Power frequency overvoltage (TOV) characteristics of zinc oxide varistors

1 Introduction
Due to the frequent occurrence of power frequency overvoltage in low voltage power supply systems, zinc oxide varistors used in low voltage power supply systems fail or even catch fire due to inability to withstand it [4]. Therefore, clear requirements are put forward for the power frequency overvoltage characteristics of SPDs using zinc oxide varistors and zinc oxide varistors used alone [4][6][8].
There are clear regulations on how to characterize and test the power frequency overvoltage characteristics of SPDs [6][8], but there are no clear methods on how to characterize and test zinc oxide varistors [9][10]. However, when zinc oxide varistors are selected and used in SPDs, requirements for TOV characteristics are put forward. In response to this situation, many methods have been proposed in the industry to describe the TOV characteristics of zinc oxide varistors, such as TOV ampere-second value, maximum thermal equilibrium voltage, TOV thermal tripping characteristics, etc. However, the TOV characteristics of zinc oxide varistors cannot be fully and scientifically explained, and no consensus has been reached.
This paper discusses the TOV characteristics of zinc oxide varistors, proposes a method to characterize the TOV characteristics of zinc oxide varistors, and discusses the factors affecting the TOV characteristics of zinc oxide varistors.

2 Characterization of TOV characteristics
of zinc oxide varistors The TOV characteristics of zinc oxide varistors are the characteristics of zinc oxide varistors during the application of TOV. During the application of TOV, zinc oxide varistors exhibit many characteristics and measurable parameters, such as TOV amplitude, TOV tolerance time, maximum temperature rise tolerance, temperature rise curve and current change, etc. So what parameters are used to characterize the TOV characteristics of products?
2.1 TOV tolerance time characteristics are the most appropriate characterization of the TOV characteristics of zinc oxide varistors
The TOV characteristics of zinc oxide varistors are essentially the ability of zinc oxide varistors to withstand power frequency overvoltage. Whether zinc oxide varistors can withstand power frequency overvoltage in low-voltage systems is related to the safety of the power system, so the TOV characteristic is an important safety characteristic. If zinc oxide varistors cannot withstand power frequency overvoltage, short circuit and breakdown failure will occur, resulting in overheating and ignition of the surrounding area of ​​zinc oxide varistors, or even fire[4]. Only when the zinc oxide varistor can withstand the overvoltage in the system, the zinc oxide varistor will not fail due to short circuit, and the power system will be safe.
Since the TOV characteristic is the ability to withstand power frequency overvoltage, to correctly measure and characterize this characteristic, we must start with the power frequency overvoltage. Common power frequency overvoltages include single-phase grounding overvoltage, high and low voltage common ground coupling transfer overvoltage and zero loss overvoltage. There are two very important parameters for power frequency overvoltage, namely amplitude and duration. Its amplitude should be higher than the normal working voltage, and its duration should be greater than the μS level of transient overvoltage, ranging from hundreds of ms to several seconds, or even longer.
The power frequency overvoltage tolerance of zinc oxide varistor is directly aimed at TOV itself, that is, what kind of TOV it can tolerate. This ability is of course directly related to the amplitude and duration of TOV, and the most appropriate measurement is the characteristics of TOV itself. Therefore, the characterization of TOV characteristics should be the tolerated TOV amplitude and tolerance time, that is, the relationship between the amplitude and tolerance time of TOV should be used to characterize and measure. In fact, the arrester, which is also used as an overvoltage protection, has made such a characterization of the TOV characteristics. GB/T 11032-2000 "AC gapless oxide arrester" [5] stipulates that "2.38 Power frequency voltage withstand time characteristics: Under specified conditions, different power frequency voltages are applied to the arrester, and the arrester is not damaged or thermally collapsed. The relationship between the maximum duration corresponding to the time when the arrester is not damaged or thermally collapsed." Based on
the above description, with the help of the characterization of the TOV characteristics of SPD in the relevant SPD specifications [6][8], "TOV withstand time characteristics" is the most appropriate quantity to characterize and measure the TOV characteristics of zinc oxide varistors. The specific definition and measurement method are as follows:
Definition of TOV withstand time characteristics: Under specified conditions, different power frequency overvoltages are applied to the zinc oxide varistor, and the zinc oxide varistor does not lose its function or before thermal breakdown occurs. The relationship between the maximum duration and the power frequency overvoltage amplitude.
In the definition, there are two overvoltage tolerance modes, one is the non-destructive "tolerance mode" of "no loss of function", and the other is the destructive "failure mode" of "thermal breakdown". Among them, "withstand mode" refers to the situation that after being subjected to overvoltage, the zinc oxide varistor can still maintain the functions and performance that meet the product design, or the reduction of its performance is within the specified range; "failure mode" refers to the situation that after being subjected to overvoltage, the zinc oxide varistor will eventually undergo thermal breakdown. Both modes can be declared by the manufacturer.
The provision of TOV withstand time characteristics can be a set of data of TOV amplitude and corresponding withstand time, or a volt-second curve composed of TOV amplitude and withstand time data group. Of course, the TOV amplitude can be expressed in absolute or relative terms.
2.2 TOV withstand ampere-second value is not enough to represent TOV characteristics .
The TOV withstand ampere-second value is a quantity based on describing the ability of zinc oxide varistor to withstand energy during TOV application. It is certain that the TOV withstand ability of the product can be improved by increasing the energy that the product can withstand during TOV, but using the TOV withstand ampere-second value to represent the TOV characteristics has its shortcomings.
First of all, under the action of TOV, the zinc oxide varistor actually directly withstands a TOV voltage of a certain amplitude. What remains unchanged during the process is the TOV voltage, while the current flowing through the zinc oxide varistor is not a direct characteristic of TOV. The current corresponding to the TOV voltage of the same amplitude varies greatly due to the different performance of the zinc oxide varistor, even when the varistor voltage is the same. In this way, for products with large TOV current, a larger energy, that is, the TOV tolerance ampere-second value, is required to withstand the same time, while products with small TOV current only need a small TOV tolerance ampere-second value. In this case, for TOVs of the same amplitude, a large TOV tolerance ampere-second value may not necessarily withstand a longer time, and the TOV tolerance characteristics may still be poor.
In addition, under the action of the TOV voltage of the same amplitude, even if two zinc oxide varistors of the same specification or even the same varistor voltage have the same TOV tolerance ampere-second value, that is, the same TOV tolerance energy, the current flowing through the zinc oxide varistor changes during the application of the TOV voltage due to product performance differences, electrical balance and self-heating semiconductor effects, and the same energy consumption will be allocated different tolerance times. In this way, under the same TOV amplitude, the same TOV tolerance ampere-second value, but different TOV tolerance time, resulting in differences in TOV tolerance characteristics.
Therefore, at a given TOV voltage amplitude, the TOV tolerance ampere-second value of the zinc oxide varistor cannot fully compare the quality of the product's TOV tolerance characteristics.
2.3 The maximum thermally stable power frequency voltage represents the TOV characteristic is a specific expression
The maximum thermally stable power frequency voltage is the maximum power frequency voltage allowed to be applied for the zinc oxide varistor to achieve thermal stability within 45 minutes (temperature rise within 10 minutes is less than 2K). This characteristic is based on the failure mechanism of thermal runaway of the zinc oxide varistor when the TOV voltage is applied. It is a good practice to use the maximum power frequency voltage that can be applied without thermal collapse to characterize the TOV tolerance characteristics of the product. It directly gives the highest TOV amplitude that can be tolerated for 45 minutes without thermal collapse. The failure mode, TOV amplitude and corresponding tolerance time are all available, giving a data point of the TOV tolerance characteristic, which is a direct expression. It provides a failure mode capability.
Of course, this is not all about the TOV tolerance characteristics of zinc oxide varistors.
The maximum thermally stable power frequency voltage, as an effective representation of the TOV tolerance characteristics of a zinc oxide varistor, is simple to operate and is a very practical method in research.
2.4 The TOV tolerance characteristics of zinc oxide varistors support the TOV characteristics of SPDs
Zinc oxide varistors are simple components, while SPDs are devices that may or may not contain zinc oxide varistors. The TOV tolerance characteristics of SPDs containing zinc oxide varistors are affected by other components in addition to the zinc oxide varistors. The TOV tolerance characteristics of SPDs that do not contain zinc oxide varistors are not affected by zinc oxide varistors. Therefore, during the TOV voltage application process, the performance of zinc oxide varistors and SPDs is different, and the TOV tolerance characteristics of zinc oxide varistors and SPDs cannot be equated, but as long as SPDs use zinc oxide varistors, the TOV tolerance characteristics of zinc oxide varistors must support the TOV tolerance characteristics of SPDs.
In IEC 61643-1[8], the TOV characteristics of SPDs used in low-voltage power distribution systems are specified in "7.4 Fault overvoltage test in high- and medium-voltage systems" and "7.6 Fault overvoltage test in low-voltage systems", both of which specify the test TOV voltage amplitude and application time in the test. This description of the TOV tolerance characteristics of SPDs can also be said to be the "TOV tolerance time characteristics", which is the same method used in this paper to characterize the TOV tolerance characteristics of zinc oxide varistors. This provides a unified method for the TOV characteristics of zinc oxide varistors to meet the TOV characteristics of SPDs.
In order to facilitate SPD design, in addition to providing the TOV tolerance time characteristics of zinc oxide varistors, what other relevant data are required?
For the "tolerance mode"[8], product failure is not allowed during the test. As long as the TOV tolerance time characteristics of zinc oxide varistors can meet the requirements of SPDs, no other data is required to support the design of SPDs.
For the “failure mode” [8] of the SPD, which involves the tripping action of the SPD, in addition to the TOV tolerance time characteristics of the zinc oxide varistor (which can be a tolerance mode or a fault mode) meeting the TOV tolerance characteristics requirements of the SPD, if it is also necessary to use the temperature increase of the zinc oxide varistor during the application of the TOV voltage to trip, the temperature increase curve of the zinc oxide varistor over time during the application of the TOV voltage is also required.

[page]3 TOV characteristics of zinc oxide varistors due to their own factors
The formation of the performance of zinc oxide varistors is of course the result of the combined effect of formula and process. We only discuss the influence of the performance of the formed zinc oxide varistors on the TOV characteristics, so as to provide direction for improving the TOV characteristics of zinc oxide varistors.
For zinc oxide varistors of the same specification, their varistor voltage will not be the same value, and there will always be a certain discrete distribution. When the TOV voltage of the same amplitude is applied, the product with a high varistor voltage will have a lower load than the product with a low varistor voltage, so that the former will show a longer TOV tolerance time, and the TOV tolerance characteristics of the former are better than the latter. However, the varistor voltage is not the essence of the performance of zinc oxide varistors that affects the TOV tolerance characteristics. In order to find out the inherent factors of zinc oxide varistors that affect the TOV characteristics from the essence, the following discussion is based on zinc oxide varistors of the same specification with the same varistor voltage, under the action of the TOV voltage of the same amplitude, the essential factors that affect the length of TOV tolerance time.
The longer the time that a given amplitude of TOV voltage can be tolerated, the better the TOV tolerance time characteristics of the zinc oxide varistor, and the stronger the TOV tolerance ability.
(1) In order to tolerate the TOV voltage for a long time without damage or thermal breakdown, it is necessary to tolerate a larger energy. Therefore, increasing the TOV energy that the zinc oxide varistor can tolerate can improve the TOV tolerance characteristics of the product. The energy that the product can tolerate is related to the uniformity and thermal stability of the product. The uniformity and thermal stability are directly related to the formulation and process. It is feasible to improve the TOV tolerance characteristics of the product by improving the uniformity and thermal stability of the product.
(2) In order to tolerate the TOV voltage for a long time, the energy consumption must be extended and the temperature rise must be slowed down. This requires reducing the TOV power consumption by reducing the current flowing after the TOV voltage is applied, that is, requiring the zinc oxide varistor to have suitable volt-ampere characteristics and volt-ampere temperature characteristics in the TOV working area. The smaller the current flowing through the zinc oxide varistor corresponding to the TOV voltage, the smaller the power; the voltage temperature coefficient of the TOV working area is very small, that is, the current temperature coefficient is very small, and the current flowing through the TOV voltage has a small trend of decreasing or increasing as the temperature rises when the TOV voltage is applied, and the power decreases or increases very little, which will increase the TOV tolerance time, thereby improving the TOV tolerance characteristics. Appropriate volt-ampere characteristics require corresponding formulas and processes to achieve.
Therefore, for the zinc oxide varistor itself, uniformity, thermal stability, volt-ampere characteristics of the TOV working area, and temperature characteristics of the volt-ampere characteristics affect the TOV tolerance characteristics.

Figure 1 Changes in the volt-ampere characteristics before and after the impact

4 Discussion of factors affecting TOV characteristics in related literature
4.1 Effect of heat treatment on the TOV tolerance performance of zinc oxide varistors
Literature [2] discusses the effect of heat treatment on the power frequency overvoltage tolerance characteristics of zinc oxide varistors. Its test data shows that when an 85%U1mA power frequency AC voltage is applied, as the heat treatment temperature increases and the time increases, its initial current increases, the tolerance time decreases, and the TOV characteristics deteriorate. It is believed that heat treatment will deteriorate the TOV tolerance characteristics.
In fact, as mentioned above, the heat treatment in literature [2] changes the volt-ampere characteristics of the product TOV voltage working area, and the volt-ampere characteristic curve moves in the direction of increasing voltage, so that the current corresponding to the TOV voltage increases, thereby shortening the tolerance time and deteriorating the TOV characteristics.
4.2 Effect of limiting voltage on TOV tolerance characteristics
Literature [1] discusses the relationship between the limiting voltage and the power frequency overvoltage tolerance performance of zinc oxide varistors. The test data shows that for 14Φ products with the same varistor voltage of 556V, when an overvoltage of 1.25 times the maximum allowable AC working voltage is applied, products with different limiting voltages show different tolerance times. The larger the limiting voltage, the longer the tolerance time and the better the TOV characteristics.
In fact, the current flowing through the zinc oxide varistor at the overvoltage working point in the test is only a few 10mA, while the current when testing the limiting voltage is 50A. The large resistance value corresponding to the large limiting voltage is only the value at 50A, which cannot directly represent the resistance value at a few 10mA. Instead, it is due to the continuity trend of the zinc oxide varistor's volt-ampere characteristic. According to the large limiting voltage at 50A, it can be inferred that the voltage of several 10mA is also large and the large resistance is obtained. In essence, the volt-ampere characteristic curve of the product with a large limiting voltage shifts to a high voltage. At the same TOV voltage, the product with a large limiting voltage shows a large resistance value, small power and good TOV tolerance characteristics.
4.3 Effect of 8/20μs pulse impact treatment on TOV characteristics
References [3] and [7] discussed the effect of 8/20μs pulse impact treatment on TOV characteristics. The article points out that the TOV tolerance characteristics of zinc oxide varistors are improved after 8/20μs pulse impact treatment.
This is also because the 8/20μs pulse impact treatment changes the volt-ampere characteristics of the product, causing the curve of the TOV voltage working area to shift to high voltage, reducing the working power and increasing the TOV tolerance time.
Table 1 is the data of the VI characteristic changes of the 14D681 product after different impact intensities. Figure 1 is a schematic diagram of the VI characteristic changes, in which it is obvious that the impact does change the volt-ampere characteristic curve, making the voltage below 1mA smaller (the resistance value smaller) and the voltage above 1mA larger (the resistance value larger). This is also the reason why the impact can improve the TOV characteristics.
When the TOV absolute amplitude is specified, the impact causes the TOV characteristics of one working point to become better, which does not mean that the TOV tolerance characteristics can be improved in all TOV voltage working sections. It can be seen from Table 1 and Figure 1 that near the lower working point, the impact causes the zinc oxide varistor to drop in voltage and reduce resistance. If the TOV working point is here, a large working current will be generated and the power will be increased, resulting in a decrease in TOV tolerance characteristics; at a larger working point, the impact may cause the zinc oxide varistor to increase in voltage and increase in resistance, resulting in a decrease in TOV tolerance characteristics. Usually, the impact will cause the voltage in the working area less than 1mA to decrease, and the power will increase when powered on, which can also explain why the impact will cause the static life to decrease.
For the TOV voltage amplitude given as a ratio of the varistor voltage, it can be seen from Figure 1 that the impact will always cause the nonlinear deterioration of the volt-ampere characteristic, so when a TOV voltage of a relative amplitude is applied, a smaller current value can always be obtained, reducing the power and improving the TOV tolerance characteristics. However, as the impact intensity increases, the damage to the varistor structure will seriously reduce the uniformity and thermal stability, making its energy absorption capacity very low. When this factor plays a decisive role, the impact will reduce the TOV tolerance characteristics, which is also consistent with the results in the literature.

[page]5 Conclusion
Through the above discussion, the following conclusions are drawn:
⑴ The best representation of the TOV tolerance characteristics of zinc oxide varistors is the TOV tolerance time characteristics;
⑵ The TOV tolerance time characteristics are related to the TOV energy tolerance capability of zinc oxide varistors, the volt-ampere characteristics of the TOV working area, and the temperature characteristics of the volt-ampere characteristics;
③ The good uniformity and thermal stability of zinc oxide varistors can improve the TOV tolerance characteristics by improving the energy tolerance capability;
⑷ The nonlinear difference of zinc oxide varistors in the TOV working area can reduce the current corresponding to the TOV voltage amplitude, thereby reducing the power, prolonging the tolerance time, and obtaining good TOV tolerance characteristics. By limiting the voltage size and the nonlinearity at (0.1~1) mA, it can be inferred that the TOV working area has high and low nonlinear indexes, but they cannot be equal;
⑸ The positive voltage temperature coefficient or the not too large negative voltage temperature coefficient in the TOV working area can obtain good TOV tolerance characteristics.
References
[1] He Xin, Wang Jianwen, Han Wei, etc. Study on the relationship between power frequency overvoltage withstand performance and limiting voltage of zinc oxide varistors. Insulators and Surge Arrester, 2005, 4 (2): 44-46
[2] He Xin. Effect of heat treatment on power frequency overvoltage withstand performance of zinc oxide resistors. Insulators and Surge Arrester, 2007, 10 (5): 16, 17, 21
[3] Pang Chi, Fei Zihao, Yang Lin, et al. Study on the correlation between power frequency overvoltage withstand and 8/20μs current carrying capacity. Semiconductor Device Application, 2008, 7: 130-132
[4] Yang Dasheng, Wu Jianwei, Guo Yaping, et al. Causes of fire and solutions for zinc oxide varistors used in low voltage power supply systems. Special Issue of the 11th Annual Varistor Academic Conference and the First Cross-Strait Technical Seminar?, 2004, 4: 25～32
[5] GB/T 11032-2000 “AC gapless oxide arrester”
[6] GB/T 18802-2002 “Surge protective devices (SPD) for low voltage power distribution systems Part 1: Performance requirements and test methods”
[7] He Xin, Wang Jianwen, Han Wei, et al. Effect of 8/20μs pulse aging on the power frequency withstand performance of ZnO zinc oxide varistors. Special Issue of Papers of the 13th Annual Conference of Voltage Sensitive Devices of Sensitive Technology Branch of China Electronics Society, 2006, 10: 86～89
[8] IEC 61643-1 Low-voltage surge protective devices –Part 1: Surge protective devices connected to low-voltage power distribution systems –Requirements and tests, 2005,3
[9] IEC 61643-331 Components for low-voltage surge protective devices –Part 331 Specification for metal oxide varistors (MOV), 2003,5
[10] GB/T 18802.331-2007 / IEC 61643-331-2003 Low-voltage surge protective device components Part 331: Metal oxide varistors (MOV) specifications. 2007,6