Energy storage system and control method thereof

Abstract

The embodiment of the invention relates to the technical field of energy storage, provides an energy storage system and a control method thereof, and can effectively suppress broadband oscillation so as to ensure safe operation of a power system. According to the control method of the energy storage system, the energy storage system comprises an energy storage converter and a filter circuit which are electrically connected, and the filter circuit is further used for being connected with a grid-connected point on the power grid side. The control method comprises the following steps: establishing a control model according to a preset principle; the step of establishing the control model according to the preset principle comprises the following steps: establishing a current control ring; the current control loop is used for controlling a current signal output by the energy storage converter and feeding back the current output by the filter circuit to the input end of the current control loop; establishing a capacitor voltage feed-forward loop; and the capacitor voltage feed-forward loop is used for feeding back a voltage signal of the capacitor of the filter circuit to the input end of the current control loop in a forward direction.

Description

Technical Field

[0001] The embodiments of the present disclosure relate to the field of energy storage technology, and in particular to an energy storage system and a control method thereof. Background Art

[0002] In recent years, wind power and photovoltaic power generation technologies have become increasingly popular. However, due to the influence of sunshine time, intensity and wind characteristics, new energy equipment such as photovoltaic and wind power are mainly distributed in deserts, mountains, grasslands and other areas, far away from the main power grid, thus showing high impedance and weak power grid characteristics. This makes the interaction between new energy equipment and weak power grid systems increasingly strong, which is easy to cause broadband oscillations, thus affecting the safe operation of the power system. How to suppress the broadband oscillation of the system is imminent. Summary of the invention

[0003] The embodiments of the present disclosure provide an energy storage system and a control method thereof, which can effectively suppress broadband oscillations, thereby ensuring the safe operation of the power system.

[0004] According to some embodiments of the present disclosure, the embodiments of the present disclosure provide, on one hand, a control method for an energy storage system, wherein the energy storage system comprises an electrically connected energy storage converter and a filter circuit, wherein the filter circuit is also used to be connected to a grid connection point on a grid side; The control method comprises: establishing a control model according to a preset principle; The control model is established according to the preset principles, including: Establishing a current control loop; the current control loop is used to control the current signal output by the energy storage converter and feed back the current output by the filter circuit to the input end of the current control loop; A capacitor voltage feedforward loop is established; the capacitor voltage feedforward loop is used to feed forward the voltage signal of the capacitor of the filter circuit to the input end of the current control loop.

[0005] According to some embodiments of the present disclosure, the energy storage converter includes a three-phase bridge arm, and each phase bridge arm is electrically connected to the filter circuit respectively; the filter circuit includes three filter units arranged in parallel, and each filter unit includes a first inductor, a capacitor, and a second inductor; one end of the first inductor is connected to the energy storage converter, and the other end is connected to the capacitor and the second inductor respectively; the end of the second inductor not connected to the first inductor is used to be connected to the grid connection point on the grid side; the ends of the capacitors of each filter unit not connected to the first inductor are connected; The establishing of the current control loop comprises: Will G c ( s )、 G d ( s )、K pwm , 1 / ( sL 1 )、1 / ( sC )、1 / ( sL 2 ) are connected in sequence; wherein, the G c ( s ) represents the transfer function of the proportional resonant controller, the G d ( s ) represents the transfer function of the data delay device, K pwm represents the equivalent gain of the energy storage converter, L 1 represents the inductance value of the first inductor, C represents the capacitance value of the capacitor, L 2 represents the inductance value of the second inductor, s represents a complex frequency domain variable; The 1 / ( sL 2 ) transmits the current signal to the grid side i g ( s ) feedback to the G c ( s ) to make the i g ( s ) with preset reference current i ref ( s ) after synthesis and then transmitted to the G c ( s ); The voltage signal of the grid connection point on the grid side u pcc ( s ) and the 1 / ( sC ) output signals are synthesized and transmitted to the 1 / ( sL 2 ); The establishing of the capacitor voltage feed-forward loop comprises: The 1 / ( sC ) The output signal is sequentially G n ( s ) and 1 / K pwm After the output, feed forward to the G d (s ) to make G c ( s ) The output signal is related to the 1 / K pwm After the output signals are synthesized, they are transmitted to the G d ( s ); wherein, the G n ( s ) represents the transfer function of the notch filter.

[0006] According to some embodiments of the present disclosure, after the capacitor voltage feedforward loop is established, the control model is established according to a preset principle, further comprising: The i g ( s ) is fed back to the 1 / ( sC ) to make the i g ( s ) and the 1 / ( sL 1 ) output signals are synthesized and transmitted to the 1 / ( sC ); The 1 / ( sC ) The output signal is fed back to the 1 / ( sL 1 ) at the front end so that the 1 / ( sC ) output signal and the K pwm After the output signal is synthesized, it is transmitted to the 1 / ( sL 1 ).

[0007] According to some embodiments of the present disclosure, G c ( s )= K p +2×20×s / (s^2+2×s+(100×pi)^2), where K p It represents the proportionality coefficient and pi is the circumference of a circle.

[0008] According to some embodiments of the present disclosure, G d ( s )=exp(-1.5×s×Ts), where Ts represents the sampling period.

[0009] According to some embodiments of the present disclosure, G n (s )=(s^2+ W n ^2) / (s^2+ W n ×s+ W n ^2) Among them, W n is the undamped natural oscillation frequency of the notch filter, W n =2×pi×frequency of the notch filter, where pi is the ratio of the circumference of a circle.

[0010] According to some embodiments of the present disclosure, after the control model is established according to the preset principle, the control method further includes: Establishing a sequence impedance model; the sequence impedance model includes:

[0011] Among them, the IR csp ( s ) represents the positive sequence impedance value, the Z gp ( s ) represents the positive sequence equivalent impedance of the grid side, the Z csp ( s ) represents the positive sequence equivalent impedance on the converter side, IR csn ( s ) represents the negative sequence impedance value, the Z gn ( s ) represents the negative sequence equivalent impedance of the grid side, the Z csn ( s ) represents the negative sequence equivalent impedance of the converter side; According to the sequence impedance model, respectively calculating the sequence impedance values ​​after applying the disturbance signal to the grid connection point on the grid side under different disturbance parameters; Establishing a sequence impedance characteristic curve according to the disturbance parameter and the sequence impedance value; According to the sequence impedance characteristic curve, based on the Nyquist stability criterion, the stability of the control model is judged.

[0012] According to some embodiments of the present disclosure, after establishing a sequence impedance characteristic curve according to the disturbance parameter and the sequence impedance value, the control method further includes: The transfer function of the control model is modified according to the sequence impedance characteristic curve.

[0013] According to some embodiments of the present disclosure, the calculating, according to the sequence impedance model, the sequence impedance value when the grid connection point on the grid side adds a disturbance signal under different disturbance parameters includes: Detecting a first phase voltage of the grid connection point and a first phase current of the grid connection point when no disturbance signal is applied to the grid connection point on the power grid side; Under the same disturbance parameters, applying a disturbance signal to the grid connection point on the grid side; Detecting a second phase voltage of the grid connection point and a second phase current of the grid connection point after a disturbance signal is applied to the grid connection point on the power grid side; calculating a first difference between the first phase voltage and the second phase voltage, and a second difference between the first phase current and the second phase current; Respectively obtaining a positive sequence component and a negative sequence component of the first difference and the second difference; The positive sequence component and the negative sequence component of the first difference and the second difference are converted from the time domain to the frequency domain, and the sequence impedance value under the disturbance parameter is calculated according to the sequence impedance model.

[0014] Another aspect of the disclosed embodiment provides an energy storage system, including: a controller, an electrically connected energy storage converter and a filter circuit, the filter circuit being used to be connected to a grid connection point on the grid side, the controller being electrically connected to the energy storage converter; The controller is used to implement any of the control methods described above.

[0015] The disclosed embodiment provides an energy storage system and a control method thereof. In the control method, a control model includes a current control loop and a capacitor voltage feed-forward loop. The cooperation of the two can effectively suppress the wide-band oscillation problem caused by capacitor voltage instability, thereby ensuring the safe operation of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] One or more embodiments are exemplarily illustrated by pictures in the corresponding drawings, and these exemplified descriptions do not constitute a limitation on the embodiments. Unless otherwise specified, the pictures in the drawings do not constitute a scale limitation. In order to more clearly illustrate the embodiments of the present disclosure or the technical solutions in the traditional technology, the drawings required for use in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present disclosure. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 A circuit structure diagram of an energy storage system provided in an embodiment of the present disclosure; Figure 2 A schematic diagram of a flow chart of establishing a control model according to a preset principle provided in an embodiment of the present disclosure; Figure 3 Another flow chart of establishing a control model according to a preset principle provided in an embodiment of the present disclosure; Figure 4 For Figure 3 The corresponding control block diagram; Figure 5 A control block diagram provided for related technology; Figure 6 In the figure, Figure a shows different amplitude curves corresponding to impedances on different power grid sides, and Figure b shows different phase curves corresponding to impedances on different power grid sides; Figure 7 Nyquist curves corresponding to positive sequence impedance values ​​and negative sequence impedance values ​​respectively; Figure 8 for Figure 1 Equivalent small signal model of the system shown; Fig. 9 for Figure 8 The equivalent closed-loop system of Fig.10 In the figure, Figure a is a curve diagram showing the amplitude of sequence impedance changing with frequency, and Figure b is a curve diagram showing the phase of sequence impedance changing with frequency; Fig.11 In the figure, Figure a is the waveform of the grid voltage, and Figure b is the waveform of the grid current; Fig.12 A schematic diagram of the structure of another energy storage system provided in an embodiment of the present disclosure. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solutions and advantages of the present disclosure clearer, the technical solutions of the present disclosure will be clearly and completely described below in combination with the specific embodiments of the present disclosure and the corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present disclosure, not all of the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present disclosure.

[0019] In the description of the embodiments of the present disclosure, “at least one” means one or more, and “a plurality of” means two or more, unless otherwise clearly and specifically defined.

[0020] In the description of the embodiments of the present disclosure, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0021] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present disclosure. The appearance of the phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0022] In related technologies, the grid-connected control of the energy storage inverter of the energy storage system mostly adopts the method of following the phase-locked grid voltage and adjusting the output current. However, for long-distance transmission lines and weak grid systems with large impedance and small short-circuit ratio, the inverter impedance and grid impedance are coupled with each other, which easily limits the power transmission capacity, resulting in broadband oscillation and even causing system instability. In order to solve the problem of broadband oscillation of the system, the engineering mainly relies on cutting off new energy equipment, but this method is not conducive to the application of new energy equipment.

[0023] Based on the above, the present disclosure provides a control method for an energy storage system. Figure 1 As shown, the energy storage system includes an electrically connected energy storage converter 1 and a filter circuit 2, and the filter circuit 2 is also used to be connected to a grid connection point on the grid side.

[0024] The control method includes: S1, establishing a control model according to a preset principle.

[0025] refer to Figure 2 As shown, step S1, establishing a control model according to a preset principle, includes: S11, establishing a current control loop; the current control loop is used to control the current signal output by the energy storage converter and feed back the current output by the filter circuit to the input end of the current control loop; S12, establishing a capacitor voltage feedforward loop; the capacitor voltage feedforward loop is used to feed forward the voltage signal of the capacitor of the filter circuit to the output end of the current control loop.

[0026] In the embodiments of the present disclosure, the topological structure of each phase bridge arm in the energy storage converter is not limited, for example, refer to Figure 1 As shown, each bridge arm may include two switch tubes S, and the type of the switch tube is not limited. For example, the switch tube may be an insulated gate bipolar transistor (IGBT), and of course, it may also be other types of transistors.

[0027] In the disclosed embodiment, the filter circuit includes at least a capacitor, and may also include an inductor, a resistor, etc., which is not limited here.

[0028] In the embodiment of the present disclosure, the grid connection point on the grid side refers to the grid connection point where the energy storage converter is connected to the grid system after passing through the filtering circuit. Figure 1 As shown, the grid connection point on the grid side can be set at the second inductor L 2 and the impedance of the grid side Z g Point of Common Coupling (PCC) between Figure 1 Marked with a dotted line. Figure 1 middle, U g Represents the AC voltage source on the grid side.

[0029] In the embodiment of the present disclosure, the control model can be represented by a control block diagram in the frequency domain, where: i ref ( s ) can be preset according to actual conditions, G c ( s ) is used to control the output voltage amplitude of the switch tube in the energy storage converter, that is, to limit the output of the switch tube; G d ( s ) is used to achieve the delay of data transmission to compensate for the opening time delay or closing time delay of the switch tube, so as to achieve synchronous data transmission; G n ( s ) is used to stabilize the capacitor voltage for feed-forward control. G c ( s )、 G d ( s )and G n ( s ) can be calculated according to the above control model and energy storage system structure, and based on relevant mathematical formulas such as time domain and frequency domain conversion. The specific expression of the transfer function can be obtained by referring to relevant technologies and will not be explained in detail here.

[0030] It should be noted that after the energy storage converter is connected to the grid, the impedance on the grid side Z g The capacitors on the converter side are mutually coupled, which leads to unstable capacitor voltage on the converter side. The unstable capacitor voltage can easily cause broadband oscillation, which leads to instability of the power grid system. It also affects the balance of the energy storage system. Therefore, by designing a capacitor voltage feedforward control loop in the control model, broadband oscillation can be effectively suppressed to ensure the safe operation of the power system. At the same time, it can also improve the balance of the energy storage system.

[0031] In the control method of the energy storage system provided in the embodiment of the present disclosure, the established control model can effectively suppress the wide-band oscillation problem caused by the unstable voltage of the capacitor, thereby ensuring the safe operation of the power system; at the same time, it can also improve the balance of the energy storage system.

[0032] In one or more embodiments, reference Figure 1 As shown, the energy storage system includes an energy storage converter 1, which includes three-phase bridge arms a, b and c, and each phase bridge arm is electrically connected to a filter circuit 2; the filter circuit 2 includes three filter units arranged in parallel, and each filter unit has a first inductor L 1 ,capacitance C and the second inductor L 2 ; First inductor L 1 One end is connected to the energy storage converter 1, and the other end is connected to the capacitor C and the second inductor L 2 Connection; Second Inductor L 2 Not with the first inductor L 1 The connected end is used to connect to the grid point on the grid side; the capacitor of each filter unit C Not with the first inductor L 1 The connected ends are connected.

[0033] Step S11, establishing a current control loop includes: S111, will G c ( s )、 G d ( s )、 K pwm , 1 / ( sL 1 )、1 / ( sC )、1 / ( sL 2 ) are connected in sequence; among them, G c ( s ) represents the transfer function of the proportional resonant controller, G d ( s ) represents the transfer function of the data delayer, K pwm represents the equivalent gain of the energy storage converter, L 1 represents the inductance value of the first inductor,C Indicates the capacitance value of the capacitor, L 2 represents the inductance value of the second inductor, s represents a complex frequency-domain variable.

[0034] S112, 1 / ( sL 2 ) current signal transmitted to the grid side i g ( s ) Feedback to G c ( s ) to make i g ( s ) with preset reference current i ref ( s ) after synthesis and then transferred to G c ( s ).

[0035] S113, the voltage signal of the grid connection point on the grid side u pcc ( s ) and 1 / ( sC ) output signals are synthesized and transmitted to 1 / ( sL 2 ).

[0036] Step S12, establishing a capacitor voltage feed-forward loop includes: 1 / ( sC ) The output signal is sequentially G n ( s ) and 1 / K pwm After output, feed forward to G d ( s ) to make G c ( s ) The output signal is related to 1 / K pwm After the output signals are synthesized, they are transmitted to G d ( s );in, G n ( s ) represents the transfer function of the notch filter.

[0037] For more accurate control, refer to Figure 3As shown, after step S12, establishing the capacitor voltage feedforward loop, step S1, establishing the control model according to the preset principle, further includes: S13. i g ( s ) feedback to 1 / ( sC ) to make i g ( s ) and 1 / ( sL 1 ) output signals are synthesized and transmitted to 1 / ( sC ).

[0038] S14, 1 / ( sC ) output signal is fed back to 1 / ( sL 1 ) at the front end so that 1 / ( sC ) output signal and K pwm After the output signal is synthesized, it is transmitted to 1 / ( sL 1 ).

[0039] The control model established by steps S11-S14 can be referred to Figure 4 shown.

[0040] It should be noted that the related art provides a Figure 5 Compared with the control model provided in the related art, the control model provided in the embodiment of the present disclosure adds an additional 1 / K pwm and G n ( s ) forms a control loop, that is, a voltage feed-forward control with an additional capacitor is added.

[0041] based on Figure 5 The control model shown in the figure has different grid impedances. Z g Under this condition, simulation analysis of the grid-connected current output by the converter side can obtain the following Figure 6 The simulation results of Figures a and b are as follows: Figure 6 In the figure a, the ordinate represents the amplitude of the grid-connected current, the abscissa represents the frequency, and different impedances on the grid side correspond to different amplitude curves; Figure 6 In the graph b, the ordinate represents the phase of the grid-connected current, and the abscissa represents the frequency. Different impedances on the grid side correspond to different phase curves. After research and analysis, it can be found that: the impedance on the grid side Z g The time can be ignored and the Figure 5The 215Kw converter of the control model shown can work stably; when Z g =0.5mH, the system works stably, but at this time the grid-connected current has high-frequency harmonics exceeding 3%; when Z g =1.25mH, the current begins to oscillate; harmonics are mainly distributed at the system open-loop cutoff frequency. Therefore, the control block diagram provided by the relevant technology is more suitable for large power grid systems with relatively stable grid frequency and amplitude, and is not suitable for complex systems with variable grid frequency and amplitude due to multiple energy storage converters in series and parallel, long-distance lines or load changes. It cannot effectively suppress wide-band oscillations caused by variable grid frequency and amplitude.

[0042] The control model provided by the embodiment of the present disclosure, because it adds additional voltage feedforward control of the capacitor, can be well adapted to complex systems with variable grid frequency and amplitude, and can effectively suppress the voltage instability of the capacitor on the converter side caused by the variable grid frequency and amplitude, which in turn causes wide-band oscillation; the control method of the energy storage system provided by the embodiment of the present disclosure has wider applicability. Z g =1.25mH, the current oscillation disappears; when Z g When the current is ≥2.5mH, there is no obvious oscillation at the beginning. In addition, the verification method based on the sequence impedance model (detailed description later) can verify that: Figure 7 As shown, IR csp ( s )’s Nyquist curve 1 crosses the negative real axis on the left side of the point (-1, j0) once. IR csn ( s ) does not surround the point (-1, j0), indicating that the system is stable. The control model provided by the embodiment of the present disclosure can enhance the adaptability of the grid-connected converter.

[0043] In the present disclosure, G c ( s )= K p +2×20×s / (s^2+2×s+(100×pi)^2), where K p Represents the proportionality coefficient, pi is the circumference of a circle G c ( s ) is expressed in second-order form, which can simplify the model, reduce the computational complexity and facilitate implementation.

[0044] In the present disclosure, G d ( s )=exp(-1.5×s×Ts), where Ts represents the sampling period. G d ( s ) is expressed in first-order form, which can simplify the model, reduce the computational complexity and facilitate implementation.

[0045] In the present disclosure, G n ( s )=(s^2+ W n ^2) / (s^2+ W n ×s+ W n ^2) Among them, W n is the undamped natural oscillation frequency of the notch filter, W n =2×pi×frequency of the notch filter, where pi is the ratio of the circumference of a circle. G n ( s ) is expressed in second-order form, which can simplify the model, reduce the computational complexity and facilitate implementation.

[0046] It should be noted that the above G c ( s )、 G d ( s )and G n ( s ) is based on Figure 1 The energy storage system structure and control model shown are calculated based on relevant mathematical formulas such as time domain and frequency domain conversion. They can be specifically calculated according to the principles of relevant technologies and will not be described in detail here.

[0047] In the disclosed embodiment, the open-loop gain of the control model is G o (s) = G c ( s ) ×G d ( s )× K pwm / (s^3× L 1 × L T ×Cs×L T × G d ( s )+s×( L 1 + L T ));in, L T represents the impedance on the grid side, L T= L 2 + Z g , Z g Represents the impedance of the grid side. It should be noted that G o In the calculation formula of (s), Figure 1 The inductor in the filter circuit shown L 2 It is equivalent to the inductance on the grid side to simplify the calculation.

[0048] In the present disclosure, K pwm = DC side voltage / (2×triangular carrier amplitude). For example, if the triangular carrier amplitude is 1 and the DC side voltage Udc is 1310V, then K pwm =1310V / 2.

[0049] After the control model is established, the stability of the control model needs to be verified. The verification principle of the control model is as follows: Figure 1 The system shown can be equivalent to Figure 8 The small signal model shown, Figure 8 middle, and Z cs are the power supply and impedance of the current-controlled converter equivalent circuit, Indicates the grid power supply, Z g Represents the impedance of the grid side. The current controlled converter inputs current to the grid as , voltage is signal. Figure 8 The equivalent closed-loop system of the small signal model shown can be referred to Fig. 9 shown.

[0050] According to the above analysis, the impedance change on the grid side will affect the voltage of the capacitor on the converter side, thus causing broadband oscillation. Fig. 9 shown Z g / Z cs If the ratio is stable, it means that the impedance of the grid side and the impedance of the converter side are changing steadily, so the voltage of the capacitor on the converter side is changing steadily, and the probability of broadband oscillation is greatly reduced, indicating that the control model has great stability. That is, the stability of the control model can be converted into Z g / Z cs stability judgment.

[0051] Based on the above analysis, in step S1, after establishing the control model according to the preset principle, the control method further includes: S20, establishing a sequence impedance model; the sequence impedance model includes: .

[0052] in, IR csp ( s ) represents the positive sequence impedance value, Z gp ( s ) represents the positive sequence equivalent impedance on the grid side, Z csp ( s ) represents the positive sequence equivalent impedance on the converter side, IR csn ( s ) represents the negative sequence impedance value, Z gn ( s ) represents the negative sequence equivalent impedance on the grid side, Z csn ( s ) represents the negative sequence equivalent impedance on the converter side.

[0053] It should be noted that Z gp ( s )、 Z csp ( s )、 Z gn ( s )and Z csn ( s The specific calculation formula of ) varies with the circuit structure of the energy storage system and is not specifically limited here. Calculate the corresponding Z gp ( s )、 Z csp ( s )、 Zgn ( s )and Z csn ( s ), which can be obtained by referring to relevant technologies and will not be described in detail here.

[0054] S21. According to the sequence impedance model, the sequence impedance values ​​after the disturbance signal is applied to the grid connection point on the grid side under different disturbance parameters are calculated respectively.

[0055] It should be noted that the disturbance parameter can be selected according to actual conditions. For example, the disturbance parameter can be a disturbance frequency, a disturbance phase or a disturbance amplitude.

[0056] S22. Obtain a sequence impedance characteristic curve according to the disturbance parameter and the sequence impedance value.

[0057] The specific expression of the sequence impedance characteristic curve is not limited here, and the corresponding curve equation can be obtained based on the sequence impedance characteristic equation.

[0058] S23. According to the sequence impedance characteristic curve and based on the Nyquist stability criterion, determine the stability of the control model.

[0059] It should be noted that, let G(s) be the open-loop transfer function of the system, and take s=jω in G(s) to obtain the open-loop frequency response G(jω). When the parameter ω changes from 0 to +∞, the trajectory of G(jω) changing with ω can be drawn on the complex plane, which is called the Nyquist plot. The basic form of the Nyquist stability criterion shows that if the system open-loop transfer function G(s) has neither poles nor zeros on the imaginary axis jω of the s complex plane, then Z=PN, where P is the number of poles of the open-loop transfer function on the right half s plane, and N is the number of times the trajectory of G(jω) moves counterclockwise around the point (-1, j0) on the real axis when the angular frequency changes from ω=0 to ω=+∞. When Z=0, the closed-loop control system is stable; when Z≠0, the closed-loop control system is unstable. The Nyquist stability analysis of the sequence impedance in the sequence impedance characteristic curve can be performed to determine the stability of the control model.

[0060] Steps S20-S23 judge the stability of the sequence impedance under different disturbance parameters based on the Nyquist stability criterion. It should be noted that steps S20-S23 can be implemented by a controller, or by a frequency sweeper, which is not limited here.

[0061] From the above analysis, it can be seen that the harmonics of the grid-connected current when oscillation occurs are mainly distributed in the system open-loop cut-off frequency. Therefore, the disturbance frequency can be used as a disturbance parameter to facilitate the accuracy of analysis and judgment; at the same time, the disturbance frequency can be set near the system open-loop cut-off frequency. For example, if the system open-loop cut-off frequency is a, the range of the disturbance frequency can be (-La, L+a), where L can be a preset deviation positive frequency. The system open-loop cut-off frequency refers to the frequency at which the amplitude-frequency characteristic curve of the open-loop system crosses 0dB. It can be calculated based on the circuit structure and control model of the energy storage system, and is not described in detail here. When the disturbance parameter is the disturbance frequency, the control method performed in steps S20-S23 can be called a frequency scanning method.

[0062] In some embodiments, Figure 1 In the energy storage system shown, L 1 =5mH, L 2 =1mH, C =3uF, K pwm =1310V / 2, sampling period Ts=1 / 20000Hz, simulation design triangle carrier amplitude is 1, switching frequency is 10kHz, sampling frequency is 20kHz, system phase margin is 56.2deg, amplitude margin is 2.46dB, stability is improved. Using impedance ratio modeling, the simulation results are: 0.25≤ ≤3 W n , 8 ≤ W n ≤38, where is the transfer function of the notch filter, W n is the undamped natural oscillation frequency of the notch filter.

[0063] In one or more embodiments, after establishing the sequence impedance characteristic curve according to the disturbance parameter and the sequence impedance value in step S22, the control method further includes: S30. According to the sequence impedance characteristic curve, the transfer function of the control model is modified.

[0064] Due to the influence of the external environment, the circuit structure of the energy storage system will also be affected. For example, the capacitance, inductance or resistance will be affected by high and low temperatures, and their own parameters will change, which will affect the accuracy and stability of the established control model. In addition, the control model established based on theoretical analysis is inevitably different from the actual situation. Therefore, according to the sweep frequency method analysis, the transfer function of the control model can be corrected to ensure the stability and accuracy of the control model.

[0065] In one or more embodiments, step S30, correcting the transfer function of the control model according to the sequence impedance characteristic curve, includes: According to the sequence impedance characteristic curve, the transfer function of the notch filter is modified G n ( s ), the transfer function of the data delay G d ( s ) and the transfer function of the proportional resonant controller G c ( s ) or any combination of any two or more.

[0066] It should be noted that, according to the sequence impedance characteristic curve, based on the stability trend, it is possible to analyze which parameters do not meet the preset requirements, thereby finding the corresponding transfer function, and then correcting the corresponding parameters in the transfer function.

[0067] Example, transfer function of a notch filter G n ( s )=(s^2+ W n ^2) / (s^2+ W n ×s+ W n ^2), where W n is the undamped natural oscillation frequency of the notch filter, W n =2×pi×the frequency of the notch filter. The transfer function of the notch filter can be corrected by correcting the frequency of the notch filter. G n ( s ) to stabilize the capacitor voltage.

[0068] Transfer function of data delayer G d ( s )=exp(-1.5×s×Ts), where Ts represents the sampling period. The transfer function of the data delayer can be corrected by correcting the sampling period. G d ( s ), thereby realizing the delay correction of data transmission to achieve data synchronization transmission. In actual engineering, the transfer function of the data delay device is generally not corrected.

[0069] Transfer Function of Proportional Resonant Controller G c ( s )= K p+2×20×s / (s^2+2×s+(100×pi)^2), where K p Represents the proportional coefficient. By correcting the proportional coefficient, the transfer function of the proportional resonant controller can be corrected. G c ( s ), thereby correcting the output voltage amplitude of the switching tube in the energy storage converter.

[0070] Of course, it is also possible to compensate for phase angle, harmonics, virtual impedance, etc. to achieve the correction of the control model. The method of compensating for phase angle, harmonics, virtual impedance, etc. can refer to the relevant technology and will not be explained here.

[0071] In one or more embodiments, step S21, respectively calculating the sequence impedance value when the grid connection point on the grid side adds the disturbance signal under different disturbance parameters according to the sequence impedance model, includes: S211, detecting a first phase voltage and a first phase current of the grid connection point when no disturbance signal is applied to the grid connection point on the grid side.

[0072] S212. Under the same disturbance parameters, a disturbance signal is applied to the grid connection point on the grid side.

[0073] S213, detecting the second phase voltage and the second phase current of the grid connection point after the disturbance signal is applied to the grid connection point on the grid side.

[0074] S214, calculating a first difference between the first phase voltage and the second phase voltage, and a second difference between the first phase current and the second phase current.

[0075] S215 , respectively obtain the positive sequence component and the negative sequence component of the first difference and the second difference.

[0076] S216 , converting the positive sequence component and the negative sequence component of the first difference and the second difference from the time domain to the frequency domain, and calculating the sequence impedance value under the disturbance parameter according to the sequence impedance model.

[0077] By executing steps S211 - S216 , the sequence impedance value under the disturbance parameter can be conveniently calculated. This method is simple and easy to implement.

[0078] In some implementations, under the same disturbance parameters, after a disturbance signal is applied to the grid connection point on the grid side, the phase voltage of the grid connection point can be expressed as: (1) The phase current at the grid connection point can be expressed as: (2) in, V 1 ,V P and V n are the amplitudes of fundamental voltage, positive sequence disturbance voltage and negative sequence disturbance voltage respectively; I 1 , I P and I n are the amplitudes of fundamental current, positive sequence disturbance current response and negative sequence disturbance current response respectively; f 1 , f P and f n They are fundamental frequency, positive sequence disturbance frequency and negative sequence disturbance frequency respectively; and are the initial phase angles of the positive-sequence disturbance voltage and the negative-sequence disturbance voltage respectively; , and They are the initial phase angles of fundamental current, positive-sequence disturbance current and negative-sequence disturbance current respectively.

[0079] Converting equations (1) and (2) from the time domain to the frequency domain yields: (3) (4) In formula (3), , , .

[0080] In formula (4), , , .

[0081] After the above conversion, the positive sequence component and the negative sequence component in the phase voltage of the grid connection point and the positive sequence component and the negative sequence component in the phase current of the grid connection point can be easily extracted.

[0082] In some implementations, in order to ensure the stability of the detected voltage and current values ​​(eg, phase voltage and phase current on the converter side), a phase-locked loop may be set for control. The transfer function of the phase-locked loop is expressed as H PLL ( s ). For example, under the same disturbance parameters, after applying the disturbance signal to the grid connection point on the grid side, the sampling is as follows: Figure 1 The DC side voltage Udc of the energy storage converter, the three-phase grid-connected capacitor voltages va, vb and vc, and the current i output through the inductor L2 in the three-phase filter circuit are shown. 2a 、i 2b 、i2c ; va, vb and vc, i 2a 、i 2b and i 2c After performing park transformation respectively, we can get vd, vq, id, and iq.

[0083]

[0084]

[0085]

[0086]

[0087] In the above expressions of vd, vq, id, iq, f 1 , f P and f n are the fundamental frequency, positive sequence disturbance frequency and negative sequence disturbance frequency respectively, , , , .

[0088] in, G p (s) and G n (s) are the transfer function between the positive sequence component of the disturbance voltage and the angle of the phase-locked loop, and the transfer function between the negative sequence component of the disturbance voltage and the angle of the phase-locked loop; G v (s) is the voltage sampling transfer function, G i (s) is the current sampling transfer function w i 、 w v are the cut-off frequencies of the current and voltage sampling filter circuits, H PLL ( s ) is the transfer function of the phase-locked loop; V 1 , V P and V n are the amplitudes of fundamental voltage, positive sequence disturbance voltage and negative sequence disturbance voltage respectively; I 1 , I P and I nare the amplitudes of fundamental current, positive sequence disturbance current response and negative sequence disturbance current response respectively; f 1 , f P and f n They are fundamental frequency, positive sequence disturbance frequency and negative sequence disturbance frequency respectively; is the phase angle of the current.

[0089] When the bandwidth of the phase-locked loop is 60Hz, based on Figure 1 The energy storage structures shown are respectively Figure 5 The control model shown in the figure and the control model provided by the present disclosure are verified by sequential impedance scanning, and the following can be obtained: Fig.10 The curves shown in Figures a and b. Fig.10 Figure a is a curve diagram of the amplitude of sequence impedance changing with frequency, where curve 1 represents the curve based on Figure 5 The control model shown in FIG. 2 is based on the control model provided by the present disclosure. Figure 4 As can be clearly seen from the control model shown in the figure, curve 2 is relatively smooth without drastic fluctuations, while curve 1 has a significant decrease between 10Hz-100Hz. Fig.10 Figure b is a graph showing the phase of the sequence impedance changing with frequency, where curve 3 represents the phase of the sequence impedance based on Figure 5 The control model shown in FIG. 4 is based on the control model provided by the present disclosure. Figure 4 The control model shown in the figure shows that curve 4 is relatively stable between 1Hz-1000Hz without drastic fluctuations, while curve 3 decreases significantly between 10Hz-100Hz. Fig.10 From Figures a and b, we can see that the bandwidth of the phase-locked loop mainly affects the impedance of the converter in the medium and low frequency bands; as the bandwidth of the phase-locked loop increases, the impedance amplitude of the converter decreases significantly, and the impedance phase also decreases slightly. Therefore, by changing the bandwidth of the phase-locked loop, the frequency scanning method can be implemented. Fig.11 Figures a and b are waveform diagrams of the grid voltage and grid current obtained by using a 60 Hz sequential impedance sweep based on the control model provided in the embodiment of the present disclosure. Fig.11 It can be seen from Figure a and Figure b that the grid voltage and grid current have basically no fluctuations and are very stable.

[0090] The present disclosure also provides an energy storage system, referring to Fig.12 As shown, it includes: a controller 3, an electrically connected energy storage converter 1 and a filter circuit 2, the filter circuit 2 is used to be connected to the grid connection point on the grid side, the controller 3 and the energy storage converter 1 are electrically connected; the controller is used to implement any of the above control methods.

[0091] refer to Figure 1As shown, the energy storage system includes an energy storage converter 1, which includes three-phase bridge arms a, b and c, and each phase bridge arm is electrically connected to a filter circuit 2; the filter circuit 2 includes three filter units arranged in parallel, and each filter unit has a first inductor L 1 ,capacitance C and the second inductor L 2 ; First inductor L 1 One end is connected to the energy storage converter 1, and the other end is connected to the capacitor C and the second inductor L 2 Connection; Second Inductor L 2 Not with the first inductor L 1 The connected end is used to connect to the grid point on the grid side; the capacitor of each filter unit C Not with the first inductor L 1 The connected ends are connected.

[0092] In this embodiment, the relevant descriptions of various structures and control methods can refer to the aforementioned embodiments and will not be repeated here.

[0093] There is no limitation on the type of controller here. The controller may include an MCU (Microcontroller Unit) chip, an FPGA (Field Programmable Gate Array) chip, a DSP (Digital Signal Processor) chip or an ARM (Advanced RISC Machines) chip. Of course, other types of chips may also be included.

[0094] The energy storage system can adapt well to complex systems with variable grid frequency and amplitude, and can effectively suppress the voltage instability of the capacitor on the converter side caused by the variable grid frequency and amplitude, thereby causing wide-band oscillation; the energy storage system provided by the embodiment of the present disclosure has strong stability and wider applicability.

[0095] Those skilled in the art can understand that the above-mentioned embodiments are specific examples for realizing the present disclosure, and in practical applications, various changes can be made to them in form and details without departing from the spirit and scope of the present disclosure. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present disclosure, so the protection scope of the present disclosure shall be based on the scope defined in the claims.

Claims

1. A control method for an energy storage system, characterized in that: The energy storage system includes an electrically connected energy storage converter and a filter circuit, wherein the filter circuit is also used to be connected to a grid connection point on the grid side; The control method comprises: establishing a control model according to a preset principle; The control model is established according to the preset principles, including: Establishing a current control loop; the current control loop is used to control the current signal output by the energy storage converter and feed back the current output by the filter circuit to the input end of the current control loop; A capacitor voltage feedforward loop is established; the capacitor voltage feedforward loop is used to feed forward the voltage signal of the capacitor of the filter circuit to the input end of the current control loop.

2. The control method according to claim 1, characterized in that: The energy storage converter includes a three-phase bridge arm, each phase bridge arm is electrically connected to the filter circuit; the filter circuit includes three filter units arranged in parallel, each of the filter units includes a first inductor, a capacitor and a second inductor; one end of the first inductor is connected to the energy storage converter, and the other end is connected to the capacitor and the second inductor respectively; the end of the second inductor not connected to the first inductor is used to be connected to the grid connection point on the grid side; The ends of the capacitors of the filter units that are not connected to the first inductor are all connected; The establishing of the current control loop comprises: Will G c ( s )、 G d ( s )、 K pwm , 1 / ( sL 1)、1 / ( sC )、1 / ( sL 2) connected in sequence; wherein, the G c ( s ) represents the transfer function of the proportional resonant controller, the G d ( s ) represents the transfer function of the data delay device, K pwm represents the equivalent gain of the energy storage converter, L 1 represents the inductance value of the first inductor, C represents the capacitance value of the capacitor, L 2 represents the inductance value of the second inductor, s represents a complex frequency domain variable; The 1 / ( sL 2) Current signal transmitted to the grid side i g ( s ) feedback to the G c ( s ) to make the i g ( s ) with preset reference current i ref ( s ) after synthesis and then transmitted to the G c ( s ); The voltage signal of the grid connection point on the grid side u pcc ( s ) and the 1 / ( sC ) output signals are synthesized and transmitted to the 1 / ( sL 2); The establishing of the capacitor voltage feed-forward loop comprises: The 1 / ( sC ) The output signal is sequentially G n ( s ) and 1 / K pwm After the output, feed forward to the G d ( s ) to make G c ( s ) The output signal is related to the 1 / K pwm The output signals are synthesized and transmitted to the G d ( s ); wherein, the G n ( s ) represents the transfer function of the notch filter.

3. The control method according to claim 2, characterized in that: After the capacitor voltage feedforward loop is established, the control model is established according to a preset principle, and further includes: The i g ( s ) is fed back to the 1 / ( sC ) to make the i g ( s ) and the 1 / ( sL 1) After the output signal is synthesized, it is transmitted to the 1 / ( sC ); The 1 / ( sC ) The output signal is fed back to the 1 / ( sL 1) at the front end so that the 1 / ( sC ) output signal and the K pwm After the output signal is synthesized, it is transmitted to the 1 / ( sL 1).

4. The control method according to claim 2, characterized in that: G c ( s )= K p +2×20×s / (s^2+2×s+(100×pi)^2), where K p It represents the proportionality coefficient and pi is the circumference of a circle.

5. The control method according to claim 2, characterized in that: G d ( s )=exp(-1.5×s×Ts), where Ts represents the sampling period.

6. The control method according to claim 2, characterized in that: G n ( s )=(s^2+ W n ^2) / (s^2+ W n ×s+ W n ^2) Among them, W n is the undamped natural oscillation frequency of the notch filter, W n =2×pi×frequency of the notch filter, where pi is the ratio of the circumference of a circle.

7. The control method according to any one of claims 1 to 6, characterized in that: After the control model is established according to the preset principle, the control method further includes: Establishing a sequence impedance model; the sequence impedance model includes: Among them, the IR csp ( s ) represents the positive sequence impedance value, the Z gp ( s ) represents the positive sequence equivalent impedance of the grid side, the Z csp ( s ) represents the positive sequence equivalent impedance on the converter side, IR csn ( s ) represents the negative sequence impedance value, the Z gn ( s ) represents the negative sequence equivalent impedance of the grid side, the Z csn ( s ) represents the negative sequence equivalent impedance of the converter side; According to the sequence impedance model, respectively calculating the sequence impedance values ​​after applying the disturbance signal to the grid connection point on the grid side under different disturbance parameters; Establishing a sequence impedance characteristic curve according to the disturbance parameter and the sequence impedance value; According to the sequence impedance characteristic curve, based on the Nyquist stability criterion, the stability of the control model is judged.

8. The control method according to claim 7, characterized in that: After establishing a sequence impedance characteristic curve according to the disturbance parameter and the sequence impedance value, the control method further includes: The transfer function of the control model is modified according to the sequence impedance characteristic curve.

9. The control method according to claim 8, characterized in that: The calculating, according to the sequence impedance model, the sequence impedance values ​​when the grid connection point on the grid side adds a disturbance signal under different disturbance parameters respectively includes: Detecting a first phase voltage of the grid connection point and a first phase current of the grid connection point when no disturbance signal is applied to the grid connection point on the power grid side; Under the same disturbance parameters, applying a disturbance signal to the grid connection point on the grid side; Detecting a second phase voltage of the grid connection point and a second phase current of the grid connection point after a disturbance signal is applied to the grid connection point on the power grid side; calculating a first difference between the first phase voltage and the second phase voltage, and a second difference between the first phase current and the second phase current; Respectively obtaining a positive sequence component and a negative sequence component of the first difference and the second difference; The positive sequence component and the negative sequence component of the first difference and the second difference are converted from the time domain to the frequency domain, and the sequence impedance value under the disturbance parameter is calculated according to the sequence impedance model.

10. An energy storage system, characterized in that: include: A controller, an electrically connected energy storage converter and a filter circuit, wherein the filter circuit is used to be connected to a grid connection point on the grid side, and the controller is electrically connected to the energy storage converter; The controller is used to implement the control method described in any one of claims 1-9.

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