Method for adjusting active power sharing of multi-inverter parallel connection based on adaptive virtual impedance

Abstract

A kind of active power sharing method based on adaptive virtual impedance adjustment multi-inverter parallel, comprising S1: constructing adaptive virtual impedance;S1.1: each inverter transmits local output power related real-time information to microgrid power control system via communication line;S1.2: power control system calculates the power reference value that each distributed power supply should be issued according to capacity proportion;S1.3: the output power actual value calculated in step S1.1 is combined with the output power reference value calculated in step S1.2, and the virtual impedance required for each distributed power supply is generated based on the power deviation of the two;S2: the resistance component in virtual impedance is introduced into active voltage droop equation, the influence of line impedance difference on multi-inverter parallel power distribution is weakened by dynamically adjusting droop coefficient, and then the power balance distribution effect of droop control is improved. Through the above scheme, the active power distribution accuracy of inverter multi-machine parallel in microgrid system is improved, and the circulating current between inverters is reduced.

Description

Technical Field

[0001] This invention relates to the field of AC / DC hybrid microgrid technology, and in particular to a method and terminal for adjusting the active power distribution of the power grid based on adaptive virtual impedance. Background Technology

[0002] With the increasing demand for renewable energy grid connection and the growing severity of AC power distribution resource shortages, distributed generation and microgrids are attracting more and more attention due to their advantages such as low pollution, high efficiency, and good controllability. To improve the reliability, scalability, and flexibility of distributed generation systems, parallel inverters are an effective technical approach.

[0003] Inverters based on droop control have the characteristic of autonomous power equalization. However, in a resistive environment, the difference between the line impedance between the inverter and the point of common coupling and the equivalent output impedance of the inverter will lead to poor power equalization in parallel systems using traditional droop control technology, resulting in circulating current in the parallel inverter system. Summary of the Invention

[0004] This invention addresses the shortcomings of existing technologies by providing a method and terminal for adjusting the active power distribution of the power grid based on adaptive virtual impedance; thereby improving the active power distribution accuracy of parallel inverters in microgrid systems and reducing circulating currents between inverters.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for sharing active power in parallel operation of multiple inverters based on adaptive virtual impedance adjustment includes the following steps:

[0007] S1: Construct an adaptive virtual impedance;

[0008] S1.1: Each inverter transmits local output power information to the microgrid power control system via communication lines;

[0009] S1.2: The power control system calculates the reference value of the power that each distributed power source should generate according to its capacity ratio;

[0010] S1.3: Combine the actual output power value calculated in step S1.1 and the reference output power value calculated in step S1.2, and generate the virtual impedance required for each distributed power source based on the power deviation between the two.

[0011] S2: The resistive component in the virtual impedance is introduced into the active voltage droop equation. By dynamically adjusting the droop coefficient, the influence of line impedance difference on inverter power distribution is reduced, thereby improving the power balance distribution effect of droop control.

[0012] To optimize the above technical solution, the specific measures also include:

[0013] Further, the local output power of the inverter mentioned in step S1.1 is expressed as follows:

[0014]

[0015] In the formula, P represents the active power output of the inverter; Q represents the reactive power output of the inverter; ω c U represents the cutoff frequency of the low-pass filter; s represents the Laplace operator; U d and U q These represent the d-axis and q-axis components of the inverter's three-phase output voltage after Parker transformation, respectively; I d I q This represents the d-axis and q-axis components of the inverter's three-phase output current after Parker transformation.

[0016] Furthermore, the specific content of step S1.2 is as follows:

[0017] Regarding the active power reference value P i * The calculation formula is as follows:

[0018]

[0019] In the formula, P oi P represents the rated active power of each inverter; i The value represents the actual value of the active power output of the inverter, i.e., P obtained in step S1.1; n represents the number of inverters.

[0020] Regarding reactive power reference values The calculation formula is as follows:

[0021]

[0022] In the formula, Q oi Q represents the rated reactive power of each inverter; i The value represents the actual reactive power output of the inverter, i.e., Q obtained in step S1.1; n represents the number of inverters.

[0023] Furthermore, the specific content of step S1.3 is as follows:

[0024] The virtual impedance includes the adaptive virtual inductance L. v and adaptive virtual resistance R V ;

[0025] Adaptive Virtual Inductance L v The calculation formula is:

[0026]

[0027] In the formula, k pl and k il These represent the proportional-integral coefficients of the virtual inductor controller; generating the adaptive virtual inductor L... V Then, to balance the line impedance ratio, the adaptive virtual resistance R is obtained according to the following formula. V :

[0028]

[0029] In the formula, k pr and k ir represents the proportional-integral coefficient of the active power controller; k is the resistance-inductance ratio.

[0030] Furthermore, the inverter introduces an adaptive virtual impedance Z. V The overall representation is as follows:

[0031] Z V =R V +jL V

[0032] In the formula, j represents the imaginary unit in the complex frequency domain.

[0033] Furthermore, the specific content of step S2 is as follows:

[0034] The resistance component R in the virtual impedance V Introducing the droop correction factor Δδ into the traditional droop control, the improved active power droop control equation is:

[0035]

[0036] In the formula, E i E represents the active voltage of a microgrid. * This indicates the rated voltage value of the microgrid; kp i This represents the corrected droop coefficient; To fix the droop coefficient; k p_R The correction coefficient for the droop equation; Δδ i This is a correction factor for the droop coefficient;

[0037] Based on the improved active power droop control equation, the droop coefficient kp is dynamically adjusted. i This reduces the impact of line impedance differences on inverter power distribution, thereby improving the power balance distribution effect of droop control.

[0038] Furthermore, a terminal includes a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that the processor, when executing the computer program, implements the steps of the method as described in any of the preceding claims.

[0039] The beneficial effects of this invention are:

[0040] 1. When the microgrid introduces the adaptive virtual impedance proposed in this invention, each inverter introduces its own constructed virtual impedance according to the virtual impedance formula of this invention. By correcting the differences between line impedances, the external characteristic curves of the inverter units are made to be nearly identical, thereby reducing the active power deviation when multiple units are connected in parallel.

[0041] 2. To improve the speed and accuracy of inverter power regulation, this invention proposes an improved droop control algorithm based on adaptive virtual impedance. By introducing the adaptive virtual impedance of the inverter unit, the droop coefficient in the active power droop equation is corrected. This dynamic correction of the droop coefficient reduces the impact of line impedance differences on parallel current sharing, while simultaneously reducing the voltage drop across the virtual impedance and improving power supply quality. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the dual closed-loop control block for adaptive virtual impedance introduced in this invention.

[0043] Figure 2 This is a schematic diagram of the active power sharing mechanism based on adaptive virtual impedance of the present invention.

[0044] Figure 3 This is a schematic diagram of the working mechanism of active power distribution after the improved droop control of the present invention.

[0045] Figure 4 This is a schematic diagram of the overall control block of the inverter based on adaptive virtual impedance and improved droop control according to the present invention. Detailed Implementation

[0046] The invention will now be described in further detail with reference to the accompanying drawings.

[0047] When the microgrid is operating in islanded mode, the inverter uses the following droop control equation:

[0048]

[0049] In the above formula, f i * , These are the frequency and voltage reference values ​​of the droop control output, respectively; f0 and U0 are the rated frequency and rated voltage values ​​of the microgrid, respectively; P i Q i These are the actual active and reactive power outputs of the inverter; k p k q These are represented as active and reactive power droop coefficients, respectively.

[0050] refer to Figure 4To address the shortcomings of droop control, which is susceptible to line impedance differences, and to improve the active power sharing accuracy of parallel inverters, this invention provides an active power sharing control method for multiple inverters in parallel in a microgrid based on adaptive virtual impedance adjustment, comprising the following steps:

[0051] Step 1: Design appropriate PI controller parameters. Both voltage and current loops use proportional control. Bode plot analysis is performed on the equivalent output impedance of the inverter to make the equivalent output impedance of the inverter exhibit resistive characteristics in the low-frequency domain. This not only fulfills the power decoupling condition but also increases the parallel damping of the system, reducing the harm of high-order harmonic currents to the system.

[0052] Since there is no integral element in the active voltage droop equation, the voltage loop tracking capability may be weakened or even the static error may be insufficient. Therefore, an additional reference voltage proportional feedforward element is designed to improve the inverter's output voltage tracking capability.

[0053] The droop control loop output voltage proportional feedforward can adjust the closed-loop gain of the system without affecting the equivalent output impedance characteristics of the inverter. The influence of different voltage feedforward coefficients on the no-load voltage gain of the inverter is analyzed from the Bode plot, and a suitable feedforward coefficient is selected as the controller parameter.

[0054] Step Two: Introduce virtual impedance to reduce the impedance differences between inverter connection lines. Adaptive virtual impedance consists of two parts: virtual inductance and virtual resistance. The values ​​of each component are determined by their respective power deviations. Each inverter transmits the actual power output calculated by its local power module to the energy management system via a communication line. The energy management system then distributes the calculated power output reference value for each inverter to each inverter, thereby generating their respective virtual impedances.

[0055] The difference between the reactive power output value and the reference value is adjusted by a PI controller to form an adaptive virtual inductance L. V Its expression is:

[0056]

[0057] In the formula, Q i * Q i These are the reference and actual values ​​of the inverter's reactive power, respectively; Q oj Rated capacity for each inverter; k pl k il These are the proportional-integral coefficients of the PI controller. The adaptive virtual inductance L is obtained. V Then, the adaptive virtual resistance R is obtained according to the following formula. V :

[0058]

[0059] In the above formula, P i * P i These are the reference value and the actual output value of the inverter's active power output, respectively; k pr k ir These are the proportional-integral coefficients of the virtual resistor PI controller. Where P... i * Expressed by the following equation:

[0060]

[0061] In the above formula, P oi Rated capacity for each inverter; P i The power output is calculated for each inverter's local power module. In summary, the adaptive virtual impedance formula is:

[0062] Z V =R V +jL V .

[0063] The inverter closed-loop control block diagram with adaptive virtual impedance is shown below. Figure 1 As shown:

[0064] Figure 1 In this invention, to improve the controller's adjustment speed, both the voltage outer loop and the current inner loop employ proportional control. kvp and kip are the proportional coefficients of the voltage and current loop controllers, respectively; kpwm is the inverter's equivalent voltage gain. To compensate for the lack of an integral term in the voltage outer loop, which leads to a static deviation in the output voltage during steady state, this invention introduces a reference voltage feedforward term on top of the ordinary dual-loop control. f The voltage feedforward coefficient significantly improves the controller's voltage tracking capability and reduces static error. By designing the controller parameters, the inverter's equivalent output impedance exhibits resistive characteristics, achieving the droop equation decoupling condition.

[0065] To explain the working mechanism of active power distribution in an adaptive virtual impedance regulation inverter, Figure 2 The following details the process of virtual impedance regulation and active power distribution between two inverters when line impedances are inconsistent:

[0066] Figure 2 In the middle, R i I is the line resistance of the inverter; i P is the output current of the inverter. iThis represents the real-time active power output of the inverter. Line ③ represents the PU droop curve. Assuming that the active power droop curves of all inverters are the same, lines ① and ② represent the active power voltage characteristic curves of each inverter, respectively. At this time, the inverters operate at the intersection points A and B of lines ①, ② and ③. From lines ① and ②, it can be seen that inverter unit A with a large line impedance receives less active power, while inverter B with a small line impedance receives more active power. The power distribution is unreasonable, and the active power is not evenly distributed among the inverters according to their respective capacities, resulting in circulating current between the parallel inverters.

[0067] When the microgrid introduces the adaptive virtual impedance proposed in this invention, each inverter introduces its own constructed virtual impedance according to the virtual impedance formula of this invention. By correcting the differences between line impedances, the external characteristic curves of the inverter units are made to be nearly identical, thereby reducing active power deviation.

[0068] Comparing the operating points A and B before adjustment and the operating points A′ and B′ after adjustment, it can be seen that the difference in active power output between the two parallel units is significantly reduced after adjustment, thus the circulating current between the parallel inverters is effectively suppressed.

[0069] When the line impedance is resistive, the resistive component R plays a major role in the distribution of active power in the virtual impedance. V Assume that the equivalent impedance adjustment of inverter unit A, which originally had a relatively small active power output, is ΔR. A The equivalent impedance adjustment of inverter unit B, which has a larger output active power, is ΔR. B According to the virtual impedance formula constructed in this invention, ΔR can be obtained. A <0, ΔR B If the value is greater than 0, then the line impedance of the inverter unit changes, and the final equivalent line impedance R... A +ΔR A With R B +ΔR B If they are equal, the adjusted voltage-power external characteristic curve will move to lines ④ and ⑤. At this time, the inverter unit will work at the new intersection points A′ and B′ of lines ④, ⑤ and ③.

[0070] Comparing the power deviation of the operating points before and after adjustment, it can be seen that the active power output deviation of the two inverter units is significantly reduced after adjustment, thus the circulating current in the system is effectively suppressed.

[0071] To further improve the speed and accuracy of inverter power regulation, this invention proposes an improved droop control algorithm based on adaptive virtual impedance.

[0072] By introducing the adaptive virtual impedance of the inverter unit, the droop coefficient k in the active power droop equation is adjusted. pThe modified equation is achieved by dynamically adjusting the droop factor to mitigate the impact of line impedance differences on parallel current sharing, while simultaneously reducing the voltage drop across the virtual impedance and improving power supply quality. The improved active voltage equation is as follows:

[0073]

[0074] In the above formula To fix the droop coefficient; k p_R k is the correction coefficient for the droop equation. pi The adaptively corrected droop coefficient; Δδ i For the previously constructed virtual impedance R V This is referred to as the correction factor for the active voltage droop coefficient.

[0075] To explain the working mechanism of the improved droop control algorithm Figure 3 The working principle of improving the active power distribution accuracy by adjusting the droop curves before and after the improvement is presented.

[0076] Figure 3 Middle U pcc AC bus voltage, k p_R The correction coefficient for the droop equation is k. p_R Δδ i This indicates the correction amount for the active power droop coefficient of each inverter unit. The solid line ① represents the PU droop characteristic curve before the improvement. At this time, the inverter unit with a larger equivalent line impedance operates at the intersection point A, and the inverter unit with a smaller equivalent line impedance operates at point B.

[0077] Taking inverter unit A with relatively low output power as an example, because the active power output is less than the active power reference value P... A * Then, according to the proposed construction formula for the adaptive virtual resistor, the correction factor Δδ A <0, based on this, set an appropriate correction coefficient k p_R At this time, the droop correction amount k of inverter unit A p_R Δδ A If <0, then the droop coefficient kp of the corrected inverter unit A is... A Increase; conversely, the droop correction amount k of inverter unit B with larger active power output. p_R Δδ A After adaptive correction, the droop coefficient kp of inverter unit B is greater than zero. B As the droop curve ① decreases, the droop curve ① is adjusted to curves ② and ③, the operating point of the inverter unit is adjusted to A′ and B′, and the active power difference is reduced from ΔP to ΔP′, thus improving the active power distribution accuracy of the parallel system.

[0078] It should be noted that the terms such as "upper", "lower", "left", "right", "front", and "back" used in the invention are only for clarity of description and are not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0079] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A method for adjusting active power sharing of multi-inverter parallel based on adaptive virtual impedance, characterized in that, Includes the following steps: S1: Construct an adaptive virtual impedance; S1.1: Each inverter transmits local output power information to the microgrid power control system via communication lines; S1.2: The power control system calculates the reference value of the power that each distributed power source should generate according to its capacity ratio; S1.3: Combining the actual output power value calculated in step S1.1 and the reference output power value calculated in step S1.2, and based on the power deviation between the two, generate the virtual impedance required for each distributed power source; the specific content of step S1.3 is as follows: The virtual impedance includes adaptive virtual inductance. and adaptive virtual resistance ; Adaptive Virtual Inductance The calculation formula is: In the formula, and These represent the proportional-integral coefficients of the virtual inductor controller, Represents the Laplace operator. This represents the actual value of the inverter's reactive power output. This indicates the reference value for reactive power. Generate adaptive virtual inductance Then, to balance the line impedance ratio, the adaptive virtual resistance is obtained according to the following formula. : In the formula, and These represent the proportional-integral coefficients of the inverter; Inductance-resistance ratio, This represents the actual value of the inverter's active power output. This represents the reference value for active power. S2: The resistive component in the virtual impedance is introduced into the active voltage droop equation. By dynamically adjusting the droop coefficient, the influence of line impedance difference on inverter power distribution is reduced, thereby improving the power balance distribution effect of droop control.

2. The method for sharing active power in parallel operation of multiple inverters based on adaptive virtual impedance adjustment according to claim 1, characterized in that, The local output power of the inverter mentioned in step S1.1 is expressed as follows: In the formula, This indicates the active power output of the inverter; This indicates the reactive power output of the inverter; This indicates the cutoff frequency of the low-pass filter; Represents the Laplace operator; and These represent the three-phase output voltages of the inverter after Parker transformation. , Axial components; , This indicates the three-phase output current of the inverter after Parker transformation. , Axial components.

3. The method for sharing active power in parallel operation of multiple inverters based on adaptive virtual impedance adjustment according to claim 2, characterized in that, The specific details of step S1.2 are as follows: Regarding active power reference value The calculation formula is as follows: In the formula, This indicates the rated active power of each inverter; This represents the actual value of the inverter's active power output, i.e., the value solved in step S1.

1. ; Indicates the number of inverters; Regarding reactive power reference values The calculation formula is as follows: In the formula, This indicates the rated reactive power of each inverter; This represents the actual value of the inverter's reactive power output, i.e., the value solved in step S1.

1. ; Indicates the number of inverters.

4. The method for sharing active power in parallel operation of multiple inverters based on adaptive virtual impedance adjustment according to claim 1, characterized in that, Adaptive virtual impedance introduced by the inverter The overall representation is as follows: In the formula, It represents the imaginary unit in the complex frequency domain.

5. The method for sharing active power in parallel operation of multiple inverters based on adaptive virtual impedance adjustment according to claim 1, characterized in that, The specific details of step S2 are as follows: The resistive component in the virtual impedance As a correction factor for droop coefficient Introduced into traditional droop control, the improved active power droop control equation is: In the formula, To represent the active voltage of a microgrid; Indicates the rated voltage value of the microgrid; This represents the corrected droop coefficient; To fix the droop coefficient; These are the correction coefficients for the drooping equation; This is a correction factor for the droop coefficient; Based on the improved active power droop control equation, the droop coefficient is dynamically adjusted. This reduces the impact of line impedance differences on inverter power distribution, thereby improving the power balance distribution effect of droop control.

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