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CT Ratio & Polarity Testing Guide: Methods, Equipment & Acceptance Criteria (IEC 61869-2, IEEE C57.13)
Meta Description: Comprehensive guide on current transformer (CT) ratio and polarity testing. Covers test methods, equipment, acceptance criteria, and compliance with IEC 61869-2 and IEEE C57.13. Includes step-by-step procedures, troubleshooting, and safety precautions for factory, site, and maintenance testing of MV/HV CTs.
1. Introduction
Current transformer (CT) ratio and polarity testing are fundamental diagnostic tests performed during factory acceptance, site commissioning, and periodic maintenance. These tests verify:
– Ratio accuracy: Ensures CT secondary current accurately reflects primary current for metering and protection
– Polarity correctness: Ensures CT secondary current direction matches relay/meter requirements for differential, directional, and distance protection
Incorrect ratio or polarity can cause:
– Metering errors: Inaccurate energy measurement, revenue loss
– Protection misoperation: False differential tripping, incorrect directional operation
– System instability: Relay miscoordination, extended fault duration
– Equipment damage: Unchecked faults, generator/transformer damage
This guide systematically covers CT ratio and polarity testing methods, equipment, acceptance criteria, procedures, and troubleshooting per IEC 61869-2:2016 and IEEE C57.13 standards.
2. CT Ratio Testing
2.1 Test Principle
Ratio Test: Apply known primary current, measure secondary current, verify ratio matches nameplate.
Primary Current (I_p) ── CT Primary
│
CT Core
│
Secondary Current (I_s) ── CT Secondary ── Ammeter/Tester
│
Ratio = I_p / I_s
Expected Ratio:
Nameplate Ratio: e.g., 1000/5A = 200:1
Measured Ratio: I_p / I_s should be ≈ 200:1
Error = (Measured Ratio - Nameplate Ratio) / Nameplate Ratio × 100%
2.2 Test Equipment
| Equipment | Description | Accuracy | Application |
|---|---|---|---|
| CT Tester | Integrated ratio/polarity/excitation tester | ±0.1% | Factory, site, maintenance |
| Primary Injection Kit | Step-down transformer, ammeter | ±0.5% | Site, maintenance |
| Clamp Ammeter | Primary current measurement | ±1% | Quick verification |
| Digital Multimeter | Secondary current measurement | ±0.5% | Backup measurement |
2.3 Test Procedure
Step 1: Safety Preparation
☐ De-energize CT primary circuit
☐ Isolate CT from relay/meter secondary circuit
☐ Short CT secondary terminals (except test terminal)
☐ Verify grounding
☐ Wear PPE (insulating gloves, safety glasses)
Step 2: Connect Test Equipment
CT Primary: Connect primary injection leads (P1, P2)
CT Secondary: Connect secondary leads (S1, S2) to tester
Ground: Connect tester ground to substation ground grid
Step 3: Perform Test
1. Select CT ratio on tester (e.g., 1000/5A)
2. Inject primary current (typically 10-50% of rated primary)
3. Measure secondary current
4. Record ratio and error
5. Repeat at multiple current levels (25%, 50%, 100% rated primary)
Step 4: Calculate Error
Error (%) = (I_s_measured - I_s_expected) / I_s_expected × 100%
Where:
I_s_expected = I_p_injected / Nameplate Ratio
2.4 Acceptance Criteria
| Application | Ratio Error Limit | Standard Reference |
|---|---|---|
| Metering (0.2S, 0.5S) | < ±0.2% (0.2S), < ±0.5% (0.5S) | IEC 61869-2 |
| Protection (5P, 10P) | < ±1.0% | IEC 61869-2 |
| Site Acceptance | < ±1.0% (all classes) | IEEE C57.13 |
| Maintenance | < ±1.0% of baseline | IEEE C57.13 |
2.5 Common Errors & Troubleshooting
| Error | Cause | Solution |
|---|---|---|
| Ratio > 1% error | Shorted turns, connection issue, wrong tap | Verify connections, check tap, replace CT |
| Unstable reading | Loose connection, interference | Tighten connections, shield cables |
| Zero secondary current | Open circuit, wrong terminal | Verify secondary circuit, check S1/S2 |
| Low secondary current | Wrong ratio selected, burden too high | Verify ratio setting, reduce burden |
3. CT Polarity Testing
3.1 Test Principle
Polarity Test: Verify CT secondary current direction matches primary current direction (additive or subtractive polarity).
Standard Polarity:
Primary Current: P1 → P2
Secondary Current: S1 → S2 (subtractive polarity, standard)
When primary current enters P1, secondary current leaves S1.
Polarity Markings:
CT Nameplate:
P1 ─── Primary (Line)
P2 ─── Primary (Load)
S1 ─── Secondary (Output)
S2 ─── Secondary (Return)
3.2 Test Methods
Method 1: Battery & Analog Meter (DC Method)
Equipment:
– 1.5V or 9V battery
– Analog voltmeter/milliammeter (center-zero)
Procedure:
1. Connect battery to primary (P1 +, P2 -)
2. Connect analog meter to secondary (S1 +, S2 -)
3. Momentarily connect battery
4. Observe meter deflection:
- Positive deflection → Additive polarity (S1 matches P1)
- Negative deflection → Subtractive polarity (standard)
Method 2: CT Tester (Automatic Polarity Test)
Equipment:
– CT tester with polarity function
Procedure:
1. Connect CT tester to primary (P1, P2) and secondary (S1, S2)
2. Select polarity test mode
3. Perform test
4. Read result: "Correct" or "Reverse"
Method 3: Secondary Injection (Relay Verification)
Equipment:
– Relay tester
Procedure:
1. Inject current into relay current input
2. Verify relay measures correct direction
3. Compare with CT polarity marking
3.3 Acceptance Criteria
| Parameter | Limit | Standard Reference |
|---|---|---|
| Polarity | Correct (S1 matches P1, subtractive) | IEC 61869-2 |
| Differential Protection | All CTs same polarity, matching | IEEE C37.112 |
| Directional Protection | Correct polarity for direction | IEEE C37.90 |
3.4 Common Errors & Troubleshooting
| Error | Cause | Solution |
|---|---|---|
| Reverse polarity | Wrong connection, CT installed backward | Reverse secondary leads (S1/S2), reinstall CT |
| Unstable reading | Loose connection, battery weak | Tighten connections, replace battery |
| No deflection | Open circuit, wrong terminal | Verify secondary circuit, check S1/S2 |
| Differential relay trips | Mismatched polarity (neutral vs terminal) | Verify all CT polarity, correct connections |
4. Combined Ratio & Polarity Testing
4.1 Integrated CT Tester Procedure
Modern CT testers perform ratio, polarity, excitation, and burden tests in one setup:
1. Connect tester to CT secondary (S1, S2)
2. Enter CT nameplate data (ratio, class, burden)
3. Select test mode (ratio, polarity, excitation)
4. Perform test
5. Record results
6. Compare with acceptance criteria
4.2 Multi-Core CT Testing
CTs with multiple cores (metering, protection, standby) require testing each core:
| Core | Purpose | Ratio Test | Polarity Test |
|---|---|---|---|
| Core 1 | Metering (0.5S) | Yes | Yes |
| Core 2 | Protection (5P20) | Yes | Yes |
| Core 3 | Standby/Reserve | Yes (if used) | Yes (if used) |
Procedure:
1. Test Core 1 (metering)
2. Short Core 1 secondary
3. Test Core 2 (protection)
4. Short Core 2 secondary
5. Test Core 3 (if used)
6. Record all results
5. Testing During Commissioning & Maintenance
5.1 Factory Acceptance Tests (FAT)
| Test | Method | Acceptance Criteria |
|---|---|---|
| Ratio Test | Primary injection, multiple taps | < ±0.2% (metering), < ±1% (protection) |
| Polarity Test | DC method or tester | Correct (subtractive) |
| Excitation Test | Secondary voltage injection | Knee-point ≥ nameplate |
| Insulation Test | Megger, withstand voltage | > 1000 MΩ, no flashover |
5.2 Site Acceptance Tests (SAT)
| Test | Method | Acceptance Criteria |
|---|---|---|
| Ratio Test | CT tester or primary injection | < ±1% of factory |
| Polarity Test | CT tester or DC method | Correct |
| Burden Test | Measure secondary circuit resistance | ≤ Rated burden |
| Secondary Injection | Relay test kit | Relay operates correctly |
5.3 Periodic Maintenance Testing
| Test | Interval | Acceptance Criteria |
|---|---|---|
| Ratio Test | 3-6 years | < ±1% of baseline |
| Polarity Test | 3-6 years (after modification) | Correct |
| Excitation Test | 3-6 years | Knee-point ≥ 90% baseline |
| Insulation Resistance | Annual | > 1000 MΩ |
6. Safety Precautions
6.1 CT Secondary Open Circuit Hazard
WARNING: Never open CT secondary circuit while primary current is flowing!
Consequences:
– High voltage (hundreds to thousands of volts) across secondary terminals
– Core saturation, overheating
– Insulation breakdown, fire hazard
– Electric shock hazard
Precautions:
– Always short CT secondary before disconnecting
– Use shorting test switches or terminal blocks
– Verify short circuit before working on secondary circuit
– Wear PPE (insulating gloves, safety glasses, arc flash suit if required)
6.2 Test Safety Checklist
☐ De-energize CT primary circuit (for FAT/SAT)
☐ Isolate CT from relay/meter secondary circuit
☐ Short CT secondary terminals (except test terminal)
☐ Verify grounding
☐ Wear PPE (insulating gloves, safety glasses)
☐ Use insulated test leads
☐ Verify test equipment calibration
☐ Follow lockout/tagout procedures
☐ Post warning signs
7. Standards & References
7.1 IEC Standards
| Standard | Title | Relevant Sections |
|---|---|---|
| IEC 61869-2 | Current Transformers | §6.3 (Ratio Test), §6.4 (Polarity Test) |
| IEC 60044-1 | CTs (Legacy) | §5 (Tests) |
7.2 IEEE Standards
| Standard | Title | Relevant Sections |
|---|---|---|
| IEEE C57.13 | Instrument Transformers | §4.2 (Ratio Test), §4.3 (Polarity Test) |
| IEEE 80 | Substation Grounding | §5 (Safety) |
8. Engineering FAQ
Q1: Can I perform ratio test on energized CT?
A: No. Ratio test requires primary injection with CT de-energized and isolated. For energized CTs, use clamp ammeter to measure primary and secondary current simultaneously and calculate ratio indirectly.
Q2: What is the difference between additive and subtractive polarity?
A:
– Subtractive polarity (standard): Primary current enters P1, secondary current leaves S1. Most common.
– Additive polarity: Primary current enters P1, secondary current enters S1. Rare, used in specific applications.
IEC and IEEE standards specify subtractive polarity as default.
Q3: How do I verify CT polarity in the field without a tester?
A: Use battery and analog meter (DC method):
1. Connect battery to primary (P1 +, P2 -)
2. Connect analog meter to secondary (S1 +, S2 -)
3. Momentarily connect battery
4. Positive deflection → Additive; Negative deflection → Subtractive (standard)
Q4: What causes CT ratio error?
A: Common causes:
– Shorted turns: Internal winding fault
– Wrong tap: Incorrect tap selection on multi-ratio CT
– Burden too high: Exceeds rated burden, causes saturation
– Core damage: Mechanical impact, overheating
Investigate and replace if error > ±1%.
Q5: How often should I test CT ratio and polarity?
A:
– Factory: Every CT, every core
– Site commissioning: Every CT, every core
– Maintenance: Every 3-6 years, or after modification/relay replacement
– After fault: If CT exposed to high fault current, verify ratio/excitation
9. Conclusion
CT ratio and polarity testing are essential diagnostic tests that verify CT accuracy and correct connection for metering and protection applications. Proper testing procedures, equipment, and safety precautions ensure reliable operation and prevent protection misoperation.
Key testing principles:
– Ratio test: Verify < ±0.2% (metering), < ±1% (protection)
– Polarity test: Verify correct (subtractive) polarity
– Multi-core testing: Test each core independently
– Safety: Never open CT secondary, short before disconnecting, wear PPE
– Frequency: Factory (100%), site (100%), maintenance (3-6 years)
Design checklist:
☐ Test equipment selected (CT tester, primary injection kit)
☐ Test procedures defined (ratio, polarity, multi-core)
☐ Acceptance criteria established (per IEC 61869-2, IEEE C57.13)
☐ Safety precautions specified (shorting, PPE, lockout/tagout)
☐ Commissioning test schedule defined
☐ Maintenance test schedule defined (3-6 years)
☐ Documentation updated (test reports, CT records)
Technical Reference: IEC 61869-2:2016, IEEE C57.13-2016
Product Reference: Duomatech LZZBJ9 series (cast-resin CTs), LJWD series (oil-immersed CTs) — optimized for ratio and polarity testing