In power systems, a "transformer short-circuit impedance" refers to the total impedance seen by a transformer during a short-circuit fault. This impedance includes the internal impedance of the transformer windings as well as the external impedance of the connected system.
The transformer short-circuit impedance has a significant impact on system protection, particularly on protective device coordination and fault current levels during a short-circuit event. Here's how it affects system protection:
Fault Current Magnitude: The short-circuit impedance of the transformer affects the magnitude of fault currents that flow through the system during a fault. Lower short-circuit impedance results in higher fault currents, while higher impedance leads to lower fault currents. This is crucial in determining the setting of protective devices like fuses, circuit breakers, and relays.
Protective Device Coordination: Protective devices throughout the power system are designed to operate in a coordinated manner. This coordination ensures that only the device closest to the fault operates, isolating the faulted section while keeping the rest of the system operational. The transformer's short-circuit impedance plays a role in determining the protection relay settings and coordination time delays.
Transformer Protection: The short-circuit impedance of the transformer affects the behavior of its own internal protective devices, such as differential relays and overcurrent relays. The impedance determines the level at which these protection devices are set to operate during a fault. Proper coordination of these protection devices is crucial to prevent unnecessary tripping or damage to the transformer.
Voltage Regulation: The short-circuit impedance of the transformer impacts voltage regulation during a short-circuit event. High short-circuit impedance can lead to significant voltage drops across the transformer, affecting the voltage levels on the secondary side.
Transient Response: During a short-circuit fault, the transformer experiences transient conditions, and the short-circuit impedance affects the magnitude and duration of these transients. Understanding these transients is important for designing protective systems that can handle such events without causing damage to equipment.
It's essential for power system engineers to consider the transformer short-circuit impedance while designing protection schemes, as it directly impacts the behavior of protective devices and system performance during fault conditions. Detailed modeling and analysis of the transformer and the entire system are necessary to ensure proper protection and reliable operation.