A multi-mode control method and system for inverter parallel system

Abstract

The invention discloses a multi-mode control method and system for an inverter parallel system, and the invention belongs to the field of microgrid control technology. The method comprises: sampling the inverter output voltage and current in the inverter parallel system, and performing dq rotation transformation; obtaining the inverter output reactive power and active power through power calculation; performing droop control calculation on the inverter output reactive power and active power to obtain the inverter output voltage reference value; comparing the inverter output voltage reference value with the actual output voltage, and then calculating the error value through the voltage control outer loop to obtain the inverter output current reference value; and comparing the inverter output current reference value with the actual output current, and then obtaining the SVPWM modulation wave through the current control inner loop. The invention not only realizes the multi-mode control of the n-unit inverter parallel system, but also provides power quality management auxiliary services for the microgrid, and provides technical support for the operation and maintenance of the microgrid system.

Description

Technical Field

[0001] The present invention belongs to the technical field of microgrid control, and in particular relates to a multi-mode control method and system for an inverter parallel system. Background Art

[0002] The increasingly mature microgrid technology has become a key component of smart grids and energy Internet. There are three main architectures commonly used in microgrids, namely, the master-slave centralized architecture with communication, the peer-to-peer architecture without communication, and the hierarchical architecture. Among them, the control mode of the inverter of the peer-to-peer architecture microgrid is the grid-type control, and the main methods include droop control, virtual synchronous machine control, etc. The peer-to-peer architecture microgrid is suitable for decentralized areas with poor communication and has plug-and-play system expansion capabilities. However, due to the inconsistent output line impedance of the inverter, there will be problems with active and reactive power coupling and power sharing between inverters. The interaction between inverters will also cause problems such as system function and frequency oscillation. Because the inverter is voltage source controlled when connected to the grid and off the grid under the peer-to-peer architecture, there is no need to change the control mode when switching between on and off the grid, and it is easier to smoothly disconnect from the grid when unplanned disconnection occurs.

[0003] There are many studies on the control strategy of peer-to-peer microgrids. For example: hierarchical control strategy, the bottom control applies a unified voltage and current control structure, introduces a current limiter, and reduces the impact of transient processes, but the control system of this control strategy is complex, computationally intensive, and highly dependent on communication. In order to solve the control deviation problem caused by droop control, an improved droop control is adopted to solve the system frequency and voltage amplitude and deviation problems and reduce the problem of power quality degradation. And an improved power control strategy is adopted to compensate for reactive power demand, harmonics and negative sequence current, and then the measurement requires a large-capacity energy storage system, which increases investment and operation and maintenance costs. Summary of the invention

[0004] In order to solve the defects of existing microgrid control technology and meet the capacity requirements of microgrid systems, the present invention proposes a multi-modal control method and system for inverter parallel systems. The present invention adopts a droop control strategy, which not only realizes the multi-modal control of n inverter parallel systems, but also provides power quality management auxiliary services for microgrids, and provides technical support for the operation and maintenance of microgrid systems.

[0005] The present invention is achieved through the following technical solutions:

[0006] A multi-mode control method for a parallel inverter system, the method comprising:

[0007] Sampling the output voltage and current of the inverters in the inverter parallel system, and performing dq rotation conversion;

[0008] The inverter outputs reactive power and active power through power calculation;

[0009] Performing droop control calculation on the reactive power and active power output by the inverter to obtain a reference value of the inverter output voltage;

[0010] The inverter output voltage reference value is compared with the actual output voltage, and the calculated error value is passed through a voltage control outer loop to obtain the inverter output current reference value;

[0011] After comparing the inverter output current reference value with the actual output current, an SVPWM modulation wave is obtained through a current control inner loop, that is, a switch control signal of the inverter is obtained;

[0012] Wherein, the inverter parallel system consists of n The multifunctional inverters are connected in parallel, and n is an integer greater than or equal to 2.

[0013] In some embodiments, the method further comprises:

[0014] The inverter outputs reactive power and active power by adopting a control technology based on inductive droop;

[0015] The control technology based on inductive droop is specifically to adjust the voltage control parameters so that the system equivalent impedance behaves as inductive above the base frequency or the specified frequency under the premise of satisfying the stability and dynamics of the inverter parallel system; the system equivalent impedance is obtained by combining the inverter equivalent output impedance and the line impedance.

[0016] In some embodiments, the method further comprises:

[0017] In off-grid mode, the voltage control outer loop adopts a composite controller in which a PI controller and a quasi-proportional resonant controller are connected in parallel, wherein the PI controller is used to control the fundamental wave of the inverter output voltage, and the quasi-proportional resonant controller is used to control the inverter output voltage harmonics and negative sequence components.

[0018] In some embodiments, the transfer function of the composite controller is for:

[0019]

[0020] in, K p is the proportional gain, K i is the integral gain, K hR is the harmonic compensation gain, ω hcnis the cut-off frequency, Nω 0 represents the angular frequency of the AC signal in the dq rotating coordinate system, s is a complex variable.

[0021] In some embodiments, the design principles of various parameters in the composite controller include:

[0022] Comprehensively consider the impact of parameters on performance: the proportional gain affects the response speed and amplitude, the integral gain affects the amplitude and phase margin of the low frequency band, and the harmonic compensation gain and cutoff frequency affect the harmonic suppression effect of the mid-frequency band;

[0023] Comprehensively consider parameter trade-offs and stability: when designing and adjusting the parameters of the composite controller, the stability of the composite controller is analyzed and ensured through Bode diagrams;

[0024] Comprehensive consideration of frequency characteristics: When designing and adjusting the parameters of the composite controller, attention should be paid to the harmonic suppression effect in the mid-frequency band.

[0025] In some embodiments, the method further comprises:

[0026] In the grid-connected mode, the current control inner loop adopts a linear self-interference rejection current inner loop based on a proportional resonant observer and combines it with an improved power detection module to achieve grid-connected power generation, and uses the remaining capacity of the inverter to manage the current and power quality.

[0027] In some embodiments, the method further comprises:

[0028] In off-grid mode, pre-synchronization control technology is used to achieve synchronization of the AC bus voltage and grid voltage amplitude and phase.

[0029] In some implementations, the pre-synchronization control technology specifically includes:

[0030] By performing phase locking on the AC bus voltage, the obtained phase angle is subjected to dq rotation transformation with the grid voltage, and the q-axis component of the grid voltage is controlled to be 0, thereby achieving the purpose of phase synchronization;

[0031] The grid voltage amplitude is used as a given reference value, and the AC bus voltage amplitude is used as feedback, and the error is controlled to achieve the purpose of amplitude synchronization.

[0032] In some embodiments, the DC side of each inverter in the inverter parallel system consists of a DC micro source and a capacitor, each inverter is connected to the AC bus through an LCL filter, nonlinear and asymmetric loads are connected to the AC bus, and the AC bus is connected to the grid via a grid-side instantaneous protection switch and a microgrid controllable protection switch.

[0033] On the other hand, the present invention provides a multi-mode control system for an inverter parallel system, the system comprising:

[0034] A sampling and conversion module, wherein the sampling and conversion module samples the output voltage and current of the inverter in the inverter parallel system and performs dq rotation conversion;

[0035] A power calculation module, wherein the power calculation module calculates the reactive power and active power output by the inverter according to the d-axis component and the q-axis component of the voltage and the current;

[0036] A droop controller, wherein the droop controller performs droop control calculation on the reactive power and active power output by the inverter to obtain a reference value of the inverter output voltage;

[0037] A voltage comparator, wherein the voltage comparator compares the inverter output voltage reference value with the actual output voltage to calculate a voltage error value;

[0038] A voltage controller, wherein the voltage controller controls the voltage error value to obtain an output current reference value of the inverter;

[0039] A current comparator, wherein the current comparator compares the inverter output current reference value with the actual output current to calculate a current error value;

[0040] and, a current controller, wherein the current controller controls the current error value to obtain an SVPWM modulation wave, that is, obtains a switch control signal of the inverter;

[0041] Wherein, the inverter parallel system consists of n The multifunctional inverters are connected in parallel, and n is an integer greater than or equal to 2.

[0042] The present invention proposes a multi-mode control method and system for an inverter parallel system. In off-grid mode, a composite controller in which a PI controller and a quasi-proportional resonant controller are connected in parallel is used to realize voltage outer loop control, which solves the problem of AC bus voltage distortion and voltage three-phase imbalance caused by nonlinear load and three-phase unbalanced load, improves bus voltage quality, and reduces harmonic distortion in its voltage. In grid-connected mode, a linear self-anti-disturbance current inner loop control technology based on a proportional resonant observer is used, and combined with an improved electric energy detection module to realize grid-connected power generation, and manage the current and electric energy quality of the microgrid. In addition, in off-grid mode, a pre-synchronization control technology is also used to solve the problem of asynchronous amplitude and phase of AC bus voltage and grid voltage.

[0043] The multi-modal control method and system of the inverter parallel system proposed in the present invention can not only realize the stable operation of the microgrid in various modes and have the ability to smoothly switch between on-grid and off-grid, but also provide power quality management auxiliary services to the microgrid, providing technical support for the operation and maintenance of the microgrid, which is of great significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0045] Figure 1 is a main circuit topology diagram of n parallel inverters according to an embodiment of the present invention;

[0046] Figure 2 is a control principle block diagram of the i-th inverter according to an embodiment of the present invention;

[0047] Figure 3 This is a simplified topology diagram of two single-phase inverters in parallel;

[0048] Figure 4 is the traditional voltage-current dual closed-loop control structure diagram of the i-th inverter;

[0049] Figure 5 This is a block diagram of the PR-LADRC current inner loop control principle used in an embodiment of the present invention;

[0050] Figure 6 is a Bode diagram of a PR controller according to an embodiment of the present invention;

[0051] Figure 7 A Bode diagram of a QPR controller according to an embodiment of the present invention;

[0052] Figure 8 is a Bode diagram of a PI-QPR controller under a first parameter example of an embodiment of the present invention;

[0053] Fig. 9 is a Bode diagram of a PI-QPR controller under a second parameter example of an embodiment of the present invention;

[0054] Fig.10 is a Bode diagram of a PI-QPR controller under a third parameter example of an embodiment of the present invention;

[0055] Fig.11 is a Bode diagram of a PI-QPR controller under a fourth parameter example of an embodiment of the present invention;

[0056] Fig.12 A control principle block diagram of the pre-synchronization control technology of an embodiment of the present invention;

[0057] Fig.13A block diagram of the multi-mode control principle of a parallel inverter according to an embodiment of the present invention;

[0058] Fig.14 A topological structure diagram of two parallel inverters according to an embodiment of the present invention;

[0059] Fig.15 This is a full-mode operation simulation diagram of a two-inverter parallel system using a traditional voltage controller in off-grid mode;

[0060] Fig.16 This is the full-mode operation simulation diagram of the two inverter parallel system using PI-QPR voltage controller in off-grid mode;

[0061] Fig.17 It is a simulation waveform diagram of the grid voltage and AC bus amplitude and phase synchronization process of the two inverter parallel system;

[0062] Fig.18 It is a simulated waveform diagram of load current, grid-connected current and inverter output current of the parallel system of two inverters when power management is not put into use;

[0063] Fig.19 It is the simulated waveform diagram of load current, grid-connected current and inverter output current of the parallel system of two inverters when putting into power management;

[0064] Fig. 20 It is the inverter output current and AC bus current simulation waveform diagram of the two inverter parallel system in the grid-connected mode;

[0065] Fig.21 It is a simulation waveform diagram of single-phase grid voltage and single-phase AC bus voltage when two inverter parallel systems smoothly switch from grid-connected to off-grid;

[0066] Fig. 22 The experimental waveform diagram of the grid voltage and AC bus voltage of the two inverter parallel system using the traditional voltage controller in the off-grid mode;

[0067] Fig.23 The experimental waveform diagram of the grid voltage and AC bus voltage of the two inverter parallel system using the PI-QPR voltage controller in off-grid mode;

[0068] Fig.24 The AC bus voltage THD measurement results of two inverter parallel systems using traditional voltage controller and PI-QPR voltage controller respectively;

[0069] Fig.25 The experimental waveform diagram of the inverter output current and AC bus current of the two inverter parallel system in off-grid mode;

[0070] Fig.26The experimental waveform diagram of the amplitude and phase synchronization process of the single-phase AC bus voltage and grid voltage in the two-inverter parallel system;

[0071] Fig. 27 The experimental waveforms of the grid-connected current and the inverter output current before and after the power quality management of the two-inverter parallel system; (a) is without power quality management, and (b) is with power quality management;

[0072] Fig.28 The grid current THD measurement results before and after power quality management for the two inverter parallel system;

[0073] Fig.29 It is the experimental waveform diagram of the inverter output current and AC bus current of the two inverter parallel system in the grid-connected mode;

[0074] Fig.30 This is the experimental waveform diagram of the single-phase AC bus and grid voltage when the two-inverter parallel system switches smoothly from grid-connected to off-grid. DETAILED DESCRIPTION

[0075] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0076] Example 1: As microgrid technology matures, the demand for microgrid system capacity is getting higher and higher. Due to the limited capacity of a single inverter, there are usually multiple parallel inverters, and the associated systems with communication are becoming more and more complex. For decentralized microgrids with poor communication, it is more suitable to adopt droop control of a non-communication peer-to-peer architecture, which greatly enhances the scalability of the microgrid system. For multiple parallel inverters in a master-slave architecture microgrid, this embodiment proposes a multi-modal control method for an inverter parallel system. This embodiment adopts a multi-modal control strategy based on droop control, which can not only realize the stable operation of the microgrid in various modes, but also provide power quality management auxiliary services to the microgrid. The control method proposed in this embodiment specifically includes: sampling the output voltage and current of the inverter in the inverter parallel system, and performing dq rotation transformation; obtaining the inverter output reactive power and active power through power calculation; performing droop control on the inverter output power and active power to obtain the inverter output voltage reference value, and comparing the voltage reference value with the actual output voltage to obtain the error value through the voltage control outer loop to obtain the inverter output current reference value, and comparing the current reference value with the actual output current to obtain the SVPWM (Space Vector Pulse Width Modulation) modulation wave through the current control inner loop, that is, obtaining the inverter switching control signal to control the inverter. The above control method is used to perform multi-mode control on each inverter in the inverter parallel system, wherein the inverter parallel system is mainly composed of n The multifunctional inverters are connected in parallel, and n is an integer greater than or equal to 2.

[0077] Among them, microgrid n The main circuit topology of parallel inverters is as follows Figure 1 As shown, the DC side is mainly composed of a DC micro source and a capacitor, with a constant DC source U dc replace, i dc is the DC side current. Each inverter is connected to the AC bus through an LCL filter (composed of inverter side inductance, grid side inductance and filter capacitor), and the bus voltage is U PCC , nonlinear load and asymmetric load are connected to the AC bus, and the total load current is i Labc , AC busbar is connected via instantaneous protection switch on the grid side S 1 and microgrid controllable protection switch S 2. Connect to the power grid. L fi , L gi , C fiRespectively i The inverter side inductor, grid side inductor and filter capacitor of each inverter. Z Li For the i The line impedance of each inverter, u gabc is the grid voltage, u oiabc For the i The output voltage of the inverter side, i oiabc For the i Output current of inverter side, u ciabc For the i The filter capacitor voltage of each inverter, i siabc For the i The output inductor current of the inverter side, i gabc is the grid-connected current. i =1,2,3,……, n . i Indicates i Inverter.

[0078] No. i The control principle of the inverter is as follows Figure 2 As shown, the inverter output voltage and current are sampled, and dq rotation transformation is performed to obtain the voltage in the dq rotation coordinate system. u oidq and current i oidq , after power calculation and low-pass filtering, the inverter output reactive power is obtained Q i Active Power P i , the inverter output voltage reference value is calculated by the droop controller u oidref , u oiqref , and the actual output voltage u oid , u oiq The error value calculated after comparison passes through the outer loop of the voltage controller to obtain the inverter output current reference value i oidref , i oiqref , and the actual output current i oid , i oiq After comparison, the SVPWM modulation wave is obtained through the inner loop of the current controller, that is, the switch control signal of the three-phase inverter is obtained.

[0079] In off-grid mode, nonlinear loads will generate harmonic currents, which will distort the current and voltage of the parallel inverters, especially the AC bus voltage. The inverter output current and voltage mainly contain 3rd, 5th, 7th, and 9th harmonic components, so the instantaneous output power of the inverter also contains harmonic power. Harmonic power is also divided into harmonic active power , and harmonic reactive power , respectively:

[0080] (1)

[0081] In the formula, , , , , , A, B, C three phases n Subharmonic voltage and current effective value.

[0082] The instantaneous output power of the inverter is:

[0083] (2)

[0084] In the formula, and are the instantaneous active power and instantaneous reactive power respectively, and They are fundamental active power and fundamental reactive power respectively.

[0085] It can be seen from equations (1) and (2) that under nonlinear load conditions, the instantaneous power output by the inverter contains both fundamental and harmonic power.

[0086] In summary, when there is an unbalanced and nonlinear load in the parallel system of n inverters, the load current contains harmonic components and negative-sequence components. The harmonic components are mainly low-order harmonics of 3, 5, and 7. In the dq rotating coordinate system, the j-th harmonic component is presented in the form of j-1 harmonic, and the negative-sequence component appears in the form of double frequency.

[0087] Because the inverter output power contains harmonic components and negative sequence components, when using the droop control strategy, it is necessary to extract and control the inverter fundamental power. Two single-phase inverters are connected in parallel to simplify the topology. Figure 3 As shown. Assume that the output voltage of the i-th inverter is , the grid voltage is , the equivalent output impedance of the i-th inverter and the line impedance are combined into the system equivalent impedance , the load is . Then the output current of the i-th inverter is and complex power for:

[0088] (3)

[0089] In the formula, , Respectively i The output voltage amplitude of each inverter and the AC bus voltage amplitude, For the i The output voltage phase angle of each inverter, , and Respectively i The inverter outputs conjugate current, active power and reactive power. , Respectively i The equivalent output resistance and reactance of each inverter.

[0090] When the parallel system operates stably, the difference between the inverter output voltage phase angle and the AC bus voltage phase angle is very small. Combining formula (3), the relationship between the inverter output active and reactive power and the frequency and phase angle can be calculated, as shown in formula (4).

[0091] (4)

[0092] In the formula, f i For the i The inverter output voltage frequency.

[0093] It can be seen from formula (4) that the inverter output power is affected by the voltage amplitude and frequency. At the same time, the inverter output power is related to the system equivalent impedance. The system equivalent impedance presents inductive or resistive characteristics, which will affect the droop equation.

[0094] If the system equivalent impedance inductance accounts for a large proportion (i.e. ), then the power equation and droop equation are:

[0095] (5)

[0096] In the formula, , are the given reference values ​​of the output voltage amplitude and frequency respectively, , are the reactive power and active power droop coefficients respectively.

[0097] In summary, the system equivalent impedance characteristics will affect the droop controller design, which is determined by the inverter equivalent output impedance and line impedance. The inverter equivalent output impedance can be changed by adjusting the LCL filter parameters, voltage and current controller gain. Among them, the current controller design depends on the bandwidth and switching frequency, and the LCL filter parameter design is based on the switching frequency. Therefore, the voltage controller directly affects the inverter output impedance.

[0098] Under the premise of satisfying system stability and dynamics, this embodiment adjusts the voltage controller parameters so that the system equivalent impedance behaves as inductive above the base frequency or the specified frequency, and thus performs droop control design for the n inverter parallel system based on inductive droop.

[0099] Furthermore, the reasons why the traditional voltage-current dual closed-loop controller has poor unbalanced and nonlinear load capacity are analyzed, and then the voltage control outer loop and current control inner loop are improved to enhance the ability of the n-unit parallel inverter system to suppress the harmonic voltage and three-phase unbalanced voltage at the grid connection point. Figure 4 In order to facilitate modeling and analysis, the grid-side inductance of the LCL filter is incorporated into the line equivalent impedance. , is the voltage controller, is the current controller, is the inverter output voltage reference value, is the output voltage of the i-th inverter, is the output current of the i-th inverter, is the inverter equivalent gain, is the inverter side inductance, is the inverter side resistance, For the filter capacitor.

[0100] based on Figure 4 , when n inverters carry nonlinear and unbalanced loads, the inverter output voltage in the fundamental domain is derived is the AC bus voltage They are:

[0101] (6)

[0102] In the formula, is the voltage loop closed-loop transfer function, is the output impedance of the inverter at fundamental wave, is the fundamental current flowing through the fundamental equivalent impedance of the line, is the line fundamental wave equivalent impedance.

[0103] Under nonlinear load conditions, the load current has harmonic current components, and the load current Also includes the hth harmonic current component , in the hth harmonic domain, the inverter output impedance is , under the influence of harmonic current, the equivalent impedance of the line is Harmonic voltage drop is generated, causing the AC bus voltage Distortion occurs, namely:

[0104] (7)

[0105] Similarly, an unbalanced load will also cause a negative sequence current component in the load current, that is, the load current Contains negative sequence current component , the inverter output negative sequence impedance is , under the influence of negative sequence current, the equivalent impedance of the line A negative sequence voltage drop is generated, causing an unbalanced AC bus voltage. n The AC bus voltage of the inverter under nonlinear and unbalanced load is:

[0106] (8)

[0107] In summary, under nonlinear and unbalanced load conditions, the traditional voltage and current double closed-loop control gives a given voltage reference value. It only contains fundamental components, and the voltage control outer loop has insufficient gain for harmonics and negative sequence voltages, and cannot completely compensate for harmonics and negative sequence voltages. From formula (8), it can be seen that the AC bus voltage will be severely distorted. Therefore, this embodiment improves the voltage control outer loop to enhance the ability of n inverters to carry nonlinear and unbalanced loads in off-grid mode.

[0108] Specifically, this embodiment improves the voltage quality of the AC bus by improving the voltage controller in the voltage control outer loop, and manages the current and power quality by improving the current control inner loop in combination with the power detection module.

[0109] Among them, for the current control inner loop of n parallel inverters, considering that the control mode of n parallel inverters in the grid-connected mode is current control, it is necessary not only to carry out grid-connected power generation, but also to locally manage the harmonics and negative sequence current of the microgrid to improve the power quality of the grid-connected current. Therefore, in the grid-connected mode, the grid-connected power generation is realized through the improved power detection module and the linear active disturbance rejection current controller (PR-LADRC) based on the quasi-proportional resonant observer, and the current power quality of the microgrid is managed by using the remaining capacity of the inverter. The simplified d-axis current inner loop control principle is as follows Figure 5 As shown in the figure, PR-LESO is a linear extended state observer based on a quasi-proportional resonant controller. is the observer gain, Express Its estimated value, is the d-axis current reference value, i sd is the actual value of the inductor current on the d-axis inverter side, For the observer i sd The estimated value of e d is the error between the inductor current reference value and the estimated value, K d is the overall bandwidth of the current control inner loop, G LCL (s) is the transfer function of the LCL filter, u iod is the control quantity after disturbance compensation, which is numerically equal to the output voltage of the inverter. d is the disturbance input of the inner current loop.

[0110] For the voltage control outer loop of n parallel inverters, in order to control the inverter output voltage fundamental, harmonics and negative sequence voltage components, it is known from the inner membrane principle that the given reference signal needs to be accurately controlled, and the controller transfer function needs to include the mathematical model corresponding to the given signal. For the fundamental voltage component, since the PI controller can make the inverter output impedance at the fundamental frequency 0 in the dq rotating coordinate system, it can achieve zero static error tracking of the DC quantity, and the controller parameter setting is simple, so the PI controller is used to control the fundamental voltage.

[0111] In the dq rotating coordinate system, the harmonics and negative sequence voltage components are both AC quantities, and the PI controller cannot perform zero-error control on the AC quantity. Therefore, the PR controller is used, and its transfer function is for:

[0112] (9)

[0113] In the formula, , , are the proportional gain, resonant gain and resonant frequency of the PR controller respectively, s is a complex variable. The Bode diagram of the PR controller is as follows Figure 6 shown.

[0114] The transfer function is shown in formula (9): Include ,and Figure 6 Display PR controller in The gain at the point is very large, so the PR controller can The AC quantity can be precisely controlled. The harmonic components of the dq rotating coordinate system are mainly composed of the 3rd, 5th and 7th harmonics, which are expressed as 2 , 4 , 6 The amount of communication, is the fundamental angular frequency. Therefore, only three resonant frequencies of 2 , 4 , 6 The PR controller can simultaneously achieve precise control of the harmonic and negative sequence voltage components of the inverter output voltage.

[0115] However, since the PR controller has a very small bandwidth and very small gain at non-resonant frequencies, if the frequency of the controlled AC signal is offset, the control gain will be insufficient and precise control will not be possible. Therefore, in order to increase the bandwidth and ensure the control effect after the AC signal is offset, the QPR controller is used to control the harmonics and negative sequence voltage components. The transfer function of this controller is for:

[0116] (10)

[0117] In the formula, is the shear frequency.

[0118] Bode diagram of QPR controller Figure 7 Compared with Figure 6 , the QPR controller is at the resonant frequency The gain of the controller is lower than that of the PR controller, but it can still be used for The QPR controller can accurately track and control the AC signal at the input, and the QPR controller has a larger bandwidth and phase margin than the PR controller, which can enhance the system stability.

[0119] Based on this, this embodiment uses two QPR controllers in parallel to accurately control the different harmonics and negative sequence voltage components of the inverter output voltage. The final transfer function of the PI-QPR controller is for:

[0120] (11)

[0121] In the formula, Nω 0 represents the angular frequency of the AC signal in the dq rotating coordinate system, and the 5th and 7th harmonics have an angular frequency of 6 in the dq rotating coordinate system. The negative sequence voltage component of the AC signal is angular frequency 2 The two are AC signals with frequencies of 300 Hz and 100 Hz respectively.

[0122] Next, the PI-QPR controller parameters are adjusted through the Bode diagram. From equation (11), we can see that the tracking performance of the PI-QPR controller is mainly determined by K p ,K i , K hR and ω hcn The following will analyze the performance of the PI-QPR controller based on four different situations.

[0123] Situation 1: When K i = 30. K hR = 35. ω hcn =10, K p =0.1, 1, 10, 20, by Figure 8 From the Bode diagram shown, we can see that as K p As the amplitude of the PI-QPR controller increases, the response speed increases, and the phase lag effect decreases.

[0124] Case 2: When K p =0.2, K hR = 35. ω hcn =10, K i =10, 20, 30, 40, by Fig. 9 From the Bode diagram shown, we can see that as K i The amplitude of the PI-QPR controller in the low frequency band increases significantly, the phase margin decreases significantly, and the controller response speed slows down. However, the amplitude of the medium and high frequency bands does not change significantly, indicating that this parameter has no effect on the harmonic suppression of this frequency band, especially the 100Hz and 300Hz frequencies.

[0125] Situation 3: When K p =0.2, K i = 30. ω hcn =10, K hR =5, 15, 25, 35, by Fig.10 From the Bode diagram shown, we can see that as K hR The amplitude of the PI-QPR controller in the low-frequency and high-frequency bands has no obvious change, but in the mid-frequency band, especially at 100Hz and 300Hz, the amplitude increases, indicating that with theK hR The increase has no effect on the controller performance in the low and high frequency bands, and the harmonic suppression effect in the mid-frequency band is continuously improved.

[0126] Situation 4: When K p =0.2, K i = 30. K hR = 35:00, ω hcn =1, 10, 20, 30, by Fig.11 From the Bode diagram shown, we can see that as ω hcn The amplitude of the PI-QPR controller in the low-frequency and high-frequency bands has no obvious change, but in the mid-frequency band, especially at 100Hz and 300Hz, the amplitude increases significantly, and the bandwidth increases, indicating that with the increase of ω hcn As the harmonic suppression capability of the controller in the mid-frequency band increases,

[0127] Based on the analysis of the above Bode diagram, the design principles of each parameter of the PI-QPR controller include:

[0128] (1) Comprehensively consider the impact of parameters on performance: proportional gain K p Affects the system's response speed and amplitude, integral gain K i Affects the amplitude and phase margin of the low frequency band, while the harmonic compensation gain K hR and cutoff frequency ω hcn Mainly affects the harmonic suppression effect in the mid-frequency band.

[0129] (2) Comprehensively consider parameter trade-offs and stability: Increase K p It can speed up the response, but it may also increase the instability of the control system; increasing K i The control effect in the low frequency band can be improved, but the response speed may be slowed down. While pursuing fast response and high performance, the stability of the controller must be guaranteed. When designing and adjusting parameters, the stability of the controller must be analyzed and guaranteed through the Bode diagram.

[0130] (3) Comprehensive consideration of frequency characteristics: harmonic compensation gain K hR and cutoff frequency ω hcnThe change of mainly affects the harmonic suppression effect in the mid-frequency band, and the performance requirements of this frequency band should be paid attention to during design.

[0131] Furthermore, in order to enable n parallel inverters to have multi-mode operation capabilities, pre-synchronization control technology needs to be applied to achieve synchronization of the AC bus voltage and the grid voltage amplitude and phase. The AC bus voltage and grid voltage of the parallel system are:

[0132] (12)

[0133] In the formula, u gn , u PPCn Respectively represent the instantaneous values ​​of the n-phase grid voltage and the AC bus voltage, U gn , U PPCn , are the corresponding grid voltage and AC bus voltage amplitude respectively, ω g , ω P are the corresponding grid voltage and AC bus voltage angular frequency, θ g It is the phase angle of the AC bus voltage lagging or leading the grid voltage.

[0134] In off-grid mode, the amplitude and phase of the AC bus voltage are different from those of the grid voltage. The moment of off-grid to grid-connected conversion is equivalent to two voltage sources connected in series, which will produce a voltage mutation. and current mutation ,Right now:

[0135] (13)

[0136] In the formula, Z L is the line impedance.

[0137] Since the line impedance is very small, when switching from off-grid to grid-connected, the grid voltage and the AC bus voltage have inconsistent amplitude and phase, which will generate a large current. In order to solve the grid-connected current impact problem, the inverter output voltage frequency f i With grid frequency f g The following conditions must be met:

[0138] (14)

[0139] In order to adjust the phase difference between the grid voltage and the AC bus voltage, the inverter output frequency can be compensated by equation (14). The control principle of the pre-synchronization technology is as follows: Fig.12As shown in the figure, two phase-locked loops are used to achieve the effect of amplitude and phase dual synchronization. For phase synchronization, firstly, the AC bus output voltage U pcc Phase lock is performed, and the phase angle obtained θ With grid voltage u gabc Perform dq rotation transformation, and finally control the q-axis component of the grid voltage to be controlled to 0 to achieve phase synchronization; for amplitude synchronization, take the grid voltage amplitude as a given reference value, and the amplitude of the AC bus voltage as feedback, and control its error to achieve amplitude synchronization.

[0140] Compared with traditional pre-synchronization technology, amplitude-phase pre-synchronization technology achieves amplitude-phase synchronization by adaptively compensating for the phase deviation caused by the LCL filter. This technology can realize seamless and smooth switching from off-grid to grid-connected mode.

[0141] Based on the above technology, the principle of multi-mode control strategy for n parallel inverters is obtained as follows: Fig.13 As shown. The control method proposed in this embodiment, in off-grid mode, improves the voltage controller by applying PI-QPR (a composite controller in which a PI controller and a quasi-proportional coordinator are connected in parallel), solves the AC bus voltage distortion and voltage three-phase imbalance caused by nonlinear loads and three-phase unbalanced loads, improves the bus voltage quality, and reduces its voltage total harmonic distortion; in grid-connected mode, grid-connected power generation is achieved through an improved power detection module and a linear self-interference current inner loop based on a proportional resonance observer, and the current and power quality of the microgrid are managed by using the remaining capacity of the inverter. On the one hand, the ability to carry nonlinear and unbalanced loads in the off-grid state is improved, and on the other hand, in the grid-connected state, the strategy not only supports grid-connected power generation, but also manages the harmonics and negative sequence current of the microgrid. In addition, the control method enables n parallel inverters to flexibly switch (smoothly switch) between off-grid mode and grid-connected mode.

[0142] Based on the above concept, this embodiment also proposes a multi-mode control system of an inverter parallel system, the system comprising:

[0143] A sampling conversion module, the sampling conversion module is used for the first i The output voltage and current of each inverter are sampled and dq rotation conversion is performed;

[0144] A power calculation module, which calculates the reactive power and active power output by the inverter according to the d-axis component and the q-axis component of the voltage and the current; it should be noted that the power calculation module not only includes a power calculation unit, but also includes a low-pass filter unit, which uses the low-pass filter unit to perform low-pass filtering on the calculated power to obtain reactive power and active power;

[0145] A droop controller performs droop control calculation on the inverter output reactive power and active power to obtain an inverter output voltage reference value;

[0146] A voltage comparator, which compares the inverter output voltage reference value with the actual output voltage to calculate a voltage error value;

[0147] A voltage controller controls the voltage error value to obtain an inverter output current reference value;

[0148] A current comparator, which compares an inverter output current reference value with an actual output current to calculate a current error value;

[0149] And a current controller, which controls the current error value to obtain an SVPWM modulation wave, that is, to obtain a switch control signal of the inverter. A control module composed of the above-mentioned module units is used to perform multi-modal control on each inverter in the inverter parallel system. It should be noted that the voltage comparator and the voltage controller together constitute the voltage control outer loop described in the above method, and the current comparator and the current controller together constitute the current control inner loop described in the above method. The specific implementation method of each module unit is as described in the above method, and will not be repeated here.

[0150] Embodiment 2:

[0151] This embodiment uses simulation technology to verify the effectiveness and feasibility of the control technology proposed in the above embodiment. Specifically, MATLAB / Simulink simulation software is used to build a parallel simulation topology of two multifunctional inverters. Fig.14 The specific simulation parameters of the parallel system are shown in Table 1.

[0152] Table 1 Simulation parameters of MFGCI

[0153]

[0154] The simulation test process specifically includes:

[0155] (1) Simulation verification of the voltage control outer loop in off-grid mode: In order to verify whether the PI-QPR harmonic voltage controller can enhance the ability of n parallel inverters to operate with nonlinear and unbalanced loads, simulation comparisons were carried out under two working conditions: traditional voltage control and harmonic voltage control. The simulation results under the two working conditions are shown in Figure 2. Fig.15 and Fig.16 As shown. Fig.15 middle, U gabc 、Upabc are the instantaneous values ​​of the three-phase voltages of the power grid and the AC bus, I sabc is the inverter output current. When t=0~1s, the two inverter parallel system operates in off-grid mode, and the AC bus three-phase voltage of the parallel inverter system using the traditional voltage controller U pabc There is a serious distortion, and it does not have the ability to carry nonlinear and unbalanced loads. At this time, the total voltage THD is 4.24%, which does not meet the electricity standard. In addition, the three-phase voltage of the AC bus has a serious three-phase imbalance, which will have a great impact on the load. Fig.16 It can be seen that when the two inverters are connected in parallel and operate in off-grid mode, based on the enlarged graph from 0.4s to 0.6s, the three-phase voltage of the AC bus U pabc The waveform quality is high, and there is no three-phase voltage imbalance, indicating that the parallel system of two inverters has enhanced its ability to carry nonlinear and unbalanced loads. At this time, the total voltage THD is 1.53%, and the THD of the voltage components containing the 3rd, 5th, and 7th harmonics does not exceed 0.4%. The simulation results prove the effectiveness of the PI-QPR harmonic voltage controller in suppressing voltage harmonics and negative sequence components, and improve the ability of the parallel inverter system to carry nonlinear and unbalanced loads.

[0156] (2) Simulation verification of the pre-synchronization operation control strategy of the multifunctional inverter parallel system: The pre-synchronization operation simulation of two inverter parallel systems using the proposed pre-synchronization control strategy was carried out. Fig.17 The amplitude and phase synchronization process of the AC bus and grid voltage in a two-inverter parallel system is given. Fig.17 It can be seen that after the pre-synchronization control is started at 0.5s, the amplitude and phase of the AC bus voltage quickly track the grid voltage. At about 0.58 seconds, the AC bus voltage and the grid voltage are completely synchronized in amplitude and phase. At t=1s, the two inverter parallel system is switched from off-grid operation mode to grid-connected operation mode. The simulation results show that the parallel system can switch from off-grid mode to grid-connected mode smoothly and without impact, verifying the effectiveness of the pre-synchronization operation control technology of the n-inverter parallel system.

[0157] (3) Simulation verification of the power quality management function of the inverter parallel system in the grid-connected mode: In order to verify the power quality management function of the multifunctional inverter parallel system in the grid-connected mode, two operating conditions, one without power quality management and the other with power quality management, were set up for comparative simulation. Fig.18 and 19 The waveforms of the load current, grid-connected current and inverter output current of the two inverter parallel system before and after the power quality management is put into operation in the grid-connected operation mode are given respectively. Fig.18As shown in the figure, the two inverter parallel system operates in the grid-connected mode from 1s to 4.5s. When t=1s, the grid is connected. In the period of 1s to 1.5s, the inverter does not inject active power into the grid. In the period of 1.5s to 3s, the inverter begins to inject active power into the grid for grid-connected power generation. In the enlarged figure from 2.6s to 2.7s, when the two inverter parallel system is in the grid-connected mode and power quality management is not put into use, the inverter output current I sabc Almost no harmonics and negative sequence current are included. Harmonics and negative sequence current of load current all flow to grid current. I gabc , which reduces the power quality of the grid-connected current. At t=2.6s, the single-phase TDH of the grid-connected current was measured to be 3.81%, and the 5th harmonic current component THD exceeded 3%, which did not meet the national grid-connected standards. In addition, the grid-connected current also showed a serious three-phase current imbalance, which affected the grid-connected power generation of the two inverter parallel systems. After the power management was put into use, the current waveform of the two inverter parallel systems was as follows: Fig.19 As shown. The system improves the power quality of the grid-connected current by outputting harmonics and negative sequence current through the inverter. In the enlarged figure at 2.6s~2.7s, I gabc The waveform is Fig.18 There has been a significant improvement, the harmonics and negative sequence currents included have been greatly reduced, the single-phase TDH of the grid-connected current measured at t=2.6s was 1.63%, the 5th harmonic THD dropped below 1.5%, and the 3rd, 7th, and 11th harmonic current components THD were all below 0.5%.

[0158] Fig. 20 The current waveforms of the two inverters sharing the harmonics and negative sequence current are given. I sabc 、I sabc1 They are the current waveforms of three-phase inverter 1 and three-phase inverter 2 respectively. I mabc is the AC bus current.

[0159] (4) Simulation verification of smooth off-grid operation of two parallel inverter systems: When the two parallel inverter systems switch from grid-connected to off-grid, the single-phase grid voltage U ga and single-phase AC bus voltage U pa The simulation waveform is as follows Fig.21As shown. Before 4.5s, the system was in the grid-connected state, and the voltage amplitude and phase were synchronized; at 4.5s, the grid-connected to off-grid switching was performed. As can be seen from the figure, during the transient process of switching between on-grid and off-grid, the AC bus voltage amplitude only showed a slight drop, indicating that the two inverter parallel system has the ability to switch between on-grid and off-grid smoothly and seamlessly. In addition, after switching to off-grid mode, the AC bus can still establish a voltage with qualified amplitude, frequency and voltage total THD.

[0160] Embodiment 3: This embodiment verifies the effectiveness of the control technology proposed in the above embodiment 1 by experiments. Fig.14 The simulation topology diagram is shown in Figure 1. The test system is built on the semi-physical experimental platform of Yuankuan Energy. The semi-physical experimental platform is mainly composed of StarSim MT3200 HIL model real-time simulator, StarSim MT6016 RCP rapid prototyping controller, physical channel, oscilloscope and host computer. Since the sampling, wave generation and internal signal interaction of the semi-physical platform are all actual electrical signals, the semi-physical experimental results are widely recognized by the power electronics industry. The specific experimental parameters of the semi-physical platform are consistent with the simulation parameters, as shown in Table 1.

[0161] The specific verification process includes:

[0162] (1) Experimental verification of the voltage control outer loop in off-grid mode. Fig. 22 and Fig.23 The grid voltage in off-grid mode using the traditional voltage controller and the PI-QPR harmonic voltage controller are given respectively. u gabc , AC bus voltage U PCC The experimental waveform of Fig. 22 As shown in the figure, the AC bus voltage using the traditional voltage controller presents harmonic distortion and serious three-phase imbalance, which does not meet the voltage quality requirements of the power load. This is mainly because the load current of the nonlinear and unbalanced load i Labc There are a lot of harmonics and negative sequence components in the inverter output voltage, which causes u oabc Distortion occurs, further causing the AC bus voltage U PCC Therefore, in off-grid mode, the grid-connected inverter using traditional voltage controller has great shortcomings in operating with nonlinear and unbalanced loads and cannot meet the power requirements of the load. Fig.23 It can be seen that the AC bus voltage of the two inverter parallel system using the PI-QPR harmonic voltage controller U PCCThe waveform quality has been greatly improved compared to traditional voltage controllers. The AC bus voltage waveform is sinusoidal, with very little harmonics and negative sequence voltage. The voltage test waveform shows almost no three-phase voltage imbalance. Fig. 22 , 23 Experimental waveforms using two different voltage controllers Fig.15 , 16 The simulation waveform is consistent with that of , indicating that the experimental and simulation results are in good agreement. Fig.24 The THD measurement results of the AC bus voltage using the traditional voltage control method and the PI-QPR harmonic voltage control method in the off-grid working mode are given. U Pa , U Pb , U Pc The THD of the power supply is 6.05%, 5.79% and 5.81% respectively. After adopting PI-QPR harmonic voltage control technology, U Pa , U Pb , U Pc The THD of the power supply was significantly reduced, reaching 2.79%, 2.41% and 2.44% respectively. Fig.25 The experimental waveforms of the harmonics and negative sequence current of the system are given in the off-grid mode when the three-phase inverter 1 and the three-phase inverter 2 share the system harmonics and negative sequence current. i o1abc , i o2abc They are the output current waveforms of three-phase inverter 1 and three-phase inverter 2 respectively. i mabc The above experimental results prove that the proposed PI-QPR harmonic voltage controller can significantly improve the AC bus voltage quality in off-grid mode, enhance the ability of the multifunctional grid-connected inverter parallel system to carry nonlinear and unbalanced loads in off-grid mode, and realize the equalization of harmonics and negative sequence currents by each inverter in the parallel system based on droop control.

[0163] (2) Experimental verification of pre-synchronization operation control of multifunctional inverter parallel system. The pre-synchronization control waveform of the two inverter parallel system starting with the proposed control strategy is shown in Figure 2. Fig.26 As shown. Taking phase a as an example, the AC bus phase a voltage u Pa The amplitude and phase of the grid phase a voltage u gaSynchronize until its amplitude and phase completely coincide with the grid voltage amplitude and phase, preparing for smooth grid connection. As can be seen from the figure, since the amplitude and phase of the AC bus and grid voltage are completely consistent, the two inverter parallel system can achieve smooth and impact-free grid connection, verifying the effectiveness of the parallel system pre-synchronization operation control technology.

[0164] Fig.26 The experimental waveform of the phase a AC bus voltage and the grid voltage amplitude phase synchronization is Fig.17 The simulation waveform in is consistent with that in , indicating that the experiment and simulation are in good agreement.

[0165] (3) Experimental verification of the power quality management function of the multifunctional inverter parallel system under the grid-connected mode. In order to verify the power quality management function of the multifunctional grid-connected inverter parallel system under the grid-connected mode, experimental comparisons were set up with and without power quality management of the inverter. Fig. 27 The waveform changes of load current, grid current and inverter output current of the two inverter parallel system before and after power quality control in the grid-connected operation mode are given respectively. Fig. 27 As shown, the inverter parallel system operates in the grid-connected mode. The two inverters inject active power into the grid for grid-connected power generation. When power quality management is not implemented, the output current of three-phase inverter 1 and three-phase inverter 2 is i o1abc , i o2abc Almost no harmonics and negative sequence current, load current i Labc The harmonics and negative sequence currents flow to the grid current i gabc , the current waveform quality is very poor, with obvious harmonics and three-phase imbalance, which reduces the power quality of the grid-connected current. After power quality management, the inverter parallel system manages the harmonics and negative-sequence current of the load current through the current detection module and the QPR-LADRC current inner loop controller, and outputs harmonics and negative-sequence current through three-phase inverter 1 and three-phase inverter 2 to compensate for the harmonics and negative-sequence current components of the grid-connected current, thereby improving the power quality of the grid-connected current. Compared with before power quality management, the grid-connected current after power quality management i gabc The waveform has been greatly improved, and the harmonics and negative sequence current components contained have been greatly reduced. Fig.28 The grid-connected current THD measurement results of the system with two inverters in parallel without power quality control and with power quality control are given.

[0166] When two inverters are connected in parallel and in grid-connected mode, and the inverters are not undergoing power quality management, the single-phase grid-connected current i ga ,i gb , i gc The THD of the single-phase grid-connected current is 4.93%, 5.87% and 4.87% respectively. i ga , i gb , i gc The THD of the power consumption was significantly reduced, reaching 3.83%, 3.69% and 3.69% respectively.

[0167] Fig.29 The experimental waveforms of the harmonics and negative sequence current of the system are given in the grid-connected mode when the three-phase inverter 1 and the three-phase inverter 2 share the system harmonics and negative sequence current. i o1abc , i o2abc They are the output current waveforms of three-phase inverter 1 and three-phase inverter 2 respectively. i mabc Represents the AC bus current. The experimental results prove the balancing effect of each inverter in the parallel system on harmonics and negative sequence current based on droop control.

[0168] Fig.29 The output current and AC bus current experimental waveforms of the parallel three-phase inverter 1 and three-phase inverter 2 in the grid-connected mode are shown in FIG. Fig. 20 The simulation waveform is consistent with that of the experiment, indicating that the experiment and simulation are in good agreement.

[0169] The above experimental results prove the effectiveness of the multi-functional grid-connected inverter multi-mode control strategy based on droop control under grid-connected mode control. It also shows that the grid-connected inverter can use the remaining capacity of the inverter to locally control the harmonic current and negative sequence current generated by nonlinear loads and three-phase unbalanced loads in the microgrid by changing the control strategy, so that the grid-connected inverter has multiple functions, saves the cost of microgrid construction, and has practical engineering significance.

[0170] (4) Experimental verification of smooth off-grid operation of two inverters in parallel. The experimental waveforms of the two inverters in parallel system switching from grid-connected to off-grid operation using the proposed control strategy are as follows: Fig.30 As shown, including the grid a phase voltage u ga , AC bus phase a voltage U pa Waveform. At the moment of switching from grid-connected to off-grid, the AC bus voltage amplitude only slightly rises, and there is no voltage shock phenomenon, and the quality is good. The experimental results show that the multifunctional grid-connected inverter parallel system has the ability to smoothly and seamlessly switch between grid-connected and off-grid under nonlinear and unbalanced load conditions.

[0171] Fig.30 The experimental waveforms of the single-phase AC bus and grid voltage when the parallel system is smoothly switched from grid-connected to off-grid are shown in Figure 1. Fig.21 The simulation waveform is consistent with that of the experiment, indicating that the experiment and simulation are in good agreement.

[0172] It should be noted that Figure 15-Figure 30 The a, b, and c involved in the figure represent the signals of phase a, phase b, and phase c respectively.

[0173] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes.

[0174] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0175] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0176] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process in the computer or other programmable device. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0177] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A multi-mode control method for a parallel inverter system, characterized in that: The method comprises: Sampling the output voltage and current of the inverters in the inverter parallel system, and performing dq rotation conversion; The inverter outputs reactive power and active power through power calculation; Performing droop control calculation on the reactive power and active power output by the inverter to obtain a reference value of the inverter output voltage; The inverter output voltage reference value is compared with the actual output voltage, and the calculated error value is passed through a voltage control outer loop to obtain the inverter output current reference value; After comparing the inverter output current reference value with the actual output current, an SVPWM modulation wave is obtained through a current control inner loop, that is, a switch control signal of the inverter is obtained; Wherein, the inverter parallel system is composed of n multifunctional inverters connected in parallel, and n is an integer greater than or equal to 2; the method further includes: The inverter outputs reactive power and active power by adopting a control technology based on inductive droop; The control technology based on inductive droop is specifically to adjust the voltage control parameters so that the system equivalent impedance is inductive above the base frequency or the specified frequency under the premise of satisfying the stability and dynamics of the inverter parallel system; the system equivalent impedance is obtained by combining the inverter equivalent output impedance and the line impedance; the method also includes: In off-grid mode, the voltage control outer loop adopts a composite controller in which a PI controller and a quasi-proportional resonant controller are connected in parallel, wherein the PI controller is used to control the fundamental wave of the inverter output voltage, and the quasi-proportional resonant controller is used to control the harmonics and negative sequence components of the inverter output voltage; The design principles of various parameters in the composite controller include: Comprehensively consider the impact of parameters on performance: proportional gain affects response speed and amplitude, integral gain affects amplitude and phase margin in the low frequency band, harmonic compensation gain and cutoff frequency affect harmonic suppression effect in the mid-frequency band; Comprehensively consider parameter trade-offs and stability: when designing and adjusting the parameters of the composite controller, the stability of the composite controller is analyzed and ensured through Bode diagrams; Comprehensive consideration of frequency characteristics: When designing and adjusting the parameters of the composite controller, attention should be paid to the harmonic suppression effect in the mid-frequency band; The method further comprises: In the grid-connected mode, the current control inner loop adopts a linear self-interference rejection current inner loop based on a proportional resonance observer and combines it with an improved power detection module to realize grid-connected power generation, and uses the remaining capacity of the inverter to manage the current and power quality; The method further comprises: In off-grid mode, pre-synchronization control technology is used to achieve synchronization of the AC bus voltage and grid voltage amplitude and phase; The pre-synchronization control technology specifically includes: By performing phase locking on the AC bus voltage, the obtained phase angle is subjected to dq rotation transformation with the grid voltage, and the q-axis component of the grid voltage is controlled to be 0, thereby achieving the purpose of phase synchronization; The grid voltage amplitude is used as a given reference value, and the AC bus voltage amplitude is used as feedback, and the error is controlled to achieve the purpose of amplitude synchronization.

2. The multi-mode control method for a parallel inverter system according to claim 1, characterized in that: The transfer function G of the composite controller is iv (s) is: Among them, K p is the proportional gain, K i is the integral gain, K hR is the harmonic compensation gain, ω hcn is the cut-off frequency, Nω0 represents the angular frequency of the AC signal in the dq rotating coordinate system, and s is a complex variable.

3. The multi-mode control method for a parallel inverter system according to claim 1, characterized in that: The DC side of each inverter in the inverter parallel system consists of a DC micro source and a capacitor. Each inverter is connected to the AC bus through an LCL filter. Nonlinear and asymmetric loads are connected to the AC bus. The AC bus is connected to the grid through a grid-side instantaneous protection switch and a microgrid controllable protection switch.

4. A multi-mode control system for an inverter parallel system, characterized in that: The system comprises: A sampling and conversion module, wherein the sampling and conversion module samples the output voltage and current of the inverter in the inverter parallel system and performs dq rotation conversion; A power calculation module, wherein the power calculation module calculates the reactive power and active power output by the inverter according to the d-axis component and the q-axis component of the voltage and the current; A droop controller, wherein the droop controller performs droop control calculation on the reactive power and active power output by the inverter to obtain a reference value of the inverter output voltage; A voltage comparator, wherein the voltage comparator compares the inverter output voltage reference value with the actual output voltage to calculate a voltage error value; A voltage controller, wherein the voltage controller controls the voltage error value to obtain an output current reference value of the inverter; A current comparator, wherein the current comparator compares the inverter output current reference value with the actual output current to calculate a current error value; and, a current controller, wherein the current controller controls the current error value to obtain an SVPWM modulation wave, that is, obtains a switch control signal of the inverter; Wherein, the inverter parallel system is composed of n multifunctional inverters connected in parallel, and n is an integer greater than or equal to 2; The droop controller uses a control technique based on inductive droop to control the reactive power and active power output by the inverter; The control technology based on inductive droop is specifically to adjust the voltage control parameters so that the system equivalent impedance is inductive above the base frequency or the specified frequency under the premise of satisfying the stability and dynamics of the inverter parallel system; the system equivalent impedance is obtained by combining the inverter equivalent output impedance and the line impedance; In the off-grid mode, the voltage comparator and the voltage controller adopt a composite controller in which a PI controller and a quasi-proportional resonant controller are connected in parallel, wherein the PI controller is used to control the fundamental wave of the inverter output voltage, and the quasi-proportional resonant controller is used to control the harmonics and negative sequence components of the inverter output voltage; The design principles of various parameters in the composite controller include: Comprehensively consider the impact of parameters on performance: proportional gain affects response speed and amplitude, integral gain affects amplitude and phase margin in the low frequency band, harmonic compensation gain and cutoff frequency affect harmonic suppression effect in the mid-frequency band; Comprehensively consider parameter trade-offs and stability: when designing and adjusting the parameters of the composite controller, the stability of the composite controller is analyzed and ensured through Bode diagrams; Comprehensive consideration of frequency characteristics: When designing and adjusting the parameters of the composite controller, attention should be paid to the harmonic suppression effect in the mid-frequency band; In the grid-connected mode, the current comparator and current controller use a linear self-interference current inner loop based on a proportional resonant observer and combine with an improved power detection module to achieve grid-connected power generation, and use the remaining capacity of the inverter to manage the current and power quality; The system further comprises: In off-grid mode, pre-synchronization control technology is used to achieve synchronization of the AC bus voltage and grid voltage amplitude and phase; The pre-synchronization control technology specifically includes: By performing phase locking on the AC bus voltage, the obtained phase angle is subjected to dq rotation transformation with the grid voltage, and the q-axis component of the grid voltage is controlled to be 0, thereby achieving the purpose of phase synchronization; The grid voltage amplitude is used as a given reference value, and the AC bus voltage amplitude is used as feedback, and the error is controlled to achieve the purpose of amplitude synchronization.

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