POWER QUALITY SYSTEM Field of the Invention The present invention relates to a power quality system having two separate turbine units integrated into the system, a first turbine unit with a first turbine shaft and a second turbine unit with a second turbine shaft. The first turbine unit comprises a first compressor stage, a first turbine stage, a first generator, and at least one combustion chamber. The second turbine unit comprises a second turbine stage and a second generator.

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 of gas turbines. In bigger stationary plants, e. g. with an electrical output of more than 500 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. By using

clutches, swash plates or valves, selectable operating modes for the two engine system are achieved.

In some stationary combined heat and power generation plants, a microturbine is used. Such a microturbine is suitable for continuous operation, but has limitations in transient response, i. e. it is slow in reaction and/or control when matching the need of generated power output. A microturbine unit has typically one turbine shaft with one compressor, one turbine, one combustor, and one high speed generator. In many applications, the microturbine unit also comprises a heat exchanger in the form of a recuperator. This is an efficient solution in for example combined heat and power plants with a continuous power demand. However, a microturbine unit has limitations when operating in a power quality application similar to an uninterrupted power supply system (UPS-system). Here, the microturbine unit can not tolerate/withstand fast variations/fluctuations in voltage and/or current due to changes in a net/grid supplied with power from the microturbine unit. This means that the microturbine unit must compensate for these variations in power, voltage and/or current very fast, i. e. from seconds down to milliseconds. A common microturbine unit does not have this- compensation capability. Thus, the power quality from a prior art micro turbine may not fulfil the high demands for a secure and reliable operation of power-operated equipment with a need for high power quality, e. g. computers and power-operated devices in hospitals. Furthermore, the earlier gas turbine systems may also generate or be affected by current peaks/surges during operation. These current peaks/surges may disturb or even harm power- operated equipment or apparatuses. They are especially harmful for equipment, e. g. computers and equipment monitoring patients at hospitals.

A 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 also 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 objects of the present invention are to facilitate the behaviour/response for a gas turbine system when transient conditions occur and eliminate any power, current and/or voltage peaks/surges in the generated power output, enhance the quality of the generated power, and maintain a high electrical quality output from the gas turbine system during operation.

This object is achieved by a power quality system comprising having two separate turbine units, a first turbine unit with a first turbine shaft and a second turbine unit with a second turbine shaft. The first turbine unit comprises a first compressor stage, a first turbine stage, a first generator, and at least one combustion chamber. The second turbine unit comprises a second turbine stage and a second generator. The power quality system also comprises a flywheel rotatably connected to the second turbine shaft. The two separate turbine units are operatively connected by means of at least one energy link.

By providing a power quality system with two separate turbine systems according to the invention, the 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; the weight and overall costs for the gas turbine system are reduced; the stress on the different components in the turbines due to current and/or voltage peaks/surges/changes is reduced; thereby enhancing the quality of the generated power output- from the gas turbine system, and, hence, is more suitable for power quality applications. Furthermore, temperature differences in the gas turbine system during transients are reduced, the life, i. e. useful life of the gas turbine system is prolonged, and a sufficient efficiency for the gas turbine system is maintained.

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 quality system comprising two turbine units on separate shafts according to the invention, and FIG 2 is a schematic diagram illustrating the transient behavior of a net comprising at least one load connected to the power quality system in FIG 1 when the at least one load in the net is changing.

Detailed Description of the Invention FIG 1 is a schematic view of a preferred embodiment of a system 10 for generating power by means of a gas turbine system comprising a first turbine unit 20 and a second turbine unit 40. The first turbine unit 20 has a first shaft 30 and the second turbine unit 40 has a second shaft 0. The two turbine units 20 and 40 are not mechanically connected, i. e. the two shafts 30 and 50 are two separate shafts. The two units 20 and 40 are controlled/driven by the system 10 so that they generate quality power, in accordance with the present invention.

The system 10 generates quality power, i. e. power that fluctuates as little as possible in regard of voltage and/or current. Power that fluctuates in voltage and/or current is a disadvantage, especially, in applications having power-operated equipment that require high-quality power, e. g. computers in voltage and/or current sensitive systems, e. g. in hospitals.

The first turbine unit 20 is shown as the upper system in FIG 1 but could of course be the lower system, as is readily understood by a skilled person. The first turbine unit 20 comprises the first shaft 30, at least one compressor 60, at least one first turbine stage 70, at least a first generator/motor 80, and at least one combustion chamber 90. The first turbine unit 20 also comprises at least one power converter 100, which is known per se. This first power converter 100 is able to convert

AC-power into DC-power and vice versa. The output from the first generator/motor 80 is connected to the first power converter 100, which is a bi-directional, four-quadrant power converter. Optionally, the first turbine unit 20 may comprise at least one heat exchanger in the form of a recuperator (not shown). The first generator/motor 80 may be used both as a generator for delivering power to a load (not shown) during normal operation at full and part load of the power quality system 10, and a motor during the startup and acceleration of the power quality system 10.

The second turbine unit 40 is shown as the lower system in FIG 1 and comprises the second shaft 50, at least one second turbine stage 110, at least a second generator/motor 120, and at least one flywheel 130. The secon Fturbine unit 40 also comprises at least one power converter, i. e. a second power converter 140 known per se, which is able to convert AC-power into DC-power and vice versa.

The output from the second generator/motor 120 in the second turbine unit 40 is coupled to the second power converter 160, which is a bi-directional, four-quadrant power converter. The second generator 120 may also be constructed and used as a generator/motor in the same way as the first generator/motor 80, as is envisaged by a skilled person. 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 combustion chamber 90 of the first turbine unit 20 is placed between the first compressor 60 and the first turbine stage 70. The first compressor supplies the combustion chamber 90 with compressed air. The power quality system 10 also comprises a fuel system (not shown) for which only the supply into the inlet of the combustion chamber 90 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 common knowledge for a skilled person.

The combustion chamber 90 shown in FIG 1 is common for the two separate turbine units 20 and 40, in this preferred embodiment, and delivers the combustion gas to the first turbine stage 70, and, in some cases, to the second turbine stage 110. This will be explained in more detail later in this description. Each of the turbine units 20 and 40 may of course have one or more turbine stages depending on the application. There could also be one combustion chamber 90 for each turbine, whereby more than one combustion chamber would have to be controlled during operation of the power quality system 10. Moreover, an additional combustion chamber (not shown) could be placed after The first combustion chamber 90. This additional combustion chamber could then be used in additional firing or heating for increasing the heat in the combustion gas from the combustion chamber 90.

The two separate turbine units 20 and 40 are connected by at least one conduit 150 leading from the outlet of the combustion chamber 90 of the first turbine unit 20 into the inlet of the second turbine stage 110 of the second turbine unit 40, and at least one conduit 160 leading from the outlet of the second turbine stage 110 of the second turbine unit 40 back into an exhaust gas conduit (not shown) after the outlet of the first turbine stage 70 of the first turbine unit 20. The at least one conduit 160 is provided with at least one control valve 170. This can be placed in the gas passage either before or after the turbine 2 (as shown in fig. l). Moreover, an optional conduit 180 shown with a dashed line leads from the outlet of the first compressor stage 60 into the inlet of the second turbine stage 110. This optional conduit 180 is an air conduit that may be used if the second turbine stage 110 is to be driven by compressed air instead of combustion

gas from the combustion chamber 90. This air drive means that the second turbine stage would have to be designed with a different shape/geometry but would be possible within the scope of the invention. Alternatively, there could be two additional turbine stages 110 with associated means, i. e. one shaft 50 and one generator 120 for each of these two second turbine stages. One of these two turbine stages would then be driven by air and the other one by combustion gas.

Each of the outputs of the power converters 100,140 is connected to a common, first control unit 190. The output of the first control unit is connected to a third power converter 200, similar to the other two power converters 100 and 140. The output of the third power converter 200 is connected to at least one load (not shown). This at least one load may be a group of loads forming a net of power users. The load/loads connected to the output of the power quality system 10 will be called the net from now on. Each of the loads can be any type of load, e. g. computers and/or monitor/surveillance equipment in hospitals. The control unit 190 is adapted to control the transferring of energy, e. g. in the form of electrical energy/power, kinetic energy, and/or thermodynamic energy, between the two turbine units 20 and 40 and all of the power converters 100,140, 200. This control is mainly performed so that the power quality system 10 can deliver or receive energy, in this case, electrical energy/power to and from the net.

The amount of fuel supplied to the power quality system 10 is controlled in relation to power/energy transfer in the system, power output and/or the rotary speed of the turbine stages 70,110. The main operational mode is such that turbine unit 1 provides continous power while turbine unit 2 stores the delivered energy in the flywheel, that can supply transient power. The flywheel can

as well absorbe energy from the net when needed. One key central parameter for valve 170 is to keep the stored energy in the flywheel at prescribed levels 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 of the first turbine stage 70, i. e. after the combustion chamber 90, and/or the inlet temperature of the second turbine stage 110, if another combustion chamber (not shown) for the second turbine stage 110 is used, may be used as an additional parameter. Moreover, the outlet temperature after the first turbine stage 70 and/or the second turbine stage 110 may also be used in combination with one or more of the above- mentioned parameters. The outlet temperature of the turbine stages-may even be used instead of the inlet temperature.

One or more of the above-mentioned parameters may also be used together with load conditions, i. e. the net performance, the quality demand on power, and/or the power output for controlling the supplied fuel, as is envisaged by a skilled person. This will not be explained further because a skilled person considers this common knowledge.

The operating principle for regulating the output power/energy of the two shafts 30,50 driven by the turbine- stages 70,110 by controlling/measuring the inlet temperature and/or the outlet temperature of the combustion chamber 90 is well known in the art and thoroughly described in e. g. US-A-5 332 959. This principle may, as mentioned earlier, also be used for operation of the turbine units 20 and 40 when each turbine stage has at least one combustion chamber 90.

The object of the power quality system 10 shown in FIG 1 is to enhance the flexibility of the power quality system by responding faster to occurring transient conditions, which affect the turbine units 20 and 40 in a negative way. Transient conditions occur when the two

turbine units 20 and 40 are operating and the net, i. e. the load or loads (not shown), rapidly changes its'required supply of power delivered from the power quality system 10.

The net is connected by means of a conduit 210 to the third power converter 200, which in turn is connected to the first control unit 190. This rapid change of power demand occurs for example when the power-operated equipment connected to or constituting the net are switched on or off rapidly. The same effect on power demand occurs when existing loads are rapidly disconnected from the net and/or new loads are rapidly connected to the net. The power- operated equipment being rapidly switched on or off can for example be a lot of devices that do not require a lot of power to operate, or a few devices that require a lot of power to operate, or a combination of such devices. If there are a few devices connected to the net requiring a lot of power to operate, a device that is switched on or off, or connected/disconnected very rapidly would affect the power quality system 10 more than one device requiring little power to operate in a similar situation. This is readily understood by a skilled person.

The flywheel 130 of the second turbine unit 40 is driven/rotated by the second turbine stage 110 in FIG 1.

The flywheel may either be continuously driven/rotated by the second turbine stage or intermittently driven when required. In the latter case, the flywheel is"charged"or "pushed"by rapidly driving/rotating/accelerating the second turbine stage 110 when the rotational speed of the second turbine stage 110 and/or the flywheel is lower than a predefined value. This"pushing"/"charging", i. e. driving is stopped when the rotational speed of the second turbine stage exceeds the predefined value. Then, the second turbine stage starts decelerating again until its rotational speed is below the predefined value and the "pushing"starts again. The power that turbine 2 feeds into

the flywheel is controlled by the valve 170. The second turbine stage 110 can be driven/rotated by compressed air supplied/bleeded from the first compressor stage 60 and delivered through the air conduit 180 if the turbine stage is designed with another shape or geometry, as mentioned earlier. This air driven turbine stage is optional but may of course be used together with the preferred embodiment of a turbine stage driven by combustion gas. Alternatively, or in combination with the bleeded air, combustion gas is supplied from the outlet of the first combustion chamber 90 through the gas conduit 150. The first control valve 170 is an electrically controlled valve and is regulated when bleeding air and/or combustion gas, whereby the control of the power quality system 10 is more flexible. In some cases, especially when using both a turbine stage 110 driven by combustion gas and a turbine stage 110 driven by air, two control valves 170 may have to be used. One control valve 170 in a separate first combustion gas conduit 150 and the other control valve 170 in a separate first air conduit 180 The flywheel can also be driven by the generator/motor. This is e. g. the case when the load on the net decreases fast. In such cases, the power quality system has to absorb excess power which otherwise would have disturbed the critical load on the net. The second turbine stage 110 may also, as a third alternative, be driven/rotated by operating the second generator/motor 120 as a motor.

The flywheel 130 may be fixedly connected to the second turbine stage 110 or connected by means of a mechanical coupling, e. g. in the form of a frictional coupling, i. e. a hydraulical friction coupling, a flexible coupling, a clutch, an electromagnetic coupling, a mechanical toroid. This means that the flywheel optionally can be connected or disconnected to/from the second turbine stage 110 depending on the operation condition for the

power quality system 10, i. e. the required power output to the net. In the preferred embodiment for this invention, the flywheel is fixedly coupled to the second turbine stage 110, i. e. the second turbine shaft 50.

The flywheel 130 must be kept rotating, i. e.

"charged", between a minimum rotary speed and a maximum rotary speed. These rotary speeds are measured and then controlled depending on the operation condition for the power quality system 10 and/or the required power output to the net, as is readily understood by a skilled person. The measurements of the rotary speeds can be done by any suitable means available on the market. The quality on the power output to the net may be controlled in relation to or by the rotary speed of the first turbine stage 70, the second turbine stage 110, the flywheel 130, required power output, the control valve 170, and the voltage and current in associted electrical conduits of the power quality system 10. The power quality is measured by suitable means available on the market so that the quality on the power output fulfills applicable standards, e. g. both international and national UPS-standards, which are common knowledge for a skilled person.

In FIG 1, the first generator 80 of the first turbine unit 20 is operatively connected to the first power converter 100 by means of a first conduit 81. The first power converter 100 is operatively coupled to the first control unit 190 by means of a second conduit 101. The second generator/motor 120 is operatively coupled to the second power converter 140 by means of a third conduit 121.

The second power converter 140 is operatively coupled to the first control unit 190 by means of a fourth conduit 141. The first control unit 190 is operatively connected to the third power converter 200 by means of a fifth conduit 191. The first control unit 190 is also operatively connected to the first control valve 170 by means of a

sixth conduit 192. The third power converter 200 is operatively connected to the net (not shown) by a seventh conduit 210. In this embodiment, each of the conduits 81, 101,121, 141,191, 192, and 210 comprises a suitable number of electrical conduits for transferring power and/or control signals. These electrical conduits available on the market could of course be a combination of electrical conduits and any other type of conduits and associated control means fulfilling the demands, e. g. optical conduits or conduits containing fluids for generating signals to the control unit 190.

The flywheel 130 can deliver and/or absorb power and thereby compensate for variations/changes/fluctuations in power demand from the net. The flywheel is efficiently charged by the first turbine unit 20, or by power from the net.

Moreover, the first control unit 190 is operatively connected to sensors (not shown) for measuring the temperatures in the first and the second turbine units 20 and 40, especially, in the combustion chamber 90 at suitable positions. These positions could be at the outlet and/or the inlet of the combustion chamber, as mentioned earlier. The first control unit is also operatively connected to sensors (not shown) at the flywheel 130 and each of the separate turbine shafts 30 and 50 for detecting and in response controlling their rotary speeds. The sensors used in this invention for detection and control of the power quality system 10 and associated components are available on the market and common knowledge for a skilled person and will hence not be explained in detail.

The function of the power quality system 10 will now be explained with reference to Fig. 1. The second turbine stage 110 and the flywheel 130 together, or separately, constitute a rotating/rotatable mass. This rotating/rotatable mass is used as a energy storage, i. e.

they are kept rotating so that their kinetic energy can be used by the power quality system 10 to"smoothen"/enhancing the quality of the power output. The second turbine stage 110 and the flywheel 130 in this embodiment of the invention are fixedly connected to each other. Instead of a fixed connection, as described earlier, the second turbine 110 and the flywheel 130 may be connected by a coupling (not shown) that could connect and disconnect the flywheel to and from the second turbine stage in a second embodiment. This would mean that, in some cases, only the second turbine stage could be used as a small"flywheel", and in other cases, both the second turbine stage and the flywheel could work together as a large"flywheel". These two cases would be applied depending on the required quality for the power output from the power quality system 10.

The quality of the power output from the power quality system 10 and the behavior of the two separate turbine units 20 and 40 are affected by two mechanisms occurring in the net (not shown). These two mechanisms occur due to rapid changes in the power demand from the net, i. e. transient conditions, as described earlier. This will be explained by using three cases or operation conditions I, II, and III for the power quality system 10, or, more specifically, the first turbine unit 20 and/or the second turbine unit 40.

I: The first turbine unit 20 operates"normally", i. e. delivers a certain power output, i. e. a certain current and voltage, with a certain quality in response of the momentarily not changing power demand from the net.

II: The power demand from the net is rapidly increased, i. e. existing loads in the net require more power or new loads are connected to the net.

III : The power demand from the net is rapidly lowered, i. e. existing loads in the net require less power or are disconnected from the net.

In the first operation condition, I, the power output from the power quality system 10, i. e. only the first turbine unit 20 is considered to operate principally static, i. e. with a"constant"rotary speed, for simplicity reasons in this embodiment. In this first operation condition, I, the second turbine unit 40 is driven/rotated with a constant rotary speed. This is done by bleeding the first compressor stage 60 on air and/or tapping combustion gas from the outlet of the combustion chamber 90. Both mediums may be used when two second turbine stages 110 is used. In this case, as explained earlier, one turbine stage would have a geometry suitable for an air drive and the other turbine stage would have a geometry suitable for a combustion gas drive. Each of the mediums, air or combustion gas, is either continuously or intermittently supplied to the associated second turbine stage 110. By doing this, both the second turbine stage and the flywheel 130 are rotated at an essentially constant rotary speed and act as storage of kinetic energy. During this first operation condition, I, the second generator/motor 120 is not used. This means that the first control unit 190 has disconnected it and the associated second power converter 140 do not supply any power to the first control unit. This is a common way of controlling a generator and is common knowledge for a skilled person.

In the second operation condition II, which is a transient condition, i. e. a fast change in power demand, the net rapidly requires more power, whereby the need of current increases and the voltage decreases/"drops" momentarily/shortly. The voltage"drop/drops"occur because the first turbine unit 20 is slow in response due to the momentum of its rotating masses, i. e. the first compressor

stage 60 and the first turbine stage 70. This means that the first turbine unit 20 is not able to deliver power fast enough to compensate for the"drop"in voltage and needs additional power from the second turbine unit 40. This additional power is provided in that the first control unit <BR> 190 connects, in other words, "turns on"the flywheel 130, the second generator/motor 120 and the associated second power converter 140, so that power stored in the flywheel is supplied to the first control unit. The first control unit 190 then supplies additional power from the flywheel 130 together with the existing power generated by the first turbine unit 20 to the third power converter 200 and the net. In its simplest form, the second turbine unit 40 is connected to the net by means of switches. The first control-unit 190 then controls the first power converter 100 and the second power converter 140 so that they"share" the load of the net. This additional amount of power is calculated in relation to how long the additional power is required for compensating the loss/decrease in voltage. As an example, the flywheel 130 should be able to deliver/generate about 5 to 100 kW during 20 seconds.

In the third operation condition III, also a transient condition, the net rapidly requires less power, whereby the need of current decreases and the voltage increases momentarily/shortly. This means that the first turbine unit 20 delivers too much power, or, more specifically, voltage, momentarily or shortly and needs to direct this additional or superfluous electric energy to the second turbine unit 40, otherwise the voltage in the output terminals of the power quality system 10 would increase rapidly. This means that the superfluous power in the form of electric energy, in a first case, is supplied to the second generator/motor 120 by means of the first power converter 100, the first control unit 190, and the second power converter 140, as will be described below. The

superfluous electric energy or power may be transformed into kinetic and/or thermodynamic energy, in a second case.

This is achieved by tapping compressed air or hot combustion gas from the first compressor stage 60 and/or the first turbine stage 70, respectively, and leading it to the second turbine stage 110. Then, the second turbine stage 110 and the flywheel 130 are accelerated. In the first case, the second generator/motor 120 is driven as a motor rotating the second turbine 110 and the flywheel 130 so that this additional electric energy is used/"stored"/ transformed into kinetic energy.

The second generator/motor 120 in this third operation condition, III, is connected a certain time and then disconnected after having transformed this additional power into kinetic energy and been working principally as a break for the first turbine unit 20. As mentioned above, the connection and disconnection of the second generator/motor 120 may be achieved by using the first control unit 190 and the power converters 100,140, or a mechanical/electromechanical coupling or device controlled by the first control unit. The mechanical coupling could for example be implemented as described in US-A-5 239 830.

The second turbine stage 110 and the flywheel 130 may be kept rotating or rotated essentially in two totally different ways, as explained above. One way is to drive the second generator/motor 120 as a motor, thereby rotating the second turbine stage, which in turn rotates the flywheel 130. The other way is to drive the second turbine stage 110 by tapping/"bleeding"the first compressor stage 60 or the first turbine stage 70 on air or gas, respectively, and leading the air/gas to the second turbine stage. This is done by using the three conduits 150,160, 180, the first control valve 170, and necessary control means (not shown), e. g. valves and/or sensors at the inlets of the first gas

conduit 150 and the first air conduit 180, and at the outlet of the combustion chamber 90 and the second turbine stage 110. These valves are electrically controlled valves, which are regulated in response of rotary speeds for the two turbine stages 70 and 110 and the power demand from the net.

The power quality system 10, i. e. the first turbine unit 20 and the second turbine unit 40 are controlled by measuring the following essential parameters: - The rotary speeds of the turbine stages 70,110 and the flywheel 130.

- The power output (calculated by measured values on voltage and current) Temperatures of the air and/or gas supplied through the conduits 150,160, and 180.

The voltage, current, and, if applicable, phase angle in the associated conduits 81,101, 121, 141,191, 192,210.

The measurements of the above-mentioned parameters may be done by using a suitable number of devices for measurements that are common on the market and common knowledge for a skilled person. Furthermore, the current and voltage have to be measured in each conduit/connection 81,101, 121,141, 191,192, and 210, transformed into suitable signals, and supplied to the first control unit 190. The first control unit controls the power quality system 10 by using these signals and necessary hard and soft ware, this is readily understood by a skilled person.

All of the conduits 81,101, 121,141, 150,160, 180, 191, 192,210 and the means for control, i. e. the first control unit 190 and the power converters 100,140, 200 together with necessary means, may together be defined as energy links or power links. In this embodiment, the energy links transfer energy in the form of electrical, kinetic,

and/or thermodynamic energy between the different components in the power quality system 10.

Any other number and type of power converters 100, 140, and 200 fulfilling the demands of the power quality system 10 may be used if more than two turbine units 20, 40, i. e. more than two shafts 30,50 and sets of turbines 70,110 are to be used. This would make the power quality system 10 more complex comprising more compressor stages 60, turbine stages 70,110 ; power converters 100,140, 200; control units 190; connections/conduits 81,121, 141,150, 160, 180, 191,192, 210, generators/motors 80,120 ; combustion chambers 90, fuel systems (not shown), and control valves 170. This would also mean that the net having more loads (not shown) has to be supplied with more power.-The loads in the net could, e. g. be in the form of accumulators and/or motors or any other type of load, as is readily understood by a skilled man. This would achieve the same function and characteristics as the present invention.

FIG 2 is a schematic diagram illustrating the transient behavior of a net comprising at least one load connected to the power quality system 10 in FIG 1 when the at least one load in the net is changing (solid lines).

Also shown is the behavior of the net when a power quality- system according to the present invention is used (dashed lines). The area between the two lines could be regarded as the energy reservoir the power quality system must be able to handle for a high quality energy supply. In other words, the dashed areas is filled with energy from the flywheel.

The upper diagram shows condition II when the power demand from the load increases very fast as the lower line, and how the voltage output, the upper line, to the net changes in response to increased power demand. The lower diagram shows condition III when the power demand from the load decreases very fast as the lower line, and how the voltage output, upper line, to the net changes in response to the

decreased power demand. Also for the lower figure, the power quality system flattens out the voltage curve (dashed line) by absorbing energy from the net and storing the energy in the flywheel. In equivalence with the upper graph, the dashed area represents the energy that must be stored in the flywheel.

Nomenclature 10 Power Quality system 20 First turbine unit 30 First turbine shaft 40 Second turbine unit 50 Second turbine shaft 60 First compressor stage 70 First turbine stage 80 First generator/motor 81 First electrical conduit 90 First combustion chamber 100 First power converter 101 Second electrical conduit 110 Second turbine stage t20 Second generator/motor 121 Third electrical conduit 130 First flywheel 140 Second power converter 141 Fourth electrical conduit 150 First gas conduit 160 First combustion gas conduit 170 First control valve 180 First air conduit 190 First control unit 191 Fifth electrical conduit 192 Sixth electrical conduit 200 Third power converter 210 Seventh electrical conduit