Factors Affecting the Selection of Rated Voltage of Non-gap Metal Oxide Lightning Arrester

The rated voltage of the arrester manufacturer is the maximum allowable effective value of the power frequency voltage applied between the terminals of the arrester. The arrester designed according to this voltage can work correctly under the temporary overvoltage determined in the specified operating load test. According to the requirements of the International Electrotechnical Commission (IEC99-4) and GB11032-2000 for gapless metal oxide arresters, the arrester must be able to withstand a temporary overvoltage equivalent to the rated voltage at a temperature of 60°C and after injecting the energy specified in the standard At least l0s, without thermal collapse.

After the arrester manufacturer is preheated to 60℃ and subjected to high current or line discharge level energy load respectively, the data of the power frequency voltage duration allowed to be applied to the arrester and the corresponding power frequency voltage value without damage or thermal collapse is called Power frequency voltage withstand time characteristics of arresters. The lightning arrester can withstand the temporary overvoltage duration of rated voltage for l0s. If the duration of the temporary overvoltage is shorter than l0s, the amplitude of the temporary overvoltage can be increased, otherwise it can be reduced. The amplitude of the temporary overvoltage is higher or lower Based on the rated voltage of the arrester, its action time can be shorter or longer than 10s. When selecting the rated voltage of the arrester, you can check with the arrester power frequency voltage withstand time characteristic curve according to different system operation modes.

In the lightning arrester product manufacturing standard "AC Gapless Metal Oxide Surge Arrester" GB11032-1989, which has been used for many years, Article 5.11 of the power frequency voltage withstand time characteristic of the arrester stipulates that "For the neutral point ineffective grounding system The lightning arresters for power stations and power distribution arresters with rated voltages of 3.8-12.7kV shall withstand 2h at 1.3 times the rated voltage and 24h at the rated voltage.” According to this clause, the medium voltage 10kV neutral point non-effective grounding system is For example, the lightning arrester should withstand a power frequency voltage of 13kV for 2h. According to the time characteristics of the non-gap metal oxide arrester withstand power frequency voltage, generally l0s is 15%-25% higher than the 2h power frequency voltage. According to the definition, the rated voltage of the arrester should be 1.3 times of 13kV, which is 17kV. This result is consistent with the IEC's definition of the rated voltage of gapless metal oxide arresters. But at the same time we noticed that the premise of this clause is the power station lightning arrester with a rated voltage of 3.8-12.7kV for the neutral point non-effective grounding system, which is obviously in contradiction with our calculation result of 17kV. This is probably the reason why this clause no longer appears in the "AC Gapless Metal Oxide Lightning Arrester" GB11032-2000/IEC60099-4: 1991 (hereinafter referred to as the "New Standard") that was implemented on August 1, 2000. In Article 6.13 of the "New Standard", it is pointed out that the power frequency voltage withstand time characteristic of the arrester is necessary for the selection of the rated voltage of the arrester, but it fails to mention the temporary overvoltage amplitude and duration requirements in detail. The power frequency voltage withstand time characteristics of the arresters given by various manufacturers are different, which causes difficulties in selection. In this way, the designer must take the grounding mode and operation mode of the medium voltage system into consideration when selecting the arrester. For different medium voltage system grounding methods, neutral point effective grounding system (directly grounded or grounded through low resistance) or ineffective grounding system (neutral point insulation through arc suppression coil or high resistance grounding), the rated voltage of metal oxide arrester is different. The reason is that for the neutral point of the medium voltage system, the neutral point is not effectively grounded. When a phase ground fault occurs, it does not immediately trip, the other two sound phase-to-ground voltages rise to line voltage, and the ground fault lasts for a period of time (maybe more than 2h, cold shrinkage) Cable terminal), according to the time characteristic of withstand power frequency voltage of non-gap metal oxide arrester, generally l0s is 15%-25% higher than 2h withstand power frequency voltage. Therefore, the neutral point of the medium voltage system is not effectively grounded. There is no gap in the neutral point of the medium voltage system. The rated voltage of the metal oxide arrester is 1.3 times of the rated voltage of the non-gap metal oxide arrester for the neutral point effective grounding system of the medium voltage system.

It can be seen from the above that the power frequency voltage withstand time characteristics of the arrester itself and the grounding method of the power system will affect the selection of the rated voltage of the arrester.

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