Neutral voltage displacement protection (ANSI 59N) — earth-fault detection via the displacement voltage in an ungrounded or resonant-earthed network

The guide on IT-system insulation monitoring (IMD) covers how a low-voltage IT system detects a first insulation fault by actively injecting a small measuring signal between the network and earth. At medium voltage — specifically an ungrounded network or a network that is resonant-earthed via a Petersen coil — a different, passive method is generally used for the same purpose: neutral voltage displacement protection, denoted by the ANSI code 59N (also referred to as 59G, or "residual overvoltage" or "neutral voltage displacement").

Why the star point of a non-solidly earthed network reveals an earth fault

As explained in the guide on VT ferroresonance, the potential of the star point of an ungrounded or resonant-earthed network is not fixed, but is determined by the balance between the capacitances of the three phases to earth. Under normal, balanced operation these three capacitances are (approximately) equal, and the star point stays close to earth potential. When an earth fault occurs on one phase, that balance shifts: the faulted phase (for a full fault) comes to earth potential, while the star point itself drifts away from earth potential, by a distance corresponding to the phase voltage. This shift of the star point — the displacement voltage or neutral voltage — is exactly the quantity a 59N relay measures.

How the measurement is performed in practice: the broken-delta connection

To measure the displacement voltage, three voltage transformers (one per phase) are used, with their secondary windings connected in series to form an open delta (broken delta): the three secondary voltages are connected such that their sum is normally (under a balanced, fault-free network) zero, so no voltage is measured across the open side of the delta. When an earth fault occurs on one phase, this balance is disturbed and a voltage proportional to the star-point displacement voltage appears across the open side of the delta — for a full earth fault on an ungrounded network, this voltage rises to the order of the full phase voltage (three times the normal zero-sequence component). A 59N relay monitors this voltage and raises an alarm or initiates a trip once the measured value exceeds a set threshold for a set time delay.

Why this method fits an ungrounded or resonant-earthed network

For a solidly earthed network, the earth-fault current in the event of a fault is large enough to be reliably detected with ordinary earth-fault current protection (for example an earth-fault relay measuring the residual current of three current transformers or a core-balance current transformer). For an ungrounded or resonant-earthed network, the earth-fault current is itself deliberately kept low — with a Petersen coil, the capacitive earth-fault current is even nearly fully compensated — so current-based earth-fault detection at that point becomes unreliable or insensitive. The star-point displacement voltage, however, remains a reliable indicator of the existence of an earth fault somewhere on the network regardless of this low fault current, offering a current-independent detection method that fits precisely with the compensation principle of this type of network.

Note: a 59N relay responding to the displacement voltage indicates that an earth fault exists somewhere on the network, but not where — selective localisation of the faulted feeder cable or bay generally requires additional, per-feeder earth-fault directional protection; this article covers the principle of the neutral voltage displacement detection itself.

Practical relevance

When inspecting a medium-voltage switchgear installation on an ungrounded or resonant-earthed network, it is important to verify that the broken-delta winding of the voltage transformer set is actually connected to a 59N relay and is functional — a broken-delta winding that is interrupted or incorrectly connected for whatever reason leaves the network without any earth-fault indication, even though the installation may appear complete on a superficial inspection.

Common mistakes

Not actually connecting the broken-delta winding of the voltage transformer set to a 59N relay — without this connection, an earth fault on an ungrounded or resonant-earthed network remains completely undetected.
Interpreting a 59N alarm as an exact fault location — the displacement voltage only indicates that a fault exists somewhere on the network, not in which bay or cable.
Forgetting or incorrectly sizing the loading resistor on the open-delta winding (see the guide on VT ferroresonance) when designing the 59N scheme — without sufficient damping, the same voltage transformer set can develop ferroresonance, separately from the 59N function itself.
Setting the 59N threshold too low on a network with a naturally, slightly unbalanced phase asymmetry — this can lead to nuisance alarms without there being an actual earth fault.