What are the transformer guaranteed values?

Like other electric equipment also transformers are generally designed in accordance with international standards. There are multiple parameters stated in the technical specification or a data sheet. However, which of them are the transformer guaranteed values? And what does the guarantee actually mean? Does it include any tolerance? This blog post looks into that topic, including some aspects of the transformers used to supply the variable frequency drives (converter transformers / VFD transformers / drive transformers).

Transformer parameters

Fundamental transformer data include parameters such as nominal voltages, winding connection/vector group, no-load and load losses, insulation level, type of cooling, insulation material, dimensions and weight etc. The parameters are stated in the technical documentation. Some of them are statements (e.g. type of insulation liquid), others are values that can be verified by measurement or test. The acceptance criteria is defined in corresponding standards.

Transformer guaranteed values

Some parameters represent guaranteed values. These values are defined in the technical specification and are verified during factory testing. Notwithstanding those guarantees may be a reason for financial compensation or even termination of the contract.

Transformer guaranteed values are:

No-load losses
Load losses
Short-circuit impedance
Sound level (if requested)

No-load losses, also called ‘iron losses’, are the losses in the magnetic circuit of the transformer. They are present all the time regardless of transformer loading, including no load condition. Thus, they play a role in the loss capitalization despite the relative low absolute value in Watt (low power loss integrated over 30-40 years of transformer lifetime can be considerable amount of energy and cost). No-load losses can be reduced by using high grade steel sheets or by using lower design flux density.

In opposite, load losses are primarily associated with losses inside the transformer windings. They are sometimes called ‘copper losses’ despite the fact that transformer winding may consist of aluminium. Load losses are strongly load dependent as the scale with the square of the load current.

Note that the guaranteed load losses are losses in case of sinusoidal operation, i.e. excluding harmonic losses. The harmonic losses can be indicated by transformer manufacturer in the technical specification but are not guaranteed (as they cannot be measured).

In applications or projects sensitive to losses and energy cost, a loss capitalization can be calculated and considered in the evaluation. It is usually stated in the tender requisition as amount of money per kW of losses. The transformer manufacturer then needs to find the best economic balance between additional material cost and ‘penalization’ due to high losses.

When it comes to transformer efficiency and losses, additional regulations such as e.g. Ecodesign regulation no. 548/2014 (European Union) may apply. So far, the interpretation is mostly such that converter transformers are excluded from the Ecodesign directive. However, that situation may change in the future.

Short-circuit impedance is another important design parameter. The short-circuit impedance corresponds to the stray reactance of the transformer (sum of reactances of primary and secondary winding). Another, commonly used definition is that the short-circuit impedance in % corresponds to the voltage (% of nominal) supplied to the primary winding so that nominal current is flowing when secondary winding is shorted (or vice versa).

As explained in [2], the choice is often a compromise between conflicting requirements (voltage drop, reduction of harmonics, magnitude of fault current etc). For every transformer design there is an optimum value or optimum range of short-circuit impedance. Extreme impedance values present one of the cost drivers [3].

Very important: multi-winding transformers [4] have more than just one value of short-circuit impedance. Each pair of windings has its short-circuit impedance. Thus, it is crucial to clearly specify what impedance value is meant.

Sound level of a transformer can also be a guaranteed parameter. Especially transformers located in vicinity of residential buildings may be specified with challenging sound level values. What talking about sound (or noise) level, it is important to differentiate between sound pressure level (Lp) and sound power level (Lw). Sound pressure level reduces as the distance from the source increases. In contrast, sound power level is an absolute value that is independent on distance.

Main source of transformer noise is mostly the magnetic core with its characteristic “humming”. Such kind of noise contains predominantly 2 x grid frequency in its spectrum (i.e. 100 Hz component on a 50 Hz grid and 120 Hz component on a 60 Hz grid) and integer multiples of it. Other sources of transformer noise can be the cooling (e.g. noise of cooling fans or cooling pumps in case of forced cooling). In case of low-noise requirements a an attention shall be paid to avoid structural vibration (e.g. applying suitable dampers). Finally, a non-sinusoidal current or (especially) voltage in converter operation can be a significant noise contributor [5].

From the contractual and testing perspective the sound level in real operation of the drive system normally cannot be tested in transformer factory. The sound level is typically tested at no-load and with sinusoidal voltage. The manufacturer cannot guarantee something that cannot be measured (and thus verified) in the test field. It is then up to the system designer to consider sufficient provision for the converter duty operation.

Tolerances

No-load losses, load losses and total losses

Generally, [S1] defines 10% tolerance on the total losses and 15% tolerance on each loss component (no-load and load losses).

Note that load losses as typically measured in the factory for sinusoidal current. Therefore, additional harmonic losses are not included and shall be considered separately.

Besides above IEC tolerance, the purchaser may request guaranteed loss values with zero positive tolerance. In that case the transformer manufacturer must include the tolerance and uncertainty in his internal calculation.

Short-circuit impedance

For transformers with tappings not exceeding +/-5% variation from the principal tapping (which is mostly fulfilled for VFD transformers), the short-circuit impedance is specified at the principal tapping only. The tolerance on short-circuit impedance at the principal tap is:

+/-10% for impedance value less than 10%
+/-7.5% for impedance value equal or larger 10%

In case of a multi-winding transformer the above tolerances apply for the first pair of windings.

Sound level

Transformer sound level is measured in a test setup specified e.g. in IEC 60076-10. The guaranteed maximum sound level is without tolerance. Note that the factory measurement is performed with sinusoidal supply. Converter transformer can emit significantly higher sound level when loaded with non-sinusoidal currents and/or voltages.