The nominal power efficiency ηpower of
a transformer is the ratio of rated real power output to rated real
power input: ηpower = Pout/Pin = 1− (Ploss/Pin). Total losses
Ploss are the sum of the no-load and load losses. No-load losses
consist of eddy-current and hysteresis losses within the core (|˜ic|2
Rc, the loss caused by the core-loss component ic of the exciting
current iφ;), ohmic loss |˜iφ|2 Rp, and dielectric loss: that is,
all losses that occur at full voltage with the secondary circuit
open.

Load losses are |˜ip(t)|2 Rp+|˜is(t)|2
Rs caused by the primary [ip(t)] and secondary [is(t)] load currents.
Eddy-current losses also occur, induced by stray fluxes within the
solid transformer structure, and similar losses are generated in the
windings, varying with the load current.

No-load losses are measured at rated
frequency and rated secondary voltage (if the secondary side is the
low-voltage side) and are considered to be independent of load. Load
losses are measured at rated frequency and rated secondary current,
but with the secondary short-circuited and with reduced voltage
applied to the primary, the high-voltage side. Load losses can be
assumed to vary as the square of the load current.

Most units are not fully loaded all the
time, and therefore one defines the energy efficiency of a
transformer, where lightly loaded periods are also taken into account
during a load cycle. For low-power-efficiency transformers (ηpower <
96%) the loss can be measured from the relatively large difference
between the input power Pin and the output power Pout.

However, for high power efficiency
units (ηpower > 96%), the errors in measuring Pin and Pout and
the small difference between the two make an efficiency determination
meaningless. If two current transformers (CTs, maximum errors εCT1 =
εCT2 = 5 mA, CT ratio = 20) and two potential transformers (PTs,
εPT1 = εPT2 = 0.24 V, PT ratio = 30) as well as two ammeters (εA1
= εA2 = 5 Ma) and voltmeters (εV1 = εV2 = 0.3 V) with full-scale
errors of 0.1% are used, then the maximum error in the measured
losses for a 25 kVA, ηpower = 98.44%, 240 V/7200 V single-phase
transformer at cos φ1 = 1 is #Ploss = (240 V ± εPT1 ±
εV1)(5.20835 A ± εCT1 ± εA1) × 20 − 30 (240 V ± εPT2 ± εV2
(3.472 A ± εCT2 ± εA2) = (240.54 V) × (104.367 A) − (7183.8 V)
× (3.462 A) = 234.1 W, so that #Ploss/Ploss = ± (234.1/390)100% ≈
60%.

This means the conventional method of
measuring the losses and therefore the power efficiency of
high-efficiency units does not produce accurate results, and other
methods must be used.

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