Impedance remodeling method for suppressing harmonic oscillation based on superconducting magnetic energy storage system

Abstract

The invention discloses an impedance remodeling method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system, and belongs to the technical field of power generation, power transformation or power distribution. The method comprises the following steps: analyzing theoretical characteristics of the superconducting magnetic energy storage system, analyzing mathematical models and working characteristics of an SMES and a storage battery, and carrying out theoretical modeling on the hybrid energy storage system; the method comprises the following steps: performing small signal modeling on a grid-connected converter control system under voltage control type virtual synchronous control, and establishing a grid-connected converter small signal sequence impedance model; a sequence impedance remodeling model is introduced, the influence of the sequence impedance remodeling model on the low-frequency impedance of the virtual synchronous control system is revealed, in order to obtain the optimal sequence impedance of the grid-connected converter, real-time and accurate power grid impedance information is obtained by injecting a multi-harmonic disturbance signal into the grid-connected inverter, and a traditional impedance remodeling strategy is dynamically adjusted according to the information. It is ensured that the system still has good stability margin when the power grid impedance fluctuates in a wide range, and the adaptability of the grid-connected inverter under different power grid working conditions is improved.

Description

Technical Field

[0001] The present invention relates to a power system control parameter setting technology, and in particular discloses an impedance reshaping method for suppressing harmonic oscillations based on a superconducting magnetic energy storage system, belonging to the technical field of power generation, transformation or distribution. Background Art

[0002] With the rapid development of the power grid, the role of energy storage systems has become increasingly important. They have the advantages of providing energy buffering, smoothing power fluctuations, and improving operational reliability. Superconducting magnetic energy storage (SMES) has attracted attention due to its high power density, fast operation, and long life, and can effectively manage short-term instantaneous power fluctuations. Technically speaking, batteries have solved the problem of long-term and sustained power changes with their huge energy density and good economic effects. The combination of superconducting magnetic energy storage and batteries can form a SMES-battery hybrid energy storage system to give full play to the functional advantages of these two energy storage devices. The SMES-battery hybrid energy storage system has the potential to assist the power system more effectively, while protecting the battery from the effects of high power requirements and fast transient cycles, mitigating various interferences, and saving system costs.

[0003] Small and medium-sized battery inverters can achieve grid-connected capabilities by utilizing virtual synchronous generator (VSG) control systems. By adjusting virtual inertia and damping to provide active voltage / frequency support, they can improve system resilience, dynamic isolation, and reconnection performance. However, the impedance of the VSG control system is primarily inductive, and impedance coupling between the VSG control system and the associated power grid must be considered. Technically, appropriate measures should be adopted to stabilize the VSG control system. Some researchers have proposed introducing a grid voltage feedforward control strategy to mitigate the impact of background harmonics in the grid voltage on the grid-connected current. This has become a common technique for suppressing grid voltage background harmonics in LCL-type grid-connected inverters. While this technique improves the system's amplitude in the low-frequency range to a certain extent, it reduces the grid-connected system's phase margin, hindering system stability under weak grid conditions. Other studies have improved the phase margin of grid-connected converter control systems by improving the grid voltage feedforward channel of the control system, but these designs are designed for fixed grid conditions. However, in actual application scenarios, with the continuous integration of a high proportion of renewable energy, the structure and operation of the power grid have undergone significant changes. The uncertain output of renewable energy and the switching of new loads cause frequent changes in grid impedance, which significantly affects the performance of traditional impedance reshaping strategies. To improve the adaptability of grid-connected inverters to different grid environments, some studies have indirectly estimated grid information through system electrical state variables to achieve adaptive reshaping of grid-connected converter impedance under virtual synchronous generator control, thereby improving the stable operation capability of grid-connected inverters. However, the grid impedance information obtained by this technical solution is not direct and accurate, so its performance in complex operating scenarios is still difficult to guarantee.

[0004] In summary, the present invention aims at a hybrid energy storage system of superconducting magnetic energy storage and batteries, and proposes an impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system to overcome the defects of traditional impedance reshaping strategies. Summary of the Invention

[0005] The purpose of the present invention is to address the deficiencies of the above-mentioned background technology and propose an impedance reshaping method for suppressing harmonic oscillations based on a superconducting magnetic energy storage system. For the hybrid energy storage system of superconducting magnetic energy storage and batteries, virtual synchronous control technology is adopted and an impedance reshaping link is added to improve the grid-connected capability of the grid-connected converter, and achieve the purpose of adaptively reshaping the impedance of the grid-connected converter, thereby solving the technical problem that the impedance reshaping technology of the grid-connected converter cannot meet the requirements of stable operation of the grid-connected converter under different working conditions.

[0006] The present invention adopts the following technical solutions to achieve the above-mentioned purpose:

[0007] An impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system, comprising:

[0008] Step 1: Establish a battery charge and discharge model and a superconducting magnetic energy storage charge and discharge model, build a superconducting magnetic energy storage-battery hybrid energy storage system, and coordinate the active power output of the superconducting magnetic energy storage and the battery based on the active power calculated by the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator. Calculate the grid-connected reference current based on the coordinated superconducting magnetic energy storage output current and the battery output current;

[0009] Step 2: Establish a small signal model of the grid-connected inverter control system based on voltage-controlled virtual synchronous control according to the grid-connected reference current, and calculate the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control;

[0010] Step 3: Identify the grid impedance based on multi-harmonic injection and reshape the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control.

[0011] As a further optimization scheme for the impedance reshaping method to suppress harmonic oscillation based on superconducting magnetic energy storage system, the battery charging and discharging model established in step 1 is ,in, is the battery capacity at time t after the charge and discharge process, is the battery output current, I1 is the capacitance between the two plates The current on , It is the overvoltage resistor, and + and - indicate whether the battery is in the charging or discharging state.

[0012] As a further optimization scheme for the impedance reshaping method of suppressing harmonic oscillation based on superconducting magnetic energy storage system, the superconducting magnetic energy storage charging and discharging model established in step 1 includes the superconducting magnetic energy storage charging model and superconducting magnetic energy storage discharge model ,in, is the superconducting magnetic energy storage charging current at time t, is the grid-side voltage, is the output current of the superconducting magnetic energy storage at time t, is the initial current flowing through the superconducting magnetic energy storage inductor, is the inductance of the superconducting magnetic energy storage inductor, is the discharge power of the superconducting magnetic energy storage inductor at time t.

[0013] As a further optimization scheme of the impedance reshaping method for suppressing harmonic oscillations based on a superconducting magnetic energy storage system, step 1 builds a hybrid energy storage system of superconducting magnetic energy storage and battery. Specifically, the DC side of the H-bridge DC chopper is connected to the DC side of the bidirectional DC chopper to form a grid-connected converter, the superconducting magnetic energy storage inductor is connected between the midpoints of the bridge arms of the H-bridge DC chopper, and the battery is connected to the DC bus formed by connecting the DC side of the H-bridge DC chopper and the DC side of the bidirectional DC chopper after passing through a DC converter.

[0014] As a further optimization scheme for the impedance reshaping method for suppressing harmonic oscillations based on the superconducting magnetic energy storage system, step 1 coordinates the active power output of the superconducting magnetic energy storage and the battery based on the active power calculated by the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator. Specifically,

[0015] When the hybrid energy storage system is in the power absorption mode, the superconducting magnetic energy storage is controlled to operate in a charging state, the battery charge is detected when the superconducting magnetic energy storage charge is higher than the maximum charge in the charging mode, and the battery is controlled to operate in a charging state when the battery charge has not reached the maximum charge in the charging mode, the active power calculated by the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator is distributed to the superconducting magnetic energy storage and the battery, the superconducting magnetic energy storage charging current is used as the superconducting magnetic energy storage output current, the battery output current is calculated in combination with the active power distributed to the battery and the battery charge and discharge model, and the cumulative sum of the superconducting magnetic energy storage output current and the battery output current is used as the grid-connected reference current;

[0016] When the hybrid energy storage system is in a power release mode, the superconducting magnetic energy storage is controlled to operate in a discharge state. When the battery charge is lower than the minimum charge in the discharge mode, the battery charge is detected. When the battery charge is higher than the minimum charge in the discharge mode, the battery is controlled to operate in a discharge state. The active power calculated by the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator is allocated to the superconducting magnetic energy storage and the battery. The superconducting magnetic energy storage output current is calculated based on the active power allocated to the superconducting magnetic energy storage and the superconducting magnetic energy storage discharge model. The battery output current is calculated based on the active power allocated to the battery and the battery charge and discharge model. The cumulative sum of the superconducting magnetic energy storage output current and the battery output current is used as the grid-connected reference current.

[0017] As a further optimization scheme of the impedance reshaping method for suppressing harmonic oscillation based on superconducting magnetic energy storage system, the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control calculated in step 2 is ,in, is the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control, is the transfer function of the current controller, is the transfer coefficient from the modulation wave to the output voltage of the bridge arm of the grid-connected converter, is the system sampling period, is the proportional gain, is the resonant gain, is the resonant angular frequency, is the bandwidth at the resonant angular frequency, are the filter capacitor current feedback coefficient and the grid current feedback coefficient, L f 、L g They are the filter inductance and the equivalent line inductance of the power grid respectively.

[0018] As a further optimization scheme for the impedance reshaping method for suppressing harmonic oscillations based on superconducting magnetic energy storage systems, the specific method for identifying the grid impedance based on multi-harmonic injection in step 3 is as follows:

[0019] Step 3-1-1: Inject a series of harmonic currents of specific frequencies into the grid-connected reference current, and detect the voltage response and current response at the common connection point after the injection of harmonics of different specific frequencies. Perform Fourier analysis on the voltage response and current response at the common connection point after the injection of harmonics of different specific frequencies to obtain the voltage component and current component corresponding to each specific frequency, and calculate the grid impedance modulus at each injected harmonic frequency. , n is the number of injected harmonics;

[0020] Step 3-1-2, convert the grid impedance modulus at each injected harmonic frequency into a relationship between resistance and inductance;

[0021] Step 3-1-3, obtain the resistive component of the grid impedance and emotional components , , where R g 、L g are the resistive and inductive components in the grid impedance, is the i-th injected harmonic frequency The grid impedance under 、 are the angular frequencies of the i-th and i+1-th injected harmonics respectively.

[0022] As a further optimization scheme for the impedance reshaping method for suppressing harmonic oscillation based on superconducting magnetic energy storage system, the specific method of step 3 for reshaping the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control is as follows:

[0023] Step 3-2-1, add the first impedance reshaping link to the voltage feedforward control and grid current feedback control of the small signal model of the grid-connected inverter control system based on voltage-controlled virtual synchronous control established in step 2 2. Second impedance reshaping step , , , the equivalent impedance of the grid-connected converter after reconstruction for ,in, Reshaping the first impedance The gain coefficient, are the first and second phase compensation coefficients, Reshape the link for the second impedance The gain coefficient, is the third phase compensation coefficient;

[0024] Step 3-2-2, reshape the link according to the first and second impedances Phase-frequency characteristics determine the gain coefficients of the first and second impedance reshaping links 、 :

[0025] 、 The maximum compensation phase frequency and maximum compensation phase of the first impedance reshaping link are: , , ,

[0026] 、 The maximum compensation phase frequency and maximum compensation phase of the second impedance reshaping link are: , , ,

[0027] Determine the intersection frequency of the grid impedance at all injected harmonic frequencies and the reconstructed grid-connected converter equivalent impedance 、 The parameter value is determined according to the actual phase compensation requirements. The parameter value after setting the parameter 、 Substitute the maximum compensation phase frequency and maximum compensation phase of the first impedance reshaping link to calculate k1, k2, , after setting the parameters Substitute the maximum compensation phase of the second impedance reshaping link and calculate 、 .

[0028] An electronic device includes a memory and a processor, wherein the memory stores a computer program that runs on the processor, and the processor executes the steps of the above-mentioned impedance reshaping method when running the computer program.

[0029] A computer-readable storage medium stores a computer program, which executes the steps of the impedance reshaping method when the computer program is run.

[0030] The present invention adopts the above technical solution and has the following beneficial effects:

[0031] (1) The present invention analyzes the theoretical mathematical model and working characteristics of SMES and batteries, constructs a hybrid energy storage system connected to the grid-connected converter, and coordinates the active power calculated by the voltage-controlled virtual synchronous generator control system with the active power output by the superconducting magnetic energy storage and the battery. The grid-connected reference current is calculated based on the coordinated control of the superconducting magnetic energy storage output current and the battery output current, providing accurate reference data for subsequent small signal modeling.

[0032] (2) The present invention performs small signal modeling on the grid-connected converter control system based on the voltage-controlled virtual synchronous control system, injects the grid-connected reference current into the control system, and reveals its influence on the low-frequency impedance of the grid-connected inverter control system, laying the foundation for the impedance reshaping of the grid-connected converter.

[0033] (3) The present invention introduces an impedance reshaping link to establish a small signal sequence impedance model of the grid-connected converter. In order to obtain the optimal sequence impedance of the grid-connected converter, the multi-harmonic disturbance signal is injected into the grid-connected inverter control system to obtain real-time and accurate grid impedance information. According to the information, the traditional impedance reshaping strategy is dynamically adjusted to realize adaptive impedance reshaping of the grid-connected converter, ensuring that the system still has a good stability margin when the grid impedance fluctuates over a wide range, thereby improving the adaptability of the grid-connected inverter under different grid working conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 A diagram of an equivalent circuit model of a battery provided in one embodiment of the present invention.

[0035] Figure 2 A battery operating characteristic curve diagram provided in one embodiment of the present invention.

[0036] Figure 3 A basic magnetic energy storage circuit diagram provided in one embodiment of the present invention.

[0037] Figure 4 A schematic diagram of virtual synchronous control based on superconducting magnetic energy storage-battery provided in one embodiment of the present invention.

[0038] Figure 5 A schematic diagram of a voltage-controlled virtual synchronous control strategy provided in one embodiment of the present invention.

[0039] Figure 6 A topological diagram of a grid-connected converter connected to a power grid provided in one embodiment of the present invention.

[0040] Figure 7 A schematic diagram of a grid-connected converter control strategy provided in one embodiment of the present invention.

[0041] Figure 8 A schematic diagram of an equivalent impedance model of a grid-connected inverter applying an improved impedance reshaping strategy provided in one embodiment of the present invention.

[0042] Figure 9 A schematic diagram of a control strategy for a grid-connected converter with an impedance reshaping step provided in one embodiment of the present invention.

[0043] Figure 10 A diagram showing the effect of suppressing harmonic oscillation using a conventional control method provided in one embodiment of the present invention.

[0044] Figure 11 An example of the present invention provides an effect diagram of the impedance reshaping control method proposed in the present invention in suppressing harmonic oscillations.

[0045] Figure 12 This is a flow chart of the impedance reshaping control method proposed in the present invention. DETAILED DESCRIPTION

[0046] The technical solution of the invention is described in detail below with reference to the accompanying drawings.

[0047] The present invention is based on a superconducting magnetic energy storage-battery hybrid energy storage system and combines constant voltage virtual synchronous control to propose an impedance reshaping method for suppressing subsynchronous oscillations of a virtual synchronous generator control system.

[0048] The present invention proposes an optimal impedance reconstruction method for suppressing subsynchronous oscillation of a virtual synchronous generator control system. Figure 12 As shown in FIG, there are mainly three steps. The specific implementation of each step is described below.

[0049] Step 1: Establish a battery charging and discharging model and a superconducting magnetic energy storage charging and discharging model, build a superconducting magnetic energy storage-battery hybrid energy storage system, and coordinate the active power output of the superconducting magnetic energy storage and the battery based on the active power calculated by the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator. Calculate the grid-connected reference current based on the coordinated control of the superconducting magnetic energy storage output current and the battery output current.

[0050] The present invention theoretically models superconducting magnetic energy storage and batteries, analyzes their working characteristics, and establishes a hybrid energy storage system model, laying the foundation for establishing a voltage-controlled virtual synchronous generator control system model and calculating the sequence impedance of the hybrid energy storage system.

[0051] Step 1-1: First, theoretical modeling is performed for the battery.

[0052] Figure 1 It is the most commonly used equivalent circuit model for lead-acid batteries. is an ideal voltage source, is the capacitance between the two plates, is the DC side capacitance of the system, is the nonlinear contact resistance between the battery plates and the electrolyte, Capacitor The voltage across the terminals, The resistance that causes loss.

[0053] (1)

[0054] In formula (1), To reduce the risk of overcharge and overdischarge of the battery and extend the battery life, it is necessary to limit the remaining power of the battery to a certain range. It can be expressed as:

[0055] (2)

[0056] In formula (2), is the initial value of the state of charge, is the battery capacity, is the battery output current, i.e., the charge and discharge current, and t is the charge and discharge time. When the battery is discharging:

[0057] (3)

[0058] In formula (3), It is the battery capacity after the charging and discharging process. The initial charge of the battery.

[0059] When the battery is charging:

[0060] (4)

[0061] Overvoltage resistance and capacitors The relationship between the current I1 and the voltage on the resistor is:

[0062] (5)

[0063] In formula (5), , and are the internal resistance and capacitance of the battery, for The voltage at both ends, from which we can get the equivalent mathematical model of the battery:

[0064] (6)

[0065] In formula (6), + and - indicate whether the battery is in the charging or discharging state.

[0066] Step 1-2: Analyze the working characteristics of the battery based on the established battery theoretical model.

[0067] according to Figure 2 It can be seen that the battery discharge process can be divided into the following three stages:

[0068] Initial discharge stage: During the initial discharge, a chemical reaction occurs at the positive and negative electrodes of the battery, and the battery terminal voltage drops rapidly.

[0069] Relatively stable stage: The battery terminal voltage decreases relatively steadily over a long period of time. At this time, the electrolyte filled and consumed in the gap space of the plate is basically balanced, and the battery discharge enters a relatively stable stage.

[0070] Rapid Decline Stage: After a relatively long period of relatively stable discharge, the battery terminal voltage drops rapidly. At this time, the electrolyte in the electrolyte has been largely consumed, and the electrolyte in the plates has not been replenished in time. As the discharge process continues, the battery terminal voltage drops rapidly.

[0071] The battery charging process can be divided into the following three stages:

[0072] Initial charging stage: At the beginning of charging, a chemical reaction occurs between the positive and negative electrodes of the battery, and the battery terminal voltage rises rapidly.

[0073] Relatively stable charging stage: The amount of electrolyte replenished and consumed in the pores of the plates is basically balanced, and the battery charging enters a relatively stable stage. At this time, the terminal voltage of the battery rises slowly.

[0074] Rapid Rise Stage: During the final stage of charging, the cathode material has been completely reduced. The excess charging current electrolyzes water, causing oxygen to escape from the electrolyte near the positive plate and hydrogen to escape from the electrolyte near the negative plate, lowering the electrolyte level. Therefore, lead-acid batteries require regular addition of distilled water. If charging continues, the battery terminal voltage will no longer increase because the electrolyte is essentially saturated.

[0075] Steps 1-3, according to Figure 3 The basic magnetic energy storage circuit of superconducting magnetic energy storage is shown, and its working characteristics are analyzed.

[0076] In the charging state, switch S is closed, K1 is closed, and K2 is closed. Assuming that the initial current flowing through the superconducting magnetic energy storage inductor is , then the voltage equation of the charging circuit is:

[0077] (7)

[0078] In formula (7), is the inductor inductance of superconducting magnetic energy storage, is the grid-side voltage, is the resistance of the superconducting magnetic energy storage inductor, t is the time, is the current flowing through the superconducting magnetic energy storage; the charging current at any time can be expressed as:

[0079] (8)

[0080] In formula (8), is the initial current flowing through the superconducting magnetic energy storage inductor. For a superconducting inductor with zero immunity, the charging current can be expressed as:

[0081] (9)

[0082] Initial current flowing through the superconducting magnetic energy storage inductor When is zero, formula (9) can be simplified to:

[0083] (10)

[0084] Superconducting magnetic energy storage inductor The voltage across the two terminals can be expressed as:

[0085] (11)

[0086] Superconducting magnetic energy storage inductor Energy consumed It can be expressed as:

[0087] (12)

[0088] In the inductive energy storage state, switch S is open, K1 is open, and K2 is closed; the storage circuit will operate in a zero-input state response, and the voltage equation of the energy storage circuit is:

[0089] (13)

[0090] (14)

[0091] In a superconducting magnetic energy storage inductor with zero resistance effect, the stored current And the stored energy can remain unchanged.

[0092] The controlled discharge mode of an inductor is a complex operating state formed by the energy storage state and the uncontrolled discharge state. According to the law of conservation of energy, the energy equation of the controlled discharge circuit should be:

[0093] (15)

[0094] In formula (15), Q SMES (t) is the energy stored in the superconducting magnetic energy storage inductor at time t, is the discharge power of the superconducting magnetic energy storage inductor at time t, then the residual current in the superconducting magnetic energy storage inductor at time t is for:

[0095] (16)

[0096] For superconducting inductors, the current when the circuit operates in constant power discharge mode is the superconducting magnetic energy storage output current. can be simplified to:

[0097] (17)

[0098] Steps 1-4: Build a hybrid energy storage system and perform theoretical modeling.

[0099] like Figure 4 As shown, the DC side of an H-bridge DC chopper is connected to the DC side of a bidirectional DC chopper to form a grid-connected converter. Based on the operating characteristics and theoretical models of superconducting magnetic energy storage and batteries, the present invention uses an H-bridge DC chopper and a bidirectional DC chopper to connect the superconducting magnetic energy storage and battery units, respectively. The superconducting magnetic energy storage inductor is connected between the midpoints of the bridge arms of the H-bridge DC chopper. The battery is connected to the DC bus formed by connecting the DC side of the H-bridge DC chopper and the DC side of the bidirectional DC chopper after passing through a DC converter, thereby designing a superconducting magnetic energy storage-battery hybrid energy storage system.

[0100] Then, if Figure 5 As shown in the figure, the hybrid energy storage system of superconducting magnetic energy storage and battery is analyzed. The active power controller of the grid-connected converter control system under the control of the virtual synchronous generator simulates the inertia and primary frequency regulation characteristics of the synchronous generator, and the reactive power controller simulates the primary voltage regulation characteristics of the synchronous generator. Therefore, the mathematical equations of the active power controller of the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator can be expressed as Equations (18) to (21), and the mathematical equation of the reactive power controller can be expressed as Equation (22):

[0101] (18)

[0102] (19)

[0103] (20)

[0104] (twenty one)

[0105] (twenty two)

[0106] In equations (18) to (22), J is the virtual moment of inertia; are the output angular frequency of the voltage-controlled virtual synchronous generator control system and the rated angular frequency of the power grid respectively; are torque reference and electromagnetic torque respectively; are the damping coefficient and voltage drop coefficient respectively; They are active power reference and reactive power reference respectively; are instantaneous active power and reactive power respectively; is the phase angle of the potential in the voltage-controlled virtual synchronous generator control system; K is the inertia coefficient of the reactive circuit; is the RMS value of the rated voltage, is the RMS value of the output voltage of the voltage-controlled virtual synchronous generator control system, is the RMS value of the internal potential of the voltage-controlled virtual synchronous generator control system.

[0107] According to the instantaneous power theory, the active power calculated by the grid-connected converter control system under the voltage-controlled virtual synchronous generator is and reactive power for:

[0108] (twenty three)

[0109] In formula (23), For voltage controlled virtual synchronous generator control system Output current in the reference frame; For voltage controlled virtual synchronous generator control system The output voltage in the reference frame.

[0110] The active power calculated by the grid-connected converter control system under the voltage-controlled virtual synchronous generator is , which can coordinate the output power of batteries and SMES in the hybrid energy storage system, including: coordinated control in power absorption mode and coordinated control in power release mode.

[0111] The specific coordinated control strategy in the power absorption mode is: when the hybrid energy storage system needs to absorb power, the SMES in the hybrid energy storage system reacts faster, so the SMES acts first and works in the charging state. If the battery is lower than 90%, it is in charging state. If the remaining power of SMES is higher than 90%, it is close to saturation and cannot be charged. At this time, it is necessary to check the charge of the battery. ;like If it is lower than 90%, the battery will work in the charging state.

[0112] The specific coordinated control strategy in the power release mode is: when the hybrid energy storage system needs to release power, the SMES will also act first. If the value is higher than 10%, SMES is in discharge state; If it is lower than 10%, the remaining power is too low to meet the discharge conditions, and the battery charge needs to be tested. ,if If it is higher than 10%, the battery is in a discharged state.

[0113] Based on the coordinated control strategy of SMES and energy storage battery in hybrid energy storage system, the active power calculated by grid-connected converter control system under voltage-controlled virtual synchronous generator can be Allocated to superconducting magnetic energy storage and batteries, the active power allocated to superconducting magnetic energy storage is recorded as P e-SMES , the active power allocated to the battery is recorded as P e-b .

[0114] In the absorption power mode, the superconducting magnetic energy storage charging current shown in formula (9) is used as the superconducting magnetic energy storage output current, and the battery output current is calculated by combining the active power allocated to the battery and the battery charging and discharging model shown in formula (6).

[0115] In the power release mode, e-SMES Assigning the value to P1(t) in formula (17) yields the SMES output current , according to P e-b Calculate the battery capacity after the charge and discharge process , the calculated Substitute into formula (6) to get the battery output current , and then get the grid reference current for:

[0116] (twenty four)

[0117] In formula (24), is the voltage at the common connection point PCC.

[0118] Step 2: Establish a small signal model of the grid-connected inverter control system based on voltage-controlled virtual synchronous control according to the grid-connected reference current, and calculate the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control.

[0119] The present invention performs small signal modeling on the grid-connected converter control system under the control of a voltage-controlled virtual synchronous generator, and reveals the influence of small signal disturbances near the power frequency signal on the low-frequency impedance of the grid-connected converter control system under the control of the virtual synchronous generator.

[0120] Step 2-1: First, it is necessary to analyze the topology of the grid-connected converter control system under the voltage-controlled virtual synchronous generator. Figure 4 The topology of the grid-connected converter control system under virtual synchronous generator control can be obtained.

[0121] Figure 4 、 Figure 6 middle, is the internal potential of the virtual synchronous generator control system; is the output current of the virtual synchronous generator control system; is the output voltage of the virtual synchronous generator control system; They are filter inductor, filter capacitor and damping resistor respectively; are the equivalent line inductance and equivalent resistance of the power grid respectively; is the grid voltage; is the grid-connected current, u dc is the DC bus voltage.

[0122] Step 2-2: The present invention derives the impedance model of the grid-connected converter under voltage-controlled virtual synchronous control based on the control logic of the grid-connected converter. Figure 7 , we can get the mathematical model of the grid-connected inverter control system under the control of the virtual synchronous generator. is the filter capacitor Current, i g (s) is the grid-connected current; are the filter capacitor current feedback coefficient and the grid current feedback coefficient respectively; For the current controller.

[0123] Depend on Figure 7 Equivalent impedance of grid-connected converter under virtual synchronous control obtained by control architecture for:

[0124] (25)

[0125] In formula (25), is the transfer coefficient from the modulation wave to the output voltage of the bridge arm of the grid-connected converter, is the system sampling period, is the proportional gain; is the resonant gain; is the resonant angular frequency; is the bandwidth at the resonant angular frequency.

[0126] Step 3: Identify grid impedance based on multi-harmonic injection and reshape the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control

[0127] Based on the sequential impedance characteristics of the grid-connected inverter control system under the control of the voltage-controlled virtual synchronous generator, the present invention takes into account the influence of system harmonic oscillation and introduces an impedance reshaping link. This improved impedance reshaping strategy adds an impedance reshaping link to the voltage feedforward control and the grid-connected current feedback control. The system compensates for the phase margin in specific frequency bands of the grid-connected converter control system, reshaping the equivalent output impedance of the grid-connected converter. When grid operating conditions change, the system extracts and calculates accurate grid impedance information in real time through multi-harmonic injection. The system then adaptively adjusts the phase advance parameters based on the compensation angle required by the grid-connected converter control system. This ensures that the grid-connected converter control system maintains a good phase margin even under varying grid impedance conditions, while also guaranteeing the system's dynamic performance.

[0128] Step 3-1, such as Figure 8 As shown, the principle of harmonic injection method is to inject harmonics into the grid-connected converter control system. The harmonic current of a specific frequency is injected into the current reference of the current loop, that is, the grid reference current. After the injection is completed, the voltage and current responses at the PCC point are obtained by using the detection element, and the voltage and current components at the PCC point are obtained after Fourier DFT / FFT analysis of the detection results of the voltage and current responses at the PCC point. Then, the grid impedance value can be calculated. Figure 8 middle, is the grid voltage.

[0129] This paper proposes a multi-harmonic injection method to identify grid impedance. This method superimposes two harmonic current frequencies onto the command current. By detecting the response components of the voltage and current amplitudes at the PCC point, the grid impedance can be calculated using a two-point method, eliminating the need to identify the phase angle. This method not only improves the accuracy of grid impedance calculations but also significantly reduces computational complexity.

[0130] The calculation process of identifying the grid impedance using the multi-harmonic injection method is as follows:

[0131] Step 3-1-1, pass Figure 8 After injecting multiple harmonics in the manner shown and performing DFT on the detection results, the injection frequency is The impedance modulus can be obtained by formula (26):

[0132] (26)

[0133] Step 3-1-2, multiple harmonic frequencies The relationship between the grid impedance modulus value converted into resistance and inductance is:

[0134] (27)

[0135] In formula (27), is the angular frequency of the injected harmonic, For The grid impedance modulus calculated at .

[0136] Step 3-1-3: The calculation formula for the resistive and inductive components in the grid impedance can be obtained as follows:

[0137] (28)

[0138] (29)

[0139] (30)

[0140] In formula (30), Frequency The grid impedance under .

[0141] Step 3-2-1: To ensure that the virtual synchronous control system maintains its performance and stable operation when the grid impedance varies over a wide range, after completing multi-harmonic injection to identify the grid impedance, the grid-connected converter control strategy under voltage-type virtual synchronous control is based on which an impedance reshaping step is added to the voltage feedforward control and grid current feedback control. The improved mathematical model of the grid-connected inverter control system is as follows: Figure 9 shown.

[0142] Impedance reshaping link of voltage feedforward control The expression is:

[0143] (31)

[0144] In formula (31), Impedance reshaping link for voltage feedforward control The gain coefficient, are the first and second phase compensation coefficients.

[0145] Impedance reshaping link of grid-connected current feedback control The expression is:

[0146] (32)

[0147] In formula (32), Impedance reshaping link for grid-connected current feedback control The gain coefficient, is the third phase compensation coefficient.

[0148] The equivalent impedance of the reconstructed grid-connected converter is:

[0149] (33)

[0150] Step 3-2-2: In order to enable the impedance reshaping link to make real-time adjustments to the changing grid impedance, the present invention designs the gain compensation parameters of this link based on the impedance method as follows:

[0151] Phase-frequency characteristics for:

[0152] (34)

[0153] if , then the maximum compensation phase frequency for:

[0154] (35)

[0155] Maximum compensation phase for:

[0156] (36)

[0157] Set the compensation link in The amplitude at is 1, that is The amplitude is 1, from which the gain compensation coefficient can be calculated for:

[0158] (37)

[0159] Phase-frequency characteristics for:

[0160] (38)

[0161] if , then the maximum compensation phase frequency for:

[0162] (39)

[0163] Set the compensation link in The amplitude at is 1, that is The amplitude of is 1, so we can calculate Gain compensation coefficient for:

[0164] (40)

[0165] In summary, the steps for designing the parameters of the impedance reshaping part are:

[0166] According to the grid resistance at all injected harmonic frequencies and the reconstructed grid-connected converter equivalent impedance The cutoff frequency Select the maximum compensation phase frequency , to achieve dynamic compensation of phase, where Obtained through the grid impedance identification link.

[0167] Maximum compensation phase The phase required for compensation should be selected based on the actual compensation phase of the grid-connected converter control system. Assuming that the original phase margin of the system is PM and the ideal phase margin after compensation is PM', the phase required for compensation of the system should be , so that the phase margin of the system is always kept good.

[0168] The designed and Substitute into equations (35) and (36) to determine the phase compensation coefficient of the impedance reshaping link of the voltage feedforward control The value of Substituting into formula (37) we can get the gain compensation coefficient .

[0169] The designed Substitute into Equation (39) to determine the phase compensation coefficient of the impedance reshaping link of the grid-connected current feedback control The value of Substituting into formula (40) we can get the gain compensation coefficient .

[0170] like Figure 10 As shown in Figure 2, when the system is connected to a weak grid, the traditional grid voltage proportional feedforward strategy can no longer effectively suppress harmonics, the grid current waveform is distorted, and the system THD increases to 8.2%. Figure 11 As shown in Figure 2, the improved impedance reshaping strategy effectively suppresses the system harmonics under the same operating conditions, and the system THD is 2.91%. This shows that the improved impedance reshaping strategy proposed in the present invention is feasible and effective.

[0171] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the specific embodiments described above. The specific embodiments and descriptions in the specification are merely intended to further illustrate the principles and preparation effects of the present invention. Various changes and improvements may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and improvements fall within the scope of the present invention. The scope of the present invention is defined by the claims and their equivalents.

Claims

1. An impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system, characterized in that: include: Step 1: Establish a battery charge and discharge model and a superconducting magnetic energy storage charge and discharge model, build a superconducting magnetic energy storage-battery hybrid energy storage system, and coordinate the active power output of the superconducting magnetic energy storage and the battery based on the active power calculated by the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator. Calculate the grid-connected reference current based on the coordinated superconducting magnetic energy storage output current and the battery output current; Step 2: Establish a small signal model of the grid-connected inverter control system based on voltage-controlled virtual synchronous control according to the grid-connected reference current, and calculate the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control; Step 3: Identify the grid impedance based on multi-harmonic injection and reshape the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control.

2. The impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system according to claim 1, characterized in that: The battery charge and discharge model established in step 1 is: ,in, is the battery capacity at time t after the charge and discharge process, is the battery output current, I1 is the capacitance between the two plates The current on , It is the overvoltage resistor, and + and - indicate whether the battery is in the charging or discharging state.

3. The impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system according to claim 2, characterized in that: The superconducting magnetic energy storage charging and discharging model established in step 1 includes a superconducting magnetic energy storage charging model and superconducting magnetic energy storage discharge model ,in, is the superconducting magnetic energy storage charging current at time t, is the grid-side voltage, is the output current of the superconducting magnetic energy storage at time t, is the initial current flowing through the superconducting magnetic energy storage inductor, is the inductance of the superconducting magnetic energy storage inductor, is the discharge power of the superconducting magnetic energy storage inductor at time t.

4. The impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system according to claim 3, characterized in that: The step 1 is to build a hybrid energy storage system of superconducting magnetic energy storage and battery, specifically: the DC side of the H-bridge DC chopper is connected to the DC side of the bidirectional DC chopper to form a grid-connected converter, the superconducting magnetic energy storage inductor is connected between the midpoints of the bridge arms of the H-bridge DC chopper, and the battery is connected to the DC bus formed by connecting the DC side of the H-bridge DC chopper and the DC side of the bidirectional DC chopper after passing through a DC converter.

5. The impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system according to claim 4, characterized in that: The step 1 coordinates and controls the active power output by the superconducting magnetic energy storage and the battery based on the active power calculated by the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator, specifically: When the hybrid energy storage system is in the power absorption mode, the superconducting magnetic energy storage is controlled to operate in a charging state, the battery charge is detected when the superconducting magnetic energy storage charge is higher than the maximum charge in the charging mode, and the battery is controlled to operate in a charging state when the battery charge has not reached the maximum charge in the charging mode, the active power calculated by the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator is distributed to the superconducting magnetic energy storage and the battery, the superconducting magnetic energy storage charging current is used as the superconducting magnetic energy storage output current, the battery output current is calculated in combination with the active power distributed to the battery and the battery charge and discharge model, and the cumulative sum of the superconducting magnetic energy storage output current and the battery output current is used as the grid-connected reference current; When the hybrid energy storage system is in a power release mode, the superconducting magnetic energy storage is controlled to operate in a discharge state. When the battery charge is lower than the minimum charge in the discharge mode, the battery charge is detected. When the battery charge is higher than the minimum charge in the discharge mode, the battery is controlled to operate in a discharge state. The active power calculated by the grid-connected converter control system under the control of the voltage-controlled virtual synchronous generator is allocated to the superconducting magnetic energy storage and the battery. The superconducting magnetic energy storage output current is calculated based on the active power allocated to the superconducting magnetic energy storage and the superconducting magnetic energy storage discharge model. The battery output current is calculated based on the active power allocated to the battery and the battery charge and discharge model. The cumulative sum of the superconducting magnetic energy storage output current and the battery output current is used as the grid-connected reference current.

6. The impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system according to claim 5, characterized in that: The equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control calculated in step 2 is: ,in, is the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control, is the transfer function of the current controller, is the transfer coefficient from the modulation wave to the output voltage of the bridge arm of the grid-connected converter, is the system sampling period, is the proportional gain, is the resonant gain, is the resonant angular frequency, is the bandwidth at the resonant angular frequency, are the filter capacitor current feedback coefficient and the grid current feedback coefficient, L f 、L g They are the filter inductance and the equivalent line inductance of the power grid respectively.

7. The impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system according to claim 6, characterized in that: The specific method of step 3 for identifying the grid impedance based on multi-harmonic injection is: Step 3-1-1: Inject a series of harmonic currents of specific frequencies into the grid-connected reference current, and detect the voltage response and current response at the common connection point after the injection of harmonics of different specific frequencies. Perform Fourier analysis on the voltage response and current response at the common connection point after the injection of harmonics of different specific frequencies to obtain the voltage component and current component corresponding to each specific frequency, and calculate the grid impedance modulus at each injected harmonic frequency. , n is the number of injected harmonics; Step 3-1-2, convert the grid impedance modulus at each injected harmonic frequency into a relationship between resistance and inductance; Step 3-1-3, obtain the resistive component of the grid impedance and emotional components , , where R g 、L g are the resistive and inductive components in the grid impedance, is the i-th injected harmonic frequency The grid impedance under 、 are the angular frequencies of the i-th and i+1-th injected harmonics respectively.

8. The impedance reshaping method for suppressing harmonic oscillation based on a superconducting magnetic energy storage system according to claim 7, characterized in that: The specific method of step 3 to reshape the equivalent impedance of the grid-connected converter under voltage-controlled virtual synchronous control is: Step 3-2-1, add the first impedance reshaping link to the voltage feedforward control and grid current feedback control of the small signal model of the grid-connected inverter control system based on voltage-controlled virtual synchronous control established in step 2 2. Second impedance reshaping link , , , the equivalent impedance of the grid-connected converter after reconstruction for ,in, Reshaping the first impedance The gain coefficient, are the first and second phase compensation coefficients, Reshape the link for the second impedance The gain coefficient, is the third phase compensation coefficient; Step 3-2-2, reshape the link according to the first and second impedances Phase-frequency characteristics determine the gain coefficients of the first and second impedance reshaping links 、 : 、 The maximum compensation phase frequency and maximum compensation phase of the first impedance reshaping link are: , , , 、 The maximum compensation phase frequency and maximum compensation phase of the second impedance reshaping link are: , , , Determine the intersection frequency of the grid impedance at all injected harmonic frequencies and the reconstructed grid-connected converter equivalent impedance 、 The parameter value is determined according to the actual phase compensation requirements. The parameter value after setting the parameter 、 Substitute the maximum compensation phase frequency and maximum compensation phase of the first impedance reshaping link to calculate k1, k2, , after setting the parameters Substitute the maximum compensation phase of the second impedance reshaping link and calculate 、 .

9. An electronic device comprising a memory and a processor, wherein the memory stores a computer program for running on the processor, and the processor executes the steps of the impedance reshaping method according to claim 1 when running the computer program.

10. A computer-readable storage medium having a computer program stored thereon, wherein the computer program executes the steps of the impedance reshaping method according to claim 1 when the computer program is executed.

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