Control method giving consideration to energy storage charge state balance and near-zero line impedance frequency self-synchronization control

Abstract

The invention provides a control method giving consideration to energy storage charge state balance and near-zero line impedance frequency self-synchronization control, and the method comprises the steps: measuring the output voltage of an ith inverter and the charge state of energy storage corresponding to the ith inverter, and calculating a current given value iiref of the ith inverter; the q-axis component of the output voltage of the ith inverter is measured, and the control phase angle of the ith inverter is determined; and controlling the inverter through a pulse width modulation (PWM) unit by using the modulation signal. According to the invention, a decentration control strategy based on a local state is introduced, frequency self-synchronization and proportional power distribution of multiple inverters under a non-communication condition are realized, and the reliability and redundancy of the system are remarkably improved; an SOC correction term is introduced into current reference, so that self-adaptive power adjustment and long-term SOC equalization between energy storage units are realized; the method overcomes the defect that traditional droop control is prone to instability in a near-zero line impedance scene, and has the advantages of being simple in structure, high in portability, suitable for a key load power supply scene and the like.

Description

Technical Field

[0001] This invention belongs to the field of power electronics and distributed power supply control, and particularly relates to a distributed control method for a modular uninterruptible power supply (UPS) system, specifically involving parallel inverter control technology based on energy storage state of charge (SOC) balancing and near-zero line impedance frequency self-synchronization. Background Technology

[0002] Critical infrastructure such as data centers, core banking systems, hospital medical equipment, life support systems, communication networks, and IT server clusters have extremely high requirements for continuous power supply. Any sudden power outage can cause significant economic losses and operational risks. Uninterruptible power supply (UPS) systems play a crucial role in ensuring continuous power supply, directly impacting the overall stability and reliability of data centers. The core function of a UPS is to provide continuous power during grid failures until a backup generator starts.

[0003] Online UPS systems continuously supply power to the load via inverters, effectively isolating the load from grid interference, and have become the mainstream choice for sensitive loads such as data centers. A typical online UPS system (such as...) Figure 2 (a) shows an AC / DC rectifier, a DC / AC inverter with an LC filter, a battery pack with a DC / DC converter, a surge suppressor (TVSS), and a static bypass switch. When a power outage disconnects the system from the grid, the bypass switch is open, and the parallel inverter operates with the battery as a backup power source. To improve the reliability and redundancy of power supply systems, the concept of modular uninterruptible power supplies (UPS) emerged in the 1990s. This system uses multiple independent power modules operating in parallel to supply power to the AC bus. Each module is equipped with complete functional components, supports independent control, and can be replaced without downtime even if a module fails. Furthermore, N+X redundancy configurations (such as...) Figure 2 As shown in (b), the N parallel modules + X redundant modules are widely used in engineering practice.

[0004] For modular UPS systems with integrated battery energy storage units (ESUs), maintaining a balanced state of charge (SOC) among the modules is crucial for ensuring long-term stable operation. An imbalance in the SOC of the energy storage units can lead to overcharging or over-discharging of some units, consequently affecting the efficiency and lifespan of the entire energy storage system. To address the SOC imbalance problem, it is necessary to coordinate the SOC values ​​of the energy storage system over a long period. Since centralized and distributed control systems rely on communication to varying degrees, the "no communication required, plug-and-play" characteristics of distributed control have made it widely used in modular UPS systems.

[0005] Mainstream decentralized control methods include droop control and virtual synchronous generator control, both widely used in multi-inverter parallel scenarios. However, grid-forming inverters are considered controlled voltage sources, and their power transmission characteristics are highly dependent on line impedance. But in modular UPS systems for critical infrastructure, the transmission distance between parallel inverters and critical loads is extremely short, with line impedance almost zero. This unique scenario presents new challenges for decentralized control: when parallel voltage sources are electrically close to each other, voltage regulation becomes significantly more difficult, potentially leading to synchronization loss in droop-controlled inverters. While adding inductors can improve the equivalent line impedance, it also increases the size and cost of the UPS system. Based on these factors, traditional decentralized control methods are no longer applicable in multi-inverter parallel scenarios where line impedance approaches zero. To overcome these technical bottlenecks, a novel decentralized control method is urgently needed to ensure that the system can efficiently achieve SOC equalization among energy storage units and inverter frequency self-synchronization.

[0006] A search revealed that Chinese invention patent CN202210671332.7 provides a SOC balancing control method for battery energy storage systems based on droop control. This method dynamically adjusts the droop coefficient within the traditional active-frequency (P–f) and reactive-voltage (Q–V) control framework of droop control, making it a function of the SOC of the energy storage unit. This approach enables high-SOC units to output more power and low-SOC units to output less power, thus gradually achieving SOC self-balancing. The control system can coordinate multiple energy storage units under certain communication conditions to achieve distributed energy dispatch and lifetime balancing.

[0007] The above-mentioned prior art documents are compared with this application as follows:

[0008] The aforementioned comparative documents still rely on the "virtual impedance" mechanism of droop control, which is highly dependent on line impedance. In near-zero impedance parallel scenarios, voltage / frequency instability is prone to occur, making synchronous operation impossible. SOC adjustment is achieved only through static correction of the droop coefficient; the response speed is affected by system inertia and filter parameters, resulting in slow convergence and difficulty in precisely controlling the dynamic process. The solution proposed in this application can maintain stable system synchronization without relying on line impedance while ensuring SOC balance.

[0009] A search revealed that Chinese invention patent CN202510869183.9 provides a state-of-charge (SOC) balancing control method and related equipment for grid-type energy storage units. This method introduces SOC-related adjustment factors into the traditional virtual synchronous generator (VSG) or grid-type inverter control framework, achieving automatic energy balancing by dynamically adjusting the active power output of the energy storage units. It maintains system synchronization through a voltage-frequency (Vf) coupling mechanism and corrects the output reference signal of the energy storage units by combining power-angle relationships. When multiple energy storage units are connected in parallel, the system can automatically allocate power according to the SOC differences of each unit, achieving SOC convergence without communication.

[0010] The above-mentioned prior art documents are compared with this application as follows:

[0011] The aforementioned prior art documents, while ensuring voltage shaping and frequency support capabilities of grid-connected inverters, also consider energy balance of energy storage units, representing an extension and optimization of the traditional droop control method. Furthermore, the grid-connected control of this invention, centered on a voltage source, relies on line impedance for power distribution. Under near-zero impedance parallel operation, this can easily lead to power oscillations or loss of synchronization, making stability difficult to guarantee. This application completely overcomes the challenge of stable frequency self-synchronization of grid-connected control under near-zero impedance parallel operation. By utilizing a first-order phase-locked loop and a controlled current source, while simultaneously introducing the influence of State of Charge (SOC), it achieves a balance between energy storage SOC balance and frequency self-synchronization.

[0012] A search revealed Chinese invention patent CN202411763054.3, which provides a droop control strategy for multi-energy storage microgrids based on the state of charge (SOC). Addressing the SOC imbalance and power distribution issues in parallel multi-energy storage unit systems (microgrids), this invention proposes an improved droop control strategy based on the SOC of energy storage. By introducing an SOC correction factor, the invention dynamically adjusts the traditional active-frequency (P–f) and reactive-voltage (Q–V) droop coefficients, allowing energy storage units with higher SOCs to handle more active power output, while those with lower SOCs output less, thus achieving SOC balance. During charge-discharge conversion, dual-mode switching logic ensures smooth control and prevents reverse energy flow. Ultimately, this distributed architecture achieves long-term energy balance among energy storage units and stable system operation within the microgrid.

[0013] The above-mentioned prior art documents are compared with this application as follows:

[0014] The aforementioned comparative documents still rely on the voltage source droop mechanism, and their control effectiveness is sensitive to line impedance and depends on the voltage-power coupling characteristics. When the system line impedance is extremely low, power distribution is prone to instability, making it unsuitable for near-zero impedance scenarios. Furthermore, although this method is known as distributed control, communication coordination may still be required during parameter calibration or abnormal conditions; operation without communication results in poor accuracy.

[0015] This application overcomes the technical bottleneck of achieving frequency self-synchronization stability in grid-connected inverters under near-zero impedance parallel scenarios. By integrating first-order phase-locked loop and controlled current source technology, and introducing state-of-charge (SOC) feedback for energy storage, it successfully achieves coordinated control of frequency self-synchronization and SOC balance among energy storage units. Furthermore, this method is a completely distributed control system, requiring no communication and achieving a new level of reliability. Summary of the Invention

[0016] To address the aforementioned technical problems, this invention proposes a control method that balances the state-of-charge (SOC) balance of energy storage units with near-zero line impedance frequency self-synchronization control. This method achieves frequency self-synchronization among multiple inverters by introducing a first-order phase-locked loop (PLL) and incorporates a SOC correction term into the current reference control to realize dynamic power distribution and equalization control of the energy storage unit (ESU) during charging and discharging. It achieves adaptive power coordination and SOC equalization among modules without the need for a communication network, offering the advantages of "no communication required, plug and play."

[0017] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0018] A control method that balances energy storage state-of-charge equilibrium and near-zero line impedance frequency self-synchronization control includes the following steps:

[0019] S1. Measure the output voltage of the i-th inverter and the state of charge of the energy stored in the i-th inverter, and calculate the current setpoint i of the i-th inverter. iref S2. Measure the q-axis component of the output voltage of the i-th inverter to determine the control phase angle of the i-th inverter.

[0020] S3. The inverter is controlled by a pulse width modulation (PWM) unit using a modulation signal. Further, the current setpoint i in step S1... iref Calculate according to the following formula: ;

[0021] in,

[0022] I idref I iqref These are the d-axis and q-axis components of the current given in the control of the i-th inverter, respectively.

[0023] k p k i These are the proportional and integral control parameters in the PI controller;

[0024] m i For inverter power output proportional system;

[0025] V ref This is the rated voltage.

[0026] V i Let i be the output voltage of the i-th inverter;

[0027] n is the SOC equilibrium coefficient;

[0028] 'a' represents charging and discharging information;

[0029] SOC i This represents the state of charge of the i-th inverter.

[0030] Furthermore, in step S2, the control phase angle is calculated according to the following formula:

[0031] ;

[0032] in,

[0033] θ i The reference angle generated for the i-th inverter control;

[0034] ɷ ref and ɷ i These are the angular frequency reference value and the output angular frequency of the i-th inverter, respectively.

[0035] V iq Let q be the q-axis component of the output voltage of the i-th inverter.

[0036] Furthermore, the q-axis component V of the output voltage of the i-th inverter iq Calculate according to the following formula: Among them, V ib V is the B-phase voltage of the i-th inverter; ic Let V be the C-phase voltage of the i-th inverter. Further, let V be the B-phase voltage of the i-th inverter. ib and the C-phase voltage V of the i-th inverter ic Calculate according to the following formula: Among them, V i Let θ be the output voltage of the i-th inverter; i The reference angle is generated for the i-th inverter control. Furthermore, the charge / discharge information a = 1 or 0, where a = 1 represents the discharge mode and a = 0 represents the charging mode.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1. This invention introduces a decentralized control strategy based on local states to achieve frequency self-synchronization and proportional power distribution among multiple inverters under conditions without communication, significantly improving the reliability and redundancy of the system. 2. By introducing a SOC correction term into the current reference, adaptive power regulation and long-term SOC balance among energy storage units are achieved, effectively preventing battery overcharging and over-discharging and extending system life. 3. This invention can maintain voltage stability and synchronous operation under near-zero line impedance conditions, overcoming the shortcomings of traditional droop control in low-impedance scenarios, and has the advantages of simple structure, strong portability, and applicability to critical load power supply scenarios. Attached Figure Description

[0039] Figure 1 This is a flowchart of the present invention;

[0040] Figure 2 This is a schematic diagram of the UPS system of the present invention;

[0041] Figure 3 This is a schematic diagram of the control scheme;

[0042] Figure 4 Diagram of the experimental platform;

[0043] Figure 5 The SOC curve of the RL load in discharge mode;

[0044] Figure 6 The active power of the RL load under the control method employed in discharge mode;

[0045] Figure 7 The reactive power of the control method proposed under RL load discharge mode;

[0046] Figure 8 The SOC curve is for the control method with RC load in discharge mode;

[0047] Figure 9 The active power is the control method for RC loads in discharge mode;

[0048] Figure 10 The reactive power of the control method proposed under RC load discharge mode;

[0049] Figure 11 The SOC curve of the proposed control method in charging mode;

[0050] Figure 12 The active power is the value of the control method proposed in the charging mode. Detailed Implementation

[0051] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. The specific implementation methods of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0052] Example 1:

[0053] like Figure 1 , 3 As shown, this embodiment is a control method that balances energy storage state-of-charge equilibrium and near-zero line impedance frequency self-synchronization control, including the following steps:

[0054] S1. Measure the output voltage of the i-th inverter and the state of charge of the energy stored in the i-th inverter, and calculate the current setpoint i of the i-th inverter. iref In step S1, the current setpoint i iref Calculate according to the following formula: ;

[0055] in,

[0056] I idref I iqref These are the d-axis and q-axis components of the current given in the control of the i-th inverter, respectively.

[0057] k p k i These are the proportional and integral control parameters in the PI controller;

[0058] m i For inverter power output proportional system;

[0059] V ref This is the rated voltage.

[0060] V i Let i be the output voltage of the i-th inverter;

[0061] n is the SOC equilibrium coefficient;

[0062] 'a' represents charging / discharging information. The charging / discharging information can be either 1 or 0. When a=1, it represents the discharging mode, and when a=0, it represents the charging mode.

[0063] SOC i This represents the state of charge of the i-th inverter.

[0064] Proportional-integral (PI) control ensures PV regulation characteristics that generate current amplitude reference values ​​for voltage support and power distribution. (The last part, "I," appears to be an unrelated fragment and is left untranslated.) iqref Setting it to 0 allows the output current to be oriented along the d-axis, a control method that helps simplify power decoupling analysis.

[0065] Battery state-of-charge (SOC) balancing is essentially about adjusting output power. In discharge mode, ESUs with a higher SOC output more power, while those with a lower SOC output less power. Similarly, in charging mode, cells with a lower SOC inject more power, while those with a higher SOC inject less power. Based on this principle, the SOC difference gradually decreases, eventually achieving SOC balance. S2. Measure the q-axis component of the output voltage of the i-th inverter to determine the control phase angle of the i-th inverter; the control phase angle is calculated according to the following formula:

[0066] ;

[0067] in,

[0068] θ i The reference angle generated for the i-th inverter control;

[0069] ɷ ref and ɷ i These are the angular frequency reference value and the output angular frequency of the i-th inverter, respectively.

[0070] V iq The q-axis component of the output voltage of the i-th inverter is calculated according to the following formula: ;

[0071] in,

[0072] V ib V is the B-phase voltage of the i-th inverter; ic V is the C-phase voltage of the i-th inverter. ib and V ic Calculate according to the following formula: Among them, V i Let θ be the output voltage of the i-th inverter; i The reference angle generated for the i-th inverter control.

[0073] S3. The inverter is controlled by the pulse width modulation (PWM) unit using the modulation signal.

[0074] Example 2:

[0075] This embodiment uses the CHIL platform of OPAL-RT 4510 and RT-BOX to conduct experiments to verify the feasibility of the proposed method. The experimental platform configuration is as follows: Figure 4 As shown in the figure. RT-BOX is responsible for the implementation of the controller, and the hardware circuit is simulated using OPAL-RT4510 with a sampling frequency of 10 kHz.

[0076] Figures 5-7 The experimental results of the proposed SOC equalization control are presented when the RL load is running in discharge mode.Figure 5 The measured SOC values ​​are presented. Figures 6-7 The output power curves are shown. In this case, the controller starts at t=3 seconds, and the initial SOC values ​​of the three energy storage units are set to 0.8, 0.75, and 0.7, respectively. Before t=5 seconds (i.e., during the controller shutdown period), the actual output power of energy storage units #1-3 remains stable. However, after applying this control strategy, the power output of each energy storage unit begins to adjust: the unit with the higher SOC value outputs more power, and the unit with the lower SOC value outputs less power. As time progresses, the power output of the three parallel inverters gradually synchronizes with their respective SOC values.

[0077] Figures 8 to 10 Experimental results of the proposed SOC equalization control under RC load conditions in discharge mode are presented. Figure 8 The measured SOC curve is presented. Figure 9 and Figure 10 The output power curve is then displayed. Similar to Case 1, the controller starts at t=3 seconds, and the initial SOC values ​​of the three energy storage units are set to 0.8, 0.75, and 0.7, respectively. By applying this control strategy, the output power of the three parallel inverters gradually synchronizes with the SOC values ​​of the energy storage units. The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any modifications or equivalent changes made based on the technical essence of the present invention shall still fall within the scope of protection claimed by the present invention.

[0078] Figures 11-12 Experimental results of the proposed SOC equalization control in charging mode are presented. Figure 11 Displays the measured SOC value. Figure 12 This will result in an output power curve. The initial SOC values ​​of the three energy storage units are set to 0.2, 0.25, and 0.3, respectively. By implementing this control strategy, the output power of the three parallel inverters will gradually synchronize with the SOC value of each energy storage unit.

[0079] The above embodiments are for illustrative purposes only and are not intended to limit the scope of this invention. Although this invention has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications, or equivalent substitutions of the technical solutions of this invention do not depart from the spirit and scope of the technical solutions of this invention and should be covered within the scope of the claims of this invention.

Claims

1. A control method that balances energy storage state-of-charge equilibrium with near-zero line impedance frequency self-synchronization control, characterized in that, Includes the following steps: S1. Measure the output voltage of the i-th inverter and the state of charge of the energy stored in the i-th inverter, and calculate the current setpoint i of the i-th inverter. iref ; S2. Measure the q-axis component of the output voltage of the i-th inverter to determine the control phase angle of the i-th inverter; S3. The inverter is controlled by the pulse width modulation (PWM) unit using the modulation signal.

2. The control method according to claim 1, which takes into account both energy storage state-of-charge balance and near-zero line impedance frequency self-synchronization control, is characterized in that, In step S1, the current given value i iref Calculate according to the following formula: ； in, I idref I iqref These are the d-axis and q-axis components of the current given in the control of the i-th inverter, respectively. m i For inverter power output proportional system; k p k i These are the proportional and integral control parameters in the PI controller; V ref This is the rated voltage. V i Let be the output voltage of the i-th inverter; n is the SOC equilibrium coefficient; 'a' represents charging and discharging information; SOC i This represents the state of charge of the i-th inverter.

3. The control method according to claim 1, which takes into account both energy storage state-of-charge balance and near-zero line impedance frequency self-synchronization control, is characterized in that... In step S2, the control phase angle is calculated according to the following formula: ； in, θ i The reference angle generated for the i-th inverter control; ɷ ref and ɷ i These are the angular frequency reference value and the output angular frequency of the i-th inverter, respectively. V iq Let q be the q-axis component of the output voltage of the i-th inverter.

4. The control method according to claim 3, which takes into account both energy storage state-of-charge balance and near-zero line impedance frequency self-synchronization control, is characterized in that, The q-axis component V of the output voltage of the i-th inverter iq Calculate according to the following formula: ; Among them, V ib V is the B-phase voltage of the i-th inverter; ic Let be the C-phase voltage of the i-th inverter.

5. The control method according to claim 4, which takes into account both energy storage state-of-charge balance and near-zero line impedance frequency self-synchronization control, is characterized in that... The B-phase voltage V of the i-th inverter ib and the C-phase voltage V of the i-th inverter ic Calculate according to the following formula: ; Among them, V i Let θ be the output voltage of the i-th inverter; i The reference angle generated for the i-th inverter control.

6. The control method according to claim 2, which takes into account both energy storage state-of-charge balance and near-zero line impedance frequency self-synchronization control, is characterized in that... The charging / discharging information a=1 or 0, where a=1 represents the discharging mode and a=0 represents the charging mode.

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