Method and system for evaluating phase angle oscillation stability of phase-locked loop

Abstract

The invention provides a phase-locked loop phase angle oscillation stability evaluation method and system, and the method comprises the steps: collecting the voltage of a grid-connected point and the output current data of a converter, calculating the complementary angle of a line impedance angle through combining with a power grid side impedance parameter, and constructing an active and reactive power coupling vector model; and quantizing the influence of the change of the active current reference value on the reactive current output to obtain a phase locking error. A PLL phase angle transfer function model comprising a negative feedback path generated by a PLL control link and a positive feedback path dynamically introduced by power coupling is further constructed, whether a phase angle has a balance point or not is judged by comparing dynamic responses of the two paths, so that the system stability is evaluated, and control parameters are adjusted according to the system stability to optimize the PLL performance. The method is suitable for various grid-connected converter systems, such as wind power, photovoltaic, pumped storage and the like, and the operation stability and the control reliability of the grid-connected system in the weak power grid environment can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of power system control and stability analysis, and in particular to a method and system for evaluating the stability of phase-locked loop phase angle oscillations. Background Technology

[0002] With the vigorous advancement of new power system construction, wind power and photovoltaic power are being integrated into the power system through voltage source grid-connected converters. Simultaneously, a large number of new energy storage technologies (such as variable-speed pumped storage and electrochemical energy storage) are being connected to the grid through grid-connected converters. The power system exhibits a "dual-high" trend of high proportion of new energy sources and high proportion of power electronic equipment, gradually replacing traditional energy sources. This has highlighted the weak grid characteristics of the system, leading to a series of new oscillation problems. Among these, the phase-locked loop (PLL) in grid-connected control is prone to phase angle oscillations in weak grids, which has attracted widespread attention. However, existing phase angle oscillation assessments do not consider the impact of the active and reactive power coupling effects of the converter in weak grids on phase angle oscillations. When the converter's active power increases, the phase angle cannot completely track the true voltage phase angle during the phase-locking process, potentially causing the converter to reduce reactive power output or even absorb reactive power from the grid. This results in a drop in the voltage at the converter's grid connection point. Since the PLL input signal is the voltage vector at that point, it inevitably affects the dynamics of the PLL's phase angle, and the voltage drop may lead to phase angle oscillations. Therefore, there is an urgent need to propose a phase angle oscillation evaluation technique that takes into account the dynamics of converter power coupling.

[0003] An existing patent, "Method and Device for Judging Synchronization Stability of Power Electronic Interface Power Supply under Small Disturbances (Application No. 202211641021.2)," addresses the inability of existing eigenvalue analysis and impedance methods to analyze the impact of internal control loops on system stability in converter systems. It provides a method and device for judging the synchronization stability of power electronic interface power supplies under small disturbances, including: calculating the damping coefficient and synchronization coefficient of the inherent oscillation mode of the phase-locked loop based on a framework for analyzing the synchronization stability of power electronic interface power supplies, and performing synchronization stability analysis using the complex torque coefficient method. However, the method's calculation methods for the damping coefficient and synchronization coefficient are overly mathematical, lacking physical interpretation, and fail to consider the impact of the converter's active and reactive power coupling dynamics on the phase angle oscillation of the phase-locked loop. Therefore, the results obtained are overly optimistic.

[0004] Existing methods for determining PLL phase angle stability mostly employ eigenvalue methods, impedance methods, and complex torque coefficient methods. While these methods can yield qualitative conclusions about PLL phase angle stability, the stability models are overly mathematical, making it difficult to grasp the influence mechanism of converter control loops on PLL phase angle stability. Furthermore, they fail to consider the impact of converter active and reactive power coupling dynamics on the voltage amplitude at the grid connection point, thus failing to reveal the underlying mechanism causing PLL phase angle oscillations. Therefore, based on the qualitative conclusions obtained from traditional analysis methods, it is impossible to determine whether phase angle instability is caused by different factors such as converter power coupling or a weak power grid. Summary of the Invention

[0005] The present invention aims to at least partially solve one of the technical problems in the related art.

[0006] This invention proposes a method for evaluating the stability of phase-locked loop (PLL) phase angle oscillations. By constructing an active / reactive power coupling vector model and a dynamic phase angle transfer function model, the method quantifies and analyzes the impact of active power changes on reactive power output and PLL phase angle stability, thereby improving the accuracy and physical interpretability of the evaluation and providing a theoretical basis for optimizing system control parameters and improving stability.

[0007] Another objective of this invention is to provide a phase-locked loop phase angle oscillation stability evaluation system.

[0008] To achieve the above objectives, this invention provides a method for evaluating the stability of phase-locked loop (PLL) phase angle oscillations, comprising:

[0009] In response to the operating status of the converter grid-connected system, collect grid connection point voltage and converter output current data;

[0010] The line impedance angle is calculated based on the grid connection point voltage and converter output current data, combined with the grid-side impedance parameters.

[0011] A vector model of active and reactive power coupling of the converter is constructed to quantify the impact of changes in active current reference value on reactive current output and obtain the phase-locked error Δθ.

[0012] Based on the phase-locked error Δθ, a phase angle transfer function model of the phase-locked loop (PLL) including power coupling dynamics is constructed. The model includes a negative feedback path generated by the PLL control loop and a positive feedback path introduced by the power coupling dynamics.

[0013] By comparing the dynamic responses of the negative feedback path and the positive feedback path, it is determined whether there is an equilibrium point in the phase angle of the PLL. If there is an equilibrium point, it is determined to be stable; otherwise, it is determined to be unstable.

[0014] Based on the stability assessment results, the converter control parameters are adjusted to optimize the PLL phase angle stability.

[0015] The phase-locked loop phase angle oscillation stability evaluation method of the present invention may also have the following additional technical features:

[0016] In one embodiment of the present invention, constructing the active and reactive power coupling vector model of the converter includes:

[0017] Based on the projection relationship of the dq axis current reference value onto the vertical direction of the voltage vector, a dynamic influence model of the change in active current reference value on reactive current output is established.

[0018] The model is used to characterize that when the active current reference value increases, the reactive current output by the converter may be less than the reference value, or even absorb reactive power from the grid.

[0019] In one embodiment of the present invention, the calculation of the phase-locked error Δθ includes:

[0020] Calculate the change in active power reference value ΔP based on the power flow equation and the Jacobian matrix. ref and reactive power reference value change ΔQ ref The impact on phase-locked loop error Δθ;

[0021] The Jacobian matrix is ​​derived from the equivalent impedance Z of the power grid. g The steady-state angle difference δ0 between the PCC voltage and the grid voltage, along with the control parameters, are used to determine the voltage.

[0022] In one embodiment of the present invention, constructing the PLL phase angle transfer function model includes:

[0023] The reactive current variation of the converter output is introduced into the PLL control model to form a dynamic model containing a positive feedback path.

[0024] The positive feedback path is used to characterize the enhancing effect of reactive current changes on the dynamic response of the PLL phase angle.

[0025] In one embodiment of the present invention, determining whether the phase angle of the PLL has an equilibrium point includes:

[0026] Compare the dynamic response of the negative feedback path generated by the PLL control loop with the dynamic response of the positive feedback path dynamically introduced by power coupling.

[0027] When the response value of the positive feedback path exceeds the response value of the negative feedback path, it is determined that the PLL phase angle has lost its balance point and the system is unstable.

[0028] To achieve the above objectives, another aspect of the present invention proposes a phase-locked loop phase angle oscillation stability evaluation system, comprising:

[0029] The data acquisition module is used to collect grid connection point voltage and converter output current data;

[0030] The parameter calculation module is used to calculate the line impedance angle based on the voltage and current data and the grid-side impedance parameters;

[0031] The power coupling modeling module is used to construct a vector model of active and reactive power coupling of the converter, quantify the impact of changes in active current reference value on reactive current output, and obtain the phase-locked error Δθ.

[0032] The PLL phase angle stability analysis module is used to construct a phase angle transfer function model of the phase-locked loop that includes power coupling dynamics based on the phase-locked error Δθ. The model includes a negative feedback path generated by the PLL control loop and a positive feedback path introduced by the power coupling dynamics.

[0033] The stability judgment module is used to determine whether there is an equilibrium point in the phase angle of the PLL by comparing the dynamic response of the negative feedback path and the positive feedback path. If there is an equilibrium point, it is determined to be stable; otherwise, it is determined to be unstable.

[0034] The control parameter adjustment module is used to adjust the converter control parameters based on the stability judgment results to optimize the PLL phase angle stability.

[0035] The phase-locked loop (PLL) phase angle oscillation stability assessment method and system of this invention realizes the stability assessment of the grid-connected system of the converter in a weak power grid environment by constructing an active / reactive power coupling vector model of the converter and a PLL phase angle transfer function model, which solves the technical defect of the prior art that ignores the power coupling effect and leads to the assessment result being too optimistic.

[0036] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0037] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

[0038] Figure 1 This is a flowchart of a phase-locked loop phase angle oscillation stability evaluation method according to an embodiment of the present invention;

[0039] Figure 2 This is a diagram of a converter grid-connected system and its power frequency side control structure according to an embodiment of the present invention;

[0040] Figure 3 This is a schematic diagram of the coupled reactive current when the voltage phase leads the phase measured by the PLL according to an embodiment of the present invention.

[0041] Figure 4 This is a model diagram of the phase angle transfer function of the converter power frequency side control PLL considering the power coupling characteristics according to an embodiment of the present invention;

[0042] Figure 5 This is a structural diagram of a phase-locked loop phase angle oscillation stability evaluation system according to an embodiment of the present invention. Detailed Implementation

[0043] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0044] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. 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 should fall within the scope of protection of the present invention.

[0045] The following describes, with reference to the accompanying drawings, a method and system for evaluating the stability of phase-locked loop phase angle oscillations according to an embodiment of the present invention.

[0046] Figure 1 This is a flowchart of a phase-locked loop phase angle oscillation stability evaluation method according to an embodiment of the present invention, such as... Figure 1 As shown, it includes:

[0047] S1, responding to the operating status of the converter grid-connected system, collects grid-connected point voltage and converter output current data;

[0048] S2, Calculate the line impedance angle based on the grid connection point voltage and converter output current data, and in combination with the grid-side impedance parameters;

[0049] S3. Construct a vector model of the coupling between active and reactive power of the converter, quantify the impact of changes in the active current reference value on the reactive current output, and obtain the phase-locked error Δθ.

[0050] S4. Based on the phase-locked loop error Δθ, construct a phase angle transfer function model of the phase-locked loop (PLL) that includes power coupling dynamics. The model includes a negative feedback path generated by the PLL control loop and a positive feedback path introduced by the power coupling dynamics.

[0051] S5. By comparing the dynamic responses of the negative feedback path and the positive feedback path, it is determined whether there is an equilibrium point in the phase angle of the PLL. If there is an equilibrium point, it is determined to be stable; otherwise, it is determined to be unstable.

[0052] S6. Based on the stability assessment results, adjust the converter control parameters to optimize the PLL phase angle stability.

[0053] In one embodiment of the present invention, the converter grid-connected system and its power frequency side control structure are as follows: Figure 2 As shown. For full-power variable-speed pumped-storage units, Figure 2The intermediate frequency converter is a modular multilevel matrix converter (M3C) phase converter. According to the literature "Research on Mathematical Model and Control Strategy of M3C Converter Based on Dual dq Coordinate Transformation", the power frequency side control and low-frequency side control of the M3C are decoupled, meaning that the dynamics of the low-frequency side control (variable-speed pumped-storage unit side) will not affect the results of the power frequency side control. For determining the active current reference value, constant active power control or constant capacitor voltage control can be selected, and its output can be used as the active current reference value i. da,ref For the reactive current reference value, either constant power frequency AC side voltage control or constant reactive power control can be selected, and its output can be used as the reactive current reference value i. qa,ref i da,ref with i qa,ref Let dq be the current flowing through the a-phase sub-converter. The dq axis is a rotating coordinate system on the power frequency side, obtained from a PLL phase-locked loop. da i qa Let i be the dq-axis current flowing through the a-phase sub-converter, whose value is 1 / 3 of the dq-axis current flowing through the power frequency transmission line. da,ref with i qa,ref As the input to the inner current loop, the reference voltage signal for the output PWM modulation wave is used to obtain the three-phase modulation signal of the a-phase sub-converter. According to the literature "Research on Mathematical Model and Control Strategy of M3C Converter Based on Dual dq Coordinate Transformation", the power frequency side control of the b-phase and c-phase sub-converters is the same as that of the a-phase, and will not be elaborated here.

[0054] Understandable, Figure 2 The illustrated full-power variable-speed pumped-storage M3C grid-connected system and its control structure are similar to those of full-power variable-speed pumped-storage back-to-back grid-side converter grid-connected systems and their control, full-power wind turbine grid-side converter grid-connected systems and their control, photovoltaic inverter grid-connected systems and their control, and doubly-fed variable-speed pumped-storage unit-side converter control, as their power coupling dynamic processes are similar. Therefore, the phase-locked loop phase angle oscillation stability evaluation method considering the power coupling dynamics of the grid-connected converter proposed in this invention is not only applicable to the analysis of full-power variable-speed pumped-storage M3C grid-connected systems, but also applicable to the aforementioned types of grid-connected systems.

[0055] In some embodiments, a coupled vector model of active and reactive power of the converter is constructed, and the coupled reactive power absorbed by the converter is quantified based on existing control and system parameters.

[0056] When a converter is connected to a weak AC power grid, the PLL bandwidth is often set relatively small to improve the stability margin under small disturbances. However, the PLL has difficulty tracking the voltage phase quickly during dynamic processes. When the converter increases its active power output, this will cause the output reactive current to be less than the reactive current reference value, which may result in a decrease in the reactive power injected into the system, or even the absorption of reactive power from the grid.

[0057] Therefore, it is urgent to construct a coupled vector model of the active and reactive power of the converter. When the active power output of the converter increases, the phase-locked loop phase θ... pll Lag the true voltage phase θ v ,like Figure 3 As shown.

[0058] a-phase converter d-axis current reference value i d,ref (For lack of generality, the subscript a is omitted) When the dq-axis current reference value i increases, d,ref i q,ref A reactive current i is generated by the projection in the direction perpendicular to the voltage vector. Q Based on the vector relationship, its expression is as follows:

[0059] i Q =i d,ref sinΔθ+i q,ref cosΔθ (1)

[0060] Where, Δθ=θ pll -θ v .

[0061] It can be seen that when Δθ=0, there is no coupled reactive current caused by active current, and the converter injects reactive power into the grid according to the reactive reference value.

[0062] When the converter outputs active and reactive power > 0, and the converter output active current increases, then Δθ < 0. If the reactive current i is made to... Q Less than the reactive current reference value i q,ref Then the following relationship is satisfied:

[0063]

[0064] Therefore, we get:

[0065]

[0066] Solve for the critical value of Δθ:

[0067]

[0068] During non-fault periods, the active current output by the converter is often greater than the reactive current, therefore i d,ref >i q,refTherefore, Δθ>0 holds true in formula (4). According to formulas (3) and (4), it can be seen that when Δθ<0, the entire domain satisfies the constraint of inequality (3). At this time, the reactive current is less than its reference value, that is, the reactive power output by the converter is less than the reference value. The reactive power output by the converter decreases as the active power increases, which weakens the voltage support capability of the grid connection point and will inevitably affect the stability of the PLL phase angle oscillation under small disturbances.

[0069] In summary, the coupling relationship between the converter's output active and reactive currents and their reference values ​​is shown in the following formula:

[0070]

[0071] In some embodiments, a phase angle stability transfer function of the converter PLL is constructed to determine the stability under small disturbances.

[0072] A grid-connected converter can be considered equivalent to a controlled current source. The current amplitude is:

[0073]

[0074] Where the proportional coefficient 3 represents the amplitude of the output current on the power frequency side, which is the current i of phase a submodule. P i Q Three times the amplitude. The phase angle of the controlled current source is shown in formula (7):

[0075]

[0076] according to Figure 2 The voltage at point PCC is:

[0077] V t ∠θ v =V g ∠θ g +I∠θ I ·Z g =V g ∠θ g +I|Z g |∠(θ I +θ Z (8)

[0078] Where V g θ g Here, I represents the voltage amplitude and phase angle of the power grid, and θ represents the amplitude of the controlled current source. Z The equivalent impedance Z of the power grid g The impedance angle, θ I The phase angle of the controlled current source. Let be the angle by which the current leads the d-axis. From (8), the output of the M3C power frequency side control PLL can be obtained as:

[0079]

[0080] In some embodiments, the PLL phase angle transfer function model is as follows: Figure 4 As shown, PI is a proportional-integral controller. Equation (5) considers the power coupling model proposed in this invention. Figure 4 As can be seen, the power coupling section provides positive feedback for the PLL phase angle, making V tq2 It increases with the increase of the phase angle, and since the PLL control dynamic is negative feedback, V tq1 It decreases as the phase angle increases. When the phase angle is stable, V tq1 +V tq2 =0, V tq There exists an equilibrium point; when unstable, V tq1 +V tq2 ≠0, that is, V tq1 Curve and V tq2 The curves do not intersect. Therefore, it can be observed that when the active power reference value P... ref When it increases, Increase, thus leading to V tq2 As the maximum value increases, when the active power reference value increases to exceed the critical value, V tq2 The maximum value will exceed V. tq1 The maximum value, thus making V tq When the equilibrium point is lost, the PLL phase angle becomes unstable. This shows that power coupling can worsen the phase angle stability of the PLL.

[0081] Furthermore, with Figure 4 Taking the full-power variable-speed pumped-storage grid-connected system shown as an example, the design and implementation shall be carried out according to the following steps.

[0082] Step 1: Define the target system structure and its control structure:

[0083] according to Figure 2 For the target system shown, the control method is defined. In this case, the converter control outer loop is selected to use active and reactive power control methods, and the control parameters are shown in Table 1.

[0084] Table 1

[0085]

[0086]

[0087] Step 2: Construct a coupled vector model of active and reactive power of the converter.

[0088] The power coupling vector model is shown in formula (5). The key point is to find Δθ = θ pll -θ v When the active power reference value and the reactive power reference value change, a Δθ will be generated, expressed as follows:

[0089]

[0090] Where ΔP=P ref –P,ΔQ=Q ref –Q, coefficient K ΔPδ ,K ΔQδ The Jacobian matrix can be inverted to obtain the result, as shown in formula (10).

[0091]

[0092] The power flow equation at PCC is given by equation (11), therefore the Jacobian matrix elements can be expressed as (12), where δ0 is the PCC voltage (v). pcc ) and grid voltage (V g The steady-state angle difference between ∠0 and ∠0. Grid impedance Z g Given that α is the complementary angle of the impedance angle of the Thevenin equivalent circuit on the AC side, obtained from formula (13).

[0093]

[0094] Combining (9)-(12), Δθ can be obtained. Therefore, the following implementation plan can be adopted: First, the voltage at the PCC point and the output current of the converter are collected through the LEM board. Combined with the AC transmission line resistance and reactance parameters, the complementary angle α of the line impedance angle is calculated by formula (13). Finally, the values ​​of each element of the Jacobian matrix are calculated, as shown in formula (12). Then, the coefficient K is obtained. ΔPδ ,K ΔQδ As shown in formula (10). When setting the active power reference value P of the converter ref When the power is increased from 200W to 500W, the PLL phase-locked error Δθ is calculated according to formula (9), and the reactive current containing the reactive coupling component is formed after the active current increases, as shown in formula (5).

[0095] Step 3: Construct the phase angle stability transfer function of the converter PLL and determine the stability under small disturbances.

[0096] against Figure 2 Medium converter grid-connected system, construction Figure 4 The transfer function model of the power frequency side control PLL is shown.

[0097] When the active reference value P ref To increase the power from 200W to 500W, the active and reactive currents can be calculated using formula (5), and then the result can be obtained. The value increases from -0.46 to -0.2, resulting in V in formula (4). tq2 Maximum value As the corresponding value increases, and the active power reference value continues to increase, it is observed that V... tq2The maximum value will exceed V. tq1 Maximum value (V) g When V tq The PLL will lose its balance point and the phase angle will become unstable. At this point, the active power reference value is critical. If the active power is exceeded, the PLL phase angle will easily oscillate.

[0098] The phase-locked loop (PLL) phase angle oscillation stability assessment method according to embodiments of the present invention first addresses the active / reactive coupling phenomenon that may be introduced by the power frequency side control of a full-power variable-speed pumped-storage (M3C) converter. It demonstrates that as active power increases, the reactive power generated by the converter decreases accordingly. Based on the PLL phase angle transfer function, the decrease in coupled reactive power introduces positive feedback, worsening the PLL phase angle oscillation stability. The proposed PLL phase angle oscillation stability assessment method and system, which considers the power coupling dynamics of the grid-connected converter, is also applicable to the PLL phase angle stability analysis of full-power variable-speed pumped-storage back-to-back grid-side converters, doubly-fed variable-speed pumped-storage converters, direct-drive wind power grid-side converters, and photovoltaic inverters, due to their similar control structures and similar power coupling mechanisms.

[0099] To achieve the above embodiments, such as Figure 5 As shown, this embodiment also provides a phase-locked loop phase angle oscillation stability evaluation system 10, including:

[0100] Data acquisition module 100 is used to acquire grid connection point voltage and converter output current data;

[0101] The parameter calculation module 200 is used to calculate the line impedance angle based on the voltage and current data and the grid-side impedance parameters.

[0102] The power coupling modeling module 300 is used to construct a vector model of active and reactive power coupling of the converter, quantify the impact of changes in active current reference value on reactive current output, and obtain the phase-locked error Δθ.

[0103] The PLL phase angle stability analysis module 400 is used to construct a phase angle transfer function model of the phase-locked loop that includes power coupling dynamics based on the phase-locked error Δθ. The model includes a negative feedback path generated by the PLL control loop and a positive feedback path introduced by the power coupling dynamics.

[0104] The stability judgment module 500 is used to determine whether there is an equilibrium point in the phase angle of the PLL by comparing the dynamic response of the negative feedback path and the positive feedback path. If there is an equilibrium point, it is determined to be stable; otherwise, it is determined to be unstable.

[0105] The control parameter adjustment module 600 is used to adjust the converter control parameters based on the stability judgment results to optimize the PLL phase angle stability.

[0106] Furthermore, the power coupling modeling module 300 includes:

[0107] The dq-axis current projection unit is used to establish a coupling model of active and reactive currents based on the projection relationship of the dq-axis current reference value in the vertical direction of the voltage vector.

[0108] The phase-locked error calculation unit is used to quantify the impact of changes in the active current reference value on the reactive current output and to calculate the phase-locked error Δθ.

[0109] Furthermore, the parameter calculation module 200 includes:

[0110] The power flow equation processing unit is used to process the power flow equations based on the equivalent impedance Z of the power grid. g Calculate the complementary angle α of the line impedance angle based on the steady-state angle difference δ0 between the PCC voltage and the grid voltage;

[0111] The Jacobian matrix calculation unit is used to calculate the elements of the Jacobian matrix based on control parameters and grid parameters, and to determine the influence coefficient K of the active / reactive power reference value changes on the phase angle. ΔPδ K ΔQδ .

[0112] Furthermore, the control parameter adjustment module 600 includes:

[0113] The feedback response comparison unit is used to compare the response values ​​of the negative feedback path generated by the PLL control loop with the positive feedback path dynamically introduced by power coupling.

[0114] The control parameter optimization unit is used to adjust the converter control parameters to reduce the active power reference value or enhance the reactive power support capability when the response value of the positive feedback path exceeds the response value of the negative feedback path.

[0115] The phase-locked loop (PLL) phase angle oscillation stability assessment system according to embodiments of the present invention first addresses the active / reactive coupling phenomenon that may be introduced by the power frequency side control of a full-power variable-speed pumped-storage (M3C) converter. It demonstrates that as active power increases, the reactive power generated by the converter decreases accordingly. Based on the PLL phase angle transfer function, the decrease in coupled reactive power introduces positive feedback, worsening the PLL phase angle oscillation stability. The proposed PLL phase angle oscillation stability assessment method and system, which considers the power coupling dynamics of the grid-connected converter, is also applicable to the PLL phase angle stability analysis of full-power variable-speed pumped-storage back-to-back grid-side converters, doubly-fed variable-speed pumped-storage converters, direct-drive wind power grid-side converters, and photovoltaic inverters, due to their similar control structures and similar power coupling mechanisms.

[0116] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0117] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

Claims

1. A method for evaluating the stability of phase angle oscillations in a phase-locked loop, characterized in that, Includes the following steps: In response to the operating status of the converter grid-connected system, collect grid connection point voltage and converter output current data; The line impedance angle is calculated based on the grid connection point voltage and converter output current data, combined with the grid-side impedance parameters. A vector model of active and reactive power coupling of the converter is constructed to quantify the impact of changes in active current reference value on reactive current output and obtain the phase-locked error Δθ. Based on the phase-locked error Δθ, a phase angle transfer function model of the phase-locked loop (PLL) including power coupling dynamics is constructed. The model includes a negative feedback path generated by the PLL control loop and a positive feedback path introduced by the power coupling dynamics. By comparing the dynamic responses of the negative feedback path and the positive feedback path, it is determined whether there is an equilibrium point in the phase angle of the PLL. If there is an equilibrium point, it is determined to be stable; otherwise, it is determined to be unstable. Based on the stability assessment results, the converter control parameters are adjusted to optimize the PLL phase angle stability.

2. The method as described in claim 1, characterized in that, The construction of the active and reactive power coupled vector model of the converter includes: Based on the projection relationship of the dq axis current reference value onto the vertical direction of the voltage vector, a dynamic influence model of the change in active current reference value on reactive current output is established. The model is used to characterize that when the active current reference value increases, the reactive current output by the converter may be less than the reference value, or even absorb reactive power from the grid.

3. The method as described in claim 2, characterized in that, The calculation of the phase-locked error Δθ includes: Calculate the change in active power reference value ΔP based on the power flow equation and the Jacobian matrix. ref and reactive power reference value change ΔQ ref The impact on phase-locked loop error Δθ; The Jacobian matrix is ​​derived from the equivalent impedance Z of the power grid. g The steady-state angle difference δ0 between the PCC voltage and the grid voltage, along with the control parameters, are used to determine the voltage.

4. The method as described in claim 3, characterized in that, The construction of the PLL phase transfer function model includes: The reactive current variation of the converter output is introduced into the PLL control model to form a dynamic model containing a positive feedback path. The positive feedback path is used to characterize the enhancing effect of reactive current changes on the dynamic response of the PLL phase angle.

5. The method as described in claim 4, characterized in that, The determination of whether the PLL phase angle has an equilibrium point includes: Compare the dynamic response of the negative feedback path generated by the PLL control loop with the dynamic response of the positive feedback path dynamically introduced by power coupling. When the response value of the positive feedback path exceeds the response value of the negative feedback path, it is determined that the PLL phase angle has lost its balance point and the system is unstable.

6. A phase-locked loop phase angle oscillation stability evaluation system, characterized in that, include: The data acquisition module is used to collect grid connection point voltage and converter output current data; The parameter calculation module is used to calculate the line impedance angle based on the voltage and current data and the grid-side impedance parameters; The power coupling modeling module is used to construct a vector model of active and reactive power coupling of the converter, quantify the impact of changes in active current reference value on reactive current output, and obtain the phase-locked error Δθ. The PLL phase angle stability analysis module is used to construct a phase angle transfer function model of the phase-locked loop that includes power coupling dynamics based on the phase-locked error Δθ. The model includes a negative feedback path generated by the PLL control loop and a positive feedback path introduced by the power coupling dynamics. The stability judgment module is used to determine whether there is an equilibrium point in the phase angle of the PLL by comparing the dynamic response of the negative feedback path and the positive feedback path. If there is an equilibrium point, it is determined to be stable; otherwise, it is determined to be unstable. The control parameter adjustment module is used to adjust the converter control parameters based on the stability judgment results to optimize the PLL phase angle stability.

7. The system as described in claim 6, characterized in that, The power coupling modeling module includes: The dq-axis current projection unit is used to establish a coupling model of active and reactive currents based on the projection relationship of the dq-axis current reference value in the vertical direction of the voltage vector. The phase-locked error calculation unit is used to quantify the impact of changes in the active current reference value on the reactive current output and to calculate the phase-locked error Δθ.

8. The system as described in claim 7, characterized in that, The parameter calculation module includes: The power flow equation processing unit is used to process the power flow equations based on the equivalent impedance Z of the power grid. g Calculate the complementary angle α of the line impedance angle based on the steady-state angle difference δ0 between the PCC voltage and the grid voltage; The Jacobian matrix calculation unit is used to calculate the elements of the Jacobian matrix based on control parameters and grid parameters, and to determine the influence coefficient K of the active / reactive power reference value changes on the phase angle. ΔPδ K ΔQδ .

9. The system as described in claim 6, characterized in that, The control parameter adjustment module includes: The feedback response comparison unit is used to compare the response values ​​of the negative feedback path generated by the PLL control loop with the positive feedback path dynamically introduced by power coupling. The control parameter optimization unit is used to adjust the converter control parameters to reduce the active power reference value or enhance the reactive power support capability when the response value of the positive feedback path exceeds the response value of the negative feedback path.

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