The theoretically ideal conditions for
paralleling transformers are:
1. Identical turn ratios and voltage
ratings.
2. Equal percent impedances.
3. Equal ratios of resistance to
reactance.
4. Same polarity.
5. Same phase angle shift.
6. Same phase rotation.
Single-Phase Transformers
For single-phase transformers, only the
first four conditions apply, as there is no phase rotation or phase
angle shift due to voltage transformation.
If the turns ratio are not same a
circulating current will flow even at no load. If the percent
impedance or the ratios of resistance to reactance are different
there will be no circulating current at no load, but the division of
load between the transformers when applied will no longer be
proportional to their KVA ratings.
Three-Phase Transformers
The same conditions hold true for three
phase transformers except that in this case the question of phase
rotation and phase angle shift must be considered.
Phase Angle Shift
Certain transformer connections as the
wye-delta or wye-zigzag produce a 30ยบ shift between the line
voltages on the primary side and those on the secondary side.
Transformers with these connections cannot be paralleled with other
transformers not having this shift such as wye-wye, delta-delta,
zigzag-delta, or zigzag-zigzag.
Phase Rotation
Phase rotation refers to the order in
which the terminal voltages reach their maximum values. In
paralleling, those terminals whose voltage maximums occur
simultaneously are paired.
Power Transformer Practice
The preceding discussion covered the
theoretically ideal requirements for paralleling. In actual
practice, good paralleling can be accomplished although the actual
transformer conditions deviate by small percentages from the
theoretical ones.
Good paralleling is considered
attainable when the percentage impedances of two winding transformers
are within 7.5% of each other. For multi-winding and
auto-transformers, the generally accepted limit is 10%.
Furthermore, in power transformers of
normal design the ratio of resistance to reactance is generally
sufficiently small to make the requirement of equal ratios of
negligible importance in paralleling.
When it is desired to parallel
transformers having widely different impedances, reactors or
auto-transformers having the proper ratio should be used. If a
reactor is used it is placed in series with the transformer whose
impedance is lower. It should have a value sufficient to bring the
total effective percent impedance of the transformer plus the reactor
up to the value of the percent impedance of the second transformer.
When an auto-transformer is used, the
relative currents supplied by each transformer are determined by the
ratio of the two sections of the auto-transformer. The
auto-transformer adds a voltage to the voltage drop in the
transformer with the lower impedances and subtracts a voltage from
the voltage drop in the transformer with the higher impedance.
Auto-transformers for use in paralleling power transformers are
specially designed for each installation. The wiring diagram showing
the method of connecting the auto-transformer is usually furnished.
In general, transformers built to the
same manufacturing specifications as indicated by the nameplate may
be operated in parallel.
Connecting transformers in parallel
when the low voltage tension is comparatively low requires care that
the corresponding connecting bars or conductors have approximately
the same impedance. If they do not, the currents will not divide
properly.
Information Courtesy of ABB Power
Transformers
