The transformer nameplate has a parameter called “impedance voltage,” typically a percentage between 5 and 10. You’ve probably all seen it. The short-circuit impedance has a profound impact on the transformer’s performance, operation, protection, and economics, making it a crucial parameter in transformer design and operation. So, what does this impedance voltage mean, and how is it used?
What Exactly is Impedance Voltage?
Impedance voltage, also known as short-circuit voltage, represents the voltage loss across the transformer’s impedance when the rated current flows through it. Short-circuit the secondary side (usually the low-voltage side) and apply a low voltage at the rated frequency to the primary side. When the rated current flows through the secondary side, the applied voltage on the primary side is commonly expressed as a percentage, such as Uz%. This definition dictates that the percentage of short-circuit voltage is equal to the percentage of short-circuit impedance, so short-circuit voltage is often called impedance voltage.
Calculation: Short-circuit the secondary side of the transformer and gradually apply voltage to the primary side. When the rated current flows through the secondary winding, the percentage ratio of the applied voltage Uz to the rated voltage Un is calculated: Uz% = Uz/Un × 100%
What is the Purpose of Impedance Voltage?
Short-circuit impedance determines the magnitude of the short-circuit current drawn by a transformer during a system short circuit, and thus the magnitude of the electrodynamic forces within the transformer. It also determines the voltage change at the load side of the transformer under varying load conditions, impacting voltage fluctuations during grid operation. Short-circuit impedance is also a necessary condition for parallel operation of transformers.
Methods for Measuring Impedance Voltage
Bridge Method: This is a relatively precise measurement method based on the bridge balance principle to determine the short-circuit impedance value.
Short-circuit the low-voltage side of the transformer.
Then use a bridge to connect the high-voltage side.
Adjust the bridge to achieve balance; the measured value gives the short-circuit impedance.
This method offers high accuracy but is complex and requires skilled personnel.
The Practical Contradiction of Impedance Voltage
From the above, it is clear that impedance voltage is somewhat contradictory in practical applications.
From the power grid operation perspective:
For transformers of the same capacity, lower impedance voltage means lower cost, higher efficiency, cheaper price, smaller voltage drop and voltage variation during operation, and better voltage quality control. Therefore, from the perspective of grid operation, a smaller impedance voltage is preferable.
From the short-circuit current limiting perspective:
However, from the perspective of limiting short-circuit current to protect electrical equipment (such as circuit breakers, isolators, cables, etc.) from damage during operation, a higher impedance voltage is preferable.
Practical application example:
In urban power grids, due to significant load fluctuations, transformers with higher short-circuit impedance are selected to ensure voltage stability. For example, in a city center area, transformers with 7% short-circuit impedance were installed to reduce voltage fluctuations caused by load changes. Compared with transformers with 5% impedance, these transformers provide better voltage regulation under load variation and maintain more stable voltage for users.
To properly balance the demands of normal operation and fault conditions, IEC specifies different impedance voltage requirements for various transformer types. Generally, the higher the voltage rating, the larger the impedance voltage value:
Power transformers at 6~10 kV level: 4% – 5.5%
Power transformers at 35 kV level: 6.5% – 8%
Power transformers at 110 kV level: 8% – 9%
Power transformers at 220 kV level: 12% – 14%
This standardization of impedance voltage helps ensure that transformers of different types and ratings can operate safely and reliably.
