Principle of live testing for zinc oxide lightning arrester
Zinc oxide arrester, abbreviated as MOA, is composed of stacked zinc oxide valve plates, completely eliminating gaps and solving various problems caused by the gap discharge time limit and discharge stability of traditional lightning arresters. And the zinc oxide valve plate is mainly made of zinc oxide and doped with other trace metal oxides sintered. When a voltage is applied, almost all of the voltage is applied to the grain boundary layer, making the current flowing through the MOA very small. When the voltage increases, the grain boundary layer changes from high resistance to low resistance, forming a large flow rate.
There are two types of leakage currents flowing through the MOA during operation under the action of AC voltage: resistive current and capacitive current. Among them, resistive current only accounts for a small part, about 5% -20%. But when the lightning arrester experiences aging, moisture, insulation degradation, and surface contamination, the capacitive current does not change much, while the resistive current increases significantly. So live testing mainly detects leakage current and its resistive component.
At the operating voltage, the total leakage current I can be decomposed into resistive current IR and capacitive current Ic. Assuming the phase difference between the operating voltage U and the total leakage current I is ∧, the relationship between each current is:
From equation 1, it can be seen that as long as MOA deteriorates, the capacitance or resistance will change, resulting in changes in parameters I, IR, and Ic.
The measurement of commonly used experimental instruments is to obtain the total resistive current and fundamental resistive current of the measured MOA by Fourier transform of current and voltage.
Live testing methods and applications
Each provincial power grid company can develop its own live testing standards based on national standards according to the local environment and conditions, making live testing more reasonable. Our bureau's standard stipulates that newly put into operation 35kV and above zinc oxide lightning arresters should be measured once before each thunderstorm season. The measured values of full current, resistive current, or power loss under operating voltage should not show significant changes compared to the initial value. When the resistive current increases by 50% (compared to the initial value), the monitoring period should be appropriately shortened. When the resistive current increases by 100%, power must be cut off for inspection and a DC test must be conducted.
However, the resistive components mainly include: surface leakage along the inner and outer surfaces of the ceramic sleeve, surface leakage along the valve plate, nonlinear resistive components of the valve plate, and leakage of the insulation support. They are affected by the operating voltage, weather, humidity, etc. Therefore, when conducting on-site testing, simply considering the magnitude of the total current and resistive current values, and comparing them with previous data, will affect the accuracy of the judgment. Therefore, after years of practical accumulation, it has been summarized that the size of the value of Φ can be used to judge the condition of MOA. According to Equation 1:
According to the national standard, the resistive current cannot exceed 25% of the total current. Therefore:
Since # is equivalent to the dielectric loss angle, the basis for determining MOA through # can be derived as follows:
4 Economic feasibility of live testing
With the development of the economy, the regular maintenance that has been formed for a long time can no longer meet the production goals of power supply enterprises, that is, to build a power network with sufficient reliability to transmit electricity at the lowest cost. How to effectively control maintenance costs, reasonably extend equipment service life, achieve mandatory and necessary repairs, save maintenance costs, and reduce power outage time? Based on China's national conditions and current situation, State Grid Corporation of China has proposed reasonable and reasonable electrical equipment condition maintenance, advocating for scientific and effective evaluation of equipment condition through live testing and other means, and determining maintenance plans based on the evaluation results.
State maintenance based on live testing and online detection has significant advantages in cost control compared to traditional maintenance systems
Overcoming the blindness of regular maintenance has strong targeting. Adopt different processing methods according to different states to reduce operation and maintenance costs. For equipment in poor condition, timely pre testing can be arranged, and for equipment in good condition, the maintenance cycle can be extended, thereby saving manpower, material resources, and financial resources, effectively reducing maintenance costs and maintenance risks.
Reducing maintenance workload and labor intensity is beneficial for reducing staff and increasing efficiency, and improving economic benefits. Reducing downtime (total maintenance), improving equipment reliability and availability, extending equipment service life, reducing equipment procurement costs for power supply enterprises, and increasing power sales for power supply enterprises.
5 Conclusion
With the rapid development of the power industry, the reliability requirements for the power grid will be higher, and the research on power equipment maintenance technology will have more economic and social benefits. It is imperative for the maintenance of power equipment to shift from planned maintenance to condition based maintenance. For MOA, the deterioration of MOA will lead to the explosion of lightning arresters and interruption of power supply. Traditional pre commissioning maintenance not only forces healthy equipment to undergo power outages for maintenance, but also cannot guarantee that lightning arresters will not have accidents in the years after pre commissioning. Therefore, it is necessary to conduct live testing on zinc oxide lightning arresters. Its testing method is simple and convenient, the test data is stable, and it can reflect the insulation condition of MOA, further strengthen insulation supervision, and promote the development and popularization of condition based maintenance, which plays an important role.