An inertia sharing control method based on hybrid five-port converter

Abstract

The application belongs to the technical field of multi-port converter control, and relates to an inertia sharing control method based on a hybrid five-port converter. Overall and phase energy balance is realized through overall and phase inertia sharing control. Inertia support for a submodule capacitor and a DAB output capacitor is realized through inertia sharing control of an LVDC port. Inertia support for the DAB output capacitor is realized through inertia sharing control of the submodule capacitor. The performance of the inertia support is improved through model prediction-based inertia sharing control of the DAB output capacitor. Power supply to a passive port is realized through MVAC port voltage and current double closed loop control. The application fully schedules joint control among multiple stages by proposing four kinds of inertia sharing controllers and three kinds of balancing controllers, increases a virtual inertia control coefficient, reduces the fluctuation of capacitor voltage, helps the system to quickly transit the transient state, optimizes the transient performance of the system, and improves the dynamic performance and operation boundary of the system.

Description

Technical Field

[0001] This invention belongs to the field of multi-port converter control technology, and relates to an inertial sharing control method based on a hybrid five-port converter. Background Technology

[0002] In high-power AC / DC power distribution systems, with the introduction of different types of loads and energy storage subsystems, the increasing complexity of actual operating conditions, the continuous improvement and enrichment of voltage and power levels, and the continuous expansion of the number of ports, energy routing at different ports involves multiple operating conditions. Flexible power scheduling between ports and seamless switching between operating conditions is a major challenge. During transient transitions, the system still experiences significant fluctuations and long recovery times, maximizing stress on power electronic devices and easily leading to system instability. Complex operating conditions and continuous load switching also significantly impact the system. Furthermore, voltage and power imbalances exist between modules within the ports. Therefore, improving the converter's ability to withstand transient shocks while achieving system switching and balancing, and designing high-dynamic-performance control strategies have become key research areas. Currently, there are three main multi-port converter control schemes: 1. Energy extension control framework; 2. Fluctuation power transfer control framework; 3. Enhanced LVDC port performance control framework.

[0003] Traditional control schemes achieve energy balance through overall and inter-arm energy control, and capacitor voltage balance through PI control. This solution employs PI-based second-harmonic circulating current suppression control for circulating current suppression, and PI decoupling control for AC port current control. At the LVDC port, the outer loop uses PI control to control the bus voltage, while the inner loop uses master-slave control to achieve current sharing among modules.

[0004] Traditional energy-extended control schemes achieve energy balance through overall, phase-to-phase, and bridge-arm-to-bridge energy control. The output of the three nested controllers serves as the reference value for the PIR circulating current suppression controller, and PI decoupling control is used to control the AC port current. At the LVDC port, only a single PI control loop is used to control the bus voltage.

[0005] Traditional fluctuating power transfer control schemes achieve capacitor voltage balance through PI control, similar to conventional control schemes, and use PI decoupling control to control AC port current. The key difference lies in the LVDC port, where the outer loop uses PI control to regulate the bus voltage, and a portion of the phase shift value generated by the fluctuating power of the submodule capacitors is superimposed to reduce capacitor voltage fluctuations.

[0006] Traditional solutions for enhancing LVDC port performance achieve energy balance through phase-to-phase and arm-to-arm energy control, with two nested controllers whose outputs serve as reference values ​​for the PI circulating current suppression controller. The outer loop of the MVAC port uses power control to generate a reference value, which is then used by the inner loop for PI decoupling control to achieve AC port current control. The key feature is the use of dual-loop PIR control at the LVDC port to achieve bus voltage control and module current sharing.

[0007] However, traditional control schemes use the MVAC port as the source, while actual high-power converters use the MVDC port. This results in a nested three-layer energy control scheme, making control parameters difficult to adjust and significantly reducing dynamic response speed. During transient transitions, the system will experience significant fluctuations and long recovery times. Furthermore, the DAB parallel port uses master-slave current sharing control, leading to high hardware costs. Additionally, this scheme only works with a single LVDC port and cannot meet the power distribution needs of multiple ports.

[0008] While traditional energy-extended control schemes can be applied to multiple LVDC ports, like conventional control schemes, they suffer from nested control and difficult parameter tuning. Furthermore, this scheme lacks control strategies for submodule capacitor voltage equalization and DAB module current equalization.

[0009] Traditional fluctuating power transfer control schemes lack circulating current suppression control and overall and phase-to-phase energy balance control, making it impossible to achieve energy balance. Furthermore, they are only applicable to situations with a single LVDC port.

[0010] Traditional solutions for enhancing LVDC port performance involve nested energy controllers between phases and bridge arms, resulting in complex control. Simultaneous sampling master-slave current sharing control also leads to a large number of hardware components. Furthermore, none of these solutions analyze or control system transients. Multi-port converters still suffer from severe inter-port interference, large fluctuations in dynamic switching system operation, long transient recovery times, and poor dynamic performance.

[0011] Therefore, a control method capable of achieving energy coordination, transient switching, and submodule balancing is needed to solve this technical problem. Summary of the Invention

[0012] To solve the above-mentioned technical problems, the present invention is implemented through the following technical solution.

[0013] An inertial sharing control method based on a hybrid five-port converter is proposed. The inertial sharing characteristics between internal modules and external ports of the converter are proposed, and the control quantity and degree of freedom of the inertial sharing characteristics are analyzed to realize the control of the five-port converter. The control method realizes the energy balance between the whole and phases through overall and phase-to-phase inertial sharing control, and realizes the transmission and regulation of inertial sharing through circulating current control. Inertial support for the submodule capacitors and DAB output capacitors is achieved through inertial shared control of the LVDC port. Inertial support for the DAB output capacitor is achieved through inertial sharing control of the submodule capacitors. The performance of inertial support is improved by inertial sharing control of DAB output capacitor based on model prediction, while an embedded current sharing controller ensures that each DAB module transmits the same power, avoiding power imbalance. Power is supplied to the passive port through dual closed-loop control of MVAC port voltage and current. At the same time, a sorting-based submodule capacitor voltage balancing method is used to reduce the embedding of the PI controller and achieve equalization of the submodule capacitor voltage. A scheme combining a second-harmonic circulating current suppression controller with a PIR controller is adopted to achieve circulating current suppression; The inertia sharing characteristics are as follows: 1) Submodule capacitors can share inertia with DAB output capacitors; 2) DAB output capacitors can share inertia with submodule capacitors; 3) MVDC ports can share inertia with submodule capacitors; 4) MVDC ports can share inertia with DAB output capacitors; 5) LVDC ports can share inertia with DAB output capacitors; 6) LVDC ports can share inertia with submodule capacitors.

[0014] Preferably, the controller for the overall inertial sharing control in the overall and inter-phase inertial sharing control is: (13) In equation (13), This is the reference value for the MVDC side current. This represents the average capacitor voltage across all submodules in the multiport converter. This refers to the rated voltage of the submodule capacitor. and For the control parameters of the overall inertial shared controller, This is the points-based system.

[0015] More preferably, the controller for the inter-phase inertial sharing control in the overall and inter-phase inertial sharing control is: (14) In equation (14), This is the reference value for the p-phase circulating current command. This represents the average capacitor voltage of all submodules in the p-phase of the MMC. and For the control parameters of the phase-to-phase inertial shared controller, This is the points-based system.

[0016] Preferably, the controller that enables the inertial sharing control of the submodule capacitors in the inertial support of the submodule capacitors and the DAB output capacitor is: (15) In equation (15), The phase shift adjustment amount is used to obtain the DAB phase shift value after passing through the PI controller. The voltage of the capacitor in the nth submodule of phase p is... and These are the control parameters for the inertial shared controller of the submodule capacitor. This is the points-based system.

[0017] Preferably, the controller for the inertial sharing control of the LVDC port is configured in steps as follows: (16) (17) In equation (16), This is a general reference value for energy storage capacity. This serves as the primary reference value for energy storage capacity. and For the control parameters of the inertial shared controller at the LVDC port, This is a points-based system; In equation (17), For LVDC port current, For the current of the LVDC port load, The voltage at the LVDC port. and For the control parameters of the inertial shared controller at the LVDC port, This is the points-based system.

[0018] Preferably, the inertial sharing control of the DAB output capacitor based on model prediction and the current sharing control in the embedded current sharing controller are as follows: (30) in (31) In equation (30), and These represent the DAB leakage inductance and the transformer turns ratio, respectively. For DAB output capacitors, The switching frequency of the DAB. This refers to the capacitor voltage of the submodule. In equation (31), Let p be the output current of the LVDC port at time k. This is the reference value for the p-phase LVDC port voltage. Let N be the output voltage of the nth DAB in phase p at time k, and N be the number of submodules in each bridge arm.

[0019] Preferably, in the scheme of using a second-harmonic circulating current suppression controller combined with a PIR controller to achieve circulating current suppression, the second-harmonic circulating current suppression controller is: (33) In equation (33), and These are the reference values ​​for the d-axis and q-axis components of the circulation, respectively. and These are the d-axis and q-axis components of the circulation, respectively. These are the parameters for the second harmonic circulating current suppression PI controller. For the points-based system, For the passive network frequency on the AC side of the MVAC port, For bridge arm inductance.

[0020] The PIR controller is: (34) In equation (34), and These are the p-phase circulating current reference value and the p-phase circulating current, respectively. For PIR controller parameters, For the points-based system, For resonant gain, For bandwidth, The selected resonant angular frequency.

[0021] Preferably, in the MVAC port voltage and current dual closed-loop control method, the outer loop control adopts voltage feedback control, which adjusts the output of the voltage controller by comparing the actual voltage and the reference voltage to achieve accurate voltage tracking; simultaneously, the voltage obtained from the outer loop control... and As a reference value for current inner loop control and The inner loop control uses current feedback control, which adjusts the output of the current controller by comparing the actual current and the reference current to achieve accurate current tracking. The variables are mutually coupled; to eliminate coupling in the dq axis, a feedforward decoupling controller design is used. The controller is as follows: (29) In equation (29), and These are the reference values ​​for the d-axis and q-axis components of the AC side output current, respectively. and These represent the d-axis and q-axis components of the AC side output current, respectively. These are the PI controller parameters for the inner current loop. For the points-based system, It is the equivalent inductance.

[0022] The beneficial effects of this invention are: 1. This invention proposes a global inertial sharing control method, designing four inertial sharing controllers and three equalization controllers. It fully utilizes multi-level joint control, increases virtual inertial control coefficients, reduces capacitor voltage fluctuations, helps the system quickly transition between transient states, optimizes the system's transient performance, and improves the system's dynamic performance and operating boundaries. Simultaneously, it reduces the mutual influence of power coupling between ports and circuit instability under unbalanced conditions. This control scheme achieves energy scheduling, transient control, and module equalization.

[0023] 2. The inertial sharing control of sub-module capacitors in the global inertial sharing control proposed in this invention clearly analyzes the energy coupling relationship between modules, reduces capacitor voltage fluctuations, and simplifies or eliminates redundant controllers. Simultaneously, the adoption of reasonable module equalization control reduces the number of control parameter adjustments and the amount of hardware circuitry required for control and sampling.

[0024] 3. The global inertial shared control proposed in this invention, specifically the inertial shared control of the LVDC port, increases the degree of freedom in energy control by introducing distributed energy storage, enriches the energy flow modes of the converter, expands the energy operating range of the system, and achieves adaptive and smooth switching between various operating conditions. Simultaneously, mode switching does not require refactoring the control algorithm, reducing control complexity. Attached Figure Description

[0025] Figure 1 This is a control block diagram of the global inertial sharing control method in the inertial sharing control method based on a hybrid five-port converter of the present invention; Figure 2 This is a topology diagram of a hybrid five-port converter; Figure 3 This is a submodule topology diagram of a hybrid five-port converter; Figure 4 This is the equivalent circuit diagram for inertial sharing mode 1; Figure 5 This is the equivalent circuit diagram for inertial sharing mode 2; Figure 6 This is the equivalent circuit diagram for inertial sharing mode 3; Figure 7 This is the equivalent circuit diagram for inertial sharing mode 4; Figure 8 This is the equivalent circuit diagram of inertial sharing mode 5; Figure 9 This is the equivalent circuit diagram for inertial sharing mode 6; Figure 10 It is the equivalent circuit diagram of inertial sharing superposition; Figure 11 This is a schematic diagram of energy transfer in a hybrid five-port converter; Figure 12 This is the flowchart of the capacitor voltage balancing algorithm for the submodule; Figure 13 It is a waveform diagram of the MVDC port power before and after applying inertial shared control to the LVDC port; Figure 14 This is a waveform diagram of the LVDC port voltage of phase a before and after applying inertial shared control to the LVDC port; Figure 15 This is a waveform diagram of the capacitor voltage of phase a submodule before and after applying inertial shared control to the LVDC port; Figure 16 This is a waveform diagram of the capacitor voltage of the b-phase submodule before and after applying inertial shared control to the LVDC port; Figure 17 This is a waveform diagram of the capacitor voltage of the c-phase submodule before and after applying inertial shared control to the LVDC port; Figure 18 This is a waveform diagram of the system circulating current before and after applying inertial shared control at the LVDC port; Figure 19 This is a graph showing the THD values ​​of the MVAC port voltage before and after applying inertial shared control to the LVDC port; Figure 20 This is a waveform comparison of the capacitor voltage of phase a submodule inertia sharing control and traditional bridge arm energy balance control. Figure 21 This is a waveform comparison of the B-phase submodule capacitor voltage between submodule capacitor inertia sharing control and traditional bridge arm energy balance control. Figure 22 This is a waveform comparison of the C-phase submodule capacitor voltage between submodule capacitor inertia sharing control and traditional bridge arm energy balance control. Figure 23 This is a diagram showing the THD values ​​of the MVAC port voltages for submodule capacitor inertia sharing control and traditional bridge arm energy balance control. Figure 24 This is a comparison waveform diagram of the transient voltage at the LVDC port of phase a when DAB employs three phase-shift control strategies. Detailed Implementation

[0026] The related technologies of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] The control strategy designed in this invention is a global inertial sharing control scheme based on a hybrid five-port converter using MMC (Modular Multilevel Converter), DAB (Dual Active Bridge), and energy storage subsystem. First, based on the characteristics of the capacitor, this scheme proposes inertial sharing characteristics between internal modules and external ports of the converter, and analyzes the control variables and degrees of freedom for controlling these inertial sharing characteristics. Next, based on the mathematical model of the converter, four inertial sharing controllers and three equalization controllers are designed for the global inertial sharing control scheme to achieve coordinated energy control, optimization of system transient performance, and equalization among submodules.

[0028] Based on the analysis of variables that adjust shared inertia, this invention proposes a global inertia sharing control strategy, the control framework of which is as follows: Figure 1 As shown, overall and phase-to-phase energy balance is achieved through overall and phase-to-phase inertial sharing control, with circulating current control used for the transmission and regulation of inertial sharing. Inertial support for submodule capacitors and DAB output capacitors is achieved through inertial sharing control at the LVDC port. Inertial support for the DAB output capacitors is also achieved through inertial sharing control of the submodule capacitors. Model-predictive inertial sharing control of the DAB output capacitors improves the performance of inertial support, while an embedded current sharing controller ensures that each DAB module transmits the same power, avoiding power imbalance. Power is supplied to the passive port through dual closed-loop control of MVAC port voltage and current, while a sorting-based submodule capacitor voltage balancing method is used to reduce the embedding of PI controllers and achieve voltage equalization of submodule capacitors. Circulating current suppression is achieved using a scheme combining a second-harmonic circulating current suppression controller and a PIR controller. Figure 1 The following icons are used to mark the controller: A - Overall and inter-phase inertia sharing controller; B - Circulating current suppression controller; C - MVAC port voltage and current controller; D - LVDC port inertia sharing controller; E - Sub-module capacitor inertia sharing control; F - Sub-module capacitor voltage equalization controller and PWM generator; G - Sequencing-based sub-module voltage equalization control loop; H - Energy collaborative management controller; I - DAB output current equalization controller; J - DAB output capacitor inertia sharing controller based on predictive control.

[0029] The control strategy designed in this invention applies a hybrid five-port converter topology as follows: Figure 2As shown. This converter contains a total of 5 ports: an MVDC port, an MVAC port, and 3 LVDC ports. , These represent the voltage and current at the MVDC port, respectively. , These represent the phase voltage and phase current at the MVAC port of the passive network, respectively. For filtering inductors; These represent the voltage and current at the LVDCp (p=a,b,c) port, respectively. This refers to the load current at the LVDCp port. This represents the output current of each energy storage port. Each port comprises three phases and six bridge arms. (x=u,l) represents the currents of the upper and lower bridge arms. These are the bridge arm resistors and inductors, respectively. The middle of the upper and lower bridge arms serves as the output terminal of the MVAC port, supplying power to the passive network. There are a total of 6N sub-modules between the bridge arms. (n=1, (2N), each submodule consists of an MMC half-bridge submodule and a DAB, with the topology as follows: Figure 3 As shown. Among them, For submodule capacitors, This refers to the capacitor voltage of the submodule. This refers to the capacitor current of the submodule. The equivalent bridge arm voltages of each submodule are... This is the equivalent input current of the submodule. The MMC and DAB are power coupled through the submodule capacitors. The LVDC port is connected to the MVDC and MVAC ports through the DAB as a power channel. The output sides of 2N DAB submodules are connected in parallel to form an input independent output parallel (IIOP) connection. and These represent the DAB leakage inductance and the transformer turns ratio, respectively. For output filter capacitor, For capacitor current, and These represent the module's output voltage and current, respectively. For the primary input current of DAB, This is the secondary output current of the DAB. Figure 2 and Figure 3The demonstrated hybrid five-port converter tightly integrates MVDC, MVAC, three LVDC ports with unbalanced power levels, and distributed energy storage. Simultaneously, it combines the advantages of series and parallel converters, enabling flexible energy flow between ports, improving operational capabilities under unbalanced energy conditions, meeting power demands under different operating conditions, and ensuring rapid switching of operating modes under uninterruptible power supply (UPS) conditions.

[0030] This invention addresses the characteristics of circuit inertia sharing in five-port converters and constructs a control scheme based on this. When the port power of a multi-port converter changes, because the switching frequency in the half-bridge submodule is much lower than that in the DAB, and the port energy changes drastically, the input current and output current on both sides of the capacitor three-port network will become inconsistent. The capacitor will spontaneously output or absorb current to maintain the node energy balance, acting as the inertial support of the system. After the circuit parameters are designed, the capacitor parameters are fixed. After a sudden change in system port power, only the change in capacitor voltage provides inertial support, which will lead to transient power mismatch and cause drastic changes in bus voltage.

[0031] According to Kirchhoff's current law and They can be expressed as follows: (1) The natural inertia provided by the submodule capacitor and the DAB output capacitor can be expressed as follows: (2) in, This represents the input power of the three-port capacitor network in the submodule. Power transmitted by DAB This refers to the power output of the DAB.

[0032] As can be seen from formula (2), the submodule capacitor inside the port and the DAB output capacitor can share inertia. Assume the submodule capacitor provides inertia to the DAB output capacitor. The inertial power support can be expressed as (3) in, A virtual capacitor shared by the submodule capacitors and the DAB output capacitors. This is the output voltage of the nth DAB module in the p-phase phase with enhanced inertia. From the above equation, it can be analyzed that... Increasing the virtual inertial control coefficient can reduce the impact of power surges on the LVDC side, which can be expressed as: (4) Combining formulas (3) and (4), the inertia sharing of the submodule capacitor to the DAB output capacitor can be intuitively represented by the equivalent circuit, such as... Figure 4 As shown.

[0033] The inertia of the DAB output capacitor sharing with the submodule capacitor can be expressed as: (5) in, The inertial power support provided by the DAB output capacitor to the submodule capacitor. This is a virtual capacitor shared by the DAB output capacitor and the submodule capacitor. For the capacitor voltage of the nth submodule in the p-phase phase with enhanced inertia, the equivalent circuit is as follows: Figure 5 As shown.

[0034] In addition, inertial support can be provided between ports, enabling inertial sharing across the entire system. MVDC ports can share inertia to enhance the capacitive inertia of submodules, which can be represented as... (6) in, To provide inertial power support to the submodule capacitors via the MVDC port. This is a virtual capacitor shared by the MVDC port and the submodule capacitors. This represents the duty cycle of the average model for the submodule. The equivalent circuit is as follows: Figure 6 As shown.

[0035] MVDC ports can share inertia to enhance the inertia of the DAB output capacitor, which can be expressed as follows: (7) in, The inertial power support provided by the MVDC port to the DAB output capacitor. The virtual capacitor shared by the MVDC port for the output capacitor to the DAB is shown in the equivalent circuit as follows: Figure 7 As shown.

[0036] LVDC ports can share inertia to enhance the inertia of the DAB output capacitor, which can be expressed as follows: (8) in, This provides inertial power support for the output capacitor from the LVDC port to the DAB. The virtual capacitor shared by the LVDC port for the DAB output capacitor is shown in the equivalent circuit as follows: Figure 8 As shown.

[0037] LVDC ports can share inertia to enhance the capacitive inertia of submodules, which can be represented as follows: (9) in, To provide inertial power support for the LVDC port to the submodule capacitors. The virtual capacitor shared by the LVDC port and the submodule capacitor is shown in the equivalent circuit as follows. Figure 9 As shown.

[0038] According to formulas (3)-(9) and Figure 4-9 It can be seen that the capacitors between ports and within ports can share a portion of their inertia, thereby increasing the virtual inertia control coefficient, reducing capacitor voltage fluctuations, and increasing the system's inherent inertia. Figure 10 As shown in the diagram, this circuit enables global inertia sharing, more efficient allocation of transient power, and improved transient response capability.

[0039] Based on the above analysis, in addition to the inertia sharing provided by the internal capacitors, the MVDC and LVDC ports can provide inertial support for the system transients. This inertial support corresponds to a balance between the steady-state and transient energy of the converter. Therefore, analyzing the inertial sharing control quantity is equivalent to analyzing the energy balance control quantity. For example... Figure 11 As shown, the power absorbed by the multi-port converter is the difference between the power on the MVDC side, the MVAC side, and the LVDC side, which can be expressed as: (10) in, This represents the amplitude of the AC phase voltage on the MVAC side. This represents the amplitude of the AC phase current on the MVAC side. This represents the phase angle difference between the AC phase voltage and phase current on the MVAC side.

[0040] The power absorbed by phase P in steady state is the difference between the power on the MVDC side, the power on the p-phase MVAC side, and the power on the p-phase LVDC side, which is... (11) in, This represents the DC component in the p-phase circulating current. According to formulas (3)-(11), it can be observed that... It can be adjusted To change its size, It can be adjusted To change its size. In a multiport converter system, energy transfer occurs between any two ports. The value is constant at the set value. The amplitude of the AC phase voltage on the MVAC side. and the amplitude of the alternating phase current It is fixed by the AC load. The voltage of each phase LVDC bus is controlled at a fixed value, and the current of each phase LVDC bus can be regulated by controlling the energy storage connected to the LVDC side bus of each phase, thereby adjusting the energy. Therefore, it is suitable for adjusting... The variables of shared inertia are and Suitable for adjustment The variables of shared inertia are and .

[0041] Meanwhile, according to formula (1), it can be observed that the characteristic quantity of the shared inertia on the primary and secondary sides of DAB is: and The two can be represented as (12) As can be seen from formula (12), in DAB control This will affect the inertia sharing performance, and consequently, the transient characteristics of the system. Therefore, the variables that the system can adjust for shared inertia include... The transient energy control degrees of freedom reach 6N+7.

[0042] The control scheme of the global inertial sharing designed in this invention will be described in detail below.

[0043] Overall inertial shared control mainly describes the use of To adjust the inertia sharing amount, because For DC signals, zero steady-state error tracking can be achieved using simple PI control. The feedback is set to the average value of the capacitor voltages of all submodules in the multi-port converter. The control objective is to control this average value to the rated value of the submodule capacitor voltage. Therefore, The value is set as a reference value, and the error between the two is used to obtain the MVDC side current reference value after passing through the PI controller. This is used to adjust the overall inertia sharing of the multi-port converter and achieve overall energy balance. Based on this rule, the controller for overall inertia sharing control is... (13) Phase-to-phase inertial sharing control mainly describes the use of To adjust the inertia sharing, PI control is used to achieve zero steady-state error tracking. The feedback is set to the average value of the capacitor voltages of all p-phase submodules in the MMC. The control objective is to control this average value to The error between the two is added after passing through the PI controller. One-third, as a reference value for the p-phase circulating current command. This is used to adjust the inter-phase inertia sharing and energy balance of the multi-port converter. Based on this rule, the controller for inter-phase inertia sharing control is... (14) Energy coupling between the MMC and DAB is achieved via capacitors. The submodule capacitor voltage exhibits low-frequency fluctuations. Utilizing the DAB as a power channel to transfer some energy to the LVDC side effectively reduces these voltage fluctuations. When the power on the LVDC side experiences sudden changes, this energy provides inertial support, ensuring the stability of the LVDC terminal voltage. The setpoint and feedback for the inertial sharing control of the submodule capacitor are as follows: and The error is processed by a PI controller to obtain the phase shift adjustment amount of the DAB phase shift value. This enables partial low-frequency power transfer and inertial sharing of submodule capacitors. Based on this rule, the controller for inertial sharing control of submodule capacitors is... (15) Besides the variables applicable to adjusting the inertia sharing of multiport converters, as well as In addition, with the introduction of energy storage systems, It can also be used as an adjustment variable for inertial sharing control, changing the LVDC port from passive to active participation in inertial sharing. The voltage on the LVDC bus is controlled by the DAB connected to the IIOP for constant voltage control, and the constant power control of the energy storage system is equivalent to constant current control. Therefore, different energy storage power reference values ​​can be set in modes where energy storage power is required. This will cause the energy storage output current to be controlled according to the reference value, thereby changing... The value is used to achieve control of energy and inertia sharing. In a mode where there is no demand for energy storage power, the setting is... The value is 0. The outer loop uses PI control, based on... and The difference generates the current adjustment for inertial sharing, dynamically adjusting the virtual control coefficients for inertial sharing to achieve transient rapid control. Based on this rule, the controller for inertial sharing control at the LVDC port can be configured step-by-step. (16) (17) Different DAB controllers Unlike steady-state transient control, high-performance control methods can quickly adjust the inertial support of other circuit components for the DAB output capacitor. Model predictive control (MDI) is a control method based on a mathematical model of the converter. It calculates the control input by predicting the future state of the system and updates the control input at each sampling time. Specifically, MDI models the system as a discrete-time dynamic model and uses this model to predict the future system state. Then, based on these predictions, it determines the optimal control input by solving an optimization problem, thereby achieving optimal control. Adjust the optimal amount of inertia sharing.

[0044] Set the equivalent output load of each DAB module to According to Kirchhoff's voltage law, the dynamic averaging model of DAB can be expressed as: (18) For a three-phase 2N DAB (Dynamic Averaging Bodies), 6N dynamic averaging models can be established. By discretizing the dynamic averaging model of the converter using the Euler forward approximation method, the discrete mathematical model of the DAB can be obtained, i.e. (19) in, This represents the output voltage of the nth DAB submodule in phase p at time k, i.e., the LVDC port voltage of phase p. This represents the predicted output voltage of the nth DAB submodule in phase p at time k+1. In multi-port converter systems, low-voltage DC port power switching is frequent, and load parameters vary greatly. The equivalent output load included in the discrete model makes the model highly inaccurate, causing the port output voltage to deviate from the reference value. By equating the voltage and load to the output current of each module port, the prediction model can be improved to... (20) To control the stability of the low-voltage DC bus voltage, the cost function is considered as follows: (twenty one) in, When the optimal phase shift value is selected, the optimal problem can be solved to minimize the cost function, i.e. (twenty two) The optimal phase shift value can be obtained. (twenty three) in (twenty four) In high-power AC / DC hybrid power supply systems, MVAC ports frequently supply power to passive receiving-end networks, while the load varies depending on operating conditions. Since the receiving-end network frequency is a fixed value, the rotational speed of the dq converter shaft remains constant, thus eliminating the need for a phase-locked loop (PLL). However, when the load changes, although the rotational speed of the dq converter shaft remains constant, The angle between the amplitude and the d-axis will change, therefore it is necessary to control... The amplitude remains constant, so the power at the MVAC port is determined by the load requirements.

[0045] The average model of the MMC submodule is (25) According to the above formula, The midpoint is considered the neutral point. Kirchhoff's voltage equations are written for the upper and lower bridge arms respectively: (26) in, This refers to the differential mode voltage of the upper and lower arms of the p-phase bridge. This refers to the common-mode voltage of the upper and lower arms of the p-phase bridge. For p-phase circulation, the following equations can be derived respectively. (27) make and Apply the first expression of formula (26) Coordinate transformation yields the mathematical model in the dq coordinate system as follows: (28) in, and These are the reference values ​​for the d-axis and q-axis components of the MVAC side output current, respectively. and They are respectively The d-axis and q-axis components, and These are the reference values ​​for the d-axis and q-axis components of the MVAC-side output voltage, respectively. For the passive network frequency on the AC side of the MVAC port.

[0046] Based on the above analysis, the outer loop control employs voltage feedback control. By comparing the actual voltage with the reference voltage, the output of the voltage controller is adjusted to achieve precise voltage tracking. Simultaneously, the voltage obtained from the outer loop control... and As a reference value for current inner loop control and The inner loop control employs current feedback control, adjusting the current controller output by comparing the actual current with a reference current to achieve precise current tracking. Since the variables are coupled, a feedforward decoupling controller design is used to eliminate coupling in the dq axis. (29) in, and These are the reference values ​​for the d-axis and q-axis components of the AC side output current, respectively. These are the parameters of the PI controller for the inner current loop.

[0047] For a DAB with 2N IIOP connections, under normal operating conditions, the output current of each DAB module should remain equal to achieve power sharing. In practical applications, to reduce system cost and enhance the flexibility of the control algorithm, it is essential to study control strategies using fewer sensors. For the output current values ​​used in the model, the low-voltage DC bus current value divided by the number of modules is used to replace the output current of each module, thereby achieving 2N degrees of freedom control while simultaneously achieving current sharing control. (30) in (31) According to the control requirements, it is necessary to detect the input voltage, output voltage, and parallel output low-voltage DC bus current of each module. Therefore, one current sensor is required, and three current sensors are required for three-phase control. Compared with master-slave control, the number of current sampling hardware is greatly reduced.

[0048] Since the capacitors of each submodule in an MMC are independent, differences in switching times, communication delays, and component parameters among submodules in practical applications can all cause inconsistent charging and discharging of the capacitors. If left uncontrolled, this will lead to voltage divergence in the submodule capacitors, increasing the difficulty of controlling the DAB converter connected to the IIOP, distorting the output voltage and current on the MVAC side, and even jeopardizing the safety and lifespan of the equipment. Therefore, capacitor voltage balancing control is fundamental to the stable operation of a multi-port converter system, and its implementation is closely related to the adopted PWM strategy. Specifically, it involves adjusting the charging and discharging power of the submodule by changing its on / off state based on the current capacitor voltage level and the corresponding PWM strategy, thereby ensuring stable and balanced capacitor voltage.

[0049] This invention combines this approach with a carrier phase-shift modulation strategy, providing a sorting-based submodule capacitor expansion balancing method. First, the capacitor voltages of all submodules in each phase are detected. The voltage magnitudes of each submodule are compared using a bubble sort method and sorted in descending order. Based on the number of submodules that need to be deployed in the bridge arm and the direction of the bridge arm current obtained from the carrier phase-shift PWM generator at the current moment, the submodules that need to be deployed are selected. The specific selection rules are as follows: Figure 12 As shown. The purpose is to allow submodules with lower capacitor voltages to operate for a longer period when the bridge arm current is positive, allowing the capacitors to charge; conversely, to allow submodules with higher capacitor voltages to operate for a longer period when the bridge arm current is negative. After calculating the required number of submodules to be operated using phase-shift modulation, the capacitor voltage balancing method based on the nearest level approximation is used to select specific submodules to be operated and removed in order to maintain capacitor voltage balance. Compared to the method of introducing voltage regulation in carrier phase shifting, this method reduces the number of embedded PI controllers and simplifies parameter tuning.

[0050] Applying the second expression to formula (26) Coordinate transformation yields the expression for this equation in a rotated coordinate system as follows: (32) in, and These are the reference values ​​for the d-axis and q-axis components of the circulation, respectively. and They are respectively The d-axis and q-axis components.

[0051] The detected three-phase circulation Using Park transformation as feedback, Perform coordinate transformation to obtain the given signal. The errors between the two are processed by a PI controller, and feedforward compensation is applied to the cross-coupling terms of the dq-axis currents. Finally, the circulating current suppression voltage commands for each phase of the MMC are obtained through Park inverse transform. The mathematical model of the circulating current shows that the variables are mutually coupled. To eliminate the coupling in the dq-axis, a feedforward decoupling controller is used. The second-harmonic circulating current suppression controller is... (33) in, and These are the reference values ​​for the d-axis and q-axis components of the circulation, respectively. These are the parameters for the PI controller to suppress circulating current.

[0052] The resonant controller is a second-order generalized integrator, capable of achieving infinite gain at a selected frequency, thus allowing selective control of harmonics at a specific frequency. The PIR controller is as follows: (34) In equation (34), and These are the p-phase circulating current reference value and the p-phase circulating current, respectively. For PIR controller parameters, For the points-based system, For resonant gain, For bandwidth, The selected resonant angular frequency.

[0053] Traditional PI control methods are insufficient for fully tracking periodic AC harmonic components, being effective only for second harmonics and unable to suppress fourth, sixth, and other subharmonics in the circulating current. Resonant controllers require setting a series of parameters, making debugging complex. This invention combines traditional PI control with a resonant controller. The PI controller acts as the master controller to generate the core common-mode component, while a basic resonant controller for suppressing the fourth harmonic in the circulating current acts as the slave controller, generating fine-tuned common-mode components to suppress the fourth harmonic, thereby achieving zero steady-state error tracking of the DC component in the circulating current.

[0054] To verify the effectiveness of the proposed control strategy, a hybrid five-port converter circuit and controller were built using MATLAB / SIMULINK. The specific circuit parameters are shown in Table 1. In multi-port converters, three-phase energy imbalance exists, especially during rapid load switching on one phase. This significantly affects the power, voltage, and current characteristics of each port, as well as the voltage of each module and the circulating current in the system. During transients, large load switching has a significant impact on both classical controllers and the controller proposed in this paper. This invention selects the condition where the LVDC load on phase a suddenly increases from 100kW to 400kW at 0.2s as a comparison to verify the superiority of the proposed control strategy. The specific operating condition changes are shown in Table 2.

[0055]

[0056]

[0057] Figure 13 The figure shows the MVDC port power waveforms before and after applying inertial sharing control to the LVDC port. It is clear from the figure that the transient fluctuations of the MVDC port power are significantly reduced after applying inertial sharing control to the LVDC port. Figure 14 The waveforms of the LVDC port voltage in phase a before and after applying inertial sharing control at the LVDC port are shown. Without inertial sharing control at the LVDC port, the voltage drops to 980V at 0.2s. Due to fluctuations at the MVDC port and abrupt changes in the submodule capacitor voltage, the voltage continues to drop and oscillate until it stabilizes after 0.08s. However, after applying inertial sharing control at the LVDC port, the voltage drops to 985V at 0.2s, but without the continuous drop and oscillation, and reaches steady state after 0.025s.

[0058] Figure 15The waveforms of the phase a submodule capacitor voltage before and after applying inertial sharing control at the LVDC port are shown. Without applying inertial sharing control at the LVDC port, the voltage drops to a maximum of 789V, and an overshoot of 1109V occurs during recovery, stabilizing after 0.15s. After applying inertial sharing control at the LVDC port, the voltage drops to a maximum of 868V, with no overshoot, and stabilizes after 0.12s. Figure 16 and Figure 17 The waveforms of the capacitor voltages of the b-phase and c-phase submodules before and after applying inertial sharing control at the LVDC port are shown. The maximum surge voltage of the capacitors after applying inertial sharing control at the LVDC port is reduced by 42V and 49V, respectively.

[0059] Figure 18 The system circulating current waveforms before and after applying inertial shared control at the LVDC port are shown. It is clear that after introducing inertial shared control at the LVDC port, the fluctuations and oscillations of the circulating current during the transient process are significantly reduced. Figure 19 The results show the THD values ​​of the MVAC port voltage before and after applying inertial sharing control at the LVDC port. The maximum THD values ​​before and after applying the inertial sharing control at the LVDC port are 0.193% and 0.149%, respectively, indicating that when the LVDC port power fluctuates significantly, the introduction of inertial sharing control at the LVDC port can reduce the impact on the MVAC port. Based on the above results, it can be concluded that when the LVDC port power switches, applying inertial sharing control at the LVDC port significantly reduces the impact on the MVDC and MVAC ports, reflected in smaller MVDC power fluctuations and lower AC voltage THD values. Simultaneously, during transients, the voltage fluctuations of the capacitor providing natural inertia within the system are also significantly reduced, the response speed is slightly faster, and the system's ability to withstand transient overshoot is improved, thus verifying the effectiveness of the proposed transient controller, namely, the inertial sharing control at the LVDC port.

[0060] Submodule capacitor inertia sharing control can effectively reduce voltage fluctuations in submodule capacitors, resulting in smaller voltage spikes and drops during transient processes and faster attainment of steady state. Simultaneously, submodule capacitor inertia sharing control can indirectly balance power between bridge arms, ensuring that the energy of the upper and lower bridge arms remains as similar as possible.

[0061] Figure 20 The waveforms of the phase a submodule capacitor voltage are compared between submodule capacitor inertia sharing control and traditional inter-arm energy balance control. Using traditional inter-arm energy balance control, the capacitor voltage fluctuation under normal operating conditions is 168V, with a maximum transient voltage drop to 778V, stabilizing after 0.2 seconds. Using submodule capacitor inertia sharing control, the capacitor voltage fluctuation under normal operating conditions is 116V, with a maximum transient voltage drop to 868V, stabilizing after 0.12 seconds.

[0062] Figure 21 The waveforms of the b-phase submodule capacitor voltage are compared between submodule capacitor inertia sharing control and traditional inter-arm energy balance control. Using traditional inter-arm energy balance control, the transient voltage drops to a maximum of 873V and surges to a maximum of 1159V, requiring a relatively long energy modulation period for the upper and lower arm voltages to reach a steady state. Using submodule capacitor inertia sharing control, the transient voltage surges to a maximum of 1079V and stabilizes after 0.06s.

[0063] Figure 22 The waveforms of the C-phase submodule capacitor voltage are compared between submodule capacitor inertia sharing control and traditional inter-arm energy balance control. Using traditional inter-arm energy balance control, the transient voltage spikes to a maximum of 1149V, and the upper and lower arm voltages take 0.1s to reach steady state. Using submodule capacitor inertia sharing control, the transient voltage spikes to a maximum of 1111V and stabilizes after 0.07s.

[0064] Figure 23 The results demonstrate the THD values ​​of the MVAC port voltage under submodule capacitor inertia-sharing control and traditional inter-arm energy balance control. Using traditional inter-arm energy balance control, the maximum THD value is 0.195%, while using submodule capacitor inertia-sharing control, the maximum THD value is 0.149%. This indicates that when the LVDC port power fluctuates significantly, submodule capacitor inertia-sharing control can better reduce the impact on the MVAC port. Based on these results, it can be concluded that under normal operating conditions, using submodule capacitor inertia-sharing control results in smaller fluctuations in the submodule capacitor value. When the LVDC port power switches, using submodule capacitor inertia-sharing control significantly reduces the voltage fluctuation during transients, greatly accelerates the recovery speed, and improves the system's ability to withstand transient overshoot, thus verifying the effectiveness of the proposed submodule capacitor inertia-sharing control.

[0065] In LVDC, the DAB output capacitor is a key component for shared inertia, and its energy and voltage stability will seriously affect the operation of the entire system. Figure 24The graph shows a comparison of the transient voltage waveforms at the LVDC port of phase a when DAB employs three phase-shift control strategies. It can be observed that the DAB output capacitor inertia-shared control based on master-slave control exhibits the largest voltage drop (78.9V) and the longest transient response time (35.1ms), along with the most pronounced voltage ripple. The DAB output capacitor inertia-shared control based on direct power control shows the second best control performance, with a voltage drop of 44.5V and a transient response time of 24.6ms. The DAB output capacitor inertia-shared control based on model predictive control demonstrates the best control performance, with a voltage drop of only 18.7V and a transient response time of 13.6ms. Based on these results, it can be verified that the DAB output capacitor inertia-shared control based on model predictive control exhibits superior control performance under sudden load changes.

[0066] In summary, this invention, through four inertial shared controllers and three equalization controllers, fully utilizes multi-level joint control, increases the virtual inertial control coefficient, reduces capacitor voltage fluctuations, helps the system quickly transition between transient states, optimizes the system's transient performance, and improves the system's dynamic performance and operating boundaries. Simultaneously, it reduces the mutual influence of power coupling between ports and circuit instability under unbalanced conditions. This control scheme achieves energy dispatch, transient control, and module equalization; therefore, this invention has broad application prospects.

[0067] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. An inertial shared control method based on a hybrid five-port converter, characterized in that: Energy balance between the whole and between phases is achieved through overall and phase-to-phase inertial sharing control, while the transmission and regulation of inertial sharing are achieved through circulating control. Inertial support for the submodule capacitors and DAB output capacitors is achieved through inertial shared control of the LVDC port. Inertial support for the DAB output capacitor is achieved through inertial sharing control of the submodule capacitors. The performance of inertial support is improved by inertial sharing control of DAB output capacitor based on model prediction, while an embedded current sharing controller ensures that each DAB module transmits the same power, avoiding power imbalance. Power is supplied to the passive port through dual closed-loop control of MVAC port voltage and current. At the same time, a sorting-based submodule capacitor voltage balancing method is used to reduce the embedding of the PI controller and achieve equalization of the submodule capacitor voltage. A scheme combining a second-harmonic circulating current suppression controller with a PIR controller is adopted to achieve circulating current suppression; The controller for the inertial sharing control of the LVDC port is configured in steps as follows: (16) (17) In equation (16), This is a general reference value for energy storage capacity. This serves as the primary reference value for energy storage capacity. and For the control parameters of the inertial shared controller at the LVDC port, This is a points-based system; In equation (17), For LVDC port current, For the current of the LVDC port load, The voltage at the LVDC port. and For the control parameters of the inertial shared controller at the LVDC port, This is a points-based system; This refers to the rated voltage of the submodule capacitor. This represents the average capacitor voltage of all submodules in the p-phase of the MMC. This is the duty cycle command value for the LVDC port converter; The inertial sharing control of the DAB output capacitor based on model prediction and the current sharing control in the embedded current sharing controller are as follows: (30) in (31) In equation (30), and These represent the DAB leakage inductance and the transformer turns ratio, respectively. For DAB output capacitors, The switching frequency of the DAB. This refers to the capacitor voltage of the submodule. In equation (31), Let p be the output current of the LVDC port at time k. This is the reference value for the p-phase LVDC port voltage. Let N be the output voltage of the nth DAB in phase p at time k, and N be the number of submodules in each bridge arm.

2. The inertial sharing control method based on a hybrid five-port converter according to claim 1, characterized in that, The controller for the overall inertial sharing control in the overall and inter-phase inertial sharing control is: (13) In equation (13), This is the reference value for the MVDC side current. This represents the average capacitor voltage across all submodules in the multiport converter. This refers to the rated voltage of the submodule capacitor. and For the control parameters of the overall inertial shared controller, This is the points-based system.

3. The inertial sharing control method based on a hybrid five-port converter according to claim 2, characterized in that, The controller for the inter-phase inertial sharing control in the overall and inter-phase inertial sharing control is: (14) In equation (14), This is the reference value for the p-phase circulating current command. This represents the average capacitor voltage of all submodules in the p-phase of the MMC. and For the control parameters of the phase-to-phase inertial shared controller, This is the points-based system.

4. The inertial sharing control method based on a hybrid five-port converter according to claim 1, characterized in that, The controller that implements the inertial shared control for the submodule capacitors in the inertial support of the submodule capacitors and DAB output capacitors is: (15) In equation (15), The phase shift adjustment amount is used to obtain the DAB phase shift value after passing through the PI controller. The voltage of the capacitor in the nth submodule of phase p is... and These are the control parameters for the inertial shared controller of the submodule capacitor. For the points-based system, This is the average value of the capacitor voltages of all submodules in the multi-port converter.

5. The inertial sharing control method based on a hybrid five-port converter according to claim 1, characterized in that, In the scheme of using a second-harmonic circulating current suppression controller combined with a PIR controller to achieve circulating current suppression, the second-harmonic circulating current suppression controller is: (33) In equation (33), and These are the reference values ​​for the d-axis and q-axis components of the circulation, respectively. and These are the d-axis and q-axis components of the circulation, respectively. These are the parameters for the second harmonic circulating current suppression PI controller. For the points-based system, For the passive network frequency on the AC side of the MVAC port, For bridge arm inductance, and They are respectively The d-axis and q-axis components; The PIR controller is: (34) In equation (34), and These are the p-phase circulating current reference value and the p-phase circulating current, respectively. For PIR controller parameters, For the points-based system, For resonant gain, For bandwidth, The selected resonant angular frequency.

6. The inertial sharing control method based on a hybrid five-port converter according to claim 1, characterized in that, In the voltage and current dual closed-loop control method of the MVAC port, the outer loop control adopts voltage feedback control. By comparing the actual voltage and the reference voltage, the output of the voltage controller is adjusted to achieve accurate voltage tracking. Simultaneously, the voltage obtained from the outer loop control... and As a reference value for current inner loop control and The inner loop control uses current feedback control, which adjusts the output of the current controller by comparing the actual current and the reference current to achieve accurate current tracking. The variables are mutually coupled; to eliminate coupling in the dq axis, a feedforward decoupling controller design is used. The controller is as follows: (29) In equation (29), and These are the reference values ​​for the d-axis and q-axis components of the AC side output current, respectively. and These represent the d-axis and q-axis components of the AC side output current, respectively. These are the PI controller parameters for the inner current loop. For the points-based system, For equivalent inductance, For the passive network frequency on the AC side of the MVAC port, and They are respectively The d-axis and q-axis components, and These are the reference values ​​for the d-axis and q-axis components of the MVAC side output voltage, respectively.

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