Asymmetric fault ride-through control method and system based on three-phase four-leg virtual synchronous generator

Abstract

The invention relates to an asymmetric fault ride-through control method and system based on a three-phase four-leg virtual synchronous generator, and belongs to the technical field of power electronics and new energy power generation. The invention provides a solution for solving the problems of asymmetric output current, transient overcurrent, weakening of system supporting capability and the like under the asymmetric fault of a power grid in the prior art. The method comprises the following steps: detecting an asymmetric fault of a power grid; freezing a reactive power ring and retaining an active power ring when a fault occurs; determining a voltage drop coefficient and a phase jump angle according to the fault type; directly calculating a three-phase asymmetric excitation electromotive force reference value based on a system phasor geometrical relationship and a current amplitude limiting constraint; and giving the reference value to a voltage loop, and controlling the output of the inverter.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of power electronics and new energy power generation, and relates to an asymmetric fault ride-through control method and system based on a three-phase four-bridge-arm virtual synchronous generator. BACKGROUND

[0002] With the transformation of energy structure, a large number of new energy is concentratedly connected to the power grid. This trend promotes green development while also brings significant problems of system damping and inertia loss, leading to severe challenges to the voltage and frequency regulation capability of the power grid. In this context, the virtual synchronous generator technology emerges as the times require. The grid-connected inverter based on the virtual synchronous generator can simulate the operating characteristics of the traditional synchronous generator and has the ability to provide active support for system voltage and frequency, so it has been increasingly widely applied.

[0003] However, when the power grid experiences asymmetric faults, the virtual synchronous generator also faces problems such as output overcurrent and asymmetric output current. These problems not only cause a decline in power quality, but more seriously, may trigger the action of overcurrent protection devices, causing the virtual synchronous generator to be disconnected from the power grid and run, thereby exacerbating the operating risk of the system. Therefore, during power grid faults, a key technical requirement is proposed for the virtual synchronous generator: it must be able to limit the amplitude of the output current and ensure that the output current remains three-phase symmetric to maintain the stability of its grid-connected operation.

[0004] Currently, the research on current limiting strategies for virtual synchronous generators under asymmetric faults is still insufficient. In response to symmetric faults, the common method to suppress output overcurrent is to directly limit the reference value of the current loop. However, the disadvantage of this method is that once the current loop reaches saturation, it will force the virtual synchronous generator to switch from voltage source mode to current source mode, thereby losing its valuable inertia and damping support capability, which is contrary to the original intention of introducing the virtual synchronous generator. In the existing technology, some documents propose to dynamically compensate the power angle output of the active loop and adjust the excitation amplitude reference value by flexibly changing the control structure and parameters of the virtual synchronous generator to achieve the purpose of suppressing short-circuit overcurrent.

[0005] Specifically, for control strategies under asymmetric faults, some documents propose to use adaptive calculation to inject virtual impedance to achieve fault current limiting. However, this method essentially changes the equivalent output impedance of the inverter, although it maintains its voltage source characteristics to some extent, it weakens the support capability for the power grid. Another document adopts positive and negative sequence decoupling control strategy to make the output current three-phase symmetric during faults. This method avoids the fault detection process required for control strategy switching, but since the excitation component is composed of positive and negative sequence components, the transient current characteristics become complex and difficult to accurately analyze, which may instead introduce new uncertainties and overcurrent risks in the transient process.

[0006] In summary, the prior art has inherent defects such as uncontrollable transient characteristics, weakened system support capability or complicated control strategy when dealing with the problem of asymmetric fault ride-through of a virtual synchronous generator. Therefore, there is an urgent need for a new control method that can ensure the symmetry and amplitude limiting of the output current of the virtual synchronous generator while maintaining the voltage source characteristics and providing grid support during asymmetric grid faults, and has excellent transient performance. SUMMARY

[0007] Therefore, the purpose of the present application is to provide an asymmetric fault ride-through control method and system based on a three-phase four-bridge-arm virtual synchronous generator.

[0008] To achieve the above purpose, the present application provides the following technical solutions:

[0009] An asymmetric fault ride-through control method based on a three-phase four-bridge-arm virtual synchronous generator, the method comprising the following steps:

[0010] Detecting whether an asymmetric fault occurs in the grid;

[0011] When detecting that an asymmetric fault occurs in the grid, freezing the reactive power loop of the virtual synchronous generator VSG and retaining the active power loop;

[0012] Determining a voltage drop coefficient k and a phase jump angle γ according to the fault type;

[0013] Based on the geometric relationship of system phasors, combined with current amplitude limiting constraints, calculating the three-phase asymmetric excitation electromotive force reference value of the VSG;

[0014] Directly giving the three-phase asymmetric excitation electromotive force reference value to the voltage loop of the VSG to control the VSG to output three-phase symmetric and amplitude limited current.

[0015] Further, the calculation of the three-phase asymmetric excitation electromotive force reference value based on the geometric relationship of system phasors specifically includes:

[0016] To eliminate the neutral point displacement, according to the fault type, the voltage drop coefficient k, the phase jump angle γ and the phasor relationship between the grid voltage and the line impedance before and after the fault, the amplitude and phase reference value of the excitation electromotive force of each phase are solved by geometric theorem.

[0017] Further, when the grid only has A-phase voltage amplitude drop, the three-phase asymmetric excitation electromotive force reference value is calculated by the following formula:

[0018] The A-phase excitation amplitude reference value E a ′ satisfies:

[0019]

[0020] A-phase power angle reference value δ a ' satisfies:

[0021]

[0022] B-phase and C-phase excitation amplitude reference values are E, and power angle reference values are and

[0023] wherein, U g is a rated amplitude of grid voltage, IZ is an impedance voltage drop amplitude, Φ is a phase difference between the impedance voltage drop and the grid voltage, and δ is a real-time output power angle of the active power loop.

[0024] Further, when the grid has a B-phase and C-phase short circuit accompanied by amplitude drop and phase jump, the three-phase asymmetric excitation electromotive force reference value is calculated by the following formula:

[0025] B-phase excitation amplitude reference value E' b satisfies:

[0026]

[0027] C-phase excitation amplitude reference value E c satisfies:

[0028]

[0029] B-phase power angle reference value δ b satisfies:

[0030] C-phase power angle reference value δ c satisfies:

[0031] A-phase excitation amplitude reference value is E, and the power angle reference value is δ;

[0032] wherein, U g is a rated amplitude of grid voltage, IZ is an impedance voltage drop amplitude, Φ is a phase difference between the impedance voltage drop and the grid voltage, δ is a real-time output power angle of the active power loop, k is a voltage drop coefficient, γ is a phase jump angle, X' b and X' c are phase compensation quantities determined by the geometric relationship of the phasor.

[0033] Further, the excitation amplitude reference value E is determined by the following formula:

[0034]

[0035] wherein, I NZ is the amplitude of the pre-fault line impedance voltage drop, which is calculated by substituting the pre-fault electrical quantities into the geometric relationship of the phasor.

[0036] Further, the voltage loop adopts a Proportional Resonant (PR) controller, whose transfer function is:

[0037]

[0038] wherein K p is the proportional coefficient, K r is the resonant gain, ω c is the cut-off frequency, and ω0 is the fundamental angular frequency of the power grid.

[0039] A system for implementing the asymmetric fault ride-through control method, the system comprising:

[0040] a three-phase four-leg inverter main circuit for connecting the DC side and the power grid;

[0041] a fault detection module for monitoring the power grid voltage in real time and determining the type of asymmetric fault;

[0042] a control module for executing the frozen reactive power loop, the reserved active power loop, calculating the three-phase asymmetric excitation electromotive force reference value, and generating a modulation signal;

[0043] a space vector modulation module for driving the three-phase four-leg inverter according to the modulation signal.

[0044] Further, the three-phase four-leg inverter main circuit comprises a filter inductance L, a filter capacitance C, a line impedance L g and r, and a fourth leg for providing a zero-sequence current path to eliminate neutral point displacement.

[0045] Further, the control module comprises an active power loop and a reactive power loop, and the active power loop is described by the following equation:

[0046]

[0047] wherein J is the virtual moment of inertia, D is the damping coefficient, P ref is the active power reference value, P e is the actual active power, ω is the VSG output angular frequency, and ω n is the rated angular frequency of the power grid.

[0048] Further, the system further comprises a hardware-in-the-loop experimental platform for verifying the effectiveness of the asymmetric fault ride-through control strategy, the platform comprising a real-time simulator and a digital signal processor (DSP), wherein the VSG main circuit runs in the real-time simulator and the control strategy is executed in the DSP.

[0049] The present application has the following advantages:

[0050] (1) The present application fundamentally avoids the problem of uncontrollable transient process caused by feedback regulation uncertainty in traditional positive and negative sequence decoupling control strategy by freezing the reactive power loop and directly giving the asymmetric excitation reference value calculated accurately. This method can significantly suppress the amplitude overshoot of output current at the moment of fault occurrence, ensure that the current is stably limited within the safety threshold from the initial moment of fault, greatly reduce the risk of triggering protection device due to transient overcurrent, and thus ensure that the virtual synchronous generator can safely and continuously operate in parallel during the fault, thereby enhancing the transient stability of the entire power system.

[0051] (2) The present application generates zero sequence and negative sequence excitation electromotive force required to offset the asymmetric component of the power grid based on the three-phase four-bridge topology and the geometric relationship of phasor. This method can accurately eliminate the neutral point potential offset, ensuring that the three-phase output current of the virtual synchronous generator remains highly symmetrical under various asymmetric faults. This not only effectively improves the power quality during the fault, but also avoids potential damage to the inverter itself and power grid equipment caused by asymmetric current.

[0052] (3) Compared with the scheme of introducing virtual impedance, the control strategy of the present application does not need to change the equivalent output impedance of the inverter. The virtual synchronous generator always works in voltage source mode during the entire fault ride-through process, and its inherent inertia and damping characteristics are fully retained. This means that the present application solves the overcurrent and asymmetry problem without sacrificing the core support of the virtual synchronous generator to the grid voltage and frequency, which is crucial for maintaining the stable operation of the power grid after the fault.

[0053] (4) The present application discards the complex positive and negative sequence decoupling calculation process and generates excitation reference by direct giving without sequence. This simple control structure reduces the dependence on the computing power of the processor and improves the real-time performance of the system. At the same time, since the dynamic adjustment process of the double closed loop is omitted, the response speed of the system is accelerated, and the output current can reach the new stable state faster, shortening the duration of the fault impact.

[0054] (5) The method principle proposed in the application is clear, and the phase quantity geometric relation model relied on is solid and reliable. Through the hardware-in-the-loop experiment verification, the strategy shows good reliability and practicability in actual operation, and provides strong technical support for the safe and stable operation of the virtual synchronous generator in the asymmetric fault scene of the real power grid.

[0055] Other advantages, objects, and features of the application will be in part apparent and in part pointed out hereinafter in the specification, and will be elucidated with reference to the accompanying drawings and claims. BRIEF DESCRIPTION OF DRAWINGS

[0056] In order to make the objects, technical solutions and advantages of the application clearer, the preferred detailed description of the application will be made below in combination with the drawings, in which:

[0057] Figure 1 is a three-phase four-bridge-arm VSG system diagram; (a) is a three-phase four-bridge-arm inverter main circuit topology; (b) is a traditional asymmetric fault VSG control block diagram based on positive and negative sequence separation;

[0058] Figure 2 is a phase quantity relation diagram of the system before and after the fault;

[0059] Figure 3 is a phase quantity relation diagram of the system when reaching a steady state after the fault;

[0060] Figure 4 is a phase quantity diagram for realizing three-phase symmetry of the output current based on the phase quantity geometric relation when only the amplitude drops;

[0061] Figure 5 is a phase quantity diagram for realizing three-phase symmetry of the output current based on the phase quantity geometric relation when both the amplitude drops and the phase jumps;

[0062] Figure 6 is an asymmetric fault ride-through control strategy block diagram;

[0063] Figure 7 is a StarSim hardware-in-the-loop experiment platform;

[0064] Figure 8 is a traditional control strategy based on positive and negative sequence separation;

[0065] Figure 9 is a control strategy for directly giving an asymmetric excitation reference; (a) is an output current; (b) is an output excitation electromotive force. DETAILED DESCRIPTION

[0066] The present application is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements, and in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Wherein, the figures are only used for example illustration, the representation is only schematic diagram, not real object figure, cannot be understood as the limitation to the present application; in order to better illustrate the embodiment of the present application, some components of the figure can be omitted, enlarged or reduced, and does not represent the size of actual product; for the person skilled in the art, some well-known structures and their description in the figure can be omitted.

[0068] The same or similar reference numerals in the figures of the embodiments of the present application correspond to the same or similar components; in the description of the present application, it needs to be understood that if there are terms such as 'upper', 'lower', 'left', 'right', 'front', 'back' and the like indicating the orientation or positional relationship, it is based on the orientation or positional relationship shown in the figure, only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore the terms describing the positional relationship in the figure are only used for example illustration, and cannot be understood as the limitation to the present application, for the person skilled in the art, the specific meaning of the above terms can be understood according to the specific situation.

[0069] 1 Transient characteristics analysis of VSG under asymmetric fault

[0070] 1.1 Three-phase four-leg VSG system description

[0071] Figure 1 It is a three-phase four-leg VSG system diagram; (a) is a typical three-phase four-leg VSG system topology diagram; (b) is a traditional asymmetric fault VSG control block diagram based on positive and negative sequence separation; wherein abc It is the grid-connected current of the inverter; L g And r constitute the line impedance, L, C are filter inductance and filter capacitance respectively; E abc And U gabc Represent the amplitude of VSG output three-phase excitation electromotive force and grid voltage respectively, i cabc It is three-phase capacitor current, u dc It is the DC side voltage.

[0072] Compared to a three-phase, three-arm topology, the additional fourth arm provides an extra path for the zero-sequence component, offering control freedom to eliminate voltage deviation between the inverter neutral point N' and the grid neutral point N. Therefore, it is widely used in asymmetrical operating conditions. Applying Kirchhoff's current law to N', the outflow current i of the fourth arm can be obtained. n for:

[0073] i n =-(i ca +i cb +i cc )=-(E a +E b +E c )ωC (1)

[0074] The neutral point offset potential difference can be expressed by Kirchhoff's voltage law as follows:

[0075]

[0076] When an asymmetrical fault occurs in the power grid, the three-phase grid voltage will become unbalanced. In order to offset the neutral point by a potential difference U... N′N Since the current is 0, it is not difficult to see from equations (1) and (2) that the outflow current i of the fourth bridge arm is 0 at this time. n The value will not be zero, meaning there is a zero-sequence current.

[0077] Figure 1 In the above, (b) is the traditional control strategy of VSG based on positive and negative sequence separation under the asymmetrical power grid condition. It achieves three-phase balance of output current by directly setting the inner loop negative sequence current reference value to zero. The power outer loop of VSG is composed of equations (3) and (4).

[0078]

[0079] In the formula: E ref It is the reference amplitude of the reactive power loop output excitation, D q U is the reactive power droop factor, and k1 is the integral coefficient of the reactive power loop; n U and Q are the reference terminal voltage and root mean square voltage of the VSG, respectively; ref With Q e These are the reference value and actual value of reactive power for VSG, respectively. P ref P is the active power reference value. e ω represents the actual active power output of the inverter; J and D are the virtual moment of inertia and damping coefficient of the system, respectively; n This is the reference angular frequency for the power grid.

[0080] 1.2 Output Current Response Characteristics

[0081] Taking a single-phase voltage drop in the power grid as an example, the inverter output current will consist of both transient and steady-state components. From Kirchhoff's voltage law, we can obtain differential equation (5), and after solving it, we can obtain the numerical solution of the inverter output current for each phase.

[0082]

[0083] Where x represents the three phases a, b, and c, and δ is the phase difference between the excitation electromotive force and the grid voltage, which is also the output power angle of the VSG.

[0084] To more intuitively analyze the inverter output excitation E x =E∠δ and grid voltage U gx =U g The effect of the potential difference between ∠0 on the output current amplitude I is plotted according to equation (5) as follows. Figure 2 The phasor diagram shown is illustrated. Here, k is the voltage sag factor, Z is the line impedance, and IZ represents the line impedance voltage drop phasor. The solid and dashed lines represent the phasors before and after the voltage sag in the power grid, respectively.

[0085] according to Figure 2 It can be seen that if the inverter's output magnetomotive force cannot respond in time when the grid voltage drops, the neutral point between the inverter and the grid will shift, i.e., U N′N It is not zero. At the same time, the amplitude and phase of the inverter output current are asymmetrical, which leads to an abnormal increase in the amplitude of the fault phase current, resulting in the risk of overcurrent.

[0086] To achieve three-phase symmetry in the output current, traditional control strategies divide the voltage and current inner loop into positive-sequence and negative-sequence controls. By setting the negative-sequence current reference to zero, the VSG needs to generate a negative-sequence excitation electromotive force to offset the negative-sequence grid voltage, which is indirectly generated through feedback regulation. This obviously introduces significant uncertainty into the negative-sequence excitation phasor during the transient regulation process before reaching steady state, and also makes the actual excitation phasor synthesized from the positive and negative-sequence components uncertain. This makes the dynamic characteristics of the output current during the transient process difficult to analyze, while also increasing the risk of overcurrent.

[0087] Assuming the phasor diagram of the system reaches steady state after a voltage dip in the grid is as follows: Figure 3 As shown, the superscripts + and - represent the positive and negative sequence components, respectively. The inverter output positive and negative sequence excitation phasors are represented by orange dashed lines and dark blue dashed lines, respectively, while the dark red solid line is the actual output excitation phasor synthesized from the positive and negative sequence excitation components.

[0088] During transient regulation before reaching steady state, due to the uncertainty of feedback regulation, the negative-sequence excitation phasor may exhibit a situation where its amplitude remains relatively unchanged compared to the steady state, but its phase shifts, i.e., it changes from a dark blue phasor to a light blue phasor, such as... Figure 3As shown in phase C, even if the positive-sequence excitation phasor remains at its steady-state value, the actual output excitation phasor resulting from the combined positive and negative-sequence excitations will change significantly. This will cause the line impedance voltage drop to change from a shorter, dark purple phasor to a longer, light purple phasor, indicating a transient overcurrent during the adjustment process. Furthermore, whether the system reaches steady state depends on whether the dynamic adjustment processes of both the positive and negative-sequence phasors have been completed. Due to the asymmetry of positive and negative-sequence control, the adjustment time to reach steady state may be prolonged.

[0089] Therefore, the traditional asymmetric control strategy based on positive and negative sequence separation will lead to increased current overshoot and settling time during transient periods, resulting in further deterioration of the current during transient periods.

[0090] 2. Fault Ride Control Method Based on Directly Given Asymmetric Excitation Reference

[0091] 2.1 Calculation method for given asymmetric excitation reference

[0092] Based on the preceding analysis, to achieve three-phase symmetry of the output current during asymmetrical grid faults, and to reduce the overshoot and settling time of the system output current during transient periods, it is necessary not only to offset the negative-sequence component of the asymmetrical grid voltage, but also to ensure that the total excitation synthesized from the positive-sequence and negative-sequence components during transient periods does not cause output overcurrent. Therefore, unlike traditional asymmetrical fault ride-through control methods that perform sequential control of the output excitation, this section, based on a three-phase four-arm inverter topology, proposes freezing the reactive power loop during asymmetrical grid faults. Utilizing the geometric relationships between system phasors, the excitation reference amplitude E is calculated through system constraints, while the power angle δ is still regulated by the active power loop output. By directly providing the required asymmetrical excitation reference value to the VSG in a non-sequence manner, three-phase symmetry of the output current and overcurrent limitation under fault conditions are achieved.

[0093] 1) The following example, taking the voltage amplitude drop of phase A of the power grid by k times, will be used to explain how to calculate the reference value of three-phase unbalanced excitation when only the voltage amplitude drops.

[0094] like Figure 4 The figure shows the transient phasor regulation process of the proposed method, where the three-phase currents are symmetrical and do not exceed the overcurrent limit, and the three-phase line voltage drop amplitude I... b Z = I c Z = IZ. Since the zero-sequence excitation component can cancel out the zero-sequence component of the grid voltage, the neutral point potential in the diagram does not shift, meaning N and N' coincide. And since the grid voltage phasors of phases B and C remain unchanged, it is easy to see that the excitation phasors of phases B and C are also symmetrical. Because the three-phase grid voltage phases are symmetrical, from... Figure 4 It is not difficult to conclude that the phase difference between the impedance voltage drop of the three-phase line and the power grid is Ф'a=Фb=Фc=Ф.

[0095] likeFigure 4 As shown in the figure, taking the deep red excitation quantity E of phase B and C as an example, in the triangle composed of the excitation electromotive force, the line impedance voltage drop and the grid voltage, the following can be obtained by the cosine theorem:

[0096]

[0097] Similarly, the amplitude of the line voltage drop of phase A at this time is:

[0098]

[0099] The phase difference between the excitation amplitude of phase A and the line voltage drop is:

[0100]

[0101] Substituting formula (6) and (7) into formula (9), the given excitation amplitude E' of phase A after the fault can be obtained: a

[0102]

[0103] Substituting the obtained E' into formula (8), the power angle of phase A after the fault can be obtained: a

[0104]

[0105] Therefore, the three-phase excitation reference value is:

[0106]

[0107] Since the reactive power ring is frozen in the method of the present application, the excitation amplitude E in formula (12) is to be determined, and the power angle δ is the real-time output of the active power ring.

[0108] 2) The following takes the short circuit of grid phases B and C as an example, where k is the amplitude drop coefficient and γ is the phase jump angle, to introduce how to calculate the three-phase asymmetric excitation reference value when there is both voltage amplitude drop and phase jump.

[0109] Taking the three-phase symmetry of the output current as the control target, combining the phasor Figure 5 It can be seen that the line impedance voltage drops IZ of each phase can be equal and the phase angle difference Φ of each phase with the grid-side voltage phasor before the fault can be equal by changing the excitation electromotive force, that is, the three-phase symmetry of the output current can be realized.

[0110] Because only the phase and amplitude of the voltage phasor of phase A of the grid are unchanged during the fault, that is, gb / k = U gcC / k = U ga = U g ​​, then the neutral displacement can be eliminated by calculating the excitation phasor of B and C phases with the reference of A phase excitation.

[0111] Firstly, the neutral displacement caused by the asymmetric drop of grid voltage is combined, so that the reference value of each phase voltage calculated by the geometric relationship of electrical quantities contains the zero sequence component for eliminating the neutral displacement. Figure 5 The reference value of each phase voltage calculated by the geometric relationship of electrical quantities contains the zero sequence component for eliminating the neutral displacement. At this time, a triangle is formed by the excitation, line impedance voltage drop and grid voltage in the non-fault phase A, and according to the geometric relationship, IZ and Ф must satisfy the following formula respectively during the adjustment process:

[0112]

[0113] Similarly, let the line voltage drop amplitudes of B and C phases at this time be respectively:

[0114]

[0115] Let the phase difference between the excitation amplitudes of B and C phases and the line voltage drop be respectively:

[0116]

[0117] IZ=1.2I N Z=IZ b ′=IZ c Substitute E b ′、E c ′ into formula (13)-(18) to obtain the corrected amplitudes E Therefore, the three-phase excitation reference value without positive and negative sequence can be obtained as:

[0118]

[0119] Similarly, since the reactive power ring is frozen in the method of the present application, the excitation amplitude E in formula (19) is to be determined, and the power angle δ is the real-time output of the active power ring.

[0120] 3) Obtain the E amplitude involved in formula (12) and formula (19)

[0121] During the transient adjustment process, the power angle δ output by the active power ring in real time can be used, and the geometric relationship in E Figure 4 and Figure 5 is considered, so that the excitation amplitude E of the non-fault phase satisfies:

[0122]

[0123] Therefore, the set excitation reference amplitude E can be obtained as:

[0124]

[0125] Among them I N Z needs to be obtained by substituting the electrical quantities before the fault into equation (6) or equation (13).

[0126] 2.3 Complete Asymmetric Fault Ride Control Strategy

[0127] In summary, the present invention ultimately adopts the following... Figure 6 The fault ride-through control strategy is illustrated. First, the grid-side fault type, drop depth k, and phase jump angle γ are determined based on the grid connection point voltage information. When no grid fault is detected, the VSG operates under normal control. However, when a fault is detected, the reactive power loop is immediately frozen while the active power loop is retained. Taking into account VSG current limiting, three-phase symmetry, and the actual output power angle of the active power loop, the excitation reference value is calculated using the aforementioned formula.

[0128] Ultimately, the asymmetric fault ride-through control method proposed in this invention can eliminate the influence of zero and negative sequence components while retaining the characteristics of the VSG voltage source, and achieve three-phase symmetry of the output current and current limiting target.

[0129] Because the proposed strategy involves actively generating zero-sequence and negative-sequence magnetomotive force (EMF) components, the inverter adopts a three-phase four-arm topology that can provide a zero-sequence loop, and the modulation method corresponds to three-dimensional space vector modulation. The magnetomotive force containing the negative-sequence component, after Park transformation, yields d-axis and q-axis components containing second-harmonic fluctuations, and a 0-axis component containing power frequency fluctuations. Therefore, the voltage loop is controlled by a quasi-PR controller, expressed as:

[0130]

[0131] Where ω c K is the cutoff frequency. r ω is the resonant gain, and ω0 is the fundamental angular frequency of the power grid.

[0132] 4. Experimental Verification

[0133] To verify the effectiveness of the proposed control strategy, a semi-physical experimental platform was built based on StarSim, using the DSP TMS320F28377D as the control core. Figure 7 As shown. The analog signals from the simulator are converted in real-time to the digital signals in the DSP via a signal adapter board. This invention relates to... Figure 7 The VSG main circuit in the image is run in the StarSim HIL real-time emulator based on NI-PXIe-786R, such as... Figure 7 The fault-crossing control strategy shown is executed in the DSP control chip. The main experimental parameters involved are shown in Table 1.

[0134] Table 1 Experimental parameters

[0135] Parameter Parameter value Direct current bus voltage U dc ]]> 1000V Grid rated voltage amplitude U g ]] 311 Filtering capacitance C 35 μF Filtering inductance L 3 mH Moment of inertia J 4 Kg / m 2 ]] Rated active power P e ]]> 56 kW Rated output voltage E 220V

[0136] VSG just starts to work under rated conditions, and then the grid A-phase voltage drops to 0.5pu, and the VSG output current and voltage characteristics of the application of the traditional control strategy based on positive and negative sequence separation and the asymmetric fault ride-through control strategy are compared.

[0137] As shown in Figure 8 The traditional control strategy based on positive and negative sequence separation can make the negative sequence component contained in the output excitation electromotive force offset the negative sequence component in the grid voltage through feedback adjustment when the fault occurs, at which time it can be obviously seen that the three-phase amplitudes of the output excitation electromotive force are different, and the output current realizes three-phase symmetry during the steady state. In order to more obviously see the inherent defects of the control strategy based on positive and negative sequence separation, no limiter is added to the current loop, the maximum transient amplitude of the output current is 1.72pu, the steady-state amplitude is 1.38pu, the current overshoot duration during the fault transient state is about 25ms, and the current amplitude from the fault occurrence to the system reaching the steady state is about 90ms.

[0138] Figure 9 The control strategy for directly giving asymmetric excitation reference; (a) is the output current; (b) is the output excitation electromotive force. It can be obtained from Figure 9 When the control strategy for directly giving asymmetric excitation reference is used, the output excitation electromotive force amplitudes of the non-fault phase are consistent, and the fault phase excitation amplitude is smaller, the inverter can actively emit appropriate negative sequence excitation electromotive force, so that the output current realizes three-phase symmetry during the steady state. The output current does not appear amplitude overshoot during the transient state, and is always controlled within the amplitude limit, and the output current can be stabilized at the amplitude limit of 1.2pu. The current amplitude from the fault occurrence to the system steady state only takes 35ms, and the adjustment time is shorter.

[0139] In order to make the VSG have the ability of outputting three-phase current symmetry and limiting the output current amplitude to a certain extent when the grid has an asymmetric fault, the application proposes a VSG fault ride-through control strategy based on a three-phase four-bridge arm circuit topology. The application obtains the following conclusions through theoretical analysis and semi-physical experiments:

[0140] 1) In the asymmetric fault, the traditional control strategy based on positive and negative sequence separation makes the actual excitation reference value uncertain during the transient state, which will lead to the output current overshoot during the transient state and increase the adjustment time required for system stabilization.

[0141] 2) the proposed fault ride-through strategy, which freezes the reactive power loop during the fault, gives the excitation amplitude by the amplitude constraint condition, and then directly calculates the total excitation reference electromotive force containing the asymmetric component in a way of not distinguishing positive and negative sequences through the geometric relationship of phasor, so as to realize the symmetry of output current and suppress the transient overcurrent.

[0142] Finally, it should be pointed out that the above embodiments are only used to illustrate the technical solutions of the present application but not limit the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the purpose and scope of the present application, and all of them should be covered in the scope of the claims of the present application.

Claims

1. A method for asymmetric fault ride-through control based on three-phase four-leg virtual synchronous generator, characterized in that: The method comprises the following steps: Detecting whether an asymmetric fault occurs in the power grid; Freezing the reactive power loop of a virtual synchronous generator (VSG) and retaining the active power loop when detecting that an asymmetric fault occurs in the power grid; Determining a voltage drop coefficient k and a phase jump angle γ according to a fault type; Calculating a three-phase asymmetric excitation electromotive force reference value of the VSG based on a system phasor geometric relationship and in combination with a current amplitude limiting constraint; Directly giving the three-phase asymmetric excitation electromotive force reference value to a voltage loop of the VSG to control the VSG to output a three-phase symmetric and amplitude limited current.

2. The method for three-phase four-leg VSG based asymmetric fault ride-through control according to claim 1, characterized in that: The three-phase asymmetric excitation electromotive force reference value is calculated based on a system phasor geometric relationship, specifically comprising: With the goal of eliminating neutral point displacement, the amplitude and phase reference values of the excitation electromotive force of each phase are solved through geometric theorems according to the fault type, the voltage drop coefficient k, the phase jump angle γ, and the phasor relationship between the grid voltage and the line impedance before and after the fault.

3. The control method of three-phase four-leg VSG based asymmetric fault ride-through according to claim 2, wherein: When only the A-phase voltage amplitude drops in the power grid, the three-phase asymmetric excitation electromotive force reference value is calculated through the following formula: A-phase excitation amplitude reference value E a ′ satisfies: A phase power angle reference value δ a satisfies: The B-phase and C-phase excitation amplitude reference values are both E, and the power angle reference values are respectively and Where, U g is the rated amplitude of grid voltage, IZis the amplitude of line impedance voltage drop, Φ is the phase difference between line impedance voltage drop and grid voltage, and δ is the power angle output by the real-time output of active power loop.

4. The control method of asymmetric fault ride-through of three-phase four-leg VSG based on claim 2, characterized in that: When B and C-phase short circuits occur in the power grid and are accompanied by amplitude drop and phase jump, the three-phase asymmetric excitation electromotive force reference value is calculated through the following formula: B-phase magnet excitation amplitude reference value E' b satisfies: C-phase excitation amplitude reference value E c satisfied: B-phase power angle reference value δ b satisfies: C-phase power angle reference value δ c satisfied: The A-phase excitation amplitude reference value is E, and the power angle reference value is δ. Wherein, U g is the rated amplitude of grid voltage, IZ is the amplitude of line impedance voltage drop, Φ is the phase difference between line impedance voltage drop and grid voltage, δ is the power angle output by active power loop in real time, k is the voltage drop coefficient, γ is the phase jump angle, X′ b and X′ c are the phase compensation quantities determined by the geometric relationship of phasor.

5. The method of asymmetric fault ride-through control based on three-phase four-leg VSG according to claim 3 or 4, characterized in that: The excitation amplitude reference value E is determined through the following formula: where I N Z is the amplitude of the pre-fault line impedance voltage drop, which is calculated by substituting the pre-fault electrical quantities into the geometric phasor relationship.

6. The method for three-phase four-leg VSG based asymmetrical fault ride-through control according to claim 1, characterized in that: The voltage loop adopts a proportional resonant (PR) controller, and the transfer function thereof is: where K p is a proportionality factor, K r is the resonance gain, ω c is the cut-off frequency, and ω0is the grid fundamental angular frequency.

7. A system for implementing the asymmetric fault ride-through control method of any one of claims 1 to 6, characterized by: The system comprises: A three-phase four-bridge arm inverter main circuit for connecting a DC side and a power grid; A fault detection module for monitoring the power grid voltage in real time and judging the asymmetric fault type; A control module for executing the freezing of the reactive power loop, the retaining of the active power loop, the calculation of the three-phase asymmetric excitation electromotive force reference value, and the generation of a modulation signal; A space vector modulation module for driving the three-phase four-bridge arm inverter according to the modulation signal.

8. The system for asymmetric fault ride through control method of claim 7, wherein: The three-phase four-bridge-arm inverter main circuit comprises a filter inductor L, a filter capacitor C and a line impedance L g and r, and a fourth bridge arm for providing a zero sequence current path to eliminate neutral point displacement.

9. The system for asymmetric fault ride through control method of claim 7, wherein: The control module comprises an active power loop and a reactive power loop, and the active power loop is described by the following equation: where J is the virtual moment of inertia, D is the damping coefficient, P ref is the active power reference value, P e is the actual active power, ω is the VSG output angular frequency, ω n is the grid rated angular frequency.

10. The system for asymmetric fault ride through control method of claim 7, wherein: The system further comprises a hardware-in-the-loop experimental platform for verifying the effectiveness of the asymmetric fault ride-through control strategy, the platform comprising a real-time simulator and a digital signal processor (DSP), wherein the VSG main circuit is run in the real-time simulator, and the control strategy is executed in the DSP.

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