With the development of modern lightning protection technology, DC high-voltage generators in the small current grounding system are gradually applied to the electrical equipment such as transformer switches, busbars, motors, generators, lines, and capacitor banks in the AC series gap zinc oxide surge arresters. As electrical equipment itself, there are also issues such as valve performance, parameter design, insulation materials, poor assembly, and sealing defects; it is also necessary to master their performance.
For the zinc oxide surge arrester in the 3-63KV power system with a non-earthed neutral point, China's power industry standard DL/T 596-1996 "Protective Test Regulations for Power Equipment" (hereinafter referred to as the "Regulations") clearly stipulates that its test items are: : 1. Insulation resistance; 2. Current at 1mA DC U1mA and 75% U1mA. As is known to all, the test item for the zinc oxide arrester of this procedure is derived from the requirements of the “Ac-free Metal Oxide Surge Arrester†(GB11032-89), and it is for the exchange of gapless zinc oxide surge arresters. It is questionable whether the zinc oxide surge arresters with series gaps are applicable to the test items specified in the Regulations.
For a single gap, when a large lightning current flows through the nonlinear resistor, the nonlinear resistance of the DC high voltage generator will show a large conductivity, so that the residual voltage U0 appearing on the arrester will not be too high. When the lightning current passes and the voltage applied to the valve is the system voltage Ux, the conductivity of the non-linear resistor suddenly drops and the freewheeling current is limited to a very small value. In fact, the gap of the arrester of the valve type consists of several or dozens of single gaps to form a capacitor chain. Due to the influence of the electrode pads on the ground and part of the capacitance of the high voltage end caps, the distribution of voltage across the gaps is not uniform. What is serious is that this kind of non-uniformity is very unstable, it is influenced by the surface conditions of the porcelain sleeve very much, make the voltage discharge voltage of the lightning arrester very unstable. Although it can be solved by connecting a shunt resistor in parallel with each gap or group of gaps, the DC high voltage generator will have long-term current (leakage current) in the shunt resistor, and the non-linear shunt resistance will gradually age after long-term operation. The performance of resistance increases, the electric conduction current decreases, and the performance of the arrester is affected.
The zinc oxide resistor has better nonlinearity in the breakdown region, which makes the zinc oxide arrester have a large resistance value under normal operating voltage, and the leakage current is very small; under the overvoltage condition, the resistance value is very small, and the overvoltage energy is released. That is, the high-resistance state is restored, and no freewheeling occurs, so the gapless zinc oxide arrester has been widely used.
However, as an overvoltage protection device, there is still a problem of overvoltage protection against the energy released by the device. For energy-limited overvoltages (such as lightning overvoltages and operating overvoltages), the arrester discharge can act as a pressure limiting protection. The overvoltage of energy is infinite (with supplementary energy), such as transient overvoltage (collectively referred to as power frequency overvoltage and resonant overvoltage), the frequency of which is either the power frequency or the integer or fractional times of the power frequency, and When the power frequency of the power supply is always matched, if for some reason the transient overvoltage is triggered, the power frequency power supply can automatically supplement the overvoltage energy even if the amplitude of the surge arrester discharge overvoltage is not attenuated or only weakly attenuated. If the voltage enters the protection zone of the arrester, it will inevitably move to a long time until the heat collapses, and the arrester will be damaged and explode. Therefore, the transient overvoltage will be fatal to the arrester. If all the transient overvoltages have been limited to lightning arresters in the protected dead zone that are not affected by them, they are said to have strong transient overvoltage withstand capability, and conversely, transient overvoltage withstand capability is poor. Silicon carbide arrester transient over-voltage withstand capability, but due to the poor stability of the operating characteristics of the operation, DC high voltage generator often due to the impact of the discharge voltage (the initial value of the protective action area) value, still may be subject to transient overvoltage hazards, action Short life of load. The gapless zinc oxide surge arrester is low due to its knee voltage (approximating the reference voltage as the knee voltage), only 2.21 to 2.56 Uxg (the maximum phase voltage), and some transient overvoltages have the maximum value of 2.5 to 3.5 Uxg. There are shortcomings such as transient overvoltage tolerance, high damage and high explosion rate, and short life.
An effective protection against transient overvoltage hazards is to add series transients with stable performance to limit all transient overvoltages in the protected dead zone and protect the arrester from damage. Series gap zinc oxide surge arresters have this unique advantage. The structure of the series gap gap zinc oxide arrester gaps and zinc oxide valve, the gap structure is different from the silicon carbide arrester. The number of gaps is small. When the overvoltage reaches the surge discharge voltage, the gap has no time delay breakdown. At the same time, due to the large gap distance and stable operating characteristics, the shortcomings of the silicon carbide arrester gap can be avoided. The gap of zinc oxide surge arresters in the series gap has limited all transient overvoltages in the protected dead zone from damage, so it can avoid the shortcomings of gapless zinc oxide surge arresters due to low knee voltage.
The "Regulations" stipulates that the test items for the silicon carbide arrester FS series are 1. Insulation resistance; 2. Power frequency discharge. FCD series test items are 1. Insulation resistance; 2. Electrically conductive current. Combination of gapless zinc oxide surge arresters and gapped silicon carbide arresters due to different structures in different tests, we believe that currently widely used in small current grounding system with zinc oxide arrester series gap test project should be 1. Insulation resistance; 2. Power frequency discharge. For some problems in order to solve the voltage distribution problem, and in parallel with the gap on both sides of the resistance gap zinc oxide surge arrester DC high voltage generator should also do the electric conduction flow.
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