Electricity: single generator systems – Automatic control of generator or driving means – Speed or frequency of generator
Patent
1995-12-12
1997-12-02
Stephan, Steven L.
Electricity: single generator systems
Automatic control of generator or driving means
Speed or frequency of generator
322 11, 322 59, 290 27, 290 47, 307153, H02P 942
Patent
active
056940262
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to electricity generator sets each comprising a turbine and a generator, e.g. a gas turbine driving an alternator.
BACKGROUND OF THE INVENTION
Gas turbines providing low or medium power (a few MW to a few tens of MW) are efficient when they rotate at high speeds (e.g. in the range 5,000 revolutions per minute (r.p.m.) to 15,000 r.p.m.). To deliver electricity at a mains frequency of 50 Hz, an alternator having 2 or 4 poles must rotate at a speed of 3,000 r.p.m. or 1,500 r.p.m. To obtain a frequency of 60 Hz with an alternator having 2 or 4 poles, the alternator must rotate at a speed of 3,600 r.p.m. or 1,800 r.p.m. Therefore, a step-down gear box must be provided between the gas turbine and the alternator. At such gas turbine powers, step-down gear boxes can be implemented industrially at acceptable costs and with good efficiency.
With higher power gas turbines (greater than 60 MW), such step-down gear boxes become very bulky and their cost is prohibitive. At very high powers, step-down gear boxes become impossible to implement, and the turbine then drives the alternator directly.
That is why, in order to use a 2-pole alternator supplying electricity directly at mains frequency, it is necessary to limit the speed of high-power gas turbines to 3,000 r.p.m. for 50 Hz, and to 3,600 r.p.m. for 60 Hz.
This reduces the overall efficiency of the gas turbine and alternator set.
It is also known that a gas turbine cannot start on its own because the air in its compressor portion must be compressed before fuel is injected into its combustion portion. Starting is obtained by means of an auxiliary electric motor or diesel engine, or by means of a starter static converter powered from the AC grid.
Turbine generator sets are known that are coupled together without a step-down gear box being provided, such sets being designed to deliver electricity to an AC electricity grid of given frequency. A static frequency converter is connected in series between the generator and the grid, which converter converts the voltage and the current delivered by the generator at a frequency that is different from the frequency of the grid into magnitudes corresponding to the frequency of the grid.
Such sets are described in Documents JP-A-58 212 367, JP-A-62 131 798, and JP-A-56 062 100. Such sets make it possible to mitigate the limit on the speed of high-power gas turbines.
SUMMARY OF THE INVENTION
An object of the present invention is to reduce the overall size of such above-described turbine generator sets by providing a turbine generator set comprising a turbine and a generator coupled to each other without a step-down gear box, and a static frequency converter connected in series between said generator and an AC electricity grid having a given operating frequency, said static frequency converter operating while the turbine generator set is generating electricity to convert the frequency of the voltage and of the current delivered by the generator into the given operating frequency of the AC electricity grid. According to the invention, while the turbine generator set is being started, said static frequency converter powers the generator which operates as a motor so as to ignite the combustion chambers of the turbine, by taking power from the AC electricity grid.
According to the invention, with the generator being an alternator including an inductor, and while the turbine generator set is generating electricity, said static frequency converter acts on said inductor so as to excite and/or regulate said alternator.
In an embodiment of the invention, the static frequency converter comprises two thyristor bridges controlled by measurement transformers for measuring the voltage and the current delivered by the generator.
Advantageously, the static frequency converter makes it possible to adjust the speed of rotation of the turbine generator set as a function of the load delivered to the AC electricity grid.
The generator may equally well be an alternator or an asynchronous gener
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European Gas Turbines SA
Ponomarenko Nicholas
Stephan Steven L.
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