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

S2

I1

I2

I2 I2

n1:11:n2

S1

I1

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

S2

I1

I2

I2 I2

n1:11:n2

S1

I1

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

S2

I1

I2

I2 I2

n1:11:n2

S1

I1

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)

S2

I1

I2

I2 I2

n1:11:n2

S1

I1

Loading back method

Source 1 requirement Variable source

– Voltage range: 0 to Us1

– Nominal current: enough to feed the total magnetizing current.

S2

I1

I2

I2 I2

n1:11:n2

S1

I1

Loading back method

Source 2 requirement Variable source

– Voltage range: 0 to total Iz voltage drop

– Nominal current Ip1

S2

I1

I2

I2 I2

n1:11:n2

S1

I1