A direct current micro-grid energy storage end power balance control method and system

Abstract

The application discloses a DC micro-grid energy storage end power balance control method and system, wherein the system comprises a photovoltaic array, a storage battery, a hydrogen fuel cell, a user end load, a hydrogen production end and a DC bus; the storage battery is combined with a Buck / Boost bidirectional DC-DC converter 2, the output end of the Buck / Boost bidirectional DC-DC converter 2 is connected with the DC bus, the input end of the Buck / Boost bidirectional DC-DC converter 2 is connected with a control loop, the control loop comprises a composite nonlinear feedback controller, the composite nonlinear feedback controller comprises a linear PI controller and a nonlinear term controller, the nonlinear term controller is connected with the linear PI controller in parallel outside a loop, and the nonlinear term controller is used for providing instantaneous correction to reduce an error when a load mutation causes a DC bus voltage error to be in a working threshold interval.

Description

Technical Field

[0001] This invention relates to a power balance control method and system for the energy storage end of a DC microgrid, belonging to the field of DC microgrid technology. Background Technology

[0002] Hydrogen energy, as a renewable energy source, differs from solar and wind energy in that it possesses excellent storage performance. Currently, renewable energy sources such as photovoltaics are entering an era of grid parity, and with advancements in industry technology, power generation costs will further decrease. The combination of hydrogen energy and photovoltaics will mitigate the impact of large-scale photovoltaic power generation on the power grid. Hydrogen electrolyzers, as controllable loads, can coordinate with the grid to solve the "bottleneck" problem of large-scale photovoltaic power generation connecting to the grid, efficiently utilizing the curtailed photovoltaic power during peak generation periods and improving the quality of grid-connected photovoltaic power. The main constraint on renewable energy power generation will shift from cost to stability and reliability. Due to their energy absorption / release characteristics, batteries are typically chosen as suitable energy storage media to regulate system power and maintain a balanced state.

[0003] Currently, new energy sources such as photovoltaics have been widely used. However, current power balance control methods for DC interconnected photovoltaic / storage / hydrogen fuel cell systems suffer from drawbacks such as poor steady-state bus voltage, static error, and slow voltage recovery. There is a lack of effective analysis and solutions regarding how to achieve effective energy management through energy storage to smoothly switch operating modes between different conditions, enabling rapid system power balance and fast bus voltage recovery. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a power balance control method and system for the energy storage end of a DC microgrid. By designing a controller for the battery Buck / Boost bidirectional DC-DC converter, the battery can quickly absorb unbalanced power in the system, reducing the fluctuation of the bus voltage during operating condition switching.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A power balance control method for the energy storage terminal of a DC microgrid, when the system is in the startup state, includes the following steps:

[0007] For DC bus voltage U dc Perform sampling.

[0008] When 0 ≤ U dc <0.95U N The photovoltaic array is in MPPT state, the battery is in maximum current discharge state, CNFC is in standby mode, and the hydrogen fuel cell is working simultaneously to ensure that the bus voltage quickly reaches the rated value. dc U is the DC bus voltage. NThis is the rated value of the DC bus voltage;

[0009] When 0.95U N ≤ U dc N The photovoltaic array remains in MPPT state, the battery is in constant voltage discharge state under composite nonlinear feedback control, and the hydrogen fuel cell is not working.

[0010] When U N = U dc At this time, the photovoltaic array is in a constant voltage output state to ensure the balance of source and load power.

[0011] Furthermore, the aforementioned also includes when U N = U dc When the system is in a stable state, subsequent U dc When the temperature is within a stable range, the battery does not operate, and the hydrogen fuel cell remains in standby mode; the stable range is 0.99U. N ≤ U dc ≤1.01U N .

[0012] Furthermore, the aforementioned also includes situations where, under steady-state conditions, the system is in a dynamic state when power fluctuations occur at the source or load end:

[0013] If at this time, 0.95U N ≤ U dc ≤0.99U N Or 1.01U N ≤ U dc ≤1.05U N At this time, the battery regulates the bus voltage through composite nonlinear feedback voltage regulation and charging control, and the hydrogen fuel cell does not work.

[0014] If at this time, U dc <0.95U N or U dc >1.05U N At this time, the battery is in the maximum current discharge and charge control state, CNFC is in standby mode, and the hydrogen fuel cell works simultaneously to ensure that the bus voltage quickly returns to the rated value.

[0015] A storage medium for storing program instructions for executing any of the aforementioned control methods.

[0016] An apparatus includes a storage medium, a processor, and a computer program stored in the storage medium and executable on the processor, wherein the processor executes the computer program to implement any of the foregoing control methods.

[0017] ​A power balance control system for the energy storage end of a DC microgrid, employing any of the aforementioned control methods, includes a photovoltaic array, a battery, a hydrogen fuel cell, a user-end load, a hydrogen production end, and a DC bus;

[0018] The photovoltaic array is connected to the DC bus via Boost converter 1;

[0019] The battery is connected to the DC bus through Buck / Boost bidirectional DC-DC converter 2. The output of Buck / Boost bidirectional DC-DC converter 2 is connected to the DC bus, and the input is connected to the control loop. The control loop includes a composite nonlinear feedback controller, which includes a linear PI controller and a nonlinear term controller. The nonlinear term controller and the outer loop of the linear PI controller are connected in parallel.

[0020] The hydrogen fuel cell is connected to the DC bus via Boost converter 3;

[0021] The user-end load and the hydrogen production end are connected to the DC bus via Buck converter 4 and Buck converter 5, respectively.

[0022] Furthermore, the aforementioned Buck / Boost bidirectional DC-DC converter 2 includes transistors S2 and S3;

[0023] When the battery is charging, transistor S3 is turned on and transistor S2 is turned off. Transistor S3 receives one or two signals from the control loop. One signal is output by the constant current charging PI controller, and two signals are output by the composite nonlinear feedback voltage regulator charging controller.

[0024] When the battery is discharging, transistor S2 is turned on and transistor S3 is turned off. Transistor S2 receives 3 or 4 signals from the control loop. The 3 signals are output by the composite nonlinear feedback regulated discharge controller, and the 4 signals are output by the constant current discharge PI controller.

[0025] Furthermore, the aforementioned composite nonlinear feedback controller function expression is as follows:

[0026]

[0027] Where x is the error between the bus voltage and the reference voltage in the system, b is the lower limit of the set threshold, a is the upper limit of the set threshold, and k p k i k n These are the proportional, integral, and nonlinear coefficients, respectively. This represents the maximum absolute value of the error x.

[0028] Furthermore, the aforementioned b=0.01U N a=0.05U N U NThis is the rated value of the DC bus voltage.

[0029] The beneficial effects achieved by this invention are as follows:

[0030] Compared to existing linear PI controllers and their droop control, the composite nonlinear feedback controller enables smaller bus voltage fluctuations and steady-state zero steady-state error tracking of the reference voltage. Attached Figure Description

[0031] Figure 1 This invention relates to an off-grid photovoltaic / storage / hydrogen fuel cell DC interconnection system structure;

[0032] Figure 2 This is a schematic diagram of the energy storage terminal and its controller structure of the present invention;

[0033] Figure 3 This invention proposes a nonlinear function curve in the nonlinear term of the Buck / Boost converter controller;

[0034] Figure 4 This is a simulated load power diagram of the system of this invention;

[0035] Figure 5 This invention provides a system simulation diagram of photovoltaic, battery, and load port currents.

[0036] Figure 6a This is a simulation bus voltage diagram of a traditional PI controller system;

[0037] Figure 6b This is a simulation bus voltage diagram of the system of this invention;

[0038] Figure 7a This is a simulation diagram of the controller output and error of a traditional PI controller system;

[0039] Figure 7b This is a diagram showing the output and error of the system simulation controller of this invention.

[0040] Meaning of the reference numerals in the figure: P PV - Photovoltaic array output power; P ES - Battery output power; P FC - Output power of hydrogen fuel cell; P Load -The sum of the user-end load power and the hydrogen production-end load power; P Load1 -User-side load power; P Load2 - Hydrogen production end load power; V bat - Battery terminal voltage; V bus -DC bus voltage; I ref V ref -Battery reference current and reference voltage, I bat V out- Battery inductor current and output voltage; Q2, Q3 - Drive signals for battery Buck / Boost converter switching transistors S2, S3; I pv - Photovoltaic output current; I load -Total load current; - Output current of hydrogen fuel cells. Detailed Implementation

[0041] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments and specific features in the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations thereof. In the absence of conflict, the embodiments and technical features in the embodiments can be combined with each other.

[0042] Example 1

[0043] This embodiment discloses a power balance control system for the energy storage end of a DC microgrid, such as... Figure 1 As shown, the system includes a DC bus, a photovoltaic array, a battery, a hydrogen fuel cell, a user-end load, and a hydrogen production terminal. Each component is connected to the DC bus via its corresponding DC-DC converter. The photovoltaic array and hydrogen fuel cell are located on the source side, while the user-end load and hydrogen production terminal are located on the load side. The battery, due to its bidirectional power flow characteristic, is responsible for absorbing unbalanced power within the system.

[0044] Specifically, the photovoltaic array is connected to the DC bus through Boost converter 1 and is regulated by two control methods: maximum power point tracking (MPPT) and constant voltage output.

[0045] The battery is combined with a Buck / Boost bidirectional DC-DC converter 2, with the output connected to the DC bus. It employs maximum input / output current control and composite nonlinear feedback control. The battery operates in two modes—charging and discharging—based on system power surplus / deficit. Different modes are achieved by controlling the drive signals Q2 and Q3 of switching transistors S2 and S3. When the battery is charging, transistor S3 is on and transistor S2 is off. The drive signal for transistor S3 comes from the auxiliary circuit. Figure 2 The 1 or 2 signals represent constant current charging control and composite nonlinear feedback voltage regulation charging control, respectively. When the battery is discharging, transistor S2 is turned on and transistor S3 is turned off. The drive signal for transistor S2 comes from the attached... Figure 2 The three or four signals in the signal represent composite nonlinear feedback regulated discharge control and constant current discharge control, respectively.

[0046] The hydrogen fuel cell is connected to the DC bus via Boost converter 3 and uses dual closed-loop control of voltage and current.

[0047] The DC bus serves as the input voltage source and is connected to the hydrogen production equipment and the equivalent load at the user end via Buck converters 4 and 5, respectively. To ensure the voltage at the load end is stable, the output adopts constant voltage closed-loop control.

[0048] Neglecting the losses incurred by the converters at each port, when the system bus voltage is stable, the power has the following relationship:

[0049]

[0050] In the formula, P PV - Photovoltaic array output power, P ES -Battery output power, P FC - Output power of hydrogen fuel cells, P Load1 -User-side load power, P Load2 - Hydrogen production end load power.

[0051] When the system is stable, the bus voltage is stable at U N When a sudden load is applied, the system power exhibits the following relationship:

[0052]

[0053] Furthermore, for the DC bus capacitor, the system unbalanced power has the following relationship with its voltage:

[0054]

[0055] In the formula, V bus DC bus voltage - DC bus capacitor.

[0056] When using a traditional PI dual-loop controller, the bus voltage drops significantly and the recovery time is long. This is because the closed-loop control output cannot simultaneously achieve both speed and overshoot. This embodiment constructs a novel CNFC (composite nonlinear feedback controller) as the control part of the battery converter, connecting the nonlinear term in parallel with the voltage PI outer loop. When voltage errors fluctuate within a threshold range, causing PI controller output fluctuations, the nonlinear term adjusts the outer loop output to keep the controller output as stable as possible. This is equivalent to increasing system damping through the control loop, enhancing the system's resistance to external interference.

[0057] Appendix Figure 2 In the controllers shown, number 2 and 3 This is the core part of CNFC, and the function is divided into two segments by setting a threshold:

[0058]

[0059] Where x is a function of time t, corresponding to the error between the bus voltage and the reference voltage in the system. b is the lower limit of the set threshold, and a is the upper limit of the set threshold. When the input error is not within the operating range, the function is a single PI output. When the input error is within the operating range, the function output is a composite output of a nonlinear function and a PI, where k... p k i k n These are the proportional, integral, and nonlinear coefficients, respectively. This represents the maximum absolute value of the error x.

[0060] The rated value of the bus voltage is set to U. N The bus error is x, and the lower threshold is b = 0.01U. N The upper limit of the threshold is a = 0.05U. N When a sudden change in load causes the bus voltage to fluctuate by 0.95U... N -0.99U N and 1.01U N -1.05U N That is, within the set operating range, the bidirectional DC-DC converter operates in Buck mode or Boost mode, and the bus voltage is controlled by its respective Composite Non-Linear Feedback Controller (CNFC).

[0061] make

[0062]

[0063] This is the nonlinear component of CNFC, whose value increases with increasing error, representing the change in system damping as the error changes. The function curve is attached. Figure 3 As shown.

[0064] When the bus voltage error is small, it can be considered a normal fluctuation. To avoid malfunction of the nonlinear term, a starting threshold b is set. When the error exceeds this threshold, the nonlinear term starts outputting. (See attached...) Figure 3 It can be seen that when the error x is small, The error is relatively small, and the ability to adjust the nonlinear term is weak. As the error x increases, The output increases rapidly, and the nonlinear term significantly enhances the ability of the PI outer loop output to adjust.

[0065] The above system was simulated using MATLAB. With the bus voltage rated at 200V, the battery state switching points were 190V, 198V, 202V, and 210V. The nonlinear terms were set to a=10, b=2, and γ=10.

[0066] As attached Figure 4 As shown, the total load power was 500W before 0.2s. At 0.2s, the user load power suddenly increased by 500W, at which point the bus voltage dropped. The battery controller switched from mode 2 to mode 3, and its converter changed from Buck charging mode to Boost discharging mode. The average output current of the photovoltaic system, battery, and load are shown in the attached figure. Figure 5 As shown, when the bus voltage is stable, the following relationship always holds:

[0067]

[0068] In the formula, I pv I load I represents the photovoltaic output current and the total load current, respectively. bat Indicates the battery inductance current. This indicates the output current of the hydrogen fuel cell.

[0069] Under the above simulation conditions, the hydrogen fuel cell does not operate. It is 0.

[0070] Comparison of dynamic response of CNFC bus voltage before and after using the present invention Figure 6a , Figure 6b As shown, the output of the PI controller before and after CNFC and the bus voltage error are as follows: Figure 7a , Figure 7b As shown.

[0071] Example 2

[0072] Based on the aforementioned system, this embodiment discloses a power balance control method for the energy storage end of an off-grid photovoltaic hydrogen fuel cell DC interconnection system, which controls the DC bus voltage U, a key power balance indicator of the system. dc Sampling is performed, and the process is divided into the following stages based on startup, steady state, and dynamic processes:

[0073] Phase 1: U dc Starting from 0, the photovoltaic array is in MPPT state, the battery is in maximum current discharge state, CNFC is in standby mode, and the drive signal for switch Q2 comes from the attached... Figure 3 The four signals in the system allow the hydrogen fuel cell to operate simultaneously, ensuring that the bus voltage quickly reaches the rated value.

[0074] Phase 2: U dc Rising to 0.95U N At this time, the photovoltaic array remains in MPPT state, the battery is in constant voltage discharge state under composite nonlinear feedback control, and the drive signal for switch Q2 comes from the attached... Figure 2 The hydrogen fuel cell is not working due to the presence of three signals.

[0075] Phase 3: U dcContinue to rise to the rated value U N At this point, the system reaches a stable state, and the photovoltaic array is in a constant voltage output state to ensure source-load power balance, and U dc At 0.99U N ~1.01U N At this time, the battery is not working. The hydrogen fuel cell remains in standby mode.

[0076] Phase 4: When power fluctuations occur at the source or load end of the system, the bus voltage will decrease or increase accordingly. When the bus voltage is at 0.95U... N -0.99U N and 1.01U N -1.05U N During this process, the battery regulates the bus voltage through composite nonlinear feedback voltage regulation and charging control. In this state, the drive signal for switch Q2 during discharge comes from the auxiliary circuit. Figure 2 The three signals in the signal source include the drive signal for the Q3 switch during charging, which comes from the attached signal source. Figure 2 The hydrogen fuel cell is not working when there are two signals in the signal; when U dc <0.95U N and U dc >1.05U N At this time, the battery is in the maximum current discharge / charge control state, and CNFC is in standby mode. During discharge in this state, the drive signal for switch Q2 comes from the attached... Figure 2 The four signals in the signal source include the drive signal for switch Q3 during charging, which comes from the attached signal source. Figure 2 One signal in the signal ensures that the bus voltage can quickly return to its rated value while the hydrogen fuel cell is operating simultaneously.

[0077] It should be understood that the functions specified in the accompanying drawings can be implemented by computer program instructions. These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means.

[0078] These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, such that the instructions, which execute on the computer or other programmable apparatus, provide steps for implementing the functions specified in the accompanying drawings.

[0079] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A power balance control method for the energy storage terminal of a DC microgrid, characterized in that, When the system is in the startup state, the following steps are included: For DC bus voltage U dc Perform sampling. When 0 ≤ U dc <0.95U N The photovoltaic array is in MPPT state, the battery is in maximum current discharge state, CNFC is in standby mode, and the hydrogen fuel cell is working simultaneously to ensure that the bus voltage quickly reaches the rated value. dc The U is the DC bus voltage. N This is the rated value of the DC bus voltage; When 0.95U N ≤ U dc N The photovoltaic array remains in MPPT state, the battery is in constant voltage discharge state under composite nonlinear feedback control, and the hydrogen fuel cell is not working.​ When U N = U dc At this time, the photovoltaic array is in a constant voltage output state to ensure source-load power balance; The battery is connected to the DC bus via a Buck / Boost bidirectional DC-DC converter 2. The output of the Buck / Boost bidirectional DC-DC converter 2 is connected to the DC bus, and the input is connected to the control loop. The control loop includes a composite nonlinear feedback controller, which includes a linear PI controller and a nonlinear term controller. The nonlinear term controller is connected in parallel with the outer loop of the linear PI controller. The function expression of the composite nonlinear feedback controller is: ； Where x is the error between the bus voltage and the reference voltage in the system, b is the lower limit of the set threshold, a is the upper limit of the set threshold, and k p k i k n These are the proportional, integral, and nonlinear coefficients, respectively. This represents the maximum absolute value of the error x.

2. The power balance control method for the energy storage terminal of a DC microgrid according to claim 1, characterized in that, Also includes when U N = U dc When the system is in a stable state, subsequent U dc When the temperature is within a stable range, the battery does not operate, and the hydrogen fuel cell remains in standby mode. This stable range is 0.99U. N ≤ U dc ≤1.01U N .

3. The power balance control method for the energy storage terminal of a DC microgrid according to claim 2, characterized in that, This also includes situations where, under steady-state conditions, the system is in a dynamic state when power fluctuations occur at the source or load end: If at this time, 0.95U N ≤ U dc ≤0.99U N Or 1.01U N ≤ U dc ≤1.05U N At this time, the battery regulates the bus voltage through composite nonlinear feedback voltage regulation and charging control, and the hydrogen fuel cell does not work. If at this time, U dc <0.95U N or U dc >1.05U N At this time, the battery is in the maximum current discharge and charge control state, CNFC is in standby mode, and the hydrogen fuel cell works simultaneously to ensure that the bus voltage quickly returns to the rated value.

4. A storage medium for storing program instructions, characterized in that, The program instructions are used to execute the control method as described in any one of claims 1-3.

5. An apparatus comprising a storage medium, a processor, and a computer program stored in the storage medium and executable on the processor, characterized in that, When the processor executes the computer program, it implements the control method as described in any one of claims 1-3.

6. A power balance control system for the energy storage terminal of a DC microgrid, employing the control method described in any one of claims 1-3, characterized in that, This includes photovoltaic arrays, batteries, hydrogen fuel cells, user-end loads, hydrogen production terminals, and DC buses; The photovoltaic array is connected to the DC bus via Boost converter 1; The battery is connected to the DC bus via a Buck / Boost bidirectional DC-DC converter 2. The output of the Buck / Boost bidirectional DC-DC converter 2 is connected to the DC bus, and the input is connected to the control loop. The control loop includes a composite nonlinear feedback controller, which includes a linear PI controller and a nonlinear term controller. The nonlinear term controller is connected in parallel with the outer loop of the linear PI controller. The hydrogen fuel cell is connected to the DC bus via Boost converter 3; The user-end load and hydrogen production end are connected to the DC bus via Buck converter 4 and Buck converter 5, respectively.

7. A power balance control system for the energy storage end of a DC microgrid according to claim 6, characterized in that, The Buck / Boost bidirectional DC-DC converter 2 includes transistors S2 and S3; When the battery is charging, transistor S3 is turned on and transistor S2 is turned off. Transistor S3 receives one or two signals from the control loop. The one signal is output by the constant current charging PI controller and the two signals are output by the composite nonlinear feedback voltage regulator charging controller. When the battery is discharging, transistor S2 is turned on and transistor S3 is turned off. Transistor S2 receives 3 or 4 signals from the control loop. The 3 signals are output by the composite nonlinear feedback voltage-regulated discharge controller, and the 4 signals are output by the constant current discharge PI controller.

8. A power balance control system for the energy storage end of a DC microgrid according to claim 6, characterized in that, The b=0.01U N a=0.05U N The U N This is the rated value of the DC bus voltage.

Images