Shunt reactor protection: 87R, 50/51, 63

Short answer

A shunt reactor is protected by a phase differential (87R), restricted earth fault, backup overcurrent, sensitive elements for turn-to-turn faults and, if it is oil-immersed, mechanical protections such as the Buchholz and sudden pressure relays. If it is connected directly to a line, its trip must open both line ends. IEEE C37.109 is the reference guide.

What is protected in a shunt reactor

A shunt power reactor absorbs reactive power to control overvoltage on long, lightly loaded lines and cables. It is connected in three ways:

To the bus, with its own breaker or a reactor switching breaker.
To the line, directly and without its own breaker in many designs, so it is switched in and out with the line.
To the tertiary of an autotransformer, at medium voltage.

It can be oil-immersed, the most common at extra-high voltage, or dry-type air-core, frequent at medium voltage and on tertiaries, and also used at higher voltages. Its typical faults are:

Ground faults from the winding to the core or the tank.
Phase-to-phase faults, in three-phase reactors.
Turn-to-turn faults, hard to detect because they change the terminal current very little. In dry-type air-core reactors they are the most frequent.
Bushing and connection faults.
Internal oil problems: gas, pressure, level and temperature.

If a turn-to-turn fault goes undetected, it develops and ends in the loss of the whole reactor phase.

Where it is applied

Connection and type	MV	HV	EHV	Typical scheme
Oil-immersed bus reactor	Not usual	Yes	Yes	87R, REF, 50/51, turn-to-turn, mechanical
Oil-immersed line reactor	Not applicable	Yes	Yes	The same, plus transfer trip to the remote end
Dry-type air-core bus reactor	Yes	Yes	Depends on design	Sensitive overcurrent and turn-to-turn elements; 87R if there are CTs at both ends
Reactor on an autotransformer tertiary	Yes	Not applicable	Not applicable	Its own differential or included in the transformer differential, plus turn-to-turn

Principle of each function

Phase differential (87R). In each phase, it compares the current entering on the line side with the current leaving on the neutral side. The reactor has no ratio and no phase shift, so the comparison is direct. It needs current transformers (CTs) at both ends of each winding.

Restricted earth fault (87N). It compares the residual current of the phases with the neutral current. It is more sensitive to ground faults close to the neutral. It works as in a transformer, as explained in restricted earth fault 87N/64REF.

Overcurrent (50/51 and 50N/51N). Backup for the previous functions, set above the reactor current at the maximum operating voltage. On reactors without neutral-side CTs it can be the main protection for solid faults.

Turn-to-turn faults. A turn-to-turn fault lowers the impedance of a single phase and creates an unbalance. The elements that detect it are:

negative-sequence elements, directional or overcurrent;
negative-sequence voltage to current ratio elements;
impedance elements, which measure the impedance of each reactor phase.

IEEE C37.109-2023 expands on this topic compared with earlier editions.

Mechanical protections (oil-immersed reactors). Buchholz, sudden pressure, oil level and temperatures, as in a transformer. They are described in mechanical protections 63 and 26.

The characteristic

Each function uses its own characteristic:

The 87R uses the differential current versus restraint current plane, with a minimum pickup and a slope. An external fault or energization stays in the restraint region; an internal fault enters the operate region.
The 51 uses an inverse-time curve or a definite time.
The turn-to-turn elements compare an unbalance quantity with a very low threshold, with a time delay to ride through switching transients.

Energization. On energization, the reactor current has a DC component that decays slowly because the reactor has very low losses. If the CTs at the two ends saturate differently, a false differential current appears. The stability of the 87R and the CT class are checked against that case.

Setting criteria

87R: pickup and slope according to the CT errors and the saturation expected on energization.
REF: sensitivity for the portion of the winding to be covered. It can be low impedance or high impedance.
51: above the reactor current at maximum voltage and below the minimum internal fault.
Turn-to-turn: above the inherent unbalance of the reactor and the system, measured during commissioning.
CTs: ratio, class and knee point according to IEEE C37.110 or IEC 61869-2.

The values come from the short-circuit study, the manufacturer’s data (IEEE C57.21 and IEC 60076-6) and the IEEE C37.109 guide.

Interaction with other functions

Transfer trip. A line reactor without its own breaker requires opening both ends of the line. Direct transfer trip (DTT) over the teleprotection channel is used.
Reclosing 79. A reactor fault is not transient: a reactor protection trip must block the line’s automatic reclosing 79.
Breaker failure 50BF. If the reactor breaker or switching breaker does not open, breaker failure 50BF opens the breakers feeding it.
Lockout 86. The internal reactor protections operate the lockout relay 86, which prevents re-energizing it.
Transformer differential. For tertiary reactors, the zone of the transformer differential 87T can include the tertiary and the reactor; turn-to-turn faults still need their own element.
Line voltage functions. After a line with a reactor is opened, the voltage oscillates and decays slowly. Dead-line logic and synchronism check 25 must take it into account.
Automatic voltage-based switching. If the reactor is switched in and out according to voltage, that logic is coordinated with the protections and with the substation voltage control.

How manufacturers implement it

Transformer relays or multifunction relays configured for the reactor are used, with differential, REF, overcurrent and negative-sequence or impedance elements. The differences are whether the relay treats the reactor as a protected object in its own right, how the turn-to-turn elements are calculated, and how many current and voltage inputs it offers. Two relays of different brands or operating principles are often installed, for redundancy.

IEC 61850 logical node

PDIF for the phase differential and, depending on the manufacturer, for the REF.
PTOC and PIOC for overcurrent and negative-sequence elements.
PDIS for impedance elements.
Mechanical protections come in as contacts and are published with supervision or generic nodes, depending on the relay.

All operations go through PTRC to the breaker (XCBR). On a line reactor, the command to the remote end goes out through the teleprotection scheme (PSCH) or the teleprotection equipment.

How it is tested

In general the test verifies:

The 87R and REF with secondary injection: pickup, slope and stability for an external fault.
The turn-to-turn elements with controlled unbalances.
The polarity of the line and neutral CTs by primary injection.
The mechanical trips from each device to the breaker.
End-to-end transfer trip, on line reactors.
On first energization, the stability of the differential, with oscillography.

Typical mistakes

Relying only on the differential and leaving turn-to-turn faults uncovered.
Reversed polarity of the neutral-side CTs.
Differential unstable on energization because of CTs of insufficient class.
Line reactor without transfer trip or without reclose blocking.
No failure protection for the reactor switching breaker.
Overcurrent set below the reactor current at the maximum operating voltage.
Reading the voltage oscillation of an open line as service voltage.

We configure your reactor protection

Are you commissioning a reactor, or did you have a trip that is not clear? We review the configuration of the reactor relays remotely, worldwide: differential, REF, turn-to-turn elements, trip logic and transfer trip. Tell us the brand and model through remote relay configuration or contact us.

References

IEEE C37.109-2023, Guide for the Protection of Shunt Reactors.
IEEE C57.21-2021, Standard Requirements, Terminology, and Test Code for Shunt Reactors Rated Over 500 kVA.
IEC 60076-6, Power transformers – Part 6: Reactors.
IEEE C37.110, Guide for the Application of Current Transformers Used for Protective Relaying Purposes.
IEC 61850-7-4, Compatible logical node classes and data object classes: PDIF, PTOC, PDIS and PTRC nodes.
K. Lawlor, S. Flores Castro, B. Cook and K. Garg, Field Experience Commissioning Reactor Projects, SDG&E and SEL, 2025.

Frequently asked questions

Why is differential protection not enough for a reactor?

Because a fault between a few turns changes the current entering and leaving the winding very little, so the differential hardly sees it. In dry-type air-core reactors these are precisely the most common faults. They are covered by sensitive negative-sequence and impedance elements and, in oil-immersed reactors, by the Buchholz and sudden pressure relays.

What happens to the line voltage when a line with a reactor is opened?

The energy left in the line capacitance and in the reactor oscillates between them and the voltage decays slowly, at a frequency different from nominal. Dead-line logic, reclosing and some voltage functions must take this into account so they do not act on a misleading reading.

Why did the reactor differential trip on energization?

Energizing a reactor produces a current with a DC component that decays slowly, because the reactor has very low losses. That component can saturate the current transformers on one side and not on the other, creating a false differential current. It is reviewed with the oscillography, the CT class and the stability of the differential.