What are the consequences of non-compliant short-circuit impedance?

What are the consequences of non-compliant short-circuit impedance?

Short-circuit impedance is a critical technical parameter for a transformer; it is closely related to voltage variation, short-circuit current, and load distribution when transformers operate in parallel. “Non-compliant short-circuit impedance” generally refers to a significant deviation between the measured value and the design value, the value stated on the nameplate, or the specified requirements. It is important to note that the implications differ depending on whether the short-circuit impedance is too high or too low. The standard GB/T 1094.1 also notes that the selection of short-circuit impedance must balance voltage drop considerations with the need to limit overcurrent during system faults; furthermore, impedance parameter matching must be considered when transformers operate in parallel.

1. What are the consequences of short-circuit impedance being too low?
When a transformer’s short-circuit impedance is too low, a short-circuit fault in the system will result in a higher short-circuit current flowing through the transformer. This increased current subjects the transformer windings to greater electrodynamic forces and thermal effects, placing higher demands on the mechanical strength of the windings, the integrity of lead connections, and the short-circuit withstand capability of associated electrical equipment. If the actual short-circuit impedance is significantly lower than the design value, the actual system short-circuit current may deviate from design calculations, potentially compromising the coordination of parameters for equipment such as circuit breakers and protection devices.

2. What are the consequences of short-circuit impedance being too high?
When short-circuit impedance is excessively high, the internal voltage drop—which occurs when the transformer is under load—typically increases. Especially under heavy loads, this can lead to a more pronounced drop in the voltage on the transformer’s secondary side, negatively affecting the quality of the power supply voltage. Therefore, a higher short-circuit impedance does not necessarily equate to greater safety; rather, it should be maintained within a reasonable range as defined by design specifications and standards. In short, a lower short-circuit impedance helps reduce load-related voltage drops but increases short-circuit current; conversely, a higher short-circuit impedance limits short-circuit current but results in a larger voltage drop during operation.

3. Impact on parallel operation of transformers
When two or more transformers operate in parallel, the matching of their short-circuit impedance values ​​is of great importance. If there is a significant discrepancy in the short-circuit impedance among transformers, the load may not be distributed reasonably in proportion to their respective capacities. Generally, a transformer with lower impedance will carry a larger share of the load, while one with higher impedance will carry a smaller share. This can lead to a situation where one transformer approaches or even exceeds its permissible load limit, while another remains underutilized—a scenario that compromises both the economic efficiency and the safety of parallel operation. Consequently, the matching of short-circuit impedance values ​​must be carefully considered when transformers are operated in parallel.

4. Changes in short-circuit impedance may indicate abnormal winding conditions
For transformers already in service, short-circuit impedance testing serves another critical function: it allows for comparisons against factory specifications, commissioning test results, and historical data. When a transformer is subjected to a high-magnitude short-circuit current, its windings may experience substantial axial and radial electromagnetic forces. If these forces cause significant displacement, deformation, or structural alteration of the windings, the leakage magnetic field distribution may shift, resulting in a change in short-circuit impedance.
Therefore, if a measured short-circuit impedance value deviates significantly from historical data, it should not be dismissed merely as a “non-compliant parameter.” Instead, further analysis—incorporating results from winding deformation tests, DC resistance measurements, turns-ratio tests, and other diagnostic procedures—is required.

Conclusion
Both excessively high and excessively low short-circuit impedance can adversely affect transformer operation. Low impedance may lead to higher short-circuit currents, subjecting the windings to greater mechanical and thermal stress, while high impedance can result in an increased voltage drop under load. Furthermore, a mismatch in short-circuit impedance can disrupt proper load distribution when multiple transformers operate in parallel. Thus, short-circuit impedance testing is not only used to verify transformer performance parameters; by comparing current measurements against factory specifications and historical data, it also provides a vital reference for assessing the condition of transformer windings and ensuring operational safety.