CN-121984086-A - Direct-current side anti-countercurrent device and method for photovoltaic inverter

CN121984086ACN 121984086 ACN121984086 ACN 121984086ACN-121984086-A

Abstract

The invention provides a direct-current side countercurrent prevention device and method for a photovoltaic inverter, and belongs to the field of distributed power generation. The device is connected in series between the photovoltaic string and the inverter and comprises a breaking unit, a parallel power adjusting unit, a blocking diode and the like. The control method adopts a hierarchical control strategy of cutting and adjusting, wherein a controller collects the reflux power of the grid-connected point in real time, judges the size of the reflux power when the reflux is detected, directly controls corresponding quantity of breaking units to cut off if the reflux power exceeds the output power of a single photovoltaic string, physically cuts off part of the photovoltaic string, controls the rest trace reflux power, calculates PWM duty ratio by using a direct calculation or incremental PID algorithm, and drives a power adjusting unit to carry out hardware chopping consumption or suppression. The invention not only avoids the long-time heating of the power device under high current, but also solves the problem of overlarge granularity of power adjustment caused by simple cutting, and realizes the smoothness and the rapid backflow prevention of the photovoltaic system.

Inventors

LIU JIANGNING
ZHANG WEI
FU JUNCHENG

Assignees

江苏百瑞自动化科技有限公司

Dates

Publication Date: 20260505
Application Date: 20260127

Claims (8)

1. The direct current side anti-reflux device of the photovoltaic inverter is configured between a photovoltaic string and a direct current input side of the inverter and is characterized by comprising a plurality of anti-reflux submodules, wherein each anti-reflux submodule is correspondingly connected with one path of photovoltaic string, and comprises a breaking unit K1, a switching unit and a switching unit, wherein the breaking unit K1 is arranged in an output loop of a negative electrode of the photovoltaic string in series and is configured to conduct or physically cut off the connection of the path of photovoltaic string according to a control signal; the input end of the power regulating unit K2 is connected to a connecting node of the breaking unit K1 and the positive electrode of the photovoltaic string and is configured to form a bypass loop in a conducting state; the blocking diode D is arranged between the positive pole of the photovoltaic string and the inverter in series; The controller is electrically connected with the circuit breaking unit K1, the power adjusting unit K2 and the grid-connected point power acquisition module of each way of anti-backflow sub-module and is configured to execute a grading anti-backflow control strategy; The grading anti-reflux control strategy is as follows: the first stage of cutting, namely calculating the number N of the photovoltaic strings to be completely cut, wherein a calculation formula is N=int (|P grid /P string |), wherein int is a rounding function, a controller controls a breaking unit K1 of the N-path reverse flow prevention sub-module to be disconnected, P grid represents the active power of a parallel network point, and P string represents the unit power of the photovoltaic strings; And the second stage of control, namely calculating the current output power of the single-path photovoltaic string represented by the residual reverse current power residual error P rem = |P grid |- N ×P pv ;P pv after cutting, calculating the PWM duty ratio D cacl required for eliminating the residual power by utilizing a closed-loop control algorithm or an open-loop control algorithm, and controlling the power regulating unit K2 in the (n+1) th reverse current prevention sub-module to conduct at the duty ratio so as to eliminate the residual reverse current.
2. The photovoltaic inverter direct current side backflow prevention device according to claim 1, wherein the power regulating unit K2 comprises a fully-controlled power semiconductor device, the controller outputs a PWM signal to control on and off of the fully-controlled power semiconductor device, when the fully-controlled power semiconductor device is on, the photovoltaic string output current bypasses the fully-controlled power semiconductor device, the output voltage is clamped to be near zero potential, and when the fully-controlled power semiconductor device is off, the photovoltaic string transmits electric energy to the inverter through the blocking diode D.
3. A photovoltaic energy storage system based on direct-current side anti-backflow is characterized by comprising a photovoltaic string, an inverter, an energy storage unit, a load, a power grid and the direct-current side anti-backflow switching device according to claim 1 or 2, wherein the direct-current side anti-backflow switching device is connected in series between the photovoltaic string and a direct-current input side of the inverter, and the inverter is connected in parallel with the energy storage unit, the load and the power grid.
4. A method of controlling reverse flow prevention based on the apparatus of claim 1, comprising the steps of: S1, collecting active power P grid of a point of connection in real time; S2, judging whether P grid is smaller than zero, if P grid is larger than 0, keeping the circuit breaking unit K1 closed and the power regulating unit K2 is turned off; S3, if P grid is less than 0, acquiring an absolute value |P grid| of current countercurrent power and current output power P pv of the single-path photovoltaic string; s4, executing a hierarchical anti-reflux control strategy: Calculating the number N of photovoltaic group strings to be completely cut, wherein the calculation formula is N=int (|P grid /P string |), wherein P string represents the unit power of the photovoltaic group strings, and int is a rounding function; And the second stage of control, namely calculating residual power P rem = |P grid |- N ×P pv of the countercurrent after the removal, calculating PWM duty ratio D cacl required by eliminating the residual power by utilizing a closed-loop control algorithm or an open-loop control algorithm, and controlling a power regulating unit K2 in the (n+1) th countercurrent prevention sub-module to conduct according to the duty ratio so as to eliminate the residual countercurrent.
5. The method according to claim 4, wherein the theoretical duty ratio D cacl is calculated in step S4 based on an inverse relation between the output power of the photovoltaic string and the duty ratio, and the specific formula relation is as follows: , wherein P pv is the current output power of the single-path photovoltaic string; The power output by the (n+1) th anti-backflow sub-module is represented, the duty cycle is set to be cut off a threshold D limit , the threshold represents the maximum duty cycle of the system allowing the photovoltaic group string to bypass through K2, and D cacl is the theoretical duty cycle calculated at the current kth moment.
6. The method according to claim 4, wherein the specific calculation of the theoretical duty ratio D cacl in step S3 uses an incremental PID algorithm, and the calculation formula is as follows: , Wherein D cacl is the theoretical duty cycle calculated at the present k-th moment, triggering the excision mode in step S4 when D cacl (k)>D limit is calculated, The adjustment value of the duty cycle at time k is shown.
7. The method according to claim 6, wherein the duty cycle at time k is adjusted to a value The following formula is calculated: , Wherein, the The scale factor is represented by a ratio of, For the target power deviation at the current moment, The integral coefficient is represented by a value of, Representing the differential coefficient.
8. The method according to claim 4, wherein the controller preferably decreases the PWM duty cycle when the grid-tie power is restored to the forward output and exceeds a preset restoration threshold, and then closes the breaking unit K1 step by step to restore the photovoltaic string power supply.

Description

Direct-current side anti-countercurrent device and method for photovoltaic inverter Technical Field The invention belongs to the technical field of distributed photovoltaic power generation and energy storage, and particularly relates to a direct-current side countercurrent prevention device and method for a photovoltaic inverter. Background With the popularization of distributed photovoltaic power generation, the anti-countercurrent requirement of 'self-power-consumption and residual power-off-grid' is increasing. The existing anti-backflow technical scheme mainly depends on a communication regulation mode, namely, an anti-backflow ammeter collects grid-connected point power and transmits data to a data collector (or directly to an inverter), and the inverter adjusts own working point (MPPT) according to a received instruction to reduce output power. However, this solution has problems of slow response speed (usually second-order delay), incompatibility of multi-vendor protocols, difficulty in system integration, and the like. In addition, existing hardware chopping anti-reflux schemes often employ a single PWM regulation mode. When the system load is extremely low (resulting in an extremely high reverse current power), the power device has to operate in a state of extremely large short-circuit duty cycle for a long time in order to suppress the photovoltaic output. This not only results in severe heating of the power device (e.g. MOSFET or IGBT), reducing the lifetime and safety of the device, but in systems equipped with energy storage units this "forced photovoltaic" strategy is not optimal. At this time, a more reasonable strategy should be to cut off the photovoltaic, directly utilize the energy storage unit to supply power, so as to optimize the energy efficiency of the system and protect the hardware circuit. Therefore, a general dc-side hard control anti-reverse solution that is fast in response speed and independent of the inverter communication protocol is needed. Disclosure of Invention The invention provides a direct current side countercurrent prevention device and method for a photovoltaic inverter, aiming at solving the problem of slow response in the prior art, introducing a judgment mechanism based on a theoretical duty ratio, and automatically switching to an energy storage power supply mode when photovoltaic output is required to be reduced greatly. The photovoltaic inverter direct current side anti-reflux device comprises a plurality of anti-reflux submodules, wherein each anti-reflux submodule is correspondingly connected with one photovoltaic string and comprises a breaking unit K1, the breaking unit K1 is arranged in an output loop of the negative electrode of the photovoltaic string in series and is configured to be connected with or physically disconnected from the photovoltaic string according to a control signal; the input end of the power regulating unit K2 is connected to a connecting node of the breaking unit K1 and the positive electrode of the photovoltaic string and is configured to form a bypass loop in a conducting state; the blocking diode D is arranged between the positive pole of the photovoltaic string and the inverter in series; The controller is electrically connected with the circuit breaking unit K1, the power adjusting unit K2 and the grid-connected point power acquisition module of each way of anti-backflow sub-module and is configured to execute a grading anti-backflow control strategy; The grading anti-reflux control strategy is as follows: the first stage of cutting, namely calculating the number N of the photovoltaic strings to be completely cut, wherein a calculation formula is N=int (|P grid/Pstring |), wherein int is a rounding function, a controller controls a breaking unit K1 of the N-path reverse flow prevention sub-module to be disconnected, P grid represents the active power of a parallel network point, and P string represents the unit power of the photovoltaic strings; And the second stage of control, namely calculating the current output power of the single-path photovoltaic string represented by the residual reverse current power residual error P rem= |Pgrid|- N ×Ppv;Ppv after cutting, calculating the PWM duty ratio D cacl required for eliminating the residual power by utilizing a closed-loop control algorithm or an open-loop control algorithm, and controlling the power regulating unit K2 in the (n+1) th reverse current prevention sub-module to conduct at the duty ratio so as to eliminate the residual reverse current. Further, in the photovoltaic inverter direct current side backflow prevention device, the power regulating unit K2 comprises a full-control type power semiconductor device, the controller outputs PWM signals to control the full-control type power semiconductor device to be turned on and off, when the full-control type power semiconductor device is turned on, the photovoltaic string output current flows through a bypass of the full-co