SOLUTION

The negative phase sequence in a three-phase synchronous motor exists when the motor is supplied with an unbalanced voltage, supplied with an unbalanced load, unbalanced system fault.

Negative phase sequence current in Synchronous motor

• 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 speed between the two is double the synchronous speed. Thus double frequency currents are induced in the rotor.
• These double-induced high-frequency currents will raise the rotor temperature very high and damages the machine it operates continuously.