Non-linear loads, such as power electronic devices, such as variable speed drives on motor systems, computers, UPS systems, TV sets and compact fluorescent lamps, cause harmonic currents on the network.
Harmonic voltages are generated in the impedance of the network by the harmonic load currents.
Harmonics increase both load and no-load losses due to increased skin effect, eddy current, stray and hysteresis losses. The most important of these losses is that due to eddy current losses in the winding; it can be very large and consequently most calculation models ignore the other harmonic induced losses. The precise impact of a harmonic current on load loss depends on the harmonic frequency and the way the transformer is designed.
In a transformer that is heavily loaded with harmonic currents, the excess loss can cause high temperature at some locations in the windings. This can seriously reduce the life span of the transformer and even cause immediate damage and sometimes fire. Reducing the maximum apparent power transferred by the transformer, often called de-rating.
To estimate the required de-rating of the transformer, the load’s de-rating factor may be calculated.
This method, used commonly in Europe, is to estimate by how much a standard transformer should be de-rated so that the total loss on harmonic load does not exceed the fundamental design loss. This de-rating parameter is known as “factor K”.
The factor K is given by:
e – the eddy current loss at the fundamental fre-quency divided by the loss due to a DC current equal to the RMS value of the sinusoidal current, both at reference temperature.
n – the harmonic order
I – the RMS value of the sinusoidal current includ-ing all harmonics given by
In – the magnitude of the n-th harmonic
I1 – the magnitude of the fundamental current
q – exponential constant that is dependent on the type of winding and frequency.
Typical values are 1.7 for transformers with round rectangular cross-section conductors in both windings and 1.5 for those with foil low voltage windings.
Developing special transformer designs rated for non-sinusoidal load currents. This process requires analysis and minimising of the eddy loss in the windings, calculation of the hot spot temperature rise, individual insulation of laminations, and/or increasing the size of the core or windings. Each manufacturer will use any or all of these techniques according to labour rates, production volume and the capability of his plant and equipment. These products are sold as ‘K rated’ transformers.
During the transformer selection process, the designer should estimate the K factor of the load and select a transformer with the same or higher K factor.
K factor is defined as:
As an example IEC 61378-1 deals with the specification, design and testing of power transformers and reactors, which are intended for integration within semiconductor converter plants; it is not designed for industrial or public distribution of AC power in general.
The scope of this standard is limited to applications of power converters, of any power rating, for local
distribution, at moderate rated converter voltage, generally for industrial applications and typically with a highest voltage for equipment not exceeding 36 kV. The converter transformers covered by this standard may be of the oil immersed or dry-type design.
The oil-immersed transformers are required to comply with IEC 60076, and with IEC 60726 for dry-type transformers.
Resource: Selecting Energy Efficient Distribution Transformers by Intelligent Energy Europe