Voltage-dependent overcurrent protection (ANSI 51V) — voltage-restrained versus voltage-controlled on a generator

The [guide on generator reactances Xd″, Xd′ and Xd](/guides/nen-3140/generator-reactantie-subtransient-transient-synchroon-xd) explains why the fault current of a synchronous generator decreases in three stages during a persisting fault, and why an ordinary time-overcurrent protection (ANSI 50/51) set to the high, subtransient initial current can miss a longer-duration fault once the current has decayed to near the normal operating current. This article covers the protection function developed specifically for this problem: voltage-dependent overcurrent protection, ANSI 51V.

The problem 51V solves

An ordinary overcurrent protection (51) on a generator output faces a dilemma: the pickup threshold must be high enough not to trip unnecessarily during normal operation (including the starting of large motors elsewhere in the network), yet low enough to detect a generator fault once the fault current — after the subtransient and transient contributions have decayed — has fallen back to near, or even below, the normal operating current. A fixed threshold cannot resolve this dilemma. 51V solves it by making the pickup threshold not fixed, but linked to the measured terminal voltage: during a generator fault the terminal voltage drops simultaneously with the decaying fault current, so a voltage-dependent threshold remains sensitive exactly when needed.

Voltage-restrained (51V-R): continuously scaling threshold

In the voltage-restrained implementation (51V-R), the pickup threshold scales continuously and proportionally with the measured voltage: at full rated voltage the normal, high overcurrent threshold applies; as the voltage drops during a fault, the threshold decreases proportionally, down to a minimum value at very low voltage. This makes the relay effectively behave as a pseudo-impedance relay: it distinguishes a nearby fault (low voltage, tripping fast at a relatively low current) from a remote fault or a normal load peak (higher voltage, tripping only at a higher current) — without an actual impedance calculation taking place.

Voltage-controlled (51V-C): switching between two fixed thresholds

In the voltage-controlled implementation (51V-C), the principle is coarser: there are two fixed pickup thresholds — a high, normal threshold for use at normal voltage, and a lower threshold that is activated as soon as the measured voltage drops below a set lower bound. Instead of a continuous scaling function, the relay simply switches between two discrete levels based on a voltage threshold. This is simpler to implement and set than 51V-R, but provides a less fine-grained match to the actual fault current at any given moment.

Implementation	Pickup threshold	Characteristic
51V-R (restrained)	Continuously proportional to voltage	More precise, behaves as pseudo-impedance relay
51V-C (controlled)	Switches between 2 fixed levels	Simpler to set, coarser

Why it is often applied as backup for differential protection

51V is often applied on small to medium synchronous generators as backup protection behind the primary stator differential protection (87G): should the 87G function or its associated current transformers fail to operate correctly for any reason, 51V — thanks to its voltage-dependent sensitivity — still catches an internal or nearby fault that a fixed 51 threshold would miss. On larger machines, actual impedance protection is often applied instead of (or alongside) 51V, which implements the "more sensitive near a nearby fault" principle more precisely through an actual Z calculation rather than a voltage approximation.

Note: 51V primarily protects the generator against external and nearby internal phase-to-phase faults; for a fault within the stator winding itself, a separate function is generally needed (for example 100% stator earth-fault protection), which is outside the scope of this article.

Practical relevance

When assessing a generator's protection settings, it is important to check whether the overcurrent function is implemented as voltage-dependent (51V) or not (ordinary 51) — an ordinary 51 setting tuned to the subtransient initial current may prove to have too high a threshold during a persisting internal fault once the current has decayed, whereas the same threshold under a 51V implementation automatically decreases along with the collapsing voltage.

Common mistakes

Applying an ordinary, fixed 51 threshold on a generator output without accounting for the decaying fault current from the decrement curve — a persisting fault can thus fall below the threshold before the protection trips.
Confusing 51V-R and 51V-C when reading a protection scheme — the former scales continuously with voltage, the latter switches between two fixed levels; the exact setting values are therefore not directly interchangeable between the two variants.
Applying 51V as the sole protection against internal faults without a primary differential protection (87G) — 51V is intended as backup, not replacement, and generally responds more slowly and less selectively than a differential function.
Deriving the voltage measurement for 51V from the wrong point (for example from a voltage transformer that is itself already affected by the fault in a non-representative way) — the accuracy of 51V stands or falls with a reliable, representative voltage measurement.