The short-circuit impedance of the transformer is also called the impedance voltage. It is defined in the transformer industry as follows: When the secondary winding of a transformer is short-circuited (steady state), the voltage applied when the primary winding flows through the rated current is called the impedance voltage Uz. Usually expressed as a percentage of the rated voltage, that is, uz=(Uz/U1e)×100%. When the transformer is running at full load, the level of the short-circuit impedance has a certain impact on the level of the secondary side output voltage. Small short-circuit impedance means less voltage drop; large short-circuit impedance means more voltage drop. When the short-circuit impedance is zero, the transformer is equivalent to being put into operation with load. This is only possible when the system is particularly small and the short-circuit impedance is relatively large. For common distribution transformers, it is more appropriate for the short-circuit impedance to be within the rated voltage transformation ratio range of 5%-10%.
If the short-circuit impedance of the transformer is selected to be small, when the system is short-circuited, the short-circuit current will be large, and the protection device may operate because it exceeds its rated setting value, cutting off the transformer, making it difficult to restore power supply to the distribution network. Therefore, it is very important to select the appropriate short-circuit impedance of the transformer so that it can not only reduce the short-circuit current but also meet the user's requirements for power supply quality.
Definition of impedance voltage
Transformers, I believe many of my friends are familiar with them. There is a parameter on the transformer nameplate called "impedance voltage", which is usually a percentage between 5-10. I guess everyone has seen it. The transformer below is also called "short-circuit impedance". The current standard name is also short-circuit impedance, but I still prefer to call it impedance voltage.
Impedance voltage is a transformer parameter expressed as a percentage. It short-circuits the secondary winding of the transformer, causing the voltage of the primary winding to slowly increase. When the short-circuit current of the secondary winding reaches the rated current, the voltage applied to the primary winding ( short circuit voltage) to the rated voltage. Expressed as a formula:
Uk%=short circuit voltage/rated voltage*100%.
This is a simplified equivalent circuit of the transformer. Rs is the resistance inside the transformer, Xs is the reactance inside the transformer, and Zs is used in the figure to represent the impedance of the entire transformer. Z\\\\'L is the equivalent impedance transformed from the load side impedance to the primary side.
According to the meaning of impedance voltage, short-circuit the secondary winding, that is, Z\\\\'L is equal to 0, and then slowly pressurize the primary winding until the current on the secondary side is equal to the voltage when the current on the secondary side of the transformer is rated. value is the short-circuit voltage value we need. Then it is easy to know the impedance voltage value according to the previous calculation.
