What is a metal oxide arrester
Metal oxide arrester (MOA) is an important protective appliance used to protect the insulation of power transmission and transformation equipment from overvoltage hazards. It has the advantages of fast response, flat volt-ampere characteristics, stable performance, large flow capacity, low residual voltage, long life, and simple structure. It is widely used in power generation, transmission, transformation, distribution and other systems. Composite jacketed metal oxide arresters are made of silicone rubber composite materials. Compared with traditional porcelain jacketed arresters, they have the advantages of small size, light weight, solid structure, strong stain resistance, and good explosion-proof performance.
Characteristics of metal oxide arresters
The main component of the nonlinear resistor valve plate of the metal oxide arrester is zinc oxide. The zinc oxide resistor plate has extremely superior nonlinear characteristics. Under normal operating voltage, its resistance value is very high, which is actually equivalent to an insulator. However, under the action of overvoltage, the resistance of the resistor sheet is very small and the residual voltage is very low. However, under normal operating voltage, the valve plate deteriorates due to long-term exposure to power frequency voltage, causing changes in resistance characteristics, resulting in an increase in leakage current flowing through the valve plate. A sharp increase in the resistive component of the current will cause the temperature on the valve plate to rise and cause thermal collapse. In severe cases, it may even cause an explosion accident of the arrester. According to the "General Technical Specifications for Metal Oxide Arresters", there are a total of 6 testing items for metal oxide arresters, namely:
(1) Insulation resistance;
(2) Leakage current at DC U1mA and 0.75U1mA;
(3) AC leakage current under operating voltage;
(4) Power frequency reference voltage under power frequency reference current;
(5) Base insulation resistance;
(6) Check the operation of the discharge counter.
Structural characteristics of metal oxide arresters
The basic structure of a metal oxide arrester is a valve plate. The valve plate is made of zinc oxide (ZnO) as the main material, mixed with a small amount of other metal oxide additives and roasted at high temperature. It has good nonlinear varistor characteristics, so it is also called a varistor.
The basic structure of this sintered body is high-conductivity zinc oxide grains with a resistivity of 1 Ω⋅ cm. The edge is surrounded by a highly resistive (mainly metal oxide additive) grain boundary layer, and the resistivity is approximately 1010 ~ 1014Ω⋅cm under low electric field strength. Under the action of higher voltage, the valence electrons in the particle boundary layer of the metal oxide addition are pulled out, or the electron avalanche is generated due to impact ionization, resulting in a large increase in carriers. When the electric field intensity reaches 104~105V/cm, its resistivity drops to 1 Ω⋅cm; when the applied voltage decreases, the resistance increases due to the reduction of carriers due to recombination, so it has good nonlinearity. And its nonlinear volt-ampere characteristics are symmetrical in forward and reverse polarity.
Under normal operating voltage, the resistive current passing through the metal oxide valve is very small, generally about 10 ~ 15μA, which is close to the insulation state. When the voltage acting on the valve plate increases, the current increases. When the resistive current passing through the valve plate is 1mA, the voltage mA U1 acting on the arrester is the starting operating voltage. Since the zinc oxide valve has good nonlinear characteristics, the ratio of residual voltage to mA U1 when passing a 10kA impulse current is generally not greater than 1.9. The smaller the pressure ratio, the better its protection performance. The value of mA U1 is approximately 1.05 to 1.15 times the allowable operating voltage peak value.
The basic working principle of zinc oxide arrester
Metal oxide arrester, also known as metal zinc oxide arrester, is a new type of arrester that appeared in the early 1970s. So far, it has been widely used in my country's power grid. The main difference between it and ordinary valve-type arresters is that the valve disc material is different. The valve disc material of ordinary valve-type arresters is silicon carbide (emery), while the valve disc material of metal oxide arresters is made of semiconductor zinc oxide and other metal oxides (such as diamond oxide, manganese oxide, etc.) sintered at high temperature (above 1000°C).
Zinc oxide valve disc, also known as varistor, has better and more ideal nonlinear resistance characteristics than silicon carbide. Under the system operating voltage, its resistance is very large, the passing current is very small, and the resistive component is only about 10~15uA. Such a small current will not burn the valve plate, so the series gap can be used to isolate the power frequency operating voltage; when the voltage rises, its resistance becomes very small, it can pass large currents, and the residual voltage is also very low, so that the equipment is protected. After the overvoltage disappears, it returns to its original state.
Advantages of metal oxide surge arresters
A new type of lightning arrester with only a varistor. The varistor is a polycrystalline semiconductor ceramic component sintered from metal oxides such as zinc oxide and has ideal valve characteristics. At the same time, it has the advantages of small nonlinear coefficient, good protection characteristics, strong energy absorption capacity, large flow capacity, simple structure and good stability.
The value of the nonlinear coefficient α is very small. When a current in the range of 1mA to 10kA passes through the metal oxide valve, the α value is generally between 0.02 and 0.06. Under the rated voltage, the current passing through is extremely small, so it can be made into a gapless arrester. Good protection performance. It does not require gap action. Once the voltage rises, it can quickly absorb overvoltage energy and inhibit the development of overvoltage; it has good steep response characteristics; the performance of the gapless oxide arrester is almost not affected by environmental conditions such as temperature, humidity, air pressure, pollution, etc., so its performance is stable.
Metal oxide arresters basically have no freewheeling, light operating load, and strong resistance to repeated operations. The volt-ampere characteristics are symmetrical and there is no polarity problem, so it can be made into a DC arrester.
Large flow capacity. The arrester easily absorbs energy and is not restricted by the series gap. It is only related to the strength of the valve plate itself. Compared with silicon carbide valve discs, the flow capacity per unit area of oxide valve discs is 4 to 4.5 times greater. Therefore, using such a valve piece to make an arrester can not only limit atmospheric overvoltage, but can also be used to limit operating overvoltage, and can even withstand short-term (power frequency) overvoltage of a certain duration.
It has simple structure, small size, easy mass production and low cost.
Suitable for a variety of special needs. Metal oxide arresters have good contamination resistance and will not affect the performance of the arrester due to contamination or changes in surface potential distribution of the jacket during electrical cleaning. At the same time, because the valve plate is not affected by the atmospheric environment and can be adapted to various insulating media, it is also suitable for various special needs such as high altitude areas and 6 SF fully enclosed combined electrical appliances.
Metal oxide arresters have a series of advantages and great development potential. They are the main direction for the development of arresters in various countries around the world. They will gradually replace traditional arresters with gaps and will also be the key overvoltage protection equipment for future UHV systems.
