Page 54 - Basic PD Theory

PD Investigations

a noticeable effect, especially if the surface contamination becomes partly conductive when damp.

* Contamination
Failure mechanism Polarity Load Effect Temperature Effect Phase Location

Contamination No predominance None Unpredictable 15°, 75°, 195°, 255°
(Can be erratic)

Symptoms Detection Tests Insulation types
PD, combination of All stator winding types
oil/grease/dust, white PD, IR, PI, Hi-pot, power factor, tan δ, tip- (High voltage coils)
powder up, visual inspection

7.3 Other Arcing and Sparking

7.3.1 Broken Strands Arcing
Poor Electrical Connections are described in Section 1.4. When excessive endwinding vibration develops or due to
manufacturing defects, some machines can experience broken copper conductors or strands of the stator winding. Such breaks
tend to occur near the outer ends of the stator bars. Violent arcing occurs at the point of failure. This is not voltage driven PD
but rather contact sparking with intensity of the activity usually dependent on a machine load, that is, current through the
conductor. Similar arcing can occur at the bar jumper connections when there is a crack in the jumper or the brazed connection
is unreliable. Although such arcing may produce plenty of high frequency activity in some cases, the detection of these PD-like
pulses could be complicated due to remoteness of the source from PD coupler locations – the fault can be anywhere in the
winding and even at the neutral ends. Sometimes useful information can be seen from the “System Activity” data plots. Please
refer to Zone of coverage, Appendix E.

C

0/180 º phase Yes Mechanical
position? vibration

* Electrical Connections

Failure mechanism Polarity Load Effect Temperature Effect Phase Location
Broken conductor Often at 0°/180°
arcing Unknown Direct Unknown but can be erratic

Symptoms Detection Tests Machine types
Partial discharge, white
powder, insulation PD, visual inspection, Infrared scan Any connection
discoloration

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