Gaseous fuel compression and control system for gas turbine engine

A gaseous fuel compression and control system for gas turbine engine (30) comprises a screw compressor (10) driven by an electric motor (11) for feeding gaseous fuel to the gas turbine engine, and a modulating device (12) for modulating the rotational speed of the motor by controlling the frequency of electric power to the motor in response to sensed output power of the gas turbine engine.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a gaseous fuel compression and control system for 
gas turbine engine. 
2. Description of the Prior Art 
A conventional gaseous fuel compression and control system for gas turbine 
engine is disclosed in, for example, U.S. Pat. No. 4,536,126 or U.S. Pat. 
No. 5,103,629. This conventional gaseous fuel control system includes a 
source of compressed gaseous fuel and many control valves which are 
interposed between the source of compressed gaseous fuel and a combustion 
chamber of a gas turbine and which control gas fuel flow to the combustion 
chamber in response to computer-generated control signals (for starting, 
for controlling engine speed and output power and so on). In general, the 
source of compressed gaseous fuel has always included a compressor of the 
reciprocating type. These traditionally specialised compressors are 
employed for duties which require the raising of the pressure of a 
flammable gas such as natural gas for combustion in the gas turbine. 
Reciprocating compressors normally operate at fixed speed and displacement. 
Therefore, in order to regulate fuel flow to a gas turbine to control the 
gas turbine output power and speed, a complex control system using the 
above mentioned control valves is required. Furthermore, a high pressure 
accumulator which is maintained between high and low pressure limits is 
also needed for controlling the fuel flow. 
There are many mechanical connections in the construction of a 
reciprocating compressor. There are also many connections in the inlet and 
outlet pipework and associated components of the above control systems. 
The integrity of these connections is threatened by the intense vibrations 
associated with reciprocating compressors. Therefore, the possibility of 
gas leaks occurring is relatively high. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved gaseous 
fuel compression and control system for a gas turbine engine which 
overcomes the above drawback. 
It is another object of the present invention to provide an improved 
gaseous fuel compression and control system for a gas turbine engine which 
can simplify the control of fuel flow to a gas turbine and which can 
improve the safety of the system. 
In order to achieve these objects, there is provided a gaseous fuel 
compression control system for a gas turbine engine comprising: 
a screw compressor driven by an electric motor for compressing gaseous fuel 
for supply to the gas turbine engine, a power sensor for sensing the 
output power of the gas turbine engine, and modulating means for 
controlling the frequency of electrical power supplied to the electric 
motor, so as to control speed of the motor, in response to a control 
signal derived directly or indirectly from the power sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A gaseous fuel compression and control system for a gas turbine engine in 
accordance with preferred embodiments of the present invention will be 
described with reference to the attached drawings. 
Referring first to FIG. 1, hydrocarbon gas, usually natural gas at low 
pressure, enters the system from a mains supply 1. Items 2 to 7 are 
required by British Gas in this embodiment and protect the mains supply 
from being under- or over-pressurized by a fault developing in the 
compressor system. The gas passes through a plug cock 2, a particulate 
filter 3, a solenoid isolating valve 4, a flexible connection 5, under and 
over pressure electronic sensors 6a and 6b and through an approved 
non-return valve 7. 
The inlet pipe passes through the wall of a compartment 8 where the gas 
passes through a check valve 9 and into the inlet of a compressor 10. The 
compressor 10 is a Lyscholm oil flooded screw type compressor which is 
driven by an electric motor 11 and an inverter device 12. The Lyscholm 
compressor 10 consists of two shafts or rotors (not shown) designated a 
female and male which interact like a long pitch multiple start thread 
squeezing the entrapped oil and gas axially from inlet to discharge. There 
is a near linear relationship between shaft speed and flow for discharge 
pressures within the normal operating range. The oil flood performs two 
basic tasks which are to lubricate and seal the rotors in the housing (not 
shown) and to absorb the heat of compression, making the compression 
polytropic and more efficient than an equivalent adiabatic process. A low 
discharge temperature for any given pressure ratio is thus realised and 
combined with the continuous presence of oil, corrosion is not a problem. 
Maintenance may indeed be zero even for an operating life in excess of 10 
years. 
The outlet of the compressor 10 is connected to a separator tank 13 which 
separates oil from the oil and gas mixture and which helps to snub any 
small pressure pulsations in the discharge. The separator tank 13 is 
connected to the inlet of the compressor 10 through a filter 14 and a 
cooler 15 so that the separated oil is recirculated to the compressor 10. 
Furthermore, the separator tank 13 is connected to the pipe between the 
inlet of the compressor 10 and the check valve 9 through a relief valve 
16. In this embodiment, when the pressure in the separator tank 13 becomes 
more than a predetermined pressure, the relief valve 16 is pneumatically 
opened and the gas is released to the inlet pipe and not to atmosphere. In 
the separator tank 13, there are provided a level switch 19 for monitoring 
the oil level, a pressure sensor 17 for monitoring the discharge (gas) 
pressure and a thermocouple 18 for monitoring the discharge (gas) 
temperature. If an oil leak occurs and the lower oil level is detected by 
the level switch 19, or if the increase of the discharge (gas) temperature 
is detected by the thermocouple 18, these error signals are sent to a 
controller 31 which shuts down the system, thereby preventing the escape 
of gas. 
The separator tank 13 is connected to a coalescing filter 20 which removes 
small droplets of oil contained in the high pressure gas. The coalescing 
filter 20 is connected to the pipe between the inlet of the compressor 10 
and the check valve 9 through a bleed hole 20a so as to recirculate the 
small amount of coalesced oil to the inlet of the compressor 10. 
In this embodiment, the coalescing filter 20 is connected to a control 
valve 21 and the control valve 21 is connected to the pilot and primary 
burners of a combustion chamber 24 of a gas turbine engine 30 through 
cut-off valves 22 and 23. The control valve 21 is a proportional flow 
control valve and is controlled by a control signal of the controller 31 
in response to an output signal of a comparator 32. The comparator 32 
compares output power of the gas turbine engine 30 with a power reference 
33 and sends the output signal to the controller 31. 
Referring next to FIGS. 2 to 4, the gaseous fuel control system is 
completely contained within the enclosed compartment 8. A ventilation fan 
(not shown) is connected and sealed to an opening 24 and air is circulated 
by the fan into the compartment 8 and exits through a second port 25. As 
the ventilation air circulates in the compartment some of it passes over 
gas detectors 26. 
As shown in FIG. 5, the inverter device 12 is interposed between the 
electric induction motor 11 and the main electric source and includes a 
converter (rectifier or controlled rectifier) 37 which transforms AC into 
DC, an inverter 38 which transforms DC into AC, a compensator 35, such as 
a proportional, integral and derivative (PID) device, a second comparator 
36, a power transducer 42, a control circuit 39 and a firing circuit 40. 
The current pressure signal of the pressure sensor 17 is compared in a 
first comparator-34 with a pressure reference 41, and an output signal of 
the first comparator 34 is sent to the compensator 35. In response to that 
output signal, the compensator 35. In response to that output signal the 
compensator 35 sends a compensating signal Iref to the second comparator 
36. The compensating signal Iref is a current signal for obtaining a fuel 
pressure the same as the pressure reference 41. 
The compensating signal Iref is compared in the second comparator 36 with a 
current signal which is sent from the power transducer 42. The power 
transducer 42 may be disposed between the converter 37 and the inverter 38 
or between the inverter 38 and the motor 11 (as illustrated) or between 
the mains electric source and the converter 37 and measures the current 
which is fed to the inverter 38. An output signal of the second comparator 
36 is sent to the control circuit 39. The control circuit 39, which may be 
analogue or digital, determines what effective voltage and frequency must 
be synthesized by the inverter 38 in order to supply current to the motor 
11 commensurate with present demand for speed and torque. The control 
circuit 39 determines the firing sequences necessary for the inverter 38 
and sends these signals which may be TTL to the firing circuits 40 and 
40'. The firing circuit 40' is designed to accept the control signals for 
the converter 37 which may be thyristors or any other type of appropriate 
semiconductor switching device. Electrical power from the mains electric 
source is thus inverted at d.c. voltage. The firing circuit 40 is designed 
to accept the control signals from the control circuits 39 and output the 
correct gate signals for the inverter 38 which may be transistors, 
mosfets, IGBTs or any other type of appropriate semiconductor switching 
device. Electrical power from the d.c. link is thus inverted at an 
appropriate voltage and a.c. frequency and can therefore modulate the 
motor speed as desired. 
The above-described embodiment of the gaseous fuel control system for gas 
turbine engine operates as follows. Referring to FIG. 1, on starting up 
the gas turbine engine 30, the gas fuel control system operates firstly to 
produce the required fuel pressure to enable gas at an elevated pressure 
to enter the combustor for ignition, as follows. A pressure reference 
signal 41, which may be from an operator or from the gas turbine 
controller 31, is sent to the first comparator 34, the output of which is 
fed to the inverter device 12 to start up the motor 11 which drives the 
compressor 10 and immediately produces pressure in the separator tank 13 
because the control valve 21 and the cut-off valves 22 and 23 are 
initially closed. The plug cock 2 and the solenoid isolating valve 4 are 
of course assumed to be opened for normal operation. When the correct 
pressure is generated in the separator tank 13, the gas turbine start 
sequence can be started. If a pilot is used, the valve 23 will be opened 
by the controller 31. When this small flow flows, the gas compressor 10 
must speed up a little to supply this small flow. When the pilot is lit, 
the primary valve 22 can be opened by the controller 31. The control valve 
21 can be opened slowly thus gradually allowing more flow to gas turbine 
to accelerate it. 
In a modification of the above described embodiment, the control valve 21 
can be omitted so that the speed of the motor 11 alone controls the supply 
of compressed hydrocarbon gas through the cut-off valves 22 and 23 to the 
gas turbine. If there is no control valve, the block valves can be opened 
before the gas compressor is energised. To start the engine the gas 
compressor can be accelerated under controlled conditions from the (gas 
turbine) controller 31. 
Alternatively the start sequence can be any that the owner, operator or 
manufacturer of the system wishes to employ. 
After the gas turbine 30 is started, the gas fuel control system operates 
continuously in pressure feedback mode. The (gas turbine) controller 31 of 
the embodiment of FIGS. 1 to 5 generates two signals in response to output 
power. One is the required gas flow rate and therefore positions the 
control valve accordingly and the other is the required gas pressure which 
therefore is the gas compression system set point pressure. 
When the present output power is higher than the power reference 33, it 
becomes necessary to reduce the gas fuel flow to the engine 30. The 
controller 31 sends a signal to the control valve 21 which begins to 
close. A natural and temporary result of this is that the gas pressure 
rises slightly. The compressor 10 is therefore slowed down by the inverter 
device 12 until a new equilibrium is achieved. 
When the present output power is lower than the power reference 33, it 
becomes necessary to increase the gas fuel flow rate to the engine 30. The 
controller 31 sends a signal to the control valve 21 which begins to open. 
A natural and temporary result is that the gas pressure falls slightly. 
The compressor 10 is therefore accelerated by the inverter device 12 until 
a new equilibrium is reached. 
FIG. 6 and FIG. 7 show a second embodiment of the invention in which the 
proportional control valve 21 of the first embodiment is eliminated. In 
FIGS. 6 and 7, the same parts as those of FIGS. 1 to 5 are identified by 
the same reference numerals. Referring to FIGS. 6 and 7, the speed of the 
motor 11 is controlled by the inverter device 12 directly in response to 
the output power of the gas turbine engine 30. The comparators 32 and 34 
of the first embodiment are replaced in this second embodiment by a single 
comparator 32 which receives an input and feedback signal representing 
output power and the power reference signal 33. This embodiment operates 
as follows. The controller 31 sends its requirement for gas fuel flow 
directly to the inverter 12 of the gas compressor 10 through a changeover 
switch 43. When gas flow is requested, the valves 4, 22 and 23 are opened. 
The compressor 10 is then started at low speed until the pilot is lit. 
When appropriate the primary valve 23 can be opened and lit by the pilot. 
To accelerate the gas turbine engine 30, the controller requires more gas 
flow by accelerating a pilot, primary and main burners whilst other 
systems may only have a main burner. When the gas turbine engine 30 starts 
to produce power, the start sequence can end and on actuation of the 
changeover switch 43 the engine 30 becomes controlled by the power feed 
back loop. If power is lower than the set point 33, the error signal 
produced tends to accelerate the gas compressor 30 whilst if the output 
power is higher than the set point 33 the error signal produced tends to 
slow down the compressor 10. Thus the engine output power is modulated by 
the compressor speed. Since the other structures are the same as the first 
embodiment, the description is omitted. 
As mentioned above, according to the present invention, since the fuel 
compressor speed is modulated by the motor inverter, compressed gas is 
supplied virtually on demand within the time it takes to accelerate the 
compressor shaft. There is no long term storage of dangerous compressed 
gas. Because the motor and inverter can be utilised as main control 
elements the control valve can be eliminated. By this method it is able to 
simplify the control of fuel flow to a gas turbine. Furthermore, since the 
connections are a few and the integrity of these connections is not 
threatened by any intense vibrations, it is able to improve the safety of 
the system.