A method for monitoring the power limit of an MMC-BESS sub-module battery

Abstract

The application discloses a kind of MMC-BESS submodule battery power limit monitoring methods, belong to the technical field of flexible direct current transmission and energy storage, comprising: obtaining the inherent parameter of MMC-BESS;Constructing MMC-BESS mathematical model, deduce the constraint condition that submodule battery power satisfies;According to the constraint that sub-module modulation wave cannot be over-modulated, deduce the power limit of single submodule battery;According to the constraint between the battery power of sub-module in bridge arm, deduce the battery power limit of multiple sub-module combination.The application provides guidance for the operation of MMC-BESS under the condition that there is battery active power difference between sub-modules, judges whether the system can normally operate when the power distribution of submodule battery is uneven according to the power limit obtained by monitoring, which helps to improve the design and implementation of MMC-BESS, and ensures its stable and safe operation.

Description

Technical Field

[0001] This invention relates to the fields of flexible DC power transmission technology and energy storage technology, and in particular to a method for monitoring the battery power limit of an MMC-BESS submodule. Background Technology

[0002] With the increasing availability of renewable energy, energy storage systems are becoming increasingly important for future power system infrastructure. The large-scale grid integration of renewable energy is gradually becoming a trend, posing new challenges to modern power systems. To mitigate the impact of the randomness and intermittency of renewable energy, battery energy storage systems (BESS) can be used in practical applications. Power electronic converters are essential for integrating BESS into the grid. Compared to traditional two-level converters, multilevel converters have significant advantages, such as modularity, excellent harmonic performance, and low switching frequency, especially in medium / high voltage applications.

[0003] Modular Multilevel Converter with Integrated Battery Energy Storage System (MMC-BESS) is one of the most promising solutions for future power systems. In the design and operation of MMC-BESS, imbalances in active power distribution among submodules can occur under various conditions, such as batteries being integrated only into some submodules of each bridge arm; unbalanced battery state of charge; and battery failures in some submodules preventing them from outputting or absorbing active power. This imbalance in active power distribution among submodules can lead to problems such as unbalanced capacitor voltage control, increased harmonic content in the output current, and three-phase asymmetry.

[0004] When MMC-BESS needs to operate under conditions where there are differences in active power between submodules, it is necessary to know the minimum permissible level of uneven power distribution, i.e., the charging and discharging power limits of the submodule batteries. Furthermore, analyzing the charging and discharging power limits between submodules helps improve the design and implementation of MMC-BESS, ensuring its stable and safe operation.

[0005] Current research on power imbalance in MMC-BESS indicates that phase-to-phase and arm-to-arm power imbalances can be addressed by controlling the arm circulating current to maintain stable system operation. However, research on power imbalance within submodules of arm bridges is mostly focused on specific application scenarios, such as submodule failures, and lacks general applicability. Studies on general MMC-BESS power imbalance within submodule arms often suffer from large errors and computational burdens. Given the various operational scenarios of MMC-BESS under submodule battery power imbalance, a general analysis of its power limits is necessary to analyze its feasibility. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for monitoring the battery power limit of an MMC-BESS submodule, which is expected to be able to conveniently and accurately monitor the maximum charge and discharge power of the submodule battery, determine whether the system can operate normally under uneven power distribution, help improve the design and implementation of MMC-BESS, and ensure that the system operates within the battery power limit.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] A method for monitoring the battery power limit of an MMC-BESS submodule includes the following steps:

[0009] S1. Obtain the inherent parameters of MMC-BESS, including: number of bridge arm sub-modules N, DC bus voltage U. dc Modulation ratio m, AC side output current i g AC side active power P ac and the output voltage angular frequency ω;

[0010] S2. Construct the MMC-BESS mathematical model and calculate the submodule port voltage u based on the inherent parameters. kji and current i kj Based on the power flow direction within the submodule, the constraints that the battery power of the submodule must satisfy are derived.

[0011] S3. Based on the constraint that the modulated wave of the submodule cannot be overmodulated, the power limit of a single submodule battery is derived through the constraint conditions that the battery power of the submodule must satisfy.

[0012] S4. Based on the constraints of battery power among sub-modules within the bridge arm, the battery power limit of a combination of multiple sub-modules is derived from the battery power limit of a single sub-module.

[0013] A further improvement to the technical solution of this invention lies in: In S2, due to the three-phase symmetry, one phase is selected, and Kirchhoff's voltage and current laws are applied to determine the submodule port voltage u. kji and current i j The formula is:

[0014]

[0015]

[0016] Where k = {a, b, c} represents phase k, j = {u, l} represents the upper and lower bridge arms respectively, the subscript i represents the i-th submodule within the bridge arm, and k i cosωt is the AC component introduced for the capacitor voltage balance control within the bridge arm submodules, I dc I represents the DC component of the circulating current. g It is the amplitude of the alternating current, and θ is the angle of the alternating power factor.

[0017] Upper arm single submodule battery power P bat_ui and the battery power P of a single submodule of the lower bridge arm bat_li Must meet:

[0018]

[0019] Among them, P bat_u and P bat_l These are the total battery charging power of the upper and lower bridge arm sub-modules, respectively.

[0020] A further improvement to the technical solution of this invention lies in: In S3, due to the characteristics of carrier phase-shift control, the charging and discharging power of the submodule battery is constrained by the fact that the modulation wave cannot be over-adjusted, and the adjustable coefficient k i satisfy:

[0021]

[0022] Maximum and minimum charging power P of a single submodule battery in the upper bridge arm batumax and P batumin for:

[0023]

[0024] in,

[0025]

[0026] Similarly, the maximum charge / discharge power P of a single submodule in the lower bridge arm batlSMmax and P batlSMmin for:

[0027]

[0028]

[0029] A further improvement to the technical solution of this invention lies in: in S4, the maximum charging power P for the combination of n sub-modules...n jmax and minimum charging / maximum discharging power P n jmin Considering the constraints imposed by the charging power of the other Nn sub-modules, when all n sub-modules are at their maximum charging power, the charging power of the other Nn sub-modules cannot be less than their minimum charging power. Therefore, the battery power limit for the combination of n sub-modules can be expressed as follows:

[0030]

[0031]

[0032] The battery charge and discharge power limits for a combination of multiple submodules are derived from the battery power limits of a single submodule.

[0033] The technological advancements achieved by this invention due to the adoption of the above technical solutions are as follows:

[0034] This invention provides a method for monitoring the battery power limit of an MMC-BESS submodule. From the perspective of the inherent constraints of the modulation characteristics of the MMC itself, it can monitor the maximum charging and discharging power of the submodule battery by combining circuit parameters. This provides guidance for the operation of MMC-BESS when there are differences in the active power of batteries among submodules. Based on the monitored power limit, it can determine whether the system can operate normally when the power distribution of the submodule batteries is uneven. This helps to improve the design and implementation of MMC-BESS and ensure its stable and safe operation. Attached Figure Description

[0035] Figure 1 This is a flowchart illustrating a method for monitoring the battery power limit of an MMC-BESS submodule provided by the present invention.

[0036] Figure 2a This is the electrical schematic diagram of the overall topology of the three-phase MMC-BESS;

[0037] Figure 2b This is the electrical schematic diagram of the sub-module topology of the integrated battery in MMC-BESS.

[0038] Figure 3 This is a schematic diagram of the power flow within a submodule of MMC-BESS;

[0039] Figure 4 This is a schematic diagram illustrating the change of battery charging power over time in an example of the present invention;

[0040] Figure 5a This is a schematic diagram illustrating the change of three-phase alternating current amplitude over time in an example of the present invention;

[0041] Figure 5bThis is a schematic diagram illustrating the variation of the three-phase alternating current from 0.55s to 0.65s in an example of the present invention.

[0042] Figure 5c This is a schematic diagram illustrating the variation of the three-phase alternating current from 1.78s to 1.88s in an example of the present invention. Detailed Implementation

[0043] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0044] like Figure 1 As shown, a method for monitoring the battery power limit of an MMC-BESS submodule includes the following steps:

[0045] S1. Obtain the inherent parameters of MMC-BESS, including: number of bridge arm sub-modules N, DC bus voltage U. dc Modulation ratio m, AC side output current i g AC side active power P ac and the output voltage angular frequency ω;

[0046] In this example, there are 5 bridge arm sub-modules, the DC bus voltage is 5000V, the modulation ratio is 0.8, the AC side current amplitude is 40A, and the frequency is 50Hz. The calculated AC active power is 40kW and the DC power is 100kW.

[0047] S2. Construct the MMC-BESS mathematical model and calculate the submodule port voltage u based on the inherent parameters. kji and current i kj Based on the power flow direction within the submodule, the constraints that the battery power of the submodule must satisfy are derived.

[0048] The topology of the three-phase MMC-BESS is shown in Figure 2. Since it is three-phase symmetrical, taking one phase as an example, applying Kirchhoff's voltage and current laws, the submodule port voltage u... kji and current i j The formula is:

[0049]

[0050]

[0051] Where k = {a, b, c} represents phase k, j = {u, l} represents the upper and lower bridge arms respectively, the subscript i represents the i-th submodule within the bridge arm, and k i cosωt is the AC component introduced for the capacitor voltage balance control within the bridge arm submodules, I dc I represents the DC component of the circulating current. g It is the amplitude of the alternating current, and θ is the angle of the alternating power factor.

[0052] According to such Figure 3 The power flow within the submodule is shown. The submodule battery power is equal to the submodule port power, which can be obtained from equations (1) and (2). Based on this, the battery power P of a single submodule in the upper arm can be derived. bat_ui and the battery power P of a single submodule of the lower bridge arm bat_li Must meet:

[0053]

[0054] Among them, P bat_u and P bat_l These represent the total battery charging power of the upper and lower bridge arm sub-modules. When the battery power of each sub-module is evenly distributed, the battery power of each sub-module is 6kW.

[0055] S3. Based on the constraint that the modulated wave of the submodule cannot be overmodulated, the power limit of a single submodule battery is derived through the constraint conditions that the battery power of the submodule must satisfy.

[0056] Due to the characteristics of carrier phase-shift control, the charging and discharging power of the submodule battery is constrained by the modulation wave, which cannot be over-adjusted; the adjustable coefficient k i satisfy

[0057]

[0058] From equations (3) and (4), the maximum and minimum charging power P of a single submodule battery in the upper bridge arm can be derived. batumax and P batumin for:

[0059]

[0060] in,

[0061]

[0062] Similarly, the maximum charge / discharge power P of a single submodule in the lower bridge arm batlSMmax and P batlSMmin for:

[0063]

[0064]

[0065] Substituting the parameters of this example into equations (5) and (6), we derive that the maximum charging power of a single submodule battery is 15kW and the minimum charging power is 5kW (maximum discharge power -5kW).

[0066] S4. Based on the constraints of battery power among sub-modules within the bridge arm, the battery power limit of a combination of multiple sub-modules is derived from the battery power limit of a single sub-module.

[0067] The maximum charging power P for the combination of n sub-modules n jmax and minimum charging / maximum discharging power P n jmin Considering the constraints imposed by the charging power of the other (Nn) submodule batteries, when all n submodule batteries are at their maximum charging power, the charging power of the other (Nn) submodule batteries cannot be less than their minimum charging power. Therefore, the battery power limit of the combination of n submodules can be expressed as follows:

[0068]

[0069]

[0070] Substituting equations (5)-(8) into equations (9) and (10), the battery charge and discharge power limits of multiple submodule combinations are derived from the battery power limits of a single submodule.

[0071] Substituting the parameters of this embodiment into the formula, we can obtain that the maximum charging power of the battery combination of the first, second, third, and fourth sub-modules is 10kW, 15kW, 20kW, and 25kW, and the minimum charging power is 5kW, 10kW, 15kW, and 20kW.

[0072] To verify the accuracy of the above monitoring results, simulations were also performed using MATLAB / Simulink in this invention example. The battery charging power of the five sub-modules within the upper arm of phase a from 0 to 3 seconds is as follows: Figure 4 As shown in Figure 5, the corresponding changes in the three-phase AC current are as follows. Before 0.6s, the charging power of all sub-module batteries is evenly distributed, with each sub-module battery having a power of 6kW, indicating that the battery power distribution is feasible. From 0.6s to 1.8s, the battery powers of the five sub-modules are 6.8kW, 6.2kW, 6kW, 5.52kW, and 5.48kW, respectively. At this time, the maximum charging power of the combined batteries of sub-modules one, two, three, and four is 6.8kW, 13kW, 19kW, and 24.52kW, and the minimum charging power is 5.48kW, 11kW, 17kW, and 23.2kW, respectively, all within the limits derived above. Therefore, the power distribution is feasible, and the AC current is normal. After 1.8 seconds, the battery power of the five sub-modules became 9.52kW, 8kW, 5.48kW, 4kW, and 3kW respectively. At this time, the maximum charging power of the combination of two sub-modules was 17.52kW, while the derived power limit was 15kW. Exceeding the limit, the power frequency amplitude of the phase a AC current decreased, the harmonic content increased, and the three-phase output became unbalanced.

[0073] In summary, the present invention provides a method for monitoring the battery power limit of an MMC-BESS submodule. From the perspective of the inherent constraints of the modulation characteristics of MMC itself, it can monitor the maximum charging and discharging power of the submodule battery by combining circuit parameters. This provides guidance for the operation of MMC-BESS when there are differences in the active power of batteries among submodules. Based on the monitored limit, it can determine whether the system can operate normally when the battery power distribution of the submodules is uneven. This helps to improve the design and implementation of MMC-BESS and ensure its stable and safe operation.

Claims

1. A kind The method for monitoring the power limit of a submodule battery is characterized by: Includes the following steps: S1, Obtain The inherent parameters include: the number of bridge arm sub-modules N, and the DC bus voltage. Modulation ratio m, AC side output current AC side active power and output voltage angular frequency ; S2, Construction Mathematical model, calculating submodule port voltage based on inherent parameters. and current Based on the power flow direction within the submodule, the constraints that the battery power of the submodule must satisfy are derived. In S2, due to the three-phase symmetry, one phase is selected, and Kirchhoff's voltage and current laws are applied to determine the submodule port voltage. and current The formula is: (1) (2) in Indicates phase k. These represent the upper and lower bridge arms, respectively, with subscripts indicating the subscripts. Indicates the first one inside the bridge arm Sub-modules It is the AC component introduced for the capacitor voltage balance control of the submodule within the bridge arm. This represents the DC component of the circulating current. It is the amplitude of alternating current. The AC power factor angle; Upper arm single submodule battery power and the battery power of a single submodule in the lower bridge arm Must meet: (3) in, and These are the total battery charging power of the upper and lower bridge arm sub-modules, respectively. S3. Based on the constraint that the modulated wave of the submodule cannot be overmodulated, the power limit of a single submodule battery is derived through the constraint conditions that the battery power of the submodule must satisfy. In S3, due to the characteristics of carrier phase-shift control, the charging and discharging power of the submodule battery is constrained by the modulation wave, which cannot be over-adjusted, and the adjustable coefficient... satisfy: (4) Maximum and minimum charging power of a single submodule battery in the upper bridge arm and for: (5) in, (6) Similarly, the maximum charging and discharging power of a single submodule in the lower bridge arm. and for: (7) (8)； S4. Based on the constraints of battery power among sub-modules within the bridge arm, the battery power limit of a combination of multiple sub-modules is derived from the battery power limit of a single sub-module. In S4, the maximum charging power for a combination of n sub-modules and minimum charging / maximum discharging power Consider other The constraint of the charging power of each submodule battery on it: when all n submodule batteries are at their maximum charging power, the others... The charging power of each submodule battery cannot be less than the minimum charging power, therefore the battery power limit of a combination of n submodules can be expressed: (9) (10) The battery charge and discharge power limits for a combination of multiple submodules are derived from the battery power limits of a single submodule.

Images