VFTO Measurement System Based on the Embedded Electrode in GIS Spacer

The method using the embedded electrode in a spacer to measure the very fast transient overvoltage (VFTO) is well applicable to the field measurement in a substation. However, the current measurement system has the disadvantage of low high-frequency cutoff frequency (2 MHz), and the problem of fallibility caused by a flashover from the embedded electrode to tank sheath. Based on the fundamental principle of the embedded electrode measurement system, a distributed parameter model has been established in this paper. With theoretical analysis and experimental verification, it is found that the major factors affecting bandwidth include the inherent inductance in external capacitors connected to an embedded electrode, the different port-distribution of external capacitors, and the nonconsistency of external capacitors on different ports. When the capacitors at four ports of an embedded electrode are highly consistent, the partial transient overvoltage of the electrode will be eliminated. In this paper, capacitors with low inductance, high stability, and highly consistency are chosen to be connected on four ports of an embedded electrode, and the consistency of four external capacitors is ensured. Hence, the high-frequency cutoff frequency increases to 10 MHz. The VFTO generated by disconnector operation in 1100-kV gas-insulated switchgear is both measured by the system based on the embedded electrode in a spacer and a broadband measurement system. The contrastive results indicate that the measuring system can meet the needs of VFTO measurement on some conditions.