A power grid segmentation recovery strategy based on a light storage combined power generation system

Abstract

The application discloses a power grid segmentation recovery strategy based on a light storage combined power generation system and relates to the technical field of power system fault recovery. When the power grid is powered off due to a fault, the energy storage system provides voltage frequency support for photovoltaic grid connection, and the energy storage system inverter adopts V / f control. After a stable grid connection bus voltage is established, the photovoltaic system adopts a control mode of fixed DC voltage and fixed reactive power for grid connection, and a virtual synchronous control is added at the same time, so that the photovoltaic system has inertia damping characteristics. After the photovoltaic system is connected to the grid, the auxiliary system is started and then the conventional unit is put into operation, meanwhile, the energy storage system is switched from V / f control to P / Q control, and an additional damping controller is configured. The application not only can complete the stable recovery of the power grid, but also can improve the stability of the photovoltaic system and inhibit the subsynchronous oscillation generated in the black start process.

Description

Technical Field

[0001] This invention relates to the field of power system fault recovery technology, specifically a grid segmentation recovery strategy based on a photovoltaic-storage combined generation system. Background Technology

[0002] As power systems grow in scale, their structures and operating modes become increasingly complex and variable. Therefore, localized system failures can cause large-scale power outages or even system collapse. For example, large-scale blackouts can occur due to unexpectedly high electricity demand during extreme weather, insufficient power output from power sources, and inadequate emergency power support from other regions; or due to human error leading to protection system malfunction. Prolonged and widespread power outages can severely impact socio-economic development. Black start, as a common measure for restoring stable power system operation, is crucial for reducing outage duration and minimizing economic losses caused by power outages.

[0003] Black start refers to the process where, after a complete power outage due to a fault, the entire system is restored without relying on other networks. It starts up using generators with self-starting capabilities, gradually expanding the recovery range and ultimately restoring the entire system. Traditional black start strategies typically utilize hydropower or gas turbines as black start power sources, but these have drawbacks due to limitations in geographical resource distribution and investment costs. For example, the uneven distribution of resources and the scarcity of water resources restrict the construction of hydropower units, while gas turbines require high-power diesel generators for starting power and regular maintenance, resulting in high investment costs and unsuitability for widespread adoption. Finding alternative black start power sources for areas with scarce hydropower resources would significantly improve the black start capability of these regions.

[0004] Distributed power sources, represented by solar and wind power, are widely used in the power industry as new energy sources and have a very broad development prospect. Utilizing them to replace traditional hydropower and gas turbine units as black-start power sources, researching black-start schemes suitable for new energy power sources has become a current hot topic. The output of photovoltaic and wind turbines is affected by the external environment, exhibiting randomness and fluctuation, and they lack the voltage and frequency regulation capabilities and overload capacity of traditional generator sets, thus lacking self-starting capability. Adding energy storage systems can provide them with a stable external voltage, assisting photovoltaic and wind turbines as black-start power sources and participating in black-start operations.

[0005] Existing technology and its disadvantages:

[0006] Measure 1: Feasibility study on the participation of photovoltaic-storage combined power generation systems in black start

[0007] Specific examples include Liu Mengjia et al.'s feasibility analysis of grid black start based on photovoltaic-storage power stations, Huang Taotao et al.'s feasibility study of a DIgSILENT-based photovoltaic-storage combined system as a black start power source for a regional power grid, and Liu Jiankun et al.'s feasibility analysis of a photovoltaic energy storage system as a grid black start power source under uncertain environmental conditions. These studies analyzed the feasibility of black start for photovoltaic-storage systems and the impact of different environmental factors on black start, but none of them detailed the coordination and control strategies for new energy sources during the start-up process.

[0008] Measure 2: Photovoltaic microgrid control switching strategy

[0009] Specific examples include the islanding / grid-connected smooth switching control strategy for microgrids with photovoltaic sources proposed by Zhang Tengfei et al. This measure can realize the switching between different control strategies under different operating modes of the microgrid, improving the traditional pre-synchronization controller. However, its research scenario is only a single microgrid with photovoltaic sources and has not been applied to grid black start.

[0010] Measure 3: Black start coordination control strategy based on photovoltaic-storage combined power generation system

[0011] Specific examples include Liu Yingpei et al.'s coordinated control strategy for a photovoltaic-storage combined power generation system applicable to black start, and Zhao Jingjing et al.'s coordinated control strategy for wind-solar-storage power stations during black start-up of adjacent thermal power plants. These measures propose a method combining load tracking and maximum power point tracking (MPPT) algorithms during the black start-up process of the photovoltaic-storage system, but neither study in detail the comprehensive control strategy for new energy sources during black start-up or the oscillation problem during the start-up process.

[0012] Measure 4: Virtual synchronization control based on photovoltaic-storage combined power generation system

[0013] Specific examples include Meng Jianhui et al.'s distributed inverter power supply control strategy for improving microgrid frequency stability, and Wang Zhenxiong et al.'s virtual synchronous generator structure applied to photovoltaic microgrids and its dynamic performance analysis. These measures combine virtual synchronous control with energy storage systems in photovoltaic-energy storage systems to improve the stability of the energy storage system. However, neither approach considers applying virtual synchronous control to an independent photovoltaic system during black start, allowing the photovoltaic system to have independent inertial damping support without relying on other systems. Summary of the Invention

[0014] To address the aforementioned problems, the present invention aims to provide a segmented grid recovery strategy based on a photovoltaic-storage combined power generation system. This strategy focuses on the reconstruction and recovery of the power source side during black-start operations, considers the oscillation problem present during black-start, and designs corresponding suppression methods. This not only achieves stable grid recovery but also improves the stability of the photovoltaic system and suppresses subsynchronous oscillations generated during black-start. The technical solution is as follows:

[0015] A grid segmentation recovery strategy based on a photovoltaic-storage combined generation system includes the following steps:

[0016] Step 1: Black boot power supply self-start

[0017] When the power grid is shut down due to a fault and the entire power grid system is in a state of complete darkness, an energy storage system is selected to provide voltage and frequency support for photovoltaic grid connection. The inverter of the energy storage system adopts V / f control to establish the frequency and voltage required for stable operation of the photovoltaic system.

[0018] Step 2: Grid connection of the photovoltaic power generation system

[0019] After the energy storage system establishes a stable grid-connected bus voltage, the photovoltaic system inverter phase-locked loop obtains the voltage signal and adopts a constant DC voltage and constant reactive power control method to connect to the grid. At the same time, a virtual synchronous generator (VSG) control is added to reduce the impact of external environmental changes on the output of the photovoltaic system and improve the damping inertial support of the photovoltaic system.

[0020] When a photovoltaic system is connected to the grid, its inverter controls the DC side voltage U. dc While maintaining a constant photovoltaic reactive power Q, MPPT control of photovoltaic cells is achieved to maximize the operating efficiency of the photovoltaic system.

[0021] Step 3: Start up conventional units and load

[0022] After the photovoltaic system is connected to the grid, to ensure the smooth startup of the conventional generating units, the auxiliary system outside the main unit is put into operation first. After the auxiliary system starts up, the conventional generating units are put into operation. At the same time as the conventional generating units are put into operation, the energy storage system switches from V / f control to P / Q control to avoid system oscillation caused by the conventional generating units and the energy storage system simultaneously maintaining the system voltage and frequency. An additional damping controller is configured at the inverter to suppress the subsynchronous oscillation that occurs during the system's black start. At this time, the grid voltage and frequency are supported by the started conventional generating units. The energy storage system adjusts the active and reactive power output of the inverter through P / Q control, thereby smoothing the system power fluctuations during startup. After the conventional generating units start up, a certain amount of load is put into operation to stabilize the system voltage and frequency.

[0023] Furthermore, the V / f control specifically involves: taking a given voltage and frequency, performing coordinate transformation on the voltage, then subtracting the measured output voltage from the voltage setting reference value to form an error input to the PI controller to obtain the current reference value for inner loop control; comparing the current reference value with the actual measured current value again, and obtaining a PWM pulse through inner loop control, which is applied to the inverter to achieve tracking of the given voltage and frequency.

[0024] Furthermore, in step 2, VSG control is introduced to improve the inertial damping support of the photovoltaic power generation system, specifically as follows:

[0025] The speed governor and excitation regulator of the synchronous generator are simulated, and the damping coefficient and inertia constant are introduced into the inverter control so that the distributed generation unit has the characteristics of a synchronous generator.

[0026] The active power-frequency regulator includes the rotor motion equation of the synchronous generator and the prime mover regulation equation; the rotor motion equation is shown in equation (1), which reflects the inertia and damping characteristics of the generator rotor:

[0027]

[0028] In the formula, J is the moment of inertia, ω n ω and T represent the rated angular frequency and the actual angular frequency, respectively. m and T e These represent mechanical and electromagnetic torques, respectively, where D is the damping coefficient and P is the electromagnetic torque. m and P e These represent mechanical and electromagnetic power, respectively, with δ being the power angle.

[0029] The speed regulation formula for the prime mover is:

[0030] P m =P ref +K(ω n -ω) (2)

[0031] In the formula, P ref The active power command is given by K, which is the droop parameter.

[0032] When the regulator detects the frequency deviation and obtains the power deviation through active power droop, it changes P m Thus controlling P e Adjust the output frequency;

[0033] For VSG control in grid-connected operation mode, the above formula can be rearranged as follows:

[0034]

[0035] In the formula, s is the Laplace operator, τ and D ρ As a user-defined variable, its calculation expression is shown in equation (4):

[0036]

[0037] The VSG control of the photovoltaic system incorporates the rotor motion equations and active power frequency regulation of the synchronous generator into the voltage outer loop of the photovoltaic system inverter; among which, the active power command P, which serves as the grid connection reference value, is... refThe maximum output power P obtained by MPPT control of the photovoltaic system MPPT ;

[0038] To avoid excessive impact on the power grid during commissioning, the photovoltaic units were commissioned in stages.

[0039] Furthermore, in step 3, the overall least squares-rotation invariant technique is used to obtain the rotation factor of the signal using the autocorrelation matrix and cross-correlation matrix of the collected signal data. The frequency and attenuation factor of the signal are then obtained from the rotation factor and combined with TLS to obtain the amplitude and phase of the signal. Based on the output signal of the time-domain simulation, the frequency and damping information of the subsynchronous oscillation are obtained as the design basis for the additional damping controller.

[0040] The closed-loop system characteristic equation of the additional damping controller is:

[0041] 1+G(s)H(s)=0 (5)

[0042] In the formula, G(s) is the transfer function of the system under study, and H(s) is the transfer function of the control system. The system under study is the overall system, including photovoltaic energy storage generators, etc., while the control system refers to the added controller. The identified transfer function of the system under study is:

[0043]

[0044] The additional damping controller is designed using the root locus method. It is set at the active outer loop control of the inverter in the energy storage system. Its input is the speed difference signal of the conventional unit. After the controller is activated, a corresponding current is generated in the system, and a corresponding additional damping torque is induced in the unit, thereby suppressing system oscillation.

[0045] The beneficial effects of this invention are:

[0046] 1) This invention adds virtual synchronization control to the inverter side of the photovoltaic system, which can improve the damping inertia support of the photovoltaic power generation system, so that the output of the photovoltaic system has inertia when the external environment changes, and the corresponding frequency change is slowed down.

[0047] 2) The additional damping controller of the energy storage system in this invention is a multi-channel additional damping controller designed based on the overall least squares-rotational invariant (TLS-ESPRIT) technology, which can suppress the subsynchronous oscillations caused by black start of the power grid and ensure the stable recovery of the power grid. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of a photovoltaic-storage combined power generation system.

[0049] Figure 2A flowchart of the grid segmented recovery strategy for a photovoltaic-storage combined power generation system.

[0050] Figure 3 This is the V / f control block diagram.

[0051] Figure 4(a) shows the waveform of the energy storage system in islanded operation – the curve of the grid-connected bus line voltage change.

[0052] Figure 4(b) shows the waveform diagram of the energy storage system in islanded operation—the system frequency variation curve.

[0053] Figure 4(c) shows the waveform diagram of the energy storage system in islanded operation – the phase voltage change curve of the grid-connected bus.

[0054] Figure 5 This is a schematic diagram of the grid connection control of a photovoltaic system.

[0055] Figure 6 This is a structural diagram of the VSG power frequency regulator.

[0056] Figure 7 This is a schematic diagram of the VSG control for a photovoltaic system.

[0057] Figure 8(a) shows the bus voltage variation curve during the grid connection of the photovoltaic system.

[0058] Figure 8(b) shows the frequency change curve of the photovoltaic system during grid connection.

[0059] Figure 9 Simulation results comparing the power output of the photovoltaic system before and after adding VSG control to the inverter side of the photovoltaic system.

[0060] Figure 10 The figure shows the simulation results of power variation of the photovoltaic system with different inertia coefficients J when the damping coefficient D = 2 is fixed.

[0061] Figure 11 The figure shows the simulation results of the power variation of the photovoltaic system with different damping coefficients D when the inertia coefficient J = 2.5 is fixed.

[0062] Figure 12 This is a schematic diagram illustrating the working principle of the additional damping controller.

[0063] Figure 13 The root locus diagram is shown after adding an additional damping controller.

[0064] Figure 14(a) shows the simulation results of the start-up of a conventional generator set—the curve of the change in line voltage of the grid-connected bus.

[0065] Figure 14(b) shows the simulation results of the start-up of a conventional generator set—a curve of system frequency variation.

[0066] Figure 15(a) shows the effect of the additional damping controller on the energy storage system—the curve of the speed difference change when the energy storage system is discharging.

[0067] Figure 15(b) shows the effect of the additional damping controller on the energy storage system—the curve of the speed difference change during the charging of the energy storage system.

[0068] Figure 16(a) shows the voltage variation curve of the grid-connected bus when conventional generator sets, photovoltaic systems, and energy storage systems jointly supply power to the load.

[0069] Figure 16(b) shows the system frequency variation curve when conventional generator sets, photovoltaic systems, and energy storage systems jointly supply power to the load. Detailed Implementation

[0070] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0071] This invention focuses on the reconstruction and recovery of the power source during black start-up. It comprehensively designs a control strategy for segmented grid recovery in a photovoltaic-storage combined power generation system, proposes coordinated optimization schemes for different control strategies at different stages, and considers the oscillation problem existing during black start-up, designing corresponding suppression methods. The proposed grid black start strategy can not only achieve stable grid recovery but also improve the stability of the photovoltaic system and suppress subsynchronous oscillations generated during black start-up.

[0072] The embodiments of the present invention are as follows: Figure 1 We will take a photovoltaic-storage combined power generation system as an example for verification.

[0073] Schematic diagram of a photovoltaic-storage combined power generation system Figure 1 As shown. The system includes a photovoltaic system, an energy storage system, a DC / DC converter, a DC / AC converter, transmission lines, conventional generating units to be started, auxiliary equipment, and loads. Based on the basic structure and typical control of the photovoltaic-energy storage system, a grid segmented recovery strategy for the photovoltaic-energy storage combined generation system is proposed, such as... Figure 2 As shown. The specific steps are as follows:

[0074] 1) Black start power supply self-start

[0075] When the system is in complete darkness, the self-starting of the black-start power supply is a crucial step to ensure a smooth system startup. Because the output of photovoltaic systems fluctuates randomly, it easily causes frequency fluctuations in the power grid and lacks the ability to start independently. Therefore, the energy storage system uses V / f control to establish the grid-connected bus voltage (230kV) and frequency (50Hz) when establishing islanding.

[0076] When the power grid experiences a power outage due to a fault, and the system is in complete darkness, the output power of photovoltaic (PV) systems is volatile, random, and intermittent due to their susceptibility to temperature and sunlight influences. Therefore, an energy storage system is selected to provide stable voltage and frequency support for PV grid connection. The energy storage system inverter uses V / f control to establish the frequency and voltage required for stable operation of the PV system. The V / f control block diagram is shown below. Figure 3 As shown, where U ref Given a voltage, the outer loop voltage reference value U is obtained through coordinate transformation. d_ref U q_ref U d U q For grid-side voltage, i dref i qref i is the reference value for the inner loop current. d i q Let L be the filter inductor current, L be the filter inductor value, and ω be the system angular frequency.

[0077] The V / f control works as follows: Given a voltage and frequency, the voltage undergoes coordinate transformation. The difference between the measured output voltage and the set voltage reference value is used to generate an error, which is then input to the PI controller to obtain the current reference value for the inner loop control. The current reference value is compared again with the actual measured current value. Through inner loop control, a PWM pulse is generated and applied to the inverter, achieving tracking of the given voltage and frequency.

[0078] Figure 4 shows the waveform of the energy storage system operating in island mode: At 0s, the energy storage system starts up with a load as a self-starting power source, with an initial active power of 27MW and reactive power of 3MVar. Figure 4 shows the simulation results of the energy storage system establishing the grid-connected bus voltage. The bus voltage rises from 0 and stabilizes at 1pu after fluctuating for about 0.03s; the energy storage system stabilizes the frequency at around 50Hz through V / f control, with a frequency deviation rate of 0.12%. The results show that the energy storage system can effectively establish a stable external voltage and frequency, providing grid-connected conditions for the photovoltaic system.

[0079] 2) Grid connection of photovoltaic power generation system

[0080] After the energy storage system establishes a stable grid-connected bus voltage, the photovoltaic system inverter's phase-locked loop acquires the voltage signal and connects to the grid using a constant DC voltage and constant reactive power control method. Simultaneously, virtual synchronization control is incorporated to reduce the impact of external environmental changes on the photovoltaic system's output and improve the system's damping inertia support. The photovoltaic system starts up, and the renewable energy source resumes its power supply output.

[0081] To ensure phase synchronization between the grid-side current and voltage during grid connection and to reduce reactive power in the grid, photovoltaic (PV) systems typically employ unity power factor output during grid connection. When a PV system is connected to the grid, its inverter controls the DC-side voltage U... dcWhile maintaining a constant photovoltaic reactive power Q, MPPT control of the photovoltaic cells is achieved to maximize system operating efficiency. The control method is as follows: Figure 5 As shown. The inverter inner loop reference signal input consists of two parts, one part i dref1 The voltage is obtained from the DC side voltage through a PI regulator, and a portion of i dref2 The maximum power P obtained by the MPPT algorithm MPPT Divide by the d-axis voltage u on the grid-connected side d get.

[0082] MPPT control can detect the generated voltage of the solar panel in real time and track the highest voltage and current value (VI), enabling the system to charge the battery at maximum power output. This invention utilizes the Perturbation Observation Method (P&O) to achieve MPPT control. The Perturbation Observation Method (P&O) involves perturbing the output voltage of the photovoltaic array and calculating the output power of the photovoltaic array before and after the perturbation using P=UI. These differences are then compared to determine the correct perturbation direction. In the next perturbation cycle, the algorithm continues to iterate in the same way until the area around the maximum power point is found.

[0083] To improve the inertial damping support of photovoltaic power generation systems, this invention introduces VSG control. The basic idea of ​​VSG control is to simulate the speed governor and excitation regulator of a synchronous generator based on the flexible and controllable characteristics of power electronic devices, and to introduce damping coefficients and inertial constants into the inverter control, so that the distributed generation unit has the characteristics of a synchronous generator.

[0084] The active power-frequency regulator includes the rotor motion equation of the synchronous generator and the prime mover regulation equation. The rotor motion equation is shown in equation (1), which reflects the inertia and damping characteristics of the generator rotor:

[0085]

[0086] In the formula, J is the moment of inertia, ω n ω and T represent the rated angular frequency and the actual angular frequency, respectively. m and T e These represent mechanical and electromagnetic torques, respectively, where D is the damping coefficient and P is the electromagnetic torque. m and P e These represent mechanical and electromagnetic power, respectively, with δ being the power angle. The speed regulation formula for the prime mover is:

[0087] P m =P ref +K(ω n -ω) (2)

[0088] In the formula, P refThe active power command is given by P, and the droop parameter is given by K. The regulator detects the frequency deviation and obtains the power deviation through the active power droop, thereby changing P. m Thus controlling P e Adjust the output frequency.

[0089] The structure of the VSG power frequency regulator is as follows: Figure 6 As shown, for VSG control in grid-connected operation mode, the above formula can be reorganized as:

[0090]

[0091] In the formula, τ and D ρ As a user-defined variable, its calculation expression is shown in equation (4):

[0092]

[0093] Figure 7 This diagram illustrates the VSG control of a photovoltaic system, incorporating the rotor motion equations and active power frequency regulation of the synchronous generator into the voltage outer loop of the photovoltaic system inverter. The grid-connected reference value P... ref The maximum output power P obtained by MPPT control of the photovoltaic system MPPT Adding VSG control to the inverter side of a photovoltaic system can improve the inertial damping characteristics of the photovoltaic system.

[0094] Once the bus voltage established by the energy storage system stabilizes, the photovoltaic system is connected to the grid. To avoid excessive impact on the grid during startup, the photovoltaic units are connected in stages: 70 units are connected at 0.25 seconds, and another 130 units are connected at 0.6 seconds. After the photovoltaic system is connected, its DC-side voltage is basically stable at 2kV.

[0095] Figures 8(a) and 8(b) show the fluctuations in bus voltage and frequency during the grid connection of the photovoltaic system. It can be observed that after grid connection, the frequency fluctuates, with a maximum deviation rate of 2.2%, before stabilizing within the range of 50 ± 0.2 Hz. The bus voltage rises slightly, with a maximum fluctuation amplitude of 0.1 pu, and stabilizes at 1 pu after adjustment. This indicates that grid connection of the photovoltaic system has a certain impact on the bus voltage and frequency, but it can return to a stable state within approximately 0.1 seconds.

[0096] Figure 9 The simulation results show the comparison of photovoltaic system output before and after adding VSG control to the inverter side of the photovoltaic system. When the ambient light decreases, the active power output of the photovoltaic system decreases. It can be seen that after adding VSG control, the photovoltaic system output exhibits inertial damping characteristics, with power oscillations and a reduced rate of change, thus mitigating system power fluctuations caused by changes in the external environment.

[0097] To further investigate the effect of adding VSG control to the photovoltaic system, simulations were performed with varying inertia coefficient J and damping coefficient D. Figure 10 The results show the power variation of the photovoltaic system under different inertia coefficients J when the damping coefficient D = 2 is fixed. When the illumination decreases, the active power output of the photovoltaic system decreases. Due to the existence of the inertia coefficient J, the rate of change of its output power decreases. As J increases, the overshoot increases, the number of power oscillations increases, and the time to recover to steady state increases, while the change slows down.

[0098] Figure 11 The diagram illustrates the power variation of a photovoltaic system with different damping coefficients D when the inertia coefficient J = 2.5 is constant. It can be observed that as the damping coefficient D increases, power fluctuations decrease, and the oscillation decay rate increases. From... Figure 10 , Figure 11 It can be seen that after adding VSG control, the output power response of the photovoltaic system has characteristics similar to those of a synchronous generator, which improves the inertia and damping of the photovoltaic system, thereby providing sufficient support for external environmental fluctuations and ensuring the stable operation of the system.

[0099] 3) Start up conventional units and load

[0100] After the photovoltaic system is connected to the grid, to ensure the smooth startup of the conventional generating units, the auxiliary systems outside the main unit are first put into operation. After the auxiliary systems start up, the conventional generating units are then put into operation. To avoid system oscillation caused by the conventional generating units and the energy storage system simultaneously maintaining system voltage and frequency, the energy storage system switches from V / f control to P / Q control at the same time as the conventional generating units are put into operation, and an additional damping controller is configured at the inverter to suppress subsynchronous oscillations that occur during the system's black start-up. At this time, the grid voltage and frequency are supported by the started-up conventional generating units. The energy storage system adjusts the active and reactive power output of the inverter through P / Q control, thereby smoothing out system power fluctuations during startup. After the conventional generating units have started up, a certain amount of load is put into operation to stabilize the system voltage and frequency.

[0101] Existing literature rarely studies the oscillation phenomenon that may occur during black start-up. This invention takes this oscillation problem into account and designs corresponding control strategies to suppress it. If the conventional generator set to be started has torsional vibration in its shaft system, it will cause a difference in rotor angular velocity after commissioning, leading to subsynchronous oscillations in the power system. Severe subsynchronous oscillations may cause significant damage to the rotor shaft system of large steam turbine generator sets, resulting in grid accidents and adversely affecting the safe and stable operation of the power grid. Therefore, this invention considers configuring an additional damping controller in the energy storage system to suppress system subsynchronous oscillations.

[0102] This invention employs the Total Least Squares-Rotation Invariant (TLS-ESPRIT) technique, which boasts high computational efficiency, stability, and disturbance rejection capabilities. The main idea of ​​the ESPRIT algorithm is to utilize the autocorrelation and cross-correlation matrices of the acquired signal data to determine the signal's rotation factor. From this rotation factor, the signal's frequency and attenuation factor are derived, and then combined with TLS to obtain the signal's amplitude and phase. Based on the output signal from time-domain simulation, this method accurately obtains the frequency and damping information of subsynchronous oscillations, which can serve as the design basis for additional damping controllers.

[0103] The additional damping controller is equivalent to adding a negative feedback loop to the system, and its working principle is as follows: Figure 12 As shown.

[0104] Figure 12 The closed-loop characteristic equation of the system shown is:

[0105] 1+G(s)H(s)=0 (5)

[0106] In the formula, G(s) is the transfer function of the system under study, and H(s) is the transfer function of the control system. The identified system transfer function is:

[0107]

[0108] This invention utilizes the root locus method to design an additional damping controller. The root locus method is a graphical method that represents the relationship between the roots of the system's characteristic equation and a certain system parameter. The fundamental characteristics of the transient response of the closed-loop system are related to the location of the closed-loop poles, which in turn depend on the selected loop gain. Adjusting the system gain can move the closed-loop poles to the desired positions, increasing the damping ratio of the dominant closed-loop poles and ensuring better damping characteristics of the system. The additional damping controller is installed at the active power outer loop control of the energy storage system inverter. Its input is the speed difference signal of a conventional generator unit. Through the controller's action, a corresponding current is generated in the system, inducing a corresponding additional damping torque on the generator unit, thereby suppressing system oscillations.

[0109] Figure 13 The root locus diagram after adding the additional damping controller shows that, after adding the controller, changing the system gain to 20 shifts the closed-loop poles to the left half of the s-plane. Furthermore, as K increases, some roots move to the left along the locus, thus facilitating system oscillation attenuation and ensuring system stability. The grid connection of a conventional unit is shown in Figure 14.

[0110] At 2.6 seconds, the conventional generator set to be started was put into operation, and simultaneously, the inverter control of the energy storage system switched to P / Q control, with the grid voltage and frequency supported by the conventional generator set. Figure 14 shows the simulation results of the conventional generator set startup. The maximum fluctuation of the effective voltage reached 0.07 pu, and the maximum fluctuation of the frequency reached 2 Hz, which stabilized after 0.5 seconds of fluctuation. The results indicate that the switching between the conventional generator set and the energy storage system control had a certain impact on the stability of the photovoltaic-storage combined power generation system, and caused fluctuations in the bus voltage and frequency. However, after adjustment, the system was able to stabilize again and complete grid connection.

[0111] Figure 15 shows the effect of the additional damping controller on the energy storage system. Due to the torsional vibration of the generator shaft system in the thermal power unit to be started, a rotor angular velocity difference will occur in the receiving-end generator unit after commissioning, leading to subsynchronous oscillation. Figure 15 illustrates the suppression effect of the additional damping controller designed in this invention on the rotor angular velocity difference. Figure 15(a) shows the energy storage in a discharging state, and Figure 15(b) shows the energy storage in a charging state. It can be seen that regardless of whether the energy storage system is in a discharging or charging state, the additional damping controller can suppress the subsynchronous oscillation caused by the torsional vibration of the conventional generator shaft system. Adding a subsynchronous damping controller during black start can improve the safety and stability of the system's black start process.

[0112] The simulation introduced a system load at 5 seconds, with conventional generators, photovoltaic systems, and energy storage systems jointly supplying power to the load. As shown in Figure 16, after the grid restored its generating capacity, the system voltage remained relatively stable at 1.0065 pu and the frequency remained relatively stable at 49.5 Hz after a certain amount of load was introduced. This indicates that the system can operate stably under load after a black start, verifying the effectiveness of the grid segmented recovery strategy based on the photovoltaic-energy storage combined generation system proposed in this invention.

[0113] Table 1

[0114]

[0115] Table 1 summarizes the voltage and frequency fluctuations at each stage of black start. According to the simulation results, it can be seen that the bus voltage deviation does not exceed 0.65% and the system frequency deviation does not exceed 1% during the grid restoration process, which meets the start-up requirements of voltage control between 0.9pu and 1.1pu and system frequency control between 49.5Hz and 50.5Hz. The start-up strategy is effective and feasible.

Claims

1. A method for grid segmented recovery strategy based on photovoltaic-storage combined generation system, characterized in that, Includes the following steps: Step 1: Black-start power supply automatically starts; When the power grid is shut down due to a fault and the entire power grid system is in a state of complete darkness, an energy storage system is selected to provide voltage and frequency support for photovoltaic grid connection. The inverter of the energy storage system adopts V / f control to establish the frequency and voltage required for stable operation of the photovoltaic system. Step 2: Connect the photovoltaic power generation system to the grid; After the energy storage system establishes a stable grid-connected bus voltage, the photovoltaic system inverter phase-locked loop obtains the voltage signal and adopts a constant DC voltage and constant reactive power control method to connect to the grid. At the same time, virtual synchronous generator control is added to reduce the impact of external environmental changes on the photovoltaic system output and improve the damping inertial support of the photovoltaic system. When a photovoltaic system is connected to the grid, its inverter controls the DC side voltage U. dc While maintaining a constant photovoltaic reactive power Q, MPPT control of photovoltaic cells is achieved to maximize the operating efficiency of the photovoltaic system. Step 3: Start up the conventional units and load; After the photovoltaic system is connected to the grid, to ensure the smooth startup of the conventional generating units, the auxiliary system outside the main unit is put into operation first. After the auxiliary system starts up, the conventional generating units are put into operation. At the same time as the conventional generating units are put into operation, the energy storage system switches from V / f control to P / Q control to avoid system oscillation caused by the conventional generating units and the energy storage system simultaneously maintaining the system voltage and frequency. An additional damping controller is configured at the inverter to suppress the subsynchronous oscillation that occurs during the system's black start process. At this time, the grid voltage and frequency are supported by the started conventional generating units. The energy storage system adjusts the active and reactive power output of the inverter through P / Q control, thereby smoothing the system power fluctuations during startup. After the conventional generating units start up, a certain amount of load is put into operation to stabilize the system voltage and frequency. In step 2, VSG control is introduced to improve the inertial damping support of the photovoltaic power generation system, specifically as follows: The speed governor and excitation regulator of the synchronous generator are simulated, and the damping coefficient and inertia constant are introduced into the inverter control so that the distributed generation unit has the characteristics of a synchronous generator. The active power-frequency regulator includes the rotor motion equation of the synchronous generator and the prime mover regulation equation; the rotor motion equation is shown in equation (1), which reflects the inertia and damping characteristics of the generator rotor: （1）； In the formula, J is the moment of inertia. and These are the rated angular frequency and the actual angular frequency, respectively, T m and T e These represent mechanical and electromagnetic torques, respectively, where D is the damping coefficient and P is the electromagnetic torque. m and P e These represent mechanical and electromagnetic power, respectively, with δ being the power angle. The speed regulation formula for the prime mover is: （2）； In the formula, P ref The active power command is given by K, which is the droop parameter. When the regulator detects the frequency deviation and obtains the power deviation through active power droop, it changes P m Thus controlling P e Adjust the output frequency; For VSG control in grid-connected operation mode, the above formula can be rearranged as follows: （3）； In the formula, s is the Laplace operator, τ and D ρ As a user-defined variable, its calculation expression is shown in equation (4): （4）； The VSG control of the photovoltaic system incorporates the rotor motion equations and active power frequency regulation of the synchronous generator into the voltage outer loop of the photovoltaic system inverter; among which, the active power command P, which serves as the grid connection reference value, is... ref The maximum output power P obtained by MPPT control of the photovoltaic system MPPT ; To avoid excessive impact on the power grid during commissioning, the photovoltaic units were commissioned in stages.

2. The method for grid segmented recovery strategy based on photovoltaic-storage combined generation system according to claim 1, characterized in that, The V / f control specifically involves taking a given voltage and frequency, transforming the voltage into coordinates, and then subtracting the measured output voltage from the voltage setting reference value to form an error input to the PI controller to obtain the current reference value for the inner loop control. The reference current value and the actual measured current value are compared again, and a PWM pulse is obtained through inner loop control. This pulse is applied to the inverter to achieve tracking of the given voltage and frequency.

3. The method for grid segmented recovery strategy based on photovoltaic-storage combined generation system according to claim 1, characterized in that, In step 3, the overall least squares-rotation invariant technique is used to obtain the rotation factor of the signal by using the autocorrelation matrix and cross-correlation matrix of the collected signal data. The frequency and attenuation factor of the signal are obtained from the rotation factor. Then, combined with TLS, the amplitude and phase of the signal are obtained. Based on the output signal of time-domain simulation, the frequency and damping information of the subsynchronous oscillation are obtained as the design basis for the additional damping controller. The closed-loop system characteristic equation of the additional damping controller is: （5）； In the formula, G(s) is the transfer function of the system under study, and H(s) is the transfer function of the control system; the identified transfer function of the system under study is: （6）； The additional damping controller is designed using the root locus method. It is set at the active outer loop control of the inverter in the energy storage system. Its input is the speed difference signal of the conventional unit. After the controller is activated, a corresponding current is generated in the system, and a corresponding additional damping torque is induced in the unit, thereby suppressing system oscillation.

Images