Selection method of zinc oxide arrester
Starting from the actual situation of my country's power system, combined with a historical review of arrester selection and the new version of the national standard for arresters, a selection method for zinc oxide arresters for parallel capacitor devices that ensures safe and reliable operation of the power system is proposed. It has certain reference value for the design of parallel capacitor devices in substations.
Zinc oxide surge arresters; rated voltage; continuous operating voltage; shunt capacitor devices
1. In the past, only operating overvoltage and lightning overvoltage levels were considered for arrester selection and disadvantages.
National standards stipulate that the system power supply terminal voltage should be slightly higher than K times the system's nominal voltage (or rated voltage) Un, that is, K=Um/Un (Um is the highest voltage of the system). The insulation of electrical equipment should be able to operate under Un for a long time. The K of systems of 220kV and below is 1.15, and the K of systems of 330kV and below is 1.1. The above principles are also followed in the initial stage of arrester design.
Zinc oxide arresters were preceded by SiC arresters. The arc-extinguishing voltage of SiC arresters of 10kV and below is designed to be 1.1 times the highest operating voltage of the system; the arc-extinguishing voltage of 35kVS SiC arresters is equal to the highest voltage of the system; the arc-extinguishing voltage of SiC arresters of 110kV and above is 80% of the highest voltage of the system. Corresponding to the above multiples are 110% arrester, 100% arrester and 80% arrester respectively.
In the early days of the use of zinc oxide arresters in my country, their rated voltage was designed based on the arc extinguishing voltage of SiC arresters. Early 6kV, 10kV and 35kV arresters all adhere to the above principles, such as: Y5WR-7.6/26, Y5WR-12.7/45, Y5WR-41/130. The maximum long-term power frequency operating voltage is the highest phase voltage of the system.
2. Ensure the selection and necessity of arresters when operating under single-phase ground overvoltage and ensuring the safety of the power system.
From the perspective of safe operation, the selection of the rated voltage of the arrester should also comply with the following principles:
①The rated voltage of the zinc oxide arrester should be higher than the power frequency transient voltage that may occur at the installation location. In neutral point grounding systems of 110kV and above, you can choose according to the above method.
② In neutral point non-directly grounded systems of 110kV and below, the power department regulations stipulate that operation is allowed for 2 hours under single-phase grounding conditions. Sometimes, faults can only be detected after running for more than 2 hours under the condition of intermittent arc grounding overvoltage. The operating characteristics of such systems pose a serious threat to the safe operation of zinc oxide arresters for 10 seconds at rated voltage. Moreover, the structures and designs of zinc oxide arresters and SiC arresters are different (the latter has a gap for arc extinguishing, while the former has no gap or only a flow isolation gap), which causes thermal collapse and even serious explosion accidents in zinc oxide arresters in practice. Faced with this situation, many power supply bureaus and electric power design institutes have proposed many types of rated voltage values (such as 14.4kV, 14.7kV, etc.) based on local power grid conditions. In the multiple national standard discussion drafts, the rated voltage requirement for withstanding 10 seconds in the operating load test has been increased to 1.2 to 1.3 times, which has improved the adaptability of zinc oxide arresters to the working conditions of neutral point non-direct grounding systems.
However, because the rated voltage of the zinc oxide arrester is selected too low, safety accidents may occur when the arrester operates under single-phase grounding overvoltage or even many transient overvoltages. The Safety Supervision and Production Coordination Department of the Ministry of Electric Power proposed modifications to the arrester as early as October 30, 1993, in the 17th safety report. The article requires that the continuous operating voltage (UC) and rated voltage (Ur) of newly installed gapless zinc oxide arresters with voltage levels of 3 to 66 kV should be selected according to the values listed in Table 1, while the protection performance cannot be reduced. (The data in brackets apply to neutral point zinc oxide arresters of generators and transformers, Um is 1.05-1.10 times the system standard voltage)
In the transitional stage between the issuance 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 zinc oxide arresters for neutral point non-grounded systems:
The rated voltage is multiplied by 1.2-1.3 times based on the arc extinguishing voltage design of the reference SiC arrester, and the continuous operating voltage is the highest line voltage of the system. In this way, the rated voltage data of arresters for various voltage levels of capacitors are as follows:
6kV rated voltage (model Y5WR-10/27):
Since there is no unified standard for the above basic data, arrester manufacturers and users may have differences in design and manufacturing.
3 The reality of implementing the 2000 version of the new standards and selecting surge arresters safely and rationally
In my country's new standard in 2000 (GB11032-2000), the above-mentioned 1.2-1.3 times principle for the selection of rated voltage has been recognized, but a new regulation has appeared for the selection of continuous operating voltage: the parameter 1.5Un//U1mA that reflects the service life of the arrester 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 arresters, the general value is 80%, so the continuous operating voltage is selected to be 0.8 times the rated voltage. This point is well-founded when we look at the small current area of the volt-ampere curve.
In this way, in practice, when performing simulation calculations based on specific conditions or selecting arresters based on empirical conventions, the overvoltage level of single-phase grounding operation for 1 hour should be considered. However, there are many rated voltage values that are 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 smaller than the U1mAAC value. The pursuit of subtle differences cannot be realized in the actual arrester design. In addition, from the discussion below, it can be seen that only by selecting according to the requirements of the new national standard can it be used safely under permitted overvoltage (this refers to an ungrounded system).
4. The scientific nature of zinc oxide arrester selection for non-grounding systems according to the 2000 version of the new standard
4.1 The rated voltage should be selected and designed according to the maximum allowable effective value of the power frequency voltage applied between the terminals of the arrester, so that it can operate correctly under the transient overvoltage determined in the specified operating load test. The selection of the continuous operating voltage must be such that it allows a permanent rms value to be applied between the arrester terminals. At this time, the power frequency discharge voltage must be high enough to prevent the insulation of the protected equipment from operating under unprotected operating overvoltage and extend the service life. The actual charge rate and residual voltage of zinc oxide arresters currently manufactured in my country must be taken into consideration.
4.2 Wherever the power frequency voltage rises seriously or the equipment insulation test voltage is high, a higher zinc oxide arrester rated voltage should be selected. The power frequency reference voltage should be selected to be equal to or greater than the rated voltage. These two points are well satisfied in the new national standard requirements, and the following calculation can also be found to meet the overvoltage requirements. The national standard requires that the single-phase grounding operation must be guaranteed to run for 2 hours without action. The most serious situation is when single-phase grounding and load shedding occur at the same time. At this time, the theoretically calculated maximum overvoltage that may occur is 1.99 times. The allowable continuous operating voltage UC (effective value) of the selected zinc oxide arrester is as follows:
The national standard selects the rated voltage (i.e. Ur≈1.25 UC) based on the charge rate of 0.8, which meets the requirements. If the arc grounding and resonant overvoltage are avoided with a high probability, the rated voltage should meet:
Press ?=0.8 again to select the continuous operating voltage, which also meets the requirements.
To sum up, the main difficulty in arrester selection is to determine the range of temporary overvoltage. It is necessary to ensure safe and reliable operation under higher operating overvoltage and atmospheric overvoltage, and to ensure that the valve plate does not operate under temporary overvoltage. At this stage, the selection and design of the arrester must ensure that the zinc oxide arrester does not operate when the system's highest overvoltage occurs when single-phase grounding occurs for 2 hours. Otherwise, the zinc oxide arrester will cause thermal collapse or even explosion accidents. Therefore, it is appropriate to choose according to the new requirements in ungrounded systems. However, in a capacitor device grounded by an arc-extinguishing coil, the ground overvoltage will be much lower. At this time, a lower rated voltage and continuous operating voltage can be selected based on actual simulation calculations to make the zinc oxide arrester operate at a lower operating overvoltage to protect the capacitor device. However, if it is inconvenient to simulate, you can also choose an ungrounded system, because the capacitor pole-to-ground insulation has been considered to meet the 2h requirement for single-phase grounding. When operating at a voltage lower than the rated voltage, if the arrester does not operate, overvoltage will not damage the capacitor device.
In short, this is because the zinc oxide valve pieces are directly connected in series without a series gap, causing the zinc oxide arrester resistor piece to be unable to withstand an overvoltage even exceeding 1.99 times. As a result, the rated voltage of the zinc oxide arrester selected with the SiC arc extinguishing voltage as a reference cannot meet the requirements and must be increased to ensure the safe operation of the arrester. If there is no actual simulation data, the recommended value reflected in the spirit of the national standard is more appropriate because it meets the extreme requirements.
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