withstand voltage testing cannot simulate the operating conditions of XLPE cables.
2.2 XLPE cables will produce a "memory" effect under DC voltage, storing and accumulating unipolar residual charges. Once the "memory" caused by DC withstand voltage testing occurs, it takes a long time to release this DC bias. If the cable is put into operation before the DC residual charge is completely released, the DC bias will be superimposed on the power frequency voltage peak, causing the voltage on the cable to far exceed its rated voltage, which may lead to insulation breakdown.
2.3 During DC withstand voltage testing, electrons will be injected into the polymer medium, forming space charges, which reduce the electric field strength at that location, making breakdown difficult. Space charges are likely to occur at semiconductor protrusions and contamination points of XLPE cables. However, if surface flashover occurs at the cable terminal head or cable accessory breakdown during the test, it will cause wave oscillations on the cable core. At locations where space charges have accumulated, due to the rapid polarity change of the oscillation voltage to opposite polarity, the electric field strength at that location significantly increases, which may damage the insulation and cause multi-point breakdown.
2.4 A fatal weakness of XLPE cables is the susceptibility to water trees in the insulation. Once water trees are generated, under DC voltage they will quickly transform into electrical trees and form discharges, accelerating insulation degradation, leading to breakdown under power frequency voltage after operation. However, pure water trees can maintain a considerable withstand voltage value under AC working voltage and can last for a period of time.
2.5 Practice has also shown that DC withstand voltage testing cannot effectively detect certain defects under AC voltage, such as mechanical damage to insulation or incorrect placement of stress cones in cable accessories. Locations where insulation is most prone to breakdown under AC voltage often do not break down under DC voltage. Breakdown under DC voltage often occurs at locations where insulation does not normally break down under AC working conditions.
3. Selection of AC Withstand Voltage Test Methods
Since DC withstand voltage testing cannot simulate the operating field strength state of XLPE cables and cannot achieve the desired inspection effect, it is natural to turn to AC withstand voltage testing to assess the installation quality of cross-linked cables and accessories. The following AC test methods are available:
3.1 Very Low Frequency 0.1Hz Withstand Voltage Test
Due to the large capacitance of the tested XLPE cable, 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 0.1Hz as the test power supply can theoretically reduce the capacity of the test transformer 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. Due to the low voltage level, it cannot yet be used for high-voltage cable testing at 110kV and above.
3.2 Oscillating Voltage Test
The oscillating voltage test charges the cable with a DC power supply, then discharges through a discharge sphere gap to a set of series resistors and reactors, obtaining a damped oscillating voltage. This method is more effective than DC withstand voltage testing, but still not as effective as power frequency testing (1).
3.3 Resonant Withstand Voltage Test
Adjustable inductance resonant test systems can meet withstand voltage requirements, but due to their heavy weight and poor mobility, they are mainly used in laboratories. Variable frequency resonant test systems not only meet the withstand voltage requirements of high-voltage XLPE cables but also have the advantages of light weight and good mobility, making them suitable for on-site testing. Through extensive investigation and demonstration, Shandong Electric Power Research Institute selected the variable frequency resonant test device developed by Changsha Electric Power Test Development Company. This device uses fixed reactors as resonant reactors and achieves resonance by frequency adjustment, with a frequency adjustment range of 30-300Hz, which complies with the recommendation of using power frequency and near power frequency (30-300Hz) AC voltage in the CIGRE WG21.09 "Recommendations for Acceptance Tests of High Voltage Extruded Insulation Cables". This AC voltage can reproduce the same field strength as under operating conditions and has been proven to be the most effective method.
4. Examples of On-site Acceptance Tests
After having the AC withstand voltage test means, we began to conduct on-site AC withstand voltage acceptance tests on newly installed 110kV cross-linked polyethylene cables in Shandong Power Grid. The following are examples of withstand voltage tests on 6 110kV cross-linked cables in a certain bureau:
Type: YJLW03
Rated voltage: 64/110kV
Specification: 1×300mm2 copper conductor cross-linked polyethylene insulated power cable,
Manufacturer: Shandong Electric Power Cable and Electrical Appliance Co., Ltd.
Capacitance per unit length: 0.139m F/km, length of a single cable: 1.495km.
Test standard: Apply U0=110 kV AC voltage between the cable core and the metal armor layer, maintain for 5 minutes.
Since the rated output voltage on the high-voltage side of the intermediate step-up transformer is 15kV, which cannot meet the 110 kV test voltage requirement, series reactors are needed for series resonance. Since the test current flowing through the series reactor exceeds its rated current, parallel reactors need to be added for compensation, actually forming a series-parallel resonant circuit. The test wiring schematic is shown in Figure 1.
Figure 1 Series-parallel resonant test wiring diagram
In the figure: L1 is a series reactor, L2 is 4 reactors in parallel, each reactor has an inductance of 200H, rated current of 4A, and rated voltage of 267kV.
In the selection of resonant frequency, we strive to make the test frequency close to power frequency by changing the number of parallel compensation reactors, under the premise of meeting the voltage and current requirements of the test equipment. The following is an estimation of the resonant frequency:
Since the reactance of L1 is much greater than that of the intermediate step-up transformer, or the test voltage is much higher than the output voltage of the high-voltage side of the intermediate transformer, the estimation of the resonant frequency can be simplified, equivalent to L1 and L2 in parallel with Cx, hence the following formula:
f=1/2p =55.2Hz
Where: L1=200H, L2=200/4=50H, Cx=0.139´ 1.495=0.207m F.
In the test, the actual resonant frequency was 55Hz, indicating that the estimation of the resonant frequency according to the above simplification is relatively accurate.
The 6 cables were tested in 6 separate tests, with a withstand voltage time of 5 minutes each, and all passed.
5. Conclusion
5.1 DC withstand voltage testing cannot simulate the operating conditions of high-voltage cross-linked cables, has poor test effectiveness, and has certain hazards. In on-site acceptance tests, the DC withstand voltage method should no longer be used.
5.2 AC withstand voltage testing is the most effective means for on-site inspection of the installation quality of cross-linked cables and accessories. The variable frequency resonant device we use complies with the relevant requirements of IEC and national standards. Through series and parallel connection of reactors, it can meet the on-site AC withstand voltage requirements for 110kV and 220kV high-voltage cross-linked cables.
