A photovoltaic medium voltage DC converter grid-connected control method

Abstract

The present invention provides a photovoltaic medium-voltage DC converter grid-connected control method, wherein the photovoltaic array input voltage sampling value and the input current sampling value are used to obtain an input voltage initial instruction through a maximum power tracking controller, the module output voltage sampling value is adjusted through P to obtain a voltage-balanced instruction, the input voltage sampling value is compared with the input voltage initial instruction, a compensation voltage instruction is generated through PI adjustment, the voltage-balanced instruction is added to the input voltage initial instruction and then the compensation voltage instruction is subtracted to obtain a final input voltage instruction of the module, the final input voltage instruction is compared with the input voltage sampling value, a current instruction is generated through PI adjustment, and a duty cycle modulation instruction is generated through PI adjustment after the current instruction is compared with the output current sampling. The generated duty cycle modulation instruction and a triangular carrier with a fixed frequency are used to generate a PWM signal through a comparator to control the module switch tube to be turned on and off.

Description

Technical Field

[0001] The invention belongs to the field of power control, and in particular relates to a photovoltaic medium-voltage DC converter grid-connected control method. Background Art

[0002] Photovoltaic medium-voltage DC converters are suitable for applications such as DC collection in large photovoltaic bases and photovoltaic DC access to DC grids. Through photovoltaic DC converters, the input voltage of several hundred volts to one thousand volts can be directly boosted to an output voltage of tens of kilovolts. In order to achieve a high boost ratio, the DC converter requires multiple modules to operate in series and parallel. Due to the inconsistency of the main circuit parameters of each module, it is easy to cause uneven power in each module, affecting the stable operation of the DC converter, and balancing control technology must be added. At present, the voltage-balancing closed loop of the DC converter module control is based on the voltage-balancing or current-balancing instructions issued by the centralized controller. If there is a problem with the voltage-balancing or current-balancing instructions, it will seriously affect the stable operation of the converter. In order to further improve the reliability of the converter module balancing control and reduce centralized control instructions, this patent proposes a module control method to realize the maximum power tracking and balancing control of the module decentralized autonomy, which is of great significance to improving the reliability of the converter and realizing modular design.

[0003] The output voltage balancing closed loop can achieve voltage balancing control between modules, but when the number of operating modules changes, the voltage balancing command needs to be adjusted in time according to the number of modules. In particular, when a module fails, if the converter does not grasp the fault information in time, it will generate an erroneous voltage balancing command and the system cannot operate normally. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a photovoltaic medium voltage DC converter grid-connected control method, so that the voltage balancing control is completed by the module itself, without the need for centralized controller instructions, avoiding the problem of erroneous voltage balancing instructions; the input voltage can effectively eliminate the input voltage error through compensation control, and realize voltage balancing control at the same time. The present invention adopts a control method combining MPPT with voltage closed loop and current balancing control, and each module adopts the same control method.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A photovoltaic medium-voltage DC converter grid-connected control method, the photovoltaic medium-voltage DC converter comprises m isolated DC conversion modules connected to a photovoltaic array, each module input is connected in parallel and output is connected in series, realizing low-voltage DC conversion into medium-voltage DC, the grid-connected control method comprises the following steps:

[0007] Step (1) The PV array input voltage sampling value V in And the input current sampling value I in Connect to the maximum power tracking controller to get the initial input voltage command Vinref ; Input voltage initial command V inref Sent to each module as the initial input voltage instruction;

[0008] Step (2) Sample the module output voltage V o The voltage equalization command V is obtained by the first regulator 1 inref_1 ;

[0009] Step (3) The PV array input voltage sampling value V in With the input voltage initial command V inref By comparison, the second regulator 2 generates a compensation voltage command V inref_2 ;

[0010] Step (4) Set the input voltage initial command V inref Add the voltage equalization instruction V inref_1 , minus the compensation voltage command V inref_2 Get the final input voltage command V of the module in_ref ;

[0011] Step (5) The final input voltage command V in_ref With input voltage sampling V in After comparison, the third regulator 3 generates a current command I oref ;

[0012] Step (6) Current command I oref With the output current sampling i o After comparison, a duty cycle modulation instruction D is generated by the fourth regulator 4;

[0013] Step (7) generates a PWM signal by using a comparator to generate the generated duty cycle modulation instruction D and a triangular carrier with a fixed frequency, and controls the switch of the module to be turned on and off.

[0014] Beneficial effects:

[0015] The present invention can realize maximum power tracking control of modules and current sharing control between modules without interconnection wires, effectively eliminate input voltage errors through input voltage compensation control, and realize accurate maximum power tracking control. The module current sharing control is simple, avoiding mutual influence between parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a structural block diagram of the photovoltaic medium voltage DC converter of the present invention;

[0017] Figure 2 The present invention is a flow chart of the photovoltaic medium voltage DC converter grid-connected control method. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0019] The present invention adopts a control method combining MPPT with voltage closed loop and voltage balancing control. Each module adopts the same control method. Each module includes an MPPT control link, an input voltage closed loop, an output voltage balancing link, an input voltage compensation link and an output current closed loop. The MPPT control link completes the maximum power tracking control. The input voltage closed loop, the output voltage balancing link and the input voltage compensation link jointly complete the input voltage control and voltage balancing control. The output current closed loop completes the output current control.

[0020] The MPPT link provides an initial input voltage instruction for the input voltage closed loop. Taking into account the inconsistency of the modules, in order to prevent the overvoltage problem caused by the unbalanced output voltage, a voltage equalization link is added to achieve voltage equalization between modules. In the present invention, after the output voltage of the sampling module is sampled, the proportional adjustment is formed to form a voltage equalization instruction, and the initial voltage instruction is added to achieve voltage equalization. When the output voltage is high, the voltage equalization instruction increases, the input voltage instruction increases, and the duty cycle decreases, thereby reducing the output voltage. And vice versa. This method can achieve voltage equalization without knowing the output voltages of other modules. Due to the addition of the voltage equalization instruction, the input voltage statically deviates from the MPPT voltage instruction. The present invention adds a voltage compensation instruction, and forms a compensation instruction through the input voltage and the instruction closed loop, and dynamically compensates the input voltage instruction in real time to avoid the existence of static errors in the input voltage.

[0021] like Figure 1 As shown, the photovoltaic medium voltage DC converter of the present invention comprises m isolated DC conversion modules connected to the photovoltaic array, namely module 1, module 2 ... module M. The input of each module is connected in parallel and the output is connected in series to realize the conversion of low voltage DC into medium voltage DC.

[0022] like Figure 2 As shown, a photovoltaic medium voltage DC converter grid-connected control method of the present invention specifically includes the following steps:

[0023] Step (1) The PV array input voltage sampling value V in And the input current sampling value I in Connect to the maximum power tracking controller to get the initial input voltage command V inref . Input voltage initial command V inref Sent to each module as the initial input voltage instruction.

[0024] Step (2) Sample the module output voltage V o The voltage equalization command V is obtained by the first regulator 1 inref_1 .

[0025] Step (3) The PV array input voltage sampling value V in With the input voltage initial command V inref By comparison, the second regulator 2 generates a compensation voltage command V inref_2 .

[0026] Step (4) Set the input voltage initial command V inref Add the voltage equalization instruction V inref_1 , minus the compensation voltage command V inref_2 Get the final input voltage command V of the module in_ref .

[0027] Step (5) The final input voltage command V in_ref With input voltage sampling V in After comparison, the third regulator 3 generates a current command I oref .

[0028] Step (6) Current command I oref With the output current sampling i o After comparison, a duty cycle modulation command D is generated by the fourth regulator 4 .

[0029] Step (7) generates a PWM signal by using a comparator to generate the generated duty cycle modulation instruction D and a triangular carrier with a fixed frequency, and controls the switch of the module to be turned on and off.

[0030] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A photovoltaic medium voltage DC converter grid-connected control method, characterized in that: The photovoltaic medium-voltage DC converter includes m isolated DC conversion modules connected to the photovoltaic array, each module has inputs connected in parallel and outputs connected in series, so as to realize the conversion of low-voltage DC into medium-voltage DC. The grid-connected control method includes the following steps: Step (1) The PV array input voltage sampling value V in And the input current sampling value I in Connect to the maximum power tracking controller to get the initial input voltage command V inref ; Input voltage initial command V inref Sent to each module as the initial input voltage instruction; Step (2) Sample the module output voltage V o The voltage equalization command V is obtained by the first regulator 1 inref_1 ; Step (3) The PV array input voltage sampling value V in With the input voltage initial command V inref By comparison, the second regulator 2 generates a compensation voltage command V inref_2 ; Step (4) Set the input voltage initial command V inref Add the voltage equalization instruction V inref_1 , minus the compensation voltage command V inref_2 Get the final input voltage command V of the module in_ref ; Step (5) The final input voltage command V in_ref With input voltage sampling V in After comparison, the third regulator 3 generates a current command I oref ; Step (6) Current command I oref With the output current sampling i o After comparison, a duty cycle modulation instruction D is generated by the fourth regulator 4; Step (7) generates a PWM signal by using a comparator to generate the generated duty cycle modulation instruction D and a triangular carrier with a fixed frequency, and controls the switch of the module to be turned on and off.

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