2. 2
10/02/2012 Slide 2
CT Testing:
Theory and Practice
Presented Tom Lawton, TESCO
For TESCO Tuesday
Tuesday, October 27, 2020
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What We Will Cover
• What are CT’s and PT’s and why do we use them?
• How to read and interpret a transformer face plate
• Why do we test CT’s and PT’s?
• Shop testing
• New Transformers
• Manufacturer’s tests
• Utility tests
• Magnetization effects and demagnetization
• Types of field tests that we will cover in greater depth
in next weeks TESCO Tuesday
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Self Contained vs. Transformer Rated
1S, 2S, 3S, 4S, 9S, 12S, 16S, 45S, etc., etc.
What’s the Difference?
Different Forms for Different Services and
Applications
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What is a CT? a PT?
“A current transformer (CT) is used for
measurement of alternating electric currents.
Current transformers, together with voltage
(or potential) transformers (VT or PT), are
known as instrument transformers. When
current in a circuit is too high to apply directly
to measuring instruments, a current
transformer produces a reduced current
accurately proportional to the current in the
circuit, which can be conveniently connected
to measuring and recording instruments. A
current transformer isolates the measuring
instruments from what may be very high
voltage in the monitored circuit. Current
transformers are commonly used in metering
and protective relays in the electrical power
industry.” - Wikipedia
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Fundamentals of Polyphase Field Meter
Testing and Site Verification
Current Transformers Conceptual
Representation
Real, with core losses
Ideal. No losses
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CT’s – Functions and Terminology
Thermal Rating factor
A value representing the amount by which the
primary current can be increased without
exceeding the allowable temperature rise.
For instance, a RF of 4.0 at 30° ambient on a
400:5 ratio CT would allow for a primary current
up to 1600A.
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• Manufacturer’s Tests
• Utility Shop Tests
– New Transformers
– Used Transformers
Typical Tests and checks
• Accuracy Testing – this is a Ratio Test
• Accuracy class determination
• Burden Testing
• Polarity checking
• Demagnetize a CT
Shop Testing
Slide 17
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Burden Rating
The burden range, present in the secondary
circuit, that the manufacturer will guarantee
their CT’s will still accurately function, in
regards to the ratio specification.
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Burden Testing
Functionality with Burden Present on the
Secondary Loop
ANSI Burden Values
0.1 Ohms
0.2 Ohms
0.5 Ohms
1 Ohms
2 Ohms
4 Ohms
8 Ohms
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CT’s – Functions and Terminology
Accuracy Classifications and Burden
Example: 0.3% @ B0.1, B0.2, B0.5
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CT’s – Functions and Terminology
Accuracy Classifications and Burden
All CT’s fall within an accuracy class.
IEEE Standards have defined accuracy classes.
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De-magnetization
CT’s can become magnetized, due to a number of reasons,
including leaving the shorting clip open, near lightning strikes,
and harmonic content.
CT’s can be demagnetized by slowly and smoothly increasing the
secondary resistance until saturation occurs, and then slowly
and smoothly decreasing the secondary resistance.
A resistance that will cause a secondary current reduction of 65%
to 75% will typically put the CT into saturation.
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• Manufacturer’s Tests
• Utility Shop Tests
– New Transformers
– Used Transformers
Typical Tests and checks
• Accuracy Testing – this is a Ratio Test
• Accuracy class determination
• Burden Testing
• Polarity checking
• Demagnetize a CT
Shop Testing
Slide 25
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Shop Testing Programs on
Equipment to be used in
Transformer Rated Services
• 100% of all Transformers
– If not possible then sample testing of all and 100% of all those over a certain size for CT’s
and all VT’s (generally not a large volume)
• Transformer testing should include
– Ratio and accuracy testing
– Polarity checking
– Accuracy class determination
• 100% of all transformer rated meters
– If not possible then sample testing of all transformer rated meters and 100% of all those
going into a certain size service and over
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Types of Field Testing and Site
Verification
Ratio Testing
Burden Testing
Admittance Testing
Polarity and wiring checks
De-magnetization
Meter Site Analyzer
Cat. 6330
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What we covered
• What are CT’s and PT’s and why do we use them?
• How to read and interpret a transformer face plate
• Why do we test CT’s and PT’s?
• Shop testing
• New Transformers
• Manufacturer’s tests
• Utility tests
• Magnetization effects and demagnetization
• Types of field tests (we will cover in greater depth
in next weeks TESCO Tuesday)
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Q&A and Upcoming
TESCO Tuesday Presentations
Presentation Date
Ratio, Burden and Admittance 11/3
Lessons Learned from AMI Deployments and Asset
Management Readiness
11/10
Harmonics 11/17
Complete Site Testing 12/1
Electric Vehicle Trends & Calibration of Commercial Chargers 12/8
Streetlights & 5G Metering and Testing 12/15
Sign up: tescometering.com/tescotuesdays
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Ratio, Burden and Admittance
November 3, 2020 | 11:00 AM Eastern
@tescometering
@TESCO_Metering
TESCO -The Eastern Specialty Company
TESCO -The Eastern Specialty Company