The correct answer is: A. Generators
Generators are the only devices in the list that can produce negative sequence currents. Negative sequence currents are caused by unbalanced loads, and can cause damage to generators if not properly protected.
Motors, transmission lines, and transformers can all be damaged by negative sequence currents, but they do not produce them.
Negative sequence currents are currents that flow in the opposite direction to the normal load current. They are caused by unbalanced loads, which can occur when there is a difference in the power factor between the phases of a three-phase system.
Negative sequence currents can cause a number of problems, including:
- Overheating of the generator
- Loss of synchronism
- Damage to the generator windings
To protect generators from negative sequence currents, a negative sequence current relay is used. This relay detects the presence of negative sequence currents and trips the generator offline if the current exceeds a certain threshold.
In conclusion, the correct answer is: A. Generators.