On a high-impedance grounded generator, the neutral overvoltage element 59N detects ground faults over most of the stator winding, but not those close to the neutral, where the voltage is almost zero. 100 % stator ground protection (64S; IEC 61850 logical node PHIZ) covers that end with the third harmonic the machine itself produces, or by injecting a low-frequency signal, which also works with the machine at standstill.
What 64S protection covers
Generators connected to the grid through a step-up transformer usually have the neutral grounded through a high impedance: a distribution transformer with a resistor on the secondary, or a resistor directly. A stator ground fault then carries a current of a few amperes and does not burn the core. The other side of it is that such a fault is hard to see.
The neutral overvoltage 59N element measures the voltage at the neutral. If the fault is at the terminals, that voltage is the phase voltage; at mid-winding, half of it; a few percent from the neutral, it is so small that it stays below the pickup. That is why 59N is called 95 % protection in practice: it leaves uncovered a band next to the neutral whose size depends on the setting.
64S protects that neutral end. A fault there does no harm by itself, but if a second ground fault appears elsewhere in the winding, the current is no longer limited by the neutral resistor and the stator damage is severe.
- ANSI/IEEE C37.2: 64, ground detector relay; the suffix S (stator) or the acronym 27TN (third-harmonic neutral undervoltage) identify the variant. The rotor variant is covered in rotor ground fault 64F.
- IEC 61850: logical node PHIZ (ground detector).
- IEC 60617: the relay symbol with ground indication, next to the generator neutral.
Where it is applied
| Generator | LV | MV | HV and EHV | Industry | Notes |
|---|---|---|---|---|---|
| Unit connected through a step-up transformer, high-impedance neutral | Not applicable | Yes, the typical case | Large units | Large cogeneration | 59N plus 64S cover the whole winding |
| Generator connected directly to an MV bus | No | Yes | Not applicable | Yes | Other ground sources limit the use of third harmonic |
| Solidly grounded LV generator | The fault is seen by its current | No | No | Standby gensets | Not used |
| Large hydro generator | No | Yes | Yes | No | Injection is common |
Measuring principle
Third harmonic. Every generator produces some third-harmonic voltage, shared between the neutral and the terminals by the winding capacitances. With a healthy machine there is third harmonic at both ends. If a fault occurs near the neutral, the third harmonic at the neutral drops almost to zero. There are two variants:
- Third-harmonic neutral undervoltage (27TN): operates when the third harmonic at the neutral drops below a threshold. It is supervised by power or voltage so that it does not operate when the machine produces little third harmonic.
- Third-harmonic differential or ratio: compares the third harmonic at the neutral with that at the terminals. A fault at either end upsets the ratio.
Its limit is the dependence on load and on the machine design.
Low-frequency injection. A low-frequency voltage is injected into the neutral circuit, through the grounding transformer or through a broken-delta VT at the terminals. The relay measures the resulting current and calculates the stator resistance to ground. Siemens, for example, uses 20 Hz with an external generator and a band-pass filter, and GE Vernova offers a 20 Hz module for its generator relays. It covers 100 % of the winding, does not depend on the third harmonic and works with the machine at standstill.
Injection needs additional equipment: the signal generator, a filter that passes the low frequency and blocks the fundamental, and measurement of the injected current. That equipment is fed from station service and is part of the protection: if injection stops, the relay must alarm, because the neutral end is unmonitored again.
The characteristic
The third-harmonic voltage profile along the winding helps to understand it: it is minimum at some intermediate point and rises towards the neutral and towards the terminals. Two zones overlap on that profile:
- 59N: from the terminals to near the neutral, where the fundamental voltage no longer reaches the pickup.
- 27TN or injection: the section next to the neutral, overlapping with 59N.
For injection, the characteristic is a resistance: alarm at an intermediate value and trip at a low value, with time delays.
- Terminals. The fault gives a high neutral voltage: 59N sees it
- Mid-winding. Intermediate neutral voltage: 59N sees it
- Near the neutral. Almost no voltage: 59N does not see it
- Third harmonic or injection. They cover the neutral end
- Overlap. The two zones overlap so no gap is left
Setting criteria
- Third harmonic: measure the third harmonic at the neutral and at the terminals over the whole range of active and reactive power before setting, and set the threshold and the supervision from that data.
- Injection: resistance thresholds as recommended by the manufacturer and according to the machine insulation, compensating for the capacitance of the winding and connections.
- Overlap with 59N verified.
- Alarm or trip: on units connected through a step-up transformer, stator ground protection usually trips, as described in IEEE C37.101.
Interaction with other functions
- 59N: covers most of the winding; 64S completes the rest.
- VT supervision (60): the terminal third harmonic depends on the VTs; a blown fuse upsets the ratio.
- Inadvertent energization: injection monitors the stator with the machine stopped, when 59N has no voltage.
- Lockout relay 86: the trip goes to the generator 86.
- Differential 87G: it does not see low-current ground faults; 59N and 64S cover them. See generator differential 87G.
How each manufacturer implements it
- Siemens SIPROTEC (DIGSI): 100 % stator ground protection by 20 Hz injection with an external generator and filter, plus the third-harmonic method.
- SEL (AcSELerator QuickSet): third-harmonic elements in the generator relays and an injection option depending on the model.
- Hitachi Energy and ABB Relion (PCM600): the REG670 offers 100 % stator ground protection by injection and by third harmonic.
- GE Vernova Multilin (EnerVista): 20 Hz injection module for its generator relays, plus third harmonic.
- Schneider MiCOM: third harmonic and low-frequency injection in the generator relays.
IEC 61850 logical node
The function is modeled with PHIZ or with the node the manufacturer defines. It publishes the start, the alarm, the operate signal and, with injection, the measured resistance. The trip goes through PTRC to XCBR and to the generator 86. Sending the measured resistance to the monitoring system lets you follow the stator insulation over time.
How it is tested
In general:
- Third harmonic: with secondary injection of voltages with controlled harmonic content, and actual measurement of the third harmonic with the machine at several load points.
- Injection: with the machine stopped, resistors of known value are connected between the neutral or the terminals and ground, and the measurement, the alarm and the trip are verified.
- Overlap with 59N and an actual trip through the 86.
Typical mistakes
- Setting the third harmonic without measuring it over the whole load range.
- Copying the setting from another machine of a different design.
- Injection filter or generator without supply after maintenance.
- Not compensating the capacitance of the injection cables.
- Leaving 64S on alarm only on a machine whose owner requires a trip.
Do you need to review the stator ground protection of your generator? At Tiza Group we review the events and settings remotely, worldwide. See our remote relay configuration service or contact us.
Related guides
References
- IEEE C37.101, Guide for Generator Ground Protection: 59N, third harmonic and injection.
- IEEE C37.102-2023, Guide for AC Generator Protection.
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: numbers 64 and 27.
- IEC 61850-7-4, Compatible logical node classes and data object classes: PHIZ node.
- Siemens, documentation of the 7XT33 20 Hz generator for 100 % stator ground protection.
Frequently asked questions
What is the difference between 95 % and 100 % stator ground protection?
The 95 % element is the 59N neutral overvoltage: it measures the voltage that appears at the neutral and cannot see faults very close to the neutral, because the voltage there is almost zero. The 100 % scheme adds a method that does see them, such as third harmonic or low-frequency injection, and together they cover the whole winding.
Why does third-harmonic protection depend on the load?
Because the third harmonic the machine produces changes with active power, reactive power and voltage, and also with the generator design. If the third harmonic is very low at some load condition, the function may block or lose sensitivity. That is why it is measured over the whole operating range before it is set.
Does 20 Hz injection work with the generator at standstill?
Yes. Injection does not depend on the voltage the machine generates, so it detects a stator ground fault with the machine stopped, starting up or shutting down. That is one of its advantages over the third-harmonic method.