A photovoltaic VSG control method based on virtual inertia and damping

Abstract

The application provides a photovoltaic VSG control method based on virtual inertia and damping, designs a topological structure of a photovoltaic virtual synchronous generator to model, obtains power and power angle equations of the virtual synchronous generator, then constructs a function expression of active power of the virtual synchronous generator, combines the function expression with the power and power angle equations of the virtual synchronous generator, obtains a second-order transfer function to obtain a step response curve of output power and angular frequency of the virtual synchronous generator, then analyzes an influence mechanism of changes of the virtual inertia and the damping on dynamic performance of the virtual synchronous generator based on the step response curve, obtains adaptive principles of the virtual inertia and the damping in different states to construct adaptive functions based on the influence mechanism, and finally adjusts parameters in the adaptive functions and designs a starting criterion to control the photovoltaic virtual synchronous generator; compared with prior art, the application can better inhibit fluctuations of power and frequency and improve stability of the system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photovoltaic virtual synchronous generator control, and particularly relates to a photovoltaic VSG control method based on virtual inertia and damping. BACKGROUND

[0002] With the intensification of energy crisis and environmental pollution problems, under the background of "double carbon", new energy power generation technologies such as photovoltaic power generation have gradually become the research focus of people. Energy generation mainly in the form of distributed power is connected to the power grid through an inverter. With the large-scale penetration of new energy distributed power, as the main interface of power electronic grid-connected technology, the popularity rate of inverters also increases. Due to the characteristics of weak damping and lack of rotational inertia support of power electronic devices such as inverters, when power disturbance or load mutation occurs, it will not be able to suppress the oscillation of the distributed power generation system, thereby threatening the operation safety and stability of the entire power grid. Therefore, virtual synchronous generator (VSG) control technology can be introduced into inverter control, which can simulate the external characteristics of synchronous generators, so that the grid-connected inverter has similar rotational inertia and damping to synchronous generators. However, the traditional VSG parameter setting is a fixed value, and the fixed value parameter setting may not be suitable for 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 that the virtual inertia and damping coefficient in the VSG control can be designed flexibly, scholars have made a lot of research on the adaptive parameters of VSG. For example, some researchers have proposed an adaptive virtual inertia control strategy that can quickly respond to load disturbance in island mode, effectively suppressing frequency fluctuations. Some researchers have proposed an improved bang-bang adaptive virtual inertia VSG control strategy that can dynamically adjust the rotational inertia according to the frequency change caused by load disturbance, avoiding rapid frequency fluctuations and 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 constraint, which adjusts the virtual inertia according to the frequency change rate of the VSG output, and then controls the value of the damping coefficient through the damping ratio, ensuring the stability of the system by working in an under-damped state. Some researchers have applied adaptive virtual inertia and damping coefficient to grid-connected inverters, effectively improving power and frequency oscillation in the transient process, but there is no start-up judgment condition for the adaptive damping. In summary, the existing technology still has poor power and frequency fluctuation suppression capability, resulting in low system stability. SUMMARY

[0004] The application provides a photovoltaic VSG control method based on virtual inertia and damping, which aims to better inhibit power and frequency fluctuations and improve system stability.

[0005] In order to achieve the above-mentioned purpose, the application provides a photovoltaic VSG control method based on virtual inertia and damping, which comprises the following steps:

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

[0007] Step 2: constructing a function expression of active power output by the virtual synchronous generator and 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;

[0008] 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 response curve 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 response curves of active power when the virtual inertia changes, the step response curves of angular frequency when the virtual inertia changes, the step response curves of active power when the damping changes and the step response curves of angular frequency when the damping changes to obtain an influence mechanism of virtual inertia change on dynamic performance of the virtual synchronous generator and an influence mechanism of damping change on dynamic performance of the virtual synchronous generator;

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

[0011] Step 6: constructing an adaptive function of the virtual inertia and the damping based on the adaptive principle, adjusting parameters in the adaptive function and determining an adaptive starting criterion of the virtual inertia and the damping to control the virtual synchronous generator.

[0012] Further, the power and power angle equations of the virtual synchronous generator are as follows:

[0013]

[0014] wherein J represents virtual inertia, P represents active power, D represents damping, ω represents angular frequency, θ represents power angle, E represents electromotive force amplitude, V represents output terminal voltage and X represents line reactance. refP represents the active power input to the virtual synchronous generator e P represents the active power output by 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, D represents the damping torque of the virtual synchronous generator, and E represents the electromotive force amplitude of the virtual synchronous generator. p D represents the damping torque of the virtual synchronous generator, and E represents the electromotive force amplitude of the virtual synchronous generator.

[0015] Further, the function expression between the active power output by the virtual synchronous generator and the line reactance, the electromotive force amplitude, the output voltage, and the power angle is constructed according to the main topology circuit, as follows:

[0016]

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

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

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

[0020]

[0021] 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 follows:

[0022]

[0023] ω represents the rated angular frequency of the virtual synchronous generator. n ω represents the rated angular frequency of the virtual synchronous generator.

[0024] Further, the first and second second-order transfer functions are respectively inverse Laplace transformed, including:

[0025] The first second-order transfer function is inverse Laplace transformed to obtain:

[0026]

[0027] The second second-order transfer function is inverse Laplace transformed to obtain:

[0028]

[0029] ω n ω

[0030] Further, the step 4 comprises:

[0031] The step 4 comprises:

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

[0033] The step 4 comprises:

[0034] 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 the stable time decrease.

[0035] 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 the stable time decrease.

[0036] Further, the adaptive principles of the virtual inertia and the damping in different states are:

[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] Further, the adaptive functions of the virtual inertia and the damping are constructed based on the adaptive principles, comprising:

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

[0043]

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

[0045]

[0046] wherein, J0 represents a basic value of virtual inertia in the virtual synchronous generator in stable operation, D0 represents a basic value of damping in the virtual synchronous generator in stable operation, K J1 , K J2 all represent adaptive adjustment coefficients of virtual inertia, K D represents an adaptive adjustment coefficient of damping, T j represents an angular frequency change rate threshold, T d represents an angular frequency offset change threshold.

[0047] Further, the parameters in the adaptive function are tuned, including:

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

[0049]

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

[0051]

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

[0053]

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

[0055]

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

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

[0058] The application firstly designs a photovoltaic VSG grid-connected topology structure to model, obtains the power and power angle equations of the virtual synchronous generator, then constructs the function expression between the active power output by the virtual synchronous generator and the line reactance, the electromotive force amplitude, the output terminal voltage and the power angle, combines the function expression with the power and power angle equations of the virtual synchronous generator, obtains the second-order transfer function to obtain the step response curve of the output power and the angular frequency of the virtual synchronous generator, then analyzes the influence mechanism of the change of the virtual inertia and the damping on the dynamic performance of the virtual synchronous generator based on the step response curve, obtains the adaptive principle of the virtual inertia and the damping in different states to construct the adaptive function based on the influence mechanism, finally adjusts the parameters in the adaptive function and designs the starting criterion to control the photovoltaic virtual synchronous generator; compared with the prior art, the application can better inhibit the fluctuation of the power and the frequency, reduces the overshoot and shortens the regulation time, thereby improving the stability of the system.

[0059] Other benefits of the application will be described in detail in the subsequent specific embodiment part. BRIEF DESCRIPTION OF DRAWINGS

[0060] Fig. 1 It is a flowchart of the embodiment of the application;

[0061] Fig. 2 It is a photovoltaic VSG grid-connected topology structure diagram in the embodiment of the application;

[0062] Fig. 3 It is a control block diagram of the active-frequency control part in the embodiment of the application;

[0063] Fig. 4 It is a step response curve diagram when the virtual inertia changes in the embodiment of the application;

[0064] Fig. 5 It is a step response curve diagram when the damping changes in the embodiment of the application;

[0065] Fig. 6 It is a change curve diagram of the angular frequency offset and the angular frequency change rate in the embodiment of the application;

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

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

[0068] In order to make the technical problems, technical solutions and advantages of the present application clearer, the following will be described in detail in conjunction with the drawings and specific embodiments. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0069] In the description of the present application, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second", "third" are only for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0070] In the description of the present application, it should be noted that unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connection" should be understood broadly, for example, it can be a locking connection, or a detachable connection, or an integral connection; it can be a mechanical connection, or an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or the internal communication of two elements. For those of ordinary skill in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

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

[0072] The present application aims at the existing problems and provides a photovoltaic VSG control method based on virtual inertia and damping.

[0073] As Fig. 1 shown, the embodiment of the present application 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, obtaining the power and power angle equations of the virtual synchronous generator, and the photovoltaic VSG grid-connected topology comprising 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, the electromotive force amplitude, the output end voltage and the 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;

[0076] 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 response curve 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: Analyze the step response curve 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 the virtual synchronous generator and the influence mechanism of damping change on the dynamic performance of the virtual synchronous generator, the adaptive principle of virtual inertia and damping in different states is obtained;

[0079] Step 6: Based on the adaptive principle, an adaptive function of the virtual inertia and damping is constructed, and the parameters in the adaptive function are adjusted to determine the adaptive starting criteria of the virtual inertia and damping for controlling the virtual synchronous generator.

[0080] Most preferably, the photovoltaic VSG grid-connected topology is as follows Fig. 2 As shown, the distributed power source on the DC side is a photovoltaic power generation system, connected to the DC bus via a boost converter. The photovoltaic power generation system uses maximum power point tracking (MPPT) control, adding an additional value to the power command to achieve a constant DC bus voltage by modifying the power command. The VSG simulates the operating characteristics of a synchronous generator, using its mechanical and electromagnetic equations to achieve frequency and voltage regulation through control. It also provides virtual inertia and damping to the grid-connected inverter, enhancing operational stability.

[0081] In the embodiment of the present invention, the photovoltaic VSG mainly consists 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] PV array output current i pv and voltage v pv Give it to the MPPT controller, the MPPT controller outputs voltage The output voltage of the photovoltaic array is v pvAfter comparison, input PI controller for processing, then input PWM modulator for modulation, after modulation, input to Boost converter for conversion, then output DC bus 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 After filtering, input to the load;

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

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

[0086] The DC bus voltage U dc Is compared with the DC bus voltage reference value U dcref , then input to the PI controller for proportional integral control, and the output result obtained after comparison with the output power p pv Of the photovoltaic system, the active power reference value P ref And the reactive power reference value Q ref of the VSG are input to the VSG control module for modulation, and the electromotive force amplitude E obtained by active-frequency modulation and the VSG power angle δ obtained by reactive-voltage modulation are input to the inverter through the voltage and current double 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 elements in the control algorithm of the grid-connected inverter of the distributed power supply, so that it has the operating characteristics of a synchronous generator, and the power and power angle equations of the virtual synchronous generator are obtained based on the modeling of the synchronous generator as shown in the following formula:

[0088]

[0089] Wherein, J represents the virtual inertia, P ref represents the active power input to the virtual synchronous generator, P e represents the active power output by 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, D pThe damping torque corresponds to a damping coefficient.

[0090] The governor in a synchronous generator can respond to the deviation of system frequency and control the mechanical torque of prime mover to achieve the regulation of active power. In a virtual synchronous generator, the active-frequency control part is responsible for this task, and its control block diagram is shown in Fig. 3 The part responsible for the excitation regulation of synchronous generator in a virtual synchronous generator is the reactive-voltage control part, which obtains the reference voltage from the droop control and reactive power variation, and then outputs the electromotive force amplitude through a control algorithm. The relationship expression of the reactive-voltage control part is:

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

[0092] wherein E0 represents the no-load electromotive force, K u represents 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 comprises:

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

[0095]

[0096] wherein Z f represents the impedance of the filter circuit, and a represents the impedance angle of the filter circuit. Generally, the reactance value X f in the impedance of the filter resistance is much larger than the resistance value R f , and because the value of a is very small, a is considered to be approximately equal to 0. Therefore, the function expression between the output active power of the virtual synchronous generator and the line reactance, the electromotive force amplitude, the output voltage, and the power angle is constructed as:

[0097]

[0098] wherein E represents the electromotive force amplitude obtained through the active-frequency modulation, U l represents the output voltage, and X f represents the reactance of the filter circuit.

[0099] According to the analysis method of the small signal model of the traditional synchronous generator, the first second-order transfer function from the input active power of the virtual synchronous generator to the output active power of the virtual synchronous generator is obtained by combining the function expression with the power and power angle equation of the virtual synchronous generator, and the second second-order transfer function from the input active power of the virtual synchronous generator to the output angular frequency of the virtual synchronous generator is obtained.

[0100]

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

[0102]

[0103] Wherein, ω n represents the rated angular frequency of the virtual synchronous generator.

[0104] According to the first second-order transfer function, the pole of the second-order model can be obtained as follows:

[0105]

[0106] It can be found from the pole obtained from the above formula that the pole will be affected by the virtual inertia J and the damping D p , if the values are not properly selected, the pole will be close to the imaginary axis, thereby causing the control system to oscillate, therefore, in order to study the influence mechanism of the virtual inertia J and the damping D p on the output active power and frequency of the virtual synchronous generator, the embodiment of the present application will analyze the dynamic response curve of the output active power and frequency of the virtual synchronous generator under different virtual inertias J and dampings D p , the process is as follows:

[0107] The Laplace inverse transform is performed on the first second-order transfer function, and the following is obtained:

[0108]

[0109] According to the above formula, the step response curve of the active power when the virtual inertia J changes is shown in Fig. Fig. 4 (a), and the step response curve shown in Fig. Fig. 4 (a) can be seen that when the damping D p is constant, the virtual inertia J increases, the stable time t s of the step response of the active power and the overshoot M P will increase, and the peak time t p will also increase, therefore, it can be concluded that when the virtual inertia J increases greatly, 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 Derivative 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, t s It is an increasing function, which can also verify that when the virtual inertia J increases, the stabilization time t s will become larger, similarly dM p / dJ>0,dt p / dJ>0.

[0115] Performing the inverse Laplace transform on the second-order transfer function yields:

[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 angular frequency when the virtual inertia changes can be obtained, such as Fig. 4 (b)

[0119] The peak time of the angular frequency at this time 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 Fig. 4 The step response curve (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 the settling time of the output frequency will increase, and the overshoot will decrease, so it can be concluded that the increase of the virtual inertia can reduce the overshoot of the frequency, make the frequency response more gentle, but the response speed will be reduced, and it is not conducive to suppress the overshoot of the active power.

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

[0125] According to the calculation expression of the peak time t p , the settling time t p and the overshoot M p when responding in the more ideal underdamped state, the peak time t p is calculated as follows:

[0126]

[0127] Therefore, dt p / dD p is an increasing function, that is, t p will increase with the increase of 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 and the influence mechanism of the damping change on the dynamic performance of the virtual synchronous generator is as follows:

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

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

[0130] Based on the above analysis, the virtual inertia and damping will affect the overshoot and settling time of the active power and frequency output of the virtual synchronous generator, and the overshoot and settling time are important factors reflecting stability. Therefore, the selection of the size of the virtual inertia and damping is important, and if it is not properly selected, it will lead to large overshoot and long adjustment time, thereby affecting the dynamic response.

[0131] In order to solve the problem of improper selection of virtual inertia and damping, the embodiments of the present application summarize the influence of parameter disturbance on the output characteristics of the virtual synchronous generator based on the foregoing analysis, as shown in Table 1 and Table 2 below:

[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 value of virtual inertia is selected in the rising region of the angular frequency oscillation and a smaller value of virtual inertia is selected in the falling region, the overshoot and response time of the output frequency will be reduced. Therefore, based on this conclusion, the embodiments of the present application propose an adaptive method for adjusting the size of virtual inertia and damping according to the change of angular frequency offset Δω and angular frequency change rate dω / dt, as shown in Fig. 6 The angular frequency offset Δω and angular frequency change rate dω / dt in a single oscillation period are divided into four state intervals, namely ① first state interval (t0-t1), ② second state interval (t1-t2), ③ third state interval (t2-t3) and ④ 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, i.e. Δω>0, and dω / dt first rapidly increases 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, so as to suppress the 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 is reversely increased from 0 to a peak value, and the angular frequency is in a deceleration state, i.e. dω / dt<0; therefore, in this process, the virtual inertia can be reduced to make the angular frequency recover to the rated value faster, and the damping is kept increasing to reduce the overshoot and settling time of the angular frequency;

[0139] In the third state interval ③ and the fourth state interval ④, the changes of J and D p are similar to those of intervals ① and ②, and are not described again.

[0140] Based on the above analysis, the adaptive principles of virtual inertia and damping in different state intervals are as follows:

[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, and 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, and 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 change of damping ① Δω > 0 dω / dt > 0 increase increase ② Δω > 0 dω / dt < 0 decrease increase ③ Δω < 0 dω / dt < 0 increase increase ④ Δω < 0 increase dω / dt > 0 decrease

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

[0148] The adaptive function of 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 in stable operation, D0 represents the basic value of damping in the virtual synchronous generator in stable operation, K J1 , K J2 all represent adaptive adjustment coefficients of virtual inertia, K D represents an adaptive adjustment coefficient of damping, T j represents the angular frequency change rate threshold, and T d represents the angular frequency offset change threshold.

[0153] For the setting of the angular frequency change rate and the angular frequency offset change threshold, when the virtual synchronous generator is in normal operation, the output frequency of the virtual synchronous generator will also have slight fluctuations in amplitude, but such fluctuations are within the allowed range of stable operation, so the virtual inertia and the damping do not need to be changed.

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

[0155] The basic value of the damping in the virtual synchronous generator in stable operation is set according to the method of the optimal second-order system, and the following is obtained:

[0156]

[0157] The basic value of the virtual inertia in the virtual synchronous generator in stable operation is set, and the following is obtained:

[0158]

[0159] The adaptive adjustment coefficient of the virtual inertia is set, and the following is obtained:

[0160]

[0161] The adaptive adjustment coefficient of the damping is set, and the following is obtained:

[0162]

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

[0164] Specifically, in order to prevent the virtual synchronous generator from frequently starting adaptation due to small frequency oscillation in the normal operation state, appropriate angular frequency change rate threshold and angular frequency offset change threshold are set as the adaptive starting criteria of the virtual inertia and the damping. The values of the angular frequency change rate threshold and the angular frequency offset change threshold should be greater than the maximum values of the angular frequency change rate and the angular frequency offset fluctuation in normal operation, and the specific values can be further confirmed through simulation experiments. In the embodiment of the application, T j =1 and T d =0.05.

[0165] In order to verify the VSG control method provided by the embodiment of the application, a grid-connected simulation model of a photovoltaic virtual synchronous generator is built by using Matlab / Simulink software, and the main parameters of the simulation are shown in Table 4 as follows:

[0166] Table 4

[0167] increase parameter value parameter value 700 Virtual inertia base value / kg m 2 ]]> 0.2 DC terminal voltage / V 380 Damping / N·m·s·rad -1 ]]> 10 grid voltage / V 0.1 filter resistance / Ω 1 angular frequency change rate threshold 3.2 filter inductance / mH 0.05 angular frequency offset change threshold 20 Adaptive tuning parameter K of virtual inertia J1 ]]> 0.1 filter capacitance / μF 0.3 Adaptive tuning parameter K of virtual inertia J2 ]]> 0.003 load resistance / Ω 3 load inductance / mH 5

[0168] According to the simulation parameters in Table 4, the following simulation results are obtained:adaptive adjustment parameter of damping (a), (b) and Fig. 7 (a) and (b) show the adaptive change curves, where Fig. 8 (a) and (b) are the virtual synchronous generator grid-connected stage, with the initial light intensity set to 600W / m 2 (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 Adaptive change curve of virtual inertia and damping under the working condition, Fig. 7 Fig. 8 (a) and (b) show the conditions when the virtual synchronous generator is connected to the grid and the illumination intensity of the photovoltaic power generation system is kept at 600W / m 2 The adaptive change curves of virtual inertia and damping under the working conditions where the power generated by the photovoltaic array is 11.65 kW) remain unchanged, the initial load is 10 kW, a 5 kW load is connected at 0.5 s, and then removed at 1.5 s, show a change trend that corresponds well to the analysis in Table 3.

[0169] In summary, the embodiment of the present invention first designs a photovoltaic VSG grid-connected topology for modeling, obtains the power and power angle equations of the virtual synchronous generator, 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. Then, based on the step response curve, the influence mechanism of the change of virtual inertia and damping on the dynamic performance of the virtual synchronous generator is analyzed, and based on the influence mechanism, the adaptive principle of virtual inertia and damping in different states is obtained to construct an adaptive function. Finally, the parameters in the adaptive function are adjusted and the start-up criterion is designed 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 fluctuations 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 within the scope of protection of the present invention.

Claims

1. A virtual inertia and damping based photovoltaic VSG control method, characterized in that, The method comprises the following steps: Step 1, modeling a virtual synchronous generator based on a designed photovoltaic VSG grid-connected topology, obtaining power and power angle equations of the virtual synchronous generator, and the photovoltaic VSG grid-connected topology comprising a main topology circuit and a control loop; Step 2, constructing a function expression of active power output by the virtual synchronous generator and line reactance, electromotive force amplitude, output terminal voltage, and power angle according to the main topology circuit, combining the function expression with the power and power angle equations of the virtual synchronous generator, obtaining a first second-order transfer function and a second second-order transfer function; the first second-order transfer function is a function from input active power of the virtual synchronous generator to active power output by the virtual synchronous generator, and the second second-order transfer function is a function from input active power of the virtual synchronous generator to an output angular frequency of the virtual synchronous generator; Step 3, respectively performing Laplace inverse transformation on the first second-order transfer function and the second second-order transfer function, obtaining a step response curve of active power when virtual inertia changes, a step response curve of angular frequency when virtual inertia changes, a step response curve of active power when damping changes, and a step response curve of angular frequency when damping changes; Step 4, analyzing the step response curve of active power when virtual inertia changes, the step response curve of angular frequency when virtual inertia changes, the step response curve of active power when damping changes, and the step response curve of angular frequency when damping changes, obtaining an influence mechanism of the virtual inertia change on dynamic performance of the virtual synchronous generator and an influence mechanism of the damping change on dynamic performance of the virtual synchronous generator; Step 5, obtaining an adaptive principle of the virtual inertia and the damping in different states based on the influence mechanism of the virtual inertia change on dynamic performance of the virtual synchronous generator and the influence mechanism of the damping change on dynamic performance of the virtual synchronous generator; Step 6, constructing an adaptive function of the virtual inertia and the damping based on the adaptive principle, adjusting parameters in the adaptive function, and determining an adaptive starting criterion of the virtual inertia and the damping, so as to control the virtual synchronous generator.

2. The virtual inertia and damping based PV VSG control method of claim 1, wherein, The power and power angle equations of the virtual synchronous generator are as follows: where J represents virtual inertia, P ref represents active power input to the virtual synchronous generator, P e represents active power output by the virtual synchronous generator, δ represents a power angle of the virtual synchronous generator, ω represents an output angular frequency of the virtual synchronous generator, ω0 represents a rated angular frequency of the power grid, D p represents a damping coefficient corresponding to the damping torque.

3. The virtual inertia and damping based PV VSG control method of claim 2, wherein, The function expression of active power output by the virtual synchronous generator and line reactance, electromotive force amplitude, output terminal voltage, and power angle according to the main topology circuit is as follows: where E represents the electromotive force amplitude obtained by active-frequency modulation, U l represents the output voltage, Z f represents the filter circuit impedance, X f represents the filter circuit reactance.

4. The virtual inertia and damping based PV VSG control method of claim 3, wherein, 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, which comprises: According to a conventional synchronous generator small signal model analysis method, 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 from input active power of the virtual synchronous generator to active power output by the virtual synchronous generator, which is as follows: A second second-order transfer function from input active power of the virtual synchronous generator to an output angular frequency of the virtual synchronous generator is as follows: where ω n denotes the rated angular frequency of the virtual synchronous generator.

5. The virtual inertia and damping based PV VSG control method of claim 4, wherein, Performing an inverse Laplace transform on the first second-order transfer function and the second second-order transfer function respectively includes: Performing an inverse Laplace transform on the first second-order transfer function yields: Performing an inverse Laplace transform on the second-order transfer function yields: where ω n denotes the rated angular frequency of the virtual synchronous generator.

6. The virtual inductance and damping based photovoltaic VSG control method according to claim 5, wherein, The step 4 comprises: By analyzing the step response curves of active power and 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 found 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 virtual inductance and damping based photovoltaic VSG control method according to claim 6, wherein, The adaptive principles of the virtual inertia and the damping in different states are: 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 a 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 virtual inductance and damping based photovoltaic VSG control method according to claim 7, wherein, Constructing an adaptive function of the virtual inertia and the damping based on the adaptive principle, including: 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: wherein J0 represents a base value of the virtual inertia in the virtual synchronous generator in stable operation, D0 represents a base value of the damping in the virtual synchronous generator in stable operation, K J1 , K J2 all represent adaptive adjustment coefficients of the virtual inertia, K D represents an adaptive adjustment coefficient of the damping, T j represents an angular frequency change rate threshold, T d represents an angular frequency offset change threshold.

9. The virtual inductance and damping based photovoltaic VSG control method of claim 8, wherein, The parameters in the adaptive function are adjusted, including: The basic value of the 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 difference between the mechanical torque T m and the electromagnetic torque T e , ΔT = T m - T e , P max represents the maximum power output by the virtual synchronous generator, J max represents the maximum value of the virtual inertia, J min represents the minimum value of the virtual inertia, D max represents the maximum value of the damping.

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