Directional overcurrent 67 (RDIR): how it works

Short answer

Directional overcurrent protection (ANSI 67; in IEC 61850, a PTOC element supervised by the RDIR direction node) trips only if the fault current exceeds the pickup and also flows in the chosen direction. To know where the current is going it compares its angle with a reference voltage, called the polarizing quantity. It is needed wherever a fault can be fed from more than one side: rings, parallel lines, sources at both ends and networks with distributed generation.

What 67 directional protection covers

Non-directional overcurrent trips on any high current, wherever it comes from. On a radial network that is enough: fault current only comes from the source. But if the fault can be fed from two sides, overcurrent loses selectivity and takes out healthy circuits.

The 67 adds a condition: the current must flow towards the protected zone. It protects lines, cables and feeders against phase faults when the network is meshed or has sources at more than one point, and prevents trips of healthy circuits that are only contributing current to someone else’s fault.

ANSI/IEEE C37.2: 67, AC directional overcurrent relay.
IEC 61850: a PTOC overcurrent element whose direction is given by the RDIR node.
IEC 60617: the overcurrent symbol with the direction arrow.

Where it is applied

Equipment or network	LV	MV	HV and EHV	Industry	Why it is directional
Parallel lines	No	Yes	Yes, as backup to 21	Double service entrances	A fault on one line is also fed through the other
Closed rings	No	Yes	Yes	Plant rings	The fault is fed from both sides of the ring
Parallel transformers	Uncommon	Low-voltage side	Low-voltage side	Plant substations	Keeps the healthy transformer from tripping on reverse contribution
Feeder with generation	No	Yes	Not applicable	Cogeneration, solar	Current reverses for external faults
Generation point of interconnection	No	Yes	Yes	Plants with on-site generation	Separates the plant according to the fault direction

At high and extra-high voltage the main line protection is usually distance or differential; the 67 remains as backup or as an element of directional comparison schemes. The ground version is covered in directional ground overcurrent 67N.

Measuring principle

The relay needs two quantities per phase: the operating current and a reference voltage whose angle does not change with the fault. The classic practice is quadrature polarization: for phase A the voltage between the other two phases (VBC) is used, which under normal conditions is 90° from VA. So, on a phase A fault, the reference keeps its angle even if VA collapses.

With those two quantities the relay calculates the angle between the current and the polarizing voltage, corrects it with the characteristic angle (or maximum torque angle) and decides whether the fault is forward or reverse.

Voltage memory: with a close-in three-phase fault, all voltages drop almost to zero. Numerical relays store the pre-fault voltage for a short time to keep deciding the direction.
Negative sequence (67Q): compares the negative-sequence current and voltage. It is useful on unbalanced faults and does not depend on the load. You can analyze those phasors with our free symmetrical components tool.

The characteristic

On the phasor plane, the polarizing voltage is drawn as the reference. The maximum torque line leaves it rotated by the characteristic angle, and the half-plane centered on that line is the forward operating zone. A current whose phasor falls inside the half-plane is a forward fault; if it falls in the opposite half-plane, it is a reverse fault.

A normal overcurrent element acts on top of that direction decision: instantaneous or time-delayed with an inverse-time curve. You can see the shape of those curves with the inverse-time curves tool.

How the 67 decides

Phase current. Magnitude and angle of the current of each phase
Polarizing voltage. Quadrature or negative-sequence voltage, with memory
Characteristic angle. Places the maximum torque line
Direction. Forward, reverse or undetermined
Overcurrent. Pickup and time curve only if the direction is the one set

Setting criteria

Characteristic angle: depends on the network impedance angle and on the polarization method; the manufacturer recommends it and the study confirms it.
Direction: towards the line or towards the bus, depending on the scheme. On two parallel lines, each relay looks into its own line.
Pickup and curve: coordinated like any overcurrent, but only with the relays that see the fault in the same direction. That is what makes selectivity possible in rings.
Voltage transformers: the VT location, line side or bus side, changes what the relay sees on energization.

The general criteria are in protection setting criteria.

Interaction with other functions

VT supervision (60): if the voltage is lost, the direction is undetermined. Depending on the setting, the 67 is blocked or becomes non-directional. The full function is in VT and CT supervision 60, and the alarm in loss-of-potential alarm.
Teleprotection: the directional elements feed the directional comparison schemes, such as the POTT scheme.
Distance 21: the 67 is usually backup and shares the directional unit with distance in many relays.
Breaker failure 50BF and reclosing 79: the 67 trip starts the 50BF and can initiate reclosing on overhead lines.

How each manufacturer implements it

Siemens SIPROTEC (DIGSI): phase directional overcurrent with a choice of polarization and angle.
SEL (AcSELerator QuickSet): negative- and positive-sequence directional elements that supervise the overcurrent elements.
Hitachi Energy and ABB Relion (PCM600): phase directional overcurrent blocks in the 615, 620, 650 and 670 series.
GE Vernova Multilin (EnerVista): phase directional element that supervises the overcurrents.
Schneider MiCOM and Easergy: overcurrent stages with a directional option per stage.

What changes is the default polarization method, the memory duration and how the element behaves without voltage.

IEC 61850 logical node

The RDIR node delivers the fault direction and PTOC the overcurrent; together they make up the 67. The start (Str) and operate (Op) of PTOC go to PTRC, which publishes the trip by GOOSE towards XCBR. The direction can also be sent by GOOSE to the remote end in directional comparison schemes.

How it is tested

With secondary injection, voltages and currents with controlled angle are injected. In general the following are verified:

The direction boundary: the current angle is rotated until both edges of the half-plane are found.
Overcurrent pickup and times in the operating direction, and no operation in the opposite one.
Voltage memory with three-phase faults of almost zero voltage.
The behavior on loss of voltage.

The test of the complete scheme with the remote end is done with end-to-end testing.

Typical mistakes

Reversed CT or VT polarity: the function looks backwards. It is the most common mistake and only shows up under load or with a real fault.
Phase sequence or VT connection wrongly declared in the relay, which changes the polarizing voltage.
Characteristic angle copied from another project without checking the polarization method.
Coordinating with relays that look in the other direction, which gives times that are never met.
Not testing the close-in three-phase fault and finding out in service that memory was not enabled.

Did your directional protection trip towards the wrong side, or do you need to review its settings? At Tiza Group we review the events and settings remotely, worldwide. See our remote relay configuration service or contact us.

References

IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: number 67.
IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines: directional elements on lines.
IEEE C37.230-2020, Guide for Protective Relay Applications to Distribution Lines: directional elements in distribution.
IEC 60255-151, Functional requirements for over/under current protection, and IEEE C37.112 for inverse-time characteristics: overcurrent element.
IEC 61850-7-4, Compatible logical node classes and data object classes: RDIR, PTOC and PTRC nodes.
J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions

What is the difference between 67 and 67N?

The 67 works with phase current and is normally polarized with a phase-to-phase voltage in quadrature. The 67N works with residual current and is polarized with the residual voltage or with a neutral current. They cover different faults and are set separately.

Why does 67 not operate on a three-phase fault very close to the relay?

Because the polarizing voltage drops almost to zero and the relay cannot measure an angle. Numerical relays use voltage memory to remember the pre-fault voltage for a short time; if memory is not available, the direction is undetermined and the function behaves as the manufacturer defines.

When is directional protection needed on a radial feeder?

When it stops being radial: if there is on-site generation, large motors or a tie to another source on the feeder or in the plant, the current can reverse for a fault on another feeder and non-directional overcurrent would trip unnecessarily.