Transformer parallel operation

Reference notes

When the load exceeds a single transformer's rating or redundancy is required, multiple transformers operate in parallel — primaries on one bus, secondaries on another. Use Next → to walk through the essential conditions for safe paralleling, the load-sharing math, the circulating-current pathology, and OLTC parallel control.

Essential conditions for safe parallel operation

1. Same voltage ratio — different ratios drive a circulating current ΔV / (Z_1 + Z_2).
2. Same phase sequence — wrong sequence creates massive opposing currents.
3. Same vector group — Dy11 cannot parallel Dy1 (60 ° secondary phase difference = catastrophic).
4. Same polarity (for single-phase units, additive vs subtractive).

Desirable (but not essential) conditions

• Same per-unit impedance Z_pu (on own rating) — for equal load sharing in proportion to ratings. IEEE 141 practice: match within ±7.5 %.
• Same X/R ratio — for matched real/reactive power sharing.

Load sharing math — common-base method

Load divides INVERSELY with per-unit impedance referred to a COMMON kVA base:

For two paralleled transformers: S_1 = S_total · Z_2 / (Z_1 + Z_2) and S_2 = S_total · Z_1 / (Z_1 + Z_2), both impedances referred to a common kVA base.

Worked example

• T1: 500 kVA, Z_pu = 5 % on own base → 5 % × 100/500 = 1.0 % on 100 kVA base.
• T2: 750 kVA, Z_pu = 6 % on own base → 6 % × 100/750 = 0.8 % on 100 kVA base.
• Total load 1000 kVA divides: T1 = 1000 × 0.8/1.8 = 444 kVA (89 % of T1 rating).
• T2 = 1000 × 1.0/1.8 = 556 kVA (74 % of T2 rating).
• T1 is more heavily loaded → it will overload first if total load rises.

The unequal-Z_pu problem

Total achievable bank capacity is NOT the sum of ratings when Z_pu values differ — it is limited by the unit that reaches 100 % loading first. Two 1000-kVA units with different Z_pu cannot actually carry 2000 kVA in parallel; ~1850 kVA might be the practical limit before the low-Z unit overloads.

Procurement mitigation: spec sheets for new transformers that will parallel an existing one should require Z_pu within ±7.5 % of the existing unit (IEEE 141 / Red Book).

Circulating current from voltage-ratio mismatch

Even small voltage-ratio mismatches drive a circulating loop current:

where ΔV is the difference in open-circuit secondary voltages. This current heats both transformers without serving load. Most common cause in service: paralleled OLTCs hunting independently → tap positions drift apart.

Vector-group errors — catastrophic

Dy11 paralleled with Dy1 puts the secondaries 60° apart electrically. Driving voltage ≈ V_pu (full per-unit). With only Z_pu (~5 %) limiting circulating current → I_circ ≈ 1/0.10 = 10 per unit = 1000 % of rated → fault-magnitude current → failure in seconds. Vector-group mismatch is an installation error that protection must catch immediately.

OLTC parallel control — ANSI 90

Paralleled OLTCs need coordinated control to prevent tap-changer hunting and circulating current. Two standard modes:

• Master-Follower: one regulator's tap commands are broadcast to all others.
• Circulating-current minimization: each regulator measures inter-transformer circulating current and biases its tap commands to null it.

Both modes implemented in modern numerical voltage regulators (SEL-2431, GE C90, ABB REG670 variants).

Switching practice

• Switching in: closing one transformer onto a loaded paralleled bus is essentially a synchronizing event. Verify tap setting, phase sequence, polarity. Close at low load, then transfer load gradually.
• Switching out for maintenance: verify remaining unit(s) can carry the full load. Some banks operate with n+1 redundancy where any one unit can be removed without overload.
• Protection: each transformer has its own differential, sudden-pressure, overcurrent. Bus tie has bus differential. Tripping one transformer must not de-energize the bus.

Economics

Two smaller paralleled units typically cost MORE in capital than one larger unit of the same total kVA. But losses at light load can be LOWER — operate one unit at high efficiency, switch the second in only when load demands. Economics depend on the load duration curve at that location. Substations with peaky load benefit from multiple smaller units; constant-load substations favor one larger unit.