51V Protection (PVOC): Voltage Restrained Overcurrent

What 51V protection is (voltage-dependent overcurrent)

The 51V is a time overcurrent protection whose pickup value changes with the measured voltage. In ANSI/IEEE C37.2, number 51 is the AC time overcurrent relay, and the letter V means voltage conditions it. In IEC 61850 it is modelled with logical node PVOC (voltage controlled time overcurrent). IEC literature often calls it voltage-dependent I>.

The idea is simple: at normal voltage, the relay needs a high current to pick up and does not mistake load for a fault. When a nearby short circuit occurs, the voltage drops and the relay becomes more sensitive. That way it detects faults whose current would be too low for conventional overcurrent.

There are two variants:

• Voltage controlled (51VC): the overcurrent element is blocked while the voltage is normal and is enabled when it drops below a threshold.
• Voltage restrained (51VR): the pickup is reduced progressively, in proportion to the voltage drop, down to a minimum defined by the manufacturer.

Where it is applied

Its classic application is the synchronous generator. For an external three-phase short circuit, the generator’s current contribution goes from the subtransient value to the transient value and then to the steady-state value, which depends on the synchronous reactance and the excitation. That sustained value can be below rated current. A plain 51 set above load would not see the fault; the 51V does. That is why IEEE C37.102 presents it as one of the generator backup options for system faults, together with distance protection (21).

It is also used in:

• Small generating plants and industrial cogeneration, where distance protection is not justified.
• Some step-up transformers and feeders with weak sources, when the short-circuit study shows fault currents close to load current.
• Installations with distributed generation, where the source contribution is limited.

How it works

The relay measures phase currents and voltages from the voltage transformer (VT) at the generator terminals or on the bus.

In the controlled version, the logic is an AND: it trips if the current exceeds pickup and the voltage is below the control threshold. At normal voltage, the function is inactive.

In the restrained version, the effective pickup is the set pickup multiplied by a factor that depends on voltage. At rated voltage the factor is 1; as the voltage falls, the factor decreases linearly down to a minimum. The exact shape of that line, and the voltage below which it stays fixed, is defined by each manufacturer.

The time characteristic is usually an inverse curve (IEC 60255-151 or IEEE C37.112), as in a normal 51, to coordinate with downstream protection.

An important detail is which voltage each phase uses. Many relays pair each phase current with the corresponding phase-to-phase voltage, so that the restraint responds correctly to phase-to-phase faults and to faults on the other side of a wye-delta transformer. Check in the manual which combination your device uses.

Setting criteria

The settings come from the short-circuit study and the protection coordination study. The general criteria are:

• Pickup at normal voltage: above the maximum expected load current, with the margin defined by the study.
• Sensitivity at depressed voltage: the reduced pickup must be below the sustained current the generator contributes to the most remote fault to be backed up, taking into account the action of the voltage regulator.
• Control threshold (51VC): below the minimum normal operating voltage and above the voltage expected during the faults it must see.
• Time: coordinated with the downstream line and transformer protection, and below the thermal capability of the generator.

Data you need: generator reactances (synchronous, transient, subtransient), time constants, excitation system behaviour, CT and VT ratios, and the equipment damage curve. None of these values is universal.

How each manufacturer implements it

All major manufacturers offer the function, but not with the same name or in the same place:

• Siemens SIPROTEC (DIGSI) presents it as voltage-dependent overcurrent, with control and restraint options.
• SEL (AcSELerator QuickSet) includes it in its generator relays with separate elements for the controlled and restrained variants.
• Relion (Hitachi Energy 670/650; ABB 615/620) (PCM600) offers it among the generator backup functions.
• GE Vernova (Multilin, EnerVista) integrates it as a voltage restraint option of phase overcurrent.
• Schneider MiCOM/Easergy (MiCOM S1 or Easergy Studio) includes it in generator relays with a choice of dependency mode.

What changes is the shape of the restraint characteristic, the reference voltage of each phase, the interaction with fuse failure supervision and the option of two stages. That is why a setting that works in one brand cannot be moved to another without review.

Why it trips or blocks

Unexpected 51V operations usually have one of these causes:

• Blown VT fuse or open miniature circuit breaker. The most common one. The relay sees zero voltage on one or more phases, the pickup drops to its minimum and load current trips it. The function must be blocked by fuse failure supervision (60, VT fuse failure); check that this logic is active and correctly assigned.
• Voltage wiring with the sequence or a phase crossed. The restraint ends up tied to the wrong voltage.
• Setting copied from a plain 51. Without accounting for the pickup reduction, the function becomes too sensitive.
• Generator start-up or synchronization at low voltage. During initial excitation the voltage may be depressed and the function enabled.
• Wrong settings group active after maintenance.

If instead the function did not operate for an external fault, check whether the voltage regulator held the voltage above the control threshold, or whether the sustained current stayed below the reduced pickup. The first thing to look at is the oscillographic record: currents, voltages and the state of VT supervision at the moment of the event.

How it is tested

The test is done by secondary injection with a test set such as the Omicron CMC 356, injecting currents and voltages at the same time. It checks:

• Pickup at rated voltage and at several reduced voltage levels, to confirm the shape of the restraint characteristic or the control threshold.
• Operating times on the curve at several points.
• Fuse failure blocking: simulate the loss of one voltage with load current and confirm there is no trip.
• The assignment of the trip output and the event record.

Before testing, confirm the ratio and polarity of the VTs and CTs, because an error there invalidates any result.

Did your relay trip or block on this function and the reason is not clear? At Tiza Group we review the events and settings remotely. See our remote relay configuration service or write to us.

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE C37.102, Guide for AC Generator Protection.
• IEC 61850-7-4, Compatible logical node classes and data object classes.
• IEC 60255-151, Functional requirements for over/under current protection.
• IEEE C37.112, Standard for Inverse-Time Characteristics Equations for Overcurrent Relays.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions