Virtual power synchronous phase-locked loop for grid-connected inverter of weak power grid and phase-locked control method

Abstract

The invention discloses a virtual power synchronous phase-locked loop for a grid-connected inverter of a weak power grid and a phase-locked control method, and the phase-locked loop comprises a virtual impedance between the output end of the inverter and the power grid; the virtual power calculation module is used for calculating virtual power generated by the virtual impedance; the active calculation branch is used for performing droop control according to the calculated active part of the virtual power to obtain an angular frequency deviation, obtaining a virtual angular frequency according to the angular frequency deviation, and obtaining a virtual phase locking angle based on the virtual angular frequency; the reactive calculation branch is used for performing droop control according to the calculated reactive part of the virtual power to obtain a voltage deviation and obtaining a virtual voltage amplitude according to the voltage deviation; and the virtual voltage calculation module is used for calculating a virtual voltage according to the virtual phase locking angle and the virtual voltage amplitude. According to the invention, the phase-locked control stability under the weak power grid can be improved, and the problem that a traditional current source type phase-locked loop is easy to lose stability under the weak power grid is effectively solved.

Description

Technical Field

[0001] The present invention relates to the technical field of weak grid phase-locked control, and particularly to a virtual power synchronization phase-locked loop and a phase-locked control method for a weak grid grid-connected inverter. Background Art

[0002] The access of distributed power sources to the grid will introduce a large number of nonlinear loads, and the combined action of nonlinear loads and transmission line impedance will cause the public grid to exhibit weak grid characteristics. A large number of distributed power sources are connected to the distribution network by means of inverters, etc., which increases the operation and control difficulty while improving the flexibility of the distribution network. For inverters applying current-type control algorithms, phase-locking is a key link for the converter to synchronize with the grid and the inverter to obtain the reference phase. However, the weak grid condition will bring a series of challenges to phase-locking. Among them, the phase-locking instability problem under the weak grid condition is the key problem leading to the instability of the inverter system.

[0003] Currently, new energy power stations usually adopt current-type control for inverter grid connection. The grid-connected inverter with current-type control needs to use a phase-locked loop to calculate the phase of the grid voltage at the grid connection point to provide a phase reference for control. For example, the widely used synchronous reference frame PLL (SRF-PLL). This type of control method can directly control the grid current and can be equivalent to a current source. Therefore, it can only work in the grid-connected mode and requires a high enough connection strength between the grid connection point and the grid. Otherwise, there will be limitations in the power transmitted by the system, or even instability may occur. Moreover, since current-type control can only passively transmit power to the grid according to the power reference command and does not participate in the frequency modulation and peak shaving of the grid, it cannot actively support the grid and cannot provide damping and inertia for the grid.

[0004] The phase-locked loops (PLLs) for inverter grid connection usually focus on the transient stability of the PLL, that is, the fault ride-through of the grid-connected inverter when a short-circuit fault occurs in the power grid. For example, the decoupled double synchronous reference-frame PLL (DDSRF-PLL), the double second-order generalized integrator PLL (DSOGI-PLL), the multiple-complex coefficient-filter-Based PLL (MCCF-PLL), etc. When a short-circuit fault occurs in the power grid, these PLLs can accurately extract the positive and negative sequence components of the grid voltage and can well solve the transient stability problem of the PLL. However, there will be a small-signal steady-state stability problem of the PLL. Under weak grid conditions, the system is still prone to instability. The traditional d-q transformation-based PLL is greatly affected by voltage imbalance and voltage harmonics and cannot work well under weak grid conditions, and its stability is also poor. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: aiming at the technical problems existing in the prior art, the present invention provides a virtual power synchronous phase-locked loop and a phase-locked control method for a weak grid-connected inverter, which can improve the phase-locked control stability under a weak grid and effectively solve the problem that the traditional current-source type phase-locked loop is prone to instability under a weak grid.

[0006] To solve the above technical problems, the technical solution proposed by the present invention is:

[0007] A virtual power synchronous phase-locked loop for a weak grid-connected inverter, comprising:

[0008] A virtual impedance, between the output terminal of the inverter and the power grid;

[0009] A virtual power calculation module, configured to calculate the virtual power generated by the virtual impedance;

[0010] An active power calculation branch, configured to perform droop control according to the active part of the calculated virtual power to obtain an angular frequency deviation, obtain a virtual angular frequency according to the angular frequency deviation, and obtain a virtual phase-locked angle based on the virtual angular frequency;

[0011] A reactive power calculation branch, configured to perform droop control according to the reactive part of the calculated virtual power to obtain a voltage deviation, and obtain a virtual voltage amplitude according to the voltage deviation;

[0012] A virtual voltage calculation module, configured to calculate a virtual voltage according to the virtual phase-locked angle and the virtual voltage amplitude.

[0013] Further, the active power calculation branch includes a first adder unit, a first droop control unit, and an angle conversion unit connected in sequence. The first adder unit inputs a power reference value and the active part of the virtual power respectively, and outputs the difference between the active part of the virtual power and the power reference value to the first droop control unit. The first droop control unit obtains the angular frequency deviation after passing through the first droop control coefficient m, and outputs it to the second adder. The second adder unit adds the angular frequency deviation and the rated angular frequency to obtain the virtual angular frequency, and outputs it to the angle conversion unit. The angle conversion unit obtains the virtual phase-locked angle according to the relationship between the phase-locked angle and the angular frequency.

[0014] Further, the reactive power calculation branch includes a third adder unit, a second droop control unit, and a fourth adder unit. The third adder unit inputs a power reference value and the reactive part of the virtual power respectively, and outputs the difference between the reactive part of the virtual power and the power reference value to the second droop control unit. The second droop control unit obtains a voltage deviation after passing through the second droop control coefficient n, and outputs it to the fourth adder unit. The fourth adder unit adds the voltage deviation and the rated voltage to obtain the virtual voltage amplitude.

[0015] Further, the power reference value is 0.

[0016] Further, the virtual power calculation module calculates the virtual power generated by the virtual impedance according to the following formula:

[0017]

[0018] where P v , Q v are the active part and the reactive part of the virtual power respectively, ΔU d , ΔU q are the voltage differences in the dq rotating coordinate system respectively, I dv , I qv are the currents generated by the voltage difference acting on the virtual impedance in the dq rotating coordinate system after introducing the virtual impedance respectively.

[0019] A virtual power synchronous phase-locked control method for a weak grid-connected inverter, the steps of which include:

[0020] Introduce a virtual impedance between the output terminal of the inverter and the grid;

[0021] Calculate the virtual power generated by the virtual impedance;

[0022] Perform droop control based on the active part of the calculated virtual power to obtain the angular frequency deviation, obtain the virtual angular frequency according to the angular frequency deviation, and obtain the virtual phase-locked angle based on the virtual angular frequency;

[0023] Perform droop control based on the reactive part of the calculated virtual power to obtain the voltage deviation, and obtain the virtual voltage amplitude according to the voltage deviation;

[0024] Calculate the virtual voltage according to the virtual phase-locked angle and the virtual voltage amplitude.

[0025] Further, the performing droop control based on the active part of the calculated virtual power to obtain the angular frequency deviation, obtaining the virtual angular frequency according to the angular frequency deviation, and generating the virtual phase-locked angle based on the virtual angular frequency includes:

[0026] Calculate the difference between the active part of the virtual power and the power reference value;

[0027] Obtain the angular frequency deviation by passing the difference between the active part of the virtual power and the power reference value through the first droop control coefficient m;

[0028] Add the angular frequency deviation to the rated angular frequency to obtain the virtual angular frequency;

[0029] Obtain the virtual phase-locked angle according to the virtual angular frequency and the relationship between the phase-locked angle and the angular frequency.

[0030] Further, the performing droop control based on the reactive part of the calculated virtual power to obtain the voltage deviation, obtaining the virtual voltage amplitude according to the voltage deviation includes:

[0031] Calculate the difference between the reactive part of the virtual power and the power reference value;

[0032] Obtain the voltage deviation by passing the difference between the reactive part of the virtual power and the power reference value through the second droop control coefficient n;

[0033] Add the voltage deviation to the rated voltage to obtain the virtual voltage amplitude.

[0034] Further, calculate the virtual power generated by the virtual impedance according to the following formula:

[0035]

[0036] where P v and Q v are the active part and the reactive part of the virtual power respectively, ΔU d and ΔU q are the voltage differences in the dq rotating coordinate system respectively, and I dv, I qv are respectively the currents generated by the voltage difference acting on the virtual impedance in the dq rotating coordinate system after introducing the virtual impedance.

[0037] Furthermore, the voltage differences ΔU d , ΔU q in the dq rotating coordinate system and the currents I dv , I qv generated in the dq rotating coordinate system are respectively calculated according to the following formulas:

[0038]

[0039]

[0040]

[0041]

[0042] where Δu a , Δu b , Δu c are respectively the three-phase voltages dropped on the virtual impedance obtained according to the terminal voltages of the virtual impedance, U g is the grid-side voltage, ω g is the grid-side angular frequency, θ g is the grid-side phase angle, U v is the output voltage of the phase-locked loop, and θ v is the virtual phase-locked angle.

[0043] Compared with the prior art, the advantages of the present invention are as follows: By introducing a virtual impedance in the phase-locked loop, calculating the virtual power based on the virtual impedance, bringing the virtual power of the virtual impedance into the droop control based on the droop control negative feedback mechanism, obtaining the virtual power phase-locked loop through the droop control of the active part of the virtual power, obtaining the voltage deviation through the droop control of the reactive part of the virtual power, and then obtaining the virtual voltage amplitude, and finally calculating the virtual voltage according to the virtual phase-locked angle and the virtual voltage amplitude, the present invention can effectively track the grid voltage amplitude, phase and frequency through the droop control, and can also eliminate the influence of voltage harmonics by adjusting the virtual impedance, effectively suppress the voltage imbalance problem, and improve the stability of the phase-locked control of the weak grid. Description of the Drawings

[0044] Figure 1 is a schematic structural principle diagram of the virtual power synchronous phase-locked loop for the weak grid grid-connected inverter in this embodiment.

[0045] Figure 2 is a schematic principle diagram of introducing the virtual impedance in this embodiment.

[0046] Figure 3 It is a schematic diagram of the implementation process for the virtual power synchronization phase-locked control of a weak grid-connected inverter in this embodiment.

[0047] Figure 4 It is a schematic diagram of the simulation waveform results obtained in a specific application embodiment of the present invention. Detailed implementation manners

[0048] The present invention will be further described below in conjunction with the accompanying drawings of the specification and specific preferred embodiments, but the protection scope of the present invention is not limited thereby.

[0049] As disclosed in the present invention, unless the context clearly indicates otherwise, words such as "a", "an", "one" and / or "the" are not specifically singular and may also include plural. The words "first", "second" and similar words used in the disclosure of the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, words such as "comprising" or "including" mean that the elements or objects appearing before this word cover the elements or objects listed after this word and their equivalents, without excluding other elements or objects. "Connection" or "connection" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect.

[0050] As Figure 1 shown, the virtual power synchronization phase-locked loop for the weak grid-connected inverter in this embodiment includes:

[0051] A virtual impedance, between the output terminal of the inverter and the power grid;

[0052] A virtual power calculation module, used to calculate the virtual power generated by the virtual impedance;

[0053] An active power calculation branch, used to perform droop control according to the active part of the calculated virtual power to obtain an angular frequency deviation, obtain a virtual angular frequency based on the angular frequency deviation, and obtain a virtual phase-locked angle based on the virtual angular frequency;

[0054] A reactive power calculation branch, used to perform droop control according to the reactive part of the calculated virtual power to obtain a voltage deviation, and obtain a virtual voltage amplitude according to the voltage deviation;

[0055] A virtual voltage calculation module, used to calculate a virtual voltage according to the virtual phase-locked angle and the virtual voltage amplitude.

[0056] This embodiment constructs a new type of phase-locked loop based on droop control and virtual power calculation. A virtual impedance is introduced in the phase-locking link. The virtual power calculation module calculates the virtual power based on the virtual impedance, and then brings the virtual power of the virtual impedance into the droop control based on the droop control negative feedback mechanism. The active calculation branch obtains the virtual phase-locked angle through droop control for the active part of the virtual power, that is, obtains the virtual power phase-locked loop based on droop control. The reactive calculation branch obtains the voltage deviation through droop control for the reactive part of the virtual power, and then obtains the virtual voltage amplitude. The virtual voltage calculation module calculates the virtual voltage according to the virtual phase-locked angle and the virtual voltage amplitude, which can effectively track the amplitude, phase, and frequency of the grid voltage through droop control, and can also eliminate the influence of voltage harmonics by adjusting the virtual impedance, effectively suppress the voltage imbalance problem, and improve the stability of the weak grid phase-locking control.

[0057] In this embodiment, the active calculation branch specifically includes a first adder unit, a first droop control unit, and an angle conversion unit connected in sequence. The first adder unit inputs the power reference value and the active part of the virtual power respectively, and outputs the difference between the active part of the virtual power and the power reference value to the first droop control unit. The first droop control unit obtains the angular frequency deviation after passing through the first droop control coefficient m and outputs it to the second adder. The second adder unit adds the angular frequency deviation and the rated angular frequency to obtain the virtual angular frequency, which is output to the angle conversion unit. The angle conversion unit obtains the virtual phase-locked angle according to the relationship between the phase-locked angle and the angular frequency, and obtains the virtual power phase-locked loop based on droop control.

[0058] In this embodiment, the reactive calculation branch specifically includes a third adder unit, a second droop control unit, and a fourth adder unit. The third adder unit inputs the power reference value and the reactive part of the virtual power respectively, and outputs the difference between the reactive part of the virtual power and the power reference value to the second droop control unit. The second droop control unit obtains the voltage deviation after passing through the second droop control coefficient n and outputs it to the fourth adder unit. The fourth adder unit adds the voltage deviation and the rated voltage to obtain the virtual voltage amplitude.

[0059] In this embodiment, by adopting the above active and reactive calculation branches, the difference between the inverter output voltage and the grid-side voltage is used to act on the virtual impedance to calculate the active and reactive powers on the virtual impedance, and then combined with droop control to realize the tracking of the amplitude, phase, and frequency of the grid voltage.

[0060] It can be understood that the values of the above first droop control coefficient m and second droop control coefficient n can be specifically configured according to actual needs.

[0061] In this embodiment, the power reference value is specifically 0, that is, through phase-locked control, the virtual power generated after introducing the virtual impedance is 0. The virtual impedance can specifically be a virtual inductor, etc. As Figure 1 shown, according to the droop control negative feedback mechanism, the phase-locked loop of this embodiment brings the virtual power of the virtual inductor into the droop control. When the virtual voltage u v obtained by phase-locking is the same as the actual grid voltage u g , the difference ΔU is 0, and the voltage drop across the virtual inductor is 0. Therefore, the active power and reactive power generated on the virtual inductor are also 0, that is, P ref =0, Q ref =0. Through the closed-loop system of droop control, the given active power and reactive power command values are both 0. In the active power calculation branch, the active part of the virtual power is subtracted from 0, and after adding the negative feedback link of the frequency, the deviation Δω of the angular frequency is obtained after passing through the droop control coefficient m. After adding it to the rated angular frequency ω g , the virtual angular frequency ω v is obtained. After passing through the phase angle and frequency relationship formula, the virtual phase-locked angle θ v is generated. The obtained angle is the phase angle locked by the virtual power phase-locked loop controlled by the virtual synchronous machine; in the reactive power calculation branch, the reactive part of the virtual power subtracts the virtual power from 0, multiplies it by the droop coefficient n to obtain the voltage deviation ΔU, and after adding it to the rated voltage U g , the virtual voltage amplitude U v is obtained. Then, due to the negative feedback mechanism of droop control, the virtual active power and virtual reactive power can be controlled to 0. Thus, the virtual angle θ v obtained after droop control is the grid voltage phase-locked angle.

[0062] In this embodiment, by adding a virtual impedance Z v between the output terminal of the inverter and the grid, as Figure 2 shown, introducing the virtual impedance can accurately obtain the parameter information of the grid voltage. The system samples the grid-side voltage u g and the voltage u v output by the phase-locked loop, which are expressed as follows:

[0063]

[0064]

[0065] Among them, U g is the grid-side voltage, ω g is the grid-side angular frequency, θ g is the grid-side phase angle, U v is the voltage output by the phase-locked loop, and θ v is the virtual phase-locked angle.

[0066] The voltage ΔU dropped across the virtual impedance can be obtained from the terminal voltage of the virtual impedance as follows:

[0067]

[0068] where Δu a 、Δu b 、Δu c are the three-phase voltages dropped across the virtual impedance obtained from the terminal voltage of the virtual impedance, respectively.

[0069] Furthermore, through Park transformation and calculation, ΔU d 、ΔU q in the dq rotating coordinate system are as follows:

[0070]

[0071] Then, the current generated by the voltage difference ΔU acting on the virtual impedance Z v in the dq rotating coordinate system, I d 、I q has the following calculation expressions:

[0072]

[0073] In the dq rotating coordinate system, the virtual power P v 、Q v generated by the virtual impedance is calculated based on the instantaneous power of the three-phase symmetric circuit as follows:

[0074]

[0075] where P v 、Q v are the active part and reactive part of the virtual power, respectively, ΔU d 、ΔU q are the voltage differences in the dq rotating coordinate system, and I dv 、I qv are the currents generated by the voltage difference acting on the virtual impedance in the dq rotating coordinate system after introducing the virtual impedance, respectively.

[0076] In this embodiment, the virtual power calculation unit can calculate the virtual power generated by the virtual impedance according to formulas (1) to (6) specifically.

[0077] As Figure 3 shown, the steps of the virtual power synchronous phase-locked control method for the weak grid-connected inverter in this embodiment include:

[0078] Step S01: Introduce a virtual impedance between the output terminal of the inverter and the power grid;

[0079] Step S02: Calculate the virtual power generated by the virtual impedance;

[0080] Step S03: Perform droop control based on the active part of the calculated virtual power to obtain the angular frequency deviation, obtain the virtual angular frequency based on the angular frequency deviation, and obtain the virtual phase-locked angle based on the virtual angular frequency; perform droop control based on the reactive part of the calculated virtual power to obtain the voltage deviation, and obtain the virtual voltage amplitude based on the voltage deviation;

[0081] Step S04: Calculate the virtual voltage according to the virtual phase-locked angle and the virtual voltage amplitude.

[0082] In this embodiment, the steps of Step S03 for performing droop control based on the active part of the calculated virtual power to obtain the angular frequency deviation, obtaining the virtual angular frequency based on the angular frequency deviation, and generating the virtual phase-locked angle based on the virtual angular frequency include:

[0083] Step S301. Calculate the difference between the active part of the virtual power and the power reference value;

[0084] Step S302. Obtain the angular frequency deviation by passing the difference between the active part of the virtual power and the power reference value through the first droop control coefficient m;

[0085] Step S303. Add the angular frequency deviation to the rated angular frequency to obtain the virtual angular frequency;

[0086] Step S304. Obtain the virtual phase-locked angle according to the virtual angular frequency and the relationship between the phase-locked angle and the angular frequency.

[0087] In this embodiment, the steps of performing droop control based on the reactive part of the calculated virtual power to obtain the voltage deviation and obtaining the virtual voltage amplitude based on the voltage deviation include:

[0088] Step S311. Calculate the difference between the reactive part of the virtual power and the power reference value;

[0089] Step S312. Obtain the voltage deviation by passing the difference between the reactive part of the virtual power and the power reference value through the second droop control coefficient n;

[0090] Step S313. Add the voltage deviation to the rated voltage to obtain the virtual voltage amplitude.

[0091] In this embodiment, in Step S02, the virtual power generated by the virtual impedance is specifically calculated according to Equation (6), where the voltage differences ΔU d , ΔU q and the currents I dv , I qv generated in the dq rotating coordinate system are respectively calculated according to Equations (1) to (5).

[0092] Since the grid voltage waveform is not always in the standard sinusoidal state of three-phase symmetry, but has dynamic characteristics along with the power system state, there are often voltage asymmetry and harmonic problems during the synchronization process between the inverter and the grid. The grid voltage containing harmonics can be expressed as:

[0093]

[0094] where \(i = a, b, c\) and \(k\) a,b,c \(= 0, 1, 2,\cdots, n\) represents the \(n\)th harmonic. The calculation of virtual power under harmonic conditions can obtain the active power \(P\) sum and the reactive power \(Q\) sum as:

[0095]

[0096] where \(P\) v and \(Q\) v are the fundamental virtual active and virtual reactive powers. Similar to Equation (6), \(P\) n and \(Q\) n are the virtual active and virtual reactive powers generated by the \(n\)th harmonic acting on the virtual inductor, and the calculation expressions can be expressed as:

[0097]

[0098] where \(L\) v is the value of the virtual inductor, is the impedance angle of the virtual inductor, that is Then there is:

[0099]

[0100] As can be seen from the above, harmonics (\(n>1\)) only generate reactive power on the virtual inductor and have no impact on the virtual active power. Therefore, the phase-locked output angle is only related to the virtual active power. The phase-locked loop of the present invention can effectively suppress harmonics by using the droop control of the virtual active power without special treatment of the harmonic voltage.

[0101] Under the condition of three-phase unbalanced voltage, the grid voltage can be expressed as:

[0102]

[0103] where \(U\) a , \(U\) b , \(U\) c are the amplitudes of the three-phase grid voltages respectively, and their values are determined according to the voltage unbalance situation.

[0104] The output voltage of the phase-locked loop is:

[0105]

[0106] The active and reactive powers on the virtual inductor can be calculated by the following formula:

[0107]

[0108] where, U i , U v are respectively the amplitude of each phase of the input voltage and the amplitude of the output voltage of the phase-locked loop. X = ω g L v is the impedance value of the virtual inductor, θ v , θ g are respectively the output phase angle of the phase-locked loop and the phase angle of the target voltage. Since the droop control part in the phase-locked loop is designed with both active and reactive commands being 0, when the virtual active power P is not 0, the output frequency of the phase-locked loop will change according to the droop characteristic until the virtual active power P Σ = 0, at this time θ v = θ n . When the virtual reactive power part is not 0, the output voltage of the phase-locked loop will change according to the droop characteristic until the virtual reactive power Q = 0, at this time U v = (U a + U b + U c ). As can be seen from the above, the phase-locked loop of the present invention can still effectively track the grid voltage phase angle under unbalanced voltage conditions.

[0109] The present invention constructs a new type of phase-locked loop based on virtual power control by using the negative feedback characteristic of the droop control itself, without the need for the inverter to perform mode switching during grid connection under weak grids, and can weaken the influence of grid voltage harmonics and voltage imbalance during the grid connection process. Combining virtual power phase-locking with current-source type virtual synchronous machine technology, a new type of virtual synchronous machine control strategy is formed, and it can also provide a certain amount of damping and inertia for the inverter to imitate a real motor, making it have stronger stability under weak grids, and can also effectively track the grid voltage phase angle under unbalanced voltage conditions.

[0110] To verify the effectiveness of the present invention, in a specific application embodiment, the method of the present invention is used for phase-locking control experiments. Among them, the ideal three-phase AC source voltage is directly collected as the input and sent into the virtual power phase-locked loop of the present invention. The sampled phase voltage effective value is 220V and the frequency is 50Hz. The simulation waveforms after the virtual power phase-locked loop of the present invention starts are as Figure 3 shown. The waveforms of (a), (b), (c), and (d) respectively correspond to the sampled phase voltage U g , virtual active and virtual reactive powers, the output phase angle θ v of the virtual power phase-locked loop, and the simulation results of the difference ΔU between the virtual power phase-locked output voltage and the input voltage.

[0111] For Figure 3 Analysis of the simulation waveforms in [the relevant context] shows that at the starting moment, both the virtual active power and the virtual reactive power of the initial phase-locked loop are not zero. The virtual active power adjusts the driving phase-locked output frequency change according to the active power droop. The integral of the difference between the phase-locked loop angular frequency and the AC source angular frequency over time causes the change in the difference between the phase-locked loop phase angle and the sampled phase angle. The phase angle difference in turn affects the virtual active power, forming a negative feedback mechanism. After being controlled by the virtual power phase-locked loop of the present invention, the virtual active power is controlled to 0 under the action of the negative feedback mechanism of the virtual power droop, that is, at this time, the phase-locked output frequency and phase angle of the virtual power are the same as the frequency and phase angle of the sampled voltage. The negative feedback mechanism of the virtual reactive power is similar to that of the active power link, and the driving makes the output voltage amplitude of the virtual power phase-locked loop approach. It can be seen from the simulation waveforms that the virtual active power and reactive power are controlled to 0 at about 12 milliseconds, and the difference between the sampled input voltage and the phase-locked loop output voltage also becomes 0 at this moment, indicating that the virtual power phase-locked loop synchronization is completed. The above also verifies that the virtual power phase-locked loop of the present invention can accurately complete the phase-locking control.

[0112] The above is only the preferred embodiment of the present invention and does not impose any form of limitation on the present invention. Although the present invention has been disclosed above with the preferred embodiment, it is not intended to limit the present invention. Therefore, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the technical solution of the present invention shall fall within the scope of the protection of the technical solution of the present invention.

Claims

1. A virtual power synchronization phase-locked loop for a weak grid-connected inverter, characterized in that, Comprising: A virtual impedance between the output terminal of the inverter and the power grid; A virtual power calculation module for calculating the virtual power generated by the virtual impedance; An active power calculation branch for performing droop control based on the active part of the calculated virtual power to obtain an angular frequency deviation, obtaining a virtual angular frequency based on the angular frequency deviation, and obtaining a virtual phase-locked angle based on the virtual angular frequency; A reactive power calculation branch for performing droop control based on the reactive part of the calculated virtual power to obtain a voltage deviation, and obtaining a virtual voltage amplitude based on the voltage deviation; A virtual voltage calculation module for calculating a virtual voltage according to the virtual phase-locked angle and the virtual voltage amplitude.

2. The virtual power synchronization phase-locked loop for the weak grid-connected inverter according to claim 1, characterized in that, The active power calculation branch includes a first adder unit, a first droop control unit, and an angle conversion unit connected in sequence. The first adder unit inputs a power reference value and the active part of the virtual power respectively, and outputs the difference between the active part of the virtual power and the power reference value to the first droop control unit. The first droop control unit obtains the angular frequency deviation after passing through a first droop control coefficient m, and outputs it to the second adder. The second adder adds the angular frequency deviation and the rated angular frequency to obtain the virtual angular frequency, and outputs it to the angle conversion unit. The angle conversion unit obtains the virtual phase-locked angle according to the relationship between the phase-locked angle and the angular frequency.

3. The virtual power synchronization phase-locked loop for the weak grid-connected inverter according to claim 1, characterized in that, The reactive power calculation branch includes a third adder unit, a second droop control unit, and a fourth adder unit. The third adder unit inputs a power reference value and the reactive part of the virtual power respectively, and outputs the difference between the reactive part of the virtual power and the power reference value to the second droop control unit. The second droop control unit obtains a voltage deviation after passing through a second droop control coefficient n, and outputs it to the fourth adder. The fourth adder adds the voltage deviation and the rated voltage to obtain the virtual voltage amplitude.

4. The virtual power synchronization phase-locked loop for a weak grid-connected inverter according to claim 2 or 3, characterized in that The power reference value is 0.

5. The virtual power synchronization phase-locked loop for the weak grid-connected inverter according to claim 1 or 2 or 3, characterized in that, The virtual power calculation module calculates the virtual power generated by the virtual impedance according to the following formula: Among them, P v and Q v are the active part and the reactive part of the virtual power respectively, ΔU d and ΔU q are the voltage differences in the dq rotating coordinate system respectively, I dv and I qv are the currents generated in the dq rotating coordinate system when the voltage difference acts on the virtual impedance after introducing the virtual impedance respectively.

6. A virtual power synchronization phase-locked control method for a weak grid-connected inverter, characterized in that the steps Comprising: Introducing a virtual impedance between the output terminal of the inverter and the power grid; Calculating the virtual power generated by the virtual impedance; Performing droop control based on the active part of the calculated virtual power to obtain an angular frequency deviation, obtaining a virtual angular frequency based on the angular frequency deviation, and obtaining a virtual phase-locked angle based on the virtual angular frequency; Performing droop control based on the reactive part of the calculated virtual power to obtain a voltage deviation, and obtaining a virtual voltage amplitude based on the voltage deviation; Calculating a virtual voltage according to the virtual phase-locked angle and the virtual voltage amplitude.

7. The virtual power synchronization phase-locked control method for a weak grid-connected inverter according to claim 6, characterized in that The performing droop control based on the active part of the calculated virtual power to obtain an angular frequency deviation, obtaining a virtual angular frequency based on the angular frequency deviation, and generating a virtual phase-locked angle based on the virtual angular frequency includes: Calculating the difference between the active part of the virtual power and the power reference value; Obtaining an angular frequency deviation by passing the difference between the active part of the virtual power and the power reference value through a first droop control coefficient m; Adding the angular frequency deviation and the rated angular frequency to obtain the virtual angular frequency; The virtual phase-locked angle is obtained according to the virtual angular frequency and the relationship between the phase-locked angle and the angular frequency.

8. The virtual power synchronization phase-locked control method for a weak grid-connected inverter according to claim 6, characterized in that, The voltage deviation is obtained by performing droop control according to the reactive part of the calculated virtual power, and the virtual voltage amplitude is obtained according to the voltage deviation, including: Calculating the difference between the reactive part of the virtual power and the power reference value; Obtaining the voltage deviation by multiplying the difference between the reactive part of the virtual power and the power reference value by the second droop control coefficient n; Adding the voltage deviation to the rated voltage to obtain the virtual voltage amplitude.

9. The virtual power synchronization phase-locked control method for a weak grid-connected inverter according to claim 6 or 7 or 8, characterized in that, The virtual power generated by the virtual impedance is calculated according to the following formula: Among them, P v and Q v are the active part and reactive part of the virtual power respectively, ΔU d and ΔU q are the voltage differences in the dq rotating coordinate system respectively, and I dv and I qv are the currents generated in the dq rotating coordinate system when the voltage difference acts on the virtual impedance after introducing the virtual impedance respectively.

10. The virtual power synchronization phase-locked control method for a weak grid-connected inverter according to claim 9, characterized in that Voltage difference ΔU in the dq rotating coordinate system d 、ΔU q and current I generated in the dq rotating coordinate system dv 、I qv are calculated respectively according to the following formulas: Among them, Δu a , Δu b , Δu c are the three-phase voltages across the virtual impedance obtained from the terminal voltages of the virtual impedance respectively, U g is the grid-side voltage, ω g is the grid-side angular frequency, θ g is the grid-side phase angle, U v is the output voltage of the phase-locked loop, θ v is the virtual phase-locked angle.

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