Partial discharge occurs when high voltage insulation breaks down, and can be detected and measured in units of charge. Partial discharge testing identifies defects in electrical insulation that could lead to failures, and can be performed on energized equipment with specialized sensors and equipment. Regular partial discharge surveys can predict failures, prioritize maintenance needs, and prevent outages by identifying problems before catastrophic failures occur.

1. I Q S e r vic e s
An Introduction to On-line
Partial Discharge Surveys

2. I Q S e r vic e s
PARTIAL DISCHARGE
When high voltage insulating material
breaks down Partial Discharges are
created, measured in units of charge
known as pico-Coulombs or millivolts.

3. I Q S e r vic e s
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.

4. I Q S e r vic e s
• 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.
PARTIAL DISCHARGE
TESTING USED FOR QUALITY
ASSURANCE

5. I Q S e r vic e s

6. I Q S e r vic e s
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

7. I Q S e r vic e s
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.

8. I Q S e r vic e s

9. I Q S e r vic e s

10. I Q S e r vic e s

11. I Q S e r vic e s

12. I Q S e r vic e s
INSULATION BREAKDOWN
Why should we be concerned with insulation
breakdown and Partial Discharges?

13. I Q S e r vic e s
• 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.
INSULATION BREAKDOWN

14. I Q S e r vic e s
Component Percentage of insulation failure
Transformers 84%
Circuit Breakers 21%
Disconnect Switches 15%
Insulated Switchgear Bus 95%
Bus duct 90%
Cable 89%
Cable Joints (splices) 91%
Cable Terminations 87%
Based on IEEE Gold Book Table 36
Total Failures due to Insulation
Breakdown

15. I Q S e r vic e s
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

16. I Q S e r vic e s
WHERE DO PARTIAL
DISCHARGES OCCUR?
• Electrical Systems > 600V (2.4KV, 4.16 KV,
12KV, etc.)
• Types of Equipment Subject to Partial
Discharge
-Cables -Transformers
-Circuit Breakers - Instrument Transformers
-Switchgear (CT’s, PT’s)
-Insulators - Bushings
-Generators - Motors
-Surge Arrestors - Capacitors

17. I Q S e r vic e s
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.

18. I Q S e r vic e s
TYPICAL MEDIUM VOLTAGE
POWER CABLE
CONSTRUCTION

19. I Q S e r vic e s
Source
Switchgear
Ground Level
Underground Cable in Conduit (not visible)
Visible Cable
Terminations
Load
Unit Substation
Visible Cable
Terminations
TYPICAL HIGH VOLTAGE
UNDERGROUND CABLE
SYSTEM

20. I Q S e r vic e s
3 STAGES OF CABLE
INSULATION FAILURE

21. I Q S e r vic e s
STAGE 1
Imperfections
and voids
voids
INSULATION
/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
INSULATION
No problems, minor voids therefore minor partial discharges
Jacket, Shield, Semi-conducting shield
Jacket, Shield, Semi-conducting shield

22. I Q S e r vic e s
INSULATION
/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
INSULATION
Tracking begins
Jacket, Shield, Semi-conducting shield
Jacket, Shield, Semi-conducting shield
STAGE 2

23. I Q S e r vic e s
INSULATION
INSULATION
CABLE FAILURE!
Jacket, Shield, Semi-conducting shield
Jacket, Shield, Semi-conducting shield
Blow-up
Phase-to-ground
/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
STAGE 3

24. I Q S e r vic e s
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

25. I Q S e r vic e s
Test Equipment & Sensors

26. I Q S e r vic e s
Partial Discharge detection of a flaw
in a dry type transformer using
capacitive coupling
flaw
Transformer Coil
Capacitive
sensor
Detector

27. I Q S e r vic e s
High Voltage Bus
Metallic Switchgear
cover (signal detected
outside)
Partial discharge
site (inside)
Propagation of Transient Earth
Voltages (TEV) in Switchgear

28. I Q S e r vic e s
Measurements
using
Capacitive PD
sensors

29. I Q S e r vic e s
Switchgear failure due to internal tracking

30. I Q S e r vic e s
Tracking on epoxy resin busbar

31. I Q S e r vic e s
Start of tracking on insulation

32. I Q S e r vic e s
Failed Switchgear Insulation

33. I Q S e r vic e s
Detecting PD Using Airborne
Acoustic (Ultrasonic) Sensors
Insulator
Busbar
Sensor

34. I Q S e r vic e s
Airborne Acoustic Sensor

35. I Q S e r vic e s
Insulator Failure Prevented

36. I Q S e r vic e s
Partial Discharge detection of a void in a
shielded cable using inductive coupling.
Insulation
+
Void
Shield (grounded)
Conductor
Detector
Inductive Sensor

37. I Q S e r vic e s

38. I Q S e r vic e s
High Frequency Current Transformer Used to Test Cable in
High Voltage Substation

39. I Q S e r vic e s
Failing Terminations

40. I Q S e r vic e s
Molded Cable Accessory Failure

41. I Q S e r vic e s
Splice Void

42. I Q S e r vic e s
Shield Delamination Causing
Partial Discharge

43. I Q S e r vic e s
Poor Workmanship Causing
Partial Discharge

44. I Q S e r vic e s
Testing 15kV cables
On-line testing of cables PD damage to termination
Case Study

45. I Q S e r vic e s
Identifying & Prioritising Circuits Most at Risk
Using On-Line PD Survey
L IV E L INE P ART IAL D IS C HARG E S URV E Y RE S UL T S
O F 6 2 9 HV F E E D E RS
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FEEDERS
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A C TIO N
C O N C E RN

46. I Q S e r vic e s
PD Survey - Worst 30 Results

47. I Q S e r vic e s
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
– 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.

48. I Q S e r vic e s
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.

49. I Q S e r vic e s
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!