Introduction #

Tap changers allow transformers to regulate secondary voltage as system conditions change. This guide explains off-load and on-load tap changers, when to adjust taps, and how to do it safely.

Tap Changer Types #

  • Off-load tap changer (NLTC): Requires de-energization; fixed position until next outage.
  • On-load tap changer (OLTC): Changes taps under load; used on primary side of medium/large transformers for dynamic regulation.

Why Taps Matter #

  • Maintain secondary voltage within code limits under varying load or feeder drop.
  • Compensate for seasonal utility voltage shifts.
  • Optimize downstream equipment life and reduce motor heating.

Typical Tap Ranges #

  • ±2 × 2.5% or ±5% on distribution transformers (off-load).
  • OLTCs often provide ±10–16 steps, typically 0.625–1.25% per step.

When to Adjust Taps (Off-Load) #

  • Persistent low voltage at MCCs or panels under normal load.
  • After major load additions or feeder length changes.
  • When utility voltage shifts seasonally and stays outside target band.

Field Checklist #

  • Measure secondary at rated load: aim for ~1.02–1.03 pu at transformer to allow feeder drop.
  • Check worst-case motor starting voltage; avoid dropping below allowable limits.
  • Confirm primary voltage stability; don’t “chase” transient sags.

OLTC Operation and Settings #

  • Control modes: line drop compensation (LDC), voltage setpoint with bandwidth and time delay.
  • Set deadband to avoid hunting; add time delay to ride through short sags.
  • Coordinate with capacitor banks to prevent interaction or overvoltage.

Protection Coordination #

  • Changing taps affects fault current (higher secondary voltage → higher fault duty).
  • Re-check relay/breaker settings after significant tap moves.
  • Verify differential and overcurrent settings still coordinate.

Maintenance Notes #

  • OLTCs: inspect contacts, oil, and mechanism per OEM intervals; carbonization increases transition resistance.
  • NLTCs: de-energize, lockout/tagout, verify position physically before re-energizing.
  • Record tap position in maintenance logs.

Integration With Calculators #

Safety Reminders #

  • Off-load tap moves require full de-energization and LOTO.
  • Verify absence of voltage on all bushings; use approved testers.
  • For OLTC work, follow OEM procedures; keep arc quenching oil quality within spec.

Conclusion #

Proper tap selection keeps voltage in the sweet spot for motors and sensitive equipment. Measure under real loads, adjust deliberately, and re-check protection and loading after any tap change.