CN106837424B - Variable-speed constant-frequency power generation system and power generation method of screw expander - Google Patents
Variable-speed constant-frequency power generation system and power generation method of screw expander Download PDFInfo
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- CN106837424B CN106837424B CN201710105190.7A CN201710105190A CN106837424B CN 106837424 B CN106837424 B CN 106837424B CN 201710105190 A CN201710105190 A CN 201710105190A CN 106837424 B CN106837424 B CN 106837424B
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- 238000010248 power generation Methods 0.000 title claims abstract description 54
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- 230000007246 mechanism Effects 0.000 claims abstract description 13
- 230000008878 coupling Effects 0.000 claims description 6
- 238000010168 coupling process Methods 0.000 claims description 6
- 238000005859 coupling reaction Methods 0.000 claims description 6
- 230000008859 change Effects 0.000 abstract description 6
- 239000007789 gas Substances 0.000 description 27
- 238000010586 diagram Methods 0.000 description 8
- 230000008569 process Effects 0.000 description 4
- 230000001360 synchronised effect Effects 0.000 description 3
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 2
- 230000004224 protection Effects 0.000 description 2
- 239000003990 capacitor Substances 0.000 description 1
- 230000006837 decompression Effects 0.000 description 1
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- 238000006073 displacement reaction Methods 0.000 description 1
- 238000004134 energy conservation Methods 0.000 description 1
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01C—ROTARY-PISTON OR OSCILLATING-PISTON MACHINES OR ENGINES
- F01C13/00—Adaptations of machines or engines for special use; Combinations of engines with devices driven thereby
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01C—ROTARY-PISTON OR OSCILLATING-PISTON MACHINES OR ENGINES
- F01C1/00—Rotary-piston machines or engines
- F01C1/08—Rotary-piston machines or engines of intermeshing engagement type, i.e. with engagement of co- operating members similar to that of toothed gearing
- F01C1/12—Rotary-piston machines or engines of intermeshing engagement type, i.e. with engagement of co- operating members similar to that of toothed gearing of other than internal-axis type
- F01C1/14—Rotary-piston machines or engines of intermeshing engagement type, i.e. with engagement of co- operating members similar to that of toothed gearing of other than internal-axis type with toothed rotary pistons
- F01C1/16—Rotary-piston machines or engines of intermeshing engagement type, i.e. with engagement of co- operating members similar to that of toothed gearing of other than internal-axis type with toothed rotary pistons with helical teeth, e.g. chevron-shaped, screw type
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01C—ROTARY-PISTON OR OSCILLATING-PISTON MACHINES OR ENGINES
- F01C20/00—Control of, monitoring of, or safety arrangements for, machines or engines
- F01C20/08—Control of, monitoring of, or safety arrangements for, machines or engines characterised by varying the rotational speed
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
- H02P9/04—Control effected upon non-electric prime mover and dependent upon electric output value of the generator
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
- H02P2101/00—Special adaptation of control arrangements for generators
- H02P2101/20—Special adaptation of control arrangements for generators for steam-driven turbines
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Abstract
The invention discloses a variable-speed constant-frequency power generation system of a screw expander, which comprises the screw expander, a transmission mechanism, an asynchronous generator, a four-quadrant frequency converter, a controller, a pressure feedback device and/or an output power feedback device, wherein the screw expander is connected with the transmission mechanism; an output shaft of the screw expander is connected with a rotor of the asynchronous generator through a transmission mechanism; the power supply side of the four-quadrant frequency converter is connected with a power grid, and the motor side of the four-quadrant frequency converter is connected with an asynchronous generator; the output end of the pressure feedback device and/or the output end of the output power feedback device is connected with the input end of the controller; the output end of the controller is connected with the signal input end of the four-quadrant frequency converter. The invention also discloses a variable-speed constant-frequency power generation method of the screw expander. The invention can make the screw expander generating system adapt to the change of the gas flow without changing the ratio of the internal pressure to the external pressure of the screw expander, has high working efficiency and can realize high-efficiency power generation when the gas flow is very small.
Description
Technical Field
The invention relates to a screw expander power generation system and a power generation method.
Background
The screw expander is a prime motor with internal expansion, is widely applied to energy recovery of pressurized gas, and can be used as a prime motor of a mechanical drive or a generator set. Screw expanders, as rotary positive displacement expanders, have an inlet volume flow that is dependent primarily on rotor diameter, internal volume ratio, rotational speed and internal clearance. The conventional screw expander, whether a synchronous generator or an asynchronous generator is matched with the screw expander, is limited by the frequency of a power grid, the rotating speed is basically kept constant, and therefore the volume flow of an inlet is relatively constant. Meanwhile, when the orifice is designed, the internal volume ratio of the expander is mainly designed according to the external pressure ratio, so that the situations of over-expansion and under-expansion can be avoided during operation, and the highest efficiency is realized.
However, in actual operation, the gas flow of the screw expander changes due to the fact that the gas flow of the low-pressure pipe network connected to the outlet of the screw expander or the gas flow of the high-pressure pipe network connected to the inlet of the screw expander changes at all times. In order to balance the gas flow of the screw expander and the gas flow of an external pipe network and ensure that the exhaust pressure and the intake pressure of the screw expander or the output power of a generator connected with the screw expander are consistent with a set value, most of the screw expanders on the market at present adopt an inlet throttling mode to adjust the gas flow of the screw expander, namely, a controller is used for adjusting the opening of an inlet adjusting valve of the expander to change the intake pressure of the screw expander so as to adjust the gas flow of the screw expander. This kind of throttle control mode can make the inlet pressure of screw expander decline, has changed the external pressure ratio, and original internal volume ratio will no longer be fit for present operating mode, and the screw expander will produce great overexpansion for screw expander generating set's efficiency sharply drops. More seriously, when the screw expander generator set is at a lower efficiency, the gas flow has to be maintained at a higher level in order to achieve synchronous speed for grid connection. If the gas flow is insufficient, the generator can not be connected into the power grid or the reverse power is split.
In addition, when the screw expander is matched with a synchronous generator, in order to realize grid connection, the rotation speed of the screw expander generator set is required to be controlled accurately, so that the grid connection condition can be met only when the power generation frequency of the screw expander generator set is consistent with the power grid frequency. Therefore, the inlet of the screw expander needs an automatic control valve with high matching precision, and the screw expander is expensive and inconvenient to operate. When the screw expander is matched with the asynchronous generator, although high-precision rotation speed control is not needed, the asynchronous generator set consumes reactive power of a power grid, so that the power factor of the power grid is reduced, and larger impact current exists at the moment of grid connection.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a variable-speed constant-frequency power generation system of a screw expander, which can adapt to the change of gas flow under the condition of not changing the internal-external pressure ratio of the screw expander, has high working efficiency, and can realize high-efficiency power generation when the gas flow is very small.
The invention provides a variable-speed constant-frequency power generation method of a screw expander.
In order to solve the technical problem, the invention provides a variable-speed constant-frequency power generation system of a screw expander, which comprises the screw expander, a transmission mechanism, an asynchronous generator, a four-quadrant frequency converter, a controller, a pressure feedback device and/or an output power feedback device, wherein the screw expander is connected with the transmission mechanism through a transmission mechanism; an output shaft of the screw expander is connected with a rotor of the asynchronous generator through a transmission mechanism; the power supply side of the four-quadrant frequency converter is connected with a power grid, and the motor side of the four-quadrant frequency converter is connected with an asynchronous generator; the output end of the pressure feedback device and/or the output end of the output power feedback device is connected with the input end of the controller; the pressure feedback device is used for feeding back the exhaust pressure and/or the intake pressure of the screw expander detected in real time to the controller, and the output power feedback device is used for feeding back the output power of the asynchronous generator detected in real time to the controller; the output end of the controller is connected with the signal input end of the four-quadrant frequency converter, and the controller is used for calculating the given frequency of the motor of the four-quadrant frequency converter according to the exhaust pressure set value, the exhaust pressure feedback value, the intake pressure set value and the intake pressure feedback value or the output power set value of the asynchronous generator and the output power feedback value of the asynchronous generator, and outputting the given frequency of the motor to the four-quadrant frequency converter so as to control the rotor speed of the asynchronous generator.
In the above screw expander variable-speed constant-frequency power generation system, the controller is configured to perform PID adjustment or PI adjustment on a difference between an exhaust pressure set value and an exhaust pressure feedback value to obtain a motor given frequency, or perform PID adjustment or PI adjustment on a difference between an intake pressure set value and an intake pressure feedback value to obtain the motor given frequency, or perform PID adjustment or PI adjustment on a difference between an output power set value of the asynchronous generator and an output power feedback value of the asynchronous generator to obtain the motor given frequency.
The invention also provides a variable-speed constant-frequency power generation method of the screw expander, which comprises the following steps:
the gaseous power generation medium enters the screw expander to do work through expansion and push the screw expander to operate, so that the asynchronous generator is driven to operate and generate power;
the electric energy output by the asynchronous generator is merged into a power grid through a four-quadrant frequency converter;
the pressure feedback device feeds back the exhaust pressure and/or the intake pressure of the screw expander detected in real time to the controller, and/or the output power feedback device feeds back the output power of the asynchronous generator detected in real time to the controller;
the controller calculates the motor given frequency of the four-quadrant frequency converter according to the exhaust pressure set value, the exhaust pressure feedback value, the intake pressure set value and the intake pressure feedback value or the output power set value of the asynchronous generator and the output power feedback value of the asynchronous generator, and outputs the motor given frequency to the four-quadrant frequency converter, and the four-quadrant frequency converter controls the rotor rotating speed of the asynchronous generator according to the motor given frequency, so that the exhaust pressure of the screw expander is stabilized at the exhaust pressure set value, or the intake pressure of the screw expander is stabilized at the intake pressure set value, or the output power of the asynchronous generator is stabilized at the output power set value.
After the technical scheme is adopted, the invention at least has the following advantages:
1. according to the invention, the exhaust pressure and the intake pressure of the screw expander or the output power of the generator connected with the screw expander are stabilized at set values by changing the rotating speed of the screw expander instead of the intake pressure, so that the gas flow of the screw expander and the gas flow of an external pipe network are balanced, the capacity of adapting to the fluctuation of the gas flow is stronger, and the external pressure ratio design value of the screw expander can be maintained under the variable flow working condition, so that the screw expander always operates at a higher efficiency point, when the gas flow of the screw expander deviates from the design, higher power generation can be still ensured, meanwhile, the minimum gas flow of the screw expander required by grid-connected power generation can be reduced, and the requirement on the grid-connected working condition is low;
2. the invention can conveniently control the rotating speed of the screw expander by setting the given frequency of the motor of the four-quadrant frequency converter, thereby realizing the control of the gas flow or the process parameters related to the gas flow, such as output power, exhaust pressure, intake pressure and the like. Because the four-quadrant frequency converter is adopted, the rotating speed of the screw expander does not need to be accurately controlled, the grid-connected splitting can be realized only by parameter design of the frequency converter, one-key starting and stopping and grid-connected splitting can be conveniently realized, the grid-connected splitting is easy, and the control is simple. The maximum impact current during grid connection is smaller than the rated current of the generator, so that impact on a power grid is avoided;
3. the asynchronous generator obtains reactive power from the four-quadrant frequency converter, and the whole system does not consume the reactive power of a power grid;
4. the four-quadrant frequency converter isolates the asynchronous generator and the power grid into mutually independent systems, so that the asynchronous generator and the power grid cannot interfere with each other; the protection function of the four-quadrant frequency converter can be utilized to conveniently realize various protections to the asynchronous generator.
Drawings
Fig. 1 shows a schematic diagram of a first embodiment of a variable speed constant frequency power generation system of a screw expander of the present invention.
Fig. 2 shows a schematic diagram of a second embodiment of a variable speed constant frequency power generation system of a screw expander of the present invention.
Fig. 3 shows a schematic diagram of a third embodiment of a variable speed constant frequency power generation system of a screw expander according to the present invention.
Fig. 4 shows a schematic diagram of a fourth embodiment of a variable speed constant frequency power generation system of a screw expander according to the present invention.
Fig. 5 is a schematic diagram showing the comparison of the power generation amount of the screw expander variable-speed constant-frequency power generation system and the conventional throttling power generation system.
Detailed Description
The invention is described in detail below with reference to the figures and the specific embodiments.
Please refer to fig. 1. The screw expander variable-speed constant-frequency power generation system comprises a screw expander 1, a transmission mechanism 2, an asynchronous generator 3, a four-quadrant frequency converter 4, a controller 5, a pressure feedback device 6 and an output power feedback device 7.
The output shaft of the screw expander 1 is connected with the rotor of the asynchronous generator 3 through a transmission mechanism 2. In a specific embodiment, the transmission 2 is a coupling. In another specific embodiment, the transmission 2 comprises a first coupling, a second coupling and a gearbox. An output shaft of the screw expander 1 is connected with an input shaft of a gear box through a first coupler, and an output shaft of the gear box is connected with a rotor of the asynchronous generator 3 through a second coupler. The gear box has the function of matching the rotating speed of the screw expander 1 and the asynchronous generator 3.
The motor side of the four-quadrant frequency converter 4 is connected with the asynchronous generator 3, and the power supply side of the four-quadrant frequency converter 4 is connected with the power grid 9 through an electric switch device 8. The active power output by the asynchronous generator 3 is fed back to an external power grid 9 through an electric switch device 8 after being inverted by an inversion unit of the four-quadrant frequency converter 4. The reactive power required by the asynchronous generator 3 is provided by the internal capacitors of the four-quadrant inverter 4 without consuming the reactive power of the grid 9. The electrical switching apparatus 8 may be, for example, a circuit breaker, but is not limited thereto.
The output of the pressure feedback device 6 and the output of the output power feedback device 7 are connected to the input of the controller 5. In the present embodiment, the pressure feedback device 6 only detects the exhaust pressure of the screw expander, the pressure feedback device 6 is configured to feed back the real-time detected exhaust pressure of the screw expander 1 to the controller 5, and the output power feedback device 7 is configured to feed back the real-time detected output power of the asynchronous generator 3 to the controller 5. In a specific embodiment, the pressure feedback device 6 is a pressure sensor; the output power feedback device 7 is an electric energy meter, which may be integrated in the four-quadrant frequency converter 4 or may be an electric energy meter independent of the four-quadrant frequency converter 4.
The output end of the controller 5 is connected with the signal input end of the four-quadrant frequency converter 4, and the controller 5 is used for calculating the motor given frequency of the four-quadrant frequency converter according to the exhaust pressure set value and the exhaust pressure feedback value, or the output power set value of the asynchronous generator and the output power feedback value of the asynchronous generator, and outputting the motor given frequency to the four-quadrant frequency converter 4 so as to control the rotor speed of the asynchronous generator 3. Because the rotor of the asynchronous generator 3 is connected with the screw expander 1, the rotating speed of the rotor of the asynchronous generator 3 is equal to that of the screw expander 1, and the rotating speed of the rotor of the asynchronous generator 3 is controlled, namely the rotating speed of the screw expander 1 is controlled. Because the air inlet pressure and the air outlet pressure of the screw expander 1 are kept unchanged, the change of the rotating speed can cause the change of the air quantity passing through the screw expander set, so that the change of the steam consumption is matched, and the air outlet pressure or the output power of the screw expander 1 is ensured to be stabilized at a set value. The controller 5 may be a PLC controller, but is not limited thereto, and may be a controller of a DCS system, for example.
The controller 5 controls the gas flow of the screw expander by adopting an exhaust pressure mode or an output power mode, and is determined by a user according to the service environment of the variable-speed constant-frequency power generation system of the screw expander. The controller 5 can switch between an exhaust pressure control mode and an output power control mode according to an external instruction, when the controller 5 is in the exhaust pressure control mode, the exhaust pressure is taken as a process variable to be adjusted, the controller 5 outputs a motor given frequency of the four-quadrant frequency converter 4 calculated according to an exhaust pressure set value and an exhaust pressure feedback value to the four-quadrant frequency converter 4, when the controller 5 is in the output power control mode, the output power is taken as a process variable to be adjusted, and the controller 5 outputs a motor given frequency of the four-quadrant frequency converter 4 calculated according to an output power set value of the asynchronous generator 3 and an output power feedback value of the asynchronous generator 3 to the four-quadrant frequency converter 4.
In this embodiment, the controller 5 performs PID adjustment on the difference between the set exhaust pressure value and the feedback exhaust pressure value to obtain the given motor frequency, or performs PID adjustment on the difference between the set output power value of the asynchronous generator 3 and the feedback output power value of the asynchronous generator 3 to obtain the given motor frequency. The controller 5 is provided with a control mode switching unit 51, the control mode switching unit 51 switches between an exhaust pressure control mode and an output power control mode according to an external instruction, the control mode switching unit 51 outputs a motor set frequency calculated from an exhaust pressure set value and an exhaust pressure feedback value to the four-quadrant inverter 4 when the controller 5 is in the exhaust pressure control mode, and the control mode switching unit 51 outputs a motor set frequency calculated from an output power set value of the asynchronous generator 3 and an output power feedback value of the asynchronous generator 3 to the four-quadrant inverter 4 when the controller 5 is in the output power control mode. In the present embodiment, although the PID adjustment method is adopted, the present invention is not limited to this, and for example, a PI adjustment method may be adopted.
Fig. 2 shows a schematic diagram of a second embodiment of a variable speed constant frequency power generation system of a screw expander of the present invention. The difference between the second embodiment and the first embodiment is that the screw expander variable speed constant frequency power generation system of the second embodiment is not provided with the output power feedback device 7, and the controller 5 controls the gas flow rate of the screw expander only by adopting the exhaust pressure control mode, that is, the controller 5 calculates the motor given frequency of the four-quadrant frequency converter 4 according to the exhaust pressure set value and the exhaust pressure feedback value, and outputs the motor given frequency to the four-quadrant frequency converter 4 to control the rotor speed of the asynchronous generator 3.
Fig. 3 shows a schematic diagram of a third embodiment of a variable speed constant frequency power generation system of a screw expander according to the present invention. The difference between the third embodiment and the first embodiment is that the screw expander variable speed constant frequency power generation system of the third embodiment is not provided with the pressure feedback device 6, and the controller 5 controls the gas flow rate of the screw expander only by adopting an output power control mode, that is, the controller 5 calculates the motor given frequency of the four-quadrant frequency converter 4 according to the output power set value of the asynchronous generator 3 and the output power feedback value of the asynchronous generator 3, and outputs the motor given frequency to the four-quadrant frequency converter 4 to control the rotor speed of the asynchronous generator 3.
Fig. 4 shows a schematic diagram of a fourth embodiment of a variable speed constant frequency power generation system of a screw expander according to the present invention. The fourth embodiment is different from the first embodiment in that the screw expander variable-speed constant-frequency power generation system of the fourth embodiment is not provided with the output power feedback device 7, the controller 5 controls the gas flow rate of the screw expander only by adopting an intake pressure control mode, that is, the pressure feedback device 6 only detects the intake pressure of the screw expander, the pressure feedback device 6 is used for feeding back the intake pressure of the screw expander 1 detected in real time to the controller 5, and the controller 5 calculates the motor given frequency of the four-quadrant frequency converter 4 according to the intake pressure set value and the intake pressure feedback value and outputs the motor given frequency to the four-quadrant frequency converter 4 to control the rotor speed of the asynchronous generator 3.
The above embodiments are merely examples, and in other embodiments, a pressure feedback device that detects only the intake pressure of the screw expander and an output power feedback device may be combined, and the controller may switch between the intake pressure control mode and the output power control mode according to an external command; alternatively, the pressure feedback device may be capable of simultaneously detecting the intake pressure and the exhaust pressure of the screw expander, and the controller may switch between the intake pressure control mode, the exhaust pressure control mode, and the output power control mode in accordance with an external command.
Take a screw type steam pressure reduction generating set as an example. The medium pressure steam parameters were assumed to be 2.5MPaG, 400 ℃ and the low pressure steam parameters to be 1.0MPaG. In order to realize energy conservation, a screw expander is adopted to recover energy generated in the decompression process. The results of comparing the power generation amounts of the screw expander variable speed constant frequency power generation systems according to examples 1 to 4 of the present invention with those of the conventional throttle power generation system using the meter-in method are shown in fig. 5. It can be clearly seen from the curve in fig. 5 that, with the existing throttling power generation system, the steam flow needs to reach about 11000kg/h to output electric energy. By adopting the variable-speed constant-frequency power generation systems according to the embodiments 1 to 3 of the invention, the steam flow can generate electric energy only by about 5000kg/h, and the generated energy of the variable-speed constant-frequency power generation system is far higher than that of the conventional throttling power generation system under the working condition of reaching the full load.
The invention also provides a variable-speed constant-frequency power generation method of the screw expander, which comprises the following steps of:
the gaseous power generation medium enters the screw expander to do work through expansion and push the screw expander to operate, so that the asynchronous generator is driven to operate and generate power; the gaseous power generation medium can be working medium steam or natural gas;
the electric energy output by the asynchronous generator is merged into a power grid through a four-quadrant frequency converter; the four-quadrant frequency converter checks the voltage, frequency and phase of the power grid in real time, controls an inversion unit of the frequency converter to work according to the check, and ensures that the output electric energy is consistent with the voltage, frequency and phase of the power grid;
the pressure feedback device feeds back the exhaust pressure and/or the intake pressure of the screw expander detected in real time to the controller, and/or the output power feedback device feeds back the output power of the asynchronous generator detected in real time to the controller;
the controller calculates a motor given frequency of the four-quadrant frequency converter according to an exhaust pressure set value, an exhaust pressure feedback value, an intake pressure set value and an intake pressure feedback value or an output power set value of the asynchronous generator and an output power feedback value of the asynchronous generator, outputs the motor given frequency to the four-quadrant frequency converter, and the four-quadrant frequency converter controls the rotor rotating speed of the asynchronous generator according to the motor given frequency to stabilize the exhaust pressure of the screw expander at the exhaust pressure set value, or stabilize the intake pressure of the screw expander at the intake pressure set value, or stabilize the output power of the asynchronous generator at the output power set value. The set value of the exhaust pressure, the set value of the intake pressure and the set value of the output power of the asynchronous generator of the screw expander are preset by a user according to the use environment of the variable-speed constant-frequency power generation system of the screw expander.
The invention ensures that the gas flow of the screw expander and the gas flow of an external pipe network are balanced by stabilizing the exhaust pressure and the intake pressure of the screw expander or the output power of a generator connected with the screw expander at set values by changing the rotating speed of the screw expander instead of the intake pressure, has stronger capability of adapting to the fluctuation of the gas flow, and can maintain the external pressure ratio design value of the screw expander under the working condition of variable flow, thereby ensuring that the screw expander always operates at a higher efficiency point and still can ensure higher generating capacity when the gas flow of the screw expander deviates from the design.
Claims (8)
1. A variable-speed constant-frequency power generation system of a screw expander is characterized by comprising the screw expander, a transmission mechanism, an asynchronous generator, a four-quadrant frequency converter, a controller and a pressure feedback device, wherein the pressure feedback device consists of a pressure sensor;
an output shaft of the screw expander is connected with a rotor of the asynchronous generator through a transmission mechanism;
the power supply side of the four-quadrant frequency converter is connected with a power grid through an electric switch device, and the motor side of the four-quadrant frequency converter is connected with the asynchronous generator;
the output end of the pressure feedback device is connected with the input end of the controller; the pressure feedback device is used for feeding back the air inlet pressure of the screw expander detected in real time to the controller;
the output end of the controller is connected with the signal input end of the four-quadrant frequency converter, the controller is used for calculating the motor given frequency of the four-quadrant frequency converter according to the air inlet pressure set value and the air inlet pressure feedback value, and outputting the motor given frequency to the four-quadrant frequency converter so as to control the rotor rotating speed of the asynchronous generator, and therefore the external pressure ratio design value of the screw expander is maintained under the variable flow working condition.
2. The screw expander variable-speed constant-frequency power generation system according to claim 1, wherein the controller is configured to perform PID adjustment or PI adjustment on a difference between an intake pressure set value and an intake pressure feedback value to obtain the motor given frequency.
3. The screw expander variable speed constant frequency power generation system according to claim 1 or 2, wherein the controller is a PLC controller.
4. The screw expander variable speed constant frequency power generation system of claim 1, wherein the transmission mechanism is a coupling.
5. The screw expander variable speed constant frequency power generation system of claim 1, wherein the transmission mechanism comprises a first coupling, a second coupling and a gear box; an output shaft of the screw expander is connected with an input shaft of the gear box through the first coupler, and an output shaft of the gear box is connected with a rotor of the asynchronous generator through the second coupler.
6. The screw expander variable speed constant frequency power generation system according to claim 1, comprising an output power feedback device for feeding back real-time detected output power of the asynchronous generator to the controller; the controller is used for switching between an air inlet pressure control mode and an output power control mode according to an external instruction, and when the controller is in the air inlet pressure control mode, the controller is used for outputting the motor given frequency calculated according to an air inlet pressure set value and an air inlet pressure feedback value to the four-quadrant frequency converter, and when the controller is in the output power control mode, the controller is used for outputting the motor given frequency calculated according to an output power set value of the asynchronous generator and an output power feedback value of the asynchronous generator to the four-quadrant frequency converter.
7. A variable-speed constant-frequency power generation method of a screw expander is characterized by comprising the following steps:
the gaseous power generation medium enters the screw expander to do work through expansion and push the screw expander to operate, so that the asynchronous generator is driven to operate and generate power;
the electric energy output by the asynchronous generator is merged into a power grid through a four-quadrant frequency converter;
the pressure feedback device feeds back the air inlet pressure of the screw expander detected in real time to the controller;
the controller calculates the motor given frequency of the four-quadrant frequency converter according to the air inlet pressure set value and the air inlet pressure feedback value, and outputs the motor given frequency to the four-quadrant frequency converter, and the four-quadrant frequency converter controls the rotor rotating speed of the asynchronous generator according to the motor given frequency, so that the air inlet pressure of the screw expander is stabilized at the air inlet pressure set value, and the external pressure ratio design value of the screw expander is maintained under the variable flow working condition.
8. The screw expander variable-speed constant-frequency power generation method according to claim 7, wherein the controller is configured to perform PID adjustment or PI adjustment on a difference between an intake pressure set value and an intake pressure feedback value to obtain the given frequency of the motor.
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