How Core Magnetization Causes False Relay Tripping

Introduction

Among the failure modes that cause protection relays to operate incorrectly in industrial plant medium voltage systems, core remanence — the residual magnetic flux that remains locked in a current transformer’s iron core after the primary current has ceased — is the most systematically misunderstood and the most frequently misdiagnosed. When an industrial plant experiences a spurious protection trip that cannot be correlated with any actual fault event, the investigation typically focuses on relay settings, relay hardware, and secondary circuit wiring. The CT core is rarely examined. Yet in a significant proportion of unexplained false trips — particularly those occurring during transformer energization, motor starting, or circuit reclosing after a fault — the CT core’s remanent flux is the root cause, and no amount of relay setting adjustment will prevent recurrence until the remanence condition is identified and corrected.

The direct answer is this: CT core remanence causes false relay tripping because the residual magnetic flux remaining in the CT core after a fault event or DC current exposure shifts the core’s operating point on its B-H magnetization curve, causing the CT to saturate earlier and more severely during the next energization transient — producing a distorted secondary current waveform that contains large DC offset and harmonic components which arc protection and overcurrent relays interpret as fault current signatures, triggering a trip decision on a circuit that is operating normally.

For industrial plant protection engineers, medium voltage maintenance teams, and arc protection system specialists troubleshooting unexplained relay operations, this guide provides the complete technical explanation of how core remanence develops, how it causes false tripping, and how to diagnose, correct, and prevent remanence-induced protection failures.

Table of Contents

• What Is CT Core Remanence and How Does It Develop in Industrial Plant Medium Voltage Systems?
• How Does Core Remanence Cause CT Saturation and False Relay Tripping?
• How to Diagnose Remanence-Induced False Tripping in Industrial Plant Protection Systems?
• How to Correct CT Core Remanence and Prevent Recurrence in Medium Voltage Arc Protection Systems?
• FAQs About CT Core Remanence and False Relay Tripping in Industrial Plant Applications

What Is CT Core Remanence and How Does It Develop in Industrial Plant Medium Voltage Systems?

The iron core of a current transformer is a ferromagnetic material whose magnetic behavior is described by its b-h magnetization curve¹ — the relationship between the magnetic flux density B in the core and the magnetizing force H applied to it. The B-H curve of a ferromagnetic material is not a simple linear relationship — it is a hysteresis loop, meaning that the flux density in the core depends not only on the current magnetizing force but also on the history of previous magnetization.

When the magnetizing force H is reduced to zero — when the primary current ceases — the flux density B does not return to zero. It remains at a residual value called the remanent flux density Br, which can be as high as 70–80% of the saturation flux density Bsat for the grain-oriented silicon steel used in CT cores. This residual flux — the remanence — is locked into the core’s magnetic domain structure and persists indefinitely until it is deliberately removed by demagnetization or overwritten by a sufficiently large opposing magnetizing force.

Remanence Development Mechanisms in Industrial Plant Medium Voltage Systems

Industrial plant medium voltage systems expose CT cores to remanence-generating conditions far more frequently than conventional distribution systems — because the combination of large motor loads, frequent fault events, and arc protection system operation creates a sequence of current conditions that systematically drive CT cores toward high remanence states.

Mechanism 1: Asymmetric Fault Current DC Offset

The most significant remanence source in industrial plant CT installations. When a fault occurs on a medium voltage system, the fault current contains a DC offset component whose magnitude depends on the point-on-wave at which the fault initiates and the system x/r ratio²:

$i_{fault}(t) = I_{peak} \times \left[\sin(\omega t + \phi) – \sin(\phi) \times e^{-t/\tau}\right]$

Where $\phi$ is the fault inception angle and$$\tau = L/R$$ is the DC time constant. For industrial plant medium voltage systems with X/R ratios of 15–30, the DC time constant is 48–95 ms — meaning the DC offset component persists for 5–10 power frequency cycles before decaying to negligible levels.

The DC component of the fault current drives the CT core’s operating point progressively toward saturation in one direction on the B-H curve. When the fault is cleared by the protection relay — typically within 60–200 ms — the DC-driven flux remains in the core as remanence. The magnitude of the remanent flux depends on the DC offset magnitude and the fault clearing time:

$B_{remanent} \approx B_{sat} \times \left(1 – e^{-t_{clearing}/\tau_{core}}\right) \times \sin(\phi)$

For a worst-case fault inception angle ($\phi$ = 90°) with a 100 ms clearing time, the remanent flux can reach 60–75% of Bsat.

Mechanism 2: Protection Relay DC Trip Current

Arc protection relays and some overcurrent relays use DC trip coil current to operate circuit breaker trip mechanisms. When the trip coil current flows through the CT secondary circuit — which can occur through inductive coupling or through shared earth connections in some industrial plant wiring configurations — it applies a DC magnetizing force to the CT core that drives it to a remanent state independent of any primary current condition.

Mechanism 3: Transformer Inrush Current

When a medium voltage transformer is energized, the inrush current contains a large DC offset component that can persist for 0.5–2 seconds — much longer than fault current DC offset. For CTs installed on the transformer primary feeder, this extended DC exposure drives the core to near-saturation remanence levels. If the transformer is subsequently de-energized and re-energized — a common occurrence during industrial plant commissioning and maintenance — the CT core accumulates remanence from each energization event.

Mechanism 4: Secondary Circuit Testing with DC Sources

Insulation resistance testing of CT secondary circuits using a 500 V or 1,000 V DC megohmmeter applies a DC voltage across the CT secondary winding. If the secondary winding is not shorted during the IR test — a common testing error — the DC test voltage drives a magnetizing current through the CT core, leaving a remanent flux state that may not be recognized as a test artifact.

Key technical parameters defining CT core remanence:

Parameter	Definition	Typical Value	Impact on Performance
Remanent Flux Density (Br)	Residual B when H = 0	0.8–1.4 T (60–80% of Bsat)	Shifts operating point toward saturation
Saturation Flux Density (Bsat)	Maximum B at high H	1.8–2.0 T for silicon steel	Defines saturation onset threshold
Coercive Force (Hc)	H required to reduce B to zero	10–50 A/m for CT core steel	Determines demagnetization current required
DC Time Constant (τ)	L/R of fault current circuit	20–100 ms for MV systems	Determines DC offset persistence duration
Remanence Factor (Kr)	Br/Bsat	0.6–0.8 for standard CT cores	iec 61869-23 defines Kr ≤ 0.1 for Class PR cores
Applicable Standard	IEC 61869-2 Class PR	Remanence-protected core specification	Kr ≤ 0.1 achieved by air gap in core

How Does Core Remanence Cause CT Saturation and False Relay Tripping?

The path from core remanence to false relay tripping involves a specific sequence of electromagnetic events that occurs during the first few cycles of primary current flow after the remanent state has been established — typically during transformer energization, motor starting, or circuit reclosing after a fault clearance.

The Remanence-to-Saturation Sequence

Stage 1: Remanent Flux Establishes Shifted Operating Point

After a fault event, the CT core retains remanent flux Br. On the B-H curve, the core’s operating point is at (H=0, B=Br) — displaced from the origin by the remanent flux. The available flux swing before saturation is now:

$\Delta B_{available} = B_{sat} – B_{remanent}$

For a core with Bsat = 1.9 T and Bremanent = 1.3 T (68% of Bsat), the available flux swing is only 0.6 T — compared to 1.9 T for a fully demagnetized core. The CT’s ability to reproduce primary current accurately is proportional to the available flux swing — a core with 68% remanence has only 32% of its normal flux capacity available for accurate current reproduction.

Stage 2: Energization Transient Drives Core to Saturation

When the circuit is re-energized — transformer energization, motor starting, or reclosing after fault clearance — the primary current contains an asymmetric component with DC offset. The DC offset drives the core flux in the same direction as the remanence (in the worst case, when the remanence polarity matches the DC offset direction). The core reaches saturation after only a fraction of the first half-cycle:

$t_{saturation} = \frac{B_{sat} – B_{remanent}}{dB/dt_{normal}}$

For a core with 68% remanence, saturation occurs approximately 3× earlier than for a fully demagnetized core — potentially within the first quarter-cycle of the energization transient.

Stage 3: Saturated CT Produces Distorted Secondary Waveform

When the CT core saturates, the magnetizing inductance collapses — the core can no longer support increasing flux, and the primary current is no longer reproduced in the secondary winding. Instead, the secondary current drops abruptly toward zero while the primary current continues to flow. The secondary waveform becomes severely distorted — containing large peaks during the unsaturated portions of each cycle and near-zero current during the saturated portions.

The distorted secondary waveform contains:

• Large DC component: From the asymmetric saturation pattern — the CT saturates more severely on one half-cycle than the other
• Large odd harmonic content: 3rd, 5th, 7th harmonics from the clipped waveform
• High di/dt transients: Rapid current transitions at the boundaries between saturated and unsaturated regions

Stage 4: Distorted Secondary Current Triggers False Relay Trip

The distorted secondary current waveform is presented to the protection relay as the measured primary current. The relay’s response depends on its measurement algorithm:

• Arc protection relay (light + current detection): Arc protection relays use instantaneous current measurement — they respond to the peak of the secondary current waveform. The high-amplitude peaks in the distorted CT secondary waveform during the unsaturated portions of each cycle can exceed the arc protection relay’s current threshold, triggering a trip decision even though no arc fault exists
• Instantaneous overcurrent relay (50 element): Responds to the peak secondary current — the distorted waveform peaks can exceed the instantaneous pickup threshold, causing false instantaneous trip
• Time-overcurrent relay (51 element): Responds to RMS current — the distorted waveform has elevated RMS content that can exceed the pickup threshold and initiate timing toward a time-delayed trip
• Differential relay (87 element): The differential relay compares secondary currents from CTs on both sides of the protected equipment; if only one CT is remanence-affected, the differential current during energization contains a large component from the remanence-induced saturation asymmetry, potentially exceeding the differential relay’s operate threshold

The mathematical relationship between remanent flux and false trip probability:

$P_{false,trip} \propto \frac{B_{remanent}}{B_{sat} – B_{remanent}} \times \frac{I_{DC,offset}}{I_{rated}} \times \frac{1}{t_{relay,pickup} \times f}$

This relationship shows that false trip probability increases with remanence level, with DC offset magnitude, and with relay speed — explaining why arc protection relays (fastest operating time: 5–10 ms) are the most vulnerable to remanence-induced false tripping.

Customer Case — 11 kV Industrial Plant Substation, Automotive Manufacturing, Central Europe:
A protection engineer at an automotive manufacturing plant contacted Bepto Electric after experiencing seven unexplained arc protection relay operations in a 14-month period — all occurring within the first 100 ms of energizing a 2 MVA transformer feeding a paint shop ventilation system. Each false trip caused a production line shutdown costing approximately €45,000 per event. Post-event oscillographic analysis from the arc protection relay showed that the relay had detected both light (from a corona discharge on the transformer bushing during energization) and overcurrent — the overcurrent element had operated on a distorted secondary current waveform with peaks 3.2× the relay’s current threshold. CT excitation curve testing revealed that the three CTs on the transformer primary feeder had remanent flux levels of 71%, 68%, and 74% of Bsat respectively — accumulated from the previous six fault events on the feeder over the preceding three years. Demagnetization of all three CTs reduced the remanence to below 5% of Bsat. In the 18 months following demagnetization, zero false arc protection trips occurred on the transformer feeder. The protection engineer stated: “Seven false trips, seven production shutdowns, and a total loss of over €300,000 — all caused by residual magnetism in three CT cores that took four hours to demagnetize. The arc protection relay was working exactly as designed. The CT was giving it false information.”

How to Diagnose Remanence-Induced False Tripping in Industrial Plant Protection Systems?

Remanence-induced false tripping produces a characteristic diagnostic signature that distinguishes it from other false trip causes — relay setting errors, secondary circuit faults, and genuine fault events. The diagnostic methodology follows a structured sequence that moves from event analysis to CT testing to confirmation.

Step 1: Analyze the False Trip Event Record

The protection relay event record and oscillographic capture provide the first diagnostic evidence:

• Timing correlation: Remanence-induced false trips occur within the first 1–5 cycles of primary current flow — during transformer energization, motor starting, or reclosing. A false trip that occurs more than 200 ms after circuit energization is unlikely to be remanence-induced
• Secondary current waveform shape: Remanence-induced saturation produces a characteristic asymmetric waveform — large peaks on one half-cycle, suppressed or clipped waveform on the other half-cycle. A symmetric distorted waveform suggests a different cause
• DC component in secondary current: Remanence-induced saturation produces a significant DC component in the secondary current waveform — visible in the oscillographic capture as a waveform that does not cross zero symmetrically
• Correlation with previous fault events: Review the protection relay event history for the 6–12 months preceding the false trip — remanence accumulates from fault events; a false trip following a period of elevated fault frequency is consistent with remanence as the cause

Step 2: Perform CT Excitation Curve Test

The excitation curve test is the definitive diagnostic for CT core remanence:

1. De-energize and isolate the CT: The excitation curve test requires the CT to be de-energized and the primary circuit open-circuited
2. Apply AC voltage to secondary winding: Increase AC voltage from zero to the knee-point voltage⁴ while measuring the magnetizing current; plot B (proportional to applied voltage) versus H (proportional to magnetizing current)
3. Compare against factory test certificate: A remanence-affected CT shows a shifted excitation curve — the knee-point occurs at a lower applied voltage than the factory certificate value, and the magnetizing current at the knee-point is higher than the factory value
4. Calculate remanence level: The shift in the excitation curve knee-point voltage from the factory value provides an estimate of the remanent flux level:

$B_{remanent} \approx B_{sat} \times \left(1 – \frac{V_{knee,measured}}{V_{knee,factory}}\right)$

Step 3: Confirm with DC Flux Measurement

For a definitive remanence measurement, the DC flux method provides a direct measurement of the remanent flux density:

1. Apply a known DC current pulse to the secondary winding in the direction that would drive the core to positive saturation
2. Measure the change in flux from the remanent state to saturation using a flux integrator (volt-second measurement)
3. Repeat in the negative direction to measure the flux change from remanent state to negative saturation
4. Calculate remanence: The asymmetry between the positive and negative flux changes directly quantifies the remanent flux:

$B_{remanent} = \frac{(\Delta\Phi_{positive} – \Delta\Phi_{negative})}{2 \times A_{core}}$

Where $A_{core}$ is the CT core cross-sectional area from the factory test certificate.

Diagnostic Decision Matrix

Observation	Remanence Indicated	Alternative Cause
False trip within first 3 cycles of energization	Strong indicator	—
Asymmetric secondary waveform with DC component	Strong indicator	CT saturation from overcurrent
False trip after previous fault event history	Strong indicator	—
Shifted excitation curve knee-point	Confirmed	Core damage (if shift >20%)
False trip at any time, symmetric waveform	Weak indicator	Relay setting, secondary circuit fault
False trip with no previous fault history	Weak indicator	Relay hardware, setting error
Relay operates on light detection only (arc relay)	Not remanence	External corona, arc flash

How to Correct CT Core Remanence and Prevent Recurrence in Medium Voltage Arc Protection Systems?

CT Core Demagnetization Procedure

CT core demagnetization — the controlled removal of remanent flux by cycling the core through progressively smaller hysteresis loops until the operating point returns to the origin of the B-H curve — is the definitive correction for remanence-induced false tripping. The procedure requires the CT to be de-energized and isolated, but does not require removal from the installation.

AC Voltage Reduction Method (Recommended):

1. Connect a variable autotransformer to the CT secondary winding with the primary circuit open-circuited; connect a current-limiting resistor in series to prevent excessive magnetizing current
2. Increase AC voltage to 120% of the CT knee-point voltage — this drives the core to saturation in both directions on each cycle, establishing a large symmetrical hysteresis loop that overwrites the remanent flux
3. Slowly reduce the AC voltage to zero at a rate of approximately 5% per second — this progressively reduces the hysteresis loop size while maintaining symmetry, walking the operating point back to the B-H curve origin
4. Verify demagnetization: Repeat the excitation curve test — the knee-point voltage should match the factory test certificate value within ±5%; the magnetizing current at the knee-point should match the factory value within ±10%
5. Document the demagnetization: Record the pre-demagnetization excitation curve, the demagnetization procedure parameters, and the post-demagnetization excitation curve in the CT maintenance record

DC Current Reversal Method (Alternative):

For CTs where AC voltage access to the secondary winding is difficult, the DC current reversal method applies a series of DC current pulses of alternating polarity and progressively decreasing magnitude — achieving the same progressive hysteresis loop reduction as the AC voltage method.

Prevention: Specifying Remanence-Protected CT Cores

For new CT installations in industrial plant arc protection applications where remanence-induced false tripping is a known risk, specify IEC 61869-2 Class PR (Remanence Protected) cores:

• Class PR definition: Remanence factor Kr = Br/Bsat ≤ 0.10 — maximum 10% remanent flux after any magnetization history
• How it is achieved: A small air gap is introduced in the CT core magnetic circuit; the air gap stores energy that forces the flux to return toward zero when the magnetizing force is removed, limiting remanence to ≤10% of Bsat
• Trade-off: The air gap reduces the CT’s magnetizing inductance, increasing the magnetizing current and slightly reducing accuracy at low primary currents; Class PR cores are typically specified for protection applications only, not for revenue metering
• Application: Mandatory specification for all CT cores connected to arc protection relays in industrial plant medium voltage systems with X/R ratio above 10

System-Level Prevention Measures

Beyond CT core specification, system-level measures reduce the remanence accumulation rate in industrial plant medium voltage arc protection systems:

• Reduce fault clearing time: Faster protection operation reduces the duration of DC offset exposure per fault event, reducing remanence accumulation per event; target fault clearing time below 80 ms for arc protection applications
• Implement point-on-wave switching⁵ for transformer energization: Controlled switching that energizes the transformer at the voltage zero-crossing minimizes the DC offset in the inrush current, reducing the remanence accumulation from each energization event
• Schedule periodic CT demagnetization: For existing installations with standard CT cores (Kr = 0.6–0.8), schedule demagnetization every 3 years or after any fault event where primary current exceeded 50% of rated short-time current — whichever occurs first
• Separate arc protection CT cores from measurement CT cores: Use dedicated CT cores for arc protection relay current measurement — cores that can be demagnetized without affecting revenue metering accuracy

Common Remanence Management Mistakes

• Demagnetizing only the CT that was identified as remanence-affected: In a three-phase installation, all three phase CTs are exposed to the same fault current history; if one CT has significant remanence, all three should be assessed and demagnetized as a set
• Performing ratio accuracy test before demagnetization: Ratio accuracy test results on a remanence-affected CT are not representative of the CT’s true accuracy class performance; always demagnetize before ratio testing
• Specifying Class PR cores for revenue metering applications: The air gap that limits remanence in Class PR cores increases magnetizing current and degrades accuracy at low primary currents; Class PR is a protection core specification — revenue metering requires standard Class 0.2S or 0.5 cores without air gap
• Adjusting arc protection relay settings to avoid false trips without addressing the CT remanence: Increasing the arc protection relay’s current threshold to avoid remanence-induced false trips reduces the relay’s sensitivity to genuine low-current arc faults — trading false trip prevention for genuine fault detection failure

Conclusion

CT core remanence is the hidden variable in industrial plant medium voltage protection system reliability — invisible to nameplate inspection, invisible to standard commissioning tests, and invisible to relay setting calculations, but fully capable of causing arc protection and overcurrent relays to operate on distorted secondary current waveforms that bear no relationship to the actual primary current during the critical first cycles of circuit energization. The mechanism is well understood, the diagnostic methodology is straightforward, and the correction — CT core demagnetization — is a four-hour maintenance activity that eliminates the remanence condition entirely. In industrial plant medium voltage arc protection systems, where a false trip costs tens of thousands of euros in production loss and a missed genuine arc fault costs lives, CT core remanence assessment and demagnetization is not a discretionary maintenance activity — it is the engineering foundation of a protection system that can be trusted to operate correctly and only correctly when it matters most.

FAQs About CT Core Remanence and False Relay Tripping

1. Provides foundational physics principles explaining how ferromagnetic materials respond to applied magnetic fields and retain residual flux. ↩

2. Explains the relationship between system reactance and resistance in determining the magnitude and duration of DC offset during electrical faults. ↩

3. Directs readers to the international standard specifying performance requirements and testing protocols for protection-class current transformers. ↩

4. Offers technical definitions and calculation methods for the critical voltage threshold where current transformer core saturation begins. ↩

5. Details the technology and operational benefits of synchronizing circuit breaker operation with voltage zero-crossings to minimize transient inrush currents. ↩