A. Supplied with an unbalanced voltage
B. Supplied with an unbalanced load
C. Unbalanced system fault
D. All of the above
Answer: D. All of the above
Explanation:
Negative sequence currents are produced because of the unbalanced currents in the power system.
The flow of negative sequence currents in electrical machines (generators and motors) is undesirable as these currents generate high and possibly dangerous temperatures in a very short time.
Phase current and voltage in the three-phase system can be represented in the form of three single-phase components.
Positive sequence components, Negative sequence components, and Zero sequence components.
Positive sequence currents exist during the balanced load condition.
Causes and effects of Negative Sequence Components :
The main cause of negative phase sequence components are:-
Unbalanced loads in the system
Unbalanced system faults (line to ground faults, two-phase faults, three-line to ground faults, double line to ground faults)
Open phases (open circuit faults).
Effects of Negative Sequence Components
When the load on the generator becomes unbalanced, negative phase sequence currents flow.
The negative sequence components produce a rotating magnetic field that rotates at synchronous speed in a direction opposite to the direction of the rotor field. Hence effectively the relative
Hence effectively the relative speed between the two is double the synchronous speed. Thus double frequency currents are induced in the rotor.
This double-induced high-frequency currents will rise the rotor temperature very high and damages the machine if operates continuously.