Transcript WHAT IS PARTIAL DISCHARGE?

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An Introduction to On-line
Partial Discharge Surveys
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PARTIAL DISCHARGE
When high voltage insulating material
breaks down Partial Discharges are
created, measured in units of charge
known as pico-Coulombs or millivolts.
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WHAT IS PARTIAL
DISCHARGE?
• Electrical discharges occurring inside medium and
high voltage insulation (flaws, cracks, voids,
irregularities).
• These imperfections create voltage stresses and
cause eventual failure of the insulation.
• Insulation failures begin with and are characterized
by small but detectable releases of energy or Partial
Discharge.
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PARTIAL DISCHARGE
TESTING USED FOR QUALITY
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• For many years partial discharge testing has been
an IEEE/ANSI standard to identify insulation
problems in electrical apparatus before it leaves
the factory floor.
• Many U.S. and International standards have been
developed for partial discharge testing.
• Until recently Partial Discharge testing was
limited to testing at manufacturer’s laboratories.
• Now computers and sophisticated equipment can
isolate and detect Partial Discharge occurrences in
the field.
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NO OUTAGE EVALUATION
These proven technologies exist today to evaluate medium or
high voltage systems while the system is energized.
1. Oil testing/DGA – limited to oil transformers
2. Infrared testing – visible loose connections only
3. Visual inspections – problems may not be visible
4. Power Quality – cannot detect insulation failures
5. Vibration – limited to motors and transformers
6. Partial Discharge Testing – single best all-around test
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PARTIAL DISCHARGE
VS. INFRARED
How are Partial Discharge Inspections
different from Infrared Inspections?
• Infrared identifies electrical current problems (loose
connections) that result in heat. The specimen must be
visible to the naked eye for the infrared camera to detect a
problem.
• Partial Discharge identifies voltage problems that result in
insulation breakdown and generation of Partial Discharges.
The specimen does not need to be visible for Partial
Discharges to be detected.
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INSULATION BREAKDOWN
Why should we be concerned with insulation
breakdown and Partial Discharges?
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INSULATION BREAKDOWN
• NFPA 70B states that insulation breakdown
is the number one cause of electrical
failures.
• In medium and high voltage equipment,
partial discharges are the first indication of
insulation breakdown.
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Total Failures due to Insulation
Breakdown
Component
Transformers
Circuit Breakers
Disconnect Switches
Insulated Switchgear Bus
Bus duct
Cable
Cable Joints (splices)
Cable Terminations
Based on IEEE Gold Book Table 36
Percentage of insulation failure
84%
21%
15%
95%
90%
89%
91%
87%
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WHAT TYPE OF FACILITES
ARE AFFECTED?
• INDUSTRIAL - Large Manufacturing,
Cogeneration
• COMMERCIAL - Research Parks, High Rises
• INSTITUTIONAL - Campuses, Hospitals
• GOVERNMENTAL - Military Bases, Large
Office Complexes
• UTILITY - Investor Owned, Municipalities
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WHERE DO PARTIAL
DISCHARGES OCCUR?
• Electrical Systems > 600V (2.4KV, 4.16 KV,
12KV, etc.)
• Types of Equipment Subject to Partial
Discharge
-Cables
-Circuit Breakers
-Switchgear
-Insulators
-Generators
-Surge Arrestors
-Transformers
- Instrument Transformers
(CT’s, PT’s)
- Bushings
- Motors
- Capacitors
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ADVANTAGES OF PARTIAL
DISCHARGE SURVEYS
• PREDICTIVE - failures can be prevented.
• NO OUTAGE NECESSARY - does not interrupt
operations.
• NON-DESTRUCTIVE - no damage to electrical
system.
• TRENDING - comparison to previous tests
possible
• PRIORITIZE MAINTENANCE ACTIVITIES determine which equipment to service first.
• PLANNING - allows time to schedule repairs.
• FINANCIAL CONTROL – repairs can be budgeted.
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TYPICAL MEDIUM VOLTAGE
POWER CABLE
CONSTRUCTION
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TYPICAL HIGH VOLTAGE
UNDERGROUND CABLE
SYSTEM
Load
Unit Substation
Source
Switchgear
Visible Cable
Terminations
Visible Cable
Terminations
Ground Level
Underground Cable in Conduit (not visible)
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3 STAGES OF CABLE
INSULATION FAILURE
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STAGE 1
No problems, minor voids therefore minor partial discharges
Jacket, Shield, Semi-conducting shield
INSULATION
Imperfections
and voids
/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
INSULATION
voids
Jacket, Shield, Semi-conducting shield
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STAGE 2
Tracking begins
Jacket, Shield, Semi-conducting shield
INSULATION
/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
INSULATION
Jacket, Shield, Semi-conducting shield
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STAGE 3
CABLE FAILURE!
Jacket, Shield, Semi-conducting shield
INSULATION
Blow-up
Phase-to-ground
/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
INSULATION
Jacket, Shield, Semi-conducting shield
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HOW CAN PARTIAL
DISCHARGES BE DETECTED?
Discharges are detected by special signal processing equipment designed to
eliminate outside interference.
Capacitive Methods - For Unshielded Components
- Dry Type Transformers
- Instrument Transformers
- Switchgear
Inductive Methods - metal enclosed components
- Shielded Cables and Components
- Oil Filled Transformers
- Rotating Apparatus
Acoustic Emission Methods
- Dry & Oil Transformers
- Switchgear
- Unshielded Cables
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Test Equipment & Sensors
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Partial Discharge detection of a flaw
in a dry type transformer using
capacitive coupling
Transformer Coil
flaw
Detector
Capacitive
sensor
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Propagation of Transient Earth
Voltages (TEV) in Switchgear
High Voltage Bus
Partial discharge
site (inside)
Metallic Switchgear
cover (signal detected
outside)
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Measurements
using
Capacitive PD
sensors
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Switchgear failure due to internal tracking
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Tracking on epoxy resin busbar
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Start of tracking on insulation
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Failed Switchgear Insulation
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Detecting PD Using Airborne
Acoustic (Ultrasonic) Sensors
Busbar
Sensor
Insulator
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Airborne Acoustic Sensor
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Insulator Failure Prevented
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Partial Discharge detection of a void in a
shielded cable using inductive coupling.
Insulation
Conductor
Shield (grounded)
Void
+
Detector
Inductive Sensor
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High Frequency Current Transformer Used to Test Cable in
High Voltage Substation
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Failing Terminations
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Molded Cable Accessory Failure
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Splice Void
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Shield Delamination Causing
Partial Discharge
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Poor Workmanship Causing
Partial Discharge
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Case Study
Testing 15kV cables
On-line testing of cables
PD damage to termination
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Identifying & Prioritising Circuits Most at Risk
Using On-Line PD Survey
L IV E L IN E P A R T IA L D IS C H A R G E S U R V E Y R E S U L T S
O F 629 HV FE E D E RS
1000
900
800
700
600
500
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300
A C TIO N
200
FEEDERS
610
589
568
547
526
484
463
442
421
400
379
358
337
316
295
274
253
232
211
190
168
147
126
105
84
63
42
21
0
505
CO NCE RN
100
1
m V D is c h a rg e
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PD Survey - Worst 30 Results
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PERFORMING A PARTIAL
DISCHARGE SURVEY
• Analyze the single line drawing to develop plan
• Onsite Measurement
– Removal of equipment covers.
– Record field data.
– Obtain signatures of atypical components.
• Offsite Data Analysis
–
–
–
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Correlate measurements to single line drawing.
Compare atypical results to like insulation from database.
Analyze signatures.
Trend Results.
• Report Generation
– Recognize immediate failure possibilities.
– Recommend possible repairs.
– Recommend possible resurvey frequency.
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SUMMARY
• Electric Insulation will fail.
• Predicting failures is now possible through
Partial Discharge Analysis.
• No outage is required to survey the
equipment.
• Results can be prioritized and trended.
• Partial Discharge Surveys should be
performed annually.
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We apologize to every electrical professional for taking 252 years
to prevent your failures.
Benjamin Franklin discovered electricity
in 1752. This year we launched an easy
to use, cost effective “no-outage” electrical
inspection technology that detects
medium and high voltage equipment
problems before they fail!