Transformer Performance Parameters Introduction – Short-Circuit Impedance.
We’ve received quite a few inquiries on our website recently, and we’ve tried our best to answer and resolve everyone’s questions. However, we’ve found that because users are located in different areas, their problems and requirements vary greatly. While we’ve answered questions, we’ve clearly not covered all the requirements. Therefore, today we’ll introduce some transformer parameters to give you more options and allow you to choose the best option when needed in the future. Today, we’ll mainly introduce the transformer’s short-circuit impedance, explaining its meaning and characteristics. If you have any further questions, please contact us.
1. Meaning: Short-circuit impedance is the impedance measured between the two terminals of the other winding when one of a pair of windings is short-circuited.
Short-circuit impedance is expressed as reactance and resistance in series, mathematically expressed as Z = R + jXk. For transformers with tap changers, when the tap range exceeds ±5%, the short-circuit impedance should be measured separately at the main tap and the maximum and minimum taps.
Below, we will introduce the detailed characteristics and provide some examples. Here, we will not distinguish between dry-type and oil-immersed transformers; we will refer to them all as transformers. Only by understanding the characteristics of transformers can we better understand their manufacturing process and make better selections. If you want to learn how to select a transformer, you can read this article: “How to Select a Transformer Based on its Short-Circuit Impedance?”
2. Characteristics: For large power transformers, the proportion of resistance to reactance in the short-circuit impedance is much smaller.
For example: For a 370MVA three-phase transformer, when the short-circuit impedance is 14%, 18%, and 20%, the percentage of resistance to reactance is 1.25%, 1.10%, and 1.05%, respectively; for a 240MVA/500kV single-phase transformer, when the short-circuit impedance is 14%, the percentage of resistance to reactance is 1.24%; for a 500kV single-phase transformer, when the short-circuit impedance is 18%, the percentage of resistance to reactance is 0.93%; for a 390MVA/500kV single-phase transformer, when the short-circuit impedance is 21%, the percentage of resistance to reactance is 0.90%.
Based on the characteristics shown above, we can clearly see that the main component of short-circuit impedance is reactance. Reactance is generated by leakage flux between windings, and is directly proportional to the square of the number of turns, directly proportional to the equivalent area of the leakage flux, and inversely proportional to the equivalent length of the leakage flux.
Therefore, increasing or decreasing short-circuit impedance is generally achieved through the following methods:
(1) Increasing short-circuit impedance involves increasing the number of winding turns. This requires reducing the core cross-sectional area and increasing the height of the core and windings, which also increases the transport height. The transport height of the transformer limits the upper limit of the short-circuit impedance; therefore, the maximum achievable value of the short-circuit impedance is limited.
(2) The most effective method to decrease short-circuit impedance is to increase the core cross-sectional area and increase the turn electromotive force, which reduces the number of winding turns and lowers the winding height. However, increasing the core cross-sectional area not only increases product cost but also may exceed the transport width limit. Therefore, the minimum achievable value of the short-circuit impedance is also limited.
Post time: Dec-11-2025
