HV Circuit-Breaker Switching Duties

Transformer-Limited and Reactor-Limited Faults

Transformer-limited and reactor-limited faults are circuit-breaker duties where a transformer or reactor limits the short-circuit current. They look easy — the current is far below the rated short-circuit breaking current — but that impression is misleading. The current is low, yet the transient recovery voltage can be severe, with a rate of rise that exceeds ordinary terminal-fault testing. The key message: a low fault current does not necessarily mean an easy interruption duty.

Reading time ≈ 20 min · Why limited fault current can still create severe TRV stress

Transformer-limited and reactor-limited faults are special circuit-breaker duties where the short-circuit current is limited by a transformer or reactor. The interrupted current may be much lower than the rated short-circuit breaking current, but the transient recovery voltage can still be severe. In these duties the current magnitude alone is not a reliable indication of circuit-breaker suitability: the TRV, RRRV, natural frequency, damping and connected capacitance must also be checked.

A terminal fault occurs directly at the circuit-breaker terminals, so the current is normally high and the rating must cover it. In transformer-limited and reactor-limited faults, a transformer or reactor is the dominant series impedance: it reduces the current but strongly shapes the TRV. The circuit-breaker may then face lower current than terminal-fault duty, a high X/R ratio and slow DC decay, a high-frequency TRV with high RRRV, a TRV dominated by equipment stray capacitances and leakage inductances, and waveforms not fully represented by ordinary terminal-fault tests. This is why limited-current duties need a separate engineering check.

Do not confuse limited current with an easy duty

A transformer or reactor can reduce the current but still create a severe TRV. A circuit-breaker can fail a transformer-limited or reactor-limited fault duty even when the interrupted current is well below its rated short-circuit breaking current.

Abbreviations and symbols used on this page

TLFTransformer-limited fault

RLFReactor-limited fault

TRVTransient recovery voltage

RRRVRate of rise of recovery voltage

RVRecovery voltage (power-frequency, after interruption)

CBCircuit-breaker

ACAlternating current

\(f\)Natural (oscillation) frequency

\(L_\text{tr}\)Transformer leakage inductance

\(C_\text{tr}\)Transformer effective stray capacitance

\(L_L,\ C_L\)Reactor inductance / stray capacitance

\(C_p\)Added parallel (mitigation) capacitance

\(C_\text{ext}\)External capacitance connected at the transformer / reactor terminal

FRAFrequency response analysis

T10 / T30Terminal-fault duties (10% / 30% of \(I_\text{sc}\))

pF / nFPicofarad / nanofarad

GISGas-insulated switchgear

CT / VT / CVTCurrent / voltage / capacitive voltage transformer

\(I_\text{sc}\)Rated short-circuit breaking current

X/RReactance-to-resistance ratio (affects DC offset / asymmetry)

\(X_0/X_1\)Zero- to positive-sequence reactance ratio

EMTElectromagnetic transient (study)

SF₆Sulphur hexafluoride (interrupting gas)

IECInternational Electrotechnical Commission

IEEEInstitute of Electrical and Electronics Engineers

Key idea

Low current is not an easy duty. Transformer- and reactor-limited faults draw a fraction of the rated current, yet the high-frequency TRV and RRRV can exceed terminal-fault severity.
TLF: leakage inductance + stray capacitance. Their natural frequency \(f = 1/(2\pi\sqrt{L_\text{tr}C_\text{tr}})\) sets a fast TRV; the duty is often verified with T10 / T30.
RLF: tiny reactor capacitance, very steep TRV. A current-limiting reactor gives a high natural frequency and high RRRV — and there is no standardised reactor-limited-fault test duty.
Parallel capacitance is a trade-off. It lowers the RRRV but can raise the TRV peak — a circuit-breaker can fail well below its rated current until the TRV is mitigated.

Key terms used on this page

01Transformer-limited fault

A short circuit whose current is dominated by the transformer impedance.

02Reactor-limited fault

A short circuit whose current is dominated by a series reactor.

03Leakage inductance

The short-circuit inductance of a transformer winding that limits the fault current.

04Stray capacitance

Winding, bushing and terminal capacitance that, with the inductance, sets the TRV frequency.

05Natural frequency

The oscillation frequency \(f = 1/(2\pi\sqrt{LC})\) of the limiting element and its capacitance.

06RRRV

Rate of rise of recovery voltage — the initial slope of the TRV.

07T10 / T30

Terminal-fault test duties at about 10% and 30% of rated short-circuit current — high-TRV cases.

08Definite-purpose circuit-breaker

A circuit-breaker rated for a specific severe duty, such as fast-TRV rise-time applications.

09FRA

Frequency response analysis — transformer impedance/admittance versus frequency.

10Parallel-capacitance mitigation

Adding capacitance to lower the natural frequency and the RRRV.

11Transformer-secondary fault

A fault on the opposite voltage side of the transformer from the circuit-breaker.

12Terminal condition

Whether the far transformer terminals are open or short-circuited — it changes the response.

Section 1

Why limited current can still be a hard duty

The intuition that a smaller current is easier to interrupt holds for the thermal and electrodynamic side of the duty, but not for the recovery voltage. Interruption is a two-part problem: the circuit-breaker must remove the arc and withstand the voltage that recovers across the gap. When a transformer or reactor limits the current, it also sets up a high-frequency oscillation that can drive a very fast recovery voltage — so a low-current fault can still overstress the circuit-breaker.

The core idea

A low fault current does not necessarily mean an easy interruption duty. Transformer- and reactor-limited faults must be judged on TRV and RRRV, not on current magnitude alone.

Section 2

Transformer-limited faults

A transformer-limited fault is a short-circuit condition where the transformer impedance is the dominant factor setting the fault current. The fault may be on one side of the transformer while the circuit-breaker being assessed is on the other side or at the same voltage level; the important point is that the transformer impedance limits the current and shapes the recovery voltage. The transformer X/R ratio sets the peak factor of the asymmetrical current, and its \(X_0/X_1\) ratio affects the relationship between single-phase-to-earth and multi-phase fault currents. At interruption, the transformer’s high-frequency behaviour — governed by leakage inductance, stray capacitance, winding resonance and connected-equipment capacitance — becomes decisive.

What X/R and \(X_0/X_1\) tell you

The transformer X/R ratio influences the DC component and the peak factor of the asymmetrical fault current. The \(X_0/X_1\) ratio influences the relative magnitude of single-phase-to-earth and multi-phase fault currents. These parameters mainly affect the current duty, while the leakage inductance, stray capacitance and connected-equipment capacitance mainly affect the TRV duty.

Transformer-secondary and transformer-fed faults

Two arrangements are commonly distinguished. A transformer-secondary fault is at the terminals or busbars on the opposite voltage side of the transformer from the circuit-breaker — for example, an HV circuit-breaker clearing a fault on the lower-voltage transformer terminals, with the transformer between circuit-breaker and fault. A transformer-fed fault is on the busbars at the same voltage level as the circuit-breaker, but with the fault current supplied mainly through a transformer. In both cases the transformer is the dominant element in the fault-current path; the difference is practical, because the physical layout affects which side of the circuit-breaker contributes most strongly to the TRV.

Section 3

Low current, but severe TRV

In a transformer-limited fault, the voltage drop across the transformer can be close to 100% during the fault, so the transformer impedance is the main current-limiting element. The resulting fault current is often roughly 10% to 30% of the rated short-circuit breaking current — for example, a 40 kA circuit-breaker may only need to interrupt a transformer-limited fault current of about 4 kA to 12 kA. That current is low; the TRV is not.

The transformer behaves as a complex high-frequency network during interruption: its leakage inductance and stray capacitance create oscillations that produce a fast transient recovery voltage. The transformer-terminal TRV can be approximated by a simple oscillating circuit using the relevant leakage inductance \(L_\text{tr}\) and effective stray capacitance \(C_\text{tr}\), giving a single natural frequency:

\[ f = \frac{1}{2\pi\sqrt{L_\text{tr}\,C_\text{tr}}} \]

\(f\): natural frequency of the transformer-terminal oscillation
\(L_\text{tr}\): relevant transformer leakage inductance
\(C_\text{tr}\): effective stray capacitance

If the effective capacitance is small, the natural frequency is high and the RRRV increases. So in a transformer-limited fault the current is relatively low, but the voltage can recover very quickly.

Section 4

Capacitance and high-frequency modelling

Transformer TRV behaviour is not controlled by leakage inductance alone — stray capacitance is equally important. The relevant capacitance includes winding-to-earth and inter-winding capacitance, bushing and terminal capacitance, and the capacitance of connected busbars, cables, GIS, instrument transformers, surge arresters and nearby conductors. Small capacitance gives a high natural frequency and high RRRV; larger capacitance lowers the frequency and RRRV but may affect the TRV peak and damping. External capacitance can therefore be beneficial or problematic depending on the complete circuit.

Typical capacitance sources to include are:

transformer winding-to-earth capacitance;
inter-winding capacitance;
bushing capacitance;
GIS busbar capacitance;
cable capacitance;
current-transformer (CT) and voltage-transformer (VT) capacitance;
capacitive voltage-transformer (CVT) capacitance;
surge-arrester capacitance;
disconnectors, earthing switches and nearby-conductor capacitance.

Transformer high-frequency modelling — see the dedicated note

A power transformer does not behave as a simple inductance and capacitance at high frequency: its winding structure contains many local inductances and capacitances, creating multiple resonances. A single-frequency \(L\)-\(C\) model is useful for conservative screening (and can sometimes be more severe than a detailed model, because the detailed model’s resonances partly interfere), but accurate work needs a proper high-frequency model — often built from frequency response analysis (FRA) data — with the correct terminal condition represented. Detailed high-frequency transformer modelling for EMT studies is covered in Transformer Modelling in EMTP® ↗. For TRV here, the key point is that the transformer must be represented over the relevant frequency range — up to about 100 kHz, with dominant frequencies of several kHz to a few tens of kHz — not only at power frequency, and that the far-terminal condition (open vs short-circuited) strongly affects the response.

Open versus short-circuited terminal condition

The transformer frequency response depends strongly on the terminal condition. An unloaded transformer response may be dominated by the magnetising inductance at low frequency, while a short-circuited terminal condition is more related to the leakage inductance. For transformer-limited-fault TRV studies the model must represent the actual condition during the fault — do not use generic FRA or capacitance data without checking whether the relevant terminals are open, short-circuited or connected to a network.

External capacitance and practical values

External capacitance \(C_\text{ext}\) — from air-insulated or GIS busbars, cables, current and voltage transformers, CVTs, bushings, surge arresters, disconnectors, earthing switches, support insulators or grading capacitors — lowers the natural frequency of the transformer-terminal oscillation and usually reduces the RRRV. But lowering the frequency does not always reduce the TRV peak: the waveform can become more single-frequency and less damped, raising the peak. Capacitance should therefore not be added blindly; its effect should be checked by simulation or against type-test evidence.

For power transformers up to a few hundred MVA, phase capacitance is often from several hundred picofarads to about 1 nF; very large transformers (above about 500 MVA) can be higher, though the effective capacitance seen by the TRV is usually still in the low-nanofarad range. Connected equipment matters too — instrument-transformer capacitance to earth is typically hundreds of picofarads, while CVTs can be in the nanofarad range — so the total capacitance between circuit-breaker and transformer in an HV substation may be of the order of a few nanofarads, enough to noticeably affect the TRV frequency. Use measured equipment data where possible, or a conservative estimate.

Section 5

Standards for transformer-limited faults

IEC 62271-100 addresses transformer-limited faults in an annex for circuit-breakers with rated voltage above 1 kV and below 100 kV (identified as Annex M in the editions where it is present). The literature states that verification of the capability to interrupt transformer-limited faults is covered by the T30 duty for circuit-breakers applied at 52 kV and below on transformer secondary terminals, and the T10 duty for other applications. The practical implication is that lower-current duties such as T10 and T30 are important here, because the TRV may be severe even when the current is only 10% or 30% of rated short-circuit breaking current.

Confirm the annex and duty split

The exact annex designation and the precise T30 / T10 duty assignment can differ between editions of IEC 62271-100. For any project, check the applicability against the specified edition and the manufacturer’s type-test documentation rather than relying on the values quoted above.

IEEE treatment and definite-purpose circuit-breakers

For IEEE-based work, IEEE C37.011 is the application guide for TRV of AC high-voltage circuit-breakers; IEEE C37.04 gives rating requirements and IEEE C37.09 the test procedures. For fast TRV rise-time duties, IEEE also uses definite-purpose approaches — IEEE C37.06.1 is a recommended practice for preferred ratings of high-voltage AC circuit-breakers designated definite-purpose for fast transient-recovery-voltage rise times. The practical point: if the transformer-limited-fault TRV is outside the standard general-purpose capability, a definite-purpose circuit-breaker or project-specific verification may be needed.

In short, the IEEE documents have distinct roles:

IEEE C37.04 — the rating basis for AC high-voltage circuit-breakers.
IEEE C37.09 — the test-procedure framework.
IEEE C37.011 — the application guide for transient recovery voltage of AC high-voltage circuit-breakers.
IEEE C37.06.1 — relevant where a definite-purpose circuit-breaker is considered for fast TRV rise-time duties.

Do not assume a general-purpose rating covers these duties

Do not imply that all transformer-limited or reactor-limited duties are automatically covered by a general-purpose circuit-breaker rating. If the calculated TRV exceeds the standard envelope or the type-tested capability, manufacturer confirmation, a definite-purpose circuit-breaker or mitigation may be required.

Section 6

Reactor-limited faults

A reactor-limited fault is a short-circuit condition where a series reactor is the dominant current-limiting element. Series reactors are installed to limit short-circuit current, manage load flow, reduce motor-starting current, control arc-furnace transients, provide neutral earthing, limit transient currents or shape system impedance. Because the reactor limits the current, the fault current may be far below the terminal-fault current — but the reactor also introduces a high-frequency TRV component. In short: a current-limiting reactor reduces current but can increase RRRV.

The reactor-limited-fault TRV frequency depends on the reactor inductance and its stray capacitance, which together form an oscillating circuit:

\[ f_R = \frac{1}{2\pi\sqrt{L_L\,C_L}} \]

\(f_R\): reactor natural frequency
\(L_L\): reactor inductance
\(C_L\): reactor stray capacitance

Many current-limiting reactors — particularly air-core dry-type units — have very small stray capacitance, so \(f_R\) is high. A high natural frequency produces a steep TRV and high RRRV, which may exceed the standard TRV envelope even when the interrupted current is much lower than the rated short-circuit breaking current. This is why reactor-limited-fault duties are important.

Section 7

Breaker position and mitigation

The severity and waveform depend on the relative location of the reactor, circuit-breaker and fault. The circuit-breaker may interrupt with the reactor on the source side (influencing the source-side TRV component) or behind the reactor with it on the load side (influencing the load-side component). In both arrangements the circuit-breaker can face high-frequency recovery voltage if the reactor natural frequency is high and damping is low, so the study must represent the actual arrangement, not only the reactor’s power-frequency impedance.

Mitigation by capacitance

A common mitigation is to add capacitance in parallel with the reactor, or from the reactor terminal to earth, increasing the effective capacitance and lowering the natural frequency:

\[ f_{Rp} = \frac{1}{2\pi\sqrt{L_L\,(C_L + C_p)}} \]

\(f_{Rp}\): reduced natural frequency after adding capacitance
\(L_L\): reactor inductance
\(C_L\): reactor stray capacitance
\(C_p\): additional parallel capacitance

Reducing the natural frequency usually reduces the RRRV, easing the duty. But adding capacitance may also increase the TRV peak, because damping at the lower frequency can be lower — so mitigation must be checked against both RRRV and peak TRV.

Worked illustration (from the literature)

The literature gives an example where a 50 nF capacitor bank in parallel with a reactor of 0.2 nF stray capacitance reduces the TRV in a 380 kV system below the IEC T30 envelope for a 420 kV circuit-breaker interrupting 0.3 times rated short-circuit current. It also describes a 12 kV SF₆ circuit-breaker tested for series-reactor-limited fault: without an added capacitor, the reactor produced a high-frequency TRV oscillation around 210 kHz; the circuit-breaker interrupted 8.1 kA but failed at 10.9 kA, even though its rated short-circuit breaking current was 20 kA. After a 100 nF capacitor was applied across the reactor, the high-frequency TRV was eliminated, only the source-side component remained, and the higher current could be interrupted. In this example the circuit-breaker failed at 10.9 kA although its rated short-circuit breaking current was 20 kA: the rated short-circuit breaking current alone does not prove suitability for reactor-limited-fault duty, because the high-frequency TRV — not the RMS current — was the limiting factor.

Section 8

No standardised reactor-limited-fault duty

The literature states that there are no standardised reactor-limited-fault test duties for circuit-breakers. This is an important practical point: if a circuit-breaker is applied near a current-limiting reactor, do not assume that standard terminal-fault or transformer-limited-fault tests automatically cover the duty.

What to do instead

Calculate the actual reactor-limited-fault TRV, or have the manufacturer confirm suitability from testing or application evidence. Where the calculated RRRV exceeds the tested values, mitigation (such as parallel capacitance) or a definite-purpose circuit-breaker may be required.

Section 9

Comparing transformer- and reactor-limited faults

The two duties share a lot: both involve a series impedance that limits the current; both can give lower current than a terminal fault; both can produce severe TRV or RRRV; both need high-frequency modelling of equipment capacitance and inductance; and both may need capacitance data and manufacturer support. Their mechanisms, however, are not identical.

In a transformer-limited fault, the TRV is mainly governed by the transformer leakage inductance, winding capacitance, stray capacitance, connected-equipment capacitance and the transformer frequency response. In a reactor-limited fault, the TRV is mainly governed by the reactor inductance and its very small stray capacitance, which can produce a very high natural frequency and high RRRV. Both are limited-current duties, but they are not the same physical phenomenon.

Table 1 — Transformer-limited versus reactor-limited faults.

Aspect	Transformer-Limited (TLF)	Reactor-Limited (RLF)
Limiting element	Transformer impedance	Series reactor
TRV governed by	Leakage inductance, winding & stray capacitance, connected-equipment capacitance, frequency response	Reactor inductance and very small reactor stray capacitance
Natural frequency	High	Often very high (tiny \(C_L\))
Typical current	~10–30% of \(I_\text{sc}\)	Low, case-dependent
Standard test duty	Often T10 / T30 (per edition)	None standardised
Modelling need	High-frequency transformer model / FRA	Reactor \(L\) and stray \(C\), plus mitigation \(C\)

Section 10

Why these duties cannot be judged by current alone

A common mistake is to compare only the interrupted current with the rated short-circuit breaking current. A circuit-breaker rated 40 kA may interrupt only 8 kA in a transformer-limited fault; a circuit-breaker rated 20 kA may interrupt only about 10 kA in a reactor-limited fault. These currents are below the rating — yet the circuit-breaker may still be overstressed because the TRV rises too fast.

Current plus recovery voltage

Circuit-breaker interruption is not only a current problem — it is a current plus recovery-voltage problem. The circuit-breaker must remove the arc and withstand the recovery voltage; if the recovery voltage rises too fast, the gap can fail even at a low current.

Section 11

Modelling requirements

A transformer-limited-fault TRV model should include:

transformer leakage inductance;
transformer stray capacitance;
bushing capacitance;
connected busbar, cable or GIS capacitance;
instrument-transformer (CT / VT / CVT) capacitance;
surge-arrester capacitance where relevant;
damping or equivalent loss;
the correct transformer terminal condition (open / short-circuited);
the correct fault location and circuit-breaker location;
the system earthing arrangement and source-side equivalent;
the pole-to-clear sequence where relevant.

Where available, FRA data improves the transformer high-frequency model; where it is not, a conservative single-frequency \(L\)-\(C\) model may be used for screening, with the uncertainty stated.

A reactor-limited-fault TRV model should include:

reactor inductance;
reactor stray capacitance;
any additional mitigation capacitance, if installed;
capacitance to earth at the reactor terminals;
the circuit-breaker position relative to the reactor;
the fault location and source-side equivalent;
damping or losses, and busbar / equipment capacitance;
TRV measurement across the circuit-breaker contacts.

The reactor should not be represented only by its power-frequency reactance if the objective is TRV assessment — its high-frequency stray capacitance must also be represented.

Section 12

Practical assessment procedure

Quick review checklist

The limiting element (transformer or reactor), the fault location and the circuit-breaker location.
The interrupted current and the X/R ratio.
The transformer or reactor inductance and its stray capacitance.
External and connected-equipment capacitance.
The natural frequency and the damping.
The RRRV and the TRV peak.
The applicable standard and the type-test evidence.
Whether manufacturer confirmation, a definite-purpose circuit-breaker or mitigation is required.

For transformer-limited faults

Identify whether the fault is transformer-secondary or transformer-fed.
Determine the transformer impedance and X/R ratio, and calculate the limited fault current.
Check whether the duty falls near T10 or T30.
Identify the connected capacitance between circuit-breaker and transformer; obtain transformer capacitance or FRA data where possible.
Calculate the TRV and RRRV, and compare with IEC/IEEE requirements and manufacturer type-test evidence.
Consider whether a definite-purpose circuit-breaker is required.

For reactor-limited faults

Identify the reactor location relative to the circuit-breaker and fault.
Obtain the reactor inductance and stray capacitance, and calculate the natural frequency.
Calculate the RRRV and TRV peak, and check whether they exceed the circuit-breaker tested envelope.
Consider mitigation by adding capacitance, and verify it reduces RRRV without an unacceptable TRV peak.
Obtain manufacturer confirmation, since no standard duty covers the case.

Section 13

Common mistakes

Common transformer- / reactor-limited-fault mistakes

Do not assume low current means low circuit-breaker stress.
Do not model the transformer or reactor only at power frequency.
Do not ignore stray capacitance.
Do not ignore connected-equipment capacitance.
Do not add capacitance for mitigation without checking the TRV peak.
Do not assume reactor-limited faults are covered by standard tests.
Do not use generic transformer capacitance without checking the terminal condition.
Do not assume a circuit-breaker rated for terminal faults is automatically suitable for TLF or RLF.

Section 14

Report wording and key message

Suggested report wording

“Transformer-limited and reactor-limited faults were considered as special circuit-breaker TRV duties. In these cases the short-circuit current is limited by the transformer or reactor impedance and may be significantly lower than the rated short-circuit breaking current; however, the transient recovery voltage may be severe because the high-frequency response is governed by leakage inductance, stray capacitance and connected-equipment capacitance. For transformer-limited faults, the transformer leakage inductance, stray capacitance, terminal condition and connected external capacitance should be represented, and the duty checked against the applicable IEC 62271-100 requirements, the relevant T10/T30 duty where applicable, IEEE TRV guidance where used, and manufacturer type-test evidence. For reactor-limited faults, the reactor inductance and stray capacitance can produce a high natural frequency and high RRRV; since these duties are not standardised in the same way, the actual TRV should be calculated and confirmed with the manufacturer, and where necessary mitigation such as parallel capacitance assessed — noting that capacitance may reduce RRRV but can increase the TRV peak. The adequacy of the circuit-breaker should be based on both current-interruption capability and TRV withstand capability, not on rated short-circuit current alone.”

Key message

Transformer-limited and reactor-limited faults are low-current but potentially high-TRV duties. For transformer-limited faults the transformer high-frequency response and connected capacitance dominate the TRV; for reactor-limited faults the reactor inductance and small stray capacitance can produce a very high-frequency TRV and high RRRV. A circuit-breaker can fail these duties even when the current is well below its rated short-circuit breaking current — so always check the actual TRV, RRRV, peak value, equipment capacitances, applicable standards and manufacturer type-test evidence.

Book cover of Switching in Electrical Transmission and Distribution Systems by Smeets, van der Sluis, Kapetanovic, Peelo and Janssen (Wiley)

Reference & further reading Switching in Electrical Transmission and Distribution Systems René Smeets, Lou van der Sluis, Mirsad Kapetanovic, David F. Peelo, Anton Janssen — Wiley

Part Three Reading now

Transformer- and Reactor-Limited Faults

Low-current but high-TRV duties: why a transformer or reactor that limits the current can still create a fast, high-frequency recovery voltage, and how to mitigate it.

Series progress 3 of 5

Part Four

Out-of-Phase Switching Duty

A moderate-current, high-recovery-voltage duty between two live, unsynchronised systems — the out-of-phase current and recovery voltage, factors and IEC / IEEE references.