IQ_Partial_Discharge_Presentation (Phân Tích Phóng Điện Cục Bộ )

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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
ASSURANCE

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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
-Transformers
-Circuit Breakers - Instrument Transformers
-Switchgear
(CT’s, PT’s)
-Insulators - Bushings
-Generators - Motors
-Surge Arrestors - 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
Source
Switchgear

Load

Unit Substation
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

/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /
INSULATION

Jacket, Shield, Semi-conducting shield

Blow-up
Phase-to-ground


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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