Page 24 - Basic PD Theory

PD Data Types

3

Chapter

3 Detection of PD

How to detect PD and only machine PD
K
nowing that PD exists is one thing, but how can you detect the PD? More importantly, how can you use the
information that you have obtained to ascertain the condition of the machine and therefore any maintenance or
repairs that are necessary? Now we just have to figure out a way to detect the PD originating within the winding,
while not detecting high frequency pulses from other sources.
PD detection involves measuring four characteristics of the PD patterns:
 PD magnitude ⇒ relates to the size or volume of the defects
 PD pulse count rate ⇒ relates to the number of defects
 PD polarity ⇒ relates to the location of defects within the slot section of the winding
 PD position relative to the phase-to-ground voltage ⇒ relates to the location of defects either in the slot or the
endwinding

3.1 PD Sensors
The first step of PD detection is the placement of a sensor somewhere near the source of the PD,  Capacitive
which is the high voltage coils. Iris uses two types of sensors: couplers used for
* Capacitive couplers, Epoxy Mica Capacitors (EMC) - for motors, hydros, and small turbos sensors are 80pF.
* Stator slot couplers (SSC) - for large turbos (>100MW)

3.1.1 Capacitive Couplers
The capacitive couplers used are 80pF +/- 4pF, sized for the electrical clearance for 6.9kV,
16kV and 25kV applications and they meet all the requirements of IEC 60034-27-2 [12]
[Figure 17]. The 50/60 Hz signal is attenuated and the high frequency PD signal can pass
more easily, as shown in Figure 18. This is obvious by comparing the impedance of an 80pF
capacitor at a typical power frequency (60Hz) to a typical partial discharge frequency
(83MHz).
X C(60 Hz) = 1/2π(60)80pF = 33 MΩ
(high impedance - blocks)
X C(83MHz) = 1/2π(83M)80pF = 24Ω
(low impedance - passes)
Elegantly simple in appearance and construction, these coupling capacitors are Figure 17: EMC couplers
rugged and reliable, presenting little risk to the machine on which they are installed.

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