Virtual synchronous machine control method for photovoltaic system without energy storage configuration at source end

Abstract

The invention discloses a virtual synchronous machine control method for a photovoltaic system with no energy storage configuration at the source end, which belongs to the field of new energy power generation control, including: integrally adjusting the difference between the actual value of the DC side capacitor voltage and the DC side capacitor voltage command value to obtain a frequency adjustment amount; the DC side capacitor voltage command value is obtained by proportionally controlling the frequency adjustment amount and introducing the DC voltage at the active standby point; calculating the sum of the frequency adjustment amount and the inverter output frequency command value to obtain the actual inverter output frequency and convert it into a phase angle command signal; detecting the output reactive power of the photovoltaic system to further calculate the command value of the filter capacitor voltage under the dq axis; generating a PWM signal according to the phase angle command signal and the command value of the filter capacitor voltage under the dq axis to control the power devices in the photovoltaic system. The new energy photovoltaic system without energy storage unit is in the active standby mode to participate in the primary frequency modulation of the power grid.

Description

Technical Field

[0001] The present invention belongs to the field of renewable energy power generation control, and more specifically, relates to a virtual synchronous machine control method for a photovoltaic system with no energy storage configuration at the source end. Background Art

[0002] In order to cope with climate change and energy crisis, the installed capacity of clean energy such as wind power and photovoltaics has been increasing in recent years. Traditional photovoltaic inverters are mostly operated in maximum power point tracking (MPPT) mode to maximize the power utilization of photovoltaic cells, but they cannot provide additional active power output during grid frequency events, cannot perform active frequency modulation, and cannot achieve flexible control of active power. Therefore, the power system is easily affected by power fluctuations and system failures under low inertia and weak damping. Based on this, virtual synchronous generator technology came into being, which can control photovoltaic inverters to simulate the operating characteristics of synchronous generators to a certain extent, and has its external characteristics of rotational inertia and damping.

[0003] Domestic and foreign researchers have proposed many control methods for applying virtual synchronous generator control technology to photovoltaic systems. At present, it is widely used to equip the DC side of the photovoltaic system with energy storage equipment to meet the absorption and release of energy in the dynamic regulation process, realize the complementary advantages of multiple energy sources, and enhance system stability. However, the installation of energy storage units in photovoltaic systems is limited by the high cost of energy storage batteries and the complexity of hardware circuit transformation. The document "A photovoltaic virtual synchronous control strategy with active standby" proposes a photovoltaic virtual synchronous generator control strategy with active standby, but it adopts a current source control method and is still subject to the weak grid stability problem caused by the phase-locked loop, and cannot be applied to island systems. Summary of the invention

[0004] In view of the defects of the prior art and the need for improvement, the present invention provides a virtual synchronous machine control method for a photovoltaic system with no energy storage configuration at the source end, which aims to enable the new energy photovoltaic system without energy storage unit to be in active standby mode and participate in the primary frequency regulation of the power grid.

[0005] To achieve the above-mentioned purpose, according to one aspect of the present invention, a virtual synchronous machine control method for a photovoltaic system with no energy storage configuration at the source end is provided, comprising: S1, integrally adjusting the difference between the actual value of the DC side capacitor voltage and the DC side capacitor voltage command value to obtain a frequency adjustment amount; wherein the DC side capacitor voltage command value is obtained by proportionally controlling the frequency adjustment amount and introducing the DC voltage at the active standby point; S2, calculating the sum of the frequency adjustment amount and the inverter output frequency command value to obtain the actual output frequency of the inverter, and converting the actual output frequency of the inverter into a phase angle command signal; S3, detecting the output reactive power of the photovoltaic system, and according to the output reactive power, sequentially performing reactive power-voltage droop control, excitation regulation control and virtual impedance control to obtain the command value of the filter capacitor voltage under the dq axis; S4, generating a PWM signal according to the phase angle command signal and the command value of the filter capacitor voltage under the dq axis, and controlling the power devices in the photovoltaic system according to the PWM signal.

[0006] Furthermore, the DC side capacitor voltage command value is obtained by proportionally controlling the frequency adjustment amount and introducing the DC voltage at the active standby point, as well as introducing an additional DC voltage command value; wherein the additional DC voltage command value is obtained by performing differential feedback control on the difference between the system frequency and the inverter output frequency command value.

[0007] Furthermore, the DC link capacitor voltage command value is:

[0008] u dcref =u dcN +K f Δf+Δu dc2

[0009]

[0010] Among them, u dcref is the DC link capacitor voltage command value, u dcN is the DC voltage at the active standby point, Δu dc2 is the additional DC voltage command value, K f is the first proportional coefficient, Δf is the frequency adjustment amount, and f ref is the inverter output frequency command value, M is the differential coefficient, τ is the first-order inertia link time constant, f g is the system frequency.

[0011] Furthermore, before S1, it also includes: according to the PV characteristic curve of the photovoltaic system, using the maximum power tracking algorithm to determine the power and voltage at the maximum power point, and controlling the photovoltaic system to operate between the voltage at the maximum power point and the open-circuit voltage of the photovoltaic power supply; calculating the power at the active standby point according to the power at the maximum power point and the proportion of the photovoltaic system's standby capacity, and adjusting the DC side capacitor voltage command value so that the photovoltaic system outputs the power at the active standby point, thereby obtaining the DC voltage at the active standby point.

[0012] Furthermore, when the photovoltaic system switches from island operation to grid-connected operation, the inverter output frequency command value and the machine-end output voltage amplitude command value obtained by the reactive power-voltage droop control are adjusted to control the common coupling point voltage to be consistent with the grid voltage in amplitude, frequency and phase.

[0013] Furthermore, the inverter output frequency command value after adjustment is:

[0014]

[0015] Among them, f r ' ef is the inverter output frequency command value after adjustment, f ref is the inverter output frequency command value before adjustment, K pt is the second proportionality coefficient, K it is the second integral coefficient, θ g is the phase of the grid voltage, θ pcc is the phase of the common coupling point voltage; the actual output frequency of the inverter is the sum of the adjusted inverter output frequency command value and the frequency adjustment amount.

[0016] Furthermore, the adjusted machine-end output voltage amplitude command value is:

[0017]

[0018] Among them, u' cref is the output voltage amplitude command value after adjustment, u cref K is the output voltage amplitude command value before adjustment, pu is the third proportionality coefficient, K iu is the third integral coefficient, u g is the amplitude of the grid voltage, u pcc is the amplitude of the voltage at the common coupling point; in said S3, excitation regulation control and virtual impedance control are performed in sequence according to the adjusted machine-end output voltage amplitude command value to obtain the command value of the filter capacitor voltage under the dq axis.

[0019] Furthermore, the command value of the filter capacitor voltage under the dq axis is:

[0020]

[0021]

[0022] Among them, u cdref 、u cqref are the command values ​​of the filter capacitor voltage on the d and q axes, respectively, L v is the virtual inductance, i od 、i oq are the values ​​of the inverter output current on the d and q axes respectively, t is the time, ω is the angular frequency of the inverter output, and E is the no-load potential of the photovoltaic system.

[0023] According to another aspect of the present invention, a computer-readable storage medium is provided, on which a computer program is stored, and when the program is executed by a processor, the virtual synchronous machine control method for a photovoltaic system with no energy storage configuration at the source end as described above is implemented.

[0024] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects:

[0025] (1) A virtual synchronous machine control method for photovoltaic systems without energy storage configuration at the source end is proposed. There is no need to configure energy storage units and hardware circuit modification at the source end of the photovoltaic system, which greatly reduces the modification cost. On this basis, the DC side voltage is regulated and the frequency-voltage ratio control is introduced to realize the photovoltaic system working in active standby mode and have the ability to participate in the primary frequency regulation of the power grid, meeting the virtual synchronous transformation requirements of the photovoltaic system. The method is simple to operate and easy to implement in engineering.

[0026] (2) By designing the frequency differential feedback link, inertial support can be provided for the system when the photovoltaic system suffers from a large power shortage, which is beneficial to the frequency stability of the system;

[0027] (3) By designing amplitude synchronization control and phase synchronization control, the output voltage of the photovoltaic system can track the grid voltage, thereby achieving seamless switching between grid-connected and off-grid modes, improving the power supply reliability of local loads and reducing the impact on the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A flowchart of a virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end provided by an embodiment of the present invention;

[0029] Figure 2 A schematic diagram of the topological structure of a photovoltaic system provided by an embodiment of the present invention;

[0030] Figure 3A control block diagram of a virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end provided by an embodiment of the present invention;

[0031] Figure 4A A frequency-DC voltage relationship diagram of a photovoltaic system provided by an embodiment of the present invention;

[0032] Figure 4B An input power-DC voltage relationship diagram of a photovoltaic system provided by an embodiment of the present invention;

[0033] Figure 4C A frequency-input power relationship diagram of a photovoltaic system provided by an embodiment of the present invention;

[0034] Figure 5 A waveform diagram of the A-phase voltage at the point of common coupling (PCC) and the A-phase voltage of the power grid after the pre-synchronization unit is put into operation provided by an embodiment of the present invention;

[0035] Figure 6 A waveform diagram showing the change of the A-phase voltage of the PCC of the photovoltaic system and the A-phase current transmitted to the power grid during the off-grid to grid switching operation provided by the embodiment of the present invention;

[0036] Figure 7 A waveform diagram of the DC voltage change of the photovoltaic system before and after the grid frequency drops when the photovoltaic system is connected to the grid and operated according to an embodiment of the present invention;

[0037] Figure 8 A frequency change waveform diagram of the photovoltaic system before and after the grid frequency drops when the photovoltaic system provided by the embodiment of the present invention is connected to the grid and is running;

[0038] Fig. 9 This is a waveform diagram of output power variation of a photovoltaic system before and after the grid frequency drops when the photovoltaic system is connected to the grid and operated according to an embodiment of the present invention. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0040] In the present invention, the terms "first", "second", etc. (if any) in the present invention and the accompanying drawings are used to distinguish similar objects but not necessarily to describe a specific order or sequence.

[0041] The embodiment of the present invention provides a virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end, which aims to solve the problems of high energy storage equipment cost and complex hardware circuit transformation. When the source end of the converter is no longer equipped with an energy storage unit, the source end is transformed from a voltage source to a power source, and the DC voltage needs to be regulated. The embodiment of the present invention starts from the two aspects of maintaining the DC side voltage stability and providing inertial support for the system, so as to achieve a new energy photovoltaic system without energy storage unit in active standby mode to participate in the primary frequency regulation of the power grid. Figure 1 , combined with Figure 2-Figure 9 , a virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end in this embodiment is described in detail, and the method includes operations S1 to S4.

[0042] The topological structure of the photovoltaic system in this embodiment is as follows Figure 2 Before executing S1, it is necessary to determine the active reserve point (u dcN ,P N ). Specifically include: according to the PV characteristic curve of the photovoltaic system, use the maximum power tracking algorithm to determine the power P at the maximum power point max and voltage U dc-mpp , with the maximum power point as the boundary, the voltage interval can be divided into the left half interval [0,U dc-mpp ] and the right half interval [U dc-mpp ,U oc ], where the left half interval dP / dU dc >0, right half interval dP / dU dc <0. In order for the system to operate stably, dP / dU is required dc <0, therefore, in this embodiment, the photovoltaic system is controlled to operate between the voltage at the maximum power point and the open circuit voltage of the photovoltaic power source [U dc-mpp ,U oc ].

[0043] Furthermore, according to the power P at the maximum power point max And the proportion of photovoltaic system reserve capacity α to calculate the power P at the active reserve point N , adjust the DC side capacitor voltage command value so that the photovoltaic system output is the active standby power P N , thus obtaining the DC voltage u at the active reserve point dcN .

[0044] Specifically, according to the specific requirements of the photovoltaic system participating in primary frequency regulation, the system's additional reserve capacity is selected as ΔP = αP N Therefore, the active reserve power P can be obtained N =P max / (1+α) and the corresponding DC voltage u at the active reserve point in the right half of the interval dcNIn this embodiment, α is, for example, 10%.

[0045] Operation S1, integrally adjust the difference between the actual value of the DC side capacitor voltage and the DC side capacitor voltage command value to obtain a frequency adjustment amount. The DC side capacitor voltage command value is obtained by proportionally controlling the frequency adjustment amount and introducing the DC voltage at the active standby point.

[0046] See also Figure 3 As shown in the control block diagram, operation S1 is implemented by the DC voltage control module to regulate the DC voltage of the photovoltaic system, and the frequency adjustment amount Δf is obtained as follows:

[0047]

[0048] Among them, K idc is the integral coefficient of the DC capacitor voltage integral regulator, u dc is the actual value of the DC side capacitor voltage, u dcref is the DC side capacitor voltage command value.

[0049] Figure 3 The DC voltage frequency proportional control module in the control block diagram shown in the figure uses the frequency adjustment value Δf as the input of the proportional controller to obtain the DC side capacitor voltage command value u dcref , so that the photovoltaic system is in active standby operation mode, with the primary frequency modulation characteristics of the synchronous generator, the DC side capacitor voltage command value u dcref for:

[0050] u dcref =K f ·Δf+U dcN

[0051] Among them, the proportional coefficient of the proportional regulator in the DC voltage frequency proportional control module is recorded as K f .

[0052] Figure 4A , Figure 4B , Figure 4C The frequency-DC voltage relationship diagram, input power-DC voltage relationship diagram, and frequency-input power relationship diagram of the photovoltaic system are shown respectively. Figure 4A-4C It can be seen that by controlling the DC voltage, the photovoltaic system can participate in the primary frequency regulation of the system.

[0053] Further preferably, an inertial support control module is introduced in the embodiment of the present invention to detect the grid frequency, introduce differential feedback control of the system frequency, and obtain an additional DC voltage command signal Δu according to the grid frequency change rate. dc2 .

[0054] The additional DC voltage command value is obtained by performing differential feedback control on the difference between the system frequency and the inverter output frequency command value. Specifically, on the basis of DC voltage and frequency proportional control, the system frequency f is introduced. g The differential link is used as feedback, and a first-order inertia link is added for buffering; further speaking, at the system frequency f g When the frequency changes, the DC voltage command value of the photovoltaic system increases by an additional DC voltage command value Δu which is proportional to the change in system frequency. dc2 :

[0055]

[0056] Accordingly, the DC side capacitor voltage command value is obtained by proportionally controlling the frequency adjustment amount and introducing the DC voltage at the active standby point and the additional DC voltage command value:

[0057] u dcref =u dcN +K f Δf+Δu dc2

[0058] Among them, u dcref is the DC side capacitor voltage command value, u dcN is the DC voltage at the active standby point, Δu dc2 is the additional DC voltage command value, K f is the first proportional coefficient, Δf is the frequency adjustment amount, f ref is the inverter output frequency command value, M is the differential coefficient, τ is the first-order inertia link time constant, f g is the system frequency, T j is the generator rotor inertia time constant.

[0059] Operation S2, calculating the sum of the frequency adjustment amount and the inverter output frequency command value to obtain the actual inverter output frequency, and converting the actual inverter output frequency into a phase angle command signal.

[0060] The actual output frequency f of the inverter is:

[0061] f=Δf+f ref

[0062] Among them, f ref Output frequency command value for the inverter.

[0063] Furthermore, the actual output frequency f of the inverter is converted into a phase angle command signal θ, such as Figure 3 shown.

[0064] Operation S3 detects the output reactive power of the photovoltaic system, and performs reactive power-voltage droop control, excitation regulation control and virtual impedance control in sequence according to the output reactive power to obtain a command value of the filter capacitor voltage under the dq axis.

[0065] See also Figure 3 As shown in the control block diagram, operation S3 is implemented through a reactive power voltage droop control module, an excitation regulation control module and a virtual impedance control module, and specifically includes sub-operations S31 to S33.

[0066] In sub-operation S31, the output reactive power of the photovoltaic system is detected, and the reactive power voltage droop control module is used to perform QU droop control, and the voltage amplitude command u is obtained according to the reactive power deviation. cref Specifically, the reactive power command value Q ref And the detected reactive power actual value Q out The difference between them is multiplied by the QU droop coefficient, plus the rated voltage U cN The command value U of the output voltage amplitude of the machine end can be generated cref :

[0067] U cref =K v (Q ref -Q out )+U cN

[0068] Among them, K v is the QU droop coefficient.

[0069] In sub-operation S32, the actual value U of the output voltage amplitude of the machine end is detected. c , virtual excitation regulation control is performed through the excitation regulation control module, and the no-load potential of the photovoltaic system is generated according to the voltage deviation. Specifically, the command value U of the output voltage amplitude of the machine end is cref The actual value of the output voltage amplitude at the machine end is U c The voltage deviation value obtained after comparison is passed through the PI regulator to obtain the no-load potential E of the photovoltaic system:

[0070]

[0071] Among them, k pE , k iE They are respectively the proportional coefficient and integral coefficient of the PI regulator in the excitation regulation control module.

[0072] In sub-operation S33, the output current of the photovoltaic system is detected, and the command value of the filter capacitor voltage under the dq axis is calculated by the virtual impedance control unit. Specifically, the virtual impedance control step of the photovoltaic system is realized by simulating the stator voltage equation of the synchronous generator under the dq axis, and the no-load potential E of the photovoltaic system and the value 0 are respectively used as the command values ​​of the d and q axes, and the detected output current is fed back to the virtual impedance control algorithm, and the command value of the filter capacitor voltage under the dq axis is obtained as follows:

[0073]

[0074]

[0075] Among them, u cdref 、u cqref are the command values ​​of the filter capacitor voltage on the d and q axes, respectively, L v is the virtual inductance, i od 、i oq are the values ​​of the inverter output current on the d and q axes respectively, t is the time, and ω is the angular frequency of the inverter output.

[0076] Operation S4 generates a PWM signal according to the phase angle command signal and the command value of the filter capacitor voltage on the dq axis, and controls the power devices in the photovoltaic system according to the PWM signal.

[0077] See also Figure 3 The control block diagram shown in the figure uses the voltage and current dual-loop control and PWM generation link to generate the PWM signal. Specifically, θ, u cdref 、u cqref After voltage and current double closed loop and dq / abc coordinate conversion, it is used as the modulation signal of SVPWM, and the SVPWM output is used as the pulse control signal of the power device.

[0078] The above is the virtual synchronous machine control process when the photovoltaic system is connected to the grid. According to the embodiment of the present invention, when the photovoltaic system switches from island operation to grid-connected operation, it is necessary to put the pre-synchronization unit (such as Figure 3 The phase synchronization module and the amplitude synchronization module shown in the figure are used to control the voltage at the common coupling point to track the grid voltage.

[0079] Specifically, the inverter output frequency command value and the machine-end output voltage amplitude command value obtained by reactive power-voltage droop control are adjusted to control the amplitude, frequency and phase of the common coupling point voltage to be consistent with the grid voltage.

[0080] The inverter output frequency command value after adjustment is:

[0081]

[0082]

[0083] Among them, f r ' ef is the inverter output frequency command value after adjustment, f ref is the inverter output frequency command value before adjustment, K pt is the second proportionality coefficient, K it is the second integral coefficient, θ g is the phase of the grid voltage, θ pcc is the phase of the voltage at the common coupling point, Δf syn is superimposed on f ref The phase synchronization signal on.

[0084] At this time, in operation S2, the inverter output frequency command value f r ' ef The sum of the frequency adjustment value Δf is taken as the actual output frequency f of the inverter, that is, f = Δf + f r ' ef .

[0085] The adjusted output voltage amplitude command value of the machine end is:

[0086]

[0087]

[0088] Among them, u' cref is the output voltage amplitude command value after adjustment, u cref K is the output voltage amplitude command value before adjustment, pu is the third proportionality coefficient, K iu is the third integral coefficient, u g is the amplitude of the grid voltage, u pcc is the amplitude of the voltage at the common coupling point, Δu syn To be superimposed on u cref The amplitude synchronization signal on.

[0089] At this time, in S3, the excitation adjustment control and virtual impedance control are performed in sequence according to the adjusted machine-end output voltage amplitude command value to obtain the command value of the filter capacitor voltage under the dq axis. Specifically, the no-load potential E of the photovoltaic system obtained in sub-operation S32 is:

[0090]

[0091] In this embodiment, a simulation model of a photovoltaic system with flexible switching between off-grid operation and grid-connected operation with local load was built in Matlab / Simulink for simulation verification. The parameters of the photovoltaic system are shown in Table 1.

[0092] Table 1

[0093]

[0094] The simulation conditions include: (1) the photovoltaic system is operated off-grid; (2) the photovoltaic system switches from off-grid operation to grid-connected operation; (3) the photovoltaic system is operated on-grid, and the infinite grid frequency drops by 0.1 Hz at 4 seconds. The simulation results are shown in Figure 2. Figure 5-Figure 9 shown.

[0095] It should be noted that before t = 0.2s, the PV system operated stably off-grid with local load (45.5MW, 0.9MVar) at the common coupling point PCC. When the system ran to t = 0.2s, the pre-synchronization unit was put into operation. Figure 5 This is the waveform of the PCC phase A voltage and the grid phase A voltage after the pre-synchronization module is put into use. It can be seen that the two voltage waveforms gradually overlap, achieving the consistency of the voltage amplitude, frequency and phase on both sides of the grid connection point.

[0096] When the system runs to t=2s, the photovoltaic system is connected to the grid and the pre-synchronization unit is removed. Figure 6 This is the waveform of the A-phase voltage of the PCC of the photovoltaic system and the A-phase current transmitted to the grid. It can be seen that the system voltage remains stable, the current increases slowly, and finally enters a steady-state operation, achieving a smooth transition from off-grid switching and avoiding impact current.

[0097] When the system runs to t=4s, the system frequency drops by 0.1Hz. Figure 7 This is the DC voltage change waveform of the photovoltaic system at this time. Figure 8 This is the output frequency change waveform of the photovoltaic system at this time. Fig. 9 This is the output power variation waveform of the photovoltaic system at this time. It can be seen that before the system frequency drops, the photovoltaic system is at the set active reserve point U dc =1200V, f=50Hz, P=50MW. When the frequency drops, the photovoltaic system quickly reduces the DC voltage command value, providing a large instantaneous power support for the system and slowing down the frequency change rate. When the steady state is restored, the primary frequency regulation of the photovoltaic system comes into play, providing continuous additional power support for the system by regulating the DC voltage to prevent the system frequency from collapsing.

[0098] The virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end in the embodiment of the present invention starts from the two aspects of maintaining the DC side voltage stability and providing inertial support for the system, and realizes that the new energy photovoltaic system without energy storage unit is in active standby mode to participate in the primary frequency modulation of the power grid. By regulating the DC side voltage, the flexible and controllable output power of the photovoltaic power source is achieved. The introduction of proportional control of frequency and DC voltage enables the photovoltaic system to have the ability to participate in the primary frequency modulation of the power grid. The introduction of differential feedback control of frequency can provide short-term inertial support power for the system. When switching between the island and grid-connected modes, only the pre-synchronization module needs to be switched, and the inverter does not need to switch the control algorithm, which effectively suppresses the impact of the current in the switching process. Simulation proves that this method can be applied to a new energy photovoltaic system without energy storage unit to regulate the DC voltage, provide inertial support for the system, and seamlessly switch off the grid, verifying the effectiveness of the control strategy.

[0099] The embodiment of the present invention further provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the following Figure 1-Figure 9 The virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end shown is not repeated here.

[0100] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end, characterized in that: include: S1, integrating and adjusting the difference between the actual value of the DC side capacitor voltage and the DC side capacitor voltage command value to obtain the frequency adjustment amount; The DC side capacitor voltage command value is obtained by proportionally controlling the frequency adjustment amount and introducing the DC voltage at the active standby point, and introducing the additional DC voltage command value; the additional DC voltage command value is obtained by differential feedback control of the difference between the system frequency and the inverter output frequency command value; the DC side capacitor voltage command value is: u dcref =u dcN +K f Δf+Δu dc2 Among them, u dcref is the DC link capacitor voltage command value, u dcN is the DC voltage at the active standby point, Δu dc2 is the additional DC voltage command value, K f is the first proportional coefficient, Δf is the frequency adjustment amount, and f ref is the inverter output frequency command value, M is the differential coefficient, τ is the first-order inertia link time constant, f g is the system frequency; S2, calculating the sum of the frequency adjustment amount and the inverter output frequency command value to obtain the actual inverter output frequency, and converting the actual inverter output frequency into a phase angle command signal; S3, detecting the output reactive power of the photovoltaic system, and performing reactive power-voltage droop control, excitation regulation control and virtual impedance control in sequence according to the output reactive power, to obtain the command value of the filter capacitor voltage under the dq axis; the command value of the filter capacitor voltage under the dq axis is: Among them, u cdref 、u cqref are the command values ​​of the filter capacitor voltage on the d and q axes, respectively, L v is the virtual inductance, i od 、i oq are the values ​​of the inverter output current on the d and q axes respectively, t is the time, ω is the angular frequency of the inverter output, and E is the no-load potential of the photovoltaic system; S4, generating a PWM signal according to the phase angle command signal and the command value of the filter capacitor voltage on the dq axis, and controlling the power devices in the photovoltaic system according to the PWM signal.

2. The virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end according to claim 1, characterized in that: Before the S1, it also includes: According to the PV characteristic curve of the photovoltaic system, the maximum power tracking algorithm is used to determine the power and voltage at the maximum power point, and the photovoltaic system is controlled to operate between the voltage at the maximum power point and the open-circuit voltage of the photovoltaic power source; The power at the active standby point is calculated according to the power at the maximum power point and the proportion of the standby capacity of the photovoltaic system, and the DC side capacitor voltage command value is adjusted so that the photovoltaic system outputs the power at the active standby point, thereby obtaining the DC voltage at the active standby point.

3. The virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end according to claim 1 or 2, characterized in that: When the photovoltaic system switches from island operation to grid-connected operation, the inverter output frequency command value and the machine-end output voltage amplitude command value obtained by the reactive power-voltage droop control are adjusted to control the common coupling point voltage to be consistent with the grid voltage in amplitude, frequency and phase.

4. The virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end according to claim 3, characterized in that: The inverter output frequency command value after adjustment is: Among them, f r ' ef is the inverter output frequency command value after adjustment, f ref is the inverter output frequency command value before adjustment, K pt is the second proportionality coefficient, K it is the second integral coefficient, θ g is the phase of the grid voltage, θ pcc is the phase of the voltage at the common coupling point; The actual output frequency of the inverter is the sum of the adjusted inverter output frequency command value and the frequency adjustment amount.

5. The virtual synchronous machine control method for a photovoltaic system without energy storage configuration at the source end according to claim 3, characterized in that: The adjusted output voltage amplitude command value of the machine end is: Among them, u' cref is the output voltage amplitude command value after adjustment, u cref K is the output voltage amplitude command value before adjustment, pu is the third proportionality coefficient, K iu is the third integral coefficient, u g is the amplitude of the grid voltage, u pcc is the amplitude of the voltage at the common coupling point; In the S3, the excitation adjustment control and the virtual impedance control are sequentially performed according to the adjusted machine-end output voltage amplitude command value to obtain the command value of the filter capacitor voltage under the dq axis.

6. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, a virtual synchronous machine control method for a photovoltaic system with no energy storage configuration at the source end is implemented as described in any one of claims 1 to 5.

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