A method for suppressing inter-loop current between network-configuration type energy storage units based on repetitive control

Abstract

The application discloses a kind of based on repeat control's network type energy storage unit inter-loop current suppression method, it is related to the technical field of power electronics and control, the method is regulated and controlled to network type multi-machine parallel inverter using power, voltage and current three rings, introduce repeater in current inner ring, accurate tracking and inhibition to periodic harmonic current, simultaneously also introduce narrowband resonant loop and LC filter state feedback and pole placement design in current loop, effectively eliminate LC filter resonance peak by LC filter state feedback and pole placement, further inhibit typical harmonic loop current in parallel system by narrowband resonant loop, to realize the efficient suppression of inter-loop current and the uniform distribution of power;The technical scheme of the application can guarantee the stable operation of the system while improving the power sharing capability and grid-connected adaptability of the energy storage unit, avoiding complex distributed communication structure, and improving the operation reliability of the network type energy storage system in complex power grid environment.

Description

Technical Field

[0001] This invention relates to the field of power electronics and control technology, and more specifically, to a method for suppressing circulating currents between grid-connected energy storage units based on repetitive control. Background Technology

[0002] With the large-scale integration of new energy sources into the power grid, energy storage systems are playing an increasingly important role in peak shaving, frequency regulation, and stable operation of the power system. Grid-based energy storage inverters, due to their virtual synchronous machine characteristics, can provide voltage support and inertial response in weak grid environments and are considered an important component of future new power systems. However, circulating currents are prone to occur when multiple energy storage units are connected in parallel. Circulating currents refer to the undesirable exchange of reactive power or harmonic currents between inverters when multiple inverters are connected in parallel, caused by differences in voltage amplitude, phase angle, or impedance parameters. These currents do not directly supply power to external loads but circulate between units, causing power distribution deviations. Circulating currents not only lead to uneven power distribution between units but also cause inverter device overload, increased losses, and decreased system stability. Therefore, how to effectively suppress circulating currents between multiple units while ensuring grid-connected stability has become a key challenge in the research of grid-based energy storage technology.

[0003] Extensive searches of databases such as CNKI, Web of Science, and the Patent Network revealed that existing circulating current suppression methods mostly focus on the fundamental component or static compensation. For example, the virtual impedance method improves power distribution by introducing equivalent impedance into the controller, but suffers from problems such as large additional losses and degraded dynamic performance. While improved droop control strategies can alleviate some circulating current to a certain extent, they are still difficult to completely eliminate under non-ideal operating conditions. Some studies have also adopted active damping, resonant compensation, or state feedback-based methods to eliminate circulating current of specific frequency components. For instance, the invention patent "Grid-connected Circulating Current Suppression Method and Device for Grid-connected Wind Turbines" (application number CN202311871256.5) discloses a grid-connected circulating current suppression method for grid-connected wind turbines. This method introduces the three-phase output voltage and three-phase output current of the grid-connected wind turbine into a virtual impedance module, thereby reconstructing the reference voltage and achieving circulating current suppression for the grid-connected wind turbine. While this method can suppress circulating currents between grid-connected units to some extent, it lacks efficient and systematic means to suppress harmonic circulating currents generated by the converter during operation, especially low-order harmonics such as the 5th and 7th harmonics.

[0004] In summary, existing technologies, particularly those based on droop control and virtual impedance, struggle to effectively suppress periodic circulating currents (especially their harmonic components) between parallel grid-connected energy storage units, leading to compromised system performance, efficiency, and reliability. Therefore, an advanced control strategy capable of actively, precisely, and broadbandly eliminating circulating currents is urgently needed. Summary of the Invention

[0005] The purpose of this invention is to provide a new circulating current suppression method to address the problems in the prior art. Without relying on high-speed global communication and centralized wide-area measurement, this method effectively suppresses the periodic circulating current and its harmonic components caused by the parallel operation of multiple energy storage inverters by deploying a repetitive controller at each inverter. This solves the problem of balancing real-time performance, delay matching, system stability, and suppression of multiple harmonic components in parallel inverter scenarios.

[0006] The technical solution of this invention is: to provide a method for suppressing circulating currents between grid-type energy storage units based on repetitive control, the method comprising:

[0007] 1. A method for suppressing circulating current between grid-connected energy storage units based on repetitive control, wherein the main circuit of the grid-connected energy storage unit includes an inverter, an LC filter, and a load, the inverter supplies power to the load through the LC filter, and the feedback control circuit of the grid-connected energy storage unit includes a power loop and a voltage-current dual closed loop, characterized in that the method includes:

[0008] Step 1: Collect the actual voltage and actual current output by the inverter, calculate the output active power and reactive power based on the actual voltage and actual current, use the active power and reactive power as the power loop input, and calculate the reference voltage.

[0009] Step 2: Using the reference voltage and the actual voltage as the outer voltage loop input, the reference current is calculated. Using the reference current and the actual current as the inner current loop input, the inverter control quantity is calculated. At the same time, a state feedback loop is introduced into the voltage and current dual closed-loop control loop. The inductor current and capacitor voltage of the LC filter are used as state variables. The state feedback gain is designed by the pole placement method and superimposed on the voltage and current dual closed-loop control loop to effectively suppress the LC filter resonance peak.

[0010] Step 3: Introduce a repetitive controller in parallel in the current inner loop control circuit. Use the reference current and the actual current as inputs to the repetitive controller. Superimpose the output of the repetitive controller onto the inverter control quantity to achieve accurate tracking and effective suppression of periodic harmonic current. The repetitive controller consists of a periodic delay inner mode, a proportional gain stage, a digital inner mode filter, and a phase lead filter.

[0011] Step 4: Connect a narrowband resonant circuit in parallel at the front and back ends of the repetitive controller to form a selective compensation channel. Superimpose the output of the narrowband resonant circuit onto the inverter control quantity to specifically suppress the circulating current of integer multiple harmonics of the fundamental frequency.

[0012] Step 5: Input the inverter control signal, which is superimposed with the output signal of the repetitive controller and the output signal of the narrowband resonant circuit, into the pulse width modulation module to generate the inverter's switching transistor drive signal, so as to accurately modulate the output voltage and current of the grid-type energy storage unit.

[0013] Furthermore, step 1 specifically includes: calculating the amplitude E of the virtual internal potential and the grid angular frequency using a power loop controller. , is represented as:

[0014]

[0015]

[0016] In the formula, J is the virtual moment of inertia, and D... p P is the virtual damping coefficient. * P is the preset active power reference value. e The active power output by the inverter. This is the rated value of the power grid angular frequency. The phase angle is the virtual internal potential. D is the difference between the actual and rated values ​​of the power grid angular frequency. q K is the reactive power droop factor. S E is the reactive power integral gain coefficient. n Q is the rated no-load internal potential. * Q is the preset reactive power reference value. e U is the reactive power output of the inverter. n U is the effective value of the system's rated voltage, U is the effective value of the actual voltage, and t is time.

[0017] Furthermore, step 1 also includes: based on the magnitude E of the virtual internal potential and the phase angle Calculate the three-phase reference voltage, and perform an abc / dq coordinate transformation on the three-phase reference voltage to obtain the two-phase reference voltage. , The transformed two-phase reference voltage , As the reference input for the outer voltage loop, the active and reactive power output of the inverter are controlled through a dual closed-loop control circuit of voltage and current.

[0018] Furthermore, step 2 specifically includes: setting the two-phase reference voltages in the dq coordinate system. , With the actual voltage v of the two phases d v q The comparison is performed, and the resulting deviation is used as the input to the voltage outer loop PI controller, which in turn controls the inductor current i. gd i gq and via the capacitor admittance stage The processed capacitor voltage u cd u cq The two-phase reference current is calculated by superimposing it onto the output of the outer voltage loop PI controller. , , where v d v q respectively with u cd u cq Equal; the two-phase reference currents in the dq coordinate system are equal. , With the actual two-phase current i d i q The comparison is performed, and the resulting deviation is used as the input to the inner-loop PI controller, which then controls the capacitor voltage u. cd u cq and via the capacitor admittance stage The processed two-phase actual current i d i q This is superimposed on the output of the inner-loop PI controller, and simultaneously, state feedback coefficients k1 and k2 are set, with k1 being related to the capacitor voltage u. cd u cq The product of k2 and the actual two-phase current i d i q The product of these two values ​​is superimposed on the output of the inner current loop PI controller to calculate the two-phase control quantity e of the inverter. d e q .

[0019] Furthermore, in step 2, the voltage-current double closed-loop representation after introducing the state feedback loop is as follows:

[0020]

[0021]

[0022] In the formula, s represents the Lagrange operator, K pv K is the proportional constant of the voltage outer loop PI controller. iv K is the integral constant of the voltage outer loop PI controller. pi K is the proportional constant of the inner loop PI controller. ii C is the integral constant of the inner loop PI controller in the current loop, and C is the filter capacitor C. f The capacitance value, where L is the filter inductance L. f The inductance value.

[0023] 6. Further, the state feedback coefficients k1 and k2 are expressed as:

[0024]

[0025] In the formula, and R represents the predetermined damping ratio and natural angular frequency, respectively. c For filter capacitor C f The equivalent resistance, R l For filter inductor L f The equivalent resistance.

[0026] Furthermore, step 3 specifically includes: setting the two-phase reference currents in the dq coordinate system. , With the actual two-phase current i d i q The obtained deviation is used as the input to the repetitive controller, and is successively passed through a digital internal model filter, a periodic delay internal model, a proportional gain stage, and a phase lead filter, and then superimposed on the two-phase control quantity e of the inverter. d e q The above is used to suppress periodic harmonic currents through an inner current control loop; the transfer function of the repetitive controller is:

[0027]

[0028] In the formula, z is a complex variable, and K rc For the proportional gain of the repetitive controller, G f (z) is the phase-lead filter, Q(z) is the digital internal-mode filter, and N s The number of sampling points with periodic delay; the sampling frequency f of the repetitive controller. s N is an integer multiple of the fundamental frequency f0. s =f s / f0, and sampling frequency f s The preferred range is 5kHz to 20kHz.

[0029] Furthermore, in step 3, the digital internal model filter Q(z) adopts a causal moving average filter, expressed as:

[0030]

[0031] Phase lead filter G f (z) Employs a pure lead element z k The low-pass filter is represented as:

[0032]

[0033] In the formula, S(z) is a second-order low-pass filter, and k is the lead phase quantity; the second-order low-pass filter S(z) is expressed as:

[0034]

[0035]

[0036] In the formula, T is the cutoff frequency. s This is the sampling period for the repetitive controller.

[0037] Furthermore, the calculation process for the lead phase quantity k in step 3 is as follows: The stability conditions of the system are set as follows:

[0038]

[0039] In the formula, Let H(z) be the grid angular frequency, and H(z) be the voltage closed-loop transfer function in the discrete domain.

[0040] The proportional gain K of the repetitive controller is obtained based on the system's stability condition. rc The range of values ​​for is:

[0041]

[0042] In the formula, The system resonant frequency, Indicates frequency response, Represented as:

[0043]

[0044] In the formula, Let Q(z) be the phase angle of the digital internal mode filter; define the phasor. for:

[0045]

[0046] Then the pure leading unit z k The leading phase quantity k that can provide the maximum stability margin is expressed as:

[0047]

[0048] In the formula, phasor The phase angle.

[0049] Further, step 4 specifically includes: the center frequency of the narrowband resonant circuit corresponds to an integer multiple of the fundamental frequency. To avoid instability of the closed-loop system due to phase delay, a damping coefficient r < 1 is introduced into the denominator of the transfer function, resulting in the narrowband resonant circuit transfer function as follows:

[0050]

[0051] In the formula, N s N represents the number of sampling points corresponding to the fundamental period. s =fs / f0, where n is the harmonic order, N n N represents the delay points. n =N s / n,K n This represents the loop gain.

[0052] The beneficial effects of this invention are:

[0053] The technical solution of this invention integrates a repetitive controller based on a periodic internal model into the control system of a grid-connected energy storage unit. Using this repetitive controller, factors (including power frequency and its multiple harmonic components) that cause circulating currents between parallel or connected energy storage units are identified and reconstructed. Corresponding compensation voltage / current signals are generated and injected into the inverter reference, thereby achieving active, precise, and systematic suppression of circulating currents, ensuring efficient suppression of circulating currents between units and uniform power distribution. Furthermore, this invention introduces a narrowband resonant circuit and LC filter state feedback and pole configuration design into the current loop. The LC filter state feedback and pole configuration effectively eliminate LC filter resonance peaks. Simultaneously, the narrowband resonant circuit further suppresses typical harmonic circulating currents such as the 5th and 7th harmonics in the parallel system, thereby improving system damping characteristics, reducing additional stress and losses in inverter devices, extending system lifespan, and achieving further efficient suppression of circulating currents between units and improved system operational stability.

[0054] The technical solution in this invention can improve the power sharing capability and grid-connection adaptability of energy storage units while ensuring stable system operation. This solution does not rely on centralized communication and global measurement. It can actively suppress the periodic circulating current generated in parallel operation by deploying repeating controllers at each inverter end. It effectively overcomes the problems of traditional virtual impedance method relying on additional losses and dynamic performance degradation. At the same time, it avoids complex distributed communication structure and has strong practicality and engineering promotion value. Attached Figure Description

[0055] The advantages of the above and additional aspects of the present invention will become apparent and readily understood in the description of the embodiments in conjunction with the following drawings, wherein:

[0056] Figure 1 This is a schematic flowchart of a method for suppressing inter-unit circulating currents in a grid-type energy storage system based on repetitive control, according to an embodiment of the present invention.

[0057] Figure 2 This is a schematic diagram of the main circuit and control circuit in a grid-type energy storage unit system according to an embodiment of the present invention;

[0058] Figure 3 This is a schematic diagram of an inverter power control circuit according to an embodiment of the present invention;

[0059] Figure 4 This is a schematic diagram of a voltage and current dual closed-loop control circuit according to an embodiment of the present invention;

[0060] Figure 5 This is a schematic diagram of a repeating controller according to an embodiment of the present invention;

[0061] Figure 6 This is a comparison chart of the active and reactive power outputs of the first two parallel inverters with circulating current suppression obtained from the simulation in the example.

[0062] Figure 7 This is a comparison chart of the active and reactive power outputs of the two parallel inverters after circulating current suppression, obtained from the simulation in the example.

[0063] Figure 8 This is a comparison chart of the output currents of the first two parallel inverters with circulating current suppression obtained from the simulation in the example.

[0064] Figure 9 This is a comparison chart of the output currents of the two parallel inverters after circulating current suppression, obtained from the simulation in the example.

[0065] Figure 10 This is a schematic diagram of the total harmonic distortion rate of the output current of the first two parallel inverters, obtained from the simulation of circulating current suppression in the example.

[0066] Figure 11 This is a schematic diagram of the total harmonic distortion rate of the output current of the two parallel inverters after circulating current suppression, obtained from the simulation in the example. Detailed Implementation

[0067] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0068] In the following description, many specific details are set forth in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0069] like Figures 1 to 5 As shown, this embodiment provides a method for suppressing circulating currents between grid-type energy storage units based on repetitive control. This method is applied to a grid-type energy storage unit system with multiple units operating in parallel. The system includes a main circuit and a control circuit. The main circuit is used to supply power to the load, and the control circuit is used to effectively suppress the periodic circulating currents and their harmonic components generated in the main circuit, so as to achieve power supply balance and ensure stable system operation.

[0070] Specifically, the main circuit consists of a voltage source inverter, an LC filter, a load, and voltage and current sensors, such as... Figure 1 As shown, the DC side voltage v of the energy storage dc1 and v dc2 Three-phase voltage source inverter 1 and three-phase voltage source inverter 2 are powered respectively. Each inverter generates three-phase AC power according to the PWM control signal, which is then filtered by inductor L. f With filter capacitor C f The LC filter device is used for filtering, taking into account the equivalent resistance R of the transmission line. g With equivalent inductance L g After removing the voltage drop across the line impedance, the remaining AC voltage supplies the load R. L The control circuit consists of an abc / dq and dq / abc coordinate transformation module, a power loop controller, a PI controller corresponding to the voltage and current dual closed loops, and a repetitive controller, such as... Figure 2 As shown, the control circuit can execute the inter-unit circulating current suppression method based on repetitive control provided in this embodiment, so that the two inverters connected in parallel can simultaneously supply power to the load in a balanced manner, thereby achieving efficient suppression of inter-unit circulating current and uniform power distribution.

[0071] like Figures 1 to 5 As shown in the figure, the circulating current suppression method between grid-type energy storage units based on repetitive control provided in this embodiment specifically includes the following steps:

[0072] Step 1: Collect the actual voltage and current output by the inverter. Calculate the active and reactive power output by the inverter based on the actual voltage and current. Use the active and reactive power as inputs to the power loop to calculate the reference voltage. Control the active and reactive power by adjusting the amplitude and phase of the output voltage using the power loop.

[0073] Specifically, the actual voltage and current output by the inverter are collected, and the active and reactive power output by the inverter are calculated based on these actual voltage and current. This includes the following steps:

[0074] An LC filter includes a filter inductor L. f and filter capacitor C f The inverter passes through the filter inductor L in sequence. f and filter capacitor C f Connected to load R L In the filter inductor L f Subsequently, the actual three-phase voltage v output by the inverter was acquired in real time using voltage and current sensors respectively. a v b v c and the actual three-phase current i a ib i c By transforming the coordinates of abc / dq (Park transformation), the voltage v in the three-phase stationary coordinate system abc is transformed respectively. a v b v c and current i a i b i c Transform to the dq two-phase rotating coordinate system to obtain the actual two-phase voltage v. d v q and the actual two-phase current i d i q The formula for calculating the Park transform is:

[0075]

[0076] In the formula, x a x b x c x represents the voltage or current components in the three-phase stationary coordinate system abc. d x q Represents the voltage or current components in the dq rotating coordinate system. This represents the angle of coordinate transformation.

[0077] The active and reactive power output of the inverter are calculated based on the transformed actual two-phase voltages and actual two-phase currents, and expressed as follows:

[0078]

[0079]

[0080] In the formula, P e Q represents the active power output of the inverter. e This refers to the reactive power output of the inverter.

[0081] It should be noted that by transforming the abc / dq coordinates, the control problem of AC quantities can be transformed into the control problem of DC quantities, thereby enabling the subsequent PI controller to control AC quantities in a simpler and more stable way.

[0082] The active power and reactive power are used as inputs to the power loop to calculate the reference voltage, specifically including:

[0083] The characteristic equations of the virtual synchronous generator and the Q-U droop control mathematical model are invoked using the power loop controller, and the magnitude E and phase angle of the virtual internal potential are calculated. , is represented as:

[0084]

[0085]

[0086] In the formula, J is the virtual moment of inertia, and D... p P is the virtual damping coefficient. * The preset active power reference value (corresponding to) Figure 2 P in ref ), This is the rated value of the power grid angular frequency (i.e., the reference angular frequency). The phase angle of the virtual internal potential (power angle of the virtual synchronous generator). The angular frequency of the power grid. D is the difference between the actual and rated values ​​of the power grid angular frequency. q K is the reactive power droop factor. S E is the reactive power integral gain coefficient. n Q is the rated no-load internal potential. * The preset reactive power reference value (corresponding to) Figure 2 Q in ref ), U n U is the effective value of the system's rated voltage, and U is the effective value of the actual voltage (corresponding to...). Figure 3 U in pcc ), t represents time.

[0087] Based on the amplitude E and phase angle of the virtual internal potential output by the power loop controller The three-phase reference voltage was calculated. , , For the three-phase reference voltage , , Perform an abc / dq coordinate transformation to obtain the two-phase reference voltage. , (Inverters connected in parallel each correspond to a set of reference voltages, such as inverter 1 corresponding to E) ref1 Inverter 2 corresponds to E ref2 E ref1 and E ref2 (Including two-phase reference voltage) The transformed two-phase reference voltage , As the reference input for the voltage outer loop, precise control of the inverter's output active and reactive power is achieved through a dual closed-loop control circuit of voltage and current, based on the virtual internal potential amplitude E and phase angle. The three-phase reference voltage is calculated as follows:

[0088]

[0089] It should be noted that the virtual synchronous generator is a control model based on the mechanism of a synchronous generator, used to introduce rotational inertia and damping response in inverter control, so as to realize the inertial support and power droop control of voltage source inverters.

[0090] Step 2: Using the reference voltage and actual voltage as the outer voltage loop input, the reference current is calculated. Using the reference current and actual current as the inner current loop input, the inverter control quantity is calculated. Simultaneously, a state feedback loop is introduced into the voltage-current dual closed-loop control circuit. The inductor current and capacitor voltage of the LC filter are used as state variables. The state feedback gain is designed using the pole placement method and superimposed on the voltage-current dual closed-loop control circuit to achieve dynamic adjustment of the inverter output voltage and current, effectively suppressing the LC filter resonance peak, eliminating system oscillations caused by the inherent characteristics of the filter, and thus improving the dynamic stability of the system.

[0091] Specifically, the two-phase reference voltages in the dq coordinate system , With the actual voltage v of the two phases d v q The comparison is performed, and the resulting deviation is used as the input to the voltage outer loop PI controller (proportional-integral), which then controls the inductor current i. gd i gq and via the capacitor admittance stage The processed capacitor voltage u cd u cq The two-phase reference current is calculated by superimposing it onto the output of the outer voltage loop PI controller. , Among them, the actual voltage v of the two phases d v q With respect to the voltage u of the two phase capacitors respectively cd u cq Corresponding and equal; the two-phase reference currents in the dq coordinate system are equal. , With the actual two-phase current i d i q The comparison is performed, and the resulting deviation is used as the input to the inner-loop PI controller, which then controls the capacitor voltage u. cd u cq and via the capacitor admittance stage The processed two-phase actual current i d i q This is superimposed on the output of the inner-loop PI controller, and simultaneously, state feedback coefficients k1 and k2 are set, with k1 being related to the capacitor voltage u. cd u cq The product of k2 and the actual two-phase current i d i qThe product of these two values ​​is then superimposed on the output of the inner current loop PI controller to calculate the two-phase control quantity e of the inverter. d e q .

[0092] With the introduction of a state feedback loop, the voltage and current dual closed-loop control loop is represented as follows:

[0093]

[0094]

[0095] In the formula, s represents the Lagrange operator, K pv K is the proportional constant of the voltage outer loop PI controller. iv K is the integral constant of the voltage outer loop PI controller. pi K is the proportional constant of the inner loop PI controller. ii C is the integral constant of the inner loop PI controller in the current loop, and C is the filter capacitor C. f The capacitance value, where L is the filter inductance L. f The inductance value.

[0096] The state feedback coefficients k1 and k2 are expressed as follows:

[0097]

[0098] In the formula, and R represents the predetermined damping ratio and natural angular frequency, respectively. c For filter capacitor C f The equivalent resistance, R l For filter inductor L f The equivalent resistance.

[0099] After introducing the state feedback loop, the reconstructed LC filter transfer function is expressed as:

[0100]

[0101] In the formula, k1 and k2 are state feedback coefficients.

[0102] In this embodiment, the voltage and current dual closed-loop control circuit after introducing a state feedback loop is as follows: Figure 4 As shown in the figure, k 1d k 1q Let k be the components of the state feedback coefficient k1 along the d-axis and q-axis, respectively. 2d k 2q These are the components of the state feedback coefficient k2 on the d-axis and q-axis, respectively.

[0103] It should be noted that the introduction of state feedback and pole configuration design of LC filter in the inner current loop enables the system to effectively weaken the resonance peak of LC filter while eliminating harmonic circulating current, improve the system damping characteristics, ensure the stability and robustness of the system under different operating conditions, adapt to actual operating conditions such as grid parameter fluctuations and line imbalances, and improve the operational reliability of grid-type energy storage system in complex grid environments.

[0104] Step 3: Introduce a repetitive controller in parallel in the inner current control loop. Use the reference current and the actual current as inputs to the repetitive controller. Superimpose the output of the repetitive controller onto the inverter control quantity output by the inner current loop to achieve accurate tracking and effective suppression of periodic harmonic currents and enhance the system's ability to compensate for periodic disturbances. The repetitive controller consists of a periodic delay inner mode, a proportional gain stage, a digital inner mode filter, and a phase lead filter.

[0105] Specifically, the two-phase reference currents in the dq coordinate system , With the actual two-phase current i d i q The obtained deviation is used as the input to the repetitive controller, and then passes sequentially through the digital internal model filter Q(z) and the periodic delay internal model filter z. -Ns / (1−z -Ns ), proportional gain stage K rc and phase lead filter G f (z), superimposed on the two-phase control quantity e of the inverter d e q The above describes a method to suppress periodic harmonic currents through an inner current control loop; the transfer function of this repetitive controller is:

[0106]

[0107] In the formula, z is a complex variable used to represent the transfer function of the discrete-time system, and K rc For the proportional gain of the repetitive controller, G f (z) is the phase-lead filter, Q(z) is the digital internal-mode filter, and N s This represents the number of sampling points corresponding to the fundamental period (the number of sampling points with period delay). To match the sampling frequency f of the repetitive controller s The corresponding periodic delay is used to achieve periodic error compensation.

[0108] The repetitive controller also satisfies the condition: sampling frequency f s It is an integer multiple of the fundamental frequency f0, and the sampling frequency f sThe preferred range is 5kHz to 20kHz; when the fundamental frequency f0 = 50Hz, the preferred sampling frequency is f s =10kHz, N s =f s / f0 corresponds to N s =200. It should be noted that in f... s When / f0 is not an integer, interpolation or phase compensation is required to achieve an equivalent periodic delay in the discrete domain to ensure internal model matching.

[0109] The digital internal model filter Q(z) of the repetitive controller is a causal moving average filter, expressed as:

[0110]

[0111] If the phase lead filter is designed as G f (z) = 1 / H(z), which can completely cancel the amplitude and phase frequency characteristics of the controlled object H(z) (i.e., completely compensate for the phase lag and amplitude attenuation of the system). H(z) is the voltage closed-loop transfer function in the discrete domain. However, considering the uncertainty of the inverter and controlled object parameters and load disturbances, G is... f (z) is set to have a pure lead unit z. k The low-pass filter is represented as:

[0112]

[0113] In the formula, S(z) is a second-order low-pass filter, and k is the lead phase quantity.

[0114] The second-order low-pass filter S(z) is expressed as:

[0115]

[0116]

[0117] In the formula, T is the cutoff frequency. s T is the sampling period of the repetitive controller. s =1 / f s .

[0118] Determine the value of the lead phase quantity k. Specifically, set the system stability conditions as follows:

[0119]

[0120] In the formula, Let H(z) be the grid angular frequency, and H(z) be the voltage closed-loop transfer function in the discrete domain, which is obtained by discretizing the voltage closed-loop transfer function H(s) in the continuous domain using ZOH. H(s) is expressed as:

[0121]

[0122]

[0123] In the formula, K pv K is the proportional constant of the voltage outer loop PI controller. iv K is the integral constant of the voltage outer loop PI controller. pi K is the proportional constant of the inner loop PI controller. ii This is the integral constant of the PI controller within the current loop.

[0124] Under the premise of satisfying the system's stability conditions, K rc A larger value is preferred to obtain a smaller damping ratio and a faster transient response. The proportional gain K of the repetitive controller is obtained based on the system's stability condition. rc The range of values ​​for is:

[0125]

[0126] In the formula, The system resonant frequency, It represents the frequency response (i.e., the z-transformation into the frequency domain). Represented as:

[0127]

[0128] In the formula, Let Q(z) be the phase angle of the digital internal mode filter; define the phasor. for:

[0129]

[0130] Then the pure leading unit z k The leading phase quantity k that can provide the maximum stability margin is expressed as:

[0131]

[0132] In the formula, phasor The phase angle.

[0133] In this embodiment, the repetitive controller circuit is as follows: Figure 5 As shown, the circuits corresponding to the d-axis and q-axis are equipped with repetitive controller circuits, and the two repetitive controllers have the same structure.

[0134] Step 3 above also includes: post-processing the output signal of the repetitive controller. Specifically, the output signal is filtered by a first-order or multi-order de-glitch low-pass filter to reduce high-frequency noise, and the amplitude of the output signal is limited by an amplitude saturation limiter to avoid over-modulation or exceeding the inverter's allowable range. At the same time, a phase correction stage is introduced to reduce phase deviation. Finally, the post-processed output signal is superimposed on the two-phase control quantities ed and eq of the inverter to achieve safe and accurate control of the inverter output.

[0135] Step 4: Connect a narrowband resonant circuit in parallel at the front and back ends of the repetitive controller to form a selective compensation channel. Superimpose the output of the narrowband resonant circuit onto the inverter control quantity to specifically suppress the circulating current of integer multiple harmonics of the fundamental frequency and compensate for the insufficient response of the repetitive control in the dynamic process. Integer multiple harmonics include at least the fifth and seventh harmonics.

[0136] A narrowband resonant circuit is connected in parallel at the front and rear ends of the repetitive controller. The center frequency of the narrowband resonant circuit corresponds to an integer multiple (e.g., 5 times, 7 times, etc.) of the fundamental frequency. To avoid instability of the closed-loop system due to phase delay, a damping coefficient r < 1 is introduced into the denominator of the transfer function, resulting in the following transfer function for the narrowband resonant circuit:

[0137]

[0138] In the formula, N s N represents the number of sampling points corresponding to the fundamental period. s =f s / f0, where n is the harmonic order, N n N represents the delay points. n =N s / n,K n This is the loop gain, used to control the resonant peak.

[0139] It should be noted that the output signal of the repetitive controller is added to the output signal of the narrowband resonant circuit to generate the final compensation signal v. comp The compensation signal v comp When superimposed on the output signal of the inner current loop PI controller, it can achieve efficient suppression of circulating current in parallel systems.

[0140] In this embodiment, a repetitive controller is introduced into the inner current loop, and this repetitive controller is fully utilized to achieve internal mode tracking and precise suppression of periodic harmonic currents. At the same time, a narrowband resonant circuit is set in parallel at the front and rear ends of the repetitive controller, which can selectively suppress circulating currents with integer multiples of the fundamental frequency that are common in multi-machine parallel systems. This compensates for the amplitude residual error and regulation lag of the repetitive controller in the dynamic response process, realizes rapid and precise suppression of specific harmonic circulating currents, improves the steady-state performance of the system, and enhances the reliability of multi-machine cooperative operation.

[0141] Step 5: Input the inverter control signal, which is superimposed with the output signal of the repetitive controller and the output signal of the narrowband resonant circuit, into the pulse width modulation (PWM) module to generate the inverter's switching transistor drive signal. Use this switching transistor drive signal to precisely modulate the output voltage and current of the grid-type energy storage unit.

[0142] Specifically, the two-phase control quantity e, which combines the output signal of the repetitive controller and the output signal of the narrowband resonant circuit, will be... d e q Perform a dq / abc coordinate transformation to obtain the three-phase control quantity e. a e b e c The three-phase control quantity e a e b e c The input pulse width modulation module is used to control the grid-type multi-machine parallel inverter system, so as to achieve stable operation of the parallel system and effective suppression of circulating current.

[0143] It should be noted that when multiple grid-type energy storage devices operating in parallel exhibit differences in output voltage amplitude, phase, or impedance characteristics at positive-sequence, negative-sequence, or harmonic frequencies, voltage inconsistencies will occur, leading to circulating currents. Through simulation analysis and experimental testing, researchers have found that positive-sequence circulating currents are mainly caused by differences in amplitude and phase angle, negative-sequence circulating currents originate from differences in the responses of each device to unbalanced components, and harmonic circulating currents are related to filter characteristics and the controller's ability to process high-frequency components. Therefore, it is concluded that to effectively suppress circulating currents, a combination of unified virtual impedance configuration, harmonic suppression control strategies, and distributed coordinated control strategies should be adopted. To address this, the aforementioned circulating current suppression method is proposed. First, a power loop controller is used to adjust the amplitude and phase of the output voltage to control active and reactive power. Second, a voltage-current dual closed-loop structure is adopted, and a PI controller is used to adjust the voltage and current separately in the dq coordinate system. A repetitive controller is introduced in the inner current loop to achieve precise tracking and suppression of periodic harmonic currents. At the same time, a narrowband resonant circuit is combined to further suppress typical harmonic circulating currents such as the 5th and 7th harmonics in the parallel system. With the addition of LC filter state feedback and pole configuration, resonance peaks are effectively eliminated, the system damping characteristics are improved, and thus efficient suppression of circulating currents between units and uniform power distribution are achieved.

[0144] Example:

[0145] To verify the effectiveness of the proposed repetitive control-based inter-unit circulating current suppression method for grid-connected energy storage units, this example establishes the following... Figure 2 The circuit model shown consists of two three-phase parallel inverters and a three-phase symmetrical load. The two parallel grid-connected inverters simultaneously supply power to the load, using a virtual synchronous machine control. By simulating this circuit model, the active power, reactive power, load voltage and current, and circulating current waveform characteristics when the two inverters operate in parallel are analyzed. The key parameters of the main circuit are shown in the table below:

[0146]

[0147] like Figures 6 to 7 The graph shows a comparison of the active and reactive power outputs of the two parallel inverters before and after circulating current suppression. As can be seen from the graph, before circulating current suppression, there was a significant difference between the effective values ​​of active and reactive power for the two inverters. After using the circulating current suppression method of this invention, the effective value of active power output by inverter 1 is P1 = 4500 W, and the effective value of reactive power is Q1 = 2300 var; the effective value of active power output by inverter 2 is P2 = 4603 W, and the effective value of reactive power is Q2 = 2250 var. This effectively achieves a balanced distribution of active and reactive power. Therefore, the circulating current suppression method of this invention can effectively achieve a balanced power distribution.

[0148] like Figures 8 to 9 The figure shows a comparison of the output current of the two parallel inverters before and after circulating current suppression. As can be seen from the figure, when the two parallel grid-connected inverters supply power to the load at the same time, the output current amplitude of inverter 1 and inverter 2 is about 11 A. Without the circulating current suppression method, the circulating current between the two parallel grid-connected inverters is about 1.3 A, and the circulating current accounts for about 11.8%. After adopting the method, the circulating current between the units is reduced to about 0.2 A, and the circulating current accounts for about 1.8%.

[0149] like Figures 10 to 11 The graph shows a comparison of the total harmonic distortion (THD) of the output current of the two parallel inverters before and after circulating current suppression. As can be seen from the graph, without the circulating current suppression method, the THD of the output current of the two parallel grid-connected inverters is 6.15%; after adopting the method, the THD of the output current is reduced to 2.46%. Therefore, the circulating current suppression method designed in this invention can not only effectively suppress circulating current between grid-connected energy storage units, but also reduce the harmonic content of the output current, thereby significantly improving the power quality of the system.

[0150] As can be seen from the above analysis, the present invention achieves circulating current suppression between grid-connected energy storage units by introducing repetitive control. Without relying on a large amount of communication or global coordination, this method can effectively reduce circulating current in parallel systems, achieve balanced power distribution, and suppress harmonic components in output voltage and current, thereby improving the system performance, reliability, and power quality of grid-connected energy storage units and ensuring the stability and efficiency of multi-unit collaborative operation.

[0151] The steps in this invention can be adjusted, combined, or deleted according to actual needs.

[0152] The units in the device of the present invention can be merged, divided, or reduced according to actual needs.

[0153] In this invention, the terms "installation," "connection," "linking," and "fixing" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; "linking" can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of these terms in this invention according to the specific circumstances.

[0154] The shapes of the components in the accompanying drawings are schematic and may differ from their actual shapes. The drawings are only used to illustrate the principles of the present invention and are not intended to limit the present invention.

[0155] Although the invention has been disclosed in detail with reference to the accompanying drawings, it should be understood that these descriptions are merely exemplary and not intended to limit the application of the invention. The scope of protection of the invention is defined by the appended claims and may include various modifications, alterations, and equivalents made to the invention without departing from the scope and spirit of the invention.

Claims

1. A method for suppressing circulating current between grid-connected energy storage units based on repetitive control, wherein the main circuit of the grid-connected energy storage unit includes an inverter, an LC filter, and a load; the inverter supplies power to the load through the LC filter; and the feedback control circuit of the grid-connected energy storage unit includes a power loop and a voltage-current dual closed loop, characterized in that... The method includes: Step 1: Collect the actual voltage and actual current output by the inverter, calculate the output active power and reactive power based on the actual voltage and actual current, use the active power and reactive power as the power loop input, and calculate the reference voltage. Step 2: Using the reference voltage and the actual voltage as the outer voltage loop input, the reference current is calculated. Using the reference current and the actual current as the inner current loop input, the inverter control quantity is calculated. At the same time, a state feedback loop is introduced into the voltage and current dual closed-loop control loop. The inductor current and capacitor voltage of the LC filter are used as state variables. The state feedback gain is designed by the pole placement method and superimposed on the voltage and current dual closed-loop control loop to effectively suppress the LC filter resonance peak. Step 3: Introduce a repetitive controller in parallel in the current inner loop control circuit. Use the reference current and the actual current as inputs to the repetitive controller. Superimpose the output of the repetitive controller onto the inverter control quantity to achieve accurate tracking and effective suppression of periodic harmonic current. The repetitive controller consists of a periodic delay inner mode, a proportional gain stage, a digital inner mode filter, and a phase lead filter. Step 4: Connect a narrowband resonant circuit in parallel at the front and back ends of the repetitive controller to form a selective compensation channel. Superimpose the output of the narrowband resonant circuit onto the inverter control quantity to specifically suppress the circulating current of integer multiple harmonics of the fundamental frequency. Step 5: Input the inverter control signal, which is superimposed with the output signal of the repetitive controller and the output signal of the narrowband resonant circuit, into the pulse width modulation module to generate the inverter's switching transistor drive signal, so as to accurately modulate the output voltage and current of the grid-type energy storage unit.

2. The method for suppressing inter-unit circulating current in a grid-type energy storage system based on repetitive control as described in claim 1, characterized in that, Step 1 specifically includes: calculating the amplitude E of the virtual internal potential and the grid angular frequency using a power loop controller. , is represented as: ； ； In the formula, J is the virtual moment of inertia, and D... p P is the virtual damping coefficient. * P is the preset active power reference value. e The active power output by the inverter. This is the rated value of the power grid angular frequency. The phase angle is the virtual internal potential. D is the difference between the actual and rated values ​​of the power grid angular frequency. q K is the reactive power droop factor. S E is the reactive power integral gain coefficient. n Q is the rated no-load internal potential. * Q is the preset reactive power reference value. e U is the reactive power output of the inverter. n U is the effective value of the system's rated voltage, U is the effective value of the actual voltage, and t is time.

3. The method for suppressing inter-unit circulating current in a grid-type energy storage system based on repetitive control as described in claim 2, characterized in that, Step 1 further includes: Based on the magnitude E of the virtual internal potential and the phase angle Calculate the three-phase reference voltage, and perform an abc / dq coordinate transformation on the three-phase reference voltage to obtain the two-phase reference voltage. , The transformed two-phase reference voltage , As the reference input for the outer voltage loop, the active and reactive power output of the inverter are controlled through a dual closed-loop control circuit of voltage and current.

4. The method for suppressing inter-unit circulating current in a grid-type energy storage system based on repetitive control as described in claim 3, characterized in that, Step 2 specifically includes: The two-phase reference voltage in the dq coordinate system , With the actual voltage of the two phases v d v q The comparison is performed, and the resulting deviation is used as the input to the voltage outer loop PI controller, which in turn controls the inductor current i. gd i gq and via the capacitor admittance stage The processed capacitor voltage u cd u cq The two-phase reference current is calculated by superimposing it onto the output of the outer voltage loop PI controller. , , where v d v q respectively with u cd u cq Equal; the two-phase reference currents in the dq coordinate system are equal. , With the actual two-phase current i d i q The comparison is performed, and the resulting deviation is used as the input to the inner-loop PI controller, which then controls the capacitor voltage u. cd u cq and via the capacitor admittance stage The processed two-phase actual current i d i q This is superimposed on the output of the inner-loop PI controller, and simultaneously, state feedback coefficients k1 and k2 are set, with k1 being related to the capacitor voltage u. cd u cq The product of k2 and the actual two-phase current i d i q The product of these two values ​​is superimposed on the output of the inner current loop PI controller to calculate the two-phase control quantity e of the inverter. d e q .

5. The method for suppressing inter-unit circulating current in a grid-type energy storage system based on repetitive control as described in claim 4, characterized in that, In step 2, the voltage and current double closed loop after introducing the state feedback loop is represented as follows: ； ； In the formula, s represents the Lagrange operator, K pv K is the proportional constant of the voltage outer loop PI controller. iv K is the integral constant of the voltage outer loop PI controller. pi K is the proportional constant of the inner loop PI controller. ii C is the integral constant of the inner loop PI controller in the current loop, and C is the filter capacitor C. f The capacitance value, where L is the filter inductance L. f The inductance value.

6. The method for suppressing inter-unit circulating current in a grid-type energy storage system based on repetitive control as described in claim 5, characterized in that, The state feedback coefficients k1 and k2 are expressed as follows: ； In the formula, and R represents the predetermined damping ratio and natural angular frequency, respectively. c For filter capacitor C f The equivalent resistance, R l For filter inductor L f The equivalent resistance.

7. The method for suppressing inter-unit circulating current in a grid-type energy storage system based on repetitive control as described in claim 5, characterized in that, Step 3 specifically includes: The two-phase reference current in the dq coordinate system , With the actual two-phase current i d i q The obtained deviation is used as the input to the repetitive controller, and is successively passed through a digital internal model filter, a periodic delay internal model, a proportional gain stage, and a phase lead filter, and then superimposed on the two-phase control quantity e of the inverter. d e q The above is used to suppress periodic harmonic currents through an inner current control loop; the transfer function of the repetitive controller is: ； In the formula, z is a complex variable, and K rc For the proportional gain of the repetitive controller, G f (z) is the phase-lead filter, Q(z) is the digital internal-mode filter, and N s The number of sampling points with periodic delay; the sampling frequency f of the repetitive controller. s N is an integer multiple of the fundamental frequency f0. s =f s / f0, and sampling frequency f s The preferred range is 5kHz to 20kHz.

8. The method for suppressing inter-unit circulating current in a grid-type energy storage system based on repetitive control as described in claim 7, characterized in that, In step 3, the digital internal model filter Q(z) adopts a causal moving average filter, expressed as: ； Phase lead filter G f (z) Employs a pure lead element z k The low-pass filter is represented as: ； In the formula, S(z) is a second-order low-pass filter, and k is the lead phase quantity; the second-order low-pass filter S(z) is expressed as: ； ； In the formula, T is the cutoff frequency. s This is the sampling period for the repetitive controller.

9. The method for suppressing inter-unit circulating current in a grid-type energy storage system based on repetitive control as described in claim 8, characterized in that, The calculation process for the leading phase quantity k in step 3 is as follows: Set the system stability conditions as follows: ； In the formula, Let H(z) be the grid angular frequency, and H(z) be the voltage closed-loop transfer function in the discrete domain. The proportional gain K of the repetitive controller is obtained based on the system's stability condition. rc The range of values ​​for is: ； In the formula, The system resonant frequency, Indicates frequency response, Represented as: ； In the formula, Let Q(z) be the phase angle of the digital internal mode filter; define the phasor. for: ； Then the pure leading unit z k The leading phase quantity k that can provide the maximum stability margin is expressed as: ； In the formula, phasor The phase angle.

10. The method for suppressing inter-unit circulating current in a grid-type energy storage system based on repetitive control as described in claim 1, characterized in that, Step 4 specifically includes: The center frequency of the narrowband resonant circuit corresponds to an integer multiple of the fundamental frequency. To avoid instability of the closed-loop system due to phase delay, a damping coefficient r < 1 is introduced into the denominator of the transfer function, resulting in the following transfer function for the narrowband resonant circuit: ； In the formula, N s N represents the number of sampling points corresponding to the fundamental frequency period. s =f s / f0, where n is the harmonic order, N n N represents the delay points. n =N s / n,K n This represents the loop gain.

Images