SERVICE PARTIAL DISCHARGE BASIC DIAGNOSTIC

1. Service
PARTIAL DISCHARGE BASIC
DIAGNOSTIC
Basics, Sensors &
Instruments, Experience
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2. BASICS
 “Partial discharges” are fast flows of electrons (discharge) due to a local concentration of
the electric field which occurs across two insulated bodies at different potential, that does
not 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 technology
utilized 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 properly
assess harmfullness
 Different insulation systems exhibits different characteristics as to PD patterns and PD
levels, therefore experience is the Key-factor
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3. BASICS
 PDs are slow degrading phenomena, therefore they can be used for early detection
of potential cause failure, and normally they allow for proper planning of corrective
actions.
 PD are at the same time both a symptom of a defect
and a new degradation process that can lead to the final breakdown.
 PDs measurement allow for local defect identification and monitoring, improving
CBM efficacy .
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4. BASICS
International Standards on PDs measurements:
IEC 60270 – High-voltage test techniques – Partial discarge measurements
IEC 60034-27 – Rotating electrical machines – Partial discharge off-line
measurements 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
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5. BASICS
Rotating Machines PD Sources
Stator core
Pressure finger
Slot PD
Surface PD (stress grading)
B2B Tracking
Sparking
Tracking
Internal PD
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6. BASICS
Internal Conductor Delamination
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7. BASICS
Surface Discharge
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8. PD Detection And Measurement
Discharges result in a electric charge displacement that generates current pulses,
which are detected as voltage pulse (Vm) at the sensor terminals.
Capacitive sensor
Ck
Cx
Va
Zm
Vm
Measurement principle
PD pulse detected by the instrument
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9. PD Detection And Measurement
NIDEC ASI’s instruments, differently from others, are not simple PD peak detectors,
but they acquire and process the complete pulse shape.
signal
UWB
Digitizer
Trigger
level
time
Pre
trigger
Recording time
RAM Bank
Personal
Computer
Insulation
system
50 W HF
resistor
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fiber optic
IEEE-488 bus
High Pass
(HP) Filter
Digitizer
Permanent
storage dev.
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10. PD Detection And Measurement
 Signals coming from different PD sources and/or locations present differents waveshapes.
 The whole shape of signals is digitally detected at the fastest sampling rate, to
avoid the frequency aliasing.
 Pulse signals are synthesized considering both their amplitude and phase of
occurrence and reported on a standardized cycle of the applied voltage to obtain a 3D
plot (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 identification
throught TF mapping.
 PD sources are identified by comparison
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11. Synthesization
Pulse signals are synthesized considering both their amplitude and phase of occurrence and
reported on a standardized cycle of the applied voltage to obtain a 3D plot (or 2D plot):
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12. TF Mapping
F
Fast pulses:
Short duration,
High frequency content
Each Pulse is processed
individually and represented by a
point on the TF map.
Slow pulses:
Long duration,
Small frequency content
PD pulses are separated into subseries each one homogeneous in
terms of shape (time-frequency
mapping).
T
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13. Separation
Signals Having Different Shape are grouped din different classes
TF Map
End-winding stress grading
deterioration, due to thermal
stress, is the defect typology
associated to pattern F1
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F2: Internal PD
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14. Identification
 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, statistical
classifiers (expert system).
 3D shape analysis by means of identification markers derived by both standard and
PD pulse height and phase distribution analysis.
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15. Diagnostic-Risk Assessment
Different defects can affect differently the insulation system reliability: it is not true that
the 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 for
reliable diagnostics
Design and manufacturing greatly affect PD patterns and levels: OEM experience is
essential for reliable diagnostics
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16. PD Sources & 3D PD Patterns
Surface Discharge
Surface PD
Tape delamination PD
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Surface PD with
tracking
Slot PD
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Bar-to-bar PD
Distributed
microvoids PD
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17. The Strength of Our Analysis
Surface Discharge
S
I
D
Separation
Identification
Diagnosis
• Potential defect
..harmfulness ..(one
source at a .. .. time)
• Risk assessment
• Noise rejection
•Source separation
• Maintenance program
• Life extension (trend of
..the weakest spots, ..time to
end point)
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18. Off-Line Measurements
PROS
CONS
 The whole winding is tested
 Voltage distribution across the winding
is different from that in service.
 Applied voltage can vary to determine
inceptions and fading voltage
 Reduced noise
Easier analysis of the data collected
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 Operating load and temp. of the
winding cannot vary
 Interphase PD cannot be detected
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19. Off-Line Measurements
PROS
CONS
 Actual PD are measured while the
machine is operating
 Applied Voltage cannot varies
 Change of load and temperature to
check the influence on PD
 Interphase PDs are measured
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 Pulsating noise is higher than in off-line
measurements
 Analysis and diagnostic is more difficult
than off-line
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20. Sensors and Accessories
A large range of sensors and filters are available to cover any possible PD acquisition need
and optimise the metering circuit. Sensors are: capacitive, inductive, antenna
Capacitive sensors family
Inductive sensors family
Antenna sensors family
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21. Sensors Installation
On Rotating Machines
Capacitive couplers
Generators/critical motors
Coupler
HFCT on surge capacitors
Large/critical motors provided with
surge capacitors
HFCT
HFCT on MV cables ground leads
Large/critical motors not provided with
surge capacitors
Clamp HFCT
HFCT on ground leads
Large/critical motors not provided with
surge capacitors
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22. PDBase
Innovative Digital PD Diagnostics System
• 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
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23. PDCheck
The Ultimate Solution for on-line PD Monitoring
• Ultra wide band acquisition unit
203x135x55 mm, 1 kg!
• 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 quantities
i.e. temperature, humidity, currents, voltages, etc.
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24. Coupling capacitors Installation
G
3~
C1
B1
A1
R
R
R
HP
Filter
PD
HP
Filter
PD
HP
Filter
PD
Ph.C In Ph.B In Ph.A In
LP
Filter
Sync
Sync In
Capacitive coupler installed
in a Generator Busbar
Derivation
Box
PD
Acquisition/
Monitoring
System
Derivation Box
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25. HFCT Installation
Sync
PD
HFCT
HP
Filter
PD
HFCT
HP
Filter
HP
Filter
PD
HFCT
LP
Filter
Ph.C In Ph.B In Ph.A In
PFCT
Sync In
M
3~
Derivation
Box
HFCTs installed on a motor
cable terminal box
PD
Acquisition/
Monitoring
System
Derivation Box
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26. Case Study: Power Generator
On-line PD monitoring was performed on a Generator in a Power plant, in order to collect
data to assess the whole machine conditions
PD measurements evidenced that the machine was affected by:
 Distributed micro-voids (all three phases)
Typical of modern thermosetting insulation systems
 Bar to bar (ground) discharges (phase U)
the high frequency currents may degrade, in the
course of time, both the stress grading system and
the slot coating protection in the region where the
bars leave the iron core
 Slot Discharges (phase V)
Dangerous phenomenon: inspection and
maintenance required !
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27. Case Study: Power Generator
Phase V
Entire Pattern acquisition
Classification map
cross talk of bar to
ground discharges in
overhang of phase U
Exciter noise
cross talk of distributed micro voids
discharges in phase U
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Distrib. Microvoids: 20%
Slot discharge 80%
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28. Case Study: Power Generator
Phase V
Results confirmed by endoscopic inspection on generator !

PD traces found in 12
of the 60 slots

(7 white traces and 8
suspecious traces)
Slot discharges:
Potential danger
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29. Services
 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 assessment
of other electrical apparatus.
 Sensors and instruments sales and installations.
 Training on PD measurements
 Remote data analysis of PD data acquired by customers.
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