Method and system for controlling frequency modulation of power grid side energy storage power station

Abstract

The invention discloses a power grid side energy storage power station frequency modulation control method and system, and the method comprises the steps: comparing the direction and power of secondary frequency modulation and primary frequency modulation when a secondary frequency modulation instruction is received in a primary frequency modulation process, and determining a frequency modulation strategy according to a comparison result. The method is applied to the field of energy storage, locking of primary frequency modulation and secondary frequency modulation of the same energy storage station in different adjustment modes is avoided, the conflict between the primary frequency modulation and the secondary frequency modulation is avoided, meanwhile, the adjustment time of the energy storage power station can be shortened, and the stability of a power system is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of optimized operation of energy storage power stations, and in particular relates to a control method and system for frequency regulation of a grid-side energy storage power station. Background Art

[0002] As a new energy service offering, energy storage is a high-quality, reliable millisecond-level control and response resource. It can provide both active and reactive power support, offering a variety of services for the power grid, including peak shaving, frequency regulation, backup, and emergency response. To alleviate regional power supply pressures during peak summer months and improve local renewable energy consumption, peak-shaving and valley-filling capabilities, local power supply capacity, and flexible regulation, a large number of grid-side energy storage power station projects have been deployed in China, leveraging the advantages of electrochemical energy storage stations, such as their short construction cycles and flexible deployment.

[0003] Grid-side energy storage power stations utilize both primary and secondary frequency regulation. Primary frequency regulation refers to frequency regulation achieved by the energy storage power station through the power station's frequency regulation system. When the grid frequency changes, the frequency regulation system detects this change and automatically adjusts the energy storage power station's charge and discharge capacity to balance the imbalance between the power system's load and power supply. Secondary frequency regulation is implemented by the power market's dispatchers, with the energy storage station's energy management system (EMS) configuring the station to automatically adjust the energy storage power station's charge and discharge capacity. To ensure that primary frequency regulation (local mode) and secondary frequency regulation (remote control) do not interfere with each other, primary frequency regulation takes precedence over secondary frequency regulation. During primary frequency regulation, the energy management system (EMS) blocks secondary frequency regulation, waiting for the primary frequency regulation to complete and return to the blocking point before the EMS can initiate secondary frequency regulation. The secondary frequency regulation system can only wait, and the brief adjustment of primary frequency regulation cannot maintain the grid frequency. Summary of the Invention

[0004] (1) Purpose of the invention

[0005] The purpose of the present invention is to provide a control method and system for frequency regulation of a grid-side energy storage power station. This method does not consider the priority of primary and secondary frequency regulation. When applied, the secondary frequency regulation is not blocked during primary frequency regulation. The secondary frequency regulation strategy is determined based on conditions such as whether the direction and magnitude of the active power commanded by the AGC (Automatic Generation Control) is consistent with the primary frequency regulation. This method can avoid conflicts between primary and secondary frequency regulation and also reduce the regulation time of the energy storage power station.

[0006] (2) Technical solution

[0007] To solve the above problems, a first aspect of the present invention provides a method for controlling frequency regulation of a grid-side energy storage power station, comprising:

[0008] The frequency modulation includes primary frequency modulation and secondary frequency modulation, and the control method includes:

[0009] During the primary frequency modulation process, when a secondary frequency modulation instruction is received, the direction and power of the secondary frequency modulation are compared with that of the primary frequency modulation;

[0010] Determining a frequency modulation strategy according to a result of the comparison includes:

[0011] If the direction of the secondary frequency modulation is consistent with that of the primary frequency modulation, the active power of the secondary frequency modulation is compared with the active power of the primary frequency modulation, and the corresponding step size instruction is determined according to the power size; the direction includes two states: charging direction and discharging direction, and charging is defined as negative and discharging as positive.

[0012] If the directions of the secondary frequency regulation are inconsistent with the primary frequency regulation, the primary frequency regulation signal will be sent to the energy storage station PCS for charging and discharging according to the principle of primary frequency regulation priority.

[0013] Furthermore, comparing the active power of the secondary frequency modulation with the active power of the primary frequency modulation and determining and issuing a corresponding step size instruction according to the power size includes:

[0014] If the discharge step length P21> the first active power P1, and P21-P1<ΔP, the AGC instruction is executed using P21;

[0015] If the charging step length P22 is less than the first active power P1 and |P22-P1| is less than ΔP, the AGC instruction is executed using P22; the power of the primary frequency modulation is the first active power P1. Under the AGC instruction, the active power of the secondary frequency modulation includes P21 and P22, where P21 and P22 are the discharge step length and the charging step length, respectively.

[0016] Furthermore, the control method further includes: during the charging and discharging process, if a new AGC instruction for secondary frequency modulation is received, the frequency modulation strategy is determined after comparing the new AGC instruction with the direction and power of the primary frequency modulation, and the cycle continues.

[0017] Furthermore, the secondary frequency modulation can also control the energy storage station PCS to perform charging and discharging.

[0018] A second aspect of the present invention provides a control system for frequency regulation of a grid-side energy storage power station, wherein the frequency regulation includes primary frequency regulation and secondary frequency regulation, and the system includes:

[0019] A comparison module is used to compare the direction and power of the secondary frequency modulation with that of the primary frequency modulation when receiving a secondary frequency modulation instruction during the primary frequency modulation process;

[0020] A frequency modulation module, configured to determine a frequency modulation strategy according to the comparison result, comprising:

[0021] The first judgment submodule is used to compare the active power of the secondary frequency modulation with the active power of the primary frequency modulation if the directions of the secondary frequency modulation and the primary frequency modulation are consistent, and determine and issue a corresponding step size instruction based on the power size;

[0022] The second judgment submodule is used to send the primary frequency modulation signal to the energy storage station PCS for charging and discharging according to the principle of primary frequency modulation priority if the directions of the secondary frequency modulation and the primary frequency modulation are inconsistent.

[0023] Furthermore, the first judgment submodule is specifically configured to:

[0024] If the discharge step length P21> the first active power P1, and P21-P1<ΔP, the AGC instruction is executed using P21;

[0025] If the charging step length P22 is less than the first active power P1 and |P22-P1| is less than ΔP, the AGC instruction is executed using P22; the power of the primary frequency modulation is the first active power P1. Under the AGC instruction, the active power of the secondary frequency modulation includes P21 and P22, where P21 and P22 are the discharge step length and the charging step length, respectively.

[0026] Furthermore, the system also includes: a circulation module, which is used to determine the frequency modulation strategy after comparing the new AGC instruction with the direction and power of the primary frequency modulation during the charging and discharging process if a new AGC instruction for the secondary frequency modulation is received, and the frequency modulation strategy is operated in a cycle.

[0027] (3) Beneficial effects

[0028] The above technical solution of the present invention has the following beneficial technical effects: The present invention provides a control method and system for frequency regulation of a grid-side energy storage power station, which is mainly used in the field of energy storage. The method avoids the locking between the primary frequency regulation and the secondary frequency regulation of the same energy storage station in different regulation modes. The design principle is: when the primary frequency regulation signal is generated, the energy storage EMS detects whether the secondary frequency regulation is in action, and the energy management system EMS detects in real time whether a new active power regulation instruction is received or whether AGC active power regulation is being executed. If not, the energy management system does not execute any strategy; if a new active power regulation instruction is detected or AGC is being executed For active power regulation, the direction and power size of the secondary frequency modulation are compared with that of the primary frequency modulation to determine the corresponding execution strategy of the secondary frequency modulation. If the direction of the secondary frequency modulation is consistent with that of the primary frequency modulation, the active power of the secondary frequency modulation is compared with that of the primary frequency modulation, and the corresponding step size instruction is determined based on the power size. If the direction of the secondary frequency modulation is inconsistent with that of the primary frequency modulation, the primary frequency modulation signal is sent to the energy storage power station PCS for charging and discharging according to the principle of primary frequency modulation priority. The present invention determines the frequency modulation strategy based on the result of the comparison to avoid the conflict between the primary and secondary frequency modulation. At the same time, it can also reduce the adjustment time of the energy storage power station and improve the stability of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a flow chart of a method for controlling frequency regulation of a grid-side energy storage power station according to the present invention;

[0030] Figure 2 This is a flow chart of a method for controlling frequency regulation of a grid-side energy storage power station according to a specific embodiment of the present invention;

[0031] Figure 3 This is a diagram of a joint regulation network structure according to a specific embodiment of the present invention;

[0032] Figure 4 Schematic diagram of a control system for frequency regulation of a grid-side energy storage power station according to the present invention. DETAILED DESCRIPTION

[0033] To make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present invention.

[0034] like Figure 1As shown, one aspect of the present invention provides a method for controlling frequency regulation of a grid-side energy storage power station. This method does not consider the priority of primary and secondary frequency regulation. The secondary frequency regulation is not blocked during primary frequency regulation. The secondary frequency regulation strategy is determined based on conditions such as whether the direction and magnitude of the active power commanded by the AGC dispatcher are consistent with the primary frequency regulation. Specifically, the method includes:

[0035] During the primary frequency modulation process, when a secondary frequency modulation instruction is received, the direction and power of the secondary frequency modulation are compared with the primary frequency modulation. When a primary frequency modulation signal is generated, the energy storage EMS is used to detect whether the secondary frequency modulation is in operation. The energy management system (EMS) collects data changes of the primary frequency modulation device in real time, including the action status signal S and the adjustment power P1. When the status signal S is activated and the adjustment power P1 also changes, it proves that the primary frequency modulation is in operation, and sends charging and discharging instructions to the PCS (bidirectional converter). The secondary frequency modulation action includes a new active power adjustment instruction or the execution of AGC active power. 。 Under the AGC instruction, the active power of the secondary frequency modulation includes P21 and P22, where P21 and P22 are the discharge step and the charging step respectively. The power of the primary frequency modulation is the first active power P1. When the primary frequency modulation signal is generated, the energy management system detects in real time whether a new active power adjustment instruction is received or whether the AGC active power is being executed. Figure 2 As shown, if the secondary frequency modulation is not in effect, no strategy is executed. The energy management system detects in real time whether it has received any new active power adjustment instructions or is executing AGC active power. If the secondary frequency modulation is in effect, the energy management system determines the appropriate strategy by comparing the direction and power of the secondary frequency modulation with the primary frequency modulation. Direction includes charging and discharging, with charging being negative and discharging being positive.

[0036] S2, determining a frequency modulation strategy according to the comparison result, including:

[0037] S21, if the direction of the secondary frequency modulation is consistent with that of the primary frequency modulation, the active power of the secondary frequency modulation is compared with the active power of the primary frequency modulation, and the corresponding step size instruction is issued according to the power size. AGC ) adjustment command or the AGC active power (P AGC ) When adjusting, record the direction and magnitude of the active power P1. This includes the following two situations:

[0038] (1) If the discharge step length P21> the first active power P1, and P21-P1<ΔP, then the AGC instruction is executed using P21. Specifically: compare the current P21 with the direction of P1, for example, charging is negative and discharging is positive. If the directions are the same, P AGCDuring the discharge process, the step size P21 of each step > P1, and P21 - P1 < ΔP. ΔP is a power fixed value that can be configured on-site. The change in ΔP is not sufficient to exceed the dead zone of the frequency detected by the primary frequency modulation device, ensuring that the primary frequency modulation device does not operate frequently. The energy management system can execute the strategy and implement the issuance of strategy P21 (corresponding to the P step in the Figure 2 discharge mode), which is sent to the energy storage converter (PCS), and the PCS completes the charging of the battery.

[0039] (2) If the charging step size P22 < the first active power P1, and |P22 - P1| < ΔP, then the AGC command is executed using P22. P AGC During the charging process, P22 is negative, P22 < P1, |P22 - P1| < ΔP. The energy management system also executes the strategy and implements the issuance of strategy P22 (corresponding to the P step in the Figure 2 charging mode), which is sent to the energy storage converter (PCS), and the PCS completes the discharge of the battery until the power regulation of the current period of the energy management system P AGC is completed.

[0040] S22. If the directions of the secondary frequency modulation and the primary frequency modulation are inconsistent, according to the principle of primary frequency modulation priority, the primary frequency modulation signal is sent to the PCS of the energy storage power station to perform charge and discharge, and the active power P1 is executed. Specifically, if it is determined that the current P AGC is in the opposite direction compared with P1, and the local primary frequency modulation is inconsistent with the remote AGC (secondary frequency modulation) command, the local priority level is high. The energy management system suspends the remote AGC, and the primary frequency modulation device first completes the local frequency modulation processing. After the status signal S operates and returns, the energy management system continues to issue the AGC command to perform the charge and discharge of the battery.

[0041] This control method also includes S23: During the charge and discharge process, if a new AGC command of the secondary frequency modulation is received, then the new AGC command is compared with the direction and power magnitude of the primary frequency modulation to determine the frequency modulation strategy, and it runs in a loop like this. During the execution of the above steps, the energy management system receives a new AGC command issued by the dispatcher, replacing the old AGC command. The energy management system issues the new AGC command to complete the charge and discharge of the battery. The secondary frequency modulation can also control the PCS of the energy storage power station to perform charge and discharge. As Figure 2 shown, the directions of the secondary frequency modulation and the primary frequency modulation are the same. In the discharge mode: If P21 - P1 ≥ ΔP, then the AGC command executes the first active power P1; in the charging mode: |P22 - P1| ≥ ΔP, then the AGC command executes the first active power P1. Figure 3 is the combined regulation network structure. Figure 3In the process, both the primary frequency modulation and the secondary frequency modulation can send adjustment commands to the PCS to control the charging and discharging of the battery. The frequency modulation device represents the primary frequency modulation and the EMS represents the secondary frequency modulation.

[0042] like Figure 4 As shown, the second aspect of the present invention provides a control system for frequency regulation of a grid-side energy storage power station, comprising:

[0043] The comparison module 21 is used to compare the direction and power of the secondary frequency modulation with that of the primary frequency modulation when receiving the secondary frequency modulation instruction during the primary frequency modulation process;

[0044] The frequency modulation module 22 is configured to determine a frequency modulation strategy according to the comparison result, including:

[0045] The first judgment submodule 221 is configured to compare the active power of the secondary frequency modulation with the active power of the primary frequency modulation if the directions of the secondary frequency modulation and the primary frequency modulation are consistent, and determine and issue a corresponding step size instruction based on the power size;

[0046] The second judgment submodule 222 is configured to send the primary frequency modulation signal to the energy storage station PCS for charging and discharging according to the principle of primary frequency modulation priority if the directions of the secondary frequency modulation and the primary frequency modulation are inconsistent.

[0047] Furthermore, the first judgment submodule 221 is specifically configured to:

[0048] If the discharge step length P21> the first active power P1, and P21-P1<ΔP, the AGC instruction is executed using P21;

[0049] If the charging step length P22 is less than the first active power P1 and |P22-P1| is less than ΔP, the AGC instruction is executed using P22; the power of the primary frequency modulation is the first active power P1. Under the AGC instruction, the active power of the secondary frequency modulation includes P21 and P22, where P21 and P22 are the discharge step length and the charging step length, respectively.

[0050] Furthermore, the system further comprises a loop module 23 for determining a frequency modulation strategy after comparing the new AGC instruction with the direction and power of the primary frequency modulation during the charging and discharging process if a new AGC instruction for the secondary frequency modulation is received, and the cycle continues.

[0051] The purpose of the present invention is to provide a control method and system for grid-side energy storage power station frequency regulation. This method is a grid-side energy management system control strategy optimization method, mainly applicable to the energy storage field. The present invention does not consider the priority of primary and secondary frequency regulation, does not block secondary frequency regulation during primary frequency regulation, and determines the secondary frequency regulation strategy based on conditions such as whether the direction and magnitude of the active power of the AGC instruction issued by the dispatcher are consistent with the primary frequency regulation. In this method, when a primary frequency modulation signal is generated, the energy storage EMS detects whether the secondary frequency modulation is in effect. If a new active power adjustment instruction is detected or AGC active power adjustment is being executed, the direction and power size of the secondary frequency modulation are compared with those of the primary frequency modulation to determine the corresponding execution strategy for the secondary frequency modulation. This includes two situations: the first situation: if the direction of the secondary frequency modulation is consistent with that of the primary frequency modulation, the active power of the secondary frequency modulation is compared with the active power of the primary frequency modulation. The power comparison process includes two types: if the discharge step size P21 is greater than the first active power P1 and P21-P1 is less than ΔP, the AGC instruction is executed using P21; if the charging step size P22 is less than the first active power P1 and |P22-P1| is less than ΔP, the AGC instruction is executed using P22; the second situation: if the direction of the secondary frequency modulation is inconsistent with that of the primary frequency modulation, the primary frequency modulation signal is sent to the energy storage power station PCS for charging and discharging according to the principle of primary frequency modulation priority. The above design can avoid the blocking between the primary and secondary frequency regulation of the same energy storage station in different regulation modes, avoid the conflict between the primary and secondary frequency regulation, and also reduce the regulation time of the energy storage station and improve the stability of the power system. The present invention has the following advantages:

[0052] 1) This method solves the mutual exclusion problem between the primary and secondary frequency regulation processes of energy storage power stations. The corresponding energy management system strategy has undergone a significant change, from focusing only on the primary frequency regulation blocking signal to not only focusing on the blocking signal, but also considering the power size of the primary frequency regulation and participating in the secondary frequency regulation system.

[0053] 2) This method determines the corresponding strategy to be executed during the primary and secondary frequency regulation processes by comparing the power direction and size, which not only meets the user's grid requirements but also improves the execution efficiency of the grid-side AGC.

[0054] It should be understood that the above-described specific embodiments of the present invention are merely illustrative or illustrative of the principles of the present invention and are not intended to limit the present invention. Therefore, any modifications, equivalent substitutions, improvements, and the like that do not depart from the spirit and scope of the present invention are intended to be included within the scope of the present invention. Furthermore, the appended claims are intended to cover all variations and modifications that fall within the bounds and metes and bounds of the appended claims, or their equivalents. The present invention has been described above with reference to the embodiments thereof. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention. The scope of the present invention is defined by the appended claims and their equivalents. Those skilled in the art may make various substitutions and modifications without departing from the scope of the present invention, and such substitutions and modifications are intended to fall within the scope of the present invention. Although the embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and modifications may be made to the embodiments of the present invention without departing from the spirit and scope of the present invention. Obviously, the above-described embodiments are merely illustrative and are not intended to limit the present invention. Those skilled in the art will readily appreciate that other variations and modifications may be made based on the above description. An exhaustive list of all embodiments is neither necessary nor possible. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention. Those skilled in the art will appreciate that the embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. The present invention is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present invention. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, may be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the functions in the process. Figure 1 a process or multiple processes and / or boxes Figure 1 These computer program instructions can also be stored in a computer-readable memory that can guide a computer or other programmable data processing device to work in a specific way, so that the instructions stored in the computer-readable memory produce a product including the instruction device, which implements the function specified in the process. Figure 1 a process or multiple processes and / or boxes Figure 1 These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process in the process. Figure 1 a process or multiple processes and / or boxes Figure 1 The steps for the function specified in one or more boxes.

[0055] Those skilled in the art will understand that all or part of the steps in the above-mentioned embodiment method can be implemented by instructing the relevant hardware through a program, and the program can be stored in a computer-readable storage medium. When the program is executed, it includes the following process of the embodiment of each method as described above. Among them, the storage medium can be a magnetic disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc. The steps in the embodiment method of the present invention can be adjusted in sequence, merged and deleted according to actual needs. The modules [units] in the system [terminal or device] of the embodiment of the present invention can be merged, divided and deleted according to actual needs.

Claims

1. A method for controlling frequency modulation of a grid-side energy storage power station, characterized in that: The frequency modulation includes primary frequency modulation and secondary frequency modulation, and the control method includes: During the primary frequency modulation process, when a secondary frequency modulation instruction is received, the direction and power of the secondary frequency modulation are compared with that of the primary frequency modulation; Determining a frequency modulation strategy according to the comparison result includes: If the direction of the secondary frequency modulation is consistent with that of the primary frequency modulation, the active power of the secondary frequency modulation is compared with the active power of the primary frequency modulation, and the corresponding step length instruction is issued according to the power size; If the directions of the secondary frequency regulation are inconsistent with the primary frequency regulation, the primary frequency regulation signal will be sent to the energy storage power station PCS for charging and discharging according to the principle of primary frequency regulation priority.

2. The control method for frequency modulation of a grid-side energy storage power station according to claim 1, characterized in that: The step of comparing the active power of the secondary frequency modulation with the active power of the primary frequency modulation and determining and issuing a corresponding step length instruction according to the power size includes: If the discharge step length P21> the first active power P1, and P21-P1<ΔP, the AGC instruction is executed using P21; If the charging step P22 is less than the first active power P1, and |P22-P1| is less than ΔP, the AGC instruction is executed using P22; the power of the primary frequency modulation is the first active power P1. Under the AGC instruction, the active power of the secondary frequency modulation includes P21 and P22, where P21 and P22 are the discharge step and the charging step respectively, and ΔP is the power constant.

3. The control method for frequency modulation of a grid-side energy storage power station according to claim 2, characterized in that: The control method further comprises: During the charging and discharging process, if a new AGC instruction for secondary frequency modulation is received, the new AGC instruction is compared with the direction and power size of the primary frequency modulation to determine the frequency modulation strategy, and the cycle continues.

4. The control method for frequency modulation of a grid-side energy storage power station according to claim 1, characterized in that: The secondary frequency modulation can also control the energy storage power station PCS to perform charging and discharging.

5. A control system for frequency modulation of a grid-side energy storage power station, characterized in that: The frequency modulation includes primary frequency modulation and secondary frequency modulation, and the system includes: A comparison module is used to compare the direction and power of the secondary frequency modulation with that of the primary frequency modulation when receiving a secondary frequency modulation instruction during the primary frequency modulation process; A frequency modulation module, used to determine a frequency modulation strategy according to the comparison result, including: The first judgment submodule is used to compare the active power of the secondary frequency modulation with the active power of the primary frequency modulation if the directions of the secondary frequency modulation and the primary frequency modulation are consistent, and determine and issue a corresponding step length instruction according to the power size; The second judgment submodule is used to send the primary frequency modulation signal to the energy storage power station PCS to execute charging and discharging according to the principle of primary frequency modulation priority if the directions of the secondary frequency modulation and the primary frequency modulation are inconsistent.

6. The control system for frequency modulation of the grid-side energy storage power station according to claim 5, characterized in that: The first judgment submodule is specifically used for: If the discharge step length P21> the first active power P1, and P21-P1<ΔP, the AGC instruction is executed using P21; If the charging step P22 is less than the first active power P1, and |P22-P1| is less than ΔP, the AGC instruction is executed using P22; the power of the primary frequency modulation is the first active power P1. Under the AGC instruction, the active power of the secondary frequency modulation includes P21 and P22, where P21 and P22 are the discharge step and the charging step, respectively.

7. The control system for frequency modulation of the grid-side energy storage power station according to claim 5, characterized in that: The system also includes: a circulation module, which is used to determine the frequency modulation strategy by comparing the new AGC instruction with the direction and power of the primary frequency modulation during the charging and discharging process, if a new AGC instruction of the secondary frequency modulation is received, and the frequency modulation strategy is operated in a circulation manner.

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