Multi-machine parallel grid-connected coordination control method for voltage source type medium-voltage energy storage system

Abstract

The invention relates to a power electronic technology and an energy storage technology, and aims to provide a multi-machine parallel grid-connected coordination control method for a voltage source type medium-voltage energy storage system. Comprising the following steps: adding virtual impedance into an original line based on a virtual impedance method, and keeping the total line impedance between each battery energy storage system and a grid-connected point to be the same; adding a self-adaptive damping module in an active frequency link according to a self-adaptive damping parameter method, and calculating an average value of output power of each virtual synchronous machine; and obtaining a new frequency reference value of an active-frequency loop through adaptive damping calculation, substituting the new frequency reference value into subsequent voltage and current double-loop control, and realizing suppression of active power oscillation in combination with treatment of virtual impedance. According to the method, the difference between the hardware parameters and the software parameters of the multi-machine parallel system is comprehensively considered, and compensation is performed by using the virtual impedance method to reduce the active oscillation problem caused by the line parameter difference; the dynamic process is improved through a self-adaptive damping method, active power oscillation can be suppressed, and the system operation stability is improved.

Description

Technical Field

[0001] The present invention relates to the fields of power electronics technology and energy storage technology, and in particular to a voltage source type medium voltage energy storage system multi-machine parallel grid-connected coordinated control method. Background Art

[0002] In recent years, with the large-scale access of renewable energy to the power grid, the inertia of the power system has been significantly reduced. When a power system lacks inertia, it is difficult for the system frequency to remain within the stable operating range when it is disturbed, which may lead to serious faults such as equipment damage and large-scale power outages.

[0003] In order to solve this problem, the battery energy storage system (BESS) is widely used, which can effectively improve the quality of the power grid and compensate for the power fluctuations caused by renewable energy. In BESS, the cascaded H-bridge (CHB) converter is widely used due to its modular structure and good harmonic characteristics. In addition, the virtual synchronous generator (VSG) technology can make the energy storage system have inertia and damping characteristics by simulating the characteristics of the synchronous machine. By applying VSG technology in CHB-BESS, it can provide inertia support for the medium-voltage power grid, thereby enhancing the stability of the power grid.

[0004] However, although VSG technology brings necessary inertia support and damping characteristics to the grid, in the scenario of multiple machines connected in parallel in large-scale energy storage systems, the power disturbance generated can easily cause power oscillations due to the inherent differences (hardware parameters or software parameters) of each VSG. Compared with microgrids, power oscillations in medium-voltage systems may cause more serious faults.

[0005] Therefore, under the premise of not changing the inertia coefficient and the steady-state value of the system power distribution, suppressing the oscillation of active power, avoiding the occurrence of power rush in the parallel system, and improving the power distribution performance are issues that cannot be ignored in the stable operation of multi-machine parallel grid-connected control of medium-voltage energy storage systems. However, no practical solutions have been seen in the current public reports. Summary of the invention

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

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

[0008] A coordinated control method for multi-machine parallel grid connection of a voltage source medium-voltage energy storage system is provided. At least two battery energy storage systems are connected to the grid bus in parallel, and each battery energy storage system (BESS) adopts virtual synchronous machine (VSG) technology. Under the premise of not changing the inertia coefficient and the steady-state value of system power distribution, the virtual impedance method is used to compensate for the difference in line parameters, and the adaptive damping method is combined to improve the active oscillation problem in the parallel grid connection operation process. 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) Add virtual impedance to the original line based on the virtual impedance method 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 to calculate the average output power of each virtual synchronous machine;

[0012] (4) Based on the average value of the virtual synchronous machine output power and the adaptive damping parameters in the adaptive damping module, a new frequency reference value of 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 treatment of the virtual impedance in step (2), the active power oscillation of the entire parallel grid-connected system is suppressed;

[0013] (5) Continuously monitor each virtual synchronous machine in each control cycle; if the output active power of the virtual synchronous machine does not change, continue to use the original parameters for control; if its output active power has a step instruction, recalculate new parameter values ​​for control according to the operations in steps (1) to (4).

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

[0015] 1. Under the premise of not changing the inertia coefficient and the steady-state value of system power distribution, the present invention comprehensively considers the differences between the hardware parameters and software parameters of the multi-machine parallel system, and uses the virtual impedance method for compensation to reduce the active oscillation problem caused by the line parameter differences.

[0016] 2. The present invention improves the dynamic process through an adaptive damping method, which can suppress the active power oscillation problem and improve the operating stability of the parallel system. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0019] Figure 3 It is the power and current dynamic response curve of each VSG when the active power command of the dual-machine parallel system in the embodiment of the present invention is stepped. DETAILED DESCRIPTION

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

[0021] Part I Implementation of the Invention

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

[0023] The medium voltage mentioned in the present invention refers to the high voltage grid above 35KV and the low voltage grid below 10KV. Generally, the battery energy storage system based on the cascade H-bridge is mostly used in the medium voltage grid. For the parallel grid-connected system running in the medium voltage grid environment, the present invention can well suppress the active power oscillation problem generated by VSG control.

[0024] In each battery energy storage system, virtual synchronous machine (VSG) technology is used. Virtual synchronous machine technology is a technology that simulates the electromechanical transient characteristics of a synchronous generator to enable the inverter to have the operating characteristics of a synchronous generator such as inertia, damping, frequency and voltage regulation. It can improve the stability of the power grid and is suitable for the centralized grid connection of renewable energy power generation and the use of energy storage inverters. Virtual synchronous machine technology is one of the important technical means for the stable operation of the power system, and it is a content that is mastered by those skilled in the art, so the content that is not involved in the improvement of this invention will not be described in detail.

[0025] The present invention applies the CHB-BESS voltage source control strategy. Figure 1An application example of the control strategy is specifically shown in the figure. As shown in the figure, its main control unit includes power calculation, power control, pre-synchronization control, state of charge (SOC) balancing control, voltage and current dual closed-loop control and modulation unit. The specific content of the control strategy can be referred to the public document "Virtual Synchronous Generator Technology and Prospects, Author Zheng Tianwen, etc." Since this part of the content also belongs to the technical means mastered by those skilled in the art, the content that is not improved by the present invention will not be described in detail.

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

[0027] The method specifically comprises the following steps:

[0028] (1) Detect the line impedance between each battery energy storage system and the grid connection point, denoted as 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 original 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 original line to make the line impedance approximately inductive and reduce the unbalanced impedance between devices. Some scholars have set up virtual impedance to make the system output impedance inductive. The virtual impedance includes virtual negative resistance (VNR) and virtual positive inductance (VPI). VNR is used to offset the influence of line resistance components, and VPI is used to increase the inductive reactance component of the inverter equivalent output impedance, thereby achieving power decoupling control and effective power distribution.

[0031] Specifically, assuming that the original line impedance between the battery energy storage system and the grid connection point is X 1i , using the virtual impedance method to add virtual impedance to each system, X vi , so that the total line impedance value 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] Where, X 1i is the line impedance corresponding to each battery energy storage system, X vi is the virtual impedance value of VSG in each system; 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 to calculate the average output power of each virtual synchronous machine.

[0035] The adaptive damping module added in the active frequency link is as follows:

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

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

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

[0039]

[0040] (4) Based on the average output power P of the virtual synchronous machine avg and the 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 brought into the subsequent voltage and current dual-loop control, and combined with the treatment of the virtual impedance in step (2), the active power oscillation of the entire parallel grid-connected system is suppressed; specifically,

[0041] The adaptive damping calculation is based on the following adaptive damping self-motion equation:

[0042]

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

[0044] The control method of the present invention is mainly aimed at the active power-frequency loop. Through the above-mentioned adaptive damping self-motion equation, a new reference value of the active power-frequency loop can be obtained, that is, the VSG output angular velocity, whose symbol is ω.

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

[0046]

[0047] Among them, J i represents the moment of inertia of each VSG, T represents a calculation cycle; K pi Indicates the active coefficient of each VSG, D i Represents the damping parameter of each VSG; i=1,2,…,N.

[0048] (5) Continuously monitor each virtual synchronous machine in each control cycle; if the output active power of the virtual synchronous machine does not change, continue to use the original parameters for control; if its output active power has a step instruction, recalculate new parameter values ​​for control according to the operations in steps (1) to (4).

[0049] The active power step command specifically means that when the entire power grid needs more active power, the superior dispatching system outputs an active power step command to the parallel grid-connected VSG system.

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

[0051] Part II: A specific application example

[0052] For ease of understanding, this example constructs a circuit topology and control model based on VSG-controlled parallel grid-connected system in PLECS simulation software to verify the specific feasibility and corresponding technical effects of the present invention. Specifically, a simulation is 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 in series. The structure of each energy storage module is the same, including an H-bridge, a filter circuit, and a series battery cluster (consisting of 300 cells in series, each with a rated voltage of 3.2V and a rated capacity of 280Ah) as an energy storage power source. The operating conditions are set as follows: total rated power P * The total power capacity is 20MW, of which each VSG has a rated power of 10MW and a rated frequency of 50Hz.

[0054] Set the system to t = 1.5s, the system power command P * From 0 to 20MW, due to power sharing, VSG operates at rated power. At t=8.5s, the system power command changes from 20MW to 30MW.

[0055] Based on the above condition settings, the voltage source medium voltage energy storage system multi-machine parallel grid-connected coordinated control method of this example is performed with reference to the operating steps of the first part:

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

[0057] (2) Add virtual impedance to the original line based on the virtual impedance method 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 to calculate the average output power of each virtual synchronous machine;

[0059] (4) Based on the average value of the virtual synchronous machine output power and the adaptive damping parameters in the adaptive damping module, a new frequency reference value of 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 treatment of the virtual impedance in step (2), the active power oscillation of the entire parallel grid-connected system is suppressed;

[0060] It can be seen from the adaptive damping self-motion equation that 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, it can be obtained:

[0061]

[0062] In addition, the frequency change of the two VSGs should not exceed the difference in initial frequency within a calculation period T. Therefore, the following relationship exists:

[0063]

[0064] According to the above two relationships, the adaptive damping parameter D of each VSG can be obtained:pi The following requirements should be met:

[0065]

[0066] The above derivation process can determine the range of the adaptive damping parameter; then, the adaptive damping function can be realized by converting the adaptive damping self-motion equation, that is, by adding -D p (P avg -P ei ) item to achieve adaptive damping.

[0067] (5) Continuously monitor each virtual synchronous machine in each control cycle; if the output active power of the virtual synchronous machine does not change, continue to use the original parameters for control; if its output active power has a step instruction, recalculate new parameter values ​​for control according to the operations in steps (1) to (4).

[0068] The simulation comparison results in this example are as follows Figure 3 In the figure: the upper left is the power variation curve without optimization control, the upper right is the power variation curve based on virtual impedance and adaptive damping control; the lower left is the VSG output current waveform without optimization control, the lower 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 time the system power changes, there will be two moments with large power differences. When the output power of VSG2 is larger than that of VSG1, the maximum power difference is 2.3MW and 1.2MW respectively. When the output power of VSG1 is larger than that of VSG2, the maximum power difference is 6.1MW and 3.1MW respectively. The difference is 61% and 31% of the rated power. The corresponding VSG output current waveform is shown in the lower left of the figure. After changing to the control method described in the present invention, the output power difference between the two machines is significantly reduced, the oscillation is suppressed, and the two VSGs smoothly reach the rated power (as shown in the upper right of the figure). When the output power of VSG2 is larger than that of VSG1, the maximum power difference is suppressed to 1.3MW, which is 56% of the case without optimized control. When the output power of VSG1 is larger than that of VSG2, the maximum power difference is suppressed to 0.2MW, which is 3.3% of the case without optimized control. At t=8.5s, the power difference between the two moments with large power difference is suppressed to 66% and 6.5% of that without optimized control, respectively. The corresponding VSG output current waveform is shown in the lower right figure.

[0070] In summary, the technical solution of the present invention comprehensively considers the hardware parameters and software parameters of the multi-machine parallel system, reduces the active power oscillation problem caused by line parameter differences by utilizing the virtual impedance method, and improves the dynamic process by using the adaptive damping method, thereby suppressing the active power oscillation problem to a certain extent and improving the operating stability of the parallel system.

Claims

1. A coordinated control method for multiple parallel-connected machines in a voltage source medium voltage energy storage system, characterized in that: At least two battery energy storage systems are connected to the grid bus in parallel, and each battery energy storage system BESS adopts virtual synchronous machine VSG technology; under the premise of not changing the inertia coefficient and the steady-state value of system power distribution, the virtual impedance method is used to compensate for the difference in line parameters, and the adaptive damping method is combined to improve the active oscillation problem during the parallel grid-connected operation; the method specifically includes the following steps: (1) Detect the line impedance between each battery energy storage system and the grid connection point; (2) Add virtual impedance to the original line based on the virtual impedance method to keep the total line impedance between each battery energy storage system and the grid connection point 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 to calculate the average output power of each virtual synchronous machine; (4) Based on the average value of the virtual synchronous machine output power and the adaptive damping parameters in the adaptive damping module, a new frequency reference value of 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 treatment of the virtual impedance in step (2), the active power oscillation of the entire parallel grid-connected system is suppressed; (5) Continuously monitor each virtual synchronous machine in each control cycle; if the output active power of the virtual synchronous machine does not change, continue to use the original parameters for control; if its output active power has a step instruction, recalculate new parameter values ​​for control according to the operations in steps (1) to (4).

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

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 X 1i , using the virtual impedance method to add virtual impedance to each system, X vi , so that the total line impedance value between each battery energy storage system and the grid connection point remains consistent; the specific formula is as follows: X 11 +X v1 =X 12 +X v2 =…=X 1i +X vi =…=X 1n +X vn In the formula, X 1i is the line impedance corresponding to each battery energy storage system, X vi is the virtual impedance value of VSG in each system; i = 1, 2, ..., N, where N is the number of battery energy storage systems in the parallel grid-connected system.

4. The method according to claim 1, characterized in that: In step (3), the adaptive damping module added to the active frequency link is as follows: D p (P avg -P ei ) Among them, D p is the adaptive damping parameter; P avg is the average output power of each VSG in the parallel grid-connected system; P ei is the output active power of each VSG; i=1,2,...,N.

5. The method according to claim 1, characterized in that In the step (4), adaptive damping calculation is performed based on the following adaptive damping self-motion equation: Among them, P m is the mechanical power; P e is the electromagnetic power; D is the damping coefficient of VSG control; ω is the VSG output angular velocity; ω s is the grid voltage angular velocity; J is the inertia coefficient of VSG control; D p is the adaptive damping parameter; P avg is the average output power of each VSG in the parallel grid-connected system; P ei is the output active power of each VSG; i=1,2,…,N.

6. The method according to claim 5, characterized in that The average output power P of each VSG avg , is calculated by the following formula: Among them, P ei is the output active power of each VSG; i=1,2,…,N.

7. The method according to claim 1, characterized in that In step (4), the adaptive damping parameter D of each VSG is pi The following requirements should be met: Among them, J i represents the moment of inertia of each VSG, T represents a calculation cycle; K pi Indicates the active power coefficient of each VSG, D i Represents the damping parameter of each VSG; i=1,2,…,N.

8. The method according to claim 1, characterized in that In step (5), the active power generates a step instruction, which specifically means that when the entire power grid needs more active power, the superior dispatching system outputs an active power step instruction to the parallel grid-connected VSG system.

9. A voltage source medium voltage energy storage system capable of realizing coordinated control of multiple machines in parallel and grid connection, characterized in that: At least two battery energy storage systems are connected to the grid bus in parallel, and each battery energy storage system BESS adopts virtual synchronous machine VSG technology; the battery energy storage system has a three-phase star structure, and each phase is connected by a filter reactor L f and 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 an 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, the control method as described in any one of claims 1 to 8 can be implemented.

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