Estimating system fault level from naturally occurring disturbances

An essential requirement in all aspects of power system design is an evaluation of the fault levels at a particular switching or control point on the network. This evaluation is covered in IEC 909. The fault level can vary according to the supply configuration and load, particularly where rotating plant is involved. Little work has been done in the past to justify the calculated values of circuit fault levels and there is some belief that the values obtained could be unduly pessimistic. At present there is no easy method of measuring the fault level at a particular point on a network. In theory an assessment of the supply impedance can be made from naturally occurring disturbances. Here, the author describes an instrument based on this principle which is being developed at EA Technology, which uses power system disturbances to make an estimate of both the source impedance, and also the contribution from induction motors downstream of the measuring point