WO2002002920A1

WO2002002920A1 - System and method for gaseous fuel control for a turbogenerator/motor

Info

Publication number: WO2002002920A1
Application number: PCT/US2001/041083
Authority: WO
Inventors: Steven W. Lampe
Original assignee: Capstone Turbine Corporation
Priority date: 2000-06-29
Filing date: 2001-06-20
Publication date: 2002-01-10
Also published as: AU2002216768A1

Abstract

A system and method for closed loop control of a gaseous fuel supply system for a turbogenerator/motor include a variable speed compressor with compressor speed controlled to reduce the error between a desired and actual compressor outlet pressure. In one embodiment, a proportional-integral controller is provided to control the compressor speed. An auxiliary energy storage device, such as a battery, provides black start capability of the fuel system and turbogenerator in the absence of available power from a utility grid. The fuel control system and method preferably include a bypass control to redirect fuel from the outlet of the compressor to the inlet when the compressor capacity exceeds the turbogenerator demand to accommodate minimum compressor speeds and maximum cycle times.

Description

SYSTEM AND METHOD FOR GASEOUS FUEL CONTROL FOR A TURBOGENERATOR/MOTOR

TECHNICAL FIELD The present invention relates to a system and method for controlling a gaseous fuel supply for turbogenerator/motor which includes a variable speed compressor.

BACKGROUND ART

Turbogenerators/motors typically include a permanent magnet motor/generator coupled to a turbine to convert heat energy produced by combustion of a fuel into electrical energy for distribution to a load, such as a utility grid or a stand-alone plant or facility. A compressor, driven by the turbine, provides compressed air which is heated by the exhaust gases of the combustion process in a recuperator (heat exchanger) prior to being combined with the fuel in the combustor. A rotor assembly of the motor/generator includes a plurality of equally spaced magnet poles which are rotatable within a stator which generally includes a plurality of windings and magnetic poles of alternating polarity. In a generator mode, rotation of the rotor causes the permanent magnets to pass by the stator poles and coils and thereby induces an electric current to flow in each of the coils. Alternatively, if an electric current is passed through the stator coils, the energized coils will cause the rotor to rotate and thus the generator will perform as a motor. Thus one of ordinary skill in the art will appreciate that the term turbogenerator as used in this application may also include operation as a motor depending upon the particular application

Turbogenerators may use a wide variety of liquid and gaseous fuels 25 which must be precisely metered to control the combustion process and resulting speed of the turbogenerator. Precise control of fuel flow into the turbogenerator is complicated by the varying properties of available fuels include heat content and contaminants, variation in fuel pressure supplied by the fuel source, location of the turbogenerator relative to the fuel source, and the relatively wide range of pressures required over the operating range of the turbogenerator from start-up to full-load operation. As such, the turbogenerator fuel system typically includes a compressor, integrated and/or external heat exchangers, and a mass flow control valve to more precisely control the pressure, temperature, and mass flow rate of the fuel supplied to the turbogenerator. The compressor motor may require an external power source, typically a three-phase, 240V, 60 Hz source, to provide fuel during start-up and may be subsequently powered by the turbogenerator once it reaches operating speed.

Natural gas is often used as a gaseous fuel for turbogenerator applications due to its widespread availability, relatively low cost, and low emissions associated with complete combustion. However, natural gas supply pressure may vary widely depending upon the location and the supplier. For example, residential areas typically have very low pressurization (around 0.2 psig) to reduce potential hazards associated with a leak or rupture, whereas industrial areas may have higher pressurization (around 20 psi) to accommodate the increased requirements of manufacturing facilities. In addition, various contaminants including water and hydrogen sulfide (present in sour gas applications) must be accommodated by the turbogenerator fuel system to provide reliability and durability.

Some turbogenerator applications have used radial flow compressors (also called helical flow, regenerative flow, and vortex compressors) to supply gaseous fuel to the turbogenerator. While radial flow compressors offer several advantages over reciprocating or centrifugal compressors for smaller turbogenerator applications, they are very low pressure ratio devices which do not scale well to larger turbogenerator applications which require higher pressures and increased flows at full load. Furthermore, radial flow compressors are only about 20% efficient.

DISCLOSURE OF INVENTION The present invention provides systems and methods for controlling a gaseous fuel supply for a turbogenerator to facilitate higher pressure ratio applications with corresponding higher flow requirements at full load. According to the present invention a closed loop turbogenerator fuel control system is provided for a variable speed compressor with the speed of the compressor motor being controlled based on a difference between a desired and measured gaseous fuel pressure. In one embodiment of the present invention, a closed loop control system is used to control motor speed of a scroll compressor by varying the frequency of the supply voltage/current to the compressor motor.

The present invention includes embodiments which utilize an auxiliary energy storage device, such as a battery, to provide black start capability for the fuel 5 system and turbogenerator start-up in the absence of a primary power supply source. In one embodiment which uses a battery bank as the auxiliary energy storage device, the battery bank can provide up to 30 kW for 15 seconds to power the variable speed motor of the gaseous fuel compressor, such as a scroll compressor. The present invention provides valving to reroute the compressor outlet to the inlet when the fuel supply to the turbogenerator is stopped.

The increased efficiency and maximum flows provided by a gaseous fuel supply system according to the present invention can effectively accommodate larger turbogenerators. Furthermore, a single compressor may be used to supply fuel to multiple turbogenerator engines in multi-pack applications. The variable speed compressor motor may be powered by a designated one of the turbogenerators, by a primary power source such as a utility grid, or switched to any one of the multiple turbogenerators to provide redundancy in the event one or more turbogenerators are shut down. Likewise, power for the compressor can be switched from a particular turbogenerator which is being shut down to the primary power- source, when available.

The present invention provides a number of advantages relativeto prior art gaseous fuel systems and methods for turbogenerators. For example, the present invention provides a closed loop pressure control system rather than a flow control system used with a radial flow compressor to accommodate larger turbogenerators. Likewise, use of a closed loop gaseous fuel control system with a variable speed compressor according to the present invention provides a higher compression (pressure) ratio of about 4.4 to 4.6 compared to a compression ratio of only about 3.2 for a radial flow compressor. Operation of a scroll compressor with a variable speed control algorithm according to the present invention eliminates the recirculation of large amounts of compressed gas when the compressor is operated in partial load situations. According to the present invention, the variable speed compressor is operated at the minimum speed required to provide the required engine fuel flow and pressure using a closed loop control system.

The above advantages and other advantages, objects, and features of the present invention, will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

FIGURE 1 is a schematic diagram illustrating a closed-loop control system for a gaseous fuel supply for a turbogenerator according to one embodiment of 10 the present invention;

FIGURE 2 is a simplified control schematic for a closed-loop control system of a gaseous fuel supply for a turbogenerator according to one embodiment of the present invention;

FIGURE 3 is a partial cut-away view illustrating operation of a 15 turbogenerator as represented in block diagram form in FIG. 1 using a gaseous fuel supply system according to the present invention;

FIGURE 4 is a schematic diagram illustrating operation of one embodiment for a gaseous fuel supply system or method using a variable speed compressor according to the present invention;

FIGURE 5 is an electrical block diagram/schematic illustrating connection of a gaseous fuel supply system to a primary power source for driving a variable speed compressor according to one embodiment of the present invention;

FIGURE 6 is an eleQtrical block diagram/schematic illustrating an embodiment utilizing an energy storage device, such as a battery, to provide a black start capability for a DC variable speed compressor according to the present invention;

FIGURE 7 is an electrical block diagram/schematic illustrating an embodiment utilizing an AC connected variable speed compressor according to the present invention; and

FIGURE 8 is a block diagram illustrating a multi-unit redundant turbogenerator arrangement having a gaseous fuel supply system using a variable speed compressor according to the present invention. BESTMODE FORCARRYINGOUTTHE INVENTION

Referring now to Figure 1, a schematic diagram illustrating a closed-loop control system for a gaseous fuel supply for a turbogenerator according to one embodiment of the present invention is shown. Turbogenerator system 20 includes a closed-loop gaseous fuel control system 22 for controlling delivery of a gaseous fuel from a corresponding supply 24 to turbogenerator 26. Supply 24 represents any available gaseous fuel supply appropriate for turbogenerator 26 and may include waste gas, natural gas, or the like, depending upon the particular application. In one preferred embodiment according to the present invention, system 22 provides appropriate pressurization of natural gas from supply 24 which may be a residential or commercial supply, for example.

Fuel from supply 24 passes through valve 28, manual inlet valve 30, and a power operated inlet isolation valve 32. Inlet isolation valve 32 includes an appropriate solenoid 34 in communication with a controller 36 to block flow from supply 24 when the compressor is off. A low pressure switch 38 may include an appropriate interface 40 to communicate with controller 36. This information may be used in controlling gaseous fuel supply system 22. Fuel passes through a check valve 42 and inlet filter 44 prior to entering variable speed compressor 46. In one embodiment, compressor 46 is a scroll compressor as described in greater detail below. Fuel entering variable speed compressor 46 may be combined with compressed fuel from the outlet of compressor 46 by appropriate operation of bypass valve 48. The bypass circuit is used for capacity control and allows 100% bypass capacity with temperature control in one embodiment of the present invention.

Variable speed compressor 46 includes a motor 50 for driving scrolls within high pressure chamber 52. Rotational speed and cycle time of motor 50 is preferably controlled by controller 36 to provide a closed-loop pressure control system according to the present invention. Variable speed compressor 46 may include various sensors to provide control and diagnostic information. For example, a low oil level switch 54 may be provided to detect a low oil level within the lower sump of variable speed compressor 46. Likewise, a high temperature switch 56 may be used to detect overheating of motor 50 or oil within the lower sump. A vacuum switch 58 may be used to detect the shelf vacuum level within compressor 46. Of course various sensors may be added or omitted depending upon the particular type of variable speed compressor, cooling system, fuel supply, etc. while keeping with the teachings of the present invention.

Variable speed compressor 46 preferably includes a heat exchanger or oil cooler 60 for cooling the oil used to cool and lubricate motor 50 and scrolls within high-pressure chamber 52. A cooling fan 62 with an associated motor may be used to provide an appropriate oil cooling capacity and acceptable packaging. Cooling circuit 64 may use any of a variety of known heat exchanger configurations. In one embodiment of the present invention, cooling circuit 64 employs a fluid-to-fluid heat exchanger with glycol used as the cooling fluid.

Fuel compressed by variable speed compressor 46 exits the high-pressure chamber 52 and is monitored by a high pressure switch 68 and pressure gauge 70, one or both of which maybe in communication with controller 36 to provide control and diagnostic information. Compressed fuel then passes through a heat exchanger or gas cooler 72. A cooling fan 74 and associated motor may be used to provide additional cooling capacity. While the oil cooler 60 and gas cooler 72 are shown as separate units with separate fans and motors 62 and 74, respectively, the heat exchangers may be integrated into a single part with shard cooling fans and or motors. If properly located, the cooling fan(s) and motor(s) may also provide the system purge function as described below.

A power operated outlet isolation valve 78 includes an associated solenoid 80 which is also preferably controlled by controller 36 to selectively block stored gas within the compressor when turbogenerator 26 is off. Compressed gaseous fuel passes through a manually controlled outlet valve 82 prior to delivery to turbogenerator 26 for combustion as explained with reference to figure 3. Turbogenerator 26 may include a separate or integrated controller in communication with controller 36 to provide a desired outlet pressure for variable speed compressor 46 based on current operating conditions of turbogenerator 26. Controller 36 implements a closed-loop control system by receiving a signal representing the actual outlet pressure of compressor 46 using an appropriate sensor represented generally by pressure gauge 70, and compares the actual pressure to desired outlet pressure to generate an error signal. The error signal is used to control rotational speed of motor 50 to reduce the steady-state error toward zero, preferably using a proportional-integral controller. However, variable speed compressor 46 typically includes minimum and maximum operating rotational speeds for motor 50 in addition to a maximum duty cycle (or minimum cycle time) which limits the number of starts per hour, for example. The present invention incorporates these factors into the control strategy to provide an appropriate fuel pressure for turbogenerator 26 according to the current operating conditions and operating mode. Excess compressor capacity may be accommodated by control of bypass valve 48 which redirects at least a portion of the compressed gaseous fuel from the outlet port to the inlet port via bypass circuit 66.

A simplified control schematic for a closed-loop control system of a gaseous fuel supply for a turbogenerator according to one embodiment of the present invention is illustrated in Figure 2. A desired fuel pressure 100 is compared to a measured or actual fuel pressure 102 in comparator 103 to generate a corresponding control error signal 105. The control error is operated on by an appropriate controller, represented by proportional-integral (P-I) controller 104 to reduce the error toward zero. As will be appreciated by those of skill in the art, the appropriate gains and delays of control 104 maybe selected and adjusted according to the particular application. The proportional-integral control 104 generates a corresponding speed signal 107 which is limited by the minimum and maximum speeds for the fuel compressor motor as represented by block 106. The resulting signal is then used to control the rotational speed of the compressor motor as represented by 108.

In one embodiment of the present invention, a variable speed drive is used to control a drive motor associated with a scroll compressor to vary the outlet pressure of the scroll compressor to achieve a desired outlet pressure. A throttling process is then used to reduce the outlet pressure to an appropriate pressure and flow demanded by the turbogenerator. The compressor motor speed may be controlled using a commercial variable speed motor drive based on the desired or commanded motor speed determined by the P-I controller. In one embodiment of the present invention, the compressor motor supply voltage/current frequency is adjusted to obtain the desired motor speed.

Figure 3 provides a partial cut-away view illustrating operation of a turbogenerator, as represented in block diagram form in figure 1, using a gaseous fuel supply system according to the present invention. Turbogenerator 126 includes a permanent magnet generator 170 driven by a power head 172. Pressurized gaseous fuel is delivered by the fuel system to injectors 142 and burned in combustor 174 with the exhaust gases driving turbine wheel 176 of turbine 178 which is connected to compressor 180 by bearing rotor (shaft) 182. Compressor 180 is in turn connected to permanent magnet rotor or sleeve 184 via a tie rod 186.

In operation, ambient air is inducted between outer sleeve 188 and 20 permanent magnet generator stator 190 as indicated generally by arrow 192. The inducted air passes through stator cooling fins 194 before reaching impeller 196 of compressor 180. Rotation of impeller 196 produces compressed air which then passes through heat transfer section 198 of recuperator 200 where it is heated by exiting exhaust gasses. A portion of the air then travels toward injectors 142 where it is delivered to combustion zone 202. The pressure of the gaseous fuel must be controlled for proper flow and mixing in combustion chamber 202 and must exceed the pressure of the compressed air generated by compressor 180 in combustion chamber 202. As the turbogenerator is loaded and the turbine speed increases, so too will the pressure of combustion air generated by compressor 180. As such, the gaseous fuel supply system of the present invention controls the speed of the fuel compressor motor to provide an appropriate supply pressure which is passed through a throttling valve prior to entering combustion chamber 202. Hot exhaust gasses from the combustion of the gaseous fuel and air mixture are expanded by turbine wheel 178 and flow between combustor dome 204 and exhaust gas dome 206 prior to passing through heat transfer section 198 and being exhausted through exhaust 208.

Figure 4 provides a more detailed schematic diagram illustrating operation of one embodiment for a gaseous fuel supply system or method using a variable speed compressor accordmg to the present invention. Gaseous fuel enters at 220 and passes tWrough inlet filter 222 and inlet restriction orifice 224 prior to entering the low-pressure portion of variable speed compressor 228. Pressurized hot oil used for cooling and lubrication of compressor 228 passes through oil orifice 252 and into a three-way heat exchanger 254 via oil inlet port 256. Heat exchanger 254 uses a cooling fluid, such as glycol which enters through cooling fluid inlet 258 and absorbs heat from the hot oil before exiting heat exchanger 254 through cooling fluid outlet 260. In addition, the cooling fluid may be used to cool the compressed gas which exits compressor 228 through gas discharge port 262, enters heat exchanger 254 through gas inlet port 264, and exits heat exchanger 254 at a reduced temperature through gas outlet port 266. The compressed and cooled gas passes through a secondary oil separator 268 which returns scavenged oil through an orifice 270 where it is combined with an oil injection inlet 272 into compressor 228. Compressed and cooled gas passing through secondary oil separator 268 is then delivered to the turbogenerator as indicated at 282. When the flow capacity of compressor 228 exceeds the requirements of the turbogenerator, bypass valve 284 is controlled to redirect the compressed gas from the outlet to the inlet where it passes through restriction orifice 224 and is recycled through compressor 228.

As will be appreciated by those of ordinary skill in the art, temperatures, pressures, and flow rates for a gaseous fuel supply system such as illustrated in figure 4 may vary depending upon the particular variable speed compressor, heat exchangers, type of fuel, supply pressure, and various other system parameters. As such, the representative values of parameters described are provided for illustration only based on use of a scroll compressor and may vary from application to application in keeping with the teaching of the present invention. Other compressors, including screw or rotating vane compressors could also be used and may have significantly different operating temperatures, pressures, and flow rates.

For a representative natural gas pressure system using a variable speed scroll compressor, a typical gas supply pressure may range between 15-30 psia with 5 a temperature on the order of 1000F/380C at inlet 220. Hot oil exiting the high pressure chamber at 262 may be about 2000F/930C. After being cooled by heat exchanger 254, the injection oil exits at 272 around 160 OF/71 0C, for example. For this embodiment, the cooling fluid is preferably a glycol which enters heat exchanger 254 as represented by reference numeral 258. The glycol is used to cool the hot oil and compressed gas and exits heat exchanger 254 as indicated at 260. Compressed gas exiting compressor 228 at 262 may have a representative pressure of between about 50 to 80 psig. After being cooled by heat exchanger 254, the gas pressure may be reduced as indicated at 266. Secondary oil separator 268 may induce a pressure drop of about 5 psia as indicated at 282.

An electrical block diagram/schematic illustrating connection of a gaseous fuel supply system according to one embodiment of the present invention is shown in figure 5. A primary power supply, such as a utility grid, may be used to provide power to the turbogenerator fuel control system according to the present invention. In one embodiment, inlet power supply 350 provides a 460 VAC, 3-phase supply to compressor variable speed drive 352, purge/cooling fan inverter 354, and power supply 356. Compressor variable speed drive 352 preferably provides a variable frequency 3-ρhase output to gaseous fuel compressor motor 358 to control the rotational speed of the motor in response to a command/control signal 360 provided by control logic 362 to achieve a desired compressor outlet pressure. Purge/cooling fan invertor 354 includes at least one motor 364 for driving the purge and cooling fan(s). As described above, a single fan and motor may be used for oil cooling, gas cooling, and purge functions in some applications. Alternatively, multiple fans driven by a single motor, or multiple fans with associated motors may be used to provide appropriate cooling and meet system packaging requirements. Likewise, the purge/cooling fan(s) and motor(s) may be driven by a single invertor or multiple inverters. Purge/cooling fan invertor 354 operates in response to a command/control signal 366 provided by control logic 362. hi operation, control logic 362 communicates with the turbogenerator controller to exchange command 368 and status 370 information. Control logic 362 monitors various sensor inputs 372 which may include oil temperature, low inlet pressure, and the like, to generate control commands for compressor variable speed drive 352 and purge/cooling fan invertor 354. According to one embodiment of the present invention, further or improved efficiency can be gained by controlling cooling fan operation (on/oft) and/or speed as a function of oil and gas temperatures. Depending upon the particular design topology, the purge fan may need to be separately controlled such that a shared fan is not desirable. Control logic 362 preferably uses a feedback signal, such as outlet pressure 374, to provide closed loop control by varying the speed of compressor motor 358 through appropriate commands communicated to variable speed drive 352. Control logic 362 also generates output commands/signals 376 to provide fuel flow control by opening and closing inlet, bypass, and outlet solenoid valves described above.

Figure 6 provides a simplified block diagram/schematic illustrating one 15 embodiment utilizing an energy storage device, such as a battery, to provide a black start capability for a gaseous fuel supply system according to the present invention. Turbogenerator system 400 includes a primary supply 402 and a secondary or auxiliary supply 404 which may be used to supply power to gaseous fuel supply system 406 which in turn powers turbogenerator system 408. The present invention provides a number of potential sources for powering the fuel system 406. Any one or more of the representative strategies may be utilized in a particular application depending upon the desired redundancy, black start capability, etc. In a typical application with black start capability, power is normally provided by primary source 402 through an AC/DC converter 420 to a variable speed drive 412. Variable speed drive 412 provides an appropriate control for compressor motor 414 to control the rotational speed and resulting outlet pressure. Primary supply 402 may also be used during startup of turbogenerator system 408 which includes AC/DC invertor 410 to convert the AC control signal to a DC bus voltage signal which is then subsequently converted to an appropriate AC signal to drive the turbogenerator until it reaches operating speed. Upon obtaining operating speed, -the turbogenerator generates sufficient power to supply load 422 in addition to providing power to variable speed drive 412 to drive compressor motor 414.

Energy storage device 404 may include one or more batteries-416-which provide power to system 400 through a DC/DC converter 418 to provide a black start capability in the absence of power from primary supply 402. Energy storage device 404 powers the DC bus 422 to start the turbogenerator and variable speed drive 412 until the turbogenerator has reached an appropriate operating speed where it can provide sufficient power to run compressor motor 414. Energy storage device 404 maybe used to absorb excess energy produced by turboge-nerator 408 and recharge batteries 416 for subsequent use.

Figure 7 is an electrical block diagram/schematic illustrating an embodiment utilizing an AC connected variable speed compressor according to the present invention. Primed reference numerals represent elements with similar structure and function as those illustrated and described with reference to Figure 6. In the embodiment of Figure 7, gaseous fuel supply

according to the present invention. System 450 includes an inlet fuel distribution line 452 to supply gaseous fuel to a plurality of variable speed compressors 454 which include a master compressor 456. One or more of the plurality of compressors 454 provides compressed gaseous fuel via outlet fuel distribution line 458 to a plurality of turbogenerators 460 which include a master turbogenerator 462. In operation, master turbogenerator 462 communicates control information via line 464 to turbogenerators 460 to start and output 480 VAC on power bus 466. In addition, master turbogenerator 462 communicates with master compressor 456 via line 468 to request an aggregate amount of fuel. Master compressor 456 communicates with compressors 454 via line 470 to provide appropriate speed commands for one or more compressors to supply the requested fuel. Once turbogenerators 460 reach desired operating speeds, load contactor 472 is closed to power load 474 via a command/signal received from master turbogenerator 462 via line 476. In this embodiment, turbogenerators 460 equally share the requirements of load 474. Provided that all turbogenerators 460 are not operating at full load to meet the demands of load 474, any single turbogenerator and/or compressor failure will not adversely affect power output of system 450.

Thus, the present invention provides various systems and methods for closed-loop control of a gaseous fuel supply for a turbogenerator having increased efficiency and maximum flows capable of supporting larger turbogenerators and multi-unit applications. In addition, the present invention provides a number of alternatives for powering the fuel supply system including a black start capability when primary power is unavailable.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method of controlling a gaseous fuel supply system for a permanent magnet turbogenerator/motor, the method comprising:

providing a variable speed compressor having an inlet for receiving gaseous fuel from a gaseous fuel supply at an inlet pressure and an outlet for delivering gaseous fuel for the turbogenerator at an outlet pressure;

determining a desired outlet pressure based on current operating conditions of the turbogenerator;

determining a value representative of an actual outlet pressure;

comparing the value representative of the actual outlet pressure with the desired outlet pressure to generate an error; and

controlling speed of the variable speed compressor so that the desired outlet pressure approaches the value indicative of the actual outlet pressure to reduce the error.

2. The method of claim 1 wherein controlling speed of the variable speed compressor comprises varying frequency of voltage and current provided to the variable speed compressor.

3. The method of claim 1 wherein the step of providing comprises providing a variable speed scroll compressor.

4. The method of claim 1 wherein controlling speed of the variable speed compressor comprises controlling compressor motor speed by applying a proportional- integral control to reduce the error between the value representative of the actual outlet pressure and the desired outlet pressure.

5. The method of claim 1 further comprising:

supplying power from at least one of a utility grid and an auxiliary energy storage device to the variable speed compressor.

6. The method of claim 1 further comprising:

providing electrical power from the turbogenerator to the variable speed compressor.

7. The method of claim 1 wherein the fuel system includes an inlet isolation valve to block flow of fuel supplied to the fuel system, the method further comprising:

controlling the inlet isolation valve based on operating conditions of the turbogenerator.

8. The method of claim 1 wherein the fuel system includes an outlet isolation valve, the method further comprising:

controlling the outlet isolation valve to block stored gas within the compressor when the turbogenerator is off.

9. The method of claim 1 wherein the step of controlling comprises controlling speed of the variable speed compressor motor within predetermined minimum and maximum limits.

10. The method of claim 1 wherein the turbogenerator is operated in an idle or start-up mode, the method further comprising:

controlling speed of the variable speed compressor to a predetermined minimum compressor motor speed; and

redirecting at least a portion of compressed gaseous fuel from the outlet of the variable speed compressor to the inlet of the variable speed compressor.

11. A system for controlling a gaseous fuel supply system for a turbogenerator, the system comprising:

a variable speed compressor having a gaseous fuel inlet for receiving gaseous fuel and a gaseous fuel outlet for providing gaseous fuel to a turbogenerator, the compressor having a motor operatively associated therewith to provide a pressure difference between the gaseous fuel inlet and the gaseous fuel outlet; a pressure transducer in communication with the gaseous fuel outlet for providing a signal indicative of actual outlet pressure;

a controller in communication with the variable speed compressor and the pressure transducer, the controller generating signals to control the variable speed compressor based on a difference between the outlet pressure and a desired outlet pressure to reduce the difference.

12. The system of claim 11 further comprising an energy storage device selectively coupleable to the controller and/or the variable speed compressor.

13. The system of claim 12 wherein the energy storage device comprises at least one battery.

14. The system of claim 11 further comprising:

an isolating valve in communication with the controller for selectively blocking fuel flow from a fuel source to the gaseous fuel supply system.

15. The system of claim 11 further comprising:

an isolating valve in communication with the controller for selectively blocking fuel flow from the outlet of the variable speed compressor when the turbogenerator is off to store compressed fuel within the variable speed compressor.

16. The system of claim 11 wherein the variable speed compressor comprises a scroll compressor.

17. The system of claim 11 wherein the controller provides proportional-integral control to the difference between the outlet pressure and the desired outlet pressure.

18. The system of claim 11 further comprising:

a controllable bypass valve in communication with the controller to selectively redirect at least a portion of compressed gaseous fuel from the outlet of the variable speed compressor to the inlet of the variable speed compressor.

19. The system of claim 11 further comprising: a variable speed motor drive in communication with the controller for generating a variable frequency signal to control rotational speed of a variable speed motor operatively couple to drive the compressor in response to a commanded speed based on the difference between the outlet pressure and the desired outlet pressure.

20. A gaseous fuel supply system for controlling pressure and flow of a gaseous fuel to a turbogenerator, the system comprising

a variable speed scroll compressor having an inlet port for receiving gaseous fuel and an outlet port for providing compressed gaseous fuel to the turbogenerator, the scroll compressor including a variable speed motor to increase pressure of the gaseous fuel at the outlet port relative to the inlet port;

an inlet isolating valve coupled to the inlet port for selectively blocking flow of the gaseous fuel a fuel source;

a variable speed drive which generates a signal to control the variable speed motor of the scroll compressor based on a difference between a desired outlet pressure of the compressed gaseous fuel and a measured outlet pressure of the compressed gaseous fuel within predetermined minimum and maximum speed limits; and

a controllable bypass valve for selectively redirecting at least a portion of the compressed gaseous fuel from the outlet port to the inlet port when the capacity of the scroll compressor exceeds current requirements of the turbogenerator.

21. The system of claim 20 further comprising:

an energy storage device selectively coupleable to the variable speed drive for providing black start capability.

22. The system of claim 20 further comprising:

a second variable speed scroll compressor having a variable speed motor with associated variable speed drive to control the variable speed motor; and a master controller in communication with the variable speed drives to generate a motor speed command for each variable speed drive based on a difference between a desired outlet pressure of the compressed gaseous fuel and a measured outlet pressure of the compressed gaseous fuel.

23. The system of claim 20 wherein the variable speed drive is connected to an AC bus powered by at least one turbogenerator.

24. The system of claim 20 wherein the variable speed drive is connected to a DC bus powered by at least one turbogenerator.