Transformer short circuit impedance
Short-circuit impedance of a transformer, impedance voltage or just impedance are terms often used as synonyms. They describe one of the most important transformer parameter: short-circuit impedance.
This article, as part of our VFD transformers series [1], presents the short-circuit impedance from VFD design perspective. After reading this post you shall understand why is the short-circuit impedance of a transformer is such an important characteristic. Finding the most suitable value or a range is an optimization process considering protection, performance and economical aspects among other factors.
Definition of transformer short circuit impedance
According IEC 60076-1 [2] the transformer short-circuit impedance is defined as equivalent series impedance Z = R + jX, in Ohms, at rated frequency and reference temperature, across the terminals of one winding of a pair, when the terminals of the other winding are short-circuited and further windings, if existing, are open circuited. For a three-phase transformer the impedance is expressed as phase impedance (equivalent star connection).
Common definition of a shot-circuit impedance is that it is a voltage at rated frequency applied to one winding (e.g. primary) to cause a rated current to flow when the other winding (e.g. secondary) is shorted.
In multi-winding transformers we differentiate short-circuit impedance when one of the secondary windings is shorted and when multiple (all) secondary windings are shorted. Note different reference power is such case.
Percentage or per unit value of impedance
Short-circuit impedance is often expressed in a relative, dimensionless form, as fraction of a reference impedance. Usually the base impedance is used as a reference, calculated from a nominal voltage and nominal current: Zb = Un/sqrt(3)/In.
zk % = Zk (Ω) / Zb (Ω)
The percentage value allows easy comparison and calculations. It might be difficult to assess whether e.g. 2.6 Ω is high or low impedance for a particular transformer XY. Instead, if the short-circuit impedance is expressed in %, e.g. 8%, it is easier to interpret the value. Obvious advantage is that percentage value is the same whether the primary winding is shorted and secondary winding supplied from a source or vice versa. In contrast, the absolute Ohm value transforms with the square of the transformer voltage ratio.
Purpose of short circuit impedance
Short-circuit impedance is an inherent property of a transformer. Each magnetic circuit has, apart from main flux, certain stray flux and this flux (in percentage of total flux) is approximately proportional to transformer short-circuit impedance.
The value of short circuit impedance can be influenced by the design, particularly the core construction and overall geometry of the transformer.
Probably most important feature of short-circuit impedance is the current limiting function. The fault currents downstream of the transformer under consideration are limited by the short-circuit impedance. The higher the value of short-circuit impedance the lower the fault current.
In VFD applications the short-circuit impedance affects many more characteristics than just the level of fault current. Such considerations are discussed in the next paragraphs.
Design value of transformer short circuit impedance
Why is the impedance value so important for medium voltage variable frequency drives? Transformer short-circuit impedance influences several system topics, for instance:
– Mechanical design
– Protection
– Grid harmonics
– Voltage drop
– Input power factor
– Noise level
– Hot spots
Therefore, the design value is a compromise of several aspects. Sometimes the value is more limited (narrow range) while in other cases the requirements are more relaxed allowing the manufacturer to choose the most economical design.
Short circuit impedance considerations
To achieve a good performance of a drive system the transformer short-circuit impedance shall be in a specified range. Such range is usually a compromise of different requirements, being partly or completely in contradiction.
1. Mechanical design
Short-circuit impedance of a transformer is important for the mechanical design and overall structural integrity. Transformer shall withstand short-circuit events without mechanical damages. The dynamic forces are proportional to the square of the faults currents. Therefore, short-circuit impedance shall not be too low. IEC 60076-5 [3] (Table 1 in 2006 issue) provides guidance on minimum recommended impedance values depending on rated power of a transformer.
2. Protection
Medium voltage VFDs preferably have fuseless protection [4]. The transformer short circuit impedance limits the fault current to a level that the VFD can withstand until e.g. the upstream main circuit breaker opens. Therefore, the higher the short circuit impedance the better for VFD protection.
For protection of transformer itself the situation looks slightly different. Especially in multi-winding transformer designs, fault in just one secondary winding may lead to rather small overcurrent seen from the primary side. The higher the short circuit impedance the lower the fault current and the bigger challenge to detect it.
3. Grid harmonics
Transformer short-circuit impedance contributes to reduction of harmonic magnitudes. Generally the higher the impedance the less distortion of grid voltage and the lower current harmonic magnitudes [5, 6]. The effect is exactly the same as for instance input (decoupling) choke frequently used in low voltage VFDs.
4. Voltage drop
From voltage drop perspective it is desirable to keep the short-circuit impedance low. This is especially valid for diode front end converters [6] that cannot actively compensate the voltage drop depending on the load. Higher impedance means larger variation of dc voltage between no-load and full load (or even overload) condition. That complicates the VFD design.
5. Input power factor
This is again a topic for diode front end converters. The larger the short circuit impedance the lower the input power factor. To be precise: the input power factor depends on the sum of grid and transformer impedance. However, the transformer short-circuit impedance is normally significantly higher than the grid impedance having major influence on the power factor [7].
6. Noise level
High value of short-circuit impedance might affect noise level of transformer under load condition [8]. While the impact is negligible in most cases, for special designs with very low noise it can play a certain role. (As explained above, higher impedance means larger stray flux).
7. Hot spots
Hot spots are locations in the transformer that tend to reach significantly higher temperature than the other parts. High transformer short-circuit impedance might increase the risk of hot spots due to losses induced by stray flux (core structure incl. joints and clamps, tank etc).
8. Economic aspects
Each transformer design has certain ‘natural impedance’ leading to the most economical design. Specifying a value far from the optimum range obviously leads to increased cost.
The percentage value of ‘natural impedance’ typically increases as the transformer rated power goes up.
Measurement of transformer short-circuit impedance
Being a guaranteed parameter, short-circuit impedance of a transformer is measured during factory tests. it is part of routine tests, i.e. the test is performed on each transformer and the impedance value is stated on the rating plate.
Summary
As you can read in this article, the short-circuit impedance of a transformer affects multiple drive system characteristics ranging from mechanical challenges and protection over network friendliness (harmonics, power factor) and performance (voltage drop, sound level etc).
Finding the optimum range of transformer short-circuit impedance is a task of drive system design.
References
[1] VFD transfomers – entire series, https://mb-drive-services.com/category/vfd_transformers/
[2] IEC 60076-1: Power transformers – Part 1: General, https://www.iec.ch/
[3] IEC 60076-5: Power transformers – Part 5: Ability to withstand short circuit, https://www.iec.ch/
[4] How to choose a medium voltage VFD: Protection concept, https://mb-drive-services.com/choosing-mv-vfd-protection/
[5] Network harmonics: Harmonic mitigation methods, https://mb-drive-services.com/harmonic_mitigation_methods/
[6] How to choose a medium voltage VFD: Line side connection and power quality, https://mb-drive-services.com/how-to-choose-mv-vfd-line-conn/
[7] Input power factor and reactive power consumption, https://mb-drive-services.com/energy-efficiency-part-6-power-factor-and-reactive-power-consumption/
[8] Acoustic noise level, https://mb-drive-services.com/acoustic-noise-level/
