In the past few decades, silicon carbide valve plate avoidance devices with gaps have been widely used in lightning protection and overvoltage protection technology in my country. This device is based on the silicon carbide valve and adds an electrical gap. The nonlinear coefficient of silicon carbide valve is large (a=0.2-0.3). Under normal system-to-ground voltage, the power frequency continuous current is hundreds of amperes. In order to protect the safety of valves and electrical systems, series gaps must be used to block the continuation of power frequency. However, the series gap, due to its difficulty in sealing, leads to unstable discharge voltage and affects the protection effect of electrical equipment. Especially in recent years, the widespread use of vacuum circuit breakers has also brought various operating voltages due to its excellent arc extinguishing performance. If previous arresters are used for overvoltage protection, they cannot meet the requirements in terms of performance or protection. People turned to look for new protective gear. This new type of protection device is a metal oxide arrester.
ZnO arresters have two structures: one is a Zn0 arrester without a gap, and the other is a Zn0 arrester with a gap.
Replace the original silicon carbide valve with a new metal oxide arrester valve. Zinc oxide valve has excellent nonlinear characteristics (a=0.04-0.05), and its volt-ampere characteristic curve is very flat. Under normal system operating voltage, the Zn0 valve only flows a few micro to tens of micro currents. Therefore, early zinc oxide arresters have fast action response, large overvoltage energy absorption, and small residual voltage. Therefore, they have excellent protective performance and protect electrical equipment from overvoltage damage. It is especially suitable for vacuum circuit breakers. Its excellent arc extinguishing performance, electrical equipment absorbs overvoltage during operation, coupled with its small size and light weight, makes it very popular among vacuum circuit breaker production units.
However, this ZnO arrester has experienced a very difficult process during its use in power systems. Due to issues such as unreasonable manufacturing quality, structure, material selection and formula, when zinc oxide arresters are used in ungrounded neutral point systems, they are often burned during operation and system operation, and explosion accidents often occur during system operation. Fire and single-phase grounding occur, so people have a lot of concerns about using ZnO arresters.
One of the key parameters to measure the performance of ZnO arresters is the nominal voltage U1mA at milliamp DC. Just add a DC voltage to its Zn0 valve block. When the current flowing through it is 1mA, the voltage applied to the zinc oxide arrester is U1mA. When there is a single-phase ground fault or arc ground overvoltage in a small ground current system, the voltage applied to the zinc oxide arrester is a good power frequency line voltage or power frequency overvoltage. This voltage is set to um, and its ratio to the nominal voltage U1mA is called the charging rate, expressed in K:
(1) The charging rate of early products has always beenAt 65%-75% (some manufacturer data says it can reach 0%), so the use conditions of N0 in small current extrusion systems are not strict. In this system, Zn0 should continue to withstand the power frequency phase voltage qm=us during normal operation. When an arc to ground occurs, the zinc oxide arrester will bear the full maximum line voltage of the system qlm=us). When arc ground overvoltage occurs, it will withstand an impact as high as 2.5-3.5 times the system phase voltage. The value of U1mA can be determined by the following formula: U1mA= × one point and five × Uo (2)
Among them u---zr0 arrester rated operating voltage
For example, the rated voltage of 6kvzn0 is 7.6kv, so UIMA(6)=×one point and five×7.6kv=11.28kv, U1mA(g)=11.3kv
The Zn0 rated voltage of the 10kV system is 12.7ky, so U1mA(10)=×one point and five×12.7kv=18.85kv U1mA(10)=18.9kv
Taking the 10kV system as an example, when a single-phase ground fault occurs in the system, considering the high operating voltage nm=1.1p and above as the average operating voltage of the system (10K is 10.5k), the perfect phase ground voltage will rise to l=1.1×10.5kv=11.5kv. At this time, the charging rate of the zinc oxide arrester is: At present, the charging rate of the zinc oxide arrester valve plate allowed for long-term operation is only about 70% - 80%. Therefore, this zinc oxide arrester will cause frequent damage when a single-phase ground fault occurs. If a single phase is grounded through an unstable arc, that is, the arc at the grounding point is intermittently extinguished and re-ignited, arc grounding overvoltage will occur in the sound phase, which can generally reach 2.5 to 3.5 times the voltage of the system phase.
