An Introduction to On-line Partial Discharge Surveys

PARTIAL DISCHARGE When high voltage insulating material breaks down Partial Discharges are created, measured in units of charge known as pico-Coulombs or millivolts.

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.

PARTIAL DISCHARGE TESTING USED FOR QUALITY ASSURANCE 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.

NO OUTAGE EVALUATION These proven technologies exist today to evaluate medium or high voltage systems while the system is energized. Oil testing/DGA – limited to oil transformers Infrared testing – visible loose connections only Visual inspections – problems may not be visible Power Quality – cannot detect insulation failures Vibration – limited to motors and transformers Partial Discharge Testing – single best all-around test

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.

INSULATION BREAKDOWN Why should we be concerned with insulation breakdown and Partial Discharges?

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.

Total Failures due to Insulation Breakdown

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

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

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.

TYPICAL MEDIUM VOLTAGE POWER CABLE CONSTRUCTION  

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)

3 STAGES OF CABLE INSULATION FAILURE

Imperfections and voids STAGE 1 No problems, minor voids therefore minor partial discharges Jacket, Shield, Semi-conducting shield INSULATION Imperfections and voids / / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / voids INSULATION Jacket, Shield, Semi-conducting shield

STAGE 2 Tracking begins Jacket, Shield, Semi-conducting shield INSULATION / / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / INSULATION Jacket, Shield, Semi-conducting shield

STAGE 3 CABLE FAILURE! Blow-up Phase-to-ground Jacket, Shield, Semi-conducting shield INSULATION Blow-up Phase-to-ground / / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / INSULATION Jacket, Shield, Semi-conducting shield

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

Test Equipment & Sensors

Partial Discharge detection of a flaw in a dry type transformer using capacitive coupling Transformer Coil Capacitive sensor Detector

Propagation of Transient Earth Voltages (TEV) in Switchgear High Voltage Bus Metallic Switchgear cover (signal detected outside) Partial discharge site (inside)

Measurements using Capacitive PD sensors

Switchgear failure due to internal tracking

Tracking on epoxy resin busbar

Start of tracking on insulation

Failed Switchgear Insulation

Detecting PD Using Airborne Acoustic (Ultrasonic) Sensors Busbar Sensor Insulator

Airborne Acoustic Sensor

Insulator Failure Prevented

Partial Discharge detection of a void in a shielded cable using inductive coupling. Insulation + Void Shield (grounded) Conductor Detector Inductive Sensor

High Frequency Current Transformer Used to Test Cable in High Voltage Substation

Failing Terminations

Molded Cable Accessory Failure

Splice Void

Shield Delamination Causing Partial Discharge

Poor Workmanship Causing Partial Discharge

Testing 15kV cables Case Study On-line testing of cables PD damage to termination

Identifying & Prioritising Circuits Most at Risk Using On-Line PD Survey

PD Survey - Worst 30 Results

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.

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.

We apologize to every electrical professional for taking 252 years   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!