A method for coordinated control of multiple parallel grid-connected voltage source medium voltage energy storage systems

Abstract

The present application relates to power electronics and energy storage technology, and aims to provide a kind of voltage source type medium voltage energy storage system multi-machine parallel grid coordination control method.It includes: based on the virtual impedance method in the original line and join virtual impedance, make the bus impedance between each battery energy storage system and grid point remain same;According to the adaptive damping parameter method, an adaptive damping module is added to the active frequency link, and the average value of the output power of each virtual synchronous machine is calculated;The new frequency reference value of active-frequency ring is obtained by adaptive damping calculation and is brought into subsequent voltage and current double loop control, and the suppression of active power oscillation is realized by the disposal of virtual impedance.The present application, considering the difference between the hardware parameters and software parameters of multi-machine parallel system, compensates by using virtual impedance method to reduce the active oscillation problem caused by line parameter difference;Through adaptive damping method, dynamic process can be improved, active power oscillation can be suppressed, and system operation stability can be improved.

Description

Technical Field

[0001] This invention relates to the fields of power electronics and energy storage technology, specifically to a coordinated control method for multi-machine parallel grid connection of a voltage source type medium-voltage energy storage system. Background Technology

[0002] In recent years, with the large-scale integration of renewable energy into the grid, the inertia of the power system has been significantly weakened. A power system lacking inertia is prone to frequency fluctuations when subjected to disturbances, making it difficult to maintain a stable operating frequency, which can lead to serious faults such as equipment damage and widespread power outages.

[0003] To address this issue, Battery Energy Storage Systems (BESS) are widely used, effectively improving grid quality and compensating for power fluctuations caused by renewable energy sources. In BESS, Cascaded H-Bridge (CHB) converters are widely employed due to their modular structure and excellent harmonic characteristics. Furthermore, Virtual Synchronous Generator (VSG) technology, by simulating the characteristics of a synchronous machine, can endow energy storage systems with inertia and damping characteristics. By applying VSG technology in CHB-BESS, inertia support can be provided for medium-voltage power grids, thereby enhancing grid stability.

[0004] However, while VSG technology provides the grid with necessary inertia support and damping characteristics, in large-scale energy storage systems with multiple units connected in parallel, the inherent differences (hardware or software parameters) of each VSG can easily lead to power oscillations. Compared to microgrids, power oscillations in medium-voltage systems can cause more severe faults.

[0005] Therefore, suppressing active power oscillations and avoiding power preemption in parallel systems, while maintaining the inertia coefficient and steady-state power distribution of the system, and improving power distribution performance, are crucial issues for the stable operation of multi-unit parallel grid-connected control in medium-voltage energy storage systems. However, no practical solutions have yet been found in publicly available reports. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a coordinated control method for multi-machine parallel grid connection of voltage source type medium-voltage energy storage system.

[0007] To solve the technical problem, the solution of the present invention is:

[0008] A method for coordinated control of multiple parallel grid-connected voltage source medium-voltage energy storage systems is provided. At least two battery energy storage systems (BESS) are simultaneously connected to the grid bus in parallel, and each BESS employs virtual synchronous machine (VSG) technology. Without altering the inertia coefficient and the steady-state value of the system power distribution, the method utilizes virtual impedance to compensate for differences in line parameters, and combines adaptive damping to improve active power oscillation during parallel grid-connected operation. The method specifically includes the following steps:

[0009] (1) Detect the line impedance between each battery energy storage system and the grid connection point;

[0010] (2) Based on the virtual impedance method, virtual impedance is added to the original line to keep the total line impedance between each battery energy storage system and the grid connection point the same.

[0011] (3) In the virtual synchronous machine of each battery energy storage system, an adaptive damping module is added to the active frequency link according to the adaptive damping parameter method, and the average value of the output power of each virtual synchronous machine is calculated.

[0012] (4) Based on the average output power of the virtual synchronous machine and the adaptive damping parameters in the adaptive damping module, a new frequency reference value for the active-frequency loop is obtained through adaptive damping calculation; the new parameter value is brought into the subsequent voltage and current dual-loop control, and combined with the handling of virtual impedance in step (2), the suppression of active power oscillation of the entire parallel grid system is realized.

[0013] (5) Monitor each virtual synchro continuously in each control cycle; if the output active power of the virtual synchro does not change, continue to use the original parameters for control; if the output active power is given a step command, recalculate the new parameter values ​​according to the operations in steps (1) to (4) for control.

[0014] Compared with the prior art, the technical advantages of the present invention are:

[0015] 1. Without changing the inertia coefficient and the steady-state value of the system power distribution, this invention comprehensively considers the differences in hardware and software parameters of a multi-machine parallel system and uses the virtual impedance method for compensation to reduce the active power oscillation problem caused by differences in line parameters.

[0016] 2. This invention improves the dynamic process through an adaptive damping method, which can suppress active power oscillation and improve the operational stability of parallel systems. Attached Figure Description

[0017] Figure 1 This is a simplified topology diagram of a multi-machine parallel system in an embodiment of the present invention;

[0018] Figure 2This is a diagram of the CHB-BESS voltage source control strategy in an embodiment of the present invention;

[0019] Figure 3 These are the dynamic response curves of power and current of each VSG in the dual-machine parallel system in this embodiment of the invention when the active power command increases by a step. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0021] Part One: Implementation Scheme of the Invention

[0022] 1. This invention relates to a coordinated control technology for multi-unit parallel grid-connected voltage source type medium-voltage energy storage systems. In this parallel grid-connected system, at least two battery energy storage systems (BESS) are simultaneously connected to the grid bus in parallel. A simplified topology diagram of the multi-unit parallel system is shown below. Figure 1 As shown. As an example, the battery energy storage system has a three-phase star topology, with each phase consisting of a filter reactor L. f It consists of N energy storage modules connected in series; each energy storage module has the same structure, including an H-bridge, a filter circuit, and a series battery cluster as the energy storage power source arranged in sequence.

[0023] The medium voltage referred to in this invention is relative to high-voltage power grids above 35kV and low-voltage power grids below 10kV. Typically, battery energy storage systems based on cascaded H-bridges are mostly used in medium-voltage power grids. For parallel grid-connected systems operating in a medium-voltage power grid environment, this invention can effectively suppress active power oscillations caused by VSG control.

[0024] Virtual synchronous generator (VSG) technology is employed in all battery energy storage systems. VSG technology is a technique that simulates the electromechanical transient characteristics of a synchronous generator, enabling the inverter to possess the inertia, damping, frequency, and voltage regulation characteristics of a synchronous generator. It can improve grid stability and is suitable for centralized grid connection of new energy power generation, as well as the use of energy storage converters. VSG technology is one of the important technical means for the stable operation of power systems and is well-known to those skilled in the art; therefore, details not involving improvements will not be described further in this invention.

[0025] This invention employs a CHB-BESS voltage source control strategy, in Figure 1The figure illustrates an application example of this control strategy. As shown, its main control units include power calculation, power control, pre-synchronization control, state of charge (SOC) equalization control, voltage and current dual closed-loop control, and a modulation unit. For details of this control strategy, please refer to the published document "Virtual Synchronous Generator Technology and Prospects," by Zheng Tianwen et al. Since this part is also a technical skill well-known to those skilled in the art, the aspects not involved in this invention will not be described further.

[0026] 2. Based on the above technical background, the multi-machine parallel grid-connected coordinated control method of the voltage source type medium-voltage energy storage system described in this invention is to compensate for the differences in line parameters by using the virtual impedance method without changing the inertia coefficient and the steady-state value of system power distribution, and at the same time combine the adaptive damping method to improve the active power oscillation problem in the parallel grid-connected operation process.

[0027] The method specifically includes the following steps:

[0028] (1) Detect the line impedance between each battery energy storage system and the grid connection point, and record it as the line impedance X. 1i , i = 1, 2, ..., N.

[0029] (2) Based on the virtual impedance method and line impedance X 1i Virtual impedance is added to the existing line to keep the total line impedance between each battery energy storage system and the grid connection point the same.

[0030] The principle of the virtual impedance method is to add a virtual impedance to the existing circuit, making the circuit impedance approximately inductive and reducing the unbalanced impedance between devices. Some researchers have used virtual impedance to make the system output impedance inductive. This virtual impedance includes a virtual negative resistance (VNR) and a virtual positive inductance (VPI). VNR is used to cancel the influence of the resistive component of the circuit, and VPI is used to increase the inductive reactance component of the inverter's equivalent output impedance, thereby achieving power decoupling control and effective power distribution.

[0031] Specifically, assume that the original line impedance between the battery energy storage system and the grid connection point is X. 1i Virtual impedance is added to each system using the virtual impedance method. vi This ensures that the total line impedance between each battery energy storage system and the grid connection point remains consistent; the specific formula is as follows:

[0032] X 11 +X v1 =X 12 +X v2 =…=X 1i +X vi =…=X 1n +X vn

[0033] In the formula, X 1i X represents the line impedance for each battery energy storage system. vi The virtual impedance value of VSG in each system is set; i = 1, 2, ..., N, where N is the number of battery energy storage systems in the parallel grid-connected system.

[0034] (3) In the virtual synchronous machine of each battery energy storage system, an adaptive damping module is added to the active frequency link according to the adaptive damping parameter method, and the average value of the output power of each virtual synchronous machine is calculated.

[0035] The adaptive damping module added to the active frequency stage is shown below:

[0036] D p (P avg -P ei )

[0037] Among them, D p It is the adaptive damping parameter; P avg P represents the average output power of each VSG in the parallel grid-connected system. ei Let i be the output active power of each VSG; i = 1, 2, ..., N.

[0038] The average output power P of each VSG avg It is calculated by the following formula:

[0039]

[0040] (4) Based on the average output power P of the virtual synchronizer avg and adaptive damping parameter D p The new frequency reference value ω of the active-frequency loop is obtained through adaptive damping calculation; the new parameter value is then incorporated into the subsequent voltage-current dual-loop control, and combined with the handling of virtual impedance in step (2), the active power oscillation of the entire parallel grid-connected system is suppressed; specifically,

[0041] Adaptive damping calculations are performed based on the following adaptive damping equations of motion:

[0042]

[0043] Among them, P m It is mechanical power; P e It is electromagnetic power; D is the damping coefficient of VSG control; ω is the output angular velocity of VSG; ω s It is the grid voltage angular velocity; J is the inertia coefficient of VSG control; D p It is the adaptive damping parameter; P avg P represents the average output power of each VSG in the parallel grid-connected system.ei Let i be the output active power of each VSG; i = 1, 2, ..., N.

[0044] The control method described in this invention is mainly aimed at the active-frequency loop. By using the above-mentioned adaptive damping self-motion equation, a new reference value for the active-frequency loop can be obtained, namely the VSG output angular velocity, which is denoted by ω.

[0045] The adaptive damping parameter D for each VSG pi The following requirements should be met:

[0046]

[0047] Among them, J i The moment of inertia of each VSG is represented by T, which represents one calculation cycle; K pi D represents the active power factor of each VSG. i This represents the damping parameter for each VSG; i = 1, 2, ..., N.

[0048] (5) Monitor each virtual synchro continuously in each control cycle; if the output active power of the virtual synchro does not change, continue to use the original parameters for control; if the output active power is given a step command, recalculate the new parameter values ​​according to the operations in steps (1) to (4) for control.

[0049] The active power step command specifically refers to the upper-level dispatching system outputting an active power step command to the parallel-connected VSG system when the entire power grid needs more active power.

[0050] 3. In the voltage source type medium-voltage energy storage system of the present invention, its controller module includes a processor and a computer-readable storage medium, on which a computer program is stored; when the computer program is executed by the processor, it can implement the control method as described above.

[0051] Part Two: A Specific Application Example

[0052] For ease of understanding, this example demonstrates the construction of a circuit topology and control model for a VSG-controlled parallel grid system in the PLECS simulation software. This method verifies the feasibility and corresponding technical effects of the invention. Specifically, a simulation was performed on two VSG-based CHB-BESS parallel systems.

[0053] In this example, each phase consists of a filter inductor and 40 energy storage modules connected in series. Each energy storage module has the same structure, including an H-bridge, a filter circuit, and a series-connected battery cluster (composed of 300 cells connected in series, each cell with a rated voltage of 3.2V and a rated capacity of 280Ah) as the energy storage power source. The operating conditions are set as follows: total rated power P * The total capacity is 20MW, with each VSG having a rated power of 10MW and a rated frequency of 50Hz.

[0054] The system power command P is set at t = 1.5s. * When the power output changes from 0 to 20MW, the VSG operates at its rated power due to power distribution. At t = 8.5s, the system power command changes from 20MW to 30MW.

[0055] Based on the above conditions, the multi-unit parallel grid-connected coordinated control method for the voltage source type medium-voltage energy storage system in this example is executed with reference to the operation steps in Part 1:

[0056] (1) Detect the line impedance between each battery energy storage system and the grid connection point;

[0057] (2) Based on the virtual impedance method, virtual impedance is added to the original line to keep the total line impedance between each battery energy storage system and the grid connection point the same.

[0058] (3) In the virtual synchronous machine of each battery energy storage system, an adaptive damping module is added to the active frequency link according to the adaptive damping parameter method, and the average value of the output power of each virtual synchronous machine is calculated.

[0059] (4) Based on the average output power of the virtual synchronous machine and the adaptive damping parameters in the adaptive damping module, a new frequency reference value for the active-frequency loop is obtained through adaptive damping calculation; the new parameter value is brought into the subsequent voltage and current dual-loop control, and combined with the handling of virtual impedance in step (2), the suppression of active power oscillation of the entire parallel grid system is realized.

[0060] As can be seen from the equations of motion derived from adaptive damping, the existence of the adaptive damping parameter affects the frequency change rate of the VSG. Taking the two-machine parallel system in this example as an example, we can obtain:

[0061]

[0062] Furthermore, the frequency variation of the two VSGs within one calculation period T should not exceed the difference in their initial frequencies. Therefore, the following relationship exists:

[0063]

[0064] Based on the two relationships above, the adaptive damping parameter D for each VSG can be obtained.pi The following requirements should be met:

[0065]

[0066] The above derivation process determines the range of values ​​for the adaptive damping parameters; then, the adaptive damping function is further realized by transforming the adaptive damping from the equation of motion, i.e., by adding -D. p (P avg -P ei The adaptive damping is achieved by using the following terms.

[0067] (5) Monitor each virtual synchro continuously in each control cycle; if the output active power of the virtual synchro does not change, continue to use the original parameters for control; if the output active power is given a step command, recalculate the new parameter values ​​according to the operations in steps (1) to (4) for control.

[0068] The simulation comparison results in this example are as follows: Figure 3 As shown in the figure: the top left is the power change curve without optimized control, and the top right is the power change curve based on virtual impedance and adaptive damping control; the bottom left is the VSG output current waveform without optimized control, and the bottom right is the VSG output current waveform based on virtual impedance and adaptive damping control.

[0069] Without optimized control, as shown in the upper left of the figure, each system power change results in two instances of significant power difference. When VSG2 outputs more power than VSG1, the maximum power difference is 2.3MW and 1.2MW, respectively. When VSG1 outputs more power than VSG2, the maximum power difference is 6.1MW and 3.1MW, respectively, representing 61% and 31% of the rated power. The corresponding VSG output current waveforms are shown in the lower left of the figure. After adopting the control method described in this invention, the power difference between the two units is significantly reduced, oscillations are suppressed, and both VSGs smoothly reach their rated power (as shown in the upper right of the figure). When VSG2 outputs more power than VSG1, the maximum power difference is suppressed to 1.3MW, which is 56% of the difference without optimized control. When VSG1 outputs more power than VSG2, the maximum power difference is suppressed to 0.2MW, which is 3.3% of the difference without optimized control. At t = 8.5s, the power difference at the two moments with large power differences was suppressed to 66% and 6.5% of that without optimization control, respectively, and the corresponding VSG output current waveform is shown in the lower right of the figure.

[0070] In summary, the technical solution of this invention comprehensively considers the hardware and software parameters of the multi-machine parallel system. It reduces the active power oscillation problem caused by line parameter differences by using the virtual impedance method. In addition, it improves the dynamic process by using an adaptive damping method, which suppresses the active power oscillation problem to a certain extent and improves the operational stability of the parallel system.

Claims

1. A coordinated control method for multi-unit parallel grid connection of a voltage source type medium-voltage energy storage system, characterized in that, At least two battery energy storage systems are connected to the grid bus in parallel. Each battery energy storage system (BESS) adopts virtual synchronous machine (VSG) technology. Without changing the inertia coefficient and the steady-state value of the system power distribution, the difference in line parameters is compensated by the virtual impedance method. At the same time, the active power oscillation problem during parallel grid operation is improved by combining the adaptive damping method. The method specifically includes the following steps: (1) Detect the line impedance between each battery energy storage system and the grid connection point; (2) Based on the virtual impedance method, virtual impedance is added to the original line so that the total line impedance between each battery energy storage system and the grid connection point remains the same. (3) In the virtual synchronous machine of each battery energy storage system, an adaptive damping module is added to the active frequency link according to the adaptive damping parameter method, and the average value of the output power of each virtual synchronous machine is calculated. The adaptive damping module is shown below: ； Among them, D p It is an adaptive damping parameter; This represents the average output power of each VSG in the parallel grid-connected system. The output active power of each VSG; ; (4) Based on the average output power of the virtual synchronous machine and the adaptive damping parameters in the adaptive damping module, a new frequency reference value for the active-frequency loop is obtained through adaptive damping calculation; the new parameter value is brought into the subsequent voltage and current dual-loop control, and combined with the handling of virtual impedance in step (2), the active power oscillation of the entire parallel grid system is suppressed. Specifically, adaptive damping calculations are performed based on the following adaptive damping equations of motion: ； Among them, P m It is mechanical power; P e It is electromagnetic power; D is the damping coefficient controlled by VSG; It is the angular velocity output by the VSG; ω is the grid voltage angular velocity; J is the inertia coefficient of the VSG control. (5) Monitor each virtual synchro continuously in each control cycle; if the output active power of the virtual synchro does not change, continue to use the original parameters for control; if the output active power is given a step command, recalculate the new parameter values ​​according to the operations in steps (1) to (4) for control.

2. The method according to claim 1, characterized in that, The battery energy storage system has a three-phase star topology, with each phase consisting of a filter reactor L. f It consists of N energy storage modules connected in series; each energy storage module has the same structure, including an H-bridge, a filter circuit, and a series battery cluster as the energy storage power source arranged in sequence.

3. The method according to claim 1, characterized in that, In step (2), it is assumed that the original line impedance between the battery energy storage system and the grid connection point is... Virtual impedance is added to each system using the virtual impedance method. This ensures that the total line impedance between each battery energy storage system and the grid connection point remains consistent; the specific formula is as follows: ； In the formula, The line impedance corresponding to each battery energy storage system, The virtual impedance value for the VSG in each system is determined; N represents the number of battery energy storage systems in the parallel grid-connected system.

4. The method according to claim 1, characterized in that, The average output power of each VSG It is calculated by the following formula: ； in, The output active power of each VSG; .

5. The method according to claim 1, characterized in that, In step (4), the adaptive damping parameters of each VSG The following requirements should be met: ； in, The moment of inertia of each VSG is represented by T, which represents one calculation cycle. This represents the active power coefficient of each VSG. This indicates the damping parameters for each VSG unit; .

6. The method according to claim 1, characterized in that, In step (5), the active power step command specifically refers to the upper-level dispatching system outputting an active power step command to the parallel grid-connected VSG system when the entire power grid needs more active power.

7. A voltage source type medium-voltage energy storage system capable of achieving coordinated control of multiple units connected in parallel and grid-connected, characterized in that, At least two battery energy storage systems are connected to the grid bus in parallel. Each battery energy storage system (BESS) uses virtual synchronous generator (VSG) technology. The battery energy storage system has a three-phase star topology, with each phase consisting of a filter reactor L. f It consists of N energy storage modules connected in series; each energy storage module has the same structure, including an H-bridge, a filter circuit, and a series battery cluster as the energy storage power source arranged in sequence. The controller module of the system includes a processor and a computer-readable storage medium on which a computer program is stored; when the computer program is executed by the processor, it is able to implement the control method as described in any one of claims 1 to 6.

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