Frequency-adaptive phase-locked loop-free grid-connected inverter control method

Abstract

The invention relates to the technical field of grid-connected inverter control, and discloses a frequency adaptive phase-locked loop-free control method based on a third-order complex vector filter, and the control method of an inverter adopts model prediction control. The method comprises the following steps: firstly, establishing a transfer function from a grid-connected voltage to a grid-connected current reference value under a non-phase-locked-loop strategy, and simply calculating the voltage of a common coupling point under a non-phase-locked-loop control method to obtain a power grid angle so as to realize grid connection; a transfer function from a grid-connected voltage to a grid-connected current reference value under the strategy without a phase-locked loop is equivalent to a constant, so that the anti-interference capability is weak; and distributed power generation is merged into the power grid, so that the power grid has weak power grid characteristics, and the impedance of the power grid cannot be neglected, and therefore, a third-order complex vector filter is introduced into the phase-locked loop-free control strategy, and the filter is designed to be frequency-adaptive, and the harmonic suppression capability of the phase-locked loop-free control strategy is improved; and meanwhile, a phase-locked loop-free control strategy can better adapt to a weak power grid. The provided frequency self-adaptive phase-locked loop-free control strategy is clear in overall control structure and easy to implement, and the dynamic performance and the steady-state tracking performance of the grid-connected system are both improved.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected inverter control, and specifically to a control method for a grid-connected inverter with frequency adaption and no phase-locked loop. Background Art

[0002] With the continuous increase in people's demand for electric energy, the gradual depletion of fossil energy, and the inherent requirements of the development of low-carbon economy, clean and renewable energy sources such as wind energy and solar energy have developed rapidly. As one of the main technologies for new energy power generation, distributed power generation technology has the characteristics of low power transmission loss, safety, and high efficiency. Photovoltaic power generation directly converts solar energy into electric energy. Compared with traditional coal-fired power generation, it has less environmental impact, is green and low-carbon, and has the characteristics of large total amount and wide distribution. Its application fields have developed rapidly.

[0003] As a key link in photovoltaic power generation, the grid-connected inverter plays a crucial role in electric energy conversion, and its working performance directly determines the quality of the grid-connected electric energy. In small-power photovoltaic power generation scenarios, two-level inverters are widely used because of their simple structure, low cost, easy control, and easy implementation. Due to the voltage withstand and overcurrent limitations of a single semiconductor device, in medium- and high-power photovoltaic power generation scenarios, a form of series and parallel combination of multiple-level inverters is often adopted.

[0004] However, this system structure is complex, with high design and maintenance costs and great control difficulty. To solve the above problems, high-power multi-level inverters have emerged. Compared with two-level inverters, three-level inverters have the advantages of large output power and high voltage withstand value, and are more suitable for medium- and high-capacity photovoltaic grid connection. Commonly used three-level inverters include NPC-type three-level inverters.

[0005] Model predictive control is an optimal control method based on the predictive model of the controlled object. Its control concept is intuitive, easy to model, and does not require complex control parameter design. In most power electronics control application fields, through the traversal method, according to the discrete mathematical model of the converter, the output states of the converter under all switching states can be calculated, and the optimal switching state can be selected to control the converter to achieve the desired control effect.

[0006] The grid-connected system requires the output signal of the inverter to be synchronized with the grid signal, including equal frequency, equal voltage amplitude, the same voltage phase sequence, and consistent voltage phase. The phase-locked loop can accurately track the voltage frequency and phase changes of the grid and use them as feedback signals for adjustment to ensure that the output alternating current is synchronized with the grid. This is crucial for the stable operation of the system because only the synchronized output alternating current can be smoothly injected into the grid and coordinated with other loads and power generation equipment on the grid.

[0007] When the system uses a traditional phase-locked loop, the reference value of the grid-connected current will no longer be an independent variable. In the case of a weak grid, the phase-locked loop will affect the output impedance through the current inner loop, resulting in a reduction in the system stability margin. At the same time, in a weak grid system, the voltage fluctuations caused by the change of the grid impedance will lead to a decline in the phase-locking performance of the phase-locked loop. Therefore, it is necessary to redesign the bandwidth of the phase-locked loop to improve the system stability and the dynamic performance of the phase-locked loop. Therefore, the phase-locked loop-free method has received more and more attention and applications in grid connection. Compared with the traditional phase-locked loop control, the phase-locked loop-free control strategy does not require a phase-locked loop, can effectively solve the above problems. At the same time, due to the simplified control structure, the system can achieve a faster tracking speed and better steady-state accuracy, and can also improve the robustness and anti-interference ability of the system. Summary of the Invention

[0008] The present invention provides a control method for a grid-connected inverter with frequency adaptive phase-locked loop-free, which solves the problem that the large calculation amount of the traditional phase-locked loop in the prior art applied to the grid-connected inverter leads to a long control response time.

[0009] The present invention provides the following technical solution: A control method for a grid-connected inverter with frequency adaptive phase-locked loop-free, the DC side of the grid-connected inverter is used to connect a DC power supply, the AC side of the grid-connected inverter is connected to the grid through an LCL filter, and the capacitor in the LCL filter is called a filtering capacitor. The method includes the following steps:

[0010] Step S1: Sample to obtain the voltage u at the point of common coupling of the grid-connected system pcc , and send the sampled value into the coordinate transformation module to transform from the three-phase stationary coordinate system to the two-phase stationary coordinate system to obtain u pccα , u pccβ ;

[0011] Step S2: Send the obtained u pccα , u pccβ into a third-order complex vector filter with frequency self-adaptation to obtain the fundamental positive sequence components of u pccα , u pccβ ; The obtained positive sequence components can be calculated to obtain the cosine function and sine function of the grid angle;

[0012] Step S3: Send the obtained cosine function and sine function into the model predictive control module to calculate the grid reference current i 2α , i 2β , and then through sampling, delay, prediction, calculate the reference voltage reference vector u * (k+1) , and judge the large sector and small sector in which it is located in the space voltage vector diagram, and select a group of optimal switching sequences to act on the inverter.

[0013] Preferably, the inverter control method used in the grid-connected system is model predictive control. By sampling, delaying, and predicting the current, the reference voltage vector u is calculated. * (k+1) Then, by judging the large sector and small sector where it is located in the space voltage vector diagram, an optimal switching sequence is selected through the prediction of the midpoint potential and applied to the inverter, improving the grid-connected current response speed.

[0014] Preferably, only by simply calculating the fundamental positive sequence component of the point of common coupling voltage extracted by the third-order complex vector filter with frequency self-adaptation the cosine function and sine function of the grid angle can be obtained:

[0015]

[0016] where θ p is the grid voltage angle.

[0017] Preferably, using a third-order complex vector filter to process the point of common coupling voltage can achieve distortion-free extraction of the fundamental positive sequence component and elimination of the fundamental negative sequence component, improving the anti-interference ability of the control strategy without a phase-locked loop. Unit power factor grid connection can still be achieved in a weak grid system; the transfer function of the third-order complex vector filter is:

[0018]

[0019] where ω 0 is the grid voltage angular frequency.

[0020] Preferably, the used filter has the characteristic of frequency self-adaptation, improving the adaptability of the filter to a weak grid. The voltage signals before and after the filter processing will be processed, and the extracted angular frequency will be fed back into the filter. The method to achieve frequency self-adaptation is:

[0021]

[0022] where T S is the switching period.

[0023] Preferably, the transfer function of the third-order complex vector filter with frequency self-adaptation is:

[0024]

[0025] where ω 0 is the grid voltage angular frequency, and ω is the angular frequency extracted according to the voltage information before and after the filter processing.

[0026] The present invention has the following beneficial effects:

[0027] 1. The grid-connected inverter control method based on a third-order complex vector filter and frequency-adaptive PLL-free can improve the response speed of grid-connected current as the control method of the inverter adopts model predictive control. Compared with traditional PLLs, the PLL-free strategy only needs to simply calculate the voltage at the point of common coupling to obtain the grid angle, without using Park coordinate transformation and designing PI parameters in the PLL, which can reduce the system design complexity and improve the system response speed.

[0028] 2. The grid-connected inverter control method based on a third-order complex vector filter and frequency-adaptive PLL-free can process the voltage at the grid point of common coupling through a third-order complex vector filter, enabling distortion-free extraction of the fundamental positive-sequence component and elimination of the fundamental negative-sequence component, and improving the harmonic suppression ability of the PLL-free control strategy. At the same time, to make the PLL-free control strategy better adapt to weak grids, the filter is designed to be frequency-adaptive. The overall control structure of the proposed frequency-adaptive PLL-free control strategy is clear and easy to implement, and both the dynamic performance and steady-state tracking performance of the grid-connected system are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the grid-connected system of the present invention;

[0030] Figure 2 Space voltage vector diagram of the present invention;

[0031] Figure 3 Space vector diagram of large sector I of the present invention;

[0032] Figure 4 Bode diagram of the present invention and traditional PLL;

[0033] Figure 5 Frequency-adaptive PLL-free control block diagram of the present invention;

[0034] Figure 6 Implementation method in Matlab / Simulink of the voltage processing method on the α-axis of the filter in the frequency-adaptive PLL-free of the present invention;

[0035] Figure 7 Detailed implementation block diagram of the present invention;

[0036] Figure 8 Phase diagram of grid voltage and grid-connected current of the present invention;

[0037] Figure 9 Grid-connected current response time diagram of the present invention;

[0038] Figure 10(a) shows the grid-connected current waveform diagram of the present invention when the grid inductance is 6 mH;

[0039] Figure 10(b) is the grid-connected current THD diagram when the grid inductance of the present invention is 6 mH;

[0040] Figure 11(a) is the grid-connected current waveform diagram when the grid inductance of the present invention is 10 mH;

[0041] Figure 11(b) is the grid-connected current THD diagram when the grid inductance of the present invention is 10 mH;

[0042] Figure 12(a) is the in-phase diagram of the grid voltage and the grid-connected current when the power grid of the present invention contains ±3 harmonics;

[0043] Figure 12(b) is the grid-connected current THD diagram when the power grid of the present invention contains ±3 harmonics; Detailed implementation manners

[0044] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0045] An embodiment of a grid-connected inverter control method with frequency self-adaptation and no phase-locked loop:

[0046] The structural diagram of the grid-connected system of the present invention is as Figure 1 shown, and it is arranged between the photovoltaic power station and the public power grid.

[0047] The grid-connected system includes a diode-clamped three-level three-phase inverter circuit (i.e., the grid-connected inverter of the present application) and an LCL filter. After the photovoltaic power station is subjected to voltage stabilization transformation through a DC converter, it is connected to the DC terminal of the diode-clamped three-level three-phase inverter circuit, and the AC terminal of the diode-clamped three-level three-phase inverter circuit is connected to the public power grid through the LCL filter.

[0048] In addition, the grid-connected system further includes a controller, a sensor, and a drive circuit.

[0049] The diode-clamped three-level three-phase inverter circuit includes an inverter bridge arm and two equalizing capacitors.

[0050] The equalizing capacitors include a first equalizing capacitor C 1 and a second equalizing capacitor C 2 ; the positive terminal of the first equalizing capacitor C 1 is connected to the positive output terminal of the DC terminal of the diode-clamped three-level three-phase inverter circuit, and the negative terminal is connected to the neutral line; the positive terminal of the second equalizing capacitor C 2Its positive terminal is connected to the neutral line, and its negative terminal is connected to the negative output terminal of the DC side of the diode-clamped three-phase three-level inverter circuit.

[0051] The inverter bridge arm includes 6 clamping diodes and 12 fully controlled switching devices. The 6 clamping diodes are clamping diode D1, clamping diode D2, clamping diode D3, clamping diode D4, clamping diode D5, and clamping diode D6 respectively;

[0052] The 12 fully controlled switching devices are fully controlled switching device S a1 , fully controlled switching device S a2 , fully controlled switching device S a3 , fully controlled switching device S a4 , fully controlled switching device S b1 , fully controlled switching device S b2 , fully controlled switching device S b3 , fully controlled switching device S b4 , fully controlled switching device S c1 , fully controlled switching device S c2 , fully controlled switching device S c3 , fully controlled switching device S c4 .

[0053] The cathodes of clamping diodes D1, D3, and D5 are respectively connected to the emitters of fully controlled switching devices S a1 , S b1 , S c1 . The anodes of clamping diodes D1, D3, and D5 are connected to the neutral line. The cathodes of clamping diodes D2, D4, and D6 are connected to the neutral line. The anodes of clamping diodes D2, D4, and D6 are respectively connected to the emitters of fully controlled switching devices S a3 , S b3 , S c3 .

[0054] The LCL filter includes filter inductor L 1 and filter inductor L 2 .

[0055] Filter inductor L 1 is the filter inductor on the inverter side, and its input terminal is connected to the AC side of the diode-clamped three-phase three-level inverter circuit. The output terminal of filter inductor L 1 is connected to the upper end of filter capacitor C f , and at the same time is connected to the input terminal of filter inductor L 2 . Filter inductor L 2 is the filter inductor on the grid side. The output terminal of filter inductor L 2 is connected to the common power grid.

[0056] The input end of the frequency adaptive PLL-free control strategy is connected to the common power grid, and the output end is connected to the input end of the controller.

[0057] The sensor module includes a plurality of voltage sensors and current sensors, which are used to detect the grid-side voltage, grid-side current, filter capacitor voltage and inverter output current, and also detect the midpoint potential and the inverter DC-side voltage.

[0058] The output end of the sensor module is connected to the input end of the controller to send the collected data to the controller for the controller to make logical judgments and process based on the collected data.

[0059] The output end of the controller is connected to the input end of the drive circuit to control each clamping diode and full-controlled switching device in the diode-clamped three-phase three-level inverter circuit.

[0060] Based on the grid-connected system introduced above, a grid-connected inverter control method with a frequency adaptive PLL-free can be realized. Its idea control block diagram and specific block diagram are respectively as Figure 1 、 Figure 5 and Figure 6 shown, and the control structure of the whole method is as Figure 7 shown;

[0061] The frequency adaptive PLL-free strategy is mainly reflected in: on the premise of realizing unity power factor grid connection, making the grid-connected current reach the steady state in a very short time, and meeting the grid connection conditions under different working conditions.

[0062] Next, the model predictive control model of the grid-connected system will be introduced first.

[0063] 1. Select the switching sequence through the predicted spatial position of the reference voltage. The predicted reference voltage at time k + 1 is:

[0064]

[0065] where i * 1α(k+2) 、i * 1β(k+2) are the reference currents of the inverter at time k + 2, is the filter capacitor voltage at time k + 1. i 1α(k+1) 、i 1β(k+1) are the inverter output currents at time k + 1, and its expression is:

[0066]

[0067] 2. The reference current of the inverter at time k + 2 is:

[0068]

[0069] where \(i\) * 2α(k+2) and \(i\) * 2β(k+2) are the grid - connected reference currents at the \((k + 2)\) - th moment, and \(i\) is the filter capacitor current at the \((k + 2)\) - th moment.

[0070] 3. The grid - connected reference current is:

[0071]

[0072] where is the given reference value of the grid - connected current.

[0073] 4. \(i\) * 2α(k+2) and \(i\) * 2β(k+2) are obtained by delaying the grid - connected reference current:

[0074]

[0075] In the above formulas, are all obtained by delaying the sampling values of the grid - connected system. The principle is the same as that of formula (5), so it will not be described again.

[0076] 5. Figure 2 For the 27 basic voltages generated by the inverter according to the distribution of the reference voltage, they can be divided into 6 large sectors with 60° as a unit. Therefore, the angle of the reference voltage vector can determine the large sector it is in, that is:

[0077]

[0078] where \(N\) is the large - sector number, and its value range is 1 - 6; \(\theta\) is the angle of the reference voltage vector;

[0079] 6. Transform the rectangular (\(\alpha\beta\)) coordinate system where the reference voltage vector is located to the 60° (\(gh\)) coordinate system, and rotate the other large sectors to large sector I through rotation transformation. Obtain the reference voltage coordinates through per - unitization, and the coordinate range can determine the small sector where the reference voltage is located.

[0080] The transformation formula for transforming the reference voltage from the rectangular (\(\alpha\beta\)) coordinate system to the 60° (\(gh\)) coordinate system is:

[0081]

[0082] The rotation formula for rotating the other sectors to large sector I is:

[0083]

[0084] where \(u\) *Ng(k+1) , u *N h(k+1) is the reference voltage in the Nth sector in the 60° coordinate system, and N is the large sector number.

[0085] The reference voltage is normalized, and its expression is:

[0086]

[0087] V * g(k+1) , V * h(k+1) are the coordinates of the normalized reference voltage vector in the gh coordinate system. As shown in Figure 3 , the judgment rules for small sectors are shown in Table 1.

[0088] Table 1 Small sector judgment rules

[0089]

[0090] 7. There are redundant vectors for small voltage vectors, and their effects on the midpoint potential are opposite. For example, for redundant vectors POO and ONN, when the switching state of the inverter is POO, the generated midpoint current is i 1a ; when the switching state of the inverter is ONN, the generated midpoint current is -i 1a . Therefore, the midpoint potential can be controlled by selecting different redundant small vectors. Thus, taking the first small sector of large sector I as an example, two groups of switching sequences can be designed, namely OOO - POO - PPO and OOO - OON - ONN. Taking large sector I as an example, its switching sequence output is shown in Table 2:

[0091] Table 2 Switching sequence of large sector I

[0092]

[0093] The selection of the switching sequence depends on the predicted value of the midpoint potential. After determining the small sector, the midpoint potentials of the two groups of switching sequences are predicted respectively, and the switching sequence with the smaller midpoint potential is selected to act on the inverter. The predicted value of the midpoint potential is:

[0094]

[0095] where ΔV (k) is the midpoint potential deviation value at the current moment, t i is the action time of the three basic voltage vectors, S a,b,c(k) is the switching state of the three-phase bridge arm obtained by sampling, i 1a,b,c(k)The three-phase current output by the inverter obtained through sampling. Since the predicted reference voltage vector is at the k+1 moment, in order to calculate the midpoint potential deviation of the predicted optimal voltage switching sequence, the midpoint potential deviation at the k+2 moment should be selected, that is:

[0096]

[0097] where t i is the action time of the three basic voltage vectors, S a,b,c(k+1) is the switching state of the three-phase bridge arm of the predicted inverter, i 1a,b,c(k+1) is the three-phase current output by the predicted inverter.

[0098] The switching state of the three-phase bridge arm is defined as:

[0099]

[0100] Determine the sector where the reference voltage is located. V * g(k+1) 、V * h(k+1) can be synthesized from the three vertex basic voltage vectors in the small sector according to the volt-second balance principle. Taking the first small sector as an example:

[0101]

[0102] where V 0 、V 3 、V 5 are the three basic voltage vectors of the first small sector, and t 1 、t 2 、t 3 are the action times of the three basic voltage vectors respectively.

[0103] The action times of the three basic voltage vectors are:

[0104] Table 3 Vector action time

[0105]

[0106] Based on the inverter model predictive control method introduced above, the implementation process of the frequency adaptive PLL-free strategy of the method of the present invention will be introduced below.

[0107] Step 1, establish the transfer function from the grid-connected voltage to the reference value of the grid-connected current under the PLL-free strategy.

[0108] The expressions of the active power P and the reactive power Q in the two-phase stationary coordinate system are as follows:

[0109]

[0110] where upccα , u pccβ are the α- and β-axis components of the common coupling point voltage, respectively, and i 2α , i 2β are the α- and β-axis components of the grid-connected current, respectively.

[0111] The α- and β-axis components of the common coupling point voltage satisfy the relationship shown in Equation (4-20).

[0112]

[0113] The unitization of the α- and β-axis components of the common coupling point voltage gives:

[0114]

[0115] Combined with the active reference current amplitude and the reactive reference current amplitude the reference values of the grid-connected current in the α- and β-axes can be obtained as:

[0116]

[0117] The PLL-free control strategy calculates the grid angle in the two-phase stationary coordinate system. The following derivations are all for the α-axis component, and the β-axis component can be obtained in the same way.

[0118] Let the reference value of the grid-connected current in the α-axis be Combined with the above formula, we get The expression is:

[0119]

[0120] It can be seen from Equation (18) that the reference value of the grid-connected current can be obtained by simply calculating the grid voltage angle only by sampling the common coupling point voltage; at the same time, since the given current is in the dq-axis coordinate system, independent control of the active current and the reactive current can be achieved.

[0121] When the system is operating normally, the α- and β-axis components of the common coupling point voltage can be expressed as:

[0122]

[0123] where U m is the grid-connected voltage amplitude.

[0124] Combined with Equations (16), (17) and (18), the reference value of the grid-connected current can be expressed as:

[0125]

[0126] In an ideal situation, the system needs to achieve unit power operation, that is Under the α-axis, upcc = U m cosθ p Then, Equation (20) can be expressed as:

[0127]

[0128] The expression of the grid-connected current reference value under the traditional phase-locked loop control is:

[0129]

[0130] where G PLL (s) is the transfer function of the traditional phase-locked loop.

[0131] Referring to Equation (21) and Equation (22), the transfer function from the grid-connected voltage to the grid-connected current reference value without a phase-locked loop can be obtained:

[0132]

[0133] The phase-locked loop-free strategy only needs to use the voltage at the point of common coupling for simple calculation to obtain the grid angle. Compared with the traditional phase-locked loop, it does not need to use the Park coordinate transformation and design PI parameters, which can reduce the system design complexity and improve the system response speed.

[0134] In the actual application environment, the phase-locked loop-free strategy appears as a constant in the grid-connected system. It can track the grid voltage phase well under ideal grid conditions. Once the grid impedance is considered, the system is no longer stable.

[0135] Step 2: Introduce a third-order complex vector filter.

[0136] Complex filtering can accurately extract the positive sequence component of the fundamental frequency, effectively separating the fundamental component from the harmonic component in the voltage at the point of common coupling, and has the characteristics of small calculation amount and easy implementation. When the voltage signal at the point of common coupling passes through the complex filter, it can ensure that the signal at the fundamental frequency passes through without attenuation, while filtering out a large number of harmonic components. This not only helps the grid-connected inverter achieve high-gain output, but also ensures that the phase-locked loop-free control strategy can accurately extract the voltage frequency information under weak grid conditions, thereby further improving the stability of the entire control system.

[0137] The transfer function of the third-order complex vector filter is:

[0138]

[0139] where, k 1 = 2.33, k 2 = 3.18, k 3 = 5.66, k 4 = 3.18.

[0140] The transfer function of the PLL-free system after adding a third-order complex vector filter is as follows:

[0141] G Z3 (s) = G Z (s)G T (s)(25)

[0142] Since the traditional phase-locked loop is not the focus of this design, the transfer function of the traditional phase-locked loop is directly given as follows:

[0143]

[0144] where K p and K i are the PI controller parameters in the traditional phase-locked loop, with values of 4.07 and 1758.58 respectively.

[0145] Based on the transfer functions of the third-order complex vector filter PLL-free system and the traditional phase-locked loop, the Bode plots of the two control strategies can be drawn, as shown in Figure 4 . Compared with the traditional phase-locked loop, the third-order complex vector filter PLL-free control strategy has stronger harmonic suppression ability.

[0146] Step 3: Add frequency adaptability to the third-order complex vector filter.

[0147] To enhance the adaptability of the third-order complex vector filter PLL-free control strategy to the power grid, a frequency adaptive link is introduced into the filter. By real-time monitoring the fluctuations of the power grid frequency and dynamically adjusting the filter output according to the actual situation, the stability and performance of the control system can be maintained.

[0148] The angular frequency fed back to the third-order complex vector filter is:

[0149]

[0150] The angular frequency used in the third-order complex vector filter is ω 0 + ω. Hereinafter, the frequency adaptive third-order complex vector filter PLL-free control strategy will be abbreviated as the frequency adaptive PLL-free control strategy.

[0151] Step 4: Calculate the power grid angle.

[0152] By simply calculating the fundamental positive sequence component of the common coupling point voltage extracted by the frequency adaptive third-order complex vector filter , the cosine function and sine function of the power grid angle can be obtained:

[0153]

[0154] where θ pis the grid voltage angle.

[0155] The implementation block diagram of the frequency adaptive PLL-free control strategy in Matlab / Simulink is as Figure 5 , Figure 6 shown.

[0156] Step 5: Feed the calculated angle into the model predictive control module, sample, calculate, and delay the grid-connected current and the inverter-side current, and select the optimal switching sequence to act on the inverter.

[0157] The following is a simulation experiment to illustrate the effectiveness of this method. The simulation parameters are shown in Table 5.

[0158] Table 5 System simulation parameters

[0159]

[0160] As Figure 8 can be seen, under the frequency adaptive PLL-free control, the grid voltage and the grid-connected current are in the same phase, achieving grid connection with unity power factor.

[0161] Figure 9 is the response time of the grid-connected current under the frequency adaptive PLL-free control. The grid-connected current can reach the steady state within 0.005 s, fully reflecting the fast characteristics of the model predictive control and the PLL-free control.

[0162] Figures 10 and 11 are the harmonic analysis of the grid current and the current when the grid inductance is 6 mH and 10 mH respectively. These two grid inductances correspond to the short circuit ratios of SCR = 3.03 and SCR = 1.82 respectively. At this time, the grid is a weak grid and an extremely weak grid. Under the frequency adaptive PLL-free control, the THD of the grid-connected current is 0.43% and 0.50% respectively, meeting the grid connection conditions.

[0163] Figure 12 is the simulation diagram of the grid phase A voltage and the grid-connected current of phase A when there are ±3rd harmonics in the grid. At this time, the two electrical quantities are in the same phase, still meeting the grid connection with unity power factor. At this time, the THD of the grid-connected current is 3.60%, meeting the grid connection conditions.

[0164] The above working conditions simulation verifies that the grid-connected system under the frequency adaptive PLL-free control can be applied to a weak grid and has a certain anti-interference ability.

[0165] It should be noted that in this text, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0166] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A frequency adaptive phase-locked loop-free grid-connected inverter control method, characterized in that: The DC end of the grid-connected inverter is used to connect a DC power supply, and the AC end of the grid-connected inverter is connected to the power grid through an LCL filter. The capacitor in the LCL filter is called a filter capacitor. The method comprises the following steps: Step S1: Sampling and obtaining the grid-connected system common coupling point voltage u pcc , the sampled value is sent to the coordinate transformation module, and the three-phase stationary coordinate system is transformed into the two-phase stationary coordinate system to obtain u pccα 、u pccβ ; Step S2: Get u pccα 、u pccβ Send it into the frequency adaptive third-order complex vector filter to get u pccα 、u pccβ The fundamental positive sequence component of The obtained positive sequence component can be calculated to obtain the cosine function and sine function of the grid angle; Step S3: Send the obtained cosine function and sine function to the model predictive control module to calculate the grid reference current i 2α 、i 2β , and then after sampling, delay, and prediction, the reference voltage vector u is calculated * (k+1) , and determine the large sector and small sector in the spatial voltage vector diagram, and select a set of optimal switching sequences to act on the inverter.

2. The frequency adaptive PLL-free grid-connected inverter control method according to claim 1, characterized in that: The inverter control method used in the grid-connected system is model predictive control, which calculates the reference voltage vector u by sampling, delaying and predicting the current. * (k+1) , and judge the large sector and small sector in the spatial voltage vector diagram, and select a set of optimal switching sequences to act on the inverter by predicting the midpoint potential.

3. The frequency adaptive PLL-free grid-connected inverter control method according to claim 1, characterized in that: Only the fundamental positive sequence component of the common coupling point voltage extracted by the frequency adaptive third-order complex vector filter is needed. A simple calculation can obtain the cosine and sine functions of the grid angle: Where θ p is the grid voltage angle.

4. The frequency adaptive non-phase-locked loop grid-connected inverter control method according to claim 3, characterized in that: Using a third-order complex vector filter to process the voltage at the common coupling point can achieve distortion-free extraction of the fundamental positive sequence component and elimination of the fundamental negative sequence component, improve the anti-interference ability of the non-phase-locked loop control strategy, and still achieve unity power factor grid connection in a weak power grid system; the transfer function of the third-order complex vector filter is: Where ω0 is the grid voltage angular frequency.

5. The frequency adaptive PLL-free grid-connected inverter control method according to claim 4, characterized in that: The filter used has the characteristic of frequency adaptation to improve the adaptability of the filter to weak power grids. The method to achieve frequency adaptation is: Where T S is the switching cycle.

6. A frequency adaptive PLL-free grid-connected inverter control method according to claim 5, characterized in that: The frequency adaptive third-order complex vector filter transfer function is: Among them, ω0 is the grid voltage angular frequency, ω is the angular frequency extracted according to the voltage information before and after the filter processing in claim 5 and then fed back to the filter, thereby achieving frequency adaptation.

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