Photovoltaic dc-dc control method and system starting method in photovoltaic off-grid system

Abstract

The application relates to a photovoltaic DC / DC control method and a system starting method in a photovoltaic parallel and off-grid system, which comprises the following steps: calculating a current limiting curve in a linear mode based on the highest working voltage, the lowest working voltage of the photovoltaic parallel and off-grid system and the full-load working current of the photovoltaic DC / DC; when the photovoltaic parallel and off-grid system is in off-grid operation, obtaining the corresponding current limiting value according to the current limiting curve based on the measured DC bus voltage value; obtaining the DC bus voltage value of the photovoltaic parallel and off-grid system and the current value of the reactor; calculating the voltage ring given value based on the MPPT algorithm, obtaining the voltage ring output voltage value through the PI regulator of the voltage ring after the voltage ring given value is subtracted from the DC bus voltage value; limiting the current given value of the current ring through the obtained current limiting value, obtaining the PWM modulation signal for controlling the photovoltaic DC / DC through the PI regulator of the current ring after the current given value after the limiting is subtracted from the current value of the reactor.

Description

Technical Field

[0001] This invention relates to the field of AC and DC microgrid technology, specifically to a photovoltaic DC-DC control method and a system startup method in a photovoltaic grid-connected and off-grid system. Background Technology

[0002] In AC / DC microgrids for solar photovoltaic power generation, the structure of on-grid and off-grid energy storage systems typically consists of photovoltaic modules, boost circuits, on-grid and off-grid inverters, energy storage batteries, and the AC power grid, such as... Figure 1 As shown, the solar panel, as a photovoltaic module, is connected to the DC bus via a photovoltaic DC-DC converter. The energy storage battery is connected to the DC bus via an energy storage DC-DC converter. The DC load is connected to the DC bus via a DC circuit breaker. The DC bus is connected to an AC-DC inverter and connected to the AC grid via an AC circuit breaker. The DC power is converted to AC power by the AC-DC inverter to supply power to the AC load.

[0003] With the widespread adoption of high-power charging piles, higher demands are being placed on the utilization rate of new energy sources in AC and DC microgrids. Improving the utilization rate of photovoltaic power generation is essential while ensuring stable DC bus voltage during AC and DC microgrid operation. Typically, when a photovoltaic grid-connected system operates in grid-connected mode, i.e. Figure 1 When the AC circuit breaker is closed, the system connects to the AC grid. The ACDC inverter operates in constant DC voltage mode, and the photovoltaic DCDC and energy storage DCDC can be scheduled by the host computer to ensure stable system operation. However, when the AC circuit breaker is open, the photovoltaic off-grid system operates in off-grid mode. Since there is no voltage control equipment on the DC bus, if the photovoltaic DCDC generates full solar power while the DC and AC loads have lower power output than the photovoltaic power generation, it will lead to an overvoltage fault in the system.

[0004] To address the issue of unstable DC bus voltage in photovoltaic (PV) grid-connected and off-grid systems, invention patent CN113162112A discloses a "Bus Voltage Control Method and PV Grid-Connected and Off-Grid System." This patent generates a current loop setpoint by using the bus loop output value and voltage loop output value, and then generates a PWM signal to control the PV DC-DC converter based on the current loop setpoint and the output current of the PV modules. While this patent adds bus loop control to the traditional voltage and current loops, ensuring stable bus voltage in both grid-connected and off-grid states, this dual-voltage loop control results in a slower rate of limiting the PV output current and requires modifications to the original loop control. Summary of the Invention

[0005] This invention first discloses a photovoltaic DC-DC control method in a photovoltaic grid-connected and off-grid system. It adopts a dual closed-loop limiting strategy, which calculates the current limiting value by detecting the DC bus voltage, thereby controlling the given value of the current loop to limit the current. This not only achieves the goal of maximizing the utilization of photovoltaic energy for power generation, but also ensures the stable operation of the system.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] Photovoltaic DC-DC control methods in photovoltaic grid-connected and off-grid systems include:

[0008] The current limiting curve is calculated linearly based on the highest and lowest operating voltages of the photovoltaic grid-connected and off-grid system and the full-load operating current of the photovoltaic DC-DC converter. When the photovoltaic grid-connected and off-grid system is running off-grid, the corresponding current limiting value is obtained based on the measured DC bus voltage value and the current limiting curve.

[0009] Obtain the DC bus voltage and reactor current values ​​of the photovoltaic grid-connected and off-grid system;

[0010] The voltage loop setpoint is calculated based on the MPPT algorithm. The voltage loop output voltage value is obtained by subtracting the voltage loop setpoint from the DC bus voltage value and then passing the voltage loop PI regulator.

[0011] The current setpoint of the current loop is limited by the obtained current limit value. The difference between the current setpoint and the current value of the reactor is used to obtain the PWM modulation signal for controlling the photovoltaic DC-DC converter through the PI regulator of the current loop.

[0012] Furthermore, when the photovoltaic grid-connected system is operating off-grid, if the measured DC bus voltage satisfies U1≥U dc >U4 or U3-ΔU - ≥U dc When the condition is ≥U2, the corresponding current limiting curve is determined by the following formula:

[0013]

[0014] Among them, U dc This is the measured DC bus voltage value;

[0015] U1 is the highest voltage at which the system can operate normally at full power.

[0016] U2 is the minimum voltage at which the system can operate at full power.

[0017] U3 is the voltage setting value for the constant DC bus voltage operation of the ACDC inverter in the system.

[0018] U4 is the set disturbance hysteresis voltage;

[0019] ΔU - The lower threshold value for the set "current limiting disturbance";

[0020] I limit This is the current limiting value;

[0021] I rate The rated output current of the photovoltaic DC-DC converter in a photovoltaic grid-connected or off-grid system.

[0022] Furthermore, when the photovoltaic grid-connected system is running in grid-connected mode, the measured DC bus voltage satisfies U3 + ΔU. + >U dc >U3-ΔU - At that time, the disturbance current limit value is increased in stages, with the stage period being the same as the MPPT adjustment period. The corresponding current limit value is as follows:

[0023] I limit =I limit_pre +ΔI limit

[0024] Among them, I limit_pre This represents the disturbance current value from the previous moment.

[0025] ΔI limit This represents the increase in the current limiting disturbance.

[0026] ΔU + The upper limit threshold for the set "current limiting disturbance".

[0027] Furthermore, when the measured DC bus voltage satisfies U4≥U dc ≥U3+ΔU + When the current limit is reached, the corresponding current limit value is determined by the following formula:

[0028] I limit =I limit_first_pre

[0029] Among them, I limit_first_pre The DC limit value for the last "current limiting disturbance" saved by the system.

[0030] Considering the potential for DC bus overvoltage issues during the startup of a photovoltaic grid-connected system disconnected from the grid, it is essential to equip the DC bus with DC-DC energy storage devices to prevent such issues. Therefore, this invention also discloses a startup method for a photovoltaic grid-connected system in off-grid mode using the dual closed-loop limiting strategy described herein, specifically implemented using the following technical solution:

[0031] The startup method for a photovoltaic grid-connected system is as follows: If the AC-DC inverter in the system is disconnected from the AC grid, the system should be started in the following order:

[0032] First, switch on the photovoltaic DC-DC converter;

[0033] Secondly, the energy storage DC-DC converter is started using a vertical control method;

[0034] Furthermore, the photovoltaic DC-DC converter operates using a dual closed-loop limiting strategy, which connects the DC circuit breaker to the DC load.

[0035] Finally, start the ACDC inverter, close the AC circuit breaker, and connect the AC load.

[0036] This invention employs a dual-closed-loop limiting strategy for photovoltaic DC-DC converters in photovoltaic grid-connected and off-grid systems. By detecting the DC bus voltage, the current limiting value is calculated, thereby controlling the given value of the current loop. Compared with the existing technology that adjusts through MPPT, this method can limit the current output at the speed of switching cycles, allowing the photovoltaic DC-DC converter to reduce the current output very quickly. Moreover, it can also achieve the goal of maximizing the utilization of photovoltaic energy for power generation, thus comprehensively ensuring the stable operation of the system. Attached Figure Description

[0037] Figure 1 This is a structural diagram of a photovoltaic grid-connected and off-grid system;

[0038] Figure 2 This is a schematic diagram of the photovoltaic DC-DC connection to the DC bus in a photovoltaic grid-connected and off-grid system.

[0039] Figure 3 This is a schematic diagram of the current limiting curve in the embodiment;

[0040] Figure 4 This is a flowchart of the dual closed-loop control used in this invention. Detailed Implementation

[0041] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0042] This embodiment discloses a photovoltaic DC-DC control method in a photovoltaic grid-connected and off-grid system, wherein the photovoltaic grid-connected and off-grid system is as follows: Figure 1 As shown, the photovoltaic DC-DC converter in the system is connected to the DC bus as follows: Figure 2 As shown, after the AC circuit breaker in this system is closed, it can be connected to the AC power grid, putting the system in a grid-connected operation state. If the AC circuit breaker is disconnected from the AC power grid, the system can be in an off-grid operation state. Regardless of whether the system is in a grid-connected or off-grid state, the photovoltaic DC-DC converter in the system can utilize this invention. Figure 4 The dual closed-loop limiting strategy shown can not only maximize the utilization of photovoltaic energy for power generation, but also ensure that the DC bus is not overvoltaged.

[0043] The dual closed-loop limiting strategy adopted in this invention can be summarized as follows: Obtain the DC bus voltage value U of the photovoltaic grid-connected / off-grid system. dc and the current value I of the reactor fb The voltage loop setpoint U is calculated based on the MPPT algorithm. ref Set the voltage loop setpoint U ref DC bus voltage value U dc After subtraction, the voltage loop output voltage value U is obtained through the PI regulator of the voltage loop. out ; through the obtained current limiting value I limit The current setpoint of the current loop is limited, and the limited current setpoint I ref With the current value I of the reactor fb The difference is then passed through a PI regulator in the current loop to obtain the PWM modulation signal for controlling the photovoltaic DC-DC converter.

[0044] For off-grid operation, to facilitate calculation, the aforementioned current limiting value I... limit Based on the highest operating voltage U1, lowest operating voltage U2, and full-load operating current of the photovoltaic DC-DC converter system, the following can be calculated linearly: Figure 3 The current limiting curve is shown, and then the measured DC bus voltage value U is used. dc and Figure 3 The intersection of the curves corresponding to the slanted solid line portion can be used to obtain the current limiting value corresponding to the measured DC bus voltage. Figure 3 The dashed lines in the diagram are not the actual current limit values; they are only used to indicate that the two diagonal solid lines are on the same straight line.

[0045] Combination Figure 3 To elaborate further on the above plan, we will first introduce... Figure 3 The meanings of the various parameter symbols given are as follows:

[0046] U dc This is the measured DC bus voltage value;

[0047] U1 is the highest voltage at which the system can operate at full power. It can be set with reference to the low-voltage power electronic devices in DC systems.

[0048] U2 is the minimum voltage at which the system can operate at full power. It can be set with reference to the minimum voltage of the ACDC inverter at full power.

[0049] U3 is the voltage setting value for the constant DC bus voltage operation of the ACDC inverter in the system.

[0050] U4 is the set disturbance hysteresis voltage;

[0051] ΔU +The upper limit threshold for the set "current limiting disturbance"

[0052] ΔU - The lower threshold value for the set "current limiting disturbance";

[0053] I limit This is the current limiting value added to the current loop reference channel in dual closed-loop control;

[0054] I rate The rated output current of the photovoltaic DC-DC converter in the photovoltaic grid-connected and off-grid system;

[0055] I limit_pre This represents the disturbance current value from the previous moment.

[0056] ΔI limit This represents the increase in the current limiting disturbance.

[0057] I limit_first_pre The DC limit value for the last "current limiting disturbance" saved by the system.

[0058] For the offline state, i.e., satisfying U1≥U dc >U4 or U3-ΔU - ≥U dc When the condition is ≥U2, the corresponding current limiting curve (such as the two slanted solid lines in Figure 3) can be determined by the following formula:

[0059]

[0060] Draw Figure 3 After the current limiting curve in the figure, based on the measured DC bus voltage value from... Figure 3 The corresponding current limit value can be obtained from this.

[0061] when Figure 1 After the AC circuit breaker in the AC circuit is disconnected from the AC grid, the ACDC inverter operates in voltage source mode to control the AC side voltage. Before the AC grid is disconnected, the photovoltaic DCDC is already operating in a state with high photovoltaic utilization. The current limiting strategy given above is used to support the DC bus without causing overvoltage on the DC bus. This period of time is sufficient for the energy storage DCDC to switch to droop mode and jointly provide power to the load.

[0062] When the photovoltaic grid-connected system is in operation, i.e., the AC circuit breaker is closed and the ACDC inverter is connected to the AC grid, it operates in constant DC voltage mode. In existing technologies, the photovoltaic DCCDC at this stage is usually scheduled by a monitoring system. In this embodiment, the dual closed-loop current limiting strategy described above in this invention is also used to control the photovoltaic DCCDC in grid-connected mode. During the grid-connected operation phase, since the ACDC inverter controls the DC bus voltage at a fixed value U3, in order to make greater use of photovoltaic energy for power generation, an upper limit threshold ΔU is set within the voltage range above and below U3. + and a lower limit threshold ΔU - Within this voltage range, the photovoltaic output power is increased by periodically increasing the disturbance current limit value. The period of the current limit disturbance is the same as the MPPT adjustment period. That is, U3 + ΔU + >U dc >U3-ΔU - The corresponding current limiting value is as follows:

[0063] I limit =I limit_pre +ΔI limit

[0064] In grid-connected operation mode, the photovoltaic DC-DC and energy storage DC-DC are not started in any order. Limitation is only applied to the given channel of the current loop. The output current of the photovoltaic DC-DC can be limited at the speed of switching cycle, which makes the photovoltaic DC-DC reduce the current output very quickly. In contrast, the traditional method is to adjust through the MPPT algorithm, which is very slow.

[0065] When the measured DC bus voltage satisfies U4≥U dc ≥U3+ΔU + At that time, when the hysteresis voltage U4 and the upper limit voltage of the limiting disturbance U3+ΔU + Within the range, no further disturbances will be made, and the DC limit value I of the last "current limit disturbance" will be saved. limit_first_pre To increase stability, disturbances to the current limiting are stopped, and the current limiting value within this voltage range is determined by the following formula:

[0066] I limit =I limit_first_pre

[0067] Considering the potential for DC bus overvoltage issues during the startup of a photovoltaic grid-connected system disconnected from the grid, a DC-DC energy storage device must be installed on the DC bus side to prevent such issues. Therefore, this invention also discloses a startup method for a photovoltaic grid-connected system in off-grid mode using the dual closed-loop limiting strategy described herein, detailed below:

[0068] when Figure 1 When the AC circuit breaker is disconnected from the AC power grid, and the DC circuit breaker is also open, and all power electronic equipment is not closed or started, the system starts in the following sequence:

[0069] First, the photovoltaic DC-DC converter is switched on; due to the minimum duty cycle of sampling and PWM, the current limiting strategy of this invention cannot completely guarantee zero power output. Figure 2 The IGBT shown has a diode connected in parallel. The energy from the solar panel charges the DC bus through the diode, eventually making the DC bus voltage equal to the open-circuit voltage of the solar panel. This process does not generate a PWM signal; it is simply a closing process of a photovoltaic DC-DC converter.

[0070] Secondly, the energy storage DC-DC converter is started using the droop control method to establish the DC bus voltage. One purpose of starting the energy storage DC-DC converter at this stage is to prevent the weak power output of the photovoltaic DC-DC converter under no-load conditions from causing the DC bus voltage to rise. The second purpose is that if the energy storage DC-DC converter is not started and the load is applied directly, it may lead to insufficient photovoltaic power and the system cannot work normally.

[0071] Furthermore, the photovoltaic DC-DC converter operates using a dual closed-loop limiting strategy, which connects the DC circuit breaker to the DC load.

[0072] Finally, start the ACDC inverter, close the AC circuit breaker, and connect the AC load.

[0073] Starting and running the system in the order given above ensures maximum utilization of photovoltaic energy for power generation while preventing overvoltage on the DC bus. This invention employs dual-closed-loop current limiting control, eliminating the need to modify the loop control of existing general-purpose DC-DC programs that use dual-closed-loop (i.e., voltage outer loop and current inner loop) control. Implementation is relatively easy; it only requires adding the current limiting control program and the system's closing and startup logic according to the current limiting value acquisition method provided in this invention.

[0074] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A photovoltaic DC-DC control method in a photovoltaic grid-connected and off-grid system, characterized in that, include: The current limiting curve is calculated linearly based on the highest and lowest operating voltages of the photovoltaic grid-connected and off-grid system and the full-load operating current of the photovoltaic DC-DC converter. When the photovoltaic grid-connected and off-grid system is running off-grid, the corresponding current limiting value is obtained based on the measured DC bus voltage value and the current limiting curve. Obtain the DC bus voltage and reactor current values ​​of the photovoltaic grid-connected and off-grid system; The voltage loop setpoint is calculated based on the MPPT algorithm. The voltage loop output voltage value is obtained by subtracting the voltage loop setpoint from the DC bus voltage value and then passing the voltage loop PI regulator. The current setpoint of the current loop is limited by the obtained current limit value. The difference between the current setpoint and the current value of the reactor is used to obtain the PWM modulation signal for controlling the photovoltaic DC-DC converter through the PI regulator of the current loop. When the photovoltaic grid-connected system is operating off-grid, if the measured DC bus voltage meets the following requirements... U 1≥ U dc > U 4 or U 3-Δ U - ≥ U dc ≥ U Under condition 2, the corresponding current limiting curve is determined by the following formula: ； in, U dc This is the measured DC bus voltage value; U 1 represents the highest voltage at which the system can operate normally at full power. U 2 represents the minimum voltage at which the system can operate normally at full power. U 3 represents the voltage setting value for the constant DC bus voltage operation of the ACDC inverter in the system; U 4 represents the set disturbance hysteresis voltage; Δ U - The lower threshold value for the set "current limiting disturbance"; I limit This is the current limiting value; I rate The rated output current of the photovoltaic DC-DC converter in the photovoltaic grid-connected and off-grid system; When the photovoltaic grid-connected system is in operation, the measured DC bus voltage meets the following requirements: U 3+Δ U + > U dc > U 3-Δ U - At that time, the disturbance current limit value is increased in stages, with the stage period being the same as the MPPT adjustment period. The corresponding current limit value is as follows: ； in, I limit_pre This represents the disturbance current value from the previous moment. Δ I limit This represents the increase in the current limiting disturbance. Δ U + The upper limit threshold for the set "current limiting disturbance"; When the measured DC bus voltage meets U 4≥ U dc ≥ U 3+Δ U + When the current limit is reached, the corresponding current limit value is determined by the following formula: ； in, I limit_first_pre The DC limit value is the last "current limiting disturbance" saved by the system.

2. A photovoltaic grid-connected / off-grid system startup method based on the control method described in claim 1, characterized in that: If the AC-DC inverter in the system is disconnected from the AC power grid, the system will start in the following order: First, switch on the photovoltaic DC-DC converter; Secondly, the energy storage DC-DC converter is started using a vertical control method; Furthermore, the photovoltaic DC-DC converter operates using a dual closed-loop limiting strategy, which connects the DC circuit breaker to the DC load. Finally, start the ACDC inverter, close the AC circuit breaker, and connect the AC load.

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