A method for controlling reverse flow protection

Abstract

This invention discloses an anti-reverse current protection control method applied to microgrids, comprising: real-time monitoring of the active power at the microgrid grid connection point; when the active power is less than the software anti-reverse current threshold, executing the first layer of protection: adjusting the output power of the energy storage converter and the photovoltaic inverter to raise the active power back above the software anti-reverse current threshold; executing the second layer of protection in parallel with the first layer of protection: controlling the photovoltaic circuit breaker and the energy storage circuit breaker to trip by simulating the tripping action of the hardware circuit breaker through software; when the active power is less than or equal to the preset hardware tripping threshold and the reverse current duration exceeds the first delay time, executing the third layer of protection: controlling the tripping of the circuit breaker at the grid connection point through the anti-reverse current integrated protection device to enable the microgrid to operate in islanded mode. Compared with the prior art, this invention ensures safe grid connection of the microgrid with zero reverse current through a three-layer collaborative anti-reverse current protection architecture, thereby improving the system operation efficiency.

Description

Technical Field

[0001] This invention relates to the field of distributed generation technology, and in particular to a reverse current protection and control method. Background Technology

[0002] Distributed energy, represented by photovoltaics, is rapidly penetrating industrial parks and other settings. While its self-consumption and surplus power-to-grid model is increasing the proportion of clean energy, it also presents new challenges to the stable operation of the power distribution network. When the instantaneous power generation of distributed energy exceeds the local load absorption capacity, a reverse current phenomenon occurs, where power is fed back to the upstream grid. This can lead to safety issues such as grid voltage exceeding limits and protection system malfunctions. Therefore, anti-reverse current control has become a core technology for ensuring the safe grid connection of distributed energy and achieving efficient local consumption.

[0003] A search revealed Chinese Patent Publication No. CN113036804A, which discloses an AC / DC microgrid control method and device. This method monitors the power at the grid connection point in real time. When preset anti-reverse current protection trigger conditions are met, it controls the disconnection of the electrical connection between the AC microgrid and the bidirectional converter, thereby cutting off the power backflow path from the DC side to the AC side and achieving anti-reverse current protection. However, this solution achieves anti-reverse current protection by disconnecting the electrical connection, which is essentially a rigid protection measure. While it can prevent reverse current, it cannot ensure the continuity of power supply and prioritize the consumption of photovoltaic power through flexible adjustment. Furthermore, frequent switching operations may affect the equipment's lifespan.

[0004] The technical problem that needs to be solved is how to reduce the frequency of operation of hardware protection devices while ensuring the safe grid connection of microgrids and avoiding reverse current, and at the same time reduce the loss of energy storage equipment during the regulation process and ensure the revenue of photovoltaic power generation. Summary of the Invention

[0005] The purpose of this invention is to overcome the defects of the prior art and provide a backflow prevention and control method.

[0006] The objective of this invention can be achieved through the following technical solutions: According to a first aspect of the present invention, a reverse current protection control method is provided, applied to a microgrid, comprising: Real-time monitoring of the active power Ppcc at the microgrid grid connection point PCC; When the active power Ppcc is less than the preset software anti-reverse current threshold Pccset, the first layer of protection is executed: the output power of the energy storage converter and the photovoltaic inverter is adjusted so that the active power Ppcc rises back to above the preset software anti-reverse current threshold Pccset. The second layer of protection is executed in parallel with the first layer of protection: the tripping action of the hardware circuit breaker is simulated by software to control the opening of the photovoltaic circuit breaker and the energy storage circuit breaker. When the active power Ppcc is less than or equal to the preset hardware tripping threshold and the reverse current duration exceeds the first delay time, the third layer of protection is executed: the grid connection point circuit breaker is tripped by the anti-reverse current integrated protection device to enable the microgrid to operate in islanded mode.

[0007] As a preferred technical solution, the first layer of protection includes: Calculate the power difference Δp that needs to be adjusted: Δp = Pccset - Ppcc; Prioritize the regulation of the energy storage converter: If the energy storage system is not fully charged, its power is adjusted according to the current state of the energy storage converter and the power difference Δp. If the energy storage system is fully charged, a remote control command is issued to put the energy storage converter into standby mode, and then adjust the output power of the photovoltaic inverter.

[0008] As a preferred technical solution, the software anti-backflow threshold Pccset is a positive value that is set by the user, and Pccset is slightly larger than the action setting value of the anti-backflow integrated protection device.

[0009] As a preferred technical solution, the power direction of the energy storage converter is specified as positive discharge and negative charge, and the adjustment of its power according to the current state of the energy storage converter includes: If the energy storage converter is in charging state, the charging power is increased and the power command is max{Ppcs-Δp,-Ppcsn}, where Ppcs is the real-time power of the energy storage converter and Ppcsn is the rated power of the energy storage converter. If the energy storage converter is in a discharging state, reduce the discharge power: When Ppcs-Δp>0, the power command issued is min{Ppcs-Δp,Ppcsn}; When Ppcs-Δp≤0, the energy storage converter is controlled to standby. If the energy storage converter is in standby mode, it will be controlled to start charging, and the power command will be -Δp.

[0010] As a preferred technical solution, the adjustment of the output power of the photovoltaic inverter includes: when the energy storage is fully charged and cannot continue to be charged, limiting the photovoltaic output and issuing a photovoltaic power command of Ppv-Δp, where Ppv is the real-time power of the photovoltaic inverter.

[0011] As a preferred technical solution, the adjustment process follows the principle of minimization adjustment. When the energy storage converter is in a discharge state and triggers the first layer of protection, the power deficit is compensated by reducing the discharge power first.

[0012] As a preferred technical solution, the method further includes: when there is no photovoltaic output and the energy storage system is fully charged, performing a load shedding operation to cut off a preset controllable load, which includes a charging pile, an air conditioning system, or a lighting system.

[0013] As a preferred technical solution, the third layer of protection further includes: When Ppcc is greater than the preset recovery threshold and the non-reverse current duration exceeds the second delay time, the anti-reverse current integrated protection device controls the circuit breaker at the grid connection point to close, so that the microgrid can be reconnected to the grid.

[0014] As a preferred technical solution, the software anti-reverse current threshold Pccset is greater than the hardware tripping threshold, and the software simulated tripping action of the second layer of protection is executed before the hardware tripping action of the third layer of protection.

[0015] As a preferred technical solution, the method is implemented based on the Acrel-2000MG energy management system.

[0016] Compared with the prior art, the present invention has the following advantages: 1. The first layer of this invention features flexible regulation that actively avoids reverse current by adjusting energy storage and photovoltaic output. The second layer uses software simulation protection for real-time monitoring and intervention when power approaches the tripping threshold. The third layer, hardware protection, serves as the last line of defense and is triggered only when regulation fails. This three-layer architecture effectively reduces the frequency of hardware protection device actions while ensuring safe grid connection of the microgrid and avoiding reverse current, thus preventing power outages and equipment damage caused by frequent tripping.

[0017] 2. In the first layer of flexible regulation, the present invention prioritizes the regulation of the energy storage converter, and only restricts the photovoltaic output when the energy storage is fully charged, effectively ensuring the photovoltaic power generation revenue. When regulating the energy storage, it tries to keep its current charging and discharging state unchanged, and makes up for the power deficit by increasing the charging power, decreasing the discharging power, or starting the standby energy storage, avoiding frequent switching between charging and discharging states of the energy storage system, and effectively extending the service life of the energy storage equipment.

[0018] 3. This invention sets the software anti-reverse current threshold to a positive value greater than the hardware tripping threshold, ensuring that the first layer of flexible regulation and the second layer of software simulation protection take effect before the hardware protection. The second layer of protection runs in parallel with the first layer of protection, monitors in real time and intervenes in advance, providing a reliable buffer for the hardware protection. The hardware protection automatically opens and closes when the conditions are met, taking into account both reliability and ease of recovery. Attached Figure Description

[0019] Figure 1 This is a flowchart of the method of the present invention; Figure 2 This is a flowchart of the first layer of protection of the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0021] Example 1: This invention provides a reverse current protection control method applied to a microgrid system. The microgrid system includes: a photovoltaic power generation system, an energy storage system, AC loads, a point of common coupling (PCC), an energy management system (EMS), a reverse current integrated protection device, and several circuit breakers.

[0022] To facilitate understanding of the technical solution of this invention, the key parameters used in this invention are first defined as follows: Ppcc: Active power value of the meter at the connection point between the microgrid and the main grid; a negative value indicates reverse flow. Pccset: The backflow prevention threshold set by the software and configured by the EMS system's built-in script. When Ppcc is less than this value, it indicates a risk of backflow. This threshold is set to a positive value and is slightly higher than the hardware tripping threshold of the subsequent backflow prevention integrated protection device. SOC: State of charge of the energy storage battery; SOCmax: The maximum charging limit for the energy storage battery set by the software, used to protect the battery from overcharging; SOCmin: The minimum discharge point of the energy storage battery set by the software, used to protect the battery from over-discharge; Ppcs: Real-time power of the power conversion system (PCS). In the embodiments of the present invention, when the energy storage system is in a discharging state, a negative Ppcs value indicates that the energy storage system is in a charging state. Ppcsn: Rated power of the energy storage converter PCS; Ppcsset: The active power that the software needs to set for the energy storage converter PCS; Δp: The calculated power difference; Ppv: Real-time output power of a photovoltaic inverter; Ppvset: The active power setting that the software needs to configure for the photovoltaic inverter.

[0023] like Figure 1As shown in the figure, the first layer of protection of the method of the present invention is software flexible reverse flow regulation. This part takes the energy storage converter and the photovoltaic inverter as the main regulating devices. By preferentially consuming the energy storage, the microgrid tries to achieve the spontaneous use of photovoltaic power and peak-valley arbitrage without reverse flow. The second layer of protection simulates the actual reverse flow prevention action principle of the circuit breaker through software, intercepting the reverse flow earlier than the actual hardware prevention and control. The third layer of protection is that the hardware reverse flow prevention device in the microgrid performs on-off protection actions according to the principle of rigid regulation. The implementation methods of each layer of protection are described in detail below.

[0024] The first layer of protection: software flexible regulation As Figure 2 shown, the first layer of protection takes the energy storage converter and the photovoltaic inverter as the main regulating objects. Its core idea is: preferentially consume the excess photovoltaic power through the energy storage system, and only when the energy storage cannot consume it, limit the photovoltaic output, so as to avoid reverse flow while effectively using clean energy.

[0025] Trigger conditions and initialization: The EMS system collects the active power Ppcc of the point of common coupling (PCC) in real time. When it is monitored that Ppcc is less than the preset software reverse flow prevention threshold Pccset, the microgrid system is about to have reverse flow, and the first layer of protection process is started.

[0026] First, calculate the power difference to be adjusted: Δp = Pccset - Ppcc, Since Ppcc < Pccset at this time, Δp > 0, indicating the amount of power that needs to be absorbed or reduced.

[0027] Energy storage priority regulation logic: The EMS system first judges the state of charge (SOC) of the energy storage system: Case A: The energy storage system is not full (SOC < SOCmax) At this time, the energy storage system has the charging ability, and the system preferentially regulates the energy storage converter. According to the current operating state of the energy storage converter, it is divided into the following sub-cases: Sub-case A1: The energy storage converter is in the charging state (Ppcs < 0). At this time, it is necessary to increase the charging power. The issued power command is: Ppcsset = max{Ppcs - Δp, -Ppcsn}. Among them, Ppcs - Δp means increasing the charging power by Δp on the basis of the original charging power. Since Ppcs is negative, subtracting a positive value of Δp will make the value more negative, indicating an increase in the charging power. Through the limit function of the max function, it is ensured that the charging power does not exceed the rated charging power -Ppcsn.

[0028] Sub-case A2: The energy storage converter is in the discharging state (Ppcs > 0). At this time, it is necessary to reduce the discharging power to release the charging ability. Further judgment is made according to the calculated value: If Ppcs-Δp>0, it means that the energy storage is still in a discharging state after reducing the discharge power. In this case, the power command issued is: Ppcsset=min{Ppcs-Δp,Ppcsn}; If Ppcs-Δp≤0, it means that after reducing the discharge power, there is no excess power left to discharge from the energy storage. According to the principle of minimization adjustment, the energy storage converter is not directly switched to the charging state at this time, but is instead controlled to standby, i.e., Ppcsset=0.

[0029] Sub-case A3: The energy storage converter is in standby mode (Ppcs=0), at which point the energy storage is neither charging nor discharging. A power command is issued to start charging: Ppcsset=Ppcs-Δp=0-Δp=-Δp, that is, charging is performed directly at the power of Δp.

[0030] Scenario B: The energy storage system is fully charged. At this point, the energy storage system can no longer absorb excess electrical energy, and the EMS system switches to regulating the photovoltaic inverter. The power command issued is: Ppvset = Ppv - Δp, which means reducing the power output by Δp based on the current photovoltaic output to prevent reverse current caused by excess photovoltaic power.

[0031] Scenario C: No solar power output and energy storage is fully charged This is a boundary case where the photovoltaic system is not generating power (e.g., at night) but the energy storage system is fully charged, and there is still a risk of reverse flow (e.g., a sudden drop in load). In this situation, the above-mentioned adjustment methods cannot be effective. In this case, the EMS system performs load shedding operations, cutting off preset controllable loads (such as user-authorized loads that can be cut off, such as charging piles) to maintain power balance.

[0032] Second layer of protection: Software simulates hardware tripping The second layer of protection is a software-simulated tripping mechanism that executes in parallel with the first layer of protection. Simple software-based flexible adjustment has a response delay and may not completely prevent instantaneous backflow during periods of severe load fluctuation. Therefore, this invention simulates the tripping action of a hardware circuit breaker through software.

[0033] To achieve the second layer of protection, the EMS system needs to acquire control of the photovoltaic circuit breakers and energy storage circuit breakers in order to simulate their opening and closing operations via software commands. These circuit breakers differ from the grid-connected point circuit breakers in the subsequent hardware protection; they are circuit breakers on the distributed power source side.

[0034] Simulated trip logic: When the EMS system detects that Ppcc continues to decrease and meets the preset simulated trip conditions (for example, Ppcc is lower than a certain intermediate threshold, which is between the software anti-backflow threshold Pccset and the hardware trip threshold, and the duration exceeds the set value), the second layer of protection is triggered.

[0035] The software executes tripping logic in advance, following a protection principle similar to that of a hardware circuit breaker (such as reverse power two-stage protection). Specifically: When the risk of reverse current intensifies and the software determines that it is about to reach the hardware protection threshold, the software commands will first control the photovoltaic circuit breaker and / or energy storage circuit breaker to trip, cutting off the possibility of reverse current at the source.

[0036] Once the system status is restored and Ppcc rises to a safe range and remains stable for a period of time, the software will then control the corresponding circuit breaker to close, restoring the connection of the distributed power source.

[0037] The significance of this layer of protection lies in the fact that it reacts faster than hardware protection, serving as a second line of defense after the failure of flexible regulation, buying time for hardware protection, and preventing the microgrid from frequently entering an islanded operation state.

[0038] Third layer of protection: Rigid hardware protection The third layer of protection is the rigid hardware protection implemented by the anti-reverse current integrated protection device. In this embodiment, the anti-reverse current integrated protection device adopts the reverse power two-stage protection principle and has automatic opening and closing functions.

[0039] Hardware configuration: The anti-reverse current integrated protection device is installed at the microgrid's grid connection point (PCC) and controls the grid connection point circuit breaker. This protection device works in conjunction with the EMS system, but executes protection logic independently to ensure reliable operation even if the EMS system fails.

[0040] The anti-backflow integrated protection device will trip when all of the following conditions are met: Power condition: Ppcc ≤ preset hardware trip threshold. This threshold is set to a positive value close to 0 to ensure operation before a reverse current actually occurs; Time condition: The duration of the above power condition exceeds the first delay time (trip delay, for example, set to 0.5 seconds, 1 second, etc. according to system requirements).

[0041] When the above conditions are met simultaneously, the anti-backflow integrated protection device determines that a backflow event has occurred and immediately issues a tripping command to the circuit breaker at the grid connection point, controlling the circuit breaker to trip and causing the microgrid to switch to islanded operation. At this time, the microgrid is physically isolated from the main grid, eliminating backflow.

[0042] During islanded operation of the microgrid, the anti-reverse current integrated protection device continuously monitors the voltage and frequency at the grid connection point. Automatic reclosing is executed when all of the following conditions are met: Power condition: Ppcc > preset recovery threshold. This recovery threshold is typically higher than the hardware trip threshold to ensure that the risk of reverse current has been completely eliminated; Time condition: The duration of the above power condition exceeds the second delay time (closing delay, which is usually set longer than the tripping delay to ensure grid stability).

[0043] When the above conditions are met simultaneously, the anti-reverse current integrated protection device sends a closing command to the circuit breaker at the grid connection point, controls the circuit breaker to close, and the microgrid is reconnected to the grid.

[0044] This invention is based on the Acrel-2000MG system. By constructing a three-layer collaborative protection architecture consisting of software flexible regulation, software-simulated hardware tripping, and hardware rigid protection, it ensures the safe operation of the microgrid with zero reverse current at the grid connection point. It also achieves efficient local photovoltaic consumption by prioritizing the use of energy storage to absorb photovoltaic power. At the same time, it adopts the principle of minimizing regulation to maintain the current operating state of energy storage as much as possible, reducing the frequency of charging and discharging state switching, thereby reducing energy storage losses and extending equipment life. The three-layer protection mechanism effectively reduces the frequency of hardware tripping, improving the reliability and overall economy of the microgrid system.

[0045] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A reverse current protection control method, applied to a microgrid, characterized in that, include: Real-time monitoring of the active power Ppcc at the microgrid grid connection point PCC; When the active power Ppcc is less than the preset software anti-reverse current threshold Pccset, the first layer of protection is executed: the output power of the energy storage converter and the photovoltaic inverter is adjusted so that the active power Ppcc rises back to above the preset software anti-reverse current threshold Pccset. The second layer of protection is executed in parallel with the first layer of protection: the tripping action of the hardware circuit breaker is simulated by software to control the opening of the photovoltaic circuit breaker and the energy storage circuit breaker. When the active power Ppcc is less than or equal to the preset hardware tripping threshold and the reverse current duration exceeds the first delay time, the third layer of protection is executed: the grid-connected circuit breaker is tripped through the anti-reverse current integrated protection device to enable the microgrid to operate in islanded mode.

2. The backflow prevention and control method according to claim 1, characterized in that, The first layer of protection includes: Calculate the power difference Δp that needs to be adjusted: Δp = Pccset - Ppcc; Prioritize the regulation of the energy storage converter: If the energy storage system is not fully charged, its power is adjusted according to the current state of the energy storage converter and the power difference Δp. If the energy storage system is fully charged, a remote control command is issued to put the energy storage converter into standby mode, and then adjust the output power of the photovoltaic inverter.

3. The backflow prevention and control method according to claim 2, characterized in that, The software backflow prevention threshold Pccset is a positive value that is set by the user, and Pccset is slightly larger than the action setting value of the backflow prevention integrated protection device.

4. The backflow prevention and control method according to claim 2, characterized in that, The power direction of the energy storage converter is specified as positive discharge and negative charge. Adjusting the power of the energy storage converter according to its current state includes: If the energy storage converter is in charging state, the charging power is increased and the power command is max{Ppcs-Δp,-Ppcsn}, where Ppcs is the real-time power of the energy storage converter and Ppcsn is the rated power of the energy storage converter. If the energy storage converter is in a discharging state, reduce the discharge power: When Ppcs-Δp>0, the power command issued is min{Ppcs-Δp,Ppcsn}; When Ppcs-Δp≤0, the energy storage converter is controlled to standby. If the energy storage converter is in standby mode, it will be controlled to start charging, and the power command will be -Δp.

5. The backflow prevention and control method according to claim 2, characterized in that, The adjustment of the output power of the photovoltaic inverter includes: when the energy storage is fully charged and cannot continue to be charged, limiting the photovoltaic output and issuing a photovoltaic power command of Ppv-Δp, where Ppv is the real-time power of the photovoltaic inverter.

6. The backflow prevention and control method according to claim 4, characterized in that, The adjustment process follows the principle of minimization. When the energy storage converter is in a discharge state and triggers the first layer of protection, the power deficit is compensated by reducing the discharge power first.

7. The backflow prevention and control method according to claim 1, characterized in that, The method further includes: when there is no photovoltaic output and the energy storage system is fully charged, performing a load shedding operation to cut off a preset controllable load, which includes charging piles, air conditioning or lighting systems.

8. The backflow prevention and control method according to claim 1, characterized in that, The third layer of protection also includes: When Ppcc is greater than the preset recovery threshold and the non-reverse current duration exceeds the second delay time, the anti-reverse current integrated protection device controls the circuit breaker at the grid connection point to close, so that the microgrid can be reconnected to the grid.

9. The backflow prevention and control method according to claim 1, characterized in that, The software anti-reverse current threshold Pccset is greater than the hardware trip threshold, and the software simulated trip action of the second layer of protection is executed before the hardware trip action of the third layer of protection.

10. The backflow prevention and control method according to claim 1, characterized in that, The method is implemented based on the Acrel-2000MG energy management system.

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