Energy storage system and control method thereof

Abstract

The invention provides an energy storage system and a control method thereof. The control method comprises the following steps: (a) providing the energy storage system comprising N energy storage modules; (b) the electric quantity of the N energy storage units is obtained, the N energy storage units are ranked from large to small according to the electric quantity to obtain a first sequence, and the N energy storage units are ranked from small to large according to the electric quantity to obtain a second sequence; (c) determining required power of the power grid according to the power grid frequency; (d) when the required power is a positive value, the first X energy storage units in the first sequence are controlled to discharge so as to jointly provide electric energy equal to the required power to the power grid; and (e) when the required power is a negative value, controlling the first Y energy storage units in the second sequence to jointly receive electric energy equal to the required power from the power grid for charging. X and Y are positive integers smaller than or equal to N.

Description

Technical Field

[0001] This case involves an energy storage system and a control method thereof, and in particular, an energy storage system and a control method thereof that can maintain a dynamic balance of electricity while adjusting the frequency of a power grid. Background Art

[0002] Grid frequency is one of the important parameters of the power system. It reflects the balance between power generation and load. Maintaining the balance between power supply and demand is to maintain the grid frequency within a certain range. At this time, the energy storage system will be the best backup for maintaining the balance of the grid. The reason is that the energy storage system has high control accuracy and fast response speed, so it can effectively adjust the grid frequency. When the power grid generates excess power, the excess electricity can be stored in the energy storage system. Conversely, when the load on the grid is too large, the energy storage system can discharge it to the grid. In recent years, countries have gradually started to use energy storage systems to participate in the auxiliary services of the grid. The purpose is to quickly respond to grid demand and maintain the balance of the grid in the long term and continuously. Summary of the invention

[0003] The purpose of this case is to provide an energy storage system and a control method thereof, which can adjust the frequency of the power grid by controlling the charging and discharging of the energy storage unit, and also take the power of each energy storage unit into consideration when deciding the energy storage unit to charge and discharge. Therefore, the energy storage system and the control method of this case can maintain the balance of the power grid by adjusting the power grid frequency, and can also maintain the dynamic balance of power to avoid the energy storage unit being in a full load or low load state, thereby improving the flexibility of power dispatching and extending the life of the energy storage unit, and improving the reliability of the energy storage system to operate for a long time and continuously.

[0004] To achieve the above-mentioned purpose, the present invention provides a control method for an energy storage system, comprising the steps of: (a) providing an energy storage system, wherein the energy storage system comprises a power grid, a parallel connection point and N energy storage modules, N being an integer greater than 1, the parallel connection point being electrically connected to the power grid, the N energy storage modules being electrically connected to the parallel connection point respectively, each energy storage module comprising a transformer, a power conditioner and an energy storage unit connected in series, the high-voltage side and the low-voltage side of the transformer being electrically connected to the parallel connection point and the power conditioner respectively; (b) obtaining the power of the N energy storage units, and sorting the N energy storage units from large to small according to the power to obtain the first (c) determining the power demand of the power grid according to the power grid frequency of the power grid; (d) when the power demand is positive, controlling the first X energy storage units in the first sequence to discharge so as to jointly provide electric energy equal to the power demand to the power grid, wherein X is a positive integer less than or equal to N; and (e) when the power demand is negative, controlling the first Y energy storage units in the second sequence to jointly receive electric energy equal to the power demand from the power grid for charging, wherein Y is a positive integer less than or equal to N.

[0005] To achieve the above-mentioned purpose, the present case further provides an energy storage system, comprising a power grid, a parallel connection point, N energy storage modules and a controller. The parallel connection point is electrically connected to the power grid. N energy storage modules are respectively electrically connected to the parallel connection point, wherein N is an integer greater than 1. Each energy storage module comprises a transformer, a power conditioner and an energy storage unit connected in series, and the high-voltage side and the low-voltage side of the transformer are respectively electrically connected to the parallel connection point and the power conditioner. The controller communicates with the N energy storage modules, wherein the controller obtains the power of the N energy storage units to sort the N energy storage units from large to small according to the power quantity to obtain a first sequence, and sorts the N energy storage units from small to large according to the power quantity to obtain a second sequence, and the controller determines the required power of the power grid according to the power grid frequency of the power grid. When the required power is a positive value, the controller controls the first X energy storage units in the first sequence to discharge to jointly provide electric energy equal to the required power to the power grid, wherein X is a positive integer less than or equal to N. When the required power is a negative value, the controller controls the first Y energy storage units in the second sequence to receive electric energy equal to the required power from the power grid for charging, where Y is a positive integer less than or equal to N. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 A schematic diagram of the circuit structure of an energy storage system according to an embodiment of the present invention;

[0007] Figure 2 A schematic diagram of a flow chart of a method for controlling an energy storage system according to an embodiment of the present invention;

[0008] Figure 3A , Figure 3B and Figure 3C Various frequency-power relationship curves of the power grid are illustrated.

[0009] Description of Figure Numbers:

[0010] 1: Energy storage system

[0011] 11: Power Grid

[0012] 12: Parallel connection

[0013] 131, 132, 13N: Energy storage module

[0014] T1, T2, TN: Transformer

[0015] 141, 142, 14N: Power regulator

[0016] 151, 152, 15N: Energy storage unit

[0017] 20: Main controller

[0018] 21, 22, 2N: Slave controller

[0019] 30: Grid meter

[0020] 311, 312, 31N: High voltage side meter

[0021] 321, 322, 32N: Low voltage side meter

[0022] S1, S2, S3, S4, S5: Steps

[0023] F0: fundamental frequency

[0024] F1, F2, F3: Grid frequency DETAILED DESCRIPTION

[0025] Some typical embodiments that embody the features and advantages of the present invention will be described in detail in the following description. It should be understood that the present invention can have various variations in different embodiments without departing from the scope of the present invention, and the descriptions and illustrations therein are essentially for illustrative purposes rather than for limiting the present invention.

[0026] Figure 1 FIG. 1 is a schematic diagram of the circuit structure of an energy storage system according to an embodiment of the present invention. Figure 1 In the figure, the solid line and the dotted line represent the power and communication connection relationships respectively. Figure 1 As shown, the energy storage system 1 includes a power grid 11, a parallel connection point 12, N energy storage modules 131-13N and a controller, wherein N is an integer greater than 1. The parallel connection point 12 is electrically connected to the power grid 11, and the N energy storage modules 131-13N are electrically connected to the parallel connection point 12 respectively. Each energy storage module (131-13N) includes a transformer (T1-TN), a power regulator (141-14N) and an energy storage unit (151-15N) connected in series in sequence, wherein the transformer (T1-TN) and the power regulator (141-14N) are respectively configured to perform voltage conversion and power regulation, thereby adjusting the power transmission between the energy storage unit (151-15N) and the power grid 11, and the high-voltage side and the low-voltage side of the transformer (T1-TN) are respectively electrically connected to the parallel connection point 12 and the corresponding power regulator (141-14N). The controller communicates with all the energy storage modules 131 - 13N and controls the operation of the power regulators 141 - 14N and the energy storage units 151 - 15N.

[0027] In some embodiments, such as Figure 1As shown, the controller includes a main controller 20 and N slave controllers 21-2N that are communicatively connected to each other, wherein the N slave controllers 21-2N are respectively communicatively connected to the components (including transformers, power regulators and energy storage units) in the N energy storage modules 131-13N. The slave controllers 21-2N provide the operation information of the energy storage modules 131-13N to the main controller 20, and the main controller 20 determines the operation of the overall energy storage system 1 in combination with the information of the power grid 11 and the energy storage modules 131-13N, and outputs instructions to the slave controllers 21-2N accordingly, so that the slave controllers 21-2N control the operation of each energy storage module 131-13N. In addition, in some embodiments, each slave controller (21-2N) can also be communicatively connected to all energy storage modules 131-13N, and the specific control object of each slave controller (21-2N) is not limited and can be adjusted according to actual conditions. Furthermore, the specific number of slave controllers is not limited, and the communication connection relationship between the slave controllers and each energy storage module can be determined according to their number.

[0028] In some embodiments, the energy storage system 1 further includes a grid meter 30, wherein the grid meter 30 is electrically connected to the parallel connection point 12 and is communicatively connected to the main controller 20. The grid meter 30 is configured to read the grid frequency of the grid 11 and provide it to the main controller 20. In some embodiments, each energy storage module (131-13N) further includes a high-voltage side meter (311-31N) and a low-voltage side meter (321-32N), wherein the high-voltage side meter (311-31N) and the low-voltage side meter (321-32N) are respectively electrically connected to the high-voltage side and the low-voltage side of the corresponding transformer (T1-TN) and measure the electric energy information at the corresponding position. In one embodiment, the high-voltage side meter (311-31N) is communicatively connected to the corresponding slave controller (21-2N) so that the slave controller (21-2N) can compensate for the error between the high-voltage side meter (311-31N) and the power regulator (141-14N).

[0029] Figure 2 is a flow chart of a control method of an energy storage system according to an embodiment of the present invention, and Figure 2 The control method shown is applicable to Figure 1 The energy storage system 1 shown. Figure 1 and Figure 2 The control method of the energy storage system in this case is described in detail. It should be noted that the control method for the energy storage system 1 in this case is executed by the controller of the energy storage system 1 and will not be repeated in the subsequent description.

[0030] like Figure 1 and Figure 2As shown, first, the power of all energy storage units 151~15N is obtained, and all energy storage units 151~15N are sorted from large to small according to the power to obtain the first order, and all energy storage units 151~15N are sorted from small to large according to the power to obtain the second order (step S1). Then, the required power of the power grid 11 is determined according to the power grid frequency of the power grid 11 (step S2). When the required power of the power grid 11 is a positive value, it means that the energy storage system 1 needs to supply power to the power grid 11 to adjust the power grid frequency. At this time, the first X energy storage units in the first order are controlled to discharge to jointly provide electric energy equal to the required power to the power grid 11, where X is a positive integer less than or equal to N (step S3). On the contrary, when the required power of the power grid 11 is a negative value, it means that the energy storage system 1 needs to receive electric energy from the power grid 11 to adjust the power grid frequency. At this time, the first Y energy storage units in the second order are controlled to jointly receive electric energy equal to the required power from the power grid 11 for charging, where Y is a positive integer less than or equal to N (step S4).

[0031] In some embodiments, the control method executes step S1 again when the time after executing step S1 exceeds a preset time. For example, the control method further includes step S5: determining whether the time since the last execution of step S1 has exceeded the preset time. If the determination result of step S5 is yes, step S1 is executed again; if the determination result of step S5 is no, step S2 is executed again.

[0032] It can be seen from this that the present case can adjust the grid frequency by controlling the charging and discharging of the energy storage units, and the power of each energy storage unit 151~15N is also taken into consideration when deciding the energy storage unit to be charged and discharged. Specifically, when the energy storage system 1 needs to provide electric energy to the grid 11, the energy storage unit with a relatively high power is discharged based on the first sequence control, and when the energy storage system 1 needs to receive electric energy from the grid 11, the energy storage unit with a relatively low power is charged based on the second sequence control. Therefore, the energy storage system 1 and its control method of the present case can maintain the balance of the grid by adjusting the grid frequency, and at the same time, maintain the dynamic balance of the power of the energy storage units 151~15N, so as to avoid the energy storage units 151~15N being in a full load or low load state, thereby improving the flexibility of power dispatching and extending the life of the energy storage units, and improving the reliability of the energy storage system 1 for long-term and continuous operation.

[0033] According to the aforementioned control method, after determining the power demand of the power grid 11, the energy storage system 1 can be controlled to provide electric energy to the power grid 11 or receive electric energy from the power grid 11 according to the power demand, thereby adjusting the power grid frequency and maintaining the balance of the power grid. It should be noted that the specific method of determining the power demand of the power grid 11 depends on the frequency-power relationship curve of the power grid 11. The following examples illustrate several frequency-power relationship curves of the power grid 11 and their corresponding specific methods of determining the power demand. However, in fact, the specifications of the power grid 11 applicable to this case are not limited to this.

[0034] In one embodiment, according to Figure 3A In the frequency-power relationship curve of the power grid 11 shown, the power grid frequency corresponds to a specific power. In this case, the power corresponding to the power grid frequency is used as the required power of the power grid 11. When the power grid frequency is less than the basic frequency F0, the required power is a positive value, and step S3 of the control method is executed until the power grid frequency rises to equal the basic frequency F0. When the power grid frequency is greater than the basic frequency F0, the required power is a negative value, and step S4 of the control method is executed until the power grid frequency drops to equal the basic frequency F0.

[0035] In another embodiment, according to Figure 3B The frequency-power relationship curve of the power grid 11 shown in FIG. 1 shows that the power grid frequency corresponds to a specific power. In this case, the power corresponding to the power grid frequency is used as the required power of the power grid 11. When the power grid frequency is less than the base frequency, the required power is positive (e.g. Figure 3B As shown, the grid frequency is between -F3 and -F1), and step S3 of the control method is executed until the grid frequency rises to the basic frequency interval -F1 to F1. When the grid frequency is greater than the basic frequency, the required power is negative (such as Figure 3B As shown, the grid frequency is between F1 and F3), and step S4 of the control method is executed until the grid frequency drops to the basic frequency interval -F1 to F1. When the grid frequency is between the basic frequency interval -F1 to F1, the required power is zero and the original grid frequency is maintained.

[0036] In another embodiment, according to Figure 3C The frequency-power relationship curve of the power grid 11 shown in FIG. 1 is a power range corresponding to the power grid frequency (i.e. Figure 3C In this case, in step S2 of the control method, the required power is determined according to the grid frequency and the average power of all energy storage units 151 to 15N, and after determining the required power, the grid frequency is adjusted to a frequency range by executing step S3 or S4 of the control method. Specifically, when the average power of all energy storage units 151 to 15N is greater than the first threshold, it means that the average power is relatively high, so the required power is determined to be equal to the maximum value in the power range (corresponding to Figure 3CThe dashed curve above the power storage unit 151 to 15N is used to prevent the energy storage units 151 to 15N from reaching full load as much as possible. When the average power of all energy storage units 151 to 15N is less than the second threshold (the second threshold is less than the first threshold), it means that the average power is relatively low, so the required power is determined to be equal to the minimum value in the power range (corresponding to Figure 3C The dashed curve below the lower part of the figure) is used to avoid the energy storage units 151 to 15N from reaching a low load state as much as possible. When the average power of all energy storage units 151 to 15N is between the first threshold and the second threshold, it means that the average power is moderate, so the required power is determined to be equal to the middle value in the power range (corresponding to Figure 3C The solid line curve in the figure, where the solid line curve can be obtained by taking the average of the two dotted line curves), so that each energy storage unit 151-15N maintains the current load state to avoid reaching a full load or low load state. The specific values ​​of the first threshold and the second threshold can be set and adjusted according to actual conditions.

[0037] Thus, the dynamic balance of the electric quantity of the energy storage units 151 - 15N can be further maintained to prevent the energy storage units 151 - 15N from being in a full load or low load state.

[0038] In addition, in some embodiments, the control method of the energy storage system 1 further includes the steps of: adding the maximum powers of all power regulators 141 to 14N to obtain the total power, and multiplying the total power by a preset percentage to obtain the total ideal power. The preset percentage is the optimal output power ratio obtained after simulation and actuarial calculation, and the specific value depends on the actual application conditions. When the magnitude of the required power is less than or equal to the total ideal power, it means that the magnitude of the required power is relatively small. At this time, if all energy storage units 151 to 15N are controlled to charge and discharge together, the energy storage system 1 will be in a low-power state, thereby making the conversion efficiency of the power regulators 141 to 14N poor. Therefore, when the magnitude of the required power is less than or equal to the total ideal power, only part of the energy storage units are controlled to charge and discharge, that is, X and Y in steps S3 and S4 of the control method are both less than N, so as to avoid the energy storage system 1 being in a low-power state, thereby improving the operating efficiency of the energy storage system 1. In addition, when the required power is greater than the total ideal power, it means that the required power is relatively large. At this time, all energy storage units 151-15N can be controlled to charge and discharge together, that is, X and Y in steps S3 and S4 are both equal to N.

[0039] The following is an example of determining specific values ​​of X and Y when the required power is less than or equal to the desired power.

[0040] Take the case where the required power is a positive value as an example. First, select the first energy storage unit in the first sequence. Then, based on the size of the required power and the maximum power and ideal power (equal to the product of the maximum power and the preset percentage) of the power regulator corresponding to the energy storage unit, determine whether the following conditions are met: (i) the sum of the ideal powers of the power regulators corresponding to the selected energy storage units is greater than the size of the required power; (ii) the sum of the ideal powers of the power regulators corresponding to the selected energy storage unit and the next energy storage unit in the first sequence is greater than or equal to the size of the required power; and (iii) the sum of the maximum powers of the power regulators corresponding to the selected energy storage units is greater than or equal to the size of the required power. When condition (i) is met (at this time, conditions (ii) and (iii) must be met) or only conditions (ii) and (iii) are met, then X is equal to the number of selected energy storage units. When condition (i) is not met and conditions (ii) and / or (iii) are also not met, add the next energy storage unit in the first sequence, and perform the above-mentioned conditional judgment again on the selected energy storage unit.

[0041] When X>1, the value of X obtained by the above-mentioned conditional judgment will make the required power greater than or equal to the product of the sum of the maximum powers of the X power regulators corresponding to the first X energy storage units in the first sequence and the preset percentage (i.e., the sum of the ideal powers), and make the required power less than or equal to the sum of the maximum powers of the X power regulators corresponding to the first X energy storage units in the first sequence. In other words, the actual output power of the power regulator corresponding to the energy storage unit being discharged will be between its maximum power and the ideal power. In this way, the energy storage system 1 can be prevented from being in a low-power state, thereby improving the operating efficiency of the energy storage system 1.

[0042] In addition, the value of Y when the required power is a negative value can also be determined in a similar manner, which will not be repeated here. When the required power is a negative value, the magnitude of the required power is greater than or equal to the product of the sum of the maximum powers of the Y power regulators corresponding to the first Y energy storage units in the second sequence and the preset percentage, and the magnitude of the required power is less than or equal to the sum of the maximum powers of the Y power regulators corresponding to the first Y energy storage units in the second sequence.

[0043] The following example illustrates how to determine the energy storage unit to be charged and discharged and its output power based on the control method described above in an actual application environment.

[0044] In the first application environment, the base frequency is 50 Hz, the total power is 10 MW, N=4, the maximum power of each power conditioner is 2.5 MW, and the preset percentage is 60%.

[0045] If the grid frequency is less than 50 Hz and the required power is 2 MW, that is, the energy storage system 1 needs to discharge 2 MW to the grid 11, then the first energy storage unit in the first sequence and its corresponding power regulator are controlled to discharge at a ratio of 80% (1*2.5 MW*80=2 MW).

[0046] If the grid frequency is >50Hz and the required power is 3MW, that is, the energy storage system 1 needs to receive 3MW from the grid 11, then the first two energy storage units and their corresponding power regulators in the second sequence are controlled to charge at a ratio of 60% (2*2.5MW*60=3MW).

[0047] If the grid frequency is less than 50 Hz and the required power is 5 MW, that is, the energy storage system 1 needs to discharge 5 MW to the grid 11, then the first three energy storage units in the first sequence and their corresponding power regulators are controlled to discharge at a ratio of 66.67% (3*2.5 MW*66.67%=5 MW).

[0048] If the power demand of the power grid 11 is greater than or equal to 6MW, all four energy storage units and their corresponding power regulators need to be charged and discharged.

[0049] In the second application environment, the base frequency is 50Hz, the total power is 5MW, N=2, the maximum power of each power regulator is 2.5MW, and the preset percentage is 60%. Ideal 1.5MW

[0050] If the grid frequency is less than 50Hz and the required power is 2.8MW, the energy storage system 1 needs to discharge 2.8MW to the grid 11. If only the first energy storage unit in the first sequence is selected, the ideal power (2.5MW*60%=1.5MW) and the maximum power of the corresponding power conditioner are both less than the required power (i.e., conditions (i) and (iii) are not met), and the sum of the ideal powers of the selected energy storage unit and the power conditioner corresponding to the next energy storage unit in the first sequence (=1.5MW+1.5MW=3MW) is greater than the required power (i.e., condition (ii) is met). Since conditions (i) and (iii) are not met and condition (ii) is met, it is necessary to add the next energy storage unit in the first sequence. Since the sum of the ideal powers of the power conditioners corresponding to the first two energy storage units in the first sequence (3MW) is greater than the required power, condition (i) is met. At this time, the first two energy storage units in the first sequence and their corresponding power conditioners are controlled to discharge at a ratio of 56% (2*2.5MW*56%=2.8MW).

[0051] In summary, this case provides an energy storage system and a control method thereof, which can adjust the frequency of the power grid by controlling the charging and discharging of the energy storage unit, and also take the power of each energy storage unit into consideration when deciding the energy storage unit to charge and discharge. Therefore, the energy storage system and the control method thereof of this case can maintain the balance of the power grid by adjusting the power grid frequency, and can also maintain the dynamic balance of power to avoid the energy storage unit being in a full load or low load state, thereby improving the flexibility of power dispatching and extending the life of the energy storage unit, and improving the reliability of the energy storage system to operate for a long time and continuously.

[0052] It should be noted that the above are only preferred embodiments for illustrating the present invention, and the present invention is not limited to the embodiments described. The scope of the present invention is determined by the attached patent claims. Moreover, the present invention may be modified in various ways by those skilled in the art, but all of them are within the scope of the protection intended by the attached patent claims.

Claims

1. A control method for an energy storage system, comprising the steps of: (a) providing an energy storage system, wherein the energy storage system comprises a power grid, a parallel connection point and N energy storage modules, N being an integer greater than 1, the parallel connection point being electrically connected to the power grid, the N energy storage modules being electrically connected to the parallel connection point respectively, each of the energy storage modules comprising a transformer, a power conditioner and an energy storage unit connected in series, the high voltage side and the low voltage side of the transformer being electrically connected to the parallel connection point and the power conditioner respectively; (b) obtaining the power of N energy storage units, and sorting the N energy storage units from large to small according to the power to obtain a first order, and sorting the N energy storage units from small to large according to the power to obtain a second order; (c) determining a power demand of the power grid according to a power grid frequency of the power grid; (d) when the required power is a positive value, controlling the first X energy storage units in the first sequence to discharge so as to jointly provide electric energy equal to the required power to the power grid, wherein X is a positive integer less than or equal to N; and (e) When the required power is a negative value, control the first Y energy storage units in the second sequence to collectively receive electrical energy equal to the required power from the power grid for charging, wherein Y is a positive integer less than or equal to N.

2. The control method according to claim 1, wherein according to the frequency-power relationship curve of the power grid, the power grid frequency corresponds to the required power; when the power grid frequency is less than the base frequency, the required power is a positive value, and the control method executes the step (d) until the power grid frequency rises to be equal to the base frequency; and when the power grid frequency is greater than the base frequency, the required power is a negative value, and the control method executes the step (e) until the power grid frequency drops to be equal to the base frequency. 3 . The control method according to claim 2 , wherein when the grid frequency is within a frequency range including the base frequency, the required power is zero, and the control method maintains the grid frequency.

4. The control method according to claim 1, wherein if the grid frequency corresponds to a power range according to the frequency-power relationship curve of the grid, then in step (c), the required power is determined according to the grid frequency and the average power of the N energy storage units, and the control method adjusts the grid frequency to within the frequency range by executing steps (d) and (e).

5. The control method according to claim 1, further comprising the steps of: adding the maximum powers of the N power regulators corresponding to the N energy storage units to obtain a total power, and multiplying the total power by a preset percentage to obtain a total ideal power; in, If the required power is less than or equal to the total ideal power, X and Y in steps (d) and (e) are both less than N, and if the required power is greater than the total ideal power, X and Y in steps (d) and (e) are both equal to N.

6. The control method according to claim 5, wherein in step (d), the magnitude of the required power is greater than or equal to the product of the sum of the maximum powers of the X power regulators corresponding to the first X energy storage units in the first sequence and the preset percentage, and the magnitude of the required power is less than or equal to the sum of the maximum powers of the X power regulators corresponding to the first X energy storage units in the first sequence.

7. The control method according to claim 5, wherein the step (d) comprises the sub-steps of: (d1) selecting the first energy storage unit in the first sequence; (d2) Determine whether conditions (i), (ii) and (iii) are met, wherein condition (i) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage units and the preset percentage is greater than the required power; condition (ii) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage unit and the next energy storage unit in the first sequence and the preset percentage is greater than or equal to the required power; condition (iii) is that the sum of the maximum powers of the power regulators corresponding to the selected energy storage units is greater than or equal to the required power; (d3) when the condition (i), the condition (ii) and the condition (iii) are all satisfied, or when the condition (i) is not satisfied and the condition (ii) and the condition (iii) are satisfied, the number of the selected energy storage units is regarded as X; as well as (d4) When the condition (i) is not met and the condition (ii) and / or the condition (iii) is not met, the next energy storage unit in the first sequence is selected and the sub-step (d2) is executed again.

8. The control method according to claim 5, wherein in step (e), the magnitude of the required power is greater than or equal to the product of the sum of the maximum powers of the Y power regulators corresponding to the first Y energy storage units in the second sequence and the preset percentage, and the magnitude of the required power is less than or equal to the sum of the maximum powers of the Y power regulators corresponding to the first Y energy storage units in the second sequence.

9. The control method according to claim 5, wherein the step (e) comprises the sub-steps of: (e1) selecting the first energy storage unit in the second sequence; (e2) Determine whether conditions (i), (ii) and (iii) are met, wherein condition (i) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage units and the preset percentage is greater than the required power; condition (ii) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage unit and the next energy storage unit in the second order and the preset percentage is greater than or equal to the required power; condition (iii) is that the sum of the maximum powers of the power regulators corresponding to the selected energy storage units is greater than or equal to the required power; (e3) when the condition (i), the condition (ii) and the condition (iii) are all satisfied, or when the condition (i) is not satisfied and the condition (ii) and the condition (iii) are satisfied, the number of the selected energy storage units is regarded as Y; and (e4) When the condition (i) is not met and the condition (ii) and / or the condition (iii) are not met, the next energy storage unit in the second sequence is selected and the sub-step (e2) is executed again.

10. The control method according to claim 1, wherein the control method executes step (b) again when the time after executing step (b) exceeds a preset time period.

11. An energy storage system comprising: Power grid; A parallel connection point electrically connected to the power grid; N energy storage modules are electrically connected to the parallel connection point, respectively, wherein N is an integer greater than 1, each of the energy storage modules comprises a transformer, a power conditioner and an energy storage unit connected in series, and the high voltage side and the low voltage side of the transformer are electrically connected to the parallel connection point and the power conditioner, respectively; and A controller is in communication with the N energy storage modules, wherein the controller obtains the power of the N energy storage units, sorts the N energy storage units from large to small according to the power, obtains a first sequence, and sorts the N energy storage units from small to large according to the power, obtains a second sequence, and the controller determines the required power of the power grid according to the power grid frequency of the power grid, in, When the required power is a positive value, the controller controls the first X energy storage units in the first sequence to discharge so as to jointly provide electric energy equal to the required power to the power grid, where X is a positive integer less than or equal to N, When the required power is a negative value, the controller controls the first Y energy storage units in the second sequence to collectively receive electrical energy equal to the required power from the power grid for charging, where Y is a positive integer less than or equal to N.

12. The energy storage system according to claim 11, wherein according to the frequency-power relationship curve of the power grid, the power grid frequency corresponds to the required power; when the power grid frequency is less than the base frequency, the required power is a positive value, and the controller controls the first X energy storage units in the first sequence to provide electrical energy to the power grid until the power grid frequency rises to be equal to the base frequency; and when the power grid frequency is greater than the base frequency, the required power is a negative value, and the controller controls the first Y energy storage units in the second sequence to receive electrical energy from the power grid until the power grid frequency drops to be equal to the base frequency. 13 . The energy storage system according to claim 12 , wherein when the grid frequency is within a frequency range including the base frequency and the required power is zero, the controller maintains the grid frequency.

14. The energy storage system according to claim 11, wherein if the grid frequency corresponds to a power range according to the frequency-power relationship curve of the grid, the controller determines the required power according to the grid frequency and the average power of the N energy storage units; when the required power is a positive value, the controller adjusts the grid frequency to within the frequency range by controlling the first X energy storage units in the first sequence to provide electrical energy to the grid; and when the required power is a negative value, the controller adjusts the grid frequency to within the frequency range by controlling the first Y energy storage units in the second sequence to receive electrical energy from the grid.

15. The energy storage system according to claim 11, wherein the controller further adds the maximum powers of the N energy storage units to obtain the total power, and multiplies the total power by a preset percentage to obtain the total ideal power; if the required power is less than or equal to the total ideal power, both X and Y are less than N, and if the required power is greater than the total ideal power, both X and Y are equal to N.

16. The energy storage system according to claim 15, wherein when the required power is a positive value, the magnitude of the required power is greater than or equal to the product of the sum of the maximum powers of the X power regulators corresponding to the first X energy storage units in the first sequence and the preset percentage, and the magnitude of the required power is less than or equal to the sum of the maximum powers of the X power regulators corresponding to the first X energy storage units in the first sequence.

17. The energy storage system according to claim 15, wherein when the required power is a positive value, the controller is configured to: Selecting the first energy storage unit in the first sequence; Determine whether conditions (i), (ii) and (iii) are met, wherein condition (i) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage units and the preset percentage is greater than the required power; condition (ii) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage unit and the next energy storage unit in the first sequence and the preset percentage is greater than or equal to the required power; condition (iii) is that the sum of the maximum powers of the power regulators corresponding to the selected energy storage units is greater than or equal to the required power; When the condition (i), the condition (ii) and the condition (iii) are all satisfied, or when the condition (i) is not satisfied and the condition (ii) and the condition (iii) are satisfied, the number of the selected energy storage units is regarded as X; and When the condition (i) is not satisfied and the condition (ii) and / or the condition (iii) are not satisfied, the next energy storage unit in the first sequence is selected, and it is determined again whether the condition (i), the condition (ii) and the condition (iii) are satisfied.

18. An energy storage system according to claim 15, wherein when the required power is a negative value, the magnitude of the required power is greater than or equal to the product of the sum of the maximum powers of the Y power regulators corresponding to the first Y energy storage units in the second sequence and the preset percentage, and the magnitude of the required power is less than or equal to the sum of the maximum powers of the Y power regulators corresponding to the first Y energy storage units in the second sequence.

19. The energy storage system according to claim 15, wherein when the required power is a negative value, the controller is configured to: Selecting the first energy storage unit in the second sequence; Determine whether conditions (i), (ii) and (iii) are met, wherein condition (i) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage units and the preset percentage is greater than the required power; condition (ii) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage unit and the next energy storage unit in the second order and the preset percentage is greater than or equal to the required power; condition (iii) is that the sum of the maximum powers of the power regulators corresponding to the selected energy storage units is greater than or equal to the required power; When the condition (i), the condition (ii) and the condition (iii) are all satisfied, or when the condition (i) is not satisfied and the condition (ii) and the condition (iii) are satisfied, the number of the selected energy storage units is regarded as Y; and When the condition (i) is not satisfied and the condition (ii) and / or the condition (iii) are not satisfied, the next energy storage unit in the second sequence is selected, and it is determined again whether the condition (i), the condition (ii) and the condition (iii) are satisfied. 20 . The energy storage system according to claim 11 , wherein when the time after the controller acquires the first sequence and the second sequence exceeds a preset time period, the controller reacquires the first sequence and the second sequence.

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