Selection method of zinc oxide arrester
Starting from the actual situation of my country's power system, combined with the historical review of arrester selection and the new version of the national standard for arresters, the selection method of zinc oxide arresters for shunt capacitor installations for safe and reliable operation of the power system is proposed. The design of the device has certain reference value.
Zinc oxide arrester; rated voltage; continuous operating voltage; parallel capacitor device
1. In the past, the selection and disadvantages of lightning arresters that only considered operating overvoltage and lightning overvoltage levels
The national standard stipulates that the voltage at the power supply terminal of the system should be slightly higher than the nominal voltage (or rated voltage) Un of the system by K times, that is, K=Um/Un (Um is the highest voltage of the system). The insulation of electrical equipment should be able to operate for a long time under Un. The K of 220kV and below system is 1.15, and the K of 330kV and below system is 1.1. The above-mentioned principles are also observed in the early stage of arrester design.
Zinc oxide arrester was formerly SiC arrester. The extinguishing voltage of the SiC arrester of 10kV and below is designed to be 1.1 times the maximum operating voltage of the system; the extinguishing voltage of the 35kV SiC arrester is equal to the maximum voltage of the system; the extinguishing voltage of the SiC arrester of 110kV and above is 80% of the maximum voltage of the system . Corresponding to the above multiples, there are 110% arresters, 100% arresters and 80% arresters.
In the initial stage of using zinc oxide arrester in my country, its rated voltage was designed based on the extinguishing voltage of SiC arrester. The early 6kV, 10kV and 35kV arresters all abide by the above principles, such as: Y5WR-7.6/26, Y5WR-12.7/45, Y5WR-41/130. The maximum long-term power frequency working voltage is the highest phase voltage of the system.
2. Ensure the selection and necessity of the arrester when operating under single-phase grounding overvoltage and the power system is safe
From the perspective of safe operation, the selection of the rated voltage of the arrester should also comply with the following principles:
① The rated voltage of the zinc oxide arrester should be higher than the power frequency transient voltage that may appear at the installation site. In the neutral grounding system of 110kV and above, it can be selected according to the above method.
②In the 110kV and below neutral point indirect grounding system, the electric power department regulations stipulate that the operation is allowed for 2h in the case of single-phase grounding, and sometimes even when the arc grounding overvoltage is generated intermittently, the fault can be found after running for more than 2h. The operating characteristics of such systems pose a serious threat to the safe operation of zinc oxide arresters under rated voltage for 10s. In addition, the structure and design of the zinc oxide arrester and the SiC arrester are different (the latter has a gap for arc extinguishing, the former has no gap or only a current isolation gap), which makes the zinc oxide arrester suffer thermal collapse or even serious explosion accidents in practice. In the face of this situation, many power supply bureaus and power design institutes have proposed many types of rated voltage values (such as 14.4kV, 14.7kV, etc.) according to the power grid conditions in each region. However, in many national standard discussion drafts, the rated voltage withstand 10s in the action load test has been increased to 1.2 to 1.3 times, which improves the adaptability of the zinc oxide arrester to the operating conditions of the neutral point indirectly grounded system. .
However, because the rated voltage of the zinc oxide arrester is too low, safety accidents occur when the arrester works under single-phase grounding overvoltage and even many transient overvoltages. The Department of Safety Supervision and Production Coordination of the Ministry of Electric Power proposed amendments to the arrester in the seventeenth safety report on October 30, 1993. The article requires that the continuous operating voltage (UC) and rated voltage (Ur) of the newly installed 3～66kV voltage level non-gap zinc oxide arrester should be selected according to the values listed in Table 1, while the protection performance cannot be reduced. (The data in brackets are applicable to the neutral point zinc oxide arresters of generators and transformers, Um is 1.05-1.10 times of the system standard voltage)
In the transitional period between the publication of the notification and the revision of the new standard, the following design rules are proposed for the selection of the rated voltage and continuous operating voltage of the zinc oxide arrester for the neutral point non-grounded system:
The rated voltage is multiplied by 1.2-1.3 times on the basis of the design of reference SiC arrester arc extinguishing voltage, and the continuous operating voltage is the highest line voltage of the system. The rated voltage data of arresters for capacitors of various voltage levels are as follows:
6kV rated voltage (model Y5WR-10/27):
Because there is no unified standard for the above basic data, surge arrester manufacturers and users will have discrepancies in the design and manufacture.
3 Implementation of the 2000 version of the new standard, the reality of safe and reasonable selection of arresters
In my country's new standard in 2000 (GB11032-2000), the 1.2-1.3 times principle for the selection of rated voltage has been recognized, but the selection of continuous operating voltage has new regulations: from the parameter 1.5Un// which reflects the service life of the arrester U1mA is used as a reference value to select (design) the continuous operating voltage of the arrester. Based on the design and manufacturing level of domestic lightning arresters, the general value is 80%, so the continuous operating voltage is selected as 0.8 times the rated voltage. We can see this from the small current area of the volt-ampere curve, and it has a basis.
In this way, in practice, when carrying out simulation calculations according to specific conditions or selecting lightning arresters according to empirical conventions, the overvoltage level of 1h of single-phase grounding operation should be considered. However, there are many rated voltage values slightly different from the above values in the technical agreements among users and even in the drawings of the Electric Power Design Institute, which I think are unnecessary (such as 16.5kV, 16.7kV, etc. in 10kV). The reason is that during the actual design of the arrester, the rated voltage value in the volt-ampere curve is in the small current area, which is less than the U1mAAC value. The pursuit of subtle differences cannot be achieved in the actual arrester design; in addition, the following discussion shows that according to the new The national standard requires the selection to be used safely under the permitted overvoltage (this refers to the ungrounded system).
4. According to the 2000 edition of the new standard, the selection of zinc oxide surge arresters for non-grounding systems is scientific
4.1 "The selection of rated voltage should be selected and designed according to the maximum allowable power frequency voltage effective value applied between the terminals of the arrester. At this time, it can work correctly under the transient overvoltage determined in the specified operating load test. The selection of the continuous operating voltage must be an effective value that allows permanent application between the terminals of the arrester. At this time, the power frequency discharge voltage must be high enough to prevent the insulation of the protected equipment from operating under the operating overvoltage that does not need protection, and prolong the service life. It must also consider the charge rate and the charge rate of the zinc oxide arrester manufactured in my country at this stage. The actual level of residual pressure.
4.2 Wherever the power frequency voltage rises more severely or the conditions allow for higher equipment insulation test voltage, a higher rated voltage of zinc oxide arrester should be selected. The choice of power frequency reference voltage should be equal to or greater than the rated voltage. These two points are well met in the new national standard requirements, and the following calculations can also be found to meet the overvoltage requirements. The national standard requires that the single-phase grounding operation should be guaranteed for 2 hours without action. The most serious situation is when single-phase grounding and load rejection occur at the same time. At this time, the maximum overvoltage that may occur in the theoretical calculation is 1.99 times. The selected zinc oxide arrester allows continuous operating voltage UC (effective value) as follows:
According to the national standard, the rated voltage is selected according to the charge rate of 0.8 (that is, Ur≈1.25 UC), which meets the requirements. If the arc grounding and resonant overvoltage with a higher probability of avoiding are used, the rated voltage should meet:
Press? = 0.8 again to select continuous operating voltage, which also meets the requirements.
In summary, the main difficulty in the selection of lightning arresters is to determine the range of temporary overvoltages. It is necessary to ensure safe and reliable operation under higher operating overvoltages and atmospheric overvoltages, and to ensure that they operate under temporary overvoltages. The valve does not move. At this stage, the selection and design of the arrester must ensure that the highest overvoltage of the system that occurs when the single-phase grounding occurs for 2 hours, the zinc oxide arrester does not operate, otherwise the zinc oxide arrester will suffer thermal collapse or even explosion. Therefore, it is appropriate to choose according to the new requirements in an ungrounded system. However, in the capacitor device grounded by the arc suppression coil, the grounding overvoltage will be much lower. In this case, a lower rated voltage and continuous operating voltage can be selected according to the actual simulation calculation to make the zinc oxide arrester operate at a lower operating overvoltage. Protect the capacitor device, but if it is not convenient to simulate, you can also choose an ungrounded system, because the capacitor pole-to-ground insulation has been considered to meet the single-phase grounding 2h requirement. Working at a voltage less than the rated voltage, the arrester will not operate and will not cause overvoltage to damage the capacitor device.
In short, this is because the zinc oxide valve plate is directly connected in series without a series gap, which causes the zinc oxide arrester resistors to withstand or even exceed 1.99 times the overvoltage, resulting in the rated voltage of the zinc oxide arrester selected with the SiC arc extinguishing voltage as a reference. To meet the requirements, it must be increased to ensure the safe operation of the arrester. If there is no actual simulation data, the recommended value embodied in the spirit of the national standard is more appropriate because it meets the limit requirements.
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