TWI844475B - Energy storage system and control method thereof - Google Patents

Abstract

The present disclosure provides an energy storage system and a control method thereof. The control method includes steps of: (a) providing an energy storage system including N energy storage modules; (b) acquiring the quantity of electricity of each of N energy storage units, soring the N energy storage units according to the quantity of electricity in descending order to obtain a first sequence, and sorting the N energy storage units according to the quantity of electricity in ascending order to obtain a second sequence; (c) determining a required power of the power grid according to the power grid frequency; (d) when the required power is positive, controlling the first X energy storage units in the first sequence to collectively discharge for providing the electric energy, which has the same magnitude with the required power, to the power grid; and (e) when the required power is negative, controlling the first Y energy storage units in the second sequence to be charged by collectively receiving the electric energy, which has the same magnitude with the required power, from the power grid. X and Y are both positive integers less than or equal to N.

Description

Energy storage system and control method thereof

This case is about 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 regulating the frequency of a power grid.

The grid frequency is one of the important parameters of the power system, which reflects the balance between power generation and load. Maintaining the balance of power grid supply and demand means maintaining 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 power 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. On the contrary, when the load of the power grid is too large, the energy storage system can discharge electricity to the power grid. In recent years, countries have gradually begun to use energy storage systems to participate in auxiliary services of the power grid. The purpose is to respond quickly to power grid needs and maintain the balance of the power grid in the long term and continuously.

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. In this way, the energy storage system and the control method of this case can maintain the balance of the power grid by adjusting the frequency of the power grid, 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 dispatch and extending the life of the energy storage unit, and improving the reliability of the energy storage system to be able to operate for a long time and continuously.

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 shunt point and N energy storage modules, N being an integer greater than 1, the shunt point being electrically connected to the power grid, the N energy storage modules being electrically connected to the shunt point respectively, each energy storage module comprising a transformer, a power regulator 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 shunt point and the power regulator 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 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 order to discharge to jointly provide power equal to the power demand to the power grid, where 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 order to jointly receive power equal to the power demand from the power grid for charging, where Y is a positive integer less than or equal to N.

To achieve the above-mentioned purpose, the present invention further provides an energy storage system, comprising a power grid, a shunt point, N energy storage modules and a controller. The shunt point is electrically connected to the power grid. The N energy storage modules are electrically connected to the shunt point, respectively, where N is an integer greater than 1. Each energy storage module comprises a transformer, a power regulator 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 shunt point and the power regulator, respectively. The controller communicates with 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 to obtain a first order, and sort the N energy storage units from small to large according to the power to obtain a second order, 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 order to discharge to jointly provide power 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 order to jointly receive power equal to the required power from the power grid for charging, where Y is a positive integer less than or equal to N.

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 aspects 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.

FIG. 1 is a schematic diagram of the circuit architecture of an energy storage system of an embodiment of the present invention. In FIG. 1, the solid line and the dashed line represent the power and communication connection relationship respectively. As shown in FIG. 1, the energy storage system 1 includes a power grid 11, a junction point 12, N energy storage modules 131~13N and a controller, wherein N is an integer greater than 1. The junction point 12 is electrically connected to the power grid 11, and the N energy storage modules 131~13N are electrically connected to the junction 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 energy storage modules 131 - 13N and controls the operation of power regulators 141 - 14N and energy storage units 151 - 15N.

In some embodiments, as shown in FIG. 1 , the controller includes a master controller 20 and N slave controllers 21-2N that are communicatively connected to each other, wherein the N slave controllers 21-2N are communicatively connected to 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 master controller 20. The master controller 20 combines the information of the power grid 11 and the energy storage modules 131-13N to determine the operation of the overall energy storage system 1, and outputs corresponding instructions to the slave controllers 21-2N, 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 the actual situation. Furthermore, the specific number of slave controllers is not limited, and the communication connection relationship between the slave controller and each energy storage module can be determined according to the number.

In some embodiments, the energy storage system 1 further includes a grid meter 30, wherein the grid meter 30 is electrically connected to the junction 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 electrically connected to the high-voltage side and the low-voltage side of the corresponding transformer (T1~TN) respectively and measure the power information of the corresponding location. In one embodiment, the high-voltage side electric 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 electric meter (311-31N) and the power regulator (141-14N).

FIG. 2 is a flowchart of a control method for an energy storage system of an embodiment of the present invention, and the control method shown in FIG. 2 is applicable to the energy storage system 1 shown in FIG. The control method for the energy storage system of the present invention is described in detail below in conjunction with FIG. 1 and FIG. 2. It should be noted that the control method for the energy storage system 1 in the present invention is executed by the controller of the energy storage system 1 and will not be described in detail in the subsequent description.

As shown in FIG. 1 and FIG. 2, 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 a first order, and all energy storage units 151~15N are sorted from small to large according to the power to obtain a 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 power 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 power demand 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 sequence are controlled to receive electric energy equal to the power demand from the power grid 11 for charging, where Y is a positive integer less than or equal to N (step S4).

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.

It can be seen that the present invention can adjust the frequency of the power grid by controlling the charging and discharging of the energy storage unit, and the power of each energy storage unit 151~15N is also taken into consideration when determining the energy storage unit to be charged and discharged. Specifically, when the energy storage system 1 needs to provide power to the power 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 power from the power grid 11, the energy storage unit with a relatively low power is charged based on the second sequence control. Thus, the energy storage system 1 and its control method of the present case can maintain the balance of the power grid by adjusting the power grid frequency while maintaining the dynamic balance of the power of the energy storage units 151~15N to avoid the energy storage units 151~15N being in a full load or low load state, thereby improving the flexibility of power dispatch and extending the life of the energy storage unit, and improving the reliability of the energy storage system 1 for long-term and continuous operation.

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 power to the power grid 11 or receive power 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.

In one embodiment, according to the frequency-power relationship curve of the power grid 11 shown in FIG. 3A , 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 F0, the required power is a positive value, and step S3 of the control method is executed until the power grid frequency rises to be equal to the base frequency F0. When the power grid frequency is greater than the base 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 be equal to the base frequency F0.

In another embodiment, according to the frequency-power relationship curve of the power grid 11 shown in FIG. 3B , the power grid frequency corresponds to a specific power, and 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 a positive value (as shown in FIG. 3B , the power grid frequency is between -F3 and -F1), and step S3 of the control method is executed until the power grid frequency rises to the base frequency range of -F1 to F1. When the grid frequency is greater than the base frequency, the power demand is negative (as shown in Figure 3B, the grid frequency is between F1 and F3), and step S4 of the control method is executed until the grid frequency drops to the base frequency range -F1 to F1. When the grid frequency is between the base frequency range -F1 to F1, the power demand is zero, and the original grid frequency is maintained.

In another embodiment, according to the frequency-power relationship curve of the power grid 11 shown in FIG. 3C , the power grid frequency corresponds to a power range (i.e., the range between the two dashed lines in FIG. 3C ). In this case, in step S2 of the control method, the required power needs to be determined based on the power grid frequency and the average power of all energy storage units 151-15N, and after the required power is determined, the power 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 the relatively upper dashed curve in Figure 3C) to avoid the energy storage units 151 to 15N from reaching a full load state 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 the relatively lower dashed curve in Figure 3C) 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 151~15N 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 the solid curve in Figure 3C, where the solid curve can be obtained by taking the average value of the two dotted 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 size of the first threshold and the second threshold can be set and adjusted according to the actual situation.

Thereby, the dynamic balance of the electricity 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.

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 141~14N 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 calculation, and the specific value depends on the actual application conditions. When the required power is less than or equal to the total ideal power, it means that the required power is relatively small. At this time, if all energy storage units 151~15N 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 141~14N poor. Therefore, when 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.

The following example illustrates an implementation of determining the specific values of X and Y when the required power is less than or equal to the total desired power.

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 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 unit is greater than the required power; (ii) the sum of the ideal powers of the selected energy storage unit and the power regulator corresponding to the next energy storage unit in the first sequence is greater than or equal to the required power; and (iii) the sum of the maximum powers of the power regulators corresponding to the selected energy storage unit is greater than or equal to the required power. When condition (i) is satisfied (at which time conditions (ii) and (iii) must be satisfied) or only conditions (ii) and (iii) are satisfied, X is equal to the number of energy storage units that have been selected. When condition (i) is not satisfied and conditions (ii) and/or (iii) are also not satisfied, the next energy storage unit in the first sequence is selected, and the aforementioned conditional judgment is performed again for the selected energy storage unit.

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 discharged energy storage unit 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.

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 order 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 order.

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.

In the first application environment, the basic frequency is 50Hz, the total power is 10MW, N=4, the maximum power of each power conditioner is 2.5MW, and the default percentage is 60%. If the grid frequency is less than 50Hz and the required power is 2MW, that is, the energy storage system 1 needs to discharge 2MW to the grid 11, then the first energy storage unit in the first sequence and its corresponding power conditioner are controlled to discharge at a ratio of 80% (1*2.5MW*80=2MW). If the grid frequency is greater than 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 in the second sequence and their corresponding power conditioners are controlled to charge at a ratio of 60% (2*2.5MW*60=3MW). If the grid frequency is less than 50Hz and the required power is 5MW, the energy storage system 1 needs to discharge 5MW to the grid 11. At this time, the first three energy storage units and their corresponding power regulators in the first sequence are controlled to discharge at a ratio of 66.67% (3*2.5MW*66.67%=5MW). If the required power of the 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.

In the second application environment, the base frequency is 50Hz, the total power is 5MW, N=2, the maximum power of each power conditioner is 2.5MW, and the default percentage is 60%. Ideal 1.5MW 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 maximum power of the corresponding power conditioner are both less than the required power (that is, 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 (that is, condition (ii) is met). Since conditions (i) and (iii) are not met and condition (ii) is met, it is necessary to select the next energy storage unit in the first sequence. Since the sum of the ideal powers (3MW) of the power regulators corresponding to the first two energy storage units in the first sequence is greater than the required power, condition (i) is met. At this time, the first two energy storage units and their corresponding power regulators in the first sequence are controlled to discharge at a ratio of 56% (2*2.5MW*56%=2.8MW).

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 energy storage unit to charge and discharge, and also take the power of each energy storage unit into consideration when deciding the energy storage unit to charge and discharge. In this way, the energy storage system and the control method of this case can maintain the balance of the power grid by adjusting the frequency of the power grid, 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 dispatch and extending the life of the energy storage unit, and improving the reliability of the energy storage system to be able to operate for a long time and continuously.

It should be noted that the above is only a preferred embodiment for illustrating the present invention. The present invention is not limited to the above embodiment. The scope of the present invention is determined by the scope of the attached patent application. Moreover, the present invention can be modified in various ways by a person skilled in the art, but it does not deviate from the scope of the attached patent application.

1: Energy storage system 11: Power grid 12: Parallel connection point 131, 132, 13N: Energy storage module T1, T2, TN: Transformer 141, 142, 14N: Power regulator 151, 152, 15N: Energy storage unit 20: Main controller 21, 22, 2N: Slave controller 30: Power grid meter 311, 312, 31N: High voltage side meter 321, 322, 32N: Low voltage side meter S1, S2, S3, S4, S5: Steps F0: Base frequency F1, F2, F3: Power grid frequency

FIG. 1 is a schematic diagram of the circuit structure of an energy storage system according to an embodiment of the present invention.

FIG. 2 is a flow chart of a control method for an energy storage system according to an embodiment of the present invention.

FIG. 3A , FIG. 3B , and FIG. 3C illustrate various frequency-power relationship curves of a power grid.

S1, S2, S3, S4, S5: Steps

Claims (20)

A control method for an energy storage system comprises the steps of: (a) providing an energy storage system, wherein the energy storage system comprises a power grid, a shunt point and N energy storage modules, N being an integer greater than 1, the shunt point being electrically connected to the power grid, the N energy storage modules being electrically connected to the shunt point respectively, each of the energy storage modules comprising a transformer, a power regulator 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 shunt point and the power regulator respectively; (b) Obtain the electricity of N energy storage units, and sort the N energy storage units from large to small according to the electricity to obtain a first order, and sort the N energy storage units from small to large according to the electricity to obtain a second order; (c) Determine a required power of the power grid according to a power grid frequency of the power grid; (d) When the required power is positive, control the first X energy storage units in the first order to discharge 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; and (e) When the required power is negative, control the first Y energy storage units in the second order to jointly 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. A control method as described in claim 1, wherein the grid frequency corresponds to the demand power according to the frequency-power relationship curve of the grid; when the grid frequency is less than a base frequency, the demand power is a positive value, and the control method executes the step (d) until the grid frequency rises to be equal to the base frequency; and when the grid frequency is greater than the base frequency, the demand power is a negative value, and the control method executes the step (e) until the grid frequency drops to be equal to the base frequency. A control method as described in 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. A control method as described in 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 based on the grid frequency and the average power of the N energy storage units, and the control method adjusts the grid frequency to a frequency range by executing steps (d) and (e). The control method as described in claim 1 further comprises 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; Wherein, if the required power is less than or equal to the total ideal power, then 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, then X and Y in steps (d) and (e) are both equal to N. A control method as described in 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. A control method as described in claim 5, wherein step (d) includes sub-steps: (d1) selecting the first energy storage unit in the first sequence; (d2) determining whether conditions (i), (ii) and (iii) are satisfied, wherein condition (i) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage unit 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 unit 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; and (d4) 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 the sub-step (d2) is executed again. A control method as described in 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. A control method as described in claim 5, wherein step (e) includes sub-steps: (e1) selecting the first energy storage unit in the second sequence; (e2) determining whether conditions (i), conditions (ii) and conditions (iii) are satisfied, wherein condition (i) is that the product of the sum of the maximum powers of the power regulators corresponding to the selected energy storage unit 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 selected energy storage unit and the power regulators corresponding to the next energy storage unit in the second 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 unit 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 quantity of the selected energy storage unit is regarded as Y; and (e4) 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 the sub-step (e2) is executed again. A control method as described in claim 1, wherein the control method executes step (b) again when the time after executing step (b) exceeds a preset time period. An energy storage system comprises: a power grid; a shunt point electrically connected to the power grid; N energy storage modules electrically connected to the shunt point, wherein N is an integer greater than 1, each of the energy storage modules comprises a transformer, a power regulator and an energy storage unit connected in series, the high voltage side and the low voltage side of the transformer are electrically connected to the shunt point and the power regulator, respectively; and A 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 to obtain a first order, and sort the N energy storage units from small to large according to the power to obtain a second order, and the controller determines a required power of the power grid according to a power grid frequency of the power grid, wherein, when the required power is a positive value, the controller controls the first X energy storage units in the first order to discharge to jointly provide power 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. An energy storage system as described in claim 11, wherein the grid frequency corresponds to the required power according to the frequency-power relationship curve of the power grid; when the grid frequency is less than a 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 power to the power grid until the grid frequency rises to equal the base frequency; and when the 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 power from the power grid until the grid frequency drops to equal the base frequency. An energy storage system as described in claim 12, wherein when the grid frequency is within a frequency range including the base frequency and the demand power is zero, the controller maintains the grid frequency. An energy storage system as described in claim 11, wherein if the frequency of the power grid corresponds to a power range according to the frequency-power relationship curve of the power grid, the controller determines the required power according to the power grid frequency and the average power of the N energy storage units; when the required power is a positive value, the controller adjusts the frequency of the power grid to a frequency range by controlling the first X energy storage units in the first sequence to provide power to the power grid; and when the required power is a negative value, the controller adjusts the frequency of the power grid to the frequency range by controlling the first Y energy storage units in the second sequence to receive power from the power grid. An energy storage system as described in claim 11, wherein the controller further adds the maximum powers of the N energy storage units to obtain a total power, and multiplies the total power by a preset percentage to obtain a total ideal power; if the required power is less than or equal to the total ideal power, then X and Y are both less than N, and if the required power is greater than the total ideal power, then X and Y are both equal to N. An energy storage system as described in 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. The energy storage system as described in claim 15, wherein when the required power is positive, the controller is configured to: select 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 unit 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 unit is greater than or equal to the required power; in the condition When condition (i), condition (ii) and condition (iii) are all satisfied, or when condition (i) is not satisfied and condition (ii) and condition (iii) are satisfied, the number of the selected energy storage units is regarded as X; and When condition (i) is not satisfied and condition (ii) and/or condition (iii) are not satisfied, the next energy storage unit in the first sequence is selected, and whether condition (i), condition (ii) and condition (iii) are satisfied is determined again. An energy storage system as described in 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. The energy storage system as described in claim 15, wherein when the required power is a negative value, the controller is configured to: select the first energy storage unit in the second order; 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 unit 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 unit is greater than or equal to the required power; in the condition When condition (i), condition (ii) and condition (iii) are all met, or when condition (i) is not met and condition (ii) and condition (iii) are met, the quantity of the selected energy storage unit is regarded as Y; and When condition (i) is not met and condition (ii) and/or condition (iii) are not met, the next energy storage unit in the second sequence is selected, and whether condition (i), condition (ii) and condition (iii) are met is determined again. An energy storage system as described in claim 11, wherein when the time after the controller obtains the first sequence and the second sequence exceeds a preset time period, the controller re-obtains the first sequence and the second sequence.

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