PD Interpretation e

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Sources of Partial Discharges in Rotating Machines:

General Interpretation Scheme for Partial Discharge Pattern Monitoring (PDM-3 and older Versions)

Im Stetterfeld 1 CH-5608 Stetten +41 (56) 496 62 80 +41 (56) 496 6578 interpretation E.doc Bernd Fruth Datum: 11.11.99 Seite 1 von 11

© PD Power Engineering AG

General Remarks

All stresses act as a TEAM on the insulation system

Thermal

Electrical

Ambient (Environmental)

Mechanical

Very often Partial Discharges indicate above stresses, they areoften not the cause but the result of the problem

often defects occur in specific locations, there is no overall averageageing, e.g. using a hammer for placing only one bar might harmthat specific bar.

Single weak points in the insulation system are indicated by partialdischarges. Partial Discharge Patterns represent the best way toidentify the defect type.

This application note gives an overview about typical partialdischarge patterns and their physical origin.

Conventional test methods as tanδ as well are sensitive to partialdischarges, but, they represent an integral value as the loss createdby the partial discharge pulses must be very high.

Partial discharges are plasma processes which take place generallyin a gas. Whenever the electrical field in a gas space exceeds theinception field of the gas, partial discharges are created: on thesurface of an insulator, due to conductive contamination, in air

inclusions of the insulation as a result of mechanical forces. Thegas plasma accelerates ageing and corrosive substances as ozoneand NO2 are created. However the mechanical wear or a surfacecontamination is usually more harmful than the partial dischargeaction. As the presence of partial discharges cannot be avoided inrotating machines insulation, mica is applied as a partial dischargeresistant medium.By-products of the partial discharge are commonly white powders

which can be found in many locations of the machines.

Typical Partial Discharge Phenomena in Rotating Machines

• loose wedges/bar vibration/slot discharges

• cracked and broken conductors

• end winding surface contamination

• damage and corrosion of the corona suppression system• connection ring external discharge due to vibration

• interphase discharge

• insulation degradation

• collector ring, brush sparking

• faults in generator high voltage accessories







Partial discharge locations: 1. internal discharges, discharge sources in the slot sections

A: highest el.field - usually pd - e.g. field calc.E = 20 to 30 kV/mm

B: delamination: winding - main

insulation, E = εE0 = 10 kV/mmC: delamination of tape layers, E see C

D: treeing in layers, unknown, very high locallyE: broken strandsG: voids, E = 1.3xE0,

F: slot discharge, semicon paint abrasion,E see C

Result: Field Strength is generally higher than typicalinception fields of gases, the complex structure with all thedifferential thermal expansion always shows partial discharge

F

A

BC

D

G

E

Slot / iron

Semiconducting Layer (corona protection paint or paper)

conductors

mica paper layers(main insulation)

winding insulation(and/or inner coronaprotection)







Partial discharge locations: 2. external discharges, discharge sources in the endwinding

H: Discharges in Cooling Duct, mainly created by vibration andabrasion of corona protection layers

I: Delamination of Insulation in Elbow (Especially whenmanually manufactured)

J: endwinding surface discharge (contamination etc.)K: Insufficient Spacing, Tracking, especially between bars with big

voltage difference, different phasesL: Connection area between slot corona protection and endwinding

corona protectionH I

J

K

L







Typical partial discharge patterns (applies to on- and off-line tests)

Typical values for sparking are around 20 nC and higher.

SLOT DISCHARGES + ENDWINDING DISCHARGES: F + J

A further typical superimposal of different discharge sources isgiven in the next example which shows the presence of slotdischarges and discharges due to an eroded endwinding coronaprotection:

The endwinding discharge pattern characteristics are explained inthe next chapter

ENDWINDING DISCHARGES: J

endwinding discharges are created by a surface contamination or ageing or inadequate design of the potential grading in theendwinding. This ist usually indicted by white powders on thesurface or within the layers of the endwinding tape.

Slot PD: F

Spark PD: J








The next Figure shows such deposits:

It is very typical that the traces of partial discharge can be seen atthe edges of the semiconducting tapes or under the tape layers.

The corresponding partial discharge pattern is shown in the nextFigure:

Typical is the asymmetry, negative halfcycle discharges are higher.But compared to slot discharges the triangular shape is differentand more oriented to the voltage maximum.Discharge magnitudes are around 10 nC.

Another modification of endwinding discharges is shown in nextexample, which is a superimposal of internal discharges in the maininsulation (C-location) and surface discharges in an initial state.








external and internal PD

INTERNAL DISCHARGES: C

If discharges prevail mainly in the mica paper layers and manydischarge sites are presents usually a pattern depicted in the nextFigure is created. The main characteristics are its symmetry withrespect to voltage half-cycle and its centre of gravity which liesaround 30 deg.

internal PD alone

INTERNAL DISCHARGES: A, B, C

The next Figures show internal discharges of A, B, C type in their initial state , these patterns are often encountered in new insulation(bars and machines and they exist in various similar patterns.

endwindingdischarge

internaldischarge








Small defects in A, B and C LocationInitial or birth state, characteristic is the symmetry and sineshaped lines and envelopes:

No asymetry: probably C Rather Symmetrical: Maybe C, probably slight thermal ageing

Slight asymmetry, positive higher: probably A or B








ENDWINDING DISCHARGES: C

Weak endwinding discharge: one or few sites maybeassociated to surface tracking

DEFECT INTERNAL STRESS GRADING: IVery strong sparking (200nC) at low repetition rate due tobadly contacted internal corona shield

Poor contact stress grading: L

Poor contact between slot corona protection and stressgrading, capacitive charging effect. Looks like a contactproblem, high magnitudes, 30 to 100nC







Some general remarks

The patterns may vary in amplitude. If no drastic changeschanges are detected over time and the shape remains similar the pd process is stable and the risk is usually low.

Patterns or signal amplitudes may vary with load andtemperature:

Decrease of PD with load/temperature:

• normal, statistical time lag decreases > lower amplitude

• insulation expands with temperature > closing of delaminations > lower amplitudes

Increase with Load alone• wedge loosens

• overhangs loose, bar vibration

Generally one might say that:

• Maximum Charge Level: reacts on defect size mainly,attention, depends on machine type!!!

surface discharges

• good: < 1 nC, medium < 10 nC, Bad > 30 nC

slot and near-conductor discharges

• good: < 1 nC, medium < 10 nC, Bad > 15Special attention: the nC calibration is ambiguous due to the

attenuation of partial discharge pulses in the winding.

Use of this report:In no event shall PDTech GmbH be liable for special, indirect or consequential loss or damages.

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