Distribution Transformer Losses: No-Load, Load and How to Reduce Them
Every transformer loses a small part of the energy passing through it. Understanding where those losses come from lets you specify a better unit and understand the numbers on the test report.
Losses fall into two measured categories, no-load and load, plus stray losses that show up inside the load loss figure.
Every transformer loses a small part of the energy passing through it. Understanding where those losses come from lets you specify a better unit and understand the numbers on the test report.
Losses fall into two measured categories, no-load and load, plus stray losses that show up inside the load loss figure.
No-load loss (iron loss)
Caused by hysteresis and eddy currents in the core. It is present whenever the transformer is energised and is essentially independent of load.
It is measured in an open-circuit test: rated voltage applied to one winding with the other open. The result is the no-load loss in watts and the no-load current, usually a small percentage of rated current.
Load loss (copper loss)
Caused by resistance in the windings, plus eddy and stray losses in windings, tank and structural parts. It rises with the square of the current, so at half load it is about a quarter of its full-load value.
It is measured in a short-circuit test at rated current and corrected to the reference temperature (75 °C for oil immersed, higher for dry type depending on insulation class).
Stray and eddy losses
Leakage flux induces circulating currents in the tank walls, clamps and other metal parts. Good design controls these, but they become significant at higher ratings and are one reason large units need careful electromagnetic design.
How to reduce losses
Better core steel. Higher grade or laser-scribed grain-oriented steel lowers no-load loss
Lower flux density. Reduces core loss at the cost of a larger core
Larger conductor section. Reduces winding resistance and load loss
Copper instead of aluminium. Lower resistance for the same rating
Good cooling. Lower operating temperature reduces resistance
Reading the test report
Insist on guaranteed and measured values for no-load loss, no-load current, load loss at rated current and impedance voltage, all at the stated reference temperature. Losses are usually subject to standard tolerances (commonly +10% on load loss and +15% on no-load loss under IEC).
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