Increased Probability of Proper Operation of Filter Protection Devices Against Phase Failure at Voltage Asymmetry in Rural Electrical Networks

Abstract

The specificity of rural electrical networks is the high level of asymmetry of supply voltage. The asymmetrical modes of the power network have randomized character. There is also a high level of drive motors failures in agricultural branch. The reasons for their failures is the voltage asymmetry and phase failure of the supply grid. Existing protection devices, including voltage filters, often do not switch off electric machines in case of such emergency modes due to the impact of accidental voltage asymmetry. A voltage vector of zero sequence could be represented as two-dimensional random continuous function, and the formula for calculation of probability of its impact in a given area in a plot is proposed. The formula for calculation of probability of proper operation of the filter protection device at occurrence of an asymmetric mode is resulted, and calculation of such probabilities is done: at operation in an asymmetric grid at level of average statistical values and break-off of a motor phase, probabilities of proper operation of a protection device on voltage of zero sequence and separate phases are in the interval from 59 % up to 86 %. It is more effective to connect the filter to the artificial zero point, which will make it possible to eliminate the influence of accidental voltage asymmetry and to reconfigure the filter to a higher threshold of triggering with reaching 100% proper operation at motor phase failure.