Noiseless PD measurements in hydrogenerators through HF grounding of the couplers and T&A calibration

Improvements of the “differential technique” of partial discharges (PD) measurements in hydrogenerators have led to noise-free PD spectra. These low-noise measurements have been obtained following the development and the implementation of the “HF grounding” technique for the couplers and of the “Time and Amplitude” (T&A) calibration technique for the PD detection systems. Following theoretical considerations on the behaviour of ungrounded coaxial lines, the “HF Grounding” technique is introduced and is shown to lead to an improved PD detection circuit having an improved response to PD impulses and an improved immunity against HF electrical noises. In order to achieve the required calibration accuracy for PD differential measurements, specially designed impulse injectors have been developed to feed the calibration impulses into the stator´s windings through the phase buses. The necessary details to implement these techniques are mentioned and PD measurements obtained from hydrogenerators in various conditions are discussed