Partial discharge measurement at high voltage test circuit having static frequency converters

For testing of high voltage (h.v.) components under so-called unconventional frequency test conditions as h.v.-test source very often static frequency converters (SFC) were applied. A typical example for that is the testing of power transformers at higher frequency. In the past this test was provided by standardized motor-generator sets, but more and more alternative power sources having SFC-units are used. In that case the performing of sensitive diagnostic test procedure like partial discharge (pd) measurement seems to be impossible caused by vast electromagnetic disturbances which are generated by the switching part of the SFC. On the other hand the performing of such sensitive diagnostic measurement is obligatory. For overcoming of these problems two possibilities will be described in the paper. At first the EMI pulses which are coupled into the test circuit must be suppressed. Unfortunately, because of the mostly unknown “current-way” of these disturbances, the optimal solution of suppression must be found by a “half-empirical” way, taking into account the well-known EMC requirements. In addition to that the right choice of the commonly used filter located after the SFC-component is very important and should be designed following the performed EMC measures. For an exemplary h.v. test circuit the appropriate measures to minimize the EMI disturbances are described, at which a pd-sensitivity of ~ 30 pC could be obtained at the full range operation of SFC. For further increasing of pd-sensitivity additional measuring methods were investigated. For a desirable measuring sensitivity of ~ 10 pC the suppression of EMI signals, reached by additional measures should have at least a value of ~ 10 (20 dB), what could be obtained either by digital or by analogous methods. Using all the described measures for h.v. test circuits having SFC a pd measuring sensitivity in the range of few pC seems to be reachable.