Figure 4.1 PD pulse measurement on motors (Red/Yellow/Blue phase notation)
a CT connected to motor neutral
b Capacitor coupler connected to high voltage phase terminal (each
measured in turn)
interference (RI) measurement [22]. In this form, it is used to assess spectra on a comparative basis among spectra measured at different time intervals e.g. annually, on
the presumption that changes in the spectrum may be indicative of discharge activity.
In use, care should be taken to make a reference measurement with the CT disconnected from the neutral prior to making the actual measurement to ensure as far as is
practicable that interference is not compromising the measurement. However, it must
be remembered that the neutral will often act to pick up extraneous signals especially
in a noisy environment, and this nullifies the validity of this approach. Only with the
CT connected to the neutral, with the plant deenergised, can a true comparison of this
type be made.
The second approach is to connect the discharge transducers to the high voltage
terminals of the plant item, e.g. the individual phase terminals of a motor, in turn
(see Figure 4.1b). Typically, this discharge transducer consists of a discharge-free
high voltage capacitor connected to a low voltage impedance circuit (RC or RLC)
which in turn is connected to an oscilloscope or similar instrument. By careful choice
of component values, the high voltage is reduced to a safe level at the low voltage