VSG control method based on frequency deviation feedforward-feedback composite control

Abstract

The invention discloses a VSG control method based on frequency deviation feedforward-feedback composite control, and belongs to the technical field of power system control, after a power coupling model based on VSG control is established, reactive power feedforward control GDQ (s) based on frequency deviation is introduced, and enough transient damping support is provided for a system; meanwhile, active feedback control GDP (s) based on frequency deviation is introduced, the active steady-state deviation of the VSG system after GDP (s) active power compensation is introduced is calculated, parameter design is carried out on feedforward-feedback control based on frequency deviation, transient damping support is achieved, and meanwhile the active steady-state deviation caused by fixed virtual damping in VSG control under power grid frequency disturbance is eliminated. The control parameter design is simple, and the stability of the virtual synchronous generator grid-connected system is improved.

Description

Technical Field

[0001] This invention relates to the field of power system control technology, specifically to a VSG control method based on frequency deviation feedforward-feedback composite control. Background Technology

[0002] With the high proportion of renewable energy generation integrated into the power system, traditional synchronous generators are gradually being replaced by power electronic converters, leading to a significant decrease in the system's inherent damping and inertia. While the rapid dynamic response characteristics of numerous power electronic devices improve system flexibility, they also introduce new oscillation risks, such as low-frequency oscillations and subsynchronous oscillations, seriously threatening the stable operation of the power grid. Virtual Synchronighters (VSG) control technology simulates the inertia and damping characteristics of synchronous generators to provide the system with necessary inertial support and damping capabilities, thereby improving the stability of renewable energy grid-connected systems. However, when the control parameters are not designed properly, the system's transient stability cannot be guaranteed; and when the system experiences a step disturbance in the grid frequency, the constant damping parameters of traditional virtual synchronous generators can also introduce a certain active power steady-state deviation into the system.

[0003] To address the aforementioned issues, traditional VSG control assumes that fixed damping D is the root cause of steady-state deviation and can only be mitigated through variable damping strategies. However, variable damping algorithms are complex and may affect transient response. Enhancing transient damping (e.g., increasing D) is likely to exacerbate steady-state deviation, while eliminating it (e.g., adaptive damping) may weaken transient response. Existing technologies generally consider that the active-frequency loop and reactive-voltage loop of a VSG are inherently coupled (e.g., power angle-voltage interaction). Adjusting both control loops simultaneously may trigger unstable oscillations or control conflicts. Under this conventional thinking, optimization is typically limited to a single loop (e.g., enhanced active damping or reactive voltage support). For example, patent application CN 118739399 A discloses a three-level VSG feedforward control method based on enhanced transient damping. This method employs a feedforward control strategy on the active control loop, increasing the system's damping ratio from the perspective of the system's closed-loop eigenvalues ​​without altering the system's steady-state characteristics, thereby changing the system's dynamic characteristics. These control methods take into account fluctuations in system output active power and frequency overshoot, enabling the system to effectively suppress fluctuations and reduce oscillations when sudden changes in active power or frequency occur. However, such control methods still do not solve the problem of active power steady-state deviation caused by constant damping parameters.

[0004] Therefore, there is an urgent need for a composite control method that can eliminate active steady-state deviations while providing sufficient transient damping, so as to improve the overall stability of VSG grid-connected systems. Summary of the Invention

[0005] This invention addresses the problem of overly simplistic solutions in existing technologies by providing a significantly different approach. It primarily offers a VSG control method based on frequency deviation feedforward-feedback composite control. By introducing reactive power feedforward control to enhance transient damping and combining it with active power feedback control to eliminate steady-state deviation, it achieves accurate active power tracking while ensuring dynamic response. This effectively solves the problem of coexistence of transient oscillations and steady-state errors in traditional VSG control.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0007] A VSG control method based on frequency deviation feedforward-feedback composite control is proposed, which involves introducing reactive power feedforward control G based on frequency deviation, based on a power coupling model. D_Q (s) and active power feedback control G D_P (s);

[0008] G D_Q The transfer function of (s) is:

[0009]

[0010] G D_P The transfer function of (s) is:

[0011]

[0012] In the formula, D and J are the virtual damping and virtual inertia coefficients of the active loop controlled by VSG, respectively; ω n is the rated frequency; s represents the Laplace operator; T, k1, and k2 are the reactive power feedforward control coefficients; k3 is an adjustable parameter used to control the steady-state deviation elimination effect.

[0013] This invention introduces reactive power feedforward control G based on frequency deviation. D_Q (s) provides sufficient transient damping support for the system; at the same time, by designing parameters k3 and T, the dynamic performance of the active power of the VSG system connected to the grid is optimized and the active power steady-state deviation caused by the original VSG constant damping under grid frequency disturbance is eliminated, thereby improving the transient stability of the VSG grid-connected system.

[0014] Further analysis shows that this invention utilizes G D_P The precise feedback compensation of (s) proves that steady-state deviation can still be eliminated under a fixed-damping architecture, overturning the traditional design concept (fixed damping D is the root cause of steady-state deviation and can only be mitigated by variable damping strategies). This invention achieves this through structured design of the transfer function (such as G). D_Q (s) and G D_P(s) Share k1, k2, T parameters), ensuring natural coordination between feedforward and feedback, breaking through the traditional understanding that "damping and steady state cannot be achieved simultaneously", proving that the synergistic effect of reactive feedforward + active feedback can simultaneously improve dynamic and steady-state performance.

[0015] Specifically, in the power coupling model, the relationship between voltage and current in the VSG system is as follows:

[0016]

[0017] In the formula, e abc i abc These represent the inverter output voltage and current, u gabc Let R be the grid voltage, and R and L be the total resistance and total inductance of the filter and the grid impedance, respectively. s represents the Laplace operator.

[0018] Furthermore, reactive power feedforward control G based on frequency deviation is introduced. D_Q (s) and active power feedback control G D_P Before (s), the system output active power and reactive power are respectively

[0019]

[0020] In the formula, P and Q represent the active and reactive power output of the VSG, respectively; e d e q These represent the voltages of the converter in the dq coordinate system; i d i q Let be the grid-connected current in the dq coordinate system.

[0021] Furthermore, a control mechanism is introduced. Then, the control equations for the active power loop and the reactive power loop are:

[0022]

[0023] In the formula, k q U is the reactive power-voltage droop factor, ω represents the VSG output frequency, and U n The rated voltage is P, where E is the inverter output voltage amplitude, and P is the rated voltage. ref Q ref These are the reference values ​​for active power and reactive power, respectively, and P and Q are the active power and reactive power output by the VSG, respectively.

[0024] Furthermore, under grid frequency disturbances, the system active power steady-state deviation is:

[0025]

[0026] In the formula, ω g This refers to the power grid frequency.

[0027] Furthermore, after introducing the active feedback control G D_P (s), the control equations of the active power loop and the reactive power loop are expressed as:

[0028]

[0029] In the formula, k q is the reactive - voltage droop coefficient, ω represents the output frequency of the VSG, U n is the rated voltage, E is the amplitude of the inverter output voltage, P ref , Q ref are the reference values of the active power and the reactive power respectively, and P and Q are the active power and the reactive power output by the VSG respectively.

[0030] Furthermore, after introducing the active feedback control G D_P (s), the steady - state active - power deviation of the VSG system is:

[0031]

[0032] In the formula, ω g is the grid frequency.

[0033] It can be seen from this formula that when k3 = k1 / k2, the steady - state active - power deviation of the system is zero, that is, the steady - state deviation caused by the fixed damping D is completely eliminated; when k3 > k1 / k2, the steady - state active - power deviation of the system has the same sign as the change in the grid frequency; when k3 < k1 / k2 in G D_P (s), the steady - state active - power deviation of the system has the opposite sign to the change in the grid frequency. At the same time, when the parameter T changes, it does not change the magnitude of the steady - state active - power deviation of the system.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] Based on the power - coupling model, the present invention introduces the reactive - power feed - forward control G D_Q (s) based on the frequency deviation and the active feedback control G D_P (s) to form a composite control strategy. In the transient process, G D_Q (s) provides fast damping support to reduce power oscillations and frequency overshoot; in the steady - state process, G D_P (s) accurately compensates the active power deviation to ensure that the power output tracks the reference value without static error. Moreover, the coefficient T is independent. By adjusting T, the dynamic response can be optimized without affecting the steady - state performance, which is beneficial to improving the control flexibility. Compared with the method of using differential feed - forward and power feed - forward in the prior art to improve the system damping, suppress the active - power fluctuation and frequency overshoot, the present invention breaks through the traditional framework of "single - loop optimization", eliminates the steady - state active - power deviation while providing sufficient transient damping, and can improve the comprehensive stability of the VSG grid - connected system.

[0036] The present invention will be explained in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0037] Figure 1 This is a structural diagram of the frequency deviation-feedforward-feedback composite control in this invention;

[0038] Figure 2 The above is a comparison chart of system frequencies under traditional VSG control and system under frequency deviation-feedforward-feedback composite control when the active power experiences a step disturbance in the embodiment.

[0039] Figure 3 The diagram shows a comparison of the active power of the system under traditional VSG control and the system under frequency deviation feedforward-feedback composite control when the grid frequency experiences a step disturbance.

[0040] Figure 4 This is a comparison chart of the active power steady-state error analysis of the system under different parameters k3 in the embodiment;

[0041] Figure 5 This is a comparison chart of system frequencies under different parameters T in the embodiment. Detailed Implementation

[0042] To facilitate understanding of the present invention, a more comprehensive description of the present invention will be given below with reference to the accompanying drawings, which illustrate several embodiments of the present invention. However, the present invention can be implemented in different forms and is not limited to the embodiments described in the text. Rather, these embodiments are provided to make the disclosure of the present invention more thorough and complete.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly associated with those skilled in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0044] Example: A VSG control method based on frequency deviation feedforward-feedback composite control, comprising the following steps:

[0045] Step 1: Establish a power coupling model based on VSG control, and introduce reactive power feedforward control G based on frequency deviation. D_Q (s) provides sufficient transient damping support for the system and improves the transient stability of the VSG grid-connected system.

[0046] Step 2: Based on Step 1, introduce active power feedback control G based on frequency deviation. D_P(s), calculate the introduced G D_P (s) The active power steady-state deviation of the VSG system after active power compensation;

[0047] Step 3, based on the introduced G calculated in Step 2 D_P (s) The active power steady-state deviation of the VSG system after active power compensation, design the parameters for the frequency deviation feedforward-feedback composite control, while achieving transient damping support, eliminate the original VSG fixed damping steady-state deviation.

[0048] Among them, introduce the reactive power feedforward control G D_Q (s) based on the frequency deviation, calculate the frequency deviation (ω - ω ref ) through the active power deviation (P n -P), and feed it forward to the reactive power control loop to form additional damping compensation; during the transient process (such as load mutation or grid frequency disturbance), this feedforward term can quickly adjust the reactive power output, enhance the damping characteristics of the system, and suppress power oscillation and frequency fluctuation. Introduce the active power feedback control G D_P (s) based on the frequency deviation, adjust the active power output through frequency deviation feedback. When k3 = k1 / k2, the steady-state deviation caused by the fixed damping D can be completely eliminated (i.e., P error2 = 0); when k3 > k1 / k2 or k3 < k1 / k2, the direction and magnitude of the active power steady-state deviation can be flexibly adjusted to adapt to different grid operation requirements.

[0049] Figure 1 The frequency deviation feedforward-feedback composite control strategy and system topology of the present invention application are given.

[0050] In Step 1, establish a power coupling model based on VSG control. The relationship between the voltage and current of the VSG system is

[0051] (1)

[0052] In the formula, e abc , i abc are the inverter output voltage and current respectively, u gabc is the grid voltage, R and L are the total resistance and total inductance of the filter and the grid impedance respectively, and s represents the Laplace operator.

[0053] The virtual synchronous generator realizes power control by controlling the inverter output voltage and phase angle. Let the phase angle of the grid voltage be zero. The expressions of the grid voltage and the converter output voltage in the dq coordinate system are

[0054] (2)

[0055] In the formula, e d , eq u d u q Let E be the voltage of the inverter and the grid in the dq coordinate system, respectively; E is the amplitude of the inverter output voltage; U is the magnitude of the grid voltage; and δ is the phase angle difference between the inverter output voltage and the grid voltage.

[0056] By performing a dq coordinate transformation on equation (1) and combining it with equation (2), we can obtain the expression for the grid-connected current in the dq coordinate system:

[0057] (3)

[0058] In the formula, i d i q Let X be the grid-connected current in the dq coordinate system, and X be the total system reactance.

[0059] From equations (2) and (3), we can obtain that the system output active power and reactive power are respectively

[0060] (4)

[0061] Small-signal linearization of equation (4) yields

[0062] (5)

[0063] In the formula, H Pδ (s), H PE (s), H Qδ (s), H QE (s) represent the transfer functions between the output active power, reactive power, phase angle difference, and voltage of the VSG grid-connected system, respectively.

[0064] Introducing reactive power feedforward control G based on frequency deviation D_Q (s), providing sufficient transient damping support for the system, G D_Q The transfer function of (s) is:

[0065] (6)

[0066] In the formula, D and J are the virtual damping and virtual inertia coefficients of the active loop controlled by VSG, respectively, and ω n Where T is the rated frequency, and k1 and k2 are the reactive power feedforward control coefficients.

[0067] Introducing control links Then, the control equations for the active power loop and the reactive power loop are:

[0068] (7)

[0069] In the formula, k qU is the reactive power-voltage droop factor, ω represents the VSG output frequency, and U n For the rated voltage, P ref Q ref These are the reference values ​​for active power and reactive power, respectively, and P and Q are the active power and reactive power output by the VSG, respectively.

[0070] Under grid frequency disturbances, the system active power steady-state deviation is

[0071] (8)

[0072] In the formula, ω g This refers to the power grid frequency.

[0073] As can be seen from formula (8), when grid frequency disturbance occurs, the grid-connected system under traditional VSG control has a certain active power stability deviation, and the magnitude of steady-state error is proportional to the magnitude of virtual damping.

[0074] In step 2, to eliminate the active steady-state deviation caused by virtual damping under grid frequency disturbances, active feedback control G based on frequency deviation is introduced. D_P (s):

[0075] (9)

[0076] In the formula, k3 is an adjustable parameter.

[0077] At this point, the control equations for the active power loop and the reactive power loop are expressed as follows:

[0078] (10)

[0079] Secondly, the calculation introduces G D_P (s) Active steady-state deviation of the VSG system after active power compensation. Combining equations (5) and (10), the closed-loop transfer function of the VSG system under active power disturbance and grid frequency disturbance is obtained:

[0080] (11)

[0081] (12)

[0082] In the formula, the expressions for G1(s), G2(s), G3(s), and G4(s) are respectively: G1(s) = Jω n s+Dω n -G D_P (s), G2(s) = 1 / (k q s), G3(s) = 1 + G2(s)H QE (s), G4(s) = s G D_Q (s) -HQδ (s);

[0083] At this time, the active power steady-state deviation of the VSG system is:

[0084] (13)

[0085] In step 3, parameter design is carried out for the feedforward-feedback composite control based on frequency deviation. While realizing transient damping support, the original VSG fixed-damping steady-state deviation is eliminated.

[0086] It can be seen from Equation (13) that when k3 = k1 / k2, the active power steady-state deviation of the system is zero; when k3 > k1 / k2, the active power steady-state deviation of the system has the same sign as the power grid frequency change; when D_P k3 < k1 / k2 in G(s), the active power steady-state deviation of the system has the opposite sign to the power grid frequency change. At the same time, when the parameter T changes, the magnitude of the active power steady-state deviation of the system is not changed.

[0087] Next, in combination with the attached Figure 2-5 , the effectiveness of a VSG control method based on feedforward-feedback composite control of frequency deviation provided by the present invention is described:

[0088] Figure 2 This is the system frequency dynamic characteristic curve of the embodiment of the present invention under fixed damping, introducing G D_Q (s), G​​​​​​​​​​​​​​Active steady-state deviation under different values ​​of parameter k3 in (s). At t=12s, the grid frequency increases by 0.05Hz, from Figure 4 It can be seen that when k3 = k1 / k2, the system's active steady-state error is zero, and the simulation results are consistent with the analysis results of formula (13). When k3 > k1 / k2, the system's active steady-state deviation is positive, and conversely, the system's active steady-state deviation is negative. Therefore, the active steady-state deviation is not only related to the grid frequency disturbance, but also proportional to the magnitude of the difference between k3 and k1 / k2.

[0091] To study the dynamic characteristics of parameter T with respect to system frequency in the control loop, Figure 5 The following are the dynamic frequency characteristic curves of the system under different parameters T, when the reference value of the system active power jumps from 20kW to 50kW according to an embodiment of the present invention. Figure 5 It can be seen that the steady-state recovery time is roughly the same under different parameters, and the system frequency overshoot decreases as the parameter T decreases. Therefore, designing the system parameters can eliminate active steady-state error while improving the system's transient stability.

[0092] From the above, it can be concluded that, with the addition of the control loop of this invention, the system exhibits better self-adjustment capability in the face of different disturbances, effectively reducing the system frequency offset and frequency change rate, resulting in good dynamic characteristics and improved system stability when subjected to disturbances. Furthermore, the proposed VSG control method based on frequency deviation feedforward-feedback composite control can effectively eliminate active steady-state deviations caused by grid frequency disturbances.

[0093] The present invention has been described by way of example in conjunction with the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvement made by adopting the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, shall be within the protection scope of the present invention.

Claims

1. A VSG control method based on frequency deviation feedforward-feedback composite control, characterized in that: Based on the power coupling model, reactive power feedforward control G based on frequency deviation is introduced. D_Q (s) and active power feedback control G D_P (s); G D_Q The transfer function of (s) is: G D_P The transfer function of (s) is: In the formula, D and J are the virtual damping and virtual inertia coefficients of the active loop controlled by VSG, respectively; ω n is the rated frequency; s represents the Laplace operator; T, k1, and k2 are the reactive power feedforward control coefficients; k3 is an adjustable parameter.

2. The VSG control method based on frequency deviation feedforward-feedback composite control according to claim 1, characterized in that: In the power coupling model, the relationship between voltage and current in the VSG system is as follows: In the formula, e abc i abc These represent the inverter output voltage and current, u gabc Let R be the grid voltage, and R and L be the total resistance and total inductance of the filter and the grid impedance, respectively. s represents the Laplace operator.

3. The VSG control method based on frequency deviation feedforward-feedback composite control according to claim 2, characterized in that: Introducing reactive power feedforward control G based on frequency deviation D_Q (s) and active power feedback control G D_P Before (s), the system output active power and reactive power are respectively In the formula, P and Q represent the active and reactive power output of the VSG, respectively; e d e q These represent the voltages of the converter in the dq coordinate system; i d i q Let be the grid-connected current in the dq coordinate system.

4. The VSG control method based on frequency deviation feedforward-feedback composite control according to claim 1, characterized in that: Introducing control links Then, the control equations for the active power loop and the reactive power loop are: In the formula, k q U is the reactive power-voltage droop factor, ω represents the VSG output frequency, and U n The rated voltage is P, where E is the inverter output voltage amplitude, and P is the rated voltage. ref Q ref These are the reference values ​​for active power and reactive power, respectively, and P and Q are the active power and reactive power output by the VSG, respectively.

5. The VSG control method based on frequency deviation feedforward-feedback composite control according to claim 1, characterized in that: Under grid frequency disturbances, the system active power steady-state deviation is In the formula, ω g This refers to the power grid frequency.

6. The VSG control method based on frequency deviation feedforward-feedback composite control according to claim 1, characterized in that: Introducing active power feedback control G D_P After (s), the control equations for the active power loop and the reactive power loop are expressed as: In the formula, k q U is the reactive power-voltage droop factor, ω represents the VSG output frequency, and U n The rated voltage is P, where E is the inverter output voltage amplitude, and P is the rated voltage. ref Q ref These are the reference values ​​for active power and reactive power, respectively, and P and Q are the active power and reactive power output by the VSG, respectively.

7. The VSG control method based on frequency deviation feedforward-feedback composite control according to claim 6, characterized in that: Introducing active power feedback control G D_P After (s), the active steady-state deviation of the VSG system is: In the formula, ω g This refers to the power grid frequency.

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