Prime-mover dynamo plants – Turbogenerators
Reexamination Certificate
1999-11-24
2001-09-04
Ponomarenko, Nicholas (Department: 2834)
Prime-mover dynamo plants
Turbogenerators
C322S010000
Reexamination Certificate
active
06285089
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to the power generation industry and, more particularly, to the field of starting turbine generators.
BACKGROUND OF THE INVENTION
For small generators in the power generation industry, particularly those that use air cooling, excitation requirements are relatively small, e.g., generally under 1 Megawatt of excitation power, and brushless excitation provides a cheaper and simpler means of providing excitation for a steam turbine generator. A brushless exciter conventionally includes a direct current (“DC”) field winding, a main revolving armature alternating current (“AC”) exciter, and a diode or rectifier wheel. The field of a rotating permanent magnet generator (“PMG”), rather than an electromagnet, is often used for the primary excitation. The pilot exciter AC output is rectified and DC power is provided to the brushless exciter field winding. The pilot exciter thereby eliminates the need for a continual external energy source. The brushless system eliminates the need for brushes and current collection components.
An example of such a shaft-driven, brushless excitation system
10
is shown in
FIG. 1
where a combustion turbine
12
is connected to a combustion turbine generator (“CTG”)
15
along a common shaft
11
. A starting package
14
is conventionally used in such a system
10
and is also connected by the common shaft
11
to the brushless excitation system
10
. A clutch
13
or torque converter is normally connected to the shaft
11
as illustrated as well and as understood by those skilled in the art. A CTG
15
, however, does not have the capability to start itself so it requires some external means of starting or a starting package such as either a motor or a static starting system.
Static exciter starting systems, such as shown in
FIG. 2
, require that the generator
15
′ have field excitation. The static excitation systems can also be quite expensive. In other words, the prior art system as shown in
FIG. 2
includes a static start
14
′ and a static excitation system
10
′ which has brushes or brush gears connected to the generator
15
′ along the common shaft
11
′ which also connects to the turbine
12
′. A common reason given for not employing brushless excitation
10
in CTGs is that brushless excitation systems
10
are not compatible with static start
14
′. This incompatibility is primarily due to the fact that the stationary field used by the existing design of brushless exciters in turbine-generators is a direct current (“DC”) field. A DC field induces no voltage in a stationary armature so the generator
15
receives no field current at zero speed and cannot be started as a synchronous motor as required in a static starting system.
Static starting also requires that excitation be provided to the generator
15
′ at all speeds from zero to synchronous speed. At zero speed, a DC field voltage generates no voltage in a rotation armature so there is no field current supplied to the generator. Accordingly, the use of DC brushless excitation with static start has previously been confined to steam turbine-generators and motor start CTGs.
SUMMARY OF THE INVENTION
In view of the foregoing, the present invention advantageously provides an alternating current (AC) induction exciter and associated methods which has induction static start for a turbine generator. The present invention also provides an AC induction exciter and associated methods which advantageously provides the use of a stationary alternating current (“AC”) field in lieu of a DC field in order to permit field generation at low speed or standstill so that brushless excitation can be used with static start. The present invention additionally advantageously provides an AC induction exciter and associated methods which more effectively control a power generator. The present invention further advantageously provides an electric power generation system which includes a combustion turbine generator and associated methods which allow a substantially lower cost excitation system to be used to start the combustion turbine generator.
More particularly, the present invention provides an electric power generation system which preferably includes a turbine, a turbine generator connected to the turbine along a common shaft, and an AC induction exciter connected to the turbine generator for starting the turbine generator. The AC induction exciter preferably includes an alternating current (AC) input, e.g., a three-phase AC input, an exciter rotor, and stationary alternating current output providing means responsive to the AC input and positioned in electrical communication with the exciter rotor for providing an alternating current output to the exciter rotor so that a voltage is generated regardless of the speed of the exciter rotor.
The present invention also provides a method for starting a turbine generator. The method preferably includes providing an alternating current (AC) input and providing a stationary alternating current to the exciter rotor so that a voltage is generated regardless of the speed of the exciter rotor. The step of providing a stationary alternating current can advantageously include rectifying the alternating current to a direct current (DC) and converting the direct current input to an alternating current output.
The electric power generation system, AC induction exciter, and associated methods of the present invention advantageously each provides a way of overcoming the objection of no field generation from conventional brushless exciters at standstill and low speed operation. By employing a stationary AC field winding in lieu of a stationary DC field winding in a static start brushless exciter, costs can be reduced by only requiring a few design changes to existing conventional brushless exciter designs. In essence, enough AC excitation can be applied to a field winding to induce a no-load field current in an AC armature. In addition to reducing the costs of a starting system for a turbine generator such as a combustion turbine generator (CTG), this design can also provide greater flexibility in the design of the rotating armature of the exciter as well, e.g., reducing the number of armature windings required.
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Ponomarenko Nicholas
Siemens Westinghouse Power Corporation
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