Power conversion system

A power conversion system (PCS) includes an alternating current (AC) power port, a first direct current (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 microcontroller unit. The microcontroller unit adjusts a frequency of an AC power output by the power conversion system based on a voltage difference across the high voltage capacitor of the power conversion system to switch the frequency of the AC power between different frequencies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a power conversion system (PCS), in particular to a power conversion system that can adjust the output AC frequency according to the charged ratio of a rechargeable battery.

2. Description of the Prior Art

Power conversion system (PCS) is a bidirectional power conversion inverter that can be used for on-grid and off-grid electrical power storage applications. The efficient operation of a power conversion system has always been an important issue in this technical field.

SUMMARY OF THE INVENTION

A power conversion system of the present invention comprises an alternating current power port, a first direct current power port, a second direct current power port, a high voltage capacitor, a first DC converter, a second DC converter, a DC/AC inverter, a microcontroller unit. The alternating current power port is coupled to a photovoltaic inverter. The first direct current power port is coupled to a rechargeable battery. The second direct current 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 DC/AC inverter is coupled between the high voltage capacitor and the AC power port. The microcontroller unit is for adjusting the frequency of an AC output from the AC power port by the power conversion system according to the voltage difference between two ends of the high voltage capacitor. When the microcontroller unit detects mains off-grid and the voltage difference is greater than a first critical value, the microcontroller unit sets a frequency of the AC output from the AC power port as a cut-off frequency, so that the photovoltaic inverter stops outputting power. When the microcontroller unit detects mains off-grid and the voltage difference is between the first critical value and a second critical value for a continuous time exceeding a predetermined time length, the microcontroller unit sets the frequency of the AC output from the AC power port as the cut-off frequency. When the microcontroller unit detects mains off-grid and the voltage difference is between the second critical value and a third critical value, the microcontroller unit increases the frequency of the AC output from the AC power port by a first predetermined value. The first critical value is greater than the second critical value, and the second critical value is greater than the third critical value.

DETAILED DESCRIPTION

FIG.1is a functional block diagram of a power conversion system (PCS) according to an embodiment of the present invention and the coupled mains10, a load60, a rechargeable battery70, a photovoltaic inverter (PV inverter)50, a solar panel80and a solar panel92. The solar panel80and the solar panel92are used to convert light into power. The PV inverter50converts the direct current power generated by the solar panel80to alternating current power, and feeds the converted alternating current into the load60and/or the power conversion system100.

The power conversion system100includes a mains connection port12, an AC power port14, a DC power port16, a high voltage capacitor C, a DC converter20, a power inverter22, a voltmeter-and-current meter30, a DC converter82, a DC power port90and a microcontroller unit (MCU)40. The power conversion system100can be connected to the mains10through the mains connection port12and receive power from the mains10. The DC power port16is coupled to the rechargeable battery7, and the power conversion system100can charge the rechargeable battery70through the DC power port16or receive power from the rechargeable battery70. The voltmeter-and-current meter30is coupled to the AC power port14to detect the voltage Va and current Ia output from the AC power port14by the power conversion system100, wherein the voltage Va and the current Ia are the AC voltage and the AC current respectively. The MCU40controls the operation of the power conversion system and receives a state-of-charge signal SOC from the rechargeable battery70. Wherein, the MCU40can obtain the current charged ratio of the rechargeable battery70according to the state-of-charge signal SOC, and obtain the output power P_Inv of the power conversion system100according to the voltage Va and current Ia detected by the voltmeter-and-current meter30. Wherein, when the output power P_Inv is positive, it means that the power conversion system100outputs power through the AC power port14; and when the output power P_Inv is negative, it means that the power conversion system100receives power from the outside through the AC power port14. The DC converter20is coupled between the high voltage capacitor C and the DC power port16for converting the DC voltage Vb output by the rechargeable battery70into a voltage difference Vbus between two ends of the high voltage capacitor C. The DC converter82is coupled between the high voltage capacitor C and the DC power port90for converting the DC voltage output by the solar panel92into a voltage difference Vbus between the two ends of the high voltage capacitor C. Therefore, the size of the voltage difference Vbus is determined by the DC voltage Vb and the DC voltage output by the solar panel92. The power inverter22is coupled between the high voltage capacitor C and the AC power port14for converting the voltage difference Vbus into an AC voltage Va, and the frequency of the AC voltage Va is F.

Please refer toFIG.2.FIG.2is a relationship diagram between the output power ratio of the photovoltaic inverter50inFIG.1and the frequency F of the alternating current output by the power conversion system100. The horizontal axis ofFIG.2represents the frequency F of the AC power output by the power conversion system100from the AC power port14, and the vertical axis ofFIG.2represents the output power ratio of the photovoltaic inverter50. The position marked100on the vertical axis inFIG.2indicates that the output of the photovoltaic inverter50is at the maximum value (i.e. 100%), and the position marked 0 on the vertical axis indicates that the output of the photovoltaic inverter50is stopped. Furthermore, when the frequency F is between F_Start and F_Stop, the output power ratio and the frequency F have a linear inverse relationship, that is, the larger the output power ratio at this time, the lower the AC frequency F will be. Wherein F_min<F_normal<F_Start<F_Stop, and F_min represents the minimum value of the frequency F of the alternating current output by the power conversion system100, and F_normal is the frequency of the power conversion system100in general normal operation. The output power ratio corresponding to F_Start is equal to 100%, and the output power ratio corresponding to F_Stop is equal to 0%. Wherein, F_min may be referred to as “minimum frequency”, F_normal may be referred to as “normal frequency”, F_Start may be referred to as “start frequency”, and F_Stop may be referred to 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 Hz. Furthermore, there is another cut-off frequency F_trip, which forces the photovoltaic inverter50to stop outputting power, so that the power conversion system100enters into over-frequency protection (F_Trip is, for example, 60.6 Hz. Since the photovoltaic inverter50stops outputting power once the frequency F of the alternating current exceeds F_Trip, the frequency F_trip may be referred to as the “cut-off frequency”).

The DC converter82can detect the voltage difference Vbus between the two ends of the high voltage capacitor C, and transmit the data of the voltage difference Vbus to the microcontroller unit40, so that the microcontroller unit40adjusts the frequency F according to the voltage difference Vbus to control the output power of the photovoltaic inverter50. Furthermore, when the microcontroller unit40detects that the mains off-grid (for example: when the connection between the connection port12and the mains10is cut off or the mains10is powered off), the microcontroller unit40can adjust the frequency F according to the voltage difference Vbus, and then adjust the output power of the photovoltaic inverter50. 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 high-voltage capacitor C has accumulated too much energy. Therefore, at this time, the microcontroller unit40will first turn off the solar panel92, and then turn off the photovoltaic inverter50. Furthermore, if the voltage difference Vbus is less than 450 volts but greater than 430 volts, the frequency F is increased so that the photovoltaic inverter50reduces its output power.

Please refer toFIG.3AandFIG.3B,FIG.3AandFIG.3Bare flowcharts of the control of the power conversion system100by the microcontroller unit40ofFIG.1. When the microcontroller unit40detects that the mains off-grid (for example: when the connection between the connection port12and the mains10is cut off or the mains10is powered off) or reconnected and feeding to the grid, the microcontroller unit40executes the process ofFIG.3AandFIG.3B, and this process includes the following steps:Step S200: the microcontroller unit40determines whether the power conversion system100is reconnected to the grid, wherein when the conversion system100is reconnected to the mains10or the photovoltaic inverter50starts to supply power; it means that the power conversion system100is reconnected to the grid. When the microcontroller unit40determines that the power conversion system100has been reconnected to the grid, execute step S201; otherwise, execute step S204;Step S201: the microcontroller unit40determines whether the current charged ratio of the rechargeable battery70is less than a predetermined ratio S1according to the state-of-charge signal SOC, wherein the predetermined ratio S1may between 10% and 80%, if the microcontroller unit40determines that the current charged ratio of the rechargeable battery70is not less than the predetermined ratio S1, then execute step S202; otherwise, execute step S203;Step S202: the microcontroller unit40resets the accumulated time T of its timer to zero (T=0), and the frequency F of the alternating current output by the power conversion system100from the alternating current power supply port14is set as the cut-off frequency F_trip, so that the photovoltaic inverter50stops outputting power, and the power conversion system100enters the over-frequency protection; return to step S201;Step S203: the microcontroller unit40lowers the frequency F from the stop frequency F_Stop by a predetermined value Min_Step (i.e. F=F_Stop−Min_Step), so that the photovoltaic inverter50can output power, and return to step S200. The predetermined value Min_Step can be equal to ((F_Stop−F_Start)/8);Step S204: the microcontroller unit40determines whether the voltage difference Vbus between the two ends of the high voltage capacitor C is greater than the critical value V2. Wherein, the critical value V2can be, for example, 445 volts to 455 volts; if the microcontroller unit40determines that the voltage difference Vbus is greater than the critical value V2, then execute step S205; otherwise, execute step S210;Step S205: the timer in the microcontroller unit40adds 1 to its accumulated time;Step S206: the microcontroller unit40determines whether the voltage difference Vbus between the two ends of the high voltage capacitor C is greater than the critical value V3. Wherein, the critical value V3can be 465 volts to 475 volts; if the microcontroller unit40determines that the voltage difference Vbus is greater than the critical value V3, then execute step S207; otherwise, execute step S208;Step S207: the microcontroller unit40increases the frequency F to (F_Trip+Max_step), so that the photovoltaic inverter50coupled to the AC power port14stops outputting power, and enters the over-frequency protection. Wherein, Max_step is, for example, 0.3 Hz or equal to ((F_Stop−F_Start)/2). Furthermore, once the frequency F of the alternating current reaches above F_Trip, the photovoltaic inverter50stops outputting power, therefore, when the frequency F of the alternating current is equal to (F_Trip+Max_step), it is more guaranteed that the photovoltaic inverter50stops outputting power; return to step S200;Step S208: the microcontroller unit40determines whether the accumulated time T of the timer is greater than the predetermined time length Th. The predetermined time length Th can be, for example, 5 seconds, and when the microcontroller unit40determines that the accumulated time T of its timer is greater than the predetermined time length Th, then execute step S209; otherwise, return to step S204;Step S209: the microcontroller unit40increases the frequency F to (F_Trip+Max_step), so that the photovoltaic inverter50coupled to the AC power port14stops outputting power, and enters the over-frequency protection. After the microcontroller unit40executes step S209, return to step S200;Step S210: the microcontroller unit40resets the accumulated time T of its timer to zero (T=0);Step S211: the microcontroller unit40determines whether the voltage difference Vbus between the two ends of the high voltage capacitor C is greater than the critical value V1. Wherein, the critical value V1can be 425 volts to 435 volts; if the microcontroller unit40determines that the voltage difference Vbus is greater than the critical value V1, then execute step S212; otherwise, execute step S204; andStep S212: the microcontroller unit40increases the frequency F by Max_step, that is, F=(F+Max_step), so that the photovoltaic inverter50reduces the output power; return to step S204.

When the photovoltaic inverter50detects that the voltage or frequency exceeds the normal operating range, it starts protection (for example: overvoltage, under voltage, over frequency, under frequency, islanding . . . etc.), and then no longer outputs power and feeds to the grid, at this time, the microcontroller unit40determines whether the photovoltaic inverter50has tripped, and adjusts the AC output frequency F of the power conversion system100according to the state to determine whether the photovoltaic inverter50can be reconnected and fed to the grid. If the photovoltaic inverter50detects that the voltage and frequency of the mains terminal meet the normal operating range, it determines that the condition for reconnecting to the grid is met, and the photovoltaic inverter50counts a certain number of seconds (for example: 300 seconds as specified by grid-connected regulations) and will be fed into the grid output.

In another embodiment of the present invention, in addition to executing the flow inFIG.3AandFIG.3Baccording to the voltage difference Vbus, the microcontroller unit40will also execute the flow inFIG.4according to the current charged ratio of the rechargeable battery70. The flow chart inFIG.4includes the following steps:

Step S200: this step is the same as step S200inFIG.3A, that is, the microcontroller unit40determines whether the power conversion system100is reconnected to the grid. When the microcontroller unit40determines that the power conversion system100is reconnected to the network, execute step S201inFIG.3A; otherwise, execute step S302;

Step S302: the microcontroller unit40determines whether the current charged ratio of the rechargeable battery70is greater than a predetermined ratio S2according to the state of charge signal SOC, wherein the predetermined ratio S2is, for example, between 20% and 90%, and when the microcontroller unit40determines that the current charged ratio of the rechargeable battery70is greater than the predetermined ratio S2, execute step S304; otherwise, execute step S200;

Step S304: the microcontroller unit40determines whether the output power P_Inv is less than the predetermined power P1, wherein the predetermined power P1(for example, 500 watts), and can be adjusted according to different control requirements. When the microcontroller unit40determines that the output power P_Inv is less than the predetermined power P1, execute step S306; otherwise, return to step S200; and

Step S306: the microcontroller unit40increases the frequency F of the alternating current output by the power conversion system100from the alternating current power supply port14, so that the photovoltaic inverter50coupled to the alternating current power supply port14stops outputting power and enters the over-frequency protection. For example: the microcontroller unit40increases the frequency F of the alternating current to (F_Trip+Max_step). Wherein, F_Trip is, for example, 60.6 Hz, and Max_step is, for example, 0.3 Hz. Furthermore, once the frequency F of the alternating current reaches above F_Trip, the photovoltaic inverter50stops outputting power and the frequency F_trip may be referred to as a cut-off frequency. Therefore, when the frequency F of the alternating current is equal to (F_Trip+Max_step), it is more guaranteed that the photovoltaic inverter50stops outputting power; when the microcontroller unit40completes step S306, return to step S200.

In another embodiment of the present invention, in addition to executing the flow inFIG.3AandFIG.3Baccording to the voltage difference Vbus, the microcontroller unit40will also execute the flow inFIG.5AandFIG.5Baccording to the current charged ratio of the rechargeable battery70. The flow inFIG.5AandFIG.5Bincludes the following steps:

Step S200: this step is the same as step S200inFIG.3A, that is, the microcontroller unit40determines whether the power conversion system100is reconnected to the grid. When the microcontroller unit40determines that the power conversion system100is reconnected to the network, execute step S201inFIG.3A; otherwise, execute step S401;

Step S401: the microcontroller unit40determines whether the current charged ratio of the rechargeable battery70is greater than a predetermined ratio S2according to the state of charge signal SOC, wherein the predetermined ratio S2may between 20% and 90%, and when the microcontroller unit40determines that the current charged ratio of the rechargeable battery70is greater than the predetermined ratio S2, execute step S402; otherwise, execute step S403;

Step S402: the microcontroller unit40increases the frequency F of the alternating current output by the power conversion system100from the alternating current power supply port14to (F_Trip+Max_Step), so that the photovoltaic inverter50coupled to the alternating current power supply port14stops outputting power and enters the over-frequency protection. Wherein, F_Trip is, for example, 62 Hz, and Max_step is, for example, 0.3 Hz. Furthermore, once the frequency F of the alternating current reaches above F_Trip, the photovoltaic inverter50stops outputting power and the frequency F_Trip may be may be referred to as a “cutoff frequency”. Therefore, when the frequency F of the alternating current is equal to (F_Trip+Max_step), it is more guaranteed that the photovoltaic inverter50stops outputting power. Furthermore, Max_Step can be equal to ((F_Stop−F_Start)/2), and F_Trip is greater than F_Stop. When the microcontroller unit40finishes executing step S402, return to step S200;

Step S403: the microcontroller unit40determines whether the current charged ratio of the rechargeable battery70is greater than the predetermined ratio S3according to the state of charge signal SOC. Wherein, the predetermined ratio S3is smaller than the predetermined ratio S2, and can range from 15% to 85%. When the microcontroller unit40determines that the current charged ratio of the rechargeable battery70is greater than the predetermined ratio S3, execute step S404; otherwise, execute step S407;

Step S404: the microcontroller unit40determines whether the negative value of the output power P_Inv (i.e. −P_Inv) is greater than the predetermined power P2. Wherein, when the negative value of the output power P_Inv is positive, it means that the power conversion system100receives power from the outside, and the predetermined power P2is, for example, 1000 watts, but not limited thereto. When the microcontroller unit40does not determine that the negative value of the output power P_Inv is greater than the predetermined power P2, execute step S405; and when the microcontroller unit40determines that the negative value of the output power P_Inv is greater than the predetermined power P2, execute Step S409;

Step S405: the microcontroller unit40determines whether the output power P_Inv is less than the predetermined power P1. Wherein, the predetermined power P1is smaller than the predetermined power P2, and the predetermined power P1is, for example, 500 watts, but not limited thereto. When it is determined that the output power P_Inv is less than the predetermined power P1, execute step S406; otherwise, return to step S401;

Step S406: the microcontroller unit40increases the frequency F by a predetermined value Min_Step (i.e. F=F+Min_Step), and return to step S401. Wherein, the predetermined value Min_Step may be equal to ((F_Stop−F_Start)/8), and the frequency F is adjusted up to F_Stop in this step, that is, the maximum value F_Max of the frequency F in this step is F_Stop. The function of step S406is: when the current charged ratio of the rechargeable battery70is greater than the predetermined ratio S3, and the output power P_Inv is lower than the predetermined power P1, the output power of the photovoltaic inverter50is reduced by increasing the frequency F;

Step S407: the microcontroller unit40determines whether the current charged ratio of the rechargeable battery70is less than the predetermined ratio S1according to the state of charge signal SOC. Wherein the predetermined ratio S1is smaller than the predetermined ratios S2and S3, and can be between 10% and 80%. When the microcontroller unit40determines that the current charged ratio of the rechargeable battery70is smaller than the predetermined ratio S1, execute step S408; otherwise, return to step S401;

Step S408: the microcontroller unit40lowers the frequency F by a predetermined value Min_Step (i.e. F=F−Min_Step), and return to step S401. Wherein the frequency F is adjusted minimum to F_Start in this step, that is, the minimum value F_Min of the frequency F in this step is F_Start. The function of step S408is: when the current charged ratio of the rechargeable battery70is less than the predetermined ratio S1, the output power of the photovoltaic inverter50is increased by lowering the frequency F; and

Step S409: the microcontroller unit40raises the frequency F by a predetermined value Mid_Step (i.e. F=F+Mid_Step), and return to step S401. Wherein the predetermined value Mid_Step can be equal to ((F_Stop−F_Start)/4), and the frequency F is adjusted up to F_Stop in this step, that is, the maximum value F_Max of frequency F in this step is F_Stop. The function of step S409is: when the current charged ratio of the rechargeable battery70is greater than the predetermined ratio S3, and the power received by the power conversion system100from the outside is greater than the predetermined power P2, by increasing the frequency F, the output power of the photovoltaic inverter50is reduced.

When the microcontroller unit40of the present invention detects mains off-grid, it allows the power conversion system100to output the AC frequency F, then induce the photovoltaic inverter50not to enter the Islanding protection and can generate power and feed the grid, its energy can be supplied to the load60and the power conversion system100. The microcontroller unit40can dynamically adjust the frequency of the alternating current output by the power conversion system100according to the voltage difference Vbus between the two ends of the high voltage capacitor C, the current charged ratio of the rechargeable battery70, and the positive or negative magnitude of the output power P_Inv. Therefore, the overall power flow of the power conversion system100can be efficiently regulated.

DESCRIPTION OF REFERENCE NUMERALS