Purpose/Aim
The very fast transient overvoltages (VFTO) generate in gas insulated switchgear (GIS) during the operation of disconnectors. The rise times can be 4-7 ns and peak levels up to 2.4 p.u., which pose a hazard to the insulation performance of electrical equipment. In contrast to conventional substations, the ground GIS is connected to the underground GIS by a long high voltage cable in pumped storage power plants. Due to this long cable, the characteristics of VFTO are currently unclear, and it has caused damage to the main transformer in the substation. Moreover, the distance between the ground and underground measuring points of the substation can reach 1 km, it is difficult to trigger the oscilloscopes of multiple measuring points synchronously. Therefore, it is necessary to achieve the trigger of oscilloscope and the accurate measurement of VFTO waveforms in pumped storage power plants by one VFTO sensor.
Experimental/Modeling methods
According to the characteristics of circuit components in the full frequency range, the circuit and PCB board of this system were designed. The PCB board divided the VFTO sensor output signal into measuring and trigger branches based on the impedance converter. The high pass filter in trigger branch removed the low-frequency part of the VFTO waveform to avoid false triggering of the oscilloscope. Using the bandwidth calibration platform, the bandwidths of measuring branch and the parallel trigger branch were calibrated. And the VFTO generated by the operation of disconnector was measured by this system in a 252 kV GIS.
Results/discussion
The bandwidth of the measuring branch was from DC to 100 MHz, and the bandwidth of the trigger branch was from 800 Hz to 100 MHz. The results of measuring VFTO on the 252 kV GIS showed that all of the VFTO parameters including the trapped charge voltage, breakdown characteristic of disconnector can be measured correctly, and the trigger branch can output high frequency trigger signal.
Conclusions
The trigger integrated VFTO measurement system based on impedance converter can accomplish VFTO measurements in full bandwidth and oscilloscope triggering stable by one VFTO sensor. The system is able to solve the difficulty in achieving multi-point triggering when measuring VFTO of pumped storage power plants and realize accurate VFTO measurement in full bandwidth.

very fast transient overvoltage (VFTO);,Gas Insulated Switchgear,impedance converter,trigger,frequency bandwidth