Power distribution and virtual inertia control method for photovoltaic hybrid energy storage system based on virtual synchronous generator

Abstract

The application relates to a virtual synchronous generator-based photovoltaic hybrid energy storage system power distribution and virtual inertia control method, which comprises the following steps: S1, determining the power distribution mode of the hybrid energy storage system; S2, determining the virtual inertia of the virtual synchronous generator and the correlation between the two energy storage states of charge; S3, determining the adaptive control strategy of the virtual inertia; and S4, determining the selection principle of the relevant parameters. The application introduces an adjustment coefficient related to the states of charge of the all-vanadium redox flow battery and the super capacitor, so that the virtual inertia is adjusted according to the inverter output frequency and the states of charge of the two energy storages, the stability of the system is analyzed through a root locus, and the selection principle of the key parameters such as the virtual inertia and the damping is determined. The application can effectively improve the stability of the voltage and frequency of the photovoltaic hybrid energy storage system, realize the reasonable distribution of the power between the hybrid energy storages, avoid the over-limit of the states of charge of the energy storages, and improve the charging and discharging performance and the service life of the energy storages.

Description

Technical Field

[0001] This invention belongs to the field of power system protection and control technology, specifically relating to a power distribution and virtual inertial control method for a photovoltaic hybrid energy storage system based on a virtual synchronous generator. Background Technology

[0002] To alleviate energy shortages, environmental pollution, and climate change, the development and utilization of renewable energy have received widespread attention. Solar energy, with its abundant reserves and wide distribution, has made photovoltaic (PV) power generation a crucial method for utilizing solar energy and a natural choice for vigorous development in many countries. However, large-scale grid-connected PV systems can negatively impact the dynamic response and stability of the power grid. On one hand, although the output frequency of PV grid-connected inverters is supported by the grid, the intermittent, random, and fluctuating nature of PV output, along with frequency or power disturbances in the grid, can cause dynamic response processes in the inverter's output frequency, posing a risk of exceeding limits. This can lead to overshoot or oscillations in output power, resulting in significant power surges and, in severe cases, triggering overcurrent protection. On the other hand, PV grid-connected inverters are power electronic components without rotational inertia. Extensive grid connection can lead to insufficient rotational inertia and damping in the overall power grid, reducing its ability to cope with power and frequency disturbances. Therefore, to improve the grid-friendliness of PV, Virtual Synchronous Generator (VSG) control technology can be introduced into the inverter control, and a certain capacity of energy storage equipment can be configured in the PV power plant to provide energy buffering for the VSG's regulation process.

[0003] In recent years, energy storage has received widespread attention in power system planning and control due to its advantages such as rapid response, bidirectional regulation, and precise tracking. To improve energy storage output performance, reduce energy storage costs, and extend cycle life, different types of energy storage can be combined based on their operating characteristics. Currently, low-pass filters are often used in photovoltaic-energy storage systems to achieve power distribution among hybrid energy storage systems. However, in practical applications, the frequency and magnitude of power fluctuations are unknown, making the design of the low-pass filter cutoff frequency difficult. Some literature uses supercapacitors to smooth high-frequency power disturbances and batteries to smooth low-frequency power disturbances during low-pass filtering, adjusting the cutoff frequency according to the energy storage's operating status and economics. However, this approach does not integrate hybrid energy storage with VSG control technology, failing to fully leverage the flexible and adjustable advantages of VSG control parameters. Some literature proposes a power distribution method for hybrid energy storage based on the VSG model and control principles, and tunes the control parameters. However, this method is only applicable to isolated microgrids operating independently from the main grid. When photovoltaic and hybrid energy storage systems are connected to the grid, the inverter output frequency is clamped by the main grid, causing changes in power frequency characteristics, making this power distribution method unsuitable.

[0004] VSG technology refers to simulating the physical model, active power frequency regulation, and reactive power voltage regulation characteristics of a traditional synchronous generator in the control strategy of a grid-connected inverter. This makes the operation control and external characteristics of a new energy power generation system with power electronic interfaces approximately equivalent to a synchronous generator, exhibiting inertial and damping characteristics. Some literature has introduced frequency variations into the active power-frequency control loop of the VSG to adaptively control the virtual inertia and damping parameters, effectively optimizing the dynamic response curves of the inverter's output frequency and power. However, most existing VSG control strategies use infinite power sources to represent distributed power sources and energy storage elements, failing to consider the impact of their actual output capacity on the VSG's virtual inertial adaptive control effect, thus hindering the practical application of new energy power generation systems.

[0005] To address the aforementioned issues, some literature has proposed optimization strategies for energy storage units in VSGs, but none has provided corresponding State of Charge (SOC) control methods. Other literature has established VSG control models based on energy storage SOC constraints and proposed a virtual inertia selection range considering both energy storage capacity and SOC constraints, but it does not consider adaptive adjustment of the virtual inertia based on changes in the energy storage unit's state of charge. Still other literature has proposed an IFVI control strategy, which adjusts the VSG virtual inertia solely based on the energy storage SOC when the remaining energy storage capacity is insufficient, avoiding overcharging and over-discharging of the energy storage. However, it does not comprehensively consider frequency changes, resulting in poor dynamic response characteristics of the inverter's output frequency and power. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a power allocation and virtual inertial control method for photovoltaic hybrid energy storage systems based on a virtual synchronous generator. According to the virtual synchronous generator model and control principles, the output power of the hybrid energy storage system is decomposed, overcoming the difficulties in designing the cutoff frequency of traditional first-order low-pass filters. Furthermore, the decomposed power is adapted to the high energy density of vanadium redox flow batteries and the high power density and high charge-discharge cycle life of supercapacitors, respectively. This invention can effectively improve the voltage and frequency stability of photovoltaic and hybrid energy storage systems, achieve reasonable power allocation among hybrid energy storage systems, avoid exceeding the state of charge limit of energy storage, and improve the charge-discharge performance and lifespan of energy storage.

[0007] The technical problem solved by this invention is achieved through the following technical solution:

[0008] A method for power distribution and virtual inertial control of a photovoltaic hybrid energy storage system based on a virtual synchronous generator, characterized in that: the method comprises the following steps:

[0009] S1. Determine the power allocation method of the photovoltaic hybrid energy storage system.

[0010] When the active power command of the virtual synchronous generator in the power grid dispatching undergoes a step change ΔP ref At that time, the total power absorbed or released by the photovoltaic hybrid energy storage system exhibits second-order oscillatory characteristics. When the input and output power of the inverter controlled by the virtual synchronous generator are unbalanced, the transfer function between the output angular frequency deviation and the power change is a first-order inertial element. Combining the above relationship with the characteristics of high energy density of vanadium redox flow batteries and high power density and multiple charge-discharge cycles of supercapacitors, the portion of the power required by energy storage that exhibits first-order inertial characteristics is allocated to the vanadium redox flow batteries, and the remaining portion is borne by the supercapacitors. That is:

[0011]

[0012]

[0013] Where J and D are the virtual inertia and virtual damping of the virtual synchronous generator, respectively;

[0014] ω0 represents the actual angular velocity and rated angular velocity corresponding to the virtual rotor of the virtual synchronous generator;

[0015] K ω This is the droop coefficient of the prime mover;

[0016] K VRB The value is Dω0+K;

[0017] When the active power command of the virtual synchronous generator undergoes a step change, the supercapacitor only bears the rapidly changing power during the dynamic response process. When the dynamic response process ends and the system enters a steady state, the supercapacitor does not participate in power exchange, and the vanadium redox flow battery fully bears the difference in power.

[0018] S2. Determine the correlation between the virtual inertia of the virtual synchronous generator and the energy storage state of charge.

[0019] When the active power command of the virtual synchronous generator in the power grid dispatching undergoes a step change ΔP ref At that time, the dynamic responses of the output power of the all-vanadium redox flow battery and the supercapacitor are respectively:

[0020]

[0021]

[0022] Where: ξ is the damping ratio;

[0023] ω d The damped natural oscillation frequency;

[0024] The state of charge and charge / discharge power of vanadium redox flow batteries and supercapacitors are related as follows:

[0025]

[0026] Wherein: S SOC and S SOC0 These represent the current and initial states of charge of the energy storage, respectively.

[0027] P represents the charging and discharging power of energy storage. A positive value indicates discharging, and a negative value indicates charging.

[0028] S n This refers to the rated capacity of the energy storage.

[0029] After the active power command of the virtual synchronous generator undergoes a step change, when the virtual inertia of the virtual synchronous generator is large, the system response time is long, the output power of the vanadium redox flow battery rises slowly, and its state of charge decreases little when it reaches steady state; the charging and discharging power of the supercapacitor oscillates more violently, releases more energy, and its state of charge decreases more when it reaches steady state.

[0030] S3. Determine the adaptive control strategy for virtual inertia.

[0031] Determine the state of charge adjustment coefficient K for each of the two energy storage states of charge. SOC_SC and K SOC_VRB When the state of charge (SCC) of the energy storage is insufficient, its output is reduced to prevent the SCC from exceeding the limit. The weight ρ of the SCC adjustment coefficient is determined based on the remaining capacity of the two types of energy storage, thus obtaining the SCC adjustment coefficient K of the hybrid energy storage consisting of supercapacitor and vanadium redox flow battery. SOC This allows the adjustment of virtual inertia to primarily consider energy storage with a state of charge closer to its limit, ensuring that the state of charge of both types of energy storage is within a reasonable range. The adaptive control strategy for the virtual inertia of the virtual synchronous generator is as follows:

[0032]

[0033] Where: J0 is the steady-state value of virtual inertia;

[0034] K J This is the adjustment coefficient for virtual inertia;

[0035] M J The threshold for the rate of change of frequency;

[0036] S4. Determine the selection principles for relevant parameters.

[0037] The effects of virtual inertia, damping, and droop coefficient of the virtual synchronous generator on the stability and dynamic performance of the photovoltaic hybrid energy storage system are analyzed, and the selection principles for these parameters are determined accordingly.

[0038]

[0039]

[0040]

[0041] Where: P max This represents the upper limit of the inverter's power.

[0042] Moreover, in step S1, the power distribution method of the photovoltaic hybrid energy storage system makes full use of the model and control principle of the virtual synchronous generator. The time constant can be represented by the virtual inertia and damping parameters of the virtual synchronous generator and is adaptively adjusted accordingly. The power carried by the vanadium redox flow battery exhibits first-order inertial characteristics, with a slow rate of change and no overshoot. The power carried by the supercapacitor exhibits damped oscillation characteristics and does not participate in power exchange in steady state, which is consistent with the working characteristics of the two types of energy storage.

[0043] Moreover, in step S3, the adaptive control strategy of the virtual inertia of the virtual synchronous generator introduces the adjustment coefficient of the hybrid energy storage state of charge. At the same time, the virtual inertia is adaptively adjusted according to the changes in the inverter output frequency and the two energy storage states of charge. While improving the output frequency and power dynamic response process of the photovoltaic-storage system, the output and state of charge of the two energy storage systems are coordinated and controlled.

[0044] The advantages and beneficial effects of this invention are as follows:

[0045] 1. The present invention provides a power distribution and virtual inertial control method for a photovoltaic hybrid energy storage system based on a virtual synchronous generator. The method decomposes the output power of the hybrid energy storage system according to the virtual synchronous generator model and control principle, overcoming the difficulty in designing the cutoff frequency of traditional first-order low-pass filters.

[0046] 2. The power distribution and virtual inertia control method of the photovoltaic hybrid energy storage system based on the virtual synchronous generator of the present invention considers the output constraint of energy storage, introduces the adjustment coefficient of hybrid energy storage state of charge on the basis of traditional control strategy, and adaptively adjusts the virtual inertia according to the changes of inverter output frequency and the two energy storage states of charge. While improving the dynamic response process of photovoltaic energy storage system output frequency and power, it coordinates the output and state of charge of the two energy storage systems.

[0047] 3. The power distribution and virtual inertial control method of the photovoltaic hybrid energy storage system based on the virtual synchronous generator of the present invention analyzes the stability of the system through root locus, determines the selection principle of key parameters, and improves the accuracy of parameter design. Attached Figure Description

[0048] Figure 1 This is a topology diagram of the photovoltaic hybrid energy storage system based on VSG according to the present invention;

[0049] Figure 2 This is a schematic diagram of the VSG control principle of the present invention;

[0050] Figure 3 This is a block diagram of the VSG active-frequency control of the present invention;

[0051] Figure 4 The figure shows the simulation verification results of the output power at each end of the photovoltaic hybrid energy storage system of the present invention;

[0052] Figure 5 The figure shows the simulation verification results of the DC bus voltage of this invention;

[0053] Figure 6 The figures show the simulation verification results under different control strategies of this invention.

[0054] Figure 7 The figures show the simulation verification results of the present invention under different working conditions. Detailed Implementation

[0055] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0056] A method for power distribution and virtual inertial control of a photovoltaic hybrid energy storage system based on a virtual synchronous generator is innovative in that the method comprises the following steps:

[0057] S1. Power allocation method for hybrid energy storage systems based on VSG

[0058] The topology of VSG-based photovoltaic and hybrid energy storage systems is as follows: Figure 1 As shown, the photovoltaic (PV) power generation units, vanadium redox flow batteries, and supercapacitors on the DC side of the system are connected to the DC bus via Boost converters and bidirectional DC / DC converters, respectively, and then connected to the grid via DC / AC converters. Specifically, the Boost converter for the PV power generation units employs Maximum Power Point Tracking (MPPT) control; the bidirectional DC / DC converter for the vanadium redox flow batteries uses constant power control, outputting the required power according to power commands; the bidirectional DC / DC converter for the supercapacitors uses constant voltage control to maintain stable DC bus voltage; and the DC / AC converter uses VSG control, enabling the PV and hybrid energy storage system to output power according to grid power dispatch commands, and possesses inertial and damping characteristics to improve system stability.

[0059] VSG control includes active-frequency control and reactive-voltage control, such as... Figure 2 As shown in the figure. C DC L is the DC-side voltage regulator capacitor.f r f C f and R f These are the filter inductor, parasitic resistance, filter capacitor, and resistor, respectively; L g and R g These are the line inductance and resistance between VSG and the power grid, respectively; E and δ are the voltage amplitude and power angle obtained from the upper-level power control, respectively, which are used to generate PWM modulation signals through voltage and current dual-loop control.

[0060] The active power-frequency control block diagram of VSG under grid-connected conditions is as follows: Figure 3 As shown in the figure. In the figure, J and D represent the virtual inertia and virtual damping of the VSG, respectively; P... e P represents the actual output power of the VSG. ref The output active power command value of the VSG is given by the grid dispatch; ω and ω0 are the actual angular velocity and rated angular velocity corresponding to the VSG virtual rotor, respectively; K ω K is the droop coefficient of the prime mover; p This is the proportionality coefficient.

[0061] Depend on Figure 3 It can be seen that when the active power command of VSG issued by the power grid dispatching undergoes a step change ΔP ref When the total power absorbed or released by the energy storage system is unbalanced, it exhibits second-order oscillatory characteristics. When the input and output power of the VSG-controlled inverter are unbalanced, the transfer function between the output angular frequency deviation and the power change is a first-order inertial element. Based on the above analysis and considering the high energy density of vanadium redox flow batteries and the high power density and numerous charge-discharge cycles of supercapacitors, a power allocation method for hybrid energy storage systems is proposed, which allocates ΔP... e The portion exhibiting first-order inertial characteristics is allocated to the vanadium redox flow battery, while the remainder is handled by the supercapacitor. The step responses of the output power of the vanadium redox flow battery and the supercapacitor are as follows:

[0062]

[0063]

[0064] To reduce the demand on supercapacitor capacity after a step change in the VSG active power command, the supercapacitor should only handle the rapidly changing power during the dynamic response. Once the dynamic response ends and the system enters steady state, the supercapacitor should not participate in power exchange, and the vanadium redox flow battery should fully handle the difference in power. The parameter K... VRB The value is Dω0+K ω At this point, we have:

[0065]

[0066]

[0067] S2. VSG Virtual Inertial Adaptive Control Considering Energy Storage SOC

[0068] When the active power command VSG issued by the power grid dispatching undergoes a step change ΔP ref The dynamic response of the output power of the vanadium redox flow battery and the supercapacitor is shown in the following equation:

[0069]

[0070]

[0071] In the formula, ξ is the damping ratio; ω d It is the damped natural oscillation frequency.

[0072] The state of charge (SOC) and charge / discharge power of vanadium redox flow batteries and supercapacitors are related as follows:

[0073]

[0074] In the formula, S SOC and S SOC0 These represent the current and initial states of charge of the energy storage, respectively; P is the charging and discharging power of the energy storage, with a positive value indicating discharging and a negative value indicating charging; S n This refers to the rated capacity of the energy storage.

[0075] It can be concluded that after a step change in the VSG active power command, when the VSG virtual inertia is large, the system response time is long, the output power of the vanadium redox flow battery rises slowly, and its SOC decreases less when it reaches steady state; the charging and discharging power of the supercapacitor oscillates more violently, releasing more energy, and its SOC decreases more significantly when it reaches steady state. Therefore, a VSG virtual inertia adaptive control strategy is considered for energy storage SOC.

[0076] Define the energy storage charge / discharge coefficient K SOC As shown in the following formula:

[0077]

[0078] In the formula, K c and K d These are the coefficients corresponding to the charging and discharging states of energy storage, respectively. The energy storage SOC is divided into 5 intervals, S... SOCmin S SOClow S SOChigh and S SOCmaxThese represent the minimum, minimum, maximum, and maximum SOC values, set according to the battery's own characteristics. Furthermore, to better meet the needs of practical engineering, a linear piecewise function is used, then K... c and K d The relationship with energy storage SOC is as follows:

[0079]

[0080]

[0081] Based on the correlation between the storage state of charge (SOC) and the virtual inertia of the VSG, the SOC adjustment coefficient K for supercapacitors and all-vanadium redox flow batteries is defined. SOC_SC and K SOC_VRB As shown in the following formula:

[0082] K SOC_SC =K SOC

[0083] K SOC_VRB =2-K SOC

[0084] When a supercapacitor or vanadium redox flow battery discharges and its remaining discharge capacity is insufficient, the lower the SOC, the lower the SOC adjustment coefficient K of the supercapacitor. SOC_SC The smaller the value, the higher the SOC regulation coefficient K of the all-vanadium redox flow battery. SOC_VRB The larger the value, the less energy the supercapacitor or vanadium redox flow battery releases, resulting in a smaller drop in SOC when reaching steady state, thus avoiding excessively low SOC. The same principle applies when a supercapacitor or vanadium redox flow battery is being charged and has insufficient remaining charge capacity.

[0085] Define the weighting coefficient ρ' as shown in the following formula. The larger the remaining charge and discharge capacity of the energy storage, the smaller the weighting coefficient, and the smaller the impact of its SOC on the virtual inertial adaptive adjustment.

[0086] ρ i ′=f i c (S SOC -0.1)+f i d (0.9-S SOC )

[0087] In the formula, i represents a vanadium redox flow battery or a supercapacitor; f i c and f i d This is a charging / discharging indicator.

[0088] The weight coefficients are normalized to obtain the normalized weight coefficients as follows:

[0089]

[0090] The SOC regulation coefficient for hybrid energy storage consisting of supercapacitors and vanadium redox flow batteries is defined as:

[0091] K SOC =ρK SOC_SC +(1-ρ)K SOC_VRB

[0092] The adaptive control strategy of VSG virtual inertia can be expressed as:

[0093]

[0094] In the formula, J0 is the steady-state value of the virtual inertia; K J M is the adjustment coefficient for virtual inertia. J The threshold for the rate of change of frequency.

[0095] When the active power command of the VSG issued by the grid dispatch changes abruptly, the virtual inertia increases rapidly with the increase of the frequency change rate. However, its value is also adaptively adjusted according to the SOC of the vanadium redox flow battery and the supercapacitor. First, the SOC adjustment coefficient is determined according to the SOC of the two types of energy storage. When the SOC of the energy storage is insufficient, its output is reduced to prevent the SOC from exceeding the limit. Then, the weight of the SOC adjustment coefficient is determined according to the remaining capacity of the two types of energy storage. This ensures that the adjustment of the virtual inertia mainly considers the energy storage with the SOC closer to the limit, guaranteeing that the SOC of both types of energy storage is within a reasonable range, and achieving coordinated control of the two types of energy storage.

[0096] The effects of VSG virtual inertia, damping, and droop coefficient on system stability and dynamic performance are analyzed, when D and K ω When J is a constant value, the larger the value of J, the closer the poles are to the origin, and the worse the system stability; when J is a constant value, D or K ω The larger the value of , the larger the absolute value of the real part of the pole, the shorter the settling time, the faster the decay rate, and the stronger the system stability. Therefore, the principle for selecting the parameters is as follows:

[0097]

[0098]

[0099]

[0100] In the formula, P max This represents the upper limit of the inverter's power.

[0101] To verify the effectiveness of the power allocation method and VSG virtual inertial adaptive control strategy for the hybrid energy storage system proposed in this invention, a photovoltaic and hybrid energy storage system based on VSG control was built in MATLAB / Simulink, and its topology is as follows. Figure 1 As shown.

[0102] Simulations were conducted comparing a single vanadium redox flow battery on the DC side and a hybrid energy storage system consisting of a vanadium redox flow battery and a supercapacitor. The changes in output power and DC bus voltage at each end of the photovoltaic and hybrid energy storage systems were analyzed. Figure 4 and Figure 5 As shown. Figure 4 In the diagram, 'a' represents a single vanadium redox flow battery, and 'b' represents hybrid energy storage.

[0103] Initially, the photovoltaic output and the VSG active power command value are equal, and the energy storage system does not participate in power exchange, with the DC bus voltage stabilizing at 750V. After a sudden increase in the VSG active power command value, the inverter's output power follows the command value. When a single vanadium redox flow battery is used on the VSG DC side, its slow response speed results in a difference between the total DC output power and the actual inverter output power, leading to system power imbalance and a significant drop in DC bus voltage. When hybrid energy storage is configured on the VSG DC side, the first-order inertial portion of the difference between the inverter output power and the photovoltaic output is handled by the vanadium redox flow battery, which exhibits a slow change rate and no overshoot. The remaining power is handled by the supercapacitor, exhibiting damped oscillation characteristics and not participating in power exchange in steady state. This aligns with the operating characteristics of both energy storage systems. Furthermore, because the supercapacitor can respond quickly to power changes, the DC bus voltage fluctuation is smaller. Therefore, the hybrid energy storage system power allocation method proposed in this invention combines the advantages of both energy storage systems, improving the response speed and lifespan of the energy storage system and enhancing the voltage stability of the photovoltaic grid-connected power generation system.

[0104] Simulations were conducted under different VSG virtual inertia control strategies, and the characteristic curves of the inverter's output frequency, power, and virtual inertia changes were shown below. Figure 6 As shown. Compared to other VSG virtual inertial control strategies, the strategy proposed in this invention can adaptively adjust the virtual inertia simultaneously according to changes in frequency and energy storage SOC, resulting in a smaller maximum deviation of the inverter output frequency, faster recovery speed, and smaller overshoot and settling time in output power. Therefore, the virtual inertial adaptive control strategy proposed in this invention can improve the dynamic response characteristics of the output frequency and power of the photovoltaic-storage system, which is beneficial to improving the stability of the photovoltaic grid-connected power generation system.

[0105] Simulations were conducted under different operating conditions to compare the state of charge (SOC) of the supercapacitor, the SOC and output power of the vanadium redox flow battery, and the characteristic curves of virtual inertia. Figure 7As shown.

[0106] Figure 7 In condition a, the supercapacitor is in the over-discharge region and the vanadium redox flow battery is in the overcharge region. When the VSG adopts the control strategy proposed in this invention, the adaptive adjustment of the virtual inertia takes into account the influence of the SOC of the supercapacitor and the vanadium redox flow battery. When the supercapacitor is discharging, it is appropriately reduced, so that the SOC of the supercapacitor decreases significantly during the simulation, effectively avoiding the problem of the supercapacitor's SOC being too low.

[0107] Figure 7 In condition b, under operating condition two, the supercapacitor is in the overcharge region and the vanadium redox flow battery is in the over-discharge region. When the VSG adopts the control strategy proposed in this invention, the virtual inertia is appropriately increased, which reduces the SOC drop of the vanadium redox flow battery during the simulation and delays the time when its SOC falls below the lower limit of operation.

[0108] Furthermore, when the supercapacitor has sufficient remaining charge and discharge capacity, the appropriate increase in virtual inertia when the VSG adopts the control strategy proposed in this invention can also reduce the rate of change of the output power of the vanadium redox flow battery, thereby making its fluctuations smoother and more in line with the working characteristics of the vanadium redox flow battery itself.

[0109] Therefore, the VSG virtual inertial adaptive control strategy proposed in this invention can improve the dynamic response of the output frequency and power of the photovoltaic energy storage system, while also adjusting the output power and SOC of the supercapacitor and the vanadium redox flow battery. This is beneficial for improving the working state of energy storage, extending the service life of energy storage, and improving the safety and stability of system operation.

[0110] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for power distribution and virtual inertial control of a photovoltaic hybrid energy storage system based on a virtual synchronous generator, characterized in that: The steps of the method are as follows: S1. Determine the power allocation method of the photovoltaic hybrid energy storage system. When the active power command of the virtual synchronous generator in the power grid dispatching undergoes a step change ΔP ref At that time, the total power absorbed or released by the photovoltaic hybrid energy storage system exhibits second-order oscillatory characteristics. When the input and output power of the inverter controlled by the virtual synchronous generator are unbalanced, the transfer function between the output angular frequency deviation and the power change is a first-order inertial element. Combining the above relationship with the characteristics of high energy density of vanadium redox flow batteries and high power density and multiple charge-discharge cycles of supercapacitors, the portion of the power required by energy storage that exhibits first-order inertial characteristics is allocated to the vanadium redox flow batteries, and the remaining portion is borne by the supercapacitors. That is: Where J and D are the virtual inertia and virtual damping of the virtual synchronous generator, respectively; ω and ω0 are the actual angular velocity and rated angular velocity corresponding to the virtual rotor of the virtual synchronous generator; K ω This is the droop coefficient of the prime mover; K VRB The value is Dω0+K ω ; When the active power command of the virtual synchronous generator undergoes a step change, the supercapacitor only bears the rapidly changing power during the dynamic response process. When the dynamic response process ends and the system enters a steady state, the supercapacitor does not participate in power exchange, and the vanadium redox flow battery fully bears this part of the differential power. S2. Determine the correlation between the virtual inertia of the virtual synchronous generator and the energy storage state of charge. When the active power command of the virtual synchronous generator in the power grid dispatching undergoes a step change ΔP ref At that time, the dynamic responses of the output power of the all-vanadium redox flow battery and the supercapacitor are respectively: Where: ξ is the damping ratio; ω d The damped natural oscillation frequency; The state of charge and charge / discharge power of vanadium redox flow batteries and supercapacitors are related as follows: Wherein: S SOC and S SOC0 These represent the current and initial states of charge of the energy storage, respectively. P represents the charging and discharging power of energy storage. A positive value indicates discharging, and a negative value indicates charging. S n This refers to the rated capacity of the energy storage. After the active power command of the virtual synchronous generator undergoes a step change, when the virtual inertia of the virtual synchronous generator is large, the system response time is long, the output power of the vanadium redox flow battery rises slowly, and its state of charge decreases little when it reaches steady state; the charging and discharging power of the supercapacitor oscillates more violently, releases more energy, and its state of charge decreases more when it reaches steady state. S3. Determine the adaptive control strategy for virtual inertia. Determine the state of charge adjustment coefficient K for each of the two energy storage states of charge. SOC_SC and K SOC_VRB When the state of charge (SCC) of the energy storage is insufficient, its output is reduced to prevent the SCC from exceeding its limit. The weight ρ of the SCC adjustment coefficient is determined based on the remaining capacity of the two types of energy storage, thus obtaining the SCC adjustment coefficient K of the hybrid energy storage consisting of supercapacitor and vanadium redox flow battery. SOC This allows the adjustment of virtual inertia to primarily consider energy storage with a state of charge closer to its limit, ensuring that the state of charge of both types of energy storage is within a reasonable range. The adaptive control strategy for the virtual inertia of the virtual synchronous generator is as follows: Where: J0 is the steady-state value of the virtual inertia; K J This is the adjustment coefficient for virtual inertia; M J The threshold for the rate of change of frequency; S4. Determine the selection principles for relevant parameters. The effects of virtual inertia, damping, and droop coefficient of the virtual synchronous generator on the stability and dynamic performance of the photovoltaic hybrid energy storage system are analyzed, and the selection principles for these parameters are determined accordingly. Where: P max This represents the upper limit of the inverter's power.

2. The power distribution and virtual inertial control method for a photovoltaic hybrid energy storage system based on a virtual synchronous generator according to claim 1, characterized in that: In step S1, the power distribution method of the photovoltaic hybrid energy storage system makes full use of the model and control principle of the virtual synchronous generator. The time constant can be represented by the virtual inertia and damping parameters of the virtual synchronous generator and is adaptively adjusted accordingly. The power carried by the vanadium redox flow battery exhibits first-order inertial characteristics, with a slow rate of change and no overshoot. The power carried by the supercapacitor exhibits damped oscillation characteristics and does not participate in power exchange in steady state, which is consistent with the working characteristics of the two energy storage systems.

3. The power distribution and virtual inertial control method for a photovoltaic hybrid energy storage system based on a virtual synchronous generator according to claim 1, characterized in that: In step S3, the adaptive control strategy of the virtual inertia of the virtual synchronous generator introduces the adjustment coefficient of the hybrid energy storage state of charge. At the same time, the virtual inertia is adaptively adjusted according to the changes in the inverter output frequency and the two energy storage states of charge. While improving the output frequency and power dynamic response process of the photovoltaic-storage system, the output power and state of charge of the two energy storage systems are coordinated and controlled.

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