Negative-sequence current creates a magnetic field rotating opposite to the positive-sequence field. In motors and generators this induces double-frequency rotor currents and can produce severe heating at modest phase-current unbalance.

Learning objectives

Calculate I2, distinguish unbalance causes and apply pickup and I2²t logic without nuisance operation during normal asymmetry.

Core engineering principles

Sequence components separate unbalance

I2 is derived from the three phase currents and is zero for a perfectly balanced positive-sequence set. Phase rotation and CT polarity must be correct.

Rotor heating can be disproportionate

Negative-sequence impedance of rotating machines is low, so a small voltage unbalance can produce a larger current unbalance and significant rotor loss.

Broken conductors can evade phase overcurrent

An open phase or bad connection can create damaging unbalance while no phase exceeds a conventional 51 pickup.

I2²t matches thermal accumulation

Time-dependent negative-sequence protection can model heating and allow short transients while clearing sustained unbalance.

System asymmetry creates standing I2

Untransposed lines, unequal loads and CT errors establish a normal baseline that settings must exceed with margin.

Engineering application method

  1. Step 1: Measure or calculate maximum healthy I2.
  2. Step 2: Obtain machine negative-sequence withstand data.
  3. Step 3: Set alarm, definite-time or I2²t stages.
  4. Step 4: Coordinate with phase-loss and thermal protection.
  5. Step 5: Test phase rotation, CT polarity and unbalanced injection.

Practical example

A loose cable termination can raise negative-sequence current without creating a high three-phase fault. Function 46 detects the sustained asymmetry before rotor or conductor heating becomes severe.

Common mistakes

  • Setting percent unbalance without naming I1 or rated-current base.
  • Ignoring standing network I2.
  • Using 46 without correct phase rotation.
  • Treating voltage and current unbalance as equal.
  • Omitting machine withstand data.

Design and review checklist

  • What healthy I2 exists?
  • What negative-sequence duty can the machine tolerate?
  • Is the setting definite-time or I2²t?
  • Are CT errors considered?
  • Are alarm and trip actions coordinated?

Standards basis and official sources

  • IEC 60255-1:2022 — Common requirements for measuring relays and protection equipment.
  • IEEE C37.2-2022 — Device function numbers, acronyms and contact designations.

Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.

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