Parallel operation of transformers: the five conditions and what happens if you ignore them

Two 1 000 kVA transformers in parallel are often a better solution than one 2 000 kVA unit: if one fails, half the plant keeps running; maintenance can be done without a full outage; and the second unit can be added later when the load grows. The price is a set of rules that must be respected.

The five conditions

Condition	Requirement	Effect of a mismatch
Same voltage ratio	Identical HV and LV rated voltages, same tap position	Circulating current even at no load
Same vector group (clock number)	Both Dyn11 or both Dyn5	Short circuit between the LV sides
Similar short-circuit impedance	uk within ±10 % of each other	Unequal load sharing; the low-uk unit overloads first
Power ratio	Not larger than 3 : 1	Small unit reaches its limit while the large one is half loaded
Same phase sequence	L1-L2-L3 connected identically on both sides	Short circuit

Load sharing in numbers

Transformers in parallel share the load in proportion to their rating and in inverse proportion to their impedance. Two 1 000 kVA units with uk 6 % and 5.4 % (a 10 % deviation) carry 47 % and 53 % of the total load: at 2 000 kVA total the second unit runs at 1 060 kVA – a 6 % overload. That is why IEC 60076 recommends the ±10 % tolerance and why the routine test report, which states the measured uk, is worth reading before paralleling.

Circulating current from a ratio mismatch

If the two units are on different taps – one at +2.5 %, the other at 0 % – the 2.5 % voltage difference drives a current through both LV windings even with no load connected. With uk = 6 % on both, the circulating current is 2.5 / (6 + 6) = 21 % of rated current. It adds vectorially to the load current, heats both transformers and is invisible to the load. Always set both tap changers to the same position and lock them.

Switchgear and protection

• Each transformer needs its own LV circuit breaker; the bus coupler between the two sections allows single-unit operation.
• The LV fault current is the sum of both contributions – recalculate the switchgear rating (for two 1 000 kVA units at 6 %: about 46 kA).
• Reverse-power or directional protection prevents one transformer from back-feeding a fault in the other.

Specifying a parallel pair

Order both units at the same time and from the same series. Two PNACHO-O 1000 kVA 21/0.42 kV units, for example, are built to the same drawing and tested in the same way; their measured impedances differ by far less than the 10 % tolerance. If you are adding a unit to an existing installation, send us the nameplate photo of the old transformer – we check ratio, group and impedance before we quote.