The zero-sequence directional element (67N) measures the residual current 3I0 and trips only if the ground fault lies in the set direction. It decides by comparing the angle between 3I0 and a polarizing quantity, usually 3V0, against a characteristic angle that depends on how the neutral is grounded.
What 67N directional ground protection is
The zero-sequence directional element is a ground overcurrent function that, besides measuring how much residual current flows, decides in which direction the fault lies. It trips only if the ground fault is in the set direction, usually toward the line or feeder it protects, and restrains if the fault is behind the relay.
In IEEE C37.2 it is device 67N: 67 is AC directional overcurrent and N indicates that it operates on residual (neutral) current. In IEC 61850 the function is modeled with the overcurrent logical node PTOC together with the directional element RDIR; for the sensitive variant in isolated or compensated neutral networks there is node PSDE.
Where it is applied
- Loops, meshed networks and parallel lines, where ground current can flow through the relay in either direction.
- Networks with several zero-sequence sources, that is, with grounded transformers at both ends, including the interconnections of solar plants and on-site generation.
- Medium-voltage networks with isolated or compensated neutral (Petersen coil), where healthy feeders contribute capacitive current and a non-directional 51N could trip them.
- Ground teleprotection at high and extra-high voltage, as a complement to distance for resistive faults, in directional comparison schemes.
How it works
Operating current
The 67N operates on the residual current 3I0 = Ia + Ib + Ic, calculated by the relay, obtained from the residual connection of the three CTs, or measured with a window (core-balance) CT. The concept is covered in zero-sequence current.
Polarization
To determine direction, the relay compares 3I0 with a reference that does not change with the fault location:
- Residual voltage 3V0, from a broken-delta voltage transformer or calculated from the three voltages. It is the most common.
- Neutral current of a grounded transformer, which always flows in the same direction for ground faults.
- Negative sequence (V2, I2), useful when 3V0 is very small or when there is zero-sequence mutual coupling between parallel lines.
- Dual polarization, which uses whichever reference is available.
The decision
The relay compares the angle between 3I0 and the polarizing quantity with a characteristic angle (called RCA or MTA depending on the manufacturer). If the current falls inside the forward zone, exceeds the pickup and the condition lasts for the set time, it trips.
The angle depends on neutral grounding
| Neutral | Ground fault current | Usual criterion |
|---|---|---|
| Solid or effective | Inductive | Angle close to that of the source zero-sequence impedance |
| Resistance | Almost resistive | Close to 0° |
| Isolated | Capacitive and small | Reactive component («sin φ» criterion) |
| Compensated | Very small active component | Active component («cos φ»), transient or admittance methods |
The table is a guide only: the sign convention (V0 or −V0, lead or lag) changes between manufacturers.
Setting criteria
The values come from the protection study. The general criteria are:
- 3I0 pickup above the normal unbalance and the residual CT error, and below the minimum ground fault. With isolated or compensated neutral, a window CT and a sensitive function are usually needed.
- Minimum polarizing voltage: below it the relay does not decide the direction. With a strong source and a remote fault, current or negative-sequence polarization may be preferable.
- Characteristic angle and zone width, according to the neutral grounding.
- Time or curve, coordinated with the other ground relays according to the protection setting criteria.
- Behavior on a blown VT fuse: losing one voltage creates a false 3V0, so the 67N is blocked or becomes non-directional, as the study decides.
How each manufacturer implements it
Every brand offers 67N, but they differ in the polarization options, the shape of the zone, the angle convention and the setting names. Siemens SIPROTEC is configured with DIGSI, SEL with AcSELerator QuickSet, Relion (670/650: Hitachi Energy; 615/620: ABB) with PCM600, GE Vernova (Multilin) with EnerVista and Schneider MiCOM and Easergy with their own software. Some brands also have specific elements for compensated networks, based on transients or admittance. Because conventions differ, the characteristic angle is never copied from one brand to another without checking the manual.
Why it trips or blocks
| Symptom | Typical cause | What to check first |
|---|---|---|
| Trips on external faults and not on internal ones | Reversed polarity of the CTs or the broken delta | Polarity at commissioning |
| Trip with no ground fault | False 3V0 from a blown VT fuse together with unbalance 3I0 above pickup | Fuse failure supervision and 3I0 pickup |
| Does not decide direction on remote faults | 3V0 below the minimum threshold | Current or negative-sequence polarization |
| Misoperation on parallel lines | Zero-sequence mutual coupling | Negative-sequence polarization |
| Does not detect faults on a compensated network | «sin φ» criterion instead of «cos φ», or high pickup | Method according to the neutral |
Keep in mind that the 67N helps with resistive faults, but it is not a high-impedance fault protection: a downed conductor on dry ground needs specific algorithms.
How it is tested
With secondary injection, forward and reverse ground faults are simulated: the 3I0 pickup and the minimum polarizing voltage are checked, the angle is swept to find the zone limits, the time is measured and the element is confirmed not to trip in the reverse direction. During commissioning, the decisive step is confirming the polarity of the CTs and of the broken delta, because a reversed polarity makes the 67N work backwards. We do this as part of our secondary injection service, in Colombia or elsewhere on request.
We review your directional ground protection
Did your 67N trip for an external fault, miss a ground fault, or are you unsure which angle to use with your grounding? At Tiza Group we review the events and settings remotely, for substations and industry at medium, high and extra-high voltage. See remote relay configuration or contact us.
Related guides
- Phase and ground overcurrent 50/51
- Zero-sequence current
- Neutral overvoltage 59N
- DCB scheme: directional comparison blocking
- Zero-sequence impedance of transformers
- Substation protection schemes
References
- IEEE Std C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
- IEC 61850-7-4, logical nodes PTOC, RDIR and PSDE.
- IEC 60255-151, Functional requirements for over/under current protection (inverse-time curves equivalent to IEEE C37.112).
- J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications, CRC Press.
Frequently asked questions
When is a 67N needed instead of a 51N?
On loops, meshed networks or parallel lines, with several zero-sequence sources, and with isolated or compensated neutrals, where the healthy feeders contribute capacitive current and a non-directional 51N could trip them.
Can the characteristic angle be copied between relay brands?
No. The sign convention changes between manufacturers, so the angle is always set with the convention in the relay manual.
What happens if the broken-delta polarity is reversed?
The 67N trips for external faults and blocks for internal ones. That is why the polarity of the CTs and of the broken-delta VT is confirmed during commissioning.