A hierarchical distributed control method and system for AC-DC microgrids

Abstract

The present invention relates to the field of microgrid control technology, and discloses a hierarchical distributed control method and system for an AC-DC microgrid, including obtaining the DC voltage output by distributed power sources in the DC microgrid and the AC frequency output by distributed power sources in the AC microgrid, and performing normalization processing; according to the voltage normalization value and the frequency normalization value, adopting a finite-time consensus algorithm to calculate the primary active power reference value of the interconnected converters; compensating the primary active power reference value according to a distributed secondary controller to obtain a secondary active power reference value; and generating control signals for each interconnected converter according to the secondary active power reference value. Through hierarchical distributed control, the present invention 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, effectively improving the safety and stability of the operation of the AC-DC microgrid.

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 absorb new energy to reduce carbon emissions. The AC-DC hybrid microgrid interconnects the AC microgrid and the DC microgrid through an interconnection 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 interconnection converter. When the AC-DC microgrid operates in island mode, the distributed power sources in the AC microgrid adopt active / frequency droop control to jointly share the AC load and maintain frequency stability; the distributed voltage in the DC microgrid adopts active / voltage droop control to jointly share the DC load 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 interconnection converter.

[0004] In the existing control strategies for AC-DC microgrids with multiple interconnection 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 overloading of the power transmission of the interconnection 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 ratios 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 interconnection converters is proportionally distributed according to the transmission capacity ratios.

[0006] In a first aspect, the present invention provides a hierarchical distributed control method for an AC-DC microgrid, the method comprising:

[0007] 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;

[0008] 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;

[0009] 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;

[0010] According to the secondary active power reference value, generate the control signals of each interconnected converter, and perform constant power control on each interconnected converter according to the control signals.

[0011] 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:

[0012] Subtract the frequency normalization value from the voltage normalization value to obtain the first difference;

[0013] Adopt the finite-time consensus algorithm to calculate the first difference to obtain the primary active power reference values of each interconnected converter.

[0014] Further, the primary active power reference value is represented by the following formula:

[0015]

[0016] 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.

[0017] 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:

[0018] Connect each interconnected converter to the distributed communication network, and calculate the secondary power difference between the interconnected converter and each neighbor converter;

[0019] The quadratic power difference is calculated using a distributed consistency algorithm to obtain the compensation power of each interconnected converter at the current moment;

[0020] 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.

[0021] Further, the step of connecting each interconnected converter to a distributed communication network and calculating the quadratic power difference between the interconnected converter and each neighbor converter includes:

[0022] Through the distributed communication network, obtain the secondary active power reference values of each neighbor converter of the interconnected converter at the previous moment;

[0023] Normalize the secondary active power reference value of the interconnected converter at the previous moment and the secondary active power reference values of each neighbor converter at the previous moment respectively;

[0024] According to the normalized secondary active power reference value of the interconnected converter and the normalized secondary active power reference values of each neighbor converter, calculate the quadratic power difference between the interconnected converter and each neighbor converter at the previous moment.

[0025] Further, the compensation power is expressed by the following formula:

[0026]

[0027] 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.

[0028] Further, 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:

[0029] Add the primary active power reference value and the compensation power to obtain the secondary active power reference value of each interconnected converter.

[0030] Further, the step of generating control signals for each interconnected converter according to the secondary active power reference value includes:

[0031] 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, and obtain the d-axis reference value and q-axis reference value of the current loop;

[0032] 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

[0033] Further, 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:

[0034] Obtain the DC voltage output by the distributed power source in the DC microgrid through the active power / voltage droop controller of the DC microgrid;

[0035] Obtain the AC frequency output by the distributed power source in the AC microgrid through the active power / frequency droop controller of the AC microgrid.

[0036] In a second aspect, the present invention provides a hierarchical distributed control system for an AC-DC microgrid, and the system includes:

[0037] A data processing module, configured 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;

[0038] 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;

[0039] An upper-layer control module, configured to compensate the primary active power reference value according to a distributed secondary controller to obtain a secondary active power reference value, and the distributed secondary controller is constructed based on a distributed communication network and the power ratio distribution of the interconnected converters;

[0040] A constant power control module, 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.

[0041] 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 proportional distribution of power transmission ratios across subnets, achieving 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. At the same time, the present invention adopts a distributed architecture, avoiding the risk of single-point failure, realizing the plug-and-play of interconnected converters, enabling the microgrid to have the function of dynamic capacity expansion, and thus further improving the scalability of the AC-DC microgrid. Brief Description of the Drawings

[0042] Figure 1 is a schematic flow chart of the hierarchical distributed control method for the AC-DC microgrid in an embodiment of the present invention;

[0043] Figure 2 is a topological structure diagram of the AC-DC microgrid in an embodiment of the present invention;

[0044] Figure 3 is a control topological structure diagram of the AC-DC microgrid in an embodiment of the present invention;

[0045] Figure 4 is a communication topological structure diagram of the AC-DC microgrid in an embodiment of the present invention;

[0046] Figure 5 is a diagram showing the change of the active power output of each distributed power source in the AC microgrid in the simulation experiment of an embodiment of the present invention;

[0047] Figure 6 is a diagram showing the change of the active power output of each distributed power source in the DC microgrid in the simulation experiment of an embodiment of the present invention;

[0048] Figure 7 is a diagram showing the change of the active power transmitted by each interconnected converter in the microgrid in the simulation experiment of an embodiment of the present invention;

[0049] Figure 8 is a diagram showing the change of the total power generated by the AC microgrid in the simulation experiment of an embodiment of the present invention;

[0050] Figure 9 is a diagram showing the change of the total power generated by the DC microgrid in the simulation experiment of an embodiment of the present invention;

[0051] Figure 10 is a schematic structure diagram of the hierarchical distributed control system for the AC-DC microgrid in an embodiment of the present invention. Detailed Embodiments

[0052] 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. Apparently, 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.

[0053] 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:

[0054] 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;

[0055] Step S20: According to the voltage normalization value and the frequency normalization value, use a finite-time consensus algorithm to calculate the primary active power reference values of each interconnected converter;

[0056] Step S30: Compensate the primary active power reference value according to a distributed secondary controller to obtain a secondary active power reference value. The distributed secondary controller is constructed based on a distributed communication network and the power ratio distribution of the interconnected converters;

[0057] 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.

[0058] The present invention provides a control optimization strategy for the interconnected converters in an AC-DC microgrid. Please 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 DG. After the distributed power sources are interconnected, they are connected to loads to supply power to the loads. The distributed power sources use PWM for pulse width modulation and realize distributed power source control through a distributed power source controller. The AC microgrid and the DC microgrid are connected by multiple interconnected converters IC, and power mutual assistance is realized between the AC side and the DC side through the interconnected converters.

[0059] When a conventional AC-DC hybrid microgrid operates in 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 during the power transmission process of a certain microgrid or a certain interconnection converter, and it is difficult to quickly return to the stable transmission state. To address this issue, please 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 interconnection converter. Among them, in the present invention, the communication topology of the AC-DC microgrid is as Figure 4 shown.

[0060] 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. The DC voltage output by the active / voltage droop controller can be expressed as:

[0061]

[0062] 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.

[0063] In the AC microgrid, its distributed power sources adopt active power and frequency droop control. The AC frequency output by the active / frequency droop controller can be expressed as:

[0064]

[0065] In the formula, represents the AC frequency output by the AC microgrid droop control, 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.

[0066] The DC voltage on the DC side and the AC frequency on the AC side are the core indicators of the microgrid operating state, which can directly reflect the power balance situation. The present invention regulates the transmission power of the interconnected AC 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.

[0067] In a preferred embodiment, through normalization processing, the DC voltage and the AC frequency are unified in scale, and its normalization formula is:

[0068]

[0069] 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.

[0070] At the steady-state operating point, when the condition that and are equal is 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.

[0071] 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:

[0072] Subtract the normalized value of the frequency from the normalized value of the voltage to obtain a first difference;

[0073] Adopt a finite-time consensus algorithm to calculate the first difference to obtain the primary active power reference values of each interconnected converter.

[0074] 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 achieve the convergence goal, this embodiment adopts a finite-time consensus algorithm to accelerate the convergence through non-linear terms.

[0075] Finite-time consensus means that agents tend to be the same for certain state variables within a finite time; the finite-time consensus algorithm refers to an algorithm in which multiple agents make corresponding reactions 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:

[0076]

[0077] 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 nodes, 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:

[0078]

[0079] 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.

[0080] 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 interconnection converters in the AC-DC microgrid to quickly achieve the power balance between the 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 reference value of the primary active power of each interconnection converter. Its formula is expressed as:

[0081]

[0082] In the formula, represents the reference value of the primary active power of the i-th interconnection converter, represents the gain coefficient of the i-th interconnection 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.

[0083] 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, which directly affects the dynamic process of power distribution. The larger the gain, the faster the convergence speed, but too large a gain 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.

[0084] 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 distributed secondary controller compensates the primary active power reference value to obtain the secondary active power reference value of the interconnected converter. The specific steps include:

[0085] Connect each interconnected converter to the distributed communication network and calculate the secondary power difference between the interconnected converter and each neighboring converter;

[0086] Use the distributed consensus algorithm to calculate the secondary power difference and obtain the compensation power of each interconnected converter at the current moment;

[0087] Compensate the primary active power reference value according to the compensation power to obtain the secondary active power reference value of each interconnected converter.

[0088] In this embodiment, each interconnected converter is connected to the distributed communication network to form a communication topology among the interconnected converters, and through data communication among the interconnected converters, the secondary power difference between the interconnected converter and each neighboring converter is calculated. The specific steps include:

[0089] Through the distributed communication network, obtain the secondary active power reference value of each neighboring converter of the interconnected converter at the previous moment;

[0090] 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;

[0091] Calculate the secondary power difference between the interconnected converter and each neighboring converter at the previous moment based on the normalized secondary active power reference value of the interconnected converter and the normalized secondary active power reference values of each neighboring converter.

[0092] In this embodiment, the distributed secondary controller realizes power compensation through the asynchrony and time decoupling of the distributed algorithm, and by means of the finite-time consensus algorithm and the power proportional distribution principle. In the initial startup stage of the microgrid system, all initial values of the compensation power are set to 0 because there is no power interaction at the initial moment, the secondary control has not been activated, so there is no need to compensate the primary reference value. At this time, the initial moment only relies on the lower-layer control (primary reference value) to maintain the basic power distribution, and the upper-layer control (secondary compensation) is in the "pending activation" state. As the communication network is established, the compensation frequency is dynamically updated through the distributed protocol. By gradually adjusting the compensation frequency, finally, the secondary active power reference values of the interconnected converters are distributed according to the rated capacity ratio. The dynamic adjustment process of the compensation frequency is described below.

[0093] Taking the discrete time step k as an example, assume that an interconnected converter obtains the secondary active power reference values of each neighboring converter at the moment k - 1 through the distributed communication network. By normalizing the secondary active power reference values of its own converter and the neighboring converters, the secondary power difference between this interconnected converter and the neighboring converters at the moment k - 1 can be obtained.

[0094] Then, based on the distributed finite-time consensus algorithm, the sign function and non-linear terms 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 between the interconnected converter and the neighboring converters at the moment 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.

[0095] At this time, the compensation power output by the distributed secondary controller can be expressed as:

[0096]

[0097] 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, denotes the rated power of the j-th neighbor converter, denotes the power gain coefficient of the i-th interconnected converter, N i denotes the set of neighbor converters of the i-th interconnected converter, γ denotes the second exponential coefficient, t denotes time, and sign denotes the sign function.

[0098] 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 and will not be elaborated here one by one.

[0099] 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:

[0100]

[0101] In this embodiment, through hierarchical distributed control, a hierarchical structure framework is established, separating the lower-level control and the upper-level control. In the lower-level control, based on the local droop characteristics, a preliminary power reference value is generated to solve the power distribution problem within the microgrid. In the upper-level 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 neighbors, 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.

[0102] 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:

[0103] 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;

[0104] 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.

[0105] In this embodiment, first, the secondary active power reference value and 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 through PQ control. 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.

[0106] The d-axis reference value and q-axis reference value of the current loop can be expressed as:

[0107]

[0108] 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 secondary active power reference value transmitted by the i-th interconnected converter, represents the actual active power value transmitted by the i-th interconnected converter, represents the reactive power reference value transmitted by the i-th interconnected converter, represents the actual reactive power value 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.

[0109] 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, thereby obtaining the d-axis and q-axis voltage reference values required for modulation. Then, the voltage reference value in the dq0 coordinate system is converted into a three-phase AC signal 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, thereby realizing the accurate transmission of active / reactive power.

[0110] Next, the effectiveness of the hierarchical distributed control method for AC-DC microgrids provided by the present invention is verified through simulation experiments. In this simulation experiment, the Simulink simulation software is used, and the Figure 2 shown microgrid topology structure is used as the simulation structure. The initial AC load of the simulation system is 120 kW, and the DC load is 80 kW. The relevant control parameters and circuit parameters are shown in Table 1 below:

[0111] Table 1 Control Parameters of AC / DC AC-DC Microgrid

[0112]

[0113] The total duration of the system simulation is 4 s, and the step size is s. Before 2 s, the system has an AC load of 120 kW and a DC load of 80 kW. At 2 s, the load suddenly changes. The AC side load suddenly changes to 80 kW, the DC side load changes to 120 kW, and at the same time, the control of three interconnected converters is added.

[0114] Through the simulation experiment, after 2 s, the load suddenly changes and reaches a steady state. The changes in the active power output of each distributed generation (DG) in the microgrid are as Figure 5 and Figure 6 shown. According to Figure 5 it can be known that the active power outputs of each distributed generation (DG) in the AC microgrid (ACMD) are respectively:

[0115] P ac1 :P ac2 :P ac3 :P ac4 =20.18 kW:30.29 kW:20.19 kW:30.30 kW = 2:3:2:3

[0116] Since the rated capacities of each distributed generation in the AC microgrid are 20 kW, 30 kW, 20 kW, and 30 kW respectively, the control goal of proportional distribution of the active power generated by each distributed generation in the AC microgrid according to its own capacity is achieved.

[0117] According to Figure 6 it can be known that the active power outputs of each distributed generation (DG) in the DC microgrid (DCMG) are respectively:

[0118] P dc1 :P dc2 :P dc3 :P dc4 =30.55 kW:20.375 kW:30.56 kW:20.377 kW = 3:2:3:2

[0119] Since the rated capacities of each DG in the DC microgrid are 30 kW, 20 kW, 30 kW, and 20 kW respectively, the control goal of proportional distribution of the active power generated by each DG in the DC microgrid according to its own capacity is achieved.

[0120] At 2 s, the controllers of each interconnected converter (IC) are also started. The active power outputs of the three interconnected converters (ICs) are as Figure 7 shown. At steady state, the active power outputs of each converter are respectively:

[0121] P IC1 :P IC2 :P IC3=-4.529 kW : -6.812 kW : -6.812 kW = 2 : 3 : 3

[0122] Since the rated active power capacity of each IC is as follows:

[0123] P IC1,max : P IC2,max : P IC3,max = 4.7 kW : 7.0 kW : 7.0 kW = 2 : 3 : 3

[0124] Therefore, we have: P IC1 : P IC2 : P IC3 = P IC1,max : P IC2,max : P IC3,max , thus achieving the control goal of distributing the active power output of each IC according to the ratio of the rated capacity. 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.

[0125] The variation diagram of the total active power output on the AC side AC at steady state is as Figure 8 shown, and the variation diagram of the total active power output on the DC side DC is as Figure 9 shown. Under the action of multiple interconnected converter IC controllers, at steady state, the ratio of the total power of the AC side to the DC side satisfies:

[0126] P ACMG : P DCMG = 100.794 kW : 100.797 kW = 1 : 1

[0127] Since the rated capacities of the AC microgrid and the DC microgrid are 100 kW respectively, the goal of distributing the total active power output of 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.

[0128] A hierarchical distributed control method for AC-DC microgrids provided in this embodiment. Through hierarchical distributed control, based on finite-time consensus and a distributed secondary controller considering the proportional allocation of the transmission power of 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 proportional allocation of power transmission ratios across subnets, achieving the goal of proportional allocation of the total power generated by the DC microgrid and the AC microgrid according to their respective rated capacities and proportional allocation of the active power transmitted by all interconnected converters according to the transmission capacity ratios, 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 failure, realizing the plug-and-play of interconnected converters, enabling the microgrid to have a dynamic expansion function, and further improving the scalability of the AC-DC microgrid.

[0129] 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:

[0130] A data processing module 10, configured to obtain the DC voltage output by distributed power sources in the DC microgrid and the AC frequency output by distributed power sources in the AC microgrid, and perform normalization processing to obtain a voltage normalization value and a frequency normalization value;

[0131] A lower-layer control module 20, 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;

[0132] An upper-layer control module 30, configured to compensate the primary active power reference value according to a distributed secondary controller to obtain a secondary active power reference value, where the distributed secondary controller is constructed based on a distributed communication network and the proportional allocation of the transmission power of interconnected converters;

[0133] A constant power control module 40, 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.

[0134] The technical features and technical effects of the hierarchical distributed control system for an AC-DC microgrid 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-mentioned hierarchical distributed control system for an AC-DC microgrid 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 a computer device in a hardware form or be independent of it, or be stored in the memory of the computer device in a software form, so that the processor can call and execute the operations corresponding to the above-mentioned modules.

[0135] In summary, a hierarchical distributed control method and system for an AC-DC microgrid proposed in an embodiment of the present invention obtain the DC voltage output by distributed power sources in the DC microgrid and the AC frequency output by distributed power sources in the AC microgrid, and perform normalization processing to obtain a voltage normalization value and a frequency normalization value; according to the voltage normalization value and the frequency normalization value, a finite-time consensus algorithm is used to calculate the primary active power reference values of each interconnected converter; according to a distributed secondary controller, the primary active power reference values are compensated to obtain secondary active power reference values, and the distributed secondary controller is constructed based on a distributed communication network and the power transmission ratio distribution of the interconnected converters; according to the secondary active power reference values, control signals for each interconnected converter are generated, and constant power control is performed on each interconnected converter according to the control signals. Through hierarchical distributed control, the present invention solves the problems of internal power distribution in the microgrid and the power transmission ratio distribution across subnets, achieves 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.

[0136] Each embodiment in this specification is described in a progressive manner. For 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 the relevant parts can refer to the partial description of the method embodiment. It should be noted that the above technical features of the embodiments can be combined arbitrarily. For the sake of concise 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 described in this specification.

[0137] The above-described embodiments only represent several preferred embodiments of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be pointed out that for those of ordinary skill in the art in this technical field, 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 should be subject to the protection scope of the claims.

Claims

1. A hierarchical distributed control method for AC / DC microgrids, characterized in that, Including: 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; According to the voltage normalization value and the frequency normalization value, adopt a 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 a secondary active power reference value, and the distributed secondary controller is constructed based on a distributed communication network and the power ratio distribution of the interconnected converters; According to the secondary active power reference value, generate control signals for each interconnected converter, and perform constant power control on each interconnected converter according to the control signals; Among them, the step of compensating the primary active power reference value according to the distributed secondary controller to obtain a 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 neighboring converter; Adopt a 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; 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: Through the distributed communication network, obtain the secondary active power reference values 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 values 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 values of each neighboring converter, calculate the secondary power difference between the interconnected converter and each neighboring converter at the previous moment.

2. The hierarchical distributed control method for AC-DC microgrid according to claim 1, characterized in that 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 a finite-time consensus algorithm includes: Subtract the frequency normalization value from the voltage normalization value to obtain a first difference; Adopt a finite-time consensus algorithm to calculate the first difference to obtain the primary active power reference values of each interconnected converter.

3. The hierarchical distributed control method for the AC-DC microgrid according to claim 2, characterized in that The primary active power reference value is expressed by the following formula: Wherein, 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.

4. The hierarchical distributed control method for AC-DC microgrid according to claim 1, characterized in that The compensation power is expressed by the following formula: Wherein, 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.

5. The hierarchical distributed control method for the AC-DC microgrid according to claim 1, wherein 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.

6. The hierarchical distributed control method for the AC-DC microgrid according to claim 1, wherein The step of generating control signals for each interconnected converter according to the secondary active power reference value includes: Obtain the reactive power reference values of each interconnected inverter, perform PQ control on the secondary active power reference value and the reactive power reference value respectively, and 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 inverter.

7. The hierarchical distributed control method for the AC-DC microgrid according to claim 1, characterized in that The steps 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 include: Obtain the DC voltage output by the distributed power source in the DC microgrid through the active power / voltage droop controller of the DC microgrid; Obtain the AC frequency output by the distributed power source in the AC microgrid through the active power / frequency droop controller of the AC microgrid.

8. A hierarchical distributed control system for AC-DC microgrids, characterized in that, The system is applied to the method according to any one of claims 1 to 7, and includes: A data processing module, configured 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-layer control module, configured to calculate the primary active power reference values of each interconnected inverter 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 value according to a distributed secondary controller to obtain a secondary active power reference value, and the distributed secondary controller is constructed based on a distributed communication network and the power ratio distribution of the interconnected inverters; A constant power control module, configured to generate control signals for each interconnected inverter according to the secondary active power reference value, and perform constant power control on each interconnected inverter according to the control signals.

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