Voltage Ratio, Tap Position & Circulating Current in Parallel Transformers
Why circulating current can exist with little or no external load
Treat the two secondaries as voltage sources. With the tie open, each maintains its own no-load voltage. When the tie closes, any voltage difference forms a closed loop through the transformers’ internal impedances.
With compatible phase relationships and similar impedance angles, the trend can be understood as circulating current increasing with voltage difference and decreasing as combined impedance increases. The point is not the simplified equation itself; it is that a small source mismatch can become a real current.
Why “both are 480 V” is not enough
480 V on the nameplate is a nominal value under rated conditions. The operating point also depends on the actual primary voltage, manufacturing ratio, current tap and any automatic voltage-control scheme.
A procurement review should therefore compare actual ratios and taps—not only the secondary-voltage line on two nameplates.
Tap position must be project data
An existing transformer may have operated for years on a non-neutral tap while the new unit is quoted at its normal tap. If the RFQ says only “13.8 kV / 480 V,” the condition that controls paralleling voltage may be missing.
For expansion and replacement projects, record existing tap position, tap range/step and normal primary-system voltage. If automatic regulation exists, the project designer should define how two regulating devices are expected to interact.
What the supplier and site should verify
The supplier should provide a complete tap table, voltage/ratio at each tap, connection/terminal drawings and routine ratio-test records.
Before closing the tie, verify both actual tap positions, compare bus-side voltage under the approved operating condition, complete phase-sequence/phase checks and confirm that no field modification changed the intended phase relationship.
Procurement / engineering execution checklist
| Role | Core information / action |
|---|---|
| Buyer / EPC | Existing/new rated voltage, normal primary voltage range, actual taps, tap range/step, long-term parallel versus transfer-only mode. |
| Supplier | Complete tap table, ratios by tap, ratio-test results and terminal/connection drawings. |
| Site / Commissioning | Record actual taps, compare bus-side voltages, perform phase checks and investigate abnormal light-load current. |
Both Transformers Read 480 V. Why Was There Current in the Bus Tie?
Frequently Asked Questions
Why can two 480 V meters still hide a circulating-current problem?
Meter accuracy and sampling may hide a small magnitude difference, and a single RMS reading does not show phase angle.
Does the same tap number on two transformers mean the same ratio?
Not necessarily. Tap numbering can differ by design; use each tap table and measured ratio.
Will circulating current always trip protection immediately?
No. It may first appear as extra current, losses, heating or unequal loading.
- Power Transformers and Protection Q&A — operation, paralleling, ratio, vector group, impedance and inrush chapters
- Electrical Engineer’s Handbook, 4th ed. — system connection, transformer impedance and substation engineering
- Relay Protection Technical Supervision Manual — transformer inrush, differential protection and commissioning checks
- Fundamentals of Power Systems — neutral grounding and transformer zero-sequence networks
This article is an engineering and procurement guide. Final paralleling, protection, grounding and switching decisions must follow the project one-line, applicable standards, utility requirements and authorized engineering/commissioning procedures.