An electromechanical transient modeling method for grid-type converters based on PSASP

Abstract

This invention belongs to the field of electromechanical transient modeling technology, specifically disclosing a method for electromechanical transient modeling of a grid-type converter based on PSASP, including: setting the grid connection point voltage and the equivalent impedance parameters of the converter transformer; establishing a grid-type converter control model in the PSASP custom modeling platform, and obtaining a reference voltage signal through the control model; performing voltage-to-current conversion to obtain the real and imaginary parts of the current injected into the bus; setting a virtual impedance element in the grid-type converter control model, calculating the virtual voltage drop using the real and imaginary parts of the current combined with the virtual impedance parameters, correcting the reference voltage signal, and obtaining the corrected real and imaginary parts of the current through voltage-to-current conversion; injecting the corrected real and imaginary parts of the current into the PSASP bus in the form of a current source to establish the electromechanical transient model of the grid-type converter.

Description

Technical Field

[0001] This invention belongs to the field of electromechanical transient modeling technology, specifically relating to an electromechanical transient modeling method for grid-type converters based on PSASP. Background Technology

[0002] With the large-scale development of new energy power generation such as wind power and photovoltaics, as well as energy storage technologies, a new type of power system is gradually taking shape. While the integration of new energy sources into the grid via power electronic converters improves grid access flexibility and system dispatchability, it also results in a low inertia characteristic for the overall power system. Under weak grid conditions, the system's ability to support frequency and voltage is insufficient, leading to stability risks during steady-state operation and fault ride-through. Therefore, researching modeling and control methods for grid-connected converters that can actively support voltage and frequency has become an important direction for ensuring the stable operation of grids with a high proportion of new energy sources.

[0003] Currently, the Power System Analysis Software Package (PSASP) is widely used as the mainstream tool for power system simulation and analysis. This software supports power flow calculations, transient stability analysis, and dynamic process simulation, meeting the needs of research on the operating characteristics of large-scale power systems. To expand the functionality of its model library, PSASP provides a User Defined Model (UDM) platform, allowing researchers to build dynamic models of power electronic devices such as wind power and photovoltaic systems, enabling them to participate in system simulations. This provides an important tool for research on grid-connected control strategies for new energy sources.

[0004] However, the current PSASP standard model library offers limited support for grid-type converters. Its bus interface only accepts injection methods for both real and imaginary current parts, while grid-type control models typically use voltage signals as outputs. This interface mismatch makes it difficult to directly call custom models. Furthermore, existing PSASP-based modeling schemes lack a unified voltage-to-current conversion mechanism, and key control elements such as virtual impedance are not fully implemented. This limits the electromechanical transient simulation of grid-type converters on the PSASP platform. Summary of the Invention

[0005] This invention addresses the problems in the prior art by providing an electromechanical transient modeling method for grid-type converters based on PSASP, thereby solving the problem in the background art where the real and imaginary parts of the current and the voltage signal are mismatched in terms of interface form, making it difficult to directly call custom models.

[0006] The technical solution adopted in this invention is as follows:

[0007] This application provides a PSASP-based electromechanical transient modeling method for grid-type converters, which includes the following steps:

[0008] Step S1: Set the grid connection point voltage and the equivalent impedance parameters of the converter transformer;

[0009] Step S2: In the PSASP custom modeling platform, establish a grid-type converter control model based on the droop control and voltage-current dual-loop control strategy, and calculate and output the reference voltage signal through the grid-type converter control model.

[0010] Step S3: Combine the reference voltage signal, grid connection point voltage, and equivalent impedance parameters of the converter transformer to perform voltage-to-current conversion to obtain the real and imaginary parts of the current injected into the bus.

[0011] Step S4: Set up a virtual impedance in the grid-type converter control model, calculate the virtual voltage drop using the virtual impedance parameters combined with the real and imaginary parts of the current, and correct the reference voltage signal based on the virtual voltage drop. The corrected real and imaginary parts of the current are obtained through voltage-current conversion.

[0012] Step S5: Inject the obtained corrected real and imaginary parts of the current into the PSASP bus in the form of current sources to establish the electromechanical transient model of the grid-type converter.

[0013] Furthermore, in step S1, the set grid connection point voltage includes amplitude and phase angle information;

[0014] The set equivalent impedance of the converter transformer includes equivalent resistance and equivalent reactance parameters.

[0015] Furthermore, in step S2, the droop control loop consists of active droop relationship and reactive droop relationship;

[0016] The angular frequency adjustment is calculated by using the active power deviation and the active power droop coefficient.

[0017] The reactive power droop relationship is calculated by using the reactive power deviation and the reactive power droop coefficient to obtain the voltage amplitude adjustment, and the angular frequency adjustment is integrated to obtain the phase angle.

[0018] Furthermore, in the droop control strategy, the active power-frequency droop stage compares the output active power of the grid converter with the reference active power to obtain the active power difference, and multiplies it with the active power droop coefficient to form the frequency offset. The frequency offset is superimposed with the rated angular frequency and accumulated by an integrator to obtain the output phase angle of the grid converter.

[0019] The reactive power-voltage droop stage compares the output reactive power of the grid-type converter with the reference reactive power to obtain the reactive power difference, and multiplies it with the reactive power droop coefficient to form the voltage amplitude adjustment. The voltage amplitude adjustment is then added to the rated voltage amplitude to form the reference voltage amplitude.

[0020] Furthermore, the voltage amplitude and phase angle of the voltage output by the droop control are used as reference inputs in the outer voltage loop. The reference voltage component is obtained through coordinate transformation and compared with the actual output voltage component. The difference is then adjusted by proportional-integral control to output the reference current.

[0021] The inner current loop takes the reference current output from the outer voltage loop as input, compares it with the actual current component, generates a pulse width modulation signal through proportional-integral regulation, and forms the output voltage signal of the grid converter through inverse coordinate transformation.

[0022] Furthermore, the outer voltage loop uses a proportional-integral controller to cumulatively adjust the deviation between the output voltage and the reference voltage;

[0023] The proportional element cancels out sudden disturbances, the integral element eliminates steady-state errors, and a limiting element is set at the controller output to limit the reference current from not exceeding a preset threshold.

[0024] The inner current loop uses a proportional-integral controller to adjust the deviation between the output current and the reference current. The output of the inner current loop is transformed by inverse coordinate transformation to obtain a pulse width modulation signal, which drives the voltage output of the converter.

[0025] Furthermore, in step S3, the voltage-to-current conversion is performed by calculating the phasor difference between the reference voltage and the grid connection point voltage, and dividing it by the equivalent impedance of the converter transformer to obtain the current phasor. The current phasor is then decomposed into real and imaginary components, which are used as the current input injected into the PSASP bus.

[0026] Furthermore, in step S4, the virtual impedance stage includes virtual resistance and virtual inductance. The virtual impedance is calculated based on the output current to obtain a virtual voltage drop. The virtual voltage drop is subtracted from the reference voltage before the reference voltage signal input voltage outer loop to obtain the corrected reference voltage signal.

[0027] Furthermore, in step S4, a temporary information exchange mechanism is used to realize the internal signal transmission of the model. The mechanism sets a temporary information exchange value number in the PSASP custom modeling platform, and exchanges the intermediate variables of each sub-module of the control model in the form of numbers to form a custom model of a grid-type converter with a block layout.

[0028] Furthermore, it also includes:

[0029] Step S6: Verify the electromechanical transient model of the grid converter. Verification is performed by setting a bus short-circuit fault in the PSASP transient stability simulation and observing the voltage, current and power changes of the grid converter before and after the fault occurs. This is used to confirm whether the model can reflect the dynamic response of the grid converter under fault conditions.

[0030] As can be seen from the above technical solutions, the advantages of the present invention are:

[0031] By constructing a grid-type converter control model in the PSASP custom modeling platform and setting up voltage-to-current conversion and virtual impedance correction mechanisms, the correct mapping of voltage signals to current signals is achieved, enabling the grid-type converter to inject current into the bus in the form of a current source. This ensures electromechanical transient modeling and simulation in the PSASP platform and solves the problem that existing models cannot be matched with the PSASP bus interface.

[0032] By clearly defining the amplitude and phase angle parameters of the grid connection point voltage, as well as the equivalent resistance and reactance of the converter transformer, in the initial stage of modeling, the input conditions for voltage-to-current conversion are ensured to be accurate and reliable, enabling the model to reflect the real operating conditions of the grid connection point and improving the simulation accuracy and applicability.

[0033] By introducing active power-frequency droop and reactive power-voltage droop relationships into the control model, the grid-type converter can simulate the frequency and voltage regulation characteristics of a synchronous machine, enhancing the converter's ability to support system voltage and frequency, and providing modeling support for stable operation under weak grid conditions.

[0034] By multiplying the active power difference by the active power droop coefficient to obtain the frequency offset, and then integrating it with the rated angular frequency to form the phase angle, and by using the reactive power difference and the reactive power droop coefficient to obtain the voltage amplitude adjustment, and then superimposing it with the rated voltage amplitude, a clear droop control calculation process is established, making the reference voltage signal more physically meaningful and operable.

[0035] By constructing a dual closed-loop control system consisting of an outer voltage loop and an inner current loop, the reference voltage generates a reference current through the voltage loop, and then the current loop generates a pulse width modulation signal. This achieves dynamic control transmission from voltage signal to current signal, improving the model's response accuracy to voltage and current changes.

[0036] By employing proportional-integral regulation in both the outer voltage loop and the inner current loop, and adding a limiting circuit at the output of the outer voltage loop, the control system is guaranteed to have both rapid dynamic response and to avoid current overshoot, thus ensuring the stability and safety of electrical quantities during the modeling process.

[0037] By calculating the phasor difference between the reference voltage and the grid connection point voltage, and combining it with the equivalent impedance of the converter transformer to calculate the current phasor, and then decomposing it into the real and imaginary parts of the current, an input quantity consistent with the PSASP bus interface was established, enabling the grid-type converter model to be seamlessly connected to power system simulation.

[0038] By setting virtual resistors and virtual inductors in the control model, calculating the virtual voltage drop based on the current signal and correcting the reference voltage, the current limiting characteristics of the model under conditions such as short-circuit faults are enhanced, making the modeling results more consistent with the operating characteristics of grid-type converters in engineering applications.

[0039] By employing the Temporary Information Exchange (TM) mechanism in the PSASP custom modeling platform, the intermediate signals of the virtual impedance correction stage and the voltage-to-current conversion stage are transmitted in the form of numbers, realizing the block layout and modular management of the model, which significantly improves the model's portability and maintainability.

[0040] By setting a bus short-circuit fault in the PSASP transient stability simulation, the response of the grid-type converter under voltage drop and frequency disturbance was observed. The current limiting and voltage support capabilities of the established model under fault conditions were verified, ensuring the effectiveness and engineering applicability of the model. Attached Figure Description

[0041] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description will be briefly introduced below. Obviously, the accompanying 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.

[0042] Figure 1 A flowchart illustrating the steps of the electromechanical transient modeling method for PSASP-based grid converters;

[0043] Figure 2 This is a topology diagram of a three-phase voltage source converter;

[0044] Figure 3 Diagram of the droop control strategy;

[0045] Figure 4 This is a schematic diagram of a control structure with virtual impedance.

[0046] Figure 5 This is a control block diagram for a grid-type converter based on droop control.

[0047] Figure 6 A schematic diagram considering the power flow distribution of the converter transformer;

[0048] Figure 7 This is a control block diagram of a voltage signal to current signal conversion module;

[0049] Figure 8 A power transfer model that takes virtual impedance into account;

[0050] Figure 9 Voltage phasor diagram considering virtual impedance;

[0051] Figure 10 Signal diagrams for each stage of the UD model using information exchange values;

[0052] Figure 11 A single-line diagram for a standalone test system;

[0053] Figure 12 To display the short-circuit current waveforms when setting different virtual impedances;

[0054] Figure 13 Waveforms of the output of a grid-type converter before and after fault ride-through;

[0055] Figure 13 (a) is a voltage waveform diagram of the output before and after fault ride-through;

[0056] Figure 13 (b) is a waveform diagram of the output frequency before and after fault crossing;

[0057] Figure 13 (c) is a waveform diagram of the active power output before and after fault ride-through;

[0058] Figure 13 (d) is a waveform diagram of reactive power output before and after fault crossing. Detailed Implementation

[0059] 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.

[0060] Please see Figures 1 to 13 As shown, this invention provides an electromechanical transient modeling method for a grid-type converter based on PSASP, comprising the following steps:

[0061] Step S1: Set the grid connection point voltage and the equivalent impedance parameters of the converter transformer;

[0062] In this step, it is necessary to clearly define the basic parameter conditions of the system operating conditions before modeling. The grid connection point voltage setting includes voltage amplitude and phase angle information, used to determine the grid-side reference voltage signal; the equivalent impedance setting of the converter transformer includes equivalent resistance and equivalent reactance, used to characterize the impedance characteristics between the converter and the grid. These parameters are directly used in subsequent modeling processes to establish the voltage-current coupling relationship, ensuring that the model's input conditions are consistent with the actual grid conditions.

[0063] In step S1, the set grid connection point voltage includes amplitude and phase angle information;

[0064] The amplitude and phase angle parameters here are used not only to define the magnitude and phase of the grid voltage, but also to determine the coordinate reference of the reference voltage signal in the voltage-to-current conversion. The amplitude setting ensures the consistency of the grid voltage level, and the phase angle setting ensures the correctness of the phasor calculation. Together, they provide the basis for the subsequent calculation of the voltage phasor difference.

[0065] The set equivalent impedance of the converter transformer includes equivalent resistance and equivalent reactance parameters;

[0066] The equivalent resistance of the converter transformer reflects the active power loss during power transmission, while the equivalent reactance characterizes the phase relationship between voltage and current. These two parameters, as crucial components of voltage-to-current conversion, determine the amplitude and phase of the injected current, ensuring that the converter model output matches the actual system operating conditions.

[0067] Step S2: In the PSASP custom modeling platform, establish a grid-type converter control model based on the droop control and voltage-current dual-loop control strategy, and calculate the output reference voltage signal through the grid-type converter control model;

[0068] In this step, a control framework is first constructed in the PSASP custom modeling platform. A droop control strategy is used to adjust the frequency and voltage amplitude. Then, a dual-loop voltage and current control strategy is employed to establish dynamic adjustment mechanisms for voltage and current. During operation, this control model can calculate a reference voltage signal, which serves as the input for subsequent voltage-to-current conversion and virtual impedance correction, ensuring the consistency between the model's control logic and electrical quantity outputs.

[0069] In step S2, the droop control loop consists of active droop relationship and reactive droop relationship;

[0070] This step incorporates two independent droop curves within the model, corresponding to active power-frequency droop and reactive power-voltage droop, respectively. The active power droop relationship reflects the impact of system power fluctuations on frequency, while the reactive power droop relationship reflects the impact of reactive power fluctuations on voltage amplitude. This hierarchical control logic enables the model to generate corresponding frequency and voltage adjustments under different power variation conditions.

[0071] The angular frequency adjustment is calculated by using the active power deviation and the active power droop coefficient.

[0072] This logic calculates the angular frequency adjustment by comparing the difference between the output active power and the reference active power, multiplying the difference by the active power droop coefficient. This adjustment reflects the system's frequency response characteristics under load fluctuations and is a crucial input for constructing the reference phase angle.

[0073] The reactive power droop relationship is calculated by using the reactive power deviation and the reactive power droop coefficient to obtain the voltage amplitude adjustment, and the angular frequency adjustment is integrated to obtain the phase angle.

[0074] This logic calculates the difference between the output reactive power and the reference reactive power, multiplies this difference by the reactive power droop coefficient, and obtains the voltage amplitude adjustment. Simultaneously, it integrates the angular frequency adjustment generated by the active power droop circuit to obtain the phase angle value of the voltage signal. These quantities collectively constitute the reference voltage signal.

[0075] In the droop control strategy, the active power-frequency droop stage compares the output active power of the grid converter with the reference active power to obtain the active power difference, and multiplies it with the active power droop coefficient to form the frequency offset. The frequency offset is superimposed with the rated angular frequency and accumulated by an integrator to obtain the output phase angle of the grid converter.

[0076] This process establishes a mapping relationship between output active power and frequency. The frequency offset is the fundamental quantity for angular frequency adjustment. After superimposing the rated angular frequency, it is integrated to obtain the phase angle, which is used to provide phase information in the reference voltage signal, thereby reflecting the dynamic correlation between the grid frequency and the converter phase angle.

[0077] The reactive power-voltage droop stage compares the output reactive power of the grid-type converter with the reference reactive power to obtain the reactive power difference, and multiplies it with the reactive power droop coefficient to form the voltage amplitude adjustment. The voltage amplitude adjustment is superimposed with the rated voltage amplitude to form the reference voltage amplitude.

[0078] This process establishes a mapping relationship between the output reactive power and the voltage amplitude. The voltage amplitude adjustment is superimposed on the rated voltage amplitude to form the amplitude component of the voltage signal, providing the amplitude input for the generation of the reference voltage signal and ensuring that the reference voltage signal is consistent with the dynamic characteristics of the system's reactive power.

[0079] The voltage outer loop uses the voltage amplitude and phase angle of the droop control output as reference inputs, obtains the reference voltage component through coordinate transformation, compares it with the actual output voltage component, and outputs the reference current after proportional-integral adjustment of the difference.

[0080] In the outer voltage loop, the voltage amplitude and phase angle generated by the droop control loop are transformed using coordinates to obtain a reference voltage component. This component is compared with the actual voltage signal, and the difference is adjusted by a proportional-integral controller to output a reference current, ensuring that the outer voltage loop can accurately track the target voltage.

[0081] The inner current loop takes the reference current output from the outer voltage loop as input, compares it with the actual current component, generates a pulse width modulation signal through proportional-integral regulation, and forms the output voltage signal of the grid converter through inverse coordinate transformation.

[0082] The inner current loop takes the reference current generated by the outer voltage loop as input and compares it with the real-time output current. The deviation is then adjusted using proportional-integral modulation to generate a pulse-width modulation (PWM) signal. This signal, after inverse coordinate transformation, is used to control the converter's output voltage signal, enabling the current loop to accurately track the current target.

[0083] The outer voltage loop uses a proportional-integral controller to cumulatively regulate the deviation between the output voltage and the reference voltage;

[0084] The proportional-integral (PI) controller design ensures the dynamic performance of the outer voltage loop. The proportional element provides fast response capability, while the integral element eliminates steady-state error by accumulating the deviation, ensuring that the output voltage can stably follow the reference voltage over a long period.

[0085] The proportional element cancels out sudden disturbances, the integral element eliminates steady-state errors, and a limiting element is set at the controller output to limit the reference current from not exceeding a preset threshold.

[0086] The proportional element responds immediately to voltage fluctuations, offsetting most of the disturbance; the integral element adjusts gradually under steady-state conditions, eliminating residual errors. Simultaneously, a limiting element is added to the controller output to constrain the reference current, preventing excessive current from causing system instability or equipment damage.

[0087] The inner current loop uses a proportional-integral controller to adjust the deviation between the output current and the reference current. The output of the inner current loop is transformed by inverse coordinate transformation to obtain a pulse width modulation signal, which drives the voltage output of the converter.

[0088] The inner current loop adjusts the output current in real time through a proportional-integral controller to ensure it remains consistent with the reference current. The proportional regulator provides rapid adjustment capability, while the integral regulator ensures steady-state tracking performance. Finally, a pulse-width modulation signal is generated through inverse coordinate transformation to control the converter's output voltage signal.

[0089] Step S3: Combine the reference voltage signal with the grid connection point voltage and the equivalent impedance parameters of the converter transformer to perform voltage-to-current conversion and obtain the real and imaginary parts of the current injected into the bus.

[0090] In this step, a mathematical relationship is established between the reference voltage signal, the grid connection point voltage, and the equivalent impedance of the converter transformer to calculate the current phasor. By decomposing the current phasor, the real and imaginary parts of the current are obtained. These quantities can be matched with the PSASP bus interface requirements, thus serving as a current source signal injected into the bus.

[0091] In step S3, the voltage-to-current conversion is performed by calculating the phasor difference between the reference voltage and the grid connection point voltage, and dividing it by the equivalent impedance of the converter transformer to obtain the current phasor. The current phasor is then decomposed into real and imaginary components, which are used as the current input injected into the PSASP bus.

[0092] This process clarifies the computational relationship between voltage and current signals. The phasor of the current is obtained through phasor difference calculation, then normalized using impedance parameters, and finally decomposed into real and imaginary parts. This process ensures the consistency between the output signal of the grid-type converter and the electrical interface of the PSASP bus.

[0093] Step S4: Set up a virtual impedance link in the grid-type converter control model, calculate the virtual voltage drop using the virtual impedance parameters combined with the real and imaginary parts of the current, and correct the reference voltage signal based on the virtual voltage drop. Obtain the corrected real and imaginary parts of the current through voltage-current conversion. Transmit the signal between the virtual voltage drop correction link and the voltage-current conversion link in the custom modeling platform through a temporary information exchange mechanism, and form a block-based control model.

[0094] In this step, the current signal and impedance parameters are first calculated using a virtual impedance stage to obtain a virtual voltage drop. This virtual voltage drop is subtracted before the reference voltage input voltage control stage to form a corrected reference voltage signal. The corrected signal then enters the voltage-to-current converter again to obtain the corrected real and imaginary parts of the current. Simultaneously, a temporary information exchange mechanism is used within the platform to transmit the signals from the virtual impedance correction stage and the voltage-to-current conversion stage using numbered sequences, enabling a modular model construction that ensures both the modular structure of the model and the clarity of signal transmission.

[0095] In step S4, the virtual impedance circuit includes virtual resistance and virtual inductance. The virtual impedance is calculated based on the output current to obtain a virtual voltage drop. The virtual voltage drop is subtracted from the reference voltage before the reference voltage signal input voltage outer loop to obtain the corrected reference voltage signal.

[0096] This logic calculates the output current using virtual resistance and virtual inductance to obtain the corresponding virtual voltage drop component. Before the reference voltage enters the outer voltage loop control, this virtual voltage drop is subtracted from the reference voltage to generate a corrected voltage signal, thereby enhancing the model's performance in current limiting and voltage support.

[0097] In step S4, a temporary information exchange mechanism is used to realize the internal signal transmission of the model. The mechanism sets the temporary information exchange value number in the PSASP custom modeling platform and exchanges the intermediate variables of each sub-module of the control model in the form of numbers, thereby forming a custom model of a grid-type converter with a block layout.

[0098] This mechanism enables signal transmission between different sub-modules through predefined signal channel numbers within a custom modeling platform, avoiding the complexity caused by excessive physical connections. This approach maintains a modular model structure, ensuring clear logic in each part and facilitating its use and maintenance within the PSASP platform.

[0099] Step S5: Based on the corrected current signal output by the control model with block layout, inject the real and imaginary parts of the corrected current into the PSASP bus in the form of current sources to realize the joint modeling of the grid-type converter and the system single-line diagram model, and establish the electromechanical transient model of the grid-type converter.

[0100] In this step, the modified current signal output from the block layout control model formed in step S4 is extracted, and the real and imaginary parts of the current are directly injected into the PSASP bus as current sources. In this way, the grid-type converter model can be jointly modeled with the system single-line diagram model, enabling the transient process of the entire system to be simulated on the PSASP platform, thus completing the establishment of the electromechanical transient model of the grid-type converter.

[0101] Step S6: Verify the electromechanical transient model of the grid converter. Verification is performed by setting a bus short-circuit fault in the PSASP transient stability simulation and observing the voltage, current and power changes of the grid converter before and after the fault occurs. This is used to confirm whether the model can reflect the dynamic response of the grid converter under fault conditions.

[0102] In this step, a bus short-circuit fault condition is artificially set during the simulation, forcing drastic fluctuations in system voltage and current, thereby observing the voltage support and current limiting behavior of the grid-connected converter. By comparing the changes in electrical quantities before and after the fault, it can be verified whether the model can accurately reflect the dynamic characteristics of the actual grid-connected converter under grid fault conditions, thus ensuring the effectiveness and reliability of the model in transient simulation.

[0103] Unlike grid-connected converters, which rely on phase-locked loops to obtain phase information at the grid connection point to synchronize their output with the grid, grid-connected converters adjust the frequency of the reference voltage through power exchange with the grid, thereby achieving synchronization with the grid. There are three main control strategies for grid-connected converters: droop control, virtual synchronizer, and matching control. This embodiment uses droop control as an example to conduct modeling research on grid-connected converters in PSASP.

[0104] A typical three-phase voltage source converter (VSC) consists of a DC side, a PWM module, and a filter, and is connected to the power grid via a point of common coupling (PCC). Figure 2 As shown.

[0105] Figure 2 middle, DC side voltage and These are the three-phase voltage and current, respectively. and These are the initial values ​​of the three-phase voltage and current, respectively. and Here are the filter inductor and filter capacitor for the low-pass filter. According to KVL and KCL laws, the voltage and current of the three-phase VSC can be expressed as follows:

[0106] (1)

[0107] (2)

[0108] in, , , These are the voltages of phase a, phase b, and phase c, respectively.

[0109] , , These are the initial voltage values ​​for phase a, phase b, and phase c, respectively.

[0110] , , These are the currents of phase a, phase b, and phase c, respectively.

[0111] , , These are the initial values ​​of the currents in phases a, b, and c, respectively.

[0112] Its state-space expression is:

[0113] (3)

[0114] Considering that a three-phase system is defined by two degrees of freedom, transforming the abc coordinate system into the dq coordinate system can reduce the order of the equations. Aligning the a-axis with the d-axis, we can obtain the transformation relationship between the abc and dq coordinate systems (i.e., the Park transformation):

[0115] (4)

[0116] From equations (3) and (4), the state-space expression of the three-phase VSC in the dq coordinate system is:

[0117] (5)

[0118] The current in the dq coordinate system can then be expressed as:

[0119] (6)

[0120] Considering the d-axis component of the current in equation (6) With q-axis components The mutual coupling between these components increases the difficulty of model debugging; therefore, it is necessary to decouple the dq-axis components of the current. Let... , Then equation (6) can be transformed into:

[0121] (7)

[0122] in, , These are the direct-axis and quadrature-axis components of the voltage, respectively.

[0123] , These are the initial values ​​of the direct-axis and quadrature-axis voltage components, respectively.

[0124] , These are the direct-axis and quadrature-axis components of the current, respectively.

[0125] , These are the initial values ​​of the direct-axis and quadrature-axis current components, respectively.

[0126] At this time, the modulation signal of the converter PWM module in the dq coordinate system is , ,in , These are the direct-axis and quadrature-axis components of the modulation signal from the converter's PWM module, respectively. , These are the direct-axis and quadrature-axis components of the filter inductor voltage, respectively. This is the PWM control gain.

[0127] Droop control adjusts the system frequency and voltage by calculating power changes. The control block diagram is shown below. Figure 3 As shown.

[0128] Power droop control satisfies the following relationship:

[0129] (8)

[0130] (9)

[0131] (10)

[0132] In the formula, There is a power setting value; This is the measured value of active power; and These are the active power droop coefficient and the reactive power droop coefficient, respectively. and These are the reference value and instantaneous value of the converter output angular frequency, respectively; No power setting value; This is the measured value of reactive power; and These are the reference value and instantaneous value of the converter output voltage amplitude, respectively. This refers to the output phase angle of the converter.

[0133] Equation (8) is the active power droop expression. When the active power of the system changes, the converter calculates the active power difference and generates the frequency adjustment amount through the active power droop coefficient to maintain the stability of the grid frequency. Equation (9) is the reactive power droop expression. When the reactive power of the system changes, the converter calculates the reactive power difference and generates the voltage adjustment amount through the reactive power droop coefficient to maintain the stability of the grid voltage.

[0134] Virtual impedance control aims to simulate the external characteristics of a physical impedance through a control algorithm, making the converter equivalent to a voltage source with internal resistance, thereby changing the output impedance of the converter and solving key problems such as power coupling and fault current limiting in the power grid system.

[0135] Typically, a calculation step based on output current feedback is introduced after the droop loop and before the voltage loop, such as... Figure 4 As shown.

[0136] The virtual impedance control circuit is based on the set virtual resistance value. Virtual inductance value (Typically, the inductive reactance is relatively large, making the impedance inductive to enhance the decoupling effect), and the voltage drop across the virtual impedance is calculated in real time:

[0137] (11)

[0138] (12)

[0139] In the formula, For the d-axis component of the virtual impedance voltage drop, This represents the q-axis component of the virtual impedance voltage drop; This is a virtual resistance value; This is the virtual inductance value.

[0140] Reference voltage of droop control output and Subtract the corresponding voltage drop component from the value to generate the corrected voltage command value. and It is then fed into the voltage and current loop for execution.

[0141] (13)

[0142] (14)

[0143] in, , These are the direct-axis and quadrature-axis components of the voltage command value corrected for droop control, respectively. , These are the direct-axis and quadrature-axis components of the reference voltage output for the droop control, respectively.

[0144] When the output current increases due to a fault, the calculated virtual voltage drop increases accordingly, which in turn leads to a decrease in the corrected output voltage command value. The active reduction of the output voltage reduces the voltage difference between the converter terminal and the fault point, thereby effectively limiting the fault current within a safe range.

[0145] This embodiment is based on the aforementioned droop control network control theory. It uses the PSASP custom modeling platform to build a network converter electromechanical model. The specific technical solution and implementation steps are as follows.

[0146] A network-based control strategy based on droop control and dual-loop voltage and current control is used to generate the system's control signal flow graph, as shown below. Figure 5 As shown, a grid-type converter model in the form of output voltage is built in the UD module of the PSASP software using the computation and control box in the toolbox.

[0147] exist Figure 5In this context, P_set represents the active power setpoint; P_mea represents the measured active power; Q_set represents the reactive power setpoint; Q_mea represents the measured reactive power; f_set represents the frequency setpoint; f_ref represents the measured frequency; Fnom represents the nominal frequency; u_set represents the voltage setpoint; u_mag represents the reference voltage amplitude; ur_Ug and ui_Ug represent the real and imaginary components of the grid voltage, respectively; ir and ii represent the real and imaginary components of the grid connection point current, respectively; id_ref and iq_ref represent the real and imaginary components of the reference current, respectively; Kp represents the proportional component of the PI integrator; Ti represents the integration time constant of the PI integrator; udc represents the DC voltage value; ir_in and ii_in represent the real and imaginary components of the converter output current, respectively; and sinref and cosref represent the sine and cosine values ​​of the reference phase, respectively.

[0148] Based on the measured data provided by the PSASP UD input and output information, the following steps are sequentially established: power comparison, droop coefficient setting, frequency calculation, angular frequency integration, and angle calculation. Specifically, the measured power and reactive power values ​​related to droop control are obtained from branch Line1, the system frequency is taken from bus Bus1, and both filters are first-order low-pass filters (transfer function is...). (The corresponding cutoff frequency is 60 rad / s). Adjustable parameters such as the droop coefficient are also set as "variables" to allow for secondary editing of the UD model's related variables in the PSASP main program after the model's calculation file is exported.

[0149] The reference voltage of the outer voltage loop comes from the voltage amplitude and phase angle of the droop control output. The real and imaginary parts of the output voltage are transformed into the dq coordinate system by Park transformation and participate in the voltage loop control.

[0150] (15)

[0151] in, This is the reference value for the phase angle of the converter droop control. , These are the real and imaginary components of the voltage, respectively.

[0152] In the voltage loop, the integrator is the core component ensuring that the converter output voltage accurately tracks its reference value. When the output voltage deviates from the reference value due to load disturbances, the proportional element can react quickly to sudden load changes, providing initial correction capability and preventing instantaneous voltage collapse. However, its effect is "not perfect" and cannot completely eliminate steady-state error. The integral element, on the other hand, accumulates the small deviation between the output voltage and the reference value, thereby generating an increasingly stronger control signal until the steady-state error is completely eliminated. Therefore, a proportional-integral (PI) controller is designed, expressed as:

[0153] (16)

[0154] In the formula, This is the integrator proportional coefficient. is the integration time constant of the integrator.

[0155] Meanwhile, to prevent excessive integrator output current due to excessive grid load disturbance or improper parameter settings, a limiting circuit is added to the voltage loop, with a limit value of [value missing]. .

[0156] The reference current of the inner current loop is the output current of the outer current loop. The grid current collected by the branch Line1 is transformed by Park to obtain the dq component. The difference between the dq component and the corresponding reference current is sent to the PI controller of the inner current loop to realize the tracking control of the current.

[0157] Considering that the converter output signal is in complex form in the time domain, the PWM module modulation signal output by the voltage and current dual-loop control module needs to be modulated first. and Transforming from the dq coordinate system to the αβ coordinate system (ab coordinate system), using the inverse Park transformation relation, we have:

[0158] (17)

[0159] in, , These are the real and imaginary components of the modulation signal of the converter's PWM module, respectively.

[0160] PWM module modulation signal and With controller gain The product of these two is the converter output voltage signal.

[0161] (18)

[0162] in, , These are the real and imaginary components of the converter output voltage signal, respectively.

[0163] According to the output information of the UD model calculated by PSASP transient stability, the signals that the bus elements can recognize are the real part "ITR" and the imaginary part "ITI" of the injected bus current. Therefore, it is necessary to construct a voltage signal to current signal model.

[0164] Figure 6 This is a schematic diagram considering the power flow distribution of the converter transformer at the converter outlet. Among them, and These are the equivalent resistance and equivalent inductance, respectively, considering the losses of the converter transformer and inverter. The grid connection point voltage and inverter output voltage are respectively set as follows: and ( For grid voltage power angle, (where the power angle is the voltage drop across the equivalent impedance) then the current injected by the converter into the grid side is:

[0165] (19)

[0166] make ,but , At this point, equation (19) can be transformed into:

[0167] (20)

[0168] definition Then we have:

[0169] (twenty one)

[0170] Therefore, the real and imaginary components of the injected AC bus current are respectively:

[0171] (twenty two)

[0172] (twenty three)

[0173] in, The equivalent resistance is taken into account for the losses of the converter transformer and converter. The equivalent inductance is taken into account for the losses of the converter transformer and converter. For grid voltage phasors; This refers to the voltage amplitude of the power grid. The grid voltage power angle; The power angle is the voltage drop across the equivalent impedance; The output current phasor of the converter; The output voltage phasor of the converter; This refers to the output voltage amplitude of the converter. The equivalent reactance is taken into account for the losses of the converter transformer and converter. The equivalent impedance that takes into account the losses of the converter transformer and converter; The impedance angle is the equivalent impedance. For equivalent admittance; , These are the real and imaginary components of the injected AC bus current, respectively.

[0174] From this, we can obtain the following... Figure 7The diagram shown is a control block diagram for converting a voltage signal into a current signal.

[0175] In PSASP UD modeling, the equivalent impedance of the converter transformer is set as a variable by the parameter definition box. At the same time, the root mean square function operation box and the arctangent function operation box are used to calculate the voltage amplitude and phase angle of the grid connection point and the converter outlet. Finally, the real and imaginary components of the current injected by the converter into the AC bus are obtained by current calculation.

[0176] Considering that the output may be unstable after the UD model is connected to the grid due to uneven line impedance or power coupling, an impedance (usually an inductive impedance) needs to be "virtually" added to the output control loop of the converter on the basis of droop control. This is to reshape the equivalent impedance characteristics from the converter end to the PCC point, thereby achieving the purposes of power decoupling, power distribution optimization and fault current limiting.

[0177] Figure 8 A power transfer model considering virtual impedance. In the figure, and Together they form a virtual impedance element. and The equivalent resistance and reactance of the external power grid.

[0178] With grid voltage ( For reference, the voltage phasor diagram considering virtual impedance is as follows: Figure 9 As shown.

[0179] Figure 9 In the middle, power angle for Voltage at point B The angle difference between them, virtual power angle Voltage at point C and voltage at point A The angle difference between them. The current flow from point A to the infinite busbar can be expressed as:

[0180] (twenty four)

[0181] (25)

[0182] In the formula, , ;

[0183] in, Let C be the voltage phasor; The voltage amplitude at point C; The voltage amplitude at point B; The voltage amplitude at point A; The active power at point A; The reactive power at point A; To account for the equivalent resistance of the grid resistance and the virtual resistance; To consider the equivalent reactance of the grid reactance and the virtual reactance;

[0184] The current-limiting effect of virtual impedance essentially simulates the voltage drop characteristics of a physical impedance for the converter, thereby reducing the converter's output voltage and decreasing the voltage difference during a fault, thus limiting the fault current. The fault current can be decomposed into periodic and aperiodic components:

[0185] (26)

[0186] (27)

[0187] In the formula, This refers to the instantaneous inrush current during a fault. This represents the steady-state current amplitude after the fault. The current amplitude at the moment before the fault occurred. This is the initial phase. The power angle after the fault. The power angle at the moment before the fault. It is a time constant. To consider the equivalent inductance of grid reactance and virtual reactance.

[0188] in, For periodic components, It is a non-periodic component, due to the instantaneous fault. Approximately 0, while Generally much greater than 1, the decay rate of the non-periodic component is proportional to the virtual inductance value.

[0189] When a fault occurs at point C in the power grid, the instantaneous inrush current... Satisfy the following formula:

[0190] (28)

[0191] In the formula, The voltage at the moment of fault at point C.

[0192] As can be seen from equation (28), the magnitude of the instantaneous inrush current during a fault is inversely proportional to the virtual inductance value.

[0193] In PSASP UDM, virtual impedance is added as a calculation module between droop control and voltage / current control (see reference). Figure 4 The following operational relationships exist between current, resistance, and inductance:

[0194] (29)

[0195] in and Define the variable form of the operation box for the parameters. and It is obtained by Park transformation of the real component I1RI and the imaginary component I1II of the positive sequence current on the Line1 i side of the branch.

[0196] Finally, combining equations (13) and (14), the current loop input reference voltage signal is obtained. and .

[0197] like Figure 10 As shown, after completing the modeling of the grid-type converter UD model, the PSASP software can be used to exchange information within the model by user-defined temporary information exchange values ​​(TM), so as to simplify line connections and realize block layout.

[0198] The Temporary Information Exchange Value (TM) number and its signal correspondence are shown in Table 1.

[0199] Table 1. Correspondence between UD model control signals and temporary information exchange values

[0200]

[0201] Based on the above technical solution, the UD model of the grid-type converter is completed. When defining the PSASP transient stability operation, the UD model is called to perform joint simulation of the PSASP single-line diagram and the UD model.

[0202] The simulation test was conducted using PSASP software. A single-line diagram simulation model of the stand-alone system was established in PSASP, such as... Figure 11 As shown.

[0203] exist Figure 11 In the single-machine infinite bus system, the generator on the left is set as an ideal power source, and the bus Bcov on the right is a bus element injected into the grid-type converter model. The system's reference voltage is 380kV and the reference capacity is 100MW. The grid-type converter is set to have an active power output of 50MW and a reactive power output of 20Mvar.

[0204] The simulation test settings for the UD model of the grid converter are as follows:

[0205] The converter generates a large inrush current during a fault. Different virtual impedance values ​​are set in the test system simulation model to verify the current-limiting effect of the virtual impedance. During stabilization operations, a three-phase ground fault is simulated on the branch between busbars Bgen and Bmid (the ground impedance is set to cause the voltage to drop to 70%). The short-circuit initiation time is 80 seconds, and the fault is not cleared. The short-circuit current waveforms under different virtual impedances are shown below. Figure 12 As shown.

[0206] Depend on Figure 12 It can be seen that when no virtual impedance is connected, the inrush current is 1.353 pu. When the inductance of the virtual impedance is connected is 0.0001 pu, 0.0003 pu, and 0.0005 pu, the inrush currents are 1.338 pu, 1.212 pu, and 1.054 pu, respectively. Therefore, after connecting the virtual impedance, the amplitude of the transient inrush current is effectively limited, and the larger the virtual inductance value, the more obvious the suppression effect on the inrush current. The current limiting effect of the virtual impedance control strategy has been verified.

[0207] Because of its voltage source characteristics, the grid-connected converter actively supports the grid voltage and frequency during grid faults, maintaining stable grid operation. By setting up a fault ride-through scenario, the grid support capability of the grid-connected converter UD model during fault ride-through is verified.

[0208] Network fault setting: A three-phase ground fault occurs in the branch between bus Bgen and Bmid at 80s, and is cleared at 100s, with a fault duration of 20s. Simulation results are shown below. Figure 13 .

[0209] Depend on Figure 13 It can be seen that at the moment the fault occurs, the voltage drops rapidly to 0.9 pu. The converter increases reactive power to support the voltage. While prioritizing reactive power output, the active power will be adjusted in time to maintain the stability of the system frequency. The above waveform is consistent with the change trend during the fault ride-through of the grid-type converter model.

[0210] In summary, simulation verification of the above UD model shows that the grid-type converter UD model built on the PSASP UDM platform has the ability to actively support grid voltage and frequency, which is in line with theoretical analysis and design expectations.

[0211] It should be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0212] The above description is merely a preferred embodiment of one or more embodiments of this specification and is not intended to limit the scope of one or more embodiments of this specification. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of one or more embodiments of this specification should be included within the protection scope of one or more embodiments of this specification.

Claims

1. A method for electromechanical transient modeling of a network-forming converter based on PSASP, characterized in that, The method comprises the following steps: Step S1, setting grid-connected point voltage and equivalent impedance parameters of the converter transformer; Step S2, in the PSASP self-defined modeling platform, a grid-connected converter control model is established based on the droop control and voltage-current double-loop control strategy, and a reference voltage signal is calculated and output through the grid-connected converter control model; Step S3, in combination with the reference voltage signal, the grid-connected point voltage and the equivalent impedance parameters of the converter transformer, voltage-current conversion is performed to obtain the real part and the imaginary part of the current injected into the bus; The voltage-current conversion is performed by calculating the phase difference between the reference voltage and the grid-connected point voltage, dividing the equivalent impedance of the converter transformer to obtain the current phase, and decomposing the current phase into real part and imaginary part as the current input injected into the PSASP bus; Step S4, in the grid-connected converter control model, a virtual impedance is set, the real part and the imaginary part of the current are combined with the virtual impedance parameters to calculate the virtual voltage drop, and the reference voltage signal is corrected based on the virtual voltage drop, and the corrected real part and the imaginary part of the current are obtained through voltage-current conversion; The virtual impedance link includes a virtual resistor and a virtual inductor, the virtual impedance calculates the virtual voltage drop based on the output current, and the virtual voltage drop is subtracted from the reference voltage before the reference voltage signal is input into the voltage outer loop to obtain the corrected reference voltage signal; Step S5, the corrected real part and the imaginary part of the current are injected into the PSASP bus in the form of a current source to establish an electromechanical transient model of the grid-connected converter.

2. The electromechanical transient modeling method of a PSASP-based network-forming converter according to claim 1, wherein, In step S1, the set grid-connected point voltage includes amplitude and phase angle information; The set equivalent impedance of the converter transformer includes equivalent resistance and equivalent reactance parameters.

3. The electromechanical transient modeling method of a PSASP-based network-forming converter according to claim 1, wherein, In step S2, the droop control link is composed of active droop relationship and reactive droop relationship; The active droop relationship is calculated by the active power deviation and the active droop coefficient to obtain the angular frequency adjustment amount; The reactive droop relationship is calculated by the reactive power deviation and the reactive droop coefficient to obtain the voltage amplitude adjustment amount, and the angular frequency adjustment amount is integrated to obtain the phase angle.

4. The electromechanical transient modeling method of a PSASP-based network-forming converter according to claim 3, wherein, In the active-frequency droop link of the droop control strategy, the output active power of the grid-connected converter is compared with the reference active power to obtain the active power difference, which is multiplied by the active droop coefficient to form the frequency deviation, the frequency deviation is superimposed with the rated angular frequency, and the output phase angle of the grid-connected converter is accumulated through the integrator; The reactive-voltage droop link compares the output reactive power of the grid-connected converter with the reference reactive power to obtain the reactive power difference, which is multiplied by the reactive droop coefficient to form the voltage amplitude adjustment amount, and the voltage amplitude adjustment amount is superimposed with the rated voltage amplitude to form the reference voltage amplitude.

5. The method of claim 3 or 4, wherein the PSASP-based electro-mechanical transient modeling of a networked power converter is characterized by, The voltage amplitude and the phase angle output by the voltage outer loop are taken as the reference input, the reference voltage components are obtained through coordinate transformation, and the difference between the reference voltage components and the actual output voltage components is output as the reference current after proportional-integral adjustment; The reference current output by the voltage outer loop is taken as the input of the current inner loop, and the difference between the reference current and the actual current component is output as the pulse width modulation signal after proportional-integral adjustment, and the output voltage signal of the grid-connected converter is formed through inverse coordinate transformation.

6. The electromechanical transient modeling method of a PSASP-based network-forming converter according to claim 5, wherein, The voltage outer loop adopts a proportional-integral controller to accumulate and adjust the deviation between the output voltage and the reference voltage. The proportional link cancels the sudden disturbance, the integral link eliminates the steady-state error, and a limiting link is arranged at the output end of the controller to limit the reference current to be less than a preset threshold value. The current inner loop adopts a proportional integral controller to adjust the deviation between the output current and the reference current, and the output of the current inner loop is subjected to coordinate inversion to obtain a pulse width modulation signal to drive the voltage output of the converter. 7.The method of claim 1, wherein, In step S5, the temporary information exchange mechanism is adopted to realize the internal signal transmission of the model, the mechanism sets the temporary information exchange value number in the PSASP self-defined modeling platform, exchanges the intermediate variables of the sub-modules of the control model in the form of numbers, and forms the self-defined model of the block-arranged network-type converter. 8.The method of claim 1, wherein, Also includes: In step S6, the electromechanical transient model of the network-type converter is tested. The test is passed by setting a bus short-circuit fault in the PSASP transient stability simulation, observing the voltage, current and power changes of the network-type converter before and after the fault occurs, and confirming whether the model can reflect the dynamic response of the network-type converter under the fault condition.

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