An adaptive virtual impedance adjustment method for droop-controlled parallel inverters

Abstract

This invention belongs to the field of inverter control technology and provides a method for adaptive virtual impedance adjustment of parallel inverters with droop control. The method comprises: each parallel inverter increases its virtual impedance value at the same time and with the same adjustment step size by receiving a synchronous trigger signal instruction; then, the virtual impedance adjustment direction is determined based on the reactive power variation trend, and the virtual impedance value is adjusted accordingly, so that the reactive power distribution of each inverter tends to be equal as the total impedance approaches. Compared with existing adjustment methods for line impedance mismatch, the proposed solution does not require PCC point voltage measurement and has lower communication bandwidth requirements for the parallel system.

Description

Technical Field

[0001] The present invention belongs to the technical field of inverter control, and in particular relates to an adaptive virtual impedance adjustment method for a droop-controlled parallel inverter. Background Art

[0002] Parallel inverters with droop control automatically distribute active and reactive power equally across all inverters without requiring communication. However, unlike active power, reactive power sharing often lacks the required accuracy due to line impedance mismatch. In extreme cases, uneven reactive power sharing can lead to excessive circulating currents between inverters, posing a significant challenge to parallel operation of inverters in islanded microgrids.

[0003] To address the problem of reactive power sharing in islanded microgrids, various improvement strategies have been proposed. These mainly fall into two categories: those based on improved droop control and those based on virtual impedance. One approach proposes indirect droop control of the point of common coupling (PCC) voltage, but in most cases, line impedance information is unavailable. This problem can be addressed using an adaptive droop control method based on online estimation of line impedance. However, the accuracy of power sharing still relies heavily on the PCC voltage measurement, and the root cause of the reactive power imbalance is not addressed. To reduce reliance on communication, equivalent communication is achieved by injecting synchronized small AC signals into each inverter. However, the injection and extraction of small AC signals complicates the implementation of this system. A common feature of these improved droop control methods is that the local controller of each inverter adjusts its voltage reference in real time based on the measured output reactive power, achieving feedback control. However, the phase delay of the low-pass filter during power measurement can degrade the system's dynamic performance. Furthermore, the constant communication required to obtain remote power information to adjust the output voltage reduces overall system reliability.

[0004] Virtual impedance-based methods can fundamentally improve dynamic performance and enhance reliability by directly compensating for mismatched line impedances. This approach achieves accurate reactive power sharing by feeding the virtual voltage drop forward to the output voltage reference. Generally, the main problem faced by virtual impedance methods is the difficulty in accurately measuring line impedance, making accurate compensation impossible. An adaptive virtual impedance method has also been proposed, combined with a consistency-based method to ensure the specified reactive power distribution. In these methods, each generating unit adjusts its virtual impedance to move toward a common reactive power target; however, this requires a communication network to obtain information from the microgrid central controller (MGCC).

[0005] To achieve equal distribution of reactive power, an improved droop control method or a virtual impedance-based method is needed. However, existing methods have the following limitations: (1) Power or PCC voltage information needs to be remotely collected and transmitted through uninterrupted communication, which affects the reliability of the system and the overall design cost; (2) Line impedance changes in real time and is difficult to measure accurately, so it is impossible to achieve accurate compensation by adjusting the virtual impedance value; (3) In order to achieve coordinated control among multiple machines, a communication network is required to obtain reactive power information of other devices from the microgrid central processor, which reduces the reliability of the system. Summary of the Invention

[0006] In view of the technical problems existing in the prior art, the present invention proposes an adaptive virtual impedance adjustment method for droop-controlled parallel inverters to achieve accurate equal distribution of reactive power.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a method for adaptively adjusting virtual impedance of a droop-controlled parallel inverter, comprising:

[0009] Each parallel inverter increases its virtual impedance value at the same time and with the same adjustment step size by receiving a synchronous trigger signal instruction. The adjustment direction of the virtual impedance is then determined according to the reactive power change trend, and the virtual impedance value is adjusted according to the adjustment direction, so that the reactive power distribution of each inverter will eventually tend to be equal as the total impedance approaches.

[0010] As a further improvement of the present invention, the inverter is a three-phase parallel inverter using droop control;

[0011] The DC side of the inverter is connected to a constant DC voltage source V dc After the three-phase inverter is driven by the modulation signal, the output port voltage v oa 、v ob and v oc Contains high-order harmonics, after the switching ripple is filtered out by the three-phase LC filter, it passes through the line inductor L g Connected to the point of common coupling PCC.

[0012] As a further improvement of the present invention, the three-phase inverter includes a switch tube S1, a switch tube S2, a switch tube S3, a switch tube S4, a switch tube S5, and a switch tube S6.

[0013] As a further improvement of the present invention, the control system is established in a dq coordinate system. First, the output capacitor voltage and output current are sampled. The voltage amplitude and angular frequency reference of the output voltage are calculated by the power outer loop. Then, the corresponding virtual impedance voltage drop is subtracted from the obtained reference voltage amplitude to serve as the reference for the inner loop.

[0014] After the voltage and current dual closed-loop control, the modulation signal is used for space vector modulation, and finally the switch tube drive signal is output; Among them, in the voltage and current dual closed-loop control, the voltage loop PI controller is K pu +K iu / s, the current inner loop proportional coefficient is K i ; The virtual impedance is composed of virtual inductance and virtual resistance, which are represented by L v and R v .

[0015] As a further improvement of the present invention, the line impedances of the parallel inverters are inconsistent, and the initial value of the virtual impedance L v Keep consistent; after the control system detects the synchronous trigger signal, it will be at the initial value L v Increase the virtual impedance value based on .

[0016] As a further improvement of the present invention, the time when each parallel inverter adjusts the virtual impedance value is determined by a synchronous trigger signal, and each synchronous trigger signal is separated by a period of time.

[0017] As a further improvement of the present invention, the adjustment direction of the virtual impedance is determined according to the reactive power change trend, including:

[0018] If the reactive power distribution increases, its virtual impedance value is reduced;

[0019] If the reactive power distribution is reduced, its virtual impedance value is increased.

[0020] As a further improvement of the present invention, after each parallel inverter receives the synchronous trigger signal, the reactive power value at the current moment is Compared with the reactive power value at the last trigger moment For comparison, if Greater than the judgment threshold Q δ , then subtract L from the virtual impedance value v_per ;like Less than -Q δ , then increase the virtual impedance value by L v_per ;

[0021] After breaking the steady-state balance of the system, the power change process of the two inverters is as follows:

[0022]

[0023] Where, is the reactive power of the i-th inverter after the N-th iteration, X1 and X2 are the line impedances connecting the two inverters, and L v is the initial value of virtual impedance, L v_add is the virtual impedance disturbance value;

[0024] After the iterative virtual impedance strategy is introduced, the power change process of the two inverters is as follows:

[0025]

[0026] Where, is the reactive power of the i-th inverter after the N-th iteration, X1 and X2 are the line impedances connecting the two inverters, and L v is the initial value of virtual impedance, L v_add is the virtual impedance disturbance value, L v_per is the virtual impedance iteration step size.

[0027] As a further improvement of the present invention, the condition for determining that the adjustment process is completed is k>threshold of the number of iteration executions.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] The present invention proposes an adaptive virtual impedance adjustment strategy to compensate for the mismatch between line impedances in order to achieve accurate reactive power distribution of parallel inverters in the microgrid. After the parallel system enters a steady state, each inverter increases its own virtual impedance value at the same time with the same adjustment step size by receiving a synchronous trigger signal instruction to break the current reactive power distribution status quo, and then determines the adjustment direction of the virtual impedance according to the reactive power change trend. Finally, the reactive power distribution of each inverter will tend to be equal as the total impedance approaches. This method achieves good power and current balancing effects and avoids potential risks of overcurrent and overheating of the device. Compared with the existing improved droop control method and virtual impedance parameter adjustment method, this solution does not require measuring the PCC point voltage, has lower requirements on the communication network, and has higher system reliability and security. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 Schematic diagram of a three-phase parallel inverter system based on droop control.

[0032] Figure 2 Flowchart of the proposed adaptive virtual impedance adjustment strategy.

[0033] Figure 3 Figure 3 shows the virtual impedance, reactive power, and active power simulation waveforms of the two parallel inverters before and after using the proposed algorithm.

[0034] Figure 4 Figure 3 is the simulated waveform of the a-phase output current of the two parallel inverters before and after using the proposed algorithm. DETAILED DESCRIPTION

[0035] In order to make the purpose and technical solution of the present invention clearer and easier to understand, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0036] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more. In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0037] The distribution of power sources and loads in microgrids is often decentralized, so distributed control strategies offer higher reliability and robustness than centralized control strategies. However, communication network costs, communication latency, and interference resistance are all important system performance indicators. Furthermore, in future, more open and autonomous microgrids, where power generation, energy storage, and loads come from multiple users, basic control strategies should not only avoid complex central computation but also reduce reliance on information exchange to achieve good compatibility. Therefore, it is necessary to pursue more decentralized distributed strategies that maximize the use of local information and reduce reliance on communication networks.

[0038] Based on the limitations of existing methods, the present invention proposes an adaptive virtual impedance adjustment method for droop-controlled parallel inverters to achieve accurate reactive power distribution. Specifically, it includes:

[0039] Each parallel inverter increases its virtual impedance value at the same time and with the same adjustment step size by receiving a synchronous trigger signal instruction. The adjustment direction of the virtual impedance is then determined according to the reactive power change trend, and the virtual impedance value is adjusted according to the adjustment direction, so that the reactive power distribution of each inverter will eventually tend to be equal as the total impedance approaches.

[0040] The adaptive virtual impedance adjustment strategy provided by the present invention compensates for the mismatch between line impedances to achieve accurate reactive power distribution of parallel inverters in the microgrid. After entering the steady state using traditional droop control, each inverter increases its own virtual impedance value at the same time with the same adjustment step size by receiving a synchronous trigger signal instruction to break the current reactive power distribution status quo, and then determines the adjustment direction of the virtual impedance based on the reactive power change trend. Ultimately, the reactive power distribution of each inverter will tend to be equal as the total impedance approaches. Compared to existing adjustment methods for dealing with line impedance mismatch, the solution proposed by the present invention does not require measuring the PCC point voltage, and has lower requirements for the communication bandwidth of the parallel system.

[0041] The droop control inverter described in this invention operates in voltage source mode, and the system includes a droop power loop, a voltage outer loop, and a current inner loop. The droop power loop samples the active and reactive power output of the inverter as feedback to provide voltage and frequency instructions for the voltage outer loop of each inverter, while also achieving power balancing between the inverters. The voltage outer loop samples the filter capacitor voltage and uses feedforward decoupled PI control to provide current instructions for the current inner loop. The current inner loop uses feedforward decoupled proportional control of the filter inductor current to output the inverter's voltage modulation wave.

[0042] In this method, each inverter receives a synchronous trigger signal and increases its virtual impedance at the same time and with the same adjustment step size to break the current reactive power distribution. The virtual impedance adjustment direction is then determined based on the reactive power trend (if the reactive power distribution increases, the virtual impedance value is reduced; if the reactive power distribution decreases, the virtual impedance value is increased). Ultimately, the reactive power distribution of each inverter tends to be equal as the total impedance approaches. Compared to existing methods for regulating uneven reactive power distribution, the strategy proposed in this paper does not require measuring the PCC point voltage and has lower communication bandwidth requirements for the parallel system.

[0043] The control technology solution proposed in this invention is mainly divided into the following parts:

[0044] The control strategy proposed in this invention is aimed at droop control of parallel inverters, such as Figure 1 As shown. The DC side input is a constant DC voltage source V dc , the inverter filters out high-order harmonics through the LC filter, and the line inductance L g Connected to the point of common coupling PCC.

[0045] The main control idea of ​​the system is that the control system is established in the dq coordinate system. First, the output capacitor voltage and output current are sampled. The voltage amplitude and angular frequency reference of the output voltage are calculated by the power outer loop. Then, the corresponding virtual impedance voltage drop is subtracted from the reference voltage amplitude, which is the reference of the inner loop. After the voltage and current double closed-loop control, the modulation signal is used for space vector modulation, and finally the switch tube drive signal is output. Among them, the voltage loop PI controller is K pu +K iu / s, the current inner loop proportional coefficient is K i The virtual impedance is composed of virtual inductance and virtual resistance, which are represented by L v and R v .

[0046] The inverter is a three-phase parallel inverter using droop control;

[0047] The DC side of the inverter is connected to a constant DC voltage source V dc After the three-phase inverter is driven by the modulation signal, the output port voltage v oa 、v ob and v oc Contains high-order harmonics, after the switching ripple is filtered out by the three-phase LC filter, it passes through the line inductor L g Connected to the point of common coupling PCC.

[0048] The three-phase inverter includes a switch tube S1, a switch tube S2, a switch tube S3, a switch tube S4, a switch tube S5, and a switch tube S6.

[0049] Optionally, the line impedances of the parallel inverters are inconsistent, but the initial value of the virtual impedance L v After the control system detects the synchronization trigger signal, it will be consistent with the initial value L v On the basis of increasing the virtual impedance value, it is L v_add .

[0050] Optionally, the time when each parallel inverter adjusts the virtual impedance value is determined by a synchronous trigger signal, and each synchronous trigger signal is separated by a period of time, so as to ensure that each parallel inverter can enter a new steady state.

[0051] After receiving the synchronous trigger signal, each parallel inverter will set the reactive power value at the current moment Compared with the reactive power value at the last trigger moment For comparison, if Greater than the judgment threshold Q δ , then subtract L from the virtual impedance value v_per ;like Less than -Q δ, then increase the virtual impedance value by L v_per .

[0052] Figure 2 The flowchart of the proposed adaptive virtual impedance adjustment strategy is shown in Figure 2. The initial value of the virtual impedance is L v At the same time, by introducing the virtual impedance disturbance value L v_add Breaking the steady-state balance of the system and prompting the redistribution of reactive power of each inverter. i When it decreases, it means that the total output impedance of this inverter is smaller than that of the other one, and L needs to be added to the virtual impedance. v_per ; When Q i If it increases, it means that its total output impedance is greater than the other one, and L needs to be subtracted from the virtual impedance. v_per When the reactive power is evenly distributed and enters a steady state, the virtual impedance is no longer adjusted.

[0053] It is worth emphasizing that each iteration is only valid when the change in reactive power is greater than the judgment threshold Q δ The iteration condition is considered to be met only when the virtual impedance is less than 0.05, otherwise the virtual impedance remains at the value of the previous iteration cycle. This is mainly to avoid the continuous change of the virtual impedance and the resulting jitter of the output reactive power.

[0054] After breaking the steady-state balance of the system, the power change process of the two inverters is as follows:

[0055]

[0056] Where, is the reactive power of the i-th inverter after the N-th iteration, X1 and X2 are the line impedances connecting the two inverters, and L v is the initial value of virtual impedance, L v_add is the virtual impedance disturbance value.

[0057] After the iterative virtual impedance strategy is introduced, the power change process of the two inverters is as follows:

[0058]

[0059] Where, is the reactive power of the i-th inverter after the N-th iteration, X1 and X2 are the line impedances connecting the two inverters, and L v is the initial value of virtual impedance, L v_add is the virtual impedance disturbance value, L v_per is the virtual impedance iteration step size.

[0060] For example, the algorithm ends when k > 10 (which can be modified based on actual debugging conditions), meaning that the virtual impedance parameter adjustment is performed 10 times. The adjustment process is considered complete when k > 10, meaning that the algorithm is iterated 10 times. In different application scenarios, the adjustment process completion criteria can be modified based on the actual reactive power sharing situation.

[0061] Figure 3 Figure 1 shows the simulated waveforms of the virtual impedance, reactive power, and active power of two parallel inverters before and after the proposed algorithm is applied. It can be seen that after the algorithm is enabled, the virtual impedance begins to change according to the iterative algorithm, with the first inverter's virtual impedance gradually increasing and the second inverter's virtual impedance gradually decreasing. The reactive power of the two inverters gradually approaches and eventually becomes evenly distributed. The active power is able to return to an evenly distributed state after each transient fluctuation. Figure 4 Figure 2 shows the simulated waveforms of the a-phase output current of two parallel inverters before and after the proposed algorithm is applied. It can be seen that before the proposed algorithm was applied, the phase and amplitude of the a-phase current of the two inverters were different. After the adaptive virtual impedance adjustment method was applied, the phase and amplitude of the a-phase current of the two inverters were the same, achieving equal sharing.

[0062] In order to enable those skilled in the art to better understand the present invention, the present invention is further described in detail below with reference to the accompanying drawings. The specific steps are as follows:

[0063] Step 1: Design the initial steady-state operating point, including determining the rated power and rated current of the parallel inverters. Then, design the LC filter based on the inverter-side current ripple requirements and the reactive power requirements absorbed by the filter capacitor.

[0064] Step 2: Design the voltage outer loop controller parameter K pu and K iu , current inner loop proportional coefficient K i , droop coefficient K p and K q , and the initial value of the virtual impedance L v (Take the virtual resistance R v =0).

[0065] Step 3: Activate the adaptive virtual impedance adjustment algorithm at t=1s, as shown in Figure 3 As shown in (a), the virtual impedance of the two inverters changes in the same direction, that is, from 2mH to 2.5mH. Compared with the reactive power distribution of the two inverters at t = 1s, the reactive power of the second inverter increases at t = 1.2s, while the reactive power of the first inverter decreases. Therefore, the adjustment direction of the virtual impedance can be determined, that is, increasing the virtual impedance of the first inverter and reducing the virtual impedance of the second inverter. The iterative step size L v_perIn this simulation, 0.5mH is used. Similarly, the algorithm is executed every 0.2s for 10 consecutive times. If the power change after impedance adjustment is less than the set threshold Q, it indicates that the impedance difference between the two units is not significant and no further adjustment is required.

[0066] like Figure 3 As shown in (b), the system reactive power will be well evenly distributed, and the final reactive power distribution error is determined by the set power judgment threshold. Although there will be disturbances in the active power during each adjustment of the virtual impedance, it will eventually return to a steady state. In practical engineering applications, a smaller virtual impedance iteration step size L can be selected to achieve the desired effect. v_per To avoid shock during adjustment.

[0067] Step 4, Figure 4 Figure 2 shows the simulated waveforms of the output current of phase a of the two inverters before and after activating the proposed adaptive algorithm. It can be seen that uneven reactive power distribution will cause differences in the inverter output currents. The inverter that bears the greater reactive power will have a higher output current, which poses the risk of output overcurrent and potential heat dissipation imbalance. When the proposed method is implemented, the inverter output currents essentially overlap, achieving excellent current sharing and avoiding potential overcurrent and overheating risks.

[0068] The above are only preferred embodiments of the present invention and do not limit the present invention in any way. Any simple modifications, changes and equivalent structural changes made to the above embodiments based on the technical essence of the present invention are still within the scope of protection of the technical solution of the present invention.

[0069] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the same. Although the present invention has been described in detail with reference to the above embodiments, a person skilled in the art may still modify or make equivalent substitutions for the specific implementation schemes of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention.

[0070] The above content is a further detailed description of the present invention, and it cannot be considered that the specific implementation methods of the present invention are limited to these. For ordinary technicians in the technical field to which the present invention belongs, they can make several simple deductions or substitutions without departing from the concept of the present invention, which should be regarded as belonging to the scope of protection of the present invention determined by the submitted claims.

Claims

1. A method for adaptive virtual impedance adjustment of droop-controlled parallel inverters, characterized in that: include: Each parallel inverter increases its virtual impedance value at the same time and with the same adjustment step size by receiving the synchronous trigger signal instruction; The virtual impedance adjustment direction is then determined based on the reactive power change trend, and the virtual impedance value is adjusted accordingly, so that the reactive power distribution of each inverter tends to be equal as the total impedance approaches. The adjustment direction of the virtual impedance is determined based on the reactive power change trend, including: If the reactive power distribution increases, its virtual impedance value is reduced; If the reactive power distribution is reduced, its virtual impedance value is increased; After receiving the synchronous trigger signal, each parallel inverter will Q i N Compared with the reactive power value at the last trigger moment Q i N-1 For comparison, if Q i N - Q i N-1 Greater than the judgment threshold Q δ , then subtract the virtual impedance value from L v_per ;like Q i N - Q i N-1 Less than - Q δ , then increase the virtual impedance value L v_per ; After breaking the steady-state balance of the system, the power change process of the two inverters is as follows: Where, Q i N For the i Inverter No. N The reactive power after iterations is X 1. X 2 are the line impedances connecting the two inverters, L v is the initial value of virtual impedance, L v_add is the virtual impedance disturbance value; After the iterative virtual impedance strategy is introduced, the power change process of the two inverters is as follows: Where, Q i N For the i Inverter No. N The reactive power after iterations is X 1. X 2 are the line impedances connecting the two inverters, L v is the initial value of virtual impedance, L v_add is the virtual impedance disturbance value, L v_per is the virtual impedance iteration step size.

2. The method for adaptive virtual impedance adjustment of droop-controlled parallel inverters according to claim 1, characterized in that: The inverter is a three-phase parallel inverter using droop control; The DC side of the inverter is connected to a constant DC voltage source V dc , after the three-phase parallel inverter is driven by the modulation signal, the output port voltage v oa 、 v ob and v oc Contains high-order harmonics, through three-phase LC After the filter removes the switching ripple, the line inductance L g Connected to the point of common coupling PCC.

3. The method for adaptive virtual impedance adjustment of droop-controlled parallel inverters according to claim 2, characterized in that: The three-phase parallel inverter includes a switch tube S1, a switch tube S2, a switch tube S3, a switch tube S4, a switch tube S5, and a switch tube S6.

4. The method for adaptive virtual impedance adjustment of droop-controlled parallel inverters according to claim 1, characterized in that: The control system is established in the dq coordinate system. First, the output capacitor voltage and output current are sampled. The output voltage amplitude and angular frequency reference are calculated by the power outer loop. The corresponding virtual impedance voltage drop is then subtracted from the reference voltage amplitude to serve as the reference for the inner loop. After the voltage and current dual closed-loop control, the modulation signal is used for space vector modulation, and the switch tube drive signal is finally output; Among them, in the voltage and current dual closed-loop control, the voltage loop PI controller is K pu + K iu / s , the current inner loop proportional coefficient is K i ; The virtual impedance is composed of virtual inductance and virtual resistance, which are expressed as L v and R v .

5. The method for adaptive virtual impedance adjustment of droop-controlled parallel inverters according to claim 4, characterized in that: The line impedances of the parallel inverters are inconsistent, and the initial value of the virtual impedance is L v Keep consistent; after the control system detects the synchronous trigger signal, it will L v Increase the virtual impedance value based on .

6. The method for adaptive virtual impedance adjustment of droop-controlled parallel inverters according to claim 1, characterized in that: The time when each parallel inverter adjusts the virtual impedance value is determined by the synchronous trigger signal, and each synchronous trigger signal is separated by a period of time.

7. The method for adaptive virtual impedance adjustment of droop-controlled parallel inverters according to claim 1, characterized in that: The conditions for judging the completion of the adjustment process are k >Threshold of number of iterations.

Images