After commissioning high voltage generators or motors, it’s recommended that you take the first baseline measurements under different load conditions on-line, or by applying voltage from an external power source off-line. 

The partial discharge (PD) measurement under off-line conditions can be taken from temporarily installed capacitive couplers, whereas with on-line measurements, permanently installed couplers are most likely required. 

Tan delta, or loss factor, measurements are commonly taken under off-line conditions with variable voltage only.

With off-line tests, the entire winding is energised with a conventional high voltage test set or with a resonant test set. In case the neutral connection is opened, each of the three phases can be tested individually. Depending on the accessibility, a coupling to the neutral side of the winding or to the line side may be chosen. The range of the 1 nF couplers (the CC25C/V) makes them a good choice. The CC25C/V offers a quadrupole connection and a high-frequency current transformer output, as well as a voltage divider output for synchronisation and voltage measurement. Permanently installed couplers of the CC20B type may be used as well, as they offer a superimposed voltage signal for synchronisation. Other types of couplers, such as cable type couplers, can be used with some restrictions regarding bandwidth and synchronisation.

On-line tests are usually made with line-side couplers permanently installed. With no permanently installed couplers available, a connection of one coupler to the neutral point may be chosen. This on-line neutral connection bypasses the neutral inductor, neutral transformer, or neutral resistor for high frequency signals. The line-side coupling requires three pieces of CC20B or CC14B couplers installed to the terminals of the machine or to the busbar.