Power frequency overvoltage (TOV) characteristics of zinc oxide varistors

Due to frequent occurrence of power frequency overvoltage in low-voltage power supply systems, zinc oxide varistors used in low-voltage power supply systems may fail or even catch fire due to their inability to withstand it. Therefore, clear requirements have been put forward for the power frequency overvoltage characteristics of SPDs using zinc oxide varistors and zinc oxide varistors used separately.

There are clear regulations on how to characterize and test the power frequency overvoltage characteristics of SPD [6] [8], but there is no clear method on how to characterize and test zinc oxide varistors [9] [10]. However, when selecting zinc oxide varistors for SPD and when applying zinc oxide varistors, requirements for TOV characteristics are proposed. 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 trip characteristics, etc. However, a comprehensive and scientific explanation of the TOV characteristics of zinc oxide varistors has not been achieved, and no consensus has been reached.

This article discusses the TOV characteristics of zinc oxide varistors, proposes a method to characterize the TOV characteristics of zinc oxide varistors, and discusses the factors that affect the TOV characteristics of zinc oxide varistors.

Characterization of TOV characteristics of 2-zinc oxide varistors

The TOV characteristics of zinc oxide varistors refer to the characteristics exhibited by zinc oxide varistors during the application of TOV. During the application of TOV, zinc oxide varistors exhibit many characteristics and measurable parameters, such as the amplitude of TOV, the time to withstand TOV, the highest temperature rise that can be tolerated, the temperature rise curve, and current changes. So what parameters are needed to characterize the TOV characteristics of the product?

2.1 TOV withstand time characteristics are the most appropriate characterization of the TOV characteristics of zinc oxide varistors
The TOV characteristic of zinc oxide varistors is essentially their ability to withstand power frequency overvoltage. The ability of zinc oxide varistors to withstand power frequency overvoltage in low-voltage systems is related to the safety of electrical systems, so TOV characteristics are an important safety characteristic. If the zinc oxide varistor cannot withstand power frequency overvoltage, it will definitely experience short circuit and breakdown failure, leading to overheating and fire around the zinc oxide varistor, and even cause a fire. Only when the zinc oxide varistor can withstand the overvoltage that occurs in the system, can the zinc oxide varistor avoid short circuit failure, and the electrical system is safe.

Since the TOV characteristic is the ability to withstand power frequency overvoltage, to accurately measure and characterize this characteristic, we must start with power frequency overvoltage. Common power frequency overvoltage includes single-phase grounding overvoltage, high and low voltage common ground coupling transfer overvoltage, and zero loss overvoltage. There are two important parameters for power frequency overvoltage, which are amplitude and duration. The amplitude should be higher than the normal operating voltage, and the duration should be greater than the transient overvoltage μ S-level, ranging from hundreds of milliseconds to several seconds, or even longer.

Power frequency overvoltage of zinc oxide varistors