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With the innovation and development of power system application technology, cross-linked polyethylene power cables (XLPE) have gradually replaced oil immersed paper insulated cables in many fields and have been widely used. To test the quality of cable laying and installation before operation and the insulation strength of cables after operation, voltage withstand tests should be conducted on cables. The power testing workshop of the control plant is responsible for insulation testing and protection testing of various high-voltage equipment in the substations and distribution stations of various levels of factories and mines in Benxi Iron and Steel. For many years, DC withstand voltage has been used to assess the insulation strength of cross-linked cables. DC withstand voltage can accumulate insulation damage to XLPE cables, accelerating the aging and deterioration of cable insulation, increasing the fault rate of cable grounding and short circuit, and threatening the safety of system operation. Practice has proven that only by applying AC withstand voltage to cables can the degree of insulation damage be effectively reduced and the service life of cables be improved. In recent years, the equivalence of the briefly applied ultra-low frequency 0.1HZ withstand voltage test still needs further research, and due to the limited capacity of the testing equipment, it can only be used for withstand voltage testing of cables of 10KV and below.
The technical means for early detection of cable insulation strength and insulation condition usually use DC withstand voltage technology. Although this technology has the advantages of low test capacity and intuitive test effect, its test mechanism does not simulate the operation of high-voltage electrical equipment and may cause potential operational hazards to the cable. The drawbacks of using DC withstand voltage technology for cross-linked polyethylene power cables (XLPE) are mainly manifested in the following aspects:
(1) Due to the different electric field distribution formed when DC voltage is applied to cable insulation compared to AC electric field, the DC withstand voltage test cannot truly reflect the overvoltage capability of the tested cable in operation.
(2) Research has shown that based on the characteristics of XLPE cable structure, it has the ability to store unipolar residual charges. If the stored residual charges cannot be effectively released under the action of a DC electric field, the peak AC voltage superimposed on the DC residual charges after operation may cause cable breakdown.
(3) When conducting DC withstand voltage tests on XLPE cables, due to the effect of space charge, the electric field strength applied to the insulation of the tested cable can be more than ten times higher than the working electric field strength of the cable insulation. Even if the DC withstand voltage test is passed, the insulation of the tested cable will be severely damaged.
(4) A fatal weakness of XLPE cables is the tendency for water branches to form inside the insulation. Once water branches are formed, they quickly transform into electrical branches under direct current voltage, causing discharge and accelerating insulation degradation, resulting in breakdown under power frequency voltage after operation.
Basic principle ofseries resonance
The equivalence of variable frequency series resonance has been widely recognized through research both domestically and internationally, and has been widely promoted and applied. The frequency conversion series resonance withstand voltage test method is to reasonably configure the capacitors and reactors in the test circuit, and change the test frequency of the test system (range 30-300HZ), artificially causing resonance in the test circuit, so that most of the current on the test sample is the current obtained by subtracting the capacitive current from the inductive current. The power consumed by the test sample is only close to the active power of the power supply, effectively reducing the capacity of the test power supply and greatly reducing the weight of the test equipment.
Advantages of series resonant power supply in power system applications
The frequency conversionseries resonance withstand voltage test devicehas the following advantages (characteristics):
(1) The required capacity of the test power supply is relatively small. According to the principle of series resonance, the test voltage on the test sample is Q times the power supply voltage (Q is the quality factor, generally greater than 30), that is, the higher the quality factor Q, the smaller the required capacity of the test power supply.
(2) The complete set of equipment is flexible and lightweight. Due to the fact that the resonant reactor of the experimental device can be used in series and parallel as needed, the individual equipment can be made lightweight and flexible, and the weight of the individual unit can be greatly reduced, making it suitable for use on mobile sites.
(3) The waveform of the output voltage is good. Due to the fact that the high-voltage resonant circuit formed by the resonant reactor L and the test sample Cx is a good filtering circuit, the voltage waveform obtained on the test sample Cx is a good sine wave.
(4) The destructive effect on the test substance is minimal. When the weak insulation part of the test sample is broken down, the resonant circuit will lose its resonance condition, and the currents of the high-voltage and low-voltage circuits will rapidly decrease, which can prevent excessive erosion of the breakdown point by large short-circuit currents and minimize the damage to the test sample.
(5) To prevent the generation of overvoltage recovery, when the test sample breaks down, the high voltage disappears immediately due to resonance, and the arc is extinguished instantly. Due to the long process of re establishing the recovery voltage, any overvoltage recovery is prevented.
