A short circuit is an unintended, low-impedance connection between two points in a circuit at different potentials. Extremely high currents flow as a result, which can cause damage within milliseconds. A well-thought-out design of transformers and protective devices is therefore essential. Short-circuit currents are already calculated during the planning phase in order to correctly size components and protection systems.
What happens during a short circuit?
In the event of a short circuit, the impedance of the circuit drops drastically, causing the current to rise to a multiple of the rated current – often 10 to 25 times. Enormous thermal and mechanical forces result. Protective devices such as fuses, circuit breakers, or protection relays must detect the fault within milliseconds and disconnect the faulty section.
Key Characteristics at a Glance
- Short-circuit currents reach 10 to 25 times the rated current
- Protection time of modern circuit breakers typically 40–80 milliseconds
- Mechanical and thermal stresses require robust components
- Transformers are designed for a specific short-circuit withstand capability
- Selectivity concept ensures that only the affected section is disconnected
- Calculation methods established according to IEC 60909
- Symmetrical and asymmetrical short circuits are distinguished
Frequently Asked Questions
How is a transformer protected against short circuits?
The design includes mechanically stable windings, internal support structures where necessary, and associated protective devices such as differential protection, a Buchholz relay, and overcurrent protection. Short-circuit withstand capability is demonstrated in type tests.
What consequences can an undisconnected short circuit have?
Without timely disconnection, fires, equipment explosions, permanent mechanical deformation of windings, and widespread power outages can result. Security of supply is therefore unthinkable without a consistent protection concept.
How is a transformer’s impedance voltage determined?
The impedance voltage (uk) is measured in a short-circuit test: the secondary side is short-circuited and the primary voltage is increased until the rated current flows. The ratio of this voltage to the rated voltage gives uk as a percentage – a key parameter for parallel operation and short-circuit behavior.
What measures reduce short-circuit risk?
Correct insulation coordination, suitable protection concepts, regular maintenance, thermographic inspections, avoidance of overloading, cleanliness of the equipment, and staff training all significantly reduce short-circuit risk. The use of current-limiting components can also be beneficial.
Would you like to review your protection concept or have transformers designed with particularly high short-circuit withstand capability? Talk to our engineers – we are here to help.