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 can generate high and possibly dangerous temperatures in a very short period of time which can damage the insulation of the machines causing them to fail prematurely.
The following is a brief overview of some of the effects of negative sequence currents can have on generators and motors.
Negative Sequence Currents
Phase current and voltage in the three-phase system can be represented in the form of three single phase components (Positive, Negative and Zero sequences).
Positive sequence components will have the sequence component rotation (vector rotation) in the same direction as the power systems voltage and current components and exist during a 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 an unbalance exists in the system, it exists in voltage and current components, both in magnitude and phase angle. and cause negative sequence components to flow in the power system. These negative sequence component will have same magnitude as positive sequence components but rotate in opposite direction to them in the power system.
Zero sequence components that flow during an unbalanced condition will cause the current to flow through the neutral of the power system.
Causes and Effects of Negative Sequence Components
There are a number of conditions which can cause the flow of three phase unbalanced currents in generators or alternators. Some of these causes include:
- Unbalanced loads in the system
- Unbalanced system faults (line to ground faults, two phase faults, triple line to ground faults, double line to ground faults)
- Open phases (open circuit faults)
These negative sequence components induce double frequency currents in the surface of the rotor, the slot wedges of the rotor, the retaining rings, and the field windings of the rotor of the machines. These doubly induced high frequency currents will raise the rotor temperature very high and damages the machine if operated continuously.
Rotor Heating in Electrical Machines
Unbalanced currents generate negative sequence components which in turn produces a reverse rotating field (opposite to the synchronous rotating field, which normally induces an EMF into the rotor windings) in the air gap between the stator and rotor of the machines. This reverse rotating magnetic field rotates at synchronous speeds but in opposite direction to the rotor of the machine. With respect to the rotor surface, these 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 current into the rotors will provide a high resistance path to the normal induced currents (generated due to synchronous rotating magnetic field) resulting in rapid heating. This heating effect in turn results in the loss of mechanical integrity and insulation failures in electrical machines within seconds. Therefore, it is not advisable to operate the machine during an unbalanced condition when negative sequence currents flow in the rotor.
Effects on Induction Motor
The flow of unbalanced currents in to the induction motor can be caused due to:
- Single phasing
- Reversal of phases
- Unbalance in the supply voltage
In case of Induction motors, a 5% unbalance can cause the reduction in the motor power by 25% even if the induction motor continues 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%. Thus proper protection should be provided against the unbalanced currents in induction motors.