HARMONICS EFFECT TO TRANSFORMER LOSSES

HARMONICS EFFECT TO TRANSFORMER LOSSES
How does harmonics contributes to transformer losses?

Harmonics or harmonic distortion in electrical definition is a type of disturbance both found in the voltage and current waveform. Most common source of this distortion is primarily associated with non-linear loads like electronic loads. These electronic loads can either be from a single phase or three-phase form. Harmonics is said to be caused by a non-sinusoidal load currents usually are blamed from commercial power supplies and motor drives like personal computers and other electronically driven devices. Harmonic distortions to either voltage or current are also reflective to the other according to the ohm’s law principle.

Previously we have discussed that transformer loss are usually categorized as no-load loss (referred sometimes as excitation loss), load loss (also called as impedance loss), and total loss (the sum of no-load loss and load loss). If you try to recall our discussions regarding transformer loss, load loss is a result from the I squared R loss and stray loss. This type of loss is directly dependent to the present loading of the transformer, the higher the load means the higher the losses. According to IEEE STD C57.110-1998, total load loss is defined by the formula

 PLL = P + PEC + POSL

where P is the I squared R portion of the load loss, PEC is the eddy-current loss in the windings and POSL is the other stray losses that can found in a transformer’s loss due to stray electromagnetic flux in the core, core clamps, magnetic shields, windings and transformer tank walls. 

Since a Harmonic Distortion is a disturbance found in the waveform of a voltage and a current, we can say that the effect of this phenomenon to the transformer loss can be directly linked. If the root-mean-square value of the load current is increased due to harmonic components, I squared R loss will be obviously increased accordingly. Likewise, eddy-current loss in the winding tends to be proportional to the square of the load current and the square of the frequency. It is in this behaviour that excessive winding loss and abnormal winding temperature rise occurs in transformers that caters non-sinusoidal load currents.

Unlike in winding eddy-current loss, stray losses will not be increased in proportion to the square of the frequency and only in the square of the load current in which stray losses will be increased proportionally. Also, harmonics in the transformer will create a domino effect to other transformer parameters like effect on top oil rise. For oil-filled transformers, the top oil rise will increase as the total load losses increases with harmonic loading. Any increase in the other stray loss will primarily affect the top oil rise.

Further discussed in IEEE STD C57.110-1998 are some of the harmonic distortions related to transformer losses topics which includes “Transformer per-unit losses”, “Transformer losses at measured currents”, “Harmonic loss factor for eddy-currents”, “Harmonic loss factor for other stray losses”, and “Design consideration for new transformer specification”.