A control method for a photovoltaic virtual synchronous generator system

Abstract

The present invention discloses a control method for a photovoltaic virtual synchronous generator system. The control method includes: inverter control, energy storage side bidirectional chopper control, and photovoltaic side boost chopper control. On the basis of virtual synchronous generator control, the inverter control adds photovoltaic power feedforward control to directly transfer the power disturbance on the photovoltaic side to the grid side. In the energy storage side bidirectional chopper control, the DC bus voltage adopts proportional integral resonant control, so that in a single-phase system or a three-phase unbalanced load, the energy storage system absorbs the double-frequency power of the DC bus. The photovoltaic side boost chopper control adopts the maximum power tracking point control method. The present invention realizes the optimization of the energy storage capacity and the DC bus capacitor capacity without affecting the VSG frequency support function.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy control, and particularly relates to a control method for a photovoltaic virtual synchronous generator system. Background Art

[0002] With the rapid development of renewable energy such as wind power and photovoltaic power, the penetration rate of distributed generators in the power system is getting higher and higher, and the dynamic characteristics of the power grid have changed significantly accordingly. In a traditional power grid dominated by synchronous generators (SGs), when there is a power imbalance between the power generation side and the load side, the SG can compensate for the power imbalance of the system by releasing or absorbing the kinetic energy of its rotor to smooth out the frequency fluctuations. However, the new power electronic power system does not have such rotational inertia. A newly emerging grid-side inverter control method called virtual synchronous generator (VSG) has received extensive attention in recent years. It realizes the transient characteristics of the synchronous machine by simulating the swing equation of the SG. Usually, the primary frequency regulation (droop) characteristics of the governor are also simulated in the VSG control scheme. In this case, the VSG control inherits the advantages of the droop control and is superior to the latter in terms of transient frequency stability due to its lower df / dt rate. Therefore, the VSG control is regarded as the key to solving the frequency stability problem of the power electronic power system.

[0003] However, previous studies on VSG have mostly focused on the design scheme of the controller, and less discussed the overall system design of VSG actually used in new energy power generation. Usually, in these studies, the power supply side is simplified to an ideal DC voltage source, which is obviously inconsistent with the actual application situation. To simulate the rotor kinetic energy of the SG and provide the primary frequency regulation ability that traditional new energy power generation does not have, the VSG requires a corresponding energy storage system. How to design a reasonable energy storage system and its coordinated control scheme has become the key to the current VSG research.

[0004] Some scholars have proposed directly using the DC bus capacitor as an energy storage system. However, due to the limited fluctuation range of the DC bus voltage, the transient energy buffer range it can provide is relatively small. To provide a significant inertial support effect, only a large-capacity DC bus capacitor can be used. At the same time, in order to take into account the primary frequency regulation function of the VSG, the team has proposed a VSG system with battery / supercapacitor hybrid energy storage, in which the DC bus capacitor uses a supercapacitor to simulate the inertia of the SG and compensate for high-frequency power fluctuations, and the battery is used to simulate the droop characteristic to provide relatively long-term slow power. However, there is no further discussion on the battery capacity and the supercapacitor capacity, and the coordinated control research between the converters of each subsystem is not sufficient. There are also scholars who have used a minute-level energy storage system to make the full-converter wind turbine have the external characteristics of the SG. Although the required energy storage system capacity is discussed therein, there is no additional design to reduce the energy storage capacity.

[0005] In summary, for the new energy VSG system with energy storage, there is currently no clear research plan on how to optimize the energy storage capacity and the DC bus capacitor capacity as much as possible. Summary of the Invention

[0006] To solve the problems existing in the prior art, the purpose of the present invention is to provide a control method for a photovoltaic virtual synchronous generator system. The present invention realizes the optimization of the energy storage capacity and the DC bus capacitor capacity without affecting the VSG frequency support function.

[0007] The purpose of the present invention is achieved by the following technical solutions:

[0008] A control method for a photovoltaic virtual synchronous generator system, the photovoltaic virtual synchronous generator system includes a photovoltaic array, an energy storage system, a boost chopper, a bidirectional DC chopper, a grid-side inverter, and an LCL filter; the photovoltaic array and the energy storage system are respectively connected to the DC bus through the boost chopper and the bidirectional DC chopper, and then are connected to the grid through the grid-side inverter and the LCL filter;

[0009] The control method includes: inverter control, bidirectional chopper control on the energy storage side, and boost chopper control on the photovoltaic side;

[0010] On the basis of virtual synchronous generator control, the inverter control adds photovoltaic power feedforward control to directly transfer the power disturbance on the photovoltaic side to the grid side;

[0011] In the bidirectional chopper control on the energy storage side, the DC bus voltage adopts proportional integral resonant control, so that in a single-phase system or a three-phase system with unbalanced loads, the energy storage system absorbs the second harmonic power of the DC bus;

[0012] The boost chopper control on the photovoltaic side adopts the maximum power tracking point control method.

[0013] As a further improvement of the present invention, the control of the two-way chopper on the energy storage side adopts a double-loop control of an outer DC bus voltage loop and an inner energy storage side current loop, and an additional resonant controller is added to the voltage outer loop; the resonant frequency is set to twice the system frequency, and the grid frequency can be used, or the virtual rotor frequency ω in the virtual synchronous generator control can be used. m When the three-phase unbalanced load is connected to the grid side or it is a single-phase system, the proposed resonant control transfers the double-frequency power generated by the system from the DC bus to the energy storage system.

[0014] As a further improvement of the present invention, the control of the two-way chopper on the energy storage side further includes a virtual rotor kinetic energy feedforward control, and the virtual rotor kinetic energy feedforward control directly transfers the grid side disturbance to the current control command on the energy storage side.

[0015] As a further improvement of the present invention, the photovoltaic power feedforward control is to add a photovoltaic power feedforward term P to the power set value P0 of the VSG. pv This feedforward control tracks the photovoltaic power by adjusting the output power of the VSG.

[0016] As a further improvement of the present invention, in the control of the two-way chopper on the energy storage side, when a frequency transient occurs in the grid due to, for example, a load change, the energy storage side needs to return to the rated state of charge, and an outer energy storage side voltage loop is set to adjust the set active power command, and its bandwidth is lower than that of the VSG control loop;

[0017] When the grid frequency is outside the droop dead zone of P-ω and the primary frequency regulation function of the photovoltaic VSG system is turned on, the outer loop on the energy storage side is turned off.

[0018] As a further improvement of the present invention, the disturbance on the grid side is quickly transferred to the energy storage side, and a feedforward term I is added to the inner current loop of the energy storage. ESff The control equation is:

[0019] P in -P out =I ESff V ES ;

[0020] Among them, P in is the virtual shaft power determined by the governor, P out is the output active power of the inverter, I ESff is the inner loop current of the feedforward, and V ES is the voltage on the energy storage side.

[0021] As a further improvement of the present invention, the VSG control of the inverter includes:

[0022] Reactive single-loop control using the bus voltage estimation scheme; when simulating the inertia of the SG and the governor characteristics in the active power loop, damping is added to attenuate the inherent oscillation; the control equation is as follows:

[0023]

[0024] Among them, J is the rotational inertia of the VSG, P d is the damping power, ω0 is the rated frequency, and ω m is the virtual angular frequency output by the VSG.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] The present invention is a control method for a photovoltaic VSG system with collaborative energy storage, which improves the traditional control method and realizes the optimization of the energy storage capacity and the DC bus capacitor capacity without affecting the VSG frequency support function. First, the DC bus voltage loop in the bidirectional DC chopper on the energy storage side adopts proportional-integral-resonant control, so that the energy storage system can absorb the second-harmonic power of the DC bus in a single-phase system or a three-phase system with unbalanced loads. Second, virtual rotor kinetic energy feedforward control is proposed to directly transfer the grid-side disturbance to the current control command on the energy storage side. Through the above two controls, the voltage fluctuation of the DC bus is greatly reduced, and the required DC bus capacitor is reduced, so that a thin-film capacitor can replace the traditional electrolytic capacitor. Finally, the inverter control adds photovoltaic power feedforward control on the basis of the virtual synchronous generator control to directly transfer the power disturbance on the photovoltaic side to the grid side, greatly reducing the voltage fluctuation on the energy storage side and thus reducing the required capacity of the energy storage system. Description of the Drawings

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention, using supercapacitors as the energy storage system, but the present invention is not limited to supercapacitor energy storage. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0028] Figure 1 is the circuit and simplified control diagram of the photovoltaic VSG grid-connected system of the present invention;

[0029] Figure 2 is the VSG control block diagram of the grid-connected inverter of the present invention, where (a) is the VSG reactive power / voltage control diagram and (b) is the VGS active power / frequency control diagram;

[0030] Figure 3It is the improved control block diagram proposed by the present invention, where (a) is the schematic diagram of photovoltaic feedforward control, and (b) is the schematic diagram of proportional-integral-resonant control and virtual rotor kinetic energy feedforward control;

[0031] Figure 4 It is the schematic diagram of the simulation waveform of the photovoltaic VSG grid-connected system of the present invention. (a) The system frequency simulation waveform diagram when the unbalanced load suddenly increases; (b) The supercapacitor voltage simulation waveform diagram when the unbalanced load suddenly increases; (c) The DC bus voltage simulation waveform diagram under steady state when the unbalanced load suddenly increases; (d) The DC bus voltage simulation waveform diagram under transient state when the balanced load suddenly increases; (e) The supercapacitor voltage simulation waveform diagram when the photovoltaic power suddenly increases. Specific implementation manners

[0032] In order to make the objectives and technical solutions of the present invention clearer and easier to understand, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0033] In the description of the present invention, it should be understood that the orientation or positional relationships indicated by the terms "center", "longitudinal", "transverse", "up", "down", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. are based on the orientation or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation of the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more. In the description of the present invention, it should be noted that unless otherwise clearly specified and limited, the terms "mounted", "connected" and "connected" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected, or indirectly connected through an intermediate medium, and it may be the internal connection of two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0034] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of the present invention.

[0035] The present invention provides a control method for a photovoltaic VSG system with collaborative energy storage, which improves the traditional control method and realizes the optimization of the energy storage capacity and the DC bus capacitor capacity without affecting the VSG frequency support function.

[0036] As Figure 1 shown, the photovoltaic virtual synchronous generator system includes a photovoltaic array, an energy storage system, a boost chopper, a bidirectional DC chopper, a grid-side inverter, and an LCL filter; the photovoltaic array and the energy storage system are respectively connected to the DC bus through the boost chopper and the bidirectional DC chopper, and then are connected to the grid through the grid-side inverter and the LCL filter; the control method includes: inverter control, energy storage side bidirectional chopper control, and photovoltaic side boost chopper control;

[0037] The photovoltaic side boost chopper control adopts the maximum power tracking point control method.

[0038] First, the DC bus voltage control in the energy storage side bidirectional DC chopper adopts the proportional integral resonant control. When the grid is a single-phase system or a three-phase system with unbalanced loads, the energy storage system can absorb the second harmonic power of the DC bus.

[0039] Secondly, the virtual rotor kinetic energy feedforward control is proposed, which directly transfers the grid-side disturbance to the current control command of the energy storage. Through the above two controls, the fluctuation of the DC bus voltage is greatly reduced, and the required DC bus capacitor is reduced, so that the thin-film capacitor can replace the traditional electrolytic capacitor.

[0040] Finally, the inverter control adds the photovoltaic power feedforward control on the basis of the virtual synchronous generator control, and directly transfers the power disturbance on the photovoltaic side to the grid side, greatly reducing the voltage fluctuation on the energy storage side, and further reducing the energy storage system capacity.

[0041] The present invention will be described below in conjunction with the accompanying drawings.

[0042] The main circuit structure topology of the photovoltaic VSG system in the present invention is as Figure 1 shown, including a photovoltaic array, an energy storage system, a boost chopper, a bidirectional DC chopper, a grid-side inverter, and an LCL filter;

[0043] The photovoltaic array and the energy storage system are respectively connected to the DC bus through a boost chopper and a bidirectional DC chopper, and then the previous-stage system is incorporated into the power grid through a grid-side inverter and an LCL filter.

[0044] The overall system control can be divided into three parts: inverter control, bidirectional chopper control on the energy storage side, and boost chopper control on the photovoltaic side. Among them, the photovoltaic boost chopper usually adopts the maximum power tracking point control technology, which is already very mature in the industry. This invention mainly focuses on the first two controls, and aims to optimize the system and the DC bus capacitor capacity through coordinated control.

[0045] For VSG control, even without any voltage or current loops, the grid-side inverter can still have the ability to form a grid and provide frequency support through the reactive power / voltage loop and the active power / frequency loop. Figure 2 (a) is the control block diagram of the reactive power and voltage loop of VSG. The single-inner-loop reactive power control realizes accurate reactive power distribution through a bus voltage estimator based on the αβ coordinate system and V-Q droop control, and is not affected by changes in active power distribution and line impedance mismatch. Among them, Q out is the output reactive power, Q0 is the reactive power command, V m is the virtual electromotive force reference value, V base is the rated voltage, is the estimated effective value of the line bus voltage, T fq is the time constant of the low-pass filter, k q is the V-Q droop coefficient, L line 、L v are the line impedance and the virtual impedance respectively. Figure 2 (b) is the control block diagram of the active power and frequency of VSG. While simulating the inertia and governor characteristics of the SG, VSG adds damping to attenuate its inherent oscillation. The control equation is where J is the rotational inertia of VSG, P out is the output active power of the inverter, P in is the virtual shaft power determined by the governor, P d is the damping power, ω0 is the rated frequency, ω m is the virtual angular frequency output by VSG. It should be noted that in the steady state, due to the effect of integration, P d is always zero, so that the damping term does not affect the steady-state operating point of primary frequency regulation. While realizing the P-ω droop characteristic, the governor model takes into account the steady-state error between the grid frequency and ω0, and P in is in the dead zone ω thr_l ≤ω m ≤ω thr_hIt is set to a constant value P0 inside. If the dead zone is not added, the PV VSG system may provide unnecessary primary frequency regulation responses for a long time due to small deviations in the system frequency, absorbing or releasing excessive energy, resulting in an increase in the cost of the ESS.

[0046] After a frequency transient occurs in the power grid, such as caused by load changes, the energy storage side voltage needs to recover to its standby value to prepare for the next power grid frequency fluctuation. Therefore, an outer loop on the energy storage side is required to adjust the set active power command, and its bandwidth is much lower than that of the VSG control loop. When the power grid frequency is outside the droop dead zone of P-ω, that is, when the primary frequency regulation function of the PV VSG system is enabled, this control loop needs to be closed to avoid affecting the frequency regulation function.

[0047] Based on the above control, the present invention adds PV power feedforward control, that is, a PV power feedforward term P is added to the power set value P0 of the VSG. pv as Figure 3 (a) shows. This feedforward control realizes the tracking of PV power by directly adjusting the VSG output power, thereby reducing the overcharging and discharging of the energy storage side caused by PV power fluctuations. Therefore, it can reduce the required capacity of the energy storage system and enable it to focus on the grid-side frequency regulation support function. The influence of the output power change of the PV array on the energy storage system is as Figure 4 (a) in shows. Compared with the existing control, through the proposed PV power feedforward control, the amplitude of the energy storage side voltage change is reduced from 94V to 39V, significantly reducing the transient voltage fluctuation on the energy storage side, which means that when the improved control system faces the same PV fluctuations, the capacity of the energy storage system can be greatly reduced.

[0048] The two-way chopper control on the energy storage side adopts a dual-loop control of a DC bus voltage outer loop control and an energy storage side current inner loop. The DC bus voltage outer loop control adopts a proportional-integral-resonant control, as Figure 3 (b) shows. The resonant frequency is set to twice the system frequency, and the grid frequency can be used, or the virtual rotor frequency ω in the virtual synchronous machine control can be used m instead. When the power grid is a single-phase system or a three-phase system with unbalanced loads, the proposed resonant control can transfer the double-frequency power generated by the system from the DC bus to the energy storage system, thereby effectively suppressing the double-frequency ripple of the DC bus voltage, so that a smaller DC bus capacitor can be used under the same ripple index. As Figure 4 shown, the waveforms of (b) and (c) show that the improved control does not affect the frequency support function of the VSG. As Figure 4 (d) in shows. Taking a three-phase unbalanced load as an example, after the DC bus voltage adopts proportional-integral-resonant control, the peak-to-peak value of the double-frequency ripple in the steady state is reduced from 36V to 1.4V, significantly suppressing the bus voltage fluctuation.

[0049] When the power grid experiences frequency fluctuations caused by factors such as load changes during the transient process, the input power P of the virtual rotor in and the output power P out will no longer be equal. The power difference between P in and P out needs to be released or absorbed by the energy storage system, that is Therefore, in order to quickly transfer this power grid-side disturbance to the energy storage side, a feedforward term I is added to the internal current loop on the energy storage side ESff , that is, the virtual rotor kinetic energy feedforward control proposed in the present invention, as shown in Figure 3 (b). This feedforward control regulates the power flow between the energy storage system and the DC bus. It helps to maintain the dynamic power balance between the power grid and the energy storage system, reduce the voltage fluctuation of the DC bus, and improve the transient response speed of the system. If this feedforward term is not added, the fluctuation amplitude of the transient DC bus voltage completely depends on the response speed of the DC bus voltage control loop, and a larger DC bus voltage offset will occur at the initial stage of the transient process. Under the combined action of the virtual rotor kinetic energy feedforward control and the proportional-integral-resonant control proposed above, the capacitance value of the DC bus capacitor can be significantly reduced, so that a thin-film capacitor with higher reliability and longer life can be used to replace the electrolytic capacitor. As shown in Figure 4 (e), through the proposed virtual rotor kinetic energy feedforward control, the maximum deviation of the DC bus voltage is greatly reduced during the transient process, and the change amplitude of the bus voltage is reduced from 9.5V to 5.7V, a decrease of 40%. Through the above two schemes, the capacity of the DC bus capacitor can be greatly reduced, so that a thin-film capacitor can be used to replace the traditional electrolytic capacitor.

[0050] The present invention can be used in all photovoltaic virtual synchronous machine systems with cooperative energy storage. The control method proposed based on this system can significantly reduce the voltage fluctuations on the DC side and the energy storage side, thereby optimizing the energy storage capacity and the DC bus capacitor capacity under the same ripple requirements.

[0051] The above are only the preferred embodiments of the present invention and do not limit the present invention in any way. Any simple modification, change, and equivalent structural change made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

[0052] The above embodiments are only used to illustrate the technical solution of the present invention and do not limit it. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art can still modify the specific implementation of the present invention or make equivalent substitutions, and any modification or equivalent substitution that does not depart from the spirit and scope of the present invention is within the protection scope of the claims of the present invention.

Claims

1. A control method for a photovoltaic virtual synchronous generator system, characterized in that, The photovoltaic virtual synchronous generator system includes a photovoltaic array, an energy storage system, a boost chopper, a bidirectional DC chopper, a grid-side inverter, and an LCL filter; the photovoltaic array and the energy storage system are respectively connected to the DC bus through the boost chopper and the bidirectional DC chopper, and then are connected to the grid through the grid-side inverter and the LCL filter; The control method includes: inverter control, energy storage side bidirectional chopper control, and photovoltaic side boost chopper control; On the basis of virtual synchronous generator control, the inverter control adds photovoltaic power feedforward control to directly transfer the power disturbance on the photovoltaic side to the grid side; In the energy storage side bidirectional chopper control, the DC bus voltage adopts proportional integral resonant control, so that in a single-phase system or a three-phase system with unbalanced loads, the energy storage system absorbs the second harmonic power of the DC bus; The photovoltaic side boost chopper control adopts the maximum power tracking point control method.

2. The control method for a photovoltaic virtual synchronous generator system according to claim 1, characterized in that, The control of the bidirectional chopper on the energy storage side adopts a dual-loop control of an outer DC bus voltage loop and an inner energy storage side current loop, and an additional resonant controller is added to the voltage outer loop; the system frequency adopts the virtual rotor frequency in the virtual synchronous machine control ; The resonant frequency is set to twice the virtual rotor frequency ; when a three-phase unbalanced load is connected to the grid side or it is a single-phase system, the proposed resonant control transfers the double-frequency power generated by the system from the DC bus to the energy storage system.

3. The control method for a photovoltaic virtual synchronous generator system according to claim 1, characterized in that, The energy storage side bidirectional chopper control further includes virtual rotor kinetic energy feedforward control, and the virtual rotor kinetic energy feedforward control directly transfers the grid-side disturbance to the current control command on the energy storage side.

4. The control method for a photovoltaic virtual synchronous generator system according to claim 3, characterized in that, The photovoltaic power feedforward control adds a photovoltaic power feedforward term to the power setpoint of the VSG ; the photovoltaic power feedforward control tracks the photovoltaic power by adjusting the output power of the VSG. ​ 5. The control method for a photovoltaic virtual synchronous generator system according to claim 2, characterized in that, In the energy storage side bidirectional chopper control, when a frequency transient occurs in the grid due to load changes, etc., the energy storage side needs to return to the rated state of charge, and an energy storage side voltage outer loop is set to adjust the set active power command, and its bandwidth is lower than the VSG control loop; When the grid frequency is outside the droop dead zone of when the primary frequency regulation function of the PV VSG system is enabled, the outer loop on the energy storage side is turned off.

6. The control method for a photovoltaic virtual synchronous generator system according to claim 1, characterized in that, The disturbances on the grid side are quickly transmitted to the energy storage side, and a feedforward term is added to the current loop inside the energy storage. ； Among them, is the virtual shaft power determined by the governor, is the active power output of the inverter, is the inner-loop current of the feedforward, is the voltage on the energy storage side.

7. The control method for a photovoltaic virtual synchronous generator system according to claim 6, characterized in that, The VSG control of the inverter includes: Reactive power single-loop control using a bus voltage estimation scheme; when the active power loop simulates the inertia of the SG and the governor characteristics, damping is added to attenuate the inherent oscillation; the control equation is: ； Among them, is the rotational inertia of the VSG, is the damping power, is the rated frequency, is the virtual angular frequency output by the VSG.

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