Page 30 - Basic PD Theory

PD Data Types

PD PT Cubicle External noise Breaker
Delay
Generator
80pF 80pF

Same
L1 Length L1

M S
TGA - B

Figure 28: Directional Time-of-Arrival Disturbance Separation
3.4.2.2.3 Directional – (SSC)
Like the Directional (BUS) installation, the SSC installation has two sensors, one on each end of the antenna. It categorizes
pulses by the direction of arrival. The delay time between pulse arrivals is part of the sensor design and does not require
calibration during installation. Pulses arriving at the “Endwinding” sensor first are classified as “Endwinding PD”, pulses
arriving at the “Slot” sensor first are classified as “Slot PD”, while those arriving at both sensors within the Delay time are called
“Under PD”. (See Appendix E. Zone of Coverage)

3.4.2.3 Pulse Shape
3.4.2.3.1 SSCs
Experiments show that internal disturbances, such as
core iron arcing, when detected by an SSC, have a
pulse width > 8ns, whereas PD in the slot has a pulse
width <6ns. The TGA-S instrument, in addition to
the directional separation described above, measures
the width of each pulse and classifies it as either PD,
wide pulses or noise pulses (width > 20ns or
oscillatory).
Figure 29: Single-ended Installation
3.4.2.4 Single-ended
In some installations, particularly motors and smaller generators, the unit is connected to the power system with a relatively long
power cable. If the power cable is longer than about 30 metres (about 100 feet), the high frequency disturbances from the power
system may be severely dispersed and attenuated when detected at the machine terminals. Thus, if a pulse observed at the
machine terminals has a fast rise time it is probably PD; whereas if it has a long rise time, it is likely a disturbance. In these cases,
only one coupler per phase, or three per unit, is necessary. This is often referred to as a “single-ended installation” as shown in
Figure 29. There is no calibration required, though a sensitivity check is recommended.
This configuration is used for PDTrac or PDTracII monitors, but can also be used with a TGA-B or PDA-IV portable
instrument.

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