Multi-port electric energy router control method and device based on MMC

Abstract

The invention relates to a multi-port electric energy router control method and device based on an MMC. The method comprises the following steps: for a five-port electric energy router based on a double-end back-to-back modular multilevel converter (MMC), connecting a middle-low voltage direct current bus by adopting an input-series output-parallel dual active bridge converter structure, and realizing module power balance of each port through input voltage balance control; in a network-following type control link of a double-end back-to-back modular multilevel converter (MMC), virtual synchronous generator control (VSG) is applied to a grid-connected power control port of the MMC, and virtual direct current motor control (VDMC) is applied to an energy storage port of the MMC; and determining an operation mode based on the power grid short-circuit ratio, and carrying out network following / network construction control switching on the MMC-based multi-port electric energy router under the strong / weak power grid condition. Through the control mode, the multi-port electric energy router can ensure the stability of direct current voltage and the balance of supply and demand of power under complex working conditions.

Description

Technical Field

[0001] The present disclosure relates to the field of new energy application control, and in particular to a method and device for controlling a multi-port power router based on MMC. Background Art

[0002] In the context of global energy transformation, with the integration of high proportions of renewable energy into the grid and the integration of diverse loads such as large-scale energy storage, electric vehicle charging stations, and data centers, traditional power systems face severe challenges in operational stability and efficient renewable energy consumption. The Energy Internet incorporates advanced power electronics and communications technologies, deeply integrating energy systems with information technology. Through intelligent and digital means, it enables efficient energy production, transmission, distribution, and consumption. As a key component of the Energy Internet, power routers enable efficient access, flexible control, and management of various energy sources at the distribution network level, providing key technical support for the optimized operation of distribution networks and the construction of new power systems.

[0003] The receiving-end MMC of the grid-connected converter uses a constant power control grid-following control method. Its external characteristics behave as a controllable current source, which distributes active and reactive power by adjusting the output current. This method is simple and easy to implement, and exhibits significant advantages in precise power control. However, with the continuous increase in the penetration of distributed generation in the power grid and the rapid popularization of power electronics in distribution stations, the limitations of traditional grid-following converters are gradually becoming apparent.

[0004] Meanwhile, grid-connected converters primarily based on grid-following control rely primarily on a phase-locked loop (PLL) to synchronize with the grid voltage, appearing as a current source. This approach offers simple control and fast response, making it suitable for precisely controlling the converter's power output under strong grid conditions. However, it cannot participate in power system regulation. With the increasing penetration of new energy sources and the proportion of power electronics in the power system, grid regulation capabilities are insufficient, resulting in a weak grid. In such conditions, grid-following control suffers from poor stability and cannot effectively support the grid. Grid-forming control strategies, primarily based on virtual synchronous generator control, simulate the external characteristics of synchronous generators, appearing as voltage sources that provide inertia. By controlling the output voltage and phase angle of the grid-connected inverter, they can actively participate in system frequency and voltage regulation, enhancing power system stability. However, under strong grid conditions, the power-frequency control loop of grid-forming control methods has low damping, potentially leading to output power oscillations. Therefore, the adaptability of grid-following and grid-forming control strategies varies significantly under varying grid strengths, and the converter control method cannot be determined based on a single operating condition.

[0005] Therefore, one or more methods are needed to solve the above problems.

[0006] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute prior art known to ordinary technicians in the field. Summary of the Invention

[0007] The purpose of the present disclosure is to provide a method and device for controlling a multi-port power router based on MMC, thereby overcoming one or more problems caused by limitations and defects of related technologies, at least to a certain extent.

[0008] According to one aspect of the present disclosure, a method for controlling a multi-port power router based on MMC is provided, comprising:

[0009] For a five-port power router based on a double-ended back-to-back modular multilevel converter (MMC), a dual active bridge converter structure with series input and parallel output is used to connect the medium and low voltage DC bus, and input voltage balancing control is used to achieve module power balancing at each port.

[0010] In the grid-following control link of a double-ended back-to-back modular multilevel converter (MMC), a virtual synchronous generator control (VSG) is applied to the grid-connected power control port of the MMC, and a virtual DC motor control (VDMC) is applied to the energy storage port of the MMC.

[0011] The operation mode is determined based on the short-circuit ratio of the power grid, and the MMC-based multi-port power router is controlled to switch between following the grid and building the grid under strong / weak grid conditions.

[0012] In an exemplary embodiment of the present disclosure, the method further includes:

[0013] Based on the characteristics of energy storage, photovoltaic, and DC loads, preset DC port control strategies, photovoltaic port control strategies, and energy storage port control strategies are established respectively, and control of each port is achieved through converters corresponding to the preset DC port control strategies, photovoltaic port control strategies, and energy storage port control strategies.

[0014] In an exemplary embodiment of the present disclosure, the method further includes:

[0015] In the grid-following control link of a double-ended back-to-back modular multilevel converter (MMC), the active-frequency control loop and reactive-voltage control loop of a virtual synchronous generator (VSG) are applied to the grid-connected power control port of the MMC.

[0016] In an exemplary embodiment of the present disclosure, the method further includes:

[0017] In the grid-following control link of a double-ended back-to-back modular multilevel converter (MMC), a virtual DC motor control (VDMC) is applied to the Buck / Boost converter at the energy storage port of the MMC by simulating the mechanical equations and armature circuit balance equations of the DC motor.

[0018] In an exemplary embodiment of the present disclosure, the method further includes:

[0019] According to the working mode of the MMC-based multi-port power router, a power distribution and dynamic adjustment strategy, a photovoltaic system output control strategy, an energy storage device charging and discharging control strategy, and a voltage stabilization control strategy are established to realize the control of the MMC-based multi-port power router.

[0020] In an exemplary embodiment of the present disclosure, the method further includes:

[0021] The power allocation and dynamic adjustment strategy includes power mutual assistance of interconnected distribution stations and local load power supply coordination control strategy;

[0022] The photovoltaic system output control strategy outputs the generated electrical energy at maximum power and feeds it into the DC bus of the power router;

[0023] The energy storage device charge and discharge control strategy includes charge state monitoring, load demand response and power balance regulation;

[0024] The voltage stabilization control strategy includes adopting a virtual synchronous generator (VSG) control strategy under weak grid conditions and a strategy for stabilizing the low-voltage DC bus voltage through a virtual DC motor control (VDMC).

[0025] In an exemplary embodiment of the present disclosure, the method further includes:

[0026] The working modes of the MMC-based multi-port power router include flexible interconnection operation mode, area power demand sudden change operation mode, load sudden change operation mode, flow reversal operation mode, new energy output fluctuation operation mode, and off-grid self-operation mode.

[0027] In an exemplary embodiment of the present disclosure, the method further includes:

[0028] The inner loop current reference value output in the control mode before switching is sampled by the sample-and-hold device and used as the initial value of the inner loop control input after switching to achieve synchronization of the current signal;

[0029] When switching from grid-following control to grid-forming control, phase and voltage amplitude pre-synchronization control is performed to complete the switching between grid-following and grid-forming control;

[0030] When switching from network-building type to network-following type control, the phase transition is achieved through phase-locked loop synchronization to complete the network-following / network-building control switch.

[0031] In one aspect of the present disclosure, a multi-port power router control device based on MMC is provided, comprising:

[0032] The power balancing module is used for a five-port power router based on a double-ended back-to-back modular multilevel converter (MMC). It uses a dual active bridge converter structure with series input and parallel output to connect to the medium and low voltage DC bus, and achieves module power balancing at each port through input voltage balancing control.

[0033] A grid-following control module, configured to apply a virtual synchronous generator control (VSG) to a grid-connected power control port of a double-ended back-to-back modular multilevel converter (MMC) and a virtual DC motor control (VDMC) to an energy storage port of the MMC in a grid-following control link of the MMC;

[0034] The control switching module is used to determine the operating mode based on the short-circuit ratio of the power grid, and to control the switching of the MMC-based multi-port power router to follow the grid / build the grid under strong / weak grid conditions.

[0035] In an exemplary embodiment of the present disclosure, a control method for a multi-port power router based on an MMC is disclosed. The method includes: for a five-port power router based on a double-ended back-to-back modular multilevel converter (MMC), a dual active bridge converter structure with series input and parallel output is used to connect the medium and low voltage DC bus, and module power balancing at each port is achieved through input voltage balancing control; in the grid-following control link of the double-ended back-to-back modular multilevel converter (MMC), a virtual synchronous generator control (VSG) is applied to the grid-connected power control port of the MMC, and a virtual DC motor control (VDMC) is applied to the energy storage port of the MMC; the operating mode is determined based on the grid short-circuit ratio, and the MMC-based multi-port power router is switched between grid-following and grid-forming control under strong / weak grid conditions. Through the above control method, the present disclosure can ensure DC voltage stability and power supply and demand balance for the multi-port power router under complex operating conditions.

[0036] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The above and other features and advantages of the present disclosure will become more apparent by describing in detail example embodiments thereof with reference to the accompanying drawings.

[0038] Figure 1 A flowchart of a method for controlling a multi-port power router based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0039] Figure 2 A three-phase MMC topology diagram of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0040] Figure 3 An equivalent topology diagram of phase A of an MMC according to a method for controlling a multi-port power router based on an MMC according to an exemplary embodiment of the present disclosure is shown;

[0041] Figure 4 A diagram showing an MMC mathematical model in a dq coordinate system of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0042] Figure 5 A block diagram of current inner loop decoupling control of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0043] Figure 6 A block diagram of a decoupled current inner loop control method of a multi-port power router based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0044] Figure 7 The figure shows an outer loop P and Q control block diagram of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure;

[0045] Figure 8 A constant DC voltage control block diagram of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0046] Figure 9 A diagram showing an MMC circulation mathematical model of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0047] Figure 10 A block diagram of an MMC circulating current suppression control method for a multi-port power router based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0048] Figure 11 FIG1 shows an NLM modulation waveform diagram of an MMC-based multi-port power router control method according to an exemplary embodiment of the present disclosure;

[0049] Figure 12 A capacitor voltage balancing control diagram based on a sorting method of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0050] Figure 13 A double-terminal back-to-back MMC overall control block diagram of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0051] Figure 14 A schematic diagram of a conductance increment method for a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0052] Figure 15 A schematic diagram of a photovoltaic port converter control strategy of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0053] Figure 16 A schematic diagram of an energy storage port converter control strategy of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0054] Figure 17 A schematic diagram of a Buck converter control strategy of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0055] Figure 18 A VSG active power-frequency control block diagram of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0056] Figure 19 A VSG reactive-voltage control block diagram of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0057] Figure 20 A schematic diagram of a virtual DC motor control principle of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0058] Figure 21 A virtual DC motor control block diagram of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0059] Figure 22 A flow chart of a coordinated control strategy of a multi-port power router based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0060] Figure 23 A control structure diagram of a typical grid-type converter according to a control method of a multi-port power router based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0061] Figure 24A control structure diagram of a typical meshed converter according to a method for controlling a multi-port power router based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0062] Figure 25 A schematic diagram showing control mode switching of a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0063] Figure 26 A schematic diagram of a smooth switching control strategy for following a network / building a network in a multi-port power router control method based on MMC according to an exemplary embodiment of the present disclosure is shown;

[0064] Figure 27 A schematic block diagram of a multi-port power router control device based on MMC according to an exemplary embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0065] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be embodied in many forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concepts of the example embodiments to those skilled in the art. Like reference numerals in the drawings represent like or similar parts, and thus repetitive description thereof will be omitted.

[0066] In addition, the described features, structures or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, many specific details are provided to provide a full understanding of the embodiments of the present disclosure. However, those skilled in the art will appreciate that the technical solutions of the present disclosure can be practiced without one or more of the specific details, or other methods, components, materials, devices, steps, etc. can be adopted. In other cases, well-known structures, methods and devices, implementations, materials or operations are not shown or described in detail to avoid obscuring various aspects of the present disclosure.

[0067] The blocks shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically separate entities. Specifically, these functional entities may be implemented in software, or in one or more software-hardened modules, or in different networks and / or processor devices and / or microcontroller devices.

[0068] In this exemplary embodiment, a multi-port power router control method based on MMC is first provided; Figure 1 As shown in , the MMC-based multi-port power router control method may include the following steps:

[0069] Step S110: For a five-port power router based on a double-ended back-to-back modular multilevel converter (MMC), a dual active bridge converter structure with series input and parallel output is used to connect the medium and low voltage DC bus, and module power balance is achieved at each port through input voltage balancing control.

[0070] Step S120, in a grid-following control link of a double-ended back-to-back modular multilevel converter MMC, applying a virtual synchronous generator control VSG to a grid-connected power control port of the MMC, and applying a virtual DC motor control VDMC to an energy storage port of the MMC;

[0071] Step S130 : determining the operation mode based on the short-circuit ratio of the power grid, and performing grid-following / grid-building control switching on the MMC-based multi-port power router under strong / weak power grid conditions.

[0072] In an exemplary embodiment of the present disclosure, a control method for a multi-port power router based on an MMC is disclosed. The method includes: for a five-port power router based on a double-ended back-to-back modular multilevel converter (MMC), a dual active bridge converter structure with series input and parallel output is used to connect the medium and low voltage DC bus, and module power balancing at each port is achieved through input voltage balancing control; in the grid-following control link of the double-ended back-to-back modular multilevel converter (MMC), a virtual synchronous generator control (VSG) is applied to the grid-connected power control port of the MMC, and a virtual DC motor control (VDMC) is applied to the energy storage port of the MMC; the operating mode is determined based on the grid short-circuit ratio, and the MMC-based multi-port power router is switched between grid-following and grid-forming control under strong / weak grid conditions. Through the above control method, the present disclosure can ensure DC voltage stability and power supply and demand balance for the multi-port power router under complex operating conditions.

[0073] Next, a control method of a multi-port power router based on MMC in this exemplary embodiment will be further described.

[0074] Example 1:

[0075] In step S110, for a five-port power router based on a double-ended back-to-back modular multilevel converter (MMC), a dual active bridge converter structure with series input and parallel output can be used to connect the medium and low voltage DC bus, and module power balance of each port can be achieved through input voltage balancing control.

[0076] In the embodiment of this example, based on the energy storage, photovoltaic, and DC load characteristics, preset DC port control strategies, photovoltaic port control strategies, and energy storage port control strategies are respectively established, and control of each port is achieved through converters corresponding to the preset DC port control strategies, photovoltaic port control strategies, and energy storage port control strategies.

[0077] In step S120, in the grid-following control link of the double-ended back-to-back modular multilevel converter MMC, the virtual synchronous generator control VSG can be applied to the grid-connected power control port of the MMC, and the virtual DC motor control VDMC can be applied to the energy storage port of the MMC.

[0078] In this exemplary embodiment, the method further includes:

[0079] In the grid-following control link of a double-ended back-to-back modular multilevel converter (MMC), the active-frequency control loop and reactive-voltage control loop of a virtual synchronous generator (VSG) are applied to the grid-connected power control port of the MMC.

[0080] In this exemplary embodiment, the method further includes:

[0081] In the grid-following control link of a double-ended back-to-back modular multilevel converter (MMC), a virtual DC motor control (VDMC) is applied to the Buck / Boost converter at the energy storage port of the MMC by simulating the mechanical equations and armature circuit balance equations of the DC motor.

[0082] In this exemplary embodiment, the method further includes:

[0083] According to the working mode of the MMC-based multi-port power router, a power distribution and dynamic adjustment strategy, a photovoltaic system output control strategy, an energy storage device charging and discharging control strategy, and a voltage stabilization control strategy are established to realize the control of the MMC-based multi-port power router.

[0084] In this exemplary embodiment, the method further includes:

[0085] The power allocation and dynamic adjustment strategy includes power mutual assistance of interconnected distribution stations and local load power supply coordination control strategy;

[0086] The photovoltaic system output control strategy outputs the generated electrical energy at maximum power and feeds it into the DC bus of the power router;

[0087] The energy storage device charge and discharge control strategy includes charge state monitoring, load demand response and power balance regulation;

[0088] The voltage stabilization control strategy includes adopting a virtual synchronous generator (VSG) control strategy under weak grid conditions and a strategy for stabilizing the low-voltage DC bus voltage through a virtual DC motor control (VDMC).

[0089] In this exemplary embodiment, the method further includes:

[0090] The working modes of the MMC-based multi-port power router include flexible interconnection operation mode, area power demand sudden change operation mode, load sudden change operation mode, flow reversal operation mode, new energy output fluctuation operation mode, and off-grid self-operation mode.

[0091] In step S130, the operation mode may be determined based on the grid short-circuit ratio, and the MMC-based multi-port power router may be controlled to switch between following the grid and building the grid under strong / weak grid conditions.

[0092] In this exemplary embodiment, the method further includes:

[0093] The inner loop current reference value output in the control mode before switching is sampled by the sample-and-hold device and used as the initial value of the inner loop control input after switching to achieve synchronization of the current signal;

[0094] When switching from grid-following control to grid-forming control, phase and voltage amplitude pre-synchronization control is performed to complete the switching between grid-following and grid-forming control;

[0095] When switching from network-building type to network-following type control, the phase transition is achieved through phase-locked loop synchronization to complete the network-following / network-building control switch.

[0096] Example 2:

[0097] In the embodiment of this example, the MMC mathematical model includes:

[0098] Three-phase MMC topology such as Figure 2 As shown, since the configuration of the three phases is exactly the same, we take phase a as an example for analysis. The equivalent circuit of phase a is as follows Figure 3 As shown, where U dc is the DC side voltage, u pa and u na They are the upper and lower bridge arm voltages of phase a, u ao is the output voltage of the AC side of phase a, i a is the a-phase AC current measurement, R0 and L0 are the a-phase bridge arm resistance and inductance.

[0099] According to Kirchhoff's voltage law, we can get

[0100]

[0101] Adding and subtracting the above formula can be obtained

[0102]

[0103] As can be seen from formula (3-2), the DC side voltage of the MMC is always equal to the sum of the voltages of the upper and lower bridge arms of each phase. The AC voltage output by the MMC is determined by the difference between the voltages of the upper and lower bridge arms of each phase. The total number of submodules put into operation in each phase remains constant at any time. Therefore, by dynamically adjusting the number of submodules put into operation in the upper and lower bridge arms of each phase, the three-phase AC output voltage can be accurately controlled. Assuming that the total number of submodules in the upper and lower bridge arms of each phase is 2N, and the number of submodules put into and removed in each phase at any time is N, the system can generate N+1 levels of output voltage waveforms. Therefore, by designing a corresponding control strategy to control the on and off of the switching tubes in the submodules in the upper and lower bridge arms of each phase, the MMC output voltage can be independently adjusted and controlled.

[0104] Write the KCL and KVL equations for the upper and lower bridge arms of phase a:

[0105] i pa =i a +i na (3)

[0106]

[0107] Add the two equations in equation (4) and combine them with equation (3) to get

[0108]

[0109] Let 1 / 2(u na -u pa )=e a , L0 / 2=L, R0 / 2=R, substitute into formula (5), and extrapolate to three phases, we can get the dynamic mathematical model of MMC in the abc three-phase stationary coordinate system:

[0110]

[0111] After the Clark-Park transformation of the equation in the three-phase rotating coordinate system in formula (6), the mathematical model in the dq coordinate system can be obtained as follows:

[0112]

[0113] Then perform Laplace transform on equation (7), and we can get the frequency domain equation in the dq coordinate system:

[0114]

[0115] The mathematical model of the MMC converter in the dq coordinate system can be obtained as follows: Figure 4 shown.

[0116] In the embodiment of this example, in the MMC controller design, the inner loop control includes:

[0117] Introducing voltage coupling compensation term u d and u q The current inner loop system block diagram after the decoupling control is introduced is as follows: Figure 5 As shown, after decoupling, the control block diagram of the current inner loop control is as follows Figure 6 shown.

[0118] In the embodiment of this example, in the MMC controller design, the outer loop control includes:

[0119] The stable operation of the common DC bus power router is highly dependent on the precise control of the DC side voltage. The converters at each end need to be reasonably configured with the outer loop control mode and ensure the reliability of the coordinated control. Taking the common DC bus power router based on the two-terminal MMC as an example, one end is usually responsible for the regulation of the DC voltage, using U dc The other end controls the power output and operates in PQ control mode. This control strategy not only simplifies the coordination mechanism of the two-end system but also improves control stability.

[0120] In the dq synchronous rotating coordinate system, the expressions of active power P and reactive power Q output by the MMC AC side are:

[0121]

[0122] If the system operates in three-phase symmetry under steady-state conditions, when the d-axis is positioned at the grid voltage, the losses of the converter and transformer are ignored. To simplify the calculation, take u q =0, we can get

[0123]

[0124] It can be seen that P, Q and i d 、i q There is a first-order linear relationship between d 、i q The tracking control realizes the independent control of P and Q. At the same time, in order to eliminate the steady-state error during calculation, the PI controller is introduced, and the outer loop active power and reactive power control are as follows: Figure 7 shown.

[0125] In an ideal state of the MMC system, the active power on the DC side is equal to that on the AC side.

[0126]

[0127] Similarly, by controlling i dThe independent control of DC voltage can be realized. Similarly, the PI controller is introduced to obtain the MMC outer loop DC voltage control as follows: Figure 8 shown.

[0128] When the system has a DC side voltage fluctuation problem caused by active power fluctuation, the inner loop current reference value is obtained by subtracting the DC voltage reference value from the actual value through PI regulation for control.

[0129] In the embodiment of this example, the MMC circulation suppression control in the MMC valve level control includes:

[0130] During MMC operation, due to the uneven distribution of capacitor voltages in the three-phase bridge arms and differences in device physical parameters, circulating currents will be generated between the bridge arms. This circulating current not only distorts the bridge arm current waveform and increases device stress, but also causes additional losses, reduces system efficiency, and exacerbates capacitor voltage fluctuations, affecting DC side voltage stability. Therefore, to ensure the efficient operation and reliability of MMC, effective circulating current suppression measures must be implemented.

[0131] Take phase a of MMC as an example, i cira is the circulating current of phase a unit. According to Kirchhoff's current law,

[0132]

[0133] The circulation expression is

[0134]

[0135] Research has shown that circulating currents exist only within the converter and are unrelated to external connections. The mechanism and characteristics of circulating currents are that the interphase circulating currents contain a double-frequency negative-sequence current component, which distorts the bridge arm current waveform and affects the capacitor voltage balance.

[0136] Assume that the voltage drop caused by the circulating current of each phase of MMC on the bridge arm reactor is u cirj , then

[0137]

[0138] To solve the problem of unbalanced voltage drop caused by the voltage difference between the upper and lower bridge arms of MMC, the goal of circulating current suppression can be achieved by controlling the voltage of each bridge arm. cirj , so that the adjusted upper and lower bridge arm voltages are consistent with the DC side voltage, which can effectively suppress the internal circulation phenomenon caused by voltage imbalance. According to KVL, we can get

[0139]

[0140] According to existing research,cirj It is composed of the DC current component and the double frequency negative sequence current component, as shown in formula (15), the three-phase circulating current expression can be obtained as follows:

[0141]

[0142] In formula (16), I 2f is the peak value of the circulating current, ω is the fundamental angular frequency, is the initial phase angle. Since the reactor exhibits low impedance characteristics to the DC component, nearly in a short-circuit state, the voltage drop caused by the double frequency component needs to be suppressed. Substituting Equation (13) into Equation (16), the three-phase circulating current is decomposed into DC components in the dq axis direction through Park transformation, as shown in Equation (17).

[0143]

[0144] Finally, we can get Figure 9 The MMC circulation model is shown.

[0145] According to the above mathematical model, the MMC circulating current suppression controller is designed, as shown in Figure 10 As shown. The circulation i is obtained by formula (13): cirj After coordinate transformation, the dq axis components of the MMC interphase circulating current are obtained. Then, the dq axis circulating current reference value is introduced and set to 0. The PI regulator is used to introduce the feedforward to eliminate the dq axis coupling, and the dq axis reference value u of the unbalanced voltage drop can be obtained. cd_ref and u cq_ref , and then obtain the reference value u of the unbalanced pressure drop caused by the circulation through coordinate transformation cirj_ref .

[0146] In the embodiment of this example, the MMC modulation and capacitor voltage balancing strategy in the MMC valve level control includes:

[0147] In the valve-level control strategy of MMC converters, unlike the valve-level control strategies of two- and three-level VSC converters, MMC modulation technology must consider coordination with submodule capacitor voltage balancing control. Nearest Level Approximation Modulation (NLM), a step-wave modulation strategy, instantly simulates a sinusoidal modulation waveform by selecting the closest level. It offers advantages such as low switching frequency, low harmonic content, simple implementation, fast dynamic response, and suitability for high-voltage and high-power scenarios. It can effectively improve the efficiency and stability of modular multilevel converters and can be used in conjunction with MMC's capacitor voltage balancing control strategy.

[0148] The waveform of MMC's nearest level approaching modulation is as follows Figure 11 As shown, where U dc is the DC side voltage of MMC, u cis the capacitor voltage of a single MMC submodule. Assuming that each phase unit's upper and lower arms each contain N submodules (N is typically an even number), then at any given moment, a total of N submodules are active in that phase unit. If these N submodules are evenly distributed between the upper and lower arms, meaning that each arm has N / 2 submodules active, the output of that phase unit will be zero level. As the instantaneous value of the modulation wave rises from zero, in order for the output voltage to dynamically follow the modulation wave, the number of active submodules in the lower arm of that phase unit needs to be gradually increased, while the number of active submodules in the upper arm needs to be correspondingly reduced.

[0149] At time t, the number of submodules required for the upper and lower arms of the MMC can be expressed as

[0150]

[0151] In formula (18), n pj 、n pj are the number of submodules that need to be put into real-time operation in the upper and lower bridge arms, u j is the instantaneous value of the modulation wave, and round(x) is the rounding function.

[0152] Since the MMC submodule capacitance is affected by differences in device characteristics, inconsistent capacitance parameters, load current fluctuations, circulating current, etc., u c There are fluctuations in size. If it is charged and discharged, the imbalance of the capacitor voltage will be magnified, which will affect the normal operation of the MMC. Capacitor voltage balancing control based on sorting algorithm is widely used and can be designed in conjunction with NLM modulation. In the NLM modulation strategy, the number of sub-modules that need to be put into the bridge arm is calculated, and the order of sub-module input needs to be determined based on the direction of the bridge arm current and the sorting results of the capacitor voltage. When the direction of the bridge arm current is positive, the current will charge these sub-modules, and these sub-modules will be selected and put into use in the order of capacitor voltage from low to high. When the direction of the bridge arm current is negative, the sub-module capacitor will discharge, and the sub-module will be selected and put into use in the order of capacitor voltage from high to low, so as to maintain the dynamic balance of the capacitor voltage. The specific flow chart is as follows Figure 12 shown.

[0153] Combining the above MMC dual-loop control, circulating current suppression, nearest level approach modulation, and sub-module voltage balancing control strategies, the design of the MMC overall control strategy is completed. Figure 13 shown.

[0154] In this exemplary embodiment, the PV port control strategy in the DC port control strategy includes:

[0155] In photovoltaic power generation systems, the output of photovoltaic arrays is affected by external environmental conditions, particularly variations in ambient light intensity and temperature. These factors act together, causing the array's output characteristics to exhibit distinct nonlinear trends. Parameters such as the array's output power, voltage, and current do not exhibit a simple linear relationship with environmental conditions. Instead, they exhibit complex and variable nonlinear characteristics as light intensity and temperature fluctuate. Under specific operating conditions, the power-voltage (PV) curve of a photovoltaic array exhibits a unique maximum power point (MPP). If a photovoltaic power generation system can operate stably at this MPP, it can maximize light-to-electricity conversion and improve photovoltaic power consumption efficiency. However, because the operating environment of photovoltaic cells is complex and variable, and these environmental changes are often difficult to accurately predict, maximum power point tracking (MPPT) technology is commonly used. The key to MPPT technology is to adjust the operation of photovoltaic cells in real time to ensure that they consistently operate near their maximum power point. MPPT technology includes various control methods, including constant voltage tracking, perturbation-and-observe, and conductance increment. The constant voltage tracking method is simple and easy, but the accuracy is slightly poor; the perturbation observation method can track the maximum power point in real time, but it may cause certain energy loss due to frequent adjustments. The conductance increment method has higher accuracy and stability. This disclosure adopts an MPPT control strategy based on the conductance increment method, such as Figure 14 As shown in the figure, the PV characteristic curve of a photovoltaic cell exhibits a single peak. In the area to the left of the maximum power point, power increases monotonically with increasing voltage, while in the area to the right, power decreases. Based on this characteristic, dynamic optimization of the operating point can be achieved by calculating the rate of change of power with respect to voltage (dP / dV) in real time. When dP / dV > 0, the output voltage is increased; otherwise, the output voltage is decreased until the convergence condition of dP / dV = 0 is met.

[0156] According to the above analysis, the control block diagram of photovoltaic MPPT through Boost converter can be obtained as follows: Figure 15 As shown, the photovoltaic module output voltage U pv and current I pv The reference voltage command U is calculated by the MPPT controller pv-ref , then U pv-ref with U pv The value of is subtracted, and then the PI regulator outputs the control signal to the Boost converter to realize the MPPT control of photovoltaic power generation.

[0157] In this exemplary embodiment, the energy storage port control strategy in the DC port control strategy includes:

[0158] The energy storage battery is connected to ISOP-DAB through a bidirectional Buck / Boost converter. When the system has power shortage, the bidirectional Buck / Boost works in Boost mode, and the energy storage battery discharges and transmits power to the system; when the system has power surplus, the bidirectional Buck / Boost works in Buck mode, and the system charges the battery. The bidirectional Buck / Boost converter uses a dual closed loop control of power outer loop and current inner loop to control its charging and discharging. Figure 16 As shown, at the same time, in order to ensure the safety of charging and discharging and extend the battery life, the charging and discharging action is not started when the battery SOC is higher than 0.9 or lower than 0.1.

[0159] In the embodiment of this example, the DC load port control strategy in the DC port control strategy includes:

[0160] The DC load port is connected to the system through a Buck converter, and the classic voltage and current dual closed-loop control strategy is adopted. The control method is simple and the effect is good. The control block diagram is as follows Figure 17 shown.

[0161] In this exemplary embodiment, the receiving-end MMC of the grid-connected converter disclosed herein employs a constant power control grid-following control scheme. Its external characteristics manifest as a controllable current source, which distributes active and reactive power by adjusting the output current. This method is simple and easy to implement, and exhibits significant advantages in precise power control. However, with the continued increase in the penetration of distributed generation in the power grid and the rapid adoption of power electronics in distribution substations, the limitations of traditional grid-following converters are becoming increasingly apparent.

[0162] Specifically, grid-following control measures the voltage at the grid connection point and uses a phase-locked loop (PLL) to track the grid voltage phase, thereby maintaining synchronous operation with the grid. However, when a weak grid experiences a large disturbance, the PLL may be unable to accurately track the grid phase, causing the grid-connected VSC to lose synchronization with the grid and resulting in transient system instability. Grid-following converters offer significant advantages over grid-following converters in several aspects. As a voltage source, they can independently provide voltage and frequency support, enhancing power system stability and simulating the inertia and damping characteristics of synchronous generators. Furthermore, grid-following converters are capable of off-grid operation, enabling seamless transitions between on-grid and off-grid modes, making them particularly suitable for weak grid conditions and renewable energy generation systems. These characteristics make grid-following converters superior to grid-following converters in terms of stability, flexibility, and adaptability, providing strong support for the reliable operation of modern power systems. As an advanced development of grid-following control technology, the VSG control strategy inherits many of the advantages of grid-following control while improving the system's frequency and voltage regulation performance. It can provide important technical support for power systems with a high proportion of renewable energy connected to the grid.

[0163] In the embodiment of this example, the MMC virtual synchronous generator control includes:

[0164] VSG control is a power electronic converter control technology that simulates the operating characteristics of synchronous generators. Its core idea is to enable the power electronic converter to provide frequency and voltage support for the power system like a traditional synchronous generator by introducing virtual inertia and damping characteristics. The present disclosure introduces VSG control in the control link of the grid-connected power control port converter MMC, so that the power router can maintain stable operation under a weak power grid. VSG control mainly includes an active-frequency control loop (Pf control loop) and a reactive-voltage control loop (QV control loop). Among them, the Pf control loop adjusts the active power output to achieve frequency regulation by simulating the speed regulator characteristics of the synchronous generator; the QV control loop adjusts the reactive power output to achieve voltage regulation by simulating the excitation control characteristics of the synchronous generator. The present disclosure applies VSG control to one end of the back-to-back MMC to cope with the problem of unstable system operation caused by load fluctuations and large-scale new energy power generation access.

[0165] The VSG's active power-frequency control loop includes:

[0166] The VSG active frequency control link simulates the power-frequency droop characteristics of the synchronous generator by introducing droop control. According to the deviation of the system frequency and the power output demand, the active power output is dynamically adjusted to achieve active support for the system frequency. Through the virtual inertia link, the rotor inertia of the synchronous generator is simulated, and a virtual inertia force proportional to the frequency change rate is introduced to suppress the rapid fluctuation of the system frequency. The primary frequency modulation formula of the VSG is:

[0167] P m =P ref +k p (ω0-ω) (19)

[0168] In formula (19), P ref For a given active power, k p is the primary frequency modulation coefficient, ω and ω0 are the actual angular velocity and rated angular velocity respectively.

[0169] The mechanical motion equation of the VSG rotor is:

[0170]

[0171] In formula (20), T m is the mechanical torque, T e is the electromagnetic torque, J is the rotor moment of inertia, D is the rotor damping coefficient, ω is the rotor mechanical speed, ω0 is the rated angular velocity, P m is the input mechanical power, P eis the output electromagnetic power. Combining equations (19) and (20), we can get Figure 18 The VSG active power-frequency control block diagram is shown.

[0172] The reactive power-voltage control loop of VSG includes:

[0173] The synchronous generator adjusts the excitation voltage through the excitation system to maintain the stability of the terminal voltage. Its dynamic behavior can be described by the first-order differential equation:

[0174]

[0175] In formula (21), T e is the time constant of the excitation system, E fd is the excitation voltage, K is the gain, U ref is the reference voltage, U o is the actual terminal voltage. This formula shows that the excitation system eliminates voltage deviations through adjustment to maintain voltage stability. Similarly, the reactive power-voltage control loop of the VSG maintains voltage stability by adjusting reactive power output, and its control characteristics are similar to reactive power-voltage droop control. Therefore, the reactive power control loop of the VSG can be designed based on the control principle of the excitation system. By simulating the dynamic response of the excitation system, the design process of the control strategy can be simplified. The reactive power-voltage control equation of the VSG can be expressed as

[0176] E = E0 + k q (Q ref - Q0) (22)

[0177] In formula (22), Q ref is the reactive power reference value, Q is the actual reactive power value, k is the voltage droop coefficient, U ref When the system voltage fluctuates due to load changes or disturbances, the reactive-voltage control loop can quickly adjust the reactive power output of the VSG, suppress voltage fluctuations, and maintain the voltage within the set range. The reactive voltage control block diagram of the VSG is shown in the figure below. Figure 19 shown.

[0178] The three-phase reference voltage U can be obtained by the angle θ output by the power frequency control loop and the voltage output by the reactive-voltage control loop. abc-ref , and then the operation of the converter is controlled by the voltage and current dual closed loop.

[0179] In this disclosure, a dual-terminal back-to-back MMC employs constant DC voltage control on one end to maintain a constant DC bus voltage, while the other end utilizes the aforementioned VSG control, simulating the inertia and damping characteristics of a synchronous generator to smooth out AC-side frequency fluctuations while supporting grid voltage through reactive power and voltage droop. Its dynamic response complements the constant DC voltage control, ensuring a coordinated balance of power transmission between the AC and DC sides.

[0180] In the embodiment of this example, the virtual DC motor control method in the energy storage port virtual DC motor control includes:

[0181] Given the inherent volatility of the output power of renewable energy sources like photovoltaics, the power input to the energy router may be in a state of continuous fluctuation. If the output power commands of the back-to-back MMCs of the grid-connected converters are not adjusted in a timely manner, the power balance will be unbalanced, causing significant fluctuations in the DC voltage. Furthermore, if the MMC controlling the DC voltage fails to operate due to a fault, the DC voltage stability of the entire system cannot be guaranteed, which may cause greater power fluctuations and even lead to safety issues. Considering the multi-port nature of the energy router topology based on a common DC bus in this paper, energy storage ports are selected to smooth out the power transmission fluctuations caused by the inherent characteristics of renewable energy generation and maintain the stability of the DC bus voltage, ensuring reliable and stable operation of the system.

[0182] Traditional methods for energy storage ports mostly use dual closed-loop control of voltage and current, but it is difficult to ensure the stability of the bus voltage when the output of new energy sources fluctuates frequently. Virtual DC Motor Control (VDMC) is a control strategy based on the dynamic characteristics of DC motors. By simulating the mechanical equations and armature circuit balance equations of DC motors, the control problem of the power electronic system is converted into the control problem of DC motors, thereby achieving rapid dynamic response and stable operation of the system. The present disclosure introduces VDMC into the Buck / Boost converter of the energy storage port, so that it has the external characteristics of a DC motor, thereby improving the inertia and damping of the system, maintaining the stability of the DC bus voltage under complex working conditions, and cooperating with the VSG control strategy of MMC to achieve stable operation of the power router system and improve the operational reliability under various complex working conditions.

[0183] like Figure 20 As shown in the figure, the virtual DC motor control equates the bidirectional Buck / Boost converter to a two-port network with the front end connected to the energy storage device and the back end connected to the DC bus. This equivalent two-port network is similar to the equivalent model of a DC motor and can simulate the inertia characteristics of a DC motor.

[0184] The mechanical equation of a DC motor is

[0185]

[0186] In formula (23), ω0 is the rated angular velocity of the DC motor, ω is the actual angular velocity, J is the moment of inertia, D is the damping coefficient, and P is the m is the mechanical power, P e is the electromagnetic power. When the load or photovoltaic power changes and the mechanical power output of the DC motor fluctuates, in order to maintain the output voltage stability, the DC motor will automatically adjust the mechanical angular velocity ω and change the induced electromotive force accordingly, thereby improving the stability and reliability of the entire system. The electromagnetic power and potential balance equation of the DC motor is

[0187]

[0188] In formula (24), E is the armature induced potential of the DC motor, C T is the torque coefficient, is the magnetic flux, I a is the armature current, R a is the armature resistance, U o is the port voltage, which is then equivalent to the energy storage system, so that the energy storage port simulates the characteristics of a DC motor.

[0189] The block diagram of the virtual DC motor control at the energy storage port of the energy router is as follows: Figure 21 As shown, first, the DC bus voltage is adjusted by the PI controller, and the mechanical power deviation ΔP is calculated. It is then substituted into the mechanical equation and the armature circuit balance equation to calculate the armature current I a , the armature current I a Converted to current reference value I ref The battery output current I is adjusted by the PI controller bat , and finally generate the switch tube drive signal through PWM modulation.

[0190] When the MMC controlling the DC voltage operates normally, the Buck / Boost converter at the energy storage port uses constant power control to provide stable active power output according to the specific needs of the power router, replenishing the system's power shortage. If the connected substation's distribution network fails or the MMC converter ceases operation, the Buck / Boost converter at the energy storage port uses virtual DC motor control to stabilize the 750V DC bus voltage and, in conjunction with photovoltaic power generation, ensure stable power supply to critical loads.

[0191] In order to achieve stable operation and efficient energy management of the power router under various complex working conditions, this paper designs a multi-mode coordinated control strategy for the power router by setting constraints and considering the actual operating conditions of the power router. This ensures that the power router can achieve multi-objective optimization of power balance, voltage stability and dynamic response under various complex working conditions, thereby improving the reliability and adaptability of the system and enhancing the stable operation capability of the power router in different scenarios.

[0192] In this exemplary embodiment, the power allocation and dynamic adjustment in the coordinated control strategy of the power router include:

[0193] Interconnecting distribution substations: Power routers achieve power sharing between distribution substations through back-to-back MMCs. The sending substation dynamically adjusts output power based on the power requirements of the receiving substation, ensuring stable power transmission.

[0194] Local load power supply: The active power demand of local loads is coordinated by the sending-end distribution station and the energy storage device. The energy storage device flexibly charges and discharges according to the state of charge (SOC) and load demand, ensuring stable power supply to local loads.

[0195] In this exemplary embodiment, the photovoltaic system output control in the power router coordinated control strategy includes:

[0196] The PV system always outputs at maximum power, feeding the generated electricity into the DC bus of the Power Router, providing clean energy support for the entire system. The Power Router uses control strategies to ensure the efficient use of PV power and prevent fluctuations in PV output from affecting system stability.

[0197] In this exemplary embodiment, the energy storage device charging and discharging control in the coordinated control strategy of the power router includes:

[0198] State of Charge Monitoring: Real-time monitoring of the state of charge (SOC) of the energy storage device, and dynamic adjustment of charging and discharging power according to SOC and load demand.

[0199] Load demand response: When the load demand increases, the energy storage device increases the discharge power if the SOC allows; if the SOC is low, the discharge power is reduced or stopped to maintain its own state and the stability of the load power supply.

[0200] Power balance regulation: When the system power demand suddenly changes or the output of new energy fluctuates, the energy storage device responds quickly and adjusts the system power balance through charging and discharging operations to ensure stable system operation.

[0201] In this exemplary embodiment, the voltage stability control in the coordinated control strategy of the power router includes:

[0202] DC bus voltage stabilization: Virtual DC motor control (VDMC) stabilizes the low-voltage DC bus voltage, ensuring that the DC bus voltage remains stable when the energy storage port is discharging or charging.

[0203] AC side voltage support: Under weak grid conditions, a virtual synchronous generator (VSG) control strategy is adopted to provide voltage and frequency support for the AC side, enhancing the stability and inertia of the system.

[0204] The coordinated control strategy process of the power router is as follows: Figure 22 As shown in the figure, when the system is running, by judging factors such as the operating conditions of the power router, power balance, photovoltaic output, DC load and energy storage battery status, each port coordinates and cooperates to achieve regional power mutual assistance and efficient consumption of renewable energy.

[0205] In this exemplary embodiment, currently, grid-connected converters primarily based on grid-following control rely primarily on a phase-locked loop (PLL) to synchronize with the grid voltage. This system appears as a current source, offering simple control and fast response. This makes it suitable for precisely controlling the converter's power output under strong grid conditions, but it cannot participate in the power system's regulation process. With the increasing penetration of new energy sources and the proportion of power electronics in the power system, grid regulation capabilities are insufficient, resulting in a weak grid. In such a weak grid, grid-following control suffers from poor stability and cannot effectively support the grid. Grid-forming control strategies, primarily based on virtual synchronous generator control, simulate the external characteristics of synchronous generators, appearing as a voltage source that provides inertia. By controlling the output voltage and phase angle of the grid-connected inverter, they can actively participate in the system's frequency / voltage regulation process, enhancing power system stability. However, under strong grid conditions, the power-frequency control loop of the grid-forming control scheme has low damping, potentially leading to output power oscillation. Therefore, the adaptability of grid-following and grid-forming control strategies varies significantly under varying grid strengths, and the converter's control method cannot be determined based on a single operating condition.

[0206] To this end, based on the previous research content, a power router grid-following / grid-forming smooth switching control strategy is proposed. The control mode is determined by the grid short-circuit capacity ratio SCR (SCR). In a strong grid with a large SCR, a grid-following P / Q control is adopted, which mainly realizes the precise transmission of power between distribution stations. At the same time, due to the small fluctuation of distributed power generation, its large-scale grid connection can be achieved; in a weak grid with a small SCR, a VSG-based grid-forming control is adopted to provide inertia and damping for the grid, actively adjust the output voltage and frequency, provide necessary support for the weak grid, reduce voltage and frequency fluctuations, and can quickly respond and adjust when the output of new energy fluctuates or the load suddenly changes, thereby improving the stability of system operation.

[0207] In the embodiment of this example, the grid-following control method includes:

[0208] This paper takes a typical grid-connected inverter system as an example to compare and analyze the two control methods of following the grid and building the grid. Figure 23 The following figure shows the control system structure of a typical grid-following converter, which mainly includes a phase-locked loop module, a power loop control module, and a current loop control module. dc is the DC side voltage, VL and I L are the output voltage and output current of the inverter respectively, i g is the AC grid side current, V g and I g P is the voltage and current at the point of common coupling (PCC). ref and Q ref are the active power and reactive power reference values, respectively. When the outer loop employs constant power control, coordinate transformation is performed based on the voltage and current at the PCC point and the phase angle from the phase-locked loop (PLL) to calculate the actual power value. Current control is then achieved through the current loop based on the d-axis and q-axis current reference values ​​calculated by the power loop, thereby generating a PWM signal to control the converter's operation. As can be seen, a converter using grid-following control relies entirely on the output signal of the PLL to track the grid. Under strong grid conditions, this allows for faster power response and better stability. However, under weak grid conditions, it is susceptible to variations in grid impedance, potentially leading to oscillation or instability, making it difficult to quickly respond to grid fluctuations. Furthermore, grid-following control lacks active support for grid voltage, resulting in poor stability under transient operating conditions.

[0209] In the embodiment of this example, the grid-following control method includes:

[0210] The structure of the networked control system taking virtual synchronous generator control as an example is as follows: Figure 24 As shown in Figure 1, the system primarily consists of an active-frequency control loop, a reactive-voltage control loop, and a voltage-current dual closed-loop system. ω is the angular frequency output by the VSG active-frequency loop, E is the internal potential amplitude output by the reactive-voltage loop, and θ is the reference angle of the power loop output. The frequency and voltage at the PCC point are obtained through a phase-locked loop (PLL). The active-frequency control loop regulates active power output, while the reactive-voltage control loop regulates reactive power to maintain voltage stability. The converter's operation is then controlled by the voltage-current dual closed-loop system. Grid-based control can provide a certain degree of inertia and damping, but strong grids already have sufficient inertia and damping. Therefore, in strong grids, grid-based control has a relatively limited contribution and results in higher control complexity and cost. However, in weak grids, grid-based control can regulate voltage and frequency, providing virtual inertia and damping, enhancing system stability and enabling rapid power output adjustments to respond to grid fluctuations or faults.

[0211] In this exemplary embodiment, the smooth switching control of the power router to follow the network / build the network includes:

[0212] like Figure 25 As shown in the figure, since both the grid-following and grid-forming control types have the same inner loop current control module, during the switching period, only the outer loop control mode needs to be switched to generate the inner loop current reference value i under each control mode. dref and iqref , and if hard switching is performed, the inner loop current reference value generated at the switching moment will have a large impact. This is because when the system is in a certain control mode, only the outer loop control corresponding to the control mode is enabled, and when the reference value output by another control mode is connected through hard switching, the inner loop reference values ​​generated by the two control modes at the switching moment are not completely equal, so i dref and i qref Large fluctuations occur. In order to prevent the inner loop current reference value from jumping before and after the switch, the inner loop current reference value output in the control mode before the switch is sampled by the sample and hold device, and is used as the initial value for the inner loop control input after the switch to complete the synchronization of the current signal. At the same time, since the grid-forming control does not rely on the phase-locked loop, when switching from the grid-following type to the grid-forming type control, the phase and voltage amplitude pre-synchronization control is required to ensure a smooth transition of the phase angle before and after the switch. When the system switches from the grid-forming type to the grid-following type control, the smooth transition of the phase is completed through the phase-locked loop synchronization, thereby completing the smooth switching process.

[0213] Based on the above research content, the present invention designs Figure 26 The smooth switching control strategy shown here uses the short-circuit ratio as the switching condition. By identifying the grid's equivalent impedance, the short-circuit ratio (SCR) is determined, and the applicable control mode is determined accordingly. When the short-circuit ratio is less than 3, the receiving-end MMC adopts grid-forming VSG control. When the short-circuit ratio is greater than or equal to 3, the receiving-end MMC adopts grid-following P / Q control. This smooth switching control strategy completes the switching of the power router's control mode.

[0214] In this example embodiment, to improve the stable operation of the power router in weak power grids and scenarios with large fluctuations in renewable energy, the energy storage port uses a virtual DC motor to control and stabilize the low-voltage DC bus voltage. A coordinated control strategy for a five-port power router based on a virtual synchronous generator (VSG) is designed. By coordinating the operating states of multiple devices, multi-objective optimization of power balance, voltage stability, and dynamic response is achieved. The five-port power router is divided into six operating modes according to the system operating conditions, enabling multi-mode coordinated operation of the system. To address the different adaptability of the grid-following and grid-forming control methods under different grid strengths, a comparative analysis of the operating performance of the two control methods under strong and weak grids is conducted. A control strategy for smooth switching between grid-following and grid-forming is designed. The control method is determined based on the grid short-circuit ratio, which can achieve the complementary advantages of the grid-following and grid-forming control strategies, effectively improving the power transmission performance and system stability of the power router under different grid strengths.

[0215] It should be noted that although the steps of the method disclosed herein are depicted in a particular order in the accompanying drawings, this does not require or imply that the steps must be performed in that particular order, or that all steps must be performed to achieve the desired result. Additionally or alternatively, certain steps may be omitted, multiple steps may be combined into one, and / or one step may be decomposed into multiple steps.

[0216] In addition, in this exemplary embodiment, a multi-port power router control device based on MMC is also provided. Figure 27 As shown, the MMC-based multi-port power router control device 200 may include: a power balancing module 210, a network control module 220 and a control switching module 230.

[0217] The power balancing module 210 is used for a five-port power router based on a double-ended back-to-back modular multilevel converter (MMC), adopting a dual active bridge converter structure with series input and parallel output to connect to the medium and low voltage DC bus, and achieving module power balancing at each port through input voltage balancing control;

[0218] A grid-following control module 220 is configured to apply a virtual synchronous generator control (VSG) to a grid-connected power control port of a double-ended back-to-back modular multilevel converter (MMC) and a virtual direct current motor control (VDMC) to an energy storage port of the MMC in a grid-following control link of the MMC.

[0219] The control switching module 230 is used to determine the operation mode based on the grid short-circuit ratio, and to control the switching of the MMC-based multi-port power router to follow the grid / build the grid under strong / weak grid conditions.

[0220] The specific details of each of the above-mentioned MMC-based multi-port power router control device modules have been described in detail in the corresponding MMC-based multi-port power router control method, and will not be repeated here.

[0221] It should be noted that although the above detailed description mentions several modules or units of an MMC-based multi-port power router control device 200, this division is not mandatory. In fact, according to the embodiments of the present disclosure, the features and functions of two or more modules or units described above can be embodied in a single module or unit. Conversely, the features and functions of a single module or unit described above can be further divided and embodied by multiple modules or units.

[0222] Furthermore, the above-described figures are merely illustrative of the processes included in the method according to exemplary embodiments of the present invention and are not intended to be limiting. It is readily understood that the processes illustrated in the above-described figures do not indicate or limit the temporal order of these processes. Furthermore, it is readily understood that these processes may be executed synchronously or asynchronously, for example, in multiple modules.

[0223] Other embodiments of the present disclosure will readily occur to those skilled in the art after considering the specification and practicing the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the present disclosure that follow from the general principles of the present disclosure and include common knowledge or customary techniques in the art not disclosed herein. The description and examples are to be considered as exemplary only, with the true scope and spirit of the present disclosure being indicated by the claims.

[0224] It should be understood that the present disclosure is not limited to the exact structures that have been described above and shown in the drawings, and that various modifications and changes can be made without departing from the scope thereof. The scope of the present disclosure is limited only by the appended claims.

Claims

1. A multi-port power router control method based on MMC, characterized in that: The method comprises: For a five-port power router based on a double-ended back-to-back modular multilevel converter (MMC), a dual active bridge converter structure with series input and parallel output is used to connect the medium and low voltage DC bus, and input voltage balancing control is used to achieve module power balancing at each port. In the grid-following control link of a double-ended back-to-back modular multilevel converter (MMC), a virtual synchronous generator control (VSG) is applied to the grid-connected power control port of the MMC, and a virtual DC motor control (VDMC) is applied to the energy storage port of the MMC. The operation mode is determined based on the short-circuit ratio of the power grid, and the MMC-based multi-port power router is controlled to switch between following the grid and building the grid under strong / weak grid conditions.

2. The method according to claim 1, wherein The method further comprises: Based on the characteristics of energy storage, photovoltaic, and DC loads, preset DC port control strategies, photovoltaic port control strategies, and energy storage port control strategies are established respectively, and control of each port is achieved through converters corresponding to the preset DC port control strategies, photovoltaic port control strategies, and energy storage port control strategies.

3. The method according to claim 1, wherein The method further comprises: In the grid-following control link of a double-ended back-to-back modular multilevel converter (MMC), the active-frequency control loop and reactive-voltage control loop of a virtual synchronous generator (VSG) are applied to the grid-connected power control port of the MMC.

4. The method according to claim 1, wherein The method further comprises: In the grid-following control link of a double-ended back-to-back modular multilevel converter (MMC), a virtual DC motor control (VDMC) is applied to the Buck / Boost converter at the energy storage port of the MMC by simulating the mechanical equations and armature circuit balance equations of the DC motor.

5. The method according to claim 1, wherein The method further comprises: According to the working mode of the MMC-based multi-port power router, a power distribution and dynamic adjustment strategy, a photovoltaic system output control strategy, an energy storage device charging and discharging control strategy, and a voltage stabilization control strategy are established to realize the control of the MMC-based multi-port power router.

6. The method according to claim 5, wherein The method further comprises: The power allocation and dynamic adjustment strategy includes power mutual assistance of interconnected distribution stations and local load power supply coordination control strategy; The photovoltaic system output control strategy outputs the generated electrical energy at maximum power and feeds it into the DC bus of the power router; The energy storage device charge and discharge control strategy includes charge state monitoring, load demand response and power balance regulation; The voltage stabilization control strategy includes adopting a virtual synchronous generator (VSG) control strategy under weak grid conditions and a strategy for stabilizing the low-voltage DC bus voltage through a virtual DC motor control (VDMC).

7. The method according to claim 5, wherein The method further comprises: The working modes of the MMC-based multi-port power router include flexible interconnection operation mode, area power demand sudden change operation mode, load sudden change operation mode, flow reversal operation mode, new energy output fluctuation operation mode, and off-grid self-operation mode.

8. The method according to claim 1, wherein The method further comprises: The inner loop current reference value output in the control mode before switching is sampled by the sample-and-hold device and used as the initial value of the inner loop control input after switching to achieve synchronization of the current signal; When switching from grid-following control to grid-forming control, phase and voltage amplitude pre-synchronization control is performed to complete the switching between grid-following and grid-forming control; When switching from network-building type to network-following type control, the phase transition is achieved through phase-locked loop synchronization to complete the network-following / network-building control switch.

9. A multi-port power router control device based on MMC, characterized in that: The device comprises: The power balancing module is used for a five-port power router based on a double-ended back-to-back modular multilevel converter (MMC). It uses a dual active bridge converter structure with series input and parallel output to connect to the medium and low voltage DC bus, and achieves module power balancing at each port through input voltage balancing control. A grid-following control module, configured to apply a virtual synchronous generator control (VSG) to a grid-connected power control port of a double-ended back-to-back modular multilevel converter (MMC) and a virtual DC motor control (VDMC) to an energy storage port of the MMC in a grid-following control link of the MMC; The control switching module is used to determine the operating mode based on the short-circuit ratio of the power grid, and to control the switching of the MMC-based multi-port power router to follow the grid / build the grid under strong / weak grid conditions.

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