Hybrid energy storage system control method based on flywheel energy storage and constant-speed pumped storage

Abstract

The invention belongs to the technical field of energy storage, and particularly provides a hybrid energy storage system control method based on flywheel energy storage and constant-speed pumped storage, which comprises the following steps that: when the frequency of a power grid is in a normal state, a flywheel energy storage system adopts a double-closed-loop PI control strategy, takes power or rotating speed as an outer loop and takes current as an inner loop; the constant-speed pumped storage unit adopts a power control mode; when the frequency of the power grid fluctuates, if the constant-speed pumped storage unit is in a power generation working condition, the flywheel energy storage system and the constant-speed pumped storage unit both participate in the frequency modulation of the power grid; when the frequency of the power grid fluctuates, if the constant-speed pumped storage unit is in a water pumping working condition, the constant-speed pumped storage unit cannot participate in the frequency modulation of the power grid as a load, and the flywheel energy storage system participates in the inertial response and primary frequency modulation stage. According to the invention, the problem of output reverse regulation when the constant-speed pumped storage unit participates in power grid frequency modulation is solved, and a more stable and reliable method is provided for the operation control of the constant-speed pumped storage unit.

Description

Technical Field

[0001] This invention belongs to the field of energy storage technology, specifically, it relates to a control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage. Background Technology

[0002] The large-scale grid connection of renewable energy sources, characterized by randomness and volatility, introduces numerous power electronic converters into the power grid, leading to reduced power system inertia and more frequent frequency disturbances. Pumped storage power stations convert excess electricity during off-peak hours into higher-priced electricity during peak hours, thus playing a role in peak shaving and frequency regulation. However, conventional pumped storage units have a slow power response, making it difficult to meet the grid's frequency inertia response and the demand for short-term high power during primary frequency regulation. Furthermore, they cannot participate in frequency regulation under pumping conditions. In addition, due to the water hammer effect, constant-speed pumped storage units exhibit reverse output regulation, meaning that in the initial stage of frequency regulation, the unit's output initially adjusts in the opposite direction, exacerbating grid frequency fluctuations.

[0003] To address the above issues, current mainstream solutions mainly include: using battery energy storage to compensate for output reversal; and improving the design of pumped-storage units to reduce output reversal. Flywheel energy storage, as an energy storage technology with advantages such as high power density and high-frequency, fast response, is inherently well-suited to solving problems such as output reversal and slow response speed of constant-speed pumped-storage units. Based on this, this invention proposes a hybrid energy storage system that utilizes flywheel energy storage and constant-speed pumped-storage units to complement each other and jointly participate in grid frequency regulation. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped storage, which solves the problem of output reversal when constant-speed pumped storage units participate in grid frequency regulation, and provides a more stable and reliable method for the control of constant-speed pumped storage unit operation.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage, comprising the following steps: When the grid frequency is normal, the flywheel energy storage system adopts a dual closed-loop PI control strategy, with power or speed as the outer loop and current as the inner loop, to ensure that the speed of the flywheel rotor operates within the specified range; the constant-speed pumped storage unit adopts a power control method to adjust the operating conditions according to the needs of other aspects of the power station. When grid frequency fluctuates, flywheel energy storage systems and / or constant-speed pumped storage units participate in the inertial response phase and primary frequency regulation phase of the frequency regulation process by absorbing or generating active power. The frequency regulation control strategy is as follows: If the constant-speed pumped storage unit is in power generation mode, both the flywheel energy storage system and the constant-speed pumped storage unit participate in grid frequency regulation. If a constant-speed pumped storage unit is in pumping mode, the constant-speed pumped storage unit, as a load, cannot participate in grid frequency regulation. Instead, the flywheel energy storage system participates in the inertial response and primary frequency regulation stages.

[0006] In the preferred embodiment, when the grid frequency is under normal conditions, the control strategy of the flywheel energy storage system is as follows: A permanent magnet synchronous motor (PMSM) used in the flywheel energy storage system is modeled. In the synchronous rotating dq coordinate system, the flux linkage equation and voltage equation of the PMSM are constructed. After dq-axis current decoupling, the PMSM adopts a dual-closed-loop PI control strategy. The outer loop selects power or speed according to the control objective, while the inner loop is a current loop. When the flywheel speed is outside the specified range, the outer loop uses a speed loop to adjust the flywheel speed to the specified range in a timely manner. When the flywheel speed is within the specified range, the outer loop uses a power loop to control the active power absorbed or output by the flywheel energy storage system.

[0007] In a preferred embodiment, the flux linkage equation of the permanent magnet synchronous motor is expressed as follows: ; In the formula, and These are the d-axis and q-axis components of the stator current in the dq coordinate system; and These are the d-axis and q-axis components of the stator flux linkage in the dq coordinate system; and These are the d-axis and q-axis inductances of the stator; It is a permanent magnet chain.

[0008] In the preferred embodiment, the voltage equation expression for the permanent magnet synchronous motor is: ; In the formula, and These are the d-axis and q-axis components of the stator voltage in the dq coordinate system; It is electric angular velocity.

[0009] In the preferred scheme, when the grid frequency fluctuates, if the constant-speed pumped storage unit is in power generation mode, during the inertial response phase, the flywheel energy storage system participates in frequency regulation using a combination of inertial control and droop control, while the constant-speed pumped storage unit participates in frequency regulation using droop control. After entering the primary frequency regulation phase, the flywheel energy storage system gradually exits frequency regulation, and the power of the flywheel energy storage system is gradually adjusted to 0 to prepare for the next frequency regulation action. Based on the droop control used in the previous phase, the constant-speed pumped storage unit makes additional compensation for the power adjustment of the flywheel energy storage system to prevent a secondary drop in grid frequency.

[0010] In the preferred scheme, when the grid frequency fluctuates, if the constant-speed pumped storage unit is in pumping mode, the flywheel energy storage system participates in frequency regulation by combining inertial control and droop control. During the inertial response phase, the flywheel energy storage system provides the grid with equivalent virtual rotational inertia through inertial control; during the primary frequency regulation phase, the flywheel energy storage system provides active power support to the grid through droop control.

[0011] In the preferred scheme, when the grid frequency fluctuates and the flywheel energy storage system is required to regulate the frequency, the grid frequency response characteristics are first modeled using the grid frequency response SFR model. The expression for the grid frequency response transfer function is as follows: ; In the formula, This refers to the power grid frequency deviation. This represents the increase in generator power. It is the power increment of the energy storage system; It is the increase in load power; H It is the equivalent inertial time constant of the power grid; s is a complex variable in the Laplace transform; D is the damping coefficient of the load; The above formula can be used to calculate the deviation of the grid frequency in real time from the load power, energy storage power and generator power, which can be used to calculate the power reference value for the flywheel energy storage system and constant speed pumped storage unit to participate in frequency regulation.

[0012] In the preferred scheme, when the constant-speed pumped storage unit is in power generation mode, the method for obtaining the power reference value for the flywheel energy storage system participating in frequency regulation during the inertial response stage is a combination of inertial control and droop control. The inertial control method uses an active power command proportional to the grid frequency change rate, while the droop control method uses an active power command proportional to the grid frequency change amount. The expressions are as follows: ; In the formula, This is the power reference value for the flywheel energy storage system participating in frequency regulation; This refers to the inertial control coefficient; It is the droop coefficient for the flywheel energy storage system participating in frequency regulation; During the first frequency regulation phase, the flywheel energy storage system gradually exits the frequency regulation process to prepare for the next frequency regulation operation. The method for the flywheel energy storage system to exit the frequency regulation is to add a first-order RC high-pass filter to the power command during the first frequency regulation, as shown in the following formula: ; In the formula, R is the power reference value for the flywheel energy storage system participating in frequency regulation during the primary frequency regulation phase; C is the resistance of the high-pass filter and C is the capacitance of the high-pass filter.

[0013] In the preferred embodiment, when the constant-speed pumped storage unit is in power generation mode, the constant-speed pumped storage unit adopts droop control to regulate active power during the inertial response phase. Its power control expression is as follows: ; In the formula, This is a power reference value for a constant-speed pumped storage unit when participating in the grid frequency inertial response; It is the droop coefficient for frequency regulation of constant-speed pumped storage units; , , These are the proportional, integral, and derivative coefficients of the speed controller; It is the water flow inertia time constant of the water turbine; When a constant-speed pumped-storage unit is in power generation mode, the reference power values ​​for frequency regulation during the primary frequency regulation phase are as follows: ; In the formula, This is the power reference value for a constant-speed pumped storage unit during the primary frequency regulation stage under power generation conditions. This is the inertial control coefficient.

[0014] In the preferred scheme, when the grid frequency fluctuates, the constant-speed pumped storage unit is in pumping mode and cannot participate in frequency regulation as a load. The flywheel energy storage system participates in the inertial response and primary frequency regulation process. The flywheel energy storage system participates in frequency regulation using a combination of inertial control and droop control. The power reference values ​​for participating in frequency regulation during the inertial response and primary frequency regulation stages are as follows: ; In the inertial response phase, the flywheel energy storage system provides equivalent virtual rotational inertia in a timely manner through inertial control related to the grid frequency change rate, thus mitigating the grid frequency change process; in the primary frequency regulation phase, droop control provides active power support to the grid for a certain period of time.

[0015] The present invention provides a control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage, which has the following beneficial effects: 1. Flywheel energy storage adopts inertial control during the inertial response phase, which can output power in a timely manner at the beginning of grid frequency fluctuations, compensate for the output reversal phenomenon of constant-speed pumped storage units under power generation conditions, and slow down the rate of grid frequency change.

[0016] 2. When the constant-speed pumped-storage unit is in power generation mode, the flywheel energy storage can gradually withdraw during the primary frequency regulation phase and perform timely charging and discharging actions to prepare for the next frequency regulation. When the constant-speed pumped-storage unit is in pumping mode, the flywheel energy storage can participate in frequency regulation during the inertial response and primary frequency regulation phases, filling the gap that traditional constant-speed pumped-storage power stations cannot participate in frequency regulation when the pumped-storage unit is in pumping mode.

[0017] 3. This invention organically combines the control advantages of flywheel energy storage and constant-speed pumped hydro storage, improving the flexibility and frequency regulation capability of the hybrid energy storage system. Attached Figure Description

[0018] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the hybrid energy storage system proposed in this invention; Figure 2 This is a block diagram of the flywheel energy storage power control strategy proposed in this invention; Figure 3 This is a block diagram of the frequency regulation control strategy for the flywheel energy storage system proposed in this invention; In the diagram: speed controller 10, electro-hydraulic servo system 20, water pump turbine 30, drive shaft one 41, drive shaft two 42, synchronous motor 50, transformer one 60, energy storage flywheel 70, permanent magnet synchronous motor 80, converter one 90, DC bus 100, converter two 110, transformer two 120, power grid 130. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0020] like Figure 1 As shown, a hybrid energy storage system based on flywheel energy storage and constant-speed pumped storage includes a flywheel energy storage system and a constant-speed pumped storage unit, as detailed below: The components of the flywheel energy storage system include: energy storage flywheel 70, permanent magnet synchronous motor 80, converter 1 90 (machine-side converter), DC bus 100, converter 2 110 (grid-side converter) and transformer 2 120.

[0021] The energy storage flywheel 70 stores kinetic energy through rotation, and changes in its rotational speed achieve the absorption and release of kinetic energy. The permanent magnet synchronous motor 80, acting as the drive motor for the energy storage flywheel, can adjust its active power by controlling converter 1 90 and converter 2 110. It is connected to the energy storage flywheel via drive shaft 2 42 to control the flywheel's rotational speed. Converter 1 90 is connected to the stator winding of the permanent magnet brushless motor 80, controlling the stator current amplitude and frequency. Converter 2 110 rectifies the AC power from the grid side into DC power, which is then transmitted to converter 1 90 via DC bus 100. Transformer 2 120 converts the AC voltage of converter 2 110 into grid voltage, enabling converter 2 110 to connect to the grid normally.

[0022] The components of a constant-speed pumped-storage unit include: a governor 10, an electro-hydraulic servo system 20, a pump-turbine 30, a synchronous motor 50, and a transformer 60. The synchronous motor 50 is an electrically excited synchronous motor. In the constant-speed pumped-storage unit, the governor 10 receives active power signals and power commands, converting the difference between the pumped-storage unit's power command and the actual power into a turbine guide vane opening command. The electro-hydraulic servo system 20 adjusts the guide vane opening of the pump-turbine 30 according to the guide vane opening command. By changing the guide vane opening, the pump-turbine 30 can control the output mechanical power. A drive shaft 41 connects the pump-turbine 30 to the synchronous motor 50, utilizing the synchronous motor 50 to achieve the mutual conversion of mechanical energy and electrical energy, thereby providing or absorbing active power to the power grid to participate in the frequency regulation process.

[0023] A control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage includes the following steps: When the grid frequency is normal, the flywheel energy storage system adopts a dual closed-loop PI control strategy, with power or speed as the outer loop and current as the inner loop, to ensure that the flywheel rotor speed operates within the specified range; the constant-speed pumped-storage unit adopts a power control method to adjust the operating conditions according to the needs of other aspects of the power station.

[0024] Specifically, when the grid frequency is under normal conditions, the control strategy of the flywheel energy storage system is as follows: A model is constructed for the permanent magnet synchronous motor used in the flywheel energy storage system. The flux linkage equation and voltage equation of the permanent magnet synchronous motor are established in the synchronous rotating dq coordinate system. Figure 2 The control strategy of the flywheel energy storage system is described below: The permanent magnet synchronous motor used in flywheel energy storage is usually modeled as an ideal motor. In the synchronous rotating dq coordinate system, its flux linkage equation and voltage equation are as follows: Magnetic flux linkage equation: ; Voltage equation: ; In the formula, and These are the d-axis and q-axis components of the stator voltage in the dq coordinate system, in V; and denoted as A, representing the d-axis and q-axis components of the stator current in the dq coordinate system. and Wb represents the d-axis and q-axis components of the stator flux linkage in the dq coordinate system. and The inductances H are the d-axis and q-axis inductances of the stator. It is a permanent magnet flux linkage, Wb; It is the electric angular velocity, in rad / s.

[0025] After decoupling the dq axis current, the permanent magnet synchronous motor adopts a dual closed-loop PI control strategy. The outer loop selects power or speed according to the control target, while the inner loop is a current loop. When the flywheel speed is outside the limit range, the outer loop uses the speed loop to adjust the flywheel speed to the limit range in time. When the flywheel speed is within the limit range, the outer loop uses the power loop to control the active power absorbed or output by the flywheel energy storage, which is consistent with the power reference value calculated by the frequency regulation control strategy, thus participating in grid frequency regulation in a timely and accurate manner.

[0026] When the power grid frequency fluctuates, such as Figure 3 As shown, a frequency regulation control strategy is adopted for frequency adjustment. The flywheel energy storage system and / or constant-speed pumped storage unit participate in the inertial response stage and the primary frequency regulation stage of the frequency regulation process. The frequency regulation control strategy is as follows: When the grid frequency fluctuates and a flywheel energy storage system is needed to regulate the frequency, the grid frequency response (SFR) model is first used to model the frequency response characteristics of the grid.

[0027] Since the inertial response and primary frequency regulation take very short times, both on the order of seconds, the modeling of the governor and prime mover in the generator set in the SFR model is ignored. The expression for the frequency response transfer function of the power grid is as follows: ; In the formula, The power grid frequency deviation is expressed in Hz. The generator power increment is W; It is the power increment of the energy storage system, in W; It is the load power increment, W; H It is the equivalent inertial time constant of the power grid, measured in seconds (s). s is a complex variable in the Laplace transform; D is the damping coefficient of the load, 1 / s.

[0028] The above formula can be used to calculate the deviation of the grid frequency in real time from the load power, energy storage power and generator power, which can be used to calculate the power reference value for the flywheel energy storage system and constant speed pumped storage unit to participate in frequency regulation.

[0029] If the constant-speed pumped storage unit is in power generation mode, both the flywheel energy storage system and the constant-speed pumped storage unit participate in grid frequency regulation.

[0030] Specifically, when the grid frequency fluctuates, if the constant-speed pumped storage unit is in power generation mode, during the inertial response phase, the flywheel energy storage system participates in frequency regulation using a combination of inertial control and droop control, while the constant-speed pumped storage unit participates in frequency regulation using droop control. After entering the primary frequency regulation phase, the flywheel energy storage system gradually exits frequency regulation, and its power is gradually adjusted to 0 to prepare for the next frequency regulation action. Based on the droop control used in the previous phase, the constant-speed pumped storage unit makes additional compensation for the power adjustment of the flywheel energy storage system to prevent a secondary drop in grid frequency.

[0031] When a constant-speed pumped-storage hydroelectric unit is in power generation mode, the method for obtaining the power reference value for frequency regulation by the flywheel energy storage system during the inertial response phase is a combination of inertial control and droop control. The inertial control method uses an active power command proportional to the grid frequency change rate, while the droop control method uses an active power command proportional to the grid frequency change. The expressions are shown below: ; In the formula, The power reference value for flywheel energy storage is in W; This refers to the inertial control coefficient; It is the droop coefficient for the flywheel energy storage system to participate in frequency regulation.

[0032] During the inertial response phase, the constant-speed pumped storage unit uses droop control to regulate active power, and its power control expression is as follows: ; In the formula, This is a power reference value for a constant-speed pumped storage unit when participating in the grid frequency inertial response; It is the droop coefficient for frequency regulation of constant-speed pumped storage units; , , These are the proportional, integral, and derivative coefficients of the speed controller; It is the time constant of the water flow inertia of the water turbine.

[0033] In the above formula, It can reflect the strength of the water hammer effect in a constant-speed pumped storage unit. The larger the value, the longer the response time of the power regulation of the constant-speed pumped storage unit, the slower the power regulation speed, and the more obvious the output reversal phenomenon caused by the water hammer effect, leading to a faster deterioration of the grid frequency. Therefore, the flywheel energy storage of this invention can not only participate in the frequency regulation process of the grid inertial response during the inertial response stage, but also compensate for the power changes caused by the output reversal phenomenon of the constant-speed pumped storage unit.

[0034] When a constant-speed pumped-storage unit is in power generation mode, because its energy storage capacity is larger than that of a flywheel pumped-storage unit, it can provide continuous active power support to the grid during the primary frequency regulation phase. Therefore, during the primary frequency regulation phase, the flywheel energy storage system gradually exits the frequency regulation to prepare for the next frequency regulation operation. The method for the flywheel energy storage system to exit the frequency regulation is to add a first-order RC high-pass filter to the power command during the primary frequency regulation, as shown in the following formula: ; In the formula, R is the power reference value for the flywheel energy storage system participating in frequency regulation during the primary frequency regulation phase; C is the resistance of the high-pass filter; and R is the capacitance of the high-pass filter. s It is a complex variable in the Laplace transform. According to the final value theorem, It will gradually approach 0 over time, achieving the effect of the flywheel exiting frequency tuning during a single tuning phase.

[0035] When a constant-speed pumped-storage unit is in power generation mode, in addition to using droop control to participate in frequency regulation during the primary frequency regulation stage, it is also necessary to compensate for the power loss caused by the flywheel energy storage exiting frequency regulation. Therefore, the reference power value for the constant-speed pumped-storage unit participating in frequency regulation during this stage is as follows: ; In the formula, This is the power reference value for a constant-speed pumped storage unit during the primary frequency regulation stage under power generation conditions. This is the inertial control coefficient.

[0036] When the power grid experiences frequency fluctuations, if the constant-speed pumped storage unit is in pumping mode, the constant-speed pumped storage unit, as a load, cannot participate in the power grid frequency regulation. Instead, the flywheel energy storage system participates in the inertial response and primary frequency regulation stages.

[0037] Specifically, when the grid frequency fluctuates, if the constant-speed pumped storage unit is in pumping mode, the flywheel energy storage system participates in frequency regulation by combining inertial control and droop control. During the inertial response phase, the flywheel energy storage system provides the grid with equivalent virtual rotational inertia through inertial control; during the primary frequency regulation phase, the flywheel energy storage system provides active power support to the grid through droop control.

[0038] When the grid frequency fluctuates, the constant-speed pumped storage unit is in pumping mode and cannot participate in frequency regulation as a load. The flywheel energy storage system participates in the inertial response and primary frequency regulation process. The flywheel energy storage system participates in frequency regulation using a combination of inertial control and droop control. The power reference values ​​for participating in frequency regulation during the inertial response and primary frequency regulation stages are as follows: ; In the inertial response phase, the flywheel energy storage system provides equivalent virtual rotational inertia in a timely manner through inertial control related to the grid frequency change rate, thus mitigating the grid frequency change process; in the primary frequency regulation phase, droop control provides active power support to the grid for a certain period of time.

[0039] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage, characterized in that, Includes the following steps: When the grid frequency is normal, the flywheel energy storage system adopts a dual closed-loop PI control strategy, with power or speed as the outer loop and current as the inner loop, to ensure that the speed of the flywheel rotor operates within the specified range; the constant-speed pumped storage unit adopts a power control method to adjust the operating conditions according to the needs of other aspects of the power station. When grid frequency fluctuates, flywheel energy storage systems and / or constant-speed pumped storage units participate in the inertial response phase and primary frequency regulation phase of the frequency regulation process by absorbing or generating active power. The frequency regulation control strategy is as follows: If the constant-speed pumped storage unit is in power generation mode, both the flywheel energy storage system and the constant-speed pumped storage unit participate in grid frequency regulation. If a constant-speed pumped storage unit is in pumping mode, the constant-speed pumped storage unit, as a load, cannot participate in grid frequency regulation. Instead, the flywheel energy storage system participates in the inertial response and primary frequency regulation stages.

2. The control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage according to claim 1, characterized in that, When the grid frequency is under normal conditions, the control strategy of the flywheel energy storage system is as follows: A model is constructed for the permanent magnet synchronous motor used in the flywheel energy storage system. In the synchronous rotating dq coordinate system, the flux linkage equation and voltage equation of the permanent magnet synchronous motor are constructed. After decoupling the dq axis current, the permanent magnet synchronous motor adopts a dual closed-loop PI control strategy. The outer loop selects power or speed according to the control target, and the inner loop is a current loop. When the flywheel speed is outside the limit range, the outer loop uses the speed loop to adjust the flywheel speed to the limit range in time. When the flywheel speed is within the limit range, the outer loop uses the power loop to control the active power absorbed or output by the flywheel energy storage.

3. The control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage according to claim 2, characterized in that, The flux linkage equation of the permanent magnet synchronous motor is expressed as follows: ； In the formula, and These are the d-axis and q-axis components of the stator current in the dq coordinate system; and These are the d-axis and q-axis components of the stator flux linkage in the dq coordinate system; and These are the d-axis and q-axis inductances of the stator; It is a permanent magnet chain.

4. The control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage according to claim 3, characterized in that, The voltage equation expression for the permanent magnet synchronous motor is as follows: ； In the formula, and These are the d-axis and q-axis components of the stator voltage in the dq coordinate system; It is electric angular velocity.

5. The control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage according to claim 1, characterized in that, When the grid frequency fluctuates, if the constant-speed pumped storage unit is in power generation mode, during the inertial response phase, the flywheel energy storage system participates in frequency regulation using a combination of inertial control and droop control, while the constant-speed pumped storage unit participates in frequency regulation using droop control. After entering the primary frequency regulation phase, the flywheel energy storage system gradually exits frequency regulation, and its power is gradually adjusted to 0 to prepare for the next frequency regulation action. Based on the droop control used in the previous phase, the constant-speed pumped storage unit makes additional compensation for the power adjustment of the flywheel energy storage system to prevent a secondary drop in grid frequency.

6. The control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage according to claim 1, characterized in that, When the grid frequency fluctuates, if the constant-speed pumped storage unit is in pumping mode, the flywheel energy storage system participates in frequency regulation by combining inertial control and droop control. During the inertial response phase, the flywheel energy storage system provides the grid with equivalent virtual rotational inertia through inertial control; during the primary frequency regulation phase, the flywheel energy storage system provides active power support to the grid through droop control.

7. The control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage according to claim 1, characterized in that, When the grid frequency fluctuates and a flywheel energy storage system is needed for frequency regulation, the grid frequency response characteristics are first modeled using the grid frequency response transfer function (SFR) model. The expression for the grid frequency response transfer function is as follows: ； In the formula, This refers to the power grid frequency deviation. This represents the increase in generator power. It is the power increment of the energy storage system; It is the increase in load power; H It is the equivalent inertial time constant of the power grid; s is a complex variable in the Laplace transform; D is the damping coefficient of the load; The above formula can be used to calculate the deviation of the grid frequency in real time from the load power, energy storage power and generator power, which can be used to calculate the power reference value for the flywheel energy storage system and constant speed pumped storage unit to participate in frequency regulation.

8. The control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage according to claim 7, characterized in that, When a constant-speed pumped-storage hydroelectric unit is in power generation mode, the method for obtaining the power reference value for frequency regulation by the flywheel energy storage system during the inertial response phase is a combination of inertial control and droop control. The inertial control method uses an active power command proportional to the grid frequency change rate, while the droop control method uses an active power command proportional to the grid frequency change. The expressions are shown below: ； In the formula, This is the power reference value for the flywheel energy storage system participating in frequency regulation; This refers to the inertial control coefficient; It is the droop coefficient for the flywheel energy storage system participating in frequency regulation; During the first frequency regulation phase, the flywheel energy storage system gradually exits the frequency regulation process to prepare for the next frequency regulation operation. The method for the flywheel energy storage system to exit the frequency regulation is to add a first-order RC high-pass filter to the power command during the first frequency regulation, as shown in the following formula: ； In the formula, R is the power reference value for the flywheel energy storage system participating in frequency regulation during the primary frequency regulation phase; C is the resistance of the high-pass filter and C is the capacitance of the high-pass filter.

9. The control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage according to claim 7, characterized in that, When a constant-speed pumped-storage unit is in power generation mode, during the inertial response phase, the unit uses droop control to regulate active power. Its power control expression is as follows: ； In the formula, This is a power reference value for a constant-speed pumped storage unit when participating in the grid frequency inertial response; It is the droop coefficient for frequency regulation of constant-speed pumped storage units; , , These are the proportional, integral, and derivative coefficients of the speed controller; It is the water flow inertia time constant of the water turbine; When a constant-speed pumped-storage unit is in power generation mode, the reference power values ​​for frequency regulation during the primary frequency regulation phase are as follows: ； In the formula, This is the power reference value for a constant-speed pumped storage unit during the primary frequency regulation stage under power generation conditions. This is the inertial control coefficient.

10. The control method for a hybrid energy storage system based on flywheel energy storage and constant-speed pumped hydro storage according to claim 7, characterized in that, When the grid frequency fluctuates, the constant-speed pumped storage unit is in pumping mode and cannot participate in frequency regulation as a load. The flywheel energy storage system participates in the inertial response and primary frequency regulation process. The flywheel energy storage system participates in frequency regulation using a combination of inertial control and droop control. The power reference values ​​for participating in frequency regulation during the inertial response and primary frequency regulation stages are as follows: ； In the inertial response phase, the flywheel energy storage system provides equivalent virtual rotational inertia in a timely manner through inertial control related to the grid frequency change rate, thus mitigating the grid frequency change process; in the primary frequency regulation phase, droop control provides active power support to the grid for a certain period of time.

Images