The main conditions that generate unbalanced stator currents are:
– One-phase opening of a line or lack of contact of a breaker pole;
– Defect unbalanced near the central and not eliminated by the relays;
– Defect in stator winding.
The reverse component of the unbalanced stator current induces a double frequency current in the rotor. If the degree of imbalance is high, severe overheating in the structural parts of the rotor may be caused. The time during which the rotor can withstand this condition is inversely proportional to the square of the negative sequence current (inverse component). Generally, the manufacturer provides the K-curve of the generator, allowing adjustment of the relay characteristic. Gates Rubber 4L880
When a synchronous generator loses its excitation, it accelerates slightly and operates as an induction generator, that is, it absorbs reactive energy from the network instead of supplying. Even during the short time that this happens, there is a magnetic imbalance in the machine, resulting in overheating.
Therefore, rapid and automatic protective equipment must act on the main and excitation circuit breakers of the generator. It is done indirectly by the overheating protection equipment of the stator, or by the excitation limitation characteristic of the voltage regulator equipment.