Power Conversion Systems

Abstract

To provide a power conversion system in which a micro control unit acquires the current charge rate of a rechargeable battery on the basis of a charge state signal and acquires the external output power of the power conversion system on the basis of the voltage and current detected by a voltage and current meter.SOLUTION: When a micro control unit detects an occurrence of an off-grid of a commercial power supply, the micro control unit adjusts the frequency of AC power output by a power conversion system and switches the frequency of the AC power between three or more different frequencies on the basis of the external output power and the current charge rate of the rechargeable battery. As the current charge rate of the rechargeable battery increases, the frequency of the AC power is switched to a lower frequency.SELECTED DRAWING: Figure 2

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 DC power port, a voltage ammeter and a microcontroller unit. The DC power port is coupled to a rechargeable battery. The voltage ammeter is coupled to the AC power port and detects the voltage and current output by the power conversion system from the AC power port. The microcontroller unit controls the operation of the power conversion system and receives a charging state signal from the rechargeable battery. The microcontroller unit also determines the current charging rate of the rechargeable battery according to the charging state signal, and calculates the external output power of the power conversion system according to the voltage and current detected by the voltage ammeter. When the micro control unit detects that the commercial power source is off-grid, the micro control unit executes the steps of: when it is determined that the current charging rate of the rechargeable battery is greater than a first scheduled ratio, adjusting the frequency of the AC power output from the AC power port of the power conversion system to a first frequency, and stopping the output of electrical energy from the photovoltaic power inverter coupled to the AC power port; when it is determined that the current charging rate of the rechargeable battery is smaller than the first scheduled ratio and greater than the second scheduled ratio, and the negative value of the external output power is greater than the first scheduled power, adjusting the frequency of the AC power output from the AC power port upward by a first scheduled value; and when it is determined that the current charging rate of the rechargeable battery is smaller than a third scheduled ratio, adjusting the frequency of the AC power output from the AC power port downward by a second scheduled 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. 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 coupled commercial power source 10, a load 60, a rechargeable battery 70, a photovoltaic inverter (PV inverter) 50, and a solar panel 80. The photovoltaic 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 voltage ammeter 30, 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, where the voltage Va and current Ia are the voltage of the AC power and the current of the AC power, respectively. The microcontroller 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 based on the state-of-charge signal SOC, and obtains the external output power P_Inv of the power conversion system 100 based on 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 electrical 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 electrical energy from the outside through the AC power port 14.

[0008] The power conversion system 100 may further include a DC converter 20 and a power supply inverter 22. The DC converter 20 converts the DC voltage Vb output by the rechargeable battery 70 into a DC voltage Vd of a different value, and the power supply inverter 22 converts the DC voltage Vd into a voltage Va in the form of an AC current.

[0009] When the microcontroller unit 40 detects an off-grid situation of the commercial power supply (for example, when the connection between the commercial power supply connection port 12 and the commercial power supply 10 is disconnected, or when the commercial power supply 10 experiences a power outage), the microcontroller unit 40 adjusts the frequency F of the alternating current output from the power conversion system 100 at the AC power port 14, and further controls the external output power P_Inv output by the power conversion system 100. Referring to FIG. 2, FIG. 2 is a relationship diagram between the external output power ratio of the photovoltaic inverter 50 in FIG. 1 and the frequency F of the alternating current output by the power conversion system 100. The horizontal axis in FIG. 2 indicates the frequency F of the alternating current output from the power conversion system 100 at the AC power port 14, and the vertical axis in FIG. 2 indicates the external output power ratio of the photovoltaic inverter 50. The place marked 100 on the vertical axis in 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, at this time, the larger the external output power ratio, the lower the frequency F of the alternating current. Here, F_min < F_normal < F_Start < F_Stop, where F_min indicates the minimum value of the frequency F of the alternating current 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 as "general frequency", F_Start as "start frequency", and F_Stop 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). In addition, there is an interruption frequency F_trip that forcibly interrupts the electrical energy output of the photovoltaic inverter 50 and causes the power conversion system 100 to shift to over-frequency protection (F_Trip is, for example, 60.6 Hertz (Hz).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 the "interruption frequency."

[0010] 3A and 3B 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 (for example, when the connection between the commercial power supply connection port 12 and the commercial power supply 10 is cut off or when the commercial power supply 10 is in a power outage) or when the micro control unit 40 reconnects to supply power on the 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. If the power conversion system 100 has been reconnected to the commercial power source 10 or the solar power generation inverter 50 has started to supply power, this means that the power conversion system 100 has been reconnected to the grid. If the micro control unit 40 judges that the power conversion system 100 has not been reconnected to the grid, it executes step S201, and conversely, it executes step S210.

[0012] Step S201: The micro control unit 40 judges whether the current charging rate of the rechargeable battery 70 is greater than the set rate S2 based on the charging state signal SOC. Note that the set rate S2 is between 20% and 90%, and when the micro control unit 40 judges that the current charging rate of the rechargeable battery 70 is greater than the set rate S2, it executes step S202, otherwise it executes step S203.

[0013] Step S202: The microcontroller 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. Here, 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, and 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. In addition, Max_Step may be equal to ((F_Stop-F_Start) / 2), and F_Trip is greater than F_Stop. After executing step S202, the microcontroller 40 returns to step S200.

[0014] Step S203: The micro control unit 40 judges whether the current charging rate of the rechargeable battery 70 is greater than the set rate S3 according to the charging state signal SOC. Note that the set rate S3 is less than the set 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 set rate S3, it executes step S204, otherwise it executes step S207.

[0015] Step S204: 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 set 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 set 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 set power P2, it executes step S205. If the micro control unit 40 judges that the negative value of the external output power P_Inv is greater than the set power P2, it executes step S209.

[0016] Step S205: The micro control unit 40 judges whether the external output power P_Inv is smaller than the set power P1. The set power P1 is smaller than the set power P2, and the set power P1 is, for example, 500 watts, but is not limited thereto. If it is judged that the external output power P_Inv is smaller than the set power P1, execute step S206, and otherwise return to step S201.

[0017] Step S206: The micro control unit 40 adjusts the frequency F upward by the set value Min_Step (i.e., F=F+Min_Step) and returns to step S201. Note that the set value Min_Step may be equal to ((F_Stop-F_Start) / 8), 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 S206 is that when the current charging rate of the rechargeable battery 70 is greater than the set ratio S3 and the external output power P_Inv is less than the set power P1, the frequency F can be adjusted upward to reduce the output power of the photovoltaic power generation inverter 50.

[0018] Step S207: The micro control unit 40 judges whether the current charging rate of the rechargeable battery 70 is less than the set rate S1 according to the charging state signal SOC. Note that the set rate S1 is less than the set 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 set rate S1, it executes step S208, otherwise it returns to step S201.

[0019] Step S208: The micro control unit 40 adjusts the frequency F downward by the set value Min_Step (i.e., F=F-Min_Step) and returns to step S201. 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 S208 is that if the current charging rate of the rechargeable battery 70 is lower than the set rate S1, the frequency F is adjusted downward to increase the output power of the photovoltaic power inverter 50.

[0020] Step S209: The micro control unit 40 adjusts the frequency F upward by the set value Mid_Step (i.e., F=F+Mid_Step) and returns to step S201. Note that the set value Mid_Step may be equal to ((F_Stop-F_Start) / 4), but the frequency F can be adjusted up to F_Stop in this step, i.e., the maximum value F_Max of the frequency F in this step is F_Stop. The effect of step S209 is that when the current charging rate of the rechargeable battery 70 is greater than the set ratio S3 and the power received by the power conversion system 100 from the outside is greater than the set power P2, the frequency F is adjusted upward to reduce the output power of the photovoltaic power generation inverter 50.

[0021] Step S210: The micro control unit 40 judges whether the current charging rate of the rechargeable battery 70 is less than the set rate S1 based on the charging state signal SOC. If the micro control unit 40 judges that the current charging rate of the rechargeable battery 70 is not less than the set rate S1, it executes step S211, otherwise it executes step S212.

[0022] Step S211: The micro control unit 40 sets the frequency F to the trip frequency F_Trip, stops the photovoltaic power inverter 50 from outputting electrical energy, and then goes into over-frequency protection, and returns to step S210.

[0023] And, step S212: the micro control unit 40 adjusts the frequency F to (F_Stop-Min_Step), and returns to step S200.

[0024] 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 or not, and adjusts the AC output frequency F of the power conversion system 100 according to the state, and determines whether the photovoltaic inverter 50 can be reconnected to supply power on the grid. 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) and then connects to the grid to output power.

[0025] In the present invention, as shown in Fig. 2, when the frequency F is between F_Start and F_Stop, the external output power ratio and the frequency F are linearly inversely correlated. Therefore, compared with the general two-stage control method (i.e., the photovoltaic inverter outputs in a full output (100%) or no output (0%) manner), the micro control unit 40 can adjust the output power of the photovoltaic inverter 50 in a manner including more stages, and can avoid the instability of the power supply system caused by the instantaneous full output (100%) or no output (0%) of the photovoltaic inverter.

[0026] In addition, as can be seen from FIG. 2 and steps S203 to S208, when the micro control unit 40 detects the occurrence of off-grid of the commercial power supply, the micro control unit 40 limits the frequency F of the AC power output from the AC power port 14 to within a predetermined range between F_Start and F_Stop, and within this predetermined range, the output power of the photovoltaic power inverter 50 and the frequency F of the AC power are inversely correlated.

[0027] 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 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.

[0028] The above are merely preferred embodiments of the present invention, and equivalent changes and modifications made within the scope of the claims also belong to the scope of the present invention. [Explanation of symbols]

[0029] 10 Commercial power supply 12 Commercial power connection port 14 AC power port 16 DC power port 20 DC Converter 22 Power Inverter 30 Voltage and current meter 40 Micro Control Unit 50 Solar power inverter 60 Load 70 rechargeable batteries 80 Solar Panels 100 Power Conversion System 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, Vd DC voltage SOC State of Charge Signal S200 to S212 Step

Claims

1. A power conversion system (PCS), comprising: An AC power port; a DC power port coupled to a rechargeable battery; a voltmeter coupled to the AC power port for detecting a voltage and a current output from the power conversion system (PCS) at the AC power port; a microcontroller unit (MCU) that controls operation of the power conversion system (PCS) and receives a state of charge (SOC) signal from the rechargeable battery; The micro control unit obtains a current charging rate of the rechargeable battery according to the charging state signal, and obtains an external output power of the power conversion system according to the voltage and the current detected by the voltage / current meter; When the micro control unit detects an occurrence of an off-grid of the commercial power supply, the micro control unit When it is determined that the current charging rate of the rechargeable battery is greater than a first predetermined rate, the power conversion system adjusts the frequency of the AC power output from the AC power port to a first frequency, and stops the output of electrical energy by a photovoltaic inverter coupled to the AC power port. When the current charging rate of the rechargeable battery is determined to be lower than the first predetermined rate and higher than a second predetermined rate that is lower than the first predetermined rate, and the negative value of the external output power is determined to be higher than the first predetermined power, adjusting the frequency of the AC power output from the AC power port upward by a first predetermined value; and when it is determined that the current charging rate of the rechargeable battery is lower than a third predetermined rate which is lower than the second predetermined rate, adjusting the frequency of the AC power output from the AC power port downward by a second predetermined value.

2. 2. The power conversion system according to claim 1, wherein when it is determined that the current charging rate of the rechargeable battery is smaller than the first scheduled rate and larger than the second scheduled rate, the negative value of the external output power is not larger than the first scheduled power, and the external output power is smaller than the second scheduled power, the frequency of the AC power output from the AC power supply port is adjusted upward by a third scheduled value.

3. The power conversion system of claim 2 , wherein the second predetermined value is equal to the third predetermined value.

4. 2. The power conversion system according to claim 1, wherein when the micro control unit detects an occurrence of off-grid of a commercial power supply, if it is determined that the negative value of the external output power is not greater than the first scheduled power, the micro control unit limits the frequency of the AC power output from the AC power port to within a scheduled range.

5. 5. The power conversion system according to claim 4, wherein when the frequency of the AC power output from the AC power supply port is within the expected range, the output power of the photovoltaic power generation inverter and the frequency of the AC power are inversely correlated.

6. The power conversion system of claim 1 , wherein the first predetermined ratio is greater than the second predetermined ratio, and the second predetermined ratio is greater than the third predetermined ratio.

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