A method and device for controlling energy storage converter under weak power grid conditions

Abstract

The present disclosure is about a control method and device for energy storage converter under weak power grid conditions. The method includes: establishing a single-stage PCS mathematical model based on a single-stage two-level topology structure of an energy storage converter PCS, and analyzing the PCS grid-connected operation control strategy and off-grid operation control strategy respectively; establishing a PCS equivalent output impedance model, and based on the PCS equivalent output impedance model, analyzing the stability of the PCS system under weak power grid conditions, and generating a PCS system stability criterion under weak power grid conditions; generating an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI to complete the control of the energy storage converter under weak power grid conditions. The present disclosure analyzes the energy storage converter as the control object, and realizes the stable control of the energy storage converter under weak power grid conditions based on the improved LC filter capacitor voltage feedforward control strategy based on the dual second-order generalized integral phase-locked loop DSOGI.

Description

Technical Field

[0001] The present disclosure relates to the field of energy storage converter control, and in particular, to a method and device for controlling an energy storage converter under weak power grid conditions. Background Art

[0002] As global energy shortages and environmental pollution become increasingly severe, building an energy architecture based on renewable clean energy has become a core strategic measure for industrial development in various countries. However, with the "high proportion" of random and volatile distributed power sources such as wind power and photovoltaic power being incorporated into the power grid and the "high proportion of power electronics" new load access, the power supply quality of microgrids is seriously affected. At the same time, microgrids gradually show the nature of "weak grids" on the basis of "double highs". Under weak grid conditions, conventional control strategies will introduce background harmonics into the grid, seriously affecting the stability of the system.

[0003] As the core equipment of renewable energy power generation system, the energy storage converter (Power Conversion System, PCS) undertakes important tasks in microgrids, such as bidirectional energy flow, power conversion, and stable system operation. Under weak grid characteristics, the control performance of the energy storage converter / grid-connected inverter will be affected, including causing grid-connected current instability and grid-connected current distortion.

[0004] Therefore, one or more methods are needed to solve the above problems.

[0005] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute the prior art known to ordinary technicians in the field. Summary of the invention

[0006] The purpose of the present disclosure is to provide a method and device for controlling an energy storage converter under weak power grid conditions, thereby overcoming one or more problems caused by limitations and defects of related technologies at least to a certain extent.

[0007] According to one aspect of the present disclosure, a method for controlling an energy storage converter under a weak power grid condition is provided, comprising:

[0008] A single-stage PCS mathematical model is established based on a single-stage two-level topology structure energy storage converter PCS, and a PCS grid-connected operation control strategy and an off-grid operation control strategy are analyzed based on the single-stage PCS mathematical model;

[0009] Establish a PCS equivalent output impedance model, analyze the stability of the PCS system under weak power grid conditions based on the PCS equivalent output impedance model, generate a PCS system stability criterion under weak power grid conditions, and analyze the LC type PCS output impedance under weak power grid conditions;

[0010] Based on the PCS system stability criterion under the weak grid condition, an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI is generated to complete the control of the energy storage converter under the weak grid condition.

[0011] In an exemplary embodiment of the present disclosure, the method further includes:

[0012] Based on the single-stage two-level topology structure of the energy storage converter PCS, a PCS mathematical model of the single-stage PCS when it is connected to the grid and a PCS AC small signal mathematical model of the PCS when it is off-grid are established respectively.

[0013] In an exemplary embodiment of the present disclosure, the method further includes:

[0014] Based on the PCS mathematical model of the single-stage PCS during grid-connected operation, the charging control strategy, discharging control strategy, charging / discharging switching control strategy, and grid-connected harmonic suppression during PCS grid-connected operation are analyzed respectively.

[0015] In an exemplary embodiment of the present disclosure, the method further includes:

[0016] Based on the PCS AC small signal mathematical model of the single-stage PCS when operating off-grid, the discharge voltage amplitude and voltage-frequency ratio V / f control strategy and the output three-phase voltage imbalance suppression control strategy when the PCS is operating off-grid are analyzed respectively.

[0017] In an exemplary embodiment of the present disclosure, the method further includes:

[0018] Based on the PCS mathematical model of a single-stage PCS when it is connected to the grid and the PCS AC small signal mathematical model when it is connected to the grid, the PCS on-grid / off-grid switching control strategy is analyzed.

[0019] In an exemplary embodiment of the present disclosure, the method further includes:

[0020] Based on the impedance analysis method, the PCS equivalent output impedance model is established to analyze the stability of the PCS system under weak power grid conditions. The PCS system stability criterion under weak power grid conditions is generated as follows: the PCS can work stably under strong power grid conditions and the ratio of the power grid impedance to the PCS impedance under weak power grid conditions is Z. g (s) / Z o (s) Whether the Nyquist stability criterion is satisfied.

[0021] In an exemplary embodiment of the present disclosure, the method further includes:

[0022] The output impedance of LC-type PCS under weak power grid is analyzed, and the expression of PCS equivalent output impedance after adding capacitor voltage feedforward is generated.

[0023] In an exemplary embodiment of the present disclosure, the method further includes:

[0024] Based on the expression of the PCS equivalent output impedance after adding capacitor voltage feedforward, an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI is generated.

[0025] In an exemplary embodiment of the present disclosure, the method further includes:

[0026] Based on the Bode plot, the stability of the improved LC filter capacitor voltage feedforward control strategy based on the dual second-order generalized integrator phase-locked loop DSOGI under different weak grid conditions is analyzed.

[0027] In one aspect of the present disclosure, a control device for an energy storage converter under a weak power grid condition is provided, comprising:

[0028] A modeling module, used for establishing a single-stage PCS mathematical model based on a single-stage two-level topology structure energy storage converter PCS, and analyzing the PCS grid-connected operation control strategy and off-grid operation control strategy based on the single-stage PCS mathematical model;

[0029] A criterion generation module is used to establish a PCS equivalent output impedance model, analyze the stability of the PCS system under weak power grid conditions based on the PCS equivalent output impedance model, generate a PCS system stability criterion under weak power grid conditions, and analyze the LC type PCS output impedance under weak power grid conditions;

[0030] The control strategy generation module is used to generate an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI based on the PCS system stability criterion under the weak power grid condition, so as to complete the control of the energy storage converter under the weak power grid condition.

[0031] A control method for an energy storage converter under weak grid conditions in an exemplary embodiment of the present disclosure includes: establishing a single-stage PCS mathematical model based on a single-stage two-level topology structure of an energy storage converter PCS, and analyzing the PCS grid-connected operation control strategy and off-grid operation control strategy respectively; establishing a PCS equivalent output impedance model, and based on the PCS equivalent output impedance model, analyzing the stability of the PCS system under weak grid conditions, and generating a PCS system stability criterion under weak grid conditions; generating an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI, and completing the control of the energy storage converter under weak grid conditions. The present disclosure analyzes the energy storage converter as the control object, and realizes the stable control of the energy storage converter under weak grid conditions based on the improved LC filter capacitor voltage feedforward control strategy based on the dual second-order generalized integral phase-locked loop DSOGI.

[0032] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The above and other features and advantages of the present disclosure will become more apparent by describing in detail example embodiments thereof with reference to the attached drawings.

[0034] Figure 1 A flow chart of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0035] Figure 2 A two-level energy storage converter topology diagram is shown according to a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure;

[0036] Figure 3 A PCS equivalent AC single-phase model circuit diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0037] Figure 4 A schematic diagram of an abc three-phase stationary coordinate system, an αβ two-phase stationary coordinate system, and a dq synchronous rotating coordinate system of a control method for an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0038] Figure 5 A PCS grid-connected AC side mathematical model diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0039] Figure 6 A PCS grid-connected DC side mathematical model diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0040] Figure 7 A PCS off-grid AC small signal mathematical model diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0041] Figure 8 A diagram showing the operation effect of a conventional control strategy of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure under a strong / weak power grid condition is shown;

[0042] Fig. 9 A three-phase LC full-bridge inverter circuit diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0043] Fig.10 An equivalent simplified circuit diagram of a three-phase LC full-bridge inverter circuit of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0044] Fig.11 A PCS Norton equivalent circuit model diagram under weak power grid of a method for controlling an energy storage converter under weak power grid conditions according to an exemplary embodiment of the present disclosure is shown;

[0045] Fig.12 A control block diagram of a three-phase LC type PCS under weak power grid conditions is shown according to a method for controlling an energy storage converter under weak power grid conditions according to an exemplary embodiment of the present disclosure;

[0046] Fig.13 A PCS circuit equivalent diagram under weak power grid of a method for controlling an energy storage converter under weak power grid conditions according to an exemplary embodiment of the present disclosure is shown;

[0047] Fig.14 FIG. 1 shows a control method for an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure. LCL Bode plot of (s);

[0048] Fig.15 A block diagram of a PCS system current control using active damping feedback of LC filter capacitor current under weak power grid conditions is shown according to a method for controlling an energy storage converter under weak power grid conditions according to an exemplary embodiment of the present disclosure;

[0049] Fig.16 A simplified schematic diagram of a current control block diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0050] Fig.17 A simplified schematic diagram of a current control block diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0051] Fig.18 A simplified schematic diagram of a current control block diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0052] Fig.19 A simplified schematic diagram of a current control block diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0053] Fig. 20 A current control block diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure with capacitor voltage feedforward added is shown;

[0054] Fig.21 A current control equivalent transformation control block diagram with capacitor voltage feedforward added to a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0055] Fig. 22 A simplified block diagram of current equivalent after adding capacitor voltage feedforward is shown for a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure;

[0056] Fig.23 A block diagram of a feedforward control strategy of an improved LC filter capacitor based on DSOGI of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0057] Fig.24 A DSOGI-PLL system structure control block diagram of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0058] Figures 25A-25D The output impedance Bode diagram under different PCS controls of a method for controlling an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown;

[0059] Fig.26 A schematic block diagram of a control device for an energy storage converter under a weak power grid condition according to an exemplary embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0060] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be comprehensive and complete and will fully convey the concepts of the example embodiments to those skilled in the art. The same reference numerals in the figures represent the same or similar parts, and thus their repeated description will be omitted.

[0061] In addition, the described features, structures or characteristics may be combined in one or more embodiments in any suitable manner. In the following description, many specific details are provided to provide a full understanding of the embodiments of the present disclosure. However, those skilled in the art will appreciate that the technical solutions of the present disclosure may be practiced without one or more of the specific details, or other methods, components, materials, devices, steps, etc. may be adopted. In other cases, known structures, methods and devices, implementations, materials or operations are not shown or described in detail to avoid blurring the various aspects of the present disclosure.

[0062] The block diagrams shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically independent entities. That is, these functional entities may be implemented in software form, or these functional entities or parts of functional entities may be implemented in one or more software hardened modules, or these functional entities may be implemented in different networks and / or processor devices and / or microcontroller devices.

[0063] In this exemplary embodiment, a method for controlling an energy storage converter under a weak power grid condition is first provided; Figure 1 As shown in , the energy storage converter control method under weak power grid conditions may include the following steps:

[0064] Step S110, establishing a single-stage PCS mathematical model based on the energy storage converter PCS with a single-stage two-level topology structure, and analyzing the PCS grid-connected operation control strategy and off-grid operation control strategy based on the single-stage PCS mathematical model;

[0065] Step S120, establishing a PCS equivalent output impedance model, analyzing the stability of the PCS system under a weak power grid condition based on the PCS equivalent output impedance model, generating a PCS system stability criterion under a weak power grid condition, and analyzing the LC type PCS output impedance under a weak power grid;

[0066] Step S130, based on the PCS system stability criterion under the weak grid condition, an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI is generated to complete the control of the energy storage converter under the weak grid condition.

[0067] A control method for an energy storage converter under weak grid conditions in an exemplary embodiment of the present disclosure includes: establishing a single-stage PCS mathematical model based on a single-stage two-level topology structure of an energy storage converter PCS, and analyzing the PCS grid-connected operation control strategy and off-grid operation control strategy respectively; establishing a PCS equivalent output impedance model, and based on the PCS equivalent output impedance model, analyzing the stability of the PCS system under weak grid conditions, and generating a PCS system stability criterion under weak grid conditions; generating an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI, and completing the control of the energy storage converter under weak grid conditions. The present disclosure analyzes the energy storage converter as the control object, and realizes the stable control of the energy storage converter under weak grid conditions based on the improved LC filter capacitor voltage feedforward control strategy based on the dual second-order generalized integral phase-locked loop DSOGI.

[0068] Next, a control method for an energy storage converter under a weak power grid condition in this exemplary embodiment will be further described.

[0069] Embodiment 1:

[0070] In step S110, a single-stage PCS mathematical model may be established based on the energy storage converter PCS with a single-stage two-level topology structure, and the PCS grid-connected operation control strategy and off-grid operation control strategy may be analyzed based on the single-stage PCS mathematical model.

[0071] In the embodiment of this example, the method further includes:

[0072] Based on the single-stage two-level topology structure of the energy storage converter PCS, a PCS mathematical model of the single-stage PCS when it is connected to the grid and a PCS AC small signal mathematical model of the PCS when it is off-grid are established respectively.

[0073] In the embodiment of this example, the method further includes:

[0074] Based on the PCS mathematical model of the single-stage PCS during grid-connected operation, the charging control strategy, discharging control strategy, charging / discharging switching control strategy, and grid-connected harmonic suppression during PCS grid-connected operation are analyzed respectively.

[0075] In the embodiment of this example, the method further includes:

[0076] Based on the PCS AC small signal mathematical model of the single-stage PCS when operating off-grid, the discharge voltage amplitude and voltage-frequency ratio V / f control strategy and the output three-phase voltage imbalance suppression control strategy when the PCS is operating off-grid are analyzed respectively.

[0077] In the embodiment of this example, the method further includes:

[0078] Based on the PCS mathematical model of a single-stage PCS when it is connected to the grid and the PCS AC small signal mathematical model when it is connected to the grid, the PCS on-grid / off-grid switching control strategy is analyzed.

[0079] In step S120, a PCS equivalent output impedance model can be established, based on which the stability of the PCS system under weak grid conditions is analyzed, a PCS system stability criterion under weak grid conditions is generated, and the LC type PCS output impedance under weak grid conditions is analyzed.

[0080] In the embodiment of this example, the method further includes:

[0081] Based on the impedance analysis method, the PCS equivalent output impedance model is established to analyze the stability of the PCS system under weak power grid conditions. The PCS system stability criterion under weak power grid conditions is generated as follows: the PCS can work stably under strong power grid conditions and the ratio of the power grid impedance to the PCS impedance under weak power grid conditions is Z. g (s) / Z o (s) Whether the Nyquist stability criterion is satisfied.

[0082] In step S130, based on the PCS system stability criterion under the weak grid condition, an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI can be generated to complete the control of the energy storage converter under the weak grid condition.

[0083] In the embodiment of this example, the method further includes:

[0084] The output impedance of LC-type PCS under weak power grid is analyzed, and the expression of PCS equivalent output impedance after adding capacitor voltage feedforward is generated.

[0085] In the embodiment of this example, the method further includes:

[0086] Based on the expression of the PCS equivalent output impedance after adding capacitor voltage feedforward, an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI is generated.

[0087] In the embodiment of this example, the method further includes:

[0088] Based on the Bode plot, the stability of the improved LC filter capacitor voltage feedforward control strategy based on the dual second-order generalized integrator phase-locked loop DSOGI under different weak grid conditions is analyzed.

[0089] Embodiment 2:

[0090] In the embodiment of this example, the two-level PCS topology has the advantages of simple structure and less switching power devices, so the subject takes the unipolar two-level topology as the research object. Figure 2 As shown, T a1 ~T c2 abc is the three-phase power switch tube; e a 、e b 、e c is the three-phase voltage of the AC power grid; u oa 、u ob 、u oc is the output phase voltage of the PCS inverter side, i oa 、i ob 、i oc is the output current of the PCS inverter side; i ga 、i gb 、i gc is the three-phase current of PCS grid-connected; i La 、i Lb 、i Lc is the three-phase load current; L a , L b , L c is the filter inductor on the output side of the PCS inverter; R d is the equivalent resistance of the filter inductor at the output side of the PCS inverter; C a , C b , C c is the filter capacitor; u dc is the DC bus voltage, C 1 , C 2 is the DC bus capacitance; QS 1 , QS 2 K is the on-grid switch; 1 is a load switch. When QS 1 , QS 2 Closed, K 1 When disconnected, PCS is in grid-connected mode; when QS 1 , QS 2 Disconnect, K 1 When closed, the PCS is in off-grid mode.

[0091] Depend on Figure 2 It can be seen that the DC side of the single-stage two-level PCS interacts with the AC side (three-phase symmetrical) power grid through the energy storage medium through the bidirectional DC / AC converter (composed of six switching devices), and is connected to the local load or power grid through the LC filter. Figure 3 The equivalent AC single-phase model circuit diagram, where U dc is the DC side voltage; u o is the output voltage of the PCS inverter side; e g is the AC side grid voltage; io The output current of the PCS inverter side. Both the DC side and the AC side of the bidirectional DC / AC converter can be controlled, and the phase relationship of the AC side grid phase voltage and phase current can be controlled at the same time, which can realize the four-quadrant operation of the PCS.

[0092] In the embodiment of this example, PCS mathematical modeling includes:

[0093] When the PCS is in grid-connected working mode, according to Figure 2 (Assume that the three-phase filter inductance on the output side of the PCS inverter is equal to L) Circuit topology and KVL theorem, the loop voltage equation of the three phases a, b, and c is,

[0094]

[0095] Assume the AC side grid voltage is,

[0096]

[0097] In the formula, E m is the peak value of the phase voltage of the power grid.

[0098] Define the switch device function as,

[0099]

[0100] Therefore, u ao 、u bo 、u co Available dc , T k It is expressed as,

[0101]

[0102] Substituting equation (4) into equation (1), we can obtain

[0103]

[0104] Substituting the three-phase three-wire power grid balance condition into equation (5), we can get, according to the KCL and KVL circuit theorems,

[0105] u dc (T a +T b +T c )=6u No (6)

[0106] Substituting formula (6) into formula (5), we get:

[0107]

[0108] PCS DC side bus current i dcThe expression is,

[0109] 2i dc =T a i oa +T b i ob +T c i oc (8)

[0110] Defining the Matrix

[0111]

[0112] By connecting equations (7) and (8) in parallel, we get the PCS switch mathematical model expression:

[0113]

[0114] Where i o_abc It is the abbreviation of the three-phase current output by the PCS inverter side; u c_abc It is the abbreviation of the three-phase filter capacitor voltage on the PCS inverter side.

[0115] The PCS switching function model of formula (9) includes the high-frequency components in the process of device switching, which increases the difficulty of controller design. The state space averaging method can be used to linearize the above model.

[0116] The basic calculation formula for the average value of the switching cycle is introduced as follows:

[0117]

[0118] Where g(t) is an electrical variable in the PCS system; T k is the on-off cycle of the power tube;<g(t)> Ts is the average value of the switching cycle at this level state.

[0119] According to formula (10), T k The average value of the switching function of the three-phase power switch tube in the PCS is the duty cycle of each phase. The duty cycle of the three phases of the PCS is expressed by D a , D b , D c Indicates. abc The average value of the switching cycle is expressed as D abc ,

[0120]

[0121] This average value only considers the fundamental signal of the electric quantity and ignores the high-frequency signal.

[0122] Similarly, it can be deduced that

[0123]

[0124] Substituting equations (10), (11) and (12) into equation (9), we can obtain the mathematical switching model represented by the period average value of the PCS three-phase power switch tube:

[0125]

[0126] In order to facilitate the controller design, the PCS mathematical model in equation (13) is processed by coordinate transformation. Figure 4 Schematic diagram of the positional relationship between the three types of coordinate systems.

[0127] The transformation matrix from the abc coordinate system to the dq coordinate system (3s / 2r) is:

[0128]

[0129] The 2r / 3s transformation matrix is,

[0130]

[0131] From formula (15), we can see that after the coordinate change, the expressions of the PCS quantities in the dq coordinate system are:

[0132]

[0133] After substituting equation (16) into equation (15) and simplifying the obtained equation, the cycle average model of the PCS power switch tube in the synchronous coordinate system can be obtained as follows:

[0134]

[0135] The mathematical model of formula (17) includes d dq dc > Ts , dT odq > Ts The nonlinear equation can be linearized using the AC small signal method. The electrical quantity d of each DC steady-state operating point of the PCS abc 、i abc Grid voltage abc , grid-side current i gabc Decomposed into dq axis components, D d , D q ,I d ,I q 、E d 、E q 、i gd 、i gq ;u dc 、i dc The DC steady-state component is U dc ​​,I dc .

[0136] Substituting the above PCS steady-state signals into equation (17), the mathematical model expression of the steady-state operating point of PCS can be obtained as follows:

[0137]

[0138] The corresponding AC small signal disturbance expression of the PCS DC steady-state operating point is:

[0139]

[0140] Substituting equation (19) into equation (18), since the battery voltage on the DC side of the PCS is relatively stable, the AC disturbance of the DC voltage can be ignored (i.e. ), we get the PCS AC small signal model, and perform Laplace transform on it to get the expression:

[0141]

[0142] From equation (20), the mathematical model block diagram of the single-stage two-level PCS grid-connected AC small signal can be obtained as follows: Figure 5 As shown; the block diagram of the mathematical model of the small signal of the PCS grid-connected DC side is as follows Figure 6 shown.

[0143] Load voltage u in off-grid working mode Labc , load current i Labc When replacing the grid-connected working mode, the grid voltage e abc , grid current i g_abc , the PCS off-grid AC small signal mathematical model can be obtained by analogy with the grid-connected mathematical modeling process.

[0144]

[0145] Similar to the linearization process in the PCS grid-connected operation mode, the periodic average model of the power switch device in the PCS off-grid operation mode is subjected to coordinate changes and disturbance input, and the AC small signal model in the PCS off-grid operation mode can be obtained.

[0146]

[0147] The off-grid PCS AC small signal mathematical model is obtained from formula (22): Figure 7 shown.

[0148] In the embodiment of this example, the characteristics of the weak power grid and its impact on the PCS system include:

[0149] 1. Weak grid characteristics

[0150] At present, microgrids present the characteristics of "double high". A large number of distributed renewable energy sources are connected to the grid through power electronic converters, resulting in the power electronics on the power generation side of the grid system; new power electronic equipment loads such as new energy vehicles, charging piles, large DC power supply systems, and large battery energy storage centers are connected to the grid, resulting in a high degree of power electronics on the user side of the grid system. In microgrids without inertial support provided by large thermal power units, the "double high" characteristics often penetrate more seriously. A large number of nonlinear power electronic equipment causes the grid to present weak grid characteristics, which has a serious impact on the stability of system operation.

[0151] The strength of the power grid is expressed by the Short Circuit Ratio (SCR), which is

[0152]

[0153] Where, S N is the rated active power of the power generation system, S gsc is the short-circuit capacity of the power grid.

[0154]

[0155] In the formula, e g is the effective value of the grid voltage; Z g is the modulus of the grid impedance at the fundamental wave.

[0156] The specific SCR corresponding to the strength of the power grid is listed in Table 1.

[0157] Table 1 Grid strength corresponding to SCR values

[0158]

[0159] The characteristics of weak grid are mainly manifested as follows: there is a non-negligible grid impedance, there are a large number of background harmonics in the grid voltage; the frequency and amplitude of the grid voltage are affected by renewable distributed power sources.

[0160] 2. Impact of weak power grid on PCS system

[0161] The aforementioned weak grid characteristics will further affect the control performance of the energy storage converter / grid-connected inverter, mainly manifested in the following aspects:

[0162] 1) The grid impedance of the weak grid causes the grid current to be unstable

[0163] The grid impedance is mainly composed of the generator impedance, transmission line impedance and the equivalent impedance of the transformer. In a weak grid, the grid impedance will have an immeasurable impact on the current loop gain of the PCS control system, resulting in system instability.

[0164] 2) The background harmonics of the grid voltage of the weak grid will cause grid-connected current distortion

[0165] Due to the existence of grid impedance, non-negligible background harmonics will be generated in the grid voltage of weak grids, causing grid-connected current distortion, heating of equipment, increased losses and other problems. In addition, due to the existence of grid impedance, when multiple machines are connected in parallel to suppress current harmonics, the grid impedance will form interactive coupling with the harmonic compensation module, and in severe cases, the power devices will be damaged. At the same time, the phase, amplitude and frequency of the PCS grid connection point voltage under weak grid conditions are prone to sudden changes, which seriously affects the grid synchronization performance of the phase-locked loop (PLL), and the real-time performance and output accuracy of the PLL directly affect the control accuracy of the harmonic compensation command current loop.

[0166] The PCS control strategy under normal power grid will no longer be applicable to weak power grid conditions. Taking the commonly used double closed-loop PI control as an example, for the AC current inner loop output, based on the preset PCS grid-connected operation control block diagram, it can be seen that the AC side grid voltage feedforward is introduced into the AC current inner loop control output terminal during PCS operation to eliminate grid voltage disturbances. However, under weak power grid conditions, the AC side grid current control and the PCC point voltage feedforward will be coupled together through the grid impedance, and a positive feedback path will be introduced into the current inner loop, which will arouse a large number of grid current harmonics, and the stability margin of the system will be seriously deteriorated.

[0167] For the outer loop, under a weak power grid, the outer loop control of the PCS will also be affected, especially the impact on the voltage control on the output side of the PCS inverter.

[0168] Taking SCR=2 as an example, the operation effect of the conventional control strategy under the weak grid condition is simulated. The three-phase current waveform on the output side of the PCS inverter is as follows: Figure 8 shown.

[0169] from Figure 8 It can be seen that the 0-0.25s stage is the conventional control strategy under a strong power grid. It can be seen that at this time, the PCS inverter side outputs a better current waveform under the conventional control strategy.

[0170] At 0.25s, the system switches to a weak grid condition (0.25-0.5s). It can be clearly seen that the current on the output side of the PCS inverter oscillates and the waveform quality deteriorates.

[0171] The main reasons for the waveform distortion of the conventional control strategy of PCS under strong grid conditions under weak grid conditions are: (1) the grid line impedance cannot be ignored under weak grid conditions, and the conventional control strategy introduces grid voltage feedforward, which will introduce a large amount of background harmonics in the weak grid and deteriorate its control effect; (2) under weak grid conditions, the introduction of a large amount of background harmonics will interfere with the accuracy of the phase-locked loop of the PCS itself, and the accuracy of the phase-locked loop is directly related to the control accuracy of the PCS, which in turn affects the stable output of the PCS under weak grid conditions.

[0172] Comprehensive waveform analysis can conclude that the conventional control strategy of PCS under strong power grid is no longer applicable to weak power grid conditions. It can be seen that the emergence of weak power grid conditions poses a huge challenge to the control of power electronic converters such as energy storage converters / grid-connected inverters. Research is conducted to improve the output current waveform of PCS under weak power grid and improve the adaptive control of PCS under weak power grid.

[0173] In this example embodiment, the PCS system stability analysis under weak power grid

[0174] The prerequisite for optimizing the output waveform quality of PCS under weak grid conditions is to analyze the factors that affect the stability of PCS operation under weak grid conditions. Among them: Impedance analysis methods include:

[0175] The sequential impedance modeling method has the advantages of direct impedance measurement, simple impedance form, clear physical meaning and wide application range. The impedance-based stability analysis method is widely used to analyze the stability of grid-connected converters under weak power grids.

[0176] The feasibility of accurately establishing a mathematical model of a weak power grid is too complicated. Therefore, the Thevenin equivalent method is often used in practical engineering applications for simplified analysis. The Thevenin equivalent treatment of the power grid at the PCS grid connection point is mainly simplified to the power grid impedance Z g (s) and voltage source e g In series. Figure 2 The circuit diagram is simplified, the equivalent resistance of LC inductance is ignored, the DC power supply is divided into two parts, and the midpoint is set as O, and we get Fig. 9 .

[0177] According to the circuit principle, the following loop equation is listed.

[0178]

[0179]

[0180] The LC filter capacitor voltage can be expressed as

[0181]

[0182] For the three-phase three-wire connection, there is

[0183]

[0184] Substituting equations (27) and (28) into equations (25) and (26), we can obtain the following expressions:

[0185]

[0186] Because u ao 、u bo 、u co , controlled by SPWM, u ao +u bo +u co ≠0, so the N' point and the O point are not at the same potential; when the three-phase power grid is balanced, e a +e b +e c =0; therefore, point N' and point N are in equal phase. Fig. 9 It can be further simplified to Fig.10 .

[0187] Depend on Fig.10 It can be seen that there is a common-mode voltage between point N and point O, and this common-mode voltage can be compared with u ao (u bo 、u co ) are superimposed, and the voltage source series impedance form is simplified to the current source parallel impedance form, and we get Fig.11 Norton equivalent circuit diagram of PCS under weak power grid, u PCC is the grid connection point voltage of the converter, i dis (s) is the PCS grid-connected current disturbance caused by the voltage at the PCC point under weak power grid; i g (s) is the grid-connected current on the PCS inverter side.

[0188] In a weak power grid, the voltage source e g It is no longer considered as an ideal sinusoidal voltage, it contains rich background harmonics, and the fundamental voltage frequency and amplitude will also change with the load; the grid impedance Z g (s) mainly includes the transmission line impedance and the leakage reactance of the distribution transformer, which is manifested as the resistance-inductance characteristic, namely Z g (s)≈R g +sL g .

[0189] according to Fig.11 , using the circuit superposition theorem, the PCS grid-connected current expression can be obtained

[0190]

[0191] Formula (30) can be further rewritten as

[0192]

[0193] In formula (31),

[0194]

[0195] Under strong power grid (SCR>3) Z g (s) = 0, then equation (30) is

[0196]

[0197] Assuming that the root locus of equation (33) is completely on the left half axis of the complex plane, the PCS system is in a stable state under a strong power grid, and in a weak power grid, Z g (s) cannot be ignored, so the stability of PCS under weak power grid depends on the term A(s) in equation (31).

[0198] From equation (32), we can see that the forward transfer function of this equation is 1, and the negative feedback transfer function of the closed-loop system is Z g (s) / Z o (s), so the stability of the PCS system under weak power grid depends on Z g (s) / Z o (s) Whether the Nyquist stability criterion is satisfied.

[0199] In summary, the conditions that PCS needs to meet for stable operation are: (1) stable operation under strong power grid; (2) stable operation under weak power grid. g (s) / Z o (s) Whether the Nyquist stability criterion is satisfied.

[0200] In the embodiment of this example, the LC type PCS output impedance analysis under weak power grid includes:

[0201] For actual megawatt-level distributed renewable energy power generation systems, there is generally a step-up isolation transformer between the inverter output side of the converter and the grid. On the one hand, the transformer may be a certain distance away from the PCS; on the other hand, in high-voltage projects, the voltage at the PCC point is relatively high after being processed by the step-up transformer, and the requirements for the sampling circuit and signal conditioning circuit are also increased synchronously, increasing the cost. Therefore, it is difficult to collect the voltage between the transformer and the grid to achieve phase-locked synchronization. In addition, because the LC filter capacitor voltage is similar to the PCC point voltage and is in a linear relationship. Therefore, under weak grid conditions, the capacitor voltage feedforward of the LC filter is used to replace the PCC point voltage as the synchronization reference voltage of the phase-locked loop.

[0202] like Fig.12 The figure shows the three-phase LC type PCS weak power grid control block diagram.

[0203] The difference compared with the PCS basic control strategy is that: Fig.12 The AC side grid voltage feedforward is not introduced into the output end of the current inner loop. The reason is that the direct voltage feedforward of the PCC point under a weak grid will introduce a positive feedback path in the current loop, which contains relevant information of the weak grid current and weak grid impedance, and will excite a large number of current harmonics, seriously reducing the stability margin of the system; on the other hand, in order to suppress the LCL resonance spike and to reduce the loss caused by the passive damping resistor connected in series with the LC inductor of the PCS, the passive damping is changed to an active damping form of LC capacitor current proportional feedback.

[0204] Depend on Fig.12 It can be seen that the step-up transformer impedance L under weak power grid T and grid impedance L g The LC filter of PCS itself can be regarded as an equivalent LCL filter, which has the problem of resonance peak.

[0205] Fig.12 The single-phase equivalent simplified circuit is as follows Fig.13 shown.

[0206] according to Fig.13 It is easy to derive the grid-connected current i from the PCS to the grid g The transfer function expression

[0207]

[0208] In formula (34), w r is the resonant angular frequency, and the mathematical expression is

[0209]

[0210] Resonance frequency f r The expression is

[0211]

[0212] At this point, it is easy to get G LCL The Bode plot of (s),

[0213] Depend on Fig.14 It can be seen that the equivalent LCL filter formed by the weak grid impedance and the PCS system filter will be r Resonance occurs at the point and the phase also jumps, resulting in a negative crossing, making the PCS system unstable. After introducing damping, it can be seen that the resonance can be effectively suppressed. Therefore Fig.12 Active damping feedback control based on LC filter capacitor current is adopted.

[0214] right Fig.13Apply the KVL circuit law and perform Laplace transform to get

[0215]

[0216] When the capacitor current active damping feedback and capacitor voltage feedforward of the LC filter are used, the control block diagram of the grid-connected current is as follows: Fig.15 shown.

[0217] in

[0218]

[0219] is the current loop controller, k Ci is the active damping feedback coefficient.

[0220]

[0221] is the system control delay function, where T s is the system switching frequency.

[0222] To perform impedance stability analysis, Fig.15 Further arrangement, the equivalent transformation process is as follows Fig.16 , Fig.17 As shown, the dotted line indicates Fig.15 The solid lines represent the original connection method, and the solid lines represent the connection after equivalent transformation.

[0223] First, the LC filter capacitor voltage u c The lead-out point is moved forward, and the feedback point is moved forward to G PI (s) output port, get Fig.16 .

[0224] Secondly, the LC filter capacitor voltage u c and current i c The feedback point is moved forward to G PI (s) front end, and simplify these two branches to obtain Fig.17 .

[0225] right Fig.17 The gray part is closed-loop simplified and the grid current i g The feedback point is moved forward to i ref The output end of the LC filter capacitor current i c , capacitor voltage u c , grid current i g The output sampling current i of the actual PCS system o The decoupling of Fig.18 .

[0226] right Fig.18The gray part in the middle is closed-loop simplified to obtain the final equivalent current control block diagram, as shown in Fig.19 shown.

[0227] Fig.19 middle:

[0228]

[0229]

[0230]

[0231] Grid current i g The loop gain is

[0232] G ig (s) = G ig1 (s)G ig2 (s)H ig (s) (42)

[0233] At this time, the grid current i g The expression can be written as

[0234]

[0235] From formula (36), we can see that the grid-connected current i g Since it includes two parts: PCS current given component (i g1 ) and the grid voltage disturbance component (i g2 ).

[0236] The equivalent output impedance of the PCS obtained at this time is

[0237]

[0238] Generally, in order to suppress the impact current when the PCS starts, grid voltage feedforward is introduced. In the case of a weak grid, this paper uses the LC filter capacitor voltage instead of the grid voltage feedforward to suppress the startup impact current. The current control block diagram is as follows: Fig. 20 shown.

[0239] right Fig. 20 Perform equivalent processing to obtain Fig.21 , from the above Figure 14-Figure 21 Simplifying the derivation process, it is easy to obtain the equivalent simplified block diagram of the current after adding capacitor voltage feedforward as shown below: Fig. 22 As shown, Fig. 22 middle

[0240]

[0241] At this time, the grid current i g The loop gain is

[0242] G' ig (s) = G ig3 (s)G ig2 (s)H ig (s) (46)

[0243] The expression of PCS equivalent output impedance after adding capacitor voltage feedforward

[0244]

[0245] from Fig.21 In the equivalent transformation diagram, it can be seen that in order to suppress the starting impact current under weak power grid, the LC filter capacitor voltage feedforward is used to replace the grid voltage feedforward containing background harmonics, which will introduce positive feedback with grid line impedance. C (s) In order to weaken the influence of grid line impedance, it is very likely to deteriorate the waveform quality and system stability of PCS under weak grid. For this reason, this paper proposes an improved LC filter capacitor voltage feedforward control strategy based on DSOGI to improve the waveform quality under weak grid.

[0246] In the embodiment of this example, the improved LC filter capacitor voltage feedforward control strategy based on DSOGI includes first analyzing the control strategy:

[0247] The overall control block diagram of the improved LC filter capacitor voltage feed-forward (D-Capacitor Voltage-Feed-forward, DCVFF) control strategy based on DSOGI is as follows: Fig.23 shown.

[0248] The DCVFF control strategy uses two second-order generalized integrator (SOGI) orthogonal generators to process the input signal by 90° phase angle shift, which can also filter out high-order harmonics. The DSOGI-PLL control structure is shown in the figure. Fig.24 As shown, from Fig.24 It can be seen that further processing of the signal processed by SOGI can obtain the fundamental positive sequence d-axis component of the LC filter capacitor voltage.

[0249] Because the control system itself needs to use DSOGI-PLL to perform grid phase synchronization locking, during which the fundamental positive sequence d-axis component of the input signal will be calculated, this method does not increase the digital calculation amount of the system software and the hardware cost.

[0250] from Fig. 20 It can be seen that the LC filter capacitor voltage passes through G C (s) function is then fed forward to G PI (s) output, while considering the effect of DSOGI phase-locked loop, G CThe expression for (s) can be written as

[0251]

[0252] where k C1 +k C2 =1, when k C2 =1 is the conventional LC filter capacitor voltage feedforward mode, which will introduce the background harmonics under the weak power grid into the PCS control system, which may cause the system to become unstable in severe cases. C1 =1(or k C1 +k C2 =1, and k C2 <1), it is the key point of DCVFF control strategy. C1 This enables the PCS system to not only continue to suppress the startup impact current under a weak power grid, but also filter out the background harmonics under a weak power grid and optimize the PCS output waveform.

[0253] Secondly, the stability analysis of the DCVFF control strategy includes:

[0254] For k without adding LC filter capacitor voltage and DCVFF C1 and k C2 The output impedance of the PCS (Z o1 and Z o2 ) for analysis, the specific Bode diagram is as follows Figures 25A-25D shown.

[0255] According to the output impedance stability criterion analysis, in order for the PCS system to be able to output stably under weak grid conditions, the following conditions need to be met:

[0256] (1) PCS is in a stable state under strong power grid conditions;

[0257] (2) Z g (s) / Z o (s) satisfies the Nyquist stability criterion, that is, Z o (s) The intersection of the amplitude curve and the SCR (grid impedance) at different levels must be within 180° in the corresponding phase Bode diagram, that is, Figure 4-18 The longer the double arrow of the phase margin marked in the middle is, the better (below 90°).

[0258] Based on the above analysis Figures 25A-25D Bode plot of PCS equivalent output impedance, Fig.25A Analysis chart below:

[0259] (1) From Fig.25A It can be seen that under weak grid conditions (SCR = 2), the PCS equivalent output impedance Z without the optimization control strategy iso1 (s) has better amplitude margin and phase margin.

[0260] (2) After adding the DCVFF control strategy (k C1 =1, k C2 =0), it can be seen that the equivalent output impedance Z of PCS at this time o2 (s) in the low frequency band and Z o1 Compared with (s), the amplitude margin and phase are improved, and the phase margin at the intersection with SCR = 2 (under weak grid conditions) is close to Z o1 Compared with (s), the phase margin is improved. It can be concluded that the DCVFF control strategy can improve the grid adaptability of the PCS system under weak grid conditions.

[0261] (3) After adding the DCVFF control strategy (k C1 =0, k C2 =1), we can see that the equivalent output impedance Z of PCS is o2 (s) in the low frequency band and Z o1 (s) Compared with the amplitude margin, there is a significant improvement, which can prove that the control under this coefficient allocation can enable the PCS system to have a better ability to suppress the background harmonics of the power grid in the low frequency band; however, the overall phase curve is no longer within the range of 180°, and the PCS system is in an unstable working mode.

[0262] Fig.25B The analysis is as follows:

[0263] The equivalent output impedance Z of the PCS after adding the DCVFF control strategy in the amplitude curve Bode diagram o2 (s) from top to bottom k C1 and k C2 The values ​​of are shown in Table 2.

[0264] Table 2 Changes in k under DCVFF control strategy C1 Coefficient, k C2 Keep it the same

[0265]

[0266] Combining Table 2 and Fig.25B , it can be seen that in k C2 When the value is fixed, k C1 The larger the value of o2 (s) The higher the amplitude margin in the low frequency band, the larger the corresponding phase margin at SCR = 2 (under weak grid conditions). This proves that the DCVFF control strategy under this value condition can make the PCS system have better weak grid adaptability.

[0267] Fig.25CThe analysis is as follows:

[0268] The equivalent output impedance Z of the PCS after adding the DCVFF control strategy in the amplitude curve Bode diagram o2 (s) from top to bottom k C1 and k C2 The values ​​of are shown in Table 3.

[0269] Table 3 k under DCVFF control strategy C1 The coefficient remains unchanged, changing k C2 coefficient

[0270]

[0271] Combining Table 3 and Fig.25C , it can be seen that in k C1 When the value is fixed, k C2 The larger the value of o2 (s) The higher the amplitude margin is in the low frequency band, the overall PCS phase curve under this value condition is no longer within the 180° range, and the PCS system is in an unstable working mode.

[0272] Fig.25D The analysis is as follows:

[0273] The equivalent output impedance Z of the PCS after adding the DCVFF control strategy in the amplitude curve Bode diagram o2 (s) from top to bottom k C1 and k C2 The values ​​of are shown in Table 4.

[0274] Table 4 Changes in k under DCVFF control strategy C1 and k C2 coefficient

[0275]

[0276] Combining Table 4 and Fig.25D , it can be seen that in k C1 The value gradually increases, k C2 When the value of Z gradually decreases, o2 (s) Although the amplitude margin in the low frequency band is reduced, it is generally higher than the Z without the DCVFF strategy. o1 The magnitude of (s) indicates that the DCVFF control strategy can improve the robustness of the PCS low frequency band; C1 The value gradually increases, k C2 The value of Z gradually decreases. o2 The phase curve of (s) also gradually enters the stable region, and as k C1 The value gradually increases, and the phase margin at SCR=2 (under weak grid conditions) also gradually increases.

[0277] Comprehensive Bode plot curve and k C1 , k C2 From the value transformation analysis, we can know that the DCVFF control strategy (k C1 =1, k C2 =0), which can ensure that the PCS has sufficient amplitude margin and phase margin in a weak power grid, improve the grid adaptability of the PCS in a weak power grid, and meet the robustness in a strong / weak power grid.

[0278] It should be noted that, although the steps of the method in the present disclosure are described in a specific order in the drawings, this does not require or imply that the steps must be performed in this specific order, or that all the steps shown must be performed to achieve the desired results. Additionally or alternatively, some steps may be omitted, multiple steps may be combined into one step, and / or one step may be decomposed into multiple steps, etc.

[0279] In addition, in this exemplary embodiment, a control device for an energy storage converter under a weak power grid condition is also provided. Figure 4 As shown, the energy storage converter control device 200 under a weak power grid condition may include: a modeling module 210, a criterion generation module 220 and a control strategy generation module 230. Among them:

[0280] A modeling module 210 is used to establish a single-stage PCS mathematical model based on a single-stage two-level topology structure energy storage converter PCS, and analyze the PCS grid-connected operation control strategy and off-grid operation control strategy based on the single-stage PCS mathematical model;

[0281] A criterion generation module 220 is used to establish a PCS equivalent output impedance model, analyze the stability of the PCS system under weak power grid conditions based on the PCS equivalent output impedance model, generate a PCS system stability criterion under weak power grid conditions, and analyze the LC type PCS output impedance under weak power grid conditions;

[0282] The control strategy generation module 230 is used to generate an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI based on the PCS system stability criterion under the weak grid condition, so as to complete the control of the energy storage converter under the weak grid condition.

[0283] The specific details of each of the above-mentioned energy storage converter control device modules under weak grid conditions have been described in detail in a corresponding energy storage converter control method under weak grid conditions, and will not be repeated here.

[0284] It should be noted that although several modules or units of the energy storage converter control device 200 under a weak power grid condition are mentioned in the above detailed description, this division is not mandatory. In fact, according to the embodiment of the present disclosure, the features and functions of two or more modules or units described above can be concretized in one module or unit. Conversely, the features and functions of one module or unit described above can be further divided into multiple modules or units to be concretized.

[0285] In addition, the above-mentioned figures are only schematic illustrations of the processes included in the method according to an exemplary embodiment of the present invention, and are not intended to be limiting. It is easy to understand that the processes shown in the above-mentioned figures do not indicate or limit the time sequence of these processes. In addition, it is also easy to understand that these processes can be performed synchronously or asynchronously, for example, in multiple modules.

[0286] Those skilled in the art will readily appreciate other embodiments of the present disclosure after considering the specification and practicing the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the present disclosure that follow the general principles of the present disclosure and include common knowledge or customary technical means in the art that are not disclosed in the present disclosure. The specification and examples are to be considered as exemplary only, and the true scope and spirit of the present disclosure are indicated by the claims.

[0287] It should be understood that the present disclosure is not limited to the exact structures that have been described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof. The scope of the present disclosure is limited only by the appended claims.

Claims

1. A method for controlling an energy storage converter under weak power grid conditions, characterized in that: The method comprises: A single-stage PCS mathematical model is established based on a single-stage two-level topology structure energy storage converter PCS, and a PCS grid-connected operation control strategy and an off-grid operation control strategy are analyzed based on the single-stage PCS mathematical model; Based on the impedance analysis method, a PCS equivalent output impedance model is established. Based on the PCS equivalent output impedance model, the stability of the PCS system under weak power grid conditions is analyzed, and the PCS system stability criterion under weak power grid conditions is generated as follows: the PCS can work stably under strong power grid conditions and the ratio of the power grid impedance to the PCS impedance under weak power grid conditions is Z. g (s) / Z o (s) Whether the Nyquist stability criterion is met, and the output impedance of the LC-type PCS under weak power grid is analyzed, and the expression of the equivalent output impedance of the PCS after adding capacitor voltage feedforward is generated; Based on the PCS system stability criterion under the weak grid condition and the expression of the PCS equivalent output impedance after adding capacitor voltage feedforward, an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI is generated to complete the control of the energy storage converter under the weak grid condition.

2. The method according to claim 1, characterized in that The method further comprises: Based on the single-stage two-level topology structure of the energy storage converter PCS, a PCS mathematical model of the single-stage PCS when it is connected to the grid and a PCS AC small signal mathematical model of the PCS when it is off-grid are established respectively.

3. The method according to claim 2, characterized in that The method further comprises: Based on the PCS mathematical model of the single-stage PCS during grid-connected operation, the charging control strategy, discharging control strategy, charging / discharging switching control strategy, and grid-connected harmonic suppression during PCS grid-connected operation are analyzed respectively.

4. The method according to claim 3, characterized in that The method further comprises: Based on the PCS AC small signal mathematical model of the single-stage PCS when operating off-grid, the discharge voltage amplitude and voltage-frequency ratio V / f control strategy and the output three-phase voltage imbalance suppression control strategy when the PCS is operating off-grid are analyzed respectively.

5. The method according to claim 4, characterized in that The method further comprises: Based on the PCS mathematical model of a single-stage PCS when it is connected to the grid and the PCS AC small signal mathematical model when it is connected to the grid, the PCS on-grid / off-grid switching control strategy is analyzed.

6. The method according to claim 1, characterized in that The method further comprises: Based on the Bode plot, the stability of the improved LC filter capacitor voltage feedforward control strategy based on the dual second-order generalized integrator phase-locked loop DSOGI under different weak grid conditions is analyzed.

7. A control device for an energy storage converter under weak power grid conditions, characterized in that: The device comprises: A modeling module, used for establishing a single-stage PCS mathematical model based on a single-stage two-level topology structure energy storage converter PCS, and analyzing the PCS grid-connected operation control strategy and off-grid operation control strategy based on the single-stage PCS mathematical model; The criterion generation module is used to establish a PCS equivalent output impedance model based on the impedance analysis method, analyze the stability of the PCS system under weak power grid conditions based on the PCS equivalent output impedance model, and generate the PCS system stability criterion under weak power grid conditions: the PCS can work stably under strong power grid conditions and the grid impedance and PCS impedance ratio Z under weak power grid conditions g (s) / Z o (s) Whether the Nyquist stability criterion is met, and the output impedance of the LC-type PCS under weak power grid is analyzed, and the expression of the equivalent output impedance of the PCS after adding capacitor voltage feedforward is generated; The control strategy generation module is used to generate an improved LC filter capacitor voltage feedforward control strategy based on a dual second-order generalized integral phase-locked loop DSOGI based on the PCS system stability criterion under the weak power grid condition and the expression of the PCS equivalent output impedance after adding capacitor voltage feedforward, so as to complete the control of the energy storage converter under the weak power grid condition.

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