A three-level converter dc microgrid hierarchical distributed control method and system

Abstract

The application discloses a three-level converter DC microgrid layered distributed control method and system, and the method comprises the following steps: performing droop control on the current d-axis component of each converter; obtaining a secondary control voltage compensation signal to compensate the droop control; converting the dq modulation voltage into a three-phase modulation voltage, injecting a modulation enhanced zero sequence component into the three-phase modulation voltage; performing midpoint potential control on the three-phase modulation voltage and three-phase current after the modulation enhanced zero sequence component is injected to obtain a midpoint balance zero sequence component, and superimposing the midpoint balance zero sequence component on the three-phase modulation voltage after the modulation enhanced zero sequence component is injected to serve as a final modulation wave signal, and the converter is controlled; the application has the advantages that the problems of midpoint potential deviation, insufficient bus voltage stability and high calculation complexity are solved.

Description

Technical Field

[0001] This invention relates to the field of DC microgrid control, specifically to a hierarchical distributed control method and system for a three-level converter DC microgrid. Background Technology

[0002] With the increasing demand for higher power density, lower switching losses, and high-quality DC power supply in scenarios such as high-power data centers and large scientific facilities, three-phase three-level converters are gradually replacing thyristors and two-level converters, and will gradually become the core interface equipment of DC microgrids. Current DC microgrid engineering control is almost entirely built for two-level AC / DC converters and DC / DC converters, lacking relevant technical reserves and applications for three-level grid construction. Furthermore, the inherent defects of its droop control can lead to bus voltage deviation and power distribution imbalance, which are difficult to predict under long-term and variable operating conditions, posing a significant threat to the entire system.

[0003] Conventional microgrid control based on the consistency principle requires periodic information exchange between the converter and the entire system. For example, the distributed event-driven hierarchical control method for AC / DC hybrid microgrids disclosed in Chinese Patent Publication No. CN115377981A configures an intelligent agent for each generation unit in the AC and DC microgrid subsystems, as well as the interconnecting converters of the AC / DC microgrid subsystems, forming a communication topology between the intelligent agents. This ensures that the network formed by the remaining intelligent agents remains connected even if any one intelligent agent fails. However, the complex converter communication topology inevitably leads to communication link delays and packet loss, thus introducing new dynamic uncertainties.

[0004] Three-level converter topologies have a unique midpoint potential offset problem, which can lead to uneven voltage distribution on the DC side capacitors. This can cause one capacitor to fail due to excessive voltage, resulting in insufficient bus voltage stability. Furthermore, the space vector pulse width modulation (SPWM) commonly used in three-level converters suffers from excessive computational complexity due to the presence of 27 space vectors. In DC microgrid systems built with three-level converters, where multiple converters are connected in parallel, the low computational efficiency can overload system computing resources, preventing the achievement of microgrid control objectives. Summary of the Invention

[0005] The technical problem to be solved by this invention is that the existing control methods for DC microgrids have insufficient bus voltage stability, and the DC microgrids constructed by three-level converters have problems such as midpoint potential shift and high computational complexity.

[0006] This invention solves the above-mentioned technical problems through the following technical means: a hierarchical distributed control method for a three-level converter DC microgrid, comprising:

[0007] S1, Current d-axis components of each converter Droop control is implemented to achieve current sharing among the converters;

[0008] S2. Obtain the virtual voltage drop of the current converter and its adjacent converters and take the average value. Subtract the average value from the previous stage secondary control voltage compensation signal of the current converter and send it to the first PI controller. Add the virtual voltage drop of the current converter to the output of the first PI controller to obtain the current stage secondary control voltage compensation signal and compensate it to the droop control point of S1.

[0009] S3. Calculate the expected value of the bus voltage. With current d-axis component The difference between the results after droop control is calculated, and this difference is summed with the secondary control voltage compensation signal of the current stage to obtain the DC voltage command. DC voltage command The dq-axis modulation voltage is obtained through modulation, and the dq-axis modulation voltage is converted into a three-phase modulation voltage. The zero-sequence component of modulation enhancement is injected into the three-phase modulation voltage.

[0010] S4. The neutral point balance zero-sequence component is obtained by using the three-phase current and the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component. The neutral point balance zero-sequence component is superimposed on the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component as the final modulation wave signal to control the converter.

[0011] Further, S1 includes:

[0012] The relationship between the DC-side currents of adjacent converters is as follows: ,in, For the first DC side current of the converter For the first The droop factor of a converter; Let j be the DC-side current of the j-th converter. Let be the droop coefficient of the j-th converter;

[0013] According to the power balance principle, when the power supplied by the power source and the power consumed by the load reach a dynamic balance, then...

[0014]

[0015] in, Indicates the first The power supply of each converter Indicates the first The load power consumption of each converter and They represent the first The d-axis and q-axis components of the three-phase voltage of the converter. and They represent the first The d-axis and q-axis components of the current of each converter. For the first DC voltage of the converter;

[0016] make Therefore The equation for the downward slope is written as ,in, For the first The DC voltage command for each converter is given; the droop coefficient of each converter is obtained through the droop equation and substituted into the relationship of the DC side current of the converter to adjust the DC side current of each converter, thereby achieving current sharing among the converters.

[0017] Furthermore, the process S2 is expressed by the following formula:

[0018]

[0019]

[0020] in, Representing the The global system average deviation of each converter Representing the The virtual voltage drop of a converter Indicates the first The average virtual voltage drop of the converter and its adjacent converters Representing the The secondary control voltage compensation signal of the converter in the previous stage. and The PI parameters represent the first PI controller. express time, This indicates the sampling time of the current within one triangular carrier cycle. Representing the The current stage secondary control voltage compensation signal of the converter.

[0021] Furthermore, the DC voltage command The dq-axis modulated voltage is obtained through modulation, and then converted into a three-phase modulated voltage, including:

[0022] Common bus load voltage of multiple converters in parallel With DC voltage command After the difference is calculated, the second PI controller generates the d-axis current reference value. d-axis current reference value The three-phase currents of the converter are respectively input to the third PI controller and the fourth PI controller; Perform a coordinate transformation from the abc coordinate system to the dq coordinate system, transforming it into the d-axis component of the current. and q-axis components ; The angular frequency of the power grid. For AC side inductance, respectively with and The cross product is then superimposed on the outputs of the third and fourth PI controllers, respectively, while the dq-axis components of the three-phase voltage are also considered. and The voltages are respectively superimposed on the outputs of the fourth PI controller and the third PI controller to obtain the dq-axis modulated voltage. and The dq modulated voltage is transformed from the dq coordinate system to the abc coordinate system to obtain the three-phase modulated voltage.

[0023] Further, the injection of modulation enhancement of the zero-sequence component into the three-phase modulation voltage includes:

[0024]

[0025] in, This represents the three-phase modulation voltage after the zero-sequence component is injected and enhanced. , express Three phases, This indicates a three-phase modulated voltage. This indicates that the modulation enhances the zero-sequence component at the current time and

[0026]

[0027] in, express The fundamental component of the three-phase output voltage.

[0028] Furthermore, the method of obtaining the neutral point balanced zero-sequence component by using the three-phase current and the three-phase modulation voltage after injecting modulation to enhance the zero-sequence component for neutral point potential control includes:

[0029] The midpoint equilibrium zero-sequence components that need to be superimposed are:

[0030]

[0031] in, These are the capacitance values ​​of the upper and lower capacitors on the DC side of the converter. This indicates the sampling time of the current within one triangular carrier cycle. and These represent the voltages of the upper and lower capacitors on the DC side of the converter, respectively. for Phase current, For symbolic functions, Represents the absolute value symbol.

[0032] This invention also provides a hierarchical distributed control system for a three-level converter DC microgrid, comprising:

[0033] The droop control module is used to control the d-axis component of the current of each converter. Droop control is implemented to achieve current sharing among the converters;

[0034] The secondary control module is used to obtain the virtual voltage drop of the current converter and its adjacent converters and take the average value. The difference between the average value and the previous stage secondary control voltage compensation signal of the current converter is sent to the first PI controller. The output of the first PI controller is superimposed on the virtual voltage drop of the current converter to obtain the current stage secondary control voltage compensation signal and compensate to the droop control point of the droop control module.

[0035] Zero-sequence injection modulation module is used to calculate the expected value of the bus voltage. With current d-axis component The difference between the results after droop control is calculated, and this difference is summed with the secondary control voltage compensation signal of the current stage to obtain the DC voltage command. DC voltage command The dq-axis modulation voltage is obtained through modulation, and the dq-axis modulation voltage is converted into a three-phase modulation voltage. The zero-sequence component of modulation enhancement is injected into the three-phase modulation voltage.

[0036] The midpoint potential control module is used to obtain the midpoint balanced zero-sequence component by using the three-phase current and the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component. The midpoint balanced zero-sequence component is superimposed on the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component as the final modulation wave signal to control the converter.

[0037] Furthermore, the droop control module is also used for:

[0038] The relationship between the DC-side currents of adjacent converters is as follows: ,in, For the first DC side current of the converter For the first The droop factor of a converter; Let j be the DC-side current of the j-th converter. Let be the droop coefficient of the j-th converter;

[0039] According to the power balance principle, when the power supplied by the power source and the power consumed by the load reach a dynamic balance, then...

[0040]

[0041] in, Indicates the first The power supply of each converter Indicates the first The load power consumption of each converter and They represent the first The d-axis and q-axis components of the three-phase voltage of the converter. and They represent the first The d-axis and q-axis components of the current of each converter. For the first DC voltage of the converter;

[0042] make Therefore The equation for the downward slope is written as ,in, For the first The DC voltage command for each converter is given; the droop coefficient of each converter is obtained through the droop equation and substituted into the relationship of the DC side current of the converter to adjust the DC side current of each converter, thereby achieving current sharing among the converters.

[0043] Furthermore, the execution process of the secondary control module is expressed by the following formula:

[0044]

[0045]

[0046] in, Representing the The global system average deviation of each converter Representing the The virtual voltage drop of a converter Indicates the first The average virtual voltage drop of the converter and its adjacent converters Representing the The secondary control voltage compensation signal of the converter in the previous stage. and The PI parameters represent the first PI controller. express time, This indicates the sampling time of the current within one triangular carrier cycle. Representing the The current stage secondary control voltage compensation signal of the converter.

[0047] Furthermore, the DC voltage command The dq-axis modulated voltage is obtained through modulation, and then converted into a three-phase modulated voltage, including:

[0048] Common bus load voltage of multiple converters in parallel With DC voltage command After the difference is calculated, the second PI controller generates the d-axis current reference value. d-axis current reference value The three-phase currents of the converter are respectively input to the third PI controller and the fourth PI controller; Perform a coordinate transformation from the abc coordinate system to the dq coordinate system, transforming it into the d-axis component of the current. and q-axis components ; The angular frequency of the power grid. For AC side inductance, respectively with and The cross product is then superimposed on the outputs of the third and fourth PI controllers, respectively, while the dq-axis components of the three-phase voltage are also considered. and The voltages are respectively superimposed on the outputs of the fourth PI controller and the third PI controller to obtain the dq-axis modulated voltage. and The dq modulated voltage is transformed from the dq coordinate system to the abc coordinate system to obtain the three-phase modulated voltage.

[0049] Further, the injection of modulation enhancement of the zero-sequence component into the three-phase modulation voltage includes:

[0050]

[0051] in, This represents the three-phase modulation voltage after the zero-sequence component is injected and enhanced. , express Three phases, This indicates a three-phase modulated voltage. This indicates that the modulation enhances the zero-sequence component at the current time and

[0052]

[0053] in, express The fundamental component of the three-phase output voltage.

[0054] Furthermore, the method of obtaining the neutral point balanced zero-sequence component by using the three-phase current and the three-phase modulation voltage after injecting modulation to enhance the zero-sequence component for neutral point potential control includes:

[0055] The midpoint equilibrium zero-sequence components that need to be superimposed are:

[0056]

[0057] in, These are the capacitance values ​​of the upper and lower capacitors on the DC side of the converter. This indicates the sampling time of the current within one triangular carrier cycle. and These represent the voltages of the upper and lower capacitors on the DC side of the converter, respectively. for Phase current, For symbolic functions, Represents the absolute value symbol.

[0058] The advantages of this invention are:

[0059] (1) The present invention relates to the d-axis components of the current of each converter. Droop control is implemented to achieve current sharing among all converters, effectively avoiding the influence of interference. Secondary control voltage compensation solves the inherent defect of bus voltage deviation caused by droop control, improving bus voltage stability. The method of modulation enhancement zero-sequence component injection to generate the final modulation wave signal significantly reduces computational complexity compared to complex SVPWM modulation strategies, providing feasibility for controlling large-scale parallel three-level converters in DC microgrid systems. For the unique midpoint potential deviation of three-phase three-level converters, a low-frequency, slowly varying midpoint balancing zero-sequence component is also introduced to control the midpoint potential. Therefore, the overall scheme can solve the problems of midpoint potential deviation, insufficient bus voltage stability, and high computational complexity.

[0060] (2) The present invention innovatively introduces a droop control gain selection mechanism based on power balance in the primary droop control, which strictly ensures that the control signal is not affected by various interferences, and adopts a combination of local droop control and voltage and current control. The secondary control is based on a distributed communication network, which uses the droop virtual voltage drop signal of adjacent converters to achieve accurate recovery of DC bus voltage, and strictly ensures the accuracy of current sharing and bus voltage regulation.

[0061] (3) By superimposing a low-frequency, slowly varying midpoint balance zero-sequence component, the present invention can effectively balance the charge changes of the upper and lower capacitors on the DC side of the three-level circuit, thereby achieving midpoint potential control and preventing midpoint potential deviation.

[0062] (4) The method proposed in this invention only requires local information and adjacent communication of the three-level converter, without the need for a global controller, which significantly reduces communication complexity and computational complexity, thereby significantly improving the computational efficiency of the three-level converter and ensuring the real-time performance of the algorithm. This method also does not require additional voltage and current measurement links, effectively avoiding most noise interference. Moreover, this hierarchical distributed control method is the first of its kind proposed for three-phase three-level DC microgrids, while ensuring scalability. For example, this hierarchical distributed control framework can be applied to DC microgrids constructed from five-level or seven-level converters. Attached Figure Description

[0063] Figure 1 This is a schematic diagram illustrating the principle of a hierarchical distributed control method for a three-level converter DC microgrid disclosed in Embodiment 1 of the present invention;

[0064] Figure 2(a) is a schematic diagram of the first communication topology in the hierarchical distributed control method of a three-level converter DC microgrid disclosed in Embodiment 1 of the present invention; Figure 2(b) is a schematic diagram of the second communication topology; and Figure 2(c) is a schematic diagram of the third communication topology.

[0065] Figure 3 The block diagram of the corresponding distributed secondary control compensation method designed based on the ring communication topology in Figure 2(a) is shown.

[0066] Figure 4 This is a schematic diagram of dual-carrier modulation for a three-phase three-level converter. Detailed Implementation

[0067] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0068] Example 1

[0069] like Figure 1 The diagram shown is a flowchart of the control method for the overall DC microgrid system. Among them, For the desired bus voltage, The virtual voltage drop is defined as the product of the droop gain selected based on power balance and the d-axis current obtained by dq transformation of the three-phase current. This is the secondary control voltage compensation signal for the previous stage. All parameters with subscript i indicate the first stage. The relevant parameters of a converter, for example... Representing the The secondary control voltage compensation signal of the converter in the previous stage. For the control of the three-phase three-level converter, a classic dual-closed-loop control structure of DC-side voltage outer loop and AC current inner loop is adopted. The voltage outer loop uses a PI controller. The DC voltage command is based on droop control (described in detail in step S1 below). The difference between the common bus load voltage of the multiple converters connected in parallel and the reference value of the d-axis current is generated by the second PI controller. d-axis current reference value The inputs are respectively fed into the third and fourth PI controllers; and based on the concept of unity power factor control, the q-axis current reference value... Equal to 0; the inner current loop is completed in the dq coordinate system to facilitate PI controller parameter tuning and flexible control of active and reactive power. abc / dq is the coordinate transformation from the three-phase abc coordinate system to the dq coordinate system, to represent the three-phase current... Transformed into the d-axis component of current and q-axis components ; Obtain the virtual voltage drop of the current converter and its adjacent converters and take the average value. Subtract the average value from the previous stage secondary control voltage compensation signal of the current converter and send it to the first PI controller. The output of the first PI controller is superimposed on the virtual voltage drop of the current converter to obtain the secondary control voltage compensation signal (described in detail in step S2 below) and compensated to the above-mentioned droop control. The angular frequency of the power grid. For AC side inductance, respectively with and Cross multiplication is used to further complete the dq-axis decoupling process; PLL is a phase-locked loop used to obtain the dq-axis components of the three-phase voltage. and and angle information The voltage dq axes are respectively superimposed on the PI controller to complete the grid voltage feedforward control, that is... respectively with and The cross product is then applied and its result is superimposed onto the outputs of the third and fourth PI controllers to obtain the dq-axis modulated voltage. and dq / abc is a coordinate transformation from the dq coordinate system to the abc coordinate system, used to convert the dq-axis modulated voltage into a three-phase modulated voltage. .

[0070] Then, a modulation enhancement zero-sequence component is injected to increase the modulation index, enabling the use of the SPWM modulation strategy, which has lower computational complexity compared to SVPWM (described in detail in step S3 below). Due to the inherent midpoint potential offset characteristic of the three-level converter, a midpoint potential control strategy must be introduced (described in detail in step S4 below). The three-phase current and the modulation voltage after the modulation enhancement zero-sequence component is injected are input to the midpoint potential control module, and the midpoint potential control signal is further superimposed on the three-phase modulation voltage after the modulation enhancement zero-sequence component is injected as the final modulation wave signal. Then, the SPWM dual-carrier modulation strategy is used to complete the turn-on and turn-off of the three-phase three-level bridge arm switching devices. SPWM is a sinusoidal pulse width modulation strategy. Since there are three switching states in the three-level converter, a dual-carrier modulation strategy is used to transform the final modulation wave signal into 12 switching signals for controlling the turn-on and turn-off of the three-phase bridge arm switching devices of the three-level converter.

[0071] Within each sampling period, the latest dq-axis components of the three-level voltage and current on the AC side of the three-level converter, as well as the latest bus voltage signal, are continuously obtained through dq transformation. All three-level converters within the microgrid continuously repeat the above process, ultimately completing the distributed control of the entire microgrid. For detailed control procedures, please refer to [link / reference needed]. Figure 1 This will not be elaborated upon here.

[0072] Based on the above principles, please refer to... Figure 1 This document details the processes of droop control, secondary control, three-level converter modulation strategy, and midpoint potential control in the hierarchical distributed control method for three-level converter DC microgrids, including the following steps:

[0073] S1, Single-layer sag control design:

[0074] Assume there are n connected three-level converters in the entire microgrid system, let and These represent the three-phase voltage and three-phase current on the AC side of the three-level converter, respectively. Then, a dq transformation is performed on the voltage and current, and the grid angular frequency is obtained via a phase-locked loop. This is used to obtain the voltage and current in the dq coordinate system. The dq transformation formula is as follows:

[0075]

[0076] in, for At time t, the d-axis components of the three-phase voltage and q-axis components and the d-axis components of the three-phase current and q-axis components All are obtained using this formula. For example, the above... Substitute three-phase current This yields the d-axis components of the three-phase current. and q-axis components The above Substitute the three-phase voltage This yields the d-axis components of the three-phase voltage. and q-axis components .

[0077] After completing the phase-locked loop and dq conversion, precise current sharing control is achieved by introducing droop control. With appropriate voltage and current control, the DC output voltage of the i-th converter can accurately track the reference voltage. Its expression is as follows:

[0078]

[0079] in, Indicates the first A converter, , Indicates the total number of converters. This represents the expected value of the bus voltage. Indicates the droop coefficient. For the first A converter droop factor This represents the DC-side current of the converter. For the first DC side current of the converter This indicates the reference voltage of the voltage control loop, i.e., the DC voltage command. For the first DC voltage command for a converter. Definition The line resistance connecting the i-th converter to the common DC bus is given by the following formula: Therefore, the load voltage of the common bus for multiple converters connected in parallel can be expressed as:

[0080]

[0081] Therefore, the following relationship exists between the i-th converter and the j-th converter:

[0082]

[0083] Due to the droop coefficient The settings are much larger than Therefore, it exists. Therefore, by setting a reasonable droop coefficient, precise current sharing can be achieved.

[0084] According to the power balance principle, the power supplied by the power source and the power consumed by the load can achieve a dynamic balance, that is:

[0085]

[0086] in, For the first The power supply of each converter For the first The load power consumption of each converter For the first The DC voltage of the converter. The current control loop is responsible for controlling current tracking stability and achieving unity power factor. Unity power factor means that only active power exists, reactive power is 0, the d-axis current is in phase with the DC voltage and is related to active power, while the q-axis current is orthogonal to the DC voltage and is related to reactive power. To ensure that the system operates at unity power factor, the q-axis command current of the current control loop is set to 0. During the control process, the q-axis current component always tracks the q-axis command current, thus the q-axis current component is 0, which yields the desired result. Therefore Therefore, based on power balance, it can be concluded that... Furthermore, the downward equation can be rewritten as:

[0087]

[0088] This can remove the noise. The current is converted into a noiseless d-axis component. Without the need to add sensors and filters, precise current sharing can be achieved simply by superimposing the droop gain. In other words, the droop coefficient of each converter can be adjusted by substituting it into the relationship of the DC side current of the converter, thereby adjusting the DC side current of each converter and achieving precise current sharing among the converters.

[0089] S2, Distributed Secondary Control Layer Design:

[0090] As shown in Figures 2(a), 2(b), and 2(c), the numbers 1 to n in Figures 2(a), 2(b), and 2(c) represent converters 1 to n, respectively. First, based on the communication topology of n converters, considering the undirected connected and weighted graph theory foundation, the secondary control layer determines the set of neighboring nodes through the adjacency matrix to achieve local data exchange.

[0091] Secondly, according to the traditional droop control method, the bus voltage will inevitably deviate due to the droop control, so it is necessary to introduce a voltage recovery control signal, and then a secondary control signal, i.e., the first... The secondary control voltage compensation signal of the converter in the previous stage The expression attached to the droop control function is as follows:

[0092]

[0093] To achieve accurate current sharing and eliminate bus voltage deviation, the first... Virtual voltage drop of a converter To compensate for the bus voltage deviation caused by droop control.

[0094] Considering the entire DC microgrid system, in the presence of bus voltage deviation, a local average virtual voltage drop is used instead of the system's global average value. The error between the secondary control signal and the average virtual voltage drop is eliminated through a first PI controller, ultimately resulting in the design of the following secondary controller:

[0095]

[0096] in, and Representing the first The bus voltage deviation of each converter and the average deviation of the entire system are obtained by summing the bus voltage deviations of the n converters. The expression is then used to calculate the average of the global system deviation. , Representing the The virtual voltage drop of the converter. and The PI parameters represent the first PI controller. This represents the average virtual voltage drop across the entire system. The upper limit of integration represents the sampling time of the current within one triangular carrier cycle. Representing the The current stage secondary control voltage compensation signal of the converter utilizes the signal obtained from the previous sampling period. (That is, the secondary control voltage compensation signal in the previous stage) Calculate the average deviation of the global system at the current stage. and utilize this The virtual voltage drop is transmitted via the first PI controller and superimposed on the i-th converter. To obtain the secondary voltage compensation signal for the current stage. Considering the communication burden of the DC microgrid system, a distributed control system is introduced, which is also the purpose of constructing an undirected interconnected communication topology. The average virtual voltage drop of adjacent converters can be defined as... Then the global system average deviation can be rewritten as .

[0097] Secondary control diagram as follows Figure 3 As shown, it assumes that the communication topology of the DC microgrid is the ring communication topology shown in Figure 2(a), and that the secondary control of converter No. 1 is applied using a distributed control system. At this time, according to... Figure 3Therefore, it is only necessary to obtain the virtual voltage drop of converter No. 1 itself and the virtual voltage drops of its adjacent converters No. 2 and n, and take the average of the three to obtain the result. Then, by subtracting its own previous stage secondary control voltage compensation signal... The difference is input to the first PI controller, and finally its own virtual voltage drop is added. To obtain the secondary control voltage compensation signal for the current stage, and to compensate to... Figure 1 As shown in Figure 2(b), if the communication topology is as shown in the local droop control point, and assuming that the control of converter No. 3 needs to be completed, the same operation needs to be performed: take the virtual voltage drop of converter No. 3 itself and converters No. 1, No. 2, No. 4, No. 5 and No. 6, and calculate the average of the six values, and then complete the subsequent operation. Similarly, if the communication topology is as shown in Figure 2(c), the same is true, and will not be elaborated here.

[0098] S3, Optimization of three-level converter modulation strategy:

[0099] The commonly used space vector modulation (SVPWM) strategy is computationally inefficient and unsuitable for DC microgrids constructed with multiple three-phase three-level converters. Traditional sinusoidal SPWM modulation, on the other hand, has lower computational complexity than SVPWM and is more suitable for this microgrid architecture. However, SVPWM's advantage lies in its higher modulation depth compared to SPWM. Injecting modulation enhancement zero-sequence components into SPWM can achieve the same effect and is equivalent to the SVPWM modulation strategy. It is important to note that the injected modulation enhancement zero-sequence component is an AC quantity with a fixed shape and frequency. The principle is as follows:

[0100] Uninjected modulation enhancement zero-sequence component hour, The fundamental component of the three-phase output voltage can be expressed as:

[0101]

[0102] in, In order to adjust the system, The DC output voltage of the converter is the average of the voltages across the upper and lower capacitors for a three-level converter. The line voltage can be expressed as (taking output ab as an example):

[0103]

[0104] Injection modulation enhances zero-sequence component ,but The fundamental component of the three-phase output voltage can be expressed as:

[0105]

[0106] At this point, the line voltage can be expressed as (taking output ab as an example):

[0107]

[0108] Because of the modulation enhancement zero-sequence component injected at this time In an unknown state, the range of values ​​for the modulation index m is not limited to (0,1). Furthermore, comparing the line voltage equation without the injected modulation-enhanced zero-sequence component reveals that the injected modulation-enhanced zero-sequence component will not appear in the line voltage, and the maximum modulation index m can reach... This indicates that injecting a suitable modulation enhancement zero-sequence component can improve the modulation of SPWM. When the injected modulation enhancement zero-sequence component is:

[0109]

[0110] It can be equivalent to a seven-segment SVPWM, but unlike SVPWM, it can greatly reduce computational complexity. The three-phase modulation voltage after injecting modulation to enhance the zero-sequence component is defined as: , can be expressed as:

[0111]

[0112] When using SPWM modulation, the three-level output has three discrete states (1, 0, -1), making single-carrier modulation unsuitable. Therefore, a dual-carrier stacked modulation scheme is needed. A schematic diagram of dual-carrier modulation is shown below. Figure 4 As shown, the orange, green, and brown sine waves represent... The three-phase modulated wave uses gray and blue triangular carriers to represent the positive and negative carriers, respectively. Each arm of the three-phase three-level converter is equipped with four switching devices, which are complementary in pairs. The new modulated wave... By directly comparing with a bipolar carrier wave, the switching timing can be determined to control the on / off state of the three-phase three-level converter's switching transistors. For example, during forward operation of the circuit, The three-phase modulated waves are directly compared with the positive carrier wave to obtain the three-phase switching state of the converter when the circuit is running in the forward direction. When the circuit is running in the reverse direction... The three-phase modulated waves are directly compared with the negative carrier wave to obtain the state of the three-phase switch of the converter when the circuit is running in reverse.

[0113] S4, Neutral point potential control of three-phase three-level converter

[0114] Due to the inherent characteristic of three-phase three-level converters—the neutral point potential—a neutral point potential control method must be introduced for DC microgrids constructed using three-level converters.

[0115] The midpoint potential shift occurs because the current flowing into the midpoint causes a change in the charge stored in the DC-side capacitor. The continuous accumulation of this charge eventually leads to a shift in the midpoint potential. It's important to understand that midpoint current only occurs when the three-level converter is in the "0" switching state. Based on this, a general expression for the midpoint current can be derived, as follows:

[0116]

[0117] in, This corresponds to the three-phase switch state of the converter. This represents the current flowing into the midpoint. for Phase current. The duration of the midpoint current can be further calculated using the following expression:

[0118]

[0119] in, and These represent the time it takes for the current to flow through the midpoint within one triangular carrier cycle and the sampling time, respectively. A new modulation wave (i.e., the three-phase modulation voltage after injecting a fixed-frequency, fixed-shape AC quantity into the three-phase modulation wave obtained in the dual-closed-loop control as described above, to enhance the modulation intensity) is introduced. Therefore, the average charge flowing into the midpoint during each switching sampling cycle... It can be represented as:

[0120]

[0121] Furthermore, by superimposing a low-frequency, slowly varying midpoint balanced zero-sequence component, the midpoint potential can be controlled without altering the original modulation wave symbol. Let the midpoint balanced zero-sequence component to be superimposed be... The new modulated wave after superimposing the midpoint balanced zero-sequence component is Then there is Therefore, the average change in charge flowing into the midpoint after superposition can be obtained as follows:

[0122]

[0123] in, This represents the switching function, also known as the symbol function, to ensure that the midpoint balance zero-sequence component of the superposition does not change the original modulation wave symbol.

[0124] To avoid charge deviation caused by the difference between the DC-side dual capacitors of the converter in the initial state, an initial charge at the midpoint is introduced. The expression is as follows:

[0125]

[0126] in, These are the capacitance values ​​of the upper and lower capacitors on the DC side of the converter. and These represent the voltages of the upper and lower capacitors on the DC side of the converter, respectively.

[0127] After superimposing the zero-sequence component at the midpoint, it is desirable that the midpoint charge is 0 in each switching cycle. Therefore, the following relationship exists:

[0128]

[0129] Therefore, the zero-sequence components at the midpoint equilibrium that need to be superimposed can be determined:

[0130]

[0131] However, the amplitude of the zero-sequence component at the superposition midpoint needs to be limited to prevent three-phase short-circuit faults caused by startup overshoot. Meanwhile, the modulation wave ultimately entering the SPWM modulation module is:

[0132]

[0133] in, This is to enhance the modulation of the zero-sequence component by injecting an equivalent SVPWM signal to improve the modulation density. A simplified representation of .

[0134] Through the above technical solution, this invention, considering power balance and unity power factor, determines the droop gain coefficient to further provide voltage loop commands for each converter based on the desired bus voltage, thereby achieving the goal of stabilizing the converter output voltage. In this process, a virtual voltage drop equation needs to be established based on the droop gain coefficient to achieve the subsequent secondary control objective. Simultaneously, based on the three-level converter communication topology, virtual voltage drop signals from adjacent converters of each converter are collected, including the converter itself. The sum of these virtual voltage drops is divided by the number of converters collected, and a secondary control signal is output via PI control. This signal is then compensated to the voltage control loop of each converter to complete distributed secondary voltage recovery. Furthermore, the duty cycle control signal is obtained through the current control loop in the dq coordinate system, and the switching on and off of the converter transistors is completed based on a modulation enhancement zero-sequence component injection SPWM modulation strategy and a midpoint potential control method based on low-frequency slowly varying midpoint balanced zero-sequence components.

[0135] Therefore, the overall solution only needs to utilize the drooping virtual voltage drop information of adjacent three-level converters to achieve bus voltage stability and precise current distribution through the voltage and current control loops of each converter. Meanwhile, the optimized modulation strategy significantly reduces computational complexity, and the simple midpoint potential control method ensures midpoint potential balance, making it suitable for multi-phase three-level parallel networking.

[0136] Example 2

[0137] Based on Embodiment 1, Embodiment 2 of the present invention provides a hierarchical distributed control system for a three-level converter DC microgrid, comprising:

[0138] The droop control module is used to control the d-axis component of the current of each converter. Droop control is implemented to achieve current sharing among the converters;

[0139] The secondary control module is used to obtain the virtual voltage drop of the current converter and its adjacent converters and take the average value. The difference between the average value and the previous stage secondary control voltage compensation signal of the current converter is sent to the first PI controller. The output of the first PI controller is superimposed on the virtual voltage drop of the current converter to obtain the current stage secondary control voltage compensation signal and compensate to the droop control point of the droop control module.

[0140] Zero-sequence injection modulation module is used to calculate the expected value of the bus voltage. With current d-axis component The difference between the results after droop control is calculated, and this difference is summed with the secondary control voltage compensation signal of the current stage to obtain the DC voltage command. DC voltage command The dq-axis modulation voltage is obtained through modulation, and the dq-axis modulation voltage is converted into a three-phase modulation voltage. The zero-sequence component of modulation enhancement is injected into the three-phase modulation voltage.

[0141] The midpoint potential control module is used to obtain the midpoint balanced zero-sequence component by using the three-phase current and the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component. The midpoint balanced zero-sequence component is superimposed on the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component as the final modulation wave signal to control the converter.

[0142] Specifically, the droop control module is also used for:

[0143] The relationship between the DC-side currents of adjacent converters is as follows: ,in, For the first DC side current of the converter For the first The droop factor of a converter; Let j be the DC-side current of the j-th converter. Let be the droop coefficient of the j-th converter;

[0144] According to the power balance principle, when the power supplied by the power source and the power consumed by the load reach a dynamic balance, then...

[0145]

[0146] in, Indicates the first The power supply of each converter Indicates the first The load power consumption of each converter and They represent the first The d-axis and q-axis components of the three-phase voltage of the converter. and They represent the first The d-axis and q-axis components of the current of each converter. For the first DC voltage of the converter;

[0147] make Therefore The equation for the downward slope is written as ,in, For the first The DC voltage command for each converter is given; the droop coefficient of each converter is obtained through the droop equation and substituted into the relationship of the DC side current of the converter to adjust the DC side current of each converter, thereby achieving current sharing among the converters.

[0148] Specifically, the execution process of the secondary control module is expressed by the following formula:

[0149]

[0150]

[0151] in, Representing the The global system average deviation of each converter Representing the The virtual voltage drop of a converter Indicates the first The average virtual voltage drop of the converter and its adjacent converters Representing the The secondary control voltage compensation signal of the converter in the previous stage. and The PI parameters represent the first PI controller. express time, This indicates the sampling time of the current within one triangular carrier cycle. Representing the The current stage secondary control voltage compensation signal of the converter.

[0152] Specifically, the DC voltage command The dq-axis modulated voltage is obtained through modulation, and then converted into a three-phase modulated voltage, including:

[0153] Common bus load voltage of multiple converters in parallel With DC voltage command After the difference is calculated, the second PI controller generates the d-axis current reference value. d-axis current reference value The three-phase currents of the converter are respectively input to the third PI controller and the fourth PI controller; Perform a coordinate transformation from the abc coordinate system to the dq coordinate system, transforming it into the d-axis component of the current. and q-axis components ; The angular frequency of the power grid. For AC side inductance, respectively with and The cross product is then superimposed on the outputs of the third and fourth PI controllers, respectively, while the dq-axis components of the three-phase voltage are also considered. and The voltages are respectively superimposed on the outputs of the fourth PI controller and the third PI controller to obtain the dq-axis modulated voltage. and The dq modulated voltage is transformed from the dq coordinate system to the abc coordinate system to obtain the three-phase modulated voltage.

[0154] Specifically, the injection of modulation enhancement of the zero-sequence component into the three-phase modulation voltage includes:

[0155]

[0156] in, This represents the three-phase modulation voltage after the zero-sequence component is injected and enhanced. , express Three phases, This indicates a three-phase modulated voltage. This indicates that the modulation enhances the zero-sequence component at the current time and

[0157]

[0158] in, express The fundamental component of the three-phase output voltage.

[0159] More specifically, the method of obtaining the neutral point balanced zero-sequence component by using the three-phase current and the three-phase modulation voltage after injecting modulation to enhance the zero-sequence component for neutral point potential control includes:

[0160] The midpoint equilibrium zero-sequence components that need to be superimposed are:

[0161]

[0162] in, These are the capacitance values ​​of the upper and lower capacitors on the DC side of the converter. This indicates the sampling time of the current within one triangular carrier cycle. and These represent the voltages of the upper and lower capacitors on the DC side of the converter, respectively. for Phase current, For symbolic functions, Represents the absolute value symbol.

[0163] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A hierarchical distributed control method for a three-level converter DC microgrid, characterized in that, include: S1, Current d-axis components of each converter Droop control is implemented to achieve current sharing among the converters; S2. Obtain the virtual voltage drop of the current converter and its adjacent converters and take the average value. Subtract the average value from the previous stage secondary control voltage compensation signal of the current converter and send it to the first PI controller. Add the virtual voltage drop of the current converter to the output of the first PI controller to obtain the current stage secondary control voltage compensation signal and compensate it to the droop control point of S1. S3. Calculate the expected value of the bus voltage. With current d-axis component The difference between the results after droop control is calculated, and this difference is summed with the secondary control voltage compensation signal of the current stage to obtain the DC voltage command. DC voltage command The dq-axis modulation voltage is obtained through modulation, and then converted into a three-phase modulation voltage. A modulation enhancement zero-sequence component is injected into the three-phase modulation voltage. The injection of the modulation enhancement zero-sequence component into the three-phase modulation voltage includes: in, This represents the three-phase modulation voltage after the zero-sequence component is injected and enhanced. , express Three phases, This indicates a three-phase modulated voltage. This indicates that the modulation enhances the zero-sequence component at the current time and in, express The fundamental component of the three-phase output voltage; S4. The neutral point balance zero-sequence component is obtained by using the three-phase current and the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component. The neutral point balance zero-sequence component is superimposed on the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component as the final modulation wave signal to control the converter.

2. The hierarchical distributed control method for a three-level converter DC microgrid according to claim 1, characterized in that, S1 includes: The relationship between the DC-side currents of adjacent converters is as follows: ,in, For the first DC side current of the converter For the first The droop factor of a converter; Let j be the DC-side current of the j-th converter. Let be the droop coefficient of the j-th converter; According to the power balance principle, when the power supplied by the power source and the power consumed by the load reach a dynamic balance, then... in, Indicates the first The power supply of each converter Indicates the first The load power consumption of each converter and They represent the first The d-axis and q-axis components of the three-phase voltage of the converter. and They represent the first The d-axis and q-axis components of the current of each converter. For the first DC voltage of the converter; make Therefore The equation for the downward slope is written as ,in, For the first The DC voltage command for each converter is given; the droop coefficient of each converter is obtained through the droop equation and substituted into the relationship of the DC side current of the converter to adjust the DC side current of each converter, thereby achieving current sharing among the converters.

3. The hierarchical distributed control method for a three-level converter DC microgrid according to claim 1, characterized in that, The process S2 is expressed by the following formula: in, Representing the The global system average deviation of each converter Representing the The virtual voltage drop of a converter Indicates the first The average virtual voltage drop of the converter and its adjacent converters Representing the The secondary control voltage compensation signal of the converter in the previous stage. and The PI parameters represent the first PI controller. express time, This indicates the sampling time of the current within one triangular carrier cycle. Representing the The current stage secondary control voltage compensation signal of the converter.

4. The hierarchical distributed control method for a three-level converter DC microgrid according to claim 1, characterized in that, The DC voltage command The dq-axis modulated voltage is obtained through modulation, and then converted into a three-phase modulated voltage, including: Common bus load voltage of multiple converters in parallel With DC voltage command After the difference is calculated, the second PI controller generates the d-axis current reference value. d-axis current reference value The three-phase currents of the converter are respectively input to the third PI controller and the fourth PI controller; Perform a coordinate transformation from the abc coordinate system to the dq coordinate system, transforming it into the d-axis component of the current. and q-axis components ; The angular frequency of the power grid. For AC side inductance, respectively with and The cross product is then superimposed on the outputs of the third and fourth PI controllers, respectively, while the dq-axis components of the three-phase voltage are also considered. and The voltages are respectively superimposed on the outputs of the fourth PI controller and the third PI controller to obtain the dq-axis modulated voltage. and The dq modulated voltage is transformed from the dq coordinate system to the abc coordinate system to obtain the three-phase modulated voltage.

5. The hierarchical distributed control method for a three-level converter DC microgrid according to claim 1, characterized in that, The method of obtaining the neutral point balanced zero-sequence component by using the three-phase current and the three-phase modulation voltage after injecting modulation to enhance the zero-sequence component for neutral point potential control includes: The midpoint equilibrium zero-sequence components that need to be superimposed are: in, These are the capacitance values ​​of the upper and lower capacitors on the DC side of the converter. This indicates the sampling time of the current within one triangular carrier cycle. and These represent the voltages of the upper and lower capacitors on the DC side of the converter, respectively. for Phase current, For symbolic functions, Represents the absolute value symbol.

6. A hierarchical distributed control system for a three-level converter DC microgrid, characterized in that, include: The droop control module is used to control the d-axis component of the current of each converter. Droop control is implemented to achieve current sharing among the converters; The secondary control module is used to obtain the virtual voltage drop of the current converter and its adjacent converters and take the average value. The difference between the average value and the previous stage secondary control voltage compensation signal of the current converter is sent to the first PI controller. The output of the first PI controller is superimposed on the virtual voltage drop of the current converter to obtain the current stage secondary control voltage compensation signal and compensate to the droop control point of the droop control module. Zero-sequence injection modulation module is used to calculate the expected value of the bus voltage. With current d-axis component The difference between the results after droop control is calculated, and this difference is summed with the secondary control voltage compensation signal of the current stage to obtain the DC voltage command. DC voltage command The dq-axis modulation voltage is obtained through modulation, and then converted into a three-phase modulation voltage. A modulation enhancement zero-sequence component is injected into the three-phase modulation voltage. The injection of the modulation enhancement zero-sequence component into the three-phase modulation voltage includes: in, This represents the three-phase modulation voltage after the zero-sequence component is injected and enhanced. , express Three phases, This indicates a three-phase modulated voltage. This indicates that the modulation enhances the zero-sequence component at the current time and in, express The fundamental component of the three-phase output voltage; The midpoint potential control module is used to obtain the midpoint balanced zero-sequence component by using the three-phase current and the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component. The midpoint balanced zero-sequence component is superimposed on the three-phase modulation voltage after the injection of modulation enhancement zero-sequence component as the final modulation wave signal to control the converter.

7. A hierarchical distributed control system for a three-level converter DC microgrid according to claim 6, characterized in that, The droop control module is also used for: The relationship between the DC-side currents of adjacent converters is as follows: ,in, For the first DC side current of the converter For the first The droop factor of a converter; Let j be the DC-side current of the j-th converter. Let be the droop coefficient of the j-th converter; According to the power balance principle, when the power supplied by the power source and the power consumed by the load reach a dynamic balance, then... in, Indicates the first The power supply of each converter Indicates the first The load power consumption of each converter and They represent the first The d-axis and q-axis components of the three-phase voltage of the converter. and They represent the first The d-axis and q-axis components of the current of each converter. For the first DC voltage of the converter; make Therefore The equation for the downward slope is written as ,in, For the first The DC voltage command for each converter is given; the droop coefficient of each converter is obtained through the droop equation and substituted into the relationship of the DC side current of the converter to adjust the DC side current of each converter, thereby achieving current sharing among the converters.

8. A hierarchical distributed control system for a three-level converter DC microgrid according to claim 6, characterized in that, The execution process of the secondary control module can be expressed by the following formula: in, Representing the The global system average deviation of each converter Representing the The virtual voltage drop of a converter Indicates the first The average virtual voltage drop of the converter and its adjacent converters Representing the The secondary control voltage compensation signal of the converter in the previous stage. and The PI parameters represent the first PI controller. express time, This indicates the sampling time of the current within one triangular carrier cycle. Representing the The current stage secondary control voltage compensation signal of the converter.

9. A hierarchical distributed control system for a three-level converter DC microgrid according to claim 6, characterized in that, The DC voltage command The dq-axis modulated voltage is obtained through modulation, and then converted into a three-phase modulated voltage, including: Common bus load voltage of multiple converters in parallel With DC voltage command After the difference is calculated, the second PI controller generates the d-axis current reference value. d-axis current reference value The three-phase currents of the converter are respectively input to the third PI controller and the fourth PI controller; Perform a coordinate transformation from the abc coordinate system to the dq coordinate system, transforming it into the d-axis component of the current. and q-axis components ; The angular frequency of the power grid. For AC side inductance, respectively with and The cross product is then superimposed on the outputs of the third and fourth PI controllers, respectively, while the dq-axis components of the three-phase voltage are also considered. and The voltages are respectively superimposed on the outputs of the fourth PI controller and the third PI controller to obtain the dq-axis modulated voltage. and The dq modulated voltage is transformed from the dq coordinate system to the abc coordinate system to obtain the three-phase modulated voltage.

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