An inverter parallel current sharing control method based on virtual reactive voltage self-adjustment

Abstract

The application discloses a kind of based on virtual reactive voltage self-adjustment's inverter parallel current sharing control method, belong to the coordinated control technical field of parallel inverter.Compared with traditional droop control, the application can well improve reactive power sharing degree, and enhance the effect and response speed of reactive power sharing when load power changes.The application analyzes the relationship between inverter output power and output impedance in depth, combines the characteristics of ring chain control and traditional droop control, introduces reactive ring chain and self-adjusting virtual reactive voltage control, so that virtual impedance can be adaptively adjusted according to the cyclic chain tracking of parallel module output reactive power, and then compensate the mismatch between parallel module output impedance, and realize the output reactive power balance of parallel module.In addition, the application does not need to measure the voltage of common coupling point, has lower requirement for communication network, and has higher system reliability and safety, which provides good reference value for engineering application.

Description

Technical Field

[0001] This invention belongs to the field of coordinated control technology for parallel inverters, specifically relating to a parallel current sharing control method for inverters based on virtual reactive voltage self-adjustment. Background Technology

[0002] With increasing public awareness of environmental protection, renewable energy sources are being developed and utilized, leading to the emergence of microgrids and distributed generation. Inverter parallel connection technology, as a key technology for microgrids and distributed generation, is also receiving increasing attention. This technology offers numerous advantages, such as modular design, high redundancy, and economic efficiency, and can enhance inverter functionality and application range, thereby improving the reliability of the power supply system.

[0003] One of the key issues in parallel inverter operation is the balanced distribution of load current to suppress circulating current between parallel systems. Load current will only be evenly distributed according to the capacity of the parallel modules when the output voltage amplitude, frequency, and phase of each parallel module are the same. However, when the output impedance and circuit parameters of the parallel modules are inconsistent, circulating current will exist between the parallel systems, leading to unbalanced output power and causing overload or damage to the parallel modules. Currently, various current sharing control strategies for parallel inverter operation have been proposed. Since power droop control does not require interconnecting wires, and the parallel modules have strong spatial dispersion and scalability, exhibiting high redundancy and reliability, power droop control is the mainstream control strategy for current sharing in parallel inverter operation. However, when the output impedances of the parallel modules are unequal, the reactive power output of the parallel modules under power droop control is unbalanced, resulting in poor current sharing performance in the parallel system.

[0004] To address the aforementioned issues, balancing the reactive power output when there is impedance mismatch in parallel modules under power droop control has become a research hotspot. One study proposed a robust droop control method that incorporates an integral term into the reactive power-amplitude droop control loop, eliminating the influence of output impedance on reactive power in steady state and achieving reactive power equalization. However, this method heavily relies on the point of common coupling (PCC) voltage to eliminate the influence of line impedance. Recent literature employs a harmonic signal injection method, which injects a small harmonic (approximately 1% to 2% of the fundamental voltage) into the reference voltage and controls reactive power based on the harmonic power to achieve reactive power equalization. However, this method increases the harmonic content of the system output voltage due to the injected harmonic voltage. Other literature uses a virtual impedance method, subtracting a current feedback term from the reference voltage to compensate for output impedance mismatch, which enhances system stability and achieves reactive power equalization. However, since the line parameters of parallel systems are difficult to measure and the output impedance of parallel modules cannot be accurately known, the traditional virtual impedance method cannot fully compensate for the output impedance mismatch, and the power sharing effect is still not ideal. Summary of the Invention

[0005] To overcome the shortcomings of the existing technology, improve the reactive power sharing degree based on power droop control, and enhance the reactive power sharing effect and response speed when the load power changes, this invention proposes a parallel current sharing control method for inverters based on virtual reactive voltage self-adjustment, which can achieve accurate reactive power sharing.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] This invention provides a parallel current sharing control method for inverters based on virtual reactive voltage self-adjustment, comprising the following steps:

[0008] 1) Several inverters are connected in parallel. The DC side of all inverters is connected to a DC source, and the AC side of all inverters is connected to an AC bus. Each inverter is controlled equally as a voltage source using the same control strategy.

[0009] 2) Each inverter samples voltage and current information for power calculation, then uses the calculated active and reactive power for droop control, and obtains the voltage reference value for droop control. Voltage phase information is obtained through a SOGI-based phase-locked loop. The phase-locked loop, combined with dual closed-loop voltage and current control, ensures the frequency reference value is near the base frequency (50Hz) and keeps all inverters synchronized, achieving stable parallel operation.

[0010] 3) Based on the reactive power calculated in step 2), perform reactive power loop control and self-adjusting virtual reactive voltage control to obtain the voltage regulation value of the self-adjusting virtual reactive voltage, and then adjust the voltage reference value obtained by the droop control in step 2) to obtain the final voltage reference value of the inverter.

[0011] 4) Based on the final voltage reference value of the inverter obtained in step 3), the input voltage and current dual closed-loop regulator generates a PWM modulation signal.

[0012] Preferably, the specific method for achieving droop control in step 2) is as follows:

[0013]

[0014] Where ω and E are the inverter voltage reference amplitude and frequency obtained from droop control, ω o and E o Here, represents the inverter's no-load output voltage amplitude and frequency; m and n are the active-frequency droop adjustment coefficient and reactive-amplitude droop adjustment coefficient, respectively; P and Q are the active power and reactive power calculated by power calculation; and i is the inverter's serial number.

[0015] Step 3) involves reactive power loop control and self-adjusting virtual reactive voltage control, the specific methods of which are as follows:

[0016] The reactive power value of the previous inverter is obtained through a reactive power loop and input into the self-adjusting virtual reactive voltage control as the reactive power reference value for this inverter. An integral control strategy is used to generate a self-adjusting virtual impedance to eliminate steady-state reactive power errors between inverters. The specific expression is as follows:

[0017]

[0018]

[0019] Among them, M v For self-adjusting virtual impedance, k v Here, is the self-adjustment coefficient, N is the number of parallel inverters, s is the Laplace operator, and u vQ i is the voltage regulation value for self-adjusting virtual reactive voltage. o This is the output current.

[0020] Preferably, for the self-adjustment coefficient k v The selection process includes the following steps:

[0021] 1) The formulas for calculating the active and reactive power output of the inverter are as follows:

[0022]

[0023] Among them, R t R is the total output resistance, and R is the inherent output resistance. o Feeder resistance R f The sum of the self-adjusting virtual resistance; X t X is the total output reactance, and X is the inherent output reactance. o Feeder Reactance X f The sum of the self-adjusting virtual resistance and the self-adjusting virtual reactance. For simplicity, the self-adjusting virtual resistance and self-adjusting virtual reactance are approximated as the self-adjusting virtual impedance M. v U o δ is the PCC voltage, and E is the inverter's no-load output voltage. o and PCC voltage U o The angle between them.

[0024] 2) Perform small-signal modeling analysis. The linearized expressions for the active and reactive power outputs of the inverter are as follows:

[0025] ΔP i =k peΔ E i +k pδ Δδ i +k pm ΔM v

[0026] ΔQi =k qe ΔE i +k qδ Δδ i +k qm ΔM v

[0027] Where k pe k pδ k pm and k qe K qδ k qm Let be the partial derivatives of active power and reactive power at the steady-state point, respectively, and their expressions are as follows:

[0028]

[0029] 3) The linearized expressions for droop control and self-adjusting virtual reactive voltage control are as follows:

[0030] Δω=-mΔP i

[0031] ΔE=-nΔQ i

[0032]

[0033] 4) The expression after adding a low-pass filter in step 3) and combining it with step 2) is as follows:

[0034]

[0035]

[0036]

[0037] Where, ω c These are the parameters for the low-pass filter.

[0038] 5) Combining step 4) and Δω=s.Δδ, the characteristic equation of the parallel system is expressed as follows:

[0039] s 4 +b1s 3 +b2s 2 +b3s+b4=0

[0040] The expressions for the coefficients b1, b2, b3, and b4 are as follows:

[0041]

[0042] Preferably, the self-adjusting coefficient k can be plotted based on the characteristic equation. vWhen designing a variable root locus diagram, the self-adjustment coefficient should not be too large in order to coordinate the stability and dynamic response of the system.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] This invention proposes a parallel current sharing control method for inverters based on virtual reactive power voltage self-adjustment. Compared with traditional droop control, this method can significantly improve reactive power sharing and enhance the reactive power sharing effect and response speed when the load power changes. This invention, through in-depth analysis of the relationship between inverter output power and output impedance, and combining the characteristics of loop control and traditional droop control, introduces a reactive power loop self-adjusting virtual impedance. This virtual impedance can adaptively adjust according to the cyclic chain tracking of the reactive power output of the parallel modules, thereby compensating for the mismatch between the output impedances of the parallel modules and achieving reactive power balance. Furthermore, this invention can still operate normally during load transitions, greatly saving adaptive adjustment time and accelerating the system's dynamic response speed. Moreover, this invention can achieve fast parallel and offline steady-state operation when the inverter is in parallel or offline mode, and has good hot-swappable functionality. Finally, this invention does not require measurement of the PCC point voltage, has lower requirements for the communication network, and offers higher system reliability and security, providing valuable reference for engineering applications. Attached Figure Description

[0045] Figure 1 This is a control block diagram of the present invention;

[0046] Figure 2 This is a diagram of the self-adjusting virtual reactive voltage structure in this invention;

[0047] Figure 3 k is the self-adjusting coefficient in this invention. v Root locus diagram of variation;

[0048] Figure 4 The output power waveforms of the three inverters under three control methods are shown.

[0049] Figure 5 The waveform diagrams of the self-adjusting virtual impedance of the three inverters under different control methods are shown.

[0050] Figure 6 The waveform of the common coupling point voltage under the three control methods is shown.

[0051] Figure 7 The simulation waveforms of reactive power, output current, and common coupling point voltage of three inverters under the control method proposed in this invention, when the load power switches from full load to light load and then to heavy load. Detailed Implementation

[0052] To further understand the content of this invention, the following detailed description is provided in conjunction with the accompanying drawings and specific embodiments. These descriptions are for illustrative purposes only and are not intended to limit the scope of the invention.

[0053] See Figure 1 and Figure 2 This invention provides a parallel current sharing control method for inverters based on virtual reactive voltage self-adjustment, comprising the following steps:

[0054] 1) Several inverters are connected in parallel. The DC side of all inverters is connected to a DC source, and the AC side of all inverters is connected to an AC bus. Each inverter is controlled equally as a voltage source using the same control strategy.

[0055] 2) Each inverter samples voltage and current information for power calculation, then uses the calculated active and reactive power for droop control, and obtains the voltage reference value for droop control. Voltage phase information is obtained through a SOGI-based phase-locked loop. The phase-locked loop, combined with dual closed-loop voltage and current control, ensures the frequency reference value is near the base frequency (50Hz) and keeps all inverters synchronized, achieving stable parallel operation.

[0056] 3) Based on the reactive power calculated in step 2), perform reactive power loop control and self-adjusting virtual reactive voltage control to obtain the voltage regulation value of the self-adjusting virtual reactive voltage, and then adjust the voltage reference value obtained by the droop control in step 2) to obtain the final voltage reference value of the inverter.

[0057] 4) Based on the final voltage reference value of the inverter obtained in step 3), the input voltage and current dual closed-loop regulator generates a PMW modulation signal.

[0058] 5) The specific method for achieving droop control in step 2) is as follows:

[0059]

[0060] Where, ω i and E i ω represents the inverter voltage reference amplitude and frequency obtained from droop control. o and E o P represents the inverter's no-load output voltage amplitude and frequency, m and n are the active-frequency droop adjustment coefficient and reactive-amplitude droop adjustment coefficient, respectively. i and Q i The active and reactive power are calculated, and i is the inverter's serial number.

[0061] 6) The specific methods for reactive power loop control and self-adjusting virtual reactive voltage control in step 3) are as follows:

[0062] The reactive power value of the previous inverter is obtained through a reactive power loop and input into the self-adjusting virtual reactive voltage control as the reactive power reference value for this inverter. An integral control strategy is used to generate a self-adjusting virtual impedance to eliminate steady-state reactive power errors between inverters. The specific expression is as follows:

[0063]

[0064]

[0065] Among them, M v For self-adjusting virtual impedance, k v Here, is the self-adjustment coefficient, N is the number of parallel inverters, s is the Laplace operator, and u vQ i is the voltage regulation value for self-adjusting virtual reactive voltage. o This is the output current.

[0066] 7) Regarding the self-adjustment coefficient k in step 6), v The selection of inverter output active and reactive power are calculated using the following formulas:

[0067]

[0068] Among them, R t R is the total output resistance, and R is the inherent output resistance. o Feeder resistance R f The sum of the self-adjusting virtual resistance; X t X is the total output reactance, and X is the inherent output reactance. o Feeder Reactance X f The sum of the self-adjusting virtual resistance and the self-adjusting virtual reactance. For simplicity, the self-adjusting virtual resistance and self-adjusting virtual reactance are approximated as the self-adjusting virtual impedance M. v U o δ is the PCC voltage, and E is the inverter's no-load output voltage. o and PCC voltage U o The angle between them.

[0069] 8) After performing small-signal modeling analysis, the linearized expressions for the inverter output active power and reactive power in step 7) are as follows:

[0070] ΔP i =k pe ΔE i +k pδ Δδ i +k pm ΔM v

[0071] ΔQ i =k qe ΔE i+k qδ Δδ i +k qm ΔM v

[0072] Where k pe k pδ k pm and k pe K qδ k qm Let be the partial derivatives of active power and reactive power at the steady-state point, respectively, and their expressions are as follows:

[0073]

[0074] 9) The linearized expressions for steps 2) and 3) are as follows:

[0075] Δω=-mΔP i

[0076] ΔE=-nΔQ i

[0077]

[0078] 10) The expression after adding a low-pass filter in step 9) and combining it with step 8) is as follows:

[0079]

[0080] Where, ω c These are the parameters for the low-pass filter.

[0081] 11) Combining step 10) and Δω=s·Δδ, the characteristic equation of the parallel system is expressed as follows:

[0082] s 4 +b1s 3 +b2s 2 +b3s+b4=0

[0083] The expressions for the coefficients b1, b2, b3, and b4 are as follows:

[0084]

[0085] 12) Based on the characteristic equation of step 11), Figure 3 The self-adjustment coefficient k is given. v By varying the root locus plot, we can draw the following conclusions:

[0086] The parallel system after introducing self-adjusting virtual reactive voltage control has four characteristic roots: a pair of conjugate complex roots and a pair of real roots, all located in the left half-plane, indicating a stable system. The conjugate complex roots exhibit an arc-shaped change as the self-adjustment coefficient increases, initially approaching the real axis, then moving away from it, and finally approaching it again. According to control theory, increasing the imaginary part of the characteristic roots leads to decreased system damping, increased overshoot, and prolonged oscillation settling time. Therefore, the self-adjustment coefficient should be selected during the initial stage when the conjugate complex roots first approach the real axis. During this stage, the system overshoot is small, the adjustment is fast, and the dynamic performance is good. The other pair of real roots gradually moves away from the imaginary axis as the self-adjustment coefficient increases, having a relatively small impact on the stability of the parallel system. Therefore, the self-adjustment coefficient should not be too large, otherwise, it will worsen the stability.

[0087] Specifically, the simulation model of this invention uses three inverters to share the load, with different feeder impedances set for each. Simulations are performed on the traditional droop control method, the virtual impedance control method, and the control method proposed in this invention. Simulation parameters are shown in Table 1.

[0088] Table 1 Simulation Parameters

[0089] parameter numerical values DC input voltage <![CDATA[V dc =400V]]> Rated capacity <![CDATA[S 1,2,3 =2.2kVA]]> Rated output voltage <![CDATA[U o =220V]]> Rated output frequency <![CDATA[f o =50Hz]]> Switching frequency <![CDATA[f s =20kHz]]> LC filter (parasitic resistance r) L = 2mH, C = 3.17μF (r = 5mΩ) Voltage controller PI parameters <![CDATA[k pu =0.0085,k iu =524.7]]> Current controller PI parameters <![CDATA[k pi =0.203,k ii =6132]]> Sag coefficient m = 0.0003, n = 0.0008 Self-adjustment coefficient <![CDATA[k v =0.01]]> Feeder impedance 1 <![CDATA[Z f1 =0.05+j0.251Ω]]> Feeder impedance 2 <![CDATA[Z f2 =0.1+j0.314Ω]]> Feeder impedance 3 <![CDATA[Z f3 =0.15+j0.377Ω]]> Load power <![CDATA[S L =(1.76+j1.32)kVA]]>

[0090] Figure 4 Output power waveforms of three inverters under three control methods are presented. The control methods at different stages, from left to right, are: traditional droop control method, virtual impedance control method, and the control method proposed in this invention. Figure 4 In the traditional droop control method (first stage), active power can be well evenly distributed, while reactive power is unevenly distributed, with its magnitude inversely proportional to the total output impedance of the inverter module. In the virtual impedance control method (second stage), active power is still evenly distributed, but the output active power decreases, while the reactive power distribution increases, though it remains unevenly distributed. In the control method proposed in this invention (third stage), both active and reactive power can be evenly distributed, distributing the load power equally.

[0091] Figure 5 The self-adjusting virtual impedance waveforms of three inverters under different control methods are presented. In steady state, the virtual impedance of the module with smaller total output impedance is positive, while the virtual impedance of the module with larger total output impedance is negative. Under its adjustment, the total output impedance of the parallel modules tends to be consistent.

[0092] Figure 6The waveforms of the common coupling point voltage under three control methods are shown. It can be seen that the common coupling point voltage is reduced under the virtual impedance control method, while there is no significant change in the common coupling point voltage under the traditional droop control and the control method proposed in this invention. Therefore, the control method proposed in this invention does not sacrifice the output voltage accuracy, achieves power equalization, and has a certain voltage regulation effect.

[0093] Figure 7 Simulated waveforms of reactive power, output current, and common coupling point voltage for three inverters under the control method proposed in this invention, during load switching from full load to light load and then back to heavy load, are presented. It can be seen that under the control method proposed in this invention, reactive power and output current quickly achieve equilibrium after load switching, exhibiting a fast dynamic response speed and good reactive power distribution. Furthermore, under the control method proposed in this invention, the common coupling point voltage fluctuates less during load changes, demonstrating high voltage regulation accuracy.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A parallel current sharing control method for inverters based on virtual reactive voltage self-adjustment, characterized in that, Includes the following steps: 1) Several inverters are connected in parallel. The DC side of all inverters is connected to a DC source, and the AC side of all inverters is connected to an AC bus. Each inverter is controlled equally as a voltage source using the same control strategy. 2) Each inverter samples voltage and current information to calculate power, and then uses the calculated active and reactive power to perform droop control, and obtains the voltage reference value for droop control. Voltage phase information is obtained through a phase-locked loop based on SOGI. The phase-locked loop, together with voltage and current dual closed-loop control, can ensure that the frequency reference value is near the base frequency of 50Hz and keep each inverter synchronized, so as to achieve stable parallel operation. 3) Perform reactive power loop and self-adjusting virtual reactive voltage control based on the reactive power calculated in step 2) to obtain the voltage regulation value of the self-adjusting virtual reactive voltage, and then adjust the voltage reference value obtained by the droop control in step 2) to obtain the final voltage reference value of the inverter. 4) Based on the final voltage reference value of the inverter obtained in step 3), the input voltage and current dual closed-loop regulator generates a PWM modulation signal; The specific method for implementing droop control in step 2) is as follows: Where ω and E are the inverter voltage reference amplitude and frequency obtained from droop control, ω o and E o The inverter's no-load output voltage amplitude and frequency are given by , m and n are the active-frequency droop adjustment coefficient and reactive-amplitude droop adjustment coefficient, respectively, P and Q are the active power and reactive power calculated by power calculation, and i is the inverter's serial number. Step 3) involves reactive power loop control and self-adjusting virtual reactive voltage control, the specific methods of which are as follows: Among them, M v For self-adjusting virtual impedance, k v Here, is the self-adjustment coefficient, N is the number of parallel inverters, s is the Laplace operator, and u vQ i is the voltage regulation value for self-adjusting virtual reactive voltage. o This is the output current.

2. The inverter parallel current sharing control method based on virtual reactive voltage self-adjustment according to claim 1, characterized in that, Regarding the self-adjustment coefficient k v The selection process includes the following steps: 1) The formulas for calculating the active and reactive power output of the inverter are as follows: Among them, R t R is the total output resistance, and R is the inherent output resistance. o Feeder resistance R f The sum of the self-adjusting virtual resistance; X t X is the total output reactance, and X is the inherent output reactance. o Feeder Reactance X f The sum of the self-adjusting virtual resistance and the self-adjusting virtual reactance is, for the sake of simplifying the analysis, approximated as the self-adjusting virtual impedance M. v U o δ is the common coupling point (PCC) voltage, and E is the inverter's no-load output voltage. o and PCC voltage U o The angle between them; 2) Perform small-signal modeling analysis. The linearized expressions for the active and reactive power outputs of the inverter are as follows: ΔP i =k pe DE i +k pδ Dd i +k pm DM v ΔQ i =k qe DE i +k qδ Dd i +k qm DM v Where k pe k pδ k pm and k qe k qδ k qm Let be the partial derivatives of active power and reactive power at the steady-state point, respectively, and their expressions are as follows: 3) The linearized expressions for droop control and self-adjusting virtual reactive voltage control are as follows: Δω=-mΔP i ΔE=-nΔQ i 4) The expression after adding a low-pass filter in step 3) and combining it with step 2) is as follows: Where, ω c These are the parameters of the low-pass filter; 5) Combining step 4) and Δω=s·Δδ, the characteristic equation of the parallel system is expressed as follows: s 4 +b1s 3 +b2s 2 +b3s+b4=0 The expressions for each phase coefficient b1, b2, b3, and b4 are as follows:

3. The inverter parallel current sharing control method based on virtual reactive voltage self-adjustment according to claim 2, characterized in that, The method for selecting the self-adjustment coefficient includes: plotting the relationship between the self-adjustment coefficient k and the characteristic equation. v When designing a variable root locus diagram, the self-adjustment coefficient should not be too large in order to coordinate the stability and dynamic response of the system.

Images