A method and system for automatically balancing power of parallel inverters in a microgrid

Abstract

The present invention discloses a method for automatically balancing the power of parallel inverters in a microgrid, comprising: step 1: each inverter obtains its own DC bus voltage, and obtains the inverter's own power matching state according to its own DC bus voltage, including: insufficient inverter power, excessive inverter power, excessive system power, excessive inverter power backflow, and inverter power matching; step 2: each inverter adjusts the frequency or amplitude of the output voltage according to its own power matching state. In the case where the output power capabilities of each parallel photovoltaic storage inverter in the microgrid are inconsistent, the present invention automatically reduces the output power of the insufficient power inverter, controls the entire system to be in a power balance state, and prolongs the stable operation time of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter power control, and in particular to a method and system for automatically balancing the power of parallel inverters in a microgrid. Background Art

[0002] With the proposal and promotion of the carbon neutrality goal, photovoltaic (PV) power generation has developed rapidly. At the same time, PV power generation and energy storage are deeply integrated to form a better photovoltaic storage power generation system. In some scenarios, multiple photovoltaic storage inverters need to be connected in parallel on the output side to form a microgrid. An example of a small microgrid containing multiple parallel photovoltaic storage inverters is as follows: Figure 1 As shown, the PV panels and batteries are connected to the DC side of the inverter, and the AC output sides of the inverters are connected in parallel to form an AC bus. The load is connected to the AC bus, and grid-connected or off-grid operation is achieved by controlling the on and off of the switch between the AC bus and the large power grid.

[0003] For the coordinated control of multiple parallel inverters when off-grid, in order to simplify the system control and decouple, droop control or VSG control is generally used for each inverter. VSG also has droop characteristics in steady state. When the inverter output inductance >> output resistance, the droop control method shown in the following two formulas is used:

[0004] ω * =ω 0 -m(PP 0 );

[0005] U * =U 0 -n(QQ 0 );

[0006] When the inverter output inductance << output resistance, the droop control method shown in the following two formulas is used:

[0007] U * =U 0 -m(PP 0 );

[0008] ω * =ω 0 -n(QQ 0 );

[0009] The corresponding relationships between active power, reactive power, frequency and voltage are interchangeable between the two methods.

[0010] Due to the differences in PV panel capacity, battery capacity, light intensity, and battery state of charge (SOC) fluctuations, the output power capacity of each PV-storage inverter is not consistent and changes at any time. The droop coefficient of traditional droop control is fixed, and the output power of each inverter is the same. On the one hand, this limits the output power of the inverter with stronger power generation capacity, resulting in a waste of PV power generation capacity; on the other hand, it may cause the output power of the inverter with weaker power generation capacity to be insufficient, thereby affecting the stability of the parallel system operation.

[0011] At present, there are some methods to solve the problem of power balance between parallel photovoltaic and energy storage inverters when off-grid. Some methods realize system control through communication or sampling signals between inverters, some methods compensate for the missing power by opening the backup power supply, and some methods adjust the droop parameters by the battery SOC. All existing methods have limitations. Some cause a large amount of coupling of interactive information between parallel inverters, some cannot fully solve the power balance problem, and some waste part of the PV power generation capacity. Summary of the invention

[0012] The present invention provides a method and system for automatically balancing the power of parallel inverters in a microgrid, so as to solve the problems in the prior art of poor system stable operation capability, power generation capability, decoupling capability and unreasonable power balancing control.

[0013] The present invention provides a method for automatically balancing power of parallel inverters in a microgrid, comprising:

[0014] Step 1: Each inverter obtains its own DC bus voltage, and obtains the inverter's own power matching status according to its own DC bus voltage, including: insufficient inverter power, excessive inverter power, excessive system power, excessive inverter power backflow, and inverter power matching;

[0015] Step 2: Each inverter adjusts the frequency or amplitude of the output voltage according to its own power matching state, including:

[0016] When the inverter power is insufficient, the frequency or amplitude of the inverter output voltage is reduced through the control loop;

[0017] When the inverter has excess power, the frequency or amplitude of the inverter output voltage is increased for the first time through the control loop;

[0018] When the system power is in excess, the PV output power is reduced through the control loop until the PV output power is zero or the actual value of the DC bus voltage approaches the DC bus voltage PV power limit threshold;

[0019] When the inverter power is over-returned, the frequency or amplitude of the inverter output voltage is increased for the second time based on the first increase through the control loop.

[0020] Furthermore, the power matching state of the inverter itself is obtained according to the bus voltage. The specific method is as follows:

[0021] when When , it is judged that the inverter power is insufficient;

[0022] when When , it is determined that the inverter has excess power;

[0023] when When , it is judged that the system has excess power;

[0024] when When the inverter is judged to be over-powered;

[0025] when When , it is judged that the inverter power matches.

[0026] Among them, U bus is the actual value of DC bus voltage; is the DC bus voltage reference value; is the DC bus voltage lower threshold; is the upper limit threshold of DC bus voltage; is the DC bus voltage PV power limit threshold; is the maximum threshold of DC bus voltage; satisfy

[0027] Furthermore, in step 2, the control loop adjusts the frequency or amplitude of the inverter output voltage by directly adjusting the frequency or amplitude of the inverter output voltage or by adjusting the droop coefficient.

[0028] Furthermore, in step 2, when the output inductance is much greater than the output resistance, the control loop adjusts the frequency of the inverter output voltage by directly adjusting the frequency or by adjusting the droop coefficient.

[0029] When the output inductance is much smaller than the output resistance, the control loop adjusts the amplitude of the inverter output voltage by directly adjusting the voltage or adjusting the droop coefficient.

[0030] Furthermore, the specific method of the direct adjustment method is:

[0031] Add three DC bus voltage control loops on the basis of the droop control system, and use the difference between the voltage frequency value or voltage amplitude output by the droop control system and the voltage frequency adjustment value or voltage amplitude adjustment value output by the three DC bus voltage control loops as the final execution parameter to adjust the frequency or amplitude of the inverter output voltage;

[0032] Among them, the three additional DC bus voltage control loops will They are input into the controller as the reference value of the DC bus voltage, and the output is the voltage frequency adjustment value or the voltage amplitude adjustment value. Among them, the output limit of the voltage lower limit loop is (0, max), max is a positive value; the output limit of the voltage upper limit loop is (min, 0), min is a negative value; the output limit of the voltage maximum loop is (min Top ,0),min Top Is a negative value.

[0033] Furthermore, the specific method of adjusting the droop coefficient is:

[0034] Add three DC bus voltage control loops on the basis of the droop control system, multiply the droop coefficient in the droop control system by the adjustment coefficients output by the three DC bus voltage control loops, and use the final control coefficient to replace the droop coefficient in the droop control system, so as to adjust the frequency or amplitude of the inverter output voltage;

[0035] Among them, the three additional DC bus voltage control loops will They are input into the controller as the reference value of the DC bus voltage, and the output is the adjustment coefficient. Among them, the output limit of the voltage lower limit loop is (1, max k ), max k It is a positive value greater than 1; the voltage upper limit loop output limit is (0,1); the voltage maximum loop output limit is (0,1).

[0036] Furthermore, the specific method of adjusting the droop coefficient is:

[0037] Add three DC bus voltage control loops on the basis of the droop control system, add the droop coefficient in the droop control system to the adjustment coefficients output by the three DC bus voltage control loops, and use the final control coefficient to replace the droop coefficient in the droop control system, so as to adjust the frequency or amplitude of the inverter output voltage;

[0038] Among them, the three additional DC bus voltage control loops will The reference values ​​of the DC bus voltage are input into the controller respectively, and the output is the adjustment coefficient. Among them, the output limit of the voltage lower limit loop is (0,m), where m is the active power droop coefficient; the output limit of the voltage upper limit loop is (-m up ,0),m up is a positive value; the maximum voltage loop output limit is (-m Top ,0),m Top is a positive value, and m Up +m Top <m。

[0039] Furthermore, a DC bus voltage control loop is added on the basis of the PV voltage control loop, and the smaller output value of the PV voltage control loop and the output value of the DC bus voltage control loop is used as the final control value to reduce the PV output power;

[0040] Among them, an additional DC bus voltage control loop will It is input into the controller as the reference value of the DC bus voltage and output as the reference value of the PV output current.

[0041] The present invention also provides a power automatic balancing control system for parallel inverters in a microgrid, which can execute the above-mentioned power automatic balancing control method for parallel inverters in a microgrid, including: a photovoltaic storage inverter power conversion circuit, a voltage and current sampling circuit, a power switch device driving circuit, and a digital control chip;

[0042] The power conversion circuit of the photovoltaic storage inverter includes: a PV DC / DC circuit, a battery DC / DC circuit, a DC bus, and a DC / AC circuit, which are used for power conversion and transmission between PV, battery and output end;

[0043] The voltage and current sampling circuit is electrically connected to the photovoltaic storage inverter power conversion circuit and is used to obtain the inverter input, output and DC bus voltage and current;

[0044] The digital control chip is electrically connected to the voltage and current sampling circuit, and executes the automatic power balancing control method of parallel inverters in the microgrid to generate a driving signal according to the inverter input, output, DC bus voltage and current obtained by the voltage and current sampling circuit;

[0045] The power switch device driving circuit is electrically connected to the digital control chip and the photovoltaic storage inverter power conversion circuit. The power switch device driving circuit drives the relays and power tubes in the photovoltaic storage inverter power conversion circuit to operate based on a driving signal.

[0046] Beneficial effects of the present invention:

[0047] When the output power capabilities of the parallel photovoltaic and energy storage inverters in the microgrid are inconsistent, the present invention automatically reduces the output power of the inverter with insufficient power, controls the entire system to be in a power balance state, and prolongs the stable operation time of the system;

[0048] The present invention converts the excess PV power generated by an inverter into charging power for the battery of the inverter with insufficient power, thus giving full play to the power generation capacity of PV;

[0049] When the total PV power is completely in excess, the present invention controls the entire system to operate stably, thereby avoiding system collapse caused by excessive power backflow and DC bus overvoltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The features and advantages of the present invention will be more clearly understood by referring to the accompanying drawings, which are schematic and should not be construed as limiting the present invention in any way. In the accompanying drawings:

[0051] Figure 1 This is an example diagram of an existing small microgrid with parallel photovoltaic and energy storage inverters;

[0052] Figure 2 It is a schematic diagram of a flow chart of a specific embodiment of the present invention;

[0053] Figure 3 A schematic diagram of a DC bus voltage threshold value in a specific embodiment of the present invention;

[0054] Figure 4 A control loop topology diagram of a direct adjustment method in a specific embodiment of the present invention;

[0055] Figure 5 A control loop topology diagram for adjusting the droop coefficient by multiplication in a specific embodiment of the present invention;

[0056] Figure 6 A control loop topology diagram for adjusting the droop coefficient by addition in a specific embodiment of the present invention;

[0057] Figure 7 A control topology diagram for adjusting PV output power in a specific embodiment of the present invention;

[0058] Figure 8 It is a schematic diagram of a system in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0059] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0060] The embodiment of the present invention provides a method for automatically balancing the power of parallel inverters in a microgrid.

[0061] like Figure 2 As shown, the power balance state of each inverter in the microgrid is first identified through the DC bus voltage:

[0062] When off-grid, each inverter in the microgrid controls its own DC bus voltage. Generally, the DC bus voltage is controlled by controlling the battery charging and discharging current. The DC bus voltage reference value is set to The actual value is Ubus , Adjust according to different operating conditions. Set the DC bus voltage lower limit threshold respectively DC bus voltage upper threshold DC bus voltage PV power limit threshold DC bus voltage maximum threshold and and The absolute value of the difference is generally selected between 10V and 20V, such as Figure 3 shown.

[0063] when When , the inverter output power is balanced with the shared load power, that is, the system power balance state can be achieved by adjusting the battery charging and discharging power;

[0064] when When the inverter is insufficient, the sum of the PV power and the battery discharge power is still less than the load power shared by the inverter.

[0065] when When the inverter is judged to have excess power, that is, the remaining power after subtracting the battery charging power from the PV power is still greater than the load power shared by the inverter;

[0066] when When the total power of the system is judged to be excessive, that is, the remaining power after deducting the total battery charging power from the total PV power generation in the microgrid is still greater than the total load consumption power;

[0067] when At this time, it is determined that the DC bus voltage of the inverter is charged high by the reverse power from the output side, that is, there is still power remaining after the power reversed from the output side is subtracted from the battery charging power.

[0068] Secondly, the power between parallel inverters in the microgrid is balanced according to the status:

[0069] a. When the inverter output inductance >> output resistance, the output power is changed by adjusting the output voltage frequency to achieve system power balance. Specifically:

[0070] when When the output frequency reference value is lowered through the control loop, the output power of the inverter is reduced to achieve power balance;

[0071] when When the output frequency reference value is increased through the control loop, the output power of the inverter is increased to achieve power balance;

[0072] when When the PV power is reduced through the control loop, it will be reduced until the PV power drops to zero or U bus Maintain Nearby, to reduce the PV output power;

[0073] when When When the output frequency reference value is increased through the control loop, the output frequency reference value is increased again through the control loop, thereby reducing the reverse power to the inverter and achieving power balance.

[0074] The output power can be increased or decreased by directly adjusting the frequency reference value of the output voltage. The control method is as follows Figure 4 As shown. Three DC bus voltage control loops are added on the basis of traditional droop control, and their reference values ​​are The output is the frequency reference value adjustment The output limit of the voltage lower limit loop is (0, max), where max is a positive value, and the output limit of the voltage upper limit loop is (min, 0), where min is a negative value. The output limit of the voltage upper limit loop is (min Top ,0),min Top For negative values, the controller generally uses proportional integral (PI) and the droop calculation frequency ω * minus Get the final execution frequency When the inverter output power is insufficient, U bus Down to Nearby, the output frequency is lowered and the output power is reduced through the control loop, and finally U bus Stable The inverter outputs at its maximum power capacity, and the insufficient power is automatically supplemented by other inverters; when the inverter outputs excess power, U bus Rise to Nearby, the output frequency is increased through the control loop, the output power is increased, and finally U bus Stable The inverter generates electricity at the maximum PV capacity, and the multi-output power automatically supplies the load or charges the batteries of other inverters; when the inverter passes Control, PV power limiting link, U bus Still rising to Nearby, the output frequency is increased again through the control loop to reduce the reverse power, and finally U bus Stable In the entire microgrid system, the battery is charged at the maximum charging power, while the load power supply is met, and the remaining PV power generation power is limited, and the system maintains stable operation.

[0075] The frequency reference value of the output voltage can also be adjusted by adjusting the active power droop coefficient m, thereby controlling the output power and achieving power balance. The adjustment coefficient k is calculated through the DC bus voltage control loop. m Then use the formula k m *m or m+k m Calculate the final active power droop coefficient m final , use m final Instead of m, droop control is performed. The method is as follows Figure 5 , 6 As shown, the control loop output limiting is also shown in the figure.

[0076] b. When the inverter output inductance << output resistance, the output power is controlled by adjusting the output voltage amplitude to achieve system power balance. The principle is exactly the same as the above two methods of adjusting the output frequency, except that the frequency is replaced by the voltage amplitude, which will not be repeated here.

[0077] c. Regardless of the inverter output inductance>>output resistance or output inductance<<output resistance, when The PV power is reduced through the control loop until the PV power drops to zero or U bus Maintain The DC bus voltage is controlled by controlling the PV DC / DC current, and its voltage reference value is The control loop output competes with the PV voltage control loop output to take the lower value as the PV DC / DC current reference value. The control method is as follows: Figure 7 shown.

[0078] like Figure 8 As shown, this embodiment also provides a parallel inverter power automatic balancing control system in a microgrid, which can execute the above-mentioned parallel inverter power automatic balancing control method in a microgrid, including: a photovoltaic storage inverter power conversion circuit, a voltage and current sampling circuit, a power switch device driving circuit, and a digital control chip;

[0079] The power conversion circuit of the photovoltaic storage inverter includes: PV DC / DC circuit, battery DC / DC circuit, DC bus, DC / AC circuit, which are used for power conversion and transmission between PV, battery and output end;

[0080] The voltage and current sampling circuit is electrically connected to the photovoltaic storage inverter power conversion circuit, and is used to obtain the inverter input, output and DC bus voltage and current;

[0081] The digital control chip is electrically connected to the voltage and current acquisition circuit, and executes the automatic power balancing control method of the parallel inverters in the microgrid to generate a driving signal according to the inverter input, output, DC bus voltage and current obtained by the voltage and current sampling circuit;

[0082] The power switch device driving circuit is electrically connected to the digital control chip and the photovoltaic storage inverter power conversion circuit. The power switch device driving circuit drives the relay and the power tube in the photovoltaic storage inverter power conversion circuit to operate based on the driving signal.

[0083] Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations are all within the scope defined by the appended claims.

Claims

1. A method for automatic power balancing control of parallel inverters in a microgrid, characterized in that: include: Step 1: Each inverter obtains its own DC bus voltage, and obtains the inverter's own power matching status according to its own DC bus voltage, including: insufficient inverter power, excessive inverter power, excessive system power, excessive inverter power backflow, and inverter power matching; Step 2: Each inverter adjusts the frequency or amplitude of the output voltage according to its own power matching state, including: When the inverter power is insufficient, the frequency or amplitude of the inverter output voltage is reduced through the control loop; When the inverter has excess power, the frequency or amplitude of the inverter output voltage is increased for the first time through the control loop; When the system power is in excess, the PV output power is reduced through the control loop until the PV output power is zero or the actual value of the DC bus voltage approaches the DC bus voltage PV power limit threshold; When the inverter power is over-returned, the frequency or amplitude of the inverter output voltage is increased for the second time based on the first increase through the control loop. The control loop adjusts the frequency or amplitude of the inverter output voltage by directly adjusting the frequency or amplitude of the inverter output voltage by adjusting the droop coefficient. The specific method of the direct adjustment method is: Add three DC bus voltage control loops on the basis of the droop control system, and use the difference between the voltage frequency value or voltage amplitude output by the droop control system and the voltage frequency adjustment value or voltage amplitude adjustment value output by the three DC bus voltage control loops as the final execution parameter to adjust the frequency or amplitude of the inverter output voltage; Among them, the three additional DC bus voltage control loops will They are input into the controller as the reference value of the DC bus voltage, and the output is the voltage frequency adjustment value or the voltage amplitude adjustment value. Among them, the output limit of the voltage lower limit loop is (0, max), max is a positive value; the output limit of the voltage upper limit loop is (min, 0), min is a negative value; the output limit of the voltage maximum loop is (min Top ,0),min Top is a negative value; The specific method of adjusting the droop coefficient is as follows: Add three DC bus voltage control loops on the basis of the droop control system, multiply the droop coefficient in the droop control system by the adjustment coefficients output by the three DC bus voltage control loops, and use the final control coefficient to replace the droop coefficient in the droop control system, so as to adjust the frequency or amplitude of the inverter output voltage; Among them, the three additional DC bus voltage control loops will They are input into the controller as the reference value of the DC bus voltage, and the output is the adjustment coefficient. Among them, the output limit of the voltage lower limit loop is (1, max k ), max k It is a positive value greater than 1; the voltage upper limit loop output limit is (0,1); the voltage maximum loop output limit is (0,1).

2. The method for automatic power balancing control of parallel inverters in a microgrid according to claim 1, characterized in that: The inverter's own power matching status is obtained according to the bus voltage. The specific method is as follows: when When , it is judged that the inverter power is insufficient; when When , it is determined that the inverter has excess power; when When , it is judged that the system has excess power; when When the inverter is judged to be over-powered; when When , it is judged that the inverter power is matched; Among them, U bus is the actual value of DC bus voltage; is the DC bus voltage reference value; is the DC bus voltage lower threshold; is the upper limit threshold of DC bus voltage; is the DC bus voltage PV power limit threshold; is the maximum threshold of DC bus voltage; satisfy 3. The method for automatic power balancing control of parallel inverters in a microgrid according to claim 1, characterized in that: In step 2, when the output inductance is much greater than the output resistance, the control loop adjusts the frequency of the inverter output voltage by directly adjusting the frequency or adjusting the droop coefficient. When the output inductance is much smaller than the output resistance, the control loop adjusts the amplitude of the inverter output voltage by directly adjusting the voltage or adjusting the droop coefficient.

4. The method for automatically balancing power of parallel inverters in a microgrid as claimed in claim 3, characterized in that: The specific method of adjusting the droop coefficient is as follows: Add three DC bus voltage control loops on the basis of the droop control system, add the droop coefficient in the droop control system to the adjustment coefficients output by the three DC bus voltage control loops, and use the final control coefficient to replace the droop coefficient in the droop control system, so as to adjust the frequency or amplitude of the inverter output voltage; Among them, the three additional DC bus voltage control loops will The reference values ​​of the DC bus voltage are input into the controller respectively, and the output is the adjustment coefficient. Among them, the output limit of the voltage lower limit loop is (0,m), where m is the active power droop coefficient; the output limit of the voltage upper limit loop is (-m up ,0),m up is a positive value; the maximum voltage loop output limit is (-m Top ,0),m Top is a positive value, and m Up +m Top <m。 5. The method for automatic power balancing control of parallel inverters in a microgrid according to claim 1, characterized in that: When the system power is in excess, the specific control method is as follows: add a DC bus voltage control loop on the basis of the PV voltage control loop, take the smaller output value of the PV voltage control loop and the output value of the DC bus voltage control loop as the final control value, and reduce the PV output power; Among them, an additional DC bus voltage control loop will It is input into the controller as the reference value of the DC bus voltage and output as the reference value of the PV output current.

6. A power automatic balancing control system for parallel inverters in a microgrid, which can execute the power automatic balancing control method for parallel inverters in a microgrid as claimed in any one of claims 1 to 5, characterized in that: The parallel inverter power automatic balancing control system in the microgrid includes: a photovoltaic storage inverter power conversion circuit, a voltage and current sampling circuit, a power switch device driving circuit, and a digital control chip; The power conversion circuit of the photovoltaic storage inverter includes: a PV DC / DC circuit, a battery DC / DC circuit, a DC bus, and a DC / AC circuit, which are used for power conversion and transmission between PV, battery and output end; The voltage and current sampling circuit is electrically connected to the photovoltaic storage inverter power conversion circuit and is used to obtain the inverter input, output and DC bus voltage and current; The digital control chip is electrically connected to the voltage and current sampling circuit, and executes the automatic power balancing control method of parallel inverters in the microgrid to generate a driving signal according to the inverter input, output, DC bus voltage and current obtained by the voltage and current sampling circuit; The power switch device driving circuit is electrically connected to the digital control chip and the photovoltaic storage inverter power conversion circuit. The power switch device driving circuit drives the relays and power tubes in the photovoltaic storage inverter power conversion circuit to operate based on a driving signal.

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