Negative sequence currents are produced because of the unbalanced currents in the power system. Flow of negative sequence currents in electrical machines (generators and motors) are undesirable as these currents generates high and possibly dangerous temperatures in very short time. This high temperatures can damage the insulation of the machines. Some of the effects of negative sequence currents on generators are motors are discussed below.
Negative Sequence Currents:
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 components will have the sequence component rotation (vector rotation) in the same direction as the power system voltage and current components. Positive sequence currents exist during the balanced load condition. In a generator if the phase currents are equal and the vectors are displaced by 120o(supplying balanced load), only positive sequence components flow in the power system. When any unbalance exist in the system, unbalance exist in voltage and current components both in magnitude and phase angle. Then negative sequence components will flow in the power system. This negative sequence components will have same magnitude as positive sequence components but rotate in opposite direction to the positive sequence components in the power system. The zero sequence components flow during unbalanced condition will cause the current to flow through the neutral of the power system.
Causes and effects of Negative Sequence Components :
There are number of conditions which can cause the flow of three phase unbalanced currents in the generators or alternators. Some of the causes includes:
- 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)
This negative sequence components induce double frequency currents in the surface of the rotor, the slot wedges of the rotor, the retaining rings, and the filed windings of the rotor of the machines. This doubly induced high frequency currents will rise the rotor temperature very high and damages the machine if operates continuously.This is explained below.
Rotor heating in electrical machines:
Unbalanced currents will generate negative sequence components which in turn produces a reverse rotating filed (opposite to the synchronous rotating filed normally induces emf in to the rotor windings) in the air gap between the stator and rotor of the machines. This reverse rotating magnetic filed rotates at synchronous speeds but in opposite direction to the rotor of the machine. With respect to the rotor surface, this reverse rotating magnetic fields induces double frequency currents into the rotor body in the case of cylindrical rotating machines (generators driven by steam turbines and motors )and induce double frequency currents in the pole faces in case of salient pole machines (generators driven by hydro turbines). This resulting induced currents into the rotors will provide high resistance path to the normal induced currents (generated due to synchronous rotating magnetic field) resulting in the rapid heating. This heating effect in turn results in the loss of mechanical integrity or insulation failures in electrical machines within seconds. Therefore it is undeniable to operate the machine during unbalanced condition when negative sequence currents flows in the rotor and has to be protected.
Effects on Induction Motor:
The flow of unbalanced currents in to the induction motor can cause due to:
- Single phasing
- Reversal of phases
- Unbalance in the supply voltage
In case of Induction motors, 5% unbalance exist can cause the reduction in the motor power by 25% even the induction motor continue to get the rated current before unbalancing. This reduced electrical power of the induction motor attributes to heating in the rotor. The unbalance present in the supply voltage by 3% can increase the rotor heating by approximately 20%. This proper protection shall be provided against the unbalanced currents in induction motors.