Measurement of off-frequency signals during live high voltage earthing system testing

During a fault on the power grid, earthing systems operate to contain the released energy keeping people and equipment safe. Unfortunately these systems can fail creating a hazard when the resistance is too high as a result of improper design, installation or maintenance. Regular on-site testing of earthing grids is essential to prevent these situations. The most commonly used method to test earthing grids is accurate and reliable, but only for installations with low background noise, a limited number of feeders and a low system impedance. These limitations in field testing have motivated this research into improving the method of measuring currents during live earthing system tests. Traditional methods of noise removal and parameter extraction do not have sufficient noise rejection capabilities for noise levels equal to and greater than the test signal magnitude. The designed method for current measurement involves active noise removal using destructive interference to improve the accuracy of small signal measurement. Passive digital filtering and phase locked loops are used to identify the parameters for each frequency before the signals are fedback to cause destructive interference. This strategy can measure phase and frequency with an error of less than 0.1% and the magnitude to less than 1% under low noise conditions. For small input magnitudes at noise levels of -60dB the phase and frequency of the signal can be extracted with errors of 1%, however the magnitude extraction can only achieve this for unity magnitude. Improvement of the measurement system in low frequency tests allows for greater confidence in the assessment of an earthing system, its subsequent models and works required for hazard reduction.