Stability analysis method for hybrid grid-connected system of network-following converter

Abstract

The invention discloses a stability analysis method for a hybrid grid-connected system of a grid-following converter, and the method comprises the steps: constructing precise impedance models of a grid-following type (GFL) converter and a grid-constructing type (GFM) converter respectively based on a phase-locked loop (PLL) mechanism and a virtual synchronous generator (VSG) control strategy, and the details are shown in an abstract figure 1; deducing the total equivalent impedance of the system at a point of common coupling (PCC) according to a circuit equivalence principle; performing stability analysis and sensitivity evaluation on different converter combination proportions, topological structures and key control parameters of the hybrid grid-connected system based on an impedance ratio criterion; and verifying the accuracy of the analysis result through frequency domain simulation. The method can be widely applied to a new energy power system, solves the problem of system stability evaluation and optimization under the high-proportion renewable energy access background, and has the characteristics of high adaptability, accurate model and comprehensive analysis.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy grid connection, and particularly relates to a stability analysis method for a hybrid grid-connected system of grid-forming converters and grid-following converters for a high-proportion new energy station, which is suitable for optimization of a converter control strategy and stability evaluation of a power electronic power system. BACKGROUND

[0002] With large-scale access of renewable energy such as wind power and photovoltaic power to the power grid, the proportion of new energy equipment with a converter as an interface in the power system is rapidly increasing, and the retirement of traditional synchronous generators is intensifying, which greatly reduces the inertia and damping of the power grid and poses significant challenges to frequency and voltage stability.

[0003] In the current mainstream converter control system, the GFL converter tracks the grid voltage phase in real time through the PLL, accurately adjusts the grid-connected current by using current vector control, and its external characteristic presents a controlled current source, which has the advantages of simple structure, mature PLL technology, and easy implementation of maximum power point tracking (MPPT), etc. It is the mainstream interface device for new energy grid connection, but the GFL control mode highly depends on the stable voltage support provided by the grid, and itself cannot provide inertia and voltage support for the grid. With the increasing penetration rate of GFL converters in the grid, the equivalent inertia of the system continues to decrease, which poses a potential threat to system stability. The stability shortcoming of single GFL large-scale grid connection makes the collaborative application of GFM converters a potential solution.

[0004] The GFM converter realizes synchronization with the grid through a power synchronization mechanism, adopts a voltage amplitude and frequency control strategy, and its external characteristic presents a controlled voltage source. This technology does not need to rely on the grid to provide a synchronization reference, has the ability to operate in island mode, and can actively provide necessary voltage and frequency support for the grid, enhancing the strength of the system.

[0005] However, the differences in external characteristics and control methods between GFL and GFM make the system stability problem complex after mixed access. Existing methods mostly use time-domain simulation, lack general, efficient and theoretical frequency-domain stability criteria, and are difficult to provide guiding suggestions for parameter perturbations. Therefore, it is of great significance to develop a systematic method based on impedance modeling, criterion analysis and parameter sensitivity quantitative evaluation. SUMMARY

[0006] To solve the above technical problems, the application provides a stability analysis method for a hybrid grid-connected system of grid-forming converters and grid-following converters, to make up for the gap in the prior art.

[0007] To achieve the above purpose, the application provides a stability analysis method for a hybrid grid-connected system of grid-forming converters and grid-following converters, comprising:

[0008] S1, constructing grid-following (GFL) converter impedance model based on harmonic linearization method, the model considering phase-locked loop (PLL) dynamic characteristics;

[0009] S2, constructing grid-forming (GFM) converter impedance model based on virtual synchronous generator (VSG) control strategy, the model embodying voltage source external characteristics;

[0010] S3, according to circuit equivalence principle, deriving total equivalent impedance of hybrid system at point of common coupling (PCC);

[0011] S4, constructing link impedance aggregation model of multiple converters in new energy station, obtaining system overall output impedance expression;

[0012] S5, analyzing system stability based on impedance ratio criterion, evaluating influence of GFM / GFL combination ratio and arrangement mode on stability margin;

[0013] S6, performing sensitivity analysis on control parameters, identifying key parameters and evaluating influence degree of disturbance on system stability.

[0014] The process of obtaining GFL impedance model in S1 includes: ignoring direct current side voltage ripple, converting modulated wave frequency domain expression into sequence impedance expression based on current inner loop control structure, and constructing positive sequence and negative sequence frequency domain impedance expression. The GFL positive sequence and negative sequence impedance expression is as follows:

[0015]

[0016] The modeling process of GFL impedance model in S1 is as follows:

[0017] S101, access of large-capacity direct current capacitor significantly suppresses dynamic fluctuation of direct current side voltage, and voltage ripple thereof in steady state operation is usually lower than 2% of rated value; based on this characteristic, dynamic response of voltage outer loop can be ignored in small signal impedance modeling of grid-connected inverter, direct current side is equivalent to ideal voltage source, and current inner loop control instruction is regarded as static reference value;

[0018] S102, for analyzing small disturbance stability of system, positive sequence and negative sequence voltage disturbance signals are injected at PCC point, and according to small signal linearization theory, disturbance amplitude needs to meet weak nonlinearity condition, and is usually limited to 5% to 10% of power frequency voltage amplitude, so as to model the same.

[0019] The process of obtaining the GFM impedance model in S2 includes: based on the VSG control structure, establishing a frequency domain relationship model of three control links, deriving the frequency domain transfer function between the output voltage, current and the modulation wave, and constructing the positive and negative sequence frequency domain impedance expressions. The GFM positive and negative sequence impedance expressions are as follows:

[0020]

[0021] Wherein:

[0022]

[0023] The modeling process of the GFM impedance model in S2 is as follows:

[0024] Injecting positive sequence disturbance frequency voltage at the grid side will generate response current at the corresponding frequency and its coupled frequency. The core of impedance modeling is to establish the transfer function relationship between the disturbance voltage at the PCC point and the response current. Considering that the main circuit establishes the relationship among the disturbance voltage, response current and bridge arm voltage, and the control circuit establishes the transfer function relationship between the disturbance voltage, response current and the modulation wave, the modulation wave and the bridge arm voltage can be connected through the DC side voltage, therefore, establishing the above three relationships, the transfer function relationship between the disturbance voltage and its response current can be obtained, and the VSG output sequence impedance is obtained.

[0025] In S3, according to the circuit equivalence principle, the total equivalent impedance of the hybrid system at the PCC (point of common coupling) is derived, and the derivation process includes: since GFL presents a controlled current source characteristic, the Norton equivalent circuit modeling is adopted; since GFM presents a controlled voltage source characteristic, the Thevenin equivalent circuit modeling is adopted. Further, the equivalent circuit of a GFL converter and a GFM converter in parallel is obtained.

[0026]

[0027] In S4, the link impedance aggregation model of multiple converters in the new energy station is constructed, and the system overall output impedance expression is obtained;

[0028] Under the premise of parallel GFM and independent stable operation of each converter, the frequency coupling effect of GFL can be effectively suppressed. Expanding the scenario to multiple converters, the parallel impedance of M GFM and N GFL can be represented as:

[0029]

[0030] The Norton equivalent model is used for all the converters in the system. The system is composed of parallel equivalent current sources and total output impedance. The grid is modeled by Thevenin, which is composed of series equivalent voltage sources. Each converter in the link and its access cable segment form an equivalent π-type network. The injection admittance of each converter is recursively superimposed in the order from far to near:

[0031]

[0032] where Z WT,m (s) is the output impedance of the mth converter.

[0033] The equivalent admittance of the link is recursively aggregated from the far end to the near end. The recursive formula is:

[0034]

[0035] where, is the equivalent admittance of the link before the mth converter is connected, is the impedance of the mth cable segment, C m is the capacitance of the mth cable segment to ground.

[0036] The final aggregated equivalent impedance of the link is:

[0037]

[0038] If the system contains M links, the overall impedance of the new energy station is the parallel relationship of the impedance of each link:

[0039]

[0040] The impedance ratio criterion in S5 is used to analyze the stability of the system and evaluate the impact of the GFM / GFL combination ratio and arrangement on the stability margin:

[0041] To analyze the impact of the GFM converter access position in the new energy station on the system stability, a new energy station system model is constructed, which contains 4 links and 8 wind power converters in each group. The number of GFM is 16, and there are 5 specific network construction methods:

[0042] 1. GFM is concentrated at the front end of each group, closest to the grid point;

[0043] 2. Concentrated at the end of each group, away from the grid point;

[0044] 3. Uniformly staggered arrangement (odd-even distribution);

[0045] 4. Central arrangement (4 central GFM);

[0046] 5. All GFM are concentrated in a separate group, and the remaining groups are GFL.

[0047] In order to study the influence of the configuration proportion of the GFM converter in the parallel new energy station on the system stability, five typical working conditions of GFM proportion of 0%, 25%, 50%, 75% and 100% are set, and the equivalent AC impedance is compared and analyzed. The type of the converter is consistent in each group.

[0048] The sensitivity analysis of the control parameters in S6 identifies the key parameters and evaluates the influence degree of the disturbance of the key parameters on the system stability:

[0049] The sensitivity analysis aims to quantitatively evaluate the influence degree of the disturbance of a specific parameter on the output characteristics (such as impedance and stability margin) of the system, so as to identify the dominant parameter and understand the action law. Let the output impedance of the system be expressed as Z(s:p), wherein s is a complex frequency domain variable, and p is any adjustable control parameter. Then the complex impedance sensitivity of the parameter p is defined as:

[0050]

[0051] Further, the normalized impedance sensitivity can be defined as:

[0052]

[0053] The expression can be expanded into the sensitivity of the module and the phase in the complex frequency domain, that is, the amplitude sensitivity |Z(jω)| and the phase sensitivity ∠Z(jω), which represent the influence degree on the impedance and the phase, respectively. When the sensitivity of a parameter p is much greater than that of other parameters in a certain frequency range, it is indicated that the parameter is a dominant factor affecting the impedance characteristics at the frequency. The stability of the system is determined by the impedance ratio in the Nyquist criterion, so the influence of a control parameter on the system stability can be converted into the sensitivity analysis of the impedance ratio L(s):

[0054] BRIEF DESCRIPTION OF DRAWINGS

[0055] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application, and are incorporated in and constitute a part of this application. The schematic embodiments of the application and the description thereof are used to explain the application, and do not constitute an improper limitation on the application. In the drawings:

[0056] Figure 1 The GFL model of the embodiment of the application is swept frequency verified;

[0057] Figure 2 The GFM model of the embodiment of the application is swept frequency verified;

[0058] Figure 3 The GFL / GFM converter parallel topology of the embodiment of the application;

[0059] Figure 4 Impedance characteristics and simulation model sweep results of a single GFL and a single GFM converter in parallel for an embodiment of the application;

[0060] Figure 5 Equivalent circuit of a single GFL / GFM link for an embodiment of the application;

[0061] Figure 6 Impedance method analysis schematic for an embodiment of the application;

[0062] Figure 7 Equivalent impedance characteristics of a new energy station under different grid structure location conditions for an embodiment of the application;

[0063] Figure 8 Equivalent impedance characteristics of a new energy station under different grid structure proportion conditions for an embodiment of the application;

[0064] Figure 9 Joint impedance sensitivity absolute value of a key frequency band for an embodiment of the application;

[0065] Figure 10 Impedance sensitivity of different GFL converters for an embodiment of the application;

[0066] Figure 11 Impedance sensitivity of different GFM converters for an embodiment of the application; DETAILED DESCRIPTION

[0067] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in combination with embodiments.

[0068] Embodiment one

[0069] First, the completed GFL impedance modeling and GFM impedance modeling are analyzed and verified according to S1 and S2.

[0070] GFL impedance modeling part:

[0071] At this time, the system operating point offset is lower than the linearization threshold, and under this condition, taking phase A as an example, its voltage and current can be expressed as the superposition form of the power frequency steady-state component and the disturbance component:

[0072]

[0073] Among them, V1, V p , V n respectively represent the amplitude of the fundamental voltage, the positive sequence disturbance voltage and the negative sequence disturbance voltage; f1, f p , f n respectively represent the corresponding fundamental frequency, positive sequence disturbance frequency and negative sequence disturbance frequency; is the initial phase angle of positive and negative sequence disturbance voltage; I1, I p , I n respectively represent the amplitude of fundamental current, positive sequence disturbance response current and negative sequence disturbance response current; respectively represent the initial phase angle of fundamental current, positive sequence disturbance response current and negative sequence disturbance response current.

[0074] To realize the efficient decoupling control of the AC current, the controlled quantity needs to be converted from the static natural coordinate system to the synchronous rotating coordinate system.

[0075]

[0076] The rotating reference angle θ output by the PLL PLL is composed of two parts: the steady-state rotating angle θ1 dominated by the fundamental positive sequence voltage, and the small signal phase angle offset Δθ introduced by the grid voltage disturbance:

[0077]

[0078] According to Figure 2 the inductance current feedforward decoupling control in the synchronous rotating coordinate system, the modulation wave m d and m q of the grid-connected inverter are:

[0079]

[0080] wherein, Hi(s) is the current PI controller: H i (s)=k pI +k iI / s

[0081] Considering the fundamental voltage and small signal disturbance components, the small signal disturbance and inter-area transfer function are derived, and the frequency domain expressions of the grid-connected inverter inductance i d , i q are obtained, which are brought into the above formula to obtain the frequency domain expressions of m d , m q , and then through dq coordinate transformation, the frequency domain expression of the modulation wave m a is obtained:

[0082]

[0083] Further, the positive and negative sequence impedance expressions of the GFL inverter can be obtained.

[0084] GFM impedance modeling part:

[0085] The grid-connected converter control system adopts a three-level control structure, which includes voltage-frequency control link, voltage outer loop and current inner loop from outside to inside. In the voltage-frequency control link, the system calculates the output active power P and reactive power Q according to the grid-connected port voltage v a 、 b 、 c and current i a 、 b 、 c , compares them with their reference values P ref 、 Q ref respectively to form the deviation. The deviation of P is outputted through a virtual rotating link to form the frequency increment Δω, which is added to the rated frequency ω0 to obtain the voltage phase angle θ through integration; and the deviation of Q is outputted through a proportional link to form the voltage amplitude, so as to realize the control of the grid-connected voltage size. This link constructs the equivalent voltage source characteristic of the synchronous generator, and has the ability of self-synchronization and frequency stabilization.

[0086]

[0087] wherein J is the virtual inertia, D is the virtual damping coefficient, P ref 、 P are the reference and measured active power, ω and ω0 are the output and rated angular frequency.

[0088] After the grid-side positive sequence small signal disturbance is applied in the time domain, the A-phase output voltage, current and filter inductance current of the VSG can be represented as:

[0089]

[0090] wherein V a(t) is the A-phase output voltage, I a(t) is the A-phase output current, and I La(t) is the A-phase filter inductance current; V1, I1 and I L are the voltage, current and filter inductance current amplitudes of the fundamental component, and ω0 is the main frequency angular frequency; V p , I p and I Lp are the voltage, current and filter inductance current amplitudes of the disturbance frequency component, and ω p is the disturbance frequency angular frequency; V n , I n and I Ln are the voltage, current and filter inductance current amplitudes of the coupling frequency component, and ω n is the coupling frequency angular frequency; are the initial phase angles of the main frequency current and filter inductance current; are the initial phase angles of the voltage of the disturbance frequency component and the coupling frequency component, are the initial phase angles of the disturbance frequency component and the coupling frequency component currents, respectively, are the initial phase angles of the disturbance frequency component and the coupling frequency component filtered inductor currents, respectively.

[0091] The VSG system output power is shown as follows:

[0092]

[0093] where G f is the transfer function of the output port low-pass filter, and G f = ω lp / (s+ω lp ), ω lp is the cut-off frequency.

[0094] The VSG modulation wave expression can be obtained as follows: Figure 3

[0095]

[0096] where Hi(s) represents the current inner loop, and Hi(s) represents the current outer loop.

[0097] The A-phase bridge arm voltage is related to the modulation ratio Km:

[0098] v la = K m E dc m a

[0099] The VSG output phase angle can be derived from the above formulas, and the reactive output voltage expression can be further derived from the frequency domain expression of the VSG system output power and the Park transformation and Clark transformation to obtain the frequency domain expressions of v d , v q , and the frequency domain expression of the corresponding filter output current:

[0100]

[0101] Thus, the GFM positive and negative sequence impedance expressions can be obtained.

[0102] The main circuit parameters are given as shown in the following table:

[0103] Table 1

[0104]

[0105]

[0106] The GFL controller parameters are given as shown in the following table:

[0107] Table 2​

[0108]

[0109] The GFM controller parameters are given as shown in the following table:

[0110] Table 3

[0111]

[0112] Under the main circuit parameters and GFL / GFM control parameters, the system is swept to verify the results as shown in Figure 1 , Figure 2 , which verifies the effectiveness of the model. At the same time, it can be seen from Figure 1 that for the GFL converter, the positive sequence impedance amplitude gradually decreases with the increase of active power, and the overall positive sequence impedance amplitude shows a downward trend, indicating that the converter controller has stronger current regulation capability when the output power is higher; the negative sequence impedance changes little, indicating that power is not its dominant factor. It can be seen from Figure 2 that for the GFM converter, it has lower and more flat impedance amplitude in the medium and low frequency band, which reflects better voltage support and dynamic stability performance. Compared with GFL, GFM impedance always maintains inductive characteristics in the medium and high frequency band, which helps to suppress system oscillation and enhance damping capability.

[0113] Example Two

[0114] Figure 3 A GFL converter and a GFM converter are shown in parallel, and based on this, the sweep analysis is carried out, and the results are shown in Figure 4 , it can be seen that the GFL converter has higher impedance amplitude in the low to super-synchronous frequency band, and the GFM impedance amplitude is slightly lower but the response is more stable, and has better phase margin. After the parallel connection of the two, the overall impedance characteristics of the system in this frequency band are obviously optimized, and the stability margin in the sub-synchronous / super-synchronous frequency band is significantly improved.

[0115] Figure 5 The equivalent circuit of a single link is shown in the case of parallel connection of multiple GFL / GFM converters, Figure 6 The schematic diagram for analyzing the stability of the system based on the impedance method is shown. When analyzing the stability of the grid-connected system, the grid-connected inverter controlled by the current is usually modeled by Norton equivalent at the PCC port, which is simplified as a structure connected in parallel by an equivalent current source and an equivalent impedance Z WFeq (s) of the new energy power station; at the same time, the sending end converter station is modeled by Thevenin equivalent at the same PCC port, which is simplified as a topology form connected in series by a voltage source equivalent impedance Z g (s), the design of which refers to the design of MMC modular multilevel, thereby constructing a complete impedance network model for stability analysis.

[0116] Based on the superposition theorem, the expression for grid-connected current can be obtained as follows:

[0117]

[0118] The open-loop gain of the system is defined as the impedance ratio, i.e.:

[0119]

[0120] Combining the two equations, we can obtain:

[0121]

[0122] The magnitude and phase characteristics of the impedance ratio L(s) directly affect the dynamic behavior of the current response and can be regarded as an open-loop gain function in the frequency domain. Therefore, the stability of the system can be judged by the Nyquist stability criterion: the interconnection system between the new energy power station and the sending-end converter station can only operate stably across the entire operating range if and only if the Nyquist curve of the impedance ratio does not encircle the critical point (-1,j0) and maintains sufficient phase margin.

[0123] In practical analysis, the impedance amplitude-frequency curves of the two subsystems are often used for discrimination. At the same frequency ω... c If the amplitude curves intersect at a certain point, it indicates that the open-loop gain magnitude is 1, i.e., |Z|. grid (jω c )|=|Z source (jω c )|.

[0124] To analyze the impact of GFM converter connection location on system stability in renewable energy power plants, a renewable energy power plant system model was constructed, comprising 4 links and 8 wind power converters in each group. The converter configuration remained consistent across groups, with 1km of connection lines between converters within the same group. The unit line inductance and capacitance values ​​were 0.499mH and 0.3305uF, respectively. A total of 16 GFM converters were used, with the specific network configuration as follows:

[0125] 1. GFM is concentrated at the front end of each group, closest to the grid connection point;

[0126] 2. Concentrated at the end of each group, far from the grid connection point;

[0127] 3. Uniformly staggered arrangement (odd-even distribution);

[0128] 4. Centered arrangement (4 units in the center are GFM);

[0129] 5. All GFMs are centrally connected to a separate group, while the remaining groups are GFLs.

[0130] Figure 7The diagram illustrates the equivalent impedance characteristics of renewable energy power plants under different grid connection locations. The red circles in the diagram indicate points where the impedance ratio of the renewable energy power plant to the grid in the supersynchronous band is 1. The phase difference at the intersection frequency reflects the stability margin at that frequency. It is evident that the "GFM centralized access link" strategy exhibits a significantly smaller phase difference at the intersection frequency compared to other strategies, resulting in a larger phase margin. The four deployment methods show minimal differences in intersection frequency but slight variations in phase margin. GFM access closer to the grid connection point yields the best results. Overall, the GFM centralized access scheme, by centrally constructing a stable impedance support channel in certain links, represents a more advantageous access structure.

[0131] To investigate the impact of the configuration ratio of GFM converters in parallel renewable energy power plants on system stability, five typical operating conditions with GFM ratios of 0%, 25%, 50%, 75%, and 100% were set up, and their equivalent AC impedances were compared and analyzed. The converter type configuration remained consistent across all groups.

[0132] Depend on Figure 8 It can be seen that as the GFM access ratio increases from 0% to 100%, the intersection point of the system impedance and the MMC impedance resonant peak gradually shifts to the left, indicating that the overall impedance amplitude characteristics of the system tend to saturate and the high-frequency interference immunity is enhanced. Simultaneously, the phase difference at each intersection point decreases significantly. After the GFM ratio exceeds 50%, the improvement in phase difference gradually slows down, and the marginal improvement in system stability decreases. Therefore, GFM resource allocation needs to be comprehensively balanced between cost and system stability requirements.

[0133] Example 3

[0134] Analyzing this sensitivity can help identify which frequency bands and which control parameters will cause significant changes in the impedance ratio trajectory. The analysis will focus on the 70-120Hz frequency band of a hybrid renewable energy power station with a GFM ratio of 25%.

[0135] First, perform a lateral sensitivity analysis on the system:

[0136] A global perturbation method was used for horizontal comparison to select the dominant parameter with the most significant impact on the system impedance characteristics from numerous candidate parameters. A 10% parameter perturbation was uniformly applied to all converters of the same type in the renewable energy power plant, while keeping other controller parameters and system structure unchanged. The results are as follows: Figure 9 As shown.

[0137] It can be observed that in pure GFL converter renewable energy power plants, the PI parameters and current loop PI parameters of the PLL are significantly more sensitive than other parameters. In pure GFM converter renewable energy power plants, K... pV K pI L fDominant. In hybrid converter renewable energy power plants, GFL's K-type is dominant. pPLL K pI K with GFM pI With L f .

[0138] Then, a longitudinal sensitivity analysis was performed on the system:

[0139] Sensitivity longitudinal analysis was performed on the previously selected dominant parameters. Sensitivity longitudinal analysis refers to studying the changing trend of the influence of a single influencing factor on a certain performance index of the system under specific system conditions and different initial values. By keeping other parameters constant and only changing the initial value of the target parameter, the changes in the system response characteristics were observed, thereby systematically evaluating the sensitivity and effect of the parameter on system performance. Figure 10 Impedance sensitivity analysis of various parameters of GFL, Figure 11 Impedance sensitivity analysis of various parameters of GFM.

[0140] K pPLL ( Figure 10 (a) A significant positive peak in amplitude sensitivity exists at specific frequency points (31Hz, 56Hz), with valleys around 43Hz and 65Hz. This indicates that at these frequency points, K... pPLL Increasing K will significantly amplify or suppress the system impedance amplitude, potentially inducing or suppressing the risk of resonance. The phase sensitivity exhibits a significant valley at 60Hz, indicating that the phase at this point is sensitive to parameter changes. With K... pPLL With increasing K, the sensitivity in the subsynchronous band (<50Hz) significantly increases, while the change in the supersynchronous band is relatively gradual, suggesting that high K... pPLL We need to be wary of the risk of subsynchronous oscillations.

[0141] K iPLL ( Figure 10 (b) Amplitude sensitivity peaks exist near 38Hz and 49Hz, and a trough exists near 60Hz. The influence pattern is similar to K. pPLL Similar but with a frequency shift.

[0142] K pi ( Figure 10 (c)): Amplitude sensitivity varies with K pi The overall effect increases with increasing K. Phase sensitivity varies drastically at 40Hz (peak), 45Hz (trough), and 50Hz (peak). This indicates that increasing K... pi This will generally enhance the system's response to disturbances and significantly alter phase characteristics in the mid-frequency range (40-50Hz), posing a risk of introducing negative damping. Appropriately reducing K... pi This may help suppress resonance and alleviate phase sensitivity issues.

[0143] K ii ( Figure 10 (d) The most significant feature is the presence of extremely high amplitude and phase sensitivity peaks near the 50Hz main frequency, indicating that the system is highly susceptible to K at this point. ii Disturbance and instability. Trend: With K ii Increasing K significantly reduces the sensitivity peak near the main frequency, and the surface becomes flatter. Appropriately increasing K... ii It can effectively reduce the system's sensitivity near the main frequency, and enhance robustness and stability margin.

[0144] K pV ( Figure 11 (a)): There are very sharp amplitude and phase sensitivity peaks at 50 Hz, and the peak height increases with K. pV It increases and rises sharply. This indicates that at higher K... pV Below this point, the system is extremely sensitive to parameter disturbances at the main frequency, easily inducing severe oscillations and posing a very high risk to stability. Sensitivity is lower at higher frequencies.

[0145] K pi ( Figure 11 (b) The amplitude sensitivity exhibits a peak at 50 Hz and a dip at 55 Hz. The phase sensitivity also shows some dependence. This indicates that K... pi The changes have a significant impact on both the impedance amplitude and phase near 50Hz, posing a potential risk of resonance and coupling.

[0146] K ii ( Figure 11 (c) Significant amplitude sensitivity peaks exist at 48Hz and 52Hz, especially when Ki is small. Phase sensitivity also fluctuates near the dominant frequency. ii Increasing K results in a significant decrease in the peak sensitivity and a smoother surface. Appropriately increasing K... ii It can effectively suppress sensitivity near the main frequency and improve phase stability.

[0147] L f ( Figure 11 (d)): Amplitude sensitivity increases with L in the frequency band above 50Hz. f The value increases significantly with increasing L, and the peak range expands. This indicates that at higher frequencies, increasing the filter inductance significantly enhances the sensitivity of the system impedance amplitude to inductance changes, potentially triggering high-frequency resonance. The phase sensitivity also shows a significant increase with increasing L. f A trend of increasing but slow rise. A larger L f It helps suppress high-frequency ripple, but it sacrifices high-frequency stability margin, and a trade-off must be made based on the specific harmonic environment and stability requirements of the system.

Claims

1. A stability analysis method for a grid-connected system with a grid-connected converter, characterized in that, Includes the following steps: S1. Construct an impedance model for a grid-type (GFL) converter based on the harmonic linearization method. The model takes into account the dynamic characteristics of the phase-locked loop (PLL). S2. Construct a grid-type (GFM) converter impedance model based on the virtual synchronous generator (VSG) control strategy. The model reflects the external characteristics of the voltage source. S3. Based on the principle of circuit equivalence, derive the total equivalent impedance of the hybrid system at the point of common coupling (PCC); S4. Construct a link impedance aggregation model for multiple converters in a new energy power station to obtain the overall output impedance expression of the system. S5. Analyze the system stability based on the impedance ratio criterion and evaluate the impact of the GFM / GFL combination ratio and arrangement on the stability margin. S6. Perform sensitivity analysis on the control parameters, identify key parameters, and assess the impact of their disturbances on system stability.

2. The stability analysis method for a hybrid grid-connected system with a grid converter according to claim 1, characterized in that, The process of establishing the GFL impedance model in S1 includes: ignoring DC-side voltage ripple, based on the current inner loop control structure, converting the frequency domain expression of the modulation wave into a sequence impedance expression, constructing positive and negative sequence frequency domain impedance expressions, and verifying the accuracy of the model using frequency sweep simulation. The GFL positive and negative sequence impedance expressions are as follows:

3. The stability analysis method for a hybrid grid-connected system with a grid converter according to claim 1, characterized in that, The process of establishing the GFM impedance model in S1 includes: based on the VSG control structure, establishing a frequency domain relationship model of the three-layer control loop, deriving the frequency domain transfer function between the output voltage, current, and modulation wave, constructing positive-sequence and negative-sequence frequency domain impedance expressions, and verifying the accuracy of the model using frequency sweep simulation. The GFM positive and negative sequence impedance expressions are as follows: in:

4. The stability analysis method for a hybrid grid-connected system with a grid converter according to claim 1, characterized in that, The derivation process of the equivalent impedance of the GFL / GFM hybrid system includes: Norton equivalent modeling of GFL is used as a current source and impedance parallel model, and GFM is modeled as a voltage source series impedance model of Thevenin equivalent. Construct a single-node parallel topology and derive the total equivalent output impedance after the converter combination. Extending to multi-link renewable energy power stations, a global equivalent impedance expression is constructed using a recursive superposition method of link admittance.

5. The stability analysis method for a hybrid grid-connected system with a grid converter according to claim 1, characterized in that, When analyzing the stability of a grid-connected system, Norton equivalent modeling is typically performed on the PCC port of the grid-connected inverter using current control, simplifying it to a model consisting of an equivalent current source and the equivalent impedance Z of the renewable energy power station. WFeq (s) Parallel structure; simultaneously, the sending-end converter station is modeled using Thevenin equivalents from the same PCC port, simplified to the voltage source equivalent impedance Z. g (s) A series topology is used to construct a complete impedance network model for stability analysis. In the system stability analysis step, the impedance ratio is defined as: Based on the generalized Nyquist criterion, the system's open-loop gain is used to determine whether it surrounds the (-1, j0) point by measuring the impedance ratio amplitude-frequency and phase characteristics, thereby evaluating the system's small-disturbance stability.

6. The stability analysis method for a hybrid grid-connected system with a grid converter according to claim 1, characterized in that, The spatial arrangement of the GFM converters, with 4 links, 8 wind power converters per group, and a total of 16 GFM converters, includes the following four methods: centralized arrangement near the grid connection point; centralized arrangement at the end; uniform staggered arrangement; and central arrangement. Single-group centralized arrangement. By comparing the system equivalent impedance and phase margin under the above arrangement methods, the optimal arrangement strategy is determined.

7. The stability analysis method for a hybrid grid-connected system with a grid converter according to claim 6, characterized in that, The GFM penetration stability analysis includes constructing five hybrid systems with GFM proportions of 0%, 25%, 50%, 75%, and 100%, analyzing the trends of impedance amplitude and phase difference changes, and determining the minimum proportion threshold to meet the stability margin requirements.

8. The stability analysis method for a hybrid grid-connected system with a grid converter according to claim 1, characterized in that, The sensitivity analysis method includes: Define the expression for the relevant complex impedance sensitivity: Conduct a horizontal global disturbance analysis to screen the key control parameters that are most sensitive to system stability.

9. The stability analysis method for a hybrid grid-connected system with a grid converter according to claim 8, characterized in that, The key control parameters include the proportional coefficient K of the PLL controller in the GFL converter. pPLL Current loop proportionality coefficient K pi The voltage controller K in the GFM converter pV Current loop PI parameter K pi / K ii Filter inductor L f The key control parameters obtained from the horizontal global disturbance analysis are analyzed vertically, and a three-dimensional response surface of sensitivity-frequency-parameter initial value is plotted to explore the adjustment direction and magnitude of key parameters.

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