Alternating current and direct current micro-grid layered distributed control method and system

Abstract

The invention relates to the technical field of micro-grid control, and discloses an AC / DC micro-grid layered distributed control method and system, and the method comprises the steps: obtaining a DC voltage outputted by a distributed power supply in a DC micro-grid and an AC frequency outputted by a distributed power supply in an AC micro-grid, and carrying out the normalization processing; according to the voltage normalization value and the frequency normalization value, a finite time consistency algorithm is adopted to calculate a primary active power reference value of the interconnected converter; compensating the primary active power reference value according to a distributed secondary controller to obtain a secondary active power reference value; and generating a control signal of each interconnected converter according to the secondary active power reference value. Through hierarchical distributed control, the purposes that the total power sent by the direct current micro-grid and the alternating current micro-grid is proportionally distributed according to respective rated capacities and the active power transmitted by all the interconnected converters is proportionally distributed according to the transmission capacities are achieved, and the operation safety and stability of the alternating current and direct current micro-grid are effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrid control, and particularly to a hierarchical distributed control method and system for an AC-DC microgrid. Background Art

[0002] A microgrid is an independent power supply system that integrates various distributed power sources, energy storage devices, and loads, and can effectively consume new energy to reduce carbon emissions. The AC-DC hybrid microgrid interconnects the AC microgrid and the DC microgrid through an interconnected converter, aggregates the advantages of the AC microgrid and the DC microgrid, can achieve high-speed conversion of electric energy, coordinate power balance, and ensure the flexible operation of the microgrid.

[0003] With more and more distributed energy sources joining the microgrid, it is bound to expand the power capacity of the AC and DC sub-grids. Therefore, there will be a greater electrical interaction between the two sub-grids, and a larger amount of power will be transmitted through the interconnected converter. When the AC-DC microgrid operates in island mode, the distributed power sources in the AC microgrid adopt active / frequency droop control to share the AC load jointly and maintain frequency stability; the distributed voltage in the DC microgrid adopts active / voltage droop control to share the DC load jointly and maintain DC voltage stability. However, when the microgrid operates in island mode, due to the lack of support from the large power grid, the frequency and voltage are more likely to fluctuate, and it is difficult to maintain the inertia between the AC microgrid and the DC microgrid. Therefore, it is necessary to optimize the control strategy of the interconnected converter.

[0004] In the existing control strategies for AC-DC microgrids with multiple interconnected converters, the control objectives are mainly to achieve the proportional distribution of active power within the microgrid and maintain the stability of voltage and frequency. Although this method can achieve power balance and state stability within the microgrid, it does not consider the power coordination problem between the AC-DC microgrids, and is prone to problems such as overload of power transmission by the interconnected converter and unstable transmission state, which brings potential safety hazards to the safe and stable operation of the AC-DC microgrid. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides a hierarchical distributed control method and system for an AC-DC microgrid, which can solve the problems of power distribution within the microgrid and proportional distribution of power transmission ratio across sub-grids, and achieve the technical effects that the total power generated by the DC microgrid and the AC microgrid is proportionally distributed according to their respective rated capacities, and the active power transmitted by all interconnected converters is proportionally distributed according to the transmission capacity ratio.

[0006] In the first aspect, the present invention provides a hierarchical distributed control method for an AC-DC microgrid, and the method includes: Obtain the DC voltage output by the distributed power sources in the DC microgrid and the AC frequency output by the distributed power sources in the AC microgrid, and perform normalization processing to obtain the voltage normalization value and the frequency normalization value; According to the voltage normalization value and the frequency normalization value, adopt the finite-time consensus algorithm to calculate the primary active power reference values of each interconnected converter; According to the distributed secondary controller, compensate the primary active power reference value to obtain the secondary active power reference value, and the distributed secondary controller is constructed based on the distributed communication network and the power ratio distribution of the interconnected converters; Generate the control signals of each interconnected converter according to the secondary active power reference value, and perform constant power control on each interconnected converter according to the control signals.

[0007] Further, the step of calculating the primary active power reference values of each interconnected converter according to the voltage normalization value and the frequency normalization value by adopting the finite-time consensus algorithm includes: Subtract the frequency normalization value from the voltage normalization value to obtain the first difference; Adopt the finite-time consensus algorithm to calculate the first difference to obtain the primary active power reference values of each interconnected converter.

[0008] Further, the primary active power reference value is expressed by the following formula: In the formula, represents the primary active power reference value of the i-th interconnected converter, represents the gain coefficient of the i-th interconnected converter, represents the voltage normalization value, represents the frequency normalization value, α represents the first exponential coefficient, t represents time, and sign represents the sign function.

[0009] Further, the step of compensating the primary active power reference value according to the distributed secondary controller to obtain the secondary active power reference value includes: Connect each interconnected converter to the distributed communication network, and calculate the secondary power difference between the interconnected converter and each neighbor converter; Adopt the distributed consensus algorithm to calculate the secondary power difference to obtain the compensation power of each interconnected converter at the current moment; Compensate the primary active power reference value according to the compensation power to obtain the secondary active power reference values of each interconnected converter.

[0010] Further, the step of connecting each interconnected converter to the distributed communication network and calculating the secondary power difference between the interconnected converter and each neighboring converter includes: Obtain the secondary active power reference values of each neighboring converter of the interconnected converter at the previous moment through the distributed communication network; Normalize the secondary active power reference value of the interconnected converter at the previous moment and the secondary active power reference values of each neighboring converter at the previous moment respectively; Calculate the secondary power difference between the interconnected converter and each neighboring converter at the previous moment according to the normalized secondary active power reference value of the interconnected converter and the normalized secondary active power reference values of each neighboring converter.

[0011] Further, the compensation power is expressed by the following formula: In the formula, represents the compensation power of the i-th interconnected converter at the current moment, represents the secondary active power reference value of the i-th interconnected converter at the previous moment, represents the secondary active power reference value of the j-th neighboring converter at the previous moment, represents the rated power of the i-th interconnected converter, represents the rated power of the j-th neighboring converter, represents the power gain coefficient of the i-th interconnected converter, N i represents the set of neighboring converters of the i-th interconnected converter, γ represents the second exponential coefficient, t represents time, and sign represents the sign function.

[0012] Further, the step of compensating the primary active power reference value according to the compensation power to obtain the secondary active power reference values of each interconnected converter includes: Add the primary active power reference value and the compensation power to obtain the secondary active power reference values of each interconnected converter.

[0013] Further, the step of generating the control signal of each interconnected converter according to the secondary active power reference value includes: Obtain the reactive power reference values of each interconnected converter, perform PQ control on the secondary active power reference value and the reactive power reference value respectively to obtain the d-axis reference value and q-axis reference value of the current loop; Perform PI control adjustment and coordinate transformation on the d-axis reference value and the q-axis reference value to obtain the control signal of each interconnected converter Further, the steps of obtaining the DC voltage output by the distributed power sources in the DC microgrid and the AC frequency output by the distributed power sources in the AC microgrid include: Obtaining the DC voltage output by the distributed power sources in the DC microgrid through the active power / voltage droop controller of the DC microgrid; Obtaining the AC frequency output by the distributed power sources in the AC microgrid through the active power / frequency droop controller of the AC microgrid.

[0014] In a second aspect, the present invention provides a hierarchical distributed control system for an AC-DC microgrid, and the system includes: A data processing module, configured to obtain the DC voltage output by the distributed power sources in the DC microgrid and the AC frequency output by the distributed power sources in the AC microgrid, and perform normalization processing to obtain a voltage normalization value and a frequency normalization value; A lower-layer control module, configured to calculate the primary active power reference values of each interconnected converter by using a finite-time consensus algorithm according to the voltage normalization value and the frequency normalization value; An upper-layer control module, configured to compensate the primary active power reference values according to a distributed secondary controller to obtain secondary active power reference values, where the distributed secondary controller is constructed based on a distributed communication network and the power transmission ratio distribution of the interconnected converters; A constant power control module, configured to generate control signals for each interconnected converter according to the secondary active power reference values, and perform constant power control on each interconnected converter according to the control signals.

[0015] The present invention provides a hierarchical distributed control method and system for an AC-DC microgrid. Through hierarchical distributed control, the present invention can solve the problems of internal power distribution in the microgrid and the power transmission ratio distribution across subnets, achieving the goal of proportionally distributing the total power generated by the DC microgrid and the AC microgrid according to their respective rated capacities and proportionally distributing the active power transmitted by all interconnected converters according to the transmission capacity ratio, thereby realizing the global optimization of the AC-DC microgrid and effectively improving the safety and stability of the operation of the AC-DC microgrid. At the same time, the present invention adopts a distributed architecture, avoiding the risk of single-point failures, realizing the plug-and-play of the interconnected converters, enabling the microgrid to have a dynamic capacity expansion function, and further improving the scalability of the AC-DC microgrid. Description of the Drawings

[0016] Figure 1 is a schematic flowchart of the hierarchical distributed control method for the AC-DC microgrid in an embodiment of the present invention; Figure 2 is a topological structure diagram of the AC-DC microgrid in an embodiment of the present invention; Figure 3It is the control topology diagram of the AC-DC microgrid in the embodiment of the present invention; Figure 4 It is the communication topology diagram of the AC-DC microgrid in the embodiment of the present invention; Figure 5 It is the graph of the active power output variation of each distributed power source in the AC microgrid in the simulation experiment of the embodiment of the present invention; Figure 6 It is the graph of the active power output variation of each distributed power source in the DC microgrid in the simulation experiment of the embodiment of the present invention; Figure 7 It is the graph of the active power transmission variation of each interconnected converter in the microgrid in the simulation experiment of the embodiment of the present invention; Figure 8 It is the graph of the total power variation emitted by the AC microgrid in the simulation experiment of the embodiment of the present invention; Figure 9 It is the graph of the total power variation emitted by the DC microgrid in the simulation experiment of the embodiment of the present invention; Figure 10 It is the structural schematic diagram of the hierarchical distributed control system of the AC-DC microgrid in the embodiment of the present invention. Specific Embodiment

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0018] Please refer to Figure 1 , a hierarchical distributed control method for an AC-DC microgrid proposed in the first embodiment of the present invention, which includes steps S10 to S40: Step S10, obtain the DC voltage output by the distributed power sources in the DC microgrid and the AC frequency output by the distributed power sources in the AC microgrid, and perform normalization processing to obtain a voltage normalization value and a frequency normalization value; Step S20, according to the voltage normalization value and the frequency normalization value, use the finite-time consensus algorithm to calculate the primary active power reference values of each interconnected converter; Step S30, compensate the primary active power reference values according to the distributed secondary controller, and obtain secondary active power reference values. The distributed secondary controller is constructed based on a distributed communication network and the transmission power ratio distribution of the interconnected converters; Step S40: Generate control signals for each interconnected converter according to the secondary active power reference value, and perform constant power control on each interconnected converter according to the control signals.

[0019] The present invention provides a control optimization strategy for interconnected converters in an AC-DC microgrid. Refer to Figure 2 , the AC-DC microgrid topology structure in the present invention includes an AC microgrid (ACMG) and a DC microgrid (DCMG). Among them, both the AC microgrid and the DC microgrid contain several interconnected distributed power sources (DGs). After the distributed power sources are interconnected, they are connected to the load to supply electrical energy to the load. The distributed power sources use PWM for pulse width modulation and realize distributed power control through a distributed power controller. The AC microgrid and the DC microgrid are connected by multiple interconnected converters (ICs), and power mutual assistance is realized between the AC side and the DC side through the interconnected converters.

[0020] When a conventional AC-DC hybrid microgrid operates in an islanding mode, due to the lack of support from the large power grid, the frequency and voltage are more likely to fluctuate, and it is difficult to maintain the inertia between the AC microgrid and the DC microgrid. Therefore, there is a risk of overload in the process of power transmission in a certain microgrid or a certain interconnected converter, and it is difficult to quickly return to a stable transmission state. To address this issue, refer to Figure 3 , the present invention provides a hierarchical distributed control method based on finite-time one-shot to optimize the control strategy of the interconnected converter. Among them, the communication topology of the AC-DC microgrid in the present invention is as Figure 4 shown.

[0021] In the present invention, first, based on the communication between the distributed power sources in the DC microgrid and the AC microgrid, the DC voltage output by the distributed power sources in the DC microgrid and the AC frequency output by the distributed power sources in the AC microgrid are obtained. In the DC microgrid, its distributed power sources adopt active power and voltage droop control, and the DC voltage output by the active / voltage droop controller can be expressed as: In the formula, represents the DC voltage output by the DC microgrid droop control, represents the rated voltage, represents the actual output power of the i-th distributed power source on the DC side, represents the rated output power of the i-th distributed power source on the DC side, represents the droop control coefficient of the i-th distributed power source on the DC side.

[0022] In the AC microgrid, its distributed power sources adopt active power and frequency droop control, and the AC frequency output by the active / frequency droop controller can be expressed as: In the formula, represents the AC frequency output by the droop control of the AC microgrid, represents the rated frequency, represents the rated active power of the i-th distributed power source on the AC side, represents the actual output active power of the i-th distributed power source on the AC side, represents the droop control coefficient of the i-th distributed power source on the AC side.

[0023] The DC voltage on the DC side and the AC frequency on the AC side are the core indicators of the operating state of the microgrid, which can directly reflect the power balance situation. The present invention regulates the transmission power of the interconnected converter based on the DC voltage and the AC frequency. Since the AC frequency and the DC voltage have different dimensions and variation ranges, it is necessary to unify the scaling to coordinate the cross-subnetwork control.

[0024] In a preferred embodiment, through normalization processing, the DC voltage and the AC frequency are unified in scale, and its normalization formula is: In the formula, represents the normalized value of the DC voltage, represents the normalized value of the AC frequency, and respectively represent the upper limit value and the lower limit value of the DC voltage in the DC microgrid, and respectively represent the upper limit value and the lower limit value of the AC frequency in the AC microgrid. In this embodiment, the maximum voltage offset is set to 5% of the rated power, and the maximum frequency offset is set to 1% of the rated frequency.

[0025] At the steady-state operating point, when the conditions of and being equal are satisfied, the total power generated by the microgrid will be proportionally distributed according to the rated capacities of the DC microgrid and the AC microgrid. In order to quickly achieve the purpose of convergence of the active power transmitted by each converter, the present invention adopts a hierarchical distributed double-layer control strategy based on finite-time consensus.

[0026] Please refer to Figure 3 . In the lower-layer control, a finite-time consensus algorithm is adopted to calculate the primary active power reference values of each interconnected converter. In the upper-layer control, the primary active power reference values are compensated by a distributed secondary controller. Among them, the specific steps of the lower-layer control include: Subtract the normalized value of the frequency from the normalized value of the voltage to obtain a first difference; The first difference is calculated using a finite-time consensus algorithm to obtain the primary active power reference values of each interconnected converter.

[0027] In this embodiment, when operating at the steady-state point, if the DC voltage on the DC side and the AC frequency on the AC side are equal, the total power generated by the microgrid will be proportionally distributed according to the rated capacities of the DC microgrid and the AC microgrid. Therefore, first, the difference between the DC voltage on the DC side and the AC frequency on the AC side is calculated. Since the traditional droop control has a slow convergence speed and cannot coordinate the cross-subnetwork power distribution, in order to enable the AC-DC sides to quickly reach the convergence goal, this embodiment uses a finite-time consensus algorithm to accelerate the convergence through a non-linear term.

[0028] Finite-time consensus means that agents tend to be the same for certain state variables within a finite time; the finite-time consensus algorithm means that multiple agents make corresponding responses based on the information obtained from their neighbors, so that the state variables reach consensus within a finite time. Its formula can be expressed as: In the formula, u i represents the control input of the i-th node, a ij is an element of the adjacency matrix, representing the communication weight between node i and neighbor node j, x represents the difference between the local node and the neighbor node, represents the non-linear term, c represents the gain coefficient, c > 0, β represents the exponential coefficient, and β ∈ (0, 1), sign(x) represents the sign function: Through the combination of the sign function and the non-linear term, the system can be made to be consistent with the states of adjacent agents within a finite time, improving the convergence speed. At the same time, the sign function can also suppress the influence of noise and enhance the robustness.

[0029] Based on the characteristics of the finite-time consensus algorithm, the present invention applies the finite-time consensus algorithm to the power calculation of the interconnected converters in the AC-DC microgrid to quickly achieve the power balance between sub-microgrids. Based on the above formula, the difference between the DC voltage and the AC frequency is calculated according to the finite-time consensus algorithm, so as to obtain the primary active power reference values of each interconnected converter. Its formula is expressed as: In the formula, represents the primary active power reference value of the i-th interconnected converter, represents the gain coefficient of the i-th interconnected converter, represents the voltage normalization value, represents the frequency normalization value, α represents the first exponential coefficient, t represents time, and sign represents the sign function.

[0030] Among them, the exponential coefficient α ranges from 0 to 1 and is used to control the convergence speed. The smaller the exponential coefficient, the stronger the non-linear term and the faster the convergence speed. The gain coefficient is used to adjust the response intensity of the finite-time consensus algorithm to the normalized frequency / voltage deviation, directly affecting the dynamic process of power distribution. The larger the gain, the faster the convergence speed, but too large will lead to oscillation or overshoot. Therefore, preferably, the coefficient is initialized according to the inverse ratio of the gain coefficient to the rated capacity of the interconnected converter IC to ensure reasonable weights of different-capacity ICs in control. Then, the allowable range of the gain is deduced through Lyapunov stability analysis or frequency-domain analysis, and finally the gain is adjusted through simulation experiments to achieve the balance between rapidity and stability. Of course, the coefficient can also be determined by manual setting or other methods, and no more limitations are made here.

[0031] Although the lower-layer control of this embodiment solves the power balance between the subnets on the DC side and the AC side, it has not solved the proportional distribution problem of the transmission power of multiple interconnected converters. Therefore, in the upper-layer control, the primary active power reference value is compensated through a distributed secondary controller to obtain the secondary active power reference value of the interconnected converter. The specific steps include: Connect each interconnected converter to the distributed communication network and calculate the secondary power difference between the interconnected converter and each neighboring converter; Use the distributed consensus algorithm to calculate the secondary power difference and obtain the compensation power of each interconnected converter at the current moment; Compensate the primary active power reference value according to the compensation power to obtain the secondary active power reference value of each interconnected converter.

[0032] In this embodiment, each interconnected converter is connected to the distributed communication network to form a communication topology among the interconnected converters, and the secondary power difference between the interconnected converter and each neighboring converter is calculated through data communication among the interconnected converters. The specific steps include: Through the distributed communication network, obtain the secondary active power reference value of each neighboring converter of the interconnected converter at the previous moment; Normalize the secondary active power reference value of the interconnected converter at the previous moment and the secondary active power reference value of each neighboring converter at the previous moment respectively; According to the normalized secondary active power reference value of the interconnected converter and the normalized secondary active power reference value of each neighboring converter, calculate the secondary power difference between the interconnected converter and each neighboring converter at the previous moment.

[0033] In this embodiment, the distributed secondary controller realizes power compensation through the finiteness-time consensus algorithm and the power proportional distribution principle based on the asynchrony and time decoupling of the distributed algorithm. In the initial startup stage of the microgrid system, all initial values of the compensation power are set to 0. Since there is no power interaction at the initial moment and the secondary control is not activated, there is no need to compensate for the primary reference value. At this time, only the lower-layer control (primary reference value) is relied on to maintain the basic power distribution at the initial moment, and the upper-layer control (secondary compensation) is in the "pending activation" state. With the establishment of the communication network, the compensation frequency is dynamically updated through the distributed protocol. By gradually adjusting the compensation frequency, the secondary active power reference values of the interconnected converters finally reach the proportional distribution according to the rated capacity. The dynamic adjustment process of the compensation frequency is described below.

[0034] Taking the discrete time step k as an example, assume that a certain interconnected converter obtains the secondary active power reference values of each neighbor converter at the moment of k - 1 through the distributed communication network. By normalizing the secondary active power reference values of its own converter and the neighbor converters, the secondary power difference of this interconnected converter and the neighbor converters at the moment of k - 1 can be obtained.

[0035] Then, based on the distributed finiteness-time consensus algorithm, a sign function and a non-linear term are introduced to the secondary power difference, and after the gain coefficient is corrected and time integration is performed, the compensation power of this interconnected converter can be obtained. The primary active power reference value is compensated by the compensation power. That is to say, in this embodiment, the secondary power difference of the interconnected converter and the neighbor converters at the moment of k - 1 is calculated to obtain the compensation power, and the primary active power reference value at the current moment is compensated according to the compensation power, so as to optimize the transmission power distribution ratio of the interconnected converter and ensure that the system can converge to the steady state within a finite time.

[0036] At this time, the compensation power output by the distributed secondary controller can be expressed as: In the formula, represents the compensation power of the i-th interconnected converter at the current moment, represents the secondary active power reference value of the i-th interconnected converter at the previous moment, represents the secondary active power reference value of the j-th neighbor converter at the previous moment, represents the rated power of the i-th interconnected converter, represents the rated power of the j-th neighbor converter, represents the power gain coefficient of the i-th interconnected converter, N i represents the set of neighbor converters of the i-th interconnected converter, γ represents the second exponential coefficient, t represents time, and sign represents the sign function.

[0037] Among them, the power gain coefficient is used to adjust the compensation amplitude of the distributed secondary controller for the power reference value, affecting the power coordination speed among multiple interconnected converters. Its setting steps can refer to the gain coefficient , which will not be elaborated here one by one.

[0038] After obtaining the compensated power, adding the compensated power to the primary active power reference value can obtain the secondary active power reference value of the interconnected converter at the current moment: In this embodiment, through hierarchical distributed control, a hierarchical structure framework is established, separating the lower-layer control and the upper-layer control. In the lower-layer control, based on the local droop characteristic, a preliminary power reference value is generated to solve the power distribution problem within the microgrid. In the upper-layer control, by compensating the preliminary power reference value, the power transmission ratio across subnets is adjusted to achieve power mutual assistance between the AC and DC subnets, thereby realizing the global optimization of the AC-DC microgrid. At the same time, adopting a distributed architecture, each interconnected converter only needs to communicate with its adjacent ones, without relying on a central node, avoiding the risk of single-point failure. When a new interconnected converter is connected, the default compensation frequency is zero, and it automatically integrates into the existing network through the protocol. Through plug-and-play, it can support the dynamic expansion of the microgrid.

[0039] After obtaining the secondary active power reference values of each interconnected converter, corresponding control signals can be generated based on the secondary active power reference values. The specific steps include: Obtain the reactive power reference values of each interconnected converter, and perform PQ control on the secondary active power reference value and the reactive power reference value respectively to obtain the d-axis reference value and q-axis reference value of the current loop; Perform PI control adjustment and coordinate transformation on the d-axis reference value and the q-axis reference value to obtain the control signals of each interconnected converter.

[0040] In this embodiment, first, through PQ control, the secondary active power reference value and the reactive power reference value of the interconnected converter are converted into the d-axis reference value and q-axis reference value of the current loop. Among them, the reactive power reference value is obtained based on the conventional control strategy of the interconnected converter, which will not be elaborated here too much.

[0041] The d-axis reference value and q-axis reference value of the current loop can be expressed as: In the formula, represents the d-axis reference value of the current loop of the i-th interconnected converter, represents the q-axis reference value of the current loop of the i-th interconnected converter, represents the reference value of the secondary active power transmitted by the i-th interconnected converter, represents the active power value actually transmitted by the i-th interconnected converter, represents the reference value of the reactive power transmitted by the i-th interconnected converter, represents the reactive power value actually transmitted by the i-th interconnected converter, and respectively represent the proportional parameter and integral parameter of the d-axis active power control loop, and respectively represent the proportional parameter and integral parameter of the q-axis reactive power control loop, represents integration.

[0042] According to the actual current values on the d-axis and q-axis and the above reference values, current loop PI control is performed to quickly track the current reference value and suppress harmonics and disturbances, so as to obtain the voltage reference values on the d-axis and q-axis required for modulation. Then, the voltage reference values in the dq0 coordinate system are converted into three-phase AC signals through Park inverse transformation, and space vector pulse width modulation SVPWM or sine pulse width modulation SPWM technology is used to compare the three-phase voltage reference value signal with the triangular carrier wave to generate the PWM drive signal of the switching device. Finally, the converter switching device is driven according to the PWM signal, so as to achieve the accurate transmission of active / reactive power.

[0043] Next, the effectiveness of the hierarchical distributed control method for AC-DC microgrids provided by the present invention is verified through simulation experiments. The Simulink simulation software is used in this simulation experiment, and the Figure 2 shown microgrid topology is used as the simulation structure. The initial AC load of the simulation system is 120kW and the DC load is 80kW. The relevant control parameters and circuit parameters are shown in Table 1 below: Table 1 AC / DC Microgrid Control Parameters The total duration of the system simulation is 4s, and the step size is s. Before 2s, the system has an AC load of 120kW and a DC load of 80kW. At 2s, the load suddenly changes. The AC side load suddenly changes to 80kW, the DC side load changes to 120kW, and at the same time, the control of three interconnected converters is added.

[0044] Through simulation experiments, after 2s, the load suddenly changes and reaches a steady state. The changes in the active power output of each distributed power source DG in the microgrid are as shown in Figure 5 and Figure 6 . According to Figure 5 , it can be known that the active power outputs of each distributed power source DG in the AC microgrid ACMD are respectively: P ac1 :P ac2 :Pac3 :P ac4 = 20.18 kW:30.29 kW:20.19 kW:30.30 kW = 2:3:2:3 Since the rated capacities of the distributed power sources in the AC microgrid are 20 kW, 30 kW, 20 kW, and 30 kW respectively, the control objective of proportional distribution of the active power generated by each distributed power source in the AC microgrid according to its own capacity is achieved.

[0045] According to Figure 6 it is known that the active powers output by the distributed power sources DG of the DC microgrid DCMG are respectively: P dc1 :P dc2 :P dc3 :P dc4 = 30.55 kW:20.375 kW:30.56 kW:20.377 kW = 3:2:3:2 Since the rated capacities of the DGs in the DC microgrid are 30 kW, 20 kW, 30 kW, and 20 kW respectively, the control objective of proportional distribution of the active power generated by each DG in the DC microgrid according to its own capacity is achieved.

[0046] At 2 s, the controllers of the interconnected converters IC are also started. The active powers output by the three interconnected converters IC are as Figure 7 shown. The active powers output by each converter at steady state are respectively: P IC1 :P IC2 :P IC3 = -4.529 kW:-6.812 kW:-6.812 kW = 2:3:3 Since the rated active power capacities of each IC are: P IC1,max :P IC2,max :P IC3,max = 4.7 kW:7.0 kW:7.0 kW = 2:3:3 Therefore, there is: P IC1 :P IC2 :P IC3 = P IC1,max :P IC2,max :P IC3,max , thus achieving the control objective of proportional distribution of the active power output by each IC according to the rated capacity ratio. At the same time, the total output active power at steady state is -18.145 kW. Therefore, the active power is transmitted from the DC side to the AC side, realizing the power mutual assistance between AC and DC.

[0047] The diagram of the total active power output by the AC side AC at steady state is as Figure 8As shown, the graph of the total active power variation output on the DC side is as Figure 9 shown. Under the action of multiple interconnected converter IC controllers, at steady state, the ratio of the total power on the AC side to the total power on the DC side satisfies: P ACMG :P DCMG =100.794kW:100.797kW = 1:1 Since the rated capacities of the AC microgrid and the DC microgrid are both 100kW respectively, the goal of proportionally distributing the total active power output by each microgrid according to its rated capacity is also achieved. At the same time, the DG units are optimally configured, avoiding the situation of uneven distribution of the generated active power, and improving the efficiency and stability.

[0048] A hierarchical distributed control method for an AC-DC microgrid provided in this embodiment. Through hierarchical distributed control, based on finite-time consensus and a distributed secondary controller considering the proportionate distribution of the transmission power of the interconnected converters, the primary active power reference values of each interconnected converter are calculated, and the primary active power control reference values are compensated, solving the problems of internal power distribution in the microgrid and the proportionate distribution of power transmission across subnets, achieving the goal of proportionally distributing the total power generated by the DC microgrid and the AC microgrid according to their respective rated capacities and proportionally distributing the active power transmitted by all interconnected converters according to the transmission capacity ratio, thereby realizing the global optimization of the AC-DC microgrid. At the same time, the present invention adopts a distributed architecture, avoiding the risk of single-point failures, realizing the plug-and-play of the interconnected converters, enabling the microgrid to have the function of dynamic capacity expansion, and further improving the scalability of the AC-DC microgrid.

[0049] Please refer to Figure 10 , based on the same inventive concept, a hierarchical distributed control system for an AC-DC microgrid proposed in the second embodiment of the present invention includes: A data processing module 10, configured to obtain the DC voltage output by the distributed power sources in the DC microgrid and the AC frequency output by the distributed power sources in the AC microgrid, and perform normalization processing to obtain a voltage normalization value and a frequency normalization value; A lower-layer control module 20, configured to calculate the primary active power reference values of each interconnected converter according to the voltage normalization value and the frequency normalization value by using a finite-time consensus algorithm; An upper-layer control module 30, configured to compensate the primary active power reference values according to a distributed secondary controller to obtain secondary active power reference values, and the distributed secondary controller is constructed based on a distributed communication network and the proportionate distribution of the transmission power of the interconnected converters; The constant power control module 40 is configured to generate control signals for each interconnected converter according to the secondary active power reference value, and perform constant power control on each interconnected converter according to the control signals.

[0050] The technical features and technical effects of the AC-DC microgrid hierarchical distributed control system proposed in the embodiments of the present invention are the same as those of the method proposed in the embodiments of the present invention, and will not be elaborated here. Each module in the above AC-DC microgrid hierarchical distributed control system can be implemented in whole or in part by software, hardware, and their combinations. The above-mentioned modules can be embedded in the processor of the computer device in the form of hardware or independent of it, or stored in the memory of the computer device in the form of software, so that the processor can call and execute the operations corresponding to the above-mentioned modules.

[0051] In summary, for an AC-DC microgrid hierarchical distributed control method and system proposed in the embodiments of the present invention, the method obtains the DC voltage output by the distributed power sources in the DC microgrid and the AC frequency output by the distributed power sources in the AC microgrid, and performs normalization processing to obtain the voltage normalization value and the frequency normalization value; according to the voltage normalization value and the frequency normalization value, the primary active power reference values of each interconnected converter are calculated by using the finite-time consensus algorithm; the primary active power reference values are compensated according to the distributed secondary controller to obtain the secondary active power reference values, and the distributed secondary controller is constructed based on the distributed communication network and the power transmission ratio distribution of the interconnected converters; according to the secondary active power reference value, control signals for each interconnected converter are generated, and constant power control is performed on each interconnected converter according to the control signals. The present invention solves the problems of internal power distribution in the microgrid and the power transmission ratio distribution across subnets through hierarchical distributed control, realizes the goal of proportional distribution of the total power generated by the DC microgrid and the AC microgrid according to their respective rated capacities, and proportional distribution of the active power transmitted by all interconnected converters according to the transmission capacity ratio, thereby realizing the global optimization of the AC-DC microgrid and effectively improving the safety and stability of the operation of the AC-DC microgrid.

[0052] Each embodiment in this specification is described in a progressive manner. For the parts that are the same or similar in each embodiment, reference can be made to each other. Each embodiment focuses on the differences from other embodiments. In particular, for the system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and reference can be made to the partial description of the method embodiment for the relevant parts. It should be noted that the above technical features of the embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the above technical features in the embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0053] The above-described embodiments merely represent several preferred embodiments of the present application. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and substitutions can be made, and these improvements and substitutions should also be regarded as the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the protection scope of the claims described above.

Claims

1. A hierarchical distributed control method for AC / DC microgrid, characterized in that: include: The DC voltage output by the distributed power source in the DC microgrid and the AC frequency output by the distributed power source in the AC microgrid are obtained, and normalized to obtain a voltage normalized value and a frequency normalized value; Calculating the primary active power reference value of each interconnected converter using a finite time consistency algorithm according to the voltage normalized value and the frequency normalized value; According to a distributed secondary controller, the primary active power reference value is compensated to obtain a secondary active power reference value, wherein the distributed secondary controller is constructed based on a distributed communication network and an interconnected converter transmission power ratio distribution; A control signal for each interconnected converter is generated according to the secondary active power reference value, and constant power control is performed on each interconnected converter according to the control signal.

2. The AC / DC microgrid hierarchical distributed control method according to claim 1, characterized in that: The step of calculating the primary active power reference value of each interconnected converter by using a finite time consistency algorithm according to the voltage normalized value and the frequency normalized value comprises: Subtracting the frequency normalized value from the voltage normalized value to obtain a first difference; The first difference is calculated using a finite time consistency algorithm to obtain a primary active power reference value of each interconnected converter.

3. The AC / DC microgrid hierarchical distributed control method according to claim 2, characterized in that: The primary active power reference value is expressed by the following formula: In the formula, represents the primary active power reference value of the i-th interconnected converter, represents the gain coefficient of the i-th interconnected converter, represents the normalized voltage value, represents the frequency normalized value, α represents the first exponential coefficient, t represents time, and sign represents the sign function.

4. The AC / DC microgrid hierarchical distributed control method according to claim 1, characterized in that: The step of compensating the primary active power reference value according to the distributed secondary controller to obtain the secondary active power reference value comprises: Connect each interconnected converter to a distributed communication network and calculate the secondary power difference between the interconnected converter and each neighboring converter; The secondary power difference is calculated by using a distributed consistency algorithm to obtain the compensation power of each interconnected converter at the current moment; The primary active power reference value is compensated according to the compensation power to obtain the secondary active power reference value of each interconnected converter.

5. The AC / DC microgrid hierarchical distributed control method according to claim 4, characterized in that: The steps of connecting each interconnected converter to a distributed communication network and calculating the secondary power difference between the interconnected converter and each neighboring converter include: Obtaining secondary active power reference values ​​of each neighbor converter of the interconnected converter at a previous moment through a distributed communication network; Normalizing the secondary active power reference value of the interconnected converter at the previous moment and the secondary active power reference value of each neighboring converter at the previous moment respectively; According to the normalized secondary active power reference value of the interconnected converter and the normalized secondary active power reference value of each neighboring converter, the secondary power difference between the interconnected converter and each neighboring converter at the previous moment is calculated.

6. The AC / DC microgrid hierarchical distributed control method according to claim 5, characterized in that: The compensation power is expressed by the following formula: In the formula, represents the compensation power of the i-th interconnected converter at the current moment, represents the secondary active power reference value of the i-th interconnected converter at the previous moment, represents the secondary active power reference value of the jth neighbor converter at the previous moment, represents the rated power of the i-th interconnected converter, represents the rated power of the jth neighbor converter, represents the power gain coefficient of the i-th interconnected converter, N i represents the neighbor converter set of the i-th interconnected converter, γ represents the second exponential coefficient, t represents time, and sign represents the sign function.

7. The AC / DC microgrid hierarchical distributed control method according to claim 4, characterized in that: The step of compensating the primary active power reference value according to the compensation power to obtain the secondary active power reference value of each interconnected converter includes: The primary active power reference value and the compensation power are added to obtain the secondary active power reference value of each interconnected converter.

8. The AC / DC microgrid hierarchical distributed control method according to claim 1, characterized in that: The step of generating a control signal for each interconnected converter according to the secondary active power reference value comprises: Obtaining reactive power reference values ​​of each interconnected converter, performing PQ control on the secondary active power reference value and the reactive power reference value, respectively, to obtain a d-axis reference value and a q-axis reference value of the current loop; The d-axis reference value and the q-axis reference value are subjected to PI control adjustment and coordinate conversion to obtain control signals of each interconnected converter.

9. The AC / DC microgrid hierarchical distributed control method according to claim 1, characterized in that: The step of obtaining the DC voltage output by the distributed power source in the DC microgrid and the AC frequency output by the distributed power source in the AC microgrid includes: The DC voltage output by the distributed power source in the DC microgrid is obtained through the active power / voltage droop controller of the DC microgrid; The AC frequency output by the distributed power source in the AC microgrid is obtained through the active power / frequency droop controller of the AC microgrid.

10. An AC / DC microgrid hierarchical distributed control system, characterized in that: include: The data processing module is used to obtain the DC voltage output by the distributed power source in the DC microgrid and the AC frequency output by the distributed power source in the AC microgrid, and perform normalization processing to obtain a voltage normalization value and a frequency normalization value; A lower control module, used to calculate the primary active power reference value of each interconnected converter using a finite time consistency algorithm according to the voltage normalization value and the frequency normalization value; An upper control module is used to compensate the primary active power reference value according to a distributed secondary controller to obtain a secondary active power reference value, wherein the distributed secondary controller is constructed based on a distributed communication network and an interconnected converter transmission power ratio distribution; The constant power control module is used to generate a control signal for each interconnected converter according to the secondary active power reference value, and perform constant power control on each interconnected converter according to the control signal.

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