Modular energy storage system and internal circulation balancing control method applicable thereto

Abstract

The present invention discloses an internal loop balancing control method suitable for a modular energy storage system. The control method implements balancing based on the DC part of the energy storage system composed of multiple half-bridge converter sub-modules connected in series. By leveraging the characteristic that the DC bus voltage of the energy storage system can be arbitrarily adjusted when idle, the balancing energy is alternately transferred between the battery group to be discharged and the DC bus capacitor, and between the DC bus capacitor and the battery group to be charged, thereby achieving indirect balancing between the battery group to be discharged and the battery group to be charged. The balancing method proposed in the present invention can also be used in other modular energy storage systems, and balancing can be achieved without the help of additional balancing circuit hardware.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochemical energy storage, and in particular to a modular energy storage system. Background Art

[0002] To achieve large-scale integration of renewable energy generation, the adoption of energy storage technology in power systems is imperative. Energy storage technology primarily focuses on the design and optimization of energy storage systems. The addition of energy storage systems can effectively suppress fluctuations in the output of renewable energy generation, ensuring the safe and reliable operation of the power grid after the integration of renewable energy. Energy storage systems can convert electrical energy into a variety of easily storable energy forms, such as chemical energy, potential energy, and electromagnetic energy, and convert it back into electrical energy when needed to supply power to the load. Currently, pumped storage and electrochemical energy storage technologies are relatively mature and widely used. Among them, electrochemical energy storage technology is not restricted by geographical environment and has flexible layout. It has developed rapidly in recent years, with lithium battery energy storage projects being the most common.

[0003] Lithium-ion battery energy storage systems typically consist of an energy storage unit and a power conversion system (PCS). The PCS manages the charging and discharging of the energy storage unit. Furthermore, through appropriate control strategies and dispatch instructions, it can track load power and control grid-side voltage. The design and control of PCSs fall within the scope of power electronics research, and their topologies vary widely. With the rapid development of power electronics technology in recent years, modular, multi-level converters have gradually gained a technological advantage in medium- and high-power applications due to their high degree of modularity, reduced voltage and current stress on power semiconductors, ease of maintenance, and low harmonic distortion. For example, modular multi-level converters (MMCs) have numerous application cases in HVDC transmission.

[0004] In lithium battery energy storage systems, due to the low voltage and capacity provided by individual cells, multiple cells are typically connected in series and parallel to form a battery pack. These packs are then connected in series and parallel to achieve the desired voltage and capacity levels. However, imbalances in state parameters (such as terminal voltage, state of charge (SOC), and state of health (SOH)) are common between individual cells within a battery pack and between battery packs. This imbalance can affect system efficiency, reduce available capacity, and, in severe cases, even cause overcharging or over-discharging of the lithium batteries, leading to system failures and potentially safety incidents.

[0005] The aforementioned imbalanced state parameters are often caused by both inherent differences in lithium-ion batteries and differences in operating conditions. These differences, often limited by process precision or cost and technical constraints, are often difficult to eliminate and often have a cumulative effect, meaning the differences gradually increase over time. This makes it impossible to completely eliminate these differences through forward design, requiring the introduction of balancing circuits or corresponding balancing control strategies for feedback regulation.

[0006] Currently, mature integrated solutions exist for internal battery pack balancing control. Therefore, this invention addresses methods for controlling SOC balancing between battery packs. Typically, both internal and inter-pack balancing circuits require additional hardware circuitry. This circuitry provides a channel for balancing energy transfer, enabling additional charging and discharging of specific unbalanced cells or battery packs to suppress SOC imbalances and ensure safe, reliable, and efficient system operation. However, the addition of additional hardware circuitry increases system cost and complexity, limiting the widespread adoption and application of balancing technology.

[0007] In traditional battery energy storage systems, battery packs are usually combined in the form of direct series and parallel connection, such as Figure 1 As shown, this type of energy storage unit is simple and reliable, but the battery packs connected directly in series are susceptible to SOC imbalance, which prevents them from fully utilizing their available capacity. A common approach to solving SOC imbalance is to add one or more balancing circuits to balance the series battery packs. Figure 2 This is a common balancing circuit diagram, in which the DC-DC converter can be any form of isolated DC-DC converter. The balancing principle is to transfer the balancing energy between unbalanced battery packs through the DC-DC converter, thereby achieving additional charging of low SOC batteries and additional discharge of high SOC batteries, and ultimately achieving SOC balancing between battery packs. However, this method will increase system cost and has low engineering value.

[0008] In summary, simple series and parallel integrated systems can lead to a "short board effect" and potential safety hazards due to the imbalance of battery state parameters. At the same time, the high voltage and current stress of power semiconductor devices limits the capacity and scale of the energy storage system. The addition of active balancing circuits can solve the imbalance of battery state parameters, but it increases the hardware cost and complexity of the system. Summary of the Invention

[0009] In response to the shortcomings of the existing technology, the present invention provides a modular energy storage system and an internal loop balancing control method applicable thereto. It does not require the addition of additional balancing circuits and does not increase the complexity of the system. At the same time, its active balancing nature is also conducive to ensuring the energy conversion efficiency of the system.

[0010] The object of the present invention is achieved as follows: A modular energy storage system comprises: n series-connected half-bridge circuit modules, a DC bus filter, a DC-AC converter, and a controller, wherein n is a positive integer;

[0011] The half-bridge circuit module includes a battery pack, a half-bridge circuit consisting of two insulated gate bipolar transistors (IGBTs), a drive circuit for driving the IGBTs, and a voltage sampling circuit for measuring the battery pack terminal voltage. In the half-bridge circuit module, the positive electrode of each battery pack is connected to the collector of the IGBT on the high-voltage side of the half-bridge circuit, and the negative electrode of each battery pack is connected to the emitter of the IGBT on the low-voltage side of the half-bridge circuit. The switch node of the half-bridge circuit and the emitter node of the low-voltage side IGBT together constitute the input / output port of the half-bridge circuit module. The drive circuit receives a pulse width modulation signal from the controller and drives the IGBTs of the half-bridge circuit to conduct complementary signals accordingly, thereby achieving charge, discharge, and balancing control of the battery pack.

[0012] The DC bus filter includes an inductor, a capacitor and a current sampling circuit for measuring charging and discharging currents;

[0013] The DC-AC converter can work bidirectionally to realize the charging and discharging operations of the energy storage system;

[0014] The controller will balance the batteries when the system is idle. At this time, the DC-AC converter does not participate in the balancing and is disconnected from the DC bus.

[0015] As a further limitation of the present invention, the controller estimates the SOC of different battery packs by acquiring results of each voltage and current sampling circuit and performing SOC estimation based on the results.

[0016] As a further limitation of the present invention, the controller determines the battery pack that needs to be balanced by comparing the SOC of each battery pack with the average SOC, including:

[0017] If the SOC of a battery pack is significantly higher than the average SOC of all battery packs, it is marked as a battery pack to be discharged. The balancing energy needs to be removed from it and periodically charged to the capacitor through the inductor to reduce its SOC value and achieve balancing control of the battery pack.

[0018] If the SOC of a battery pack is significantly lower than the average SOC of all battery packs, it is marked as a battery pack to be charged. Energy in the capacitor needs to be transferred into it through the inductor to increase its SOC value and achieve balanced control of the battery pack.

[0019] A method for controlling an internal circulation balance of a modular energy storage system comprises the following steps:

[0020] 1) Sample the voltage and current of each half-bridge circuit module battery pack;

[0021] 2) Estimate the SOCi of each half-bridge circuit module and calculate the average SOC of all battery packs;

[0022] 3) If SOCi < (average SOC-ε), the battery pack is marked as a battery pack to be charged; if SOCi > (average SOC+ε), the battery pack is marked as a battery pack to be discharged; if (average SOC-ε) ≤ SOCi ≤ (average SOC+ε), the battery pack is marked as a balanced battery pack, where ɛ is a threshold that measures the degree of deviation of SOC from its average value;

[0023] 4) Control the battery pack to be discharged to charge the bus capacitor until it reaches the set value Vset;

[0024] 5) Control the bus capacitor to charge the battery pack to be charged until Vc=0;

[0025] 6) Recalculate the SOC of the balanced battery. If it is not balanced, return to step 4);

[0026] 7) All battery packs have been balanced and the process is complete.

[0027] As a further limitation of the present invention, if the number of battery packs to be charged and the number of battery packs to be discharged are the same, that is, n groups, then step 4) is specifically as follows:

[0028] 0~nT / 2: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be charged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged increases linearly until it reaches the set value Dset;

[0029] nT / 2~nT: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be discharged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be discharged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be charged decreases linearly from the set value Dset' to 0.

[0030] As a further limitation of the present invention, step 4) is specifically as follows: if the number of battery groups to be charged is greater than the number of battery groups to be discharged, and the number of battery groups to be charged, group a, is greater than the number of battery groups to be discharged, then step 4) is specifically as follows:

[0031] 0~bT / 2: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be charged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged increases linearly until it reaches the set value Dset;

[0032] bT / 2—bT: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be discharged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be discharged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be charged decreases linearly from the set value Dset' to 0.

[0033] As a further limitation of the present invention, step 4) is specifically as follows: if the number of battery groups to be charged is less than the number of battery groups to be discharged, and battery group a to be charged is less than battery group b to be discharged, then step 4) is specifically as follows:

[0034] 0~aT / 2: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be charged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged increases linearly until it reaches the set value Dset;

[0035] aT / 2~aT: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be discharged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be discharged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be charged decreases linearly from the set value Dset' to 0.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) The inner loop balancing control method disclosed in the present invention is essentially an active balancing method with high energy utilization. Compared with the passive balancing method, it can effectively improve the comprehensive performance of the system in terms of conversion efficiency, available capacity, etc.

[0038] (2) The internal loop balancing control method implemented by the present invention does not require the addition of an additional balancing circuit to the energy storage system. Compared with the traditional active balancing method, it can effectively reduce the hardware cost of the system while ensuring the balancing effect;

[0039] (3) The internal loop balancing control method implemented in the present invention is portable and is also applicable to other modular energy storage systems in principle, and has simple and reliable control logic. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0041] Figure 1 This is a circuit diagram of a traditional energy storage system.

[0042] Figure 2 This is a schematic diagram of a traditional battery active balancing circuit.

[0043] Figure 3 It is a circuit diagram of a modular energy storage system applicable to the present invention.

[0044] Figure 4 It is a schematic diagram of a special circuit example of the balancing part of the energy storage system with four half-bridge circuit modules applicable to the present invention.

[0045] Figure 5 It is a control flow chart of the present invention.

[0046] Figure 6 It is a schematic diagram of circuit state 1 of the "one-to-one" equalization process of the present invention.

[0047] Figure 7 This is a schematic diagram of circuit state 2 of the "one-to-one" equalization process of the present invention.

[0048] Figure 8 1 is a schematic diagram of circuit state 1 of the "two-to-two" equalization process of the present invention.

[0049] Figure 9 This is a schematic diagram of circuit state 2 of the "two-to-two" equalization process of the present invention.

[0050] Figure 10 This is a typical circuit waveform diagram of the "one-to-one" equalization process of the present invention.

[0051] Figure 11 This is a typical circuit waveform diagram of the "two-to-two" equalization process of the present invention. DETAILED DESCRIPTION

[0052] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0053] like Figure 3 A modular energy storage system is shown, comprising: n series-connected half-bridge circuit modules, a DC bus filter, a DC-AC converter, and a controller, where n is a positive integer;

[0054] The half-bridge circuit module includes a battery pack, a half-bridge circuit consisting of two insulated gate bipolar transistors (IGBTs), a drive circuit for driving the IGBTs, and a voltage sampling circuit for measuring the battery pack terminal voltage. In the half-bridge circuit module, the positive electrode of each battery pack is connected to the collector of the IGBT on the high-voltage side of the half-bridge circuit, and the negative electrode of each battery pack is connected to the emitter of the IGBT on the low-voltage side of the half-bridge circuit. The switch node of the half-bridge circuit and the emitter node of the low-voltage side IGBT together constitute the input / output port of the half-bridge circuit module. The drive circuit receives a pulse width modulation signal from the controller and drives the IGBTs of the half-bridge circuit to conduct complementary conduction accordingly, realizing charge, discharge, and balancing control of the battery pack.

[0055] The DC bus filter includes an inductor, a capacitor, and a current sampling circuit for measuring the charge and discharge currents;

[0056] The DC-AC converter can work bidirectionally to realize the charging and discharging operations of the energy storage system;

[0057] The controller balances the batteries when the system is idle. During this time, the DC-AC converter does not participate in balancing and is disconnected from the DC bus. The controller obtains the results of each voltage and current sampling circuit and uses this to estimate the SOC of different battery packs. The controller compares the SOC of each battery pack with the average SOC to determine the battery pack that needs balancing, including:

[0058] If the SOC of a battery pack is significantly higher than the average SOC of all battery packs, it is marked as a battery pack to be discharged. The balancing energy needs to be removed from it and periodically charged to the capacitor through the inductor to reduce its SOC value and achieve balancing control of the battery pack.

[0059] If the SOC of a battery pack is significantly lower than the average SOC of all battery packs, it is marked as a battery pack to be charged. Energy in the capacitor needs to be transferred into it through the inductor to increase its SOC value and achieve balanced control of the battery pack.

[0060] like Figure 6 An internal cycle balancing control method applicable to the above modular energy storage system is shown, comprising the following steps:

[0061] 1) Sample the voltage and current of each half-bridge circuit module battery pack;

[0062] 2) Estimate the SOCi of each half-bridge circuit module and calculate the average SOC of all battery packs;

[0063] 3) If SOCi < (average SOC-ε), the battery pack is marked as a battery pack to be charged; if SOCi > (average SOC+ε), the battery pack is marked as a battery pack to be discharged; if (average SOC-ε) ≤ SOCi ≤ (average SOC+ε), the battery pack is marked as a balanced battery pack, where ɛ is a threshold that measures the degree of deviation of SOC from its average value;

[0064] 4) Control the battery pack to be discharged to charge the bus capacitor until it reaches the set value Vset;

[0065] 5) Control the bus capacitor to charge the battery pack to be charged until Vc=0;

[0066] 6) Recalculate the SOC of the balanced battery. If it is not balanced, return to step 4);

[0067] 7) All battery packs have been balanced and the process is complete.

[0068] In step 3), if the number of battery groups to be charged and the number of battery groups to be discharged are the same, that is, n groups, then step 4) is specifically as follows:

[0069] 0~nT / 2: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be charged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged increases linearly until it reaches the set value Dset;

[0070] nT / 2~nT: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be discharged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be discharged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be charged decreases linearly from the set value Dset' to 0.

[0071] In step 3), if the number of battery groups to be charged is greater than the number of battery groups to be discharged, and the number of battery groups to be charged, group a, is greater than the number of battery groups to be discharged, then step 4) is specifically as follows:

[0072] 0~bT / 2: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be charged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged increases linearly until it reaches the set value Dset;

[0073] bT / 2-bT: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be discharged is neither charged nor discharged. The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged decreases linearly from the set value Dset' to 0. This ensures balanced distribution when supply exceeds demand, avoiding the problem of uneven discharge caused by the discharge of excess battery packs.

[0074] In step 3), if the number of battery groups to be charged is less than the number of battery groups to be discharged, and battery group a to be charged is less than battery group b to be discharged, then step 4) is specifically as follows:

[0075] 0~aT / 2: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be charged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged increases linearly until it reaches the set value Dset;

[0076] From aT / 2 to aT: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit module of the battery pack to be discharged remains at zero, meaning that the half-bridge circuit module is bypassed and the battery pack to be discharged is neither charged nor discharged. The duty cycle of the high-side IGBT drive signal of the half-bridge circuit module of the battery pack to be charged decreases linearly from the set value Dset' to zero. This ensures balanced distribution when supply exceeds demand, preventing overcharging of the charging battery pack.

[0077] The above technical solution will be further explained below with reference to specific examples.

[0078] like Figure 4 Figure 2 shows a schematic diagram of the DC portion of an energy storage system with four half-bridge circuit modules, including four half-bridge circuit modules connected in series, a DC bus filter, and a controller. The half-bridge circuit module includes a battery pack, a half-bridge circuit consisting of two IGBTs, a drive circuit for driving the IGBTs, and a voltage sampling circuit for measuring the battery pack terminal voltage. The DC bus filter includes an inductor, a capacitor, and a current sampling circuit for measuring the charge and discharge currents.

[0079] The battery packs in the half-bridge circuit module serve as the energy storage units and balancing targets of this embodiment. These battery packs are connected in series through a half-bridge converter to form a battery cluster, providing the high voltage required by the system while also ensuring low voltage stress on the IGBTs within each half-bridge circuit module. The positive electrode of each battery pack is connected to the collector of the IGBT on the high-voltage side of the half-bridge circuit, and the negative electrode of each battery pack is connected to the emitter of the IGBT on the low-voltage side of the half-bridge circuit. The switch node of the half-bridge circuit and the emitter node of the low-voltage side IGBT together constitute the input / output port of the half-bridge circuit module, where the switch node is defined as the positive electrode of the input / output port of the half-bridge circuit module, and the emitter node of the low-voltage side IGBT is defined as the negative electrode of the input / output port of the half-bridge circuit module. The drive circuit receives a PWM signal from the controller and, based on this signal, drives the two IGBTs of the half-bridge circuit into complementary conduction, achieving charge and discharge control and balancing control of the battery pack.

[0080] One end of the inductor in the DC bus filter is connected to the positive electrode of the input / output port of the half-bridge circuit module 1, and is also connected to the positive electrode of the DC bus filter capacitor. The negative electrode of the DC bus filter capacitor is connected to the negative electrode of the input / output port of the half-bridge circuit module 4, thereby forming the DC part of the modular energy storage system and also a loop for balanced energy transfer.

[0081] The balance control method disclosed in the present invention is a digital control method. The required control program is stored in the controller. The balance control process is as follows: Figure 5 As shown in the figure, the controller performs analog-to-digital conversion on the output signals of each voltage and current sampling circuit to quantify parameters such as the battery pack voltage and inductor current. The controller then uses an SOC estimation algorithm to estimate the SOC of each battery pack (defined as SOCi, where i=1, 2, 3, 4) and the average SOC of all battery packs. The controller compares the degree to which SOCi deviates from the average SOC to determine whether battery pack i needs to be balanced. Finally, using the DC bus filter capacitor as a medium, the controller first transfers the excess charge of the battery pack to be discharged to the capacitor, and then transfers it to the battery pack to be charged, achieving balancing between battery packs. During the balancing process, the SOC of each battery pack is still estimated and monitored, and the balancing process ends when the SOC of all battery packs is close to the average SOC.

[0082] The following example takes the case where the SOC of battery pack i (i=1, 2, 3, 4) is too high and the SOC of battery pack j (j≠i; j=1, 2, 3, 4) is too low. Combined with the circuit modal diagram ( Figure 6 , Figure 7 ) and typical circuit waveforms ( Figure 10 ), and give a detailed introduction to the “one-to-one” equilibrium process.

[0083] Sample the voltage Vi and inductor current I of all battery packsL , the SOC of each battery pack is estimated through the SOC estimation algorithm, and the average value of the SOC of all battery packs, SOCave, is calculated. If the SOC of battery pack i is high (i.e., SOCi > SOCave + ɛ), the SOC of battery pack j is low (i.e., SOCj < SOCave - ɛ), and the SOC of other battery packs does not differ much from SOCave (i.e., SOCave - ɛ < other SOC < SOCave + ɛ), then mark battery pack i as the battery pack to be discharged, mark battery pack j as the battery pack to be charged, and mark other battery packs as balanced battery packs. Here, ɛ is the threshold for measuring the degree of deviation of the SOC from its average value. Once the quantity deviating from its average value exceeds ɛ, balancing is required.

[0084] The typical circuit waveforms during one balancing cycle of this balancing process are as Figure 10 shown:

[0085] 0 - T / 2: The duty cycle Dj of the high - voltage - side IGBT drive signal of the half - bridge circuit of half - bridge circuit module j is always 0, that is, half - bridge circuit module j is bypassed, and battery pack j neither charges nor discharges; the duty cycle Di of the high - voltage - side IGBT drive signal of the half - bridge circuit of half - bridge circuit module i linearly rises until Dset; the inductor current I L tends to be stable after a short climbing process, that is, the bus capacitor is charged approximately at a constant current until V C = Vset. Here,

[0086] Vset ≤ Vj, Dset = Vset / Vi

[0087] This inequality ensures that there will be no over - modulation and current overshoot during the process of the bus capacitor switching from charging to discharging. The circuit modes of the above process are as Figure 6 shown.

[0088] T / 2 - T: The duty cycle Di of the high - voltage - side IGBT drive signal of the half - bridge circuit of half - bridge circuit module i is always 0, that is, half - bridge circuit module i is bypassed, and battery pack i neither charges nor discharges; the duty cycle Dj of the high - voltage - side IGBT drive signal of the half - bridge circuit of half - bridge circuit module j linearly decreases from Dset’ until 0; the inductor current I L tends to be stable after a short polarity - switching process, and the bus capacitor discharges approximately at a constant current until V C = 0. Here,

[0089] Dset’ = Vset / Vj

[0090] Since Vset ≤ Vj, so Dset’ ≤ 1, which ensures that there will be no over - modulation and current overshoot during the process of the bus capacitor switching from charging to discharging. The circuit modes of the above process are as Figure 7 shown.

[0091] Taking the example that the SOC of battery packs h and i (h ≠ i; h, i = 1, 2, 3, 4) is on the high side and the SOC of battery packs j and k (j ≠ h, j ≠ i, j ≠ k, k ≠ h, k ≠ i; j, k = 1, 2, 3, 4) is on the low side, combined with the circuit mode diagram ( Figure 8 , Figure 9 ), and the typical circuit waveform diagram ( Figure 11 ), the "two - to - two" equalization process will be introduced in detail.

[0092] Sample the voltage Vi of all battery packs and the inductor current I L . Estimate the SOC of each battery pack through the SOC estimation algorithm, and calculate the average value SOCave of the SOC of all battery packs. If the SOC of battery packs h and i is on the high side (i.e., SOCh > SOCave + ɛ, SOCi > SOCave + ɛ), and the SOC of battery packs j and k is on the low side (i.e., SOCj < SOCave - ɛ, SOCk < SOCave - ɛ), then mark battery packs h and i as the battery packs to be discharged, and mark battery packs j and k as the battery packs to be charged. Here, ɛ is the threshold for measuring the deviation degree of the SOC from its average value. Once the quantity deviating from its average value exceeds ɛ, equalization is required.

[0093] The typical circuit waveform of this equalization process within one equalization period is as shown in Figure 11 :

[0094] 0 - T: The duty cycles Dj and Dk of the high - voltage - side IGBT drive signals of the half - bridge circuits of half - bridge circuit module j and half - bridge circuit module k are always 0, that is, half - bridge circuit module j and half - bridge circuit module k are bypassed, and battery packs j and k neither charge nor discharge; the duty cycles Dk and Di of the high - voltage - side IGBT drive signals of the half - bridge circuits of half - bridge circuit module h and half - bridge circuit module i linearly increase until Dset; the inductor current I L tends to be stable after a short climbing process, that is, the bus capacitor is charged approximately at a constant current until V C = Vset. Among them,

[0095] Vset ≤ Vj + Vk, Dset = Vset / (Vh + Vi)

[0096] This inequality ensures that over - modulation and current overshoot do not occur during the process of the bus capacitor changing from charging to discharging. The circuit mode of the above process is as shown in Figure 8 . ​T—2T: The duty ratios Dh and Di of the high-side IGBT drive signals of the half-bridge circuit modules h and i are always 0, that is, the half-bridge circuit modules h and i are bypassed, and the battery packs h and i are neither charged nor discharged; the duty ratios Dj and Dk of the high-side IGBT drive signals of the half-bridge circuit modules j and k decrease linearly from Dset' to 0; the inductor current I L After a short polarity switching process, it tends to be stable, and the bus capacitor discharges at a constant current until V C =0. Among them,

[0098] Dset'=Vset / (Vj+Vk)

[0099] Since Vset≤Vj+Vk, Dset'≤1, thus ensuring that there will be no overmodulation and current overshoot in the process of busbar capacitance switching from charging to discharging. The circuit mode of the above process is as follows: Figure 9 shown.

[0100] In addition to the two balancing cases mentioned above, "one-to-two" balancing and "two-to-one" balancing can also be implemented in the manner of battery group a to be charged < battery group b to be discharged, or battery group a to be charged > battery group b to be discharged.

[0101] An embodiment of the present invention provides an internal loop balancing control method for a modular energy storage system. This method implements balancing based on the DC portion of the energy storage system, which is composed of four series-connected half-bridge converter circuit modules. By leveraging the arbitrarily adjustable DC bus voltage of the energy storage system when idle, balancing energy is alternately transferred between the battery pack to be discharged and the DC bus capacitor, and between the DC bus capacitor and the battery pack to be charged, thereby achieving indirect balancing between the battery pack to be discharged and the battery pack to be charged. The balancing method proposed in the present invention can also be used in other modular energy storage systems. Since it does not require additional balancing circuit hardware to achieve balancing, this method is superior to traditional balancing control methods.

[0102] The above embodiments are only intended to help understand the method and core concept of the present invention. It should be noted that, without departing from the principles of the present invention, a number of improvements and modifications may be made to the present invention by those skilled in the art, and such improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. A method for controlling an internal circulation balance of a modular energy storage system, the modular energy storage system comprising: n series-connected half-bridge circuit modules, a DC bus filter, a DC-AC converter, and a controller, wherein n is a positive integer; The half-bridge circuit module includes a battery pack, a half-bridge circuit consisting of two insulated gate bipolar transistors (IGBTs), a drive circuit for driving the IGBTs, and a voltage sampling circuit for measuring the battery pack terminal voltage. In the half-bridge circuit module, the positive electrode of each battery pack is connected to the collector of the IGBT on the high-voltage side of the half-bridge circuit, and the negative electrode of each battery pack is connected to the emitter of the IGBT on the low-voltage side of the half-bridge circuit. The switch node of the half-bridge circuit and the emitter node of the low-voltage side IGBT together constitute the input / output port of the half-bridge circuit module. The drive circuit receives a pulse width modulation signal from the controller and drives the IGBTs of the half-bridge circuit to conduct complementary signals accordingly, thereby achieving charge, discharge, and balancing control of the battery pack. The DC bus filter includes an inductor, a capacitor and a current sampling circuit for measuring charging and discharging currents; The DC-AC converter can work bidirectionally to realize the charging and discharging operations of the energy storage system; The controller will balance the batteries when the system is idle. At this time, the DC-AC converter does not participate in balancing and is disconnected from the DC bus. The controller obtains the results of each voltage and current sampling circuit and estimates the SOC accordingly to estimate the SOC of different battery packs. The controller determines the battery pack that needs to be balanced by comparing the SOC of each battery pack with the average SOC, including: If the SOC of a battery pack is significantly higher than the average SOC of all battery packs, it is marked as a battery pack to be discharged. The balancing energy needs to be removed from it and periodically charged to the capacitor through the inductor to reduce its SOC value and achieve balancing control of the battery pack. If the SOC of a battery pack is significantly lower than the average SOC of all battery packs, it is marked as a battery pack to be charged. Energy in the capacitor needs to be transferred to it via the inductor to increase its SOC value and achieve balanced control of the battery pack. The method is characterized by including the following steps: 1) Sample the voltage and current of each half-bridge circuit module battery pack; 2) Estimate the SOCi of each half-bridge circuit module and calculate the average SOC of all battery packs; 3) If SOCi < (average SOC-ε), the battery pack is marked as a battery pack to be charged; if SOCi > (average SOC+ε), the battery pack is marked as a battery pack to be discharged; if (average SOC-ε) ≤ SOCi ≤ (average SOC+ε), the battery pack is marked as a balanced battery pack, where ɛ is a threshold that measures the degree of deviation of SOC from its average value; 4) Control the battery pack to be discharged to charge the bus capacitor until it reaches the set value Vset; 5) Control the bus capacitor to charge the battery pack to be charged until Vc=0; 6) Recalculate the SOC of the balanced battery. If it is not balanced, return to step 4); 7) All battery packs have been balanced and the process is complete.

2. The internal circulation balancing control method of the modular energy storage system according to claim 1, characterized in that: If the number of battery packs to be charged and battery packs to be discharged is the same, that is, n groups, then step 4) is specifically as follows: 0~nT / 2: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be charged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged increases linearly until it reaches the set value Dset; nT / 2~nT: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be discharged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be discharged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be charged decreases linearly from the set value Dset' to 0.

3. The internal circulation balancing control method of the modular energy storage system according to claim 1, characterized in that: Step 4) is specifically as follows: if the number of battery groups to be charged is greater than the number of battery groups to be discharged, and the number of battery groups to be charged, group a, is greater than the number of battery groups to be discharged, then step 4) is specifically as follows: 0~bT / 2: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be charged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged increases linearly until it reaches the set value Dset; bT / 2—bT: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be discharged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be discharged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be charged decreases linearly from the set value Dset' to 0.

4. The internal circulation balancing control method of the modular energy storage system according to claim 1, characterized in that: Step 4) is specifically as follows: if the number of battery groups to be charged is less than the number of battery groups to be discharged, and battery group a to be charged is less than battery group b to be discharged, then step 4) is specifically as follows: 0~aT / 2: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be charged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be charged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the battery pack to be discharged increases linearly until it reaches the set value Dset; aT / 2~aT: The duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be discharged is always 0, that is, the half-bridge circuit module is bypassed, and the battery pack to be discharged is neither charged nor discharged; the duty cycle of the high-side IGBT drive signal of the half-bridge circuit of the half-bridge circuit module of the battery pack to be charged decreases linearly from the set value Dset' to 0.

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