Grid-connected inverter grid-following / grid-building flexible switching method, system, device and medium

Abstract

The present invention discloses a method, system, device and medium for flexible switching of grid-connected inverters for following the grid / forming the grid, wherein the method includes: establishing a control model of following the grid control and forming the grid control for the grid-connected inverter of the photovoltaic storage system; using hysteresis switching control to establish a switching criterion for following the grid / forming the grid control according to the grid strength; using an integrator, voltage pre-synchronization and a state follower to realize flexible switching of the phase angle and the dq axis current reference value in the control of the grid-connected inverter, so as to realize flexible switching control of following the grid / forming the grid. According to the stable complementary characteristics of following the grid control and forming the grid control under different grid strengths, the present invention uses flexible switching control of following the grid / forming the grid to realize stable operation of the photovoltaic storage system under a wide range of changes in grid strength.

Description

Technical Field

[0001] The present invention relates to the technical field of power system operation control, and in particular to a method, system, equipment and medium for flexible switching of a grid-connected inverter to follow the grid / build the grid. Background Art

[0002] With the rapid rise of renewable energy such as photovoltaics and wind power, the proportion of traditional fossil fuel energy generation has declined, and the penetration rate of photovoltaics has continued to increase. The access of photovoltaic and storage hybrid systems to the grid depends on inverters. With the increase in the penetration rate of renewable energy based on power electronic equipment connected to the grid, the power system has gradually changed from synchronous generator power generation to power electronic equipment power generation, resulting in a lack of inertia and damping in the grid, and the strength of the grid has shown weak grid characteristics, which has driven the development of grid control. Grid control such as droop control and its improved control, power synchronization control and virtual synchronous generator can simulate the operating characteristics of synchronous generators, and have the flexibility of power electronic equipment, which can effectively solve the low inertia and damping problems of the grid.

[0003] As the penetration rate of renewable energy increases, the strength of the power grid further decreases, and the power system gradually becomes a weak power grid. The short circuit ratio (SCR) is used to describe the strength of the system. When SCR<3, the system can be regarded as a weak power grid. Grid control uses a phase locked loop (PLL) to achieve phase synchronization with the grid voltage. Under a weak grid, the PLL generates negative damping near the grid base frequency, which causes system oscillation, and the asymmetric output of the PLL itself will produce frequency coupling. When there are multiple PLLs in the grid, there will also be strong coupling between control loops, which will deteriorate the stable operation of the system. Grid control is based on the synchronization of power signals with the grid, which can provide frequency and voltage support for the power system, avoid the adverse effects of PLL, and have good stability in weak grids. However, since grid control reduces the system damping ratio, when the grid strength is large, the system stability performance decreases. As the SCR decreases, the inverter of grid control gradually becomes unstable, while the stability of the inverter of grid control increases. It can be seen that the stability of grid control is stronger under weak grid conditions and weaker under strong grid conditions, while the stability of grid control is the opposite. In a hybrid photovoltaic and energy-storage system, the grid strength is constantly changing due to the influence of power supply or load. Single grid-following or grid-building control is difficult to maintain system stability when the grid strength changes over a large range. Summary of the invention

[0004] In view of the deficiencies in the above-mentioned prior art, the present invention provides a method, system, device and medium for flexible switching of grid-connected inverters to follow the grid / build the grid, so as to solve the problem that single grid-following type or grid-building type control is difficult to maintain system stability with large-scale changes in grid strength.

[0005] In a first aspect, a method for flexible switching of a grid-connected inverter to follow the grid / build the grid is provided, comprising the following steps:

[0006] S1: For the grid-connected inverter of the photovoltaic storage system, a control model for grid-following control and grid-building control is established;

[0007] S2: Based on the strength of the power grid, hysteresis switching control is used to establish the switching criteria for following the grid / forming the grid control;

[0008] S3: Use integrator, voltage pre-synchronization and state follower to realize flexible switching of phase angle and dq axis current reference value in grid-connected inverter control, so as to realize flexible switching control of following the grid / forming the grid.

[0009] Furthermore, the photovoltaic storage system includes a photovoltaic power generation system, an energy storage power generation system and a grid-connected inverter. The energy storage power generation system and the photovoltaic power generation system are respectively connected in parallel to the DC side of the grid-connected inverter through a DC / DC converter, and then supply power to the AC grid through the grid-connected inverter; wherein the photovoltaic power generation system adopts a boost converter to boost the voltage and adopts maximum power tracking control; the energy storage power generation system adopts a buck-boost converter to boost the voltage, and the energy storage power generation system adopts a fixed DC bus control, and obtains a DC / DC converter drive signal through a voltage and current dual PI loop control. The specific energy storage control is expressed as follows:

[0010]

[0011] In the formula, u dcref is the DC voltage reference value; u dc is the actual value of DC voltage; i dcref is the DC current reference value; v dcref is the output DC voltage reference value; k p1 , k i1 are the proportional coefficient and integral coefficient, k p2 , k i2 They are the proportional coefficient and integral coefficient of the current loop respectively.

[0012] Furthermore, the control structure of the grid-following control is divided into a phase-locked loop, a power outer loop and a current inner loop. The control model of the grid-following control is expressed as follows:

[0013]

[0014] Where θ is the phase angle; ω and ω n Represent the actual angular frequency and the rated angular frequency respectively; k ppll and k ipll Respectively represent the proportional coefficient and integral coefficient in the phase-locked loop PI coefficient; i dref and i qref Indicates the dq axis reference current value output by the grid-connected inverter; P ref and Q refRespectively represent the reference values ​​of active power and reactive power output by the grid-connected inverter; u d and u q represents the dq axis component of the voltage at PCC, i d and i q represents the dq axis component of the current at the PCC; e dref and e qref Indicates the dq axis reference voltage value output by the grid-connected inverter; k pi and k ii Respectively represent the proportional coefficient and integral coefficient in the current inner loop; L f represents the filter inductance; s represents the complex frequency domain variable; The subscript 1 in indicates that the grid-connected inverter is in the grid-following control state.

[0015] Furthermore, the network control structure is similar to the network control structure in the inner loop, but there are significant differences in the outer loop control and synchronization method. The control model of network control is expressed as follows:

[0016]

[0017] In the formula, m n and n n Respectively represent the active and reactive droop coefficients; ω LFP represents a low-pass filter; ω c Represents the cutoff frequency of the low-pass filter; P 0 and Q 0 Respectively represent the actual value of the active power and reactive power output by the grid-connected inverter; U n Indicates the rated voltage of the power grid; u dref and u qref Respectively represent the reference voltage u of the voltage outer loop ref The dq axis component of k pv and k iv They represent the proportional coefficient and integral coefficient in the voltage outer loop respectively; The subscript 2 in indicates that the grid-connected inverter is in the grid-following control state.

[0018] Furthermore, in step S2, the grid strength is quantified by the short circuit ratio SCR, which is expressed as follows:

[0019]

[0020] In the formula, S ac is the short-circuit capacity of the AC system; S N is the rated capacity of new energy equipment; U N is the rated voltage of the grid-connected inverter output port; P dN is the rated power of the grid-connected inverter output port; Z gis the grid line impedance;

[0021] To avoid repeated switching between the two control strategies when the operating conditions change, the switching method is set to hysteresis switching. 1 The grid-connected inverter adopts grid-following control, and when the short-circuit ratio is reduced to SCR 2 When the grid-connected inverter adopts grid control, and SCR 1 Greater than SCR 2 .

[0022] Furthermore, step S3 specifically includes:

[0023] When the grid-connected inverter is initially running in the grid-following control mode, when the grid-connected inverter switches from the grid-following control mode to the grid-forming control mode, the voltage pre-synchronization starts and the reference voltage u output by the grid-forming control mode is ref Follow the actual changes of the power grid; the phase angle switching uses the continuity of the integrator and places the switch before the integrator. The phase angle will not produce a sudden change. When switching, the phase angle θ under the control of the grid 1 The phase angle θ under network control 2 equal; the state follower always controls the reference current i under the network control dqref2 Reference current i under follow-up grid control dqref1 , when switching to network control, the reference current i under network control dqref2 With the reference current i under grid control dqref1 equal;

[0024] When the grid-connected inverter is initially running in grid-building control, when the grid-connected inverter switches from grid-building control to grid-following control, since the phase-locked loop is always working and synchronized with the grid phase, the phase angle θ under grid-following control during switching is 1 Always with the phase angle θ under network control 2 equal; the state follower always controls the reference current i under the grid control dqref1 Reference current i under follower network control dqref2 , when switching to grid-following control, the reference current i under grid-following control dqref1 The reference current i under network control dqref2 equal.

[0025] Furthermore, the state tracker outputs the compensation amount The expression is:

[0026]

[0027] In the formula, k p3 , k i3 They are the proportional coefficient and integral coefficient of the PI loop respectively.

[0028] In a second aspect, a grid-connected inverter grid-following / grid-building flexible switching system is provided, comprising:

[0029] The control model building module is used to build the control model of grid-following control and grid-building control for the grid-connected inverter of the photovoltaic storage system;

[0030] A switching criterion establishment module is used to establish a switching criterion for following the grid / forming the grid by adopting hysteresis switching control according to the grid strength;

[0031] The flexible switching module is used to realize the flexible switching of the phase angle and the dq axis current reference value in the grid-connected inverter control by using the integrator, voltage pre-synchronization and state follower, so as to realize the flexible switching control of following the grid / forming the grid.

[0032] In a third aspect, an electronic device is provided, including:

[0033] Memory on which computer programs or instructions are stored;

[0034] The processor is used to load and execute the computer program or instructions to implement the flexible switching method of the grid-connected inverter to follow the grid / build the grid as described above.

[0035] In a fourth aspect, a computer-readable storage medium is provided, on which a computer program is stored. When the computer program is executed by a processor, the method for flexible switching of the grid-connected inverter to follow the grid / build the grid as described above is implemented.

[0036] The present invention proposes a method, system, device and medium for flexible switching of grid-following / grid-building of a grid-connected inverter, which has the following beneficial effects: in a photovoltaic storage system, energy storage can smooth out photovoltaic power fluctuations and stabilize the DC bus voltage; based on the photovoltaic storage system, an integrator and a state follower are used to realize flexible switching control of following / grid-building, and the inverter adopts grid-following control under a strong power grid, and adopts grid-building control under a weak power grid, which effectively ensures the stability of the system when the power grid strength changes over a large range. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0038] Figure 1 It is a schematic diagram of the main circuit topology and control principle of the photovoltaic storage system provided by an embodiment of the present invention;

[0039] Figure 2 is a schematic diagram of hysteresis switching control provided by an embodiment of the present invention;

[0040] Figure 3 Schematic diagram of power response of SCR under different controls provided by an embodiment of the present invention; wherein (a) is the power response of different SCR values ​​during grid-following control, and (b) is the power response of different SCR values ​​during grid-building control;

[0041] Figure 4 is a schematic diagram of a state follower provided by an embodiment of the present invention;

[0042] Figure 5 is a schematic diagram of a flexible switching control strategy provided by an embodiment of the present invention;

[0043] Figure 6 Schematic diagram of the influence of different SCRs on the small signal stability and transient stability of the system under working condition 1 provided by an embodiment of the present invention; wherein (a) and (c) are schematic diagrams of the output power of the grid-connected inverter and the voltage change at the PCC under working condition 1 using grid-following control, respectively, and (b) and (d) are schematic diagrams of the output power of the grid-connected inverter and the voltage change at the PCC under working condition 1 using grid-building control, respectively;

[0044] Figure 7 Schematic diagram of the influence of different SCRs on the small signal stability and transient stability of the system under working condition 2 provided by an embodiment of the present invention; wherein (a) and (c) are schematic diagrams of the output power of the grid-connected inverter and the voltage change at the PCC under working condition 2 using grid-following control, and (b) and (d) are schematic diagrams of the output power of the grid-connected inverter and the voltage change at the PCC under working condition 2 using grid-building control, respectively;

[0045] Figure 8 1 is a schematic diagram of simulation results of flexible switching and direct switching of following the grid / building the grid provided by an embodiment of the present invention, wherein (a) is the simulation result of the d-axis reference current, (b) is the simulation result of the q-axis reference current, (c) is the simulation result of the phase angle of the grid-connected inverter, (d) is the simulation result of the output power of the grid-connected inverter, and (e) is the simulation result of the phase angle difference between the inverter and the grid;

[0046] Fig. 9 1 is a schematic diagram of an inverter output power waveform for flexible switching control of following the grid / building the grid provided by an embodiment of the present invention, wherein (a) is a schematic diagram of an inverter output power waveform when following the grid is switched to building the grid, and (b) is a schematic diagram of an inverter output power waveform when building the grid is switched to following the grid;

[0047] Fig.10 Schematic diagram of simulation results of flexible switching of the photovoltaic energy storage system with / into the grid provided by an embodiment of the present invention, wherein (a) is a schematic diagram of the output power of the grid connection point, and (b) is a schematic diagram of the frequency of the grid connection point. DETAILED DESCRIPTION

[0048] To make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described in detail below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention.

[0049] The embodiment of the present invention provides a method for flexible switching of a grid-connected inverter to follow the grid / build the grid, comprising the following steps:

[0050] S1: For the grid-connected inverter of the photovoltaic energy storage system, a control model for grid-following control and grid-building control is established.

[0051] The main circuit topology of the solar storage system is as follows: Figure 1 As shown, the photovoltaic storage system mainly includes a photovoltaic power generation system (PV), a energy storage power generation system (BESS) and a grid-connected inverter. The energy storage power generation system and the photovoltaic power generation system are respectively connected in parallel to the DC side of the grid-connected inverter through a DC / DC converter, and then supply power to the AC grid through the grid-connected inverter. Among them, the photovoltaic power generation system converts solar energy into electrical energy and outputs a constant DC voltage, which is boosted by a boost converter and controlled by maximum power point tracking (MPPT). The energy storage power generation system uses a lithium battery pack and a buck-boost converter for boosting. The energy storage power generation system uses a fixed DC bus control, and the DC / DC converter drive signal is obtained through the voltage and current dual PI loop control. The specific energy storage control is expressed as follows:

[0052]

[0053] In the formula, u dcref is the DC voltage reference value; u dc is the actual value of DC voltage; i dcref is the DC current reference value; v dcref is the output DC voltage reference value; k p1 , k i1 are the proportional coefficient and integral coefficient, k p2 , k i2 are the proportional coefficient and integral coefficient of the current loop respectively, and s represents the complex frequency domain variable.

[0054] In Figure 1, L f and C f Respectively represent the filter inductance and capacitance; L g Represents line inductance; i L Indicates the three-phase current output of the grid-connected inverter; u pcc and i pcc Respectively represent the three-phase voltage and current at the common coupling point (PCC); Pref and Q ref Respectively represent the active and reactive power reference values ​​output by the grid-connected inverter; u d and u q represents the dq axis component of the voltage at PCC, i d and i q represents the dq axis component of the current at the PCC, i ld and i lq Represents the dq-axis component of the grid-connected inverter output current; u PV and i PV Respectively represent the photovoltaic output voltage and current; u dcref and i dcref Respectively represent the DC bus reference voltage and current; u dc and i dc They represent the DC bus voltage and current respectively; θ is the phase angle; u dqref and i dqref Respectively represent the dq axis reference voltage and current values ​​output by the grid-connected inverter. Subscript 1 indicates that the grid-connected inverter is in the grid-following control state, and subscript 2 indicates that the grid-connected inverter is in the grid-building control state. In this embodiment, the main simulation parameters are shown in Table 1.

[0055]

[0056] Specifically, the control structure of the grid-following control is divided into a phase-locked loop, a power outer loop and a current inner loop. The control model of the grid-following control is expressed as follows:

[0057]

[0058] Where θ is the phase angle; ω and ω n Represent the actual angular frequency and the rated angular frequency respectively; k ppll and k ipll Respectively represent the proportional coefficient and integral coefficient in the phase-locked loop PI coefficient; i dref and i qref Indicates the dq axis reference current value output by the grid-connected inverter; P ref and Q ref Respectively represent the reference values ​​of active power and reactive power output by the grid-connected inverter; u d and u q represents the dq axis component of the voltage at PCC, i d and i q represents the dq axis component of the current at the PCC; e dref and e qref Indicates the dq axis reference voltage value output by the grid-connected inverter; k pi and k ii Respectively represent the proportional coefficient and integral coefficient in the current inner loop; L frepresents the filter inductance; The subscript 1 in indicates that the grid-connected inverter is in the grid-following control state.

[0059] The structure of network control is similar to that of network control in the inner loop, but is quite different in the outer loop control and synchronization mode. The control model of network control is as follows:

[0060]

[0061] In the formula, m n and n n Respectively represent the active and reactive droop coefficients; ω LFP represents a low-pass filter; ω c Represents the cutoff frequency of the low-pass filter; P 0 and Q 0 Respectively represent the actual value of the active power and reactive power output by the grid-connected inverter; U n Indicates the rated voltage of the power grid; u dref and u qref Respectively represent the reference voltage u of the voltage outer loop ref The dq axis component of k pv and k iv They represent the proportional coefficient and integral coefficient in the voltage outer loop respectively; The subscript 2 in indicates that the grid-connected inverter is in the grid-following control state.

[0062] S2: According to the strength of the power grid, hysteresis switching control is used to establish the switching criteria for following the grid / forming the grid control.

[0063] The grid strength is quantified by the short circuit ratio SCR, which is expressed as follows:

[0064]

[0065] In the formula, S ac is the short-circuit capacity of the AC system; S N is the rated capacity of new energy equipment; U N is the rated voltage of the grid-connected inverter output port; P dN is the rated power of the grid-connected inverter output port; Z g is the grid line impedance.

[0066] Based on the difference in stability characteristics between grid-following control and grid-forming control, their control advantages can be utilized. Grid-forming control is used under weak grid conditions, and grid-following control is used under strong grid conditions. Therefore, the switching boundary between the two controls needs to be determined. To avoid repeated switching between the two control strategies when the operating conditions change, the switching method is set to hysteresis switching. When the short-circuit ratio increases to SCR, the switching method is set to hysteresis switching. 1 The grid-connected inverter adopts grid-following control, and when the short-circuit ratio is reduced to SCR2 When the grid-connected inverter adopts grid control, and SCR 1 Greater than SCR 2 . Figure 2 This is a schematic diagram of hysteresis switching control.

[0067] Figure 3 is the power response of different SCRs under different controls in this embodiment, Figure 3 (a) shows the power response of different SCRs during grid-following control. Figure 3 (b) shows the power response of different SCRs during grid control. Figure 3 It can be seen that when the grid-following control is used, the system is unstable when SCR = 1.4, and the system is stable when SCR = 1.5; under the grid-building control, the system is unstable when SCR = 8.6, and the system is stable when SCR = 8.2. The grid-following control system has good stability when the SCR is large, and the grid-building control has better stability when the SCR is small. Considering the stability of the system when a fault occurs, a certain margin needs to be left at the switching boundary. Theoretically, SCR 1 and SCR 2 The value of satisfies the range [1.5, 8.2], and SCR 1 Greater than SCR 2 In this embodiment, SCR is preferred 1 =3, SCR 2 =1.8.

[0068] S3: Integrator, voltage pre-synchronization and state follower are used to realize flexible switching of phase angle and dq-axis current reference value in grid-connected inverter control, so as to realize flexible switching control of following / building the grid, so that when the grid strength of the system changes, the flexible control strategy can meet the needs of stable operation of the system under strong and weak grids.

[0069] Through the structural analysis of the grid-following control and grid-forming control modes, it can be seen that there are obvious differences in the methods of synchronizing the grid with the grid. The grid-following control uses a phase-locked loop to synchronize with the grid, and the grid-forming control uses a power signal to synchronize with the grid. From the mathematical models of the grid-following control and the grid-forming control, it can be seen that the current inner loop control structure of the two controls is the same. The phase angle and the dq axis reference current value are the key to ensure smooth switching between the grid-following and grid-forming control. Therefore, a grid-following / grid-forming switching control strategy with limited phase angle and dq axis reference current value is introduced.

[0070] The state follower realizes the switching of the reference current values ​​of the dq axes. The control structure for the transition from network following control to network building control is as follows: Figure 4 As shown in Figure 1, the state follower maintains the current input of the two controlled inner loops. The control principle includes compensating the difference between the voltage outer loop and the power outer loop through a PI regulator. In the grid-following control, the reference current of the system is the i output of the power outer loop.dqref1 When the two modes are switched, the current reference value i output by the network control voltage outer loop is dqref2 with i dqref1 Mismatch. Switching the state follower before will make i dqref2 Follow me dqref1 , so that when the two control modes are switched, the dq axis reference current will not change. The method for switching from network construction control to network following control is the same. dqref1 Follow me dqref2 To ensure that the current does not change suddenly. The state tracker output compensation The expression is:

[0071]

[0072] In the formula, k p3 , k i3 They are the proportional coefficient and integral coefficient of the PI loop respectively.

[0073] Flexible switch control strategy such as Figure 5 As shown, Figure 5 (a) is the phase angle switching. Figure 5 (b) is the dq axis reference current switching. As the system strength changes, frequency and phase differences will appear between the grid-connected inverter and the grid voltage. The measures taken include voltage pre-synchronization and flexible phase angle switching. The phase angle switching uses the continuity of the integrator and places the switching before the integrator to ensure a smooth transition. In addition, the switching of the dq axis reference current value adopts a state follower to track the state current before switching, thereby stabilizing the reference current under the dual control switch. Figure 5 In position 1, the grid-connected inverter operates under grid-following control. During the switch from grid-following control to grid-forming control, the voltage starts pre-synchronization, and the reference voltage u output by the grid-forming control is ref Track actual grid changes. Phase angle switching utilizes the continuity of the integrator and places the switch before the integrator. The phase angle will not produce a sudden change. When switching, the phase angle θ under grid control 1 The phase angle θ under network control 2 The reference current is equal to the reference current, ensuring that the phase angle remains stable during the entire switching process. During the grid-following operation, the state follower always tracks the reference current, and during the switching process, the reference current remains consistent between the two control modes. In position 2, the grid-connected inverter operates under grid-building control. When switching from grid-building control to grid-following control, due to the continuous operation of the PLL, it is synchronized with the grid phase, so the phase angle θ under grid-following control during switching is 1 Always with the phase angle θ under network control 2 equal; by using a state follower, the continuity of the dq axis reference current is also maintained.

[0074] The technical solution of the present invention is further described below in conjunction with specific simulation experiments.

[0075] The initial state of charge (SOC) of the energy storage power generation system is 50%, the rated capacity is 200 Ah, and the photovoltaic capacity is 14.92kW. The stability difference between grid-following control and grid-building control is studied to determine the switching boundary. Under different SCR setting values, the stability difference between grid-following control and grid-building control is reflected in the change of power. At working condition 1:1 s, the terminal load of the grid-connected inverter suddenly increases, causing small signal interference to the grid-connected inverter in the photovoltaic storage system. Figure 6 (a) and (b) show the small signal stability characteristics of different SCR values ​​under grid-following control and grid-building control, respectively. Condition 2: A single-phase short circuit fault occurs in the PCC at 1 second and lasts for 0.01 seconds. Figure 7 Figures (a) and (b) show the transient stability trend of the power grid strength when the power grid strength changes during the grid following control and grid building control process. The voltage change curves under these two conditions are shown in Figure 2. Figure 6 (c), (d) and Figure 7 As shown in (c) and (d) in Figure 1, the drive signal of the energy storage DC / DC inverter is generated through voltage and current dual-loop control, ensuring the stability of the DC bus voltage. The safety range of the energy storage system is considered under operating conditions.

[0076] Figure 6 (a) shows that under grid-following control, when SCR=2.3, the system's inverter output active power fluctuates slightly. However, at SCR=1.5, the power gradually deviates over time, causing system instability. On the contrary, when SCR is 3.8 or 5, the system remains stable, and the stability improves with the increase of SCR. Figure 6 (b) shows that under grid control, a smaller SCR value can reduce the power fluctuation during disturbances, thereby improving system stability as the SCR decreases. Figure 7 Figures (a) and (b) show that when SCR = 2.3, the system with grid-following control experiences a significantly longer power oscillation period than in the case of a strong grid, resulting in a larger oscillation amplitude. However, after adopting grid-building control, the power oscillation amplitude decreases, and the stability gradually improves as the SCR value decreases. Figure 7 It can be seen from (c) and (d) in Figure 2 that under the condition 2 with grid-following control, when SCR=2.3, the duration of the voltage drop to 0 is longer, about 0.04 s longer than other SCR values. As the SCR increases, the interference effect weakens. On the contrary, under grid-building control, when SCR=3.8, the voltage remains unchanged from 0 to about 1.04 s after the disturbance, and gradually recovers after several oscillation cycles. Figure 6 and Figure 7It can be seen that the stability characteristics of the grid-connected inverters with grid-following and grid-forming control vary with the change of grid strength. Under strong grid conditions, the stability of grid-following control is good, and under weak grid conditions, the stability of grid-forming control is better.

[0077] The control strategy described in the above embodiment can realize flexible switching of the system when the SCR changes, and the switching of following the network / building the network control is verified in the simulation. Figure 8 This is the simulation result of flexible switching between following the network and building the network. Figure 8 (a) is the d-axis reference current, Figure 8 (b) is the q-axis reference current, Figure 8 (c) is the phase angle of the grid-connected inverter. Figure 8 (d) is the output power of the grid-connected inverter. Figure 8 (e) is the phase angle difference between the inverter and the grid. Under the initial condition, the system is in grid-following control, and the SCR is 3.5. At 1 second, the SCR changes to 1.5, and the inverter control switches to grid-building control. At 2 seconds, the SCR returns to 3.5, and the control switches to grid-following.

[0078] Depend on Figure 8 It can be seen that during the switching process between following the grid / building the grid, the dq axis reference current value changes significantly at 1s and 2s when direct switching is used, the phase angle changes suddenly when the grid is switched from building the grid to following the grid at 2s, the power decreases from 1pu to 0.1pu at 1s, and there is obvious fluctuation at 2s; when flexible switching is used, the sudden change of the dq axis reference current and phase angle is significantly suppressed during switching, the maximum fluctuation of the d axis is 0.05pu, the maximum fluctuation of the q axis is 0.0025pu, and the maximum fluctuation of the power during the switching process is 0.04pu. Therefore, the flexible switching strategy proposed in this paper can effectively suppress the fluctuation of the phase angle and the dq axis reference current, and the fluctuation is small during switching, ensuring the stable operation of the system when the SCR changes.

[0079] In order to further verify the accuracy and feasibility of the flexible switching control method for following the network / building the network proposed in the present invention, C-HIL (Computer-Hardware in the Loop) was used for simulation verification, and the specific experimental parameters were consistent with those in Table 1. Fig. 9 The inverter output power waveform diagram for flexible switching control of grid-following / grid-building. Fig. 9 (a) shows the inverter output power fluctuation when switching from grid-following to grid-building. Fig. 9 (b) shows the output power fluctuation of the inverter when the grid-connected inverter switches to the grid-following inverter. In the initial state, the system strength is SCR=3.5, and the grid-connected inverter is in the grid-following control. At 11s, the line is disconnected, the system strength becomes SCR=1.5, and the inverter switches to the grid-connected control; at 21s, the line returns to normal, the system strength returns to SCR=3.5, and the inverter switches back to the grid-following control. Fig. 9 It can be seen that the maximum power fluctuation is 0.118 when the network following is switched to network building, and the maximum power fluctuation is 0.59 when the network building is switched to network following, which effectively suppresses the power fluctuation during switching.

[0080] The control strategy proposed in the present invention is used in a photovoltaic and energy storage system with a DC bus. To further verify the flexibility and applicability of the proposed control strategy, it is applied to a photovoltaic and energy storage system with an AC bus. The photovoltaic system adopts grid-following control, and the energy storage system adopts the proposed control strategy. When the system SCR changes, the grid-connected inverter of the energy storage system switches the control strategy to ensure the stable operation of the system.

[0081] Under the initial conditions, the energy storage system is in grid-following control, and the system SCR is 3.5. At 1 second, the SCR changes to 1.5, and the grid-connected inverter control of the energy storage system switches to grid-building control. At 2 seconds, the SCR returns to 3.5, and the control switches to grid-following. Fig.10 The simulation results of the optical storage system following the network / building the network are shown. Fig.10 (a) is the grid-connected point output power, Fig.10 (b) is the grid connection point frequency. For different energy storage access methods, the output power and frequency fluctuations of the inverter at the switching moment are relatively small. When the grid is switched from following the grid to building the grid, that is, at 1s, the maximum power fluctuation of the shared DC bus access is 0.02pu, and the maximum frequency fluctuation is 50.23Hz. The maximum power fluctuation of the shared AC bus access is 0.04pu, and the maximum frequency fluctuation is 50.25Hz; when the grid is switched from building the grid to following the grid, that is, at 2s, the maximum power fluctuation of the shared DC bus access is 0.027pu, and the maximum frequency fluctuation is 49.80Hz. The maximum power fluctuation of the shared AC bus access is 0.03pu, and the maximum frequency fluctuation is 49.79Hz.

[0082] The above-mentioned embodiment proposes a method for flexible switching control of grid-connected inverters for following the grid / forming the grid, which has the following beneficial effects: in the photovoltaic storage system, energy storage can smooth out photovoltaic power fluctuations and stabilize the DC bus voltage; based on the photovoltaic storage system, an integrator and a state follower are used to realize flexible switching control of following the grid / forming the grid. In a strong grid, the inverter adopts grid-following control, and in a weak grid, the inverter adopts grid-forming control, which effectively ensures the stability of the system when the grid strength changes over a large range.

[0083] The embodiment of the present invention further provides a grid-connected inverter grid-following / grid-building flexible switching system, comprising:

[0084] The control model building module is used to build the control model of grid-following control and grid-building control for the grid-connected inverter of the photovoltaic storage system;

[0085] A switching criterion establishment module is used to establish a switching criterion for following the grid / forming the grid by adopting hysteresis switching control according to the grid strength;

[0086] The flexible switching module is used to realize the flexible switching of the phase angle and the dq axis current reference value in the grid-connected inverter control by using the integrator, voltage pre-synchronization and state follower, so as to realize the flexible switching control of following the grid / forming the grid.

[0087] It should be understood that the functional unit modules in various embodiments of the present invention may be concentrated in one processing unit, or each unit module may exist physically separately, or two or more unit modules may be integrated in one unit module, and may be implemented in the form of hardware or software.

[0088] An embodiment of the present invention further provides an electronic device, including:

[0089] Memory on which computer programs or instructions are stored;

[0090] The processor is used to load and execute the computer program or instruction to implement the grid-connected inverter following the grid / forming the grid flexible switching method described in the above embodiment.

[0091] An embodiment of the present invention further provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the method for flexible switching of the grid-connected inverter to follow the grid / build the grid as described in the above embodiment is implemented.

[0092] It can be understood that the same or similar parts of the above embodiments can be referenced to each other, and the contents not described in detail in some embodiments can refer to the same or similar contents in other embodiments.

[0093] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes.

[0094] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0095] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0096] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process in the computer or other programmable device. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0097] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for flexible switching of a grid-connected inverter to a grid / grid, characterized in that: The steps include: S1: For the grid-connected inverter of the photovoltaic storage system, a control model for grid-following control and grid-building control is established; S2: Based on the strength of the power grid, hysteresis switching control is used to establish the switching criteria for following the grid / forming the grid control; S3: Using integrator, voltage pre-synchronization and state follower to realize flexible switching of phase angle and dq axis current reference value in grid-connected inverter control, so as to realize flexible switching control of following the grid / forming the grid; Step S3 specifically includes: When the grid-connected inverter is initially running in the grid-following control mode, when the grid-connected inverter switches from the grid-following control mode to the grid-forming control mode, the voltage pre-synchronization starts and the reference voltage output by the grid-forming control mode is u ref Follow the actual changes of the power grid; the phase angle switching uses the continuity of the integrator and places the switch before the integrator. The phase angle will not produce a sudden change. When switching, the phase angle is controlled by the grid. θ 1 and the phase angle under network control θ 2 are equal; the state follower always controls the reference current under network control i dqref2 Reference current under follow-up grid control i dqref1 , when switching to network control, the reference current under network control i dqref2 With the reference current under grid control i dqref1 equal; When the grid-connected inverter is initially running in grid-building control, when the grid-connected inverter switches from grid-building control to grid-following control, since the phase-locked loop is always working and synchronized with the grid phase, the phase angle under grid-following control during switching is θ 1 is always the phase angle with the network control θ 2 are equal; the state follower always controls the reference current under the grid control i dqref1 Reference current under follower network control i dqref2 , when switching to grid-following control, the reference current under grid-following control i dqref1 Reference current under network control i dqref2 equal; In step S2, the grid strength is quantified by the short circuit ratio SCR, which is expressed as follows: ； In the formula, S ac is the short-circuit capacity of the AC system; S N is the rated capacity of new energy equipment; U N is the rated voltage of the grid-connected inverter output port; P dN is the rated power of the grid-connected inverter output port; Z g is the grid line impedance; The switching method is set to hysteresis switching. When the short-circuit ratio increases to SCR1, the grid-connected inverter adopts grid-following control. When the short-circuit ratio decreases to SCR2, the grid-connected inverter adopts grid-forming control, and SCR1 is greater than SCR2.

2. The method for flexible switching of grid-connected inverters to / from the grid according to claim 1, characterized in that: The photovoltaic storage system includes a photovoltaic power generation system, an energy storage power generation system and a grid-connected inverter. The energy storage power generation system and the photovoltaic power generation system are respectively connected in parallel to the DC side of the grid-connected inverter through a DC / DC converter, and then supply power to the AC power grid through the grid-connected inverter; wherein the photovoltaic power generation system adopts a boost converter to boost the voltage and adopts maximum power tracking control; the energy storage power generation system adopts a buck-boost converter to boost the voltage, and the energy storage power generation system adopts a fixed DC bus control, and obtains the DC / DC converter drive signal through the voltage and current dual PI loop control.

3. The method for flexible switching of grid-connected inverters to follow the grid / build the grid according to claim 1, characterized in that: The control model of the network control is expressed as follows: ； Where θ is the phase angle; ω and ω n Represent the actual angular frequency and the rated angular frequency respectively; k ppll and k ipll Respectively represent the proportional coefficient and integral coefficient in the phase-locked loop PI coefficient; i dref and i qref Indicates the dq axis reference current value output by the grid-connected inverter; P ref and Q ref Respectively represent the reference values ​​of active power and reactive power output by the grid-connected inverter; u d and u q represents the dq axis component of the voltage at PCC, i d and i q represents the dq axis component of the current at the PCC; e dref and e qref Indicates the dq axis reference voltage value output by the grid-connected inverter; k pi and k ii Respectively represent the proportional coefficient and integral coefficient in the current inner loop; L f represents the filter inductance; s represents the complex frequency domain variable; The subscript 1 in indicates that the grid-connected inverter is in the grid-following control state.

4. The method for flexible switching of grid-connected inverters to follow the grid / build the grid according to claim 3, characterized in that: The control model of the network control is expressed as follows: ； In the formula, m n and n n Respectively represent the active and reactive droop coefficients; ω LFP represents a low-pass filter; ω c represents the cut-off frequency of the low-pass filter; P0 and Q0 represent the actual values ​​of the active power and reactive power output by the grid-connected inverter respectively; U n Indicates the rated voltage of the power grid; u dref and u qref Respectively represent the reference voltage u of the inverter voltage outer loop ref The dq axis component of k pv and k iv They represent the proportional coefficient and integral coefficient in the voltage outer loop respectively; The subscript 2 in indicates that the grid-connected inverter is in the grid-following control state.

5. The method for flexible switching of grid-connected inverters to follow the grid / build the grid according to claim 1, characterized in that: State follower output compensation The expression is: ； In the formula, k p3 , k i3 are the proportional coefficient and integral coefficient of the PI loop respectively, and s represents the complex frequency domain variable.

6. A grid-connected inverter grid-following / grid-building flexible switching system, characterized in that: include: The control model building module is used to build the control model of grid-following control and grid-building control for the grid-connected inverter of the photovoltaic storage system; A switching criterion establishment module is used to establish a switching criterion for following the grid / forming the grid by adopting hysteresis switching control according to the grid strength; The flexible switching module is used to realize the flexible switching of the phase angle and the dq axis current reference value in the grid-connected inverter control by using the integrator, voltage pre-synchronization and state follower, so as to realize the flexible switching control of following the grid / building the grid; specifically, it includes: When the grid-connected inverter is initially running in the grid-following control mode, when the grid-connected inverter switches from the grid-following control mode to the grid-forming control mode, the voltage pre-synchronization starts and the reference voltage output by the grid-forming control mode is u ref Follow the actual changes of the power grid; the phase angle switching uses the continuity of the integrator and places the switch before the integrator. The phase angle will not produce a sudden change. When switching, the phase angle is controlled by the grid. θ 1 and the phase angle under network control θ 2 are equal; the state follower always controls the reference current under network control i dqref2 Reference current under follow-up grid control i dqref1 , when switching to network control, the reference current under network control i dqref2 With the reference current under grid control i dqref1 equal; When the grid-connected inverter is initially running in grid-building control, when the grid-connected inverter switches from grid-building control to grid-following control, since the phase-locked loop is always working and synchronized with the grid phase, the phase angle under grid-following control during switching is θ 1 is always the phase angle with the network control θ 2 are equal; the state follower always controls the reference current under the grid control i dqref1 Reference current under follower network control i dqref2 , when switching to grid-following control, the reference current under grid-following control i dqref1 Reference current under network control i dqref2 equal; The grid strength is quantified by the short circuit ratio SCR, which is expressed as follows: ； In the formula, S ac is the short-circuit capacity of the AC system; S N is the rated capacity of new energy equipment; U N is the rated voltage of the grid-connected inverter output port; P dN is the rated power of the grid-connected inverter output port; Z g is the grid line impedance; The switching method is set to hysteresis switching. When the short-circuit ratio increases to SCR1, the grid-connected inverter adopts grid-following control. When the short-circuit ratio decreases to SCR2, the grid-connected inverter adopts grid-forming control, and SCR1 is greater than SCR2.

7. An electronic device, characterized in that: include: Memory on which computer programs or instructions are stored; A processor is used to load and execute the computer program or instruction to implement the grid-connected inverter following the grid / forming the grid flexible switching method as described in any one of claims 1 to 5.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the method for flexible switching of a grid-connected inverter to follow the grid / build the grid as described in any one of claims 1 to 5 is implemented.

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