Negative-sequence voltage protection (ANSI 47) — incorrect phase sequence and voltage unbalance

The guide on negative-sequence protection (ANSI 46) covers a protection function that monitors the negative-sequence current of a motor or generator — that is, on the load side. ANSI 47 ("Phase-Sequence or Phase-Balance Voltage Relay") is the mirror image of that: this function monitors negative-sequence voltage on the supply side, and thereby signals problems that are not caused by the load, but by the grid or the supply connection itself.

What ANSI 47 detects

A three-phase voltage can, just like a three-phase current, be decomposed into symmetrical components: a positive-sequence, a negative-sequence and a zero-sequence component (see the guide on symmetrical components for the background of this decomposition). ANSI 47 specifically monitors the negative-sequence voltage (V2) and trips when it exceeds a set threshold. Typical causes:

• Swapped phases after (re)wiring or after maintenance — the phase sequence is then reversed (for example L1-L3-L2 instead of L1-L2-L3), which means a reversed rotation direction for a three-phase motor.
• A blown fuse in the voltage-transformer measuring circuit (VT fuse) — if one of the three voltage measurement signals drops out, an artificial, strong negative-sequence component arises that is not caused by the grid itself, but by the measuring circuit.
• Severe grid unbalance — for example due to a large single-phase load elsewhere in the grid that is unevenly distributed across the phases, or due to a fault on one phase that causes the voltage on that phase to sag.

Difference from ANSI 46, ANSI 27 and ANSI 60

It is important not to confuse these four functions, despite the shared emphasis on unbalance or voltage:

An installation can apply ANSI 27 (undervoltage) and ANSI 47 (negative-sequence) simultaneously: ANSI 27 catches a simple voltage drop across all phases, whereas ANSI 47 is specifically sensitive to the case where the voltages have become skewed relative to each other or are in the wrong sequence — a situation ANSI 27 alone can miss if the average voltage still stays within the normal margin.

Practical relevance

ANSI 47 is often applied as a closing interlock: before a motor or installation is connected or switched in, the relay checks whether the phase sequence is correct and whether the negative-sequence voltage stays within the permitted limit. In the event of a reversed phase sequence, this prevents a three-phase motor from starting in the wrong direction — relevant for example with emergency generators that are switched back onto the grid via a transfer switch (see also the guide on four-pole switching for emergency generators), where an incorrect phase sequence during transfer can cause serious damage to connected three-phase machinery.

Common mistakes

1. Confusing ANSI 47 with ANSI 46 — ANSI 47 monitors voltage on the supply side, ANSI 46 monitors current on the load side; both respond to unbalance, but they flag a different part of the installation.
2. Replacing ANSI 47 with ANSI 27 alone — a simple undervoltage protection misses an incorrect phase sequence or a skewed voltage distribution as long as the average voltage still appears acceptable.
3. Overlooking a VT fuse failure as a possible cause of an ANSI 47 trip — not every ANSI 47 alarm indicates a genuine grid problem; check the measuring circuit itself first.

Related

• Negative-sequence protection (ANSI 46) — phase unbalance in motors/generators
• Symmetrical components and asymmetric fault analysis
• Three-phase voltage unbalance
• Emergency generator — four-pole switching of the neutral conductor