Negative Sequence Protection 46: How It Works

What 46 protection is (negative sequence)

In IEEE C37.2, device 46 is the reverse-phase or phase-balance current relay: it operates when the polyphase currents are in reverse sequence, are unbalanced, or contain negative-sequence components above a given value.

A balanced three-phase system carries only positive-sequence current. Any unbalance (a large single-phase load, an open phase, a phase-to-phase or ground fault) produces negative-sequence current, I2. That component sets up a magnetic field that rotates opposite to the rotor.

In a rotating machine that field induces currents at twice the system frequency in the rotor iron, the wedges and the retaining rings. The heat concentrates in a few spots and can damage the rotor in a short time. The 46 limits that heating.

Where it is applied

• Synchronous generators: the classic application. It protects the rotor against sustained unbalance and uncleared external faults.
• Induction and synchronous motors: it detects single phasing and supply voltage unbalance, which in a motor produces a much larger current unbalance.
• Medium-voltage feeders and lines: broken conductor detection and sensitive backup for phase-to-phase faults.
• Transformers: backup for phase-to-phase faults on the low-voltage side which, seen from the high-voltage side, may fall below the phase setting.
• Industry: arc furnaces and large single-phase loads that unbalance the supply.

How it works

The relay calculates I2 from the three phase currents with the symmetrical components operator. Depending on the application it uses two kinds of element:

Definite time. A threshold on I2 (or on the ratio I2/I1) and a timer. It is used as an alarm in generators and as a broken conductor or phase loss element in motors and feeders.

Inverse thermal time (I2²t). For generators, the machine capability is expressed as I2² · t = K, where I2 is in per unit of rated current and K is a rotor constant. The relay integrates I2² over time and trips when the accumulated energy approaches K. The machine standards (IEEE C50.12, IEEE C50.13 and IEC 60034-1) cover continuous and short-time negative-sequence capability; the value for each generator comes from its manufacturer.

Many relays include a thermal memory with a cooling time: if the unbalance disappears and returns, the relay remembers the accumulated heat.

Motor applications often use the ratio I2/I1, which is more stable with load changes than the absolute value of I2.

Setting criteria

• Generator: the manufacturer’s continuous negative-sequence capability and K constant. The alarm is set below the continuous level and the trip characteristic below the damage curve.
• Motor: sensitivity to supply voltage unbalance, starting current and how long the motor tolerates an open phase. The setting accounts for the normal unbalance of the supply at the site.
• Feeder or transformer: coordination with downstream protection, like any overcurrent element. It is defined in the protection coordination study.
• CT ratios and the base current the relay uses for per unit.

There is no universal value. It is set from the machine data, the coordination study and the relay manual.

How each manufacturer implements it

• Siemens SIPROTEC offers it as unbalanced load or negative-sequence protection, with definite-time and thermal elements; it is set in DIGSI.
• SEL includes it in generator, motor and feeder relays; it is set with AcSELerator QuickSet, with definite-time and curve I2 elements.
• Relion (Hitachi Energy 670/650; ABB 615/620) provides separate function blocks for machines and for feeders, configured in PCM600.
• GE Vernova (Multilin) includes it in generator and motor relays and as negative-sequence overcurrent elements in feeder relays; it is set in EnerVista.
• Schneider MiCOM / Easergy offers it as negative-sequence overcurrent and as a broken conductor element; it is configured in MiCOM S1 or Easergy Studio.

The differences are in the per-unit base, whether the element uses I2 or I2/I1, the form of the thermal memory and how it resets. The same number can mean different things in two brands.

Why it trips or blocks

1. Wrong phase rotation setting in the relay or two CT phases swapped. The most frequent cause during commissioning: with balanced load the relay sees high I2.
2. An open phase upstream: blown fuse, a breaker pole that did not close, a broken conductor.
3. Supply unbalance larger than the setting assumed, especially in motors.
4. Uncleared external fault: the generator 46 acts as backup and trips correctly.
5. Different CT ratios between phases, or one CT on a different tap.
6. Single-pole reclosing on a nearby line: during the open-pole period, negative sequence appears in the generators of the area.
7. Thermal memory holding heat from an earlier event, which shortens the time of the next trip.

What to check first: the I1 and I2 values the relay shows under load, the phase rotation setting, the event oscillography and the state of the breaker poles.

How it is tested

By secondary injection with a test set such as the Omicron CMC 356, injecting pure negative-sequence current systems or controlled unbalances.

The test checks:

• The pickup of each element.
• The I2²t curve at several points and the definite-time delays.
• The thermal memory: the time of a second trip and the cooling time.
• Correct I2 measurement with a balanced positive-sequence injection (it must read zero).

At the end the thermal memory is reset to zero before the relay goes back into service.

Did your relay trip on the 46?

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 remote relay configuration or write to us with the brand and model of the device.

Related guides

• Thermal overload 49 (PTTR): how it works
• Motor differential 87M (PDIF): how it works

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE C37.102, Guide for AC Generator Protection.
• IEEE C37.96, Guide for AC Motor Protection.
• IEEE C50.12 and IEEE C50.13, requirements for salient-pole and cylindrical-rotor synchronous generators.
• IEC 60034-1, Rotating electrical machines: rating and performance.
• J. Lewis Blackburn and Thomas J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions