01
Nominal discharge current of arrester
If the generator and substation are not equipped with lightning rods, they will be struck by lightning once in decades in general areas. If they are equipped with lightning rods or lightning wires, and the operation experience proves that they will be struck by lightning once in hundreds of years.
The external overvoltage suffered by the electrical equipment in the generator and substation is often the lightning wave uploaded from the line.
For example, 7 porcelain insulators are generally used for 110kV lines, and its insulation level can only withstand 700kv impulse voltage. When the lightning wave voltage on the line is higher than 700kv, it will cause flashover to the insulator, so 700kv shock wave will be transmitted to the substation. Due to economic reasons, the insulation level of electrical equipment is usually lower than that of the line. For example, 110kV transformer can only withstand 480kv impulse voltage.
Now there are 700kv lightning waves, and there is no doubt that the transformer will be damaged. Therefore, all current equipment in the generator and substation should be protected by lightning arresters.
However, lightning arrester alone is not enough. Due to the limitation of zinc oxide material and manufacturing level, zinc oxide valve can only pass the lightning current below 20KA, and the vast majority of zinc oxide valve can only pass the lightning current of 5ka.
We know that in China, more than 60% of the lightning current exceeds 20KA and more than 80% of the lightning exceeds 10kA. Therefore, we must find other ways to limit the lightning current entering the line to 20KA or 10kA or even 5ka. Secondly, let these filtered lightning currents pass through the arrester, which is the nominal discharge current of the arrester.
According to the provisions of Chinese standards: the nominal discharge current of the arrester is 20, 10, 5, 3 and 1ka according to different voltage levels, that is, the zinc oxide valve plate can work reliably under this current without damage.
The reason why it is called nominal is that through other lightning protection measures, the actual lightning current flowing through the arrester will not reach the above specified value.
For example, for 110kV zinc oxide arrester, the lightning current flowing through the arrester is only about 4KA, while the nominal discharge current of the corresponding arrester is 10kA.
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02
Residual voltage of nominal lightning wave of lightning arrester
If the meaning of the nominal discharge current of the arrester is understood, the voltage drop caused on its nonlinear resistor when the current flows through the arrester is the nominal lightning wave residual voltage of the arrester.
03
Protection level of lightning arrester
Any kind of electrical equipment has its protection level.
For example, if a 110kV transformer can withstand 480kv impulse voltage, its impulse insulation level is called 480kv.
If the lightning wave exceeds 480kv, the equipment will be damaged. If we limit the nominal lightning wave residual voltage of the arrester below 480kv in the design, that is, lower than the insulation level of the equipment, the equipment is safe. Therefore, the nominal lightning wave residual voltage of the arrester is called the protection level of the arrester, and the difference between the protection level and the insulation level of the protected equipment is the protection margin.
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04
Rated voltage ur of lightning arrester
The rated voltage of general power equipment is best understood. Most of them are consistent with the rated voltage of the system, but the rated voltage of lightning arrester is different. Firstly, it is not equal to the rated voltage of the system, and the rated voltage of silicon carbide lightning arrester and zinc oxide lightning arrester is different, even whether it is necessary for the rated voltage of the latter, So far, experts at home and abroad still have different opinions.
The main reason for the above situation is that the lightning arrester is an overvoltage protection equipment.
Such protective equipment must:
1) Conduct electricity immediately when an overvoltage of a certain value higher than the normal working voltage of the system occurs.
2) When the overvoltage is lower than a certain value, immediately stop the conduction and restore the insulation state.
The rated voltage of the arrester is related to the overvoltage higher than the normal working voltage of the system by a certain value, and also to whether the insulation level can be restored normally after the arrester stops conducting electricity. Therefore, the rated voltage of the arrester is not a simple problem, it involves the rated voltage of the system and the neutral grounding mode of the system, How does the arrester act after the lightning nominal discharge current acts?
For example, for the valve arrester, its action depends on the conduction of the air gap, and then the lightning current can reliably extinguish the arc after passing through the gap, so that the arrester can return to the insulation state normally.
For zinc oxide arrester, because it has no gap and no arc extinguishing problem, what is used to measure that the arrester can return to the normal insulation state after action?
At this time, the concept of rated voltage will be used, that is, the zinc oxide arrester will operate for 10 seconds under its rated voltage, and then reduce to the continuous operating voltage for 30 minutes, and the arrester will not have thermal collapse.
Therefore, the rated voltage of zinc oxide is the test of the thermal load of the arrester, and a voltage value higher than the rated voltage of the system is used to test the heat resistance of the arrester.
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Further explanation:
Generally, the voltage acting on electrical equipment can be divided into: power frequency voltage under normal operating conditions, transient overvoltage (the most common cause of this overvoltage is single-phase grounding fault), operating overvoltage (the main cause of this overvoltage is the operation of circuit breaker) and lightning overvoltage.
Since the zinc oxide arrester has no gap, it should not only directly bear the effects of normal working voltage and temporary overvoltage, but also bear the effects of lightning overvoltage and operating overvoltage.
Because the arrester should be able to withstand the effect of temporary overvoltage, people have to consider the aging, service life and thermal stability of the arrester under temporary overvoltage. The ability of the arrester to withstand temporary overvoltage should be measured by the rated voltage of the arrester, so the rated voltage of the arrester is often greater than the rated voltage of the system. The arrester can normally cool down without thermal collapse after running for a period of time (10 seconds according to the standard) under its rated voltage. Therefore, using the rated voltage of lightning arrester to measure its ability to withstand temporary overvoltage is a very complex problem, because the operating conditions involved are complex. There are many problems in determining the rated voltage of lightning arrester and whether the pressurization time of rated voltage is equivalent and reasonable to the actual system.
For example, in standard gb11032-89, for gapless zinc oxide arresters used in neutral indirectly grounded systems, usually 1.1 times the maximum working line voltage is taken as the rated voltage of the arrester, so the rated voltage of the arrester used in 10kV systems is:
one point one five × ten × 1.1＝12.65≈12.7（kV）
This is why in the national standard of zinc oxide arrester in 1989, the rated voltage of 10kV products is 12.7kv. In the above formula, 1.15 is because it is normal that the maximum operating voltage of the system is 5 ~ 15% higher than the normal system voltage, and the maximum operating voltage of 10kV system is often 15% of its upper limit.
However, there is another opinion that in order to ensure the safe operation of the system, it is recommended to increase the rated voltage of lightning arrester. For example, the 17th safety bulletin issued by the work safety department of the Ministry of electric power on December 30, 1993 put forward the requirements of "improving the rated voltage and continuous operation voltage of 3-66KV gapless metal oxide arrester". In the machinery industry standard in 1997, the rated voltage and continuous operation voltage of zinc oxide arrester with indirect neutral grounding were also increased.
Now we know that increasing the rated voltage of the arrester is to improve the ability of the product to withstand temporary overvoltage, that is, to increase its service life. However, for zinc oxide materials, the rated voltage increases and the residual voltage will increase, so the protection margin of the arrester is reduced.
Some foreign companies simply correspond the rated voltage of lightning arrester to the rated line voltage of the system. For example, in the product catalogue of G.E. company of the United States, the rated voltage of lightning arrester for neutral point direct grounding system is set as 75 ~ 90% of the rated line voltage of the system, and the rated voltage of lightning arrester for neutral point indirect grounding system is set as 1.05 ~ 1.25% of the rated line voltage of the system.
In our country, for the neutral point direct grounding system at 220kV and below, the rated voltage of lightning arrester is traditionally taken as 1.4 times of phase voltage. For 500kV power grid, due to the long line and great influence of capacitance effect, the temporary Overvoltage on the bus side and line side is different, and the rated Overvoltage on the line side is larger, Therefore, the standard stipulates that the rated voltage of lightning arrester is 1.3 times and 1.4 times of the maximum phase voltage on the bus side and line side respectively. Therefore, several rated voltages of lightning arresters with the same voltage level are often obtained in this way.
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05
Continuous operating voltage Uc of lightning arrester
The continuous operation voltage of the arrester is basically equivalent to the maximum phase voltage of the system, but for the system with neutral point not directly grounded, the continuous operation voltage of the arrester is greater than the maximum phase to ground rated power frequency voltage during fault.
Since the zinc oxide arrester has no gap, the power frequency voltage of the power grid has been added to it. The continuous effect of this voltage may cause the aging of zinc oxide products (gradual deterioration of performance). Therefore, in the arrester test, when the dangerous overvoltage passes, the rated voltage shall be used to simulate the temporary overvoltage and added to the arrester, The time is 10 seconds (this time is determined by the action time of the circuit breaker connected to the power grid, but in the EHV system, even considering the action of the backup relay protection, it does not exceed 1 second), which is not enough, because the lightning arrester also bears the operating voltage of the system after the temporary overvoltage, At this time, the continuous operating voltage is used to simulate the maximum operating phase voltage of the system, that is, the maximum operating line voltage of the system is divided by root 3, the pressurization time is 30 minutes, and then observe whether the temperature of the arrester decreases or continues to rise, resulting in thermal collapse. Therefore, in a sense, the rated voltage and continuous operation voltage of lightning arrester are of great significance in test, but not in operation.
To illustrate this, let's take another example. In i.e.c. standards and national standards, the accelerated aging test of lightning arrester shall be carried out.
Because the zinc oxide arrester is connected to the power grid for a long time and continuously bears the voltage of the power grid, the product may age. The aging of the product connected to the actual power grid is natural aging. If we think that the normal operation time of a certain arrester in the power grid is 100 years, if we use the method of natural aging, we will wait 100 years, This kind of waiting is unbearable for the development of any new product, so it is necessary to conduct artificial accelerated aging test, that is, under human factors, accelerate the aging of the product for a short time (of course not too short), and then equivalent this time to the natural aging time.
In the accelerated aging test of the arrester, the voltage shall be added first. This voltage stipulates that the continuous operation voltage shall be added, and the pressurization time is 1000 hours (about 42 days). If the surface temperature of the zinc oxide valve sheet can always be maintained within 115 ± 4 o C, it is considered that the arrester can operate for 100 years under the continuous operation voltage, which is the role of artificial accelerated aging, It connects the artificial pressure time of 1000 hours with the natural pressure time of 100 years, which greatly improves the efficiency. It can be said that there is almost no electrical equipment without artificial accelerated aging.
Of course, as for whether it can be so equivalent, countries are still further studying, but being able to withstand the continuous operating voltage for 1000 hours is the minimum requirement for the zinc oxide valve piece, but can it really be equivalent to 100 years?
