Network-forming industrial microgrid energy router and its control method

Abstract

The present invention provides a network-forming industrial microgrid energy router and its control method. The photovoltaic port is connected to the DC bus through a boost DC / DC converter. The battery energy storage unit port is connected to the DC bus through a boost / buck bidirectional DC / DC converter. The DC load port is directly connected to the DC bus. One end of the grid connection port is connected to the DC bus, and the other end is connected to the AC bus. The photovoltaic port is used to collect renewable energy. The battery energy storage unit port is used to stabilize the energy changes caused by load and power fluctuations and provide backup energy when renewable energy is insufficient. The DC load port is used to meet the power demands of DC sensitive loads. The grid connection port is used for power transmission between the energy router and the grid and actively supports the grid. It can build a power grid in the network-forming mode, support the grid in the grid-connected mode and transient support mode, and also ensure the reliability and quality of power supply.

Description

Technical Field

[0001] The present invention relates to the technical field of energy storage systems, and more particularly to a grid-forming industrial microgrid energy router and its control method. Background Art

[0002] With the accelerating global energy transition process, the proportion of renewable energy sources (RESs) in industrial microgrids is increasing continuously. However, the power electronic devices of RESs lack the physical rotating shaft of synchronous generators, making the new power system gradually show the characteristics of low inertia, weak damping, and low short-circuit ratio, and weakening its grid-forming and stable operation capabilities. In addition, due to the critical operation requirements of sensitive equipment, the direct impact on productivity and safety, the potential cost impact of downtime, and the necessity to comply with strict regulatory standards, power quality problems such as voltage sags, swells, and unbalances have become particularly important in industrial microgrids. Therefore, RES inverters can be incorporated into the grid and provide auxiliary services such as active support to meet the high-quality power supply for sensitive loads.

[0003] In order to achieve the high-quality operation of microgrids and flexible regulation of power, the concept of an energy router (ER) has been proposed, which has characteristics such as power conversion, energy management, and plug-and-play. The ER consists of a series of controlled components and can serve as an intelligent interface for devices such as RESs, energy storage units, power grids, and loads. Compared with traditional multi-converter architectures, the ER can achieve multi-level power distribution and flow control, while reducing the number of components, size, cost, and power consumption. The unique features and capabilities of the ER make it suitable for building and supporting modern power grids characterized by increasing renewable energy penetration and distributed generation. Currently, the research on ER mainly focuses on structural design and energy management strategies, and the potential for supporting the power grid has yet to be explored.

[0004] In addition, some studies have explored the possibility of RESs providing active support to the power grid. Generally, the active support of RESs is achieved by controlling the output current of the grid-connected converter. The phase of the grid-connected converter is synchronized with the power grid through a phase-locked loop, and the current loop ensures the power injection accuracy and current quality issues. Some studies have proposed strategies for the active support of photovoltaic inverters, obtaining the reference current by calculating the support power. Some studies have calculated the positive and negative sequence reference currents respectively for active support under unbalanced faults. Some studies have proposed a reference current generator that can eliminate active power oscillations and compensate for voltage imbalance. However, the above active support strategies are only applicable to grid-following control and not to grid-forming control without a phase-locked loop. Droop control is a typical grid-forming control strategy with advantages such as fast control response, and it has also been studied in the field of active support for the power grid. Some studies have investigated an active support strategy based on hierarchical droop control. The secondary control mainly calculates the positive and negative sequence powers under voltage sags and sends voltage support signals to the primary control. However, the experimental results show that the power quality control strategy of the inverter is not accurate, and there will still be voltage deviations even when the inverter supports the power grid. Some studies have introduced a hybrid control strategy that combines frequency droop with a passive-based switching function, which can restore the voltage level by injecting positive / negative sequence active / reactive power. However, the voltage sag depth set in the simulation and experiment is only 20%, which cannot verify the support effect in more extreme cases. The above studies mainly focus on the control of the inverter and do not fully consider the system connected to the DC side of the inverter, which may lead to instability of the DC bus voltage. In addition, the capacity limitation of the inverter has not been fully resolved.

[0005] Some studies have endued ER with the ability to improve the power quality of the power grid. Some studies have proposed an ER with 5 ports, which can integrate a photovoltaic system, an energy storage system, and AC / DC loads. It has an advanced energy management strategy that can improve some power quality of the power grid. However, the mechanism of power quality improvement is achieved by isolating the interference of the load and photovoltaic from the power grid and reducing potential interference sources, rather than actively supporting the power grid. In some studies, the proposed ER can mitigate harmonics and ensure the three-phase power balance of the microgrid. However, it cannot provide voltage support during faults.

[0006] In summary, the current ER models do not have the ability to form a grid and actively support the power grid simultaneously, and there are also certain limitations in the research on active support control. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a grid-forming industrial microgrid energy router and its control method, which can not only construct and support the power grid but also ensure the reliability and quality of power supply.

[0008] In a first aspect, an embodiment of the present invention provides a network-forming industrial microgrid energy router, including: a photovoltaic port, a battery energy storage unit port, a DC load port, and a grid connection port; the photovoltaic port is connected to a DC bus through a boost DC / DC converter, the battery energy storage unit port is connected to the DC bus through a boost / buck bidirectional DC / DC converter, the DC load port is directly connected to the DC bus, one end of the grid connection port is connected to the DC bus, and the other end of the grid connection port is connected to an AC bus; the photovoltaic port is used to collect renewable energy; the battery energy storage unit port is used to stabilize the energy change caused by load and power fluctuations and provide backup energy when renewable energy is insufficient; the DC load port is used to meet the power demand of DC sensitive loads; the grid connection port is used for power transmission between the energy router and the grid and actively supports the grid.

[0009] In a second aspect, an embodiment of the present invention further provides a hierarchical cooperative control method for a network-forming industrial microgrid energy router, which is applied to the control method of the above-mentioned network-forming industrial microgrid energy router. The control method of the network-forming industrial microgrid energy router includes: obtaining an instruction from an upper controller; determining control strategies for the photovoltaic system and the battery energy storage unit port based on the instruction from the upper controller; wherein, the control strategy of the photovoltaic system includes: constant voltage control and maximum power point tracking control, and the control strategy of the battery energy storage unit port includes: constant voltage control and standby mode.

[0010] In an optional embodiment of the present application, in the above-mentioned maximum power point tracking control, the admittance increment method is used to achieve maximum power tracking; the maximum power point tracking control is provided with a power limit threshold, so that the maximum output power of the photovoltaic system is not greater than the power limit threshold; in the constant voltage control, a double-loop strategy is adopted, and the double loop includes a voltage outer loop and a current inner loop; in the standby mode, the reference current of the battery energy storage unit port is 0.

[0011] In an optional embodiment of the present application, the control method of the above-mentioned network-forming industrial microgrid energy router further includes: controlling the DC / AC converter by using a droop control method based on positive and negative sequence separation; wherein, the droop control based on positive and negative sequence separation includes: a primary control module and a power calculation module.

[0012] In an optional embodiment of the present application, the above-mentioned primary control module includes a current inner loop, a voltage outer loop, and a droop control loop, and the current inner loop, the voltage outer loop, and the droop control loop are implemented in a two-phase rotating reference frame; the active power and reactive power in the primary control module are processed by a first-order low-pass filter; the decoupling of negative sequence in the primary control module is based on a biquadratic generalized integrator; a virtual impedance is set in front of the voltage outer loop, and a secondary voltage frequency amplitude control and grid connection pre-synchronization control are integrated in the droop control.

[0013] In an alternative embodiment of the present application, the above power calculation module is used to calculate a power reference value according to the degree of asymmetrical voltage dip.

[0014] In an alternative embodiment of the present application, the control method of the above grid-forming industrial microgrid energy router further includes: initializing variables, processing monitoring data to calculate the power of the photovoltaic system, battery energy storage unit, local load, and grid connection port; determining the operating modes of the photovoltaic system and battery energy storage unit ports according to the internal energy management strategy; actively switching the operating mode of the grid-forming industrial microgrid energy router according to the degree of grid voltage sag; wherein, in the grid connection mode and transient support mode, grid support is optimized through the power distribution module; and the operating mode of the grid-forming industrial microgrid energy router is sent to the upper controller.

[0015] In an alternative embodiment of the present application, the above variables include: the state of charge threshold and rated capacity of the battery energy storage unit port, and the monitoring data includes: the real-time value of voltage, the real-time value of current, and the real-time state of charge of the battery energy storage unit port.

[0016] In an alternative embodiment of the present application, the operating modes of the above grid-forming industrial microgrid energy router include: grid connection mode, transient support mode, and grid-forming mode; in the grid connection mode, the grid-forming industrial microgrid energy router exchanges power with the grid and also provides steady-state support; in the transient support mode, the grid-forming industrial microgrid energy router actively supports the grid by providing specified positive and negative sequence active and reactive power responses; in the grid-forming mode, the grid-forming industrial microgrid energy router reduces the amplitude and frequency deviation of the output voltage through secondary voltage amplitude and frequency control.

[0017] In an alternative embodiment of the present application, the above power distribution module adjusts the power transmitted to the grid and the power reference value based on the total required capacity, the capacity required for voltage support, and the rated capacity of the converter.

[0018] The embodiments of the present invention bring the following beneficial effects:

[0019] The embodiments of the present invention provide a grid-forming industrial microgrid energy router (GFMER) and its control method. The proposed GFMER can not only construct and support the grid, but also ensure the reliability and quality of power supply.

[0020] Other features and advantages of the present disclosure will be described in the subsequent specification, or, some features and advantages can be inferred from the specification or determined without doubt, or can be learned by implementing the above technologies of the present disclosure.

[0021] To make the above objects, features, and advantages of the present disclosure more apparent and understandable, the following provides preferred embodiments in conjunction with the accompanying drawings and makes a detailed description as follows. Brief Description of the Drawings

[0022] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0023] Figure 1 It is a schematic diagram of the basic topology of a grid-forming industrial microgrid energy router provided by an embodiment of the present invention;

[0024] Figure 2 It is a schematic diagram of a PV and BESU control strategy provided by an embodiment of the present invention;

[0025] Figure 3 It is a schematic diagram of a secondary control provided by an embodiment of the present invention;

[0026] Figure 4 It is a schematic diagram of a DC / AC converter control strategy provided by an embodiment of the present invention;

[0027] Figure 5 It is a schematic diagram of an upper-layer control flow provided by an embodiment of the present invention;

[0028] Figure 6 It is a schematic diagram of a simulation result under a 25% voltage sag provided by an embodiment of the present invention;

[0029] Figure 7 It is a schematic diagram of another simulation result under a 25% voltage sag provided by an embodiment of the present invention;

[0030] Figure 8 It is a schematic diagram of the PCC voltage under different grid voltage sags provided by an embodiment of the present invention;

[0031] Figure 9 It is a schematic diagram of a simulation result of a grid-forming mode provided by an embodiment of the present invention;

[0032] Figure 10 It is a schematic diagram of another simulation result of a grid-forming mode provided by an embodiment of the present invention;

[0033] Figure 11 It is a schematic diagram of a simulation result of grid-forming to grid-connected provided by an embodiment of the present invention;

[0034] Figure 12Schematic diagram of grid-connected to islanded simulation results provided by an embodiment of the present invention;

[0035] Figure 13 Schematic diagram of a CHIL platform provided by an embodiment of the present invention;

[0036] Figure 14 Schematic diagram of transient support experiment results provided by an embodiment of the present invention;

[0037] Figure 15 Schematic diagram of islanded mode experiment results provided by an embodiment of the present invention;

[0038] Figure 16 Schematic diagram of phase A voltage during mode switching provided by an embodiment of the present invention. Detailed implementation manners

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Apparently, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0040] The energy router has the ability to effectively absorb renewable energy sources (RESs) in the industrial microgrid and ensure the reliability of user power consumption. The current ER models do not simultaneously possess the capabilities of islanded operation and actively supporting the power grid, and there are also certain limitations in the research on active support control.

[0041] Based on this, an islanded industrial microgrid energy router and its control method provided by an embodiment of the present invention specifically provide a multi-functional grid-forming energy router (GFMER) using a hierarchical cooperative control strategy. The lower-layer control includes the operation strategies of photovoltaic (PV), battery energy storage unit (BESU), and DC / AC converters. In the lower-layer control, an improved multi-objective droop control is proposed to actively regulate the voltage imbalance and deviation on the grid side. At the same time, the upper-layer control is used to maintain the DC bus voltage, and a power distribution module is added to improve the dynamic support ability for the power grid. Finally, the practicability and effectiveness of the proposed control strategy are verified through MATLAB / Simulink and hardware-in-the-loop experiments.

[0042] This embodiment proposes a grid-forming multi-functional energy router (GFMER) and equips it with a hierarchical cooperative control scheme. The proposed GFMER can not only construct and support the power grid, but also ensure the reliability and quality of power supply. This embodiment mainly proposes:

[0043] 1) The proposed GFMER supports the multi-port access of the power grid, battery energy storage unit (BESU), photovoltaic and load, and provides three operating modes: grid connection, transient support, and grid formation.

[0044] 2) In the lower-level controller, an improved multi-objective droop control is proposed, which can actively adjust the AC-side grid voltage imbalance and deviation under various operating modes.

[0045] 3) The upper-level controller is designed to maintain the stability of the DC bus voltage and energy routing. A power distribution module is also deployed in the upper-level controller to improve the dynamic support ability of the GFMER for the power grid in the grid connection and transient support modes.

[0046] For the convenience of understanding this embodiment, first, a grid-forming industrial microgrid energy router disclosed in the embodiments of the present invention will be introduced in detail.

[0047] Embodiment 1:

[0048] The embodiment of the present invention provides a grid-forming industrial microgrid energy router. Refer to Figure 1 the schematic diagram of the basic topology of a grid-forming industrial microgrid energy router shown. The grid-forming industrial microgrid energy router includes: a photovoltaic port, a battery energy storage unit port, a DC load port, and a grid connection port; the photovoltaic port is connected to the DC bus through a boost DC / DC converter, the battery energy storage unit port is connected to the DC bus through a boost / buck bidirectional DC / DC converter, the DC load port is directly connected to the DC bus, one end of the grid connection port is connected to the DC bus, and the other end of the grid connection port is connected to the AC bus; the photovoltaic port is used to collect renewable energy; the battery energy storage unit port is used to stabilize the energy change caused by load and power source fluctuations and provide backup energy when renewable energy is insufficient; the DC load port is used to meet the power demand of DC sensitive loads; the grid connection port is used for power transmission between the energy router and the power grid and actively supports the power grid.

[0049] As Figure 1 shown, the parallel connection structure of the GFMER system includes 4 ports:

[0050] 1) PV (Photovoltaic System) Port: That is, the photovoltaic port. To collect renewable energy, the photovoltaic system is connected to the 750V DC bus through a boost DC / DC converter.

[0051] 2) BESU (Battery Energy Storage Unit) Port: In the GFMER, the BESU can stabilize the energy variations caused by load and power fluctuations and provide backup power when renewable energy is insufficient. To maintain energy balance between the DC side and the AC side of the GFMER, the BESU is connected to the DC bus through a boost / buck bidirectional DC / DC converter.

[0052] 3) DC Load Port: The DC load port is directly connected to the 750V DC bus to meet the power demands of DC sensitive loads and comply with the specified voltage level standards.

[0053] 4) Grid-Connected Port: The grid-connected port is an important part of the GFMER, which helps with power transfer between the energy router and the grid and provides active support to the grid. In addition, in the event of a severe grid fault, the energy router can be isolated from the grid by disconnecting this port.

[0054] In this system, U bus1 and U busn are the DC bus voltages of the first and the nth GFMERs respectively. I pv1 and I pvn are the PV output currents of the first and the nth GFMERs, U pv is the output voltage of the PV. I Lb1 and I Lbn are the BESU output voltages of the first and the nth GFMERs. The grid-connected converter consists of a three-phase voltage source SPWM inverter and a filter. In addition, R g and L g are the resistance and inductance of the grid impedance respectively, u g is the grid voltage. v o1 and v on are the output voltages of the first and the nth GFMERs. i o1 , i on , i g and iload They are the output currents of the first and the nth GFMER, the current injected into the grid, and the load current respectively. The 380V AC sensitive load is a local load, which can be a resistive, inductive, capacitive load, or a linear or non-linear load.

[0055] The relationships among the current injected into the grid, the output current of the GFMER, and the load current are as follows:

[0056] (1)

[0057] The relationships among the output voltages of each GFMER are as follows:

[0058] (2)

[0059] Since the GFMER studied in this embodiment is connected to a 380V / 50Hz low-voltage distribution network, electrical isolation is not required for the main circuit, and the voltage level and electrical energy form are converted through a power electronic converter. In addition, the integrated all-power electronic circuit contributes to the modularization and miniaturization of the GFMER.

[0060] The embodiment of the present invention provides a grid-forming industrial microgrid energy router, which can not only construct and support the grid, but also ensure the reliability and quality of power supply.

[0061] Embodiment 2:

[0062] This embodiment provides a control method for a grid-forming industrial microgrid energy router. This method is implemented on the basis of the above embodiment, and focuses on describing the lower-layer control strategy. This method is applied to the grid-forming industrial microgrid energy router provided in the foregoing embodiment. In this embodiment, the PV and BESU control strategies are first provided. The control method for this grid-forming industrial microgrid energy router includes: obtaining the instructions of the upper-layer controller; determining the control strategies for the ports of the photovoltaic system and the battery energy storage unit based on the instructions of the upper-layer controller; wherein, the control strategy for the photovoltaic system includes: constant voltage control and maximum power point tracking control, and the control strategy for the battery energy storage unit port includes: constant voltage control and standby mode.

[0063] Refer to Figure 2 As shown in the schematic diagram of a PV and BESU control strategy, the control mode of the PV port needs to be adjusted according to the light energy and the power balance requirements of the GFMER.

[0064] In some embodiments, the maximum power point tracking control uses the admittance increment method to achieve maximum power tracking; the maximum power point tracking control is provided with a power limit threshold, so that the maximum output power of the photovoltaic system is not greater than the power limit threshold; the constant voltage control uses a dual-loop strategy, and the dual loop includes an external voltage loop and an internal current loop; in the standby mode, the reference current at the port of the battery energy storage unit is 0.

[0065] In this embodiment, the control strategy of the photovoltaic system selects constant voltage (CV) control and maximum power point tracking (MPPT) control, as Figure 2 shown. For MPPT control, the admittance increment method is used to achieve maximum power tracking, and at the same time a power limit ( P limit ) is imposed. When the maximum output power ( P pv_mppt ) of the photovoltaic system exceeds P limit , the photovoltaic output power is limited to P limit .

[0066] The BESU port also has a variety of control methods, as Figure 2 shown. Among them, the CV control uses a dual-loop strategy, including an external voltage loop and an internal current loop, to ensure the precise stability of the DC bus voltage. In some cases, the BESU will switch to the standby mode. In the standby mode, the reference current is 0.

[0067] It should be noted that the control strategies of the photovoltaic and BESU need to be switched and operated according to the instructions of the upper-layer controller.

[0068] This embodiment also provides a control strategy for the DC / AC converter. In some embodiments, the DC / AC converter can also be controlled by a droop control method based on positive and negative sequence separation; among them, the droop control based on positive and negative sequence separation includes: a primary control module and a power calculation module.

[0069] Droop control is a method of controlling the converter by simulating the external droop characteristic curve of a synchronous generator. In order to meet the requirements of self-regulation, independent communication and network-forming ability, the DC / AC converter uses droop control. It should be noted that during a voltage sag, the GFMER not only needs to output positive-sequence active / reactive power, but also negative-sequence active / reactive power to restore and balance the voltage at the point of common coupling (PCC). In order to meet the needs of grid dynamic support, an improved droop control based on positive and negative sequence separation is proposed, and this control includes two modules: primary control and power calculation.

[0070] In some embodiments, the primary control module includes a current inner loop, a voltage outer loop, and a droop control loop, which are implemented in a two-phase rotating reference frame; the active power and reactive power in the primary control module are processed by a first-order low-pass filter; the decoupling of negative sequence in the primary control module is based on a double second-order generalized integrator; a virtual impedance is set before the voltage outer loop, and a secondary voltage-frequency-magnitude control and grid connection pre-synchronization control are integrated in the droop control.

[0071] 1. Primary control module

[0072] The primary control module includes a current inner loop, a voltage outer loop, and a droop control loop. These control loops are implemented in a two-phase rotating reference frame (d-q) for subsequent decoupling. In addition, to filter out power fluctuations and improve control accuracy, the active and reactive powers are processed by a first-order low-pass filter. It should be noted that the decoupling of positive and negative sequences is based on a double second-order generalized integrator (DSOGI), which has high precision, high reliability, and strong anti-interference ability.

[0073] Traditional droop control methods are developed based on inductive line impedance, which is inconsistent with the resistive impedance often encountered in low-voltage distribution networks. This difference may lead to active-reactive power coupling. To solve this problem, a virtual impedance is added before the voltage outer loop to improve the power sharing accuracy between GFMERs and make the system have better damping characteristics without sacrificing system efficiency.

[0074] In addition, to reduce the deviation of the frequency and magnitude of the port output voltage, a secondary voltage-frequency-magnitude control is integrated in the droop control. Its main principle is to translate the operating point of the droop curve, restoring the voltage frequency and magnitude to the set values while maintaining a constant active and reactive output. The control principle can be seen in Figure 3 the schematic diagram of a secondary control shown. At the same time, a grid connection pre-synchronization control is added to enable GFMER to seamlessly switch between grid-connected and off-grid modes. This control uses the difference between the amplitude and phase of the GFMER output voltage and the grid voltage as a feedback signal, which is adjusted by a proportional-integral (PI) controller to generate the compensation amounts of the converter voltage and frequency.

[0075] In some embodiments, the power calculation module is used to calculate the power reference value according to the degree of asymmetric voltage dip.

[0076] 2. Power calculation module

[0077] When a voltage asymmetric sag occurs in the power grid, the primary control module lacks the ability to adjust the power reference value, resulting in a deviation in the supported voltage. To solve this problem, a power calculation module is developed based on the primary droop module, which can be seen inFigure 4 Schematic diagram of a control strategy for a DC / AC converter. This module calculates an appropriate power reference value according to the degree of asymmetric voltage dip to achieve dynamic adjustment of the power reference value.

[0078] During an asymmetric fault, the voltage and current at the PCC can be expressed as the sum of positive, negative, and zero-sequence components:

[0079] (3)

[0080] For a three-phase three-wire system, since its neutral point is not grounded, the zero-sequence component can be ignored. Through coordinate transformation, both the voltage and current at the PCC can be represented by the positive and negative sequence components in the αβ coordinate system, where the voltage is expressed as shown in Equation (4), and the current is expressed as shown in Equation (5).

[0081] (4)

[0082] (5)

[0083] Among them, are the positive / negative sequence voltages respectively; are the positive / negative sequence currents respectively. is the amplitude of the positive / negative sequence voltage at the PCC, and are the amplitudes of the positive / negative sequence active and reactive currents respectively; is the phase angle of the positive / negative sequence voltage phasor; ω is the angular frequency.

[0084] Then, convert the , , in Equations (4) and (5) to the d-q coordinate system, and we can get:

[0085] (6)

[0086] (7)

[0087] (8)

[0088] (9)

[0089] The relationship between the voltage at the PCC point and the grid voltage in the αβ coordinate system is:

[0090] (10)

[0091] (11)

[0092] Substituting Eqs. (4)-(5) into (8)-(11), the voltage support equations can be obtained:

[0093] (12)

[0094] (13)

[0095] where are the amplitudes of the positive and negative sequence components of the grid-side voltage, respectively.

[0096] It can be seen from Eqs. (12) and (13) that if the low-voltage grid is regarded as resistive rather than purely inductive, the influence of the active current component on the grid voltage support cannot be ignored.

[0097] Substituting the positive and negative sequence voltage reference values into the above equations and considering the load current, the initial current reference values of each GFMER ( and ) can be obtained as follows:

[0098] (14)

[0099] (15)

[0100] (16)

[0101] (17)

[0102] where I loadd and I loadq are the load currents in the d-q coordinate system; S n1 , S nk and S nn are the capacities of the 1st, kth, and nth converters, respectively.

[0103] The formulas for calculating the positive / negative sequence active / reactive power outputs of GFMER using a first-order low-pass filter (P + / - and Q + / - ) are as follows:

[0104] (18)

[0105] (19)

[0106] (20)

[0107] (21)

[0108] where ωc is the cut-off frequency of the low-pass filter.

[0109] Substituting the current reference values in Eqs. (14)-(17) into Eqs. (18)-(21), the initial reference powers of positive and negative sequence active / reactive power for droop control can be obtained ( and ). It should be noted that the final reference values ( and ) need to be obtained from the upper-level controller. Then the calculated reference values are sent to the primary control module. By combining the two control modules, the proposed droop control enables the GFMER to provide accurate support for the power grid.

[0110] In summary, after adding the secondary control, virtual impedance, and grid connection pre-synchronization, the proposed droop control expressions are:

[0111] (22)

[0112] (23)

[0113] (24)

[0114] where are the rated positive and negative sequence angular frequencies; and are the positive / negative sequence active-power - frequency and reactive-power - voltage droop coefficients respectively; and are the virtual resistance and reactance respectively; and are the differences between the measured value and the reference value of the positive / negative sequence voltage angular frequency and amplitude respectively; , are the differences between the amplitude and phase of the positive / negative sequence voltage of the GFMER output and the power grid respectively. , , and can be calculated by the following equations:

[0115] (25)

[0116] (26)

[0117] (27)

[0118] (28)

[0119] where Output the positive / negative sequence angular frequency detection value for GFMER; Is the positive / negative sequence angular frequency detection value of the power grid.

[0120] Embodiment 3:

[0121] This embodiment provides a control method for a grid-forming industrial microgrid energy router. This method is implemented on the basis of the above embodiment, focusing on describing the upper-layer control strategy. This method is applied to the grid-forming industrial microgrid energy router provided in the foregoing embodiment. The control method of the grid-forming industrial microgrid energy router includes: initializing variables, processing monitoring data to calculate the power of the photovoltaic system, battery energy storage unit, local load, and grid-connected port; determining the operating modes of the photovoltaic system and battery energy storage unit ports according to the internal energy management strategy; actively switching the operating mode of the grid-forming industrial microgrid energy router according to the degree of grid voltage sag; wherein, in the grid-connected mode and transient support mode, optimize grid support through the power distribution module; send the operating mode of the grid-forming industrial microgrid energy router to the upper-layer controller.

[0122] The upper-layer control process proposed in this embodiment can be referred to Figure 5 As shown in the schematic diagram of an upper-layer control process, its main functions are as follows: 1) Ensure that the DC bus voltage is stable at the rated level in all cases; 2) Actively adjust the voltage imbalance and deviation on the AC grid side to the maximum extent; 3) Actively switch the operating mode according to different degrees of grid voltage drop.

[0123] Its control process is as follows: First, initialize variables and process monitoring data. Second, determine the operating modes of the photovoltaic and BESU according to the internal energy management strategy. Then, actively switch the operating mode according to the degree of grid voltage sag. In the grid-connected mode and transient support mode, the power distribution module is used to optimize grid support. Finally, send the command to the lower-layer controller.

[0124] In some embodiments, the variables include: the state of charge threshold and rated capacity of the battery energy storage unit port, and the monitoring data includes: the real-time value of voltage, the real-time value of current, and the real-time state of charge of the battery energy storage unit port.

[0125] 1. Data processing

[0126] First, initialize variables, including the state of charge (SoC) thresholds (SoCmax and SoCmin) and rated capacity (Sn) of the BESU. Subsequently, monitor Figure 1 The real-time values of voltage and current indicated by the red marks, and the real-time SoC of the BESU. Finally, process the measurement data to calculate the power of the PV, BESU, local load, and grid-connected port.

[0127] 2. Internal Energy Management Solution

[0128] For the internal energy management of GFMER, it is first necessary to analyze the possible energy management states of GFMER. The energy balance relationship inside the energy router is as follows:

[0129] (29)

[0130] In the formula P g is the exchange power between GFMER and the power grid; P l is the DC load power inside GFMER; P pv is the PV output power; P b is the BESU output power.

[0131] According to formula (29) and with the SoC of BESU as the constraint condition, the internal energy management process of the energy router proposed in this embodiment can be obtained, as Figure 5 shown. The core of the internal energy management is to make full use of the PV output power while keeping the DC bus voltage constant. The operating modes of PV and BESU are shown in Table 1.

[0132] Table 1 Operating Modes of PV and BESU

[0133] Energy relationship SoC PV operating mode BESU operating mode Ppv = 0 SoC > SoCmin MPPT CV Ppv = 0 SoC < SoCmin MPPT CV Ppv > Pl SoC < SoCmax MPPT CV Ppv > Pl SoC > SoCmax CV Standby Ppv < Pl SoC > SoCmin MPPT CV Ppv < Pl SoC < SoCmin CV Standby

[0134] At night, since the PV output power is 0, the control target of maintaining the DC bus voltage is entirely borne by BESU, and at this time BESU works in the CV mode. When there is a power demand from the DC load, if it is greater than SoC min , BESU performs CV control and supplies power to the load, and at this time P g =P l -P b . If it is less than SoC min , BESU still adopts CV control, and the power required by the load is mainly obtained from the power grid, and at this time P g =P l . When there is no load power demand, the energy router is in the standby state and outputs power to the external power grid when needed to meet the requirements of power quality control.

[0135] During the day, first determine whether the PV output power P pv is less than the load demand power Pl If P pv >P l during the period when the SoC of the energy storage system is less than SoC max the PV system operates in the MPPT mode and the BESU operates in the CV mode. At this time P g =P l -P pv - P b When the SoC reaches SoC max the control of the PV switches to the CV mode and the energy storage enters the standby state. At this time P g =P l -P pv -P b If P pv <P l when the SoC is greater than SoC min the BESU is in the CV control state and the PV system is in the MPPT operation state. When the SoC is less than SoC min the PV system switches from the MPPT mode to the CV mode.

[0136] It should be noted that when the trigger mechanism detects an abnormal grid operation, it will appropriately adjust the instructions transmitted from the internal energy management to the lower-level controller to implement the functions of each operation mode of the GFMER.

[0137] In some embodiments, the operation modes of the grid-forming industrial microgrid energy router include: grid-connected mode, transient support mode, and grid-forming mode; the power distribution module adjusts the power transmitted to the grid and the power reference value based on the total required capacity, the required capacity for voltage support, and the rated capacity of the converter.

[0138] 3. Mode Division

[0139] The proposed GFMER has three different operation modes: grid-connected mode, transient support mode, and grid-forming mode. These modes respectively achieve power exchange, active support, and local load optimization, and ensure reliable and efficient power distribution.

[0140] 1) Grid-connected mode: When the grid operates normally, the GFMER is in the grid-connected mode. In this mode, the GFMER not only exchanges power with the grid, but also can provide steady-state support to improve the power quality of the grid.

[0141] 2) Transient support mode: When a grid voltage sag is detected and the sag depth is less than 50%, the GFMER will operate in the transient support mode. In this mode, the GFMER actively supports the grid by quickly responding with specified positive and negative sequence active and reactive powers.

[0142] 3) Grid-forming mode: When the grid voltage sags by more than 50%, due to the capacity limitation of the GFMER, even with active support, it is impossible to boost the PCC voltage to the rated range. To ensure the high-quality operation of local AC and DC loads, the GFMER will disconnect from the grid and operate in the grid-forming mode. In this mode, secondary voltage amplitude and frequency control can reduce the amplitude and frequency deviations of the output voltage.

[0143] 4. Power distribution module

[0144] Due to the limited capacity of the GFMER, how to reasonably distribute the connected power to maximize the support for the grid has become a challenge. Based on this, a power distribution module is added in the grid-connected mode and the transient support mode. Considering the power demand for active support, this module redistributes the power inside the GFMER by dividing the power priorities, and the capacity of the GFMER is fully considered. By further adjusting the reference power of the droop control through this module, the active support effect can be optimized.

[0145] This module takes high-quality power supply and active voltage support as the primary and secondary objectives respectively, and sorts the power outputs of each port according to the priority. Among them, the power priority of the load output port is the highest to ensure the quality of the load power supply under any circumstances. The second priority is for the converter to output active and reactive powers to support the grid, thereby enhancing the active support ability in various scenarios. Finally, the power exchange between the GFMER and the grid is set as the third level.

[0146] The active power and reactive power for supporting the voltage can be expressed as:

[0147] (30)

[0148] (31)

[0149] In the formula, P s and Q s are the active power and reactive power for supporting the grid respectively.

[0150] The exchanged power can be expressed as:

[0151] (32)

[0152] In the formula Pout The power transmitted by GFMER to the power grid.

[0153] Total required capacity S t And the capacity required for voltage support S r Can be calculated as:

[0154] (33)

[0155] (34)

[0156] According to S t 、 S r The relationship with the converter rated capacity S n Can be divided into the following three scenarios:

[0157] 1) When S t >S n >S r The total demand capacity exceeds the converter rated capacity, which may lead to overload. To ensure the voltage support effect, the required active and reactive reference powers remain unchanged. The power transmitted to the power grid will be adjusted to:

[0158] (35)

[0159] 2) When S t >S r >S n The converter needs to output more power to actively support the power grid. However, due to the capacity limitation of the converter, it cannot provide the required power. Therefore, the power reference value needs to be adjusted to:

[0160] (36)

[0161] (37)

[0162] (38)

[0163] 3) When S n >S t >S rWhen the required active support power is small and the total demand capacity is lower than the rated capacity of the converter. Therefore, there is no need to adjust the reference power, and the final reference power is equal to the initial reference power.

[0164] Combining Eqs. (29) and (32), it can be obtained that P out Subject to P pv the influence of. If P out adjustment is required, the PV upper-layer controller issues a specific reference power ( P limit ) to operate in the power limit mode, and its calculation formula is:

[0165] (39)

[0166] Example 4:

[0167] This example provides an analysis of the simulation results for the foregoing example. To verify the feasibility and effectiveness of the proposed GFMER and its control method, a system simulation model as shown in Figure 1 is built based on MATLAB / Simulink, where the number of parallel GFMERs is 2. The simulation verifies the performance of the GFMER in each working mode. The main system parameters are shown in Table 2.

[0168] Table 2 Main System Parameters

[0169] Parameter Expression Specific value Distribution network voltage vg 380 V AC frequency fg 50 Hz GFMER DC bus voltage Ubus 750 V BESU voltage ULb 450 V BESU capacity SLb 20 Ah Grid-side line inductance Lg 12 mH Grid-side line resistance Rg 0.8 Ω Grid-connected port LC filter Lf, Cf 12 mH, 10μF Grid-connected port capacity Sn 10kVA PV maximum power Pmpp 30 kW Local AC load Pl-AC 12 kW DC load connected to each GFMER Pl 14 kW Switching frequency fs 10 kHz

[0170] 1. Simulation Verification of Transient Support

[0171] The transient support mode operates under grid-connected conditions. If the transient support ability can be verified, the steady-state support ability under grid-connected mode can also be verified. Therefore, this section focuses on the simulation verification of the transient support ability. The illumination intensity in the grid-connected simulation fluctuates between 600 - 1000 W / m 2 . The active support performance of the proposed strategy is tested under a 25% two-phase voltage sag. Refer to Figure 6 a schematic diagram of the simulation results during a 25% voltage sag as shown in Figure 6 (a). Under normal conditions, the phase voltage amplitude is 311 V. The voltage sag occurs at t = 0.2 s and is eliminated at t = 0.5 s. Obviously, during the grid voltage sag, the PCC voltage instantaneously rises to 311 V, and the grid voltage drop has little effect on it. The unbalance degrees of the PCC voltage and the grid voltage are as shown in Figure 6(c). It can be seen that the three-phase unbalance coefficient of the PCC voltage is always almost 0, much lower than that of the grid voltage during the fault period. Therefore, under the condition of grid voltage sag, the GFMER can effectively support the PCC voltage and protect the AC sensitive load from damage.

[0172] See Figure 7 Another schematic diagram of the simulation results during a 25% voltage sag as shown. The output powers of GFMER 1 and GFMER 2 during the grid voltage sag are as Figure 7 (a). Obviously, under the combined action of the proposed droop control and power distribution module, the GFMER can adaptively output appropriate active / reactive power according to the depth of the voltage sag, and the parallel GFMERs can share the output power well. As can be seen from Fig. 7(b), under the condition of changing light intensity and actively supporting the grid, the DC bus voltage always remains near 750 V, ensuring the normal operation of the DC load and verifying the effectiveness of the upper-layer controller in the transient support mode. Due to the influence of the negative sequence component, the DC side voltage will oscillate when the GFMER actively supports the grid. However, its oscillation amplitude is only 2.8 V, and the influence on the DC load can be ignored.

[0173] Finally, the active support performance under different grid voltage dips was tested. See Figure 8 A schematic diagram of the PCC voltage under different grid voltage sags as shown. When the grid voltage sag depths are 12%, 20% and 33% respectively, due to the power distribution module and the proposed droop control, the PCC voltage can be supported above 0.99 p.u. In the case of a 48% voltage sag, due to the need for higher power support and the limited capacity of the converter, the distribution module appropriately reduces the reference power, and the PCC voltage is supported to 0.96 p.u.

[0174] 2. Simulation verification of the grid-forming mode

[0175] Due to the limited capacity of the converter, when the grid voltage drops severely, even if the GFMER actively provides support, the PCC voltage cannot meet the load operation requirements. At this time, the GFMER will operate in the grid-forming mode. In this simulation, it is set that the photovoltaic power increases once every 0.1 s, and the BESU is charged to SoC at t = 0.5 s max .

[0176] Verification of the secondary voltage amplitude and frequency control can be seen in Figure 9 A schematic diagram of the simulation results in the grid-forming mode as shown. The secondary control is put into operation at t = 0.1 s. Obviously, adding the secondary control to the droop control by the proposed control strategy can reduce the frequency and amplitude deviation of the AC voltage output by the GFMER and optimize the power quality.

[0177] Figure 10 Schematic diagram of the simulation results of another network formation mode shown. The active power curves of the PV, BESU, and AC and DC loads in the network formation mode are as Figure 10 (a) shown. Before t = 0.5 s, as the PV power gradually increases, the BESU switches from the charging state to the discharging state, and the charge-discharge power changes with the PV power. After t = 0.5 s, assuming that the SoC of the BESU has reached SoC max , it will switch to the standby mode at this time, and its output power is 0. At the same time, the PV switches from the MPPT mode to the CV mode and supplies power to the load simultaneously. During this period, both the AC and DC loads can operate stably. From Figure 10 (b), it can be seen that the DC bus voltage always remains around 750 V, and the maximum deviation is only 3 V.

[0178] 3. Simulation verification of mode switching

[0179] Refer to the simulation results of grid connection pre-synchronization Figure 11 Schematic diagram of the simulation results of a network formation to grid connection shown. The GFMER is connected to the grid at t = 0.8 s. It can be clearly seen that the voltage and frequency fluctuations during pre-synchronization grid connection are significantly reduced. Refer to the amplitude and frequency waveforms of the PCC voltage when the GFMER grid connection mode switches to the network formation mode Figure 12 Schematic diagram of the simulation results of a grid connection to network formation shown. At t = 1.5 s, the GFMER switches to the network formation mode. The frequency and amplitude fluctuations of the PCC voltage during the switching process are almost negligible. The simulation results show that the proposed GFMER can smoothly switch the working mode.

[0180] Example 5:

[0181] This example provides an analysis of the experimental results for the foregoing example. To more fully verify the effectiveness of the proposed GFMER and control strategy, a GFMER system connected to the distribution network was built on the StarSim CHIL platform. The system parameters are shown in Table 2, and the only difference is that the local AC load becomes 6 kW. The main circuit was simulated on the MT 6020 simulator with a time step of 1 μs. The hierarchical active control of the GFMER was implemented using the MT1050 rapid control prototype (RCP) with a time step of 10 μs. Due to the limited number of I / O ports, the proposed control strategy was only implemented in one GFMER. However, all parallel GFMERs are based on the same control strategy and parameters. If the corresponding performance in one GFMER scenario can be verified, then the scenario of multiple GFMERs can also be verified. Refer to the schematic diagram of the CHIL platform Figure 13 Schematic diagram of a CHIL platform shown.

[0182] The experimental results of GFMER transient active support can be seen in Figure 14 the schematic diagram of the experimental results of a transient support shown in Figure 14 (a). It can be seen that a two-phase voltage sag with a duration of 1 second and a sag depth of 25% occurred in the power grid at 0.44 s. The PCC voltage under the support of GFMER is as Figure 15 (b) shown. Comparing Figure 14 (a) and Figure 14 (b), due to the accurate injection of positive / negative sequence power, the PCC voltage is supported and the three-phase unbalance is reduced. Figure 14 (c) shows the DC bus voltage inside GFMER during the support. During this period, the voltage oscillation is only 10 V, which proves the power balance inside GFMER.

[0183] The experimental results of the grid-forming mode can be seen in Figure 15 the schematic diagram of the experimental results of a grid-forming mode shown in Figure 15 (a) is the output power of GFMER in the grid-forming mode. Obviously, GFMER can accurately and stably output power to meet the needs of AC loads. The DC bus voltage inside GFMER is as Figure 15 (b) shown. Obviously, the DC bus voltage always remains at 750 V, and the maximum oscillation amplitude does not exceed 3 V.

[0184] When GFMER switches between grid-connected and grid-forming operations, the PCC voltage can be seen in Figure 16 the schematic diagram of the phase A voltage during a mode switch shown in

[0185] In summary, the embodiment of the present invention proposes a grid-forming energy router with a hierarchical cooperative control strategy for the high power quality requirements of industrial microgrids. GFMER realizes the energy complementarity between photovoltaic and energy storage to obtain renewable energy. The droop control of the DC / AC converter is improved to realize the generation of power reference values based on the voltage sag degree, enabling GFMER to provide more accurate support for the power grid in the grid-connected mode and the transient support mode. In addition, secondary control and grid-connected pre-synchronization are added to the droop control, improving the power supply quality of GFMER in the grid-forming mode and mode switching. The power distribution module of the upper controller can provide the maximum support for the power grid. The internal energy management scheme stabilizes the DC bus voltage and maintains power balance under any circumstances. The various working modes of GFMER and the switching between modes are simulated and experimentally verified. The results show that GFMER has good performance, verifying the feasibility and effectiveness of the proposed GFMER and control strategy.

[0186] Those skilled in the art can clearly understand that for the convenience and brevity of description, the specific working processes described above can refer to the corresponding processes in the foregoing embodiments and will not be elaborated herein.

[0187] Finally, it should be noted that the above embodiments are only specific embodiments of the present invention, used to illustrate the technical solutions of the present invention, rather than limiting it. The protection scope of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that any person skilled in the art within the technical scope disclosed by the present invention can still modify the technical solutions recorded in the foregoing embodiments or easily conceive of changes, or make equivalent replacements for some of the technical features; and these modifications, changes or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention and should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A control method for an energy router of a networked industrial microgrid, characterized in that Applied to the grid-forming industrial microgrid energy router, the grid-forming industrial microgrid energy router includes: a photovoltaic port, a battery energy storage unit port, a DC load port, and a grid connection port; the photovoltaic port is connected to the DC bus through a boost DC / DC converter, the battery energy storage unit port is connected to the DC bus through a boost / buck bidirectional DC / DC converter, the DC load port is directly connected to the DC bus, one end of the grid connection port is connected to the DC bus, and the other end of the grid connection port is connected to the AC bus; the photovoltaic port is used to collect renewable energy; the battery energy storage unit port is used to stabilize the energy changes caused by load and power fluctuations and provide backup energy when the renewable energy is insufficient; the DC load port is used to meet the power demand of DC sensitive loads; the grid connection port is used for power transmission between the energy router and the grid and actively supports the grid; The control method of the grid-forming industrial microgrid energy router includes: obtaining instructions from the upper controller; determining the control strategies of the photovoltaic system and the battery energy storage unit based on the instructions of the upper controller; wherein, the control strategies of the photovoltaic system include: constant voltage control and maximum power point tracking control, and the control strategies of the battery energy storage unit include: constant voltage control and standby mode; The control method of the grid-forming industrial microgrid energy router further includes: initializing variables, processing monitoring data to calculate the powers of the photovoltaic system, the battery energy storage unit, local loads, and the grid connection port; determining the control strategies of the photovoltaic system and the battery energy storage unit according to the internal energy management strategy; actively switching the operation mode of the grid-forming industrial microgrid energy router according to the degree of grid voltage dip; wherein, in the grid connection mode and the transient support mode, the grid support is optimized through the power distribution module; sending the operation mode of the grid-forming industrial microgrid energy router to the upper controller; The operation modes of the grid-forming industrial microgrid energy router include: grid connection mode, transient support mode, and grid-forming mode; in the grid connection mode, the grid-forming industrial microgrid energy router exchanges power with the grid and also performs steady-state support; in the transient support mode, the grid-forming industrial microgrid energy router actively supports the grid by providing specified positive and negative sequence active and reactive powers; in the grid-forming mode, the grid-forming industrial microgrid energy router reduces the amplitude and frequency deviation of the output voltage through secondary voltage frequency amplitude control; The power distribution module adjusts the power transmitted to the grid and the power reference value based on the total required capacity, the capacity required for voltage support, and the rated capacity of the converter; When S t >S n >S r , the active and reactive reference powers remain unchanged, and the power transmitted to the power grid is adjusted to: ; When S t >S r >S n , , it is necessary to adjust the active and reactive reference powers to: , ; When S n >S t >S r , there is no need to adjust the reference power, and the final reference power is equal to the initial reference power; where S t , S n , S r are the total required capacity, the capacity required for voltage support, and the converter rated capacity respectively, and are the active power and reactive power for supporting the power grid respectively, and are the initial positive and negative sequence active reference powers and the initial positive and negative sequence reactive reference powers respectively.

2. The control method of the network-constructing industrial microgrid energy router according to claim 1, characterized in that, In the maximum power point tracking control, the admittance increment method is used to achieve maximum power tracking; the maximum power point tracking control is provided with a power limit threshold to make the maximum output power of the photovoltaic system not greater than the power limit threshold; In the constant voltage control, a double-loop strategy is adopted, and the double loop includes an external voltage loop and an internal current loop; In the standby mode, the reference current of the battery energy storage unit port is 0.

3. The control method of the grid-forming industrial microgrid energy router according to claim 1, characterized in that, The control method of the network-constructing industrial microgrid energy router further includes: Controlling the DC / AC converter by means of droop control based on positive and negative sequence separation; wherein, the droop control based on positive and negative sequence separation is implemented by a primary control module and a power calculation module.

4. The control method of the network-constructing industrial microgrid energy router according to claim 3, characterized in that, The primary control module includes a current inner loop, a voltage outer loop and a droop control loop, and the current inner loop, the voltage outer loop and the droop control loop are implemented in a two-phase rotating reference frame; The active power and reactive power in the primary control module are processed by a first-order low-pass filter; The decoupling of positive and negative sequences in the power calculation module is based on a bi-second-order generalized integrator; A virtual impedance is provided in front of the voltage outer loop, and a secondary voltage frequency amplitude control and grid connection pre-synchronization control are integrated in the droop control.

5. The control method of the grid-forming industrial microgrid energy router according to claim 3, characterized in that The power calculation module is used to calculate a power reference value according to the degree of asymmetric voltage dip.

6. The control method of the network-constructing industrial microgrid energy router according to claim 1, wherein, The variables include: the state of charge threshold and rated capacity of the battery energy storage unit, and the monitoring data includes: the real-time value of voltage, the real-time value of current, and the real-time state of charge of the battery energy storage unit.

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