IEEE C57.123-2010 pdf download IEEE Guide for Transformer Loss Measurement
3. Transformer no-load losses
3.1 General
No-load losses (also referred to as excitation losses, core losses, and iron losses) are a very small part of thepower rating of the transformer, usually less than 1%. However, these losses are essentially constant overthe lifetime of the transformer (do not vary with load), and hence they generally represent a sizeableoperating expense, especially if energy costs are high. Therefore, accurate measurements are essential inorder to evaluate individual transformer performance accurately.
No-load losses are the losses in a transformer when it is energized but not supplying load. They includelosses due to magnetization of the core, dielectric losses in the insulation, and winding losses due to theflow of the exciting current and any circulating currents in parallel conductors. Load-tap-changingtransformers may use preventive autotransformers, series transformers,or occasionally, both, In mosidesigns the no-load losses of these auxiliary transformers add to the no-load losses of the main transformerwhen the tap changer is not in the neutral position. For example, the additional no-load losses of preventiveautotransformers depend on whether the tap changer is bridging or non-bridging, For series transformershe additional no-load losses depend on tap position, No-load losses are affected by a number of variablesdiscussed in 3.2.
3.2 Parameters affecting magnitude of no-load losses3.2.1 Induction
Losses in the core vary with the level of induction in the core (flux density), and thus the base no-load lossis established by the rated level of the design core flux density of the transformer.
3.2.2 Excitation voltage magnitude
Since the core flux density is a direct function of the magnitude of the excitation voltage, it then follows that the no-load losses are also a function of this voltage. For example, a 1% change in voltage causes a corresponding change in core loss generally in the 1%- 3% range. The design and material used for the core determine the magnitude of the change in losses. It is, therefore, essential to have an accurate measurement of the magnitude of the excitation voltage.
3.2.3 Excitation voltage waveform No-load losses are usually quoted and reported based on a sine-wave volage exciation. Even with a sinusoidal source voltage, the non-linearity of the transformer core introduces significant harmonics into the excitation current and could result in distorted excitation voltage and flux waveforms. The magnitude of the voltage waveform distortion is usually determined by the output impedance of the voltage source and the magnitude and harmonics of the excitation current. The higher these parameters are, the greater will be . the magnitude of the voltage waveform distortion. Figure I ilustrates the supply transformer circuit at the no-load test.