CN118611158B

CN118611158B - Meshing type industrial microgrid energy router and control method thereof

Info

Publication number: CN118611158B
Application number: CN202411079640.6A
Authority: CN
Inventors: 郑子萱; 李世杰; 青叶平; 贺岩; 张名顺; 姜山; 任杰; 徐彬翔
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2024-08-07
Filing date: 2024-08-07
Publication date: 2025-07-18
Anticipated expiration: 2044-08-07
Also published as: CN118611158A

Abstract

The invention provides a grid-built industrial micro-grid energy router and a control method thereof, wherein a photovoltaic port is connected with a direct current bus through a boost DC/DC converter, a battery energy storage unit port is connected with the direct current bus through a boost/buck bidirectional DC/DC converter, a direct current load port is directly connected with the direct current bus, one end of a grid-connected port is connected with the direct current bus, the other end of the grid-connected port is connected with an alternating current bus, the photovoltaic port is used for collecting renewable energy, the battery energy storage unit port is used for stabilizing energy changes caused by load and power supply fluctuation and providing standby energy when the renewable energy is insufficient, the direct current load port is used for meeting the power demand of a direct current sensitive load, and the grid-connected port is used for transmitting power between the energy router and a power grid and actively supporting the power grid. The power grid can be constructed in a grid-connected mode, the power grid can be supported in a grid-connected mode and a transient support mode, and the reliability and the quality of power supply can be ensured.

Description

Network construction type industrial micro-grid energy router and control method thereof

Technical Field

The invention relates to the technical field of energy storage systems, in particular to a network construction type industrial micro-grid energy router and a control method thereof.

Background

As global energy conversion progresses, the specific gravity of renewable energy sources (renewable energy sources, RESs) in industrial micro-grids is increasing. However, the power electronic equipment of RESs lacks the physical rotating shaft of the synchronous generator, so that the novel power system gradually has the characteristics of low inertia, weak damping and low short-circuit ratio, and the networking and stable operation capabilities of the novel power system are weakened. Moreover, due to critical operational requirements of sensitive equipment, direct impact on productivity and safety, potential cost impact of downtime, and the necessity to comply with strict regulatory standards, power quality issues such as voltage sag, sag and imbalance become particularly important in industrial micro-grids. Therefore, RESs inverters can be added to the grid and provide auxiliary services such as active support to meet the high quality power supply of sensitive loads.

In order to realize high-quality operation of the micro-grid and flexible regulation of electric power, an Energy Router (ER) concept is proposed, and the micro-grid has the characteristics of power conversion, energy management, plug and play and the like. ER is composed of a series of controlled elements and can be used as intelligent interfaces of RESs, energy storage units, power grids, loads and other devices. ER can achieve multi-stage distribution of power and flow control while reducing the number of components, size, cost and power consumption compared to conventional multi-converter architectures. The unique characteristics and capabilities of ER make it suitable for constructing and supporting modern power grids featuring constantly increasing renewable energy permeability and decentralized power generation. Current research into ER is focused mainly on structural design and energy management strategies, and the potential for supporting the grid is still lacking.

In addition, some studies have investigated RESs the possibility of providing active support for the grid. Usually, the active support of RESs is realized by controlling the output current of the grid-connected converter, the phase of the grid-connected converter is synchronous with the power grid through the phase-locked loop, and meanwhile, the current loop ensures the problems of power injection precision and current quality. Some studies have proposed strategies for active support of photovoltaic inverters, with reference currents obtained by calculation of the support power. Some studies calculate positive and negative sequence reference currents, respectively, for active support in unbalanced faults. Some studies have proposed a reference current generator capable of eliminating active power oscillation and compensating for voltage imbalance. However, the active support strategy described above is only applicable to heel-net type control, and is not applicable to net-type control without a phase-locked loop. Droop control is a typical network-structured control strategy, has the advantages of quick control response and the like, and is also studied in the field of active support of a power grid. Some studies have investigated an active support strategy based on layered droop control. The secondary control mainly performs positive and negative sequence power calculation under the condition of voltage sag and sends a voltage supporting signal to the primary control. However, experimental results indicate that the power quality control strategy of the inverter is not accurate, and even if the inverter supports the grid, there is a voltage deviation. Some studies introduce a hybrid control strategy combining frequency droop with a passive-based switching function, which can restore voltage levels by injecting positive/negative sequence active/reactive power. However, the voltage dip depth of the simulation and experimental setup is only 20%, which does not verify the supporting effect in more extreme cases. The above-mentioned researches have focused mainly on the control of the inverter, but do not sufficiently consider the system of the dc side connection of the inverter, which may cause the dc bus voltage to be unstable. Further, the capacity limitation of the inverter has not been sufficiently solved.

Some studies have given the ER the ability to improve the power quality of the grid. Some studies have proposed ER with 5 ports that enable integration of photovoltaic systems, energy storage systems, and ac-dc loads. The system has an advanced energy management strategy and can improve part of the electric energy quality of the power grid. However, the mechanism of improvement in power quality is achieved by isolating the load and the disturbance of the photovoltaic from the grid, reducing potential sources of disturbance, rather than actively supporting the grid. In some studies, the proposed ER can handle harmonics and guarantee three-phase power balance of the microgrid. However, it does not provide voltage support during a fault.

In summary, the current ER model does not have the capability of constructing a network and actively supporting the network at the same time, and there is a certain limitation in the research of active support control.

Disclosure of Invention

In view of the above, the present invention aims to provide a network-structured industrial micro-grid energy router and a control method thereof, which can not only construct and support a grid, but also ensure the reliability and quality of power supply.

In a first aspect, the embodiment of the invention provides a grid-built industrial micro-grid energy router, which comprises a photovoltaic port, a battery energy storage unit port, a direct current load port and a grid connection port, wherein the photovoltaic port is connected with a direct current bus through a boost DC/DC converter, the battery energy storage unit port is connected with the direct current bus through a boost/buck bidirectional DC/DC converter, the direct current load port is directly connected with the direct current bus, one end of the grid connection port is connected with the direct current bus, the other end of the grid connection port is connected with an alternating current bus, the photovoltaic port is used for collecting renewable energy, the battery energy storage unit port is used for stabilizing energy changes caused by load and power supply fluctuation and providing standby energy when the renewable energy is insufficient, the direct current load port is used for meeting the power demand of a direct current sensitive load, and the grid connection port is used for power transmission between the energy router and a power grid and actively supporting the power grid.

In a second aspect, the embodiment of the invention further provides a layered cooperative control method of the grid-built industrial micro-grid energy router, which is applied to the control method of the grid-built industrial micro-grid energy router, and comprises the steps of obtaining an instruction of an upper controller, determining a control strategy of a photovoltaic system and a battery energy storage unit port based on the instruction of the upper controller, wherein the control strategy of the photovoltaic system comprises constant voltage control and maximum power point tracking control, and the control strategy of the battery energy storage unit port comprises constant voltage control and standby mode.

In an alternative embodiment of the application, the maximum power point tracking control is implemented by adopting an admittance increment method, the maximum power point tracking control is provided with a power limiting threshold value so that the maximum output power of the photovoltaic system is not greater than the power limiting threshold value, the constant voltage control adopts a double-ring strategy, the double ring comprises a voltage outer ring and a current inner ring, and the reference current of a port of the battery energy storage unit is 0 in a standby mode.

In an alternative embodiment of the application, the control method of the grid-structured industrial micro-grid energy router further comprises the step of controlling the DC/AC converter in a droop control mode based on positive and negative sequence separation, wherein the droop control based on positive and negative sequence separation comprises a primary control module and a power calculation module.

In an alternative embodiment of the present application, the primary control module includes a current inner loop, a voltage outer loop and a droop control loop, where the current inner loop, the voltage outer loop and the droop control loop are implemented in a two-phase rotation reference system, active power and reactive power in the primary control module are processed by a first-order low-pass filter, decoupling of a negative sequence in the primary control module is performed based on a bi-second-order generalized integrator, virtual impedance is set in front of the voltage outer loop, and two-stage voltage frequency amplitude control and grid-connected presynchronization control are integrated in the droop control.

In an alternative embodiment of the present application, the power calculation module is configured to calculate the power reference value according to an asymmetric voltage drop degree.

In an optional embodiment of the application, the control method of the grid-built industrial micro-grid energy router further comprises initializing variables, processing monitoring data to calculate power of the photovoltaic system, the battery energy storage unit, the local load and the grid connection port, determining operation modes of the photovoltaic system and the battery energy storage unit port according to an internal energy management strategy, actively switching the operation modes of the grid-built industrial micro-grid energy router according to the degree of grid voltage sag, optimizing grid support through a power distribution module in the grid connection mode and the transient support mode, and transmitting the operation modes of the grid-built industrial micro-grid energy router to an upper controller.

In an alternative embodiment of the application, the variables include a state of charge threshold and a rated capacity of the battery energy storage unit port, and the monitored data includes a real-time value of the voltage, a real-time value of the current, and a real-time state of charge of the battery energy storage unit port.

In an alternative embodiment of the application, the operation modes of the grid-built industrial micro-grid energy router comprise a grid connection mode, a transient support mode and a grid-built mode, wherein in the grid connection mode, the grid-built industrial micro-grid energy router exchanges power with a power grid and performs steady-state support, in the transient support mode, the grid-built industrial micro-grid energy router actively supports the power grid by providing specified positive and negative sequence active and reactive power responses, and in the grid-built mode, the grid-built industrial micro-grid energy router reduces amplitude and frequency deviation of an output voltage through secondary voltage amplitude and frequency control.

In an alternative embodiment of the present application, 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 of the voltage support and the rated capacity of the converter.

The embodiment of the invention has the following beneficial effects:

The embodiment of the invention provides a grid-forming industrial micro-grid energy router (grid-forming multi-functional energy router, GFMER) and a control method thereof, and the GFMER not only can construct and support a grid, but also can ensure the reliability and quality of power supply.

Additional features and advantages of the disclosure will be set forth in the description which follows, or in part will be obvious from the description, or may be learned by practice of the techniques of the disclosure.

The foregoing objects, features and advantages of the disclosure will be more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are needed in the description of the embodiments or the prior art will be briefly described, and it is obvious that the drawings in the description below are some embodiments of the present invention, and other drawings can be obtained according to the drawings without inventive effort for a person skilled in the art.

Fig. 1 is a schematic diagram of a basic topology of a network-structured industrial micro-grid energy router according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a PV and BESU control strategy provided in an embodiment of the present invention;

FIG. 3 is a schematic diagram of a two-stage control according to an embodiment of the present invention;

Fig. 4 is a schematic diagram of a DC/AC converter control strategy according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of an upper layer control flow provided in an embodiment of the present invention;

FIG. 6 is a schematic diagram of simulation results at 25% voltage sag according to an embodiment of the present invention;

FIG. 7 is a schematic diagram of simulation results at a 25% voltage sag according to another embodiment of the present invention;

FIG. 8 is a schematic diagram of a voltage sag PCC of a different power grid according to an embodiment of the present invention;

FIG. 9 is a schematic diagram of a simulation result of a networking mode according to an embodiment of the present invention;

FIG. 10 is a schematic diagram of another simulation result of a networking mode according to an embodiment of the present invention;

FIG. 11 is a schematic diagram of a simulation result of converting a network configuration into a grid connection according to an embodiment of the present invention;

FIG. 12 is a schematic diagram of simulation results of a grid-connected to-grid conversion network according to an embodiment of the present invention;

FIG. 13 is a schematic diagram of a CHIL platform according to an embodiment of the present invention;

FIG. 14 is a schematic diagram of a transient support experimental result provided by an embodiment of the present invention;

FIG. 15 is a schematic diagram of a networking mode experiment result provided in an embodiment of the present invention;

Fig. 16 is a schematic diagram of a phase a voltage during mode switching according to an embodiment of the present invention.

Detailed Description

For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The energy router has the ability to achieve efficient digestion of renewable energy sources (renewable energy sources, RESs) in an industrial microgrid and to ensure consumer electricity reliability. The current ER model does not have the capability of constructing a network and actively supporting the power grid at the same time, and has certain limitation on the research of active support control.

Based on the above, the embodiment of the invention provides a network-structured industrial micro-grid energy router and a control method thereof, in particular provides a multifunctional network-structured energy router (grid-forming energy router, GFMER) using a layered cooperative control strategy. The underlying control includes operating strategies for Photovoltaic (PV), battery energy storage units (battery energy storage unit, BESU), and DC/AC converters. In the lower control an improved multi-objective droop control is proposed to actively adjust for grid side voltage imbalance and deviation. Meanwhile, the upper control is used for maintaining the voltage of the direct current bus and adding a power distribution module to improve the dynamic supporting capacity of the power grid. Finally, the practicability and effectiveness of the provided control strategy are verified through MATLAB/Simulink and hardware in-loop experiments.

The embodiment provides a network-structured multi-functional energy router (GFMER), and a hierarchical cooperative control scheme is provided for the network-structured multi-functional energy router. The GFMER not only can construct and support a power grid, but also can ensure the reliability and the quality of power supply. The embodiment mainly provides that:

1) The proposed GFMER supports multi-port access to the grid, battery storage units (battery energy storage unit, BESU), photovoltaics and loads, and provides three modes of operation, grid-connected, transient support and grid-formation.

2) In the lower level controller, an improved multi-objective droop control is proposed that actively adjusts for ac side grid voltage imbalances and deviations in various modes of operation.

3) The upper layer controller is designed to keep the voltage of the direct current bus stable and the energy is routed. The power distribution module is further deployed in the last controller to improve GFMER dynamic support capability to the power grid in grid-connected and transient support modes.

For the sake of understanding the present embodiment, a detailed description will be first given of a network-structured micro-grid energy router disclosed in the present embodiment.

Embodiment one:

The embodiment of the invention provides a grid-built industrial micro-grid energy router, which is shown in a basic topological schematic diagram of the grid-built industrial micro-grid energy router in fig. 1, and comprises a photovoltaic port, a battery energy storage unit port, a direct current load port and a grid-connected port, wherein the photovoltaic port is connected with a direct current bus through a boost DC/DC converter, the battery energy storage unit port is connected with the direct current bus through a boost/buck bidirectional DC/DC converter, the direct current load port is directly connected with the direct current bus, one end of the grid-connected port is connected with the direct current bus, the other end of the grid-connected port is connected with the alternating current bus, the photovoltaic port is used for collecting renewable energy sources, the battery energy storage unit port is used for stabilizing energy changes caused by load and power supply fluctuation, and providing standby energy sources when the renewable energy sources are insufficient, the direct current load port is used for meeting the power requirements of direct current sensitive loads, and the grid-connected port is used for power transmission between the energy router and the grid and actively supporting the grid.

As shown in fig. 1, the parallel connection structure of GFMER system contains 4 ports:

1) The PV (photovoltaic system) ports, i.e. photovoltaic ports, are connected to 750V DC bus through a boost DC/DC converter for collecting renewable energy.

2) BESU (battery storage unit) port in GFMER, BESU is able to stabilize energy changes caused by load and power fluctuations and provide backup power when renewable energy is not available. To maintain energy balance between the DC side and the ac side of GFMER, BESU is connected to the DC bus through a boost/buck bi-directional DC/DC converter.

3) DC load port-DC load port is directly connected to 750V DC bus to meet power demand of DC sensitive load and to comply with prescribed voltage class standard.

4) And the grid connection port is an important component of GFMER, and is beneficial to power transmission between the energy router and the power grid and active support of the power grid. Furthermore, in severe grid faults, the energy router may be isolated from the grid by disconnecting this port.

In this system, U _bus1 and U _busn are dc bus voltages of 1 st and n GFMER, respectively. I _pv1 and I _pvn are photovoltaic output currents of 1 st and n GFMER th, and U _pv is an output voltage of photovoltaic. I _Lb1 and I _Lbn are BESU output voltages of the 1 st and n GFMER th. The grid-connected converter consists of a three-phase voltage source SPWM inverter and a filter. Further, R _g and L _g are the resistance and inductance, respectively, of the grid impedance, and u _g is the grid voltage. v _o1 and v _on are the output voltages of the 1 st and n GFMER th. i _o1、i_on、i_g and i _load are the output current of the 1 st and n GFMER th, respectively, the current injected into the grid and the load current. The 380V alternating current sensitive load is a local load, and can be a resistive, inductive and capacitive load or a linear or nonlinear load.

The relationship between the current injected into the grid, the output current of GFMER and the load current is:

(1)

the relationship of the output voltages of each GFMER is:

(2)

Since GFMER of the present embodiment was connected to a 380V/50Hz low voltage distribution network, the main circuit does not need to be electrically isolated and the voltage class and power form are converted by a power electronic converter. Furthermore, the integrated all-power electronic circuit contributes to the modularization and miniaturization of GFMER.

The embodiment of the invention provides a network construction type industrial micro-grid energy router which not only can construct and support a grid, but also can ensure the reliability and quality of power supply.

Embodiment two:

The embodiment provides a control method of a network construction type industrial micro-grid energy router, which is realized on the basis of the embodiment, and mainly describes a lower control strategy. The embodiment firstly provides a PV and BESU control strategy, the control method of the grid-built industrial micro-grid energy router comprises the steps of obtaining an instruction of an upper controller, determining a control strategy of a photovoltaic system and a battery energy storage unit port based on the instruction of the upper controller, wherein the control strategy of the photovoltaic system comprises constant voltage control and maximum power point tracking control, and the control strategy of the battery energy storage unit port comprises constant voltage control and standby mode.

Referring to the schematic of one PV and BESU control strategy shown in fig. 2, the control mode of the PV ports needs to be adjusted according to the light energy and power balance requirements of GFMER.

In some embodiments, the maximum power point tracking control is implemented by adopting an admittance increment method, the maximum power point tracking control is provided with a power limiting threshold value, so that the maximum output power of the photovoltaic system is not greater than the power limiting threshold value, the constant voltage control adopts a double-loop strategy, the double loop comprises an external voltage loop and an internal current loop, and the reference current of a port of the battery energy storage unit is 0 in a standby mode.

In this embodiment, the control strategy of the photovoltaic system selects Constant Voltage (CV) control and maximum power point tracking (maximum power point tracking, MPPT) control, as shown in fig. 2. For MPPT control, maximum power tracking is achieved using admittance delta methods, while a power limit is imposed (P _limit). When the maximum output power (P _{pv_mppt}) of the photovoltaic system exceeds P _limit, the photovoltaic output power is limited to P _limit.

The BESU ports likewise have a variety of control methods, as shown in figure 2. The CV control adopts a double-ring strategy, comprising an external voltage ring and an internal current ring, so that the voltage of the direct current bus is ensured to be accurate and stable. In some cases BESU will switch to standby mode. In standby mode, the reference current is 0.

It should be noted that both the photovoltaic and BESU control strategies need to be switched and operated by the commands of the upper controller.

The embodiment also provides a DC/AC converter control strategy. In some embodiments, the DC/AC converter can also be controlled by adopting a droop control mode based on positive and negative sequence separation, wherein the droop control mode based on positive and negative sequence separation comprises a primary control module and a power calculation module.

Sag control is a method of controlling a converter by simulating the external sag characteristics of a synchronous generator. To meet the requirements of self-regulation, communication independence and networking capability, the DC/AC converter employs droop control. It is noted that GFMER need not only output positive-sequence active/reactive power at voltage sag, but also negative-sequence active/reactive power to restore and balance the voltage of the common junction (point of common coupling, PCC). In order to meet the requirement of dynamic support of the power grid, an improved droop control based on positive and negative sequence separation is proposed, and comprises a primary control module and a power calculation module.

In some embodiments, the primary control module comprises a current inner ring, a voltage outer ring and a droop control ring, wherein the current inner ring, the voltage outer ring and the droop control ring are realized in a two-phase rotating reference system, active power and reactive power in the primary control module are processed through a first-order low-pass filter, decoupling of negative sequences in the primary control module is performed based on a two-second-order generalized integrator, virtual impedance is arranged in front of the voltage outer ring, and secondary voltage frequency amplitude control and grid-connected presynchronization control are integrated in the droop control.

1. Primary control module

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) to facilitate subsequent decoupling. In addition, in order to filter out power fluctuations and improve control accuracy, the active and reactive power is processed through a first order low pass filter. It should be noted that the decoupling of positive and negative sequences is based on a biquad generalized integrator (DSOGI), and the method has high precision, high reliability and strong anti-interference capability.

Conventional droop control methods were developed based on inductive line impedance, which is inconsistent with the resistive impedance often encountered in low voltage distribution networks. This difference may result in 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 GFMER and to give the system better damping characteristics without sacrificing the system efficiency.

In addition, in order to reduce the deviation of the frequency and amplitude of the output voltage of the port, a secondary voltage frequency amplitude control is integrated in droop control, the main principle of which is to shift the working point of the droop curve, restore the voltage frequency and amplitude to the set values while maintaining constant active and reactive outputs. The control principle can be seen from a schematic diagram of a two-stage control shown in fig. 3. Meanwhile, grid-connected presynchronization control is added, so that GFMER can be switched seamlessly in a grid-connected mode and a grid-off mode. The control uses GFMER output voltage amplitude and phase difference with the grid voltage as feedback signals, and the feedback signals are regulated by a Proportional Integral (PI) controller to generate compensation quantities of the converter voltage and frequency.

In some embodiments, the power calculation module is configured to calculate the power reference value based on an asymmetric voltage sag level.

2. Power calculation module

When an asymmetric voltage dip occurs in the power grid, the primary control module lacks the ability to adjust the power reference value, resulting in a deviation of the supported voltage. To solve this problem, a power calculation module was developed based on the primary droop module, and reference may be made to a schematic diagram of a DC/AC converter control strategy shown in fig. 4. The module calculates a proper power reference value according to the asymmetric voltage drop degree, and realizes the dynamic adjustment of the power reference value.

In asymmetric faults, the voltage and current at the PCC may be expressed as the sum of positive, negative, zero sequence components:

(3)

For a three-phase three-wire system, the zero sequence component is negligible because the neutral point is not grounded. Through coordinate transformation, the voltage and the current of the PCC point can be represented by positive and negative sequence components in an alpha beta coordinate system, wherein the voltage is represented by a formula (4), and the current is represented by a formula (5).

(4)

(5)

Wherein, the Positive sequence/negative sequence voltages, respectively; Positive sequence/negative sequence currents, respectively. Is the positive/negative sequence voltage amplitude of the PCC,AndThe magnitudes 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.

Then, the compounds of the formulae (4) and (5) are mixed、、The method can be converted into a d-q coordinate system to obtain:

(6)

(7)

(8)

(9)

the relation between the PCC point voltage and the power grid voltage in the alpha beta coordinate system is as follows:

(10)

(11)

bringing formulae (4) - (5) into (8) - (11), a voltage support equation can be obtained:

(12)

(13)

In the middle of The positive and negative sequence component amplitudes of the grid side voltages are respectively obtained.

From equations (12) and (13), it can be seen that the effect of the active current component on the grid voltage support is not negligible if the grid is considered resistive rather than purely inductive.

Reference value of positive and negative sequence voltageTaking into account the above equations and the load current, the initial current reference value of each GFMER #And) The method can obtain:

(14)

(15)

(16)

(17)

Wherein I _loadd and I _loadq are load currents under d-q coordinate system, and S _n1,S_nk and S _nn are capacities of the 1 st, k th and n th converters respectively.

The formula for calculating the positive/negative sequence active/reactive outputs (P ^+/- and Q ^+/-) of GFMER using a first order low pass filter is as follows:

(18)

(19)

(20)

(21)

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

The current reference values in the formulas (14) - (17) are brought into the formulas (18) - (21) to obtain the droop control positive and negative sequence active/reactive initial reference powerAnd). It should be noted that the final reference value [ ]And) It is required to be obtained from an upper controller. The calculated reference value is then sent to the primary control module. By combining two control modules, the proposed droop control enables GFMER to provide accurate support for the grid.

In summary, after adding the secondary control, virtual impedance and grid-connected presynchronization, the proposed droop control expression is:

(22)

(23)

(24)

In the middle of Is rated positive and negative sequence angular frequency; And Positive/negative sequence active-frequency, reactive-voltage droop coefficients, respectively; And Virtual resistance and reactance respectively; And Positive/negative sequence voltage angular frequency and amplitude difference of the measured value and the reference value respectively;、 The positive/negative sequence voltage amplitude and phase difference with the grid are respectively GFMER outputs. ,,AndCan be calculated as follows:

(25)

(26)

(27)

(28)

In the middle of Outputting a positive/negative sequence angular frequency detection value for GFMER; Is a grid positive/negative sequence angular frequency detection value.

Embodiment III:

The embodiment provides a control method of a network construction type industrial micro-grid energy router, which is realized on the basis of the embodiment, and mainly describes an upper control strategy. The control method of the grid-built industrial micro-grid energy router comprises the steps of initializing variables, processing monitoring data to calculate power of a photovoltaic system, a battery energy storage unit, a local load and a grid connection port, determining operation modes of the photovoltaic system and the battery energy storage unit port according to an internal energy management strategy, actively switching the operation modes of the grid-built industrial micro-grid energy router according to the sinking degree of grid voltage, optimizing grid support through a power distribution module in the grid connection mode and the transient support mode, and sending the operation modes of the grid-built industrial micro-grid energy router to an upper controller.

The upper control flow mentioned in this embodiment can be referred to as a schematic diagram of an upper control flow shown in fig. 5, and its main functions are that 1) it is ensured that the dc bus voltage is stabilized at the rated level in all cases, 2) the voltage unbalance and deviation on the ac power grid side are actively regulated to the maximum extent, and 3) the operation mode is actively switched according to different degrees of the power grid voltage drop.

The control flow is as follows, firstly, initializing variables and processing monitoring data. Next, the operating modes of the photovoltaic sum BESU are determined according to the internal energy management strategy. Then, the operation mode is actively switched according to the degree of the grid voltage sag. In grid-tie mode and transient support mode, the power distribution module is used to optimize grid support. Finally, the instruction is sent to the lower controller.

In some embodiments, the variables include a state of charge threshold and a rated capacity of the battery energy storage unit portal, and the monitored data includes a real-time value of the voltage, a real-time value of the current, and a real-time state of charge of the battery energy storage unit portal.

1. Data processing

First, variables including state of charge (SoC) thresholds (SoCmax and SoCmin) and rated capacity (Sn) of BESU are initialized and then real-time values of voltage and current, shown in fig. 1 by red marks, and real-time SoC of BESU are monitored. Finally, the measurement data is processed to calculate the power of the PV, BESU, local load, and grid-tie ports.

2. Internal energy management scheme

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

(29)

Wherein P _g is the exchange power of GFMER and the power grid, P _l is GFMER internal DC load power, P _pv is PV output power, and P _b is BESU output power.

According to the formula (29) and taking the SoC of BESU as a constraint condition, the energy management flow inside the energy router proposed by the embodiment can be obtained, as shown in FIG. 5. The core of the internal energy management is to fully utilize the photovoltaic output power while keeping the voltage of the direct current bus constant. The modes of operation of PV and BESU are shown in table 1.

TABLE 1PV and BESU modes of operation

Energy relation	SoC	PV mode of operation	BESU mode of operation
				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

At night, since the photovoltaic output power is 0, the control target for maintaining the dc bus voltage is fully assumed by BESU, at which time BESU operates in CV mode. When the direct current is loaded with power demand, if it is greater than SoC _min, BESU performs CV control and supplies power to the load, at this point P _g=P_l-P_b. If it is smaller than SoC _min, BESU still employs CV control, the power required by the load is taken mainly from the grid, at which point P _g=P_l. When no load power is required, the energy router is in a standby state and outputs power to an external power grid when required to meet the requirements of power quality control.

During the daytime, it is first determined whether the photovoltaic output power P _pv is less than the load demand power P _l. If P _pv>P_l, during the time that the SoC of the energy storage system is less than SoC _max, the photovoltaic system is operated in MPPT mode and BESU is operated in CV mode, at which point P _g=P_l-P_pv-P_b. When the SoC reaches SoC _max, the control of the photovoltaic is switched to CV mode, and the stored energy enters a standby state, at this time P _g=P_l-P_pv-P_b. If P _pv<P_l, when the SoC is larger than SoC _min, BESU is in CV control state and photovoltaic system is in MPPT running state. When the SoC is smaller than SoC _min, the photovoltaic system switches from MPPT mode to CV mode.

It should be noted that when the trigger mechanism detects an abnormal operation of the power grid, the command transmitted from the internal energy management to the lower controller is appropriately adjusted to implement the function of each operation mode GFMER.

In some embodiments, the operating modes of the grid-formation type industrial micro-grid energy router include a grid-connection mode, a transient support mode and a grid-formation mode, and the power distribution module adjusts power and power reference values transmitted to the grid based on the total required capacity, the voltage support required capacity and the rated capacity of the converter.

3. Pattern partitioning

The proposed GFMER has three different modes of operation, grid-tie mode, transient support mode and grid-tie mode. These modes enable power exchange, active support and local load optimization, respectively, and ensure reliable and efficient power distribution.

1) And in a grid-connected mode, GFMER is in the grid-connected mode when the power grid normally operates. In this mode GFMER not only exchanges power with the grid, but also performs steady-state support to improve the power quality of the grid.

2) Transient support mode-GFMER will operate in transient support mode when a grid voltage dip is detected and the drop depth is less than 50%. In this mode GFMER actively supports the grid by providing a specified positive and negative sequence active and reactive power fast response.

3) Network mode-when the grid voltage drops by more than 50%, the PCC voltage cannot be lifted to the rated range even if active support is performed due to the limit of GFMER capacity. To ensure good operation of the local ac and dc loads, GFMER will be off grid and operate in grid mode. In this mode, the secondary voltage amplitude and frequency control may reduce amplitude and frequency deviations of the output voltage.

4. Power distribution module

Due to the limited capacity of GFMER, it is a challenge to distribute the access power reasonably to maximize the support to the grid. Based on which one power distribution module is added in grid-tie mode and transient support mode. The module considers the power requirements of the active support and reallocates GFMER internal power by prioritizing the power, the capacity of GFMER is fully accounted for. The reference power of sagging control is further adjusted by the module to optimize the active support effect.

The module prioritizes the power output of each port with the premium power supply and active voltage support as primary and secondary targets, respectively. Wherein the power priority of the load output port is highest to ensure the quality of the load power supply in any case. The second priority is to output active and reactive power for the converter to support the grid, thereby enhancing active support capability in various scenarios. Finally, the power exchange between GFMER and the grid is set to the third stage.

The active and reactive power for the support voltage can be expressed as:

(30)

(31)

Where P _s and Q _s are the active and reactive power, respectively, of the supporting grid.

The switching power can be expressed as:

(32)

Where P _out is GFMER the power delivered to the grid.

The total required capacity S _t and the voltage support required capacity S _r can be calculated as:

(33)

(34)

According to the relation between S _t、S_r and the rated capacity S _n of the converter, the following three scenarios can be classified:

1) When S _t>S_n>S_r, the total required capacity exceeds the rated capacity of the converter, an overload may result. To ensure the voltage supporting effect, the required active and reactive reference power is unchanged. The power transmitted to the grid will be adjusted to:

(35)

2) At S _t>S_r>S_n, the converter needs to output more power to actively support the grid. However, the capacity of the converter is limited, so the required power cannot be provided, and therefore the power reference value needs to be adjusted as follows:

(36)

(37)

(38)

3) When S _n>S_t>S_r, the required active support power is small, and the total required capacity is lower than the rated capacity of the converter. Thus, there is no need to adjust the reference power, and the final reference power is equal to the initial reference power.

Combining equations (29) and (32) it can be concluded that P _out is affected by P _pv. If P _out needs to be adjusted, the specific reference power (P _limit) sent by the photovoltaic upper layer controller operates in a power limiting mode, and the calculation formula is as follows:

(39)

Embodiment four:

The present embodiment provides simulation result analysis for the previous embodiment. In order to verify feasibility and effectiveness of GFMER and a control method thereof according to the present embodiment, a system simulation model as shown in fig. 1 is built based on MATLAB/Simulink, wherein the number of parallel GFMER is 2. Simulations verify GFMER performance in each mode of operation. The main parameters of the system are shown in table 2.

TABLE 2 System principal parameters

Parameters (parameters)	Expression type	Specific values
			Power 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 of	SLb	20 Ah
Network side line inductance	Lg	12 mH
			Network side line resistance	Rg	0.8 Ω
Grid-connected port LC filter	Lf, Cf	12 mH, 10μF
			Grid-connected port capacity	Sn	10kVA
Maximum photovoltaic power	Pmpp	30 kW
			Local ac load	Pl-AC	12 kW
Each GFMER is connected with a DC load	Pl	14 kW
			Switching frequency	fs	10 kHz

1. Transient support simulation verification

The transient support mode operates under grid-tie conditions, and if the transient support capability can be verified, the steady state support capability in the grid-tie mode can be verified as well. Therefore, this section focuses on transient support capability simulation verification. The illumination intensity in the grid-connected simulation fluctuates between 600 and 1000W/m ². The active support performance of the proposed strategy was tested at 25% two-phase voltage sag. Referring to fig. 6, a schematic diagram of a simulation result at 25% voltage sag is shown in fig. 6 (a), where the phase voltage amplitude is 311V under normal conditions, and the voltage sag occurs at t=0.2 s and is eliminated at t=0.5 s. It is apparent that during a grid voltage sag, the PCC voltage rises instantaneously to 311V, and the grid voltage sag has little effect. The unbalance of the PCC voltage and the grid voltage is shown in fig. 6 (c). It can be seen that the three-phase imbalance factor of the PCC voltage is always almost 0, well below the three-phase imbalance factor of the grid voltage during the fault period. Therefore, under the condition of grid voltage sag, GFMER can effectively support PCC voltage and protect the AC sensitive load from damage.

Referring to another schematic diagram of simulation results at 25% voltage sag shown in fig. 7, the output power of GFMER and GFMER 2 at grid voltage sag is shown in fig. 7 (a). It is apparent that under the combined action of the proposed droop control and power distribution modules GFMER can adaptively output the appropriate active/reactive power according to the depth of the voltage sag, and the output power can be well shared between the parallel GFMER. As can be seen from fig. 7 (b), under the condition that the illumination intensity changes and the power grid is actively supported, the voltage of the dc bus is always kept near 750V, so that the normal operation of the dc load is ensured and the effectiveness of the upper controller in the transient support mode is verified. Due to the negative sequence component, the dc side voltage oscillates when the GFMER actively supports the grid. However, the oscillation amplitude is only 2.8V, and the influence on the direct current load is negligible.

Finally, the active supporting performance under different grid voltage drop conditions is tested. Referring to the schematic of a different grid voltage sag PCC voltage shown in fig. 8, when the grid voltage sag depths are 12%, 20% and 33%, respectively, the PCC voltage can be supported above 0.99p.u due to the power distribution module and the proposed sag control. At 48% voltage sag, the power distribution module properly reduces the reference power due to the need for higher power support, and limited converter capacity, the PCC voltage is supported to 0.96 p.u..

2. Networking mode simulation verification

Due to limited capacity of the converter, when the grid voltage drops severely, the PCC voltage cannot meet the load operation requirements even if GFMER is actively providing support. At this point GFMER will operate in the mesh mode. In this simulation, the photovoltaic power is set to increase once every 0.1 seconds, BESU charges to SoC _max at t=0.5 s.

Verification of secondary voltage amplitude and frequency control referring to a schematic diagram of a net pattern simulation result shown in fig. 9, secondary control is put into operation at t=0.1 s. Obviously, the control strategy adds secondary control in droop control, so that the frequency and amplitude deviation of GFMER output alternating voltage can be reduced, and the electric energy quality can be optimized.

Fig. 10 is a schematic diagram of another simulation result of a network mode, and active power curves of the photovoltaic, BESU and the ac/dc load in the network mode are shown in fig. 10 (a). Before t=0.5 s, BESU switches from the charge state to the discharge state as the photovoltaic power gradually increases, and the charge-discharge power varies with the photovoltaic power. After t=0.5 s, it is assumed that the SoC BESU has reached SoC _max, at which point it will switch to standby mode with an output power of 0. At the same time the photovoltaic switches from MPPT mode to CV mode while powering the load. During this time, both ac and dc loads can be operated stably. As can be seen from fig. 10 (b), the dc bus voltage was always kept around 750V, and the maximum deviation was only 3V.

3. Mode switching simulation verification

The grid-connected pre-synchronization simulation result is shown in a schematic diagram of a grid-connected to-grid simulation result in FIG. 11. GFMER is grid connected at t=0.8 s. It is evident that the voltage and frequency fluctuations during pre-synchronization grid connection are significantly reduced. The amplitude and frequency waveforms of the PCC voltage when the GFMER grid-tie mode is switched to grid-tie mode are shown in a schematic diagram of a grid-tie-to-grid simulation result shown in fig. 12. At t=1.5 s, GFMER switches to the mesh mode. The frequency and amplitude fluctuations of the PCC voltage during switching are almost negligible. Simulation results show that the proposed GFMER can smoothly switch the operating mode.

Fifth embodiment:

This example provides an analysis of experimental results for the previous examples. In order to more fully verify the effect of the proposed GFMER and control strategy, a GFMER system connected with the power distribution network is built on a STARSIM CHIL platform. The system parameters are shown in Table 2, with the only difference that the local AC load becomes 6kW. The main circuit was simulated on an MT 6020 simulator with a time step of 1 mus. Hierarchical active control of GFMER was achieved using MT1050 Rapid Control Prototype (RCP) with a time step of 10 mus. The proposed control strategy is implemented in only one GFMER due to the limited number of I/O ports. But all parallel GFMER are based on the same control strategy and parameters, multiple GFMER scenarios can be verified if the corresponding performance in one GFMER scenario can be verified. CHIL platform schematic view referring to a schematic view of a CHIL platform shown in fig. 13.

The experimental results of GFMER transient active support can be seen in a schematic diagram of one of the transient support experimental results shown in fig. 14. As can be seen from fig. 14 (a), the grid developed a two-phase voltage dip with a duration of 1 second at 0.44s, with a dip depth of 25%. GFMER support the lower PCC voltage as shown in fig. 15 (b). Comparing fig. 14 (a) and 14 (b), the PCC voltage is supported and the three-phase imbalance is reduced due to the precise injection of positive/negative sequence power. Fig. 14 (c) shows the dc bus voltage inside the support period GFMER. During this time the voltage oscillation is only 10V, which demonstrates the power balance inside GFMER.

The experimental results of the network formation mode can be referred to as a schematic diagram of the experimental results of one network formation mode shown in fig. 15. Fig. 15 (a) shows the output power GFMER in the mesh mode. Obviously GFMER can accurately and stably output power to meet the requirement of alternating current load. GFMER the internal dc bus voltage is shown in fig. 15 (b). It is obvious that the dc bus voltage is always kept at 750V and the maximum oscillation amplitude is not more than 3V.

When GFMER is switched between grid-tied and grid-tied operation, the PCC voltage may be referred to as a schematic diagram of the A-phase voltage at the time of one mode switch as shown in FIG. 16. Obviously, the voltage hardly fluctuates at the moment of mode switching, and it is verified that the proposed GFMER can smoothly switch the operation mode.

In summary, the embodiment of the invention provides a network construction type energy router with a layered cooperative control strategy aiming at the high power quality requirement of an industrial micro-grid. GFMER enable energy complementation between photovoltaic and stored energy to obtain renewable energy sources. Droop control of the DC/AC converter is improved, generation of a power reference value based on voltage sag degree is achieved, and GFMER can provide more accurate support for a power grid in a grid-connected mode and a transient support mode. In addition, secondary control and grid-connected presynchronization are added in droop control, so that the power supply quality of GFMER in the grid-connected mode and mode switching is improved. The power distribution module of the upper controller may provide maximum support for the grid. The internal energy management scheme stabilizes the dc bus voltage, maintaining power balance in any case. Simulation and experimental verification were performed on GFMER modes of operation and switching between modes. The results show that GFMER has good performance, verifying the feasibility and effectiveness of the proposed GFMER and control strategy.

It will be clear to those skilled in the art that, for convenience and brevity of description, reference may be made to the corresponding process in the foregoing embodiment for the specific working process described above, and thus, no further description is given here.

It should be noted that the foregoing embodiments are merely illustrative embodiments of the present invention, and not restrictive, and the scope of the invention is not limited to the foregoing embodiments, but it should be understood by those skilled in the art that any modification, variation or substitution of some technical features of the foregoing embodiments may be made within the scope of the present invention without departing from the spirit and scope of the technical solutions of the embodiments. Therefore, the protection scope of the invention is subject to the protection scope of the claims.

Claims

1. A control method for a meshed industrial microgrid energy router, characterized in that it is applied to a meshed industrial microgrid energy router, wherein the meshed industrial microgrid energy router comprises: a photovoltaic port, a battery energy storage unit port, a DC load port and a grid-connected 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-connected port is connected to the DC bus, and the other end of the grid-connected 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 energy changes caused by load and power supply 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-connected port is used for power transmission between the energy router and the power grid, and actively supports the power grid;

The control method of the network-type industrial microgrid energy router includes: obtaining instructions from an upper-level controller; determining control strategies of a photovoltaic system and a battery energy storage unit based on the instructions of the upper-level 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 includes: constant voltage control and standby mode;

The control method of the grid-type industrial microgrid energy router also includes: initializing variables, processing monitoring data to calculate the power of the photovoltaic system, the battery energy storage unit, the local load and the grid-connected port; determining the control strategy 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-type industrial microgrid energy router according to the degree of grid voltage drop; wherein, in the grid-connected mode and the transient support mode, the grid support is optimized by the power distribution module; and sending the operation mode of the grid-type industrial microgrid energy router to the upper controller;

The operation modes of the meshed industrial microgrid energy router include: grid-connected mode, transient support mode and meshed mode; in the grid-connected mode, the meshed industrial microgrid energy router exchanges power with the grid and also provides steady-state support; in the transient support mode, the meshed industrial microgrid energy router actively supports the grid by providing specified positive and negative sequence active and reactive power responses; in the meshed mode, the meshed 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 power 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;

When S _t >S _n >S _r , the active and reactive reference powers remain unchanged, and the power transmitted to the grid is adjusted to: ; When S _t >S _r >S _n , , the active and reactive reference powers need to be adjusted as follows: , ; When Sn >St _> Sr _, there is no need _to adjust the reference power, _and the final reference power is equal to the initial reference power; where St , Sn _, _and Sr are the total required capacity, the required capacity for voltage support, and the rated capacity of the converter, respectively. and are the active power and reactive power supporting the power grid, and They are respectively the positive and negative sequence active initial reference power and the positive and negative sequence reactive initial reference power.

2. The control method of the network-type industrial microgrid energy router according to claim 1 is characterized in that the maximum power point tracking control adopts the admittance increment method to realize the 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;

A dual-loop strategy is adopted in constant voltage control, which includes an outer voltage loop and an inner current loop;

In the standby mode, the reference current of the battery energy storage unit port is 0.

3. The control method of the meshed industrial microgrid energy router according to claim 1 is characterized in that the control method of the meshed industrial microgrid energy router further comprises:

The DC/AC converter is controlled by adopting a droop control method 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 networked industrial microgrid energy router according to claim 3 is 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 system;

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 performed based on a double second-order generalized integrator;

A virtual impedance is arranged before the voltage outer loop, and secondary voltage frequency amplitude control and grid-connected pre-synchronization control are integrated in the droop control.

5. The control method of the meshed industrial microgrid energy router according to claim 3 is characterized in that the power calculation module is used to calculate the power reference value according to the degree of asymmetric voltage drop.

6. According to the control method of the meshed industrial microgrid energy router according to claim 1, it is characterized in that the variables include: the charge state threshold and rated capacity of the battery energy storage unit, and the monitoring data includes: the real-time value of the voltage, the real-time value of the current, and the real-time charge state of the battery energy storage unit.