A monitoring device of leakage currents flowing through ZnO surge arresters

In this study, we developed a device for monitoring resistive leakage current and a technique for the diagnosis of the deterioration. of zinc oxide (ZnO) surge arresters. The resistive leakage current that increases with time results in a thermally unstable state that may even lead to a serious problem. We developed the leakage current monitoring device for the ageing test and durability evaluation for ZnO blocks. The third-harmonic component of the leakage current of ZnO blocks may be used as an indicator of whether ZnO surge arresters are in a good state or in a bad state. Two methods for the diagnosis of the deterioration of surge arresters were assessed. Both the measurement instruments for the resistive leakage current with a compensation circuit arid the third-harmonic current component were designed and fabricated. For higher precision and greater reliability of current detectors, the current transducer with a ferrite core, which has a very high relative permeability, was used to improve detection sensitivity. Laboratory tests for ZnO blocks were performed by measuring the resistive leakage current and third-harmonic current components, together with the fast Fourier transform (FFT) analysis. The third-harmonic current component was clearly separated from the leakage currents flowing through ZnO surge arresters for power frequency ac applied voltages. As a result, it was known that the proposed monitoring systems for both the resistive leakage current and the third-harmonic current would be useful for developing a device for predicting the defect and performance deterioration of ZnO surge arresters in power system applications.