TESTING Current Transformers

8/13/2019 TESTING Current Transformers

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Current Transformers

Bonneville Power Administration Steve Laslo

For the Hands On Relay School (3-12)

Revision 1.1 (Basic)


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Objective of the presentation:

For learners to increase their knowledgelevel of Current Transformers in thefollowing areas: Basic Theory

Application

Terminology Safety Hazards

Safe Work Practices


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Examples of CTs


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Examples of CTs


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Examples of CTs


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Basic Theory:

CT as a Voltage Transformer


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CT as a Voltage Transformer


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Working Range of (relative) flux levels on core:

Saturation Curves


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CT with varying burden


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March 7, 2012 12From ABB/Kuhlman 15kV Instrumentation Brochure


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Meter Accuracy:

Secondary Current will be within 0.3% accuracy at ratedcurrent and Burden levels of 1.8 ohms or less. Relay Accuracy:

Secondary Current will be within 10% accuracy at 1-20 xrated current with burden levels of 3 ohms or less.

Full winding output of the CT is essentially 300V, whichcan drive 100A secondary current through a 3 ohmburden (or less).

C indicates accuracy can be calculated based on designof this CT. C ratings are the most common. Less common letter classes: K, T, H, L

Rating Factor: Up to 2 x rated current can be applied continuously withthe CT staying within its accuracy and thermal ratings.

Ratings of CTs


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0.3% onlybetween 100%

and its TRF

Note: No

accuracystandards

between

0% and

10%

By the IEEE Standard, does a 0.3% CT mean its 0.3%accurate?

IEEE Standard


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New Extended Accuracy Range CTs

New supplement to IEEE Standard C57.13.6-2005 Creates a new 0.15% accuracy class

With an extended, c o n s i s t e n t range between 5% and TRF


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C800 CT Working Limits


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Multi-ratio issues


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Multi-ratio issues


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March 7, 2012 22Testing CTs

Ratio Test Two commonly used out-of-service methods totest:

Voltage Method CT is essentially tested as a voltage transformer by

applying voltage to the CT Secondary and measuringthe primary voltage.

The turns ratio is approximately equal to the voltageratio.


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Ratio Test Current Method

Some form of loading gear is used to push current throughthe CT primary. Secondary current is compared to primarycurrent, usually through a Reference CT.


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Polarity Test Four commonly used out-of-service methods totest:

DC Flash Method A lantern battery or equivalent DC source is

momentarily connected to the CT Secondary and theprimary voltage is monitored with a voltmeter.


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Classical Polarity Test Voltage Method

This test is performed in the same manner as avoltage transformer polarity test.


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Excitation Test using Primary Current Injection Primary Current is measured along with secondary

voltage Primary exciting current is divided by the CT ratio to determine

equivalent secondary exciting current to compare to manufacturers

diagrams.


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Secondary

Induced Voltage

Spike

Core Flux During

Saturation

Secondary open circuit wave shapes

Rapid flux state

change causes

high voltagespikes


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Demonstration of high voltagesTo verify that such voltages can be produced

we will spark a pre-set gap at 1,000 volts

Click Video to Run


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A CT can easily supply currents above lethal levels!

Death

Th i it it ti l t hi h lt d l th l


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The open circuit situation resolves to high voltages and lethal

currents WATCH THIS!!

Click Video to

Run


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Working around CT Circuits: Circuit Identification

Wiring Diagram / AC Schematic Information.

Testing for Energized Circuits Secondary Current measurement.

Audible/Visual arcing check.

Safe work practices Use of Safety Gloves, Blankets, and InsulatedTools.

Job Briefings Methods of shorting at CT Shorting Blocks

Practical / Safety Considerations


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March 7, 2012 35Circuit Identification

Circuit Identification was a primary factor in a CT

accident at BPA. CT leads were lifted on the wrong terminalblock. Instead of being a circuit that wasshorted and isolated, the circuit had live

current flowing.

If you arent 100% sure of the identity andfunction of the circuit you are about to work on,

dont work on it research and get assistance ifnecessary until you are Circuit Identification isan important part of safe CT work.


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Most utilities have standards for wiring specific

circuits like those associated with CTs and/orPTs. Use your knowledge of your companys standards to help

identify CT circuits so that you can treat themappropriately. When in doubtresearch until you are

sure.

Example BPA standard for CTs: Normally Color Coded on BPA run cabling:

Black (1A) A-Phase Green (2A) B-Phase Red (3A) C-Phase White (0A) CT Common / Neutral

Normally designated 1A, 2A, 3A, and 0A (if Wye-connected)

Circuit Identification


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Circuit IdentificationExample


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Wiring Diagrams and/or Layout Prints should show

actual placement of Terminal Blocks seen onSchematic Diagrams.

Schematic Diagrams show Circuit Functionality andmay have some Wiring Diagram information

shown on them. Schematics and Wiring Diagrams should agree

with each other. Your company may have standards for typical CT

configurations that may aid in the identificationprocess.

Circuit Identification


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Example of wiring information on

AC Circuit Schematic

Wiring Diagram Information


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Example of Cluesthat wiring is part

of a CT Circuit(continued onnext page).



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Example of Clues to physicallocation of CT Circuits.

Diagram shows physical location ofCTs relative to the PCB and eachother.



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g g

Example of criticalinformation pertinent tophysical location of CTCircuits.


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Example of using Clamp-onammeter to test forsecondary current.


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CT Secondary Wires should be liftedslowly while listening for arcing as a finalverification check.

The visual/audible Arcing Check is also not

100% reliable as a test for an open-circuitCT condition. In brightly lit and/or noisy areas, it may be

difficult to detect the arcing condition. With low values of CT secondary current, theremay be little-to-no arcing when wiring is lifted.

Visual/Audible Arcing Check

Using Test Instruments


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Fluke Current

Measuring

Devices



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Make sure to check the Current TestInstrument on a known circuit first, thenthe unidentified circuit, then the knowncircuit again, just like voltage test

devices Verify test instrument operation before

relying on its results.



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Another item to consider when using normal

hand-tools or placing your fingers on secondarywiring insulation is whether the insulationbetween you and the secondary conductor istruly adequate to protect you from the possible

voltages a CT can produce on the secondarywiring.

If that insulation is normally rated at 600VAC or1000VAC, are you protected from voltages that canhave peaks well over 4kV?

Can that screwdriver protect you from that samevoltage? Its going to be directly connected to theconductor as you remove that screw on the terminationblock

Safe Work Practices


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Ostrander Open-Circuit CT Incident

Note only 4 wiresfrom CT pocket.

Note there is no

wire on the CTside of the block.

Damaged CTs from 500kV PCB


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Damaged CT s from 500kV PCB

Damaged CTs from 500kV PCB


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Damaged CT s from 500kV PCB


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Re-enactment of 1st Contact Accident


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ARC EXITED

HERE

ARC ENTERED

HERE

CABLE

62E-04

Quote from

the Accident

Report:

The

Electriciansaid he

could smell

his f lesh

burning


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Sample items that can be covered at aJob Briefing or Tailgate: Hazards associated with the Job.

Work Procedures.

Special Precautions. Energy Source Controls

Personal Protective Equipment.

Clearances, Work Permits, Hold Orders

Job Briefings

CT Open-Circuit Secondary Arcing


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ClickVideo

to

Run

p y g


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When shorting CT secondaries at CTShorting Blocks, care must be taken toproperly short the CT Circuit.

Depending on the connection made at theblock it may take anywhere from 2 to 6shorting screws to fully short the CT

secondaries.

CT Shorting Blocks

Single Phase Type CT Shorting Block


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Single-Phase Type CT Shorting Block

CT ratio tap

wiring from one

individual CT

Shorting Block

Ground

Single-Phase

wiring from

individual CT

to relays,

instruments,

etc.

Ground screw

connection

normally left in-

place.

Note that a

minimum of two

screws are

needed to short

this 1 CT if the

full winding is

shorted (Y1-Y5

in this case).

Three-Phase Type CT Shorting Block


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Three-Phase Type CT Shorting Block

CT wiring from

three single-

phase individual

CTs (three pairs)

Shorting Block

Ground

Three-Phasewiring to relays,

instruments, etc.

Ground screwconnection

normally left in-

place.

Note that it takesa minimum of

fourscrews to

short this set of 3

CTs (2Y1, 4Y1,

6Y1, and one of2Y5, 4Y5, and

6Y5 in this case)

as long as the

Wye connection is

intact whitewiring here.


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March 7, 2012 59Three-Phase Block Analysis

Questions?


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Questions?

TESTING Current Transformers

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