A front high-pressure cylinder thermal system for a coal-fired steam turbine generator unit and a method of operation

Abstract

This invention relates to the field of steam turbine technology, specifically to a front-mounted high-pressure cylinder thermal system and its operation method for a coal-fired steam turbine generator set. The front-mounted high-pressure cylinder of this invention can be rigidly connected to the unit coaxially, eliminating the need for a clutch and a separate generator via a split-shaft design, thus greatly simplifying the system. Under low-load conditions, the front-mounted high-pressure cylinder is connected in series with the high-pressure cylinder, allowing full-flow main steam through both, increasing the turbine inlet pressure and improving the unit's cycle efficiency. This invention ensures safe and controllable operation during transitional periods, avoiding blade blowout and overload conditions, while also ensuring boiler safety and controllability. The front-mounted high-pressure cylinder and the high-pressure cylinder operate at full flow under all conditions, without affecting the efficiency within the high-pressure cylinder. This invention can be fully implemented under pure condensing conditions without requiring external steam supply from industrial users.

Description

Technical Field

[0001] This invention relates to the field of steam turbine technology, specifically to a front-mounted high-pressure cylinder thermal system and its operation method for a coal-fired steam turbine generator set. Background Technology

[0002] During operation, the main steam flow and pressure of coal-fired power units change with the load. A decrease in main steam pressure leads to a reduction in cycle efficiency. With the establishment of a new power system primarily based on new energy sources, the peak-shaving depth and duration of coal-fired power units are increasing. If the operating conditions of the units deviate significantly from the design conditions for an extended period, the actual operating efficiency of the coal-fired power units will decrease significantly. Therefore, improving unit efficiency under medium and low loads and achieving high-efficiency operation under wide load conditions has become a very meaningful research topic. The "Implementation Plan for Upgrading New Generation Coal-fired Power (2025-2027)" stipulates that the increase in coal consumption for power supply under pure condensing conditions at 30% load compared to rated load for existing units and new generation coal-fired power pilot demonstration units should, in principle, not exceed 25% and 15% respectively, and for newly built units, the increase should be controlled within 20%. Increasing the initial steam pressure is an important measure to improve the cycle efficiency of the unit. Various research institutions have carried out related research, and setting up a front-mounted high-pressure cylinder is an important research direction. Among the many research results, most of them focus on steady-state operating conditions. However, there are still many problems to be solved in terms of how to complete the operating condition transition and how not to affect the efficiency of the original cylinder. Summary of the Invention

[0003] To improve the efficiency of units operating under medium and low load conditions, this invention includes a pre-positioned high-pressure cylinder connected in series before the high-pressure cylinder to increase the main steam pressure, thereby improving the unit's cycle efficiency.

[0004] The specific technical solution is to install a front-mounted high-pressure cylinder on the basis of a conventional steam turbine generator set. The front-mounted high-pressure cylinder and the generator set can be arranged coaxially and rigidly connected, without the need to install a clutch or a separate generator using a split-shaft scheme, which greatly simplifies the equipment system.

[0005] Under low-load conditions, the front high-pressure cylinder is connected in series with the high-pressure cylinder, and the main steam flows through the front high-pressure cylinder and the high-pressure cylinder to increase the turbine inlet steam pressure and improve the unit's cycle efficiency.

[0006] Under high unit load, the pressure stage group before the third-stage extraction of the intermediate pressure cylinder is connected in parallel. The steam source of the front high-pressure cylinder is switched from main steam to reheat steam, and the exhaust steam from the front high-pressure cylinder is discharged to the third-stage extraction steam pipeline of the intermediate pressure cylinder. Under this condition, the enthalpy drop of the front high-pressure cylinder is close to its design enthalpy drop, and it can still maintain a high variable operating condition efficiency. Moreover, it is close to the enthalpy drop of the pressure stage group before the third-stage extraction of the intermediate pressure cylinder. Its exhaust steam parameters (pressure, temperature) are close to those of the third-stage extraction steam (i.e., the heating steam of the No. 3 high-pressure heater). Since the specific volume of reheat steam is relatively large, the mass flow rate of the front high-pressure cylinder is close to the mass flow rate of the third-stage extraction steam under this condition, and the flow rate ratio is small. The steam flow rate of the intermediate pressure cylinder is partially diverted by the front high-pressure cylinder. The diverted flow rate is close to the flow rate of the third-stage extraction steam. That is, the flow rate of the pressure stage group before the third-stage extraction of the intermediate pressure cylinder is slightly reduced, while the flow rate of the pressure stage group after the third-stage extraction of the intermediate pressure cylinder is not affected. Therefore, it will not have a significant impact on the efficiency of the intermediate pressure cylinder.

[0007] Because the inlet steam temperature of the high-pressure cylinder changes to some extent during the switching operation, the main steam pressure and temperature must be adjusted in conjunction with the switching operation of this invention. This should be done under constant main steam pressure conditions. When switching from series to parallel, the main steam pressure is reduced first, and the main steam temperature is reduced first and then restored. When switching from parallel to series, the main steam pressure is increased first, and then the switching is done, and the main steam temperature is reduced first and then restored. The front high-pressure cylinder and the high-pressure cylinder share the burden of the inlet steam temperature change and maintain a reasonable temperature change range and rate. The exhaust temperature of the front high-pressure cylinder does not change much before and after the switching, while the exhaust temperature of the high-pressure cylinder changes significantly. Switching to series operation will reduce the temperature considerably. The rate of change of the high-pressure cylinder exhaust temperature should also be monitored and controlled during the switching operation. The reduction in the high-pressure cylinder exhaust temperature increases the heat absorption of the reheater, which also contributes to the improvement of cycle efficiency.

[0008] Choosing a reasonable load switching point (this load switching point is related to the position of the three-stage extraction steam in the intermediate pressure cylinder; based on calculations for a conventional single-stage reheat unit, the load switching point is around 65% of the rated load. The exact load switching point may vary slightly depending on the specific manufacturer's model) involves optimizing the flow rate and design enthalpy drop (thermal stage) of the front high-pressure cylinder. At low and medium loads, it is matched with the high-pressure cylinder in series, resulting in higher internal efficiency. At high loads, it is matched with the pressure stage group before the three-stage extraction steam in the intermediate pressure cylinder, still maintaining high internal efficiency. At this time, the steam flowing through the front high-pressure cylinder still performs work normally.

[0009] The beneficial effects of this invention.

[0010] The present invention allows the front-mounted high-pressure cylinder to be rigidly connected to the generator unit on the same axis, without the need for a clutch or a separate generator, thus greatly simplifying the equipment system.

[0011] In this invention, under low-load conditions in the unit, the front high-pressure cylinder and the high-pressure cylinder are connected in series, and the main steam flows through the front high-pressure cylinder and the high-pressure cylinder in full flow, thereby increasing the turbine inlet steam pressure and improving the unit's cycle efficiency.

[0012] The front-mounted high-pressure cylinder set in this invention is different from the existing disclosed similar technical solutions (202511039641.2, 202521560136.8, 202510589803.3). Different coupling targets are selected for different load conditions of the unit, namely, the high-pressure cylinder is coupled in series and the intermediate-pressure cylinder is coupled in parallel.

[0013] This invention does not opt ​​for direct switching (from the pre-high pressure cylinder's non-steam-inlet and vacuum state to the full-flow-inlet series high pressure cylinder operating condition, or vice versa; this operation is similar to, but different from, the unit's intermediate pressure cylinder startup. When the intermediate pressure cylinder is started and the high pressure cylinder is engaged, or when the high pressure cylinder is reverse-switched, there are high-pressure bypass and low-pressure bypass to stabilize the main steam and reheat steam pressures and match the steam flow. Moreover, since this operating condition exists during the unit's start-up and shutdown phase, the unit load and steam pressure are not high, and the high pressure cylinder's exhaust pressure is also not high. Nevertheless, the cylinder switching operation still needs to be performed quickly to avoid the forced draft condition; however, there is no similar high- or low-pressure bypass measure for the pre-high pressure cylinder and the high pressure cylinder's series-parallel switching, and it can only be completed by relying on the cylinder's own steam inlet and exhaust valves. Furthermore, the unit switching condition is in the medium-to-high load range, the steam pressure is high, and the back pressure faced by the pre-high pressure cylinder is also high, which is not only difficult to control, but also very dangerous for the blades).

[0014] This invention ensures safety and controllability during the switching process through an intermediate transition condition, avoiding blade blasting and overload conditions, while also considering boiler safety and controllability (maintaining stable boiler combustion conditions and main steam pressure during switching requires either increasing pressure before switching from parallel to series connection or decreasing pressure after switching from series to parallel connection, and the impact of switching on the boiler cannot be ignored; boiler pressure-bearing components have rate requirements for steam and water pressure changes, and excessively rapid steam pressure changes are detrimental to boiler steam temperature control and boiler hydrodynamic safety, and the steam drum water level control of the steam drum boiler cannot accept rapid changes in main steam pressure). Under high-load conditions, the front-end high-pressure cylinder is not shut down, but rather embedded in an appropriate position within the thermodynamic cycle (connected in parallel with the pressure stage group before the third stage of steam extraction in the intermediate-pressure cylinder), continuing to operate efficiently. Existing publicly available solutions, which use a front-end high-pressure cylinder without steam intake and open the ventilation valve for vacuuming, still incur some ventilation losses, which increase over time. Due to heat transfer within the main body, the temperature fields of the cylinder and rotor of the front-mounted high-pressure cylinder will undergo convergent changes, failing to maintain the temperature gradient required for normal operation. However, this invention not only enables the front-mounted high-pressure cylinder to operate normally but also allows it to maintain a better temperature field and internal efficiency.

[0015] Furthermore, compared to the scheme where two high-pressure cylinders operate in parallel under high-load conditions and one high-pressure cylinder is deactivated under medium and low loads, the parallel operation of two high-pressure cylinders under high-load conditions results in both cylinders handling the entire main steam flow, while each cylinder only handles a portion of the steam flow. This significantly increases blade height losses, leading to a marked decrease in efficiency within the high-pressure cylinders. In contrast, the pre-high-pressure cylinder and the high-pressure cylinder of this invention operate at full flow under all conditions, without affecting the efficiency within the high-pressure cylinders. This invention can be fully implemented under pure condensing conditions without requiring external steam supply from industrial steam users.

[0016] The series of operating condition switching control methods proposed in this invention can safely and conveniently realize operating condition conversion. In peak-shaving mode, the unit typically operates at the lowest peak load most of the time. When the grid experiences a load shortfall, the unit will increase to high or full load operation. Peak-shaving mode generally allows ample time for operating condition switching operations, and the switching can be completed in advance based on the unit load curve provided by the grid dispatch. In frequency regulation mode, the unit typically operates at intermediate load most of the time. The load command from the grid to the unit fluctuates slightly and frequently. The unit should operate in series mode to improve operating efficiency. If the load command from the grid exceeds the maximum load of series mode and there is not enough time to switch to parallel mode, the high-pressure cylinder inlet steam regulating valve can be opened directly, allowing some main steam to directly enter the high-pressure cylinder to do work, increasing the unit output. In this case, the high-pressure cylinder inlet steam regulating valve acts as a supplementary steam valve, and the unit operating in series + supplementary steam mode can also meet the maximum output of frequency regulation mode.

[0017] Whether switching from series to parallel or from parallel to series, this invention adopts a strategy of "first reducing the main steam temperature and then restoring it". Furthermore, the front-end high-pressure cylinder and the high-pressure cylinder share the burden of changes in the inlet steam temperature, maintaining a reasonable temperature change range and rate, and ensuring the safety and stability of the switching process.

[0018] In series mode, if the load command from the power grid exceeds the maximum load of the series mode and there is no time to switch to parallel mode, the high-pressure cylinder inlet steam regulating valve can be directly opened, allowing a portion of the main steam to directly enter the high-pressure cylinder to perform work. At this time, the high-pressure cylinder inlet steam regulating valve functions as a supplementary steam valve, and the unit operates in a "series + supplementary steam" mode, which can meet the maximum output requirements of the frequency regulation mode. This design allows the unit to operate beyond the design limit of the series mode within a certain range (e.g., 65%-75% of rated load), enhancing the unit's frequency regulation capability and operational flexibility.

[0019] This invention achieves automatic switching and isolation of series and parallel operating conditions by rationally arranging the front-mounted high-pressure drain A check valve, front-mounted high-pressure drain B check valve, and front-mounted high-pressure drain C check valve, and utilizing the automatic opening and closing characteristics of the check valves. It eliminates the need for complex valve linkage control, simplifies the control system, and improves reliability. Attached Figure Description

[0020] Figure 1 Schematic diagram of the connection of the pre-high pressure cylinder thermal system equipment. In the diagram: 1-High pressure cylinder, 2-Medium pressure cylinder, 3-Pre-high pressure cylinder, 4-Main steam valve, 5-High pressure regulating valve, 6-High pressure exhaust check valve, 7-Medium pressure combined steam valve, 8-Three-stage extraction steam isolation valve, 9-Three-stage extraction steam check valve, 10-Pre-mounted main steam valve, 11-Pre-mounted high pressure regulating valve, 12-Pre-mounted high pressure exhaust A shut-off valve, 13-Pre-mounted high pressure exhaust A check valve, 14-Reheat steam shut-off valve, 15-Reheat steam regulating valve, 16-Pre-mounted high pressure exhaust B shut-off valve, 17-Pre-mounted high pressure exhaust B check valve, 18-Pre-mounted high pressure exhaust C shut-off valve, 19-Pre-mounted high pressure exhaust regulating valve, 20-Pre-mounted high pressure exhaust C check valve, 21-Make-up steam valve, 22-Pre-mounted make-up steam shut-off valve.

[0021] Figure 2 A schematic diagram of the steam flow rate of the pre-pressure cylinder and the high-pressure cylinder connected in series and parallel. In the figure: the vertical axis G is the steam mass flow rate, the horizontal axis t is time, the broken line 1-5-6-7-10 is the steam flow rate curve of the pre-pressure cylinder, the broken line 1-2-3-4 is the steam flow rate curve of the high-pressure cylinder, and the broken line 11-12-13-14 is the steam flow rate curve of the intermediate-pressure cylinder.

[0022] Figure 3 A schematic diagram of steam pressure and temperature in a series-parallel connection between the front high-pressure cylinder and the high-pressure cylinder. In the diagram: the vertical axis P / T represents the main steam pressure and temperature, the horizontal axis t represents time, line 1-2-3-4 represents the main steam temperature curve, line 8-9-10 represents the main steam pressure curve, line 5-2-3-4 represents the high-pressure cylinder inlet temperature curve, and line 5-6-12 represents the front high-pressure cylinder exhaust temperature curve.

[0023] Figure 4 A schematic diagram of the steam flow rate of the front high-pressure cylinder and the high-pressure cylinder connected in parallel and series. In the figure: the vertical axis G is the steam mass flow rate, the horizontal axis t is the time, the broken line 1-2-3-4 is the steam flow rate curve of the high-pressure cylinder, the broken line 9-6-7-8-4 is the steam flow rate curve of the front high-pressure cylinder, and the broken line 11-12-13-14 is the steam flow rate curve of the intermediate-pressure cylinder.

[0024] Figure 5 This is a schematic diagram of the steam pressure and temperature of the front high-pressure cylinder and the high-pressure cylinder connected in parallel and series. In the diagram: the vertical axis P / T represents the main steam pressure and temperature, the horizontal axis represents time, line 1-2-3-4-5 represents the main steam temperature curve, line 10-11-12 represents the main steam pressure curve, line 2-3-4-8 represents the high-pressure cylinder inlet temperature curve, line 9-3-4-5 represents the front high-pressure cylinder inlet temperature curve, and line 6-7-8 represents the front high-pressure cylinder exhaust temperature curve.

[0025] Figure 2 and Figure 3The time stamps are consistent. Figure 4 and Figure 5 The time stamps are consistent. Detailed Implementation

[0026] Taking a 660 MW ultra-supercritical once-through reheat unit with an additional pre-installed high-pressure cylinder 3 as an example, this unit is a full-circumference steam inlet + make-up steam valve type. The make-up steam valve opening point is THA condition, the rated main steam pressure is 28.0 MPa, the rated main steam temperature is 600℃, and the rated reheat steam temperature is 620℃. The pre-installed high-pressure cylinder 3 is set with a flow capacity of 65% of the high-pressure cylinder, that is, the switching point is set at 65% of the unit's rated load (switching is also possible below 65% of the rated load, and the main steam pressure does not need to be raised to the rated pressure).

[0027] like Figure 1 As shown, when the pressure stages before the three-stage steam extraction of the front high-pressure cylinder 3 and the intermediate-pressure cylinder 2 are connected in parallel, the main steam pressure is 18.27 MPa. When the front high-pressure cylinder 3 and the high-pressure cylinder 1 are connected in series, the main steam pressure will increase to the rated pressure of 28.0 MPa. When the front high-pressure cylinder 3 is set with 3 pressure stages, the average pressure ratio of each stage is 0.867, which is basically consistent with the enthalpy drop and pressure ratio of each pressure stage of the high-pressure cylinder. When the front high-pressure cylinder 3 is connected in series with the high-pressure cylinder 1, it can operate efficiently. When the front high-pressure cylinder 3 is connected in parallel with the pressure stage group before the three-stage extraction of the intermediate-pressure cylinder 2, the enthalpy drop is slightly greater and the pressure ratio (0.816) is slightly less. In the case of being connected in series with the high-pressure cylinder 1, it is still within the reasonable range of variable operating conditions, and the internal efficiency does not change much. If the 660MW ultra-supercritical once-reheat unit is redesigned, the optimization objectives can incorporate the above factors, appropriately reduce the enthalpy drop of the pressure stage group before the three-stage extraction and increase the pressure ratio, so that when the unit is operating at high load, the front high-pressure cylinder 3 and the intermediate-pressure cylinder 2 can achieve higher internal efficiency when the three-stage extraction is connected in parallel.

[0028] The typical process for a single reheat unit is as follows: Main steam enters high-pressure cylinder 1 through main steam valve 4 and high-pressure regulating valve 5. In the supplementary steam condition, main steam also enters high-pressure cylinder 1 through supplementary steam valve 21. After the steam does work, it goes to the boiler reheater through high-pressure exhaust check valve 6. The reheated steam enters intermediate-pressure cylinder 2 through intermediate-pressure combined steam valve 7. After the steam does work, it is discharged to low-pressure cylinder. Intermediate-pressure cylinder 2 is equipped with three-stage extraction steam, which goes to high-pressure heater No. 3 through extraction steam isolation valve 8 and extraction steam check valve 9.

[0029] Based on the conventional process, under high-load conditions, the pressure stages before the three-stage extraction of the front high-pressure cylinder 3 and the intermediate-pressure cylinder 2 operate in parallel. The added steam flow is as follows: reheat steam enters the front high-pressure cylinder 3 through the reheat steam shut-off valve 14 and the reheat steam regulating valve 15. After the steam performs work, it is discharged through the front high-pressure exhaust B shut-off valve 16 and the front high-pressure exhaust B check valve 17 to the stage before the three-stage extraction isolation valve 8. Under medium- and low-load conditions, the front high-pressure cylinder 3 and the high-pressure cylinder 1 operate in series. The added steam flow is as follows: main steam enters the front high-pressure cylinder 3 through the front main steam valve 10 and the front high-pressure regulating valve 11. After the steam performs work, it is discharged through the front high-pressure exhaust A shut-off valve 12 and the front high-pressure exhaust A check valve 13 to the stage after the high-pressure regulating valve 5, and then enters the high-pressure cylinder to continue performing work. A front supplementary steam shut-off valve 22 is also added. The exhaust steam from the front high-pressure cylinder 3 enters the high-pressure cylinder 1 to perform work through the front supplementary steam shut-off valve 22 and the supplementary steam valve 21.

[0030] Random Start-up: Regardless of the start-up method used (high-pressure cylinder start-up, intermediate-pressure cylinder start-up, or combined high- and intermediate-pressure start-up), the front high-pressure cylinder 3 and high-pressure cylinder 1 start as a whole. The front high-pressure exhaust A shut-off valve 12 is open, and the front high-pressure exhaust A check valve 13 is in a free state [if there is an actuator, it can be forcibly opened; if there is no actuator, and the intermediate-pressure cylinder is used for start-up, the front high-pressure cylinder 3 needs to be equipped with a ventilation valve (the existing high-pressure cylinder 1 is equipped with a ventilation valve)]. The main steam valve 4, high-pressure regulating valve 5, reheat steam shut-off valve 14, reheat steam regulating valve 15, front high-pressure exhaust B shut-off valve 16, and front high-pressure exhaust C shut-off valve 18 are closed. The front main steam valve 10 is opened and controlled by the front high-pressure regulating valve 11.

[0031] High load (load increase) of the unit / series to parallel connection ( Figure 1 , Figure 2 , Figure 3 When the unit load reaches 65%, the main steam pressure reaches the rated value. If the load continues to increase, it is necessary to switch the operation of the front high-pressure cylinder 3 and high-pressure cylinder 1 from series operation to parallel operation with the pressure stage group before the three-stage steam extraction of the intermediate-pressure cylinder 2.

[0032] Main steam cooling: To maintain stable boiler combustion, the unit's CCS (Coordinated Control System) switches to BF (Boiler Follower) to maintain stable main steam pressure. The main steam temperature setpoint is gradually reduced in the DCS to lower the main steam temperature, and the switching operation is initiated simultaneously.

[0033] The front-mounted high-pressure cylinder 3 and the high-pressure cylinder 1 are connected in parallel from series: (e.g.) Figure 1 As shown, before the switch, the valve states are as follows: valves 10, 11, 12, 13, 6, 7, 8, and 9 are in the open state; valves 4, 5, 14, 15, 16, 17, 18, 19, 20, 21, and 22 are in the closed state. The steam flow rates of the pre-pressure cylinder 3, high-pressure cylinder 1, and intermediate-pressure cylinder 2 are as follows: Figure 2As shown, the steam pressure and temperature are as follows: Figure 3 As shown. Figure 2 Under the 65% THA condition of the unit, the main steam flow rate is 100%, and the reheat steam flow rate is 80%. At time t1, the main steam valve 4 and the pre-high pressure exhaust C shut-off valve 18 are opened. The high pressure regulating valve 5 and the pre-high pressure exhaust regulating valve 19 are gradually opened. The pre-high pressure exhaust C check valve 20 will open due to the forward and backward pressure difference, and the pre-high pressure regulating valve 11 will close simultaneously. During the switching process, the main steam flow rate and the steam flow rate to the boiler reheater remain unchanged, so the exhaust pressure of high pressure cylinder 1 also remains unchanged. During the switching process, the pressure after the pre-high pressure regulating valve 11 gradually decreases, the flow rates of the pre-high pressure exhaust A shut-off valve 12 and the pre-high pressure exhaust A check valve 13 gradually decrease, the steam flow rate of the high pressure regulating valve 5 gradually increases, the steam flow rate of the pre-high pressure cylinder 3 gradually decreases, and the steam flow rate from the pre-high pressure cylinder 3 to the high pressure cylinder 1 also gradually decreases. Figure 2 As shown, the steam flow rate of the pre-pressure cylinder 3 decreases from point 1 (100%) at time t1 to point 5 (approximately 42.5%) at time t2. The exhaust steam from the pre-pressure cylinder 3 gradually changes from being entirely discharged to the high-pressure cylinder 1 to being partially discharged to the high-pressure cylinder 1 and partially discharged directly to the boiler reheater. The steam flow rate of the high-pressure cylinder 1 decreases from point 1 (100%) at time t1 to point 2 (approximately 57.5%) at time t2. The steam flow rate of the high-pressure cylinder 1 gradually changes from being entirely from the pre-pressure cylinder 3 to being partially from the exhaust steam from the pre-pressure cylinder 3 and partially from the high-pressure regulating valve 5. By point 2 at time t2, all the exhaust steam from the pre-pressure cylinder 3 is directly discharged to the boiler reheater. After connecting the boiler reheater, the steam intake of high-pressure cylinder 1 comes entirely from high-pressure regulating valve 5. At this time, the pressure after the front high-pressure regulating valve 11 is about 13.92 MPa, and the pressure before and after the front high-pressure exhaust A check valve 13 is about 10.76 MPa. The exhaust pressure of the front high-pressure cylinder 3 and the steam intake pressure of high-pressure cylinder 1 are exactly equal (in this condition, the front high-pressure cylinder 3 and high-pressure cylinder 1 have similar degree of change in operating conditions, and the pressure ratio is about 0.918. This condition is the most unfavorable condition in the switching process. Therefore, the pressure is shared by the front high-pressure cylinder 3 and high-pressure cylinder 1 to minimize the degree of change in operating conditions and make both safe operating conditions, which is the best strategy for switching). Continue to open the high-pressure regulating valve 5, and simultaneously close the front high-pressure regulating valve 11. The steam differential pressure and flow direction before and after the front high-pressure exhaust A check valve 13 will reverse, and the front high-pressure exhaust A check valve 13 will close accordingly. Starting from time t2, as the high-pressure regulating valve 5 continues to open, the steam flow rate of the high-pressure cylinder 1 will continuously increase from point 2. As the front high-pressure regulating valve 11 continues to close, the steam flow rate of the front high-pressure cylinder 3 will continuously decrease from point 5 to point 6 at time t3 (approximately 15%, maintaining the same pressure ratio as at time t2). The opening degree of the front high-pressure exhaust regulating valve 19 is adjusted according to the enthalpy drop and pressure ratio of the front high-pressure cylinder 3 to keep it in a reasonable and safe operating condition.

[0034] Pressure and temperature changes as follows Figure 3As shown, from time t1 to time t5, the main steam pressure remains constant at 28 MPa (from 8 to 9 o'clock). The main steam temperature decreases from 1 o'clock (600℃) at time t1 to 2 o'clock (562℃) at time t2, and then rises back to 4 o'clock (600℃) at time t5. The exhaust temperature of the front high-pressure cylinder 3 is 5 o'clock (approximately 524℃) at time t1. As the inlet main steam temperature decreases (from 1 to 2 o'clock) and the steam flow rate decreases, resulting in reduced efficiency, the exhaust temperature tends to rise. The combined effect of these two factors leads to 6 o'clock (approximately 522℃) at time t2, and at time t5... At 12 o'clock, the steam inlet of the front high-pressure cylinder 3 is switched to reheat steam, and the exhaust steam is switched to the third-stage extraction steam of the intermediate-pressure cylinder 2. The exhaust steam temperature is slightly higher than the third-stage extraction steam temperature of the intermediate-pressure cylinder 2. The inlet steam temperature of the high-pressure cylinder 1 increases from 5 o'clock at time t1 (all exhaust steam from the front high-pressure cylinder 3) to 2 o'clock at time t2 (all main steam). During the process from t1 to t2, the steam is a mixture of the two, until 4 o'clock at time t5 (all main steam) when the rated temperature is restored. All of the above operating conditions are allowed to be steady-state operating conditions. The switching time can be determined according to the allowable pressure and temperature change rate of the unit.

[0035] The exhaust steam from the front high-pressure cylinder 3 is routed through the high-pressure cylinder exhaust pipe to the boiler reheater, then switched to the intermediate-pressure cylinder 2 via a three-stage extraction process. Figure 2 At time t3, the pre-high pressure exhaust B shut-off valve 16 is opened. Since the exhaust pressure of the pre-high pressure cylinder 3 is higher than the three-stage extraction pressure of the intermediate pressure cylinder 2, the pre-high pressure exhaust B check valve 17 is immediately opened by the steam flow. The differential pressure reversal valve before and after the pre-high pressure exhaust C check valve 20 closes accordingly. Then, the pre-high pressure exhaust C shut-off valve 18 is closed. The exhaust steam from the pre-high pressure cylinder 3 is switched from the exhaust pipe of the high pressure cylinder 1 to the three-stage extraction pipe of the intermediate pressure cylinder 2, where the pressure is lower. The pre-high pressure regulating valve 11 is further closed to further reduce the steam flow rate of the pre-high pressure cylinder 3. Figure 2 At 7:00 AM (approximately 10%) of time t4, the exhaust pressure of the pre-high-pressure cylinder 3 decreases from the exhaust pressure of the high-pressure cylinder 1 to the third-stage extraction pressure of the intermediate-pressure cylinder 2. At this time, the pressure ratio of the pre-high-pressure cylinder 3 is the design pressure ratio. Besides the required steam flow rate (approximately 6.7%) for the No. 3 high-pressure heater, excess steam will flow back into the intermediate-pressure cylinder 2. During this process, the steam inlet flow rate of the intermediate-pressure cylinder 2 is... Figure 2 The steam pressure decreases from 12:00 at time t3 to 13:00 at time t4 in the diagram (10% of the steam enters the three-section extraction pipe of the intermediate-pressure cylinder 2, causing its pressure to increase and reducing the steam flow rate of the intermediate-pressure cylinder 2 by about 4%, which corresponds to about 76% of the steam flow rate in the diagram). Since this part of the steam does not pass through the boiler reheater, this process will have a certain impact on the heat absorption ratio of the boiler superheater and reheater, as well as on the reheat cycle and the regenerative cycle. After this process is completed, the next step of switching should be carried out immediately.

[0036] The front-mounted high-pressure cylinder 3 steam source is switched from main steam to reheat steam: such as Figure 1As shown, open the reheat steam shut-off valve 14, gradually open the reheat steam regulating valve 15, and simultaneously close the front high-pressure regulating valve 11 until it is fully closed. Then close the front main steam valve 10, and the steam source of the front high-pressure cylinder 3 is switched from main steam to reheat steam. This process is as follows: Figure 2 As shown, from time t4 to time t5, the steam in the front high-pressure cylinder 3 is switched from main steam to reheat steam, with the flow rate remaining constant. That is, from 7 o'clock (about 10%) to 10 o'clock (about 10%), the main steam flow rate of the front high-pressure cylinder 3 decreases from 7 o'clock to 8 o'clock (0%), the reheat steam flow rate of the front high-pressure cylinder 3 increases from 9 o'clock (0%) to 10 o'clock, the steam flow rate of the high-pressure cylinder 1 increases from 3 o'clock (about 90%) to 4 o'clock (100%), and the steam flow rate of the intermediate-pressure cylinder 2 decreases from 13 o'clock (about 76%) at time t4 to 14 o'clock at time t6 (about 70%, which is affected by the split flow of the front high-pressure cylinder 3 and the volume of the reheater). Because the specific volume of reheat steam is much larger than that of main steam, the reheat steam regulating valve 15 can be opened to a relatively large degree, reducing throttling losses. The pre-pressure cylinder 3 can maintain a high volumetric flow rate while maintaining a small mass flow rate (it can work normally under this condition; the exhaust flow rate of the pre-pressure cylinder 3 is slightly greater than the steam flow rate required by the No. 3 high-pressure heater, and the excess steam will flow back into the intermediate-pressure cylinder 2. If it is desired to reduce the steam flow back into the intermediate-pressure cylinder 2, the reheat steam regulating valve 15 can be appropriately closed. The pre-pressure cylinder 3 has a wide range of reasonable variable operating conditions). After the switching is completed, the operating conditions of the high-pressure cylinder 1 are exactly the same as the design conditions, so there is no impact on the internal efficiency. The steam flow rate of the pressure stage group before the three-stage extraction of the intermediate-pressure cylinder 2 is reduced by about 12.5% ​​compared with the design conditions, while the stage group after the three-stage extraction is not affected. The impact on the overall internal efficiency of the intermediate-pressure cylinder is also very small, and there is no impact on the regenerative cycle and reheat cycle. At this time, the pressure stage group before the three-stage steam extraction of the front high-pressure cylinder 3 and the intermediate-pressure cylinder 2 enters the parallel operation state, and the enthalpy drop of the two is close. The enthalpy drop of the front high-pressure cylinder 3 remains close to the design condition, so it can work efficiently.

[0037] Main steam pressure reduction: The main steam pressure setpoint is gradually reduced. As the main steam pressure decreases, the high-pressure regulating valve 5 will gradually open to a reasonable opening to reduce throttling losses. The BF (boiler follow-up) switches the unit CCS (coordinated control). As the unit load increases to the rated load, the main steam pressure will also reach the rated pressure again.

[0038] The pressure and temperature changes from time t4 to time t7 are as follows: Figure 3 As shown, the inlet temperature of high-pressure cylinder 1 increases from 3 points at time t4 to 4 points at time t5 (600℃), the inlet temperature of front high-pressure cylinder 3 increases from 3 points at time t4 to 7 points at time t5 (620℃), and the main steam pressure decreases from 9 points at time t5 (28MPa) to 10 points at time t7 (18.27MPa).

[0039] Units at low load (load reduction) / parallel to series switching ( Figure 1 , Figure 4 , Figure 5 When the unit reduces the load to 65%, the main steam pressure is about 65% of the rated pressure (18.2MPa, full-circumference steam inlet + make-up steam valve model). If the load continues to be reduced, it is necessary to switch the front high-pressure cylinder 3 from parallel operation with the pressure stage group before the three-stage steam extraction of the intermediate-pressure cylinder 2 to series operation with the high-pressure cylinder 1.

[0040] Main steam pressure boost: The main steam pressure setpoint in the DCS is increased to the rated pressure. The high-pressure regulating valve 5 will gradually close, thus increasing the main steam pressure to the rated pressure. Figure 5 As shown, from time t0 to time t1, the main steam pressure increases from 10 o'clock (18.2 MPa) to 11 o'clock (28 MPa). Then, to maintain stable boiler combustion, the unit's CCS (Coordinated Control System) switches to BF (Boiler Follower) to maintain stable main steam pressure in order to meet subsequent switching conditions.

[0041] The steam source for the front-mounted high-pressure cylinder 3 is switched from reheat steam to main steam: such as Figure 1 As shown, open the pre-installed main steam valve 10, gradually open the pre-installed high-pressure regulating valve 11, and simultaneously close the reheat steam regulating valve 15 until it is fully closed. Then close the reheat steam shut-off valve 14. During the switching process, maintain a stable steam flow rate in the pre-installed high-pressure cylinder 3. Figure 4 From time t1 to time t2, the steam flow rate in the front high-pressure cylinder 3 changes from... Figure 4 At point 9, the steam flow rate drops to point 6, maintaining a constant flow rate (approximately 10%). The reheat steam flow rate of the front high-pressure cylinder 3 decreases from point 9 to point 10 (0%), while the main steam flow rate of the front high-pressure cylinder 3 increases from point 5 (0%) to point 6. The steam source for the front high-pressure cylinder 3 is switched from reheat steam to main steam. To maintain stable main steam flow rate and pressure, the high-pressure regulating valve 5 closes synchronously, causing the steam flow rate of high-pressure cylinder 1 to decrease from point 1 to point 2 (approximately 90%). The reheat steam pressure begins to rise slowly (affected by the reheater volume), and the steam flow rate of intermediate-pressure cylinder 2 slowly increases from point 11 to point 12. Subsequent processes will have a certain impact on the heat absorption ratio of the boiler superheater and reheater, as well as on the reheat and regenerative cycles. The next step should be switched immediately. From time t1, the main steam temperature setpoint is gradually reduced in the DCS, decreasing until it reaches 562℃ at time t5, at which point it begins to rise, gradually restoring to 600℃.

[0042] The pressure and temperature changes during this process are as follows: Figure 5 As shown, from time t1 to time t2, the steam source of the front high-pressure cylinder 3 is switched from reheat steam to main steam, and the temperature drops from point 9 (reheat steam) to point 3 (main steam) in the figure. The inlet steam temperature of the high-pressure cylinder 1 drops from point 2 to point 3.

[0043] The exhaust of the front-mounted high-pressure cylinder 3 is achieved through a three-stage extraction and high-pressure exhaust system: (e.g.) Figure 1 As shown, continue to open the pre-pressure regulating valve 11 and simultaneously close the high-pressure regulating valve 5 to maintain stable main steam flow and pressure. When the steam flow of the pre-pressure cylinder 3 increases to... Figure 4 At approximately 7:00 AM (around 15%), open the pre-high-pressure exhaust C shut-off valve 18 and gradually open the pre-high-pressure exhaust regulating valve 19. At this time, the pre-high-pressure exhaust C check valve 20 will be opened by the steam flow under the action of steam differential pressure. Close the pre-high-pressure exhaust B shut-off valve 16, keeping the steam flow of the pre-high-pressure cylinder 3 constant. Since the exhaust steam of the pre-high-pressure cylinder 3 is switched from the three-stage extraction steam of the intermediate-pressure cylinder 2 to high-pressure exhaust steam, the pressure of the three-stage extraction steam is reduced, and the steam flow of the intermediate-pressure cylinder 2 will increase. Figure 4 From time t2 to time t3, the steam flow rate of intermediate pressure cylinder 2 increases from 12 o'clock at time t2 to 13 o'clock at time t3, and stabilizes at 14 o'clock at time t4 (affected by the reheater volume). The opening of the front high-pressure discharge regulating valve 19 is adjusted according to the enthalpy drop and pressure ratio of the front high-pressure cylinder 3 to keep it in a reasonable and safe operating condition. The front high-pressure cylinder 3 and the high-pressure cylinder 1 enter a parallel operation state.

[0044] The front-mounted high-pressure cylinder 3 and the high-pressure cylinder 1 are connected in parallel and then in series: (e.g.) Figure 1 As shown, open the pre-high pressure exhaust A shut-off valve 12, gradually open the pre-high pressure regulating valve 11, and simultaneously close the high pressure regulating valve 5 and the pre-high pressure exhaust regulating valve 19, gradually increasing the inlet pressure to approach the outlet pressure of the pre-high pressure exhaust A check valve 13. When the steam flow rate of the pre-high pressure cylinder 3 increases to approximately 42.5% of the flow rate, and the steam flow rate of the high pressure cylinder 1 decreases to approximately 57.5% of the flow rate, the exhaust pressure of the pre-high pressure cylinder 3 and the inlet pressure of the high pressure cylinder 1 are exactly equal, with a pressure of approximately 10.76 MPa (in this condition, the pre-high pressure cylinder 3 and the high pressure cylinder 1 have similar variable operating conditions, with a pressure ratio of approximately 0.918. This is the most unfavorable condition during the switching process, so it is shared by the pre-high pressure cylinder 3 and the high pressure cylinder 1 to minimize the variable operating conditions and ensure that both are in safe operating conditions, which is the optimal switching strategy). Figure 4At time t5, the steam flow rate of high-pressure cylinder 1 decreases to 3 points (approximately 57.5%), while the steam flow rate of the front high-pressure cylinder 3 increases from 7 points at time t3 to 8 points at time t4 (42.5%). The front high-pressure regulating valve 11 is opened further, while the high-pressure regulating valve 5 and the front high-pressure exhaust regulating valve 19 are simultaneously closed. The exhaust pressure of the front high-pressure cylinder 3 continues to rise. When the pressure before the front high-pressure exhaust check valve 13 exceeds the pressure after the valve, it will be forced open by the steam flow, and the exhaust steam from the front high-pressure cylinder 3 will begin to enter the high-pressure cylinder 1. After the front high-pressure regulating valve 11 is fully open and the high-pressure regulating valve 5 and the front high-pressure exhaust regulating valve 19 are closed, all the exhaust steam from the front high-pressure cylinder 3 enters the high-pressure cylinder 1. The high-pressure main steam valve 4 and the front high-pressure exhaust C shut-off valve 18 are closed. At this time, the exhaust pressure of the front high-pressure cylinder 3 and the inlet pressure of the high-pressure cylinder 1 both reach 18.27 MPa. Figure 4 From time t5 to time t6, the steam flow rate of high-pressure cylinder 1 increases from 3 to 4 (100%), and the steam flow rate of the front high-pressure cylinder 3 increases from 7 to 4. During this process, the main steam pressure (flow rate) and the high-pressure exhaust pressure (flow rate) remain unchanged. At this point, the front high-pressure cylinder 3 and high-pressure cylinder 1 switch from parallel to series connection, and then the boiler follower (BF) switches to the control system (CCS). As the unit load decreases, the main steam pressure will gradually decrease. However, compared to the conventional method of steam intake through high-pressure cylinder 1, the presence of the front high-pressure cylinder 3 significantly increases the main steam pressure, which will significantly improve the unit's cycle efficiency.

[0045] The pressure and temperature changes during this process are as follows: Figure 5 As shown, from time t2 to time t5, the inlet steam temperature of both the front high-pressure cylinder 3 and the high-pressure cylinder 1 decreases from point 3 to point 4 (562℃). From time t5 to time t6, the main steam temperature (the inlet temperature of the front high-pressure cylinder 3) rises from point 4 to point 5 (600℃). The exhaust temperature of the front high-pressure cylinder 3 rises from point 6 (slightly higher than the third-stage extraction temperature of the intermediate-pressure cylinder 2) through point 7 (approximately 522℃) to point 8 (approximately 524℃). The inlet temperature of the high-pressure cylinder 1 decreases from point 4 to point 8. The main steam pressure rises from point 3 to point 12, maintaining the rated pressure. All of the above operating conditions are allowed as steady-state conditions, and the switching time can be determined according to the allowable pressure and temperature change rate of the unit.

[0046] Gasoline replenishment conditions: such as Figure 1As shown, the supplementary steam valve 21 can be used in both parallel and series operation of the front high-pressure cylinder 3 and the high-pressure cylinder 1. Specifically, when the unit is under high load, the pressure stage group before the three-stage extraction of steam from the front high-pressure cylinder 3 and the intermediate-pressure cylinder 2 is connected in parallel, the front supplementary steam shut-off valve 22 is in the closed state, and the supplementary steam valve 21 can be opened at any time during the dynamic process of the unit increasing load. The steam source is taken from after the main steam valve 4. When the unit is under medium and low load, the front high-pressure cylinder 3 and the high-pressure cylinder 1 are connected in series, the front supplementary steam shut-off valve 22 is in the open state, and the supplementary steam valve 21 can be opened at any time during the dynamic process of the unit increasing load. The steam source is taken from after the front high-pressure exhaust A check valve 13. In this way, the supplementary steam valve 21 has the ability to quickly increase load under any operating condition.

[0047] Frequency regulation mode: Compared to peak shaving mode (AGC automatic generation control), where the unit tracks the load curve given by the grid dispatch, it can predict the load situation in the subsequent time and accurately grasp the switching timing of the front high-pressure cylinder 3. However, when the unit operates in frequency regulation mode (such as the ACE regional control deviation mode of the Shanxi power grid), the typical load change range is 30%-70% of the rated load, and the command change frequency is on the minute level. If the load command reaches and exceeds 65% of the rated load, the front high-pressure cylinder 3 will not have a chance to switch. At this time, the unit must supplement steam in series mode. Figure 1 As shown, the front high-pressure cylinder 3 and high-pressure cylinder 1 are in series operation. When the unit load reaches 65% of the rated load, the main steam has reached the rated pressure. When the unit load continues to increase, the main steam valve 4 is opened (in this condition, the flow capacity of the front high-pressure cylinder 3 has reached its upper limit, and opening the front supplementary steam shut-off valve 22 is no longer effective). The supplementary steam valve 21 or the high-pressure regulating valve 5 is gradually opened (or after the supplementary steam valve 21 is fully opened, if the unit output is still insufficient and the front high-pressure cylinder 3 is still within a reasonable range of variable operating conditions, the high-pressure regulating valve 5 is continued to be opened) to increase the unit output and meet the grid demand. Under this condition, the steam flow of high-pressure cylinder 1 increases, the inlet steam pressure increases, and the inlet steam temperature also increases, resulting in increased output. The output of the front high-pressure cylinder 3 decreases due to the increased back pressure, but the total unit output increases, which can meet the unit's operation above 65% of the rated load within a certain range. If we consider the front high-pressure cylinder 3 and the high-pressure cylinder 1 as a whole, then the steam replenishment valve 21 or the high-pressure regulating valve 5 are equivalent to the steam replenishment valve of the front high-pressure cylinder 3 and the high-pressure cylinder 1 as a whole, which can meet the upper limit output conditions of the unit in frequency regulation mode.

[0048] In the above embodiment, opening the steam replenishment valve 21 can instantly increase the steam intake and respond to the power grid frequency regulation command.

[0049] In the above embodiments, check valves 13 and 17 can also be regulating valves. After the check valves are changed to regulating valves, the switching process cannot be decoupled, involving multi-valve linkage. Flow and pressure measuring points must be added and automatically completed by the control system.

[0050] In the above embodiments, a clutch can be provided between the front high-pressure cylinder 3 and the high-pressure cylinder 1.

[0051] In the above embodiments, the front-mounted high-pressure cylinder 3 can also drive a generator independently.

[0052] The above embodiments are also applicable to nozzle steam distribution units.

Claims

1. A thermal system for a front-mounted high-pressure cylinder and its operation method for a coal-fired steam turbine generator set, characterized in that... A front-mounted high-pressure cylinder is installed on the basis of a conventional steam turbine generator set. The front-mounted high-pressure cylinder is rigidly connected to the generator set on the same axis. No clutch is installed. No split-shaft scheme is used. A separate generator is installed. Under low-load conditions in the unit, the front high-pressure cylinder is connected in series with the high-pressure cylinder, and the main steam flows through the front high-pressure cylinder and the high-pressure cylinder in full. Under high load conditions, the pressure stages before the three-stage extraction of steam from the front high-pressure cylinder and the intermediate high-pressure cylinder are connected in parallel. The steam source of the front high-pressure cylinder is switched from main steam to reheat steam. The exhaust steam from the front high-pressure cylinder is discharged to the three-stage extraction steam pipeline of the intermediate high-pressure cylinder. During the switching operation, the main steam pressure and temperature are adjusted in coordination, and the operation is carried out under constant main steam pressure. When switching from series to parallel, the main steam pressure is reduced first and then the main steam temperature is reduced first and then restored. When switching from parallel to series, the main steam pressure is increased first and then the switching is performed, and the main steam temperature is reduced first and then restored. The front high-pressure cylinder and the high-pressure cylinder share the burden of changes in inlet steam temperature and maintain a reasonable temperature change range and rate.

2. The thermal system and operating method of the front-mounted high-pressure cylinder for a coal-fired steam turbine generator set according to claim 1, characterized in that... The load switching point is near 65% of the rated load.

3. The thermal system and operating method of the front-mounted high-pressure cylinder for a coal-fired steam turbine generator set according to claim 1, characterized in that... In series mode, to improve operating efficiency; once the load command from the power grid to the unit exceeds the maximum load of the series mode, and there is no time to switch to parallel mode, the high-pressure cylinder steam inlet regulating valve is opened directly to allow a portion of the main steam to directly enter the high-pressure cylinder to do work, increasing the unit output. At this time, the high-pressure cylinder steam inlet regulating valve is equivalent to the function of the supplementary steam valve. The unit operates in series + supplementary steam mode to meet the maximum output requirements of the frequency regulation mode.

4. The system and operating method according to claim 1, characterized in that, The thermal system includes: a main steam valve, a high-pressure regulating valve, a high-pressure cylinder, a high-pressure exhaust check valve, a medium-pressure combined steam valve, a medium-pressure cylinder, a three-stage extraction steam isolation valve, and a three-stage extraction steam check valve. Pre-positioned main steam valve, pre-positioned high-pressure regulating valve, pre-positioned high-pressure cylinder, pre-positioned high-pressure exhaust A shut-off valve, pre-positioned high-pressure exhaust A check valve; Reheat steam shut-off valve, reheat steam regulating valve, front-mounted high-pressure exhaust B shut-off valve, front-mounted high-pressure exhaust B check valve, front-mounted high-pressure exhaust C shut-off valve, front-mounted high-pressure exhaust regulating valve, front-mounted high-pressure exhaust C check valve; Steam replenishment valve, pre-positioned steam replenishment shut-off valve; Among them, the front high-pressure exhaust A check valve is set on the flow path from the front high-pressure cylinder exhaust to the high-pressure cylinder, the front high-pressure exhaust B check valve is set on the flow path from the front high-pressure cylinder exhaust to the three-stage extraction steam pipeline of the intermediate-pressure cylinder, and the front high-pressure exhaust C check valve is set on the flow path from the front high-pressure cylinder exhaust to the boiler reheater. The mechanical automatic (passive) opening and closing characteristics of the check valves are used to realize the automatic (adaptive) switching and isolation of series and parallel operating conditions.

5. The system and operating method according to claim 4, characterized in that, The aforementioned high-pressure drain A check valve, high-pressure drain B check valve, and high-pressure drain C check valve are either free-state check valves or forced-opening check valves with actuators.

6. The system and operating method according to claim 1, characterized in that, Based on the case, the pre-pressure cylinder is equipped with three pressure stages, with an average pressure ratio of 0.867, which is consistent with the enthalpy drop and pressure ratio of each pressure stage of the high-pressure cylinder.

7. The system and operating method according to claim 1, characterized in that, The specific steps for switching from series to parallel connections include: Main steam cooling: To maintain stable boiler combustion, the unit coordinates and controls the boiler to follow suit, maintains stable main steam pressure, and gradually reduces the main steam temperature setpoint. Switching the exhaust steam from the front-mounted high-pressure cylinder: Close the front-mounted high-pressure regulating valve slightly and open it wider. When the exhaust steam pressure of the front-mounted high-pressure cylinder equals the inlet steam pressure of the high-pressure cylinder, the pressures before and after the front-mounted high-pressure exhaust A check valve are balanced. Continue to open the high-pressure regulating valve wider and close the front-mounted high-pressure regulating valve slightly. The front-mounted high-pressure exhaust A check valve will automatically close, and the exhaust steam from the front-mounted high-pressure cylinder will switch to the front-mounted high-pressure exhaust C shut-off valve, the front-mounted high-pressure exhaust regulating valve, and the front-mounted high-pressure exhaust C check valve. Then, close the front-mounted high-pressure regulating valve slightly and open the front-mounted high-pressure exhaust B shut-off valve. The front-mounted high-pressure exhaust B check valve will automatically open. The front-mounted high-pressure exhaust C-type check valve automatically closes, and the front-mounted high-pressure cylinder exhaust steam is redirected to the intermediate-pressure cylinder three-stage extraction steam pipeline; Steam source switching: Close the front main steam valve and the front high pressure regulating valve, open the reheat steam shut-off valve and the reheat steam regulating valve, and the steam source of the front high pressure cylinder is switched from main steam to reheat steam; Main steam pressure reduction: Gradually reduce the main steam pressure setpoint, gradually open the high-pressure regulating valve, and coordinate the boiler control accordingly. When the unit load rises to the rated load, the main steam pressure reaches the rated pressure again.

8. The system and operating method according to claim 1, characterized in that, The specific steps for switching from parallel to series connection include: Main steam pressure boosting: The main steam pressure setpoint in the coordinated control is raised to the rated pressure, the high-pressure regulating valve is gradually closed to raise the main steam pressure to the rated pressure, maintain stable boiler combustion, and the coordinated control switches the boiler to follow suit to maintain stable main steam pressure; Main steam cooling: Gradually reduce the main steam temperature setpoint; Steam source switching: Open the front main steam valve and the front high pressure regulating valve, close the reheat steam shut-off valve and the reheat steam regulating valve, and the steam source of the front high pressure cylinder is switched from reheat steam to main steam; Front-mounted high-pressure cylinder exhaust switching: The exhaust steam from the front-mounted high-pressure cylinder enters the high-pressure cylinder through the front-mounted high-pressure exhaust A shut-off valve and the front-mounted high-pressure exhaust A check valve. The front-mounted high-pressure exhaust regulating valve adjusts its opening according to the enthalpy drop and pressure ratio of the front-mounted high-pressure cylinder to maintain a reasonable and safe operating condition. Parallel-to-series switching: Gradually open the front high-pressure regulating valve until it is fully open, and simultaneously close the high-pressure regulating valve until it is fully closed. When the steam flow of the high-pressure cylinder increases to 100%, the steam flow of the front high-pressure cylinder increases to 100%, and the front high-pressure cylinder and the high-pressure cylinder switch from parallel to series connection. The boiler follows suit and coordinates the control.

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