Phase Sequence Protection 47: How It Works

What 47 protection is (phase sequence voltage relay)

In IEEE C37.2, device 47 is the phase sequence or phase balance voltage relay: it operates on a predetermined value of polyphase voltage in the desired phase sequence, when the voltages are unbalanced, or when the negative-sequence voltage exceeds a given value.

In numerical relays the 47 usually does two jobs:

• Detect reverse rotation: the supply arrives with ACB sequence when the installation expects ABC, or the other way round. It is the typical result of a wrong connection after maintenance or a change of service entrance.
• Detect voltage unbalance: it measures the negative-sequence voltage component, V2, or the ratio V2/V1, and operates if it exceeds a threshold for a set time.

A balanced system with the correct sequence has only positive-sequence voltage. A lost phase, a phase with very low voltage or a reversed sequence make V2 appear.

Where it is applied

• Motors: the most common application. A motor that starts with reversed sequence turns backwards and can damage pumps, compressors, conveyors or the coupled load. Voltage unbalance also produces a larger current unbalance and heats the rotor.
• Generators: as a check before synchronizing and as backup against system unbalance. It complements synchronism check 25.
• Automatic transfer schemes and industrial switchgear with two sources: it prevents a transfer to a source with the wrong sequence.
• Supervision of other functions: in some schemes the 47 is combined with undervoltage 27 to permit a start or to tell a lost phase from a general voltage dip.
• Solar plants and sensitive loads: as voltage quality monitoring at the point of interconnection.

How it works

The relay takes the three voltages from the voltage transformer (VT) and calculates the symmetrical components:

• V1, positive-sequence voltage.
• V2, negative-sequence voltage.

Reverse sequence element. If V2 is much larger than V1, rotation is reversed. The relay flags it or blocks closing or starting.

Negative-sequence overvoltage element. If V2 (or V2/V1) exceeds the set threshold for the set time, the relay alarms or trips. It usually has a minimum positive-sequence voltage supervision so it does not operate with the bus de-energized.

The measurement depends on the relay knowing the installation rotation. Almost every relay has a phase rotation setting (ABC or ACB) that changes the V1 and V2 calculation of all functions, not just the 47.

Setting criteria

• Phase rotation of the installation, verified in the field with a phase rotation meter.
• Allowable voltage unbalance for the load. For motors, the machine standards (for example NEMA MG 1 and IEC 60034-1) cover the effect of voltage unbalance and the derating it requires; the limit is set by the motor manufacturer or the study.
• Normal system unbalance at the site, measured or taken from a power quality study, so the element does not operate under normal conditions.
• Time delay: long enough not to operate on transient faults cleared by other protection, and short enough to protect the machine. It is set according to the coordination study.
• Coordination with VT supervision (60 or VTFF).

How each manufacturer implements it

• Siemens SIPROTEC separates phase rotation supervision and negative-sequence overvoltage protection; both are set in DIGSI.
• SEL offers negative-sequence voltage and phase rotation elements in motor, generator and feeder relays; they are set in AcSELerator QuickSet.
• Relion (Hitachi Energy 670/650; ABB 615/620) provides negative-sequence overvoltage and sequence supervision blocks, configured in PCM600.
• GE Vernova (Multilin) includes negative-sequence voltage and phase reversal elements; they are set in EnerVista.
• Schneider MiCOM / Easergy offers it as time-delayed negative-sequence overvoltage; it is configured in MiCOM S1 or Easergy Studio.

In some relays the 47 is a named function; in others it has to be built from a V2 overvoltage element and logic. The threshold may also be expressed in volts, in per unit or as the ratio V2/V1.

Why it trips or blocks

1. Wrong phase rotation setting in the relay. The number one cause during commissioning.
2. Crossed VT phases in the secondary wiring or at the terminal block.
3. Blown VT fuse or open miniature circuit breaker: V2 shows up in the measurement with nothing wrong in the supply.
4. Open phase upstream: blown medium-voltage fuse, a disconnect switch or breaker pole that did not close, a broken conductor.
5. Real unbalance in the supply from large single-phase loads or a remote fault that was not cleared.
6. Change of service entrance or work on the network that reversed the sequence.

What to check first: the V1 and V2 measurement in the relay, the rotation setting, the VT fuses, and a sequence measurement at the relay terminals and on the primary side.

How it is tested

Secondary injection with a three-phase test set such as the Omicron CMC 356 generates voltage systems with the desired sequence and unbalance.

The test checks:

• That with correct sequence and rated voltage the relay measures almost zero V2 and does not operate.
• That with reversed sequence the relay detects the condition.
• The V2 or V2/V1 threshold and the operating time.
• Minimum voltage supervision and blocking on VT failure.

At commissioning the most valuable test is a primary one: confirming with the installation energized that the relay measures the correct sequence and that the motor turns in the expected direction.

Did your relay block on the 47?

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.

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE C37.96, Guide for AC Motor Protection.
• IEC 60034-1, Rotating electrical machines: rating and performance.
• NEMA MG 1, Motors and Generators.
• J. Lewis Blackburn and Thomas J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions