The Quadripole – an Underrated Component of Partial Discharge Measurement

The quadripole, also known as power separation filter serves as measurement impedance typically in the ground lead of a coupling capacitor or a test object. The quadripole, along with the coupling capacitor, forms the first filter in the measurement chain to separate the test voltage of several hundred kV and the partial discharge signal as low as 100µV, while the frequencies to be separated are only about three decades apart, which makes the choice of the filter characteristics demanding. Here, in frequency domain, an optimized stopband performance as well as a low passband ripple is required, while in time domain the step response shall show low ringing. Accordingly, further filtering in the measurement chain needs to match and maintain the strategy chosen. Besides the high frequency properties for partial discharge testing, a quadripole must withstand a sudden breakdown of the test voltage and the related surge current and transient voltage. Moreover, the chosen pass-band impedance has a strong impact on sensitivity, signal cable matching, and reflection properties. Various design principles and examples, as well as component properties are discussed.