Virtual synchronous generator improved fault ride-through control method considering current amplitude limiting

Abstract

The invention discloses a virtual synchronous generator improved fault ride-through control method considering current amplitude limiting, and the method comprises the steps: building an equivalent model of a virtual synchronous generator considering a current amplitude limiting link, and enabling new energy power generation to be connected to a power grid through the virtual synchronous generator; merging the virtual synchronous generator into a power grid equivalent circuit, and drawing an isoline representation relationship between the power angle change and the virtual synchronous generator by using a vector model; and when the virtual synchronous generator is in constant voltage control or the droop coefficient is in a relatively small value, reactive voltage characteristics during voltage change are analyzed. According to the method, under the condition of grid voltage drop considering virtual synchronous generator current amplitude limiting, virtual impedance is input, a band-pass filter is added, the voltage supporting link of a system can be further improved, the fault ride-through capacity of the system is enhanced, the virtual impedance is removed after a fault is eliminated, and the fault ride-through capacity of the system is improved. And the output characteristics of the virtual synchronous generator in normal operation are not influenced.

Description

Technical Field

[0001] The invention relates to the field of virtual synchronous generators, and in particular discloses an improved fault ride-through control method for a virtual synchronous generator taking current limiting into account. Background Art

[0002] Nowadays, the field of wind power and photovoltaic power generation has ushered in large-scale development. Large-scale grid connection of wind, solar and storage depends on power electronic converters, so the study of flexible control mode of converters has become the current focus. Since the grid-following inverter has the characteristics of negative resistance in the low-frequency area, it is suitable for strong grid conditions (high short-circuit ratio). Under weak grid conditions (low short-circuit ratio), it is easy to couple with the grid and oscillate, which is not conducive to the stability of the grid. Therefore, the simulation of virtual synchronous generator control, that is, grid-building control, is now the mainstream discussion hotspot, because it can provide autonomously constructed AC voltage amplitude and frequency under weak grid conditions, or provide frequency and voltage support under grid disturbance and fault conditions.

[0003] Existing literature has studied adding virtual impedance to the grid-connected inverter to improve the stability of the system, or introducing a transient droop compensator to reduce the time the virtual synchronous generator power angle enters the transient period. At the same time, the introduction of virtual impedance changes the impedance characteristics of the power grid. Although it can limit the fault current when the power grid fails, the voltage drop of the current in the virtual impedance during normal operation will cause the PCC (Point of Common Coupling) point voltage to decrease, and the power output of the virtual synchronous generator will decrease.

[0004] Therefore, when the existing simulated virtual synchronous generator control is working normally, the voltage drop of the current in the virtual impedance will cause the PCC point voltage to decrease, and the power output of the virtual synchronous generator will decrease, which is a technical problem that needs to be solved urgently. Summary of the invention

[0005] The present invention provides an improved fault ride-through control method for a virtual synchronous generator taking current limiting into account, aiming to solve the technical problem that in the existing simulated virtual synchronous generator control, the voltage drop of the current in the virtual impedance during normal operation will cause the PCC point voltage to decrease, and the power output of the virtual synchronous generator will be reduced.

[0006] The present invention and an improved fault ride-through control method for a virtual synchronous generator taking current limiting into account include the following steps:

[0007] Establish an equivalent model of a virtual synchronous generator taking into account the current limiting link, and connect renewable energy generation to the power grid through the virtual synchronous generator;

[0008] The virtual synchronous generator is incorporated into the grid equivalent circuit, and the contour lines representing the relationship between the power angle change and the virtual synchronous generator are drawn using a vector model.

[0009] When the virtual synchronous generator is in constant voltage control or the droop factor is at a small value, the reactive voltage characteristics when the voltage changes are analyzed.

[0010] Furthermore, an equivalent model of a virtual synchronous generator taking into account the current limiting link is established. In the step of connecting the new energy power generation to the power grid through the virtual synchronous generator, the virtual synchronous generator includes active frequency control. In the active frequency control, the virtual synchronous generator includes reactive voltage control. Assuming that the voltage angle of the power grid is 0 and the reference voltage angle of the output is θ, in the reactive voltage control, the actual reactive power Q s With reference reactive Q ref , and the actual voltage U s With reference voltage U ref The integration of the difference weights generates the potential E inside the virtual synchronous generator.

[0011] Furthermore, an equivalent model of a virtual synchronous generator taking into account the current limiting link is established. In the step of connecting the renewable energy power generation to the power grid through the virtual synchronous generator, in the active frequency control, when the reactive power and voltage at the grid connection point are in a steady state, the following conditions are satisfied:

[0012] K q (Q ref -Q s )+K u (U ref -U s )=0

[0013] Among them, K q is the reactive power regulation coefficient; Q ref is the reference reactive power of the grid-connected point; Q s is the actual reactive power at the grid-connected point; K u is the voltage regulation coefficient; U ref is the reference voltage; U s is the actual voltage; when K q = 0 is constant voltage amplitude control; when K u =0 is constant reactive power control; when K q ≠0, K u When ≠0, it is reactive power droop control.

[0014] Furthermore, in reactive power droop control, the droop coefficient is:

[0015]

[0016] Among them, K qu is the droop coefficient, K u is the voltage regulation coefficient; K q is the reactive power regulation coefficient.

[0017] Furthermore, an equivalent model of a virtual synchronous generator taking into account the current limiting link is established. In the step of connecting renewable energy power generation to the power grid through the virtual synchronous generator, the virtual synchronous generator includes voltage and current loop control. In the voltage and current loop control, the current limiting link is implemented with an equivalent virtual admittance. According to the singular perturbation principle, the internal potential dynamics of the virtual synchronous generator and the output voltage and current reach a steady-state relationship.

[0018] Furthermore, an equivalent model of a virtual synchronous generator taking into account the current limiting link is established. In the step of connecting the renewable energy power generation to the power grid through the virtual synchronous generator, on the basis of current limiting, the equivalent circuit equation containing the virtual synchronous generator is:

[0019] I s =(EU g ) / (kZ v +Z g )

[0020] Among them, I s is the output current of the virtual synchronous generator; E is the internal potential amplitude; U g is the grid voltage; k is the adaptive parameter of the virtual impedance when the current reaches the limit value when the current controller adopts equal limit of dq axis; Z v The output impedance value of the virtual synchronous generator when the current does not reach the limit value; Z g is the grid impedance.

[0021] Furthermore, an equivalent model of a virtual synchronous generator taking into account the current limiting link is established. In the step of connecting renewable energy power generation to the power grid through a virtual synchronous generator, it is assumed that when the sensor equipment detects grid voltage fluctuations, the control system invests in virtual impedance and bandpass filter in the reactive power control link. The bandpass filter mainly filters out harmonics and is not included in the calculation. The equivalent circuit equation after only considering the virtual impedance investment is:

[0022] I s '=(EU g -Z LV I s ) / (kZ v +Z g )

[0023] Among them, I s ' is the output current of the virtual synchronous generator after the virtual impedance is put into use; E is the internal potential amplitude; U g is the grid voltage; Z LV The value of the virtual impedance when the power grid fails; I s is the output current of the virtual synchronous generator; k is the adaptive parameter of the virtual impedance when the current controller adopts equal limiting of dq axes and the current reaches the limiting value; Z vThe output impedance value of the virtual synchronous generator when the current does not reach the limit value; Z g is the grid impedance.

[0024] Furthermore, in the step of connecting the virtual synchronous generator to the grid equivalent circuit and using the vector model to draw contour lines representing the relationship between the power angle change and the virtual synchronous generator, it is assumed that the voltage amplitude at the grid connection point is U s The coordinates of the end point C are (x U ,y U ), δ U For U s , U g The angle between them, U s The amplitude expression is:

[0025]

[0026] Among them, x U is the voltage amplitude at the grid connection point U s Projection value on the horizontal axis; U g is the grid voltage; y U is the voltage amplitude at the grid connection point U s Projection value on the ordinate; X g is the grid impedance; Q s is the reactive voltage at the grid connection point;

[0027] If the voltage adopts constant reactive power control, assuming X g , U g is a constant value, then the reactive power contour line C2 is is the center of the circle, A circle with radius U s The end of the always remains on the reactive power contour line C2, satisfying Q s =Q ref .

[0028] Furthermore, in the step of incorporating the virtual synchronous generator into the grid equivalent circuit and using the vector model to draw contour lines representing the relationship between the power angle change and the virtual synchronous generator, if the bandpass filter and the virtual impedance are put into use, and the bandpass filter is mainly used to filter out harmonics and is not included in the calculation, and only the constant reactive power control is used when the virtual impedance is put into use, U s The amplitude expression is:

[0029]

[0030] Among them, x U is the voltage amplitude at the grid connection point U s Projection value on the horizontal axis; U g is the grid voltage; y U is the voltage amplitude at the grid connection point U sProjection value on the ordinate; X g is the grid impedance; X LV is the input virtual impedance value; Q s is the reactive voltage at the grid connection point;

[0031] It can be seen that the radius of reactive contour line C2 after the virtual impedance is put into use is The radius of reactive power equivalent C2 is enlarged, U s The end is kept at a radius of On the circle.

[0032] Further, in the step of analyzing reactive voltage characteristics when voltage changes when the virtual synchronous generator is in constant voltage control or the droop coefficient is at a small value,

[0033] If the voltage drops sharply, the current reaches the limit value. On the basis of the active reference value limit, the virtual impedance and bandpass filter of the reactive voltage control link are added to further limit the current amplitude. The voltage amplitude U s ' s , voltage fluctuation amplitude |G'F'|<|GF|;

[0034] When the grid voltage changes slightly, the virtual synchronous generator current reaches the limit value. If the virtual impedance and bandpass filter are put into the reactive voltage control link, although the current amplitude is further limited, the radius of the reactive contour is expanded, and the current limit loop and the Qs-U curve obtain a new intersection point U s '>U s Among them, U s ' is the voltage value of the grid connection point taking into account the virtual impedance; U s is the voltage value of the grid-connected point without taking into account the virtual impedance; G'F' is the maximum range of voltage fluctuation of the grid-connected point taking into account the virtual resistance; GF is the maximum range of voltage fluctuation of the grid-connected point without taking into account the virtual resistance.

[0035] The beneficial effects achieved by the present invention are:

[0036] ​The present invention provides an improved fault ride-through control method for a virtual synchronous generator taking into account current limiting. By establishing an equivalent model of a virtual synchronous generator taking into account the current limiting link, the new energy generation is connected to the power grid through the virtual synchronous generator; the virtual synchronous generator is incorporated into the power grid equivalent circuit, and the contour line representation relationship between the power angle change and the virtual synchronous generator is drawn using a vector model; when the virtual synchronous generator is in constant voltage control or the droop coefficient is at a small value, the reactive voltage characteristics when the voltage changes are analyzed. The improved fault ride-through control method for a virtual synchronous generator taking into account current limiting provided by the present invention studies that under the condition of a power grid voltage drop taking into account the current limiting of the virtual synchronous generator, a virtual impedance is put into operation, and a bandpass filter is added, which can further improve the voltage support link of the system, enhance the system fault ride-through capability, and cut off the virtual impedance after the fault is eliminated, without affecting the output characteristics of the virtual synchronous generator when it is working normally. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a flow chart of an improved fault ride-through control method of a virtual synchronous generator taking current limiting into account according to the present invention;

[0038] Figure 2 This is a typical virtual synchronous generator control block diagram of the present invention;

[0039] Figure 3 It is an equivalent circuit diagram of virtual synchronization and grid connection under the current limiting condition of the present invention;

[0040] Figure 4 The virtual synchronous motor equivalent circuit vector diagram considering current limitation for the present invention;

[0041] Figure 5 A schematic diagram of the virtual impedance and bandpass filter control strategy of the present invention is provided;

[0042] Figure 6 This is a schematic diagram of current saturation caused by a sharp drop in grid voltage in the present invention;

[0043] Figure 7 This is a schematic diagram of current saturation caused by a small drop in grid voltage in the present invention. DETAILED DESCRIPTION

[0044] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0045] like Figure 1 and Figure 2 As shown, the first embodiment of the present invention proposes an improved fault ride-through control method for a virtual synchronous generator taking into account current limiting, comprising the following steps:

[0046] Step S100: Establish an equivalent model of a virtual synchronous generator taking into account a current limiting link, and connect the new energy power generation to the power grid through the virtual synchronous generator.

[0047] Renewable energy generation is connected to the grid through virtual synchronous generators. The typical control block diagram is as follows: Figure 2 As shown, it includes three main parts: active frequency control, reactive voltage control and voltage and current loop control. All electrical quantities are in per unit system. In the active control frequency, H and D represent the inertia time constant and droop coefficient, ω n is the rated angular frequency of the power grid, assuming that the voltage angle of the power grid is 0, the output reference voltage angle is θ, and E represents the internal potential of the virtual synchronous generator.

[0048] Step S200: Incorporate the virtual synchronous generator into the grid equivalent circuit, and use a vector model to draw contour lines representing the relationship between the power angle change and the virtual synchronous generator.

[0049] The characterization method of reactive voltage characteristics of virtual synchronous generator under current limitation is analyzed. On the time scale, since the dynamic speed of internal potential is much faster than the change of angular frequency, when analyzing the change of power angle, the internal potential amplitude E has reached steady state, and the vector method can be used for analysis.

[0050] Step S300: When the virtual synchronous generator is in constant voltage control or the droop coefficient is at a small value, the reactive voltage characteristics when the voltage changes are analyzed.

[0051] When the virtual synchronous generator is in constant voltage control or the droop factor K qu When it is at a smaller value, the reactive voltage characteristics when the voltage changes are analyzed.

[0052] Furthermore, in the improved fault ride-through control method of the virtual synchronous generator taking into account the current limit proposed in this embodiment, in step S100, the virtual synchronous generator includes active frequency control. In the active frequency control, the virtual synchronous generator includes reactive voltage control. Assuming that the voltage angle of the power grid is 0 and the reference voltage angle of the output is θ, in the reactive voltage control, the actual reactive power Q of the grid connection point is calculated. s With reference reactive Q ref , and the actual voltage U s With reference voltage U ref The integration of the difference weights generates the potential E inside the virtual synchronous generator. In active frequency control, when the reactive power and voltage at the grid connection point are in steady state, they satisfy:

[0053] K q (Q ref -Q s )+K u (U ref -U s )=0 (1)

[0054] In formula (1), K q is the reactive power regulation coefficient; Q ref is the reference reactive power of the grid-connected point; Q s is the actual reactive power at the grid-connected point; K u is the voltage regulation coefficient; U ref is the reference voltage; U s is the actual voltage.

[0055] When K q = 0 is constant voltage amplitude control; when K u =0 is constant reactive power control; when K q ≠0, K u When ≠0, it is reactive power droop control. In reactive power droop control, the droop coefficient is:

[0056]

[0057] In formula (2), K qu is the droop coefficient, K u is the voltage regulation coefficient; K q is the reactive power regulation coefficient.

[0058] Furthermore, in the improved fault ride-through control method of the virtual synchronous generator taking into account the current limiting proposed in this embodiment, in step S100, the virtual synchronous generator includes a voltage and current loop control, in which the current limiting link is implemented by an equivalent virtual admittance, and according to the singular perturbation principle, the internal potential dynamics of the virtual synchronous generator and the output voltage and current reach a steady-state relationship, and its equivalent circuit vector diagram is as follows: Figure 3 shown.

[0059] When the current does not reach the limit value, the expression is:

[0060] I s =Z v -1 (EU s ) (3)

[0061] In formula (3), I s is the output current of the virtual synchronous generator; Z v is the impedance value simulated by the virtual synchronous machine through the virtual admittance control; E is the potential inside the virtual synchronous generator; U s is the grid connection point voltage.

[0062] When the current reaches the limit value, the expression is:

[0063] I s =(kZ v ) -1 (EU s ) (4)

[0064] In formula (4), I s is the output current of the virtual synchronous generator; k is the adaptive parameter of the virtual impedance when the current reaches the limit value when the current controller adopts equal limit of dq axis; Z v is the impedance value simulated by the virtual synchronous machine through the virtual admittance control; E is the potential inside the virtual synchronous generator; U s is the grid connection point voltage.

[0065] According to formula (4), the equivalent circuit equation of the virtual synchronous generator including the current limiting link is written as:

[0066] I s =(EU g ) / (kZ v +Z g ) (5)

[0067] In formula (5), I s is the output current of the virtual synchronous generator; E is the internal potential amplitude; U g is the grid voltage; k is the adaptive parameter of the virtual impedance when the current reaches the limit value when the current controller adopts equal limit of dq axis; Z v The output impedance value of the virtual synchronous generator when the current does not reach the limit value; Z g is the grid impedance.

[0068] On the basis of current limiting, assuming that the sensor equipment detects grid voltage fluctuations, the control system invests in virtual impedance and bandpass filter in the reactive power control link. The bandpass filter mainly filters out harmonics and is not included in the calculation. The equivalent circuit equation after only considering the virtual impedance investment is:

[0069] I s '=(EU g -Z LV I s ) / (kZ v +Z g ) (6)

[0070] In formula (6), I s ' is the output current of the virtual synchronous generator after the virtual impedance is put into use; E is the internal potential amplitude; U g is the grid voltage; Z LV The value of the virtual impedance when the power grid fails; I s is the output current of the virtual synchronous generator; k is the adaptive parameter of the virtual impedance when the current controller adopts equal limiting of dq axes and the current reaches the limiting value; Z v The output impedance value of the virtual synchronous generator when the current does not reach the limit value; Z g is the grid impedance.

[0071] This embodiment discusses that when the current reaches the limit value, the above circuit equation is expressed by vector method as follows: Point A represents vector E, U s ,U g The starting end, B is U g The end of C is U s The end of the inner potential E, D is the end of the inner potential E, δ = θ-θ g For E and U g The phase difference between the output current I s The maximum current limit I lim When δ increases, the impedance in the circuit increases to k times with the adaptive parameter k, and satisfies the voltage equation U s =U g +Z g I s constraints, while ensuring the internal potential E and grid voltage U under current limitation s Satisfying the circuit vector triangle relationship, its equivalent circuit vector diagram is as follows Figure 5 shown.

[0072] Preferably, in the improved fault ride-through control method for virtual synchronous generators taking into account current limiting proposed in this embodiment, in step S200, it is assumed that the grid connection point voltage amplitude U s The coordinates of the end point C are (x U ,y U ), δ U For U s , U g The angle between them, U s Amplitude expression:

[0073]

[0074] In formula (7), U s is the grid connection point voltage; x U is the voltage amplitude at the grid connection point U s Projection value on the horizontal axis; y U is the voltage amplitude at the grid connection point U s Projection value on the ordinate.

[0075] If the virtual synchronous machine adopts constant voltage control, U s The amplitude will be maintained at a circle with A as the center and a radius of On the voltage contour line C1, U s =U ref .

[0076] The coordinate expression of the reactive power at the grid connection point is:

[0077]

[0078] In formula (8), Q s is the reactive power at the grid connection point; U s is the grid connection point voltage; U g is the grid voltage; cos is the cosine function; δ U is the phase difference between the grid connection point voltage and the grid voltage; x U is the voltage amplitude at the grid connection point U s Projection value on the horizontal axis; y U is the voltage amplitude at the grid connection point U s Projection value on the ordinate; U g is the grid voltage; X g is the grid impedance.

[0079] Further simplification yields:

[0080]

[0081] In formula (9), x U is the voltage amplitude at the grid connection point U s Projection value on the horizontal axis; U g is the grid voltage; y U is the voltage amplitude at the grid connection point U s Projection value on the ordinate; X g is the grid impedance; Q s is the reactive voltage at the grid connection point.

[0082] If the voltage adopts constant reactive power control, assuming X g , U g is a constant value, then the reactive power contour line C2 is is the center of the circle, A circle with radius U s The end of the always remains on the reactive power contour line C2, satisfying Q s =Q ref .

[0083] If the bandpass filter and virtual impedance are put into use, and the bandpass filter is mainly used to filter out harmonics, it is not included in the calculation here. Only the constant reactive power control is considered when the virtual impedance is put into use, U s The amplitude expression is:

[0084]

[0085] In formula (10), x U is the voltage amplitude at the grid connection point U s Projection value on the horizontal axis; U g is the grid voltage; y U is the voltage amplitude at the grid connection point U s Projection value on the ordinate; X gis the grid impedance; X LV is the input virtual impedance value; Q s is the reactive voltage at the grid connection point.

[0086] It can be seen that the radius of reactive contour line C2 after the virtual impedance is put into use is The radius of reactive power equivalent C2 is enlarged, U s The end is kept at a radius of On the circle.

[0087] Furthermore, the improved fault ride-through control method for a virtual synchronous generator taking into account current limiting proposed in this embodiment includes two types of current saturation in step S300:

[0088] 1. Current saturation caused by low grid voltage

[0089] When a serious voltage drop occurs in the power grid, the grid voltage U' g From point B to point B', point C represents the grid-connected voltage U s The maximum value that can be reached within the current limiting circle. If better voltage support is to be obtained, point C should be near point F. Since the current limiting circle is far away from Q s -U curve, there is no intersection, so it cannot meet the required reactive voltage output characteristics. s -U curve and the change of the internal potential amplitude is controlled by the reactive voltage with a faster time scale. Then the internal potential E increases to point D in a short time. The maximum active power of the virtual synchronous generator is at point G P in the figure. max =U' g I lim Therefore, in order to ensure that the power angle does not continue to increase, the grid voltage U s At point J (the voltage minimum point U J ) and point F (the maximum voltage point U F ) The voltage is unstable due to the back and forth fluctuations. The active reference value should be less than the maximum value of the output active power P ref <P max , according to the above constraints, the maximum voltage fluctuation amplitude is |GF|.

[0090] If the voltage drops sharply, the current reaches the limit value. On the basis of the active reference value limit, the virtual impedance and bandpass filter of the reactive voltage control link are added to further limit the current amplitude. The control strategy is as follows: Figure 5 As shown in the figure, the voltage amplitude U at the grid connection point is compared before and after the virtual impedance is put into use. s ' s , the voltage fluctuation amplitude |G'F'|<|GF|. It can be seen that after the virtual impedance is put into use, the grid voltage will be slightly reduced, but the voltage fluctuation amplitude will be reduced, preventing the voltage from further deteriorating. The equivalent circuit vector is as follows: Figure 6 shown.​

[0091] 2. Current saturation caused by a small change in grid voltage

[0092] When the grid voltage changes slightly, the current of the virtual synchronous generator reaches the limit value. Assuming that the grid voltage U' g From point B to point B', the grid voltage amplitude U s The end is at point C on the current limiting circle. Due to a small change in the grid voltage, point C is closer to Q s -U curves intersect, which can meet the required reactive voltage output characteristics. If virtual impedance and bandpass filter are added to the reactive voltage control link, although the current amplitude is further limited, the radius of the reactive contour is expanded, and the current limit loop and Q s -U curve gets a new intersection point U s '>U s Grid connection point voltage amplitude U s Compared with the case where no control strategy is adopted, the supporting voltage of the grid connection point is increased, which shows the effectiveness of the control strategy. Its equivalent circuit vector is as follows: Figure 7 As shown, where U s ' is the voltage value of the grid connection point taking into account the virtual impedance; U s is the voltage value of the grid connection point without considering the virtual impedance; G'F' is the maximum range of voltage fluctuation of the grid connection point with considering the virtual resistance; GF is the maximum range of voltage fluctuation of the grid connection point without considering the virtual resistance.

[0093] The improved fault ride-through control method for a virtual synchronous generator taking into account current limiting provided in this embodiment, compared with the prior art, establishes an equivalent model of a virtual synchronous generator taking into account the current limiting link, connects new energy power generation to the power grid through the virtual synchronous generator; incorporates the virtual synchronous generator into the power grid equivalent circuit, and uses a vector model to draw the contour line representation relationship between the power angle change and the virtual synchronous generator; when the virtual synchronous generator is in constant voltage control or the droop coefficient is at a small value, analyzes the reactive voltage characteristics when the voltage changes. The improved fault ride-through control method for a virtual synchronous generator taking into account current limiting provided in this embodiment studies that under the condition of a grid voltage drop taking into account the current limiting of the virtual synchronous generator, virtual impedance is put into operation, and a bandpass filter is added, which can further improve the voltage support link of the system, enhance the system fault ride-through capability, and remove the virtual impedance after the fault is eliminated, without affecting the output characteristics of the virtual synchronous generator during normal operation.

[0094] Although preferred embodiments of the present invention have been described, additional changes and modifications may be made to these embodiments by those skilled in the art once the basic inventive concepts are known. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention. Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. An improved fault ride-through control method for a virtual synchronous generator taking into account current limiting, characterized in that: The following steps are involved: Establishing an equivalent model of a virtual synchronous generator taking into account a current limiting link, and connecting the renewable energy generation to the power grid through the virtual synchronous generator; Incorporating the virtual synchronous generator into a power grid equivalent circuit, and using a vector model to draw contour lines representing the relationship between the power angle change and the virtual synchronous generator; When the virtual synchronous generator is in constant voltage control or the droop coefficient is at a small value, the reactive voltage characteristics when the voltage changes are analyzed.

2. The improved fault ride-through control method for a virtual synchronous generator taking into account current limiting as claimed in claim 1, characterized in that: In the step of establishing an equivalent model of a virtual synchronous generator taking into account the current limiting link and connecting the renewable energy power generation to the power grid through the virtual synchronous generator, the virtual synchronous generator includes active frequency control. In the active frequency control, the virtual synchronous generator includes reactive voltage control. Assuming that the voltage angle of the power grid is 0 and the reference voltage angle of the output is θ, in the reactive voltage control, the actual reactive power Q s With reference reactive Q ref , and the actual voltage U s With reference voltage U ref The integration of the difference weights generates the potential E inside the virtual synchronous generator.

3. The improved fault ride-through control method for a virtual synchronous generator taking into account current limiting as claimed in claim 2, characterized in that: In the step of establishing an equivalent model of a virtual synchronous generator taking into account the current limiting link and connecting the renewable energy power generation to the power grid through the virtual synchronous generator, in the active frequency control, when the reactive power and voltage at the grid connection point are in a steady state, the following conditions are satisfied: K q (Q ref -Q s )+K u (U ref -U s )=0 Among them, K q is the reactive power regulation coefficient; Q ref Q is the reference reactive power of the grid-connected point; s is the actual reactive power at the grid-connected point; K u is the voltage regulation coefficient; U ref is the reference voltage; U s is the actual voltage; when K q = 0 is constant voltage amplitude control; when K u =0 is constant reactive power control; when K q ≠0, K u When ≠0, it is reactive power droop control.

4. The improved fault ride-through control method for a virtual synchronous generator taking into account current limiting as claimed in claim 3, characterized in that: In reactive power droop control, the droop coefficient is: Among them, K qu is the droop coefficient, K u is the voltage regulation coefficient; K q is the reactive power regulation coefficient.

5. The improved fault ride-through control method for a virtual synchronous generator taking current limiting into account according to claim 1, characterized in that: In the step of establishing an equivalent model of a virtual synchronous generator taking into account the current limiting link and connecting the renewable energy power generation to the power grid through the virtual synchronous generator, the virtual synchronous generator includes a voltage and current loop control. In the voltage and current loop control, the current limiting link is implemented with an equivalent virtual admittance. According to the singular perturbation principle, the internal potential dynamics of the virtual synchronous generator and the output voltage and current reach a steady-state relationship.

6. The improved fault ride-through control method for a virtual synchronous generator taking into account current limitation as claimed in claim 5, characterized in that: In the step of establishing an equivalent model of a virtual synchronous generator taking into account the current limiting link and connecting the renewable energy power generation to the power grid through the virtual synchronous generator, on the basis of current limiting, the equivalent circuit equation containing the virtual synchronous generator is: I s =(EU g ) / (kZ v +Z g ) Among them, I s is the output current of the virtual synchronous generator; E is the internal potential amplitude; U g is the grid voltage; k is the adaptive parameter of the virtual impedance when the current reaches the limit value when the current controller adopts equal limit of dq axis; Z v The output impedance value of the virtual synchronous generator when the current does not reach the limit value; Z g is the grid impedance.

7. The improved fault ride-through control method for a virtual synchronous generator taking into account current limitation as claimed in claim 6, characterized in that: In the step of establishing an equivalent model of a virtual synchronous generator taking into account the current limiting link and connecting the renewable energy power generation to the power grid through the virtual synchronous generator, it is assumed that when the sensor device detects the voltage fluctuation of the power grid, the control system invests in the virtual impedance and the bandpass filter in the reactive power control link, and the bandpass filter mainly has the function of filtering out harmonics and is not included in the calculation. The equivalent circuit equation after only considering the investment of the virtual impedance is: I s '=(EU g -Z LV I s ) / (kZ v +Z g ) Among them, I s ' is the output current of the virtual synchronous generator after the virtual impedance is put into use; E is the internal potential amplitude; U g is the grid voltage; Z LV The value of the virtual impedance when the power grid fails; I s is the output current of the virtual synchronous generator; k is the adaptive parameter of the virtual impedance when the current reaches the limit value when the current controller adopts equal limit of dq axis; Z v The output impedance value of the virtual synchronous generator when the current does not reach the limit value; Z g is the grid impedance.

8. The improved fault ride-through control method for a virtual synchronous generator taking into account current limiting as claimed in claim 7, characterized in that: In the step of incorporating the virtual synchronous generator into the grid equivalent circuit and using a vector model to draw contour lines representing the relationship between the power angle change and the virtual synchronous generator, it is assumed that the grid connection point voltage amplitude U s The coordinates of the end point C are (x U ,y U ), δ U For U s , U g The angle between s The amplitude expression is: Among them, x U is the voltage amplitude at the grid connection point U s Projection value on the horizontal axis; U g is the grid voltage; y U is the voltage amplitude at the grid connection point U s Projection value on the ordinate; X g is the ground impedance; Q s is the reactive power at the grid connection point; If the voltage adopts constant reactive power control, assuming X g , U g is a constant value, then the reactive power contour line C2 is is the center of the circle, A circle with radius U s The end of the always remains on the reactive power contour line C2, satisfying Q s =Q ref .

9. The improved fault ride-through control method for a virtual synchronous generator taking into account current limitation as claimed in claim 8, characterized in that: In the step of incorporating the virtual synchronous generator into the grid equivalent circuit and using a vector model to draw contour lines representing the relationship between the power angle change and the virtual synchronous generator, if a bandpass filter and a virtual impedance are put into use, and the bandpass filter is mainly used to filter out harmonics and is not included in the calculation, and only the constant reactive power control is used when the virtual impedance is put into use, U s The amplitude expression is: Among them, x U is the voltage amplitude at the grid connection point U s Projection value on the horizontal axis; U g is the grid voltage; y U is the voltage amplitude at the grid connection point U s Projection value on the ordinate; X g is the grid impedance; X LV is the input virtual impedance value; Q s is the reactive voltage at the grid connection point; It can be seen that the radius of reactive contour line C2 after the virtual impedance is put into use is The radius of reactive equivalent C2 is enlarged, U s The end is kept at a radius of On the circle.

10. The improved fault ride-through control method for a virtual synchronous generator taking into account current limitation as claimed in claim 9, characterized in that: In the step of analyzing reactive voltage characteristics when voltage changes when the virtual synchronous generator is in constant voltage control or the droop coefficient is at a small value, If the voltage drops sharply, the current reaches the limit value. On the basis of the active reference value limit, the virtual impedance and bandpass filter of the reactive voltage control link are added to further limit the current amplitude. The voltage amplitude U s ' s , voltage fluctuation amplitude |G'F'|<|GF|;​ When the grid voltage changes slightly, the virtual synchronous generator current reaches the limit value. If the virtual impedance and bandpass filter are put into the reactive voltage control link, the current amplitude is further limited, but the radius of the reactive contour is expanded, and the current limit loop and Q s -U curve gets a new intersection point U s '>U s ; Among them, U s ' is the voltage value of the grid connection point taking into account the virtual impedance; U s is the voltage value of the grid-connected point without taking into account the virtual impedance; G'F' is the maximum range of voltage fluctuation of the grid-connected point taking into account the virtual resistance; GF is the maximum range of voltage fluctuation of the grid-connected point without taking into account the virtual resistance.

Images