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Disconnector switching operations in large gas-insulated switchgear (GIS) systems may produce very fast transient overvoltages (VFTOs), causing high electric stress to other apparatuses in the substation, such as power transformers. This paper presents a VFTO measuring technique based on a transformer bushing sensor, which consists of the transformer bushing (C1) and a capacitor sensor (C2) installed at the test tap of transformer bushing. In order to solve the problem of the frequency limitation of the transformer bushing sensor for measuring VFTO, a convolution model on the square-wave response of the transformer bushing sensor is introduced to obtain the frequency response of the transformer bushing sensor, and a deconvolution model based on the incremental Wiener filter is applied to reconstruct the input transient voltage waveform at the tip of the bushing by using the output signals from the bushing sensor. A 110-kV transformer bushing sensor and a fast transient overvoltage source are built in our laboratory. The results from the laboratory indicate that a transient voltage can be measured correctly based on our technique with the 110-kV transformer bushing sensor. A 750-kV transformer bushing sensor is built in a 750-kV substation. The measuring results from the 750-kV substation by using our technique show that the peak value of the VFTO produced by the disconnector (AIS) switching off operations is about 775 kV, and the dominant oscillating frequencies of the VFTO are 3.5 MHz, 1.6 MHz, and 650 kHz.

Disconnector operations in GIS may generate very fast transient overvoltage (VFTO), which is characterized by its high peak value, short rise time, and high frequencies. Mostly, researches on VFTO are focusing on the apparatus involving in the active circuit. However, VFTO may pass through open devices by electromagnetic coupling. To measure VFTO under this situation, lots of field experiments have been carried out in a 750 kV substation. VFTO is measured at the test tap of the high voltage reactor bushing, using the sensor we developed. In one of the experiments, the breaker is kept open, VFTO is generated by disconnector operations (in AIS) on one side of the breaker and measured on the other side. Results show that under this situation, VFTO could reach the peak values of about 55 kV and - 70 kV, the main oscillation frequencies are about 236 kHz, 535 kHz and 1.59 MHz. To verify this case further, computer simulation has also been carried out in the laboratory with the help of ATP. The results agree with each other well.

A capacitive sensor has been developed for the measurements of very fast transient over-voltages (VFTO) in GIS. Step response measurement was performed on the sensor. It is shown from the test that the settling time is less than 20 ns and the response time is less than 2ns, suitable to the IEC1321-1. The sensor has been calibrated by power frequency voltage, impulse voltage, the division ratio of the power frequency voltage is almost as the same as that of the impulse voltage, the maximum error was less than 1.3%. The sensor was mounted in a 126KV GIS test setup to measure the transients produced by the air gap discharges. The measurement results demonstrated that the sensor is able to measure the transients with 10ns rise-time successfully.

Disconnector switching operations in large gas insulated switchgear (GIS) systems may cause very fast transients overvoltages (VFTO), which produce dielectric stress to transformer connecting with the disconnector. A VFTO bushing sensor is developed to measure the VFTO at the transformer entrance. Since the high frequency ratio of the bushing sensor is not constant, calibration before the field measurement is needed. Based on the convolution model and step response, a new calibration method is used to identify the frequency response of the bushing sensor. Calibration of an 110 kV rated bushing sensor is carried on in our laboratory to investigate the effectiveness of this method, and the calibrated results agree well with the amplitude-response gained by frequency sweep test, which demonstrates the calibrating method can be applied in VFTO bushing sensor calibration. Calibration of an UHV rated bushing sensor is processed in a factory and the frequency response of the sensor is given at the end of the paper.