Pole-slip / out-of-step protection (ANSI 78) — asynchronous operation of a generator

The guide on generator field-failure protection (40) — loss of excitation covers what happens when a generator loses its excitation. This article covers a related, but fundamentally different failure mechanism: a generator that still has its excitation, but loses synchronism with the grid due to a severe, prolonged fault or a too-slow voltage recovery — known as pole-slipping or out-of-step operation, denoted by ANSI device number 78.

What is pole-slipping?

To deliver power to the grid, a synchronous generator must rotate at the same electrical angular speed as the grid it is connected to — the rotor angle relative to the grid (the power angle δ) then stays within a stable range. During a severe, prolonged fault near the generator, or with insufficient damping/too slow a return to equilibrium after a disturbance, the power angle can increase so far that the generator loses synchronism: the rotor "slips" a full pole revolution relative to the grid field, instead of settling back into a new equilibrium. This causes high mechanical shock loading on the shaft and coupling, large current and voltage spikes, and — if left undetected — can affect the stability of a larger part of the grid because the generator repeatedly falls out of phase.

The difference from a stable power swing

Not every disturbance of the power angle is a pole slip: after a fault, the rotor angle can temporarily deviate and then, damped, return to a new stable equilibrium — this is called a stable power swing. It is essential that a protection relay distinguishes this from an actual pole slip:

With a stable swing, the power angle returns within a normal range after one or a few oscillations — no tripping is needed, and an unwanted trip would actually undermine stability rather than protect it.
With an unstable pole slip, the power angle continues through a full 360° (or a multiple thereof) — the generator must then be disconnected quickly and in a controlled manner, preferably close to the moment the impedance passes through the electrical center of the system, to limit the mechanical shock and the voltage dip for other grid users.

Recognition via the R-X impedance diagram

A relay with ANSI device number 78 does not detect pole-slipping via current or voltage alone, but via the trajectory of the measured impedance in an R-X diagram as the generator falls out of phase with the grid:

During normal operation, the measured impedance stays within a limited area around the operating impedance.
During a pole slip, the measured impedance moves along a characteristic trajectory that sweeps across the diagram, from one quadrant to another, as the power angle δ increases from 0° to 360°.

The relay uses two or more configured characteristics (often a lens-shaped zone plus one or two straight "blinder" lines through it) to determine: if the impedance crosses both blinders within a certain time (indicating a fast, unstable trajectory), this is recognised as a pole slip and a trip follows; if the impedance stays within the lens without crossing both blinders, this is regarded as a stable swing and the relay does not intervene.

Note: alongside ANSI 78 (the relay that actually trips on a pole slip), practice also uses ANSI 68 (power-swing blocking): a function that instead blocks other, faster protections (for example distance protection 21) during a stable power swing, to prevent those faster-responding relays from misinterpreting the swing as a fault and tripping unnecessarily.

Practical relevance

When assessing the protection of a larger synchronous generator (industrial CHP, an emergency generator running in parallel with the grid, or a decentralised generation unit), it is important to check whether pole-slip protection (78) is present and correctly configured alongside the usual functions such as field-failure protection (40) and negative-sequence protection (46) — a generator that pole-slips once without a timely, controlled disconnection runs a real risk of mechanical shaft damage, even if the electrical quantities on their own do not immediately activate a classic overcurrent or earth-fault protection.

Common mistakes

Confusing pole-slip protection (78) with power-swing blocking (68) — the former actually trips on an unstable condition, the latter prevents other relays from tripping unnecessarily on a stable condition.
Not distinguishing between a stable swing and an actual pole slip when configuring the blinder/lens characteristics, resulting in unnecessary trips during normal, damped grid disturbances.
Tripping at an arbitrary moment during the pole slip instead of as close as possible to the passage through the electrical center — this unnecessarily increases the mechanical shock and the voltage disturbance for other grid users.
Considering pole-slip protection unnecessary for a smaller decentralised generator, while a smaller machine too can suffer significant shaft damage from an undetected pole slip, even though the grid-wide impact is smaller than for a large central unit.