Generator field-failure protection (ANSI 40) — recognizing loss of excitation with an offset-mho impedance relay

The guide on under- and overvoltage protection (ANSI 27/59) and the guide on reverse-power protection (ANSI 32) cover generator protections that react to terminal voltage and the direction of active power, respectively. This article covers a third protection function specifically aimed at the excitation itself: field-failure protection, or loss-of-excitation protection (LOE), designated by ANSI code 40.

What happens when excitation is (partially) lost

A synchronous generator running in parallel with the grid delivers its active power because the rotor turns in synchronism with the grid, while the excitation (field) current in the rotor winding creates the magnetic field needed to also supply or absorb reactive power. If excitation is lost entirely or partially — due to a fault in the automatic voltage regulator (AVR), an open circuit in the excitation system, or a short circuit in the field winding — the generator can lose synchronism with the grid: the rotor starts to "slip" relative to the rotating field, and the machine begins to behave like an induction generator that absorbs a significant amount of reactive power from the grid instead of supplying it. This carries two risks: the absorbed reactive current can thermally overload the stator winding, and on a cylindrical (non-salient-pole) rotor, the slip-related currents induced in the rotor and damper winding cause fast, concentrated heating that can damage the rotor within tens of seconds to a few minutes.

The offset-mho impedance principle

Loss of excitation cannot be reliably recognized from terminal voltage or current alone: depending on the remaining load and grid strength, terminal voltage may initially remain reasonably normal while the generator is already absorbing reactive power. Field-failure protection therefore monitors terminal impedance (voltage divided by current, as a complex quantity) using a so-called offset-mho relay: an impedance characteristic in the shape of a circle that does not pass through the origin of the R-X diagram but is shifted from it by a fixed offset. When the generator loses excitation, the measured terminal impedance moves into a region that falls inside this offset-mho circle, and the relay picks up. The offset is typically derived from the generator's direct-axis transient reactance (X'd): a common practical value for the offset is on the order of half of X'd, so that the characteristic is sensitive enough to a genuine field failure without picking up spuriously during normal operation or during an external fault.

Coordination with the capability curve and the underexcitation limiter

The exact placement of the mho circle is a compromise. If the circle is set to cover only the third and fourth quadrant of the impedance diagram (i.e. it does not pass through the origin), the relay responds only to a clear, substantial field failure and remains insensitive to normal operating points. However, if the field-failure protection also needs to coordinate with the generator's capability curve and with the excitation regulator's underexcitation limiter (UEL) — which already protects the generator against operating too far under-excited, before an actual field failure occurs — the characteristic is sometimes set to pass through the origin instead. This makes the relay more sensitive to a gradual loss of excitation close to the normal operating limit, but also increases the risk of spurious operation during a power swing or an external system fault, and therefore requires a careful system study.

Note: the exact offset, the radius of the mho circle, and the time delay follow from the generator data (in particular X'd and the capability curve) and from the system study of the specific plant; this article covers the principle, not a ready-made setting table for every generator.

Practical relevance

When commissioning a generator that will run in parallel with the grid, it is important to verify that field-failure protection is actually set and tested, rather than left on a generic factory default — an offset-mho setting not tuned to the machine's actual X'd and capability curve can either respond too late to a genuine field failure, or trip spuriously during normal underexcited operation.

Common mistakes

Relying on an undervoltage relay (27) alone for field-failure protection — during a partial loss of excitation, terminal voltage may initially remain normal enough that a 27 relay does not pick up.
Not tuning the offset-mho setting to the generator's actual X'd — a generic setting may be too insensitive, or conversely too sensitive, for the specific machine.
Not coordinating field-failure protection with the excitation regulator's underexcitation limiter (UEL) — this can lead to an unnecessary trip while the UEL was already correcting the situation, or conversely to a missed detection.
Not accounting for power swings when using a mho characteristic that passes through the origin — without additional measures (such as a time delay or a supplementary blocking criterion), a temporary swing following an external system fault can cause spurious operation.