Generator stator differential protection (ANSI 87G) — why it needs no inrush restraint, and its blind spot near the neutral

The transformer differential protection (87T) guide covers percentage-differential protection with harmonic restraint against nuisance tripping on magnetising inrush. This article covers the closely related but distinct application to a synchronous generator's stator windings: ANSI 87G.

The basic principle: the same comparison, a simpler CT problem

Like 87T, 87G compares the current entering a protected zone with the current leaving it, phase by phase, and trips on a significant imbalance. For a generator, that zone is each stator phase winding: one set of CTs sits at the generator neutral (star) end of each winding, and a second set sits at the terminal end, where the winding connects to the generator circuit breaker and the rest of the plant. Under normal operation and for any fault external to this zone, the current measured at both ends of a given winding is equal; an internal winding-to-winding or winding-to-earth fault inside the protected zone creates the imbalance the relay detects.

Unlike a power transformer, a generator has no turns-ratio or vector-group difference to correct for between the two CT locations — both CT sets see the same nominal current magnitude and the same phase, on the same winding, simply at its two ends. This makes CT specification and matching for 87G considerably more straightforward than for 87T, where CTs of different ratio must be compensated and a phase shift from the vector group must be corrected for, typically in software.

Why 87G needs no harmonic-restraint inrush blocking

87T's harmonic restraint exists specifically to prevent nuisance tripping on the second-harmonic-rich magnetising inrush current that occurs when an unloaded transformer is energised. A generator's stator winding is not energised in that same way from an external source — it is the generator itself that builds up voltage as it is brought up to speed and excited — so there is no equivalent inrush transient to guard against. This is one of the reasons 87G relays are commonly set with a lower percentage-differential slope (more sensitive) than a comparable 87T relay: the restraint margin that 87T reserves for inrush immunity is largely unnecessary for 87G, though a slope is still applied to accommodate ordinary CT ratio and burden errors during heavy external faults.

The blind spot near the star point, and why a separate function covers it

A percentage-differential 87G relay is most sensitive to a fault that produces a large difference between the two winding-end currents. For an earth fault very close to the winding's own star point, the fraction of the winding between the fault and the neutral is small, so the resulting fault current — and therefore the differential signal 87G can see — is correspondingly small, in some cases too small for the relay to detect reliably, especially on a generator whose neutral is earthed through a high-impedance grounding resistor or transformer, which itself limits the maximum earth-fault current. This is the same structural weakness that motivates restricted earth fault (REF) protection on a power transformer (see the REF guide), and on a generator it is why 100% stator earth-fault coverage is usually completed with a separate, dedicated function — most commonly a third-harmonic-based scheme (comparing the naturally occurring third-harmonic voltage at the neutral and at the terminals, which collapses characteristically for a fault near the star point) — layered on top of the ordinary neutral-displacement voltage protection (ANSI 59N) that covers the rest of the winding.

Practical relevance

When reviewing a generator protection scheme, 87G should be recognised as the primary, fast-acting protection against phase-to-phase and most-of-the-winding earth faults, but not as complete stand-alone stator protection: confirming that a separate 100% stator earth-fault scheme (third-harmonic-based, or an injection-based scheme at standstill) is present and correctly coordinated with 87G is part of a full generator protection review, alongside the other generator-specific functions such as loss-of-field (40), reverse power (32) and negative- sequence (46) protection.

Common mistakes

1. Assuming 87G alone gives 100% stator earth-fault protection — its sensitivity collapses for a fault very close to the neutral, and a separate function is needed to cover that last stretch of the winding.
2. Applying an inrush-restraint slope copied from an 87T setting to an 87G relay without adjusting for the absence of a comparable inrush phenomenon, resulting in an unnecessarily desensitised generator differential.
3. Overlooking the CT ratio/matching advantage 87G has over 87T and applying transformer-style vector-group compensation where none is needed, adding unnecessary complexity to the relay configuration.
4. Confusing 87G (stator winding differential) with 87GN/64REF-style restricted schemes used for the generator's own earth-fault coverage — the two are complementary, not interchangeable.

Related

• Transformer differential protection (87T)
• Restricted earth fault (REF) protection — 64N/87N
• Generator field-failure protection (40) — loss of excitation
• Reverse-power protection (32) — generator anti-motoring