K-Factor monitoring through Multifunction Meter

The use of VFD’s in industries were limited until two decades ago. Simple AC Induction motors were more in prominence during that era i.e. the loads were linear in nature. However, due to continuous modernization efforts and energy saving initiatives many applications now operate with VFD driven motors instead of old technology. Due to this, various feeders now handle a higher percentage of nonlinear loads and this results in harmonic currents getting injected into the supply system.

The transformers that feed the present-day power electronic devices exhibit additional losses due to high level of harmonic currents manifested as distorted current and voltage waveforms. The transformer manufacturer used to test their product only under ideal conditions i.e with linear load and therefore a substantial gap exists between published loss data and actual losses incurred after installation.

Losses in transformers are primarily due to eddy current losses and stray magnetic losses in the core and resistive losses in the windings. Of these, eddy current loss would play a major role because of the presence of harmonics. This loss is directly proportional to the square of the frequency. Higher harmonic content (in buildings, industries with more non-linear loads and so on) would lead to higher eddy current loss. This in turn would rise the operating temperature of transformer and would result in premature failure of transformer.

Underwriter’s Laboratory had brought out the standards for the transformers handling higher frequency harmonic currents. This is described under UL1561 based on K factor concept derived from ANSI/IEEE C57.110 standard – Recommended practices for establishing Transformer capability when supplying non sinusoidal load currents.

K-Factor is the weighting of the harmonic load currents according to their effects on transformer heating.

K-Factor of 1.0 indicates a linear load (no harmonics). The higher the K-Factor, greater is the harmonic heating effects.

The K-Factor transformers have additional thermal capacity to tolerate the heating effects of the harmonic currents. K-factor indicates the multiples of 50 Hz winding eddy current losses that the transformer can safely dissipate, without requirement of de-rating.

Transformer load losses consist of winding I2R losses plus eddy current and stray losses. For example, a transformer with K-4 rating that has winding I2R loss of 2000W and 50 Hz eddy current and stray losses of 100W will be required to dissipate the 2000 watts of I2R losses plus 4 times the 50 Hz value of 100 watts i.e a total load loss of 2400 watts without exceeding the maximum winding temperature rise.

UL has chosen standard K factors of K-4, K-9, K-13, K-20 etc. K factors greater than 9 are infrequent. K factors above 4 normally find use with single phase transformer supplies used to feed computer loads in office, etc.