Transformer heat run test:
The loading back method
IEEE transformer Committee
meeting, March 2008
By Marcel Fortin, eng.,
Consultant
Standard Requirement:
C57.12.90 clause 11
Tests shall be made by one of the following
methods:
– a) Actual loading
– b) Simulated loading
1) The short-circuit method, in which appropriate total
losses are produced by the effect of short-circuit current
2) The loading back (opposition) method, in which rated
voltage and current are induced in the transformer under
test
Standard Requirement:
C57.12.90 clause 11
Duplicate (not nessary) transformers may be
tested by connecting their respective high-
voltage and low-voltage windings in parallel
(see Figure 27 and Figure 28). Transformers
shall be tested with the combination of
connections and taps that give the highest
average winding temperature rise. This will
generally involve those connections and taps
resulting in the highest losses.
Loading back method
I1
I1
1:n2 n1:1
S1
I2 S2 I2
I2
Loading back method
Transformer 1: I1
I1
transformer under test S1
1:n2 n1:1
Transformer 2: I2 S2 I2
« source » transformer I2
S1: variable source,
provides essentially the
total iron losses
S2: variable source,
provides essentially the
copper losses
Loading back method
Tranformer 1 I1
I1
P1 : nomiral kVA S1
1:n2 n1:1
Ip1: nominal primary current I2
I2 S2
Is1: nominal secondary I2
current
Up1: nominal primary
voltage
Us1: nominal secondary
voltage
Iz1: percentage impedance
N1: Up1/ Us1
Loading back method
Tranformer 2 requirements I1
I1
P2 P1 S1
1:n2 n1:1
Ip2 Ip1 I2 S2 I2
Is2 Is1 I2
Up2 Up1
Us2 Us1
Iz2: percentage impedance
n2 = n1 (Iz1+Iz2) (base
power: P1)
Loading back method
Source 1 requirement I1
I1
1:n2 n1:1
Variable source S1
– Voltage range: 0 to Us1 I2 S2 I2
I2
– Nominal current: enough
to feed the total
magnetizing current.
Loading back method
Source 2 requirement I1
I1
1:n2 n1:1
Variable source S1
– Voltage range: 0 to I2 S2 I2
total Iz voltage drop I2
– Nominal current Ip1