A coordinated control strategy for wind-solar-storage DC microgrid

Abstract

The present invention provides a coordinated control strategy for a wind, solar and storage DC microgrid, which belongs to the field of grid control technology, including S1: analyzing the operation mode of the DC microgrid in the grid-connected free mode and the operation mode of the DC microgrid in the grid-connected dispatching mode respectively; S2: establishing a DC microgrid control model for grid-connected operation, the DC microgrid control model for grid-connected operation includes a photovoltaic power generation system control model, a wind power generation system control model, an energy storage system control model, a grid-connected converter control model, a load management control model within the microgrid, and an energy coordination control model for the grid-connected DC microgrid; S3: establishing a simulation model for the grid-connected wind, solar and storage DC microgrid. Through the application of MPPT and LPTC control modes, the photovoltaic power generation system and the wind power generation system can adjust the power output according to actual working conditions and optimize energy utilization.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grid control, and in particular relates to a coordinated control strategy for a wind-solar-storage DC microgrid. Background Art

[0002] A DC microgrid is a microgrid composed of direct current. It is an important component of the future intelligent power distribution system and is of great significance to promoting energy conservation and emission reduction and achieving sustainable energy development. Compared with an AC microgrid, a DC microgrid can more efficiently and reliably accommodate distributed renewable energy generation systems such as wind and light, energy storage units, electric vehicles and other DC power loads.

[0003] At present, in the power flow of DC microgrid, the energy storage system and grid-connected inverter maintain the system balance by exchanging power with the DC bus. Therefore, the power of micro-source generation Power consumption with load It is a one-way flow. and It flows in both directions, but the output power of the micro source , dispatching grid power and load power When changes occur, the DC bus voltage will fluctuate. Therefore, it is necessary to study the coordinated control strategy of the DC microgrid to ensure the stability of the DC bus voltage and reduce power fluctuations within the grid. Summary of the invention

[0004] Based on the above technical problems, the present invention provides a coordinated control strategy for a wind-solar-storage DC microgrid.

[0005] The specific technical solution is:

[0006] A coordinated control strategy for a wind-solar-storage DC microgrid includes the following steps:

[0007] S1: Analyze the operation mode of the DC microgrid in the grid-connected free mode and the operation mode of the DC microgrid in the grid-connected dispatching mode respectively;

[0008] S2: Establishing a grid-connected DC microgrid control model, wherein the grid-connected DC microgrid control model includes a photovoltaic power generation system control model, a wind power generation system control model, an energy storage system control model, a grid-connected converter control model, a microgrid internal load management control model, and an energy coordination control model of a grid-connected DC microgrid;

[0009] S3: Establish a simulation model for the grid-connected wind, solar, and energy storage DC microgrid.

[0010] In addition, a coordinated control strategy of a wind-solar-storage DC microgrid in the above technical solution provided by the present invention may also have the following additional technical features:

[0011] In the above technical solution, two control modes, MPPT and LPTC, are used in the photovoltaic power generation system control model to cope with the working conditions in the DC microgrid;

[0012] Among them, when there is power surplus in the microgrid and the energy storage system and the power grid cannot fully absorb it, the photovoltaic power generation system virtually switches from the MPPT control mode to the LPTC control mode to ensure power balance in the microgrid.

[0013] In the above technical solution, in the wind power generation system control model, when the photovoltaic power generation unit cuts off all power in the LPTC control mode, if the power generation in the microgrid Still greater than the load power consumption , Allowable access power Energy storage absorption power When the three are combined, the wind power system also needs to switch from MPPT control mode to LPTC control mode to limit the power generation of the wind power system and ensure power balance within the microgrid.

[0014] In the above technical solution, the energy storage system control model includes a grid-connected free mode and a grid-connected dispatching mode;

[0015] In the grid-connected free mode, the energy storage system is charged at a constant current of 0.5C. When the lithium battery SOC exceeds 90%, the battery is changed to trickle charging to compensate for the power loss of battery self-discharge.

[0016] In the grid-connected dispatching mode, when the lithium battery When the limit is not exceeded, the energy storage system charges and discharges according to the power command issued by the microgrid controller, and the energy storage system participates in the coordinated control of the microgrid. When the time limit is exceeded, the energy storage system switches to floating charge mode.

[0017] In the above technical solution, conventional vector control with voltage and current double closed loops is adopted in the grid-connected converter control model.

[0018] In the above technical solution, the energy coordination control model of the grid-connected DC microgrid includes a grid-connected free mode and a grid-connected dispatching mode;

[0019] In the grid-connected free mode, both wind and light micro-sources operate in MPPT control mode; the lithium battery energy storage system is charged at a constant current. When the power is turned off, the energy storage system switches to floating charge mode; the grid-connected inverter operates in constant voltage control mode; all loads in the microgrid are powered; coordinated control is performed based on the power flow inside the microgrid and the power exchange with the large power grid to maintain the DC bus voltage stable;

[0020] In the grid-connected dispatch mode, coordinated control responds to dispatch requirements according to the power flow inside the microgrid.

[0021] Compared with the prior art, the coordinated control strategy of a wind-solar-storage DC microgrid of the present invention has the following beneficial effects:

[0022] 1. By analyzing the different operating conditions of the microgrid during grid-connected operation, and dividing the microgrid operation process into different operation modes according to its characteristics, a DC microgrid control model for grid-connected operation is established, and verified according to the simulation model that in the DC microgrid control model for grid-connected operation of this application, the DC microgrid can switch smoothly and sensitively between various operation modes under different operating conditions.

[0023] 2. Through the application of MPPT and LPTC control modes, photovoltaic power generation systems and wind power generation systems can adjust power output according to actual working conditions and optimize energy utilization; and when there is power surplus, by switching the control mode, the power balance in the microgrid can be ensured to avoid causing instability to the power grid.

[0024] 3. Under different grid-connected modes, the energy storage system and grid-connected converter can automatically adjust the working status to adapt to the grid demand and maintain system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a flow chart of the conductance increment method;

[0026] Figure 2 This is the operation control strategy block diagram of the DC microgrid photovoltaic power generation system;

[0027] Figure 3 This is the program flow chart of the variable step size hill climbing method;

[0028] Figure 4 This is the operation control strategy block diagram of the DC microgrid wind power generation system;

[0029] Figure 5 This is the operation control strategy block diagram of the DC microgrid energy storage system;

[0030] Figure 6 This is the control block diagram of the DC microgrid grid-connected inverter;

[0031] Figure 7 is the switching hysteresis loop of the load in the DC microgrid;

[0032] Figure 8 The control flow chart of the coordinated control strategy for the DC microgrid in grid-connected free mode;

[0033] Fig. 9 The control flow chart of the coordinated control strategy for the DC microgrid grid-connected dispatching mode;

[0034] Fig.10 It is the output characteristic curve of photovoltaic modules under variable light intensity. DETAILED DESCRIPTION

[0035] The following is a combination of specific implementation cases and attached Figure 1-10 The present invention is further described below, but the present invention is not limited to these embodiments.

[0036] in, Generates power for the photovoltaic system; Generates power for wind power system; is the total power generated by micro-sources, ; Transmit power to the energy storage battery; It represents the charging and discharging power of bus capacitor; The power consumed by important loads and non-important loads in the network. ; The energy storage system and the grid-connected inverter maintain the system power balance by exchanging power with the DC bus. Power consumption with load It is a one-way flow. and It flows in both directions.

[0037] When the DC microgrid operates in a stable operating state, the system power balance equation is:

[0038]

[0039] The DC bus voltage The relationship with the power flow in the grid is:

[0040]

[0041] To ensure the DC bus voltage Stability, need to be guaranteed ,but:

[0042]

[0043] From the above three formulas, we can know that the DC microgrid bus voltage It directly shows the internal power balance of the microgrid. , dispatching grid power and load power When the voltage of the DC bus changes, it will cause the DC bus voltage to fluctuate. Therefore, it is necessary to study the coordinated control strategy of the DC microgrid to ensure the stability of the DC bus voltage and reduce the power fluctuation in the grid.

[0044] 1. Analysis of the operation mode of DC microgrid in grid-connected free mode

[0045] When the DC microgrid operates in the grid-connected free operation mode, the power exchanged between the microgrid and the large grid is adjustable. The distributed generation unit operates in the maximum power point tracking (MPPT) control mode; the energy storage battery is in the charging state. When the battery enters the floating charge mode, the grid-connected inverter has the ability to quickly respond to micro-source disturbances and load mutations within the microgrid. This converter works in the constant voltage control mode and stabilizes the DC bus voltage of the microgrid through rectification or inversion.

[0046] Based on the discussion of the power balance relationship within the system, the coordinated control conditions of the microgrid in the free mode are designed. Define the system net power ;Battery charging power ; The corresponding relationship between the operating conditions and coordinated control of the wind-solar-storage DC microgrid free mode is shown in the following table.

[0047] Table 1 Energy management and coordination control table of wind-solar-storage DC microgrid in free operation mode

[0048]

[0049] 2. Analysis of the operation mode of DC microgrid under grid-connected dispatching mode

[0050] When the microgrid control detects the command power sent by the dispatcher, the microgrid switches from the grid-connected free mode to the grid-connected dispatching mode.

[0051] In the grid-connected dispatching operation mode, the power grid will give the dispatching power value that the microgrid needs to output to the distribution network. . After giving priority to meeting the load demand within the grid, try to meet the dispatching demand. The grid-connected inverter works in the inverter state and transmits power to the grid; the distributed generation unit is in the maximum power point tracking (MPPT) or limited power tracking control (LPTC) control mode; the energy storage battery charges and discharges according to the command power, cooperates with the dispatching requirements as much as possible, and reserves a certain capacity for the island mode; except for individual operating modes, most operating modes ensure that the load is fully powered. The microgrid controller coordinates the various subsystems in the grid to work together to support the stability of the bus voltage and ensure the power balance of the microgrid system.

[0052] When the microgrid operates in island mode, the energy storage unit needs to have sufficient capacity to stabilize the bus voltage. Therefore, the upper limit of the state of charge of the lithium battery is specified in the dispatching operation mode. , lower limit of state of charge In the general operation mode of the scheduling operation mode, when When the limit is exceeded, the energy storage switches to floating charge mode. When the energy storage unit is in operation, it only supplies power to the load and no longer responds to the dispatching instructions.

[0053] Defining the discharge power of energy storage batteries , maximum discharge power , charging power , maximum charging power ; Battery state of charge upper limit , battery state of charge lower limit ; The difference between the micro-source power and the load power is the micro-grid net power ,pass , Battery power With the command power value The relative relationship between the power supply and the state of charge jointly determines the coordinated control strategy in the grid-connected dispatching operation mode.

[0054] Mode 5, when and When the microgrid is in operation, it can respond to part or all of the grid-connected power required by the dispatcher according to the working status of the energy storage unit in the grid.

[0055] Mode 5.1, when When the micro-sources are running in MPPT control mode, the energy storage battery is discharged, and the microgrid only responds to the dispatching request to access the grid. Partially, no longer responding Partial power requirement, full power supply for the load.

[0056] Mode 5.2, when When the power is off, the micro-sources all operate in the MPPT control mode, the energy storage battery is discharged, and all loads are powered.

[0057] Mode 5.3, when When the micro-sources are running in MPPT control mode, the energy storage is floating charged, and the microgrid only responds to the dispatching request to access the grid. part, no longer responding to the remaining power requirements, and all loads are powered.

[0058] Mode 6, when and In addition to meeting the dispatching requirements, the microgrid can charge the energy storage unit in the grid. When the lithium battery SOC exceeds the upper limit, continuing to charge it will cause damage to the battery due to overcharging. When the energy storage unit enters the floating charge mode due to charging, the microgrid needs to limit the power generation of the micro source according to the net power value in the grid. Considering that the long-term operation of rotating mechanical equipment under conditions of frequent changes in speed will affect its service life, in the micro-source LPTC control mode, the power generation of the photovoltaic unit is limited first. At this time, the wind power generation system is still working in the MPPT control mode. If the photovoltaic unit stops generating power, the microgrid , the power generation capacity of the wind power generation system will continue to be limited.

[0059] Mode 6.1, when When the micro source is running in MPPT control mode, the energy storage battery is charged, and the battery charging power , all loads are powered.

[0060] Mode 6.2, when When the micro source enters the LPTC control mode, the energy storage battery is float charged, and all loads are powered.

[0061] Mode 7, when and When the power generated by micro-sources cannot meet the power supply demand of the load, the energy storage has a certain capacity to respond to the remaining demand of the load and respond to part or all of the grid-connected power required by the dispatcher.

[0062] Mode 7.1, when When the micro-sources are running in MPPT control mode, the microgrid part responds to the dispatching requirements. The energy storage battery supplies power to the load first, and the remaining discharge power of the battery responds to the dispatching. At this time, the battery discharge power , all loads are powered.

[0063] Mode 7.2, when When the micro source is running in MPPT control mode, the energy storage battery will supply power to the load first, and the remaining discharge power of the battery can meet the dispatching requirements. , all loads are powered.

[0064] Mode 8, when and When the micro-sources are running in MPPT control mode, the microgrid does not respond to the dispatching requirements, and the energy storage battery only supplies power to the load. At this time, the energy storage unit is over-discharging. If the battery state of charge is close to 20%, it will cause the lithium battery to be over-discharged, which will seriously affect the battery life. When the non-important loads in the microgrid need to be removed If removed After that, the power generated by micro-sources still cannot meet the needs of important loads. If the power supply demand is met, it is necessary to remove the important loads in the microgrid. , using the surplus power to charge the energy storage. By setting the SOC hysteresis loop for load switching, frequent load switching can be prevented, thus improving the stability of microgrid operation.

[0065] Mode 8.1, when and When the micro-sources are running in MPPT control mode, the microgrid does not respond to the dispatching requirements, the energy storage is discharged, and the battery discharge power , ensuring that all loads are powered.

[0066] Mode 8.2, when and When the micro-sources are running in MPPT control mode, the microgrid does not respond to the dispatching requirements, the energy storage unit stops discharging, and part of the load in the microgrid is removed according to the power shortage until all loads are out of operation. After the load is removed, the remaining power in the grid supplies power to the energy storage. When the battery is in a charging state, it switches to a discharging state and continues to supply power to the load.

[0067] 3. Photovoltaic power generation system control strategy

[0068] This application adopts two control methods, MPPT control and LPTC, in the photovoltaic power generation system to cope with various working conditions in the DC microgrid.

[0069] (1) Photovoltaic maximum power point tracking control

[0070] According to the working characteristics of photovoltaic cells, the photovoltaic power generation system needs to be controlled to track the maximum power point of photovoltaic cells at all times. Common photovoltaic MPPT algorithms include constant voltage method, perturbation observation method, conductance increment method, etc.

[56] . Compared with other MPPT algorithms, the conductance increment method has a simple control structure and good tracking effect when environmental parameters change. Therefore, the conductance increment method is selected as the MPPT algorithm for photovoltaic power generation system.

[0071] The output power of photovoltaic modules is:

[0072] (4-4)

[0073] Formula (4-4) is It can be derived that:

[0074] (4-5)

[0075] when When Voltage Gradient , at this time, formula (4-5) can be transformed into:

[0076] (4-6)

[0077] From equations (4-5), (4-6) and the output characteristic curve of the photovoltaic cell Fig.10 It can be seen that at light intensity ,temperature When a certain period of time, the ideal photovoltaic module The curve is a single-peak smooth curve.

[0078] When the derivative of power with respect to voltage When the power point is Left side, , the output voltage of the photovoltaic module needs to be increased; when the gradient of power to voltage When the power point is Right side, , the output voltage of the PV module needs to be reduced.

[0079] In summary, the reference voltage when the photovoltaic module outputs maximum power The optimization algorithm is

[0080] (4-7)

[0081] In the formula, is the sampling time; The step size of the set tracking voltage change;

[0082] The flow chart of the conductance increment method is as follows: Figure 1 shown.

[0083] (2) Photovoltaic power generation system control strategy

[0084] When there is a power surplus in the microgrid and the energy storage system and the power grid cannot fully absorb it, the photovoltaic power generation system needs to switch from the MPPT control mode to the LPTC control mode to ensure power balance in the microgrid. The input of the LPTC control mode power regulator is the command power issued by the microgrid controller to the photovoltaic power generation system. Photovoltaic power generation The difference between the output voltage and the output voltage of the photovoltaic module Reference value , and then control the output power of the photovoltaic power generation system. The operation strategy block diagram of the photovoltaic power generation system is as follows Figure 2 shown.

[0085] 4. Wind power generation system control strategy:

[0086] (1) Maximum power tracking control of wind power generation system

[0087] In order to achieve the maximum power output of the wind power system, it is necessary to perform maximum power tracking control on it. Without considering the variable pitch control, according to the working characteristics of the wind turbine, the reference speed of the PMSG needs to be calculated in real time through the MPPT algorithm. , to ensure that the wind turbine can capture wind energy to the maximum extent.

[0088] At present, there are three main MPPT algorithms used in small wind power systems: hill climbing search method, power feedback method, and optimal tip speed ratio method. The optimal curve method is represented by the power feedback method and the optimal tip speed ratio method. This type of algorithm requires the installation of wind measurement equipment and has low practicality in small wind power systems.

[0089] The optimization algorithm is represented by the fixed-step hill climbing method and the variable-step hill climbing method, which are suitable for systems with small moment of inertia. However, since the search step size of the fixed-step hill climbing method is not easy to select accurately, it is easy to produce oscillations near the maximum power point of the wind turbine. Therefore, this paper uses the variable-step hill climbing method to calculate the reference speed of the PMSG. , its flow chart is as follows Figure 3 shown.

[0090] (2) Wind power generation system control strategy

[0091] When the photovoltaic power generation unit cuts off all power in the LPTC control mode, if the power generation in the microgrid Still greater than the load power consumption , Allowable access power Energy storage absorption power When the sum of the three is reached, the wind power system also needs to switch from MPPT control mode to LPTC control mode to limit the power generation of the wind power system and ensure power balance within the microgrid. The input of the power regulator in the LPTC control mode is the command power sent by the microgrid controller to the wind power system. Wind power generation The difference is output as the reference value of the PMSG speed regulator , and then control the output power of the wind power generation system. The operation strategy block diagram of the wind power generation system is as follows Figure 4 shown.

[0092] 5. Energy storage system control strategy

[0093] When the wind, solar and energy storage microgrid is connected to the grid, the grid-connected inverter is responsible for maintaining the DC bus voltage, and the lithium battery energy storage system switches from the voltage control mode in the island state to the current or power control mode.

[0094] (1) Grid-connected free mode control of energy storage system

[0095] In the grid-connected free mode of the DC microgrid, the energy storage system is charged at a constant current of 0.5C. When the SOC of the lithium battery exceeds 90%, the battery is changed to trickle charging to compensate for the power loss of battery self-discharge.

[0096] (2) Energy storage system grid-connected dispatching mode control

[0097] In the DC microgrid grid-connected dispatching mode, when the lithium battery When the limit is not exceeded, the energy storage system charges and discharges according to the power command issued by the microgrid controller, and the energy storage system participates in the coordinated control of the microgrid. When the time limit is exceeded, the energy storage system switches to floating charge mode.

[0098] (3) Energy storage system control strategy

[0099] The control block diagram of the energy storage system when the DC microgrid is connected to the grid is as follows: Figure 5 shown.

[0100] 6. Grid-connected converter control strategy

[0101] When connected to the grid, the grid-connected inverter adopts conventional vector control with voltage and current double closed loops. The three controllers of the axis all use PI regulators. The virtual inertia of the grid-connected inverter of this application is to add a simplified VDCM control unit after the voltage loop to improve the inertial support capacity of the DC interface of the grid-connected inverter, and after using the differential compensation VDCM control strategy, the grid-connected inverter's ability to suppress DC bus voltage fluctuations is further improved, and the stable operation capability of the DC microgrid is enhanced.

[0102] 7. Load management strategy within microgrid

[0103] In a DC microgrid, the load that runs for a long time after calculating the loss is called the basic load. So far, there is no unified standard for load classification in the field of microgrids. According to the requirements of power supply reliability, the basic load can be divided into important loads and non-critical loads There are two types. When the power available in the microgrid cannot meet the power consumed by all loads In order to ensure the power supply to important loads as much as possible, non-important loads need to be automatically switched off. Frequent switching near the removal point causes power fluctuations in the microgrid. It is necessary to switch on and off non-important loads according to the lithium battery A hysteresis loop is set to improve the stability of system operation.

[0104] When wind and light sources provide insufficient power and lithium batteries are used When the load switching hysteresis loop sends a cut-off signal, all loads will stop running. For example, when the circuit breaker on the non-important load branch receives a "0" signal, the circuit breaker trips and the load is removed from the microgrid. When the load is reset to the system, the hysteresis loop sends a “1” signal again to reconnect the load to the system.

[0105] 8. Energy coordination control strategy for grid-connected DC microgrid

[0106] (1) Coordinated control strategy of DC microgrid in free mode

[0107] The flow chart of the coordinated control strategy in the DC microgrid free mode is as follows: Figure 8 When the DC microgrid operates in the grid-connected free mode, the wind and light micro-sources operate in the MPPT control mode; the lithium battery energy storage system is charged at a constant current. When the power is off, the energy storage system switches to floating charge mode; the grid-connected inverter operates in constant voltage control mode; all loads in the microgrid are powered. Coordinated control exchanges power with the large power grid according to the power flow inside the microgrid to maintain the DC bus voltage stable.

[0108] (2) Coordinated control strategy for DC microgrid grid-connected dispatching mode

[0109] The flow chart of the coordinated control strategy in the DC microgrid grid-connected dispatching mode is as follows: Fig. 9 When the DC microgrid operates in the grid-connected dispatching mode, the coordinated control responds to the dispatching requirements according to the power flow inside the microgrid.

[0110] (3) Overall control strategy of DC microgrid grid-connected dispatching mode

[0111] It is the switching signal of the photovoltaic system control mode; A switching signal for the control mode of the wind power generation system; It is the switching signal of the control mode of the lithium battery energy storage system; It is the switching signal of important load and non-important load. The coordination controller in the system outputs the switch signal to each system of the microgrid according to the formulated coordination control strategy, so as to realize the smooth switching of each mode.

[0112] 9. Simulation Model

[0113] A DC microgrid simulation model consisting of a photovoltaic power generation system, a permanent magnet direct-drive wind power generation system, a lithium battery energy storage system, a grid-connected converter and a DC load is established through simulation software, and the effectiveness of the designed coordinated control strategy is verified through simulation.

[0114] The photovoltaic power generation system consists of a photovoltaic array and a Boost converter; the wind power generation system consists of a wind turbine, a PMSG and a PWM rectifier; the lithium battery capacity in the energy storage system is , rated voltage , set lithium battery exist Within the range, the energy storage system can charge and discharge according to the command power or charge at a constant current. After exceeding the limit, the operation control of the energy storage system switches to floating charge mode; the rated power consumption of a group of non-critical loads , the rated power consumption of a group of important loads .

[0115] 10. Simulation Results

[0116] (1) Simulation results in grid-connected free mode

[0117] In order to shorten the simulation time, the capacity of the lithium battery was appropriately scaled during the simulation. , the light intensity is , wind and solar power generation units are both operated in MPPT control mode; the initial charge state of the lithium battery , in grid-connected free mode, the battery is charged with constant current, After that, the battery operation control switches to floating charge mode; the power consumption of the load in the network is initially The grid-connected inverter interacts with the large power grid in real time, Stabilize the DC bus voltage when there is a sudden change or battery charging mode change to ensure microgrid power balance.

[0118] (2) Simulation results of grid-connected dispatching mode

[0119] 1) Switch from Mode 5 to Mode 6

[0120] The system initially operates in mode 5.2. The micro-sources are all operating in MPPT control mode, the output power of the photovoltaic unit is 15kW, the output power of the wind power generation system is 5kW, the load power consumption is 18kW, the dispatching power is 10kW, the initial SOC of the lithium battery is 88%, and the battery discharge power is 8kW. At 2 seconds, the light intensity suddenly changes to , the output power of the photovoltaic power generation system suddenly drops to 12kW, then the battery discharge power Increased to 11kW. At 3 seconds, the load consumes power The power suddenly drops from 18kW to 6kW, and the coordinated control switches to mode 6, charging the battery at 1kW. At 4 seconds, the wind speed decreases to , the power generation of the wind power generation system suddenly drops to 4kW, at which time the battery is in standby mode and does not exchange power with the DC bus temporarily. At 5 seconds, the dispatching power is reduced to 5kW, and the battery is charged at 5kW.

[0121] 2) Switch from mode 7 to mode 8

[0122] The system initially operates in mode 7.2. The micro-sources are all running in MPPT control mode, the output power of the photovoltaic unit is 12kW, the output power of the wind power generation system is 5kW, the load power consumption is 18kW, the dispatching power is 8kW, the initial SOC of the lithium battery is 55%, and the battery discharge power is 9kW. At 0.7 seconds, the battery SOC exceeds the limit, and the coordinated control switches to mode 8.1. The microgrid no longer responds to the dispatching power request. The microgrid only supplies power to the load, and the battery discharge power becomes 1kW. At 2 seconds, the light intensity suddenly drops to , the output power of the photovoltaic power generation system suddenly drops to 9kW, then the battery discharge power At 3 seconds, the total power consumed by the load suddenly increases to 24kW, and the battery discharges at a power of 10kW. At 4 seconds, the wind speed decreases to , the wind power generation system power suddenly dropped to 4kW, and the battery discharge power increased to 11kW. At 5.7 seconds, the battery SOC was close to 20%, and the coordinated control switched to mode 8.2, cutting off 12kW of non-important loads in the grid, and the micro-source power supply was given priority to the 12kW important loads, and the remaining power was supplied to the battery, and the lithium battery was charged at a power of 1kW.

[0123] 3) Switch from mode 6 to mode 7

[0124] The system initially operates in mode 6.1. All micro sources operate in MPPT control mode, with an output power of 12kW for the photovoltaic unit, 5kW for the wind power system, 6kW for the load consumption, 5kW for the dispatching power, 85% for the initial SOC of the lithium battery, and 6kW for the battery charging power. Due to the continuous charging of the lithium battery, its SOC rises to 90% at 1s and cannot continue to absorb the surplus power of the microgrid. The coordinated control of the microgrid switches to mode 6.2. At this time, the photovoltaic power generation system switches to the LPTC control mode, and the output power of the photovoltaic power generation system is limited to 6kW. The wind power generation system still operates in the MPPT control mode, and the wind power output power is 5kW. At 3s, the load consumption power in the microgrid suddenly increases to 18kW, and the coordinated control of the microgrid switches to mode 7.2. The photovoltaic power generation unit switches from the LTPC control mode to the MPPT control mode, and the energy storage system switches from the floating charge state to the discharge state. The battery discharge power is 6kW. At 4 seconds, the wind speed decreases to , the wind power generation system power suddenly dropped to 4kW, and the energy storage system discharge power increased to 7kW. At 5 seconds, the dispatching power changed from 5kW to 10kW, and the energy storage system discharge power suddenly increased to 12kW.

[0125] During the simulation process, excluding the losses within the microgrid system, the energy flow within the microgrid is balanced, and the DC bus voltage is stable at 750V under disturbance, verifying that the coordinated control strategy can ensure the safe and stable operation of the DC microgrid under various operating conditions.

[0126] Summarize:

[0127] This invention designs a coordinated control strategy for the wind, solar, and storage DC microgrid connected to the grid. First, the power balance relationship of the DC microgrid is briefly derived, and the different working conditions of the microgrid during the grid-connected operation are analyzed. According to the characteristics of each working condition, the operation process of the microgrid is divided into different operation modes. Secondly, the control strategies of the wind, solar, storage, and grid-connected inverter systems in the microgrid are analyzed, and the load management strategy is designed, and a selection flow chart under different coordinated control modes is given. Finally, a simulation model of the wind, solar, and storage DC microgrid connected to the grid is established, and the working conditions of switching between several operation modes of the microgrid are simulated. The simulation results show that the photovoltaic power generation system, wind power generation system, lithium battery energy storage system, grid-connected inverter, and in-grid load can operate normally and stably as expected, and the DC microgrid can switch smoothly and sensitively between various operation modes under different operating conditions, which verifies the feasibility and effectiveness of the coordinated control strategy designed in this chapter.

[0128] In the description of the present invention, the term "plurality" refers to two or more than two. Unless otherwise clearly defined, the orientation or positional relationship indicated by the terms "upper" and "lower" is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as a limitation on the present invention. The terms "connection", "installation", "fixation", etc. should be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected or indirectly connected through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0129] In the description of the present invention, the description of the terms "one embodiment", "some embodiments", "specific embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In the present invention, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0130] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A coordinated control strategy for a wind-solar-storage DC microgrid, characterized in that: The steps include: S1: Analyze the operation mode of the DC microgrid in the grid-connected free mode and the operation mode of the DC microgrid in the grid-connected dispatching mode respectively; S2: Establishing a grid-connected DC microgrid control model, wherein the grid-connected DC microgrid control model includes a photovoltaic power generation system control model, a wind power generation system control model, an energy storage system control model, a grid-connected converter control model, a microgrid internal load management control model, and an energy coordination control model of a grid-connected DC microgrid; S3: Establish a simulation model for the wind-solar-storage DC microgrid connected to the grid and define the discharge power of the energy storage battery , maximum discharge power , charging power , maximum charging power ; Battery state of charge upper limit , battery state of charge lower limit ; The difference between the micro-source power and the load power is the micro-grid net power ,pass , Battery power With the command power value The relative relationship between the state of charge and the grid-connected dispatching operation mode jointly determines the coordinated control strategy. and When the microgrid is in operation, it can respond to the dispatching request for part or all of the grid-connected power according to the working status of the energy storage unit in the grid; Mode 6, when and In addition to meeting the dispatching requirements, the microgrid can charge the energy storage unit in the grid; when the lithium battery SOC exceeds the upper limit, if it continues to be charged, the battery will be damaged due to overcharging; when the energy storage unit enters the floating charge mode due to charging, the microgrid needs to limit the power generation of the micro source according to the net power value in the grid; considering that the long-term operation of rotating mechanical equipment under the condition of frequent changes in speed will affect its service life, the power generation of the photovoltaic unit is limited first in the micro source LPTC control mode, and the wind power generation system is still working in the MPPT control mode at this time; if the photovoltaic unit stops generating power, the microgrid , then continue to limit the power generation of wind power generation system; Mode 7, when and When the power generated by the micro-source cannot meet the power supply demand of the load; Energy storage has a certain capacity to respond to the remaining load demand and part or all of the grid-connected power required by the dispatcher; Mode 8, when and When the micro-sources are running in MPPT control mode, the microgrid does not respond to the dispatching requirements, and the energy storage battery only supplies power to the load; at this time, the energy storage unit discharges beyond the limit. If the battery charge state is close to If not managed, the lithium battery will be over-discharged, which will seriously affect the battery life. When the non-important loads in the microgrid need to be removed If removed After that, the power generated by micro-sources still cannot meet the needs of important loads. If the power supply demand is met, it is necessary to remove the important loads in the microgrid. , use the surplus power to charge the energy storage; by setting the SOC hysteresis loop for load switching, frequent load switching is prevented to improve the stability of microgrid operation.

2. A coordinated control strategy for a wind-solar-storage DC microgrid according to claim 1, characterized in that: The photovoltaic power generation system control model adopts two control modes, MPPT and LPTC, to cope with the working conditions in the DC microgrid; Among them, when there is power surplus in the microgrid and the energy storage system and the power grid cannot fully absorb it, the photovoltaic power generation system virtually switches from the MPPT control mode to the LPTC control mode to ensure power balance in the microgrid.

3. The coordinated control strategy of a wind-solar-storage DC microgrid according to claim 1 is characterized in that: In the wind power generation system control model, when the photovoltaic power generation unit cuts off all power in the LPTC control mode, if the power generation in the microgrid Still greater than the load power consumption , Allowable access power Energy storage absorption power When the three are combined, the wind power system also needs to switch from MPPT control mode to LPTC control mode to limit the power generation of the wind power system and ensure power balance within the microgrid.

4. The coordinated control strategy of a wind-solar-storage DC microgrid according to claim 1 is characterized in that: The energy storage system control model includes a grid-connected free mode and a grid-connected dispatching mode; In the grid-connected free mode, the energy storage system is charged at a constant current of 0.5C. When the lithium battery SOC exceeds 90%, the battery is changed to trickle charging to compensate for the power loss of battery self-discharge. In the grid-connected dispatching mode, when the lithium battery When the limit is not exceeded, the energy storage system charges and discharges according to the power command issued by the microgrid controller. The energy storage system participates in the coordinated control of the microgrid. When the time limit is exceeded, the energy storage system switches to floating charge mode.

5. The coordinated control strategy of a wind-solar-storage DC microgrid according to claim 1 is characterized in that: The grid-connected converter control model adopts conventional vector control with voltage and current double closed loops.

6. The coordinated control strategy of a wind-solar-storage DC microgrid according to claim 1 is characterized in that: The energy coordination control model of the grid-connected DC microgrid includes a grid-connected free mode and a grid-connected dispatching mode; In the grid-connected free mode, both wind and light micro-sources operate in MPPT control mode; the lithium battery energy storage system is charged at a constant current. When the power is turned off, the energy storage system switches to floating charge mode; the grid-connected inverter operates in constant voltage control mode; all loads in the microgrid are powered; coordinated control is performed based on the power flow inside the microgrid and the power exchange with the large power grid to maintain the DC bus voltage stable; In the grid-connected dispatch mode, coordinated control responds to dispatch requirements according to the power flow inside the microgrid.

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