Short-circuit impedance is a critical parameter of electrical equipment such as power transformers and reactors. It characterizes the impedance characteristics of the equipment under short-circuit conditions and is crucial for equipment operation, protection design, and grid stability analysis.
Core Definition
Short-circuit impedance (also known as short-circuit voltage) refers to the ratio (usually expressed as a percentage) of the voltage applied to the rated voltage when one winding on a device is short-circuited while the other winding is flowing with rated current. It reflects the impedance generated by the internal winding resistance and leakage reactance of the device and is a key indicator of the device’s ability to limit short-circuit current.
Main Functions
1. Limiting Short-Circuit Current: The greater the short-circuit impedance, the greater the impedance generated by the device during a short-circuit, effectively limiting the short-circuit current and reducing the impact on the power grid and equipment.
2. Impact on Voltage Regulation: In a transformer, short-circuit impedance affects the voltage fluctuation under load. The greater the impedance, the higher the voltage regulation.
3. Equipment Selection and Parallel Operation: When operating transformers in parallel, the short-circuit impedance of each device must be within an acceptable range to ensure even load distribution.
4. Protection Design Basis: Short-circuit impedance is a key parameter for calculating short-circuit current and configuring relay protection settings.
Measurement Method
It is typically measured through a short-circuit test: short-circuit one winding on the equipment and slowly apply voltage to the other. When the current on the short-circuit side reaches the rated value, record the applied voltage. The percentage of this voltage to the rated voltage is the short-circuit impedance value.