A grid-type control method and system for a high-voltage direct-mounted cascade energy storage system

Abstract

The present invention relates to the grid-connected control technology of the converter, and aims to provide a grid-connected control method and system for a high-voltage direct-mounted cascade energy storage system. The method includes three parts: grid-connected control, equivalent internal voltage control, and output current control; the internal voltage phase and amplitude reference values ​​are obtained by sampling and calculating the active power and reactive power output by the system; the equivalent internal voltage is adjusted by a proportional-integral controller, and the output current is adjusted by a proportional-integral controller; the internal voltage phase reference value is used for coordinate transformation to obtain a three-phase modulation voltage reference value, and the H-bridge of each module in the high-voltage direct-mounted cascade energy storage system is driven to generate voltage based on the reference value. The method can improve the current control capability of the system, enhance the stability of the system, and improve the overall performance of the system; the present invention constructs the internal voltage equivalently by feedback of the modulation voltage reference value, and can be directly applied without the need for additional internal voltage sampling or other hardware changes, which can effectively reduce costs and losses.

Description

Technical Field

[0001] The present invention relates to the technical field of converter grid-connected control, and in particular to a grid-connected control method and system for a high-voltage direct-mounted cascade energy storage system. Background Art

[0002] As the proportion of renewable energy generation in the power system gradually increases, large-capacity energy storage systems are widely used to solve the volatility and intermittency problems of renewable energy generation. In order to improve the single-unit capacity and voltage level of the energy storage system, researchers proposed a high-voltage direct-mounted converter based on the cascaded H-bridge topology; because of its large capacity, high efficiency, redundancy and scalability, it has become the preferred solution for grid-connected converters for large-capacity energy storage systems.

[0003] Current grid-connected converters usually use grid-following control based on a phase-locked loop, and their operation depends on stable grid voltage. However, this type of grid-following converter has insufficient support for the grid, and is prone to losing stability, especially under weak grid conditions. Therefore, the access of large-scale grid-following converters will cause the power system to face stability problems such as reduced inertia and weakened strength. To meet this challenge, grid-forming control operates the converter as a voltage source, enabling it to have the ability to actively support the grid frequency and voltage similar to a synchronous generator, becoming a key technology for solving power system stability problems. At present, some researchers have proposed some grid-forming control schemes for high-voltage direct-mounted converters:

[0004] For example, Chinese patent application CN115051404A proposes an AC voltage control method for a high-voltage grid-type converter. In view of the problem that the traditional dq dual-loop voltage control cannot be applied to a high-voltage grid-type converter without an AC capacitor filter, a voltage-current proportional control method is proposed. This method introduces a voltage-current proportional relationship and forms a cascade control with an inner-loop current controller, uses an inner-loop positive-sequence current controller to limit the positive-sequence current, and uses an inner-loop negative-sequence current controller to suppress the negative-sequence current to limit the fault current. However, since this method lacks integral control, it may affect the accuracy of power control.

[0005] For another example, Chinese patent application CN116961055A proposes a control method and system for a high-voltage direct-mounted battery energy storage system that supports the operation of a wind farm voltage source. For a high-voltage direct-mounted battery energy storage system that adopts a single-inductor filter topology, voltage inner loop control and current limiting control are proposed to achieve grid-connected point voltage control and fault current limiting. When the current does not reach the limit value, the voltage loop controls the grid-connected point voltage to track the internal potential amplitude and phase; when the current reaches the limit value, the current is limited by the maximum and minimum value functions, and the current control is switched to ensure that the grid-side current does not over-current. However, this method involves switching between voltage and current control, and its control complexity is relatively high.

[0006] For another example, Chinese patent application CN117674174A proposes a control method and system for a meshed STATCOM, which combines meshed technology with STATCOM, controls the voltage amplitude and phase angle at the grid connection point, and makes it have voltage source characteristics. It generates an adaptive virtual impedance based on the output current measurement value of the meshed STATCOM and the maximum allowable overload multiple, calculates the virtual impedance voltage based on the adaptive virtual impedance, and limits the short-circuit current during the fault. However, this adaptive virtual impedance is difficult to completely replace the current closed-loop control in the traditional dual-loop control.

[0007] In summary, although existing research results have proposed some grid-type control methods for high-voltage direct-mounted converters with single inductance grid connection, so that they do not rely on high-voltage capacitor filters; however, they still have the following disadvantages when facing actual application scenarios:

[0008] (1) Current control problem: Although existing methods can limit the current of the converter during faults, they lack effective direct current control during normal operation of the converter. In addition, methods such as virtual impedance to increase current feedback are limited by control accuracy and cannot completely replace current closed-loop control.

[0009] (2) Stability problem: The existing grid-connection control method mainly realizes the grid-connection characteristics by feedback adjustment of the grid connection point voltage, and its stability is heavily dependent on the grid inductance. Under the condition of a rigid grid with small grid impedance and grid connection point voltage clamped by the grid, the system faces stability problems.

[0010] The present invention intends to propose a new type of grid-connected control technology for a high-voltage direct-mounted cascade energy storage system to solve the above technical problems. Summary of the invention

[0011] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a grid-type control method and system for a high-voltage direct-mounted cascade energy storage system.

[0012] To solve the technical problem, the solution of the present invention is:

[0013] A grid-type control method for a high-voltage direct-mounted cascade energy storage system is provided, comprising the following steps:

[0014] (1) Network control

[0015] Sampling and calculating the output active power and reactive power of the high-voltage direct-mounted cascade energy storage system; obtaining the internal voltage phase reference value and the internal voltage amplitude reference value respectively through the calculation of the active power control law and the reactive power control law;

[0016] (2) Equivalent internal voltage control

[0017] According to the modulation voltage reference value of the previous control cycle, the equivalent internal voltage is calculated; the virtual impedance voltage is calculated; the internal voltage amplitude reference value and 0 are used as the given values ​​of the internal voltage d-axis and q-axis respectively; the equivalent internal voltage is used as voltage feedback, and the equivalent internal voltage is adjusted by a proportional integral controller to obtain a first reference value of the current;

[0018] (3) Output current control

[0019] The second reference values ​​of the current of the d-axis and the q-axis are calculated according to the first reference value of the current and the current limit value as the current given value; the output current is used as the current feedback, and the output current is adjusted by the proportional integral controller to obtain the modulation voltage reference value; the internal voltage phase reference value is used to perform coordinate transformation to obtain the three-phase modulation voltage reference value, and based on the reference value, the H-bridge of each module in the high-voltage direct-mounted cascade energy storage system is driven to generate voltage.

[0020] As a preferred embodiment of the present invention, the high voltage direct-mounted cascade energy storage system has a three-phase star structure, each phase of which 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.

[0021] As a preferred solution of the present invention, the equivalent internal voltage control and output current control are vector controls in the same dq rotating coordinate system, and the d-axis direction of the dq rotating coordinate system is consistent with the direction of the equivalent internal voltage.

[0022] As a preferred embodiment of the present invention, the step (1) specifically comprises:

[0023] (1.1) Sampling and power calculation:

[0024] The three-phase voltage of the acquisition system grid connection point , , And the system's three-phase output current , , ; According to the collected data, calculate the system output active power and reactive power ;

[0025] (1.2) Active power control:

[0026] Given system active power , combined with the system output active power and the grid rated angular frequency , the internal voltage phase reference is calculated by the active power control law ;

[0027] (1.3) Reactive power control:

[0028] Given system reactive power , combined with the system output reactive power , reactive power reference value and rated output voltage amplitude , the internal voltage amplitude reference value is calculated by the reactive power control law .

[0029] As a preferred embodiment of the present invention, in step (1),

[0030] The active power control law is shown as follows:

[0031]

[0032]

[0033] in, is the internal voltage angular frequency reference value; is the internal voltage phase reference value; is the inertia coefficient; is the Laplace operator; is the damping coefficient;

[0034] Reactive power reference value The control law is as follows:

[0035]

[0036] The reactive power control law is as follows:

[0037]

[0038] Among them, K VQ is the reactive power droop coefficient; V m is the voltage amplitude at the grid connection point; K pQ is the reactive power control proportional coefficient; K iQ is the reactive power control integral coefficient; s is the Laplace operator.

[0039] As a preferred embodiment of the present invention, the step (2) specifically comprises:

[0040] (2.1) Extract the modulation voltage reference value of the previous control cycle in the high-voltage direct-mounted cascade energy storage system controller module, and calculate the equivalent internal voltage according to the following formula:

[0041]

[0042] in, and denote the equivalent internal voltages of the d-axis and q-axis respectively; and Respectively represent the modulation voltage reference values ​​of the d-axis and q-axis of the previous control cycle; represents the modulation equivalent gain;

[0043] (2.2) Set the system virtual resistance and virtual inductance, and calculate the virtual impedance voltage according to the following formula:

[0044]

[0045] in, and represent the virtual impedance voltage of d-axis and q-axis respectively; and Represent the d-axis and q-axis output currents respectively; represents a virtual resistor; represents virtual inductance;

[0046] (2.3) Equivalent internal voltage control:

[0047] The closed-loop regulation of the equivalent internal voltage is realized according to the following proportional-integral control law, and the first reference value of the current is calculated:

[0048]

[0049] in, and Respectively represent the first reference values ​​of the current of the d-axis and the q-axis; and They represent the proportional coefficient and integral coefficient of the equivalent internal voltage control respectively; s is the Laplace operator; It is the internal voltage amplitude reference value.

[0050] As a preferred embodiment of the present invention, the step (3) specifically comprises:

[0051] (3.1) Current reference value limit:

[0052] Given the maximum current limit value , calculate the second reference value of current according to the following formula:

[0053]

[0054] in, and Respectively represent the first reference values ​​of the current of the d-axis and the q-axis; and denote the second reference values ​​of the current of the d-axis and the q-axis respectively; the superscript T denotes the matrix transposition;

[0055] (3.2) Output current control:

[0056] The calculated second reference value of the current is used as the given value of the d-axis and q-axis current respectively, and the output current of the d-axis and q-axis is used as the current feedback. The output current is adjusted by the proportional integral controller, and its control law is expressed as:

[0057]

[0058]

[0059] in, and Respectively represent the modulation voltage reference values ​​of the d-axis and q-axis; and Represent the output current of d-axis and q-axis respectively; and They represent the proportional coefficient and integral coefficient of output current control respectively; s is the Laplace operator;

[0060] (3.3) Three-phase modulation wave calculation:

[0061] Using the internal voltage phase reference and d-axis and q-axis modulation voltage reference values and , the three-phase modulation voltage reference value is calculated by coordinate transformation; the three-phase modulation voltage reference value is obtained by coordinate transformation modulation, and the H-bridge of each module in the high-voltage direct-mounted cascade energy storage system is driven to generate voltage based on the reference value.

[0062] As a preferred solution of the present invention, the H-bridge of each energy storage module in the system is driven to generate voltage through a carrier phase shift modulation method.

[0063] The present invention further provides a high-voltage direct-mounted cascade energy storage system with grid-type control, wherein the cascade 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;

[0064] 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 aforementioned networking control method can be implemented.

[0065] Compared with the prior art, the present invention has the following beneficial effects:

[0066] 1. The present invention realizes the grid-type control for the high-voltage direct-mounted cascade energy storage system by combining the grid-type control, equivalent internal voltage control and output current control; this method can not only improve the system current control capability, but also enhance the system stability, thereby improving the overall performance of the grid-type high-voltage direct-mounted cascade energy storage system.

[0067] 2. The present invention enables the high-voltage direct-mounted cascade energy storage system to operate as a voltage source through grid construction control, and has the grid construction characteristics of actively supporting the frequency and voltage of the power grid, which helps to improve the stability of the power system.

[0068] 3. The present invention integrates current closed-loop control, which can not only effectively damp resonance and improve output current quality, but also provide current limiting function through current reference limiting.

[0069] 4. The present invention directly adjusts the internal voltage of the system through equivalent internal voltage control, reuses the filter inductor as a coupling inductor, and enhances the coupling inductance between the grid-forming converter and the grid. This improvement can improve the stability of the system, so that it can still maintain stability under conditions of low grid impedance and high grid strength. Therefore, the method of the present invention can be directly applied without additional configuration of the wiring inductor, which can reduce costs and losses in practical applications.

[0070] 5. In the process of controlling voltage, the present invention constructs the internal voltage by modulating the voltage reference value feedback equivalently, so there is no need to add additional internal voltage sampling or other hardware changes. It can be achieved by simply adjusting the control strategy, which is easy to apply. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 It is the topology diagram of the high-voltage direct-mounted cascade energy storage system.

[0072] Figure 2 It is a block diagram of the grid-type control structure of a high-voltage direct-mounted cascade energy storage system.

[0073] Figure 3 The present invention is a waveform diagram of multiple parameters of a high-voltage direct-mounted cascade energy storage system that adopts the control method of the present invention under rigid grid conditions when the grid frequency drops. DETAILED DESCRIPTION

[0074] In order to make the purpose, technical scheme and advantages of the present invention more clear, the present invention is further described clearly and completely in conjunction with the accompanying drawings and embodiments. It should be understood that the described embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. Any modification, equivalent replacement made to the present invention under the inspiration of the present invention, or a product similar to the present invention obtained by combining the present invention with other technical features, all fall within the protection scope of the present invention.

[0075] The embodiments of the present invention provide a grid-connected control method and system for a high-voltage direct-mounted cascade energy storage system, in a preferred but non-limiting implementation manner of the present invention.

[0076] First, the high-voltage direct-mounted cascade energy storage system of the grid-type control described in the present invention is as follows Figure 1 As shown, it has a three-phase star structure, and each phase is connected by 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.

[0077] 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 networking control method described below can be implemented.

[0078] Secondly, the grid-forming control method of the high-voltage direct-mounted cascade energy storage system proposed in the present invention includes three main operation steps: grid-forming control, equivalent internal voltage control, and output current control; the details are as follows:

[0079] 1. Network control

[0080] Sampling and calculating the output active power and reactive power of the high-voltage direct-mounted cascade energy storage system (hereinafter referred to as the system); obtaining the internal voltage phase reference value and the internal voltage amplitude reference value respectively through the calculation of the active power control law and the reactive power control law; specifically including:

[0081] (1.1) Sampling and power calculation:

[0082] The three-phase voltage of the acquisition system grid connection point , , And the system's three-phase output current , , ; According to the collected data, calculate the system output active power and reactive power ;

[0083] (1.2) Active power control:

[0084] Given system active power , combined with the system output active power and the grid rated angular frequency , the internal voltage phase reference is calculated by the active power control law ;

[0085] The active power control law is shown as follows:

[0086]

[0087]

[0088] in, is the internal voltage angular frequency reference value; is the internal voltage phase reference value; is the inertia coefficient; is the Laplace operator; is the damping coefficient;

[0089] (1.3) Reactive power control:

[0090] Given system reactive power , combined with the system output reactive power , reactive power reference value and rated output voltage amplitude , the internal voltage amplitude reference value is calculated by the reactive power control law .

[0091] Reactive power reference value The control law is as follows:

[0092]

[0093] The reactive power control law is as follows:

[0094]

[0095] Among them, K VQ is the reactive power droop coefficient; V m is the voltage amplitude at the grid connection point; K pQ is the reactive power control proportional coefficient; K iQ is the reactive power control integral coefficient; s is the Laplace operator.

[0096] 2. Equivalent internal voltage control

[0097] The equivalent internal voltage control is a vector control in the same dq rotating coordinate system, and the d-axis direction of the dq rotating coordinate system is consistent with the direction of the equivalent internal voltage. According to the modulation voltage reference value of the previous control cycle, the equivalent internal voltage is calculated; the virtual impedance voltage is calculated; the internal voltage amplitude reference value and 0 are used as the given values ​​of the internal voltage d-axis and q-axis respectively; the equivalent internal voltage is used as voltage feedback, and the equivalent internal voltage is adjusted by the proportional integral controller to obtain the first reference value of the current; specifically including:

[0098] (2.1) Extract the modulation voltage reference value of the previous control cycle in the controller module of the system, and calculate the equivalent internal voltage according to the following formula:

[0099]

[0100] in, and denote the equivalent internal voltages of the d-axis and q-axis respectively; and Respectively represent the modulation voltage reference values ​​of the d-axis and q-axis of the previous control cycle; represents the modulation equivalent gain;

[0101] (2.2) Set the system virtual resistance and virtual inductance, and calculate the virtual impedance voltage according to the following formula:

[0102]

[0103] in, and represent the virtual impedance voltage of d-axis and q-axis respectively; and Represent the d-axis and q-axis output currents respectively; represents a virtual resistor; represents virtual inductance;

[0104] (2.3) Equivalent internal voltage control:

[0105] The closed-loop regulation of the equivalent internal voltage is realized according to the following proportional-integral control law, and the first reference value of the current is calculated:

[0106]

[0107] in, and Respectively represent the first reference values ​​of the current of the d-axis and the q-axis; and They represent the proportional coefficient and integral coefficient of the equivalent internal voltage control respectively; s is the Laplace operator; It is the internal voltage amplitude reference value.

[0108] 3. Output current control

[0109] The output current control is a vector control in the same dq rotating coordinate system, where the d-axis direction of the dq rotating coordinate system is consistent with the direction of the equivalent internal voltage. The second reference value of the current of the d-axis and q-axis is calculated based on the first reference value of the current and the current limit value as the current given value; the output current is used as the current feedback, and the output current is adjusted by the proportional integral controller to obtain the modulation voltage reference value; the internal voltage phase reference value is used for coordinate transformation to obtain the three-phase modulation voltage reference value, and the H-bridge of each module in the system is driven to generate voltage based on the reference value. Specifically including:

[0110] (3.1) Current reference value limit:

[0111] The current reference value of the system should be configured with limit control to prevent the output current from exceeding the current limit threshold during transient conditions. , calculate the second reference value of current according to the following formula:

[0112]

[0113] in, and Respectively represent the first reference values ​​of the current of the d-axis and the q-axis; and denote the second reference values ​​of the current of the d-axis and the q-axis respectively; the superscript T denotes the matrix transposition;

[0114] (3.2) Output current control:

[0115] The calculated second reference value of the current is used as the given value of the d-axis and q-axis current respectively, and the output current of the d-axis and q-axis is used as the current feedback. The output current is adjusted by the proportional integral controller, and its control law is expressed as:

[0116]

[0117]

[0118] in, and Respectively represent the modulation voltage reference values ​​of the d-axis and q-axis; and Represent the output current of d-axis and q-axis respectively; and They represent the proportional coefficient and integral coefficient of output current control respectively; s is the Laplace operator;

[0119] (3.3) Three-phase modulation wave calculation:

[0120] Using the internal voltage phase reference and d-axis and q-axis modulation voltage reference values and , the three-phase modulation voltage reference value is calculated by coordinate transformation; the three-phase modulation voltage reference value is obtained by coordinate transformation modulation, and the H-bridge of each module in the high-voltage direct-mounted cascade energy storage system is driven to generate voltage based on the reference value.

[0121] The following is a specific verification example:

[0122] This example constructs the circuit topology and control model of a high-voltage direct-mounted cascade energy storage system in the PLECS simulation software, in order to verify the specific feasibility and corresponding technical effects of the present invention.

[0123] Figure 1 This is the topology diagram of the high-voltage direct-mounted cascade energy storage system of the present invention. The system has a three-phase star structure, each phase contains cascade connected modules, each phase through the filter inductor After filtering, it is directly connected to the public connection point of the AC power grid; the structures of each module are the same, including an H-bridge, a filtering circuit and an energy storage power supply consisting of a series-connected battery cluster.

[0124] In this embodiment, the number of modules per phase is The number of series batteries is 40, and the series battery cluster in each module is composed of 300 cells connected in series. The rated voltage of each cell is 3.2V and the rated capacity is 280Ah. The relevant operating conditions are set as follows: the rated voltage of the grid is 35 kV, the rated frequency is 50 Hz, the grid condition is a rigid grid (the system short-circuit ratio is 10), the rated capacity of the high-voltage direct-mounted cascade energy storage system is 20 MVA, and the filter inductor is is 19 mH.

[0125] Figure 2 It is a block diagram of the grid-type control structure of the high-voltage direct-mounted cascade energy storage system in the present invention. According to the figure, it can be understood that the grid-type control method described in the present invention mainly includes three parts: grid control, equivalent internal voltage control, and output current control. The cascade energy storage system of the present invention has a controller module (including a processor and a computer-readable storage medium), and a computer program is stored on its computer-readable storage medium; when the computer program is executed by the processor, the grid-type control method can be implemented, and the specific steps are exemplified as follows:

[0126] Step 1: Build network control

[0127] (1.1) Sampling and power calculation

[0128] Collect the following data: Three-phase voltage at the grid connection point , , ; Three-phase output current , , ; Calculate the system output active power according to the three-phase voltage and three-phase output current at the grid connection point and reactive power .

[0129] (1.2) Active power control

[0130] Given system active power According to the system output active power in step 1.1 , System active power given value and the grid rated angular frequency , the internal voltage angle frequency reference is obtained through active power control .

[0131] The control law of active power control adopted in the embodiment of the present invention is:

[0132]

[0133] in, is the internal voltage angular frequency reference value; is the Laplace operator; , .

[0134] Calculate the internal voltage phase reference value based on the internal voltage angular frequency reference , and its calculation formula is:

[0135]

[0136] (1.3) Reactive power control

[0137] Given system reactive power According to the output reactive power in step (1.1) , System reactive power reference value and rated output voltage amplitude , the internal voltage amplitude reference is obtained through reactive power control.

[0138] The control law of reactive power control adopted in the embodiment of the present invention is:

[0139]

[0140] The reactive power reference value The control law is:

[0141]

[0142] in, ;

[0143] Step 2: Equivalent internal voltage control

[0144] The equivalent internal voltage control is a vector control in a dq rotating coordinate system, and the d-axis direction of the dq rotating coordinate system is consistent with the direction of the equivalent internal voltage.

[0145] (2.1) Calculation of equivalent internal voltage:

[0146] The equivalent internal voltage is calculated based on the modulation voltage reference value of the previous control cycle in the controller module. The calculation formula is:

[0147]

[0148] in, and denote the equivalent internal voltages of the d-axis and q-axis respectively; and Respectively represent the modulation voltage reference values ​​of the d-axis and q-axis of the previous control cycle; , represents the modulation equivalent gain.

[0149] (2.2) Virtual impedance voltage calculation:

[0150] Set the system virtual resistance and virtual inductance, and calculate the virtual impedance voltage according to the d-axis and q-axis output currents, virtual resistance and virtual inductance. The calculation formula is:

[0151]

[0152] in, and represent the virtual impedance voltage of d-axis and q-axis respectively; and Represent the d-axis and q-axis output currents respectively; , represents virtual resistance; , represents the virtual inductance.

[0153] (2.3) Equivalent internal voltage control: The internal voltage amplitude calculated in step (1.3) is referenced to As the internal voltage d-axis given value. 0 is taken as the internal voltage q-axis given value. According to the voltage given value and the virtual impedance voltage calculated in step (2.2), the equivalent internal voltage is closed-loop regulated by the proportional-integral controller, and its control law is expressed as:

[0154]

[0155] in, and Respectively represent the first reference values ​​of the current of the d-axis and the q-axis; and They represent the proportional coefficient and integral coefficient of the equivalent internal voltage control respectively; s is the Laplace operator; It is the internal voltage amplitude reference value.

[0156] Step 3: Output Current Control

[0157] The output current control is vector control and is in the same dq rotating coordinate system as the voltage control.

[0158] (3.1) Current reference value limitation

[0159] Configure the current limit control at the current reference value to prevent the output current from exceeding the current limit threshold during transient conditions. =560. The current reference value amplitude limit is calculated according to the first current reference value and the current amplitude limit value, and the calculation formula is:

[0160]

[0161] in, and Respectively represent the first reference values ​​of the current of the d-axis and the q-axis; and denote the second reference values ​​of the current of the d-axis and the q-axis respectively; the superscript T denotes the matrix transposition;

[0162] (3.2) Output current control

[0163] The second reference values ​​of the d-axis and q-axis currents calculated in step 3.1 are used as the given values ​​of the d-axis and q-axis currents, respectively, and the output current is adjusted by the proportional-integral controller. The control law is expressed as:

[0164]

[0165] in, and Respectively represent the modulation voltage reference values ​​of the d-axis and q-axis; and Represent the output current of d-axis and q-axis respectively; and They represent the proportional coefficient and integral coefficient of output current control respectively; s is the Laplace operator;

[0166] (3.3) Three-phase modulation wave calculation

[0167] Using the calculation in step 1.2 and in step 3.2 and , the three-phase modulation voltage reference value is calculated through coordinate transformation.

[0168] The three-phase modulation voltage reference value is calculated by coordinate transformation, and the calculation formula is:

[0169]

[0170] in, , and They respectively represent the modulation voltage reference values ​​of system phase A, phase B and phase C. Then, modulation is performed using the three-phase modulation voltage reference values.

[0171] In this embodiment, the H-bridges of each module of the high-voltage direct-mounted cascade energy storage system are driven to generate voltage by means of carrier phase shift modulation.

[0172] Figure 3 The waveform diagram is of the grid frequency, three-phase output current, active power and reactive power of the high-voltage direct-mounted cascade energy storage system using the method of the present invention under rigid grid conditions when the grid frequency drops, wherein the sub-diagrams are numbered (a), (b), (c) and (d) respectively.

[0173] like Figure 3 As shown in (a) in Figure 2, at t = 3 s, the grid frequency decreases from 50.0 Hz to 49.8 Hz at a frequency change rate of -2 Hz / s. Figure 3 It can be seen from (b), (c), and (d) in that the system can remain stable before and after the frequency drop disturbance and achieve a stable dynamic response; and Figure 3 As shown in (c), in the event of frequency drop, the system actively provides 8.0 MW of active power to support the grid frequency.

[0174] In summary, the grid-forming control method of the present invention has a current direct closed-loop control function similar to that in traditional dual-loop control, and provides an overcurrent protection function in combination with the current reference limit; the stability of the system is enhanced by equivalent internal voltage control, so that the system can operate stably and maintain the grid-forming characteristics in a rigid power grid without adding hardware.

Claims

1. A grid-type control method for a high-voltage direct-mounted cascade energy storage system, characterized in that: The following steps are involved: (1) Network control Sampling and calculating the output active power and reactive power of the high-voltage direct-mounted cascade energy storage system; By calculating the active power control law and the reactive power control law, an internal voltage phase reference value and an internal voltage amplitude reference value are obtained respectively; This step specifically includes: (1.1) Sampling and power calculation: The three-phase voltage v of the acquisition system grid connection point a ,v b ,v c And the system's three-phase output current i a ,i b ,i c ; According to the collected data, calculate the system output active power P and reactive power Q respectively; (1.2) Active power control: Given the system active power P set , combined with the system output active power P and the grid rated angular frequency ω0, the internal voltage phase reference θ is calculated by the active power control law ref ; The active power control law is shown as follows: , , Among them, ω ref is the internal voltage angular frequency reference value; θ ref is the internal voltage phase reference value; J is the inertia coefficient; s is the Laplace operator; D is the damping coefficient; (1.3) Reactive power control: Given the system reactive power Q set , combined with the system output reactive power Q and reactive power reference value Q ref And the rated output voltage amplitude V0, the internal voltage amplitude reference value E is calculated by the reactive power control law ref； Reactive power reference value Q ref The control law is as follows: , The reactive power control law is as follows: , Among them, K VQ is the reactive power droop coefficient; V m is the voltage amplitude at the grid connection point; K pQ is the reactive power control proportional coefficient; K iQ is the reactive power control integral coefficient; s is the Laplace operator; (2) Equivalent internal voltage control According to the modulation voltage reference value of the previous control cycle, the equivalent internal voltage is calculated; the virtual impedance voltage is calculated; the internal voltage amplitude reference value and 0 are used as the given values ​​of the internal voltage d-axis and q-axis respectively; the equivalent internal voltage is used as voltage feedback, and the equivalent internal voltage is adjusted by a proportional integral controller to obtain a first reference value of the current; (3) Output current control The second current reference values ​​of the d-axis and q-axis are calculated according to the first current reference value and the current limit value as the current given value; the output current is used as the current feedback, and the output current is adjusted by the proportional integral controller to obtain the modulation voltage reference value; the internal voltage phase reference value is used to perform coordinate transformation to obtain the three-phase modulation voltage reference value, and the H-bridge of each module in the high-voltage direct-mounted cascade energy storage system is driven to generate voltage based on the three-phase modulation voltage reference value.

2. The method according to claim 1, characterized in that The high-voltage direct-mounted cascade energy storage system has a three-phase star structure, and each phase is connected by 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 The equivalent internal voltage control and the output current control are vector controls in the same dq rotating coordinate system, and the d-axis direction of the dq rotating coordinate system is consistent with the direction of the equivalent internal voltage.

4. The method according to claim 1, characterized in that: The step (2) specifically includes: (2.1) Extract the modulation voltage reference value of the previous control cycle in the high-voltage direct-mounted cascade energy storage system controller module, and calculate the equivalent internal voltage according to the following formula: , Among them, e d,EQ and e q,EQ denote the equivalent internal voltages of the d-axis and q-axis respectively; and Respectively represent the modulation voltage reference values ​​of the d-axis and q-axis of the previous control cycle; k m represents the modulation equivalent gain; (2.2) Set the system virtual resistance and virtual inductance, and calculate the virtual impedance voltage according to the following formula: , Among them, v vird and v virq Represent the virtual impedance voltage of d-axis and q-axis respectively; i d and i q Represent the d-axis and q-axis output currents respectively; R vir Indicates virtual resistance; X vir represents virtual inductance; (2.3) Equivalent internal voltage control: The closed-loop regulation of the equivalent internal voltage is realized according to the following proportional-integral control law, and the first reference value of the current is calculated: , Among them, i dref and i qref k represents the first reference value of the current of the d-axis and the q-axis respectively; pv and k iv They represent the proportional coefficient and integral coefficient of the equivalent internal voltage control respectively; s is the Laplace operator; E ref It is the internal voltage amplitude reference value.

5. The method according to claim 1, characterized in that The step (3) specifically includes: (3.1) Current reference value limit: Given the maximum current limit value I max , calculate the second reference value of current according to the following formula: , Among them, i dref and i qref Respectively represent the first reference values ​​of the current of the d-axis and the q-axis; and denote the second reference values ​​of the current of the d-axis and the q-axis respectively; the superscript T denotes the matrix transposition; (3.2) Output current control: The calculated second reference value of the current is used as the given value of the d-axis and q-axis current respectively, and the output current of the d-axis and q-axis is used as the current feedback. The output current is adjusted by the proportional integral controller, and its control law is expressed as: , , Among them, e dref and e qref Respectively represent the modulation voltage reference values ​​of the d-axis and q-axis; i d and i q Represent the output current of d-axis and q-axis respectively; k pi and k ii They represent the proportional coefficient and integral coefficient of output current control respectively; s is the Laplace operator; (3.3) Three-phase modulation wave calculation: Using the internal voltage phase reference θ ref and d-axis and q-axis modulation voltage reference values ​​e dref and e qref , the three-phase modulation voltage reference value is calculated by coordinate transformation; the three-phase modulation voltage reference value is obtained by coordinate transformation modulation, and the H-bridge of each module in the high-voltage direct-mounted cascade energy storage system is driven to generate voltage based on the reference value.

6. The method according to claim 5, characterized in that The H-bridge of each energy storage module in the system is driven to generate voltage through carrier phase shift modulation.

7. A high-voltage direct-mounted cascade energy storage system with grid-type control, characterized in that: The cascade energy storage system has a three-phase star structure, and each phase is composed of a filter reactor Lf 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 network-forming control method as described in any one of claims 1 to 6 can be implemented.

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