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Generators

Generators are synchronous machines that convert mechanical energy into electrical energy. Its main components are the stator and the rotor. The exciter coil that is laid in the groove of the rotors is supplied with a direct current via brushes and sliding rings and thus magnetic poles are created. To induce electrical tension in the stator coils, the rotor must be spun. To connect the generator to a network, certain conditions must be fulfilled, the frequency of the network and the generator must be the same; this is achieved by spinning the rotor in synchrony with the network. The tension of the generator and the grid must be the same and this is achieved by exciting the rotor. The phase angle for both voltages in the three-phase system must also be the same. The synchronisation of the generator with the network occurs when the main circuit breaker connects to the high voltage transmission line.

The generator load is raised by increasing the flow of steam on the turbine. This in turn raises the torque on the turbine shaft and the rotor of the generator starts to overtake the rotating fields of other generators. Because of the rotor’s tendency to overtake other generators, the production of operating energy is increased and the generator takes on a greater load from the network. Because of the increased operating current the necessary nominal exciter flux is reduced, but because the generator must be nominally excited at all times the lacking flux is provided by the generator's own exciter. The strong magnetic field of the rotor harnesses the torque provided by the turbine. Because of the overtaking of the rotor the generator cannot fall out of synch, it can only take over or reduce the load.

Along with the basic monitoring of the generator, it is necessary to take care of many auxiliary areas. Because of the losses in the generator, a great amount of heat is created and it needs to be conveyed away. The electrical generator has up to 2 % of power loss, which reduces the efficiency of the generating unit.

There are different closed systems for generator cooling. Generators one and two are ventilated by air, which then passes the heat to water coolers; this heat is then conveyed to the main cooling system. Generators three, four and five are cooled by hydrogen, which flows into the generator via dedicated ventilation and then transfers the heat to the main cooling system. The cooling efficiency is determined by hydrogen pressure, which is in turn determined by machine size which is the amount of heat generated. Hydrogen cooling results six times more efficient than air-cooling, as hydrogen is a good thermal conductor. We have our own hydrogen production facility that supplies the needs of the generators. The negative aspect of hydrogen cooling is that hydrogen is explosive if mixed with air. To prevent it escaping into the environment each generator has an oil sealing system and hydrogen purity and consumption are also monitored. The generators have fail-safe mechanisms that prevents damage spreading to other equipment or prevents damage altogether.