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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.

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Abstract-Switched-mode power supplies in variable-frequency drives often combine good electrical efficiency with generous production of conducted emission. The common-mode (CM) current through the output-to-motor cable may perturb nearby systems via cable-to-cable crosstalk. Parameters relevant for the coupling are the rise-fall times and amplitude of the output current and voltage, and types and lengths of the cables. Of the many techniques to reduce the crosstalk, we investigated a particular one: reduction of the CM current by the armor of the motor cable. The armor is intended for mechanical protection, but may also substantially reduce the crosstalk. In an actual installation, we measured the transient current through the three phase leads of a motor cable. The cable length was 85 m, and it was buried in wet soil for most of its length. We also measured the transfer impedance of the armor up to 10 MHz, and used this to calculate the overall CM current in a coupled cable model using assumed reasonable values for the cable and soil parameters. Measurements and model results agreed well on the amplitude of the dominant resonance at about 300 kHz. The ratio of the inside transients to CM current was a factor of 20.

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