Power distribution method in off-grid state and energy storage system

Abstract

Disclosed in the present disclosure are a power distribution method in an off-grid state and an energy storage system. The method comprises: on the basis of a droop control principle and by means of a target output active power corresponding to each power conversion system to be distributed, calculating a droop coefficient of each said power conversion system, so that each said power conversion system outputs the corresponding target output active power by adjusting the droop coefficient of each said power conversion system. The present disclosure solves the technical problems in the prior art of battery stacks corresponding to power conversion systems to be distributed being unable to be fully charged or discharged at the same time or the large influence of voltage fluctuation on the output precision, and achieves the technical effect of controlling the battery stacks corresponding to multiple said power conversion systems to be fully charged or discharged at the same time and improving the anti-interference capability.

Description

A power distribution method and energy storage system in an off-grid state

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure claims priority to Chinese patent application number 2024101845672, filed with the Patent Office of China on February 19, 2024, entitled “A power distribution method and energy storage system in an off-grid state,” the entire contents of which are incorporated by reference into this disclosure. Technical Field

[0003] The present disclosure relates to the technical field of energy storage systems, and in particular to an off-grid power distribution method and an energy storage system. Background Art

[0004] When the energy storage system is in an off-grid state, multiple energy storage converters (Power Conversion System, PCS) of the energy storage system control the output power of the corresponding battery stack respectively. Among them, the multiple energy storage converters include a master energy storage converter and a slave energy storage converter. The master energy storage converter operates in the off-grid mode VF to ensure the stability of the output voltage and frequency of the energy storage system. The slave energy storage converter operates in the grid-connected mode PQ to control the output active power and reactive power. As a result, when voltage fluctuations occur, it is difficult to smoothly control the output power of the slave energy storage converter, and due to the differences in battery capacity between the battery stacks, it is impossible to fully charge or discharge the battery at the same time, which affects the consistency of the battery stack.

[0005] Summary of the Invention

[0006] The problem solved by the present disclosure is that in the prior art, the battery stacks corresponding to the energy storage converters to be allocated cannot be fully charged or fully discharged at the same time, and voltage fluctuations have a significant impact on output accuracy.

[0007] In order to overcome the above-mentioned deficiencies in the prior art, the present disclosure aims to provide a method for distributing power in an off-grid state, the method comprising:

[0008] Determine the target output active power corresponding to each of the multiple energy storage converters to be allocated in the energy storage system in an off-grid state;

[0009] Based on the droop control principle, the droop coefficient of each energy storage converter to be allocated is calculated by the target output active power corresponding to each energy storage converter to be allocated, so that each energy storage converter to be allocated outputs the corresponding target output active power by adjusting the droop coefficient of each energy storage converter to be allocated.

[0010] Furthermore, in the above method, determining the target output active power corresponding to each of the plurality of energy storage converters to be allocated in the off-grid state of the energy storage system includes:

[0011] Obtaining the active power to be allocated of the energy storage system in an off-grid state to construct a first equation in which the active power to be allocated is equal to the sum of the target output active powers corresponding to the plurality of energy storage converters to be allocated;

[0012] Constructing a second equation for expressing the target output active power corresponding to each to-be-allocated energy storage converter by using the input power of the reference energy storage converter, wherein the reference energy storage converter is one of the plurality of to-be-allocated energy storage converters;

[0013] The target output active power of each energy storage converter to be allocated is calculated by combining the first equation with the second equation corresponding to each energy storage converter to be allocated.

[0014] Furthermore, in the above method, the second equation includes:

[0015] The target output active power is obtained by expressing the input power of each energy storage converter to be allocated by referring to the input power of the energy storage converter and expressing the corresponding target output active power by expressing the input power of each energy storage converter to be allocated.

[0016] Furthermore, in the above method, the second equation corresponding to each energy storage converter to be allocated is determined in the following manner:

[0017] Based on the battery capacity relationship between the energy storage converter to be allocated and the reference energy storage converter, constructing a first conversion relationship formula that expresses the input power of the energy storage converter to be allocated by using the input power of the reference energy storage converter;

[0018] Based on the conversion relationship between the input power and the output active power of the energy storage converter to be allocated, a second conversion relationship is obtained in which the target output active power of the energy storage converter to be allocated is expressed by the input power;

[0019] By combining the first conversion relationship and the second conversion relationship corresponding to the energy storage converter to be allocated, a second equation is obtained in which the target output active power of the energy storage converter to be allocated is expressed by the input power of the reference energy storage converter.

[0020] Furthermore, in the above method, a first conversion relationship expressing the input power of the energy storage converter to be allocated by using the input power of the reference energy storage converter is constructed in the following manner:

[0021] The battery capacity relationship is described by assuming that the ratio of the input current between the energy storage converter to be allocated and the reference energy storage converter is equal to the ratio of the battery capacities, so as to construct the first conversion relationship corresponding to the energy storage converter to be allocated, wherein the input current includes: obtained by the input power and input DC voltage of the energy storage converter.

[0022] Furthermore, in the above method, the target output active power of each energy storage converter to be allocated is calculated in the following manner:

[0023] Based on the first equation and the second equation corresponding to each energy storage converter to be allocated, the input power of the reference energy storage converter is calculated; based on the input power of the reference energy storage converter and the second equation corresponding to each energy storage converter to be allocated, the target output active power corresponding to each energy storage converter to be allocated is calculated.

[0024] Furthermore, in the above method, the energy storage system also includes a target energy storage converter in addition to the multiple energy storage converters to be allocated, and the target energy storage converter refers to the energy storage converter corresponding to the largest battery capacity among the multiple energy storage converters in the energy storage system. The obtaining of the active power to be allocated of the energy storage system in an off-grid state includes: determining the active power to be allocated based on the total output active power of the energy storage system in an off-grid state and the rated output active power of the target energy storage converter.

[0025] Furthermore, in the above method, the droop coefficient of each energy storage converter to be allocated is calculated in the following manner:

[0026] Based on the impedance performance of each energy storage converter to be allocated, the target output voltage value of each energy storage converter to be allocated is calculated using the target output active power corresponding to each energy storage converter to be allocated;

[0027] Based on the power-voltage droop relationship of each energy storage converter to be allocated, the droop coefficient of each energy storage converter to be allocated is calculated using the target output active power and the target output voltage value corresponding to each energy storage converter to be allocated.

[0028] Furthermore, in the above method, the plurality of energy storage converters to be allocated are all connected to a common connection point, and the target output voltage value of each energy storage converter to be allocated is calculated by the following method:

[0029] Based on a voltage-to-power conversion expression corresponding to the impedance performance of the energy storage converter to be allocated, the impedance coefficient corresponding to the energy storage converter to be allocated is calculated using a preset voltage value of the common connection point, a no-load output voltage value of the energy storage converter to be allocated when not connected to the common connection point, and a no-load output active power value;

[0030] Based on the impedance coefficient corresponding to the energy storage converter to be allocated and the voltage-to-power conversion expression, the target output voltage value is calculated using the target output active power corresponding to the energy storage converter to be allocated.

[0031] Furthermore, in the above method, the voltage-to-power conversion expression is used to characterize the relationship between the output active power and the output voltage of the energy storage converter to be allocated when the impedance coefficient of the energy storage converter to be allocated is constant, wherein the output voltage value is equal to the product of the impedance coefficient and the output active power corresponding to the energy storage converter to be allocated plus the preset voltage value.

[0032] Furthermore, in the above method, the plurality of energy storage converters to be allocated are all connected to a common connection point, and the power voltage droop relationship includes:

[0033] Based on the no-load output voltage value of the energy storage converter to be allocated when it is not connected to the common connection point, the relationship between the output active power and the output voltage of the energy storage converter to be allocated is described, wherein the output voltage value is equal to the product of the droop coefficient and the output active power corresponding to the energy storage converter to be allocated plus the no-load output voltage value of the energy storage converter to be allocated when it is not connected to the common connection point.

[0034] Furthermore, in the above method, the power-voltage droop relationship of each energy storage converter to be allocated is determined by the following method:

[0035] Based on the impedance property of the energy storage system, the AC power calculation formula of each energy storage converter to be allocated is simplified to obtain the power-voltage droop relationship of each energy storage converter to be allocated, wherein the impedance property of the energy storage system is resistive.

[0036] Another object of the present disclosure is to provide an energy storage system, comprising:

[0037] Multiple battery stacks; multiple energy storage converters to be allocated, each energy storage converter to be allocated corresponds to a battery stack, and is used to convert the direct current of the corresponding battery stack into alternating current for output; an energy management system, used to execute the steps of the power distribution method in an off-grid state described in the above-mentioned first aspect or any possible implementation method of the first aspect.

[0038] Another object of the present disclosure is to provide a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the steps of the power distribution method in an off-grid state described in the first aspect or any possible implementation of the first aspect are executed.

[0039] Compared with the prior art, the present disclosure has the following beneficial effects:

[0040] An embodiment of the present disclosure provides a power distribution method and energy storage system in an off-grid state, the method comprising: determining the target output active power corresponding to each of a plurality of energy storage converters to be allocated of the energy storage system in an off-grid state; based on the droop control principle, calculating the droop coefficient of each energy storage converter to be allocated by using the target output active power corresponding to each energy storage converter to be allocated, so as to adjust the droop coefficient of each energy storage converter to be allocated so that each energy storage converter to be allocated outputs the corresponding target output active power. After determining the target output active power of each to-be-allocated energy storage converter in the off-grid state of the energy storage system, the droop coefficient corresponding to the target output active power of the to-be-allocated energy storage converter is determined through the droop control principle. Then, the droop coefficient of the to-be-allocated energy storage converter is adjusted so that the to-be-allocated energy storage converter outputs the target output active power. This solves the technical problems in the prior art that the battery stacks corresponding to the to-be-allocated energy storage converters cannot be fully charged or fully discharged at the same time and that voltage fluctuations have a significant impact on output accuracy. This achieves the technical effect of controlling the battery stacks corresponding to multiple to-be-allocated energy storage converters to achieve simultaneous full charging or full discharge and increasing anti-interference capability.

[0041] The beneficial effects of other technical solutions in the above technical solutions will be described in detail in the subsequent specific implementation methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG1 shows a flow chart of a power distribution method in an off-grid state provided by an embodiment of the present disclosure.

[0043] FIG2 shows a flowchart of the steps of determining target output active powers corresponding to a plurality of energy storage converters to be allocated in an energy storage system in an off-grid state, provided by an embodiment of the present disclosure.

[0044] FIG3 shows a schematic diagram of adjusting the droop coefficient provided by an embodiment of the present disclosure. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solutions and advantages of the embodiments of the present disclosure clearer, the technical solutions in the embodiments of the present disclosure will be clearly and completely described below in conjunction with the drawings in the embodiments of the present disclosure. It should be understood that the drawings in the present disclosure are only for the purpose of illustration and description and are not used to limit the scope of protection of the present disclosure. In addition, it should be understood that the schematic drawings are not drawn to scale. The flowcharts used in the present disclosure illustrate operations implemented according to some embodiments of the present disclosure. It should be understood that the operations of the flowchart can be implemented out of sequence, and steps that do not have a logical context relationship can be reversed in order or implemented simultaneously. In addition, those skilled in the art, guided by the contents of the present disclosure, can add one or more other operations to the flowchart, or remove one or more operations from the flowchart.

[0046] In addition, the described embodiments are only a part of the embodiments of the present disclosure, rather than all of the embodiments. The components of the embodiments of the present disclosure generally described and shown in the drawings herein can be arranged and designed in a variety of different configurations. Therefore, the following detailed description of the embodiments of the present disclosure provided in the drawings is not intended to limit the scope of the disclosure claimed for protection, but merely represents selected embodiments of the present disclosure. Based on the embodiments of the present disclosure, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present disclosure.

[0047] In existing technologies, energy storage systems typically utilize grid-connected control. Off-grid operation is rare, and off-grid operation is difficult. Furthermore, stabilizing output voltage is difficult in this state, and off-grid power control is even more difficult. In most cases, the master power conversion system (PCS) controls voltage stability, while the slave power converters operate in grid-connected mode. This makes it difficult to smoothly control the power output of the slave power converters when voltage fluctuations occur.

[0048] Based on this, the embodiments of the present disclosure provide a power distribution method and energy storage system in an off-grid state. After determining the target output active power of each to-be-allocated energy storage converter of the energy storage system in the off-grid state, the droop coefficient corresponding to the target output active power of the to-be-allocated energy storage converter is determined through the droop control principle. Then, the droop coefficient of the to-be-allocated energy storage converter is adjusted so that the to-be-allocated energy storage converter outputs the target output active power. This solves the technical problems in the prior art that the battery stacks corresponding to the to-be-allocated energy storage converters cannot be fully charged or fully discharged at the same time and that voltage fluctuations have a significant impact on output accuracy. This achieves the technical effect of controlling the battery stacks corresponding to multiple to-be-allocated energy storage converters to achieve simultaneous full charging or full discharging and increasing anti-interference capability, as described below.

[0049] Please refer to Figure 1, which is a flow chart of a power distribution method in an off-grid state provided by an embodiment of the present disclosure. As shown in Figure 1, the power distribution method in an off-grid state provided by an embodiment of the present disclosure includes the following steps:

[0050] S101: Determine target output active powers corresponding to a plurality of energy storage converters to be allocated in an off-grid state of an energy storage system.

[0051] The energy storage system includes multiple energy storage converters, each of which includes multiple to-be-allocated energy storage converters and a target energy storage converter. Each energy storage converter corresponds to a battery stack, and the target energy storage converter is the converter with the largest battery capacity among the multiple energy storage converters. In other words, the battery capacity of the battery stack corresponding to the target energy storage converter is the maximum value among the battery capacities of the battery stacks corresponding to all energy storage converters.

[0052] The battery capacity of each battery stack in the energy storage device is different each time it is charged or discharged. Therefore, before each charge or discharge, the target energy storage converter for this charge or discharge can be determined based on the battery capacity of the battery stack corresponding to each energy storage converter. As a result, the target energy storage converter corresponding to each charge or discharge may be different. This avoids the problem of long-term fixed target energy storage converter, which may lead to uncontrollable output active power of the target energy storage converter, resulting in large differences between the battery stacks corresponding to each energy storage converter, and thus reducing the system discharge depth.

[0053] The target energy storage converter operates in an off-grid state, outputting power at rated voltage and frequency. Theoretically, the target energy storage converter's output power is rated power. The sum of the output active power of multiple to-be-allocated energy storage converters is the to-be-allocated active power of the energy storage system in an off-grid state. The sum of the target energy storage converter and the to-be-allocated active power is the total output active power of the energy storage system in an off-grid state.

[0054] The total output active power is determined by the active power required by the electrical equipment. Both the total output active power and the rated output active power of the energy storage converter are known. In other words, the difference between the total output active power of the energy storage system in an off-grid state and the rated output active power of the target energy storage converter is used as the active power to be allocated by the energy storage system in the off-grid state.

[0055] Please refer to Figure 2, which is a flow chart of the steps for determining the target output active power corresponding to multiple energy storage converters to be allocated in an off-grid energy storage system according to an embodiment of the present disclosure. As shown in Figure 2, the steps for determining the target output active power corresponding to multiple energy storage converters to be allocated in an off-grid energy storage system include:

[0056] S1011: Obtain the active power to be allocated of the energy storage system in an off-grid state to construct a first equation in which the active power to be allocated is equal to the sum of the target output active powers corresponding to the plurality of energy storage converters to be allocated.

[0057] Among them, the first equation includes:

[0058] In formula (1), P0 refers to the active power to be distributed, P AC,i It refers to the target output active power corresponding to the i-th energy storage converter to be allocated, and I refers to the total number of multiple energy storage converters to be allocated.

[0059] That is to say, since each energy storage converter to be allocated corresponds to a battery stack, and therefore, the input power of each energy storage converter to be allocated is the power input from the corresponding battery stack to the energy storage converter to be allocated, the energy storage converter to be allocated converts the DC power of the corresponding battery stack into AC power and then outputs it, so that the output power of the energy storage converter to be allocated is composed of active power and reactive power.

[0060] S1012: Construct a second equation that expresses the target output active power corresponding to each energy storage converter to be allocated by referring to the input power of the energy storage converter.

[0061] The reference energy storage converter is one of the multiple energy storage converters to be allocated. The reference energy storage converter can be any one of the multiple energy storage converters to be allocated or a preset one.

[0062] That is, one of the multiple energy storage converters to be allocated is used as the reference energy storage converter, and the target output active power of each energy storage converter to be allocated is expressed by the input power of the reference energy storage converter, thereby reducing unknown quantities for easy calculation.

[0063] The second equation includes: expressing the input power of each energy storage converter to be allocated by referring to the input power of the energy storage converter and expressing the corresponding target output active power by the input power of each energy storage converter to be allocated.

[0064] The second equation corresponding to each energy storage converter to be allocated is determined in the following manner: based on the battery capacity relationship between the energy storage converter to be allocated and the reference energy storage converter, a first conversion relationship is constructed to express the input power of the energy storage converter to be allocated through the input power of the reference energy storage converter; based on the conversion relationship between the input power and the output active power of the energy storage converter to be allocated, a second conversion relationship is obtained to express the target output active power of the energy storage converter to be allocated through the input power; combining the first conversion relationship and the second conversion relationship corresponding to the energy storage converter to be allocated, a second equation is obtained to express the target output active power of the energy storage converter to be allocated through the input power of the reference energy storage converter.

[0065] A first conversion relationship expressing the input power of the energy storage converter to be allocated through the input power of the reference energy storage converter is constructed in the following manner: the battery capacity relationship is described by setting the ratio of the input current between the energy storage converter to be allocated and the reference energy storage converter to be equal to the ratio of the battery capacities, so as to construct the first conversion relationship corresponding to the energy storage converter to be allocated, wherein the input current includes: obtained by the input power of the energy storage converter and the input DC voltage.

[0066] Specifically, the first conversion relationship includes:

[0067] In formula (2), Cap i Refers to the battery capacity of the battery stack corresponding to the i-th energy storage converter to be allocated, Cap j Refers to the battery capacity of the battery stack corresponding to the reference energy storage converter, P DC,i Refers to the input power of the i-th energy storage converter to be allocated, P DC,j Refers to the input power of the reference energy storage converter, U i Refers to the output DC voltage of the battery stack corresponding to the i-th energy storage converter to be allocated, that is, the input DC voltage of the i-th energy storage converter to be allocated, U j Refers to the output DC voltage of the battery stack corresponding to the reference energy storage converter. That is, It refers to the output current of the battery stack corresponding to the i-th energy storage converter to be allocated, or in other words, it refers to the input current of the i-th energy storage converter to be allocated.

[0068] In other words, the output current of the corresponding battery stack is calculated by the ratio of the output power of the battery stack corresponding to the energy storage converter to be allocated to the output DC voltage, and the first conversion relationship is determined by the battery capacity ratio of the battery stack being equal to the input current value ratio. In other words, the first conversion relationship expresses the input power relationship between the energy storage converter to be allocated and the reference energy storage converter by considering the battery capacity of the reference energy storage converter.

[0069] Specifically, the second conversion relationship includes:

[0070] In formula (3), P AC,i Refers to the target output active power of the i-th energy storage converter to be allocated, P DC,i Refers to the input power of the i-th energy storage converter to be allocated, η i Refers to the conversion coefficient between the input power and output active power of the i-th energy storage converter to be allocated.

[0071] The conversion coefficient may also be the working efficiency of the energy storage converter, which represents how much input power the energy storage converter converts into output active power.

[0072] Then, we get the second equation:

[0073] That is to say, for each energy storage converter to be allocated, the input power of the reference energy storage converter is obtained by assuming that the input current ratio between the energy storage converter to be allocated and the reference energy storage converter is equal to the battery capacity ratio to express the input power of the energy storage converter to be allocated, and then the target output active power of the energy storage converter to be allocated is expressed by the conversion coefficient and input power of the energy storage converter to be allocated to obtain the second equation of the energy storage converter to be allocated.

[0074] The target output active power of each energy storage converter to be allocated is calculated in the following manner: based on the first equation and the second equation corresponding to each energy storage converter to be allocated, the input power of the reference energy storage converter is calculated; based on the input power of the reference energy storage converter and the second equation corresponding to each energy storage converter to be allocated, the target output active power corresponding to each energy storage converter to be allocated is calculated.

[0075] That is to say, the second equation corresponding to each energy storage converter to be allocated is substituted into the first equation to calculate the input power of the reference energy storage converter; the input power of the reference energy storage converter is substituted into the second equation corresponding to each energy storage converter to be allocated to calculate the target output active power corresponding to each energy storage converter to be allocated.

[0076] That is, after substituting the second equation corresponding to each energy storage converter to be allocated into the first equation, the first equation only contains one unknown quantity, namely the input power of the reference energy storage converter. Thus, the input power of the reference energy storage converter can be calculated.

[0077] S102: Based on the droop control principle, the droop coefficient of each energy storage converter to be allocated is calculated through the target output active power corresponding to each energy storage converter to be allocated, so that each energy storage converter to be allocated outputs the corresponding target output active power by adjusting the droop coefficient of each energy storage converter to be allocated.

[0078] The droop coefficient of each energy storage converter to be allocated is calculated in the following manner: based on the impedance performance of each energy storage converter to be allocated, the target output active power corresponding to each energy storage converter to be allocated is used to calculate the target output voltage value of each energy storage converter to be allocated; based on the power-voltage droop relationship of each energy storage converter to be allocated, the target output active power and the target output voltage value corresponding to each energy storage converter to be allocated are used to calculate the droop coefficient of each energy storage converter to be allocated.

[0079] The impedance of each energy storage converter to be allocated is constant. Based on the impedance performance and target output active power of each energy storage converter to be allocated, the target output voltage value of each energy storage converter to be allocated can be calculated.

[0080] The plurality of energy storage converters to be distributed are all connected to a common connection point, that is, the output end of each energy storage converter to be distributed is connected to the same point, and the voltage value of the common connection point is stable.

[0081] The target output voltage value of each energy storage converter to be allocated is calculated in the following manner: based on the voltage-power conversion expression corresponding to the impedance performance of the energy storage converter to be allocated, the impedance coefficient corresponding to the energy storage converter to be allocated is calculated through the preset voltage value of the common connection point, the no-load output voltage value and the no-load output active power value of the energy storage converter to be allocated when not connected to the common connection point; based on the impedance coefficient and the voltage-power conversion expression corresponding to the energy storage converter to be allocated, the target output voltage value is calculated through the target output active power corresponding to the energy storage converter to be allocated.

[0082] The voltage-to-power conversion expression is used to characterize the relationship between the output active power and the output voltage of the energy storage converter to be allocated when the impedance coefficient of the energy storage converter to be allocated is constant, wherein the output voltage value is equal to the product of the impedance coefficient and the output active power corresponding to the energy storage converter to be allocated plus the preset voltage value.

[0083] That is to say, the target output voltage value of each energy storage converter to be allocated is calculated in the following manner: the preset voltage value of the common connection point, the no-load output voltage value of the energy storage converter to be allocated when not connected to the common connection point, and the no-load output active power value are substituted into the voltage-power conversion expression to calculate the impedance coefficient of the energy storage converter to be allocated, and the impedance coefficient is used to describe the impedance performance of the energy storage converter to be allocated; and then the target output active power corresponding to the energy storage converter to be allocated is substituted into the voltage-power conversion expression with a known impedance coefficient to calculate the target output voltage value.

[0084] Specifically, the voltage-to-power conversion expression includes: V i =λ i ×P AC,i +V pcc (5)

[0085] In formula (5), λ i Refers to the impedance coefficient of the i-th energy storage converter to be allocated, V i Refers to the target output voltage value of the i-th energy storage converter to be allocated, PAC,i Refers to the target output active power of the i-th energy storage converter to be allocated, V pcc Refers to the preset voltage value of the common connection point.

[0086] That is to say, since the voltage output by the energy storage converter after grid connection is clamped by the host or the grid, the impedance characteristic curve of the energy storage converter cannot be observed. The impedance characteristic curve is the inherent property of each energy storage converter and needs to be obtained through testing when it is not grid connected. Then, the impedance coefficient of the energy storage converter is calculated by the no-load output voltage value and no-load output active power value of the energy storage converter when it is not connected to the public connection point. And no-load output active power P AC,i,k Substituting into the voltage-power conversion expression we get Thus, the impedance coefficient of the i-th energy storage converter to be allocated can be obtained. Furthermore, after the target output active power of the i-th energy storage converter to be allocated is known, the target output voltage value can be calculated.

[0087] The power-voltage droop relationship includes: describing the relationship between the output active power and the output voltage of the energy storage converter to be allocated based on the no-load output voltage value when the energy storage converter to be allocated is not connected to the common connection point, wherein the output voltage value is equal to the product of the droop coefficient and the output active power corresponding to the energy storage converter to be allocated plus the no-load output voltage value of the energy storage converter to be allocated when it is not connected to the common connection point.

[0088] The power-voltage droop relationship of each energy storage converter to be allocated is determined in the following manner: based on the impedance properties of the energy storage system, the AC power calculation formula of each energy storage converter to be allocated is simplified to obtain the power-voltage droop relationship of each energy storage converter to be allocated, wherein the impedance properties of the energy storage system are resistive.

[0089] The AC power calculation formula includes:

[0090] In formula (6), P a Refers to the output active power of the ath grid-connected point in the energy storage system, Q a Refers to the output reactive power of the ath grid-connected point in the energy storage system, V a Refers to the output voltage value of the ath grid-connected point in the energy storage system, V0 refers to the output voltage value of the reference grid-connected point, R a Refers to the resistance between the ath grid connection point and the reference grid connection point, X aIt refers to the reactance between the ath grid-connected point and the reference grid-connected point. δ refers to the voltage phase angle difference between the ath grid-connected point and the reference grid-connected point. The voltage phase angle of the reference grid-connected point is set to 0.

[0091] The energy storage system disclosed in the present invention is applied in a low-voltage microgrid less than 10 kV. The impedance of the energy storage system in the low-voltage state is resistive. a Much smaller than R a , X can be ignored a , and the value of δ is also small, so cosδ≈1 and sinδ≈δ.

[0092] Furthermore, the AC power calculation formula can be simplified to:

[0093] Furthermore, from formula (7), we can see that under low voltage conditions, active power is related to the voltage peak, and reactive power is related to the phase angle or frequency. Generally, energy storage converters have reactive power compensation. This solution does not discuss the reactive power part for the time being, and only considers the case of active power control.

[0094] In this disclosure, the output voltage value of the energy storage converter to be allocated can be understood as the maximum voltage value of the output alternating current of the energy storage converter to be allocated.

[0095] The power voltage droop relationship is expressed by the following formula:

[0096] In formula (8), V i Refers to the output voltage value of the i-th energy storage converter to be allocated, It refers to the no-load output voltage value of the energy storage converter to be distributed when it is not connected to the common connection point, m refers to the droop coefficient, P AC,i Refers to the output active power corresponding to the i-th energy storage converter to be allocated.

[0097] Furthermore, by substituting the target output active power and target output voltage value of each energy storage converter to be allocated into formula (8), the droop coefficient of each energy storage converter to be allocated can be obtained. Thus, by adjusting the droop coefficient corresponding to each energy storage converter to be allocated, each energy storage converter to be allocated can output the target active power, thereby achieving the effect of controlling the active power output.

[0098] For example, please refer to FIG3 , which is a schematic diagram of adjusting the droop coefficient provided in an embodiment of the present disclosure. As shown in FIG3 , since the impedance coefficient of the i-th energy storage converter to be allocated is constant, the intersection of the power-voltage droop equation and the voltage-to-power conversion expression with a slope equal to the impedance coefficient is changed by adjusting the droop coefficient m in the power-voltage droop equation. This results in the i-th energy storage converter to have an output voltage value of V1 and an output active power of P1 when m is not adjusted, while the i-th energy storage converter to have an output voltage value of V2 and an output active power of P2 after m is adjusted.

[0099] Furthermore, the present disclosure controls the output current of each energy storage converter to be allocated by controlling the active power output by each energy storage converter to be allocated, thereby achieving simultaneous full or emptying of the controlled battery stacks. Unlike the grid-connected power allocation strategy, the present disclosure uses the energy storage converter corresponding to the battery stack with the largest battery capacity as the master energy storage converter. The master energy storage converter is used to stabilize the voltage amplitude and frequency of the off-grid island system. Its output power can be determined by the load and the output power of the energy storage converter to be allocated. The energy storage converter to be allocated operates in a droop control mode. Unlike the high-voltage power grid, the reactance in the low-voltage power grid is much smaller than the resistance, the system is resistive, and the phase difference of each voltage is very small. Therefore, the present disclosure is based on PV droop control (power voltage droop control) and combines the impedance characteristic curve of the energy storage converter to be allocated to find its intersection. After the AC and DC power of the energy storage converter to be allocated can be calculated based on the capacity ratio of the battery stack being equal to the current ratio of each DC bus, the new droop coefficient of each energy storage converter to be allocated can be calculated. Therefore, the target active power output of the energy storage converter to be allocated is achieved by adjusting the droop coefficient of the energy storage converter to be allocated, thereby achieving the inter-stack balancing function of the battery stacks corresponding to all the energy storage converters to be allocated.

[0100] Based on the same application concept, the embodiments of the present disclosure also provide an energy storage system corresponding to the power distribution method in an off-grid state provided in the above embodiments. Since the principle of solving the problem by the energy storage system in the embodiments of the present disclosure is similar to the power distribution method in an off-grid state provided in the above embodiments of the present disclosure, the implementation of the energy storage system can refer to the implementation of the method, and the repeated parts will not be repeated.

[0101] The energy storage system provided by the embodiments of the present disclosure includes: multiple battery stacks; multiple energy storage converters to be allocated, each energy storage converter to be allocated corresponds to a battery stack, and is used to convert the direct current of the corresponding battery stack into alternating current for output; an energy management system, which is used to execute the power distribution method in an off-grid state as described in any of the above embodiments.

[0102] Based on the same application concept, an embodiment of the present disclosure further provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the steps of the power distribution method in an off-grid state provided in the above embodiment are executed.

[0103] Specifically, the storage medium can be a general storage medium, such as a mobile disk, a hard disk, etc. When the computer program on the storage medium is run, it can execute the above-mentioned power distribution method in the off-grid state. After clarifying the target output active power of each to-be-allocated energy storage converter of the energy storage system in the off-grid state, the droop coefficient corresponding to the target output active power of the to-be-allocated energy storage converter is determined through the droop control principle. Then, the droop coefficient of the to-be-allocated energy storage converter is adjusted so that the to-be-allocated energy storage converter outputs the target output active power. This solves the technical problems in the prior art that the battery stacks corresponding to the to-be-allocated energy storage converters cannot be fully charged or fully discharged at the same time and that the voltage fluctuation has a greater impact on the output accuracy. This achieves the technical effect of controlling the battery stacks corresponding to multiple to-be-allocated energy storage converters to achieve simultaneous full charging or full discharge and increasing the anti-interference capability.

[0104] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working process of the system and device described above can refer to the corresponding process in the aforementioned method embodiment, and will not be repeated here. In the several embodiments provided in the present disclosure, it should be understood that the disclosed system, device and method can be implemented in other ways. The device embodiments described above are merely schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some communication interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

[0105] The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of these units may be selected to achieve the purpose of this embodiment according to actual needs.

[0106] In addition, each functional unit in each embodiment of the present disclosure may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit.

[0107] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a non-volatile computer-readable storage medium that is executable by a processor. Based on this understanding, the technical solution of the present disclosure, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product, and the computer software product is stored in a storage medium, including a number of instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the method described in each embodiment of the present disclosure. The aforementioned storage medium includes: various media that can store program codes, such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disk.

[0108] The above are only specific embodiments of the present disclosure, but the scope of protection of the present disclosure is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this disclosure should be included in the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure should be based on the scope of protection of the claims. Industrial Applicability:

[0109] The present disclosure provides a power distribution method and energy storage system in an off-grid state. After determining the target output active power of each to-be-allocated energy storage converter of the energy storage system in the off-grid state, the droop coefficient corresponding to the target output active power of the to-be-allocated energy storage converter is determined through the droop control principle. Then, the droop coefficient of the to-be-allocated energy storage converter is adjusted so that the to-be-allocated energy storage converter outputs the target output active power. This solves the technical problems in the prior art that the battery stacks corresponding to the to-be-allocated energy storage converters cannot be fully charged or fully discharged at the same time and that voltage fluctuations have a significant impact on output accuracy. This achieves the technical effect of controlling the battery stacks corresponding to multiple to-be-allocated energy storage converters to achieve simultaneous full charging or simultaneous full discharge and increasing anti-interference capability.

Claims

1. A power distribution method in an off-grid state, characterized in that: The method comprises: Determine the target output active power corresponding to each of the multiple energy storage converters to be allocated in the energy storage system in an off-grid state; Based on the droop control principle, the droop coefficient of each energy storage converter to be allocated is calculated by the target output active power corresponding to each energy storage converter to be allocated, so that each energy storage converter to be allocated outputs the corresponding target output active power by adjusting the droop coefficient of each energy storage converter to be allocated.

2. The method according to claim 1, characterized in that The determining of the target output active powers corresponding to the plurality of energy storage converters to be allocated in the energy storage system in an off-grid state includes: Obtaining the active power to be allocated of the energy storage system in an off-grid state to construct a first equation in which the active power to be allocated is equal to the sum of the target output active powers corresponding to the plurality of energy storage converters to be allocated; Constructing a second equation for expressing the target output active power corresponding to each to-be-allocated energy storage converter by using the input power of the reference energy storage converter, wherein the reference energy storage converter is one of the plurality of to-be-allocated energy storage converters; The target output active power of each energy storage converter to be allocated is calculated by combining the first equation with the second equation corresponding to each energy storage converter to be allocated.

3. The method according to claim 2, characterized in that The second equation comprises: expressing the input power of each to-be-allocated energy storage converter by referring to the input power of the energy storage converter and expressing the corresponding target output active power by the input power of each to-be-allocated energy storage converter.

4. The method according to claim 3, characterized in that The second equation corresponding to each energy storage converter to be allocated is determined in the following manner: Based on the battery capacity relationship between the energy storage converter to be allocated and the reference energy storage converter, constructing a first conversion relationship formula that expresses the input power of the energy storage converter to be allocated by using the input power of the reference energy storage converter; Based on the conversion relationship between the input power and the output active power of the energy storage converter to be allocated, a second conversion relationship is obtained in which the target output active power of the energy storage converter to be allocated is expressed by the input power; By combining the first conversion relationship and the second conversion relationship corresponding to the energy storage converter to be allocated, a second equation is obtained in which the target output active power of the energy storage converter to be allocated is expressed by the input power of the reference energy storage converter.

5. The method according to claim 4, characterized in that A first conversion relationship expressing the input power of the energy storage converter to be allocated by using the input power of the reference energy storage converter is constructed in the following manner: The battery capacity relationship is described by assuming that the ratio of the input current between the energy storage converter to be allocated and the reference energy storage converter is equal to the ratio of the battery capacities, so as to construct the first conversion relationship corresponding to the energy storage converter to be allocated. The input current includes: obtained through the input power and input DC voltage of the energy storage converter.

6. The method according to claim 4, characterized in that The target output active power of each energy storage converter to be allocated is calculated as follows: Calculating the input power of the reference energy storage converter based on the first equation and a second equation corresponding to each energy storage converter to be allocated; The target output active power corresponding to each energy storage converter to be allocated is calculated based on the input power of the reference energy storage converter and the second equation corresponding to each energy storage converter to be allocated.

7. The method according to claim 2, characterized in that The energy storage system further includes a target energy storage converter other than the plurality of energy storage converters to be allocated. The target energy storage converter refers to the energy storage converter corresponding to the largest battery capacity among the multiple energy storage converters in the energy storage system. The obtaining of the active power to be allocated of the energy storage system in an off-grid state includes: determining the active power to be allocated based on the total output active power of the energy storage system in an off-grid state and the rated output active power of the target energy storage converter.

8. The method according to claim 1, characterized in that The droop coefficient of each energy storage converter to be allocated is calculated in the following way: Calculating a target output voltage value of each energy storage converter to be allocated based on the impedance performance of each energy storage converter to be allocated and the target output active power corresponding to each energy storage converter to be allocated; Based on the power-voltage droop relationship of each energy storage converter to be allocated, the droop coefficient of each energy storage converter to be allocated is calculated using the target output active power and the target output voltage value corresponding to each energy storage converter to be allocated.

9. The method according to claim 8, characterized in that The multiple energy storage converters to be allocated are all connected to a common connection point, and the target output voltage value of each energy storage converter to be allocated is calculated by the following method: Based on the voltage-power conversion expression corresponding to the impedance performance of the energy storage converter to be allocated, through the common connection The impedance coefficient corresponding to the energy storage converter to be allocated is calculated based on the preset voltage value of the contact, the no-load output voltage value and the no-load output active power value of the energy storage converter to be allocated when not connected to the common connection point; Based on the impedance coefficient corresponding to the energy storage converter to be allocated and the voltage-to-power conversion expression, the target output voltage value is calculated using the target output active power corresponding to the energy storage converter to be allocated.

10. The method according to claim 9, characterized in that The voltage-power conversion expression is used to characterize the relationship between the output active power and the output voltage of the energy storage converter to be distributed when the impedance coefficient of the energy storage converter to be distributed is constant. The output voltage value is equal to the product of the impedance coefficient and the output active power corresponding to the energy storage converter to be allocated plus the preset voltage value.

11. The method according to claim 8, characterized in that The plurality of energy storage converters to be allocated are all connected to a common connection point, and the power voltage droop relationship includes: describing the relationship between the output active power and the output voltage of the energy storage converter to be allocated based on the no-load output voltage value when the energy storage converter to be allocated is not connected to the common connection point, The output voltage value is equal to the product of the droop coefficient and the output active power corresponding to the energy storage converter to be allocated plus the no-load output voltage value of the energy storage converter to be allocated when it is not connected to the common connection point.

12. The method according to claim 11, characterized in that The power-voltage droop relationship of each energy storage converter to be allocated is determined by: Based on the impedance property of the energy storage system, the AC power calculation formula of each energy storage converter to be allocated is simplified to obtain the power-voltage droop relationship of each energy storage converter to be allocated, wherein the impedance property of the energy storage system is resistive.

13. An energy storage system, characterized in that: The energy storage system comprises: multiple battery stacks; Multiple energy storage converters to be allocated, each corresponding to a battery stack, for converting the direct current of the corresponding battery stack into alternating current for output; An energy management system, configured to execute the off-grid power distribution method according to any one of claims 1 to 12.

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