Direct-current voltage control method for photovoltaic power generation direct-current power transmission system

Abstract

The invention belongs to the technical field of direct-current voltage control, and particularly relates to a direct-current voltage control method for a photovoltaic power generation direct-current power transmission system, which comprises the following steps of: 1, improving the efficiency of a solar photovoltaic system by utilizing an MPPT (maximum power point tracking) control strategy, so that the solar photovoltaic system always works at the maximum power point, and the maximum power point tracking speed is increased; therefore, the power generation efficiency of the photovoltaic system is maximized; according to the method, an MPPT (maximum power point tracking) control strategy is adopted, so that the solar photovoltaic system keeps the maximum power point, the difference between voltages is reduced, a photovoltaic power generation system model is established, a power grid loss optimization model is established, voltage optimization control is utilized, the deviation voltage generated by droop control can return to a set rated value, and the purpose of DC voltage error adjustment is achieved; and meanwhile, voltage fluctuation is stabilized by using sliding mode control, DC voltage fluctuation in the photovoltaic power generation DC power transmission system is suppressed, and the system voltage is stabilized, so that the operation of the photovoltaic power generation DC power transmission system is safer and more reliable.

Description

Technical Field

[0001] The present invention belongs to the technical field of DC voltage control, and particularly relates to a method for controlling the DC voltage of a photovoltaic power generation DC transmission system. Background Art

[0002] With the increasing depletion of fossil energy and the growing environmental pressure, new energy sources such as wind energy and solar energy will account for an increasing proportion in the energy structure. In terms of new energy access, compared with traditional AC power grids, DC power grids have the characteristics of simple structure, good controllability, large transmission capacity, and low line cost, and are more advantageous technically and economically. Therefore, DC transmission systems will become the trend of future power grid development. In a DC transmission system, the DC voltage is the only indicator reflecting the power balance within the system.

[0003] Currently, for multi-terminal (photovoltaic power generation system) DC power grid systems, voltage droop control is often used to balance the system power. Voltage droop control does not require communication and has higher reliability. However, after power adjustment, there is a deviation in the DC voltage and it cannot be maintained at the rated voltage value for a long time, which is likely to affect the operation of each terminal in the DC transmission system. Summary of the Invention

[0004] To solve the problems raised in the above background art, the present invention provides a method for controlling the DC voltage of a photovoltaic power generation DC transmission system, which has the characteristics of restoring the voltage within the DC transmission system and maintaining it at the rated value.

[0005] To achieve the above object, the present invention provides the following technical solution: A method for controlling the DC voltage of a photovoltaic power generation DC transmission system, comprising the following steps:

[0006] Step 1: By using the MPPT (maximum power point tracking) control strategy, improve the efficiency of the solar photovoltaic system so that it always operates at the maximum power point, thereby maximizing the power generation efficiency of the photovoltaic system;

[0007] Step 2: Establish a photovoltaic power generation system model and collect the output voltage data of the photovoltaic power grid;

[0008] Step 3: Set the rated voltage value, and establish an optimization model with the goal of minimizing grid losses based on the voltage of the output current of the photovoltaic power grid and the losses generated during voltage transmission;

[0009] Step 4: According to the deviation of the voltage within the DC transmission system, start voltage optimization control, change the DC voltage of the interconnected converter, charge and discharge the DC network side capacitor by increasing or absorbing power, and make the voltage recover and be maintained at the rated value;

[0010] Step 5: Construct the integral terminal synovial surface, introduce the hyperbolic tangent function and add the sign function, calculate the composite reaching law, and perform sliding mode control on the DC voltage flowing in Step 4 to stabilize the DC output voltage fluctuation.

[0011] Preferably, in Step 1, the MPPT control enables each end of the photovoltaic grid to reach the maximum power generation, makes its output power tend to be maximized, and reduces the voltage difference between each end of the photovoltaic grid.

[0012] Preferably, in Step 3, the positive input terminal of the comparator is connected to the DC line, and the negative input terminal of the comparator is connected to the DC voltage optimization control module, which is used to determine the voltage error based on the DC voltage output voltage and the DC voltage measurement value.

[0013] Preferably, in Step 4, the voltage optimization control adjusts the DC voltage through the intercept of the capping droop curve.

[0014] Preferably, in Step 5, for the sliding mode control, a sliding mode controller is used to control the on-off state of the switching tube in the circuit according to the DC voltage to stabilize the voltage fluctuation.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] 1. In the present invention, the MPPT (maximum power point tracking) control strategy is adopted to keep the solar photovoltaic system at the maximum power point, reduce the voltage difference, establish a photovoltaic power generation system model and optimize the grid loss model. By using voltage optimization control, the deviation voltage generated by droop control can be returned to the set rated value to achieve the purpose of regulating the DC voltage error. At the same time, the sliding mode control is used to suppress the voltage fluctuation, stabilize the DC voltage in the DC power transmission system of photovoltaic power generation, and make the operation of the DC power transmission system of photovoltaic power generation safer and more reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the present invention and do not constitute a limitation to the present invention. In the drawings:

[0018] Figure 1 It is a schematic diagram of the system flow structure of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0020] Example 1

[0021] Please refer to Figure 1 , the present invention provides the following technical solutions: A method for controlling the DC voltage of a photovoltaic power generation DC transmission system, including the following steps:

[0022] Step 1: By using the MPPT (Maximum Power Point Tracking) control strategy, improve the efficiency of the solar photovoltaic system, so that it always operates at the maximum power point, thereby maximizing the power generation efficiency of the photovoltaic system;

[0023] Step 2: Establish a photovoltaic power generation system model and collect the output voltage data of the photovoltaic grid;

[0024] Step 3: Set the rated voltage value, and establish an optimization model with the goal of minimizing the grid loss according to the voltage of the output current of the photovoltaic grid and the loss generated during voltage transmission;

[0025] Step 4: According to the voltage deviation generated in the DC transmission system, start the voltage optimization control, change the DC voltage of the network-connected converter, charge and discharge the DC network side capacitor by increasing or absorbing power, and make the voltage recover and remain at the rated value;

[0026] Step 5: Construct an integral terminal sliding mode surface, introduce the hyperbolic tangent function and add the sign function, calculate the composite reaching law, and perform sliding mode control on the DC voltage flowing in Step 4 to stabilize the DC output voltage fluctuation.

[0027] Specifically, in Step 1, the MPPT control enables each end of the photovoltaic grid to reach the maximum power generation, makes its output power tend to be maximized, and reduces the voltage difference between each end of the photovoltaic grid.

[0028] Specifically, in Step 3, the positive input terminal of the comparator is connected to the DC line, and the negative input terminal of the comparator is connected to the DC voltage optimization control module, which is used to determine the voltage error based on the DC voltage output voltage and the DC voltage measurement value.

[0029] Specifically, in Step 4, the voltage optimization control adjusts the DC voltage through the intercept of the droop curve of the cover surface.

[0030] Specifically, in Step 5, for the sliding mode control, a sliding mode controller is used to control the on-off state of the switching tubes in the circuit according to the DC voltage to stabilize the voltage fluctuation.

[0031] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for controlling direct current voltage of a photovoltaic power generation direct current transmission system, comprising the following steps, characterized in that: Step 1: Improve the efficiency of the solar photovoltaic system by using the MPPT (maximum power point tracking) control strategy so that it always works at the maximum power point, thereby maximizing the power generation efficiency of the photovoltaic system; Step 2: Establish a photovoltaic power generation system model and collect photovoltaic power grid output voltage data; Step 3: Set the rated voltage value, and establish an optimization model with the goal of minimizing grid losses based on the voltage of the photovoltaic grid output current and the losses generated during voltage transmission; Step 4: According to the voltage deviation in the DC transmission system, start voltage optimization control, change the DC voltage of the interconnected converter, charge and discharge the DC network side capacitor by increasing or absorbing power, so that the voltage is restored and maintained at the rated value; Step 5: Construct the integral terminal sliding film surface, introduce the hyperbolic tangent function and add the sign function, calculate the composite reaching law, and perform sliding mode control on the DC voltage flowing in step 4 to stabilize the DC output voltage fluctuation.

2. A method for controlling direct current voltage of a photovoltaic power generation direct current transmission system according to claim 1, characterized in that: In the step 1, the MPPT control enables the photovoltaic power grid at each end to reach the maximum power generation, maximizes its output power, and reduces the voltage difference of the photovoltaic power grid at each end.

3. A method for controlling direct current voltage of a photovoltaic power generation direct current transmission system according to claim 1, characterized in that: In step three, the positive input terminal of the comparator is connected to the DC line, and the negative input terminal of the comparator is connected to the DC voltage optimization control module, so as to determine the voltage error between the DC voltage output voltage and the DC voltage measurement value.

4. A method for controlling direct current voltage of a photovoltaic power generation direct current transmission system according to claim 1, characterized in that: In the step 4, the voltage optimization control adjusts the DC voltage by the intercept of the cap droop curve.

5. A method for controlling direct current voltage of a photovoltaic power generation direct current transmission system according to claim 1, characterized in that: The sliding mode control in step five adopts a sliding mode controller to control the on-off state of the switch tube in the claim circuit according to the DC voltage to stabilize the voltage fluctuation.

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