Turbogenerator

A turbo-generator, also referred to as Vollpolmaschine, is a type of a synchronous generator, which is predominantly driven by high-speed gas or steam turbines. The combination of turbo-generator and the turbine is called a turbo kit.

The main scope of this machine is in the range of medium to large thermal power plants such as coal or nuclear power plants for the generation of electrical energy. More than 75 % of the electrical energy worldwide is generated by turbo-generators.

Construction

The essential distinction between the salient, which also represents a synchronous generator, is the relatively high speed of 3,000 or 1,500 min-1 in networks with 50 Hz or 3,600 or 1,800 min-1 in networks with 60 Hz is to control the resulting centrifugal forces running the rotor as slender long Vollpolrotor and operated in a horizontal position. Currently, the maximum feasible diameter are approximately 1.25 m for 3000 min -1 and 1.15 m for 3600 min -1. On four-pole machines, the possible rotor diameters are significantly larger ( by 2 m for 1500 min -1). The limits are derived from the currently technically feasible maximum peripheral speeds of up to 235 m / s, where the interpretation (in terms of 20% over speed to the rated speed) an overspeed factor of 1.2 is used as a basis.

On the rotor, the grooves are milled to accommodate the excitation winding directly from the solid rotor body ( forging ). The rotor winding is inserted and wedged in said grooves. At the end of a ball cap ring is shrunk each. This avoids flying out of the coming together at the end of copper windings (rotor winding ) by the centrifugal forces. The cap ring is in contrast to the rotor itself from high-alloy steel, and non-magnetic forms of the slot wedges of the surface of the rotor, an electrically conductive cage which is referred to as a damper winding. The damper winding is used to reduce shock loads ( phase swings ) and the heating of the rotor with load imbalance. In the forged from solid steel rotor will not occur alternating fields and hence no eddy currents in symmetrical load. With an unbalanced load occurs but at a rotating magnetic field in the steel core of the rotor, which can lead to eddy currents and excessive overheating and, in extreme cases destroy the turbo generator.

To supply the exciting coil with direct current are in older turbine generators to the fixed axis coupled DC machines, which are referred to in this case as an exciter used. The supply of the direct current to the rotor of the turbo generator must then via brushes and slip rings.

Today, mainly two types of excitation for large turbo-generators usual:

• Brushless excitation with rotating exciter (eg Außenpolmaschine with on or attached to the rotor shaft diodes for rectification). The exciter is fed from the outside with a direct current, which is usually provided by a converter plant.
• Static Excitation: A power converter system supplies the DC current which is transmitted to the rotor winding of the revolving field brushes over a bridge ( slip rings and carbon brushes ). The brushes are easily replaceable in this variant during operation.

Which of the two methods mentioned in each case is used, depends not only on the respective manufacturers philosophy primarily on the requirements of the power plant operator from. Both methods have advantages and disadvantages:

• The rotating exciter is usually low maintenance, but it can be done in an emergency maintenance / repair only when machine is stopped. The to be processed by the associated rectifier system currents are relatively low, but tracking the excitation current in fast changing the operating state is rather slow due to the exciter time constants. To compensate for voltage drops at the generator terminals, the converter system has to provide with respect to the normal operation of very large voltage reserves (so-called ceiling excitation).
• The static converter excitation is overall somewhat more complex in structure, however, can largely be maintained online, ie during operation of the generator. The processed streams meet the required excitation currents and are in large machines in the range of up to 10 kA. Therefore, the static excitation react very quickly to changes in load, the ceiling voltage can be considerably less than in the rotating exciter. This dynamic advantage is today due to the increasing load flow dynamics in networks with many renewable energy producers, more and more into play. In general, network operators have specific minimum requirements for the generator systems of power plant operators regarding dynamics and reliability transient disturbances, which are often not feasible with rotating exciters.

The excitation is very important for the performance of the generator, because of the setting of the excitation current, the amplitude of the terminal voltage, and thus, the reactive power is controlled, which can provide the generator to the network is available ( the active power is determined by the turbine rotational speed or the torque). The excitation power is at turbogenerators about 0.5 % to 3% of the generator power.

Furthermore, Turbo generators, in contrast to the slowly rotating salient, not idling-proof and only allow a small overspeed. With a sudden drop in load ( in the worst case by an unforeseen mains) must be made to avoid mechanical damage immediately to an automatic turbine trip. For the steam turbines have so-called quick-closing valves, which block the full mass flow of steam to the turbines in less than a second and lead on diverter into the condenser. Thus, the turbines can not generate torque. In parallel, the deenergization of the turbo-generator is carried out.

The voltage generator of a turbine generator is in service in the area of 40 MVA at 6.3 kV, in large turbo-generators in excess of 1000 MVA up to 27 kV achieved. The currents are around 10 kA at larger plants. About a generator circuit breaker, the generator voltage is supplied to the conditions laid down in close proximity to the machine shop machine transformer, which they stepped up to the usual high voltage in the mains voltage of 400 kV, for example.

Cooling

Depending on the output size of the turbo-generators of cooling is selected.

• For machines with power ratings up to 300 MW, the cooling of the machine is carried out mainly with fresh air.
• In the power range of 250 MW to 450 MW, the cooling is done mostly by means of hydrogen, its large specific heat capacity than air allows more effective cooling.
• With the most powerful turbo-generators up to 1,800 MW, the cooling is carried out in combination with hydrogen and clean water. For the specific heat dissipation the windings of turbogenerator be performed with waveguides and flows through the respective cooling media.

Turbo-generators include an efficiency of up to 99% of the most efficient energy converters.

Importance for electrical power generation

In 2000, the electric power generation amounted to 55,440 PJ (equivalent to 15,400 TWh). Approximately 64 % came from fossil energy sources ( coal, gas, oil), a further 17 % nuclear power plants. In both areas of thermal power plants are used only turbo-generators to produce electricity.