Online en offline tests
After commissioning of high voltage generators or motors it is recommended to take first base line measurements under different load conditions online, or by applying voltage from an external power source offline. The PD measurement under offline condition can be taken from temporarily installed capacitive couplers, whereas with online measurements permanently installed couplers are required most likely. Loss factor measurements are commonly taken under offline conditions with variable voltage only.
With off-line tests the entire winding is energized 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 1nF-couplers is 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 synchronization and voltage measurement. Permanently installed couplers of the CC20B type may be utilized as well, as they offer a superimposed voltage signal for the synchronization. Other types of couplers, such as the cable type couplers may be used with some restrictions regarding bandwidth and synchronization.
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 bus bar.
ICMsystem Operation Manual
IEC TS 60034-27
"Off-line partial discharge measurements on the stator winding
insulation of rotating electrical machines"