Network-formation type hybrid energy storage power optimization control method based on response priority

Abstract

The application discloses a network construction type hybrid energy storage power optimization control method based on response priority, relates to the field of hybrid energy storage power optimization control and comprises the following steps: the application iteratively calculates the grid impedance by injecting non-characteristic sub-harmonic sequence components, judges the strength of the grid, generates a response priority power limiting coefficient, and generates a power distribution instruction after limiting, which is distributed to a lithium battery converter and a super capacitor converter; a continuous moving track of the grid impedance in a complex plane is constructed in a continuous control period, and when the track intersects with a boundary line, the crossing effectiveness is confirmed; after detecting that the grid state crosses the switching boundary, the potential phase in the virtual synchronous generator is first adjusted to be consistent with the grid voltage phase, and after the target control mode is ready, the switching is executed, and at the switching effective moment, the power instruction value is latched to be forced to be kept until the target mode is stably operated; the dynamic matching of the converter power output and the grid carrying capacity is realized.

Description

Technical Field

[0001] This invention belongs to the field of hybrid energy storage power optimization control technology, and relates to a grid-type hybrid energy storage power optimization control method based on response priority. Background Technology

[0002] With the expansion of new energy grid connection, grid-connected converters are increasingly used in weak grids and islanded scenarios. Hybrid energy storage systems, through the combination of lithium batteries and supercapacitors, achieve complementarity in energy capacity and power response speed. In actual operation, the dynamic changes in grid conditions require converters to adaptively switch control modes according to grid impedance and ensure the smoothness of the switching process.

[0003] Existing technologies have proposed solutions for switching between grid-connected and grid-following control modes. For example, CN121461499A proposes a method for smooth switching between grid-connected and grid-following control modes of converters. First, the short-circuit ratio of the new energy grid-connected system is obtained to determine the current grid type. If it is determined that the current grid is a strong grid and the converter is operating in grid-connected control mode, the converter phase angle, converter current reference value, and active power reference command value are controlled respectively.

[0004] However, while the above scheme achieves grid connection and grid-following mode switching based on short-circuit ratio, it still has the following problems: 1. The above scheme uses short-circuit ratio for a one-time judgment. However, in actual operation, the grid impedance does not change abruptly, but rather exhibits a continuous and gradual shifting trend. The grid state may fluctuate near the boundary between a strong grid and a weak grid. If the judgment is based solely on the short-circuit ratio at a single moment, frequent switching is likely to occur when the grid impedance fluctuates near the threshold, causing the converter to repeatedly switch between the two control modes, increasing the risk of system oscillation.

[0005] 2. Before switching, perform special phase alignment preparation and power preservation mechanism during the switching process. The output voltage phase may be different at the moment of switching. If switching is performed directly when the phase is inconsistent, an instantaneous inrush current will be generated, which may damage the power device or trigger the grid protection action, increasing the risk of switching failure. Summary of the Invention

[0006] In view of this, in order to solve the problems mentioned in the background art, the present invention provides a power optimization control method for grid-type hybrid energy storage based on response priority.

[0007] The objective of this invention can be achieved through the following technical solution: a power optimization control method for grid-connected hybrid energy storage based on response priority, comprising: S1, detecting the voltage and current at the grid connection point, iteratively calculating the grid impedance by injecting non-characteristic subharmonic sequence components, determining whether the current grid is a strong grid or a weak grid based on the grid impedance, and generating a response priority power limiting coefficient.

[0008] S2. The total power demand of the hybrid energy storage system is limited by a power limiting factor, and a power allocation command after limiting is generated and allocated to the lithium battery converter and the supercapacitor converter.

[0009] S3. Monitor whether the grid status crosses the switching boundary. If it crosses from a strong grid to a weak grid, or from a weak grid to a strong grid, the mode switching preparation process is triggered: the current converter output voltage phase is collected, the phase of the internal potential of the virtual synchronous generator is continuously adjusted until it is consistent with the grid voltage phase, and the phase alignment flag is output.

[0010] S4. Based on the phase alignment flag, during the soft switching transition, the converter control architecture is gradually transitioned from the current mode to the target mode, and the power allocation command before the switch is used as the execution command for the first cycle after the switch.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention calculates the grid impedance by injecting non-characteristic subharmonic sequence components, judges the strength of the grid, generates response priority power limiting coefficient, and generates power allocation command after limiting, which is allocated to lithium battery converter and supercapacitor converter. This overcomes the problem that the power limiting coefficient automatically decreases under weak grid conditions, compresses the total power command proportionally, and ensures that the actual power injected into the grid does not exceed the safe range that the current grid can withstand, thus avoiding the voltage limit of the grid connection point or system oscillation caused by excessive power injection.

[0012] (2) This invention constructs a continuous movement trajectory of the grid impedance in the complex plane within a continuous control cycle. When the trajectory intersects with the boundary line, the validity of the crossing is confirmed. This overcomes the defects caused by making a one-time judgment based on the short-circuit ratio at a single moment. This makes the judgment of the grid state no longer dependent on isolated single-point measurement values, avoids misjudgment caused by measurement noise, and also avoids the risk of system oscillation caused by the converter repeatedly switching between two control modes.

[0013] (3) After the present invention detects that the grid state crosses the switching boundary, it first adjusts the phase of the internal potential of the virtual synchronous generator to make it consistent with the phase of the grid voltage. After the target control mode is ready, the switching is performed. At the same time, the power command value is latched at the moment of switching and is forced to be maintained until the target mode is stable. This overcomes the phase alignment before switching, eliminates the phase difference at the moment of switching, avoids the generation of inrush current, and prevents damage to power devices or grid protection action. In addition, the power command is forced to be maintained during the switching process to avoid instantaneous power jump caused by control parameter switching and filtering state, and realizes smooth and shock-free switching of control mode. Attached Figure Description

[0014] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 This is a diagram illustrating the implementation steps of the method of the present invention;

[0016] Figure 2 This is the process for generating the response priority power limiting coefficient of the present invention;

[0017] Figure 3 This is the process for verifying the effectiveness of the energy storage impedance trajectory crossing in this invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Please see Figure 1 As shown, the present invention provides a power optimization control method for grid-connected hybrid energy storage based on response priority, including: S1, detecting the voltage and current at the grid connection point, iteratively calculating the grid impedance by injecting non-characteristic subharmonic sequence components, determining whether the current grid is a strong grid or a weak grid based on the grid impedance, and generating a response priority power limiting coefficient.

[0020] The method for obtaining the voltage and current at the grid connection point is as follows: voltage transformers and current transformers are installed at the grid connection point to collect analog signals of three-phase voltage and three-phase current in real time, and the analog signals are converted into digital signals through an analog-to-digital converter.

[0021] Since grid impedance cannot be directly measured, it must be inferred by applying a known disturbance and observing the response. Based on this, the iterative calculation of grid impedance by injecting non-characteristic subharmonic sequence components includes: selecting a non-characteristic subharmonic that does not overlap with the grid characteristic harmonic frequency as the injection frequency, and superimposing the positive sequence current disturbance of the injection frequency in the current control loop of the hybrid energy storage converter.

[0022] Non-characteristic harmonics refer to harmonic components of orders other than the characteristic harmonics. They arise because the actual operating conditions of the power electronic converter deviate from ideal conditions, such as three-phase voltage imbalance, unequal AC impedances of each phase, asymmetrical trigger pulses, or control angle errors. For example, 175Hz corresponds to the 3.5th harmonic, and 325Hz corresponds to the 6.5th harmonic; both are non-characteristic harmonic frequencies.

[0023] Under normal operating conditions, the power grid contains abundant characteristic subharmonics. If a frequency that is the same as or similar to these characteristic harmonics is selected as the injection signal, the frequency voltage and current responses collected from the grid connection point will simultaneously contain both the injection signal component and the grid background harmonic components. These two components overlap in the frequency domain and cannot be effectively separated, resulting in impedance calculations that deviate significantly from the true values. Choosing a non-characteristic subfrequency as the injection frequency, however, allows for the extraction of a response signal that originates almost entirely from the converter's active injection, since the harmonic content in the grid background is extremely low.

[0024] The specific implementation process of superimposing the positive sequence current disturbance of the injection frequency in the current control loop of the hybrid energy storage converter is as follows: First, in the digital signal processor, the three-phase positive sequence current disturbance command value is generated in real time according to the injection frequency. The command value is a three-phase symmetrical sine wave with an amplitude of 3% to 5% of the rated current of the converter and the three phases are sequentially different by 120 electrical degrees.

[0025] Secondly, the generated three-phase positive sequence current disturbance command values ​​are superimposed on the original three-phase fundamental current setpoints in the inner current loop, and the sum is used to obtain a comprehensive current command value that includes the fundamental component and the injected disturbance component.

[0026] Then, the superimposed comprehensive current command value is input to the current regulator, which uses a proportional resonant controller, and the center frequency of the resonant controller is set to the injection frequency.

[0027] Finally, the output of the current regulator generates a switching signal through the pulse width modulation stage, which drives the power devices of the converter to operate, so that the converter outputs a three-phase current superimposed with the positive sequence current component of the injected frequency to the grid connection point.

[0028] The three-phase voltage and three-phase current at the grid connection point are collected, and the three-phase voltage phasors and three-phase current phasors at the injection frequency are extracted by bandpass filtering. The positive sequence voltage and positive sequence current are obtained by symmetrical component transformation.

[0029] The implementation steps for extracting the three-phase voltage phasors and three-phase current phasors at the injection frequency using bandpass filtering are as follows: After acquiring the digital signals of the three-phase voltage and three-phase current at the grid connection point, the voltage and current signals of each phase are first input into a bandpass filter with the center frequency set as the injection frequency. The fundamental frequency component, DC component, harmonic components, and high-frequency switching noise are filtered out, leaving only the narrowband signal near the injection frequency, thus obtaining the filtered signals of the three-phase voltage and three-phase current at the injection frequency. Then, a discrete Fourier transform is performed on the filtered signals of each phase to extract the spectral components at the injection frequency, and the three-phase voltage phasors and three-phase current phasors at the injection frequency are calculated.

[0030] The specific steps for performing symmetrical component transformation are as follows: Obtain the three-phase voltage phasors at the injection frequency, denoted as follows: , , The three-phase current phasors are denoted as follows: , , Then, perform the following operations: First, define the symmetric component transformation matrix T, which has the following specific form: .

[0031] Where α is the rotation factor, and its value is... ,Right now , .

[0032] Secondly, the three-phase voltage phasors , , Arranged into column matrix Transform the three-phase current phasors , , Arranged into column matrix .

[0033] Then, multiply the transformation matrix T on the left by the two column matrices mentioned above, that is: , .

[0034] After the matrix multiplication operation described above, the first row of elements in the result and These are the positive-sequence voltage phasors and positive-sequence current phasors, the elements in the second row. and For negative order components, the third row element and It is the zero-order component.

[0035] The positive sequence voltage and positive sequence current were measured before and after the disturbance injection, and the difference between the two measurements was used as the voltage increment and current increment caused by the disturbance.

[0036] Phasor subtraction can cancel out the fundamental steady-state component and background harmonic component that coexist before and after the disturbance, so that only the voltage increment and current increment caused by the injected disturbance are retained in the difference, thereby isolating the influence of the grid background signal on the response to be measured.

[0037] Calculate the ratio of voltage increment to current increment to obtain the positive sequence impedance of the power grid at the injection frequency.

[0038] The ratio of voltage increment to current increment follows the frequency domain form of Ohm's law, that is, the equivalent impedance of the power grid at the injection frequency is equal to the voltage response phasor divided by the current excitation phasor at that frequency. Since both voltage and current increments originate from the response to the same injected disturbance, the ratio between them represents the inherent impedance characteristic of the power grid at the corresponding frequency. Furthermore, this ratio is unaffected by the fundamental operating state of the converter or the background harmonics of the power grid, and thus reflects the positive-sequence impedance value of the power grid at that frequency.

[0039] The convergent grid impedance is obtained by taking the average value of multiple measurements of the positive sequence impedance of the grid.

[0040] Random errors are inevitably introduced in a single measurement. By taking the average of multiple measurements, positive and negative errors can be canceled out, and the calculation result can be gradually brought closer to the true value, thereby suppressing the impact of random interference on the accuracy of impedance identification.

[0041] Considering that when the grid impedance is small, the grid exhibits low-impedance voltage source characteristics, and when the grid impedance is large, the grid exhibits high-impedance characteristics, a strong grid can allow for a rapid response to power commands, while a weak grid must limit the rate of power change to avoid voltage exceeding the limit at the grid connection point or power angle instability caused by power fluctuations.

[0042] Based on this, determining whether the current power grid is a strong or weak grid according to the grid impedance includes: obtaining the effective value of the rated output voltage and the rated output capacity of the energy storage converter, and calculating the reference impedance modulus of the converter.

[0043] The steps to obtain the rated output voltage RMS value and rated output capacity of the energy storage converter are as follows: read the rated data marked on the equipment nameplate parameters or product technical specifications of the energy storage converter, and obtain the rated output voltage RMS value and rated output capacity from them.

[0044] The formula for calculating the magnitude of the reference impedance is as follows: Among them, U rated S is the effective value of the rated voltage. rated Z is the rated output capacity. base This is the reference impedance magnitude.

[0045] Divide the reference impedance magnitude by the grid impedance magnitude to obtain the grid short-circuit ratio.

[0046] The strength of a power grid is not determined solely by the absolute value of its impedance, but rather by the relative characteristics of that impedance with respect to the capacity of the energy storage converter. The same power grid may exhibit a strong performance for a low-power converter and a weak performance for a high-power converter.

[0047] In the standard definition of power systems, the short-circuit ratio is the ratio of the grid's short-circuit capacity to the converter's rated capacity. A larger ratio indicates a stronger grid relative to the converter, meaning the grid connection voltage is less susceptible to fluctuations in the converter's power output, classifying it as a strong grid. Conversely, a smaller ratio indicates a weaker grid, where changes in the converter's power output can easily cause significant voltage fluctuations at the grid connection point, classifying it as a weak grid.

[0048] If the short-circuit ratio of the power grid is greater than the threshold for determining whether it is a strong or weak power grid, it is determined to be a strong power grid; otherwise, it is determined to be a weak power grid.

[0049] The threshold for determining strong and weak power grids is obtained through the following steps: During the long-term operation of the energy storage converter, the grid short-circuit ratio calculated iteratively in each operating interval is recorded periodically, and whether overvoltage oscillation alarm or instability protection action occurs in that interval is also recorded.

[0050] The maximum short-circuit ratio corresponding to the section where an alarm or protection action has occurred is marked as the instability boundary value of the section.

[0051] Statistical analysis was performed on the instability boundary values ​​of multiple operating ranges. After removing outliers, the lower statistical limit was taken as the critical short-circuit ratio for system stability.

[0052] The critical short-circuit ratio for stability is multiplied by a safety factor to obtain the threshold for determining whether the power grid is strong or weak. The safety factor is directly set to a fixed value of 0.8, based on the principle of leaving a 20% stability margin.

[0053] Because the impedance is relatively large under weak grid conditions, power changes can affect the amplitude and phase of the voltage at the grid connection point, and may even cause power angle instability or oscillation. Therefore, the converter needs to generate a response priority power limiting coefficient based on the measured grid impedance in order to dynamically adjust the upper limit of power output under the current operating conditions and ensure stable system operation.

[0054] refer to Figure 2 As shown, the generation of response priority power limiting coefficients includes: real-time acquisition of the virtual power angle between the internal potential of the virtual synchronous generator and the phase of the grid connection point voltage.

[0055] The zero-crossing times of the output voltage of the virtual synchronous generator and the zero-crossing times of the grid connection point voltage are directly detected. The time difference between the two detected zero-crossing signals is divided by the time of one power frequency cycle, and then multiplied by the full-cycle phase angle to obtain the phase difference value corresponding to the time difference, which is the virtual power angle.

[0056] Calculate the rate of change of the virtual power angle within a preset time window. When the rate of change exceeds the power angle stabilization threshold, subtract the first adjustment step from the current power limiting coefficient.

[0057] The virtual power angle of a virtual synchronous generator is the phase difference between its internal electromotive force and the grid voltage. This phase difference determines the amount of active power transmitted between the converter and the grid. When the rate of change of the virtual power angle exceeds the stability threshold, it indicates that the system is rapidly losing synchronization. If operation continues according to the current power command, the power angle may continue to increase, exceeding the static stability limit and causing power angle instability. Subtracting the first adjustment step size from the power limiting coefficient actively reduces the upper limit of the converter's power output, suppressing further acceleration of the power angle increase by reducing the power command, and allowing the rate of change of the power angle to fall back to a safe range. Essentially, this step actively reduces the power response, providing sufficient damping time for the system to regain synchronization.

[0058] The preset time window adopts a sliding window method, and its length is an integer multiple of the power frequency cycle, usually 5 to 10 power frequency cycles.

[0059] There are two ways to determine the power angle stability threshold. First, if there is a relevant national standard to follow, multiply the limit of the frequency change rate of the grid connection point voltage in the national standard for grid-connected operation of converters by the phase change per unit time corresponding to the rated frequency of the grid connection point voltage to obtain the upper limit of the allowable power angle change rate. Then, multiply this upper limit by a safety margin coefficient of 0.7 to 0.9 to obtain the power angle stability threshold. If there is no clear standard to follow, during the on-site commissioning of the converter, an active power step disturbance is artificially applied, and the power angle response waveform is observed. The rate of change when the power angle begins to show an oscillating divergence trend is taken as the critical value, and 60% to 80% of the critical value is taken as the power angle stability threshold.

[0060] The first adjustment step size is set to 1% to 5% of the rated power of the converter. The specific principle is as follows: the greater the degree to which the measured power angle change rate exceeds the stability threshold, the larger the value of the first adjustment step size should be to quickly suppress the power angle instability trend; when the degree of exceedance is small, the value of the first adjustment step size should be small to avoid overshoot in the power command adjustment.

[0061] Once the rate of change falls below the power angle stability threshold and the duration threshold is reached, the current power limiting coefficient is increased by the second adjustment step.

[0062] When the rate of change of the power angle falls below the power angle stability threshold and remains below it for a certain period of time, it indicates that the system has returned to a synchronous and stable operating state. The power limiting coefficient, which was proactively reduced due to the risk of power angle instability, can be gradually restored. Adding the second adjustment step size to the current power limiting coefficient is equivalent to gradually replenishing the upper limit of power output after confirming that the system has escaped the risk of instability, so that the power response capability of the converter gradually returns to a normal level. In essence, this step is: after the system's synchronization capability is restored, a gradual power increase strategy, the opposite of proactive power reduction, is adopted to allow the power limiting coefficient to return to its normal value asymptotically, avoiding a new power angle surge caused by a one-time full recovery.

[0063] The time threshold is determined as follows: the adjustment time required from the sudden change in power command to the power angle response reaching a new steady-state value is obtained. This adjustment time is usually 3 to 5 times the response time constant of the converter active power control loop, and this value is taken as the time threshold.

[0064] The second adjustment step size is set to 0.5% to 2% of the converter's rated power, and the value of the second adjustment step size is smaller than that of the first adjustment step size. The determination principle is: when the rate of change of the power angle falls back below the stability threshold, the system has escaped the risk of instability but may not have fully recovered the stability margin. Therefore, a smaller step size is used to gradually restore the power limiting coefficient to ensure a smooth recovery process and to avoid causing new power angle shocks.

[0065] The adjusted power limiting factor is restricted to between the minimum limiting value and 1, and is used as the response priority power limiting factor.

[0066] The power limiting factor represents the maximum power ratio that the converter is allowed to execute under current grid conditions. A factor of 1 indicates that 100% of the total power command is allowed, and a smaller factor indicates a greater degree of compression of the power command. The upper limit is set to 1 because the power limiting factor cannot exceed 1; otherwise, it would be equivalent to amplifying the total power command, causing the actual executed power to exceed the original demand issued by the upper layer. A minimum limiting value is set to prevent the power limiting factor from being reduced indefinitely to zero or even negative values ​​under extreme conditions in weak grids, which would cause the converter to barely respond to power commands, lose its basic power regulation function, and be unable to provide frequency and voltage support to the grid.

[0067] The minimum limit value is the lowest power response ratio that the converter must maintain under the worst grid conditions, and the value range is usually 5% to 20% of the rated power.

[0068] S2. The total power demand of the hybrid energy storage system is limited by a power limiting factor, and a power allocation command after limiting is generated and allocated to the lithium battery converter and the supercapacitor converter.

[0069] Given that power injection under weak grid conditions can easily cause voltage fluctuations, and that lithium batteries and supercapacitors have slow response but high energy density and fast response but low energy density, respectively, the total power command is first limited according to the grid impedance to ensure that the power is within a safe range. Then, the smooth power is distributed to the lithium battery and the fluctuating power is distributed to the supercapacitor according to the frequency, so that the power output matches the grid carrying capacity and the physical characteristics of the energy storage medium.

[0070] Therefore, the generated power allocation command after limiting includes: obtaining the total power demand command of the hybrid energy storage system and the response priority power limiting coefficient.

[0071] The total power demand instruction for the hybrid energy storage system is issued by the upper-level energy management system or grid dispatching system through the communication network. After receiving the instruction message, the communication interface of the energy storage converter parses and verifies it to obtain the active power dispatch value and the reactive power dispatch value. The two together constitute the total power demand instruction for the current period.

[0072] Multiply the total power demand command by the power limiting factor to obtain the limited total power command.

[0073] The magnitude of the grid impedance determines the sensitivity of the grid connection point voltage to power injection. Under weak grid conditions, directly executing an unlimited total power command issued from the upper level may lead to grid connection point voltage exceeding limits or triggering power oscillations. Multiplying the total power demand command by a power limiting factor is equivalent to scaling the power command proportionally according to the current grid voltage support capacity, ensuring that the actual executed power command always remains within the safe range that the grid can withstand. In weak grid conditions, the power injection amount is proactively reduced to mitigate the impact of power fluctuations on the grid connection point voltage; in strong grid conditions, the original command remains unchanged to fully utilize the system's power response capability.

[0074] The total power command after limiting is low-pass filtered to separate the low-frequency power component and the high-frequency power component.

[0075] Specifically, A1, based on the difference in response characteristics between lithium batteries and supercapacitors, sets the cutoff frequency of the low-pass filter, with a value range of 0.1Hz to 2Hz, so that power changes below the cutoff frequency are classified as low-frequency components, and those above the cutoff frequency are classified as high-frequency components.

[0076] A2. Input the limited total power command to the digital low-pass filter. Using the digital implementation of a first-order inertial element, iteratively calculate the low-frequency filter output value of the current cycle by using the filter output value of the previous control cycle and the total power command input value of the current cycle.

[0077] A3. Take the output value of the low-pass filter as the low-frequency power component, and subtract the low-frequency power component from the total power command after limiting to obtain the high-frequency power component.

[0078] The low-frequency power component is used as the power distribution command for the lithium battery converter, and the high-frequency power component is used as the power distribution command for the supercapacitor converter, and power boundary clamping is performed separately.

[0079] Lithium-ion batteries have high energy density but slow power response, and frequent power fluctuations accelerate their lifespan degradation. Supercapacitors, on the other hand, have high power density and fast response but small energy capacity, making them suitable for handling rapidly fluctuating power. Low-frequency components, which are filtered and have a smooth transition, are allocated to lithium-ion batteries, allowing them to operate in a relatively stable charge and discharge state. High-frequency components, which fluctuate dramatically, are allocated to supercapacitors, leveraging their rapid response to handle power spikes and fluctuations.

[0080] The specific process of power boundary clamping is as follows: Step 1: Obtain the maximum allowable charging power limit and the maximum allowable discharging power limit of the lithium battery converter, as well as the maximum allowable charging power limit and the maximum allowable discharging power limit of the supercapacitor converter.

[0081] Step 2: Input the low-frequency power component allocated to the lithium battery converter and the high-frequency power component allocated to the supercapacitor converter to their respective power clamping units.

[0082] Step 3: If the input power command value is greater than the maximum allowable discharge power limit of the converter, the command value is forcibly limited to the maximum allowable discharge power limit; if the input power command value is less than the maximum allowable charging power limit of the converter, the command value is forcibly limited to the maximum allowable charging power limit; if the input power command value is between the two, it remains unchanged and passes directly.

[0083] Step 4: Output the clamped power command value to the lithium battery converter and the supercapacitor converter respectively as the final execution command.

[0084] The clamped power distribution command is output to the lithium battery converter and the supercapacitor converter respectively.

[0085] S3. Monitor whether the grid status crosses the switching boundary. If it crosses from a strong grid to a weak grid, or from a weak grid to a strong grid, the mode switching preparation process is triggered: the current converter output voltage phase is collected, the phase of the internal potential of the virtual synchronous generator is continuously adjusted until it is consistent with the grid voltage phase, and the phase alignment flag is output.

[0086] Considering that when the power grid state switches between a strong grid and a weak grid, the converter needs to switch the control architecture accordingly to ensure stable operation. Direct switching will cause the internal potential phase to be inconsistent with the grid voltage phase, resulting in inrush current. Therefore, the above-mentioned monitoring of whether the power grid state crosses the switching boundary includes: within a continuous control cycle, comparing the measured value of the grid impedance in the current cycle with the measured value of the grid impedance in the previous cycle to obtain the change in impedance magnitude and the change in impedance angle. The two together constitute the displacement vector of impedance on the complex plane of resistance and reactance.

[0087] The measured values ​​of power grid impedance are usually expressed in polar coordinates, that is, impedance magnitude and impedance angle. Therefore, the power grid impedance magnitude and impedance angle measured in the current cycle are converted into the resistance component (real part of impedance) and the reactance component (imaginary part of impedance) according to the trigonometric function relationship. That is, resistance is equal to the impedance magnitude multiplied by the cosine of the impedance angle, and reactance is equal to the impedance magnitude multiplied by the sine of the impedance angle. The impedance value of the previous cycle is converted in the same way.

[0088] Subtracting the resistance component of the current cycle from the resistance component of the previous cycle yields the change in resistance; subtracting the reactance component of the current cycle from the reactance component of the previous cycle yields the change in reactance. Plotting the change in resistance as the horizontal axis increment and the change in reactance as the vertical axis increment, a vector is constructed on the resistance-reactance complex plane pointing from the impedance point of the previous cycle to the impedance point of the current cycle; this vector represents the displacement vector of impedance on the complex plane.

[0089] On the resistivity-reactance complex plane, the trajectory of the impedance point reflects the changes in the power grid structure or operating state. When the impedance point moves continuously in a certain direction, it indicates that the power grid state is undergoing a trend change.

[0090] Using the complex value of the grid impedance obtained at the initial measurement moment as the starting point of the trajectory, the displacement vector of each control cycle is extended sequentially from the end point of the impedance trajectory of the previous cycle to the measured impedance point of the current cycle, forming a continuous moving trajectory of the grid impedance on the complex plane.

[0091] The movement trajectory is compared point by point with the pre-divided boundary lines of the strong power grid characteristic area and the weak power grid characteristic area to determine whether the movement trajectory intersects with the boundary line. If there is an intersection, the validity of the crossing is confirmed.

[0092] The specific division method for the strong power grid characteristic region and the weak power grid characteristic region is as follows: the critical impedance magnitude is the quotient obtained by dividing the reference impedance magnitude by the strong and weak power grid determination threshold. An arc is drawn with the critical impedance magnitude as the radius and the origin of the complex plane as the center. The arc is the boundary line between the strong power grid characteristic region and the weak power grid characteristic region. The side with the impedance magnitude less than the critical impedance magnitude is the strong power grid characteristic region, and the side with the impedance magnitude greater than the critical impedance magnitude is the weak power grid characteristic region.

[0093] When determining the crossing point, it is only necessary to compare the measured impedance magnitude of the current control cycle with the measured impedance magnitude of the previous cycle with the critical impedance magnitude. If the two cycles are located on opposite sides of the boundary line, it is determined that the movement trajectory intersects with the boundary line.

[0094] refer to Figure 3 As shown, the above process for confirming the validity of the crossing is as follows: when an intersection point is detected between the moving trajectory and the boundary line, a preset number of continuous control cycles are delayed to obtain the position distribution of the impedance trajectory points relative to the boundary line within a preset number of cycles; the preset number is 3 to 5 control cycles.

[0095] If a preset number of trajectory points are all located within the area on one side of the boundary line corresponding to the crossing direction, it is determined to be a valid crossing, and a switching trigger signal is output.

[0096] When an intersection between the moving trajectory and the boundary line is detected, it may be caused by a variety of reasons: First, the power grid state has undergone a fundamental change, and the impedance point moves steadily from one side to the other side; second, measurement noise or instantaneous disturbance causes the impedance point to briefly cross the boundary line and then immediately return.

[0097] If there are intersections on the movement trajectory where the device moves from one side of a strong power grid characteristic area to the other side of a weak power grid characteristic area, or in the opposite direction, then the power grid state is determined to have crossed the switching boundary.

[0098] Specifically, the process for determining the crossing direction is as follows: The measured impedance magnitude of the current control cycle and the measured impedance magnitude of the previous cycle are compared with the critical impedance magnitude. If the impedance magnitude of the previous cycle is less than the critical impedance magnitude, and the impedance magnitude of the current cycle is greater than the critical impedance magnitude, it is determined that the crossing is from a strong power grid to a weak power grid. Conversely, if the impedance magnitude of the previous cycle is greater than the critical impedance magnitude, and the impedance magnitude of the current cycle is less than the critical impedance magnitude, it is determined that the crossing is from a weak power grid to a strong power grid.

[0099] When the converter switches control modes, if the phase of the internal electromotive force of the virtual synchronous generator is inconsistent with the phase of the grid voltage, the phase difference at the moment of switching will generate an inrush current, which may damage the power devices. Therefore, the output phase alignment flag includes: after receiving the mode switch, obtaining the current phase of the grid connection point voltage through a phase-locked loop and locking it as the target phase, and synchronously reading the current phase of the internal electromotive force of the virtual synchronous generator.

[0100] Calculate the phase deviation between the target phase and the current phase of the internal potential. When the phase deviation is positive, adjust in the acceleration direction; when it is negative, adjust in the deceleration direction.

[0101] When the deviation is positive, it indicates that the internal potential phase lags behind the grid voltage phase, requiring adjustment in the acceleration direction to increase the virtual rotor speed and allow the internal potential phase to catch up. When the deviation is negative, it indicates that the internal potential phase leads the grid voltage phase, requiring adjustment in the deceleration direction to reduce the virtual rotor speed and allow the internal potential phase to fall back. The determination of the adjustment direction is based on the sign of the phase deviation, ensuring that the internal potential phase always moves in the direction of decreasing deviation until the phase deviation approaches zero, achieving phase alignment.

[0102] Apply a phase adjustment bias to the virtual mechanical power command of the virtual synchronous generator according to the adjustment direction, driving the internal electromotive force phase to approach the target phase.

[0103] The calculated phase deviation is multiplied by a scaling factor, and the product is used as the phase adjustment bias. The scaling factor is selected based on the following principle: the larger the scaling factor, the faster the phase adjustment speed, but too large a value will cause phase overshoot and oscillation; the smaller the scaling factor, the smoother the adjustment process, but the convergence speed will be slower. The specific value range is 0.1 to 0.5.

[0104] When the absolute value of the phase deviation is continuously reduced to below the phase alignment threshold, the internal potential phase is identified as being consistent with the grid voltage phase, and the phase alignment flag is set and output.

[0105] The phase alignment threshold value is determined based on the maximum withstand current of the converter power devices and the allowable operating current of the grid-connected protection device. The maximum allowable switching inrush current value is determined by dividing the current value by the product of the effective value of the grid-connected point voltage and the system impedance. The maximum allowable phase deviation angle is obtained by taking 50% to 80% of this angle value as the phase alignment threshold value.

[0106] After the phase alignment flag is set and before the target control mode is ready, the grid impedance trajectory is continuously monitored; if the impedance trajectory crosses the original switching boundary again and stabilizes at the original side, the current mode switching process is terminated, the phase alignment flag is reset and the process returns to S3.

[0107] S4. Based on the phase alignment flag, during the soft switching transition, the converter control architecture is gradually transitioned from the current mode to the target mode, and the power allocation command before the switch is used as the execution command for the first cycle after the switch.

[0108] The above-mentioned gradual transition of the converter control architecture from the current mode to the target mode includes: after the phase alignment flag is set, the control of the target control mode is started synchronously and run in parallel. The parallel control follows the state with reference to the internal potential amplitude and phase output by the current control mode.

[0109] When both the internal potential amplitude tracking error and the internal potential phase tracking error of the target control mode are lower than their respective threshold values, the target control mode is considered ready.

[0110] During parallel operation, the target control mode follows the current control mode with its internal potential amplitude and phase as state variables. Only when the amplitude error and phase error of both are continuously lower than their respective threshold values ​​does it indicate that the output state of the target control mode is basically consistent with the actual state of the current system. At this time, switching the control signal source will not cause a jump in the amplitude or phase of the converter output voltage.

[0111] The amplitude tracking error threshold is obtained by multiplying the effective value of the grid connection point rated voltage by a proportional coefficient, with a value ranging from 0.5% to 2%. The default value is 1% of the grid connection point rated voltage.

[0112] The phase tracking error threshold is consistent with the phase alignment threshold, directly taking a fixed value within the range of 1 to 3 electrical degrees, with a default of 2 electrical degrees. This is determined based on the fact that the switching inrush current within the phase deviation range has been suppressed to an acceptable level, avoiding misjudgments caused by measurement noise.

[0113] In the next control cycle after the target control mode is ready, the control signal source of the converter is switched from the current control mode to the target control mode, with the switching moment as the end point of the gradual transition, to complete the mode switch.

[0114] The above-mentioned execution instruction for the first cycle after the switch, which uses the power allocation instruction before the switch as the execution instruction, includes: latching the current lithium battery power instruction value and the supercapacitor power instruction value when the mode switch trigger signal takes effect.

[0115] The mode switching trigger signal takes effect at the instant when the controller determines that all switching preparation conditions have been met after the validity of the grid state crossing the switching boundary has been confirmed and the phase alignment flag has been set.

[0116] From the moment it takes effect until the soft switch is completed and the target mode control architecture has run for a full control cycle, the output commands of the power distribution and limiting processing stages are frozen, and the input commands of the two converters are forced to remain latched. This ensures that the power output of the converters remains constant throughout the entire switching transition and is not affected by the intermediate state fluctuations during the control architecture switching process, thereby enabling the system to smoothly transition to the target control mode.

[0117] At the start of the second control cycle in target mode, the forced hold is released, and the normal power distribution and limiting process is restored.

[0118] During mode switching, the power command is forcibly latched from the moment it takes effect, and the converter maintains a constant power value until the soft switch is complete. After the switch is completed, the control architecture has switched to the target mode, but the switching action may cause changes in the internal control state variables. If the power allocation process is resumed immediately after the switch is completed, the power command calculated in that cycle may be unstable due to sudden changes in the state variables. Therefore, by waiting for the target mode to run for a full first control cycle, allowing the control state variables to be initialized and stabilized, the forced latch is released at the beginning of the second control cycle, and the normal allocation process is resumed, which can avoid the power command jump at the moment of recovery.

[0119] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented using software, the above embodiments can be implemented, in whole or in part, in the form of a computer program product.

[0120] Those skilled in the art will recognize that the modules and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0121] In addition, the functional modules in the various embodiments of this application can be integrated into one processing module, or each module can exist physically separately, or two or more modules can be integrated into one module.

[0122] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

[0123] Finally, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A power optimization control method for grid-type hybrid energy storage based on response priority, characterized in that: include: Detect the voltage and current at the grid connection point, iteratively calculate the grid impedance by injecting non-characteristic subharmonic sequence components, determine whether the current grid is a strong grid or a weak grid based on the grid impedance, and generate a response priority power limiting coefficient. The total power demand of the hybrid energy storage system is limited by a power limiting factor, and a power allocation command after limiting is generated and allocated to the lithium battery converter and the supercapacitor converter. Monitor whether the grid status crosses the switching boundary. If it crosses from a strong grid to a weak grid, or from a weak grid to a strong grid, the mode switching preparation process is triggered: the current converter output voltage phase is collected, the phase of the internal potential of the virtual synchronous generator is continuously adjusted until it is consistent with the grid voltage phase, and the phase alignment flag is output. Based on the phase alignment flag, during the soft switching transition, the converter control architecture is gradually transitioned from the current mode to the target mode, and the power allocation command before the switch is used as the execution command for the first cycle after the switch.

2. The power optimization control method for grid-type hybrid energy storage based on response priority according to claim 1, characterized in that: The iterative calculation of grid impedance by injecting non-characteristic subharmonic sequence components includes: Select a non-characteristic subharmonic that does not overlap with the characteristic harmonic frequency of the power grid as the injection frequency, and superimpose the positive sequence current disturbance of the injection frequency in the current control loop of the hybrid energy storage converter. The three-phase voltage and three-phase current at the grid connection point are collected, and the three-phase voltage phasors and three-phase current phasors at the injection frequency are extracted by bandpass filtering. The positive sequence voltage and positive sequence current are obtained by symmetrical component transformation. The positive sequence voltage and positive sequence current were measured before and after the disturbance injection, and the difference between the two measurements was used as the voltage increment and current increment caused by the disturbance. Calculate the ratio of voltage increment to current increment to obtain the positive sequence impedance of the power grid at the injection frequency; The convergent grid impedance is obtained by taking the average value of multiple measurements of the positive sequence impedance of the grid.

3. The power optimization control method for grid-type hybrid energy storage based on response priority according to claim 1, characterized in that: The method of determining whether the current power grid is a strong or weak power grid based on the power grid impedance includes: Obtain the RMS value of the rated output voltage and the rated output capacity of the energy storage converter, and calculate the reference impedance modulus of the converter; Divide the reference impedance magnitude by the grid impedance magnitude to obtain the grid short-circuit ratio; If the short-circuit ratio of the power grid is greater than the threshold for determining whether it is a strong or weak power grid, it is determined to be a strong power grid; otherwise, it is determined to be a weak power grid.

4. The power optimization control method for grid-type hybrid energy storage based on response priority according to claim 1, characterized in that: The generated response priority power limiting coefficient includes: Real-time acquisition of the virtual power angle between the internal electromotive force of the virtual synchronous generator and the phase of the grid connection point voltage; Calculate the rate of change of the virtual power angle within a preset time window. When the rate of change exceeds the power angle stabilization threshold, subtract the first adjustment step from the current power limiting coefficient. Once the rate of change falls below the power angle stability threshold and the duration threshold is reached, the current power limiting coefficient is increased by the second adjustment step. The adjusted power limiting factor is restricted to between the minimum limiting value and 1, and is used as the response priority power limiting factor.

5. The power optimization control method for grid-type hybrid energy storage based on response priority according to claim 1, characterized in that: The generated power allocation command after limiting includes: Obtain the total power demand command and response priority power limiting coefficient of the hybrid energy storage system; Multiply the total power demand command by the power limiting factor to obtain the limited total power command; The total power command after limiting is low-pass filtered to separate the low-frequency power component and the high-frequency power component. The low-frequency power component is used as the power distribution command for the lithium battery converter, and the high-frequency power component is used as the power distribution command for the supercapacitor converter, and power boundary clamping is performed separately. The clamped power distribution command is output to the lithium battery converter and the supercapacitor converter respectively.

6. The power optimization control method for grid-type hybrid energy storage based on response priority according to claim 1, characterized in that: Whether the monitored power grid status crosses the switching boundary includes: Within a continuous control cycle, the measured value of the grid impedance in the current cycle is compared with the measured value of the grid impedance in the previous cycle to obtain the change in impedance magnitude and the change in impedance angle. The two together constitute the displacement vector of impedance on the complex plane of resistance and reactance. Using the complex value of the grid impedance obtained at the initial measurement moment as the starting point of the trajectory, the displacement vector of each control cycle is extended sequentially from the end point of the impedance trajectory of the previous cycle to the measured impedance point of the current cycle in chronological order, forming a continuous moving trajectory of the grid impedance on the complex plane. The movement trajectory is compared point by point with the pre-divided boundary lines of the strong power grid characteristic area and the weak power grid characteristic area to determine whether the movement trajectory intersects with the boundary lines. If there is an intersection, the validity of the crossing is confirmed. If there are intersections on the movement trajectory where the device moves from one side of a strong power grid characteristic area to the other side of a weak power grid characteristic area, or in the opposite direction, then the power grid state is determined to have crossed the switching boundary.

7. The power optimization control method for grid-type hybrid energy storage based on response priority according to claim 6, characterized in that: The confirmation of the validity of the crossing includes: When an intersection between the moving trajectory and the boundary line is detected, the position distribution of the impedance trajectory points relative to the boundary line is obtained within a preset number of consecutive control cycles after a preset number of cycles. If a preset number of trajectory points are all located within the area on one side of the boundary line corresponding to the crossing direction, it is determined to be a valid crossing, and a switching trigger signal is output.

8. The power optimization control method for grid-type hybrid energy storage based on response priority according to claim 1, characterized in that: The output phase alignment flag includes: Upon receiving the mode switch, the current phase of the grid connection point voltage is obtained through the phase-locked loop and locked as the target phase, while the current phase of the internal electromotive force of the virtual synchronous generator is read synchronously. Calculate the phase deviation between the target phase and the current phase of the internal potential. When the phase deviation is positive, adjust in the acceleration direction; when it is negative, adjust in the deceleration direction. Apply a phase adjustment bias to the virtual mechanical power command of the virtual synchronous generator according to the adjustment direction, and drive the internal electromotive force phase to approach the target phase; When the absolute value of the phase deviation is continuously reduced to below the phase alignment threshold, the internal potential phase is identified as being consistent with the grid voltage phase, and the phase alignment flag is set and output.

9. The power optimization control method for grid-type hybrid energy storage based on response priority according to claim 1, characterized in that: The gradual transition of the converter control architecture from the current mode to the target mode includes: After the phase alignment flag is set, the control of the synchronously started target control mode runs in parallel. The parallel control follows the state with reference to the internal potential amplitude and phase output by the current control mode. When both the internal potential amplitude tracking error and the internal potential phase tracking error of the target control mode are lower than their respective threshold values, the target control mode is considered to be ready. In the next control cycle after the target control mode is ready, the control signal source of the converter is switched from the current control mode to the target control mode, with the switching moment as the end point of the gradual transition, to complete the mode switch.

10. The power optimization control method for grid-type hybrid energy storage based on response priority according to claim 1, characterized in that: The method of using the power allocation command before the handover as the execution command for the first cycle after the handover includes: At the moment the mode switching trigger signal takes effect, the current lithium battery power command value and supercapacitor power command value are latched. From the effective time until the soft switch is completed and the target mode control architecture has run for a full control cycle, the output commands of the power distribution and limiting processing links are frozen, and the input commands of the two converters are forced to remain latched values. At the start of the second control cycle in target mode, the forced hold is released, and the normal power distribution and limiting process is restored.

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