Power Conversion Systems

Abstract

To provide a power conversion system that can adjust its output AC frequency according to a charged ratio of a rechargeable battery.SOLUTION: A power conversion system 100 includes an AC power port, a first DC power port 16, a second DC power port 90, a high voltage capacitor C, a first DC converter, a second DC converter, a power inverter, and a microcontroller unit. The microcontroller unit adjusts a frequency F of AC power output by the power conversion system on the basis of a voltage difference Vbus across both ends of the high voltage capacitor of the power conversion system.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power conversion system (PCS), and more particularly to a power conversion system capable of adjusting the frequency of the AC power it outputs based on the charging rate of a rechargeable battery. [Background technology]

[0002] A power conversion system (PCS) is a bidirectional power conversion inverter that can be used for on-grid and off-grid electrical energy storage. How to operate a power conversion system effectively has been a key issue in the field. Summary of the Invention [Problem to be solved by the invention]

[0003] The present invention provides a power conversion system that can adjust the frequency of the AC power it outputs based on the charging rate of a rechargeable battery. [Means for solving the problem]

[0004] The power conversion system of the present invention includes an AC power port, a first DC power port, a second DC power port, a high-voltage capacitor, a first DC converter, a second DC converter, a power inverter, and a microcontrol unit. The AC power port is coupled to a first solar power inverter. The first DC power port is coupled to a rechargeable battery. The second DC power port is coupled to a solar panel. The first DC converter is coupled between the high-voltage capacitor and the first DC power port. The second DC converter is coupled between the high-voltage capacitor and the second DC power port. The power inverter is coupled between the high-voltage capacitor and the AC power port. The microcontrol unit is for adjusting the frequency of the AC power output from the AC power port of the power conversion system according to a voltage difference between both ends of the high-voltage capacitor. When the microcontrol unit detects the occurrence of an off-grid commercial power supply and the voltage difference is greater than a first critical value, the microcontrol unit sets the frequency of the AC power output from the AC power port to an interruption frequency to stop the output of electrical energy from the solar power inverter. When the micro control unit detects the occurrence of off-grid of the commercial power supply and the continuous time during which the voltage difference is between the first critical value and the second critical value exceeds the length of the preset time, the micro control unit sets the frequency of the AC power output from the AC power port to an interruption frequency. When the micro control unit detects the occurrence of off-grid of the commercial power supply and the voltage difference is between the second critical value and the third critical value, the micro control unit adjusts the frequency of the AC power output from the AC power port to increase by a first preset value. The first critical value is greater than the second critical value, and the second critical value is greater than the third critical value. [Brief description of the drawings]

[0005] [Figure 1] FIG. 1 is a functional block diagram of a power conversion system according to an embodiment of the present invention, and a coupled commercial power source, load, rechargeable battery, solar power generation inverter, and solar panel. [Diagram 2]Figure 1 shows the relationship between the external output power ratio of the solar power inverter and the frequency of the AC power output by the power conversion system. [Figure 3A] 2 is a flowchart showing how the microcontroller unit in FIG. 1 controls the power conversion system. [Figure 3B] 2 is a flowchart showing how the microcontroller unit in FIG. 1 controls the power conversion system. [Figure 4] 2 is another flowchart showing how the micro control unit in FIG. 1 controls the power conversion system. [Figure 5A] 2 is another flowchart showing how the micro control unit in FIG. 1 controls the power conversion system. [Figure 5B] 2 is another flowchart showing how the micro control unit in FIG. 1 controls the power conversion system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] 1 is a functional block diagram of a power conversion system (PCS) 100 according to an embodiment of the present invention, and a commercial power source 10, a load 60, a rechargeable battery 70, a photovoltaic inverter (PV inverter) 50, a solar panel 80, and a solar panel 92 coupled thereto. The solar panel 80 and the solar panel 92 convert irradiated light into electrical energy. The solar power inverter 50 converts DC power generated by the solar panel 80 into AC power, and provides the converted AC power to the load 60 and / or the power conversion system 100.

[0007] The power conversion system 100 includes a commercial power connection port 12, an AC power port 14, a DC power port 16, a high-voltage capacitor C, a DC converter 20, a power inverter 22, a voltage ammeter 30, a DC converter 82, a DC power port 90, and a microcontroller unit (MCU) 40. The power conversion system 100 is connected to a commercial power source 10 via the commercial power connection port 12 and can receive power from the commercial power source 10. The DC power port 16 is coupled to a rechargeable battery 70, and the power conversion system 100 can charge the rechargeable battery 70 or receive power from the rechargeable battery 70 via the DC power port 16. The voltage ammeter 30 is coupled to the AC power port 14 and detects the voltage Va and current Ia output by the power conversion system 100 from the AC power port 14. Here, the voltage Va and current Ia are the voltage of the AC power and the current of the AC power, respectively. The microcontrol unit 40 controls the operation of the power conversion system and receives a state-of-charge signal SOC from the rechargeable battery 70. The microcontrol unit 40 obtains the current charging rate of the rechargeable battery 70 according to the state-of-charge signal SOC, and obtains the external output power P_Inv of the power conversion system 100 according to the voltage Va and current Ia detected by the voltmeter 30. If the external output power P_Inv is positive, it indicates that the power conversion system 100 outputs electric energy to the outside through the AC power port 14, and if the external output power P_Inv is negative, it indicates that the power conversion system 100 receives electric energy from the outside through the AC power port 14. The DC converter 20 is coupled between the high-voltage capacitor C and the DC power port 16, and converts the DC voltage Vb output by the rechargeable battery 70 into a voltage difference Vbus across the high-voltage capacitor C. The DC converter 82 is coupled between the high-voltage capacitor C and the DC power port 90, and converts the DC voltage output by the solar panel 92 into a voltage difference Vbus across the high-voltage capacitor C. Therefore, the magnitude of the voltage difference Vbus is determined by both the DC voltage Vb and the DC voltage output by the solar panel 92.The power inverter 22 is coupled between the high-voltage capacitor C and the AC power port 14, converts the voltage difference Vbus into an AC-form voltage Va, and the frequency of the AC voltage Va is F.

[0008] Referring to FIG. 2, FIG. 2 is a relational diagram between the external output power ratio of the photovoltaic inverter 50 in FIG. 1 and the frequency F of the AC power output by the power conversion system 100. The horizontal axis of FIG. 2 indicates the frequency F of the AC power output from the AC power port 14 by the power conversion system 100, and the vertical axis of FIG. 2 indicates the external output power ratio of the photovoltaic inverter 50. The place marked 100 on the vertical axis of FIG. 2 indicates that the photovoltaic inverter 50 outputs at the maximum value (i.e., 100%), and the place marked 0 on the vertical axis indicates the output stop of the photovoltaic inverter 50. Also, when the frequency F is between F_Start and F_Stop, the external output power ratio and the frequency F have a linear inverse correlation, that is, the larger the external output power ratio at this time, the lower the frequency F of the AC power. Here, F_min < F_normal < F_Start < F_Stop, where F_min indicates the minimum value of the frequency F of the AC power output by the power conversion system 100, F_normal is the frequency of the general normal operation of the power conversion system 100, the external output power ratio corresponding to F_Start is equivalent to 100%, and the external output power ratio corresponding to F_Stop is equivalent to 0%. Note that F_min may be abbreviated as "minimum frequency", F_normal may be abbreviated as "general frequency", F_Start may be abbreviated as "start frequency", and F_Stop may be abbreviated as "stop frequency". The start frequency F_Start is, for example, 60 Hertz (Hz), and the stop frequency F_Stop is, for example, 60.5 Hertz (Hz). Furthermore, there is an interruption frequency F_trip for forcibly interrupting the electrical energy output of the photovoltaic inverter 50 and shifting the power conversion system 100 to over-frequency protection (F_Trip is, for example, 60.6 Hertz (Hz). Once the frequency F of the AC power reaches F_Trip or more, the photovoltaic inverter 50 stops the output of electrical energy, so the frequency F_trip is also called the "interruption frequency").

[0009] The DC converter 82 detects the voltage difference Vbus between both ends of the high-voltage capacitor C, and transmits the data of the voltage difference Vbus to the micro control unit 40, which adjusts the frequency F according to the voltage difference Vbus to control the output power of the solar power inverter 50. In addition, when the micro control unit 40 detects that the utility power is off-grid (for example, when the connection between the utility power connection port 12 and the utility power 10 is cut off or the utility power 10 is powered off), the micro control unit 40 adjusts the frequency F according to the voltage difference Vbus, and further adjusts the output power of the solar power inverter 50. For example, the normal value of the voltage difference Vbus is 400 to 430 volts, and when the voltage difference Vbus exceeds 450 volts, it means that the energy stored in the high-voltage capacitor C is excessive, so at this time, the micro control unit 40 turns off the solar panel 92 first, and then turns off the solar power inverter 50. Also, when the voltage difference Vbus is less than 450 volts but greater than 430 volts, the frequency F is increased to cause the solar power inverter 50 to reduce the electrical energy it outputs.

[0010] 3A and 3B, which are flow charts showing how the micro control unit 40 in FIG. 1 controls the power conversion system 100. When the micro control unit 40 detects that the commercial power supply is off-grid (e.g., when the connection between the commercial power supply connection port 12 and the commercial power supply 10 is cut off or the commercial power supply 10 is in a power outage) or when the micro control unit 40 reconnects to supply power on-grid, the micro control unit 40 executes the process shown in FIG. 3A and 3B, which includes the following steps:

[0011] Step S200: The micro control unit 40 judges whether the power conversion system 100 has been reconnected to the grid. When the power conversion system 100 is reconnected to the commercial power source 10 or when the photovoltaic power generation inverter 50 starts supplying power, it means that the power conversion system 100 has been reconnected to the grid. When the micro control unit 40 judges that the power conversion system 100 has been reconnected to the grid, it executes step S201, and when the opposite is true, it executes step S204.

[0012] Step S201: The micro control unit 40 judges whether the current charging rate of the rechargeable battery 70 is less than the preset rate S1 according to the charging state signal SOC. Note that the preset rate S1 is between 10% and 80%. When the micro control unit 40 judges that the current charging rate of the rechargeable battery 70 is not less than the preset rate S1, it executes step S202, otherwise it executes step S203.

[0013] Step S202: The micro control unit 40 resets the time T counted by the timer (T=0), and sets the frequency F of the AC power output from the AC power port 14 of the power conversion system 100 to the interruption frequency F_trip, stops the output of electrical energy from the photovoltaic power generation inverter 50, and switches the power conversion system 100 to over-frequency protection, and then returns to step S201.

[0014] Step S203: The microcontroller 40 adjusts the frequency F to be lower than the stop frequency F_Stop by the preset value Min_Step (i.e., F=F_Stop-Min_Step), resumes the output of electrical energy from the photovoltaic power inverter 50, and returns to step S200. The preset value Min_Step may be equal to ((F_Stop-F_Start) / 8).

[0015] Step S204: The micro control unit 40 determines whether the voltage difference Vbus across the high-voltage capacitor C is greater than a critical value V2. The critical value V2 may be, for example, 445 volts to 455 volts. If the micro control unit 40 determines that the voltage difference Vbus is greater than the critical value V2, it executes step S205; otherwise, it executes step S210.

[0016] Step S205: The time counted by the timer in the micro control unit 40 is incremented by one.

[0017] Step S206: The micro control unit 40 judges whether the voltage difference Vbus across the high voltage capacitor C is greater than the critical value V3. Note that the critical value V3 may be between 465V and 475V. If the micro control unit 40 judges that the voltage difference Vbus is greater than the critical value V3, it executes step S207, otherwise it executes step S208.

[0018] Step S207: The microcontroller unit 40 increases the frequency F to (F_Trip+Max_step), stops the output of electrical energy of the photovoltaic inverter 50 coupled to the AC power port 14, and enters into over-frequency protection. Here, Max_step is, for example, 0.3 Hz, or equal to ((F_Stop-F_Start) / 2). Furthermore, once the frequency F of the AC power reaches or exceeds F_Trip, the photovoltaic inverter 50 stops outputting electrical energy, so that the output of electrical energy of the photovoltaic inverter 50 can be more reliably stopped when the frequency F of the AC power is equal to (F_Trip+Max_step). Return to step S200.

[0019] Step S208: The micro control unit 40 judges whether the current count time T of the timer is greater than the preset time length Th. The preset time length Th is, for example, 5 seconds. If the micro control unit 40 judges that the current count time T of the timer is greater than the preset time length Th, it executes step S209, otherwise it returns to step S204.

[0020] Step S209: The micro control unit 40 increases the frequency F to (F_Trip+Max_step), stops the output of electrical energy from the photovoltaic inverter 50 coupled to the AC power port 14, and enters into over-frequency protection. After the micro control unit 40 executes step S209, it returns to step S200.

[0021] Step S210: The micro control unit 40 resets the current counted time T of the timer (T=0); Step S211: The micro control unit 40 judges whether the voltage difference Vbus across the high voltage capacitor C is greater than the critical value V1. Here, the critical value V1 may be from 425V to 435V. If the micro control unit 40 judges that the voltage difference Vbus is greater than the critical value V1, it executes step S212, otherwise it returns to step S204.

[0022] Step S212: The micro control unit 40 increases the frequency F to Max_step, i.e. F=(F+Max_step), and makes the photovoltaic power inverter 50 reduce the electric energy output therefrom. Then, return to step S204.

[0023] When the photovoltaic inverter 50 detects that the voltage or frequency exceeds the normal working range, it activates protection (e.g., overvoltage, undervoltage, overfrequency, underfrequency, islanding, etc.) and does not output any more power to the grid. Here, the microcontroller unit 40 judges whether the photovoltaic inverter 50 is cut off, and determines whether the photovoltaic inverter 50 can be reconnected to supply power on the grid by adjusting the AC output frequency of the power conversion system 100 according to the state. When the photovoltaic inverter 50 detects that the voltage and frequency of the commercial power supply terminal are within the normal working range, it judges that the conditions for reconnecting to the grid and supplying power are established, and the photovoltaic inverter 50 counts a predetermined number of seconds (e.g., 300 seconds as stipulated in the On-Grid Law) before connecting to the grid and outputting power.

[0024] In another embodiment of the present invention, the micro control unit 40 performs the process of Figures 3A and 3B according to the voltage difference Vbus, and also performs the process of Flowchart 4 of Figure 4 according to the current charging rate of the rechargeable battery 70. The process of Figure 4 includes the following steps:

[0025] Step S200: This step is the same as step S200 in FIG. 3A, that is, the micro control unit 40 judges whether the power conversion system 100 has been reconnected to the grid, and if the micro control unit 40 judges that the power conversion system 100 has been reconnected to the grid, it executes step S201 in FIG. 3A, and vice versa.

[0026] Step S302: The micro control unit 40 judges whether the current charging rate of the rechargeable battery 70 is greater than the preset rate S2 according to the charging state signal SOC. Here, the preset rate S2 (for example, between 20% and 90%), if the micro control unit 40 judges that the current charging rate of the rechargeable battery 70 is greater than the preset rate S2, it executes step S304, otherwise it returns to step S200.

[0027] Step S304: The micro control unit 40 judges whether the external output power P_Inv is less than the preset power P1. It should be noted that the preset power P1 (for example, 500 watts) can be adjusted according to different control requirements. If the micro control unit 40 judges that the external output power P_Inv is less than the preset power P1, it executes step S306, otherwise it returns to step S200.

[0028] Step S306: The microcontrol unit 40 increases the frequency F of the AC power output from the AC power port 14 of the power conversion system 100, stops the output of electrical energy of the solar power inverter 50 coupled to the AC power port 14, and proceeds to over-frequency protection. For example, the microcontrol unit 40 increases the frequency F of the AC power to (F_Trip+Max_step). Here, F_Trip is, for example, 60.6 Hertz (Hz), and Max_step is, for example, 0.3 Hz. In addition, once the frequency F of the AC power reaches or exceeds F_Trip, the solar power inverter 50 stops outputting electrical energy, so the frequency F_trip can be called an interruption frequency. Therefore, when the frequency F of the AC power is equal to (F_Trip+Max_step), the output of electrical energy of the solar power inverter 50 can be more reliably stopped. After executing step S306, the microcontrol unit 40 returns to step S200.

[0029] In another embodiment of the present invention, in addition to performing the process of Figures 3A and 3B based on the voltage difference Vbus, the micro control unit 40 also performs the process of Figures 5A and 5B based on the current charging rate of the rechargeable battery 70. The process of Figures 5A and 5B includes the following steps:

[0030] Step S200: This step is the same as step S200 in FIG. 3A, that is, the micro control unit 40 judges whether the power conversion system 100 has been reconnected to the grid, and if the micro control unit 40 judges that the power conversion system 100 has been reconnected to the grid, it executes step S201 in FIG. 3A, and vice versa.

[0031] Step S401: The micro control unit 40 judges whether the current charging rate of the rechargeable battery 70 is greater than the preset rate S2 according to the charging state signal SOC. The preset rate S2 may be between 20% and 90%. If the micro control unit 40 judges that the current charging rate of the rechargeable battery 70 is greater than the preset rate S2, it executes step S402, otherwise it executes step S403.

[0032] Step S402: The microcontrol unit 40 increases the frequency F of the AC power output from the AC power port 14 of the power conversion system 100 to (F_Trip+Max_Step), stops the output of electrical energy from the solar power inverter 50 coupled to the AC power port 14, and moves to over-frequency protection. Note that F_Trip is, for example, 62 Hertz (Hz), and Max_step is, for example, 0.3 Hz. Furthermore, once the frequency F of the AC power reaches or exceeds F_Trip, the solar power inverter 50 stops outputting electrical energy, so the frequency F_Trip may be called an "interruption frequency". Therefore, when the frequency F of the AC power is equal to (F_Trip+Max_step), the output of electrical energy from the solar power inverter 50 can be more reliably stopped. Also, Max_Step may be equal to ((F_Stop-F_Start) / 2), but F_Trip is greater than F_Stop. After executing step S402, the microcontrol unit 40 returns to step S200.

[0033] Step S403: The micro control unit 40 judges whether the current charging rate of the rechargeable battery 70 is greater than the preset rate S3 according to the charging state signal SOC. Note that the preset rate S3 is less than the preset rate S2 and may be between 15% and 85%. If the micro control unit 40 judges that the current charging rate of the rechargeable battery 70 is greater than the preset rate S3, it executes step S404, otherwise it executes step S407.

[0034] Step S404: The micro control unit 40 judges whether the negative value of the external output power P_Inv (i.e., -P_Inv) is greater than the preset power P2. If the negative value of the external output power P_Inv is positive, it indicates that the power conversion system 100 receives power from the outside, and the preset power P2 is, for example, but not limited to, 1000 watts. If the micro control unit 40 does not judge that the negative value of the external output power P_Inv is greater than the preset power P2, it executes step S405. If the micro control unit 40 judges that the negative value of the external output power P_Inv is greater than the preset power P2, it executes step S409.

[0035] Step S405: The micro control unit 40 judges whether the external output power P_Inv is smaller than the preset power P1. Here, the preset power P1 is smaller than the preset power P2, and the preset power P1 is, for example, but not limited to, 500 watts. If it is judged that the external output power P_Inv is smaller than the preset power P1, execute step S406; otherwise, return to step S401.

[0036] Step S406: The micro control unit 40 adjusts the frequency F to increase by the preset value Min_Step (i.e., F=F+Min_Step), and returns to step S401. Note that the preset value Min_Step may be equal to ((F_Stop-F_Start) / 8), but the frequency F can be adjusted to the highest value F_Stop in this step, that is, the maximum value F_Max of the frequency F in this step is F_Stop. The effect of step S406 is that when the current charging rate of the rechargeable battery 70 is greater than the preset ratio S3 and the external output power P_Inv is less than the preset power P1, the output power of the photovoltaic power generation inverter 50 can be reduced by adjusting the frequency F to increase.

[0037] Step S407: The micro control unit 40 judges whether the current charging rate of the rechargeable battery 70 is less than the preset rate S1 according to the charging state signal SOC. Note that the preset rate S1 is less than the preset rates S2 and S3, and may be between 10% and 80%. If the micro control unit 40 judges that the current charging rate of the rechargeable battery 70 is less than the preset rate S1, it executes step S408, otherwise it returns to step S401.

[0038] Step S408: The micro control unit 40 adjusts the frequency F down by the preset value Min_Step (i.e., F=F-Min_Step), and returns to step S401. Note that in this step, the frequency F can be adjusted to the lowest F_Start, that is, the minimum value F_Min of the frequency F in this step is F_Start. The effect of step S408 is that when the current charging rate of the rechargeable battery 70 is lower than the preset ratio S1, the output power of the photovoltaic power generation inverter 50 can be increased by adjusting the frequency F down.

[0039] Step S409: The micro control unit 40 adjusts the frequency F to increase by the preset value Mid_Step (i.e., F=F+Mid_Step) and returns to step S401. Note that the preset value Mid_Step may be equal to ((F_Stop-F_Start) / 4), but the frequency F can be adjusted to the highest F_Stop in this step, that is, the maximum value F_Max of the frequency F in this step is F_Stop. The effect of step S409 is that when the current charging rate of the rechargeable battery 70 is greater than the preset ratio S3 and the power received by the power conversion system 100 from the outside is greater than the preset power P2, the output power of the photovoltaic power generation inverter 50 can be reduced by adjusting the frequency F to increase.

[0040] When the microcontrol unit 40 of the present invention detects that the commercial power source is off-grid, it causes the power conversion system 100 to output an AC frequency F, guides the solar power inverter 50 not to be in isolated (islanding) protection, generates power and provides it to the grid, and provides the energy to the load 60 and the power conversion system 100. The microcontrol unit 40 dynamically adjusts the frequency of the AC output by the power conversion system 100 based on the voltage difference Vbus between both ends of the high-voltage capacitor C, the current charging rate of the rechargeable battery 70, and the positive / negative and magnitude of the external output power P_Inv, thereby efficiently controlling the power flow of the entire power conversion system 100.

[0041] The above is merely a preferred embodiment of the present invention, and equivalent modifications and variations within the scope of the claims also belong to the scope of the present invention. [Explanation of symbols]

[0042] 10 Commercial power supply 12 Commercial power connection port 14 AC power port 16, 90 DC power port 20, 82 DC converter 22 Power Inverter 30 Voltage and current meter 40 Micro Control Unit 50 Solar power inverter 60 Load 70 rechargeable batteries 80, 92 Solar panels 100 Power Conversion System C High voltage capacitor F frequency F_min Minimum frequency F_normal General frequency F_Start Starting frequency F_Stop Stop frequency F_Trip Interruption frequency Ia current P_Inv Power Va Voltage Vb DC voltage Vbus voltage difference SOC State of Charge Signal S200~S212, S302~S306, S401~S409 Steps

Claims

1. A power conversion system (PCS), comprising: an AC power port coupled to a solar power inverter; a first DC power port coupled to the rechargeable battery; a second DC power port coupled to the solar panel; High voltage capacitors, a first DC converter coupled between the high voltage capacitor and the first DC power port; a second DC converter coupled between the high voltage capacitor and the second DC power port; a DC / AC inverter coupled between the high voltage capacitor and the AC power port; a microcontroller unit (MCU) that adjusts the frequency of the AC power output from the AC power port of the power conversion system based on a voltage difference between both ends of the high-voltage capacitor; When the micro control unit detects an occurrence of off-grid of the commercial power supply and the voltage difference is greater than a first critical value, the micro control unit sets the frequency of the AC power output from the AC power port to an interruption frequency to stop the output of electrical energy from the solar power inverter; When the micro control unit detects that the utility power source is off-grid, and the continuous time during which the voltage difference is between the first critical value and the second critical value exceeds a preset time length, the micro control unit sets the frequency of the AC power output by the AC power port to the interruption frequency; When the micro control unit detects that the utility power source is off-grid and the voltage difference is between the second critical value and the third critical value, the micro control unit adjusts the frequency of the AC power output from the AC power port to increase by a first preset value; The first critical value is greater than the second critical value, and the second critical value is greater than the third critical value.

2. 2. The power conversion system of claim 1, wherein the first critical value is between 465 volts and 475 volts, the second critical value is between 445 volts and 455 volts, and the third critical value is between 425 volts and 435 volts.

3. 2. The power conversion system according to claim 1, wherein when the micro-control unit detects that the commercial power source is off-grid, and the current charging rate of the rechargeable battery is greater than a first preset rate and the external output power is less than a first preset power, the micro-control unit increases the frequency of the AC power and stops the photovoltaic power inverter from outputting electrical energy.

4. 4. The power conversion system of claim 3, wherein when the micro control unit detects an occurrence of off-grid commercial power and determines that the current charging rate of the rechargeable battery is greater than the first preset rate, the external output power is less than the first preset power, and the voltage difference is less than the first critical value, the power conversion system receives electrical energy from the rechargeable battery and provides it to a load.

5. 4. The power conversion system of claim 3, wherein when the micro control unit detects an occurrence of off-grid commercial power and determines that the current charging rate of the rechargeable battery is greater than the first preset rate, the external output power is less than the first preset power, and the voltage difference is less than the first critical value, the power conversion system receives electrical energy from the rechargeable battery and provides it to a load.

6. 2. The power conversion system of claim 1, wherein when the micro control unit detects an occurrence of off-grid commercial power and determines that the current charging rate of the rechargeable battery is greater than a first preset rate, the micro control unit adjusts the frequency of the AC power to a first frequency and stops the solar power inverter from outputting electrical energy.

7. 7. The power conversion system according to claim 6, wherein when the micro control unit detects that the commercial power source is off-grid, and determines that the current charging rate of the rechargeable battery is smaller than the first preset rate and larger than the second preset rate, and the negative value of the external output power is larger than the first preset power, the micro control unit adjusts the frequency of the AC power output from the AC power port to increase by a first preset value.

8. 8. The power conversion system according to claim 7, wherein when the micro control unit detects an occurrence of off-grid commercial power and determines that the current charging rate of the rechargeable battery is lower than a third preset rate, the frequency of the AC power output from the AC power port is adjusted to be lowered by a second preset value.

9. 9. The power conversion system according to claim 8, wherein when the micro control unit detects that the commercial power source is off-grid, and determines that the current charging rate of the rechargeable battery is smaller than the first preset rate and larger than the second preset rate, the negative value of the external output power is not larger than the first preset power, and the external output power is smaller than the second preset power, the micro control unit adjusts the frequency of the AC power output from the AC power port to increase by a third preset value.

10. The power conversion system of claim 9 , wherein the second preset value is equal to the third preset value.

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