Fused salt energy storage auxiliary photo-thermal unit frequency modulation control system and method

Abstract

The invention discloses a fused salt energy storage auxiliary photo-thermal unit frequency modulation control system and method.The system comprises a hot salt tank and a cold salt tank, high-temperature fused salt in the hot salt tank flows through a steam generation system, and after heat exchange is completed in the steam generation system, the high-temperature fused salt becomes low-temperature fused salt to be stored in the cold salt tank; the low-temperature fused salt in the cold salt tank is reheated into high-temperature fused salt through a heat absorption system and returns to the hot salt tank again; the steam generation system drives the turbine system to operate, and the turbine system drives the power generation system to output electric energy. One part of electric energy output by the power generation system is transmitted to a power grid, and the other part is supplied to a plant through a high-voltage plant transformer; the variable-frequency electric heater obtains electric energy from the high-voltage station transformer and heats low-temperature fused salt in the cold salt tank, and the fused salt frequency modulation control system receives frequency modulation signals from a power grid and adjusts power output of the variable-frequency electric heater according to requirements. The method can improve the frequency modulation response performance of the photo-thermal unit, and improves the operation stability of a high-proportion new energy power grid.

Description

Technical Field

[0001] The invention belongs to the technical field of frequency modulation optimization control of solar thermal power units, and particularly relates to a molten salt energy storage assisted solar thermal power unit frequency modulation control system and method. Background Art

[0002] With its unique heat storage system, solar thermal power generation reduces the coupling between some systems during the operation of the unit, and to a certain extent, enables the unit to have good frequency modulation potential. Participating in frequency modulation by solar thermal power generation can not only improve the frequency stability of the power grid, but also reduce the dependence on traditional fossil energy for frequency modulation and promote the consumption of renewable energy.

[0003] However, in the field of frequency modulation optimization control of solar thermal power generation units, the existing technologies mainly rely on traditional frequency modulation methods. By directly changing the opening of the steam admission control valve of the steam turbine, the output of the unit is changed, and the molten salt flow is adjusted to change the steam inlet pressure to maintain the balance and stability of the entire unit operation. Although these methods can achieve the load regulation of the unit following AGC (Automatic Gain Control) to a certain extent, for primary frequency modulation with more stringent requirements for the load change rate, the following deficiencies will be exposed: 1) Slow response speed. The conventional island part of the solar thermal power unit is the same as that of the thermal power unit, and the steam turbine is driven to do work by high-temperature and high-pressure steam. The process of generating steam still involves heat exchange, and the large delay and large inertia of the heat exchange system will inevitably affect the frequency modulation rate of the unit. 2) Insufficient frequency modulation accuracy. The solar thermal power unit is a complex system with multiple variables, and the output of the unit is also affected by multiple parameters. The fluctuation of one or more of these parameters often causes deviation in the frequency modulation amount of the unit. 3) Lack of flexibility. The existing systems and technologies only utilize the conventional energy exchange of the solar thermal power unit, lack the flexible scheduling of multiple energy forms, and it is difficult to make full use of the advantages of renewable energy. Summary of the Invention

[0004] The purpose of the invention is to provide a molten salt energy storage assisted solar thermal power unit frequency modulation control system and method for the above problems in the existing technologies, significantly improving the frequency modulation response performance of the solar thermal power unit, including the rapidity and accuracy of the solar thermal power unit frequency modulation; and making full use of renewable energy, suppressing the frequency fluctuation of the power grid, and improving the operation stability of the high-proportion new energy power grid.

[0005] In order to achieve the above purpose, the invention has the following technical solutions:

[0006] In a first aspect, the present invention provides a molten salt energy storage assisted solar thermal power unit frequency modulation control system, which includes a hot salt tank and a cold salt tank. The high-temperature molten salt in the hot salt tank flows through a steam generation system. After the high-temperature molten salt completes heat exchange in the steam generation system, it becomes low-temperature molten salt and is stored in the cold salt tank. The low-temperature molten salt in the cold salt tank is reheated to high-temperature molten salt through an endothermic system and then returns to the hot salt tank again. The steam generation system drives the steam turbine system to operate, and the steam turbine system drives the power generation system to output electric energy. A part of the electric energy output by the power generation system is transmitted to the power grid, and the other part is supplied to the plant through a high-voltage station service transformer. The variable frequency electric heater obtains electric energy from the high-voltage station service transformer and heats the low-temperature molten salt in the cold salt tank. The molten salt frequency modulation control system receives a frequency modulation signal from the power grid and adjusts the power output of the variable frequency electric heater according to the demand.

[0007] As a preferred solution, the steam generation system is supplied with water through a feed water system. The water is heated in the steam generation system to generate steam, which drives the steam turbine system to operate, and the steam turbine system then drives the power generation system to output electric energy.

[0008] As a preferred solution, a regulating valve and a temperature measuring element are arranged on the connecting branch of the variable frequency electric heater. The regulating valve adjusts the opening according to the molten salt temperature measured by the temperature measuring element, and the regulating valve controls the variable frequency electric heater to keep the temperature of the molten salt after heating stable and meet the requirements of high-temperature molten salt.

[0009] As a preferred solution, a hot salt inlet valve is arranged on the pipeline between the regulating valve and the hot salt tank, and a cold salt recovery valve is arranged on the pipeline between the regulating valve and the cold salt tank. If the temperature of the molten salt after being heated by the variable frequency electric heater meets the requirements of high-temperature molten salt, the hot salt inlet valve is opened and the cold salt recovery valve is closed. If the temperature of the molten salt after being heated by the variable frequency electric heater does not meet the requirements of high-temperature molten salt, the hot salt inlet valve is closed and the cold salt recovery valve is opened.

[0010] As a preferred solution, the frequency modulation signal from the power grid includes an automatic gain control AGC signal and a primary frequency modulation signal.

[0011] In a second aspect, a molten salt energy storage assisted solar thermal power unit frequency modulation control method is provided, including:

[0012] Obtain the output power Pg of the power generation system, the basic plant electricity consumption P0, and the operating power Ph of the variable frequency electric heater, calculate the power P of the power plant feeding into the grid, and adjust the power P of the power plant feeding into the grid by controlling the operating power Ph of the variable frequency electric heater. The calculation expression of the power P of the power plant feeding into the grid is as follows:

[0013] P = Pg - (P0 + Ph)

[0014] The molten salt frequency modulation control system receives the frequency modulation signal ΔP from the power grid and calculates the power control range of the variable frequency electric heater according to the following formula:

[0015] P + ΔP = Pg - [P0 + Ph']

[0016] Ph' = Ph - ΔP

[0017] Within the power control range of the variable frequency electric heater, when the power grid requires an increase in load, that is, when ΔP is positive, the variable frequency electric heater reduces the heating load on the basis of the current operating power, reduces Ph to Ph', and increases the power P of the power plant going online; when the power grid requires a decrease in load, that is, when ΔP is negative, the variable frequency electric heater increases the heating load on the basis of the current operating power, increases Ph to Ph', and reduces the power P of the power plant going online.

[0018] As a preferred solution, during the process of changing the operating power of the variable frequency electric heater, a feedforward-feedback composite control strategy is adopted to adjust the opening of the regulating valve set on the connecting branch of the variable frequency electric heater to maintain the stability of the molten salt temperature. In the feedforward-feedback composite control strategy, the change trend of the regulating valve opening is predicted through the feedforward link, and the temperature deviation is corrected in combination with the feedback link;

[0019] The opening is calculated according to the following formula:

[0020] VP = VP0 + VP1 = FF(Ph') + PID(Et)

[0021] In the formula, VP is the regulating valve opening command, VP0 is the basic regulating valve opening command, that is, FF(Ph'), which is also equal to FF(Ph - ΔP), representing the opening command calculated by a non-linear function according to the power of the variable frequency electric heater during frequency modulation; VP1 is the fine-tuning opening command of the regulating valve, that is, PID(Et), representing the deviation between the molten salt temperature measured by the temperature measuring element set on the connecting branch of the variable frequency electric heater and the molten salt temperature set value, and the opening command obtained after PID operation; the sum of VP0 and VP1 is the final regulating valve opening command VP, which guides the actual opening of the regulating valve.

[0022] As a preferred solution, when the variable frequency electric heater is operating stably, the low-temperature molten salt is heated and after heating, it meets the inlet salt temperature requirements of the high-temperature molten salt and enters the hot salt tank through the hot salt inlet valve; if the molten salt temperature after heating by the variable frequency electric heater does not meet the high-temperature molten salt requirements, the hot salt inlet valve is closed and the cold salt recovery valve is opened to recover it to the cold salt tank;

[0023] The following temperature interlock control logic is set for the hot salt inlet valve:

[0024] IF(T ≥ Ts), OPEN; IF(T < Ts), CLOSE

[0025] In the formula, T represents the molten salt temperature at the outlet of the variable-frequency electric heater measured by the temperature measuring element, and Ts represents the molten salt temperature that meets the requirement for the hot salt tank to receive salt;

[0026] Set the following temperature interlock control logic for the cold salt recovery valve:

[0027] IF(T < Ts), OPEN; IF(T ≥ Ts), CLOSE.

[0028] As a preferred solution, in the case of insufficient energy supply from the optical field due to extreme weather, the variable-frequency electric heater is used to consume external electric energy to maintain the molten salt circulation, so as to realize the uninterrupted operation of the solar thermal power unit.

[0029] As a preferred solution, by preferentially adjusting the operating power of the variable-frequency electric heater, the fluctuation of the steam turbine system is reduced.

[0030] Compared with the prior art, the present invention has at least the following beneficial effects:

[0031] The molten salt energy storage assisted solar thermal power unit frequency modulation control system of the present invention provides a system design and control strategy for the solar thermal power generation unit to flexibly utilize its own energy form. The present invention significantly improves the frequency modulation response performance of the solar thermal power unit. Compared with the lag of the traditional solar thermal power unit relying on the thermal inertia regulation of the optical field, the present invention directly realizes load regulation at the "electric-electric" level through the reverse linkage between the electric heating power and the power grid frequency modulation, avoiding other energy exchange processes, thereby significantly improving the rapidity and accuracy of the frequency modulation of the solar thermal power unit, and effectively meeting the stringent requirements of the new power system for flexible regulation resources. The present invention enhances the adaptability of the power grid for bidirectional frequency modulation based on the solar thermal power generation unit through the dual coupling mechanism of the power dynamic regulation of the variable-frequency electric heater and the closed-loop control of the molten salt temperature. Especially in the scenario where the fluctuations of photovoltaic or wind power are severe, this characteristic can significantly suppress the power grid frequency fluctuations and improve the operation stability of the high-proportion new energy power grid.

[0032] Furthermore, the present invention provides a molten salt temperature stable control technology of "feedforward-feedback" compound control (FF+PID) during the molten salt assisted frequency modulation process, which cooperates with the intelligent interlock logic of the hot salt inlet valve and the cold salt recovery valve to automatically switch the path, which can not only accurately control the molten salt temperature and ensure the energy storage quality of the hot salt tank, but also reduce the energy loss caused by the repeated heating of the low-temperature molten salt.

[0033] Furthermore, the present invention constructs a multiple safety guarantee mechanism, which can consume external electric energy through the variable-frequency electric heater to maintain the molten salt circulation when the energy supply from the optical field is insufficient due to extreme weather (continuous cloudy or rainy and snowy weather), realize the long-term uninterrupted operation of the solar thermal power unit, avoid the risk of unit shutdown caused by energy interruption, and significantly improve the power supply reliability of the system.

[0034] Furthermore, through the frequency modulation power distribution algorithm, the present invention preferentially utilizes the fast adjustment characteristics of the electric heating system to reduce the frequent fluctuations of the steam turbine system, and can significantly optimize the equipment life and operation and maintenance costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and those of ordinary skill in the art can obtain other related drawings based on these drawings without creative efforts.

[0036] Figure 1 Structure block diagram of the molten salt energy storage assisted solar thermal power generation unit frequency modulation control system of the present invention;

[0037] In the drawings: 1 - hot salt tank; 2 - steam generation system; 3 - cold salt tank; 4 - heat absorption system; 5 - feed water system; 6 - steam turbine system; 7 - power generation system; 8 - power grid; 9 - high-voltage auxiliary transformer; 10 - variable-frequency electric heater; 11 - regulating valve; 12 - temperature measuring element; 13 - hot salt inlet valve; 14 - cold salt recovery valve; 15 - molten salt frequency modulation control system. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, those of ordinary skill in the art can obtain other embodiments without creative efforts.

[0039] Please refer to Figure 1 , an embodiment of the present invention provides a molten salt energy storage assisted solar thermal power generation unit frequency modulation control system. In Figure 1In it, solid lines represent pipelines, dashed lines represent electric wires, and dotted lines represent control lines. In the system structure of the present invention, there are involved a hot salt tank 1, a steam generation system 2, a cold salt tank 3, an endothermic system 4, a feed water system 5, a steam turbine system 6, a power generation system 7, a power grid 8, a high-voltage auxiliary transformer 9, a variable-frequency electric heater 10, a regulating valve 11, a temperature measuring element 12, a hot salt inlet valve 13, a cold salt recovery valve 14, and a molten salt frequency modulation control system 15. Among them, the variable-frequency electric heater 10, the regulating valve 11, the temperature measuring element 12, the hot salt inlet valve 13, the cold salt recovery valve 14, and the molten salt frequency modulation control system 15 together constitute a molten salt auxiliary frequency modulation system. The high-temperature molten salt in the hot salt tank 1 flows through the steam generation system 2. After the high-temperature molten salt completes heat exchange in the steam generation system 2, it becomes low-temperature molten salt and is stored in the cold salt tank 3; the low-temperature molten salt in the cold salt tank 3 is reheated to high-temperature molten salt through the endothermic system 4 and then returns to the hot salt tank 1 again; the steam generation system 2 drives the steam turbine system 6 to operate, and the steam turbine system 6 drives the power generation system 7 to output electric energy; a part of the electric energy output by the power generation system 7 is transmitted to the power grid 8, and the other part is supplied to the plant through the high-voltage auxiliary transformer 9; the variable-frequency electric heater 10 obtains electric energy from the high-voltage auxiliary transformer 9 and heats the low-temperature molten salt in the cold salt tank 3. The molten salt frequency modulation control system 15 receives a frequency modulation signal from the power grid 8 and adjusts the power output of the variable-frequency electric heater 10 according to the demand.

[0040] Further, the steam generation system 2 is supplied with water through the feed water system 5. The water is heated in the steam generation system 2 to generate steam, which drives the steam turbine system 6 to operate, and the steam turbine system 6 then drives the power generation system 7 to output electric energy.

[0041] In a possible implementation manner, a regulating valve 11 and a temperature measuring element 12 are arranged on the connection branch of the variable-frequency electric heater 10 in the embodiment of the present invention. The regulating valve 11 adjusts the opening according to the molten salt temperature measured by the temperature measuring element 12, and the regulating valve 11 controls the variable-frequency electric heater 10 to keep the temperature of the molten salt after heating stable and meet the requirements of high-temperature molten salt.

[0042] In a possible implementation manner, a hot salt inlet valve 13 is arranged on the pipeline between the regulating valve 11 and the hot salt tank 1, and a cold salt recovery valve 14 is arranged on the pipeline between the regulating valve 11 and the cold salt tank 3. If the temperature of the molten salt after being heated by the variable-frequency electric heater 10 meets the requirements of high-temperature molten salt, the hot salt inlet valve 13 is opened and the cold salt recovery valve 14 is closed, so that the heated high-temperature molten salt enters the hot salt tank 1; if the temperature of the molten salt after being heated by the variable-frequency electric heater 10 does not meet the requirements of high-temperature molten salt, the hot salt inlet valve 13 is closed and the cold salt recovery valve 14 is opened to recover the molten salt to the cold salt tank 3.

[0043] Further, the frequency modulation signal from the power grid 8 includes an automatic gain control AGC signal and a primary frequency modulation signal.

[0044] Another embodiment of the present invention further provides a frequency modulation control method for a molten salt energy storage assisted solar thermal power unit, including:

[0045] Obtain the output power Pg of the power generation system 7, the basic plant power consumption P0, and the operating power Ph of the variable frequency electric heater 10, calculate the power P of the power plant feeding into the grid, and adjust the power P of the power plant feeding into the grid by controlling the operating power Ph of the variable frequency electric heater 10; the calculation expression of the power P of the power plant feeding into the grid is as follows:

[0046] P = Pg - (P0 + Ph)

[0047] The molten salt frequency modulation control system 15 receives a frequency modulation signal ΔP from the power grid 8 and calculates the power control range of the variable frequency electric heater 10 according to the following formula:

[0048] P + ΔP = Pg - [P0 + Ph']

[0049] Ph' = Ph - ΔP

[0050] Within the power control range of the variable frequency electric heater 10, when the power grid 8 requires an increase in load, that is, when ΔP is positive, the variable frequency electric heater 10 reduces the heating load on the basis of the current operating power, reduces Ph to Ph', and increases the power P of the power plant feeding into the grid; when the power grid 8 requires a decrease in load, that is, when ΔP is negative, the variable frequency electric heater 10 increases the heating load on the basis of the current operating power, increases Ph to Ph', and reduces the power P of the power plant feeding into the grid.

[0051] During the process of changing the operating power of the variable frequency electric heater 10, a feedforward-feedback composite control strategy (FF + PID) is adopted to adjust the opening of the regulating valve 11 arranged on the connecting branch of the variable frequency electric heater 10 to maintain the stability of the molten salt temperature. In the feedforward-feedback composite control strategy, the change trend of the opening of the regulating valve 11 is predicted through the feedforward link, and the temperature deviation is corrected in combination with the feedback link;

[0052] The opening is calculated according to the following formula:

[0053] VP = VP0 + VP1 = FF(Ph') + PID(Et)

[0054] In the formula, VP is the opening command of the regulating valve 11, VP0 is the basic opening command of the regulating valve 11, that is, FF(Ph'), and is also equal to FF(Ph - ΔP), which represents the opening command calculated by a non-linear function according to the power of the variable-frequency electric heater 10 during the frequency modulation process; VP1 is the fine-tuning opening command of the regulating valve 11, that is, PID(Et), which represents the opening command obtained after PID operation on the deviation between the molten salt temperature measured by the temperature measuring element 12 provided on the connecting branch of the variable-frequency electric heater 10 and the set value of the molten salt temperature; the sum of VP0 and VP1 is the final opening command VP of the regulating valve 11, which guides the actual opening of the regulating valve 11.

[0055] When the variable-frequency electric heater 10 operates stably, the low-temperature molten salt is heated and after heating, it meets the inlet salt temperature requirement of the high-temperature molten salt, and enters the hot salt tank 1 through the hot salt inlet valve 13; if the molten salt temperature after the variable-frequency electric heater 10 is heated does not meet the requirement of the high-temperature molten salt, the hot salt inlet valve 13 is closed and the cold salt recovery valve 14 is opened, and it is recovered to the cold salt tank 3;

[0056] The following temperature interlock control logic is set for the hot salt inlet valve 13:

[0057] IF(T≥Ts), OPEN; IF(T<Ts), CLOSE

[0058] In the formula, T represents the molten salt temperature at the outlet of the variable-frequency electric heater 10 measured by the temperature measuring element 12, and Ts represents the molten salt temperature that meets the inlet salt requirement of the hot salt tank 1;

[0059] The following temperature interlock control logic is set for the cold salt recovery valve 14:

[0060] IF(T<Ts), OPEN; IF(T≥Ts), CLOSE

[0061] Due to the particularity of the energy acquisition method of the solar thermal power unit, there may be a situation of insufficient energy supply during continuous cloudy, rainy or snowy weather. In the molten salt energy storage assisted solar thermal power unit frequency modulation control system according to the embodiment of the present invention, when the light field energy supply is insufficient due to extreme weather, the external electric energy is consumed by the variable-frequency electric heater 10 to maintain the molten salt circulation, so as to realize the uninterrupted operation of the solar thermal power unit.

[0062] In a possible implementation manner, the present invention can significantly optimize the equipment life and operation and maintenance cost by preferentially adjusting the operating power of the variable-frequency electric heater 10 and reducing the fluctuation of the steam turbine system 6.

[0063] For a 100MW solar thermal power unit, a 2×5MW variable-frequency electric heater group is configured. The variable-frequency electric heaters are set in the total operation control mode, and overall receive the power grid frequency modulation command, and distribute the frequency modulation command according to the operating states of the 2 variable-frequency electric heaters.

[0064] At a certain moment, the solar thermal power unit of this embodiment operates stably at a load of 60 MW. The operating power of both 2 variable-frequency electric heaters is 4 MW, that is, Ph = 2 × 4 = 8 MW. After the heaters work, the opening of the regulating valve is 38%. The temperature measuring element measures the molten salt temperature to be 572 °C. The hot salt inlet valve is opened and the cold salt recovery valve is closed. The power grid 8 issues an AGC command of ΔP = +5 MW to the solar thermal power unit, and the target grid-connected power of the solar thermal power unit becomes P + ΔP = 60 MW + 5 MW = 65 MW.

[0065] S1. The molten salt frequency modulation control system 15 performs the calculation of power redistribution of the variable-frequency electric heater group. The total power of the variable-frequency electric heater group is adjusted to Ph' = Ph - ΔP = 8 - 5 = 3 MW, and is evenly distributed to the 2 heaters, and the operating power is respectively reduced to 1.5 MW.

[0066] S2. Synchronously trigger the opening control of the regulating valve 11.

[0067] VP = VP0 + VP1 = FF(Ph') + PID(Et)

[0068] 1) The VP0 part

[0069] VP0 = FF(Ph') = FF(3)

[0070] The functional relationship between VP0 and the power of the variable-frequency electric heater is as follows:

[0071] Ph'(MW) VP0(%) 0 0 2 15 4 25 6 35 8 40 10 45

[0072] By interpolation, VP0 = FF(3) = 20%

[0073] Before frequency modulation, VP0 = FF(8) = 40%. Therefore, when the power of the variable-frequency electric heater group changes, the opening of the regulating valve 11 rapidly decreases by 40% - 20% = 20%.

[0074] 2) The VP1 part

[0075] The temperature measuring element 12 detects that the outlet temperature drops from 572 °C to 565 °C. The PID performs proportional-integral operation, and the opening of the regulating valve 11 is adjusted within a small range based on the 20% opening, and finally tends to be stable.

[0076] S3. Temperature interlock detection.

[0077] The system sets Ts = 568 °C.

[0078] During the process that the molten salt temperature at the outlet of the heater group drops from 572 °C to 565 °C, when the detected temperature T is lower than 568 °C, i.e., T < Ts, the hot salt inlet valve 13 is automatically closed, and at the same time, the cold salt recovery valve 14 is automatically opened to recover the unqualified molten salt to the cold salt tank 3; after the opening of the regulating valve 11 is adjusted and the temperature rebounds and is higher than 568 °C, i.e., T ≥ Ts, the hot salt inlet valve 13 is automatically opened, and at the same time, the cold salt recovery valve 14 is automatically closed to convey the qualified molten salt to the hot salt tank 1.

[0079] S4. The frequency modulation process ends and the system rebounds.

[0080] The system rebound process is similar to the load shedding frequency modulation process, and the system control is executed according to steps S1 - S3.

[0081] In the above embodiments, the descriptions of the various embodiments have their own emphases. For the parts not detailed or recorded in a certain embodiment, reference may be made to the relevant descriptions of other embodiments.

[0082] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A molten salt energy storage assisted solar thermal power unit frequency modulation control system, characterized in that, It includes a hot salt tank (1) and a cold salt tank (3). The high-temperature molten salt in the hot salt tank (1) flows through a steam generation system (2). After the high-temperature molten salt completes heat exchange in the steam generation system (2), it becomes low-temperature molten salt and is stored in the cold salt tank (3). The low-temperature molten salt in the cold salt tank (3) is reheated to high-temperature molten salt through an endothermic system (4) and then returns to the hot salt tank (1) again. The steam generation system (2) drives the steam turbine system (6) to operate, and the steam turbine system (6) drives the power generation system (7) to output electric energy. A part of the electric energy output by the power generation system (7) is transmitted to the power grid (8), and the other part is supplied to the plant through a high-voltage auxiliary transformer (9). The variable-frequency electric heater (10) obtains electric energy from the high-voltage auxiliary transformer (9) and heats the low-temperature molten salt in the cold salt tank (3). The molten salt frequency modulation control system (15) receives a frequency modulation signal from the power grid (8) and adjusts the power output of the variable-frequency electric heater (10) according to the demand.

2. The molten salt energy storage assisted solar thermal power unit frequency modulation control system according to claim 1, characterized in that, The steam generation system (2) is supplied with water through a feed water system (5). The water is heated in the steam generation system (2) to generate steam, which drives the steam turbine system (6) to operate, and the steam turbine system (6) then drives the power generation system (7) to output electric energy.

3. The molten salt energy storage-assisted solar thermal power unit frequency modulation control system according to claim 1 is characterized in that: A regulating valve (11) and a temperature measuring element (12) are arranged on the connecting branch of the variable-frequency electric heater (10). The regulating valve (11) adjusts the opening according to the molten salt temperature measured by the temperature measuring element (12). The regulating valve (11) controls the variable-frequency electric heater (10) to keep the temperature of the molten salt after heating stable and meet the requirements of high-temperature molten salt.

4. The molten salt energy storage-assisted solar thermal power unit frequency modulation control system according to claim 3 is characterized in that: A hot salt inlet valve (13) is arranged on the pipeline between the regulating valve (11) and the hot salt tank (1), and a cold salt recovery valve (14) is arranged on the pipeline between the regulating valve (11) and the cold salt tank (3). If the temperature of the molten salt after being heated by the variable-frequency electric heater (10) meets the requirements of high-temperature molten salt, the hot salt inlet valve (13) is opened and the cold salt recovery valve (14) is closed. If the temperature of the molten salt after being heated by the variable-frequency electric heater (10) does not meet the requirements of high-temperature molten salt, the hot salt inlet valve (13) is closed and the cold salt recovery valve (14) is opened.

5. The molten salt energy storage assisted solar thermal power unit frequency modulation control system according to claim 1, characterized in that, The frequency modulation signal from the power grid (8) includes an automatic gain control AGC signal and a primary frequency modulation signal.

6. A molten salt energy storage assisted solar thermal power unit frequency modulation control method, characterized in that: It includes: Obtain the output power Pg of the power generation system (7), the basic auxiliary power consumption P0 of the plant, and the operating power Ph of the variable-frequency electric heater (10), calculate the power P fed into the grid by the power plant, and adjust the power P fed into the grid by the power plant by controlling the operating power Ph of the variable-frequency electric heater (10). The calculation expression of the power P fed into the grid by the power plant is as follows: P = Pg - (P0 + Ph) The molten salt frequency modulation control system (15) receives the frequency modulation signal ΔP from the power grid (8) and calculates the power control range of the variable-frequency electric heater (10) according to the following formula: P + ΔP = Pg - [P0 + Ph'] Ph' = Ph - ΔP Within the power control range of the variable-frequency electric heater (10), when the power grid (8) requires an increase in load, i.e., when ΔP is positive, the variable-frequency electric heater (10) reduces the heating load on the basis of the current operating power, reducing Ph to Ph', so that the power P fed into the power grid by the power plant increases; when the power grid (8) requires a decrease in load, i.e., when ΔP is negative, the variable-frequency electric heater (10) increases the heating load on the basis of the current operating power, increasing Ph to Ph', so that the power P fed into the power grid by the power plant decreases.

7. The frequency modulation control method of the molten salt energy storage assisted solar thermal power unit according to claim 6, characterized in that, During the process of changing the operating power of the variable-frequency electric heater (10), a feedforward-feedback composite control strategy is adopted to adjust the opening of the regulating valve (11) provided on the connecting branch of the variable-frequency electric heater (10) to maintain the stability of the molten salt temperature. In the feedforward-feedback composite control strategy, the change trend of the opening of the regulating valve (11) is predicted through the feedforward link, and the temperature deviation is corrected in combination with the feedback link. The opening is calculated according to the following formula: VP = VP0 + VP1 = FF(Ph') + PID(Et) In the formula, VP is the opening command of the regulating valve (11), VP0 is the basic opening command of the regulating valve (11), that is, FF(Ph'), which is also equal to FF(Ph - ΔP), representing the opening command calculated by a non-linear function according to the power of the variable-frequency electric heater (10) during the frequency modulation process; VP1 is the fine-tuning opening command of the regulating valve (11), that is, PID(Et), representing the opening command obtained after PID operation on the deviation between the molten salt temperature measured by the temperature measuring element (12) provided on the connecting branch of the variable-frequency electric heater (10) and the set value of the molten salt temperature; the sum of VP0 and VP1 is the final opening command VP of the regulating valve (11), guiding the actual opening of the regulating valve (11).

8. The method for frequency modulation control of a molten salt energy storage-assisted solar thermal power unit according to claim 6, characterized in that: When the variable-frequency electric heater (10) is operating stably, the low-temperature molten salt is heated and after heating, it meets the inlet salt temperature requirements of the high-temperature molten salt and enters the hot salt tank (1) through the hot salt inlet valve (13); if the molten salt temperature after heating by the variable-frequency electric heater (10) does not meet the requirements of the high-temperature molten salt, the hot salt inlet valve (13) is closed and the cold salt recovery valve (14) is opened, and it is recovered to the cold salt tank (3). The following temperature interlock control logic is set for the hot salt inlet valve (13): IF(T≥Ts), OPEN; IF(T<Ts), CLOSE In the formula, T represents the molten salt temperature at the outlet of the variable-frequency electric heater (10) measured by the temperature measuring element (12), and Ts represents the molten salt temperature that meets the inlet salt requirements of the hot salt tank (1). The following temperature interlock control logic is set for the cold salt recovery valve (14): IF(T<Ts), OPEN; IF(T≥Ts), CLOSE.

9. The method for frequency modulation control of a molten salt energy storage-assisted solar thermal power plant according to claim 6, characterized in that: In the case of insufficient energy supply from the optical field due to extreme weather, the variable-frequency electric heater (10) is used to consume external electric energy to maintain the molten salt circulation, so as to realize the uninterrupted operation of the solar thermal power generation unit.

10. The frequency modulation control method of the molten salt energy storage assisted solar thermal power unit according to claim 6, wherein, By preferentially adjusting the operating power of the variable-frequency electric heater (10), the fluctuation of the steam turbine system (6) is reduced.

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