Method and device for improving the stability of a virtual synchronous generator

Abstract

The present invention discloses a method and device for improving the stability of a virtual synchronous generator. The method includes: performing impedance modeling on the VSG to obtain a VSG impedance model; generating an adjustment voltage based on a variable transient virtual damping control scheme; superimposing the output voltage of the current loop and the adjustment voltage, and applying the superimposed voltage to the VSG impedance model to adjust the three-phase voltage output by the VSG. In the variable transient virtual damping control scheme: according to the similarity principle of the power system, the VSG impedance model is analogized with the operating principle of a synchronous motor, the transient virtual resistance is calculated using the angular frequency change and d-axis current change output by the VSG, and the steady-state virtual resistance and virtual inductance are calculated using the DC bus voltage fluctuation and DC bus output current change of the VSG; the adjustment voltage is determined using the transient virtual resistance, steady-state virtual resistance, and virtual inductance. The present invention can improve the stability of the virtual synchronous generator.

Description

Technical Field

[0001] The present invention relates to the technical field of power system stability analysis and control, and particularly to a method and device for improving the stability of a virtual synchronous generator. Background Art

[0002] This section aims to provide background or context for the embodiments of the present invention described in the claims. The description herein is not admitted to be prior art merely because it is included in this section.

[0003] Currently, wind power generation and photovoltaic power generation will gradually become important components of a new power system. As an energy conversion interface between renewable energy and the power grid, the stable operation of the grid-connected converter is of great significance to the new power system. The virtual synchronous generator (VSG), as a way of grid-forming control for the grid-connected converter, can provide frequency support to a weak grid and has the ability to operate in a network, which can provide effective support for the large-scale application of renewable energy. However, under the conditions of DC bus voltage fluctuation or AC voltage frequency fluctuation, the output power and output frequency of the VSG will oscillate, which is an urgent problem to be solved.

[0004] In the existing field of converter control, damping control is mainly used to eliminate power oscillation. Traditional damping control mainly includes methods such as virtual damping control or active damping control. This method can effectively suppress the output power oscillation of the VSG when the DC bus voltage fluctuates or the AC voltage frequency fluctuates, but there are also corresponding disadvantages. For example: virtual damping control cannot suppress power according to the specific cause of power oscillation, but suppresses power in all working conditions. Although it can effectively suppress the output power oscillation of the VSG, it also reduces the output power of the VSG. At the same time, since the specific oscillation cause is not analyzed, simply increasing the system damping will reduce the system stability. Under the condition of large external condition changes, the system will become unstable, which has a negative impact on the power generation efficiency of the grid-connected converter. Active damping control extracts the corresponding frequency of power oscillation, generates a voltage control quantity opposite to the power oscillation frequency, and adjusts the active power reference value of the VSG through a feed-forward compensation method to achieve the purpose of power oscillation suppression. The above two mainstream control methods both suppress power oscillation by sacrificing the overall power generation efficiency, and may cause the stability of the VSG to deteriorate under large disturbance conditions such as faults. Summary of the Invention

[0005] An embodiment of the present invention provides a method for improving the stability of a virtual synchronous generator, which is used to improve the stability of the virtual synchronous generator when the DC voltage and the AC voltage frequency fluctuate. The method includes:

[0006] Perform impedance modeling on the virtual synchronous generator (VSG) to obtain the VSG impedance model. Among them, the VSG control includes an active power loop, a reactive power loop, a voltage loop, a current loop, a speed governor, and a DC voltage loop.

[0007] Based on the variable transient virtual damping control scheme, generate a regulating voltage. The regulating voltage is used to regulate the three-phase voltage output by the VSG. In the variable transient virtual damping control scheme: According to the similarity principle of the power system, analogize the VSG impedance model with the operating principle of the synchronous motor to construct a virtual impedance control structure diagram. Based on the virtual impedance control structure diagram, calculate the transient virtual resistance using the angular frequency change and d-axis current change of the VSG output, calculate the steady-state virtual resistance and virtual inductance using the DC bus voltage fluctuation and DC bus output current change of the VSG, and determine the regulating voltage using the transient virtual resistance, steady-state virtual resistance, and virtual inductance.

[0008] Superimpose the output voltage of the current loop and the regulating voltage, and apply the superimposed voltage to the VSG impedance model to regulate the three-phase voltage output by the VSG.

[0009] An embodiment of the present invention also provides a device for improving the stability of a virtual synchronous generator, which is used to improve the stability of the virtual synchronous generator when the DC voltage and the AC voltage frequency fluctuate. The device includes:

[0010] An impedance modeling module, which is used to perform impedance modeling on the virtual synchronous generator (VSG) to obtain the VSG impedance model. Among them, the VSG control includes an active power loop, a reactive power loop, a voltage loop, a current loop, a speed governor, and a DC voltage loop.

[0011] A variable transient virtual damping control module, which is used to generate a regulating voltage based on the variable transient virtual damping control scheme. The regulating voltage is used to regulate the three-phase voltage output by the VSG. Superimpose the output voltage of the current loop and the regulating voltage, and apply the superimposed voltage to the VSG impedance model to regulate the three-phase voltage output by the VSG. In the variable transient virtual damping control scheme: According to the similarity principle of the power system, analogize the VSG impedance model with the operating principle of the synchronous motor to construct a virtual impedance control structure diagram. Based on the virtual impedance control structure diagram, calculate the transient virtual resistance using the angular frequency change and d-axis current change of the VSG output, calculate the steady-state virtual resistance and virtual inductance using the DC bus voltage fluctuation and DC bus output current change of the VSG, and determine the regulating voltage using the transient virtual resistance, steady-state virtual resistance, and virtual inductance.

[0012] An embodiment of the present invention also provides a computer device, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, the above method for improving the stability of the virtual synchronous generator is implemented.

[0013] An embodiment of the present invention also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-mentioned method for improving the stability of a virtual synchronous generator.

[0014] An embodiment of the present invention also provides a computer program product including a computer program, which, when executed by a processor, implements the above-mentioned method for improving the stability of a virtual synchronous generator.

[0015] In the embodiment of the present invention, the variable transient virtual impedance control scheme takes into account the influence of DC voltage fluctuations and system frequency fluctuations on the VSG output, calculates the transient virtual resistance using the angular frequency change and d-axis current change of the VSG output, and calculates the steady-state virtual resistance and virtual inductance using the DC bus voltage fluctuation and DC bus output current change of the VSG; determines the regulation voltage using the transient virtual resistance, steady-state virtual resistance and virtual inductance, and superimposes the current loop output voltage and the regulation voltage and applies them to the VSG impedance model to regulate the three-phase voltage output by the VSG. Using the steady-state virtual resistance, virtual inductance, and transient virtual resistance to suppress the VSG power oscillation problem caused by DC bus voltage fluctuations and system frequency fluctuations, the embodiment of the present invention effectively suppresses the causes of VSG power oscillation in a targeted manner, and can also have good effects during sudden power increase, avoiding the unstable phenomenon of VSG that may occur in traditional damping control. The embodiment of the present invention can effectively solve the problem of insufficient stability of the virtual synchronous generator under the condition of DC voltage and AC voltage frequency fluctuations, and has a positive effect on the popularization and application of virtual synchronous generator control. Description of the Drawings

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings. In the drawings:

[0017] Figure 1 It is a schematic flowchart of the method for improving the stability of a virtual synchronous generator in an embodiment of the present invention;

[0018] Figure 2 It is a schematic diagram of the main circuit of the VSG in an embodiment of the present invention;

[0019] Figure 3 It is a control block diagram of the VSG in an embodiment of the present invention;

[0020] Figure 4 It is an analog schematic diagram of the VSG and a synchronous motor in an embodiment of the present invention;

[0021] Figure 5 It is the variable transient virtual impedance control structure diagram in the embodiment of the present invention;

[0022] Figure 6 It is the schematic structural diagram of the variable transient virtual impedance control for regulating voltage generation in the embodiment of the present invention;

[0023] Figure 7 It is the modulation coefficient calculation block diagram after being processed by the variable transient virtual impedance control scheme in the embodiment of the present invention;

[0024] Figure 8 It is the Bode diagram of the VSG impedance without adding suppression measures under the existing operating condition - 1;

[0025] Figure 9 It is the Bode diagram of the VSG impedance with active damping control added under the existing operating condition - 1;

[0026] Figure 10 It is the Bode diagram of the VSG impedance with active damping control added under the existing operating condition - 2;

[0027] Figure 11 It is the Bode diagram of the VSG impedance with variable transient virtual impedance control added under the operating condition - 2 in the embodiment of the present invention;

[0028] Figure 12 It is the verification schematic diagram of the method for improving the stability of the virtual synchronous generator in the embodiment of the present invention Figure 1 ;

[0029] Figure 13 It is the verification schematic diagram of the method for improving the stability of the virtual synchronous generator in the embodiment of the present invention Figure 2 ;

[0030] Figure 14 It is the verification schematic diagram of the method for improving the stability of the virtual synchronous generator in the embodiment of the present invention Figure 3 ;

[0031] Figure 15 It is the verification schematic diagram of the method for improving the stability of the virtual synchronous generator in the embodiment of the present invention Figure 4 ;

[0032] Figure 16 It is the schematic diagram of the device for improving the stability of the virtual synchronous generator in the embodiment of the present invention. Detailed implementation manners

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer and more understandable, the following further elaborates on the embodiments of the present invention with reference to the accompanying drawings. Herein, the illustrative embodiments of the present invention and their descriptions are used to explain the present invention, but not to limit the present invention.

[0034] In the technical solution of this application, the acquisition, transmission, storage, use, processing, etc. of data all comply with the relevant provisions of national laws and regulations.

[0035] In the prior art, damping control is mainly used in the field of converter control to eliminate power oscillation. Traditional damping control mainly focuses on methods such as virtual damping control or active damping control. This method can effectively suppress the output power oscillation of the VSG when the DC bus voltage fluctuates or the AC voltage frequency fluctuates. However, there are also corresponding disadvantages. For example, virtual damping control cannot suppress power according to the specific cause of power oscillation, but suppresses power under all operating conditions. Although it can effectively suppress the output power oscillation of the VSG, it also reduces the output power of the VSG. At the same time, since the specific oscillation cause is not analyzed, simply increasing the system damping will reduce the system stability. Under the condition of large changes in external operating conditions, the system will become unstable, which has a negative impact on the power generation efficiency of the grid-connected converter. Active damping control extracts the corresponding frequency of power oscillation, generates a voltage control quantity opposite to the power oscillation frequency, and suppresses the power oscillation of the VSG through a feed-forward compensation method. The above two mainstream control methods cannot suppress the specific cause of power oscillation, but suppress power oscillation by sacrificing the overall power generation efficiency, resulting in negative impacts.

[0036] Therefore, aiming at the disadvantages of traditional damping control in the prior art, the embodiments of the present invention can take targeted measures against the VSG power oscillation caused by the DC bus fluctuation and AC voltage frequency fluctuation of the grid-connected converter, and this design idea can be extended to the use in the improvement of the stability of the grid-connected converter under various abnormal operating conditions.

[0037] Figure 1 It is a schematic flow chart of the method for improving the stability of the virtual synchronous generator in the embodiments of the present invention. As Figure 1 shown, the method includes:

[0038] Step 101, perform impedance modeling on the virtual synchronous generator VSG to obtain the VSG impedance model; wherein, the VSG control includes an active power loop, a reactive power loop, a voltage loop, a current loop, a speed governor, and a DC voltage loop;

[0039] Step 102: Generate an adjustment voltage based on the variable transient virtual damping control scheme; the adjustment voltage is used to adjust the three-phase voltage output by the VSG; in the variable transient virtual damping control scheme: according to the principle of power system similarity, the VSG impedance model is analogized with the operating principle of a synchronous motor to construct a virtual impedance control structure diagram; based on the virtual impedance control structure diagram, calculate the transient virtual resistance using the angular frequency change and d-axis current change of the VSG output, and calculate the steady-state virtual resistance and virtual inductance using the DC bus voltage fluctuation and DC bus output current change of the VSG; determine the adjustment voltage using the transient virtual resistance, steady-state virtual resistance, and virtual inductance.

[0040] Step 103: Superimpose the current loop output voltage and the adjustment voltage, and apply the superimposed voltage to the VSG impedance model to adjust the three-phase voltage output by the VSG.

[0041] The method for improving the control stability of the virtual synchronous generator in the embodiments of the present invention will be explained in detail below.

[0042] The solution adopted in the embodiments of the present invention is based on a variable transient virtual impedance control method with angular frequency, DC bus voltage, and output current as input quantities. First, impedance modeling is performed on the VSG to obtain the VSG impedance model. Second, the specific implementation method and theoretical derivation of the variable transient virtual impedance control are described, and the Bode diagram of the impedance model is used to analyze the influence of active damping control and variable transient virtual control on stability under changing external working conditions. Finally, the effectiveness of the proposed method is verified through an experimental platform under two working conditions of DC bus voltage fluctuation and output voltage frequency fluctuation, and a comparison is made with the existing active damping control.

[0043] Figure 2 is the schematic diagram of the VSG main circuit in the embodiments of the present invention, as Figure 2 shown, Figure 2 where, u dc is the DC bus voltage, i dc is the DC bus current, C dc is the DC bus capacitor, u cabc is the three-phase voltage output by the VSG, L c and R c are the inductor and resistor of the VSG output filter, u 0 is the grid connection point voltage, i Labc is the grid connection current, that is, the VSG output current, L g and R g are the grid line inductor and line resistor, u g is the grid voltage, and the dashed box indicates PWM modulation of the VSG through the variable transient virtual damping control scheme, S abc is the switching signal, and PWM represents pulse width modulation. , are the modulation coefficients to be applied to the VSG, d abc is the duty cycle, dq and abc represent the two-phase rotating coordinate system and the three-phase stationary coordinate system, and θ is the coordinate transformation conversion angle.

[0044] Figure 3 is the VSG control block diagram in the embodiment of the present invention. As Figure 3 shown, the VSG control mainly includes: active power loop, reactive power loop, voltage and current loop, and sampling circuit. In the active power loop, ω g is the grid angular frequency, ω 0 is the VSG angular frequency, k pv is the primary frequency modulation coefficient, ΔP is the primary frequency modulation power change amount, P e is the VSG electromagnetic power, P ref is the VSG active power reference value, J is the virtual inertia coefficient, D p is the virtual damping coefficient, ω is the transient value of the VSG angular frequency, s is the differential operator, θ is the VSG power angle, and also serves as the coordinate transformation conversion angle for coordinate transformation, Δω 1 is the DC voltage control output, G dc is the DC voltage control transfer function, u dc is the DC voltage sampled value, u dc.ref is the DC voltage reference value. In the reactive power loop, Q e is the reactive power sampled value, Q ref is the reactive power reference value, k q is the reactive power droop coefficient, s is the differential operator, is the d-axis component of the VSG output voltage reference value, is the q-axis component of the VSG output voltage reference value. In the voltage loop and current loop, , are the d-axis component and q-axis component of the VSG output voltage sampled value respectively, G vc is the voltage control transfer function, , , , are the d-axis component of the VSG output current sampled value, the q-axis component of the sampled value, the d-axis component of the reference value, and the q-axis component of the reference value respectively, G cc is the current control transfer function, G f is the feed-forward link transfer function, L c is the output filter inductor. In the sampling circuit, u ca , u cb , u cc are the VSG output a, b, and c phase voltages respectively, i La , i Lb , i LcThey are the phase a, b, and c currents output by the VSG respectively.

[0045] In one embodiment, the active power loop is used to control the active power of the VSG by simulating the swing equation of a synchronous generator. The active power loop has the frequency modulation ability of the synchronous generator governor and introduces the DC bus voltage variation into the active power loop for active power control, enhancing the dynamic response ability of the VSG to DC voltage variations. Refer to Figure 3 The active power loop in [reference] simulates the swing equation of a synchronous generator, enabling the grid-connected converter to have the external characteristics of a synchronous generator. The active power loop also has the frequency modulation ability of the synchronous generator governor and takes into account the response ability to DC bus voltage variations at the same time. The output of the reactive power loop is used as the d-axis reference value of the voltage loop In the embodiments of the present invention, unity power factor control is adopted, so . The voltage loop and the current loop regulate the output voltage and output current of the VSG. The sampling circuit completes the functions of signal sampling and coordinate transformation.

[0046] In one embodiment, impedance modeling is performed on the virtual synchronous generator (VSG) to obtain a VSG impedance model, which may include: determining the output voltage expression and DC current expression of the VSG according to the power balance principle; establishing the angular frequency expression of the VSG output; introducing DC bus voltage disturbance and AC voltage frequency disturbance into the angular frequency expression, and deriving the active power expression and reactive power expression considering DC voltage disturbance and AC voltage frequency disturbance; performing a transformation of the voltage expression, DC current expression, angular frequency expression, active power expression, and reactive power expression from the VSG system coordinate system to the VSG control coordinate system to obtain the VSG impedance model considering DC voltage disturbance and AC voltage frequency disturbance; where the VSG system coordinate system is established based on the VSG main circuit, and the VSG control coordinate system is based on the VSG control block diagram; the VSG control block diagram is used to control the VSG main circuit.

[0047] During implementation, it can be based on Figure 2 to derive the VSG output voltage expression as:

[0048] (1)

[0049] In the formula, the character Δ represents that the variation degree of the involved variable is very small, that is, it varies within a small range near the operating point. i Ld , i Lq are respectively the d-q axis components of the VSG output current sampling values, and u od , u oq , u cd , u cq are respectively the d-q axis components of the VSG output voltage, the d-q axis components of the VSG output voltage in the control coordinate system, and Z fis the impedance of the VSG output filter. According to the power balance principle, the VSG output voltage and DC current can be derived as follows:

[0050] (2)

[0051] (3)

[0052] In formula (2), Δm d and Δm q are the d-q small-signal variations of the modulation coefficient respectively, m d0 and m q0 are the steady-state d-q small-signal variations of the modulation coefficient respectively, that is, the fixed modulation coefficient, u dc0 is the steady-state value of the DC voltage, and Δu dc is the small-signal variation of the DC voltage.

[0053] In formula (3), i dc0 and Δi dc are the steady-state value of the DC current and the small-signal variation of the DC current respectively, u cd0 and u cq0 are the steady-state voltage d-q axis components of the VSG output respectively, i Ld0 and i Lq0 are the steady-state value d-q components of the VSG output current respectively, that is, the steady-state value d-q components of the grid-connected current.

[0054] Let , , . The current loop can be expressed as:

[0055] (4)

[0056] where , , , , .

[0057] where and are the d-q components of the modulation coefficient in the control coordinate system respectively, and T s is the switching period.

[0058] Similarly, the voltage loop can be expressed as:

[0059] (5)

[0060] The VSG output angular frequency can be expressed as:

[0061] (6)

[0062] Adding DC bus voltage disturbance and AC voltage frequency disturbance to formula (6) and performing small-signal linearization, the small-signal model of the active power loop output of the VSG can be obtained as follows:

[0063] (7)

[0064] In the formula, J is the virtual inertia coefficient. Similarly, the reactive power loop can also be expressed as:

[0065] (8)

[0066] Adding disturbance to formula (8) and performing small-signal linearization, the small-signal model of the reactive power loop output of the VSG is obtained as follows:

[0067] (9)

[0068] The main circuit model and control model of the VSG can be represented by formulas (1)-(9), and the conversion relationship between the VSG control coordinate system and the VSG system coordinate system is represented by formula (10), that is:

[0069] (10)

[0070] In formula (10), , , Δf d , Δf q , f d0 , f q0 , are the d-q axis components of different variables, the d-q axis components of different components, the d-q axis components of the steady-state values of different components, and the d-axis component of the VSG output voltage reference value in the VSG control coordinate system, respectively.

[0071] f is a different variable within the VSG system, and T 1 , T 2 are transformation matrices and can be expressed as:

[0072] (11)

[0073] (12)

[0074] Among them, θ 0 is the deviation between different coordinate systems under steady state. At the same time, from formulas (7) and (9), we can obtain:

[0075] (13)

[0076] Among them, , They are the proportional coefficient and integral coefficient for DC voltage control respectively.

[0077] In addition, the small-signal expressions of the DC bus voltage and angular frequency can be obtained as follows:

[0078] (14)

[0079] (15)

[0080] where C dc is the DC bus capacitor.

[0081] Under steady state, let , , .

[0082] According to the above analysis, the conversion relationship between the system d-q coordinate system and the control d-q coordinate system can be obtained as:

[0083] (16)

[0084] (17)

[0085] (18)

[0086] where G m1 , G i1 , G v1 can be expressed by the following formula:

[0087] (19)

[0088] (20)

[0089] (21)

[0090] Therefore, the active power and reactive power output by the VSG can be expressed as:

[0091] (22)

[0092] After adding perturbations to Equation (22) and linearizing it, we can obtain:

[0093] (23)

[0094] where , , and further we can obtain:

[0095] (24)

[0096] The following can be obtained in the VSG system coordinate system The expression is:

[0097] (25)

[0098] The matrix E - H can be expressed as:

[0099] (26)

[0100] The modulation coefficient The small - signal model of is:

[0101] (27)

[0102] Among them, can be expressed as:

[0103] (28)

[0104] (29)

[0105] Therefore, the small - signal model of the modulation coefficient in the VSG system coordinate system is:

[0106] (30)

[0107] Among them, L, P, Q can be expressed as:

[0108] (31)

[0109] (32)

[0110] (33)

[0111] Therefore, it can be obtained that:

[0112] (34)

[0113] Combining the above analysis, the VSG impedance model can be obtained as:

[0114] (35)

[0115] Next, the specific implementation method and theoretical derivation of the variable transient virtual impedance control are described.

[0116] The rotor motion equation of the VSG can be analogized to the dynamic equation of the DC voltage change. The DC - side voltage equation of the VSG can be expressed as:

[0117] (36)

[0118] Among them, Hc is the DC capacitance inertia time constant, P m is the active power on the DC side. And it satisfies: H c =(C dc ×u dc.ref ) / (2×S n ), where S n is the capacity of the VSG. Meanwhile, the rotor motion equation of the VSG is:

[0119] (37)

[0120] H J is the inertia time constant of the VSG rotor. By comparing Eq. (36) and Eq. (37), we can obtain:

[0121] (38)

[0122] According to Eq. (38), it can be seen that the dynamic characteristics of the DC voltage u dc are similar to those of the angular frequency ω 0 . And through the similarity principle, it can be deduced that the inertia time constant H c of the DC capacitance is similar to the rotor time constant H J . Therefore, the active power of the VSG can be expressed in another form:

[0123] (39)

[0124] where , X 1 is the impedance between the VSG and the power grid, is the phase angle error between the VSG output voltage and the power grid voltage, is the rotor flux linkage of the VSG, X 2 is the equivalent reactance of the VSG, is the torque angle of the VSG.

[0125] Figure 4 is the analogy schematic diagram of the VSG and the synchronous motor in the embodiments of the present invention. As Figure 4 shown, in the variable transient virtual damping control scheme: According to the similarity principle of the power system, the modulation coefficient of the VSG impedance model is analogized to the rotor flux linkage of the synchronous motor, the inertia time constant of the DC capacitance of the VSG impedance model is analogized to the rotor time constant H J of the synchronous motor, and the DC voltage of the VSG impedance model is analogized to the angular frequency ω 0 of the synchronous motor.

[0126] The torque angle of the VSG can be expressed as:

[0127] (40)

[0128] ω Bg is the base value of the grid angular frequency, and the relationship between the output voltage phase angle and the DC voltage of the VSG is as follows:

[0129] (41)

[0130] Through the above analysis, a variable transient virtual impedance control structure diagram is constructed. Figure 5 This is the variable transient virtual impedance control structure diagram in the embodiment of the present invention. Figure 5 In it, k 1 is the active coefficient of the variable transient virtual damping control, and satisfies k 1 = 1 / (u 0 - u g ). k 2 is the damping coefficient of the variable transient virtual damping control, k 3 is the virtual inertia coefficient of the variable transient virtual damping control, and take k 3 = 0.5. is the output voltage phase angle of the VSG under steady state. R VTVR , R VI , L VI are the transient virtual resistance, steady-state virtual resistance, and virtual inductor respectively, and Z VI is the virtual impedance. ω 1 , ω 2 represent the cut-off frequencies of the filter. Through these two cut-off frequencies, the effective working range of the filter can be determined. Take ω 1 = ω 2 = 1e 3 rad / s.

[0131] The variable transient virtual impedance control scheme in the embodiment of the present invention takes into account the influence of DC voltage fluctuations and system frequency fluctuations on the VSG output. According to the angular frequency change and the d-axis output current change, the value of the transient virtual resistance R VTVR can be obtained, and its size can be adjusted through k 2 , k 3 . At the same time, by adjusting the sizes of ω 1 , ω 2 , the effective frequency range of the transient virtual resistance can be controlled. By calculating the steady-state virtual resistance R VI and the virtual inductor L VI from the DC bus voltage fluctuation and the output current change, and using the steady-state virtual resistance, virtual inductor, and transient virtual resistance to suppress the VSG power oscillation problem caused by DC bus voltage fluctuations and system frequency fluctuations, the embodiment of the present invention effectively suppresses the causes of the VSG power oscillation in a targeted manner, and can also have good effects during sudden power increase, avoiding the VSG instability phenomenon that may occur in traditional damping control.

[0132] According to Figure 5 the steady-state damping ratio σ ss and the transient damping ratio σ tr can be obtained and expressed as:

[0133] (42)

[0134] Meanwhile, the damping coefficient k 2 , can be obtained through the limit as follows:

[0135] (43)

[0136] The parameters for the variable transient virtual impedance control can be obtained from Equations (36) - (43). After obtaining all the parameters, the corresponding regulating voltage needs to be generated.

[0137] Figure 6 FIG. is a schematic diagram of the regulating voltage generation for the variable transient virtual impedance control in the embodiment of the present invention. Combining Figure 6 , the transient virtual resistance, steady-state virtual resistance, and virtual inductance are calculated according to the following formula:

[0138] (44)

[0139] where R VTVR , R VI , L VI are the transient virtual resistance, steady-state virtual resistance, and virtual inductance respectively, is the d-axis component of the VSG output current reference value, is the d-axis component of the VSG output current sampled value, ω 0 is the VSG angular frequency, ω 1 , ω 2 are both the cut-off frequencies of the filter, ω g is the grid angular frequency, k 1 , k 2 , k 3 are the active coefficient, damping coefficient, and virtual inertia coefficient of the variable transient virtual damping control respectively, u dc_pu is the per-unit value of the DC voltage, ω g_pu is the per-unit value of the grid angular frequency, ω Bg is the base value of the grid angular frequency, s is the differential operator, is the output voltage phase angle of the VSG under steady state;

[0140] Combining Figure 6 , according to the following formula, the regulating voltage is determined using the transient virtual resistance, steady-state virtual resistance, and virtual inductance:

[0141] (45)

[0142] Wherein, and are respectively the d-q components of the output voltage of the variable transient virtual damping control, and T 1 is the time constant of the variable transient virtual damping control.

[0143] Finally, the output voltage of the current loop is superimposed with the regulating voltage, and the superimposed voltage is applied to the VSG impedance model to regulate the three-phase voltage output by the VSG.

[0144] Figure 7 is the calculation block diagram of the modulation coefficient processed by the variable transient virtual impedance control scheme in the embodiment of the present invention. As Figure 7 shown, the regulating voltage is superimposed with the output voltage of the current loop and used as a new control quantity in the VSG control structure to control the converter, achieving the control objective, that is, eliminating the influence of DC voltage and AC frequency fluctuations on the stability of the VSG.

[0145] Figure 8 is the Bode diagram of the VSG impedance without suppression measures under the existing operating condition -1. Δu dc = ±10V, Δω g = 0.628rad / s, P out = 25kW. It can be seen from Figure 8 that the Bode diagram of the impedance model corresponds to with a phase angle of 0185.95° at f = 50.8Hz, indicating that negative resistance behavior will occur, which will affect the stable operation of the VSG. Similarly, the phase margin is insufficient when f < 10Hz. In short, the VSG will be unstable without additional power oscillation suppression measures.

[0146] Figure 9 is the Bode diagram of the VSG impedance with active damping control under the existing operating condition -1. Δu dc = ±10V, Δω g = 0.628rad / s, P out = 25kW. It can be seen from Figure 9 that after adding active damping control, the phase at f = 50.8Hz is within range, indicating that there is no negative resistance behavior at this time, and There is also sufficient phase margin in the low - frequency band. Therefore, adding active damping control can ensure the stable operation of the VSG when there are fluctuations in the DC bus voltage or system frequency. However, due to the phase - lag effect brought by the active damping control, if the impact of sudden power increase is considered, the VSG will become unstable.

[0147] Figure 10 Another VSG impedance Bode plot with active damping control added under the existing operating condition - 2, Δu dc = ± 10V, Δω g = 1.884rad / s, P out = 40kW, from Figure 10 it can be seen that when the system frequency fluctuation increases from 0.628rad / s to 1.884rad / s, negative - resistance behavior will appear in the f < 100Hz frequency band and the phase margin is insufficient. Similarly, , , obvious phase lag also appears in the middle - frequency band, fully indicating that the phase - lag effect of the active damping control will make the VSG unstable when the external disturbance is large.

[0148] Figure 11 The VSG impedance Bode plot with variable transient virtual impedance control added under operating condition - 2 in the embodiment of the present invention, Δu dc = ± 10V, Δω g = 1.884rad / s, P out = 40kW, from Figure 11 it can be seen that under the same operating conditions as Figure 10 the variable transient virtual impedance control has no negative - resistance behavior and has sufficient phase margin. At the same time , , the phase lag in the middle - frequency band is also significantly weakened. Therefore, it can be seen that the variable transient virtual impedance control can improve the stability of the VSG under large changes in external conditions.

[0149] The following introduces the verification process of the method for improving the stability of the virtual synchronous generator in the embodiment of the present invention.

[0150] Operating condition - 1:

[0151] DC bus voltage fluctuation: Δu dc = ± 10V, system angular - frequency fluctuation: Δω g = 0.628rad / s, test duration: 6 s, load (15 kW) is connected at 3 s.

[0152] Operating condition - 2:

[0153] DC bus voltage fluctuation: Δu dc = ±10V, system angular frequency fluctuation: Δω g = 1.884 rad / s, test duration: 6 s, load (30 kW) is connected at 3 s.

[0154] Figure 12 Schematic diagram for verifying the method of improving the control stability of the virtual synchronous generator in the embodiment of the present invention Figure 1 , such as Figure 12 shown, showing the output waveforms of the VSG without additional control under Condition-1, (a) output current, (b) output frequency, (c) active power, (d) reactive power.

[0155] Figure 13 Schematic diagram for verifying the method of improving the stability of the virtual synchronous generator in the embodiment of the present invention Figure 2 , Figure 13 showing the output waveforms of the VSG with active damping control under Condition-1, (a) output current, (b) output frequency, (c) active power, (d) reactive power. It can be seen from Figure 13 that after the load is connected at 3 s, under the action of active damping control, the VSG can operate stably, but the frequency also fluctuates greatly.

[0156] Figure 14 Schematic diagram for verifying the method of improving the control stability of the virtual synchronous generator in the embodiment of the present invention Figure 3 , Figure 14 showing the output waveforms of the VSG with active damping control under Condition-2, (a) output current, (b) output frequency, (c) active power, (d) reactive power. It can be seen from Figure 14 that after the load is connected at 3 s, although active damping control is added, the VSG still has power oscillation and the frequency also fluctuates greatly.

[0157] Figure 15 Schematic diagram for verifying the method of improving the control stability of the virtual synchronous generator in the embodiment of the present invention Figure 4 , Figure 15 showing the output waveforms of the VSG with time-varying transient virtual impedance control under Condition-2, (a) output current, (b) output frequency, (c) active power, (d) reactive power. It can be seen from Figure 15 that after the load is connected at 3 s, after adding time-varying transient virtual impedance control, the VSG can operate stably and the frequency does not fluctuate.

[0158] In summary, in the prior art, the active damping control can improve the stability of the VSG under condition - 1, but condition - 2 will cause the VSG to oscillate. By adopting the variable transient virtual impedance control scheme proposed in the embodiments of the present invention, the stable operation of the VSG can be ensured even in condition - 2 with large external changes, highlighting the effectiveness and advancement of the embodiments of the present invention. Therefore, the scheme proposed in the embodiments of the present invention can effectively solve the problem of insufficient stability of the virtual synchronous generator under the conditions of DC voltage and AC voltage frequency fluctuations, and has a positive effect on the popularization and application of the virtual synchronous generator.

[0159] In the embodiments of the present invention, a device for improving the stability of a virtual synchronous generator is also provided, as described in the following embodiments. Since the principle of solving problems by this device is similar to that of the method for improving the stability of a virtual synchronous generator, the implementation of this device can refer to the implementation of the method for improving the stability of a virtual synchronous generator, and the repeated parts will not be elaborated.

[0160] Figure 16 is a schematic diagram of the device for improving the stability of a virtual synchronous generator in the embodiments of the present invention, as Figure 16 , the device 1600 includes:

[0161] An impedance modeling module 1601 is configured to perform impedance modeling on a virtual synchronous generator (VSG) to obtain a VSG impedance model; wherein, the VSG control includes an active power loop, a reactive power loop, a voltage loop, a current loop, a governor, and a DC voltage loop;

[0162] A variable transient virtual damping control module 1602 is configured to generate an adjustment voltage based on a variable transient virtual damping control scheme; the adjustment voltage is used to adjust the three - phase voltage output by the VSG; the voltage output by the current loop is superimposed on the adjustment voltage, and the superimposed voltage is applied to the VSG impedance model to adjust the three - phase voltage output by the VSG; in the variable transient virtual damping control scheme: according to the similarity principle of the power system, the VSG impedance model is analogized with the operation principle of a synchronous motor to construct a variable transient virtual impedance control structure diagram; based on the variable transient virtual impedance control structure diagram, the transient virtual resistance is calculated using the angular frequency change and d - axis current change of the VSG output, and the steady - state virtual resistance and virtual inductor are calculated using the DC bus voltage fluctuation and DC bus output current change of the VSG; the adjustment voltage is determined using the transient virtual resistance, the steady - state virtual resistance, and the virtual inductor.

[0163] In one embodiment, the active power loop is configured to control the active power of the VSG by simulating the swing equation of a synchronous generator, the active power loop has the frequency modulation ability of the governor of the synchronous generator, and the change of the DC bus voltage is introduced into the active power loop to improve the response ability of the VSG to the change of the DC voltage.

[0164] In one embodiment, the impedance modeling module 1601 is specifically configured to: determine the output voltage expression and the DC current expression of the VSG according to the power balance principle; establish the angular frequency expression of the VSG output; introduce the DC bus voltage disturbance and the AC voltage frequency disturbance into the angular frequency expression, and derive the active power expression and the reactive power expression under the consideration of the two disturbances; perform the transformation of the VSG system coordinate system to the VSG control coordinate system for the voltage expression, the DC current expression, the angular frequency expression, the active power expression and the reactive power expression, so as to obtain the VSG impedance model under the consideration of the DC voltage disturbance and the AC voltage frequency disturbance; wherein, the VSG system coordinate system is established based on the VSG main circuit, and the VSG control coordinate system is based on the VSG control block diagram; the VSG control block diagram is used to control the VSG main circuit.

[0165] In one embodiment, in the variable transient virtual damping control scheme: according to the similarity principle of the power system, the modulation coefficient of the VSG impedance model is analogized to the rotor magnetic flux of the synchronous motor, the DC capacitance inertia time constant of the VSG impedance model is analogized to the rotor time constant of the synchronous motor, and the DC voltage of the VSG impedance model is analogized to the .

[0166] In one embodiment, the variable transient virtual damping control module is specifically configured to:

[0167] calculate the transient virtual resistance, the steady-state virtual resistance and the virtual inductance according to the following formula:

[0168] ;

[0169] In the formula, R VTVR , R VI , L VI are the transient virtual resistance, the steady-state virtual resistance and the virtual inductance respectively, , are the d-axis component of the output current reference value and the d-axis component of the output current sampled value respectively, ω 0 is the VSG angular frequency, ω 1 , ω 2 are the cut-off frequencies of the filter, ω g is the grid angular frequency, k 1 is the active coefficient of the variable transient virtual damping control, k 2 is the damping coefficient of the variable transient virtual damping control, k 3 is the virtual inertia coefficient of the variable transient virtual damping control, u dc_pu is the per-unit value of the DC voltage, ω g_pu is the per-unit value of the grid angular frequency, ω Bg is the base value of the grid angular frequency, s is the differential operator, is the output voltage phase angle of the VSG under steady state;

[0170] Determine the regulation voltage using the transient virtual resistance, steady-state virtual resistance, and virtual inductance according to the following formula:

[0171] ;

[0172] In the formula, 、 are the d-q components of the output voltage of the variable transient virtual damping control respectively, and T 1 is the time constant of the variable transient virtual damping control.

[0173] An embodiment of the present invention also provides a computer device, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, the above method for improving the stability of the virtual synchronous generator is implemented.

[0174] An embodiment of the present invention also provides a computer-readable storage medium. The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the above method for improving the stability of the virtual synchronous generator is implemented.

[0175] An embodiment of the present invention also provides a computer program product. The computer program product includes a computer program, and when the computer program is executed by a processor, the above method for improving the stability of the virtual synchronous generator is implemented.

[0176] In the embodiment of the present invention, the variable transient virtual impedance control scheme takes into account the influence of DC voltage fluctuations and system frequency fluctuations on the output of the VSG. The transient virtual resistance is calculated using the angular frequency change and d-axis current change of the VSG output, and the steady-state virtual resistance and virtual inductance are calculated using the DC bus voltage fluctuation and DC bus output current change of the VSG; the regulation voltage is determined using the transient virtual resistance, steady-state virtual resistance, and virtual inductance, and the output voltage of the current loop is superimposed with the regulation voltage and then applied to the VSG impedance model to adjust the three-phase voltage output by the VSG. The steady-state virtual resistance, virtual inductance, and transient virtual resistance are used to suppress the VSG power oscillation problem caused by DC bus voltage fluctuations and system frequency fluctuations. The embodiment of the present invention effectively suppresses the causes of VSG power oscillation, and can also have good effects during sudden power increase, avoiding the unstable phenomenon of the VSG that may occur in traditional damping control. The proposed scheme in the embodiment of the present invention can effectively solve the problem of insufficient stability of the virtual synchronous generator under the conditions of DC voltage and AC voltage frequency fluctuations, and has a positive effect on the popularization and application of the virtual synchronous generator.

[0177] Those skilled in the art should understand that the embodiments of the present invention can be provided as a method, a system, or a computer program product. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk memory, CD-ROM, optical memory, etc.) that contain computer-usable program code.

[0178] The present invention is described with reference to the flowcharts and / or block diagrams of methods, apparatuses (systems), and computer program products according to the embodiments of the present invention. It should be understood that each flow and / or block in the flowchart and / or block diagram, and the combination of flows and / or blocks in the flowchart and / or block diagram, can be realized by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate means for realizing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0179] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing devices to work in a specific manner, such that the instructions stored in the computer-readable memory generate a manufactured article including instruction means that realizes the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0180] These computer program instructions can also be loaded onto a computer or other programmable data processing devices, such that a series of operation steps are executed on the computer or other programmable devices to generate a computer-implemented process, and thus the instructions executed on the computer or other programmable devices provide steps for realizing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0181] The specific embodiments described above further elaborate on the objectives, technical solutions, and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for improving the stability of a virtual synchronous generator, characterized in that: include: The impedance modeling of the virtual synchronous generator VSG is carried out to obtain the VSG impedance model; wherein, the VSG control includes active power loop, reactive power loop, voltage loop, current loop, speed regulator, and DC voltage loop; Based on the variable transient virtual damping control scheme, a regulating voltage is generated; the regulating voltage is used to regulate the three-phase voltage output by the VSG; in the variable transient virtual damping control scheme: according to the principle of power system similarity, the VSG impedance model is compared with the operation principle of the synchronous motor to construct a virtual impedance control structure diagram; based on the virtual impedance control structure diagram, the transient virtual resistance is calculated using the angular frequency change and d-axis current change of the VSG output, and the steady-state virtual resistance and virtual inductance are calculated using the DC bus voltage fluctuation and DC bus output current change of the VSG; the regulating voltage is determined using the transient virtual resistance, steady-state virtual resistance and virtual inductance; The current loop output voltage is superimposed with the adjustment voltage, and the superimposed voltage is applied to the VSG impedance model to adjust the three-phase voltage output by the VSG; The impedance modeling of the virtual synchronous generator VSG is performed to obtain the VSG impedance model, including: According to the power balance principle, determine the output voltage expression and DC current expression of VSG; Establish the angular frequency expression of VSG output; Introduce DC bus voltage disturbance and AC voltage frequency disturbance into the angular frequency expression, and derive the active power expression and reactive power expression considering the two disturbances; The voltage expression, DC current expression, angular frequency expression, active power expression and reactive power expression are transformed from the VSG system coordinate system to the VSG control coordinate system to obtain the VSG impedance model considering DC voltage disturbance and AC voltage frequency disturbance; wherein, the VSG system coordinate system is established based on the VSG main circuit, and the VSG control coordinate system is established based on the VSG control block diagram; the VSG control block diagram is used to control the VSG main circuit.

2. The method according to claim 1, characterized in that The active power loop is used to control the VSG by simulating the swing equation of the synchronous generator. The active power loop has the frequency regulation capability of the synchronous generator speed regulator to perform active power control.

3. The method according to claim 1, characterized in that In the variable transient virtual damping control scheme: according to the power system similarity principle, the modulation coefficient of the VSG impedance model is analogized to the rotor flux of the synchronous motor, the DC capacitor inertia time constant of the VSG impedance model is analogized to the rotor time constant of the synchronous motor, and the DC voltage of the VSG impedance model is analogized to the angular frequency of the synchronous motor.

4. The method according to claim 1, characterized in that Generate regulated voltage based on variable transient virtual damping control scheme, including: The transient virtual resistance, steady-state virtual resistance and virtual inductance are calculated according to the following formula: ； In the formula, R VTVR 、R VI 、L VI They are transient virtual resistance, steady-state virtual resistance and virtual inductance, , They are the d-axis component of the output current reference value and the d-axis component of the output current sampling value, respectively. ω 0 is the VSG angular frequency, ω 1. ω 2 is the cutoff frequency of the filter, ω g is the grid angular frequency, k 1 is the active coefficient of transient virtual damping control, k 2 is the damping coefficient of transient virtual damping control, k 3 is the virtual inertia coefficient of transient virtual damping control, u dc_pu is the DC voltage per unit value, ω g_pu is the per unit value of the grid angular frequency, ω Bg is the grid angular frequency base value, s is the differential operator, is the output voltage phase angle of VSG in steady state; The regulation voltage is determined using the transient virtual resistance, steady-state virtual resistance, and virtual inductance as follows: ； In the formula, , are the dq components of the output voltage of the transient virtual damping control, T 1 is the time constant of transient virtual damping control.

5. A virtual synchronous generator stability improvement device, characterized in that: include: The impedance modeling module is used to perform impedance modeling on the virtual synchronous generator VSG to obtain a VSG impedance model; wherein the VSG control includes an active power loop, a reactive power loop, a voltage loop, a current loop, a speed regulator, and a DC voltage loop; A variable transient virtual damping control module is used to generate a regulating voltage based on a variable transient virtual damping control scheme; the regulating voltage is used to regulate the three-phase voltage output by the VSG; the current loop output voltage is superimposed on the regulating voltage, and the superimposed voltage is applied to the VSG impedance model to regulate the three-phase voltage output by the VSG; in the variable transient virtual damping control scheme: according to the power system similarity principle, the VSG impedance model is compared with the operation principle of the synchronous motor to construct a virtual impedance control structure diagram; based on the virtual impedance control structure diagram, the transient virtual resistance is calculated using the angular frequency change and d-axis current change of the VSG output, and the steady-state virtual resistance and virtual inductance are calculated using the DC bus voltage fluctuation and DC bus output current change of the VSG; the regulating voltage is determined using the transient virtual resistance, steady-state virtual resistance and virtual inductance; The impedance modeling module is specifically used for: According to the power balance principle, determine the output voltage expression and DC current expression of VSG; Establish the angular frequency expression of VSG output; Introduce DC bus voltage disturbance and AC voltage frequency disturbance into the angular frequency expression, and derive the active power expression and reactive power expression considering the two disturbances; The voltage expression, DC current expression, angular frequency expression, active power expression and reactive power expression are transformed from the VSG system coordinate system to the VSG control coordinate system to obtain the VSG impedance model considering DC voltage disturbance and AC voltage frequency disturbance; wherein, the VSG system coordinate system is established based on the VSG main circuit, and the VSG control coordinate system is established based on the VSG control block diagram; the VSG control block diagram is used to control the VSG main circuit.

6. The device according to claim 5, characterized in that The active power loop is used to control the VSG by simulating the swing equation of the synchronous generator. The active power loop has the frequency regulation capability of the synchronous generator speed regulator to perform active power control.

7. The device according to claim 5, characterized in that In the variable transient virtual damping control scheme: according to the power system similarity principle, the modulation coefficient of the VSG impedance model is analogized to the rotor flux of the synchronous motor, the DC capacitor inertia time constant of the VSG impedance model is analogized to the rotor time constant of the synchronous motor, and the DC voltage of the VSG impedance model is analogized to the angular frequency of the synchronous motor.

8. The device according to claim 5, characterized in that The transient virtual damping control module is specifically used for: The transient virtual resistance, steady-state virtual resistance and virtual inductance are calculated according to the following formula: ； In the formula, R VTVR 、R VI 、L VI They are transient virtual resistance, steady-state virtual resistance and virtual inductance, , They are the d-axis component of the output current reference value and the d-axis component of the output current sampling value, respectively. ω 0 is the VSG angular frequency, ω 1. ω 2 is the cutoff frequency of the filter, ω g is the grid angular frequency, k 1 is the active coefficient of transient virtual damping control, k 2 is the damping coefficient of transient virtual damping control, k 3 is the virtual inertia coefficient of transient virtual damping control, u dc_pu is the DC voltage per unit value, ω g_pu is the per unit value of the grid angular frequency, ω Bg is the grid angular frequency base value, s is the differential operator, is the output voltage phase angle of VSG in steady state; The regulation voltage is determined using the transient virtual resistance, steady-state virtual resistance, and virtual inductance as follows: ； In the formula, , are the dq components of the output voltage of the transient virtual damping control, T 1 is the time constant of transient virtual damping control.

9. A computer device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the method according to any one of claims 1 to 4 is implemented.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the method according to any one of claims 1 to 4 is implemented.

11. A computer program product, characterized in that The computer program product comprises a computer program, and when the computer program is executed by a processor, the method according to any one of claims 1 to 4 is implemented.

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