Generator inadvertent energization protection 50/27

What 50/27 protection protects

A generator out of service can be energized from the system by mistake: a breaker that closes because of a control failure, a switching error, a stuck contact in a breaker-and-a-half scheme, or a flashover across the open contacts of the breaker. If the machine is at standstill or turning slowly, at that instant it behaves like an induction motor with a locked rotor.

The stator current is high and the rotating field induces currents in the rotor body, the wedges and the amortisseur windings. The rotor heats up within seconds, and the bearings, without the oil film that rotation provides, can be damaged. The 50/27 protects the rotor, the stator and the bearings from that event by clearing the current with no intentional delay.

When it happens. Accidental closures occur mostly during maintenance and startups: control tests with the disconnector closed, switching in breaker-and-a-half or ring bus arrangements where the generator breaker is left connected to a live bus, or interlocks bypassed for a test. That is why the function is included on large units even when the machine has other backup protection.

• ANSI/IEEE C37.2: a combination of 50 (instantaneous overcurrent) and 27 (undervoltage). It has no device number of its own.
• IEC 61850: there is no dedicated logical node for inadvertent energization; it is built from PTUV and PIOC with logic between them.
• IEC 60617: no dedicated symbol; it is drawn with the 50 and 27 symbols and the logic.

Where it is applied

IEEE C37.102 treats inadvertent energization as a risk specific to large units and describes several schemes, including undervoltage-supervised overcurrent.

Operating principle

The logic combines two quantities with two timers:

1. Arming by undervoltage (27). When the generator terminal voltages stay low for an arming time, the logic concludes that the machine is out of service and enables the overcurrent.
2. Instantaneous overcurrent (50). While enabled, a current above pickup trips without delay.
3. Delayed disarming. When voltage returns, the enable is held for a dropout time. This is essential: in an inadvertent energization the voltage appears at the same time as the current, and without that time the logic would disarm before tripping.

Some schemes add supervision by breaker position, frequency or field status. The idea is the same: to know that the machine was not supposed to have voltage.

The characteristic

It is drawn as gate logic and a timeline:

• AND gate: undervoltage sustained for the arming time (pickup timer) and instantaneous overcurrent.
• Dropout timer: holds the enable for a short time after voltage returns.

The timeline is: machine stopped and de-energized → the logic arms → accidental breaker close → voltage and high current appear → instantaneous trip before the timer disarms.

Setting criteria

• 50 pickup: below the current the machine draws during inadvertent energization with the weakest system, calculated from the machine, transformer and system reactances.
• 27 threshold: below the minimum normal operating voltage, so it does not arm in service.
• Arming time: longer than the clearing time of system faults including backup, so that a voltage dip caused by an external fault does not arm it.
• Dropout time: long enough for the 50 to trip when voltage returns with the close.

The current calculation comes from the short-circuit study and the machine data.

Interaction with other functions

• Breaker failure 50BF: if the energization comes from a flashover across the open contacts, opening the breaker does not help. The 50BF, with its flashover logic, trips the adjacent breakers. See breaker failure 50BF.
• Reverse power 32 and loss of excitation 40: they may see the event, but with time delays or blocks. See reverse power 32.
• Negative sequence 46 and distance 21: backups that were not designed for this case.
• VT supervision (60): a blown fuse in service looks like a machine without voltage; the logic must be blocked so it does not arm with the machine in service.
• Machine starting: with static frequency converter or reduced-frequency starts, the logic must not mistake the start for an energization.
• Lockout relay 86: the trip goes to the generator 86.

How each manufacturer implements it

• Siemens SIPROTEC (DIGSI): inadvertent energization function in the generator relays, with supervision by voltage and breaker position.
• SEL (AcSELerator QuickSet): inadvertent energization logic in the generator relays, configurable in the equations.
• Hitachi Energy and ABB Relion (PCM600): accidental energizing function in the REG670.
• GE Vernova Multilin (EnerVista): accidental energization element in the generator relays.
• Schneider MiCOM: inadvertent energization logic in the generator relays.

IEC 61850 logical node

There is no dedicated node. Undervoltage is modeled with PTUV, overcurrent with PIOC, and the arming and disarming logic with the IED logic blocks or a manufacturer-specific node. The trip goes through PTRC to the generator XCBR and to the 86. For later analysis, the recorder should capture the armed logic as a binary signal.

How it is tested

The complete sequence is simulated with secondary injection. In general the test verifies:

• That the logic arms only after the arming time with low voltage.
• That a high current with the logic armed trips without delay.
• That disarming waits for the set time when voltage returns, by applying voltage and current at the same time.
• That a brief voltage dip, like that of an external fault, does not arm the logic.
• The block on a blown VT fuse and the actual trip through the 86.

Typical mistakes

• No dropout time, or one that is too short: the logic disarms with the voltage from the close and never trips.
• Short arming time, which arms the logic during external faults.
• Testing the 50 and the 27 separately instead of the complete sequence.
• Not blocking on a blown VT fuse.
• Leaving the function disabled after a startup in which it got in the way.

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

Related guides

• Generator protection scheme
• Breaker failure 50BF
• Undervoltage protection 27
• Reverse power 32

References

• IEEE C37.102-2023, Guide for AC Generator Protection: inadvertent energization and protection schemes.
• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: devices 50 and 27.
• IEC 60255-151, Functional requirements for over/under current protection.
• IEC 60255-127, Functional requirements for over/under voltage protection.
• IEC 61850-7-4, Compatible logical node classes and data object classes: PTUV, PIOC and PTRC nodes.

Frequently asked questions