Optical storage grid-connected system control method

Abstract

The invention discloses an optical storage grid-connected system control method, and relates to the technical field of photovoltaic system control. Comprising the following steps: acquiring a first reference current signal when the VSG inverter is in a normal operation condition and a second reference current signal when the VSG inverter is in a fault condition; performing coordinate transformation on the three-phase abc static coordinate system to obtain a two-phase dq rotating coordinate system; performing coordinate transformation on the two-phase dq rotation coordinates to obtain alpha-axis components and beta-axis components of the first reference current signal and the second reference current signal; discretizing the output current equation of the VSG inverter to obtain a current prediction model, and inputting the output current of the VSG inverter at the current moment into the current prediction model to obtain a predicted output current at the next moment; and constructing an outer loop value function and an inner loop value function, and controlling the alternating current side output voltage of the VSG inverter so as to control the optical storage grid-connected system. According to the invention, the static and dynamic performance of the photovoltaic grid-connected system can be improved, and the reliability and stability of the system are enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic storage system control, and in particular to a photovoltaic storage grid-connected system control method. Background Art

[0002] The photovoltaic storage system consists of photovoltaic power generation equipment and energy storage equipment. Photovoltaic power generation, as one of the forms of new energy power generation, has developed rapidly, especially in the northwest region, where it has great advantages in power generation due to its long sunshine time and high sunshine intensity. However, when large-scale photovoltaic power stations are connected to the grid, it will have a certain adverse effect on the power grid. The main reason is that the output of the photovoltaic power generation system is intermittent and volatile. It is mainly composed of power electronic devices, which do not have damping characteristics and mechanical inertia. It cannot provide frequency and voltage support for the photovoltaic storage system by storing mechanical energy like a synchronous generator. Therefore, when a large number of photovoltaic systems are connected to the grid, the anti-disturbance ability of the grid is reduced, the AC grid has limited absorption capacity for the DC grid, and the photovoltaic storage system is prone to transient processes.

[0003] In the prior art, a dual-loop control mode is used to control the inverter of the photovoltaic storage system, which includes an inner current loop and an outer voltage loop. The inner current loop is responsible for quickly responding to current changes, and the outer voltage loop is responsible for adjusting the output voltage. The control process is: the voltage set value is compared with the feedback value through the outer voltage loop, and a current set signal is generated through the PI regulator. Then, the inner current loop compares the current set signal with the inductor current feedback signal, generates a control amount through the PI regulator, and controls the inverter output voltage.

[0004] The defects of the above-mentioned prior art are: the voltage outer loop and the current inner loop all adopt PI control, and there are problems such as the adjustment coefficient is greatly affected by the system performance, the design is complex, and the adjustment range is limited. It is impossible to simulate the strong overcurrent capacity of the virtual synchronous machine when the system is strongly disturbed, resulting in the control effect of the inverter failing to meet expectations. Summary of the invention

[0005] Based on this, it is necessary to provide a control method for a photovoltaic energy storage grid-connected system to address the above technical issues.

[0006] The embodiment of the present invention provides a control method for a photovoltaic storage grid-connected system. A photovoltaic storage device at a sending end and a receiving end power grid of the photovoltaic storage grid-connected system are connected via a VSG inverter. In the current loop control of the VSG inverter, a current predictor is used to replace the PI control and the PWM regulator.

[0007] A control method for a photovoltaic energy storage grid-connected system, comprising:

[0008] Obtaining a first reference current signal when the VSG inverter is in normal operation and a second reference current signal when the VSG inverter is in a fault condition;

[0009] According to the topological structure of VSG inverter, the output current equation of VSG inverter in three-phase abc stationary coordinate system is established;

[0010] Perform coordinate transformation on the three-phase abc stationary coordinate system to obtain a two-phase dq rotating coordinate system; perform coordinate transformation on the first reference current signal and the second reference current signal under the two-phase dq rotating coordinate system to obtain the α-axis component and β-axis component of the first reference current signal and the α-axis component and β-axis component of the second reference current signal under the two-phase αβ stationary coordinate system;

[0011] The output current equation of the VSG inverter is discretized in a two-phase αβ stationary coordinate system to obtain a current prediction model; the α-axis output current and the β-axis output current of the VSG inverter at the current moment are input into the current prediction model to obtain the α-axis predicted output current and the β-axis predicted output current at the next moment;

[0012] An inner loop value function C2 for controlling the α-axis variable and an outer loop value function C1 for controlling the β-axis variable are constructed by using the α-axis predicted output current, the β-axis predicted output current, and the α-axis component and the β-axis component of the first reference current signal or the α-axis component and the β-axis component of the second reference current signal; wherein, the α-axis component and the β-axis component of the first reference current signal are used to construct the function in a normal state, and the α-axis component and the β-axis component of the second reference current signal are used in a fault state;

[0013] The AC side output voltage u of the VSG inverter is controlled by the outer loop control value function C1 and the inner loop value function C2 inabc , to control the photovoltaic storage grid-connected system.

[0014] Optionally, obtaining a first reference current signal when the VSG inverter is in normal operation and a second reference current signal when the VSG inverter is in a fault state includes:

[0015] The operating conditions of the VSG inverter are judged according to the following criteria:

[0016]

[0017] When the output voltage of the VSG inverter is in the normal operating range, the VSG inverter operates in the normal operating state, and the first reference current signal is taken as the voltage loop output current and

[0018] When an asymmetric fault occurs in the external power grid, causing the voltage at the converter station connection point to drop, the inverter VSG works in a fault state, and the second reference current signal is taken, which can be expressed as:

[0019]

[0020] Among them, IN is the rated current of the VSG inverter, U N is the rated voltage of the power grid, U g(1) It is the positive sequence voltage at the grid-connected point of the VSG inverter.

[0021] Optionally, an output current equation of the VSG inverter in the three-phase abc stationary coordinate system is established, including:

[0022]

[0023] Where a, b, and c are the phases of the three-phase bridge circuit in the VSG inverter, j is numerically equal to a, b, or c, and i inj is the phase current on the AC output side of the VSG inverter, u inj is the phase voltage on the AC output side of the VSG inverter, u gj is the phase voltage at the grid connection point, u dc is the DC side voltage of the VSG inverter, a is the rotation factor;

[0024] g j is the on / off state of the j-phase bridge arm switch of the VSG inverter, g j When it is 1, it means that the upper arm of one phase in the three-phase bridge circuit is turned on and the lower arm is turned off; g j When it is 0, it means that the upper arm of one phase in the three-phase bridge circuit is turned off and the lower arm is turned on.

[0025] Optionally, coordinate transformation is performed on the three-phase abc stationary coordinate system, including:

[0026]

[0027] Where a, b, and c are the phases of the three-phase bridge circuit, j is numerically equal to a, b, or c, and i inj is the phase current on the AC output side of the VSG inverter, u inj is the phase voltage on the AC output side of the VSG inverter, u gj is the phase voltage at the grid connection point, u dc is the DC side voltage of the VSG inverter.

[0028] Optionally, obtaining a current prediction model includes:

[0029]

[0030] Among them, i inα (t k ) is the tth k The α-axis output current of the VSG inverter at the moment, i inβ (t k ) is the tth k The β-axis output current of the VSG inverter at the moment, iinα (t k+1 ) is the tth k+1 The α-axis predicted output current of the VSG inverter at time i inβ (t k+1 ) is the tth k+1 The β-axis predicted output current of the VSG inverter at time T S is the sampling period, L is the inductance of the three-phase bridge circuit, R f is the resistance of the three-phase bridge circuit.

[0031] Optionally, an inner loop cost function C2 for controlling the α-axis variable and an outer loop cost function C1 for controlling the β-axis variable are constructed, which include:

[0032] Under normal operation, the α-axis component and the β-axis component of the first reference current signal are used to construct the current signal, and the specific expression is:

[0033]

[0034] In the case of a fault, the α-axis component and the β-axis component of the second reference current signal are used to construct the expression:

[0035]

[0036] Among them, g j (t k ) is the jth phase bridge arm switch of the three-phase inverter at the tth phase k The opening and closing state at the moment, g j (t k +1) is the jth phase bridge arm switch of the three-phase inverter at the tth phase k+1 The opening and closing state at the moment, a, b, c are the phases of the three-phase AC circuit, j is equal to a, b or c in value, is the α-axis component of the reference current signal in the two-phase αβ stationary coordinate system, It is the β-axis component of the reference current signal in the two-phase αβ stationary coordinate system.

[0037] Compared with the prior art, the above-mentioned control method for a photovoltaic energy storage grid-connected system provided by an embodiment of the present invention has the following beneficial effects:

[0038] In the current loop control of the VSG inverter, the present invention replaces the PI and PWM regulators with a current predictor, and the output is the switching state of the power device, with a small amount of calculation. On the one hand, the static and dynamic performance of the photovoltaic grid-connected system is improved, thereby enhancing the reliability and stability of the system; on the other hand, the input reference signal of the current loop can be quickly switched according to the unbalanced current or fault generated by the system, so as to suppress the voltage drop at the grid connection point of the VSG inverter, which causes the VSG inverter to output excessive current.

[0039] The present invention also controls the AC side output voltage u of the VSG inverter by constructing an outer loop value function C1 and an inner loop value function C2. inabc , in order to control the photovoltaic storage grid-connected system, it can suppress the increase of VSG inverter output current when the external power grid has a symmetrical fault and the imbalance of VSG inverter output current when the power grid is unbalanced, ensure the safe operation of VSG inverter when the power grid is strongly disturbed, and improve the reliability and stability of photovoltaic grid-connected system. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A structure diagram of a photovoltaic-storage grid-connected system according to a photovoltaic-storage grid-connected system control method provided in one embodiment;

[0041] Figure 2 A schematic diagram of the structure of an inverter VSG control strategy of a photovoltaic energy storage grid-connected system control method provided in an embodiment;

[0042] Figure 3 A schematic diagram of an improved predictive control method for a current loop multi-objective value function of a photovoltaic energy storage grid-connected system control method provided in one embodiment;

[0043] Figure 4 A DC bus voltage waveform diagram of a photovoltaic storage system at different light intensities at 25° Celsius in a photovoltaic storage grid-connected system control method provided in one embodiment;

[0044] Figure 5 An output power waveform diagram of a photovoltaic power generation unit, an energy storage unit, and a VSG inverter under different light intensities in a photovoltaic energy storage grid-connected system control method provided in an embodiment;

[0045] Figure 6 A waveform diagram of VSG inverter output power and DC bus voltage under different light intensities in a control method for a photovoltaic energy storage grid-connected system provided in an embodiment;

[0046] Figure 7 A control method for a photovoltaic energy storage grid-connected system provided in one embodiment is a waveform diagram of VSG inverter output power and grid frequency when the load increases by 50% in 2-4s, the illuminance increases from 900 to 1400 in 2s, and increases from 1400 to 1800 in 4s;

[0047] Figure 8 A control method for a photovoltaic energy storage grid-connected system provided in one embodiment is a waveform diagram of VSG inverter output power and DC bus voltage when the load increases by 50% in 2-4s, the illuminance increases from 900 to 1400 in 2s, and increases from 1400 to 1800 in 4s;

[0048] Fig. 9is a comparison diagram of frequency modulation results of a control method for a photovoltaic energy storage grid-connected system provided in an embodiment, Fig. 9 (a) is a comparison diagram of power grid frequency response. Fig. 9 (b) is a comparison chart of photovoltaic inverter output power. DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solution 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.

[0050] In one embodiment, a control method for a photovoltaic energy storage grid-connected system is provided, the method comprising:

[0051] The photovoltaic storage grid-connected system is mainly composed of the photovoltaic storage equipment at the sending end and the receiving end power grid. The photovoltaic storage equipment includes: photovoltaic panels and their front-stage BOOST converters, rear-stage DC-AC grid-connected inverters, energy storage devices and their bidirectional BUCK-BOOST converters.

[0052] 1. Photovoltaic power supply DC system and control strategy

[0053] (1) The photovoltaic power generation system adopts Figure 1 The bipolar structure shown in the figure has a photovoltaic power supply BOOST circuit at the front pole and an inverter circuit at the back pole. The BOOST converter uses the Maximum Power Point Tracking (MPPT) control method: the voltage U is extracted from the photovoltaic cell side. PV and current value I PV Through the MPPT algorithm, the deviation between the actual voltage and the voltage at the maximum power point is kept to a minimum. From the PV curve, it can be seen that the voltage at this time corresponds to the maximum output power of the photovoltaic cell.

[0054] (2) When a high proportion of photovoltaic power is connected to the grid, the power grid fluctuations caused by changes in light intensity or ambient temperature and load changes can be absorbed by the grid, and this method has certain applicability. When the grid's absorption capacity is limited or close to saturation, VSG coordinated control is performed through the back-stage inverter side of the photovoltaic storage system.

[0055] 2. Energy storage device and coordinated control

[0056] The energy storage device can be a lithium iron phosphate battery or a large capacitor, and the energy storage device is physically connected to the photovoltaic system through a bidirectional BUCK-BOOST converter. The function of the energy storage device is to maintain the stability of the DC side voltage and provide power support for the system. When the light intensity changes or the system load changes, the inverter energy storage device can flexibly output power through the VSG control unit to ensure the stability of the DC voltage bus and the power balance of the inverter output power; when a system failure occurs, it can improve the low voltage ride-through capability of the inverter to ensure the stable operation of the inverter. The VSG control structure of the inverter circuit is as follows: Figure 2 As shown:

[0057] The power command output by the energy storage device to the VSG inverter is divided into two parts:

[0058] ①Multiply the deviation between the SOC collected in the system and the expected SOC value by the proportional coefficient to obtain a power: P SOC =(SOC ref -SOC)k soc ρ, used to adjust the system SOC;

[0059] ② VSG output ω g and the expected value ω ref The deviation is multiplied by the proportionality factor to get a power: P ω =(ω ref -ω g ) ω (1-ρ), used to adjust the frequency of the system;

[0060] The sum of the two parts is used to adjust the active power input P of the VSG inverter ref , that is, P ref =P SOC +P ω , where 0<ρ<1 is the adjustment coefficient. This power has the functions of regulating energy storage and primary frequency regulation: when ρ is larger, P SOC The larger the value of ρ, the more the energy storage system is used to adjust the SOC. ω The larger the value is, the more the energy storage system tends to regulate the system frequency.

[0061] 3. VSG inverter and its control strategy

[0062] The VSG inverter is used to realize the parallel operation of the photovoltaic storage system and the power grid, and to support the voltage and frequency changes of the power grid when the photovoltaic power generation system accounts for a high proportion, so as to ensure the stable operation of the power grid. Figure 2 The inverter circuit VSG control includes three parts: power droop control, voltage loop control and current loop control; the energy storage device shown in the figure plays the role of VSG coordination control.

[0063] (1) Active / reactive power droop control

[0064] When the photovoltaic storage system is connected to the grid for power generation, the VSG inverter needs to maintain the stability of voltage and frequency under grid fluctuations. Among them, the active / reactive droop control allows the photovoltaic storage power generation system to operate flexibly in island mode and grid-connected mode, and can effectively cope with fluctuations and load mutations caused by factors such as weather and environment, and realize the plug-and-play of distributed power sources.

[0065] ① Active power-frequency droop control: Set the active power P of the energy storage coordinated output ref As a reference value, collect the active power P in the power grid g , multiply the difference between the two by the Pf droop coefficient K P , is superimposed with the rated angular frequency of the grid introduced by the feedforward link, and the difference between the angular frequency output by the VSG system after PI adjustment and the rated angular frequency, and the reference value of the voltage phase of the VSG inverter is obtained after integration. Figure 2 We can get:

[0066]

[0067] Active power droop control loop and is the PI frequency adjustment parameter. When the system is disturbed, this control can adjust and correct the active power and frequency. m is the mechanical angular frequency of the VSG inverter, ω n is the synchronous angular frequency of the power grid, ω g is the actual operating frequency of VSG.

[0068] ② Reactive power - voltage droop control: The reference value of reactive power in the system is Q ref , collect reactive power Q in the power grid g , multiply the difference between the two by the QV droop coefficient K Q , and the rated voltage introduced by the feedforward link, as well as the difference between the system voltage and the rated voltage after PI adjustment, to obtain the reference value of the VSG inverter voltage amplitude. Figure 2 We can get:

[0069]

[0070] Reactive power droop control and It is the PI voltage regulation parameter. When disturbance occurs in the system, this process can adjust and correct the voltage by reactive power.

[0071] When disturbances and failures occur in the power grid, the above process can correct the fluctuations of the system frequency and voltage through the VSG control strategy, so that it can quickly recover to the expected value and improve the anti-interference ability of the power grid. On the basis of droop control, the control algorithm is improved, and the simulation of the mechanical motion part and the electromagnetic part is added, so that the VSG inverter has the inertia and damping characteristics of the synchronous generator, which effectively slows down the transient process during disturbances and reduces the impact current of the power grid, which is suitable for the current complex power grid environment.

[0072] (2) Voltage loop control

[0073] The input reference signal of the voltage loop is u gdref and u gqref , are the voltage E output by the active / reactive droop control link m ∠θ m The voltage u obtained after coordinate transformation indref and u inqref The output voltage u of the virtual impedance regulator vd and u vq The actual voltage at the grid point is u g After the coordinates are changed, we can get u gd and u gq , calculate the deviation between it and the reference voltage, and after the voltage loop PI control, the reference current value of the current inner loop can be obtained and

[0074] (3) Current loop control - improved current prediction model control

[0075] In the current loop control of the VSG inverter, the current predictor replaces the PI and PWM regulators, and the output is the switching state of the power device, which reduces the amount of calculation. On the one hand, it improves the static and dynamic performance of the photovoltaic grid-connected system, thereby enhancing the system reliability and stability; on the other hand, it can quickly switch the input reference signal of the current loop when the system generates unbalanced current or faults, so as to suppress the voltage drop at the grid connection point of the VSG inverter, which causes the VSG inverter to output excessive current. The predictive control strategy of the current loop is as follows: Figure 3 As shown, the control process is:

[0076] A. Obtain a first reference current signal when the VSG inverter is in normal operation and a second reference current signal when the VSG inverter is in a fault condition.

[0077] Specifically, the reference signal is divided into two conditions: normal operation and fault condition. The operation condition of the VSG inverter is judged, and the judgment standard is shown in the following formula:

[0078]

[0079] When the output voltage of the VSG inverter is in the normal operating range, the VSG inverter operates in the normal operating state, and the first reference current signal is taken as the voltage loop output current and When an asymmetric fault occurs in the external power grid, causing the voltage at the converter station connection point to drop, that is, when the positive sequence voltage of the VSG inverter drops by more than 10%, the inverter VSG works in a fault state, and the second reference current signal is taken, which is expressed as:

[0080]

[0081] Among them, I N is the rated current of the VSG inverter, U N is the rated voltage of the power grid, U g(1) It is the positive sequence voltage at the grid-connected point of the VSG inverter.

[0082] B. According to the topological structure of the VSG inverter, establish the output current equation of the VSG inverter in the three-phase abc stationary coordinate system. Perform coordinate transformation on the three-phase abc stationary coordinate system to obtain the two-phase dq rotating coordinate system; perform coordinate transformation on the first reference current signal and the second reference current signal in the two-phase αβ stationary coordinate system to obtain the α-axis component and β-axis component of the first reference current signal and the α-axis component and β-axis component of the second reference current signal in the two-phase αβ stationary coordinate system. Discretize the output current equation of the VSG inverter in the two-phase αβ stationary coordinate system to obtain the current prediction model; input the α-axis output current and β-axis output current of the VSG inverter at the current moment into the current prediction model to obtain the α-axis predicted output current and β-axis predicted output current at the next moment.

[0083] The output current equation of the VSG inverter in the three-phase abc stationary coordinate system is:

[0084]

[0085] Where a, b, and c are the phases of the three-phase bridge circuit in the VSG inverter, j is numerically equal to a, b, or c, and i inj is the phase current on the AC output side of the VSG inverter, u inj is the phase voltage on the AC output side of the VSG inverter, u gj is the phase voltage at the grid connection point, u dc is the DC side voltage of the VSG inverter, a is the rotation factor, indicating a 120-degree counterclockwise rotation;

[0086] g j is the on / off state of the j-phase bridge arm switch of the VSG inverter, g j When it is 1, it means that the upper arm of one phase in the three-phase bridge circuit is turned on and the lower arm is turned off; g jWhen it is 0, it means that the upper arm of one phase in the three-phase bridge circuit is turned off and the lower arm is turned on. The coordinate transformation is expressed as:

[0087]

[0088] Where a, b, and c are the phases of the three-phase bridge circuit, j is numerically equal to a, b, or c, and i inj is the phase current on the AC output side of the VSG inverter, u inj is the phase voltage on the AC output side of the VSG inverter, u gj is the phase voltage at the grid connection point, u dc is the DC side voltage of the VSG inverter.

[0089] The output current equation of the VSG inverter is discretized in the two-phase αβ stationary coordinate system, and the control system is based on T S is the sampling period, the expression of the current prediction model is:

[0090]

[0091] Where a, b, and c are the phases of the three-phase bridge circuit, j is numerically equal to a, b, or c, and i inj is the phase current on the AC output side of the VSG inverter, u inj is the phase voltage on the AC output side of the VSG inverter, u gj is the phase voltage at the grid connection point, u dc is the DC side voltage of the VSG inverter.

[0092] The control strategy of the current prediction model is expressed as follows: j When the values ​​in the three phases are different, there are 8 different combinations of g a g b g c , which are 8 switch states: 000, 001, 010, 011, 100, 101, 110, 111. From equation (4), we can get the corresponding u in the 8 states: inα ,u inβ and inα ,i inβ Signal value, after substituting this value into formula (5), we can get the current position of t k The current signal at time i inα (t k ),i inβ (t k ), calculate the current prediction signal i at the next moment according to the prediction model inα (t k+1 ),i inβ (t k+1), substitute the predicted value and the corresponding reference current into the cost function to find the minimum value of the cost function. The switch state corresponding to the minimum value is g a g b g c The optimal action state of the three-phase bridge arm switch of the inverter is obtained, thereby achieving the control goal of the prediction model: that is, predicting the action state of the inverter switch at the next moment, and then using the value function to optimize the action state of the inverter switch to achieve the purpose of controlling the inverter output voltage.

[0093] C. Construct an inner loop value function C2 for controlling the α-axis variable and an outer loop value function C1 for controlling the β-axis variable through the α-axis predicted output current, the β-axis predicted output current, and the α-axis component and β-axis component of the first reference current signal or the α-axis component and β-axis component of the second reference current signal; wherein, the α-axis component and β-axis component of the first reference current signal are used to construct the function under normal operation, and the α-axis component and β-axis component of the second reference current signal are used to construct the function under fault conditions. The AC side output voltage u of the VSG inverter is controlled through the outer loop control value function C1 and the inner loop value function C2 inabc , to control the photovoltaic storage grid-connected system.

[0094] The construction process specifically includes:

[0095] Under normal operation, the α-axis component and the β-axis component of the first reference current signal are used to construct the current signal, and the specific expression is:

[0096]

[0097] In the case of a fault, the α-axis component and the β-axis component of the second reference current signal are used to construct the expression:

[0098]

[0099] Among them, g j (t k ) is the jth phase bridge arm switch of the three-phase inverter at the tth phase k The opening and closing state at the moment, g j (t k +1) is the jth phase bridge arm switch of the three-phase inverter at the tth phase k+1 The opening and closing state at the moment, a, b, c are the phases of the three-phase AC circuit, j is equal to a, b or c in value, is the α-axis component of the reference current signal in the two-phase αβ stationary coordinate system, It is the β-axis component of the reference current signal in the two-phase αβ stationary coordinate system.

[0100] The value function requires that the tth k+1The error between the predicted value and the reference value is the smallest. When the sampling period is small, the value function can be considered to be 0, and there is When the system is operating normally, the reference value of the current loop is the α-axis component and β-axis component of the first reference current, which is the value of the difference between the output current of the voltage loop and the actual operating current of the inverter after coordinate transformation. When a symmetrical fault occurs, the α-axis component and β-axis component of the second reference current are used. This reference current function can ensure that the transformer inverter always outputs a three-phase symmetrical current within the allowable value range, and can suppress the increase of the output current of the VSG inverter when the external power grid has a symmetrical fault and the imbalance of the output current of the VSG inverter when the power grid is unbalanced, ensuring the safe operation of the VSG inverter when the power grid is strongly disturbed, and improving the reliability and stability of the photovoltaic grid-connected system.

[0101] When the system is disturbed, the output voltage and current of the inverter will also fluctuate to a certain extent, so a master-slave value function control method is adopted, such as Figure 3 As shown: the outer loop value function C1 plays a major control role, and on this basis, the inner loop value function C2 is used for auxiliary control to achieve tracking control of the corresponding reference current by the predicted current of the β axis and the α axis, and to achieve reactive power and voltage control of the system as well as active power and frequency adjustment. The constraint conditions restrict the sequence of the 6 switch actions of the three-phase bridge arm of the VSG inverter to ensure that only one phase of the three-phase switch of the VSG inverter can change state at a time, which is used to suppress the high-order harmonic current generated by the frequent action of the IGBT of the VSG inverter.

[0102] The VSG control method is used to simulate the external characteristics of the synchronous generator mathematically and physically, which can ensure the stable operation characteristics of the photovoltaic energy storage grid-connected inverter, so that the VSG inverter can balance the difference between photovoltaic power generation and load power while ensuring the stability of the DC bus voltage, and play a frequency regulation function. In view of the defects of the current loop PI control that the parameter changes have a great impact on the system performance and the calculation is complex, the method proposed in the present invention uses the current sampling value to predict the output current of the VSG inverter at the next moment, which is simple to control, has high tracking accuracy, fast response speed, and better stability.

[0103] For example, this embodiment provides a simulation result diagram:

[0104] (1) DC bus voltage of the solar energy storage system under different light intensities at 25°C

[0105] like Figure 4 As shown, the purple curve is the illumination at different time periods, and the green curve is the voltage of the DC bus.

[0106] (2) Output power of VSG inverter, photovoltaic power generation unit, and energy storage unit under different light intensities

[0107] like Figure 5 As shown, the green curve is the output power of the front-stage photovoltaic power generation unit, the blue curve is the output power of the auxiliary energy storage battery, and the purple curve is the output power of the VSG inverter.

[0108] (3) VSG inverter output power and DC bus voltage under different light intensities

[0109] like Figure 6 As shown, the red curve is the output power of the VSG inverter, and the blue curve is the voltage of the DC bus.

[0110] (4) When the grid load increases by 50% between 2s and 4s, and the illuminance increases from 900 to 1400 at 2s and from 1400 to 1800 at 4s, the output power of the VSG inverter and the frequency of the grid

[0111] like Figure 7 As shown, the red curve is the output power of the VSG inverter, and the blue curve is the frequency of the grid.

[0112] (5) The VSG inverter output power and DC bus voltage when the grid load increases by 50% between 2s and 4s, and the illuminance increases from 900 to 1400 at 2s and from 1400 to 1800 at 4s

[0113] like Figure 8 As shown, the red curve is the output power of the VSG inverter, and the blue curve is the DC bus voltage.

[0114] (6) Comparison of output power and frequency modulation results between two-stage control and VSG control of the photovoltaic storage system of the present invention

[0115] The comparison of FM results is shown in the figure below: Fig. 9 As shown, Fig. 9 (a) is the grid frequency comparison diagram, and (b) is the inverter output power comparison diagram.

[0116] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A control method for a photovoltaic storage grid-connected system, wherein the photovoltaic storage device at the sending end and the receiving end power grid of the photovoltaic storage grid-connected system are connected via a VSG inverter; characterized in that: In the current loop control of the VSG inverter, the PI control and PWM regulator are replaced by the current predictor; The control method of the photovoltaic energy storage grid-connected system comprises: Obtaining a first reference current signal when the VSG inverter is in normal operation and a second reference current signal when the VSG inverter is in a fault condition; According to the topological structure of VSG inverter, the output current equation of VSG inverter in three-phase abc stationary coordinate system is established; Perform coordinate transformation on the three-phase abc stationary coordinate system to obtain a two-phase dq rotating coordinate system; perform coordinate transformation on the first reference current signal and the second reference current signal under the two-phase dq rotating coordinate system to obtain the α-axis component and β-axis component of the first reference current signal and the α-axis component and β-axis component of the second reference current signal under the two-phase αβ stationary coordinate system; The output current equation of the VSG inverter is discretized in a two-phase αβ stationary coordinate system to obtain a current prediction model; the α-axis output current and the β-axis output current of the VSG inverter at the current moment are input into the current prediction model to obtain the α-axis predicted output current and the β-axis predicted output current at the next moment; An inner loop value function C2 for controlling the α-axis variable and an outer loop value function C1 for controlling the β-axis variable are constructed by using the α-axis predicted output current, the β-axis predicted output current, and the α-axis component and β-axis component of the first reference current signal or the α-axis component and β-axis component of the second reference current signal; wherein, the α-axis component and β-axis component of the first reference current signal are used to construct the function under normal operation, and the α-axis component and β-axis component of the second reference current signal are used under fault conditions; The AC side output voltage u of the VSG inverter is controlled by the outer loop control value function C1 and the inner loop value function C2 inabc , to control the photovoltaic storage grid-connected system.

2. A control method for a photovoltaic energy storage grid-connected system as claimed in claim 1, characterized in that: The obtaining of a first reference current signal when the VSG inverter is in normal operation and a second reference current signal when the VSG inverter is in a fault state comprises: The operating conditions of the VSG inverter are judged according to the following criteria: When the output voltage of the VSG inverter is in the normal operating range, the VSG inverter operates in the normal operating state, and the first reference current signal is taken as the voltage loop output current and When an asymmetric fault occurs in the external power grid, causing the voltage at the converter station connection point to drop, the inverter VSG works in a fault state, and the second reference current signal is taken, which can be expressed as: Among them, I N is the rated current of the VSG inverter, U N is the rated voltage of the power grid, U g(1) It is the positive sequence voltage at the grid-connected point of the VSG inverter.

3. A control method for a photovoltaic energy storage grid-connected system as claimed in claim 1, characterized in that: The method of establishing the output current equation of the VSG inverter in the three-phase abc stationary coordinate system includes: Where a, b, and c are the phases of the three-phase bridge circuit in the VSG inverter, j is numerically equal to a, b, or c, and i inj is the phase current on the AC output side of the VSG inverter, u inj is the phase voltage on the AC output side of the VSG inverter, u gj is the phase voltage at the grid connection point, u dc is the DC side voltage of the VSG inverter, a is the rotation factor; g j is the on / off state of the j-phase bridge arm switch of the VSG inverter, g j When it is 1, it means that the upper arm of one phase in the three-phase bridge circuit is turned on and the lower arm is turned off; g j When it is 0, it means that the upper arm of one phase in the three-phase bridge circuit is turned off and the lower arm is turned on.

4. A control method for a photovoltaic energy storage grid-connected system as claimed in claim 3, characterized in that: The coordinate transformation of the three-phase abc stationary coordinate system includes: Where a, b, and c are the phases of the three-phase bridge circuit, j is numerically equal to a, b, or c, and i inj is the phase current on the AC output side of the VSG inverter, u inj is the phase voltage on the AC output side of the VSG inverter, u gj is the phase voltage at the grid connection point, u dc is the DC side voltage of the VSG inverter.

5. A control method for a photovoltaic energy storage grid-connected system as claimed in claim 1, characterized in that: The obtaining of the current prediction model comprises: Among them, i inα (t k ) is the tth k The α-axis output current of the VSG inverter at the moment, i inβ (t k ) is the tth k The β-axis output current of the VSG inverter at the moment, i inα (t k+1 ) is the tth k+1 The α-axis predicted output current of the VSG inverter at time i inβ (t k+1 ) is the tth k+1 The β-axis predicted output current of the VSG inverter at time T S is the sampling period, L is the inductance of the three-phase bridge circuit, R f is the resistance of the three-phase bridge circuit.

6. A control method for a photovoltaic energy storage grid-connected system as claimed in claim 1, characterized in that: The inner loop cost function C2 for controlling the α-axis variable and the outer loop cost function C1 for controlling the β-axis variable are constructed, which includes: Under normal operation, the α-axis component and the β-axis component of the first reference current signal are used to construct the current signal, and the specific expression is: In the case of a fault, the α-axis component and the β-axis component of the second reference current signal are used to construct the expression: Among them, g j (t k ) is the jth phase bridge arm switch of the three-phase inverter at the tth phase k The opening and closing state at the moment, g j (t k +1) is the jth phase bridge arm switch of the three-phase inverter at the tth phase k+1 The opening and closing state at the moment, a, b, c are the phases of the three-phase AC circuit, j is equal to a, b or c in value, is the α-axis component of the reference current signal in the two-phase αβ stationary coordinate system, It is the β-axis component of the reference current signal in the two-phase αβ stationary coordinate system.

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