Neutral Overvoltage 59N (3V0): How It Works

Short answer

The 59N measures the residual voltage (3V0) and operates when it exceeds a setting for a set time. It detects ground faults where the current is very small, as on ungrounded networks or high-impedance grounded generators, although on its own it does not show which circuit has the fault.

What 59N protection is (residual overvoltage)

The 59N is an overvoltage protection that measures the residual or neutral voltage, that is, the phasor sum of the three phase-to-ground voltages (3V0). In a healthy, balanced system that sum is almost zero. When one phase contacts ground, the neutral shifts and a significant residual voltage appears.

In ANSI/IEEE C37.2 it is number 59 with the suffix N (neutral). In IEC 61850 most relays model it with the same overvoltage logical node PTOV, applied to the residual quantity. IEC literature often shows it as “U0>” or “UN>”. On generators the designation 59GN or 64G is sometimes used.

Its advantage is that it does not depend on fault current. That is why it works exactly where that current is very small and overcurrent protection cannot see it.

Where it is applied

Medium-voltage networks with an ungrounded or high-impedance grounded neutral, common in industry and in some distribution systems. It detects the ground fault, usually as an alarm, because the network can keep running for a while with one phase grounded.
High-impedance grounded generators, with a distribution transformer and a resistor in the neutral. The 59N measures the voltage across that resistor and protects the stator against ground faults. IEEE C37.102 describes it as the basic stator ground fault protection for this scheme.
Buses and transformers with delta windings, where there is no source of zero-sequence current.
Backup for feeder ground protection in substations.
Solar plants with step-up transformers delta-connected on the grid side, where ground faults on the high-voltage side must be detected.

How it works

The relay can obtain the residual voltage in two ways:

Measured directly from a broken-delta (open-delta) VT, whose three secondaries are connected in series. The voltage across its terminals is proportional to 3V0.
Calculated from the three phase-to-ground voltages, which must come from wye-connected VTs with grounded neutral.

On generators, the voltage across the neutral resistor is measured through the grounding transformer.

The characteristic is almost always definite time, with one or two stages. Some relays offer an inverse curve. Many can filter the fundamental component only, which is necessary on generators, because third-harmonic voltage appears at the neutral under normal conditions.

In a generator, the neutral voltage during a ground fault is proportional to the distance from the fault to the neutral. A fault at the terminals produces full phase voltage; a fault near the neutral produces very little. That is why the 59N covers most of the winding and leaves uncovered a band near the neutral, whose size depends on the setting. That band is protected with third-harmonic functions or subharmonic voltage injection, depending on the manufacturer.

Setting criteria

Pickup: above the residual voltage present in normal operation (capacitance unbalance, VT asymmetries, harmonics) and below the voltage produced by the most remote or highest-resistance fault to be detected.
Time: on generators, coordinated with the ground protection of the connected system, because a ground fault on the high-voltage side of the step-up transformer can transfer some residual voltage through the interwinding capacitance. On ungrounded networks, coordinated with the selective feeder protection.
Alarm or trip: on ungrounded networks it is usually an alarm with delayed trip; on generators, a trip.
VT ratio: with a broken delta the secondary voltage depends on the ratio of each VT; an error in that data is common.

These values come from the protection coordination study and from the capacitance data of the network or generator.

How each manufacturer implements it

Siemens SIPROTEC (DIGSI) offers it as residual or neutral voltage overvoltage, with a choice between measured and calculated quantity.
SEL (AcSELerator QuickSet) distinguishes calculated zero-sequence elements from measured neutral elements, especially in generator relays.
Relion (Hitachi Energy 670/650; ABB 615/620) (PCM600) includes it as a multi-stage residual overvoltage function.
GE Vernova (Multilin, EnerVista) offers it as neutral or auxiliary overvoltage, depending on the input used.
Schneider MiCOM/Easergy (MiCOM S1 or Easergy Studio) includes it as neutral voltage displacement.

The big difference is the source of the measurement (dedicated input or calculated) and how the relay scales the broken-delta voltage. A correct setting in one brand can be three times too high or too low in another if that scaling is not checked.

Why it trips or blocks

Blown VT fuse when 3V0 is calculated: the voltage of that phase disappears and a large residual voltage appears. Fuse failure supervision must block the function.
Ferroresonance in inductive VTs connected to an ungrounded network, typically after switching or when a ground fault clears. It is prevented with a damping resistor across the broken delta, as specified by the VT manufacturer.
Capacitive unbalance in networks with long cables or filters, which raises the normal residual voltage.
Third harmonic on generators, if the fundamental filter is not active.
Wrong polarity or connection of the broken delta: an inverted secondary produces a permanent residual voltage.
Intermittent ground faults in cables, which cause repeated pickups without reaching the trip time.

The first thing to check is the record of the three phase voltages. On ungrounded or high-impedance grounded networks, if one falls to zero and the other two do not rise, the problem is in the voltage circuit, not in the network. On a solidly grounded network a bolted ground fault produces the same picture: compare with the 3I0 current; if there was 3I0, it was a real fault.

First reading of a 59N trip or alarm

What do the three phase voltages show?
- One falls to zero and the others do not riseHow is the network grounded?
  - Ungrounded or high impedanceVoltage circuit problem, such as a blown VT fuse with calculated 3V0
  - Solidly groundedCompare with the 3I0 current; if there was 3I0 it was a real ground fault; if not, check the voltage circuit
- One falls and the others riseGround fault on the network, with the neutral shifted
- Permanent 3V0 with no faultBroken-delta polarity or connection, capacitive unbalance or unfiltered third harmonic

How it is tested

With secondary injection, for example with an Omicron CMC 356, the test checks:

Pickup, reset and time of each stage, injecting directly into the residual input, or unbalancing the three phase voltages if the quantity is calculated.
The response to third harmonic, if the relay must ignore it.
Fuse failure blocking.
Consistency between the calculated and the measured voltage, when the relay has both.

At commissioning, verify the VT polarity and the broken-delta connection.

Did your relay trip or block on this function and the reason is not clear? At Tiza Group we review the events and settings remotely. See our remote relay configuration service or write to us.

References

IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
IEEE C37.102, Guide for AC Generator Protection.
IEEE C37.101, Guide for Generator Ground Protection.
IEC 60255-127, Functional requirements for over/under voltage protection.
IEC 61850-7-4, Compatible logical node classes and data object classes.
J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions

Why do residual voltage and a 59N alarm appear with no ground fault?

The usual causes are a blown voltage transformer fuse when 3V0 is calculated, ferroresonance in VTs on ungrounded networks, and the natural unbalance of capacitances. First check whether the three phase voltages are consistent and whether there was a switching operation just before.

Does the 59N tell me which feeder has the fault?

No. The whole galvanically connected network sees the same residual voltage. To select the circuit, a directional ground element (67N) or sensitive residual current protection is used on each feeder.

What part of a generator stator does the 59N protect?

It protects most of the winding, from the terminals towards the neutral. Near the neutral the fault voltage is low and the 59N cannot see it; that zone is covered by another function, for example one based on the third harmonic or on injection.