Impedance analysis method based on digital energy storage system

Abstract

The invention discloses an impedance analysis method based on a digital energy storage system, and belongs to the technical field of energy storage of a new energy power system, in the m-parallel and n-series digital energy storage system, a battery monomer, a digital energy network card and a cable are included; calculating the internal resistance of the single battery module and the internal resistance of the digital energy network card; calculating cable resistance and cable parasitic inductance; and calculating the equivalent resistance R of the system according to the internal resistance of the single battery module and the internal resistance of the digital energy network card. Based on the calculation of the equivalent resistance and the equivalent inductance of the digital energy storage system, the method can be applied to the working condition evaluation of a large-current short-circuit fault, provides a basis for the calculation of the maximum short-circuit current upper limit and the current rise rate during short circuit in the system, and reasonably designs a protection circuit and corresponding time according to the short-circuit current upper limit and the current rise rate. The protection action can be carried out in time when the fault occurs, the safety of related equipment in the digital energy storage system is effectively protected, and the safe and stable operation of the digital energy storage system is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of energy storage of new energy power systems, and in particular to a method based on impedance analysis of digital energy storage systems. Background Art

[0002] The main performance indicators of battery energy storage systems, such as efficiency, reliability and safety, are crucial to the safe operation and successful commercial deployment of energy storage systems. Digital energy storage systems discretize and digitize the battery analog energy flow, transforming the physical hard connection between battery cells into a flexible connection controlled by a program, thereby transforming analog battery energy storage systems into digital battery energy storage systems.

[0003] like Figure 1 As shown in the figure, the common topology of digital energy storage system is 3 parallel and 14 series mode. Specifically, 3 parallel means that 3 battery modules are connected in parallel as a unit through the smart network card. According to the system power, by controlling the action of the smart network card, different numbers of battery modules are connected to meet the power requirements of the circuit; 14 series means that 3 parallel battery modules are used as a unit, and 14 units are connected in series in the circuit to meet the voltage parameters of the entire cluster DC system.

[0004] In addition to the conventional 3-in-14-string application scenario, based on current control technology, 6-in-20-string or m-in-n-string application scenarios can also be realized to meet the needs of larger current and higher voltage for single-cluster DC systems.

[0005] Among them, the topology of 6 parallel and 20 series is as follows Figure 2 As shown: During the operation of the system, if there is a system operation failure, a short-term increase in current, or even a short circuit caused by direct connection between the positive and negative poles, it will cause system damage, property loss, or even personal danger.

[0006] The impedance parameters in the system will affect the short-term rise rate of the current. By evaluating the system impedance parameters and estimating the current rise time under different working conditions, through reasonable hardware design, related equipment can be disconnected in time when a fault occurs to protect the system safety.

[0007] Based on this, the present invention provides a method based on impedance analysis of a digital energy storage system. Summary of the invention

[0008] In view of the deficiencies in the prior art, the present invention provides a method for impedance analysis based on a digital energy storage system.

[0009] The technical solution of the present invention to solve the above technical problems is as follows: A method based on impedance analysis of a digital energy storage system, in which an m-parallel and n-series digital energy storage system comprises a battery cell, a digital energy network card and a cable; Calculate the internal resistance of a single battery module. The internal resistance of a single battery module is , where the battery internal resistance is expressed as , a single battery module consists of k battery cells connected in series, is the single-pole contact resistance of the battery cell; Calculate the internal resistance of a digital energy network card , the internal resistance of the digital energy network card is ; Calculate the cable resistance and cable parasitic inductance of the cable; The equivalent resistance R of the system is calculated based on the internal resistance of the single battery module and the internal resistance of the digital energy network card, as shown in the following formula: ; Where R represents the equivalent resistance of the system; It represents the DC resistance of the cable; W represents the total length of the main cable, that is, the total length of the cable from the positive and negative poles of the battery cluster to the positive and negative poles of the PCS, in meters; n represents the number of batteries connected in series.

[0010] Furthermore, in the m-parallel and n-series digital energy storage system, the equivalent inductance per meter of the cable is set to In the main circuit, the total length of the cable from the positive and negative electrodes of the battery cluster to the positive and negative electrodes of the PCS is W, so the equivalent inductance is ; The electrical connection points in the digital energy storage system include the electrical connection points of the battery cells, the network card and the electrical connection points of the entire cluster. The parasitic inductance of each electrical connection point The calculation is as follows: ; in, It represents the parasitic inductance of the electrical joint; represents the magnetic permeability of vacuum; D represents the distance between conductors; r represents the radius of the conductor; According to the parasitic inductance of the junction, the system equivalent inductance L is obtained. The calculation formula of the system equivalent inductance L is as follows: ; in, Represents the parasitic inductance of the battery cell.

[0011] Furthermore, in the digital energy storage system, each electrical connection point is a copper busbar of the same specification overlapped in parallel.

[0012] Furthermore, when calculating the internal resistance of a battery cell, the calculation is based on the initial internal resistance value of the battery. When calculating the current, the single-pole contact resistance of the battery cell electrical connection position needs to be considered. , the single-pole contact resistance of the battery cell is set to , and each battery cell has two contact points, the equivalent resistance of the battery cell is .

[0013] Furthermore, the minimum internal resistance of the battery cell is 0.15mΩ~0.25mΩ.

[0014] Furthermore, the positive and negative electrodes of the battery cluster are connected to the positive and negative electrodes of the PCS through cables, and the DC resistance of the cables The calculation of is as follows: ; in, is the DC resistance of the cable, in Ω; is the resistivity of the cable material, in Ω•m; A represents the cross-sectional area, in square meters.

[0015] Furthermore, the cable is a multi-conductor cable.

[0016] Furthermore, the parasitic inductance of multi-conductor cables The calculation of is as follows: ; in, represents the center distance between the ith and jth conductors.

[0017] Furthermore, the digital energy network card is used to control the connection and disconnection of battery modules in the system.

[0018] In summary, compared with the prior art, the above technical solution has the following beneficial effects: The present invention can be applied to the working condition evaluation of large current short-circuit faults by analyzing the impedance of the digital energy storage system and calculating the equivalent resistance and equivalent inductance of the digital energy storage system based on m-parallel and n-series. It provides a basis for calculating the maximum short-circuit current upper limit in the system and the current rise rate during short circuit. According to the short-circuit current upper limit and the current rise rate, the protection circuit and the corresponding time are reasonably designed. When such a fault occurs, the protection action can be performed in time, and the safety of related equipment in the digital energy storage system can be effectively protected, which is conducive to the safe and stable operation of the digital energy storage system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the topology diagram of the digital energy storage system with 3 parallel and 14 series; Figure 2 This is the topology diagram of the digital energy storage system with 6 parallel and 20 series; Figure 3 Schematic diagram of the impedance analysis method based on the digital energy storage system. DETAILED DESCRIPTION

[0020] The principles and features of the present invention are described below in conjunction with all the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0021] The embodiment of the present invention discloses a method based on impedance analysis of a digital energy storage system.

[0022] In the embodiment of the present invention, the impedance evaluation of the digital energy storage system mainly considers the equipment that affects the current rise time when the system fails. In the embodiment of the present invention, the impact caused by the short-circuit fault on the DC side is mainly considered. In the present invention, in the short-circuit condition of the digital energy storage system, for impedance evaluation, the key components include batteries, digital energy cards and cables.

[0023] Reference Figures 1 to 3 ,A method based on impedance analysis of digital energy storage system.,In the m-parallel and n-series digital energy storage system, key components include battery monomers, digital energy network card and main cable; Calculate the internal resistance of a single battery module. The internal resistance of a single battery module is , where the battery internal resistance is expressed as , a single battery module consists of k battery cells connected in series, It is the resistance of one electrical point of a single cell, that is, the single-pole contact resistance of a battery cell; a battery cell has two positive and negative contact points, the internal resistance of one contact point is 0.025mΩ, and the internal resistance of two contact points is 0.05mΩ, that is, under ideal conditions, the single-pole contact internal resistance of a battery cell is The ideal value is 0.025mΩ; Calculate the internal resistance of the digital energy network card. The internal resistance of the digital energy network card is When the digital energy network card is working, there will be two electrical points (contact points) connected, each electrical point has an internal resistance of 0.025mΩ, and when two electrical points are connected, the internal resistance is 0.05mΩ; Calculate the cable resistance and cable parasitic inductance of the cable; The equivalent resistance R of the system is calculated based on the internal resistance of the single battery module and the internal resistance of the digital energy network card, as shown in the following formula: ; Substitution The specific value of can be expressed as: ; Where R represents the equivalent resistance of the system; It represents the DC resistance of the cable; W represents the total length of the cable, that is, the total length of the cable from the positive and negative electrodes of the battery cluster to the positive and negative electrodes of the PCS (bidirectional energy storage converter, or bidirectional converter), in meters.

[0024] Among them, the battery cluster is usually composed of battery cells connected in series, in parallel, or in series and parallel.

[0025] Specifically, the internal resistance of the battery cell is evaluated as follows: Battery impedance evaluation is mainly based on the initial internal resistance of the battery before leaving the factory. The present invention collects statistics on the factory internal resistance of 314Ah battery cells from several mainstream battery suppliers, as shown in Table 1.

[0026] Table 1 Initial internal resistance of batteries before leaving the factory

[0027] From Table 1, we can see that the minimum internal resistance of a single-cell battery from mainstream manufacturers is 0.15mΩ~0.25mΩ. If the internal resistance of the battery is designed to , when calculating the current, it is necessary to consider the contact resistance of the battery cell electrical connection position, the single-pole contact resistance (one contact point) The ideal value is about 0.025mΩ, so the equivalent resistance of the battery cell is about mΩ.

[0028] k battery cells are connected in series to form a battery module. The internal resistance of a single battery module is .

[0029] Specifically, the internal resistance of the digital energy network card is evaluated as follows: The digital energy network card is used to control the connection and disconnection of the battery module in the system. The parameters of the digital energy network card in the embodiment of the present invention at different currents are shown in Table 2.

[0030] Table 2 Internal resistance of digital energy network card

[0031] As shown in Table 2, when power is flowing in the digital energy network card, the short-circuit current is far more than 400A, and the maximum internal resistance is , considering the contact resistance of the electrical connection position, when evaluating the internal resistance, follow ,Right now mΩ is used as a reference.

[0032] Cable impedance evaluation includes cable resistance and cable parasitic inductance. Specifically, the calculation of cable resistance is as follows: Considering the problems of bending and insulation, the battery cluster is currently connected to the PCS mainly through cables. The DC resistance of the cable is It can be calculated by formula (1): (1); in, is the DC resistance of the cable, in Ω; is the resistivity of the cable material, in Ω•m; It represents the total length of the cable in meters; A represents the cross-sectional area in square meters.

[0033] Among them, different materials have different resistivities, for example: Copper: about 1.68×10 -8 Ω•m; Aluminum: about 2.82×10 -8 Ω•m.

[0034] According to the on-site conditions, the currently commonly used cable is 185mm 2 and 240mm 2 Copper conductor, according to formula (1), 1m length is 185mm 2 and 240mm 2 The DC resistance is about 9mΩ and 7mΩ respectively.

[0035] The calculation of cable parasitic inductance is as follows: Commonly used cables 185mm 2 and 240mm 2 Usually it is a multi-conductor cable, and its parasitic inductance can be calculated by formula (2): (2); in, represents the center distance between the i-th and j-th conductors in meters; represents the vacuum magnetic permeability, which is approximately 4π×10 -7 H / m; r represents the conductor radius.

[0036] For multi-conductor cables, the equivalent parasitic inductance value of a conventional 185 square cable is about 1uH per 1m, and the parasitic inductance value of a 240 square cable is about 0.9uH per 1m.

[0037] The system equivalent resistance is evaluated as follows: In the digital energy storage system, the resistance value in the series direction determines the maximum current in the system when a short circuit occurs. At the same time, as a new type of digital energy storage system, there is a complex series-parallel relationship on the DC side.

[0038] For example, in a 3-in-parallel 14-in-series system, there are 42 battery modules in total, each of which contains 16 battery cells connected in series.

[0039] In the three-parallel connection, the equivalent resistance of each parallel connection is the sum of the internal resistance of 16 battery cells and one smart network card, that is, , considering the maximum power of the system operation, all three battery modules are charged and discharged, then the equivalent resistance of the three battery modules and the smart network card in parallel is .

[0040] Considering that the series copper busbar selects the shortest route in the series connection of 14 3-parallel battery cells, and ignoring the influence of its equivalent resistance, the equivalent resistance of the 14 3-parallel cells in series is: .

[0041] Combined with the 3-in-14-in-parallel solution, in an m-in-parallel and n-in-string system, a total of k battery cells are connected in parallel inside the battery module, and the cable is selected to be 185 square meters long in parallel. The calculation of the equivalent resistance value R is shown in formula (3): (3); Where R represents the equivalent resistance of the system, represents the DC resistance of the cable. In formula (3), R1=9, and W represents the total length of the cable, that is, the length of the cable from the positive and negative electrodes of the battery cluster to the positive and negative electrodes of the PCS.

[0042] The calculation of the system equivalent inductance is as follows: According to the above content, the equivalent parallel parasitic inductance value of 185 square cable 1m is 1uH, and the equivalent inductance of 30m long cable is 30uH.

[0043] In the m-parallel and n-series digital energy storage system, the equivalent inductance per meter of the cable is set to (This parameter can be based on the manufacturer's factory parameters). In the main circuit, the total length of the cable from the positive and negative electrodes of the battery to the positive and negative electrodes of the PCS is W, then the equivalent inductance is Considering the parasitic inductance of the electrical connection position, there are many electrical connection points in the digital energy storage system, including the electrical connection points of the battery cell, the network card and the electrical connection points of the entire cluster. The calculation is as follows: (4); in, It represents the parasitic inductance of the electrical connection, in H; represents the vacuum magnetic permeability, which is approximately 4π×10 -7 H / m; D represents the distance between conductors in meters; r represents the radius of the conductor in meters.

[0044] Considering two copper bars of the same specification overlapped, the parasitic inductance of the overlapped surface is calculated by formula (4) to be about 0.3uH. Among them, there is a copper bar for each electrical node. We describe two pieces because we consider that the battery has positive and negative poles, and a battery cell has two electrical connection points, so two copper bars of the same specification are overlapped.

[0045] Each battery cell has two electrical connection points. The parasitic inductance of a single point is about 0.3uH, so the parasitic inductance of the battery cell is about 0.6uH. Based on the parasitic inductance of the connection point, the system equivalent inductance L is obtained. The calculation of the system equivalent inductance L is shown in formula (5): (5); in, represents the parasitic inductance of the battery cell, which is taken as 0.6uH in formula (5); Furthermore, in the m-parallel and n-series digital energy storage system, the equivalent inductance per meter of the cable is set to In the main circuit, the total length of the cable from the positive and negative electrodes of the battery cluster to the positive and negative electrodes of the PCS is W, so the equivalent inductance is ; Furthermore, when calculating the internal resistance of a battery cell, the calculation is based on the initial internal resistance value of the battery. When calculating the current, the contact resistance of the electrical connection position of the battery cell needs to be considered, and the single-pole contact resistance of the battery cell is set. The ideal value is 0.025mΩ, and the equivalent resistance of the battery cell is ,Right now mΩ.

[0046] Furthermore, the minimum internal resistance of the battery cell is 0.15mΩ~0.25mΩ.

[0047] The embodiment of the present invention analyzes the impedance of the digital energy storage system, calculates the equivalent resistance and equivalent inductance of the digital energy storage system based on the expansion of the conventional 3-parallel 14-string and 6-parallel 20-string to m-parallel n-string, and is applied to the working condition evaluation of large current short-circuit faults, providing a basis for the calculation of the maximum short-circuit current upper limit in the system and the current rise rate during short circuit. According to the short-circuit current upper limit and the current rise rate, the protection circuit and the corresponding time are reasonably designed, and the protection action can be carried out in time when such a fault occurs, effectively protecting the safety of related equipment in the digital energy storage system, and facilitating the safe and stable operation of the digital energy storage system.

[0048] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method based on impedance analysis of a digital energy storage system, characterized in that: In the m-parallel and n-series digital energy storage system, it includes battery cells, digital energy network cards and cables; Calculate the internal resistance of a single battery module. The internal resistance of a single battery module is , where the battery internal resistance is expressed as , a single battery module consists of k battery cells connected in series, is the single-pole contact resistance of the battery cell; Calculate the internal resistance of a digital energy network card , the internal resistance of the digital energy network card is ; Calculate the cable resistance and cable parasitic inductance of the cable; The equivalent resistance R of the system is calculated based on the internal resistance of the single battery module and the internal resistance of the digital energy network card, as shown in the following formula: ; Where R represents the equivalent resistance of the system; It represents the DC resistance of the cable; W represents the total length of the main cable, that is, the total length of the cable from the positive and negative poles of the battery cluster to the positive and negative poles of the PCS, in meters; n represents the number of batteries connected in series.

2. The method for impedance analysis based on a digital energy storage system according to claim 1, characterized in that: In the m-parallel and n-series digital energy storage system, the equivalent inductance per meter of the cable is set to In the main circuit, the total length of the cable from the positive and negative electrodes of the battery cluster to the positive and negative electrodes of the PCS is W, so the equivalent inductance is ; The electrical connection points in the digital energy storage system include the electrical connection points of the battery cells, the network card and the electrical connection points of the entire cluster. The parasitic inductance of each electrical connection point The calculation is as follows: ; in, It represents the parasitic inductance of the electrical joint; represents the magnetic permeability of vacuum; D represents the distance between conductors; r represents the radius of the conductor; According to the parasitic inductance of the junction, the system equivalent inductance L is obtained. The calculation formula of the system equivalent inductance L is as follows: ； in, Represents the parasitic inductance of the battery cell.

3. The method for impedance analysis based on a digital energy storage system according to claim 2, characterized in that: In the digital energy storage system, each electrical connection point is a copper busbar of the same specification connected in parallel.

4. The method for impedance analysis based on a digital energy storage system according to claim 1, characterized in that: When calculating the internal resistance of a battery cell, the calculation is based on the initial internal resistance value of the battery. When calculating the current, the single-pole contact resistance of the battery cell electrical connection position needs to be considered. , the single-pole contact resistance of the battery cell is set to , and each battery cell has two contact points, the equivalent resistance of the battery cell is .

5. The method for impedance analysis based on a digital energy storage system according to claim 4, characterized in that: The minimum internal resistance of a battery cell is 0.15mΩ~0.25mΩ.

6. The method for impedance analysis based on a digital energy storage system according to claim 2, characterized in that: The positive and negative electrodes of the battery cluster are connected to the positive and negative electrodes of the PCS through cables. The DC resistance of the cables The calculation of is as follows: ； in, is the DC resistance of the cable, in Ω; is the resistivity of the cable material, in Ω•m; A represents the cross-sectional area, in square meters.

7. The method for impedance analysis based on a digital energy storage system according to claim 6, characterized in that: The cable is a multi-conductor cable.

8. The method for impedance analysis based on a digital energy storage system according to claim 7, characterized in that: Parasitic Inductance of Multi-Conductor Cables The calculation of is as follows: ; in, represents the center distance between the ith and jth conductors.

9. The method for impedance analysis based on a digital energy storage system according to claim 1, characterized in that: The digital energy network card is used to control the connection and disconnection of battery modules in the system.

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