Device and method for preventing countercurrent on direct current side of photovoltaic inverter

Abstract

The invention provides a photovoltaic inverter DC side anti-reflux device and method, and belongs to the field of distributed power generation. The device is connected in series between a photovoltaic string and an inverter, and comprises a circuit breaking unit, a parallel power adjusting unit, a blocking diode and the like. The control method adopts a hierarchical control strategy of removal and adjustment: a controller collects the countercurrent power of a grid-connected point in real time, and when countercurrent is detected, the countercurrent power is judged; if the countercurrent power exceeds the output power of the single-path photovoltaic string, cutting off is executed, the corresponding number of circuit breaking units are directly controlled to be disconnected, and part of the photovoltaic string is physically cut off; and controlling the residual trace countercurrent power, calculating a PWM duty ratio by using a direct calculation or incremental PID algorithm, and driving a power adjusting unit to carry out hardware chopping consumption or suppression. According to the invention, long-time heating of the power device under large current is avoided, the problem of overlarge power regulation granularity caused by pure cutting is solved, and smooth and rapid countercurrent prevention of the photovoltaic system is realized.

Description

Technical Field

[0001] This invention belongs to the field of distributed photovoltaic power generation and energy storage technology, specifically relating to a reverse current protection device and method for the DC side of a photovoltaic inverter. Background Technology

[0002] With the increasing popularity of distributed photovoltaic power generation, the demand for anti-reverse current technology, which aims for "self-consumption and no grid connection of surplus power," is growing. Existing anti-reverse current technology solutions mainly rely on communication regulation methods, namely: the anti-reverse current meter collects the power at the grid connection point and transmits the data to the data acquisition unit (or directly to the inverter). The inverter then adjusts its operating point (MPPT) according to the received instructions to reduce the output power.

[0003] However, this solution suffers from slow response times (typically with delays in the order of seconds), incompatibility with multiple vendor protocols, and difficulties in system integration. Furthermore, some existing hardware chopper anti-reverse current solutions often employ a single PWM regulation mode. When the system load is extremely low (resulting in extremely high reverse current power), power devices must operate at extremely high short-circuit duty cycles for extended periods to suppress photovoltaic output. This not only leads to severe overheating of power devices (such as MOSFETs or IGBTs), reducing the lifespan and safety of the device, but also, in systems equipped with energy storage units, this "forced suppression of photovoltaics" strategy is not the optimal solution. A more reasonable strategy in this situation would be to disconnect the photovoltaic system and directly utilize the energy storage unit for power supply, thereby optimizing system energy efficiency and protecting the hardware circuitry.

[0004] Therefore, there is an urgent need for a universal, fast-responding DC-side hard-control anti-reverse current solution that does not depend on the inverter communication protocol. Summary of the Invention

[0005] This invention addresses the problems existing in the prior art by providing a device and method for preventing reverse current on the DC side of a photovoltaic inverter. It aims to solve the problem of slow response in the prior art and introduces a decision mechanism based on the "theoretical duty cycle" to automatically switch to energy storage power supply mode when the photovoltaic output needs to be significantly reduced.

[0006] To solve the above technical problems, the present invention provides the following technical solution: a DC-side anti-reverse current device for a photovoltaic inverter, configured between the photovoltaic string and the DC input side of the inverter, comprising: a multi-channel anti-reverse current submodule, each channel anti-reverse current submodule being connected to one photovoltaic string, the submodule comprising: a circuit breaker unit K1, which is connected in series in the output circuit of the negative terminal of the photovoltaic string, configured to conduct or physically disconnect the connection of the photovoltaic string according to a control signal;

[0007] The power regulation unit K2 has its input terminal connected to the connection node between the circuit breaker unit K1 and the positive terminal of the photovoltaic string, and is configured to form a bypass loop in the on state.

[0008] The blocking diode D is connected in series between the positive terminal of the photovoltaic string and the inverter.

[0009] The controller is electrically connected to the circuit breaker unit K1, power regulation unit K2, and grid connection point power acquisition module of each anti-backflow submodule; the controller is configured to execute a hierarchical anti-backflow control strategy.

[0010] The graded anti-backflow control strategy is as follows:

[0011] First-level cutoff: Calculate the number N of photovoltaic strings that need to be completely cut off, using the formula N = int(|P grid / P string |), where int is the integer function; the controller controls the circuit breaker unit K1 of the N-channel anti-reverse current submodule to open; P grid P represents the active power at the grid connection point. string This represents the unit power of the photovoltaic string;

[0012] Second-level control: Calculate the residual reverse power P after the cut-off. rem = |P grid |- N ×P pv ;P pv This represents the current output power of a single photovoltaic string; the PWM duty cycle D required to eliminate this residual power is calculated using a closed-loop control algorithm or an open-loop control algorithm. cacl And control the power regulation unit K2 in the N+1th anti-backflow submodule to conduct with this duty cycle to eliminate the remaining backflow.

[0013] Furthermore, in the aforementioned photovoltaic inverter DC-side anti-reverse current device, the power regulation unit K2 includes a fully controlled power semiconductor device, and the controller outputs a PWM signal to control the on and off of the fully controlled power semiconductor device; when the fully controlled power semiconductor device is on, the output current of the photovoltaic string is bypassed by it, and the output voltage is clamped to near zero potential; when it is off, the photovoltaic string supplies power to the inverter through the blocking diode D.

[0014] Furthermore, the aforementioned photovoltaic energy storage system based on DC-side anti-reverse current includes: a photovoltaic string, an inverter, an energy storage unit, a load, a power grid, and a DC-side anti-reverse current switch device; the DC-side anti-reverse current switch device is connected in series between the photovoltaic string and the DC input side of the inverter; the inverter is connected in parallel with the energy storage unit, the load, and the power grid.

[0015] Furthermore, the aforementioned backflow prevention control method includes the following steps:

[0016] S1. Real-time acquisition of active power P at the grid connection point grid ;

[0017] S2. Determine P grid Is it less than zero? If P grid >0, keep circuit breaker unit K1 closed and power regulation unit K2 off;

[0018] S3. If P grid < 0, obtain the absolute value of the current reverse current power |P grid| and the current output power P of a single photovoltaic string pv ;

[0019] S4. Implement a tiered anti-backflow control strategy:

[0020] First-level cutoff: Calculate the number N of photovoltaic strings that need to be completely cut off, using the formula N = int(|P grid / P string |), where P string This represents the unit power of the photovoltaic string, and int is the integer function; the controller controls the circuit breaker unit K1 of the N-channel anti-reverse current submodule to disconnect;

[0021] Second-level control: Calculate the residual reverse power P after the cut-off. rem = |P grid |- N ×P pv The PWM duty cycle D required to eliminate the residual power is calculated using either a closed-loop or open-loop control algorithm. cacl And control the power regulation unit K2 in the N+1th anti-backflow submodule to conduct with this duty cycle to eliminate the remaining backflow.

[0022] Furthermore, in the aforementioned anti-backflow control method, step S4 involves calculating the theoretical duty cycle D. cacl The basis for this is the inverse relationship between the output power of a photovoltaic string and its duty cycle, as shown in the following formula:

[0023] ,

[0024] Among them, P pv This represents the current output power of a single photovoltaic string. This represents the output power of the (N+1)th anti-reverse submodule, with the duty cycle set to the cutoff threshold D. limit This threshold represents the maximum duty cycle that the system allows for bypassing of the photovoltaic string via K2, D cacl This is the theoretical duty cycle calculated at the current k-th time.

[0025] The theoretical duty cycle D in step S3 cacl The specific calculation uses an incremental PID algorithm, and the calculation formula is as follows:

[0026] ,

[0027] Among them, D cacl The theoretical duty cycle calculated at time k; when the calculated D cacl (k)>D limit At that time, the resection mode in step S4 is triggered. This represents the adjustment value of the duty cycle at time k.

[0028] Calculate as follows:

[0029] ,

[0030] in, This represents the proportionality coefficient. The target power deviation at the current moment. Represents the integral coefficient. This represents the differential coefficient.

[0031] Furthermore, in the aforementioned anti-reverse current control method, when the grid connection point power recovers to positive output and exceeds the preset recovery threshold, the controller first reduces the PWM duty cycle, and then closes the circuit breaker unit K1 step by step to restore the power supply of the photovoltaic string.

[0032] Compared with the prior art, the beneficial technical effects of the present invention using the above technical solution are as follows:

[0033] First, the control strategy is scientific and efficient. It introduces duty cycle calculation as a pre-judgment condition, balancing fine adjustment during low-power reverse current and system protection during high-power reverse current, effectively preventing overheating of power devices under long-term high-current short-circuit conditions. Second, it boasts fast response and strong compatibility. Employing DC-side hardware chopper control, its response speed reaches millisecond levels, unaffected by communication delays. Furthermore, the device operates independently of the inverter, eliminating the need for RS485 / Modbus communication, thus solving the problem of protocol incompatibility between multiple inverter brands. Third, it is safe and reliable. It incorporates blocking diodes to prevent capacitor backflow and utilizes the inherent short-circuit safety characteristics of photovoltaic modules, preventing damage to the modules. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the overall structure of the photovoltaic energy storage system provided in an embodiment of the present invention.

[0035] Figure 2 This is a circuit topology diagram of the DC-side anti-reverse current switch device provided by the present invention.

[0036] Figure 3 This is a control flowchart based on duty cycle calculation and determination provided by the present invention.

[0037] Figure 4These are timing waveform diagrams of switch changes and mode switching under different load conditions. In the figure, (a) is the system power change curve and (b) is the timing diagram of key switch control. Detailed Implementation

[0038] To better understand the technical content of the present invention, specific embodiments are described below in conjunction with the accompanying drawings.

[0039] In this invention, various aspects of the invention are described with reference to the accompanying drawings, in which numerous illustrative embodiments are shown. Embodiments of the invention are not limited to those depicted in the drawings. It should be understood that the invention is implemented through any of the various concepts and embodiments described above, as well as the concepts and embodiments described in detail below, because the concepts and embodiments disclosed herein are not limited to any particular implementation. Furthermore, some aspects of the invention disclosed may be used alone or in any suitable combination with other aspects of the invention disclosed.

[0040] Example 1: As Figure 1 and 2 As shown, a DC-side anti-reverse current device for a photovoltaic inverter is configured between the photovoltaic string and the DC input side of the inverter, including: a multi-channel anti-reverse current submodule, each channel anti-reverse current submodule being connected to one photovoltaic string, the submodule including: a circuit breaker unit K1, which is connected in series in the output circuit of the negative terminal of the photovoltaic string, configured to conduct or physically disconnect the connection of the photovoltaic string according to a control signal;

[0041] The power regulation unit K2 has its input terminal connected to the connection node between the circuit breaker unit K1 and the positive terminal of the photovoltaic string, and is configured to form a bypass loop in the on state.

[0042] The blocking diode D is connected in series between the positive terminal of the photovoltaic string and the inverter.

[0043] The controller is electrically connected to the circuit breaker unit K1, power regulation unit K2, and grid connection point power acquisition module of each anti-backflow submodule; the controller is configured to execute a hierarchical anti-backflow control strategy.

[0044] The graded anti-backflow control strategy is as follows:

[0045] First-level cutoff: Calculate the number N of photovoltaic strings that need to be completely cut off, using the formula N = int(|P grid / P string |), where P string This represents the unit power of the photovoltaic string, and int is the integer function; the controller controls the circuit breaker unit K1 of the N-channel anti-reverse current submodule to disconnect;

[0046] Second-level control: Calculate the residual reverse power P after the cut-off. rem= |P grid |- N ×P pv The PWM duty cycle D required to eliminate the residual power is calculated using either a closed-loop or open-loop control algorithm. cacl And control the power regulation unit K2 in the N+1th anti-backflow submodule to conduct with this duty cycle to eliminate the remaining backflow.

[0047] As a preferred embodiment of Example 1, the power regulation unit K2 includes a fully controlled power semiconductor device. The controller outputs a PWM signal to control the on and off of the fully controlled power semiconductor device. When the fully controlled power semiconductor device is on, the photovoltaic string output current is bypassed by it, and the output voltage is clamped to near zero potential. When it is off, the photovoltaic string supplies power to the inverter through the blocking diode (D).

[0048] Example 2: A photovoltaic energy storage system based on DC-side anti-reverse current is also provided, including: a photovoltaic string, an inverter, an energy storage unit, a load, a power grid, and a DC-side anti-reverse current switch device as in Example 1; the DC-side anti-reverse current switch device is connected in series between the photovoltaic string and the DC input side of the inverter; the inverter is connected in parallel with the energy storage unit, the load, and the power grid.

[0049] Example 3: As Figure 3 and Figure 4 As shown, Figure 3 The control process based on duty cycle calculation and determination is demonstrated. Figure 4 The timing waveforms of switching changes and mode switching under different load conditions are shown. Figure (a) shows the system power change curve, and (b) shows the timing diagram of key switch control. An example provides an anti-reverse current control method for a photovoltaic inverter DC-side anti-reverse current device based on Example 1. The control system uses a sampling period Ts = 5ms. A duty cycle cutoff threshold D is set. limit =50%. The specific control steps are as follows:

[0050] S1. Real-time acquisition of active power P at the grid connection point grid ;

[0051] S2. Determine P grid Is it less than zero? If P grid >0, keep circuit breaker unit K1 closed and power regulation unit K2 off;

[0052] S3. If P grid < 0, obtain the absolute value of the current reverse current power |P grid |and the current output power P of a single photovoltaic string. pv ;

[0053] S4. Implement a tiered anti-backflow control strategy:

[0054] First-level cutoff: Calculate the number N of photovoltaic strings that need to be completely cut off, using the formula N = int(|P grid / P string |), where P string This represents the unit power of the photovoltaic string, and int is the integer function; the controller controls the circuit breaker unit K1 of the N-channel anti-reverse current submodule to disconnect;

[0055] Second-level control: Calculate the residual reverse power P after the cut-off. rem = |P grid |- N ×P pv The PWM duty cycle D required to eliminate the residual power is calculated using either a closed-loop or open-loop control algorithm. cacl And control the power regulation unit K2 in the N+1th anti-backflow submodule to conduct with this duty cycle to eliminate the remaining backflow.

[0056] As a preferred embodiment of Example 3, the theoretical duty cycle D is calculated. cacl The basis for this is the inverse relationship between the output power of a photovoltaic string and its duty cycle, as shown in the following formula:

[0057] ,

[0058] Among them, P pv This represents the current output power of a single photovoltaic string. This represents the output power of the (N+1)th anti-reverse submodule, with the duty cycle set to the cutoff threshold D. limit This threshold represents the maximum duty cycle that the system allows for bypassing of the photovoltaic string via K2, D cacl This is the theoretical duty cycle calculated at the current k-th time.

[0059] As a preferred embodiment of Example 3, the theoretical duty cycle D cacl The specific calculation uses an incremental PID algorithm, and the calculation formula is as follows:

[0060] ,

[0061] Among them, D cacl The theoretical duty cycle calculated at time k; when the calculated D cacl (k)>D limit At that time, the resection mode in step S4 is triggered. This represents the adjustment value of the duty cycle at time k.

[0062] As a preferred embodiment of Example 3, Calculate as follows:

[0063] ,

[0064] in, This represents the proportionality coefficient. The target power deviation at the current moment. Represents the integral coefficient. This represents the differential coefficient.

[0065] As a preferred embodiment of Example 3, when the grid connection point power recovers to positive output and exceeds the preset recovery threshold, the controller first reduces the PWM duty cycle, and then closes the circuit breaker unit K1 step by step to restore the power supply of the photovoltaic string.

[0066] While the present invention has been described above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A DC-side anti-reverse current device for a photovoltaic inverter, configured between the photovoltaic string and the DC input side of the inverter, characterized in that, include: A multi-channel anti-reverse current submodule, each channel anti-reverse current submodule is connected to one photovoltaic string, the submodule includes: a circuit breaker unit K1, which is connected in series in the output circuit of the negative terminal of the photovoltaic string, and is configured to turn on or physically disconnect the connection of the photovoltaic string according to the control signal; The power regulation unit K2 has its input terminal connected to the connection node between the circuit breaker unit K1 and the positive terminal of the photovoltaic string, and is configured to form a bypass loop in the on state. The blocking diode D is connected in series between the positive terminal of the photovoltaic string and the inverter. The controller is electrically connected to the circuit breaker unit K1, power regulation unit K2, and grid connection point power acquisition module of each anti-backflow submodule; the controller is configured to execute a hierarchical anti-backflow control strategy. The graded anti-backflow control strategy is as follows: First-level cutoff: Calculate the number N of photovoltaic strings that need to be completely cut off, using the formula N = int(|P grid / P string |), where int is the integer function; the controller controls the circuit breaker unit K1 of the N-channel anti-reverse current submodule to open; P grid P represents the active power at the grid connection point. string This represents the unit power of the photovoltaic string; Second-level control: Calculate the residual reverse power P after the cut-off. rem = |P grid |- N ×P pv ;P pv This represents the current output power of a single photovoltaic string; the PWM duty cycle D required to eliminate this residual power is calculated using a closed-loop control algorithm or an open-loop control algorithm. cacl And control the power regulation unit K2 in the N+1th anti-backflow submodule to conduct with this duty cycle to eliminate the remaining backflow.

2. The anti-reverse current device for the DC side of a photovoltaic inverter according to claim 1, characterized in that, The power regulation unit K2 includes a fully controllable power semiconductor device. The controller outputs a PWM signal to control the on and off of the fully controllable power semiconductor device. When the fully controllable power semiconductor device is on, the photovoltaic string output current bypasses it, and the output voltage is clamped to near zero potential. When it is turned off, the photovoltaic string supplies electrical energy to the inverter through the blocking diode D.

3. A photovoltaic energy storage system based on DC-side anti-reverse current, characterized in that, include: Photovoltaic strings, inverters, energy storage units, loads, power grids, and the DC-side anti-reverse current switch device as described in claim 1 or 2; The DC-side anti-reverse current switch is connected in series between the photovoltaic string and the DC input side of the inverter; the inverter is connected in parallel with the energy storage unit, the load, and the power grid.

4. A backflow prevention control method based on the device described in claim 1, characterized in that, Includes the following steps: S1. Real-time acquisition of active power P at the grid connection point grid ; S2. Determine P grid Is it less than zero? If P grid >0, keep circuit breaker unit K1 closed and power regulation unit K2 off; S3. If P grid < 0, obtain the absolute value of the current reverse current power |P grid| and the current output power P of a single photovoltaic string pv ; S4. Implement a tiered anti-backflow control strategy: First-level cutoff: Calculate the number N of photovoltaic strings that need to be completely cut off, using the formula N = int(|P grid / P string |), where P string This represents the unit power of the photovoltaic string, and int is the integer function; the controller controls the circuit breaker unit K1 of the N-channel anti-reverse current submodule to disconnect; Second-level control: Calculate the residual reverse power P after the cut-off. rem = |P grid |- N ×P pv The PWM duty cycle D required to eliminate the residual power is calculated using either a closed-loop or open-loop control algorithm. cacl And control the power regulation unit K2 in the N+1th anti-backflow submodule to conduct with this duty cycle to eliminate the remaining backflow.

5. The anti-backflow control method according to claim 4, characterized in that, In step S4, the theoretical duty cycle D is calculated. cacl The basis for this is the inverse relationship between the output power of a photovoltaic string and its duty cycle, as shown in the following formula: ， Among them, P pv This represents the current output power of a single photovoltaic string. This represents the output power of the (N+1)th anti-reverse submodule, with the duty cycle set to the cutoff threshold D. limit This threshold represents the maximum duty cycle that the system allows for bypassing of the photovoltaic string via K2, D cacl This is the theoretical duty cycle calculated at the current k-th time.

6. The anti-backflow control method according to claim 4, characterized in that, In step S3, the theoretical duty cycle D cacl The specific calculation uses an incremental PID algorithm, and the calculation formula is as follows: ， Among them, D cacl The theoretical duty cycle calculated at time k; when the calculated D cacl (k)>D limit At that time, the resection mode in step S4 is triggered. This represents the adjustment value of the duty cycle at time k.

7. The anti-backflow control method according to claim 6, characterized in that, Adjustment value of duty cycle at time k Calculate as follows: ， in, This represents the proportionality coefficient. The target power deviation at the current moment. Represents the integral coefficient. This represents the differential coefficient.

8. The method according to claim 4, characterized in that, When the grid connection point power recovers to positive output and exceeds the preset recovery threshold, the controller first reduces the PWM duty cycle, and then closes the circuit breaker unit K1 step by step to restore the power supply of the photovoltaic string.

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