DOWDING & MILLS

Condition Monitoring Of High Voltage

Machines

By The Analysis of

Electrical Partial Discharges

22 April 04

Dino KouadriaPhD MSc BEng CEng MIEE MIEEE

DOWDING & MILLS

CONTENT

What is Partial Discharge (PD) Analysis.

Stator winding Common Faults.

Basics Of Why & How To Measure PD.

Case Studies.

Open Discussions.

DOWDING & MILLS

It is an electrical condition monitoring

technique.

Used to assess the condition of hv

insulation

Wider applications

static items i.E. Switchgear, cables

transformers, etc.

Dynamic items i.E. Electrical machines

What is Partial Discharges Analysis

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Apply to HV motors & generators (>5kv)

Online & off line tests.

Predominantly on-line tests - no shut

down & tests under

normal stress condition.

PDs Analysis On HV Windings

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Assess the current & development in

condition of the

windings

Maintenance planning based on machines’

health.

Elimination of unwanted/unexpected

shutdowns.

Assistance in targeting maintenance effort

effectively.

Leave Maintain

Replace

PDs Analysis On HV Windings

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Why Do HV Stators Fail?

Failures due to windings’ insulation failure caused

by the combined

electrical, thermal, environmental and mechanical

stress factors.

Most common causes in modern insulation are

mechanically induced defects i.E. Loose wedges,

erosion due to bar vibration etc. - Short time

degradation process.

Discharge between phases due to cracks / low

spacing

Contamination of end windings.

Others: internal voids, insulation ageing,

delamination.

Electrical partial discharges

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Regardless of the type of defects, they

all result in the generation of

Electrical partial discharges.

So what are partial discharges?

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What is a partial dischargeLocal breakdown that short circuits part of the Insulation.

Formation of corrosive ions, acids, nitrogen oxides, ozone.

Detected using of chemical sensors for detection.

Emission of light, sound & electromagnetic waves.

Detected using ultrasonic devices, antennae, cameras..

Injection of high frequency current pulses.

Detected using electrical partial discharges measurement

(pd).

Vs = Applied voltage

CavityVs

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-

+ +

Patterns & location of pulses within AC cycleIs the key in defects identification

Partial Discharges Basic Theory

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Importance Of Monitoring PDs

Different defects produce different PD patterns.

Complex PD patterns are produced when multiple defects

are present.

Failures are due to localized defects.

PD pattern development over time represents developing

degradation processes.

Provide an early warning of potential problems

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Discharge levels

Time

3 – 6 months ?? Years

Curing Process Fabrication Defects

Contamination

Mechanical failure

Partial discharge monitoring over a machine life cycle

Repair/action required

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PD Detection

Two main type of permanently installed sensors.

Capacitive couplers first developed in north

america. Main users power industry.

Rogowski coils first developed for the north sea

oil industry.

North sea oil industry – mainly Rogowski coils

Ease of installation: no disconnection of supply

cables, less installation time.

Atex certified

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Rogowski Coils: Typical Installation

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Partial Discharge Analysis/Display: Typical Cycle

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Partial Discharge Analysis/Display: Total Discharge Activity

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Partial Discharge Analysis/Display: Pulse Height Analysis

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CASE STUDIES

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Case studies

1 Contamination on old pump motor 11KV-3MW

2 New motor manufacturing problem 13.8KV-12MW

3 Coil Movement old motor 6.6KV-9MW

4 Plastics plant old motor high PD 11KV-2.5MW

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Total Discharge Activity

3MW Pump Motor

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White power deposits clear signs of discharge activity

Typical Detected Faults Damage due to movement and contamination3MW Pump Motor

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13.8 kV 12.7 MW Gas Compressor

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13.8 kV 12.7 MW Gas Compressor

0ver 70% of wedges loose or missing

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Typical Detected Faults Damage due to wedge movement

13.8 kV 12.7 MW Gas Compressor

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Banded Discharge is modulated which is due to coil movement .....

9MW Gas Compression Motor

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9MW Gas Compression Motor

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Good Coil

Air Duct Impressions

9MW Gas Compression Motor

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Damaged Coil

Lamination groves due to movement

Air Ducts (no abrasion)

9MW Gas Compression Motor

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Heavily Damaged Coil Due to impacting on slot bottom

9MW Gas Compression Motor

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2420KW 11kV ASEA synchronous compressor motor

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Detect any potential insulation problems.

The contribution of the different problems.

Location of problems within the winding.

Development in windings condition (through regular

monitoring)

Early detection allows scheduling of remedial

work/repairs.

“Provide the maintenance engineer with the

information to make

The most cost effective decisions on planned

maintenance”

Outcome of A PD Analysis Survey

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Partial Discharge Analysis

Not an exact Science.

Does not predict when plant is likely to

fail.

BUT will highlight which machines are

most likely to fail.

Does not detect Inter-turn Faults

More room for improvement in defect

recognition.

Future development in insulation/motor

designs.

DOWDING & MILLS

Condition Monitoring Of High Voltage

Machines

By The Analysis of

Electrical Partial Discharges

22 April 04

Dino KouadriaPhD MSc BEng CEng MIEE MIEEE