How to Read and Interpret a Current Transformer Excitation Curve for Instrument Transformer Health?

The excitation curve is the single most revealing diagnostic signature a current transformer can produce — yet it remains one of the most misread tests in medium voltage substation commissioning and maintenance practice. A CT’s V-I characteristic curve encodes the complete health story of its magnetic core: knee-point voltage integrity, residual flux condition, insulation degradation, and turn-to-turn fault indicators — all visible to an engineer who knows how to read the shape. For electrical engineers, protection relay specialists, and procurement managers specifying instrument transformers for power distribution systems, mastering excitation curve interpretation is the difference between catching a failing CT before it compromises a protection scheme and discovering the problem only after a catastrophic misoperation. This article walks through the physics behind the curve, the step-by-step test procedure, and the diagnostic patterns that reveal exactly what is happening inside your CT core.

Table of Contents

• What Is a Current Transformer Excitation Curve and What Does It Measure?
• How Do You Interpret the Key Features of a CT V-I Characteristic Curve?
• How Do You Perform a CT Excitation Test in the Field for Medium Voltage Applications?
• What Do Abnormal Excitation Curve Patterns Reveal About CT Health and Reliability?

What Is a Current Transformer Excitation Curve and What Does It Measure?

The excitation curve — formally called the V-I characteristic or magnetization curve — is a graphical representation of the relationship between the voltage applied to a CT secondary winding and the resulting magnetizing current drawn by the core, with the primary circuit open. It is measured entirely from the secondary terminals, making it one of the safest and most accessible diagnostic tests available in the field.

The physics behind the curve is rooted in the core’s b-h hysteresis¹ behavior. When AC voltage is applied to the secondary winding, it drives a magnetic flux in the core proportional to the applied voltage (by faraday’s law²: $V = N \times \frac{d\Phi}{dt}$). The magnetizing current required to sustain that flux is determined by the core’s magnetic permeability at that operating point. As applied voltage increases, the core progressively saturates, permeability drops sharply, and magnetizing current rises steeply — producing the characteristic knee shape that defines every CT excitation curve.

Key parameters encoded in the excitation curve:

• Knee-point voltage (Vk): The voltage at which a 10% increase in applied voltage produces a 50% increase in magnetizing current — the critical boundary between linear and saturated core operation per IEC 61869-2
• Magnetizing current at Vk (Imag): Defines the CT’s exciting current burden; directly impacts ratio and phase angle accuracy at low primary currents
• Curve slope in the linear region: Reflects core permeability and material quality — steeper slope indicates higher permeability grain-oriented silicon steel
• Saturation behavior above Vk: Rate of current rise above knee-point determines how quickly the CT saturates under fault current transients

Parameter	Definition	IEC 61869-2 Reference	Engineering Significance
Knee-point Voltage (Vk)	10% ΔV → 50% ΔI crossover point	Clause 5.6.201	Minimum Vk determines protection CT suitability
Magnetizing Current (Imag)	RMS current at Vk	Clause 5.6.201	High Imag = accuracy degradation at low currents
Saturation Flux Density (Bsat)	Maximum core flux before full saturation	Material specification	Determines available flux swing for fault transients
Remanence Factor (Kr)	Br/Bsat ratio	IEC 61869-2 TPY/TPZ	Governs residual flux susceptibility
Secondary Winding Resistance (Rct)	DC resistance of secondary winding	Clause 5.6.201	Used in protection CT dimensioning calculations

The excitation curve is the foundation of every CT health assessment — from factory acceptance testing to post-fault field diagnostics. Without a factory baseline curve on file, field comparison testing loses most of its diagnostic value, which is why Bepto Electric provides full excitation curve documentation with every CT shipment.

How Do You Interpret the Key Features of a CT V-I Characteristic Curve?

Reading a CT excitation curve correctly requires understanding three distinct regions of the plot and what each region reveals about core condition and protection performance. The curve is almost always plotted on a log-log scale to compress the wide dynamic range of both voltage and current into a readable format.

Region 1 — The Linear Region (Below Knee-Point) In this region, the core operates within its linear permeability range. Applied voltage and magnetizing current increase proportionally, producing a straight line on the log-log plot. The slope of this line reflects core material quality:

• A steep, well-defined linear region indicates high-permeability grain-oriented silicon steel³ in good condition
• A shallow or irregular slope suggests core degradation, inter-lamination shorts, or contamination

Region 2 — The Knee-Point The knee-point is the most diagnostically important single feature of the excitation curve. Per IEC 61869-2, it is defined as the point where the tangent to the curve makes a 45° angle with the horizontal axis on a log-log plot — equivalently, where a 10% voltage increase produces a 50% current increase.

• Vk must meet or exceed the minimum value specified in the protection CT dimensioning formula: $V_k \geq I_f \times (R_{ct} + R_{\text{burden}}) \times ALF$
• A knee-point that has shifted downward compared to the factory curve indicates core degradation or residual flux
• A knee-point that appears at a higher current than the factory baseline suggests turn-to-turn winding shorts

Region 3 — The Saturation Region (Above Knee-Point) Above the knee-point, the curve bends sharply upward as the core saturates and magnetizing current rises steeply for small voltage increments. The shape of this saturation region reveals:

• Gradual saturation curve: Healthy core with expected silicon steel behavior
• Abrupt, near-vertical saturation: Possible core damage or severe residual flux condition
• Irregular humps or inflection points: Strong indicator of turn-to-turn winding faults or inter-lamination shorts

Healthy vs. Degraded CT Excitation Curve Comparison

Curve Feature	Healthy CT	Residual Flux Present	Turn-to-Turn Fault	Core Degradation
Linear region slope	Consistent, steep	Reduced slope	Irregular, shifted	Shallow, inconsistent
Knee-point voltage	Matches factory Vk	Shifted lower	Higher current at Vk	Significantly reduced
Saturation onset	Gradual above Vk	Premature saturation	Abrupt transition	Early, irregular
Magnetizing current at Vk	Matches factory Imag	Similar to factory	Higher than factory	Significantly higher

Customer Case — Quality-Focused Utility Engineer, 110kV Substation, North Africa: A utility engineer in Morocco responsible for commissioning a new 110kV substation extension received a batch of twelve protection CTs from a previous supplier. During factory acceptance testing, three units showed knee-point voltages 22–35% below the specified minimum — a defect invisible without excitation curve testing. The engineer contacted Bepto Electric, and our replacement units were shipped with full excitation curve documentation matching IEC 61869-2 Class 5P20 specifications. Post-installation commissioning confirmed all twelve positions met protection scheme dimensioning requirements — preventing what could have been a systematic protection under-reach condition across an entire substation section.

How Do You Perform a CT Excitation Test in the Field for Medium Voltage Applications?

The excitation test is performed from the CT secondary terminals with the primary circuit open — making it executable during planned outages without primary circuit access. The procedure is standardized under IEC 61869-2 and IEEE C57.13.1, with minor procedural variations between the two standards.

Step 1: Isolate and Prepare the CT

• Confirm primary circuit is de-energized and isolated — verify with approved voltage tester
• Open all secondary burden connections (disconnect relays, meters, and wiring) — the test must be performed on the bare secondary winding only
• Short-circuit any unused secondary cores on multi-core CTs to prevent induced voltage hazards
• Record CT nameplate data: ratio, accuracy class, rated Vk, rated Imag, Rct, and ALF

Step 2: Select Test Equipment

• Preferred: Dedicated CT analyzer (e.g., Megger MRCT, Omicron CT Analyzer) — automatically plots the full excitation curve and calculates Vk per IEC 61869-2 definition
• Alternative: Variable AC voltage source (Variac) + true-RMS voltmeter + true-RMS ammeter — manual point-by-point curve plotting
• Ensure test equipment voltage range covers at least 120% of the expected Vk value
• Confirm ammeter range covers from 1mA (low-current linear region) to at least 5× rated Imag

Step 3: Execute the Excitation Test

1. Connect test voltage source across S1–S2 secondary terminals
2. Starting from zero, increase applied voltage in small increments — suggested steps: 10% of expected Vk up to 50% Vk, then 5% steps from 50% to 110% Vk, then 2% steps around the knee-point region
3. Record both applied voltage (V) and magnetizing current (I) at each step — allow 3–5 seconds stabilization per point
4. Continue increasing voltage until clear saturation behavior is observed (current rising steeply with minimal voltage increase)
5. Reduce voltage slowly back to zero — this also serves as a partial demagnetization step
6. Plot V on the Y-axis and I on the X-axis on a log-log scale

Step 4: Determine Knee-Point Voltage

• Using the plotted curve, locate the point where the tangent angle equals 45° on the log-log plot
• For automated CT analyzers, the instrument calculates Vk directly per IEC 61869-2 Clause 5.6.201
• Compare measured Vk against: factory baseline value, nameplate specification, and protection scheme minimum Vk requirement

Step 5: Document and Compare Results

• Record: Vk measured, Imag at Vk, Rct (DC resistance measurement), and full V-I data table
• Compare against factory excitation curve — deviations >10% in Vk or >20% in Imag warrant further investigation
• For protection CTs, verify: Vk ≥ If(max) × (Rct + Rburden) per IEC 61869-2 dimensioning

Application-Specific Excitation Test Considerations

• Industrial switchgear panels: Test during scheduled maintenance windows; document baseline curves at commissioning for future comparison
• Power grid protection CTs: Mandatory post-fault excitation test after any fault current exceeding 10× rated primary current
• Substation differential protection zones: Test all CTs in the differential zone simultaneously; compare curves for symmetry — asymmetric curves indicate mismatched CT characteristics that can cause false differential current
• Solar farm grid connection CTs: Verify Vk adequacy for inverter fault current contribution, which may have significant DC offset components

What Do Abnormal Excitation Curve Patterns Reveal About CT Health and Reliability?

Abnormal excitation curve patterns are the CT’s way of communicating specific internal failure modes. Each defect type produces a characteristic curve signature that an experienced engineer can identify and diagnose without dismantling the unit.

Diagnostic Pattern Recognition Guide

Pattern 1 — Knee-Point Voltage Shifted Downward (Vk reduced vs. factory)

• Primary cause: Residual flux from previous fault or open-circuit event
• Secondary cause: Core lamination damage from mechanical shock or improper handling
• Action: Perform full demagnetization procedure; re-test excitation curve; if Vk remains low after demagnetization, CT requires replacement

Pattern 2 — Magnetizing Current Higher Than Factory Baseline at Same Voltage

• Primary cause: Turn-to-turn short circuit in secondary winding — shorted turns reduce effective turns count, increasing magnetizing current requirement
• Secondary cause: eddy current losses⁴ in core increasing eddy current losses
• Action: Measure secondary winding DC resistance (Rct) — reduced Rct confirms shorted turns; CT requires replacement

Pattern 3 — Irregular Inflection Points or Humps in Linear Region

• Primary cause: Multiple turn-to-turn faults creating multiple magnetic circuit paths with different saturation characteristics
• Secondary cause: Core mechanical damage creating non-uniform flux distribution
• Action: CT is unreliable for protection duty — remove from service immediately

Pattern 4 — Curve Shifted Uniformly Higher (Higher voltage required for same current)

• Primary cause: Increased winding resistance due to connection corrosion or partial conductor failure
• Secondary cause: Measurement error — verify test lead resistance and connection quality before concluding
• Action: Measure Rct; inspect secondary terminal connections; clean or replace corroded terminals

Common Field Mistakes in Excitation Curve Testing

• Using average-responding voltmeter instead of true-RMS: Harmonic content in the magnetizing current waveform near saturation causes significant reading errors with average-responding instruments — always use true-RMS⁵ meters
• Testing with secondary burden still connected: Connected impedance adds to the measured voltage, shifting the apparent knee-point higher and masking real core degradation
• Insufficient voltage range: Stopping the test before reaching clear saturation prevents accurate knee-point identification — always test to at least 120% of expected Vk
• Single-point comparison instead of full curve: Comparing only the knee-point value misses diagnostic information encoded in the curve shape — always compare the full V-I characteristic against the factory baseline

Conclusion

The CT excitation curve is the most comprehensive single-test diagnostic available for current transformer health assessment in medium voltage power distribution systems. From knee-point voltage integrity to turn-to-turn fault detection, residual flux identification, and core degradation monitoring, every critical reliability indicator is encoded in the V-I characteristic shape. For protection engineers and maintenance teams responsible for substation reliability, establishing factory baseline excitation curves at commissioning and comparing them systematically after every significant fault event is not best practice — it is the minimum standard for a protection system you can trust. At Bepto Electric, every CT ships with a full factory excitation curve certificate to IEC 61869-2, giving your team the diagnostic baseline that makes field health assessment meaningful from day one.

FAQs About CT Excitation Curve Interpretation

1. Technical explanation of magnetic core behavior and energy loss during cycles. ↩

2. Scientific principles explaining how voltage is induced in transformer windings. ↩

3. Material science properties that determine the efficiency and permeability of transformer cores. ↩

4. Understanding the circulating currents that cause heat and efficiency loss in iron cores. ↩

5. Comparison of measurement methods for non-linear or distorted electrical waveforms. ↩