Step-by-step guide to performing knee point voltage tests on protection-class current transformers
CT knee point voltage testing is a critical procedure for verifying the performance of
protection-class current transformers. The knee point voltage (Vk) indicates the saturation characteristic of the CT
core, which directly affects protection relay operation during fault conditions.For CT fundamentals, see our Complete CT Guide.
This guide covers the complete CT knee point voltage testing procedure, including both IEC and ANSI
test methods, test equipment requirements, and result interpretation.
increase in voltage causes a 50% increase in excitation current (IEC definition). This marks the onset of core
saturation.
What is Knee Point Voltage?
Knee point voltage (Vk) is the voltage applied to the CT secondary terminals (with primary
open-circuited) at which the CT core begins to saturate. Above this point, the CT can no longer accurately transform
the primary current.
Why Knee Point Matters
| Application | Importance |
|---|---|
| Differential Protection | Both CTs must have matching Vk for balanced operation |
| REF Protection | Adequate Vk ensures proper operation during internal faults |
| High-Impedance Protection | Vk must exceed calculated stability voltage |
| Distance Protection | Prevents CT saturation during zone 1 faults |
Knee Point Formula
Vk ≥ If × (Rct + Rl + Rr)
Where:
- If = Maximum fault current (secondary)
- Rct = CT secondary winding resistance
- Rl = Lead wire resistance (round trip)
- Rr = Relay burden resistance
IEC vs ANSI Knee Point Definitions
IEC 61869-2 Definition
The knee point is defined as the voltage at which a 10% increase in voltage causes a 50% increase in
excitation current.
ANSI C57.13 Definitions
| CT Type | Definition |
|---|---|
| Non-Gapped Core | Point where tangent line forms 45° angle with horizontal |
| Air-Gapped Core | Point where tangent line forms 30° angle with horizontal |
Comparison Chart
| Standard | Method | Application |
|---|---|---|
| IEC 61869-2 | 10%/50% voltage-current relationship | International standard |
| ANSI 45° | Tangent line on log-log plot | US, non-gapped CTs |
| ANSI 30° | Tangent line on log-log plot | US, gapped CTs |
CT Knee Point Voltage Testing Procedure
Equipment Required
| Equipment | Specification |
|---|---|
| Variable AC source | 0-2000V, 50/60Hz, adjustable |
| Voltmeter | True RMS, 0.5% accuracy |
| Ammeter | Range 1mA-10A, 0.5% accuracy |
| Test leads | Rated for test voltage |
| Safety equipment | Insulated gloves, barriers |
Step-by-Step Test Procedure
Step 1: Safety Preparation
- Isolate CT from system
- Ground CT primary winding
- Short-circuit all unused secondary windings
Step 2: Connect Test Equipment
Step 3: Perform Excitation Test
- Start with zero voltage
- Gradually increase voltage in small steps
- Record voltage (V) and excitation current (Ie) at each step
- Continue until current increases rapidly (past knee point)
- Reduce voltage to zero before disconnecting
Data Recording Table
| Step | Voltage (V) | Current (mA) | Notes |
|---|---|---|---|
| 1 | 10 | 2.1 | Linear region |
| 2 | 20 | 4.3 | Linear region |
| 3 | 50 | 11 | Linear region |
| 4 | 100 | 24 | Approaching knee |
| 5 | 150 | 42 | Near knee point |
| 6 | 180 | 85 | Knee point |
| 7 | 200 | 180 | Saturation region |
Interpreting Test Results
Determining Knee Point (IEC Method)
- Plot voltage vs. current on log-log graph
- Find the point where:
- V2 = V1 × 1.10 (10% increase)
- I2 = I1 × 1.50 (50% increase)
- This intersection defines Vk
Acceptance Criteria
| Parameter | Requirement |
|---|---|
| Knee Point Voltage | ≥ Specified Vk on nameplate |
| Excitation Current at Vk/2 | ≤ Specified Ie on nameplate |
| Curve Shape | Smooth, no discontinuities |
Troubleshooting Failed Tests
| Issue | Possible Cause | Action |
|---|---|---|
| Low Vk | Shorted turns in winding | Replace CT |
| High Ie | Core degradation | Investigate further |
| Erratic curve | Poor test connections | Check connections, retest |
| No clear knee | Air-gapped core (normal) | Use ANSI 30° method |
PS Class Current Transformers
Protection Special (PS) class CTs are specified by:
| Parameter | Symbol | Meaning |
|---|---|---|
| Knee Point Voltage | Vk | Minimum voltage at knee point |
| Excitation Current | Ie | Maximum current at Vk/2 |
| Secondary Resistance | Rct | Maximum DC resistance at 75°C |
CT Rating: 1000/1A, PS Class, Vk ≥ 650V, Ie ≤ 30mA at Vk/2This means knee point voltage must be at least 650V, and at 325V (Vk/2), excitation current must not exceed 30mA.
DC Saturation Testing Method
For CTs with very high knee point voltage, DC testing is an alternative:
Advantages of DC Testing
- Lower voltage source required
- Smaller, lighter test equipment
- Can test very high Vk CTs
reducing AC voltage (from above Vk to zero) to remove residual magnetism before returning CT to service.
🏭 Factory Experience:
- Always Ground Primary: During knee point testing, the CT primary must be solidly grounded.
Floating primary can cause dangerous voltages. - Demagnetization After Testing: The CT core may retain residual magnetism after testing.
Apply gradually reducing AC voltage to demagnetize. - Matching CTs for Differential: When testing CT pairs for differential protection, both CTs
should have Vk within ±10% of each other and similar excitation characteristics. - Temperature Effects: CT resistance increases with temperature. For accurate comparison with
nameplate values, correct Rct to 75°C reference temperature.
Frequently Asked Questions
What is CT knee point voltage?
Knee point voltage (Vk) is the voltage applied to CT secondary terminals at which the core begins to saturate. Per
IEC 61869-2, it’s defined as the voltage where a 10% increase causes a 50% increase in excitation current.
Why is knee point voltage testing important?
CT knee point voltage testing verifies that the CT will perform correctly during fault conditions.
If Vk is too low, the CT may saturate during faults, causing incorrect relay operation.
What is the difference between IEC and ANSI knee point?
IEC uses a 10%/50% voltage-current definition, while ANSI uses tangent line methods (45° for non-gapped cores, 30°
for gapped cores). The resulting Vk values may differ by 10-20%.
What is PS class in current transformers?
PS (Protection Special) class specifies a CT by its knee point voltage (Vk) and excitation current (Ie) rather than
by accuracy class. It’s used for differential and high-impedance protection schemes.
How do I calculate required knee point voltage?
Calculate Vk ≥ If(sec) × (Rct + 2Rl + Rr), where If(sec) is maximum secondary fault current, Rct is CT resistance, Rl
is lead resistance per conductor, and Rr is relay burden.
Related Articles
- Current Transformer (CT):
Complete Guide - CT Accuracy Class Explained
- CT Burden Calculation
Guide - CT Ratio Calculation &
Selection
References
- IEC 61869-2 – Instrument Transformers: Current
Transformers - IEEE C57.13 – Standard Requirements for
Instrument Transformers - Megger – What is a Knee Point?
Disclaimer: This information is provided for educational purposes. Always follow applicable standards and
manufacturer guidelines when performing CT testing. High voltages are involved – only qualified personnel should
conduct these tests.