Technical Level Training

Instrument Transformers

Current Transformers
Theory & Testing

Prepared by
Hashem Talukder

High Voltage (HV) Service Team

Reverie Power & Automation Engineering Limited
Copyright ©2020 by Reverie Power & Automation Engineering Limited. All rights reserved
 Why Instrument Transformers

▪ Supervision during normal conditions

Current Transformer
▪ Energy Metering for buying and selling
▪ Registration of abnormal conditions- Faults

June , 2020 | Slide 02

 Main Tasks of Current Transformers

▪ Transform currents from high levels to measureable levels

Current Transformer
▪ Insulate the secondary circuits from the high voltage network
▪ Standardize meters and relays

June , 2020 | Slide 03

 Convert Primary Power Signals to Manageable Values for

▪ Indicating Meters

Current Transformer
▪ Revenue Metering
▪ Protective Relay Systems
▪ Power Generation
▪ Plant Monitoring Systems
▪ Fault Recorders
▪ SCADA
▪ Overall Electric Grid Monitoring (Local Dispatch & ISO Level)
▪ Building (Energy) Management Systems
▪ Load Control

June , 2020 | Slide 04

 Basic Transformer

Current Transformer
June , 2020 | Slide 05
 In a Real CT

Current Transformer
June , 2020 | Slide 06
 CT Construction Types

▪ Window or Bus Type

Current Transformer
▪ Split Core
▪ Outdoor ( Live & Dead Tank)

June , 2020 | Slide 07

 Current Transformer
June , 2020 | Slide 08
 Current Transformer
June , 2020 | Slide 09
 Live Tank vs Dead Tank CT

Current Transformer
June , 2020 | Slide 10
 Live Tank vs Dead Tank CT

▪ The core with secondary winding is housed in the ▪ The core with secondary winding is housed in the
top tank which is live ( At high voltage) bottom tank which is earthed (Dead)
▪ Core and Secondary winding - isolated ▪ Only primary winding is isolated

Current Transformer
▪ Insulation is robust and reliable ▪ Insulation is not robust

▪ Primary winding is of shortest length possible ▪ Primary winding has to pass through the porcelain
which offers high strength against the short time insulator and the larger length of primary
dynamic force conductor produces maxm mechanical force
during short time dynamic force
▪ Minimum heat generation, maximum dissipation ▪ Maximum heat generation, minimum dissipation

▪ Compact and economical ▪ Bulky and costly

June , 2020 | Slide 11

 CT Core

Current Transformer
June , 2020 | Slide 12
 IEC Standards

Current Transformer
June , 2020 | Slide 13
 Definition as per IEC 60044-1
▪ Accuracy Class : a designation assigned to a current transformer the error of which remain within
specified limits under prescribed conditions of use

▪ Burden : the impedance of secondary circuit in ohms and power-factor. Usually expressed in VA

Current Transformer
▪ Instrument Security factor (FS): the ratio of rated instrument limit primary current to the rated
primary current

▪ Accuracy Limit Factor (ALF) : the ratio of the rated accuracy limit primary current to the rated
primary current

▪ Rated Knee-Point e.m.f : the minimum sinusoidal e.m.f at rated power frequency when applied to
the secondary terminals of the transformer, all others terminals being open circuited , which when
increased by 10% causes the r.m.s exciting current to increase by no more than 50%. The actual
knee point emf will be equal or greater rated knee point emf.

June , 2020 | Slide 14

 Major Technical Specification of a Current Transformer

Current Transformer
June , 2020 | Slide 15
 CT Terminal Markings
▪ Expressed in 5A or 1A Ratio
▪ 1200/5 or 1200/1

Current Transformer
June , 2020 | Slide 16
 CTs for Protection and Metering Applications

▪ A distinction has to be made between a Metering and Protection Class CT

Current Transformer
▪ The designs of the magnetic cores are different
▪ This ensures that they perform according to the needs of the particular device
connected

June , 2020 | Slide 17

 Properties of Metering Core

▪ High Accuracy in a smaller range ( 5-120% of rated current / 1-120% for class S)

Current Transformer
▪ Low Burden
▪ Low Over Current Factor (FS) (At FS, Core saturated and Accuracy isn’t guaranteed)
▪ Leads to lower Saturation Voltage ( Protects Metering Devices from overloading)

June , 2020 | Slide 18

 Standard Accuracy Classes for Metering

▪ 0.1 – 0.2 – 0.5 – 1

Current Transformer
▪ 0.2S – 0.5S
▪ 3–5

June , 2020 | Slide 19

 IEC 60044 – 1 Metering Accuracy Requirements

Current Transformer
June , 2020 | Slide 20
 IEC 60044 – 1 Metering Accuracy Requirements

Current Transformer
June , 2020 | Slide 21
 Properties of Protection Core

▪ Designed to transform a distortion-free signal even well into the overcurrent

range

Current Transformer
▪ Current range 100% of rated current to ALF x rated current
▪ Relays are required to perform in fault current type situations
▪ Low accuracy requirements
▪ High saturation voltage
▪ More core material is needed

June , 2020 | Slide 22

 Standard Accuracy Classes for Protection

▪ 5P and 10P

Current Transformer
▪ PX (User defined CT performance)
• Turn Ratio error should not exceed ± 0.25%

• Rated Knee point emf (𝐸𝑘 )

• Maximum exciting current (𝐼𝑒 ) at rated (𝐸𝑘 )

• Maximum Resistance (𝑅𝑐𝑡 ) at 75°C

June , 2020 | Slide 23

 IEC 60044 – 1 Protection Accuracy Requirements

Current Transformer
June , 2020 | Slide 24
 Current Transformer
June , 2020 | Slide 25
 Testing CTs

When ?

▪ Initial Commissioning

Current Transformer
▪ Investigation
▪ Scheduled

Why ?

▪ Verify Factory Tests

▪ Ordered/Delivered Correctly
▪ Ensure no Damages

June , 2020 | Slide 26

 Test Methods
▪ Primary Injection
▪ Secondary Injection – Fixed Frequency
▪ Secondary Injection – Variable Frequency

Current Transformer
Test Requirements

▪ Determination of Knee/Saturation Point

▪ Insulation
▪ Polarity
▪ Winding Resistance
▪ Primary/Secondary Ratio ( %Ratio Error )
▪ Burden Check
▪ Tan-Delta
▪ Documentation / Visual Check

June , 2020 | Slide 27

 Why so many Tests

▪ Ensure proper Relay Operations

▪ Certify Billing Accuracy

Current Transformer
▪ Reduce Possibility of Failure when Energized

▪ Manufacturing Defects Do Happen

▪ Installation Error Do Happens

June , 2020 | Slide 28

 Before Testing Starts

▪ Ask Concerned Personnel to isolate CTs ( Electrically & Mechanically )

▪ DON’T Operate Yourself

Current Transformer
▪ Ensure Isolation Yourself - Physically

▪ Use Caution Tape to avoid enthusiastic people from entering premises

▪ Ground One-End (P1 / P2 ) to avoid Induced Voltage

▪ Take notes if removal of secondary connection requires

▪ Ensure Proper Grounding of Testing Kits

▪ Collect FAT / Previous Test Reports

▪ Revert to Original Connection after Tests end ( Ensure Yourself )

June , 2020 | Slide 29
 Testing Excitation / Knee Point
▪ Secondary kept Opened
▪ Voltage increased until knee-
point reached

Current Transformer
June , 2020 | Slide 30
 Testing Excitation / Knee Point

Current Transformer
June , 2020 | Slide 31
 Testing Turn Ratio Error%
▪ Current Injected at Primary
▪ Transformed Current is
measured from Secondary

Current Transformer
June , 2020 | Slide 32
 Testing Turn Ratio Error%

𝐾𝑛 𝐼𝑠 −𝐼𝑝 ×100
Current Error% = Kn = Rated Transformation Ratio
𝐼𝑝 Is = Actual / Measured Secondary Current
Ip = Actual / Measured Primary Current

Current Transformer
June , 2020 | Slide 33
 Measuring Secondary Winding Resistance
▪ Usually done with AVO Meter but recommended way is to do with Test Kit / CT Analyzer

▪ Convert Resistance Value at 75°C and compare with Reference / Rated Value

Current Transformer
▪ It’s the simplest test for diagnosing CT Secondary . It should be the 1st test to perform on CT if Core
Demagnetization option is available, otherwise, perform it at last.

▪ As DC current leads to Residual Magnetism, it may jeopardize other tests Accuracy

June , 2020 | Slide 34

 How CTs Secondary are Connected

Current Transformer
▪ Always Connected in SERIES

▪ Don’t Connect Burden Beyond its Rating

▪ Connect Cores as per Designation of

Devices
▪ Don’t Interchange CT Cores

June , 2020 | Slide 35

 CTs Performance at Through-Fault

Current Transformer
▪ CTs are designed to sustain a through-fault condition without being saturated
▪ To avoid saturation CT should develop adequate voltage at secondary
▪ Extra burden should not be imposed beyond its rating

▪ For Differential Protection, PX class CTs are recommended to be used as they are
calculated to provide sufficient voltage during a through-fault

For Protection Purposes, CTs are NEVER Expected to be Saturated

June , 2020 | Slide 36
 Dos and Don’ts at Commissioning

▪ Do Primary Injection and check Continuity throughout the secondary circuit

▪ Measure Secondary Current as per transformation ratio at each junction point with Clamp Meter

Current Transformer
▪ Ensure proper Core Allocation & Polarity

▪ Avoid Double-Grounding as it leads to Wrong Measurement

▪ Tighten CT secondary with Utmost care. Double Check !

▪ NEVER Leave CTs Secondary OPENED

▪ Once Primary Injection is done successfully, DON’T Touch CT secondary or Change Connections

▪ If Changes are made, Perform Primary Injection AGAIN

June , 2020 | Slide 37

 Current Transformer
June , 2020 | Slide 38
References

Most of the information is gathered from multiple online sources that are accessible to the public