Direct-current micro-grid control method in collaborative cooperation mode

Abstract

The invention relates to a DC micro-grid control method in a cooperation mode. The DC micro-grid control method comprises the following steps: step 1, providing a DC micro-grid photovoltaic power generation system control strategy; step 2, constructing a direct current micro-grid system; and step 3, constructing a simulation model according to the DC micro-grid system established in the step 2, and verifying whether the control strategy in the step 1 is feasible or not. According to the invention, the whole DC micro-grid system can be controlled.

Description

Technical Field

[0001] The present invention belongs to the field of energy technology and relates to a direct current microgrid control method, in particular to a direct current microgrid control method in a coordinated cooperation mode. Background Art

[0002] As we all know, energy has played an extremely important role in the development of human society. The rapid development of society in recent years is inseparable from the development of energy-related technologies. At present, fossil energy is the most widely used energy in society, but what is worrying is that fossil energy is basically a non-renewable energy source. As the process of human development continues to accelerate, energy consumption is also increasing, which has led to a continuous decrease in the global reserves of non-renewable energy. At the same time, the problem of environmental damage in the process of using fossil energy is becoming increasingly acute. Energy shortages and increasingly severe ecological environment have become the focus of attention of countries around the world.

[0003] Because of these unavoidable shortcomings of fossil energy, the calls for protecting the environment and saving energy are growing louder, and clean, renewable distributed power generation technology is receiving more and more attention. With its own advantages such as little damage to the environment, flexible and changeable power generation forms, and low construction costs, it has attracted more and more attention in the hope of replacing fossil energy.

[0004] Distributed generation uses modular power generation units to generate electricity. The power generation site is generally close to the user, which can avoid long-distance transmission, reduce power loss, and improve the reliability of power transmission. Compared with centralized power generation that requires long-distance transmission, it can better guarantee the quality of power and the power needs of users. However, everything has two sides. The power generated by distributed generation is easily affected by environmental changes, and a system may use more than one module to generate electricity. The overall control between different power generation modules is also an issue that needs to be considered. Many experts have proposed the concept of microgrids to address the shortcomings of distributed generation.

[0005] In addition to distributed power sources, microgrids are also equipped with energy storage devices. The role of energy storage devices is to reduce the impact of fluctuating power on the stable operation of the system, neutralize these fluctuating powers, and maintain the power of the system in a balanced state. Microgrids are integrated systems that can control the generation, distribution, and consumption of power. Microgrids can operate in isolation in some cases, or multiple microgrids can work together in the overall dispatching control strategy, so the control strategy of DC microgrids is particularly important.

[0006] The current mainstream control strategy of DC microgrid is the control strategy based on droop characteristics, which is a control method that simulates the primary frequency adjustment of traditional power systems and is achieved by controlling power electronic inverters. In DC microgrids, droop control is usually used to adjust the output voltage and current of distributed generation (DG) to achieve power distribution and stable operation of the system.

[0007] However, the existing DC microgrid control method based on droop characteristics still has the defect and deficiency that droop control has high requirements on the controller, and is mainly applicable to the case where the inductance of the grid is much larger than the resistance, and its applicability is limited in other cases.

[0008] After searching, no public documents of the prior art that are identical or similar to the present invention were found. Summary of the invention

[0009] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a DC microgrid control method in a coordinated manner, which has relatively smaller requirements on the controller and a more comprehensive and concise control strategy. In addition, the design of the DC microgrid control strategy in the coordinated manner of each part studied in this paper can meet the requirements of the DC microgrid system to operate smoothly in both grid-connected and islanded conditions.

[0010] The present invention solves the practical problem by adopting the following technical solutions:

[0011] A DC microgrid control method in a coordinated manner includes the following steps:

[0012] Step 1: Propose a control strategy for a DC microgrid photovoltaic power generation system;

[0013] Step 2: Build a DC microgrid system;

[0014] Step 3: Build a simulation model based on the DC microgrid system established in step 2 to verify whether the control strategy in step 1 is feasible.

[0015] Moreover, the DC microgrid photovoltaic power generation system control strategy of step 1 includes: photovoltaic power generation system control strategy, energy storage system control strategy and overall control strategy;

[0016] Moreover, the photovoltaic power generation system control strategy of step 1 includes: maximum power control and constant voltage control; wherein the maximum power control is applicable to the case where the power generated by the photovoltaic power generation system is less than the power required by the system, and the constant voltage control is applicable to the case where the power generated by the photovoltaic power generation system is greater than the power required by the system;

[0017] Moreover, the maximum power control method is:

[0018] The output voltage of the photovoltaic cell is randomly changed by an amount, and then the output current and output voltage of the photovoltaic cell are sampled in real time, and then the power is calculated and compared with the power in the previous state: if the power is smaller than the power in the previous state, it means that the control this time makes the output power decrease, so the output voltage of the photovoltaic cell is changed by an opposite amount; if the power is larger than the power in the previous state, it means that the control this time makes the output power increase, so the output voltage of the photovoltaic cell maintains the same control direction. Repeated disturbance observation and comparison can achieve the maximum output power of the photovoltaic cell at all times.

[0019] Moreover, the specific method of the constant pressure control is:

[0020] The output voltage of the photovoltaic power generation system is controlled to maintain it at a constant level; this is usually achieved through a closed-loop control system. The DC bus voltage is measured by a voltage sensor, and the measured DC bus voltage is compared with a preset DC voltage reference value. Based on the comparison result, the controller generates a current reference value, which is used to guide the output current of the photovoltaic power generation system. By adjusting the power electronic devices in the photovoltaic system, the output current of the photovoltaic system reaches the preset current reference value, thereby achieving constant voltage control.

[0021] Moreover, the energy storage system control strategy of step 1 includes: charging control and discharging control;

[0022] The above charging control includes: constant current charging and constant voltage charging; wherein, the constant current charging adopts single closed-loop current control, by collecting the actual charging current of the battery and comparing it with the set reference charging current value, and then passing the difference through the PI link to obtain the duty cycle signal; wherein, the constant voltage charging adopts the voltage outer loop and current inner loop method for control. The voltage outer loop compares the collected voltage at both ends of the energy storage battery with the reference voltage value, sends the obtained difference into the PI link, and enters the inner loop current loop after appropriate limiting, and uses this value as the reference value and the charging current of the energy storage battery to make a difference, and outputs the difference through the PI link to form a duty cycle signal.

[0023] The discharge control method comprises:

[0024] The discharge control method is similar to the constant voltage control method, which controls the output voltage of the battery to keep it at a constant level; it is usually implemented through a closed-loop control system, in which the battery voltage is measured by a voltage sensor, and the measured battery voltage is compared with a preset voltage reference value. According to the comparison result, the controller generates a current reference value, which is used to guide the output current of the battery. By adjusting the power electronic devices in the photovoltaic system, the output current of the battery reaches the preset current reference value, thereby realizing discharge control.

[0025] Moreover, the overall control strategy of step 1 includes:

[0026] (1) The voltage of the DC bus in the DC microgrid is classified into multiple working modes:

[0027] The normal voltage fluctuation range of the DC bus in the DC microgrid is ±2%, and the maximum allowable voltage fluctuation is ±5%. The voltage classification is as follows: +5%, +2%, -2%, -5%;

[0028] It is divided into 5 modes: Mode 1: -2% to +2%, Mode 2: +2% to +5%, Mode 3: -5% to -2%, Mode 4: bus voltage greater than +5%, Mode 5: bus voltage less than -5%.

[0029] (2) The grid operation mode is divided into two operation modes: off-grid operation and grid-connected operation. According to the working modes divided in step (1), the working states of the photovoltaic power generation system, the energy storage system and the load under the corresponding working mode are obtained.

[0030] Moreover, the DC microgrid system of step 2 includes: a photovoltaic power generation system, an energy storage system, a large power grid, a load and a DC bus with a voltage level of 1kV; the output end of the photovoltaic power generation system is connected to the 1kV DC bus through a DC / DC module, which is used to access the DC bus through a DC / DC conversion circuit to power the photovoltaic power generation system; the energy storage system is connected to the DC bus through a bidirectional DC / DC conversion circuit to power the system or absorb excess electric energy; the large power grid is connected to the DC bus through a bidirectional DC / AC conversion circuit for energy flow between the DC microgrid; the input end of the load is connected to the DC bus through a DC / DC converter to obtain electric energy.

[0031] Moreover, the simulation model of step 3 includes: a battery constant current charging simulation circuit model, a battery constant voltage charging simulation circuit model and a battery constant voltage discharging simulation circuit model;

[0032] The battery constant current charging simulation circuit model includes: a reference current Current_ref_change_I generation link, a current input current and a reference current current_ref_change_I are negatively feedback regulated to ultimately achieve the purpose of constant current charging;

[0033] The battery constant voltage charging simulation circuit model adopts a voltage outer loop and a current inner loop double closed loop control, the input voltage Ui and the reference value are subjected to voltage outer loop control to generate a reference current value Current_ref_change_U, and then Current_ref_change_U and the input current Current are subjected to current inner loop control to finally form voltage constant voltage charging;

[0034] The battery constant voltage discharge simulation circuit model includes: the principle used is similar to the constant voltage charging, the bus voltage U is collected, and the reference value 1000 is compared and controlled in the outer loop to form a reference current Current_ref_dis, and then Current_ref_dis and the input current Current are controlled in the current inner loop to finally form a voltage constant voltage discharge.

[0035] Advantages and beneficial effects of the present invention:

[0036] The present invention provides a DC microgrid control method in a coordinated cooperation mode, which takes the system voltage as the standard and realizes the control of the entire DC microgrid system through the coordinated cooperation of various parts in the microgrid. The present invention has relatively small requirements on the system controller, a relatively comprehensive control strategy, and can allow the DC microgrid system to operate correctly and reasonably regardless of whether it is off-grid or on-grid according to different voltage conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is the algorithm flow chart of the MPPT disturbance observation method of the present invention;

[0038] Figure 2 It is a constant current charging control diagram of the present invention;

[0039] Figure 3 is a constant voltage charging control diagram of the present invention;

[0040] Figure 4 is a voltage classification diagram of the present invention;

[0041] Figure 5 It is a topological architecture diagram of a DC microgrid of the present invention;

[0042] Figure 6 It is a diagram of equivalent circuit models of various photovoltaic cells of the present invention;

[0043] Figure 7 is an output characteristic curve diagram of a photovoltaic cell of the present invention;

[0044] Figure 8 It is a battery constant current charging simulation circuit diagram of the present invention;

[0045] Fig. 9 It is a simulation result diagram of constant current charging of a storage battery of the present invention;

[0046] Fig.10 It is a battery constant voltage charging simulation circuit diagram of the present invention;

[0047] Fig.11 This is a simulation result diagram of constant voltage charging of a battery according to the present invention;

[0048] Fig.12 It is a battery constant voltage discharge simulation circuit diagram of the present invention;

[0049] Fig.13 It is a diagram of the simulation result of constant voltage discharge of the battery of the present invention. DETAILED DESCRIPTION

[0050] The embodiments of the present invention are further described in detail below with reference to the accompanying drawings:

[0051] A DC microgrid control method in a coordinated manner includes the following steps:

[0052] Step 1: Propose a control strategy for a DC microgrid photovoltaic power generation system;

[0053] The DC microgrid photovoltaic power generation system control strategy of step 1 includes: photovoltaic power generation system control strategy, energy storage system control strategy and overall control strategy;

[0054] The photovoltaic power generation system control strategy of step 1 includes: maximum power control and constant voltage control; wherein, the maximum power control is applicable to the case where the power generated by the photovoltaic power generation system is less than the power required by the system, and the constant voltage control is applicable to the case where the power generated by the photovoltaic power generation system is greater than the power required by the system;

[0055] The maximum power control method is:

[0056] The output voltage of the photovoltaic cell is randomly changed by an amount, and then the output current and output voltage of the photovoltaic cell are sampled in real time, and then the power is calculated and compared with the power in the previous state: if the power is smaller than the power in the previous state, it means that the control this time makes the output power decrease, so the output voltage of the photovoltaic cell is changed by an opposite amount; if the power is larger than the power in the previous state, it means that the control this time makes the output power increase, so the output voltage of the photovoltaic cell maintains the same control direction. Repeated disturbance observation and comparison can achieve the maximum output power of the photovoltaic cell at all times.

[0057] The specific method of the constant pressure control is:

[0058] The output voltage of the photovoltaic power generation system is controlled to maintain it at a constant level; this is usually achieved through a closed-loop control system. The DC bus voltage is measured by a voltage sensor, and the measured DC bus voltage is compared with a preset DC voltage reference value. Based on the comparison result, the controller generates a current reference value, which is used to guide the output current of the photovoltaic power generation system. By adjusting the power electronic devices in the photovoltaic system, the output current of the photovoltaic system reaches the preset current reference value, thereby achieving constant voltage control.

[0059] The energy storage system control strategy of step 1 includes: charging control and discharging control;

[0060] The above charging control includes: constant current charging and constant voltage charging; wherein, the constant current charging adopts single closed-loop current control, by collecting the actual charging current of the battery and comparing it with the set reference charging current value, and then passing the difference through the PI link to obtain the duty cycle signal; wherein, the constant voltage charging adopts the voltage outer loop and current inner loop method for control. The voltage outer loop compares the collected voltage at both ends of the energy storage battery with the reference voltage value, sends the obtained difference into the PI link, and enters the inner loop current loop after appropriate limiting, and uses this value as the reference value and the charging current of the energy storage battery to make a difference, and outputs the difference through the PI link to form a duty cycle signal.

[0061] The discharge control method comprises:

[0062] The discharge control method is similar to the constant voltage control method, which controls the output voltage of the battery to keep it at a constant level; it is usually implemented through a closed-loop control system, in which the battery voltage is measured by a voltage sensor, and the measured battery voltage is compared with a preset voltage reference value. According to the comparison result, the controller generates a current reference value, which is used to guide the output current of the battery. By adjusting the power electronic devices in the photovoltaic system, the output current of the battery reaches the preset current reference value, thereby realizing discharge control.

[0063] The overall control strategy of step 1 includes the following steps:

[0064] (1) The voltage of the DC bus in the DC microgrid is classified into multiple working modes:

[0065] The normal voltage fluctuation range of the DC bus in the DC microgrid is ±2%, and the maximum allowable voltage fluctuation is ±5%. The voltage classification is as follows: +5%, +2%, -2%, -5%;

[0066] It is divided into 5 modes: Mode 1: -2% to +2%, Mode 2: +2% to +5%, Mode 3: -5% to -2%, Mode 4: bus voltage greater than +5%, Mode 5: bus voltage less than -5%.

[0067] (2) The grid operation mode is divided into two operation modes: off-grid operation and grid-connected operation. According to the working modes divided in step (1), the working states of the photovoltaic power generation system, the energy storage system and the load under the corresponding working mode are obtained.

[0068] In this embodiment, there are two control strategies for the photovoltaic power generation system, one is maximum power control (MPPT control), and the other is constant voltage control. Maximum power control is applicable to the case where the power generated by the photovoltaic power generation system is less than the power required by the system, and constant voltage control is applicable to the case where the power generated by the photovoltaic power generation system is greater than the power required by the system. The maximum power control is implemented by the perturbation observation method, and the constant voltage control adopts the control strategy of the voltage outer loop and the current inner loop.

[0069] (1) Maximum power control: Photovoltaic cells are affected by natural conditions such as ambient temperature and light intensity, so the output power of photovoltaic cells is very unstable. In order to ensure the maximum power generation efficiency, maximum power control (MPPT control) is required. Without maximum power tracking technology, the output power of the battery module cannot reach the optimal (maximum) value under any circumstances, which reduces the utilization rate of the solar cell module and improves the economy of the system.

[0070] The methods for implementing maximum power control include conductivity increment method, perturbation observation method, and neural network control method. The most commonly used method is the perturbation observation method, because this method has a simple structure, requires few measurement parameters, and is easy to implement. Of course, everything has two sides, and its inevitable disadvantage is that because the output voltage is constantly changing in the perturbation observation method, the system cannot stabilize at the maximum power point, and will oscillate back and forth within a very small range at the maximum power point. The oscillation range depends on the step size, but this error is acceptable.

[0071] Its working principle is to first introduce a small change, compare it with the previous state, and continuously adjust the output working condition of the photovoltaic cell through the comparison result. The specific operation method is: arbitrarily change the output voltage of the photovoltaic cell by an amount, and then sample the output current and output voltage of the photovoltaic cell in real time, and then calculate the power and compare it with the power in the previous state: if it is smaller than the power in the previous state, it means that this control has reduced the output power, so let the output voltage of the photovoltaic cell change by an opposite amount; if it is larger than the power in the previous state, it means that this control has increased the output power, so let the output voltage of the photovoltaic cell maintain the same control direction. By repeatedly performing disturbance observation and comparison in this way, the purpose of making the output power of the photovoltaic cell always at the maximum value can be achieved. The specific flow chart is as follows Figure 1As shown: collect voltage U(k) and I(k) in real time, calculate power P(k), and compare it with the previously collected power P(k-1). If P(k)>P(k-1), do not change the input voltage reference value. If P(k)<P(k-1), compare the magnitude relationship between the two voltages U(k) and U(k-1). If U(k)>U(k-1), reduce the reference value of the input voltage, that is, Uref-△U, otherwise increase it. Similarly, we can get the situation of U(k)<U(k-1).

[0072] (2) Constant pressure control

[0073] Under normal circumstances, in order to make photovoltaic cells work at maximum efficiency and obtain higher economic benefits, maximum power control is often used. However, in special cases where the power required by the system is less than the maximum power output of the photovoltaic system, if the power load in the DC microgrid suddenly decreases due to some reason during island operation, the energy storage device cannot be adjusted due to insufficient capacity, or a fault occurs during grid-connected operation, and the PV photovoltaic power generation system cannot send power to the grid, the photovoltaic system should enter the constant voltage mode, which can maintain the system voltage stability. At this time, the DC bus voltage value should be the reference value of the photovoltaic system output voltage, thereby ensuring the stability of the DC bus voltage. The specific operation method is: the core of the constant voltage control principle is to control the output voltage of the photovoltaic system to keep it at a constant level. This is usually achieved through a closed-loop control system. The DC bus voltage is measured by a voltage sensor, and the measured DC bus voltage is compared with the preset DC voltage reference value. According to the comparison result, the controller generates a current reference value, which is used to guide the output current of the photovoltaic system. By adjusting the power electronic devices in the photovoltaic system, the output current of the photovoltaic system reaches the preset current reference value, thereby achieving constant voltage control.

[0074] In this embodiment, the energy storage system control strategy design includes:

[0075] (1) Charging control: The principle of charging control is fast and safe, that is, charging as quickly as possible while ensuring that the charging current is limited to a safe range and preventing damage to the battery due to excessive voltage or current values.

[0076] ① Constant current charging: In order to achieve the above-mentioned purpose, the present invention takes the state of charge SOC as the research object. When the SOC is less than 85%, constant current charging is adopted. This is because the battery power is low at this time, and the numerical difference between the voltage at both ends of the battery and the charging voltage is large, which will generate a large charging current. At the least, it will affect the battery life, and at the worst, it will directly scrap the battery. Therefore, when the SOC is less than 85%, constant current charging is adopted, and the charging current is the maximum charging current allowed by the energy storage device. This can ensure rapid charging without damaging the battery. Implementation method: Constant current charging adopts single closed-loop current control, by collecting the actual charging current Ib of the battery and comparing it with the set reference charging current value Icb-ref, and then the difference is passed through the PI link to obtain the duty cycle signal, such as Figure 2 shown.

[0077] ② Constant voltage charging: When SOC is greater than or equal to 85%, constant voltage charging is used. This is because the energy storage battery has a high power level at this time, and the voltage across the battery and the charging voltage are basically the same. If the constant current charging method is continued at this time, it may cause the battery body to be overcharged and affect the battery life. Therefore, constant voltage charging is used at this time until the battery is fully charged. Implementation method: Constant voltage charging is controlled by the voltage outer loop and current inner loop method. The voltage outer loop is to collect the voltage V at both ends of the energy storage battery. cb With reference voltage V cb-ref The difference is compared and sent to the PI link. After appropriate limiting, it enters the inner current loop and takes this value as the reference current value I ref The charging current I of the energy storage battery b Make a difference, and output the difference through the PI link to form a duty cycle signal, such as Figure 3 shown.

[0078] (2) Discharge control: The principle is to ensure bus voltage stability under certain conditions (the energy stored in the energy storage device is greater than the required energy). The control strategy is the same as the constant voltage control of the photovoltaic power generation system.

[0079] 3. Overall control strategy

[0080] Compared with AC microgrid, DC microgrid has no frequency and phase. The only indicator of whether the system power is balanced is the bus voltage. As long as the voltage is within the normal fluctuation range, the system power is balanced and the entire system is stable. The DC bus voltage level in the DC microgrid is U0, and the normal voltage fluctuation range is ±2% (that is, U in the figure). H1 with U L1 ), the maximum allowable voltage fluctuation is ±5%

[16] (That is, U H2 with U L2 ), voltage classification as Figure 4 shown.

[0081] It is divided into 5 modes: Mode 1: -2% to +2%, Mode 2: +2% to +5%, Mode 3: -5% to -2%, Mode 4: bus voltage greater than +5%, Mode 5: bus voltage less than -5%.

[0082] The grid operation mode is divided into two modes: off-grid operation and grid-connected operation:

[0083] ① Off-grid state: The PV photovoltaic power generation system, energy storage system and load together form a small off-grid power grid with balanced power and stability. The maintenance of the DC microgrid bus voltage mainly depends on the photovoltaic power generation system and energy storage system. If the power generated by the photovoltaic power generation system and energy storage system is not enough to meet the needs of the load, then it is necessary to cut off the unimportant load to ensure the power supply to the important load. The working status of each part of each mode under off-grid operation is shown in Table 1:

[0084] Table 1 Operation status of each part in off-grid state

[0085]

[0086]

[0087] ②Grid-connected state: On the basis of off-grid operation, the entire system is connected to the large power grid to achieve grid-connected operation. The large power grid is an infinite battery relative to the DC microgrid. Therefore, when controlling the bus voltage to be stable, it is only necessary to control the converter between the microgrid and the large power grid to adjust the bus voltage, without the need to control the photovoltaic power generation system and the energy storage system separately. Under normal circumstances, as long as grid connection is allowed, energy can be transmitted to the large power grid. The photovoltaic power generation system can output the maximum power and deliver the electricity to the grid for sale to obtain economic benefits. The working status of each part of each mode under grid-connected operation is shown in Table 2:

[0088] Table 2 Operation status of each part in grid-connected state

[0089]

[0090] Step 2: Build a DC microgrid system;

[0091] The DC microgrid system of step 2 includes: a photovoltaic power generation system, an energy storage system, a large power grid, a load and a DC bus with a voltage level of 1 kV;

[0092] In this embodiment, the DC microgrid topology is designed as follows:

[0093] In the study of DC microgrids, radial topology is widely used in DC microgrids. The present invention also adopts this radial topology, as follows: Figure 5As shown: the direction of the arrow in the figure is the direction of power flow. The DC microgrid in the present invention is composed of a photovoltaic power generation system, an energy storage system, a large power grid, a load and a DC bus with a voltage level of 1kV. The output end of the photovoltaic power generation system is connected to the 1kV DC bus through a DC / DC module, which is used to access the DC bus through a DC / DC conversion circuit to power the photovoltaic power generation system; the energy storage system is connected to the DC bus through a bidirectional DC / DC conversion circuit to power the system or absorb excess electric energy; the large power grid is connected to the DC bus through a bidirectional DC / AC conversion circuit for energy flow between the DC microgrid; the input end of the load is connected to the DC bus through a DC / DC converter to obtain electric energy. Each part needs to use conversion circuits to realize the conversion of energy between different voltage levels. The photovoltaic power generation system and the energy storage system are both connected to the DC bus through DC / DC conversion circuits, but the conversion circuit of the energy storage system is bidirectional, and power can flow bidirectionally between the energy storage system and the DC bus. The large power grid is connected to the DC bus through a bidirectional DC / AC conversion circuit, and power can flow bidirectionally between the large power grid and the DC bus. The load is connected to the DC bus through a DC / DC converter, and power can only flow from the DC bus to the load.

[0094] In this embodiment, PV photovoltaic power generation system design: There are currently three main mathematical models of PV photovoltaic cells: Figure 6 Equivalent circuit diagrams of three models: Model 1 is an ideal circuit model, that is, the battery is equivalent to a current source Iph, Model 2 takes into account the internal resistance Rsh of the photovoltaic, and Model 3 takes into account the external resistance Rs and internal resistance Rsh of the battery. The accuracy and complexity of the model are increasing. The present invention adopts a more accurate model 3, as follows Figure 6 shown.

[0095] The output characteristics of photovoltaic cells mainly refer to the relationship between output voltage and output power. The relationship between the output power and terminal voltage of photovoltaic cells is not a linear relationship. Before approaching the open circuit voltage, the output power increases with the increase of output voltage. After exceeding the open circuit voltage, the output power decreases with the increase of output voltage. It has a maximum power point, such as Figure 7 This is why there is maximum power control in photovoltaic power generation systems.

[0096] In this embodiment, the energy storage system is designed by using battery simulation. The battery is the most widely used energy storage device and energy storage technology.

[0097] Step 3: Build a simulation model based on the DC microgrid system established in step 2 to verify whether the control strategy in step 1 is feasible;

[0098] The simulation model of step 3 includes: a battery constant current charging simulation circuit model, a battery constant voltage charging simulation circuit model and a battery constant voltage discharging simulation circuit model;

[0099] The battery constant current charging simulation circuit model includes: a reference current Current_ref_change_I generation link, a current input current and a reference current current_ref_change_I are negatively feedback regulated, and finally the purpose of constant current charging is achieved, such as Figure 8 shown.

[0100] The battery constant voltage charging simulation circuit model adopts voltage outer loop and current inner loop double closed loop control. The input voltage Ui and the reference value are controlled by the voltage outer loop to generate the reference current value Current_ref_change_U. Then, Current_ref_change_U and the input current Current are controlled by the current inner loop to finally form a voltage constant voltage charging. Fig.10 shown.

[0101] The battery constant voltage discharge simulation circuit model includes: the principle used is similar to the constant voltage charging, the bus voltage U is collected, and the reference value 1000 is compared and controlled in the outer loop to form a reference current Current_ref_dis, and then Current_ref_dis and the input current Current are controlled in the current inner loop to finally form a voltage constant voltage discharge, such as Fig.12 shown.

[0102] In this embodiment, a simulation model is established: Simulink is used to perform simulation verification on the above model.

[0103] ①Constant current charging

[0104] When the battery SOC is less than 85%, constant current charging is adopted, and the charging current is the maximum charging current of the battery, 5A. This has high charging efficiency and fast charging speed. Fig. 9 shown.

[0105] It can be seen that the charging current has been charging at a maximum value of 5A (lower left figure, negative numbers indicate charging), and the battery SOC (upper left) and terminal voltage (upper right) have been rising.

[0106] ②Constant voltage charging

[0107] When the battery SOC≧85%, constant voltage charging is adopted. The constant voltage value is 409V, the battery voltage when SOC=95%. Such charging can prevent the battery from being overcharged.

[0108] The constant voltage charging simulation results are as follows Fig.11 shown.

[0109] The simulation is of the battery charging process when SOC=90. Because the voltage across the battery U_barry (lower right) is close to the given charging voltage 409V, the charging current Current is very small (lower left), which can prevent the battery from overcharging.

[0110] ③Constant voltage discharge

[0111] The battery discharge adopts constant voltage discharge. When the system power is insufficient, the battery discharges to ensure power balance and make the bus voltage normal.

[0112] The constant voltage discharge simulation results are as follows Fig.13 shown.

[0113] In the absence of power supply, the battery discharge can maintain the bus voltage U_load at 1kV to ensure power consumption of the load.

[0114] Through the above simulation experiments, we can see that the control strategy studied in the present invention can have a good control effect on the DC microgrid system.

[0115] The working principle of the present invention is:

[0116] With the development of the times, energy shortage and environmental pollution issues have become more and more concerned by people. Renewable distributed energy provides a solution to the problems of energy shortage and environmental pollution, but its own defects of instability, high cost and difficult control have always restricted its development. With the application of DC microgrid technology, these problems have been well solved. At present, in the research on microgrids, DC microgrids have gradually become the focus of everyone's attention due to their advantages such as small power fluctuations, small energy loss, and easy and flexible control.

[0117] In this context, the present invention takes the DC microgrid as the research object, designs a DC microgrid control method in which each part cooperates with each other, takes the voltage on the DC bus as the control object, and combines constant current charging, constant voltage charging, and constant voltage discharge control of the energy storage system, ultimately realizing the control of the DC microgrid system.

[0118] The present invention also takes the DC microgrid control strategy as the research object and builds a DC microgrid system, including a photovoltaic power generation system, a battery energy storage system, a load, a large power grid and the conversion circuits required for connecting their various parts with the DC bus. After the modeling of each part is completed, the PV and battery energy storage control strategy design is determined. Finally, the designed DC microgrid and control strategy are simulated and analyzed through Simulink simulation software to verify the correctness of the designed PV system control strategy, energy storage system control strategy and DC microgrid.

[0119] In the control strategies of each part, the photovoltaic power generation system adopts maximum power control and constant voltage control realized by the disturbance observation method, the energy storage system adopts constant current charging, constant voltage charging, and constant voltage discharging based on the battery state of charge SOC (State of Change) to meet the requirements of fast and safe charging, and the large power grid adopts constant voltage control. The overall control strategy determines which control strategy to adopt for each part based on factors such as whether the system is grid-connected and the bus voltage, and performs voltage mode control, and then combines the coordinated control of each module to finally achieve the requirements for DC microgrid control.

[0120] It should be emphasized that the embodiments of the present invention are illustrative rather than restrictive. Therefore, the present invention includes but is not limited to the embodiments described in the specific implementation modes. Any other implementation modes derived by those skilled in the art based on the technical solutions of the present invention also fall within the scope of protection of the present invention.

Claims

1. A DC microgrid control method in a coordinated manner, characterized by: The following steps are involved: Step 1: Propose a control strategy for a DC microgrid photovoltaic power generation system; Step 2: Build a DC microgrid system; Step 3: Build a simulation model based on the DC microgrid system established in step 2 to verify whether the control strategy in step 1 is feasible.

2. The DC microgrid control method in a coordinated cooperation mode according to claim 1, characterized in that: The DC microgrid photovoltaic power generation system control strategy of step 1 includes: a photovoltaic power generation system control strategy, an energy storage system control strategy and an overall control strategy.

3. The DC microgrid control method in a coordinated cooperation mode according to claim 2 is characterized in that: The photovoltaic power generation system control strategy of step 1 includes: maximum power control and constant voltage control; wherein, the maximum power control is applicable to the situation where the power generated by the photovoltaic power generation system is less than the power required by the system, and the constant voltage control is applicable to the situation where the power generated by the photovoltaic power generation system is greater than the power required by the system.

4. The DC microgrid control method in a coordinated cooperation mode according to claim 3 is characterized in that: The maximum power control method is: The output voltage of the photovoltaic cell is changed by an amount at random, and then the output current and output voltage of the photovoltaic cell are sampled in real time, and then the power is calculated and compared with the power in the previous state: if the power is smaller than the power in the previous state, it means that this control has reduced the output power, so the output voltage of the photovoltaic cell is changed by an opposite amount; if the power is larger than the power in the previous state, it means that this control has increased the output power, so the output voltage of the photovoltaic cell maintains the same control direction; in this way, the disturbance observation and comparison are repeated to achieve the goal of making the output power of the photovoltaic cell at the maximum value at all times.

5. The DC microgrid control method in a coordinated cooperation mode according to claim 3 is characterized in that: The specific method of the constant pressure control is: The output voltage of the photovoltaic power generation system is controlled to maintain it at a constant level; this is usually achieved through a closed-loop control system. The DC bus voltage is measured by a voltage sensor, and the measured DC bus voltage is compared with a preset DC voltage reference value. Based on the comparison result, the controller generates a current reference value, which is used to guide the output current of the photovoltaic power generation system. By adjusting the power electronic devices in the photovoltaic system, the output current of the photovoltaic system reaches the preset current reference value, thereby achieving constant voltage control.

6. The DC microgrid control method in a coordinated cooperation mode according to claim 2 is characterized in that: The energy storage system control strategy of step 1 includes: charging control and discharging control; The charging control includes: constant current charging and constant voltage charging; wherein, the constant current charging adopts single closed loop current control, by collecting the actual charging current of the battery and comparing it with the set reference charging current value, and then passing the difference through the PI link to obtain the duty cycle signal; wherein, the constant voltage charging adopts the voltage outer loop and current inner loop method for control; the voltage outer loop compares the collected voltage at both ends of the energy storage battery with the reference voltage value, sends the obtained difference into the PI link, and enters the inner loop current loop after appropriate limiting, and uses this value as the reference value and the charging current of the energy storage battery to make a difference, and outputs the difference through the PI link to form a duty cycle signal; The discharge control method includes: the discharge control method is similar to the constant voltage control method, by controlling the output voltage of the battery to keep it at a constant level; it is usually implemented through a closed-loop control system, the battery voltage is measured by a voltage sensor, the measured battery voltage is compared with a preset voltage reference value, and according to the comparison result, the controller generates a current reference value, which is used to guide the output current of the battery, and the output current of the battery reaches the preset current reference value by adjusting the power electronic devices in the photovoltaic system, thereby realizing discharge control.

7. The DC microgrid control method in a coordinated cooperation mode according to claim 2 is characterized in that: The overall control strategy of step 1 includes: (1) The voltage of the DC bus in the DC microgrid is classified into multiple working modes: The normal voltage fluctuation range of the DC bus in the DC microgrid is ±2%, and the maximum allowable voltage fluctuation is ±5%. The voltage classification is as follows: +5%, +2%, -2%, -5%; There are five modes: Mode 1: -2% to +2%, Mode 2: +2% to +5%, Mode 3: -5% to -2%, Mode 4: bus voltage greater than +5%, Mode 5: bus voltage less than -5%; (2) The grid operation mode is divided into two operation modes: off-grid operation and grid-connected operation. According to the working modes divided in step (1), the working states of the photovoltaic power generation system, the energy storage system and the load under the corresponding working mode are obtained.

8. The DC microgrid control method in a coordinated cooperation mode according to claim 1, characterized in that: The DC microgrid system of step 2 includes: a photovoltaic power generation system, an energy storage system, a large power grid, a load and a DC bus with a voltage level of 1kV; the output end of the photovoltaic power generation system is connected to the 1kV DC bus through a DC / DC module, which is used to access the DC bus through a DC / DC conversion circuit to power the photovoltaic power generation system; the energy storage system is connected to the DC bus through a bidirectional DC / DC conversion circuit to power the system or absorb excess electric energy; the large power grid is connected to the DC bus through a bidirectional DC / AC conversion circuit for energy flow between the DC microgrid; the input end of the load is connected to the DC bus through a DC / DC converter to obtain electric energy.

9. The DC microgrid control method in a coordinated cooperation mode according to claim 1, characterized in that: The simulation model of step 3 includes: a battery constant current charging simulation circuit model, a battery constant voltage charging simulation circuit model and a battery constant voltage discharging simulation circuit model; The battery constant current charging simulation circuit model includes: a reference current Current_ref_change_I generation link, a current input current and a reference current current_ref_change_I are negatively feedback regulated to ultimately achieve the purpose of constant current charging; The battery constant voltage charging simulation circuit model adopts a voltage outer loop and a current inner loop double closed loop control, the input voltage Ui and the reference value are subjected to voltage outer loop control to generate a reference current value Current_ref_change_U, and then Current_ref_change_U and the input current Current are subjected to current inner loop control to finally form voltage constant voltage charging; The battery constant voltage discharge simulation circuit model includes: the principle used is similar to the constant voltage charging, the bus voltage U is collected, and the reference value 1000 is compared and controlled in the outer loop to form a reference current Current_ref_dis, and then Current_ref_dis and the input current Current are controlled in the current inner loop to finally form a voltage constant voltage discharge.

Images