A method for analyzing transient stability of a follow-net and construct-net hybrid system considering capacity matching

Abstract

A transient stability analysis method for grid-connected and grid-connected hybrid systems considering capacity allocation, belonging to the field of transient stability analysis technology for new energy power systems, includes the following steps: S1, establishing a hybrid system model considering capacity allocation for grid-connected and grid-connected converters; S2, based on the hybrid system models, establishing a synchronous machine-like equivalent model of the hybrid system considering capacity allocation; S3, based on the synchronous machine-like equivalent model, analyzing the influence mechanism of capacity allocation on transient synchronization characteristics. This method is used to reveal the influence mechanism of the capacity configuration of the two types of converters on the transient synchronization stability of hybrid systems, and provides a basis for the optimized configuration and stable operation of converter capacity in high-proportion new energy scenarios.

Description

Technical Field

[0001] This invention relates to the field of transient stability analysis technology for new energy power systems, and in particular to a transient stability analysis method for grid-connected and hybrid grid-connected systems that takes into account capacity matching. Background Technology

[0002] In high-proportion renewable energy power systems, grid-following voltage source converters (GFL-VSC) and grid-forming voltage source converters (GFM-VSC) coexist for extended periods. GFL-VSC relies on phase-locked loops to track the grid voltage phase for synchronization, but its synchronization stability margin is insufficient under weak grid conditions. GFM-VSC actively establishes voltage amplitude and frequency references to provide voltage and frequency support for the system, but during large disturbances, it may trigger current limiting due to current exceeding limits, thus altering its voltage source characteristics.

[0003] Existing methods for analyzing the transient stability of hybrid systems connected to and separated from the grid primarily model from the perspectives of control strategies, control parameters, and network conditions, without incorporating the capacity ratio of the two types of converters as an explicit variable into the transient synchronization dynamic equations. However, the capacity ratio simultaneously produces two opposing effects: increasing the proportion of GFL-VSC capacity enhances its current injection capability, while decreasing the proportion of GFM-VSC capacity weakens its voltage support capability and current limiting margin during faults. This competition makes the impact of the capacity ratio on system transient stability non-monotonic. Existing technologies cannot characterize the aforementioned competitive relationship and the coupling effect between converters, making it difficult to provide a theoretical basis for capacity configuration in hybrid systems.

[0004] Therefore, it is necessary to propose a transient stability analysis method for hybrid grid-connected and grid-connected systems that takes into account capacity ratio. Summary of the Invention

[0005] The purpose of this invention is to provide a transient stability analysis method for grid-connected and grid-connected hybrid systems that takes into account capacity allocation. This method is used to reveal the influence mechanism of the two types of converter capacity configuration on the transient synchronization stability of the hybrid system, and to provide a basis for the optimized configuration and stable operation of converter capacity in high-proportion renewable energy scenarios.

[0006] This invention provides a transient stability analysis method for hybrid grid-connected and grid-connected systems that takes into account capacity allocation, comprising the following steps:

[0007] S1. Establish a hybrid system model of grid-connected converters and grid-connected converters that takes into account capacity matching.

[0008] S2. Based on the hybrid system model of the grid-connected converter and the grid-connected converter, establish an equivalent synchronous machine model of the hybrid system that takes into account the capacity ratio.

[0009] S3. Based on the equivalent model of the aforementioned synchronous machine, the influence mechanism of capacity allocation on transient synchronization characteristics is analyzed.

[0010] Furthermore, S1 includes the following steps:

[0011] S1.1, Based on the hybrid system topology, the capacity ratio of grid-connected converters and network-connected converters is defined; the hybrid system includes grid-connected converters, network-connected converters and AC power grid, and the three are connected to a common connection point through line impedance;

[0012] Among them, the grid-type converter is connected to the line impedance. The grid-type converter is connected to the PCC point via the line impedance. Connecting to the PCC point, the AC power grid passes through the equivalent impedance Connect to point PCC;

[0013] Define the port voltage and output current of the grid converter as follows: , The port voltage and output current of the grid-connected converter are respectively , The voltage at point PCC is The grid voltage is Line impedance is expressed as , , Using the grid voltage phase angle as the reference phase angle, the power angles of the grid-connected converter and the grid-connected converter are defined as follows: , ,in, To match the phase angle of the phase-locked loop output of the grid converter, To control the output phase angle of a grid-type converter, The phase angle of the grid voltage;

[0014] The ratio of the capacity of the grid converter to the total capacity is:

[0015]

[0016] in, To match the rated capacity of the grid-type converter, This refers to the rated capacity of the grid-type converter;

[0017] The total system capacity is defined as follows:

[0018]

[0019] The rated capacity of the grid-connected converter is:

[0020]

[0021] The rated capacity of the grid-type converter is:

[0022]

[0023] The system voltage reference is The rated current of the grid converter is:

[0024]

[0025] The rated current of the grid-type converter is:

[0026]

[0027] in, It depends on the selected voltage base value, when When taking the effective value of line voltage, ;when When taking the effective value of the phase voltage, .

[0028] Furthermore, S1 also includes the following steps:

[0029] S1.2, Establish the control strategy model for the grid-connected converter;

[0030] In the grid-connected converter's own synchronous coordinate system, its output current is:

[0031]

[0032] in, and These are the output currents of the grid-connected converter. shaft and Axial components;

[0033] The current component is:

[0034]

[0035] The output current amplitude of the grid-connected converter is:

[0036]

[0037] The output phase angle of the grid converter is:

[0038]

[0039] During non-fault operation, the grid-connected converter generates a current reference value based on the power reference value. If voltage-oriented control is used, after considering the capacity matching ratio, the current reference value is:

[0040]

[0041] in, To represent the d-axis component of the grid-connected converter port voltage vector in the dq coordinate system, which rotates synchronously with its PLL; the subscript 'd' indicates the d-axis, and the subscript 'L' indicates the grid-connected converter. When voltage-oriented control is used, the d-axis of the PLL coordinate system is aligned with the direction of the grid-connected converter port voltage vector, therefore... , and have ; , These are the per-unit forms of the active and reactive power reference values ​​for grid-connected converters, respectively.

[0042] set up To track the port voltage of the grid converter in the PLL coordinate system Axial components, with Let the power angle relative to the grid converter be represented, then the PLL dynamics can be expressed as:

[0043]

[0044] in, This is the PLL proportional gain coefficient. This is the integral gain coefficient of the PLL.

[0045] Furthermore, S1 also includes the following steps:

[0046] S1.3, Establish the control strategy model for the grid-type converter;

[0047] The VSG control of a grid-type converter is represented as follows:

[0048]

[0049] in, The equivalent inertia coefficient, The damping coefficient is... The synchronization angular frequency, and These are the reference value for active power and the actual output active power of the grid-type converter, respectively.

[0050] After considering capacity allocation, the active power reference value of the grid-type converter is expressed as:

[0051]

[0052] in, This is the per-unit form of the active power reference value for grid-type converters;

[0053] The dynamic equations of the grid-type converter, taking into account capacity matching, are obtained:

[0054]

[0055] The reactive power-voltage control of a grid-connected converter can be expressed as:

[0056]

[0057] in, This is a reference value for voltage amplitude. Given the no-load voltage, This is the reactive power-voltage regulation coefficient. This refers to the actual reactive power output of the grid-type converter.

[0058] Let the per-unit value of the maximum allowable current of the grid converter be... Therefore, its actual current limiting threshold is:

[0059]

[0060] When a grid-connected converter enters current-limiting mode, its output current is expressed as:

[0061]

[0062] in, The saturation current phase angle under current-limited conditions is used to describe the current-limiting current in... , The allocation between axes.

[0063] Furthermore, S2 includes the following steps:

[0064] S2.1, Deriving the terminal voltage of the grid-connected converter Axial components;

[0065] Define network equivalent quantities:

[0066]

[0067]

[0068]

[0069]

[0070] in, , and These are the line impedance of the grid-connected converter, the line impedance of the grid-type converter, and the equivalent impedance of the AC power grid, respectively. The parallel equivalent impedance of the grid-connected line impedance and the grid equivalent impedance; and These are the complex allocation coefficients corresponding to the power grid branch and the grid-type converter branch, respectively, and satisfy the following conditions: ; This refers to the series equivalent impedance from the port of the grid-connected converter to the AC power grid. The series equivalent impedance from the grid-connected converter port to the AC power grid;

[0071] The output current amplitude of the grid-connected converter is:

[0072]

[0073] Its current phase angle is ;

[0074] After considering the capacity ratio, the current limiting current amplitude of the grid-type converter is:

[0075]

[0076] in, This is the per-unit value of the maximum allowable current for a grid-connected converter; the output current of a grid-connected converter is expressed as:

[0077]

[0078] in, The current limiting capability of a grid-type converter is related to the saturation current phase angle. Proportional;

[0079] With the terminal voltage of the grid converter The axis components are represented as:

[0080] .

[0081] Furthermore, S2 also includes the following steps:

[0082] S2.2, Derive the terminal voltage and output active power of the grid-type converter;

[0083] The port voltage of a grid-connected converter is expressed as:

[0084]

[0085] Based on the complex power relationship, the output active power of the grid-type converter is:

[0086] .

[0087] Furthermore, S2 also includes the following step: S2.3, establishing a synchronous machine equivalent model for the hybrid system;

[0088] Synchronous machines are represented as:

[0089]

[0090] For a grid-type converter, its angular dynamics are expressed as:

[0091]

[0092] The synchronous machine-like model is:

[0093] .

[0094] Furthermore, S3 includes the following steps:

[0095] S3.1, Analyze the impact of capacity allocation ratio on the dynamic synchronization of grid-type converters;

[0096] The fault caused the grid voltage to drop to When the grid-type converter enters the current-limiting state, the synchronization equation with the grid-type converter is expressed as:

[0097]

[0098] At this point, the power terms are: , , , , ;

[0099] Two of them and Respected and The effects are represented as follows:

[0100]

[0101]

[0102] The mutual damping term is:

[0103] .

[0104] Furthermore, S3 also includes the following steps:

[0105] S3.2, Analyze the impact of capacity allocation on the synchronous dynamics of grid-type converters;

[0106] The motion equations of a grid converter are:

[0107]

[0108] The active power reference value of the grid-type converter during a fault is expressed as follows:

[0109]

[0110] The output power of the grid converter is:

[0111] .

[0112] Furthermore, S3 also includes the following steps:

[0113] S3.3, Analyze the impact of capacity ratio on coupling terms;

[0114] The mutual damping term in the equations for a grid converter is:

[0115]

[0116] The coupling power term in the equations of a grid converter is:

[0117] .

[0118] The present invention discloses the following technical effects: In the scenario of a hybrid system consisting of GFL-VSC, GFM-VSC and AC power grid, the present invention introduces the rated capacity ratio of the two types of converters as explicit variables into the analysis process of converter output current, power reference value and current limiting characteristics, establishes an equivalent synchronous machine model of the hybrid system considering the capacity ratio, and reveals the mechanism of the capacity ratio on the transient synchronization stability of the system through synchronous dynamic equation analysis. Attached Figure Description

[0119] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0120] Figure 1 This is a topology diagram of the root / structure network hybrid system in this invention;

[0121] Figure 2 This is a block diagram of the GFL-VSC control in this invention;

[0122] Figure 3 This is a block diagram of the GFM-VSC control in this invention;

[0123] Figure 4The equivalent circuit of the hybrid system in this invention and the current-limiting phasor diagram of GFM-VSC are shown. Detailed Implementation

[0124] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0125] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0126] Example:

[0127] like Figures 1-4 As shown, to address the shortcomings of existing transient stability analysis methods, such as difficulty in characterizing the impact of converter capacity configuration on transient synchronization dynamics, inability to describe changes in the external characteristics of grid-connected converters under current-limited conditions, and lack of system analysis on inter-converter coupling effects, this invention provides a transient stability analysis method for grid-connected and grid-connected hybrid systems that takes capacity matching into account, comprising the following steps:

[0128] S1. Establish a hybrid system model for grid-connected and grid-connected converters, taking into account capacity matching:

[0129] S1.1, Based on the hybrid system topology, define the capacity ratio of grid-connected converters and network-connected converters.

[0130] like Figure 1 As shown, the hybrid system includes GFL-VSC, GFM-VSC and AC power grid, and the three are connected to a common coupling point through line impedance.

[0131] Among them, GFL-VSC via line impedance The connection point is PCC, and the GFM-VSC is connected via line impedance. Connecting to the PCC point, the AC power grid passes through the equivalent impedance Connected to point PCC.

[0132] Define the GFL-VSC port voltage and output current as follows: , The GFM-VSC port voltage and output current are respectively , The voltage at point PCC is The grid voltage is Line impedance is expressed as , , Using the grid voltage phase angle as the reference phase angle, the power angles of GFL-VSC and GFM-VSC are defined as follows: , ,in, The output phase angle of the GFL-VSC phase-locked loop. To control the output phase angle of the GFM-VSC network, This represents the phase angle of the grid voltage.

[0133] To describe the capacity share of GFL-VSC and GFM-VSC, we introduce the GFL-VSC capacity share:

[0134] (1)

[0135] in, For the rated capacity of GFL-VSC, Rated capacity for GFM-VSC.

[0136] The total system capacity is defined as follows

[0137] (2)

[0138] The rated capacities of GFL-VSC and GFM-VSC are expressed as follows:

[0139] (3)

[0140] (4)

[0141] From the above formula, it can be seen that when As the capacity increases, the proportion of GFL-VSC in the system increases, while the proportion of GFM-VSC decreases. Since GFL-VSC primarily exhibits current source characteristics, GFM-VSC mainly provides voltage and frequency support.

[0142] Further define the rated current values ​​for GFL-VSC and GFM-VSC; if the system voltage reference is... The rated current of GFL-VSC and GFM-VSC is uniformly expressed as:

[0143] (5)

[0144] in, It depends on the selected voltage base value; when When taking the effective value of line voltage, ;when When taking the effective value of the phase voltage, Simplifying, we get:

[0145] (6)

[0146] Therefore, the capacity ratio not only changes the rated capacity of the two types of converters, but also their current base values. In subsequent modeling, the output current of GFL-VSC, the power reference value of GFM-VSC, and the current limiting threshold of GFM-VSC are all determined by... Decide.

[0147] S1.2, Establish the control strategy model for the grid-connected converter.

[0148] like Figure 2 As shown, in the GFL-VSC's own synchronous coordinate system, its output current is expressed as:

[0149] (7)

[0150] in, and The output current of GFL-VSC is respectively shaft and Axial components;

[0151] To account for the influence of capacity ratio, the current component is expressed as:

[0152] (8)

[0153] Therefore, the output current amplitude and phase angle of the GFL-VSC are:

[0154] (9)

[0155] (10)

[0156] Under the same per-unit current command, the actual output current and capacity ratio of GFL-VSC This is directly proportional. Therefore, as the capacity ratio of GFL-VSC increases, the actual current injected into the PCC point increases, thereby enhancing the network coupling between GFL-VSC and the power grid.

[0157] During non-fault-prone operation, the GFL-VSC can generate a current reference value based on the power reference value. If voltage-oriented control is used, then:

[0158] (11)

[0159] in, Let be the d-axis component of the GFL-VSC port voltage vector in its PLL synchronously rotating dq coordinate system; the subscript d denotes the d-axis, and the subscript L denotes GFL-VSC. When voltage-oriented control is used, the d-axis of the PLL coordinate system is aligned with the direction of the GFL-VSC port voltage vector, therefore... , and have .

[0160] Therefore, the current reference value can be obtained:

[0161] (12)

[0162] After considering capacity matching, the power reference value of GFL-VSC can be expressed as:

[0163] (13)

[0164] in, , These are the per-unit forms of the active and reactive power reference values ​​for GFL-VSC, respectively.

[0165] Substituting, we get:

[0166] (14)

[0167] The power command of GFL-VSC varies with the capacity percentage in terms of actual value; when using the same per-unit power command, the higher the capacity percentage, the greater the actual current command corresponding to GFL-VSC.

[0168] The synchronization dynamics of GFL-VSC are determined by the PLL. Let... GFL-VSC port voltage in PLL coordinate system If the axis component is used, then the PLL dynamically represents it as follows:

[0169] (15)

[0170] Differentiate the above expression and use Representing the relative power angle between GFL and VSC, we get:

[0171] (16)

[0172] in, This is the PLL proportional gain coefficient. This is the integral gain coefficient of the PLL.

[0173] The synchronous movement of GFL-VSC depends on the port voltage. Axial component, and The capacity ratio is determined by the GFL-VSC's self-injected current, the GFM-VSC's output voltage or current, and the grid voltage. By changing the actual injection current of GFL-VSC This further affects the PLL synchronization dynamics.

[0174] S1.3, Establish the control strategy model for the grid-type converter;

[0175] like Figure 3 As shown, the VSG control of GFM-VSC is represented as follows:

[0176] (17)

[0177] in, The equivalent inertia coefficient, The damping coefficient is... The synchronization angular frequency, and These are the active power reference value and the actual output active power of the GFM-VSC, respectively.

[0178] After considering capacity allocation, the active power reference value of GFM-VSC is expressed as follows:

[0179] (18)

[0180] in, This is the per-unit form of the active power reference value for GFM-VSC.

[0181] The dynamic equation of GFM-VSC, taking into account capacity ratio, is further obtained:

[0182] (19)

[0183] As can be seen from the above formula, when the total system capacity is constant, An increase means a decrease in the capacity ratio of GFM-VSC, and its actual active power reference value decreases accordingly; at the same time, due to the increase in the output current of GFL-VSC, the coupled power disturbances that GFM-VSC is subjected to may be enhanced.

[0184] The reactive-voltage control of GFM-VSC can be expressed as:

[0185] (20)

[0186] in, This is a reference value for voltage amplitude. Given the no-load voltage, This is the reactive power-voltage regulation coefficient. and These are the GFM-VSC reactive power reference value and the actual output reactive power, respectively.

[0187] After considering the capacity allocation, the reactive power reference value can be expressed as:

[0188] (twenty one)

[0189] Further simplification yields:

[0190] (twenty two)

[0191] The reactive power support capability of GFM-VSC is directly related to its capacity ratio. When the capacity ratio of GFM-VSC decreases, its actual reactive power regulation capacity under the same per-unit reactive power command decreases.

[0192] During large disturbances or low-voltage ride-throughs, the GFM-VSC may trigger current limiting if the current exceeds the allowable range. Before current limiting is triggered, the GFM-VSC can be considered equivalent to a voltage source; after current limiting is triggered, its output current is restricted, and its external characteristics change to a controlled current source. Let the per-unit value of the maximum allowable current of the GFM-VSC be... Therefore, its actual current limiting threshold is:

[0193] (twenty three)

[0194] The controller calculates the current command amplitude before limiting in real time. ,when When the voltage source is maintained, the GFM-VSC maintains its operating characteristics; when When this occurs, the GFM-VSC is determined to be in current-limiting mode, and the output current amplitude is limited to a certain value. At this point, the GFM-VSC can be considered equivalent to a controlled current source, and its output current is expressed as:

[0195] (twenty four)

[0196] in, The saturation current phase angle under current-limited conditions is used to describe the current-limiting current in... , The distribution between axes. Therefore, the capacity allocation not only affects the power reference value of the GFM-VSC, but also changes its maximum output current capability during faults. When As the current increases, the rated capacity of the GFM-VSC decreases, and the current limiting threshold decreases accordingly. Under the same fault disturbance, the GFM-VSC is more likely to enter the current limiting state, and its current support capability is weakened after current limiting.

[0197] S2, Based on the hybrid system model of the grid-connected converter and the grid-connected converter, establish an equivalent synchronous machine model of the hybrid system that takes into account the capacity matching ratio:

[0198] like Figure 4 As shown, the network equivalent quantities are defined as follows:

[0199] ,

[0200] ,

[0201] ,

[0202] ,

[0203] .

[0204] in, , and These are the GFL-VSC access line impedance, GFM-VSC access line impedance, and AC grid equivalent impedance, respectively. The equivalent impedance of the GFM-VSC access line impedance and the equivalent impedance of the power grid is the parallel equivalent impedance. and These are the complex allocation coefficients corresponding to the power grid branch and the GFM-VSC branch, respectively, and satisfy the following conditions: ; This is the equivalent series impedance from the GFM-VSC port to the AC power grid. This is the series equivalent impedance from the GFL-VSC port to the AC power grid.

[0205] The output current magnitude of the GFL-VSC is expressed as a function of the capacity percentage:

[0206] (25)

[0207] Its current phase angle is .

[0208] When the output current of the GFM-VSC exceeds the current limiting threshold, its voltage source characteristics are no longer maintained, and the port output is constrained by the current limiting circuit. At this time, the GFM-VSC can be equivalent to a controlled current source, and its output current amplitude is determined by the current limiting threshold, while the phase angle is jointly determined by the network phase angle and the saturation current phase angle.

[0209] After considering the capacity ratio, the current limiting current amplitude of GFM-VSC is

[0210] (26)

[0211] in, This is the per-unit value of the maximum allowable current for the GFM-VSC.

[0212] The output current of GFM-VSC is expressed as follows:

[0213] (27)

[0214] in, The phase angle is the saturation current. The current limiting capability of GFM-VSC is related to... Proportional.

[0215] S2.1, Deriving the terminal voltage of the grid-connected converter Axial components

[0216] GFL-VSC terminal voltage The axial component consists of the GFL-VSC's own current voltage drop, the grid voltage component, and the GFM-VSC current-limiting current coupling component, expressed as:

[0217] (28)

[0218] Among them, the self-impedance voltage drop term is ;

[0219] The grid voltage term is ;

[0220] GFM-VSC current-limiting coupling term is .

[0221] Substituting the above equation, we get:

[0222] (29)

[0223] in, The capacity increases with the proportion of GFL-VSC, while It decreases as the proportion of GFM-VSC capacity decreases. Therefore, when As the current increases, the current drop of the GFL-VSC increases, while the current support capability of the GFM-VSC after current limiting decreases.

[0224] S2.2, Deriving the terminal voltage and output active power of the grid-type converter.

[0225] The GFM-VSC port voltage is determined by its current-limiting current, the GFL-VSC injection current, and the grid voltage. Based on the network superposition relationship, this can be expressed as:

[0226] (30)

[0227] According to the complex power relationship, the active power output of GFM-VSC consists of the grid-related power terms and the GFL-VSC coupled power terms. Among them, the power grid-related power items are:

[0228] (31)

[0229] The GFL-VSC coupling power term is:

[0230] (32)

[0231] Substituting, we get

[0232] (33)

[0233] GFM-VSC output power and Directly related, a decrease in the capacity ratio of GFM-VSC will reduce its output power under current-limited conditions; simultaneously, the coupling term contains This indicates that the capacity ratio has a dual impact on the power interaction between the two types of converters.

[0234] S2.3, Establish a synchronous machine equivalent model for the hybrid system.

[0235] The PLL dynamics of GFL-VSC are as follows:

[0236] (34)

[0237] definition , , .

[0238] Substituting the expression for voltage, we get:

[0239]

[0240] PLL dynamics can be simplified to:

[0241] (35)

[0242] Organize it into a synchronous machine-like form:

[0243] (36)

[0244] in,

[0245] The equivalent mechanical power and mutual damping term of GFL-VSC are both affected Impact, and The capacity ratio is determined by the proportion of GFM-VSC. Therefore, the capacity ratio not only affects the injected current of GFL-VSC itself, but also changes the synchronization dynamics of GFL-VSC through the current limiting current of GFM-VSC.

[0246] For GFM-VSC, its angular dynamics can be expressed as:

[0247] (37)

[0248] in .

[0249] Further simplification yields:

[0250] (38)

[0251] definition , .

[0252] Furthermore, the synchronous machine-like model of GFM-VSC is obtained:

[0253] (39)

[0254] The output power of the GFM-VSC is no longer determined by the voltage source power angle characteristics, but by the current limiting current. and its phase angle Decision. Capacity matching also appears... , and The results show that the capacity ratio has a more direct impact on the power angle stability of GFM-VSC after current limiting is triggered.

[0255] S3. Based on the aforementioned equivalent model of a synchronous machine, analyze the mechanism by which capacity allocation affects transient synchronization characteristics:

[0256] S3.1, Analyze the impact of capacity allocation on the dynamic synchronization of the grid-type converter.

[0257] The fault caused the grid voltage to drop to When the GFM-VSC enters current-limiting mode, the GFL-VSC synchronization equation is rewritten as follows:

[0258] (40)

[0259] At this point, the power terms are: , , , , .

[0260] Two of them and Respected and The effects are represented as follows:

[0261] (41)

[0262] (42)

[0263] Follow Increase and increase, Follow Increase and decrease, the two are in competition. When When smaller, Dominant, GFL-VSC Mainly affected by the coupling effect of the GFM-VSC current limiting current; when When it is large, The self-impedance voltage drop of the GFL-VSC's own current is the dominant factor, and it is the main contributor to the voltage drop.

[0264] Only the magnitude of the grid fault voltage, and Irrelevant. Therefore, the electromagnetic power of the GFL-VSC under current-limited conditions... and Irrelevant.

[0265] The mutual damping term is:

[0266] (43)

[0267] because by Influence, Follow The rate of increase is linearly reduced. This means that as the capacity ratio of GFM-VSC decreases, the synchronization damping that GFL-VSC receives from GFM-VSC during a fault also decreases.

[0268] S3.2, Analyze the impact of capacity allocation on the synchronous dynamics of grid-type converters.

[0269] The equation of motion for GFM-VSC is:

[0270] (44)

[0271] in: , .

[0272] The active power reference value of GFM-VSC is also affected during the fault. Influence:

[0273] (45)

[0274] The output power of GFM-VSC is:

[0275] (46)

[0276] Simplified to

[0277] (47)

[0278] The above formula includes The first and second terms; about It exhibits nonlinear decay.

[0279] S3.3, Analyze the impact of capacity ratio on coupling terms.

[0280] In a hybrid system, the coupling between the two types of converters is a key factor affecting transient stability. Capacity matching is achieved by changing... and It simultaneously affects the strength of multiple coupling terms.

[0281] Mutual damping term in the GFL-VSC equation

[0282] (48)

[0283] exist In larger systems, GFL-VSCs exhibit weaker synchronous damping during faults, making their angular dynamics more susceptible to disturbances.

[0284] Coupled power term in GFM-VSC equations

[0285] (49)

[0286] The amplitude is proportional to ,exist The maximum value is obtained at this point. This indicates that when the capacities of GFL-VSC and GFM-VSC are similar, the coupling power between the two types of converters is strongest during a fault, and the dynamic behavior of the system is also the most complex.

[0287] The analytical method of this invention is implemented according to the following logic: First, a detailed model is established; then, an equivalent model is constructed; and finally, the influence mechanism is analyzed based on the model. Specifically, firstly, based on the hybrid system topology, the capacity ratios of grid-connected and network-connected converters are defined, and control and current-limiting models for the two types of converters are established respectively to obtain the relationship between the capacity ratio and the output current, power reference value, and current-limiting threshold. Secondly, using network superposition and impedance equivalence methods, the above detailed model is transformed into a synchronous machine-like equivalent model considering the capacity ratio, obtaining the synchronous motion equations for the two types of converters. Finally, based on the established synchronous motion equations, the influence of the capacity ratio on the equivalent power term, damping term, and coupling term is analyzed.

[0288] Each coupling term is followed The trends of change are summarized in Table 1:

[0289] Table 1 Key Coupling Term and Relationship

[0290]

[0291] The transient stability analysis method was verified by simulation:

[0292] To verify the feasibility of the transient stability analysis method for grid-connected and grid-connected hybrid systems that takes into account capacity matching, this embodiment builds a simulation model of a hybrid system consisting of GFL-VSC, GFM-VSC, and an AC power grid in MATLAB / Simulink. The total rated capacity of the system is... Set to 2MVA, mains voltage reference The voltage is 220kV, the system rated frequency is 50Hz, and the grid equivalent impedance is... It is 1.68 m Access line impedance of GFL-VSC and GFM-VSC , Both are j0.2m The proportional and integral coefficients of the GFL-VSC phase-locked loop were set to 0.8 and 200, respectively; the equivalent inertia and damping coefficients of the GFM-VSC were set to 0.6 pu and 80 p.u., respectively; and the per-unit value of the maximum allowable current was set to 1.25. During the simulation, the total system capacity, network parameters, and control parameters were kept constant, and only the capacity ratio of the GFL-VSC was changed. .

[0293] Set separately The values ​​are 0.2, 0.4, 0.5, 0.6, and 0.8, corresponding to the rated capacities of GFL-VSC (0.4 MVA, 0.8 MVA, 1.0 MVA, 1.2 MVA, and 1.6 MVA) and GFM-VSC (1.6 MVA, 1.2 MVA, 1.0 MVA, 0.8 MVA, and 0.4 MVA), respectively. A three-phase short-circuit fault is set at t=1s, lasting for 1.6ms before being cleared. The voltage at the PCC point, the output current of both types of converters, the power angle, and the frequency changes are recorded during the fault. The recovery time is defined as the time required for the system's key state variables to enter a steady-state range of ±5% after the fault is cleared and remain stable.

[0294] Simulation results show that when When the capacity ratios are 0.2, 0.4, 0.5, 0.6, and 0.8, the maximum power angle offset of the GFL-VSC is 9.190 rad, 0.730 rad, 3.002 rad, 2.989 rad, and 2.966 rad, respectively, with corresponding recovery times of 0.162 s, 0.177 s, 0.178 s, 0.138 s, and 0.172 s, and maximum frequency deviations of 0.281 Hz, 0.2816 Hz, 0.282 Hz, 0.285 Hz, and 0.291 Hz. Under different capacity ratios, the maximum power angle offset of the GFM-VSC remains around 0.9 rad, indicating that changes in capacity ratio have a more significant impact on the synchronization dynamics of the GFL-VSC than on the power angle dynamics of the GFM-VSC itself.

[0295] when When the current injection capability of the GFL-VSC is relatively weak, its synchronization dynamics rely heavily on the voltage and power support provided by the GFM-VSC; as the current injection capability of the GFL-VSC decreases, its synchronization dynamics become more dependent on the voltage and power support provided by the GFM-VSC. Increasing the capacity of the GFL-VSC enhances its actual output current capability, but correspondingly reduces the capacity ratio and current limiting threshold of the GFM-VSC, weakening its voltage and current support capabilities during faults. These two effects compete with each other, causing the system's transient performance to exhibit non-monotonic characteristics with changes in capacity ratio. Considering the combined effects of power angle offset, recovery time, and frequency deviation, it can be seen that when... When the current regulation capability of GFL-VSC and the network support capability of GFM-VSC are in the range of 0.4 to 0.6, the system can effectively balance the current regulation capability of GFL-VSC and the network support capability of GFM-VSC. The simulation results are consistent with the previous theoretical analysis, which verifies the feasibility of the analysis method.

[0296] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since it corresponds to the method disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0297] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A transient stability analysis method for hybrid grid-connected and grid-connected systems considering capacity allocation, characterized in that, Includes the following steps: S1. Establish a hybrid system model of grid-connected converters and grid-connected converters that takes into account capacity matching. S2. Based on the hybrid system model of the grid-connected converter and the grid-connected converter, establish an equivalent synchronous machine model of the hybrid system that takes into account the capacity ratio. S3. Based on the equivalent model of the aforementioned synchronous machine, the influence mechanism of capacity allocation on transient synchronization characteristics is analyzed.

2. The transient stability analysis method for a hybrid grid-connected and grid-connected system considering capacity allocation as described in claim 1, characterized in that, S1 includes the following steps: S1.1, Based on the hybrid system topology, the capacity ratio of grid-connected converters and network-connected converters is defined; the hybrid system includes grid-connected converters, network-connected converters and AC power grid, and the three are connected to a common connection point through line impedance; Among them, the grid-type converter is connected to the line impedance. The grid-type converter is connected to the PCC point via the line impedance. Connecting to the PCC point, the AC power grid passes through the equivalent impedance Connect to point PCC; The capacity ratio of grid-type converters is: in, To match the rated capacity of the grid-type converter, This refers to the rated capacity of the grid-type converter; The total system capacity is defined as follows: The rated capacity of the grid-connected converter is: The rated capacity of the grid-type converter is: The system voltage reference is The rated current of the grid converter is: The rated current of the grid-type converter is: in, It depends on the selected voltage base value, when When taking the effective value of line voltage, ;when When taking the effective value of the phase voltage, .

3. The transient stability analysis method for a hybrid grid-connected and grid-connected system considering capacity allocation according to claim 2, characterized in that, S1 further includes the following steps: S1.2, Establish the control strategy model for the grid-connected converter; In the grid-connected converter's own synchronous coordinate system, its output current is: in, and These are the output currents of the grid-connected converter. shaft and Axial components; The current component is: The output current amplitude of the grid-connected converter is: The output phase angle of the grid converter is: During non-fault operation, the grid-connected converter generates a current reference value based on the power reference value. Using voltage-oriented control and considering capacity matching, the resulting current reference value is: in, To represent the d-axis component of the grid-connected converter port voltage vector in the dq coordinate system, which rotates synchronously with its PLL; the subscript 'd' indicates the d-axis, and the subscript 'L' indicates the grid-connected converter. When voltage-oriented control is used, the d-axis of the PLL coordinate system is aligned with the direction of the grid-connected converter port voltage vector, therefore... , and have ; , These are the per-unit forms of the active and reactive power reference values ​​for grid-connected converters, respectively. set up To track the port voltage of the grid converter in the PLL coordinate system Axial components, with Let the power angle relative to the grid converter be represented, then the PLL dynamics can be expressed as: in, This is the PLL proportional gain coefficient. This is the integral gain coefficient of the PLL.

4. The transient stability analysis method for a hybrid grid-connected and grid-connected system considering capacity allocation according to claim 3, characterized in that, S1 further includes the following steps: S1.3, Establish the control strategy model for the grid-type converter; The VSG control of a grid-type converter is represented as follows: in, The equivalent inertia coefficient, The damping coefficient is... The synchronization angular frequency, and These are the reference value for active power and the actual output active power of the grid-type converter, respectively. After considering capacity allocation, the active power reference value of the grid-type converter is expressed as: in, This is the per-unit form of the active power reference value for grid-type converters; The dynamic equations of the grid-type converter, taking into account capacity matching, are obtained: The reactive power-voltage control of a grid-connected converter can be expressed as: in, This is a reference value for voltage amplitude. Given the no-load voltage, This is the reactive power-voltage regulation coefficient. This refers to the actual reactive power output of the grid-type converter. Let the per-unit value of the maximum allowable current of the grid converter be... Therefore, its actual current limiting threshold is: When the grid-type converter enters the current-limiting state, its output current is: in, The saturation current phase angle under current-limited conditions is used to describe the current-limiting current in... , The allocation between axes.

5. The transient stability analysis method for a hybrid grid-connected and grid-connected system considering capacity allocation according to claim 4, characterized in that, S2 includes the following steps: S2.1, Deriving the terminal voltage of the grid-connected converter Axial components; Define network equivalent quantities: in, , and These are the line impedance of the grid-connected converter, the line impedance of the grid-type converter, and the equivalent impedance of the AC power grid, respectively. The parallel equivalent impedance of the grid-connected line impedance and the grid equivalent impedance; and These are the complex allocation coefficients corresponding to the power grid branch and the grid-type converter branch, respectively, and satisfy the following conditions: ; This refers to the series equivalent impedance from the port of the grid-connected converter to the AC power grid. The series equivalent impedance from the grid-connected converter port to the AC power grid; The output current amplitude of the grid-connected converter is: Its current phase angle is ; After considering the capacity ratio, the current limiting current amplitude of the grid-type converter is: in, This is the per-unit value of the maximum allowable current for a grid-connected converter; the output current of a grid-connected converter is expressed as: in, The phase angle is the saturation current. With the terminal voltage of the grid converter The axis components are represented as: 。 6. The transient stability analysis method for a hybrid grid-connected and grid-connected system considering capacity allocation according to claim 5, characterized in that, S2 further includes the following steps: S2.2, Derive the terminal voltage and output active power of the grid-type converter; The port voltage of a grid-connected converter is expressed as: Based on the complex power relationship, the output active power of the grid-type converter is: 。 7. The transient stability analysis method for a hybrid grid-connected and grid-connected system considering capacity allocation according to claim 6, characterized in that, S2 further includes the following step: S2.3, establishing a synchronous machine equivalent model for the hybrid system; Synchronous machines are represented as: For a grid-type converter, its angular dynamics are expressed as: The synchronous machine-like model is: 。 8. The transient stability analysis method for a hybrid grid-connected and grid-connected system considering capacity allocation according to claim 7, characterized in that, S3 includes the following steps: S3.1, Analyze the impact of capacity allocation ratio on the dynamic synchronization of grid-type converters; The fault caused the grid voltage to drop to When the grid-type converter enters the current-limiting state, the synchronization equation with the grid-type converter is expressed as: At this point, the power terms are: , , , , ; Two of them and Respected and The effects are represented as follows: The mutual damping term is: 。 9. A transient stability analysis method for a hybrid grid-connected and grid-connected system considering capacity allocation, as described in claim 8, is characterized in that... S3 further includes the following steps: S3.2, Analyze the impact of capacity allocation on the synchronous dynamics of grid-type converters; The motion equations of a grid converter are: The active power reference value of the grid-type converter during a fault is expressed as follows: The output power of the grid converter is: 。 10. A transient stability analysis method for a hybrid grid-connected and grid-connected system considering capacity allocation, as described in claim 9, is characterized in that... S3 further includes the following steps: S3.3, Analyze the impact of capacity ratio on coupling terms; The mutual damping term in the equations for a grid converter is: The coupling power term in the equations of a grid converter is: in, To match the power angle of the grid converter, The power angle of the grid-type converter, where, , , To match the phase angle of the phase-locked loop output of the grid converter, To control the output phase angle of a grid-type converter, This represents the phase angle of the grid voltage.

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