1. Transformer heat run test: The loading back method
IEEE transformer Committee meeting, March 2008
By Marcel Fortin, eng., Consultant

2. 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

3. 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.

4. Loading back method

5. Loading back method Transformer 1: transformer under test
Transformer 2: « source » transformer
S1: variable source, provides essentially the total iron losses
S2: variable source, provides essentially the copper losses

6. Loading back method Tranformer 1 P1 : nomiral kVA
Ip1: nominal primary current
Is1: nominal secondary current
Up1: nominal primary voltage
Us1: nominal secondary voltage
Iz1: percentage impedance
N1: Up1/ Us1

7. Loading back method Tranformer 2 requirements P2  P1 Ip2  Ip1
Is2  Is1
Up2  Up1
Us2  Us1
Iz2: percentage impedance
n2 = n1  (Iz1+Iz2) (base power: P1)

8. Loading back method Source 1 requirement Variable source
Voltage range: 0 to Us1
Nominal current: enough to feed the total magnetizing current.

9. Loading back method Source 2 requirement Variable source
Voltage range: 0 to  total Iz voltage drop
Nominal current  Ip1