Expander overspeed prevention system and method

Abstract

A power generation system is disclosed that includes a power generation turbine. The turbine drives at least two generators that are drivingly coupled to the power generation turbine and electrically coupled to an electrical power distribution grid and / or local loads. In the event of a failure of one generator, the surviving generator prevents uncontrolled overspeed of the shaft. A method for controlling a power generation system is also disclosed herein.

Description

Technical Field

[0001] This disclosure relates to improvements to power generation systems. The embodiments disclosed herein relate to power generation systems including power turbines, such as expanders, for example, supercritical carbon dioxide expanders; and electric generator arrangements driven by the turbines and adapted to convert mechanical power into electrical power. Background Technology

[0002] Fossil fuels are the primary source of chemical energy used to generate mechanical power. Fossil fuels are mixed with air and burned to produce high-pressure, high-temperature combustion gases that expand in a turbine or expander. The expander or turbine converts the enthalpy of the combustion gases into mechanical power that can be obtained on the output shaft of the expander or turbine and used to drive loads, such as compressors or compressor units, or to rotate generators and convert mechanical power into electrical power.

[0003] A major concern regarding the combustion of fossil fuels involves the production of carbon dioxide, a greenhouse gas considered one of the main causes of global warming and climate change.

[0004] To reduce the environmental impact of power generation through the combustion of fossil fuels, post-combustion carbon dioxide (CO2) capture options have been investigated. CO2 capture facilities have been developed to treat flue gas from gas turbines and remove CO2 from it before it is released into the environment. However, CO2 capture facilities are costly in terms of both CAPEX and the energy required to operate them, which reduces the overall thermodynamic efficiency of the system. The percentage of CO2 in the flue gas is also low. This necessitates processing large volumes of flue gas through CO2 capture facilities, making the capture process particularly inefficient.

[0005] In recent years, the oxy-fuel cycle (also known as the oxy-fuel cycle) has been developed, in which fuel (such as natural gas or another fossil fuel) is blended under high pressure into a mixture of oxidant consisting primarily of oxygen (O2) and carbon dioxide (CO2). The blend of fuel, oxygen, and carbon dioxide is burned in the burner of an expander, producing pressurized flue gas consisting only or almost entirely of carbon dioxide and water.

[0006] The flue gas expands in an expander to generate mechanical power, which is ultimately converted into electrical power by a generator driven by the expander. The exhaust gas exiting the expander is cooled in a regenerative heat exchanger and further refrigerated into condensate, which can then be removed from the cooled flue gas. The cryogenic flue gas, primarily or solely composed of carbon dioxide, is pressurized and recirculated through the regenerative heat exchanger toward the expander's burner. A portion of the anhydrous flue gas that is not recirculated to the burner can be efficiently treated in a carbon dioxide capture facility.

[0007] The oxygen supplied to the expander burner can be obtained by separating from ambient air and removing nitrogen from it, so that the working fluid supplied to the burner consists mainly of oxygen and carbon dioxide and does not include nitrogen.

[0008] Oxygen-fuel cycles (such as those described above) are of particular interest in terms of efficiency and reduction of harmful emissions. However, they operate under supercritical CO2 conditions at the expander inlet and are characterized by a high pressure drop across the expander.

[0009] Given the high pressure of the process gas in the expander, managing the oxygen-fuel circuit can become critical in the event of load loss on the generator driven by the expander. When the load on the generator is lost, for example because the generator is isolated from the electric power distribution grid by disconnecting a circuit breaker, the expander accelerates suddenly. To prevent the expander from overspeeding, the flow control valve upstream of the expander must be closed, which can lead to water hammer.

[0010] Similar problems may occur in other thermodynamic cycles that use expanding turbines to drive generators.

[0011] The implementation schemes disclosed herein are intended to mitigate or overcome the aforementioned disadvantages of the prior art. Summary of the Invention

[0012] According to one aspect, this document discloses a power generation system comprising a power generation turbine and a generator array. In some embodiments, the generator array includes a first generator and a second generator. The two generators are drivenly coupled to the power generation turbine and electrically connected to a power distribution network, a local load, or both. The first and second generators are controlled such that, in response to a load loss on one of the first and second generators, a load is transferred to the other (the surviving generator), and the power generation turbine is braked by the other of the first and second generators.

[0013] Therefore, in the event of a generator failure, the surviving generator still applies a braking load to the turbine, thereby preventing or limiting the turbine's angular acceleration. The braking torque applied to the generator turbine shaft by the surviving generator avoids the need to abruptly stop the turbine by closing the flow control valve, and allows for smooth control of the turbine's rotational speed by acting on the flow control valve to gradually reduce the power generated by the generator turbine.

[0014] If the generator turbine includes a fuel delivery line with a fuel control valve, the reduction in power generated by the generator turbine's rotational speed can be controlled by acting on the fuel control valve.

[0015] Therefore, under load loss conditions, the turbine does not stop and there is sufficient time available to control its speed by acting on one or more fluid flow control valves, particularly the fuel control valve (if present).

[0016] This avoids the water hammer effect, which would otherwise be harmful to turbines and pipelines, especially in supercritical carbon dioxide cycles or any cycle with a high pressure drop, such as oxy-fuel cycles.

[0017] In some embodiments, the first generator, the second generator, and the generating turbine (e.g., an expander) can be mounted on a common axis. In some embodiments, the flywheel can also be drivenly coupled to the generating turbine.

[0018] According to another aspect, this document discloses a method for limiting overspeed of a generator turbine driven to a generator array in the event of load loss. In the disclosed embodiments, the method includes the steps of: rotating the generator turbine and thereby generating mechanical power; using the mechanical power generated by the generator turbine to rotate a first generator and a second generator, and feeding the electrical power generated by the first and second generators to: a power distribution network, or a local load, or both the local load and the power distribution network. In response to a load loss on one of the first and second generators, overspeed of the generator turbine is prevented or limited by continuing to convert mechanical power into electrical power using the other of the first and second generators.

[0019] In some implementations, in response to a load loss in a generator, steps can be anticipated to increase the power generated by the surviving generator, thereby braking the generator turbine.

[0020] Further embodiments and features of the methods and systems disclosed herein are summarized below and set forth in the appended claims. Attached Figure Description

[0021] Now, please briefly refer to the attached diagram, in which:

[0022] Figure 1 An example is illustrated of a power generation system comprising an oxygen-fuel thermodynamic cycle according to a first embodiment;

[0023] Figure 2 A schematic diagram of a power generation system according to the second embodiment is illustrated; and

[0024] Figure 3 A schematic diagram of a power generation system according to the third implementation scheme is shown. Detailed Implementation

[0025] The following description relates to embodiments of oxygen-fueled supercritical carbon dioxide thermodynamic cycles (such as the Allam thermodynamic cycle) that utilize supercritical carbon dioxide expanders. However, it should be understood that the features disclosed herein can also be advantageous when combined with different thermodynamic cycles (such as the Brayton cycle), for example, using power generation turbines (e.g., gas turbines) that differ from expanders.

[0026] Now turn to the attached image, Figure 1 The first embodiment of a power generation system according to this disclosure is illustrated. Specifically, Figure 1 The schematic diagram illustrates a simplified supercritical carbon dioxide cycle, such as the Allam cycle or a similar oxygen-fuel cycle, in which the novel features disclosed herein are particularly useful.

[0027] Figure 1 The power generation system 1 shown includes an expander 3, which includes an expansion section 5 and a burner 7. The burner 7 is supplied with an oxidant stream from an oxidant source. The oxidant can be oxygen (O2) or a mixture containing oxygen and carbon dioxide (CO2) or primarily composed of oxygen and carbon dioxide. The oxidant stream can be generated by an air separation unit 9, which represents the oxidant source. The air separation unit 9 can remove nitrogen or nitrogen and carbon dioxide from ambient air to generate the desired oxidant stream, which is supplied to the burner 7 of the expander 3 via an oxidant line 11. An oxidant control valve 11.1 is positioned on the oxidant line to control the oxidant flow rate and can be used to regulate the oxidant flow rate to the expander 3.

[0028] In some embodiments, the oxidant stream may contain approximately 20% by volume oxygen and 80% by volume carbon dioxide. The CO2 and O2 percentages described above are merely illustrative and should not be construed as limiting the scope of this description. Carbon dioxide may be added to the oxygen via recirculation line 12, as explained in more detail below.

[0029] Reference numeral 13 indicates a fuel supply line, which is adapted, for example, to supply natural gas, such as methane, to the burner 7. A fuel control valve 13.1 is arranged along the fuel supply line 13 to regulate the fuel flow rate.

[0030] The oxidizer fuel blend is burned in burner 7. The pressurized, heated combustion gases produced by combustion expand in expansion section 5 of expander 3. The exhaust gases expand at the exhaust side of expander 3 and are then discharged through exhaust line 15.

[0031] The circuit also includes a regenerative heat exchanger 17, through which hot exhaust gas flowing through the hot side 17.1 of the regenerative heat exchanger 17 is cooled in heat exchange with cooled exhaust gas flowing through the cold side 17.2 of the regenerative heat exchanger 17. The exhaust gas from the hot side 17.1 of the regenerative heat exchanger 17 is further cooled in a refrigeration heat exchanger 19 to a temperature that causes the vapor contained in the exhaust gas to condense. Condensate is removed from the exhaust gas in a water / gas separator 21.

[0032] The dehydrated and refrigerated exhaust gas, mainly or solely composed of carbon dioxide, is compressed in the flue gas compressor 23 to the pressure at the inlet side of the expander 3. Although in Figure 1 In the schematic diagram, flue gas compressor 23 is illustrated as a single compressor, but in some embodiments, multiple compressors may be used. For example, flue gas compressor 23 may be a multi-stage compressor or a compressor unit. In some embodiments, the compressor may be an intercooled compressor. In some embodiments, the main compressor may be connected in series with two sequentially arranged pumps.

[0033] Compressed flue gas delivered by flue gas compressor 23 is partially removed from the circulation via discharge line 24. If compressor 23 includes multiple compressor turbines arranged in series, discharge line 24 may be connected between two sequentially arranged turbines and / or at the discharge side of the most downstream compressor turbine.

[0034] The remaining compressed flue gas is conveyed through the cold side 17.2 of the regenerative heat exchanger 17 and heated by heat exchange with the hot flue gas flowing through the hot side 17.1 of the regenerative heat exchanger 17, and then recirculated to the expander 3 via the recirculation line 25. The flue gas recirculated through the recirculation line 25 is mixed with the combustion gases generated in the burner 7, or with the oxidant flow from the oxidant line 11. The recirculation flow control valve 25.1 on the recirculation line 25, i.e., the carbon dioxide flow control valve, controls the flow rate of the recirculated flue gas.

[0035] The side flow of cooled flue gas is delivered via cooling line 27 to the components of the expander 3 that require cooling, bypassing the regenerative heat exchanger 17. The side flow of cooled flue gas may be partially fed through line 12 and mixed with oxygen from the air separation unit 9, and partially fed through line 28 to the expander 3 that requires cooling. A coolant flow control valve 28.1 in line 28 is adapted to control the flow rate of the cooled flue gas used as coolant in the expander 3.

[0036] The expander 3 may include an output shaft 31 that drivesly connects the expander 3 to a load. Figure 1 In some embodiments, the load includes a generator array 33. In some embodiments, the generator array 33 includes multiple generators. Figure 1 In one embodiment, the generator arrangement 33 includes a first generator 35 and a second generator 37. In other embodiments, more than two generators may be driven by the same expander 3.

[0037] exist Figure 1 In one embodiment, the first generator 35 and the second generator 37 are coaxial and arranged along a common axis 39, which is driven to the output shaft 31, for example, via a joint 41.

[0038] In some implementations, the flywheel 43 may be drivably coupled to the same axis 39.

[0039] exist Figure 1 In the schematic diagram, the first generator 35 and the second generator 37 are sequentially positioned on the same side of the expander 3 along axis 39, and in the illustrated example, on the rear side of the expander 3. In other embodiments, the first generator 35 may be connected to the front side of the expander 3, and the second generator 37 may be connected to the rear side of the expander 3 via a through shaft extending through the expander 3. As used herein, "front" and "rear" refer to the flow direction of the process gas through the expander 3. Thus, "front" indicates the position on the gas inlet side of the expander 3, and "rear" indicates the position on the gas outlet side (i.e., the discharge side of the expander 3).

[0040] In another embodiment, both the first generator 35 and the second generator 37 can be positioned on the front side of the expander 3. For example... Figure 1 As shown, arranging the generator arrangement 33 on the rear side of the expander 3 is beneficial in reducing the thermal load on the generator arrangement 33. Arranging the two generators 35 and 37 on opposite sides of the expander 3 can provide a better distribution of the mechanical load along the axis 39.

[0041] The first generator 35 and the second generator 37 may be identical, similar, or different from each other. For example, the two generators 35 and 37 may have different rated power. However, in some embodiments, the first generator 35 and the second generator 37 have the same rated power.

[0042] Flywheel 43 (if it exists) can be placed with Figure 1 The flywheel 43 can be placed in different positions as shown. For example, it can be placed on the front side of the expander 3, or between the first generator 35 and the second generator 37, or between the expander 3 and the first generator 35.

[0043] The first generator 35 is electrically connected to the transformer 47 and the local load 49 via a circuit breaker 45. The high-voltage side of the transformer 47 is electrically connected to the power distribution network 51. The second generator 37 is electrically connected to the transformer 47 and the local load 49 via a circuit breaker 53. An additional circuit breaker 55 may be placed between the transformer 47 and the power distribution network 51.

[0044] In some implementations, if necessary, the additional emergency load 57 can be electrically connected to the first generator 35 or the second generator 37, or both, via an emergency switch 59.

[0045] The control unit or controller 61 can be functionally connected to the flow control valves (i.e., oxidant control valve 11.1, fuel control valve 13.1, recirculation flow control valve 25.1 (also known as carbon dioxide control valve) and coolant flow control valve 28.1) of the first generator 35, the second generator 37, the emergency switch 59, and the expander 3.

[0046] The aforementioned power generation system 1 can operate as follows: Under normal operating conditions, the mechanical power generated by the expander 3 causes the first generator 35 and the second generator 37 to rotate. Typically, both generators 35 and 37 are electrically excited to each generate a portion of the total power output. Under certain operating conditions, one generator can be excited while the other can idle.

[0047] Under certain operating conditions, the two generators 35 and 37 can produce the same power, but this is not mandatory. The control unit 61 can control the amount of power produced by each generator 35 and 37 as needed.

[0048] The electrical power generated by generator array 33 can be partially used to power local load 49, and the remaining power can be fed into power distribution network 51. Circuit breakers 45, 53, and 55 are closed, and switch 59 is open. No power is fed into emergency load 57. If local load 49 is not anticipated, the total power is fed into power distribution network 51. In some embodiments, the generation system can be islanded. In this case, all power generated by the generator is fed into the local load, and no connection to power distribution network 51 is provided.

[0049] Two generators 35 and 37 work together to apply braking torque to expander 3 via shaft 39. This braking torque is balanced by the torque generated by the expansion of flue gas passing through expander 3. The torque is a function of the electrical power generated by the first generator 35 and the second generator 37. Therefore, the flow rates of fuel, recirculated flue gas (recirculated carbon dioxide), and oxidant are regulated by corresponding flow control valves 11.1, 25.1, and 13.1, so that the driving torque generated by expander 3 and the braking torque applied by generators 35 and 37 are balanced, and the rotational speed of shaft 39 is maintained near a preset value. Gradual changes in the load applied by generators 35 and 37 are balanced by corresponding adjustments to the flow rates of fuel, oxidant, and recirculated flue gas.

[0050] If the load on one of the first generator 35 and the second generator 37 is lost, for example because one of the circuit breakers 45 and 53 disconnects after a corresponding generator failure, the mechanical load applied to shaft 39 will suddenly decrease. For example, if the first generator 35 and the second generator 37 produce the same power, a failure of one of the generators 35 and 37 will suddenly reduce the braking torque applied to the rotor of the expander 3 through shaft 39 by 50%.

[0051] A sudden drop in braking torque will cause the expander 3 to accelerate, leading to overspeed of the train unit, which includes generators 35 and 37, flywheel 43, and expander 3. Acceleration of expander 3 cannot be prevented by reducing fuel flow, as load loss occurs faster than a reduction in fuel flow and the resulting power loss; this can be achieved by acting on fuel control valve 13.1.

[0052] According to the method disclosed herein, in order to prevent or limit the acceleration of shaft 39, after the failure of one generator, the braking torque applied by the surviving generator is increased by increasing its excitation current. Thus, the increased braking torque applied by the surviving generator is suddenly achieved. The increased braking torque at least partially balances the load reduction caused by the failure of the other generator and allows for the timing of fuel flow regulation via the throttling fuel control valve 13.1.

[0053] In one example, the first generator 35 and the second generator 37 have the same rated power, for example, 150MW each. Let us assume that at a given moment, each generator produces 100MW, such that a total of 200MW is produced by system 1.

[0054] If one of the two generators (e.g., the first generator 35) fails, the braking torque applied to shaft 39 decreases by 50%. Control unit 61 is adapted to detect the generator failure and trigger a load transfer routine.

[0055] The load transfer routine involves increasing the power generated by the surviving generator, i.e., transferring at least a portion of the load to the surviving generator, which in this example is generator 37. By increasing the excitation current of the surviving generator (the second generator 37 in this example), the load on the surviving generator is increased, i.e., the amount of electrical power generated, and consequently, the resistance torque applied to the expander 3 by the surviving generator 37. As a result of the increased excitation current, the surviving generator 37 begins to generate more power.

[0056] For example, the power generated by generator 37 can be increased to a rated power of 150MW. The higher power generated by the surviving generator 37 corresponds to the increased braking torque applied to shaft 39 by generator 37.

[0057] Therefore, the decrease in braking torque due to the loss of the first generator 35 is at least partially balanced by the increased braking torque generated by the second generator 37.

[0058] Even if the surviving generator 37 is switched to full power without fully balancing the losses of generator 35, the total drop is limited to 50MW[2]. [100-(100+50)=50MW], and the resulting acceleration of axis 39 is less than the acceleration that would occur if the second generator 37 continued to produce 100MW.

[0059] The speed increase caused by the decrease in braking torque is balanced by reducing the fuel flow rate via an action on fuel control valve 13.1. This can be achieved by initiating a fuel flow reduction routine. Since the decrease in braking torque is small, the time available to act on the fuel control valve is longer if the load on the surviving generator remains constant. By reducing the fuel flow rate, as well as the flow rates of oxidizer and recirculated flue gas, overspeed of expander 3 can be controlled, and the speed of shaft 39 can be quickly returned to rated speed. The reduction in fuel flow rate also involves a reduction in the flow rates of residual fluids (i.e., coolant, recirculated flue gas (recirculated carbon dioxide), and oxidizer), which is achieved by acting on the corresponding control valves 28.1, 25.1, and 11.1.

[0060] The inconvenience of water hammer that occurs in the event of a single generator failure, as is commonly seen in existing technology lines, is also due to... Figure 1 The publicly available systems were avoided.

[0061] If the loss of one generator occurs when both generators are operating at rated power (e.g., 150MW each), the overspeed of shaft 39 remains under control compared to what would happen in a prior art system with a single generator. In effect, at least half the load is still applied to shaft 39 by the surviving generator. Sudden closure of control valves 11.1, 13.1, 25.1, and 28.1, which would be necessary in a prior art system, is avoided. Smooth action on valves, such as fuel control valve 13.1, to return shaft 39 to its rated rotational speed is possible. Avoiding water hammer is particularly important when the generator turbine operates at high pressures, such as in the case of an oxy-fuel expander or a supercritical carbon dioxide expander.

[0062] If the total power generated before the fault is less than the rated power of the surviving generator, the load loss can be fully balanced by shifting the load to the surviving generator. For example, if the total power generated before the fault is 120MW, such as 60MW generated by each generator 35, 37, the decrease in drag torque caused by the fault of generator 35 can be fully balanced by doubling the power generated by the surviving generator 37. If, for any reason, it is not desirable to operate the surviving generator at full power, the flow control valve can be activated to reduce the power generated by the surviving generator.

[0063] A fault in one of generators 35 or 37 can be detected by control unit 61, for example, by monitoring at least one parameter indicating load loss. Such parameters may include the rotational speed of shaft 39, the torque transmitted to shaft 39 by expander 3, the acceleration of shaft 39, or the total power output by generator arrangement 33. For this purpose, suitable speed sensors, acceleration sensors, torque sensors, power sensors, or current sensors can be anticipated. Figure 1 In the schematic diagram, speed sensor 63 is arranged on axis 39 to detect its rotational speed and generate a signal that is transmitted to control unit 61. More than one parameter can be detected by control unit 61.

[0064] Based on signals from sensors used to detect parameters indicating load loss, control unit 61 can trigger an increase in the excitation current of the surviving generator in the event of another generator failure. Control unit 61 also controls flow control valves 11.1, 13.1, 25.1, and 28.1, and particularly fuel control valve 13.1, to initiate partial throttling in response to a sudden increase in the excitation current of the surviving generator. Therefore, the reduction in mechanical power generated by the expander after the fuel flow decreases will balance the load loss.

[0065] The acceleration of shaft 39 after the loss of one of the generators can be further reduced by flywheel 43, so that the additional time can be used to balance the load loss by partially throttling the fuel, oxidant and recirculated flue gas flow rates via the corresponding control valves 13.1, 11.1, 25.1.

[0066] In some cases, generators 35 and 37 can be isolated from the power distribution network 51 by disconnecting circuit breaker 55. To avoid or reduce the subsequent acceleration of shaft 39 caused by sudden load loss due to the disconnection of circuit breaker 55, emergency load 57 can be electrically connected to generator arrangement 33 by closing switch 59.

[0067] exist Figure 2 A schematic diagram of another embodiment of the power generation system 1 according to this disclosure is shown. Figure 2 Some components of the system were omitted and... Figure 1 The same reference numerals are present in the figures. Figure 1 The same components shown and described above. These components will not be described again. Figure 2 Implementation plan and Figure 1 The main difference in the implementation scheme is that the two generators 35 and 37 are arranged on opposite sides of the expander 3, i.e., at the front and rear ends. The flywheel 43 can be positioned behind the second generator 37. Alternative positions of the flywheel 43 are shown as between the expander 3 and the first generator 35 (position 43X), and between the expander 3 and the second generator 37 (position 43Y).

[0068] exist Figure 3 Another embodiment of the power generation system 1 according to this disclosure is shown. Figure 3 Some components of the system were omitted and... Figure 1 The same reference numerals are present in the figures. Figure 1 The same components shown and described above are not described again. Figure 3 Implementation plan and Figure 1The main difference in the implementation scheme is that the generator and flywheel are not arranged along the same axis. That is, in Figure 3 In this configuration, the first generator 35 has a shaft 39A; the second generator 37 has an additional shaft 39B, and the two shafts 39A and 39B are drivenly connected to the expander 3 via a common shaft 39 and a gearbox 39C. The flywheel 43 can be placed on the shaft 39.

[0069] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. Those skilled in the art will understand that various changes, omissions, and additions may be made to the specific disclosure herein without departing from the scope of the invention as defined in the following claims.

Claims

1. A power generation system, the power generation system comprising: Power generation turbine; A first generator is driven to be connected to the generator turbine and electrically connected to the power distribution network, a local load, or both the power distribution network and the local load. A second generator is driven to the generator turbine and electrically connected to the power distribution network, the local load, or both the power distribution network and the local load. A control unit, the control unit being adapted to detect at least one parameter indicating load loss on the first generator and the second generator; The first generator and the second generator are controlled by the control unit such that, in response to a load loss on one of the first generator and the second generator, a load is transferred to the other of the first generator and the second generator, and the generator turbine is braked by the other of the first generator and the second generator; wherein the control unit is adapted to, in response to a detected load loss, cause an increase in the power generated by the other of the first generator and the second generator by increasing the excitation current of the other of the first generator and the second generator.

2. The power generation system of claim 1, wherein the control unit is further adapted to reduce the fluid flow to the power generation turbine by means of a control valve arrangement, such that the power generation turbine does not stop and its speed is controlled by closing the flow control valve.

3. The power generation system according to claim 1 or 2, wherein the at least one parameter indicating load loss is selected from the group consisting of: the rotational speed of the power generation turbine, the first generator, or the second generator; the angular acceleration of the power generation turbine, the first generator, or the second generator; the power generated by the first generator and the second generator; the torque on the mechanical connection between the power generation turbine and the first generator and the second generator; and combinations thereof.

4. The power generation system according to any one of the preceding claims, wherein the control unit is adapted to adjust the fuel flow rate in response to detected load loss.

5. The power generation system according to claim 4, further comprising: A fuel supply line adapted to supply fuel to a generator; and a fuel control valve on the fuel supply line, the fuel control valve being controlled by the control unit and adapted to reduce the fuel flow to the turbine in response to a detected load loss.

6. The power generation system according to claim 4 or 5, further comprising an oxidant supply line having an oxidant control valve; and a carbon dioxide supply line having a carbon dioxide control valve; wherein the oxidant control valve and the carbon dioxide control valve are controlled by the control unit and are adapted to reduce the oxidant flow rate and the recirculated carbon dioxide flow rate in response to a detected load loss.

7. The power generation system according to any one of the preceding claims, wherein the power generation turbine comprises an oxygen fuel expander.

8. The power generation system according to any one of the preceding claims, wherein the power generation turbine comprises a supercritical carbon dioxide expander.

9. The power generation system according to any one of the preceding claims, wherein the first generator, the second generator, and the power generation turbine are mounted on a common axis.

10. The power generation system according to any one of the preceding claims, the power generation system further comprising a flywheel drivenly connected to the power generation turbine.

11. The power generation system of claim 10, wherein the first generator, the second generator, the flywheel, and the power generation turbine are mounted on a common axis.

12. The power generation system according to any one of the preceding claims, wherein the local load includes a dissipative resistor.

13. A method for limiting overspeed of a rotating power generation turbine driven to a generator array under load loss conditions, the method comprising the steps of: To make the power generation turbine rotate and thereby generate mechanical power; The mechanical power generated by the power generation turbine is used to rotate the first generator and the second generator, and the electrical power generated by the first generator and the second generator is fed to: the power distribution network, or the local load, or both the local load and the power distribution network; In response to load loss on one of the first and second generators, overspeed of the generator turbine is prevented or limited by increasing the power generated by the other of the first and second generators, thereby braking the generator turbine, and the other of the first and second generators continues to convert mechanical power into electrical power.

14. The method of claim 13, further comprising the following steps: In response to the load loss, the fluid flow rate to the power generation turbine is reduced by controlling the valve arrangement.

15. The method of claim 14, further comprising the following steps: In response to the load loss, the fuel flow to the power generation turbine is reduced via a fuel flow control valve.

16. The method of claim 15, further comprising the following steps: In response to load loss, the oxidant flow rate to the power generation turbine is reduced via an oxidant flow control valve; and the carbon dioxide flow rate to the power generation turbine is reduced via a carbon dioxide control valve.

17. The method according to any one of claims 13 to 16, the method further comprising the following steps: Detect at least one control parameter that indicates the load loss on one of the first generator and the second generator; When the control parameters indicate a condition of load loss in one of the first and second generators, a load transfer routine is initiated, thereby increasing the load on the other of the first and second generators; and a fluid flow reduction routine is initiated to reduce the flow to the generator turbine.

18. The method of claim 17, wherein the fluid flow reduction routine includes the step of reducing the fuel flow rate to the power generation turbine.

19. The method according to any one of claims 13 to 18, wherein the power generation turbine is an oxygen fuel expander, such as a supercritical carbon dioxide expander.

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