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PARTIAL DISCHARGE BASICDIAGNOSTIC

Basics, Sensors &Instruments, Experience

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BASICS

“Partial discharges” are fast flows of electrons (discharge) due to a local concentration ofthe electric field which occurs across two insulated bodies at different potential, that doesnot breakdown the whole insulation.

They normally occurs within gases (air or other).

PD’s normally occur within electric machines and other apparatus due to the technologyutilized in the insulation systems (termosetting materials).

There are different kinds of discharges, some are harmful some are not.

It is not true that the most dangerous PD phenomena exhibits the larger amplitude

PD quantities and PD intensity must be associated with the defect typology to properlyassess harmfullness

Different insulation systems exhibits different characteristics as to PD patterns and PDlevels, therefore experience is the Key-factor

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PDs are slow degrading phenomena, therefore they can be used for early detectionof potential cause failure, and normally they allow for proper planning of correctiveactions.

PD are at the same time both a symptom of a defectand a new degradation process that can lead to the final breakdown.

PDs measurement allow for local defect identification and monitoring, improvingCBM efficacy .

BASICS

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International Standards on PDs measurements:

IEC 60270 – High-voltage test techniques – Partial discarge measurements

IEC 60034-27 – Rotating electrical machines – Partial discharge off-linemeasurements on the stator winding insulation of rotating machinery

STANDARDS do not give acceptance criteria for new machines or machine in service

At the moment, there are not end-of-life criteria based on PD

Diagnostic is based on trending and experience

BASICS

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Slot PD

Surface PD (stress grading)

B2B Tracking

Tracking

Internal PD

Sparking

Pressure finger

Stator core

BASICSRotating Machines PD Sources

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BASICSInternal Conductor Delamination

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BASICSSurface Discharge

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Discharges result in a electric charge displacement that generates current pulses,which are detected as voltage pulse (Vm) at the sensor terminals.

PD pulse detected by the instrument

VmVa

Cx

Ck

Zm

Measurement principle

Capacitive sensor

PD Detection And Measurement

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signal

Triggerlevel

Pretrigger

UWBDigitizer

RAM Bank

Permanentstorage dev.

time

Recording time

NIDEC ASI’s instruments, differently from others, are not simple PD peak detectors,but they acquire and process the complete pulse shape.

Insulation

HFresistor50W

High Pass(HP) Filter

Digitizer

system

PersonalComputer

fiber opticIEEE-488 bus

PD Detection And Measurement

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Signals coming from different PD sources and/or locations present differents wave-shapes.

The whole shape of signals is digitally detected at the fastest sampling rate, toavoid the frequency aliasing.

Pulse signals are synthesized considering both their amplitude and phase ofoccurrence and reported on a standardized cycle of the applied voltage to obtain a 3Dplot (or 2D plot).

Pulse signals are recorded in a number high enough to perform statistical analysis.

Different PD sources are separated through TF mapping.

Noise by different sources (power electronics switches, broadcast signals, arcing,AC/DC rectifiers, random noise, PD cross-talk, etc.) is rejected by identificationthrought TF mapping.

PD sources are identified by comparison

PD Detection And Measurement

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Pulse signals are synthesized considering both their amplitude and phase of occurrence andreported on a standardized cycle of the applied voltage to obtain a 3D plot (or 2D plot):

Synthesization

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T

F Fast pulses:Short duration,

High frequency content

Slow pulses:Long duration,

Small frequency content

Each Pulse is processedindividually and represented by apoint on the TF map.

PD pulses are separated into sub-series each one homogeneous interms of shape (time-frequencymapping).

TF Mapping

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TF Map

F2: Internal PD

End-winding stress gradingdeterioration, due to thermalstress, is the defect typologyassociated to pattern F1

SeparationSignals Having Different Shape are grouped din different classes

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Visual comparison between the given 3D PD plot and a collection of known patterns(databases).

Automatic pattern matching by means of Neural Net., distance classifiers, statisticalclassifiers (expert system).

3D shape analysis by means of identification markers derived by both standard andPD pulse height and phase distribution analysis.

Identification

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Different defects can affect differently the insulation system reliability: it is not true thatthe most dangerous PD phenomenon exhibits the larger amplitude

PD quantities relevant to PD intensity must be associated with the defect typology

At the moment, there are not end-of-life criteria based on PD

A complete collection of 3D PD plots and the relevant defect typologies is essential forreliable diagnostics

Design and manufacturing greatly affect PD patterns and levels: OEM experience isessential for reliable diagnostics

Diagnostic-Risk Assessment

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Surface PDSurface PD with

tracking Bar-to-bar PD

Tape delamination PD Slot PDDistributed

microvoids PD

PD Sources & 3D PD PatternsSurface Discharge

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• Noise rejection

•Source separation

D

Diagnosis

S I

• Potential defect..harmfulness ..(onesource at a .. .. time)

• Risk assessment

• Maintenance program

• Life extension (trend of..the weakest spots, ..time toend point)

Separation Identification

The Strength of Our AnalysisSurface Discharge

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PROS

The whole winding is tested

Applied voltage can vary to determineinceptions and fading voltage

Reduced noise

Easier analysis of the data collected

CONS

Voltage distribution across the windingis different from that in service.

Operating load and temp. of thewinding cannot vary

Interphase PD cannot be detected

Off-Line Measurements

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PROS

Actual PD are measured while themachine is operating

Change of load and temperature tocheck the influence on PD

Interphase PDs are measured

CONS

Applied Voltage cannot varies

Pulsating noise is higher than in off-linemeasurements

Analysis and diagnostic is more difficultthan off-line

Off-Line Measurements

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A large range of sensors and filters are available to cover any possible PD acquisition needand optimise the metering circuit. Sensors are: capacitive, inductive, antenna

Capacitive sensors family Inductive sensors family

Antenna sensors family

Sensors and Accessories

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Capacitive couplersGenerators/critical motors

HFCT on surge capacitorsLarge/critical motors provided withsurge capacitors

HFCT on ground leadsLarge/critical motors not provided withsurge capacitors

HFCT on MV cables ground leadsLarge/critical motors not provided withsurge capacitors

Clamp HFCT

HFCT

Coupler

Sensors InstallationOn Rotating Machines

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• Ultra wide band acquisition unit• IEC60270 standard feature• Pulse shape analysis• Fuzzy logic tools and statistical processing• Noise Rejection• PD Source Identification• Database application• Location of defects• Fiber optic transmission• External or line synchronization• Fully remote controlled• SMART gating options• Spectrum Analyzer• Quality control test management• Immediate diagnosis and alert• Preprocessing-Post Processing

250x250x100 mm, 7 kg

PDBaseInnovative Digital PD Diagnostics System

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• Ultra wide band acquisition unit• Pulse shape analysis• Fuzzy logic tools and statistical processing• Noise Rejection• PD Source Identification• Waveform Analyser• Spectrum Analyser• Database application• Location of defects• Ethernet interface• Fully remote controllable (internet/intranet)• Stand alone or part of centralized complex monitoring system• Possibility of connecting several instruments to the same network• Large data storage capability (with memory cards)• Ideal for permanent on-line monitoring of PD and other quantitiesi.e. temperature, humidity, currents, voltages, etc.

203x135x55 mm, 1 kg!

PDCheckThe Ultimate Solution for on-line PD Monitoring

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RA1

Derivation

Box

LP

Filter

HP

Filter

HP

Filter

HP

Filter PD

Acquisition/

Monitoring

System

Sync

InP

h.A

InR

B1

RC1

Ph.B

InP

h.C

In

Sync

PD

PD

PD

G

3~

Capacitive coupler installedin a Generator Busbar

Derivation Box

Coupling capacitors Installation

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PFCT

HFCT

HFCT

HFCT

M

3~

Derivation

Box

LP

Filter

HP

Filter

HP

Filter

HP

Filter

Sync

InP

h.A

InP

h.B

InP

h.C

In

PD

Acquisition/

Monitoring

SystemS

ync

PD

PD

PD

HFCTs installed on a motorcable terminal box

Derivation Box

HFCT Installation

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PD measurements evidenced that the machine was affected by:

Distributed micro-voids (all three phases)

Bar to bar (ground) discharges (phase U)

Slot Discharges (phase V)

Typical of modern thermosetting insulation systems

the high frequency currents may degrade, in thecourse of time, both the stress grading system andthe slot coating protection in the region where thebars leave the iron core

Dangerous phenomenon: inspection andmaintenance required !

On-line PD monitoring was performed on a Generator in a Power plant, in order to collectdata to assess the whole machine conditions

Case Study: Power Generator

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Classification mapEntire Pattern acquisition

Exciter noise

Distrib. Microvoids: 20%Slot discharge 80%

cross talk of bar toground discharges inoverhang of phase U

cross talk of distributed micro voidsdischarges in phase U

Case Study: Power GeneratorPhase V

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Results confirmed by endoscopic inspection on generator !

PD traces found in 12of the 60 slots

(7 white traces and 8suspecious traces)

Slot discharges:Potential danger

Case Study: Power GeneratorPhase V

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Off-line and on-line PD testing for on-site diagnostics on rotating machines.

Off-line and on-line PD testing for the commissioning and condition assessmentof other electrical apparatus.

Sensors and instruments sales and installations.

Training on PD measurements

Remote data analysis of PD data acquired by customers.

Services

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