Field of the Invention The present invention generally relates to gas turbine power units, and more particularly to a two-shaft gas turbine power system with improved response time and part load characteristics when the required power is changed.

Description of the Prior Art A gas turbine can be used in a stationary combined heat and power generation plant or in a mobile application, e. g. in vehicles used on land, at sea or in the air. The turbines and compressors used in the gas turbine can be of an axial or radial type and one or more. compressor and/or turbine stages, depending on the power and heat requirement, and available space. Different power requirements and heat outputs lead to different sizes and types ofgas turbines. In bigger stationary plants, e. g. with an electrical output of more than 150 kWe, turbine units of an axial type are often used. Smaller plants often have to fulfil not just demands on required output but also limitations in space and noise due to their location, e. g. in hospitals, hotels, small industries and small-scale district heating installations.

US-A 5 239 830 discloses a two engine system with two gas turbines, which are interconnected through a pneumatic, mechanical, hydraulic or power link. This power link runs from the gas producer shaft of one engine to the gas producer shaft of the other engine either delivering or absorbing power directly to or from each other. Through the use of clutches, swash plates or valves, selectable operating modes for the two engine system are achieved.

One problem with gas turbine systems having more than one shaft, e. g. one shaft for a turbine wheel driving a

compressor wheel and another shaft for another turbine wheel driving a generator or the wheels of a car, is that a gearbox and a coupling have to be built in together with the gas turbine system. This increases the weight, cost, and space for the gas turbine systems and their emitted sound. Another problem concerns the maintenance and replacement of these gearboxes and couplings, because their construction makes these operations more difficult with subsequent high costs, due to a complicated maintenance and replacement procedure when maintaining them in an existing system. Moreover, additional equipment, e. g. oil pumps for supplying oil to the gearboxes are necessary further increasing costs and required space for the gas turbine system. Furthermore, these earlier gas turbine systems have a slow response when transient conditions occur. Another problem with the earlier gas turbine systems is high fuel consumption at part load unless some kind of variable geometry is used for suitable parts, e. g. the compressors and/or the turbines, and/or at appropriate positions in the air/gas channels. Such a complicated geometry also increases manufacture and maintenance costs for the gas turbine system.

Summary of the Invention The main object of the present invention is to provide a fast and reliable distribution system for controlling the transfer of power between different operating parts in a two-shaft gas turbine system.

This object is achieved by a power distribution system. The power distribution system comprises a first high-speed shaft having a first compressor, a first turbine and a first generator/motor ; and a second high-speed shaft having a second turbine and a second generator. The power distribution system also comprises a control unit being operatively coupled to the first generator/motor, the second generator and to a load. The control unit is adapted

to control the transfer of power between the first generator/motor, the second generator and the load during operation of the gas turbine system.

By providing a gas turbine system with a power distribution system according to the invention, following advantages are achieved: the part load performance for the gas turbine system is enhanced, thereby reducing the fuel consumption; the performance at transient conditions for the gas turbine system is improved; the manufacture, construction, start-up/normal operation, and maintenance of the gas turbine system are simplified; and the weight and overall costs for the gas turbine system are reduced.

Brief Description of the Drawings The present invention will now be described in further detail, reference being made to the accompanying drawings, in which: FIG 1 is a schematic view of a preferred embodiment of a power distribution system according to the invention showing the different parts of the power distribution system, and FIG 2 is a schematic view of a preferred embodiment of a control unit of the power distribution system in FIG 1.

Detailed Description of the Invention FIG 1 is a schematic view of a preferred embodiment of a distribution system 1 for generating power, by means of a gas turbine system, and, especially, transferring power between the different parts of the gas turbine system, below simply called the system 1, in accordance with the present invention. The system 1 may be used for power applications in stationary combined heat and/or power generation plants as well as in a variety of vehicles, such as ships, aircraft and automobiles.

The system 1 utilizes, preferably, a first high-speed shaft 2 comprising a first compressor 4, a first turbine 5 and a permanent-magnet motor/generator 6. The system also comprises a second high-speed shaft 3 with one or more turbines 9 and a second permanent-magnet generator 10. The first motor/generator 6 is used both as a generator for delivering power to a load 7 during normal operation at full and part load of the system 1 and a motor during the startup and acceleration of the system 1. Thus, as will be described below, the first motor/generator will, preferably, be used for optimizing the performance of the system 1. Furthermore, the second generator 10 may also be used alone or together with the first motor/generator 6 for optimizing the performance of the system.

The system 1 also comprises a combustion chamber 2' placed between the first compressor 4 and the first turbine 5. The first compressor supplies the combustion chamber 2' with compressed air. The system 1 also comprises a fuel system (not shown) for which only the supply into the inlet of the combustion chamber 2'is shown. The function of the combustion chamber supplied by air and fuel is not explained further because the function for such a part in a gas turbine system is a common knowledge for a skilled person. The combustion chamber 2'is common for the two shafts 2 and 3 in this preferred embodiment and delivers the exhaust gas first to the first turbine 5 and then to a second set of turbines 9. The exhaust gas connection between the two sets of turbines 5 and 9 is not shown for clarity reasons. The second set of turbines may be one or more than two turbines and there could also be one combustion chamber 2'for each turbine, whereby more than one combustion chamber would have to be controlled during operation of the system 1. This is readily understood by a skilled person and is therefore not explained further.

The output from the motor/generator 6 is coupled to a bi-directional, four-quadrant power converter 8 known per se, which is able to convert AC-power into DC-power and vice versa. Various embodiments of four-quadrant power converters are thoroughly disclosed in US-A-6 031 294, US- A-5 428 522, and WO/9215148.

The second shaft 3 comprises, preferably, two turbines 9 and the second generator 10, which generator is also used for delivering power to the load 7. The second generator is coupled to a power converter 11 in the same way as the first motor/generator 6. However, the power converter 11 is not necessary of a bi-directional design, but rather of a simpler unidirectional design, such as an active or passive rectifier. This second generator 10 may also be constructed and used as a generator/motor in the same way as the first generator/motor 6, as is envisaged by a skilled person.

The outputs from the first and second power converter 8,11 are coupled to a common DC-bus in a control unit 12, which is further disclosed below. The control unit 12 and the associated DC-bus is also coupled to a third bi- directional power converter 13 for producing AC power to the load 7, which may be a three-phase synchronous motor in e. g. a vehicle. It is, however, understood that the power converter 13 may as well deliver DC-power instead of AC- power, if required by another type of load using DC-power.

A fourth bi-directional power converter 14 is also coupled to the control unit 12, the fourth bi-directional power converter is used to convert the varying voltage of the DC-bus into a regulated DC-voltage. The power converter 14 is coupled to an accumulator 15 for storing energy in case the turbines 5,9 produce more energy than the load 7 consumes. The accumulator 15 is also used for providing power during startup of the turbines 5,9.

Any other number and type of power converters fulfilling the demands of the system 1 may be used if more than two shafts 2,3 and sets of turbines 5,9 are to be used. This makes the system 1 more complex comprising more compressors 4, turbines 5,9 ; power converters 8,11, 13, 14; control units 12,21 ; connections, generators/motors 6, 10; combustion chambers 2', fuel systems, and even more loads 7. The loads could, e. g. be in the form of more accumulators 15 and/or motors, as is readily understood by a skilled man. This would achieve the same function and characteristics as the present invention.

The amount of fuel supplied to the system 1 is controlled in relation to power transfer in the system and/or the speed of the turbines 5,9. In addition, other parameters may be used in various combinations with the above-mentioned parameters for controlling the supplied amount of fuel. The inlet temperature T1 of the first turbine 5, i. e. after the combustion chamber 2', and/or the inlet temperature T1 of the second turbines 9, if another combustion chamber (not shown) for the second turbines is used, may be used as an additional parameter. Moreover, the outlet temperature T2 after the first turbine 5 and/or the second turbines 9 may also be used in combination with one or more of the above-mentioned parameters, the outlet temperature T2 may even be used instead of the inlet temperature T1. One or more of the above-mentioned parameters may also be used together with load conditions for controlling the supplied fuel, as is envisaged by a skilled person. This will be explained further below in the preferred embodiment referring only to one combustion chamber 2'common for both set of turbines 5,9.

Fig 2 illustrates the function of the control unit 12. The common DC-bus 16 is coupled to the first, second, third and fourth power converter 8,11, 13,14 by means of the connections Al, A2, A3, and A4 respectively. The inlet

temperature T1 of the first turbine 5, i. e. the temperature after the combustion chamber 2', is sensed in a manner known per se by means of a thermocouple and is received in the control unit 12 where it is compared with a pre- determined value Tri in the comparison means 17 residing in the control unit 12. An output signal F is generated in dependence on the difference between the inlet temperature T1 and the reference value Trl, which signal will control the amount of fuel supplied to the turbine 5 by actuating the fuel system. The function of the fuel system is well known per se in the art and is not explained further for clarity reasons. However, the fuel system will act to reduce the fuel supply to the turbine 5 when the inlet temperature T1 increases so that the inlet temperature of the turbine 5 will remain at a constant value corresponding to Trl.

At some instances, e. g. at very low power outputs, the temperature drop across the first turbine 5is reduced resulting in an increased value of the outlet (exhaust gas) temperature T2 after the first turbine 5. Therefore, it may be advantageous to reduce the fuel supply in dependence of the outlet temperature T2. The outlet temperature T2 is sensed, in the same way as the inlet temperature T1, by means of a thermocouple and is compared with a reference value Tr2 in the comparison means 17. Hence, the output signal F will, at some instances, also depend on the difference between the exhaust gas temperature T2 and the second reference value Tr2.

The operating principle of regulating the output power of the two shafts 2,3 driven by the turbines 5,9 by controlling the inlet temperature Tl and/or the outlet temperature T2 is well known in the art and is thoroughly described in e. g. US-A-5 332 959. This principle may also, as mentioned earlier, be used for operation of turbines 5, 9 when each turbine has at least one combustion chamber 2'.

The power P1 generated by the first motor/generator 6, when it is used as a generator, as well as the power supplied to it, when it is used as a motor, are sensed by measuring both the current Il floating through a current sensor 18a and the voltage U of the DC-bus by means of a voltage sensor 18b, 18c. Similarly, the power P2 generated by the second generator 10 and the power P3 transferred to/from the load 7 is sensed by means of current sensors 19a, 20a and voltage sensors 19b, 19c, 20b, 20c.

The measured power levels P1, P2, and P3 are received in a main control unit 21 residing in the control unit 12.

Based on the received power values, the main control unit 21 is able to control the transfer of power throughout the system 1. More specifically, by monitoring the three power levels Pl, P2, and P3, the main control unit 21 is able to make the system 1 operate in five different modes, namely: 1. The power from the first motor/generator 6 and the second generator 10 is supplied to the load 7. This is the basic mode of operation. The load from the first E motor/generator may then be controlled in such a way that the operating temperature, e. g. the inlet temperature T1 and/or the outlet temperature T2 for each turbine 5 and/or 9, is kept at a prescribed level. In this operating mode, the main control unit 21 senses that the power required by the load is greater or equal to the power supplied by the first motor/generator 6 and the second generator 10 and keeps the contactors S1, S2, and S3 closed so that both the first motor/generator and the second generator may supply power to the load 7. In this context it is understood that the bi-directional power converters comprises the necessary sensing and controlling means for being able to direct the power in the right direction. Otherwise, the main control unit 21 may comprise additional circuitry and sensing lines coupled to the different power converters.

2. Power from the load 7 is fed back into the first motor/generator 6. This mode will be used when fast decrease in power is needed. The main control unit 21 will open contactor S2 and then affect the output signal F from the comparison means 17 by a selector 22 as to reduce the fuel flow to the turbine 5. The compressor 4 will then act as a brake and consume power from the load. This operating mode is particularly useful in the case when the two shafts 2,3 are used as power units in a vehicle and the load 7 is an electric motor driving a set of wheels.

3. The main control unit opens contactor S1 and consequently no power will be supplied by the first motor/generator 6. This mode will be used when a fast acceleration is needed for the load 7. In this mode, it may as well, in some cases, be useful to take power from the second generator 10 if an even faster acceleration for the load is required.

4. Power is supplied to an energy reserve or storage device, such as the accumulator 15. The accumulator 15 may then operate both to improve acceleration of the shafts 2, 3 by feeding power into the first motor/generator 6 or for deceleration by absorbing power from the first motor/generator 6 and, in some cases, the second generator 10.

5. During extreme part load conditions (idle), the power from the first motor/generator 6 and/or the second generator 10, and/or the load 7 will be used to drive auxiliaries of the system (not shown), e. g. by connecting the auxiliaries to the accumulator 15.

Consequently, throughout the five different operating modes, the control unit 12 is able to transfer power between the different parts of the system 1 described above, whereby both an improved working economy and response time for the system are obtained.

Moreover, this control unit 12 may also be used in a gas turbine power system 1 for operating and controlling more complicated and larger gas turbine plants having more than two turbines 5, 9 with more than two shafts 2,3 as explained earlier. In this case, an appropriate number of power converters, as explained earlier, together with appropriate numbers and types of sensors, sensing lines, contactors, selectors, and control means would of course be required for fulfilling the demands of controlling and operating a more complex power system 1.