FM fault location on short power cables

A method was developed for determining the fault distance on electrically short power cables using swept-frequency FM techniques. The method is based upon the observation that the envelope of the composite (transmitted plus reflected) signal at the sending end of the cable has the same pattern as the magnitude of the input impedance function of the cable over the range covered by the frequency sweep. When the cable is electrically short, only a fraction of a cycle of this impedance magnitude function is observed so that the coefficients of the first three Legendre polynomials suffice to fit the impedance curve. By comparing Legendre coefficients measured on a faulted cable with those computed in advance for a cable having identical parameters, the fault distance can be inferred. The algorithm was evaluated in terms of fidelity and sensitivity to parameter variations by means of both digital computer simulation and laboratory tests. A prototype hardware implementation of a Legendre coefficient measuring system which was designed and built during the study is described.