Photovoltaic VSG control method based on virtual inertia and damping

Abstract

The invention provides a photovoltaic VSG control method based on virtual inertia and damping, and the method comprises the steps: designing a topological structure of a photovoltaic virtual synchronous generator for modeling, and obtaining a power and power angle equation of the virtual synchronous generator; constructing a function expression of the active power of the virtual synchronous generator and combining the function expression with a power and power angle equation of the virtual synchronous generator to obtain a second-order transfer function so as to obtain a step response curve of the output power and the angular frequency of the virtual synchronous generator; secondly, analyzing an influence mechanism of changes of virtual inertia and damping on the dynamic performance of the virtual synchronous generator based on a step response curve, obtaining self-adaptive principles of the virtual inertia and damping in different states based on the influence mechanism to construct a self-adaptive function, and finally setting parameters in the self-adaptive function and designing a starting criterion. Therefore, the photovoltaic virtual synchronous generator can be controlled. Compared with the prior art, the fluctuation of power and frequency can be better suppressed, and the stability of the system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic virtual synchronous generator control, and in particular to a photovoltaic VSG control method based on virtual inertia and damping. Background Art

[0002] Photovoltaic power generation and other new energy generation technologies have gradually become a research hotspot. Energy generation is mainly connected to the large power grid through inverters in the form of distributed power sources. With the large-scale penetration of new energy distributed power sources, the popularity of inverters as the main interface of power electronic grid-connected technology has also increased. Due to the characteristics of weak damping and lack of rotational inertia support of power electronic devices such as inverters, when power disturbances or load mutations occur, the oscillation of the distributed power generation system cannot be suppressed, thus threatening the operation safety and stability of the entire power grid. Therefore, virtual synchronous generator (VSG) control technology can be introduced in inverter control, which can simulate the external characteristics of synchronous generators and enable grid-connected inverters to have similar rotational inertia and damping as synchronous generators. However, the traditional VSG parameters are set to fixed values, and the fixed value parameter settings may not be applicable to all working scenarios. In some cases, the power oscillation amplitude may exceed the range, causing the VSG to stop working. The virtual inertia and damping parameters in the VSG motion equation are virtual quantities, which are not constrained by physical characteristics and can be dynamically adjusted.

[0003] Based on the characteristics of flexible design of virtual inertia and damping coefficient in VSG control, scholars have done a lot of research on parameter adaptation of VSG. For example, some researchers have proposed an adaptive virtual inertia control strategy that responds quickly to load disturbances in island mode, which effectively suppresses frequency fluctuations; some researchers have proposed an improved bang-bang adaptive virtual inertia VSG control strategy, which can dynamically adjust the rotational inertia in real time according to the frequency change caused by load disturbances, avoiding rapid frequency fluctuations, thereby improving the frequency response characteristics, but the influence of the damping coefficient is not considered; some researchers have proposed an adaptive control strategy based on damping ratio constraints, which adjusts the virtual inertia according to the rate of change of VSG output frequency, and then controls the value of the damping coefficient through the damping ratio, and ensures the stability of the system by making the system work in an under-damped state; some researchers have proposed to apply adaptive virtual inertia and damping coefficient to grid-connected inverters, which effectively improves the power and frequency oscillations in transient processes, but no start-up judgment conditions are set for damping adaptation; in summary, the existing technology still has poor ability to suppress power and frequency fluctuations, resulting in low system stability. Summary of the invention

[0004] The present invention provides a photovoltaic VSG control method based on virtual inertia and damping, the purpose of which is to better suppress power and frequency fluctuations and improve system stability.

[0005] In order to achieve the above object, the present invention provides a photovoltaic VSG control method based on virtual inertia and damping, comprising:

[0006] Step 1, modeling the virtual synchronous generator based on the designed photovoltaic VSG grid-connected topology structure to obtain the power and power angle equations of the virtual synchronous generator, and the photovoltaic VSG grid-connected topology structure includes a main topology circuit and a control loop;

[0007] Step 2, constructing a function expression between the active power output by the virtual synchronous generator and the line reactance, electromotive force amplitude, output terminal voltage, and power angle according to the main topology circuit, and combining the function expression with the power and power angle equations of the virtual synchronous generator to obtain the first second-order transfer function and the second second-order transfer function;

[0008] Step 3, performing inverse Laplace transform on the first second-order transfer function and the second second-order transfer function respectively, to obtain a step effect of active power when the virtual inertia changes, a step response curve of angular frequency when the virtual inertia changes, a step response curve of active power when the damping changes, and a step response curve of angular frequency when the damping changes;

[0009] Step 4, analyzing the step effect of active power when the virtual inertia changes, the step response curve of angular frequency when the virtual inertia changes, the step response curve of active power when the damping changes, and the step response curve of angular frequency when the damping changes, to obtain the influence mechanism of the virtual inertia change on the dynamic performance of the virtual synchronous generator and the influence mechanism of the damping change on the dynamic performance of the virtual synchronous generator;

[0010] Step 5, based on the influence mechanism of virtual inertia change on the dynamic performance of virtual synchronous generator and the influence mechanism of damping change on the dynamic performance of virtual synchronous generator, the adaptive principle of virtual inertia and damping in different states is obtained;

[0011] Step 6: construct an adaptive function of virtual inertia and damping based on the adaptive principle, adjust the parameters in the adaptive function, and determine the adaptive starting criterion of the virtual inertia and damping for controlling the photovoltaic virtual synchronous generator.

[0012] Specifically, the power and angle equations of the virtual synchronous generator are:

[0013]

[0014] Where J represents virtual inertia, P refrepresents the active power input to the virtual synchronous generator, represents the active power output from the virtual synchronous generator, δ represents the power angle of the virtual synchronous generator, ω represents the output angular frequency of the virtual synchronous generator, ω0 represents the rated angular frequency of the power grid, and D p Indicates the damping coefficient corresponding to the damping torque.

[0015] Furthermore, the functional expression between the active power output of the virtual synchronous generator constructed according to the main topology circuit and the line reactance, electromotive force amplitude, output terminal voltage, and power angle is:

[0016]

[0017] Where E represents the electromotive force amplitude obtained by active-frequency modulation, U l Represents the output voltage, Z f Indicates the filter circuit impedance, X f Represents the reactance of the filter circuit.

[0018] Furthermore, the function expression is combined with the power and power angle equations of the virtual synchronous generator to obtain the first second-order transfer function and the second second-order transfer function, including:

[0019] According to the analysis method of the traditional synchronous generator small signal model, combined with the function expression and the power and power angle equations of the virtual synchronous generator, the first and second order transfer functions from the active power input to the virtual synchronous generator to the active power output of the virtual synchronous generator are obtained as follows:

[0020]

[0021] The second-order transfer function from the active power of the input virtual synchronous generator to the output angular frequency of the virtual synchronous generator is obtained as:

[0022]

[0023] Among them, ω n Indicates the rated angular frequency of the virtual synchronous generator.

[0024] Specifically, the first second-order transfer function and the second second-order transfer function are respectively subjected to an inverse Laplace transform, including:

[0025] Performing inverse Laplace transform on the first and second order transfer functions yields:

[0026]

[0027] Taking the inverse Laplace transform of the second-order transfer function, we get:

[0028]

[0029] Among them, ω n Indicates the rated angular frequency of the virtual synchronous generator.

[0030] More specifically, step 4 includes:

[0031] The step effect of active power when the virtual inertia changes and the step response curve of angular frequency when the virtual inertia changes are analyzed, and the influence mechanism of virtual inertia change on the dynamic performance of virtual synchronous generator is obtained as follows:

[0032] When the virtual inertia increases, the peak time and stabilization time of the output frequency increase, and the overshoot decreases;

[0033] The step response curve of active power when damping changes and the step response curve of angular frequency when damping changes are analyzed, and the influence mechanism of damping change on the dynamic performance of virtual synchronous generator is obtained as follows:

[0034] For the step response curve of active power, when the damping increases, the peak time of active power increases, and the overshoot and stabilization time decrease;

[0035] For the step response curve of angular frequency, when the damping increases, the peak time, overshoot and settling time are reduced.

[0036] To further explain, the adaptive principle of virtual inertia and damping in different states is:

[0037] In the first state interval, the angular frequency offset and the angular frequency change rate are both greater than 0, the virtual inertia increases, and the damping increases;

[0038] In the second state interval, the angular frequency offset is greater than 0, the angular frequency change rate is less than 0, the virtual inertia decreases, and the damping increases;

[0039] In the third state interval, the angular frequency offset and the angular frequency change rate are both less than 0, the virtual inertia increases, and the damping increases;

[0040] In the fourth state interval, the angular frequency offset is less than 0, the angular frequency change rate is greater than 0, the virtual inertia decreases, and the damping increases.

[0041] Furthermore, the adaptive functions of virtual inertia and damping are constructed based on the adaptive principle, including:

[0042] The adaptive function of constructing virtual inertia based on the adaptive principle is:

[0043]

[0044] The adaptive function of damping based on the adaptive principle is:

[0045]

[0046] Where J0 represents the basic value of virtual inertia in the virtual synchronous generator during stable operation, D0 represents the basic value of damping in the virtual synchronous generator during stable operation, and K J1 , K J2 Both represent the adaptive adjustment coefficient of virtual inertia, K D Represents the adaptive adjustment coefficient of damping, T j Indicates the angular frequency change rate threshold, T d Indicates the angular frequency offset change threshold.

[0047] Furthermore, the parameters in the adaptive function are adjusted, including:

[0048] The basic value of damping in the virtual synchronous generator during stable operation is adjusted to obtain:

[0049]

[0050] The basic value of the virtual inertia in the virtual synchronous generator during stable operation is adjusted to obtain:

[0051]

[0052] The adaptive adjustment coefficient of virtual inertia is adjusted to obtain:

[0053]

[0054] The adaptive adjustment coefficient of damping is adjusted to obtain:

[0055]

[0056] Where ΔT represents the mechanical torque T of the virtual synchronous generator m and electromagnetic torque T e The difference between ΔT and T m -T e , P max Indicates the maximum power output of the virtual synchronous generator, J max Indicates the maximum value of virtual inertia, J min Indicates the minimum value of virtual inertia, D max Indicates the maximum value of damping.

[0057] The above scheme of the present invention has the following beneficial effects:

[0058] The present invention first designs a photovoltaic VSG grid-connected topology structure for modeling, obtains the power and power angle equations of the virtual synchronous generator, then constructs a function expression between the active power output by the virtual synchronous generator and the line reactance, electromotive force amplitude, output terminal voltage, and power angle, and combines the function expression with the power and power angle equations of the virtual synchronous generator to obtain a second-order transfer function to obtain a step response curve of the output power and angular frequency of the virtual synchronous generator, then analyzes the influence mechanism of the change of virtual inertia and damping on the dynamic performance of the virtual synchronous generator based on the step response curve, and obtains the adaptive principle of virtual inertia and damping in different states based on the influence mechanism to construct an adaptive function, finally adjusts the parameters in the adaptive function and designs a start-up criterion to control the photovoltaic virtual synchronous generator; compared with the prior art, the power and frequency fluctuations can be better suppressed to reduce overshoot, shorten the adjustment time, and thus improve the stability of the system.

[0059] Other beneficial effects of the present invention will be described in detail in the subsequent specific implementation section. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 It is a schematic diagram of a flow chart of an embodiment of the present invention;

[0061] Figure 2 This is a topological diagram of the photovoltaic VSG grid-connected structure in an embodiment of the present invention;

[0062] Figure 3 is a control block diagram of an active power-frequency control part in an embodiment of the present invention;

[0063] Figure 4 Schematic diagram of a step response curve when the virtual inertia changes in an embodiment of the present invention;

[0064] Figure 5 Schematic diagram of a step response curve when damping changes in an embodiment of the present invention;

[0065] Figure 6 Schematic diagram of a curve showing a change in angular frequency offset and angular frequency change rate in an embodiment of the present invention;

[0066] Figure 7 It is the waveform diagram of the virtual synchronous generator output when the photovoltaic power generation power changes suddenly;

[0067] Figure 8 This is the waveform diagram of the virtual synchronous generator output when the load changes suddenly. DETAILED DESCRIPTION

[0068] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following will be described in detail with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0069] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0070] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a locking connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0071] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0072] In view of the existing problems, the present invention provides a photovoltaic VSG control method based on virtual inertia and damping.

[0073] like Figure 1 As shown, an embodiment of the present invention provides a photovoltaic VSG control method based on virtual inertia and damping, comprising:

[0074] Step 1, modeling the virtual synchronous generator based on the designed photovoltaic VSG grid-connected topology structure to obtain the power and power angle equations of the virtual synchronous generator, and the photovoltaic VSG grid-connected topology structure includes a main topology circuit and a control loop;

[0075] Step 2, constructing a function expression between the active power output by the virtual synchronous generator and the line reactance, electromotive force amplitude, output terminal voltage, and power angle according to the main topology circuit, and combining the function expression with the power and power angle equations of the virtual synchronous generator to obtain the first second-order transfer function and the second second-order transfer function;

[0076] Step 3, respectively performing inverse Laplace transform on the first second-order transfer function and the second second-order transfer function to obtain a step effect of active power when the virtual inertia changes, a step response of angular frequency when the virtual inertia changes, a step response of active power when the damping changes, and a step response of angular frequency when the damping changes;

[0077] Step 4, analyzing the step effect of active power when the virtual inertia changes, the step response of angular frequency when the virtual inertia changes, the step response of active power when the damping changes, and the step response of angular frequency when the damping changes, to obtain the influence mechanism of the virtual inertia change on the dynamic performance of the virtual synchronous generator and the influence mechanism of the damping change on the dynamic performance of the virtual synchronous generator;

[0078] Step 5, based on the influence mechanism of virtual inertia change on the dynamic performance of virtual synchronous generator and the influence mechanism of damping change on the dynamic performance of virtual synchronous generator, the adaptive principle of virtual inertia and damping in different states is obtained;

[0079] Step 6: construct an adaptive function of virtual inertia and damping based on the adaptive principle, adjust the parameters in the adaptive function, and determine the adaptive starting criterion of the virtual inertia and damping for controlling the photovoltaic virtual synchronous generator.

[0080] Most preferably, the photovoltaic VSG grid-connected topology is as follows Figure 2 As shown in the figure, the distributed power source on the DC side is a photovoltaic power generation system, which is connected to the DC bus through a boost converter. The photovoltaic power generation system adopts maximum power point tracking control, adds an additional amount to the power command, and achieves a constant DC bus voltage effect by correcting the power command. VSG simulates the operating characteristics of the synchronous generator, refers to the mechanical equations and electromagnetic equations of the synchronous generator, and realizes the frequency and voltage regulation functions through control. At the same time, the grid-connected inverter has virtual inertia and damping to enhance the operating stability.

[0081] In the embodiment of the present invention, the photovoltaic VSG is mainly composed of a main topology circuit and a control loop, wherein:

[0082] The main topology circuit includes: photovoltaic array, MPPT controller, PWM modulator, PI controller, Boost converter, inverter, filter inductor, filter capacitor, filter resistor, and load;

[0083] Photovoltaic array output current i pv and voltage v pv Give it to the MPPT controller, the voltage output by the MPPT controller The voltage v of the photovoltaic array output pvAfter comparison, it is input into the PI controller for processing and then into the PWM modulator for modulation. After modulation, it is input into the Boost converter for conversion and then outputs the DC terminal voltage U dc Through the DC bus capacitor C dc Input to the inverter for inversion, after inversion, through the filter inductor L f , filter resistor R f And filter capacitor C f Input load after filtering;

[0084] The control loop includes: SVPWM modulator, voltage and current double closed-loop controller, VSG control module, active and reactive power calculation module, and PI controller;

[0085] The active and reactive power calculation module receives the output voltage u of the virtual synchronous generator from the input end of the load. abc and the output current i abc , calculate active and reactive power, and get active power P e and reactive power Q and input into VSG control module;

[0086] DC terminal voltage U dc and DC bus voltage reference value U dcref After comparison, the output result obtained by inputting the PI controller for proportional integral control is consistent with the output power p of the photovoltaic system. pv By comparison, the active power reference value P of the input VSG is obtained. ref and reactive power reference value Q ref They are input into the VSG control module for modulation together to obtain the electromotive force amplitude E obtained by active-frequency modulation and the VSG power angle δ obtained by reactive-voltage modulation, and then input into the inverter through the voltage-current dual closed-loop control module and the SVPWM modulator in turn to control the inverter.

[0087] The design idea of ​​modeling the virtual synchronous generator based on the designed photovoltaic VSG grid-connected topology is to introduce virtual inertia and damping links into the control algorithm of the distributed power grid-connected inverter to make it have the operating characteristics of the synchronous generator. The virtual synchronous generator is modeled based on the synchronous generator, and the power and power angle equations of the virtual synchronous generator are shown as follows:

[0088]

[0089] Where J represents virtual inertia, P ref represents the active power input to the virtual synchronous generator, represents the active power output from the virtual synchronous generator, δ represents the power angle of the virtual synchronous generator, ω represents the output angular frequency of the virtual synchronous generator, ω0 represents the rated angular frequency of the power grid, and D p Indicates the damping coefficient corresponding to the damping torque.

[0090] The speed regulator in the synchronous generator can respond when the system frequency deviates and control the mechanical torque of the prime mover to adjust the active power. The active-frequency control part completes this task in the virtual synchronous generator. Its control block diagram is shown in the figure. Figure 3 As shown in the figure, the part of the virtual synchronous generator that completes the synchronous generator excitation regulation is the reactive power-voltage control part. The reference voltage is obtained from the droop control and reactive power change, and then the electromotive force amplitude is output through the control algorithm. Among them, the relationship expression of the reactive power-voltage control part is:

[0091] E=E0+K u (U ref -U)+K q (Q ref -Q)

[0092] Among them, E0 represents the no-load electromotive force, K u Indicates the voltage droop coefficient, K q represents the reactive power droop coefficient, U represents the output voltage, Q ref represents the reactive power reference value, and Q represents the reactive power output value.

[0093] Specifically, step 2 includes:

[0094] The output active power and reactive power of the virtual synchronous generator are calculated according to the main topology circuit, as shown in the following formula:

[0095]

[0096] Among them, Z f represents the impedance of the filter circuit, α represents the impedance angle of the filter circuit, and the reactance value X in the general filter resistor impedance f Much larger than the resistance value R f , and because the value of α is very small, it is considered that α≈0, so the functional expression between the active power output of the virtual synchronous generator and the line reactance, electromotive force amplitude, output terminal voltage, and power angle is constructed as follows:

[0097]

[0098] Where E represents the electromotive force amplitude obtained by active-frequency modulation, U l Indicates the output voltage, X f Indicates the reactance of the filter circuit;

[0099] According to the analysis method of the traditional synchronous generator small signal model, combined with the function expression and the power and power angle equations of the virtual synchronous generator, the first and second order transfer functions from the active power input to the virtual synchronous generator to the active power output of the virtual synchronous generator are obtained as follows:

[0100]

[0101] The second-order transfer function from the active power of the input virtual synchronous generator to the output angular frequency of the virtual synchronous generator is obtained as:

[0102]

[0103] Among them, ω n Indicates the rated angular frequency of the virtual synchronous generator.

[0104] According to the first and second order transfer functions, the poles of the second order model can be found as:

[0105]

[0106] From the poles obtained from the above formula, it can be found that the poles are affected by the virtual inertia J and damping D p If its value is not properly selected, the pole will be close to the imaginary axis, which will cause the control system to oscillate. Therefore, in order to study the virtual inertia J and damping D p The embodiment of the present invention will observe the influence mechanism of the virtual synchronous generator output active power and frequency under different virtual inertia J, damping D p The dynamic response curve below is analyzed, and the process is as follows:

[0107] Performing inverse Laplace transform on the first and second order transfer functions yields:

[0108]

[0109] According to the above formula, the virtual inertia can be obtained. When the active power changes, the step response curve is as follows: Figure 4 As shown in (a), Figure 4 (a) shows the step response curve, when the damping D p When the virtual inertia J increases, the stabilization time t of the active power step response is s and overshoot M P will increase, the peak time t p It will also increase, so it can be concluded that when the virtual inertia increases significantly, it is not conducive to the stability of the output power of the virtual synchronous generator;

[0110] In the embodiment of the present invention, it is assumed that the virtual synchronous generator responds in an ideal underdamped state, that is, 0<ξ<1, ξ is the damping ratio, and the error is selected as ±2%. At this time, its peak time t p , stabilization time t s and overshoot M P It can be expressed as:

[0111]

[0112] The stabilization time t s By taking the derivative of the virtual inertia J, we get the following formula:

[0113]

[0114] According to the above formula, we can get dt s / dJ>0, that is, ts is an increasing function. It can also be verified that when the virtual inertia J increases, the stable time t s will become larger, and similarly dM p / dJ>0,dt p / dJ>0.

[0115] Taking the inverse Laplace transform of the second-order transfer function, we get:

[0116]

[0117] Among them, ω n represents the rated angular frequency of the virtual synchronous generator;

[0118] According to the above formula, the step response curve of the angular frequency when the virtual inertia changes can be obtained, such as Figure 4 (b)

[0119] At this time, the peak time of the angular frequency is t p1 , stabilization time t s1 and overshoot M P1 It can be expressed as:

[0120]

[0121] Similarly, dt can be calculated s1 / dJ>0,dM p1 / dJ>0,dt p1 / dJ>0;

[0122] according to Figure 4 The step response curve of (b) and the calculation of the above formula can deduce the influence mechanism of virtual inertia change on the output frequency of virtual synchronous generator:

[0123] When the virtual inertia increases, the peak time and stabilization time of the output frequency will increase, while the overshoot will decrease. It can be concluded that the increase of virtual inertia can reduce the overshoot of the frequency and make the frequency response smoother, but the response speed will decrease and it is not conducive to suppressing the overshoot of active power.

[0124] In the embodiment of the present invention, in addition to the virtual inertia, another parameter affecting the output active power and frequency of the virtual synchronous generator, damping, is analyzed; when the virtual inertia is constant, the step response curves of the output active power and angular frequency under different damping are as follows: Figure 5 (a) and Figure 5 (b) as shown.

[0125] The peak time t is calculated based on the calculation expressions of peak time, stable time and overshoot in the ideal underdamped state. p About Damping D p The derivative of , we can get:

[0126]

[0127] Therefore, dt p / dD p is an increasing function, that is, t p Will change with the damping D p Similarly, dM p / dD p <0,dt s / dD p <0,dt p1 / dD p <0,dt s1 / dD p <0,dM p1 / dD p <0. Therefore, it can be obtained that the influence mechanism of damping change on the dynamic performance of virtual synchronous generator is:

[0128] For the step response of active power, when the damping D p When it increases, the peak time of active power will increase, and the overshoot and stabilization time will decrease;

[0129] For the step response of angular frequency, when the damping D p When the damping value is increased, the peak time, overshoot and stabilization time will decrease; therefore, if the damping value is set too small, the active power and angular frequency will take longer to stabilize, and the overshoot will be too large; when the damping value is increased, the overshoot and stabilization time will decrease, which can effectively suppress oscillation and improve stability.

[0130] Based on the above analysis, virtual inertia and damping will affect the overshoot and stabilization time of the virtual synchronous generator output active power and frequency, and overshoot and stabilization time are important factors reflecting stability. Therefore, the selection of virtual inertia and damping size is very important. If the selection is inappropriate, it will lead to large overshoot and long adjustment time, thus affecting the dynamic response.

[0131] In order to solve the problem of improper selection of virtual inertia and damping, the embodiment of the present invention summarizes the influence of parameter disturbance on the output characteristics of the virtual synchronous generator based on the above analysis, as shown in the following Table 1 and Table 2:

[0132] Table 1

[0133]

[0134] Table 2

[0135]

[0136] According to the conclusions of Table 1 and Table 2, for a fixed virtual inertia, if a larger virtual inertia can be selected in the rising region of the angular frequency oscillation and a smaller virtual inertia can be selected in the falling region, the overshoot and response time of the output frequency will be reduced. Therefore, based on this conclusion, the embodiment of the present invention proposes an adaptive method for adjusting the virtual inertia and damping size according to the change of the angular frequency offset Δω and the angular frequency change rate dω / dt, such as Figure 6 As shown, the angular frequency offset Δω and the angular frequency change rate dω / dt within a single oscillation period are divided into four state intervals, namely ① the first state interval (t0~t1), ② the second state interval (t1~t2), ③ the third state interval (t2~t3) and ④ the fourth state interval (t3~t4).

[0137] In the first state interval ①, the angular frequency of the virtual synchronous generator is greater than the angular frequency of the power grid, that is, Δω>0, dω / dt first increases rapidly to a peak value and then gradually decreases to 0; therefore, in this process, the virtual inertia and damping can be increased to reduce the angular frequency offset and change rate to suppress overshoot;

[0138] In the second state interval ②, the angular frequency of the virtual synchronous generator is still greater than the angular frequency of the power grid, but dω / dt increases in the reverse direction from 0 to reach the peak value, and the angular frequency is in a deceleration state, that is, dω / dt<0; therefore, in this process, the virtual inertia can be reduced to make the angular frequency return to the rated value faster, and the damping keeps increasing, reducing the overshoot and stabilization time of the angular frequency;

[0139] In the third state interval ③ and the fourth state interval ④, J and D p The changes in are similar to those in intervals ① and ② and will not be repeated here.

[0140] Based on the above analysis, it can be concluded that the adaptive principle of virtual inertia and damping in different state ranges is:

[0141] In the first state interval, the angular frequency offset and the angular frequency change rate are both greater than 0, the virtual inertia increases, and the damping increases;

[0142] In the second state interval, the angular frequency offset is greater than 0, the angular frequency change rate is less than 0, the virtual inertia decreases, and the damping increases;

[0143] In the third state interval, the angular frequency offset and the angular frequency change rate are both less than 0, the virtual inertia increases, and the damping increases;

[0144] In the fourth state interval, the angular frequency offset is less than 0, the angular frequency change rate is greater than 0, the virtual inertia decreases, and the damping increases, as shown in Table 3:

[0145] Table 3

[0146] Interval Angular frequency offset Angular frequency change rate Change of virtual inertia Changes in damping ① Δω>0 dω / dt>0 Increase Increase ② Δω>0 dω / dt<0 Reduce Increase ③ I see <0 dω / dt<0 Increase Increase ④ I see <0 dω / dt>0 Reduce Increase

[0147] According to the adaptive principle of virtual inertia and damping in Table 3 above, the adaptive functions of virtual inertia and damping can be constructed, including:

[0148] The adaptive function of constructing virtual inertia based on the adaptive principle is:

[0149]

[0150] The adaptive function of damping based on the adaptive principle is:

[0151]

[0152] Where J0 represents the basic value of virtual inertia in the virtual synchronous generator during stable operation, D0 represents the basic value of damping in the virtual synchronous generator during stable operation, and K J1 , K J2 Both represent the adaptive adjustment coefficient of virtual inertia, K D Represents the adaptive adjustment coefficient of damping, T j Indicates the angular frequency change rate threshold, T d Indicates the angular frequency offset change threshold.

[0153] Regarding the setting of the angular frequency change rate and the angular frequency offset change threshold, when the virtual synchronous generator operates normally, the output frequency of the virtual synchronous generator will also fluctuate slightly, but this fluctuation is within the allowable range of stable operation, so there is no need to change the virtual inertia and damping.

[0154] Specifically, the parameters in the adaptive function are adjusted, including:

[0155] According to the method of "optimal second-order system", the basic value of damping in the virtual synchronous generator during stable operation is adjusted, and the following is obtained:

[0156]

[0157] The basic value of the virtual inertia in the virtual synchronous generator during stable operation is adjusted to obtain:

[0158]

[0159] The adaptive adjustment coefficient of virtual inertia is adjusted to obtain:

[0160]

[0161] The adaptive adjustment coefficient of damping is adjusted to obtain:

[0162]

[0163] Where ΔT represents the mechanical torque T of the virtual synchronous generator m and electromagnetic torque T e The difference between ΔT and T m -T e , P max Indicates the maximum power output of the virtual synchronous generator, J max Indicates the maximum value of virtual inertia, J min Indicates the minimum value of virtual inertia, D max Indicates the maximum value of damping.

[0164] Specifically, in order to prevent the virtual synchronous generator from frequently starting the adaptation due to small frequency oscillations in the normal operation state, the embodiment of the present invention sets a suitable angular frequency change rate threshold and angular frequency offset change threshold as the adaptive start criterion of the virtual inertia and damping. The values ​​of the angular frequency change rate threshold and the angular frequency offset change threshold should be greater than the maximum value of the angular frequency change rate and the angular frequency offset fluctuations in normal operation. Specifically, it can be further confirmed by simulation experiments. In the embodiment of the present invention, T j =1,T d =0.05.

[0165] In order to verify the VSG control method provided by the embodiment of the present invention, a grid-connected simulation model of a photovoltaic virtual synchronous generator was built using Matlab / Simulink software. The main parameters of the simulation are shown in Table 4 below:

[0166] Table 4

[0167] parameter Numeric parameter Numeric DC terminal voltage / V 700 <![CDATA[Base value of virtual inertia / kg·m 2 > 0.2 Grid voltage / V 380 <![CDATA[Damping / N·m·s·rad -1 > 10 Filter resistance / Ω 0.1 Angular frequency change rate threshold 1 Filter inductance / mH 3.2 Angular frequency offset change threshold 0.05 Filter capacitor / μF 20 <![CDATA[Adaptive adjustment parameter K of virtual inertia J1 > 0.1 Load resistance / Ω 0.3 <![CDATA[Adaptive adjustment parameter K of virtual inertia J2 > 0.003 Load inductance / mH 3 Adaptive adjustment parameters of damping 5

[0168] According to the simulation parameters set in Table 4 above, we can get Figure 7 (a), (b) and Figure 8 (a) and (b) show the adaptive change curves, where Figure 7 (a) and (b) are the virtual synchronous generator grid-connected stage, and the initial light intensity is set to 600W / m2 (The power generated by the photovoltaic array is 11.65kW). At 0.5s, the light intensity increases to 1200W / m 2 (The power generated by the photovoltaic array is 23.44kW), and then recovers to 600W / m in 1.5s. 2 The adaptive change curve of virtual inertia and damping under the working condition, Figure 8 (a) and (b) show that the illumination intensity of the photovoltaic power generation system is maintained at 600W / m during the virtual synchronous generator grid-connected stage. 2 (The power generated by the photovoltaic array is 11.65kW) remains unchanged, the initial load is 10kW, a 5kW load is connected at 0.5s, and then removed at 1.5s. The adaptive change curve of the virtual inertia and damping under the working condition, its change trend can correspond well to the analysis in Table 3.

[0169] In summary, the embodiment of the present invention first designs a photovoltaic VSG grid-connected topology structure for modeling, obtains the power and power angle equations of the virtual synchronous generator, and then constructs a function expression between the active power output of the virtual synchronous generator and the line reactance, electromotive force amplitude, output terminal voltage, and power angle, and combines the function expression with the power and power angle equations of the virtual synchronous generator to obtain a second-order transfer function to obtain a step response curve of the output power and angular frequency of the virtual synchronous generator, and then analyzes the influence mechanism of the change of virtual inertia and damping on the dynamic performance of the virtual synchronous generator based on the step response curve, and obtains the adaptive principle of virtual inertia and damping in different states based on the influence mechanism to construct an adaptive function, and finally adjusts the parameters in the adaptive function and designs the start-up criterion to control the photovoltaic virtual synchronous generator; compared with the traditional non-adaptive VSG and virtual inertia adaptive VSG control strategies in the prior art, it can better suppress the fluctuation of power and frequency to reduce overshoot, shorten the adjustment time, and thus improve the stability of the system.

[0170] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A photovoltaic VSG control method based on virtual inertia and damping, characterized in that: include: Step 1, modeling a virtual synchronous generator based on a designed photovoltaic VSG grid-connected topology structure to obtain power and power angle equations of the virtual synchronous generator, wherein the photovoltaic VSG grid-connected topology structure includes a main topology circuit and a control loop; Step 2, constructing a function expression between the active power output by the virtual synchronous generator and the line reactance, electromotive force amplitude, output terminal voltage, and power angle according to the main topology circuit, and combining the function expression with the power and power angle equations of the virtual synchronous generator to obtain a first second-order transfer function and a second second-order transfer function; Step 3, performing inverse Laplace transform on the first second-order transfer function and the second second-order transfer function respectively, to obtain a step effect of active power when the virtual inertia changes, a step response curve of angular frequency when the virtual inertia changes, a step response curve of active power when the damping changes, and a step response curve of angular frequency when the damping changes; Step 4, analyzing the step effect of active power when the virtual inertia changes, the step response curve of angular frequency when the virtual inertia changes, the step response curve of active power when the damping changes, and the step response curve of angular frequency when the damping changes, to obtain the influence mechanism of the virtual inertia change on the dynamic performance of the virtual synchronous generator and the influence mechanism of the damping change on the dynamic performance of the virtual synchronous generator; Step 5, based on the influence mechanism of the virtual inertia change on the dynamic performance of the virtual synchronous generator and the influence mechanism of the damping change on the dynamic performance of the virtual synchronous generator, obtain the adaptive principle of the virtual inertia and the damping in different states; Step 6: construct an adaptive function of the virtual inertia and the damping based on the adaptive principle, adjust the parameters in the adaptive function, and determine the adaptive starting criterion of the virtual inertia and the damping for controlling the photovoltaic virtual synchronous generator.

2. The photovoltaic VSG control method based on virtual inertia and damping according to claim 1 is characterized in that: The power and power angle equations of the virtual synchronous generator are: Where J represents virtual inertia, P ref represents the active power input to the virtual synchronous generator, represents the active power output from the virtual synchronous generator, δ represents the power angle of the virtual synchronous generator, ω represents the output angular frequency of the virtual synchronous generator, ω0 represents the rated angular frequency of the power grid, and D p Indicates the damping coefficient corresponding to the damping torque.

3. The photovoltaic VSG control method based on virtual inertia and damping according to claim 2 is characterized in that: The functional expression between the active power outputted by the virtual synchronous generator constructed according to the main topology circuit and the line reactance, electromotive force amplitude, output terminal voltage and power angle is: Where E represents the electromotive force amplitude obtained by active-frequency modulation, U l Represents the output voltage, Z f Indicates the filter circuit impedance, X f Represents the reactance of the filter circuit.

4. The photovoltaic VSG control method based on virtual inertia and damping according to claim 3 is characterized in that: The function expression is combined with the power and power angle equations of the virtual synchronous generator to obtain a first second-order transfer function and a second second-order transfer function, including: According to the analysis method of the small signal model of the traditional synchronous generator, the first and second order transfer functions from the active power input to the virtual synchronous generator to the active power output of the virtual synchronous generator are obtained by combining the function expression with the power and power angle equations of the virtual synchronous generator: The second second-order transfer function from the active power input to the virtual synchronous generator to the output angular frequency of the virtual synchronous generator is obtained as: Among them, ω n Indicates the rated angular frequency of the virtual synchronous generator.

5. The photovoltaic VSG control method based on virtual inertia and damping according to claim 4 is characterized in that: Performing inverse Laplace transform on the first second-order transfer function and the second second-order transfer function respectively, comprising: Performing an inverse Laplace transform on the first second-order transfer function yields: Performing an inverse Laplace transform on the second second-order transfer function yields: Among them, ω n Indicates the rated angular frequency of the virtual synchronous generator.

6. The photovoltaic VSG control method based on virtual inertia and damping according to claim 5 is characterized in that: The step 4 comprises: By analyzing the step effect of active power when the virtual inertia changes and the step response curve of angular frequency when the virtual inertia changes, it is found that the influence mechanism of the virtual inertia change on the dynamic performance of the virtual synchronous generator is: When the virtual inertia increases, the peak time and the stabilization time of the output frequency increase, and the overshoot decreases; By analyzing the step response curve of active power when the damping changes and the step response curve of angular frequency when the damping changes, it is obtained that the influence mechanism of the damping change on the dynamic performance of the virtual synchronous generator is: For the step response curve of the active power, when the damping increases, the peak time of the active power increases, and the overshoot and stabilization time decrease; For the step response curve of the angular frequency, when the damping increases, the peak time, overshoot and settling time are reduced.

7. The photovoltaic VSG control method based on virtual inertia and damping according to claim 6 is characterized in that: The adaptive principle of the virtual inertia and the damping in different states is: In the first state interval, the angular frequency offset and the angular frequency change rate are both greater than 0, the virtual inertia increases, and the damping increases; In the second state interval, the angular frequency offset is greater than 0, the angular frequency change rate is less than 0, the virtual inertia decreases, and the damping increases; In the third state interval, the angular frequency offset and the angular frequency change rate are both less than 0, the virtual inertia increases, and the damping increases; In a fourth state interval, the angular frequency offset is less than 0, the angular frequency change rate is greater than 0, the virtual inertia decreases, and the damping increases.

8. The photovoltaic VSG control method based on virtual inertia and damping according to claim 7 is characterized in that: Constructing an adaptive function of the virtual inertia and the damping based on the adaptive principle includes: The adaptive function of the virtual inertia constructed based on the adaptive principle is: The adaptive function of the damping constructed based on the adaptive principle is: Where J0 represents the basic value of virtual inertia in the virtual synchronous generator during stable operation, D0 represents the basic value of damping in the virtual synchronous generator during stable operation, and K J1 , K J2 Both represent the adaptive adjustment coefficient of virtual inertia, K D Represents the adaptive adjustment coefficient of damping, T j Indicates the angular frequency change rate threshold, T d Indicates the angular frequency offset change threshold.

9. The photovoltaic VSG control method based on virtual inertia and damping according to claim 8 is characterized in that: The parameters in the adaptive function are adjusted, including: The basic value of damping in the virtual synchronous generator during stable operation is adjusted to obtain: The basic value of the virtual inertia in the virtual synchronous generator during stable operation is adjusted to obtain: The adaptive adjustment coefficient of the virtual inertia is adjusted to obtain: The adaptive adjustment coefficient of the damping is adjusted to obtain: Where ΔT represents the mechanical torque T of the virtual synchronous generator m and electromagnetic torque T e The difference between ΔT and T m -T e , P max Indicates the maximum power output of the virtual synchronous generator, J max Indicates the maximum value of virtual inertia, J min Indicates the minimum value of virtual inertia, D max Indicates the maximum value of damping.

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