How to Perform a Demagnetization Procedure for Current Transformers After a Fault Event?

A fault event in a medium voltage power distribution system does more than trip a breaker — it can leave an invisible but dangerous legacy inside your current transformer core: residual magnetism. Residual flux trapped in a CT core after a fault or DC offset transient directly degrades electromagnetic induction accuracy, causes premature core saturation, and can trigger false protection relay operations or dangerous under-reach during the next fault. For electrical engineers and maintenance teams responsible for substation reliability, knowing how to correctly demagnetize a CT core is not optional maintenance knowledge — it is a frontline protection system integrity task. This article details the physics of residual flux, the step-by-step field demagnetization procedure, and the selection criteria that determine whether your CT core is even susceptible to remanence in the first place.

Table of Contents

• What Is Residual Flux and Why Does It Form in CT Cores?
• How Does Residual Magnetism Affect CT Induction Performance and Reliability?
• How Do You Perform a Field Demagnetization Procedure on a Current Transformer?
• What Are Common Mistakes That Cause Demagnetization to Fail in Medium Voltage CTs?

What Is Residual Flux and Why Does It Form in CT Cores?

Residual flux — also called remanent magnetism or remanence — is the magnetic flux density that remains locked inside a CT core’s grain-oriented silicon steel structure after the magnetizing force has been removed. Understanding why it forms requires a brief look at the b-h hysteresis loop¹ that governs all ferromagnetic core behavior.

When a CT experiences a fault current with a significant DC offset component, the primary current does not oscillate symmetrically around zero. Instead, it drives the core flux along the hysteresis curve into a region of high magnetic flux density². When the fault is cleared and current drops to zero abruptly — as happens during a circuit breaker interruption — the core does not return to zero flux. It remains at the remanent flux density (Br), which for grain-oriented silicon steel can reach 60–80% of saturation flux density³ (Bsat).

Key technical characteristics of CT core remanence:

• Core material sensitivity: Grain-oriented silicon steel (used in high-accuracy CTs) has high permeability but also high remanence. Nickel-iron alloy cores exhibit even higher remanence levels.
• Air-gap cores: CTs designed with a small deliberate air gap in the core (TPY and TPZ classes per IEC 61869-2) have significantly lower remanence — typically less than 10% of Bsat — because the air gap provides a magnetic reset mechanism.
• Triggering events: DC offset fault currents, CT secondary open-circuit events, and improper demagnetization after testing are the three primary causes of significant residual flux buildup.

Core Type	Remanence Level	IEC Class	Typical Application
Grain-oriented Si-Steel (no air gap)	60–80% Bsat	5P, 10P, TPS	Standard protection CTs
Nickel-Iron Alloy (no air gap)	Up to 90% Bsat	Class X, TPS	High-sensitivity differential protection
Gapped Core (small air gap)	<10% Bsat	TPY	Auto-reclose protection schemes
Large Air Gap Core	~0% Bsat	TPZ	High-speed protection, transient performance

The core type installed in your switchgear panel directly determines your remanence risk profile — and whether a demagnetization procedure is periodically mandatory or merely precautionary.

How Does Residual Magnetism Affect CT Induction Performance and Reliability?

Residual flux does not cause immediate visible failure — it is a hidden degradation mechanism that silently compromises your protection system’s reliability until the next fault event exposes it catastrophically. The impact operates through one primary mechanism: reduced available flux swing before saturation.

A CT core can only support a finite change in flux density before it saturates. The total available flux swing is:
$\Delta B = B_{\text{sat}} – B_{r}$

If Br is already at 70% of Bsat due to residual magnetism, the core has only 30% of its normal flux capacity available for the next fault current transient. This means the CT saturates far earlier than its rated Accuracy Limit Factor (ALF) would suggest, producing a severely distorted secondary current waveform that protection relays cannot correctly interpret.

Practical consequences of unaddressed residual flux:

• Distance relay under-reach: Saturated CT output causes the relay to see a higher apparent impedance than actual, potentially failing to trip for in-zone faults
• Differential protection maloperation: Asymmetric saturation between CTs on opposite sides of a protected zone generates false differential current, causing unwanted tripping
• Overcurrent relay delayed operation: Distorted secondary waveform extends relay operating time beyond designed trip curves
• Energy metering errors: Even at normal load currents, a partially saturated core introduces ratio and phase angle errors exceeding Class 0.5 limits

Customer Case — Power Contractor, 35kV Substation Retrofit, Middle East: A power contractor managing a 35kV substation retrofit in Saudi Arabia reported repeated nuisance trips on a feeder differential protection scheme following a nearby bus fault. After consulting Bepto’s technical team, CT secondary waveform analysis revealed severe asymmetric saturation consistent with high residual flux in two of the six CTs in the differential zone. Following a structured demagnetization procedure on all six units, differential protection stability was fully restored — eliminating three weeks of intermittent nuisance trips that had been misattributed to relay settings.

How Do You Perform a Field Demagnetization Procedure on a Current Transformer?

The demagnetization procedure works by driving the CT core through progressively smaller hysteresis loops until the residual flux converges to near zero. There are two accepted field methods — AC voltage injection and DC current injection with reversal — each suited to different site conditions and CT designs.

Step 1: Isolate and Prepare the CT Circuit

• De-energize the primary circuit and confirm isolation with a voltage tester
• Short-circuit all unused CT secondary cores before beginning — open-circuit secondary terminals under any residual flux condition can generate dangerous induced voltages
• Disconnect the protection relay and metering burden from the secondary terminals being demagnetized
• Document the CT nameplate: rated ratio, accuracy class, knee-point voltage (Vk), and magnetizing current (Imag)

Step 2: Select the Demagnetization Method

Method	Equipment Required	Best For	Limitation
AC Voltage Injection (Degaussing)	Variable AC source (Variac), ammeter	Standard 5P/10P silicon steel cores	Requires access to variable voltage source
DC Current Injection with Reversal	DC power supply, reversing switch, ammeter	TPY / gapped cores, high-inductance CTs	Requires careful current reversal sequencing
Dedicated CT Analyzer	CT analyzer with built-in demagnetization function	All core types — most reliable	Equipment cost; not always available on-site

Step 3: AC Injection Demagnetization Procedure (Most Common Field Method)

1. Connect a variable ac voltage source⁴ (Variac) across the CT secondary terminals (S1–S2)
2. Slowly increase AC voltage from zero until the magnetizing current reaches approximately 120–150% of the rated knee-point magnetizing current — this drives the core into saturation, establishing a known starting point on the hysteresis loop
3. Slowly and continuously reduce the AC voltage back to zero — do not stop or reverse; the reduction must be smooth and uninterrupted over 30–60 seconds
4. The core flux traces progressively smaller hysteresis loops, converging to near-zero remanence as voltage approaches zero
5. Measure the magnetizing current at the original test voltage — compare against the pre-demagnetization baseline to confirm flux reduction

Step 4: Verify Demagnetization Success

• Perform a CT excitation curve⁵ test (V-I characteristic) and compare against the factory magnetizing curve
• A successfully demagnetized core will show magnetizing current within ±5% of the factory baseline at the same applied voltage
• For protection CTs, verify the knee-point voltage (Vk) is restored to nameplate specification
• Record all test results in the substation maintenance log per IEC 61869-2 commissioning requirements

Step 5: Restore Secondary Circuits

1. Reconnect protection relay and metering burden in correct polarity (S1→S2 orientation)
2. Remove secondary short-circuit links only after all burden connections are confirmed
3. Re-energize primary circuit and monitor CT secondary output during first load cycle
4. Verify protection relay current inputs match expected values based on primary load current and CT ratio

What Are Common Mistakes That Cause Demagnetization to Fail in Medium Voltage CTs?

Demagnetization is a precision procedure — small execution errors can leave significant residual flux in the core or, worse, introduce new remanence at a different polarity. These are the most critical field mistakes observed across medium voltage substation maintenance operations.

Critical Errors to Avoid

• Stopping the voltage reduction mid-procedure: Interrupting the AC voltage sweep at any non-zero level freezes the core at a new remanence point — potentially worse than the original condition. The reduction must be continuous and uninterrupted to zero.
• Applying excessive initial voltage: Over-driving the core beyond 150% of knee-point magnetizing current risks insulation stress on the secondary winding. Always calculate the safe injection voltage limit before starting.
• Demagnetizing with secondary burden connected: Connected relay impedance alters the effective circuit inductance, preventing the core from completing full hysteresis loops. Always disconnect burden before the procedure.
• Skipping the excitation curve verification: Visual inspection cannot confirm successful demagnetization. Only a post-procedure V-I characteristic test against the factory curve provides objective confirmation.
• Ignoring adjacent CT cores in multi-core units: In dual-core CTs, demagnetizing one core can induce flux changes in the adjacent core through magnetic coupling. Both cores must be tested and demagnetized sequentially.

Post-Procedure Checklist

1. ✔ Excitation curve matches factory baseline within ±5%
2. ✔ Knee-point voltage restored to nameplate value
3. ✔ Secondary polarity markings verified before burden reconnection
4. ✔ All short-circuit links removed after burden reconnection
5. ✔ Test results documented in maintenance records

Conclusion

Residual flux in a current transformer core is a silent reliability threat that fault events routinely create and maintenance teams routinely overlook. The demagnetization procedure — whether by AC voltage sweep or DC current reversal — restores the core’s full available flux swing, ensuring your protection relays operate within designed accuracy limits when the next fault occurs. For medium voltage power distribution systems where protection reliability is non-negotiable, demagnetization is not a corrective action — it is a mandatory post-fault commissioning step. At Bepto Electric, our CTs are manufactured to IEC 61869-2 with full factory excitation curve documentation, giving your maintenance team the baseline data needed to verify successful demagnetization every time.

FAQs About CT Demagnetization Procedure

1. Understanding how ferromagnetic materials retain magnetism through the hysteresis cycle. ↩

2. Technical definitions of flux density and its role in transformer core performance. ↩

3. The physical limits of magnetic flux a transformer core can support before saturation. ↩

4. How variable autotransformers (Variacs) control voltage for electrical testing. ↩

5. A guide to interpreting V-I characteristic curves for instrument transformer health. ↩