Methodology and accuracy for non‐invasive detection of switching transient overvoltages from compensation coils connected to power transformers

Abstract

Transient voltages from (dis)connecting compensation coils near high voltage transformers are studied. Non-invasive sensors are needed in order to exclude any influence by the measurement equipment on the overvoltage magnitude. This study describes results from open-air capacitive sensors as part of a differentiating/integrating measurement system. Its bandwidth (up to 10 MHz) allows to record large detail in the switching events. However, the coupling strengths and the cross-couplings inherent to the use of air capacitive sensors need to be determined. Calibration based on symmetric phase voltages during normal operation supplies part of the information. Three methods are compared to supplement this knowledge for establishing the complete coupling matrix between phase conductors and sensors. One method employs estimates from the system configuration, in particular, its symmetry. The other techniques extract information from fast transient events within the responses for both switching on and off. The decoupled waveforms from different methodologies align within their confidence bounds and predict the maximum overvoltage magnitudes within an accuracy of 5% when employing information from the fast transients. Based on measurements performed on two compensation coils, both tested three times, larger overvoltages occurred during energisation compared to de-energisation with maximum amplitude exceeding three per unit. (a) The sensor signals, waveforms after decoupling based on (b) Symmetry, (c) Energisation, (d) De-energisation. The arrows indicate instances where the crosscoupled oscillation from phases 1 and 3 start dominating the (faster) oscillation in the signal representing the interrupted phase 2 IET Sci. Meas. Technol.

On-site Measurement of Complex Waveforms on Transmission Network Components

“…It is characteristically terminated at the receiving end and the response will be the time derivative up to the cut-off frequency given by 1/2πR1(C1+C2). The capacitor C2 can either be the parasitic capacitance from the sensor to ground or part of the circuit design aiming for a defined high-frequency cut-off frequency [4]. An integrator restores the waveform.…”

“…A drawback is that a sensor not only receives a signal from the intended phase conductor, but also from the other phases. Signal decoupling methods together with their accuracies are described in [4].…”

“…The coupling of the sensors to the phase conductors can be formulated in terms of a 3 by 3 matrix [4]. The nine elements are extracted from the following considerations.…”

“…The relative response of a sensor responding to the energization of a neighbouring phase, compared to the response of the sensor near that phase, is a measure for their relative coupling strength. With this information applied to the contact instances of the three phases, the coupling matrix can be established [4]. The decoupled phase voltages upon energization are shown in Fig.…”