Page 25 - Basic PD Theory

PD Data Types

They are made to close electrical tolerances and of necessity, they are partial discharge free well beyond voltage levels
normally encountered in machine operation. These capacitors are permanently attached to the phase circuit ring or
the isolated phase bus [
& Figure 28]. By connecting the couplers to these high voltage points they are close to the points in the winding that have the
highest partial discharge activity. Remember, partial discharge is dependent upon the applied voltage so that only the line-end
coils are likely to have discharge activity.

1.5kΩ

Figure 18: Capacitive Coupler Location

3.1.2 Stator Slot Couplers (SSC)
On some large (>100MW) high-speed turbo generators there may be
PD-like pulses that occur because of contact sparking at the generator
terminal flexible leads and/or core lamination arcing. In some
circumstances it may difficult to discriminate between these pulses and
true (insulation) PD using external capacitive couplers, especially for
hydrogen-cooled machines. Therefore, for these applications we
recommend using an antennae type sensor in these situations. This
antenna sensor, called stator slot coupler (SSC), is comprised of an
electrode structure printed on an epoxy-fiber glass laminate [Figure 19].
The SSC is placed in the slot embedded either between the top and Figure 19: Stator Slot Couplers (SSC)
bottom coils or directly underneath the wedge, as shown in Figure 20.
Like the capacitive couplers, the SSC attenuates the low-frequency noise pulses but detects the high-frequency PD pulses. Since
the SSC makes no electrical connection to the winding, it is only sensitive to PD occurring within the slot containing the SSC.
Though this limits the amount of coverage for the sensor, it does guarantee that only PD from the slot and not high-frequency
disturbances from the core, brushgear or other local sources will be detected. When the SSCs are installed within a hydrogen
environment, a pressure penetration flange is required. In addition, an external voltage reference is needed to locate the PD
pulses relative to the AC phase-to-ground voltage since the SSC is not connected to the winding.

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