Rotor ground fault protection 64F: how it works

Short answer

Rotor ground fault protection (ANSI 64F or 64R; IEC 61850 logical node PHIZ) monitors the insulation to ground of the field winding of a synchronous generator. The field circuit is ungrounded, so a first ground causes no current and no immediate damage; the danger is the second one, which shorts part of the winding and unbalances the magnetic field. The function measures the field insulation resistance by injecting a signal or with a resistive bridge, and gives alarm and trip in stages.

What 64F protection protects

The rotor field winding of a synchronous generator is fed with direct current from the excitation system, and its circuit is ungrounded. The insulation between that winding and the rotor body suffers from temperature, vibration, carbon dust contamination and moisture. When that insulation fails at one point, the machine does not notice. When it fails at a second point, part of the field is short-circuited.

The 64F protects the generator rotor and bearings against the consequences of that second fault: magnetic unbalance, severe vibration, localized heating and burns in the rotor iron. It does so by detecting the first fault, while there is still time to schedule a shutdown of the machine.

ANSI/IEEE C37.2: 64, ground detector relay; the suffix F (field) or R (rotor) indicates where. The stator variant is covered in 100% stator ground fault 64S.
IEC 61850: logical node PHIZ (ground detector).
IEC 60617: the relay symbol with the earth indicator, next to the field winding.

Where it is applied

Machine	LV	MV	HV and EHV	Industry	Notes
Generator with static excitation and brushes	Not applicable	Medium-size generators	Large units connected through a step-up transformer	Cogeneration	Direct access to the field through the brushes
Brushless generator	Small sets, uncommon	Yes	Yes	Yes	Measurement on the rotor by telemetry or measuring brushes
Synchronous motor	No	Large synchronous motors	Not applicable	Compressors, mills	Same principle when the field is accessible
Standby generator set	Uncommon	Uncommon	No	Yes	Depends on the controller and the size

The application and level of detail depend on the size and criticality of the machine, as described in IEEE C37.101 and IEEE C37.102.

Operating principle

The relay has to measure the insulation resistance between the field winding and ground while the machine runs with the excitation voltage present. There are three main methods:

DC injection: a DC voltage is applied between the field and ground through a resistor and the current is measured. If the insulation drops, the current rises. Its sensitivity depends on where the fault is along the winding, because the field voltage adds to or subtracts from the injected voltage.
Low-frequency square-wave injection: a voltage that slowly reverses its polarity is applied. The relay measures the current in each half cycle once the transient due to the rotor capacitance has died out, and calculates a resistance that is independent of the field voltage and of the fault position.
Resistive bridge: a divider connected across the field poles with its midpoint grounded through the relay. A ground unbalances the bridge. It has a blind spot near the center of the winding, which is covered by varying the bridge.

Coupling is done with a unit connected to the field and to ground, and on machines with brushes, shaft grounding brushes are used to close the circuit.

Brushless machines. In generators with a brushless exciter, the field winding rotates together with the rectifier diodes and there is no static access to it. Some machine manufacturers mount a measuring module on the rotor that transmits the insulation resistance by telemetry; others fit measuring brushes that touch only during a periodic check. In that case monitoring is no longer continuous, and that should be known before defining the alarm and trip philosophy.

The characteristic

The characteristic is insulation resistance versus time: the relay displays the calculated resistance and compares it with two thresholds.

Alarm: at an intermediate resistance, indicating insulation deterioration or a high-resistance first fault.
Trip: at a low resistance, with a time delay, when the first fault is solid and the machine is exposed to the second.

On the R–t chart, the rotor insulation resistance falls over time and crosses first the alarm threshold and then the trip threshold.

How a rotor ground is detected

Coupling. Injection unit connected between the field and ground
Signal. DC, low-frequency square wave or bridge
Measurement. Insulation resistance calculated by the relay
Alarm. Resistance below the first threshold
Trip. Sustained low resistance, through the 86

Setting criteria

Alarm threshold: below the normal in-service insulation resistance of the rotor, which changes with temperature and cleanliness.
Trip threshold: at a resistance already regarded as a solid fault, as recommended by the machine manufacturer.
Time delays: long compared with excitation transients.
Alarm or trip on the first fault: some owners trip; others alarm and schedule the shutdown. IEEE C37.102 describes both philosophies.

Interaction with other functions

Loss of excitation 40: a rotor with part of the field shorted can lead to insufficient excitation. See loss of excitation 40.
Vibration: vibration monitoring of the turbine-generator helps confirm a second ground.
Lockout relay 86: the 64F trip goes to the generator 86, which opens the breaker, the field and the turbine according to the trip matrix.
Excitation: a fault in the exciter or its cables is seen as a rotor fault. See generator excitation and AVR.

How each manufacturer implements it

Siemens SIPROTEC (DIGSI): rotor ground fault protection with an external coupling unit and an injection method.
SEL (AcSELerator QuickSet): field ground modules connected to the generator relay.
Hitachi Energy and ABB Relion (PCM600): the REG670 offers injection-based rotor ground fault protection.
GE Vernova Multilin (EnerVista): field ground module for its generator relays.
Schneider MiCOM: generator relays with injection modules for the rotor.

What changes is the injection method, the coupling unit and how the rotor capacitance is compensated.

IEC 61850 logical node

The function is modeled with PHIZ (ground detector) or with the node the manufacturer defines. It publishes start, alarm and operate signals, and the measured resistance as an analog value. The trip goes through PTRC to the XCBR and to the generator 86. Check the ICD file to find which node publishes the resistance, which is worth bringing into the monitoring system.

How it is tested

In general:

With the machine stopped, resistors of known value are connected between the field and ground to verify that the relay measures them, alarms and trips at the thresholds.
The injection unit and its coupling are tested.
The measurement is checked at several points of the winding, if the method allows it.
The signal to the SCADA and the trip through the 86 are checked.
The rotor insulation resistance is also measured with an insulation tester, as explained in insulation resistance and polarization index.

The relay part is done with secondary injection and the machine part during commissioning of the unit.

Typical mistakes

Worn or dirty shaft grounding brushes, which open the measuring circuit.
High alarm threshold, which alarms on the moisture of a cold start.
Coupling unit left disconnected after field maintenance.
Ignoring the first-ground alarm for months.
Not compensating the rotor capacitance in low-frequency injection.

Do you need the rotor ground fault protection of your generator reviewed? At Tiza Group we review events and settings remotely, worldwide. See our remote relay configuration service or contact us.

References

IEEE C37.101, Guide for Generator Ground Protection: field ground detection methods.
IEEE C37.102-2023, Guide for AC Generator Protection: alarm and trip philosophy.
IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: device 64.
IEC 61850-7-4, Compatible logical node classes and data object classes: PHIZ node.

Frequently asked questions

Why does a first ground on the rotor not damage the generator?

Because the field circuit is isolated from ground. With a single point to ground there is no closed path for current, so the machine keeps running as before. The problem is that the first ground turns any second ground into a short circuit across part of the field winding.

What happens if a second ground appears on the rotor?

Part of the field winding is short-circuited. The flux is no longer symmetrical, unbalanced magnetic forces and vibration appear that can damage the bearings and the rotor itself, and the fault current can burn the iron and the winding.

How is the rotor of a brushless generator protected?

In brushless machines the field winding rotates with the exciter and the diodes, and there is no direct access through brushes. Rotor-mounted measuring systems that transmit the result by telemetry are used, or measuring brushes that touch the shaft only during the check, depending on what the machine manufacturer offers.