Converter seamless switching control method between grid-connected mode and grid-forming mode under asymmetric fault

Abstract

The application discloses a converter seamless switching control method under asymmetric fault, and belongs to the technical field of power electronic control. The converter adopts positive and negative sequence double control loop decoupling control. If switching from the network construction mode to the network following mode, phase angle initial value synchronization is performed, and then preset alignment of the current reference instruction of the network following mode is performed by intercepting actual physical current measurement values. If switching from the network following mode to the network construction mode, phase angle initial value synchronization is first performed, then based on the voltage reference value and the voltage and current measurement values before switching, the initial value of the virtual impedance is inversely solved, and finally, the integral term initial value of the positive and negative sequence voltage loop PI controller is reset to the actual current measurement value. The method eliminates the reference instruction deviation caused by the sudden change of the control structure under asymmetric fault from the bottom mathematical logic, greatly improving the fault ride-through safety and flexible support potential of the unit in the new power system.

Description

Technical Field

[0001] This invention belongs to the field of power electronic control technology, specifically relating to a seamless switching control method for converter grid connection and grid construction modes under asymmetrical faults. Background Technology

[0002] With a high proportion of new energy sources, such as wind and solar power, being connected to the power grid through grid-connected converters, the traditional power system is evolving into a dual-high system with a high proportion of renewable energy and a high proportion of power electronic equipment. Under this trend, the control strategy of grid-connected converters plays a crucial role in the safe and stable operation of the power grid.

[0003] Currently, the control modes of grid-connected converters are mainly divided into two categories: grid-following and grid-connecting. Grid-following control is characterized by a controlled current source, primarily relying on a phase-locked loop (PLL) to track the grid voltage phase. It offers fast dynamic response and easily achieves maximum power point tracking for renewable energy sources, but it cannot autonomously establish voltage and is prone to instability risks under weak grid conditions or faults. Grid-connecting control, on the other hand, is characterized by a controlled voltage source, capable of actively providing voltage and frequency support to the grid. However, when dealing with grid faults, it is prone to severe transient overcurrents due to its voltage support characteristics. Clearly, relying on a single control mode is insufficient to cope with the complex and ever-changing operating environment of modern power systems.

[0004] In recent years, some research has been conducted on grid-connected and grid-connected mode switching technologies. However, most of this research focuses on steady-state operating conditions or planned switching scenarios such as planned grid connection and islanding switching of microgrids. The main purpose is to optimize the system's operating efficiency and power quality under normal operating conditions. However, when a transient fault occurs in the grid, the converter often faces a more urgent need for mode switching. In the case of a deep fault causing the phase-locked loop to lose synchronization, the converter needs to switch from grid-connected mode to grid-connected mode to provide active voltage support and maintain system synchronization. Conversely, when a fault causes a severe transient overcurrent that endangers the safety of equipment hardware, it is necessary to switch from grid-connected mode to grid-connected mode to utilize its direct current control advantage to achieve rapid and safe current limiting.

[0005] Among various power grid faults, asymmetrical faults are the most frequent and complex type of fault in actual operation. They refer to the electrical conditions in which the amplitude and phase of the three-phase voltage at the converter connection point are unequal when a single-phase ground fault, a two-phase short circuit, or a two-phase ground fault occurs in the power system. Under these conditions, the grid voltage contains both positive-sequence and negative-sequence components.

[0006] Therefore, when an asymmetrical fault occurs, the grid voltage drops along with a large number of unbalanced negative sequence components. If the existing mode switching technology is used to deal with this condition, it usually only considers the alignment of the original voltage reference value with the actual value and the reset of the positive sequence control integrator, and simply forces the negative sequence current reference command to 0, while directly hard switching the control loop.

[0007] However, this conventional approach has serious logical flaws when dealing with asymmetrical operating conditions: During asymmetrical drops, the integrators of the positive and negative sequence dual PI controllers within the control system accumulate complex transient state values, and the positive and negative sequence voltage outer loop introduces virtual impedance to correct the voltage reference value. If the original voltage command is simply aligned with the actual value without considering the virtual impedance, or if the command is set to zero and a direct switch is performed, the voltage outer loop reference value and integrator state will become severely out of sync with the actual physical quantities. Simultaneously, the significant delay introduced by the traditional positive and negative sequence dual phase-locked loop under asymmetrical operating conditions will also cause a phase angle jump at the moment of switching. These problems directly lead to severe runaway of the positive and negative sequence currents at the moment of switching, causing severe transient overcurrent impacts, easily triggering equipment protection trips, and making it impossible to achieve a seamless and smooth transition under complex transient processes.

[0008] Therefore, conducting research on smooth mode switching during asymmetric faults is of great significance for improving the transient support and stability capabilities of high-proportion renewable energy systems. Summary of the Invention

[0009] To address the technical shortcomings of existing mode switching technologies and traditional positive- and negative-sequence dual phase-locked loops (PLLs) in handling asymmetrical faults, which suffer from severe transient overcurrent surges and phase jumps during switching due to insufficient consideration of negative-sequence component control, virtual impedance matching, and phase-locked delay, this invention provides a seamless switching control method for converters and network architecture under asymmetrical faults. It aims to construct a unified switching framework compatible with both positive- and negative-sequence dual loops and voltage loop virtual impedance. Through innovative introduction of a synchronization mechanism with a single positive-sequence phase angle inversion, precise virtual impedance matching based on inverse calculations of existing physical quantities, and a positive- and negative-sequence dual integrator collaborative reset mechanism, it completely eliminates the problems of control command errors and phase angle jumps under asymmetrical transient conditions from the underlying control logic.

[0010] The specific technical solution is as follows:

[0011] S1, determine whether the converter's mode has changed. If it has changed from grid-connecting mode to grid-following mode, proceed to S2; if it has changed from grid-following mode to grid-connecting mode, proceed to S3.

[0012] S2, when switching to grid-connected mode, firstly, the positive-sequence virtual synchronous phase generated by the virtual synchronous generator before switching is captured and written as the initial value into the phase integrator of the positive-sequence phase-locked loop; secondly, the current reference values ​​of the positive-sequence control loop and the negative-sequence control loop in grid-connected mode are reset to the actual measured current values ​​of the corresponding loops in grid-connected mode before switching.

[0013] After the switching is completed, the Park transform reference phase of the positive sequence control loop uses the positive sequence phase-locked loop output, and the Park transform reference phase of the negative sequence control loop is obtained by inverting the positive sequence phase.

[0014] S3, when switching to the grid-connected mode, firstly, the positive-sequence phase output of the positive-sequence phase-locked loop before switching is captured and written into the phase integrator of the virtual synchronous generator as the initial value; secondly, based on the measured voltage value, measured current value and voltage reference value before switching, the positive and negative sequence virtual impedances are obtained and used as the initial values ​​of the virtual impedance in the grid-connected mode; finally, the initial values ​​of the integral terms of the voltage loop PI controllers of the positive-sequence control loop and the negative-sequence control loop are reset to the measured positive and negative sequence current values ​​before switching.

[0015] After the switch is completed, the Park transformation reference phase of the positive sequence control loop uses the positive sequence virtual synchronous phase output by the virtual synchronous generator, and the Park transformation reference phase of the negative sequence control loop is obtained by inverting the positive sequence virtual synchronous phase.

[0016] Furthermore, in S1, the determination of whether the mode of the converter has switched is achieved by using the edge detection value.

[0017] Furthermore, in S2, the process of extracting the positive-sequence virtual synchronization phase generated by the virtual synchronous generator before the switchover is as follows:

[0018] Obtain the AC voltage and AC current at the converter outlet at the sampling moment before the mode switch occurs, and obtain the measured value of active power;

[0019] In the active-frequency control loop of the virtual synchronous generator, the virtual angular velocity is generated autonomously by integrating the swing equation of the synchronous generator rotor based on the active power reference value and the measured active power value.

[0020] In the phase integrator of the virtual synchronous generator, the virtual angular velocity is integrated to obtain the positive-sequence virtual synchronous phase.

[0021] Furthermore, in S2, the process of obtaining the positive-sequence phase output of the positive-sequence phase-locked loop is as follows:

[0022] Obtain the AC voltage at the converter outlet after switching and input it into the positive sequence control loop;

[0023] In the positive sequence control loop, the AC voltage is transformed by Park coordinates to obtain the measured value of the positive sequence voltage. The measured value of the positive sequence q-axis voltage is then sent to the PI controller of the positive sequence phase-locked loop, and the adjustment value is obtained after proportional-integral adjustment.

[0024] The adjustment amount is superimposed with the rated angular frequency of the power grid to obtain the angular frequency of the positive sequence phase-locked loop;

[0025] In the phase integrator of the positive-sequence phase-locked loop, the angular frequency is integrated to obtain the positive-sequence phase.

[0026] Furthermore, in S3, the process of extracting the positive-sequence phase of the positive-sequence phase-locked loop output before switching is as follows:

[0027] Obtain the AC voltage at the converter outlet at the sampling moment before the mode switch occurs, and input it into the positive sequence control loop;

[0028] In the positive sequence control loop, the AC voltage is transformed by Park coordinates to obtain the measured value of the positive sequence voltage. The measured value of the positive sequence q-axis voltage is then sent to the PI controller of the positive sequence phase-locked loop, and the adjustment value is obtained after proportional-integral adjustment.

[0029] The adjustment amount is superimposed with the rated angular frequency of the power grid to obtain the angular frequency of the positive sequence phase-locked loop;

[0030] In the phase integrator of the positive-sequence phase-locked loop, the angular frequency is integrated to obtain the positive-sequence phase.

[0031] Furthermore, in step S3, the calculation process for the positive and negative sequence virtual impedances, based on the measured voltage and current values ​​before switching and the voltage reference value, is as follows:

[0032] Based on the measured voltage, measured current and voltage reference value, and according to Ohm's law, establish the equivalent relationship between the product of virtual impedance and measured current and the difference between voltage reference value and measured voltage value;

[0033] In the positive sequence control loop, the measured voltage, measured current and voltage reference values ​​of the positive sequence control loop before switching are substituted into the above-mentioned equal relationship to obtain the positive sequence virtual impedance;

[0034] In the negative sequence control loop, the voltage reference value is set to 0. The measured voltage and current values ​​of the negative sequence control loop before switching are substituted into the above-mentioned equal relationship to obtain the negative sequence virtual impedance.

[0035] Furthermore, the expression for the equality relationship is:

[0036] ;

[0037] Where the superscript x represents the positive or negative order component. and These are the real and imaginary parts of the virtual impedance, respectively. and These are the d-axis and q-axis voltage reference values, respectively. and These are the measured voltage values ​​for the d-axis and q-axis, respectively. To switch to the previous sampling time, For the moment of switching, and These are the measured values ​​of the d-axis and q-axis currents, respectively.

[0038] Furthermore, in the positive sequence control loop, the voltage reference value includes a positive sequence d-axis voltage reference value and a positive sequence q-axis voltage reference value. The positive sequence q-axis voltage reference value is 0, and the positive sequence d-axis voltage reference value is obtained from a virtual synchronous generator, specifically:

[0039] In the reactive power-voltage droop control loop of the virtual synchronous generator, the measured reactive power value is obtained based on the AC voltage and AC current at the converter outlet; combined with the reactive power reference value and the rated voltage amplitude, the positive sequence d-axis voltage reference value is obtained.

[0040] Furthermore, in step S3, the process of obtaining the positive-sequence virtual synchronization phase output by the virtual synchronous generator is as follows:

[0041] Obtain the AC voltage and AC current at the converter output after switching, and get the measured value of active power;

[0042] In the active-frequency control loop of the virtual synchronous generator, the virtual angular velocity is generated autonomously by integrating the swing equation of the synchronous generator rotor based on the active power reference value and the measured active power value.

[0043] In the phase integrator of the virtual synchronous generator, the virtual angular velocity is integrated to obtain the positive-sequence virtual synchronous phase.

[0044] The beneficial effects of this invention are:

[0045] (1) By separating the positive and negative sequences and then performing seamless switching control on them respectively, the technical bottleneck of switching between grid-following and grid-building modes under asymmetrical faults is overcome. Most existing grid-following and grid-building switching methods are limited to three-phase symmetrical steady-state operation. This invention innovatively extends the seamless switching architecture between grid-following and grid-building modes to the transient scenario of asymmetrical faults. At the same time, it proposes a single positive sequence phase angle inversion orientation mechanism, which effectively reduces the phase delay introduced by the traditional positive and negative sequence dual phase-locked loop. By reconstructing the positive and negative sequence dual-loop control architecture, it fills the technical gap of the converter's inability to switch smoothly under unbalanced grid drops.

[0046] (2) By accurately calculating and setting the virtual impedance, the transient impact introduced by the virtual impedance is eliminated. The traditional voltage outer loop state reset method does not consider the voltage drop of the virtual impedance used for current limiting in the network control under fault conditions. The present invention simultaneously sets the virtual impedance for matching while resetting the initial value of the integrator, which not only eliminates the sudden change in current command caused by the error of the outer loop reference command, but also ensures the instantaneous seamless connection of the current limiting impedance, and completely eliminates the current transient impact caused by mode switching.

[0047] (3) Compared with the traditional steady-state switching technology, the present invention achieves flexible support under all operating conditions, which greatly improves the fault ride-through and stability control capabilities of the converter in the new power system. The present invention enables the converter to switch seamlessly in all directions between the grid-following mode, which provides efficient following and transient overcurrent protection, and the grid-building mode, which provides active voltage and frequency support. This maximizes the potential of the converter to provide transient support for the new power system and comprehensively improves the fault ride-through capability of the dual-high voltage system. Attached Figure Description

[0048] Figure 1 This diagram shows the main circuit topology and overall control architecture of a converter grid-connected system under asymmetrical fault conditions.

[0049] Figure 2 A flowchart illustrating the control method for seamless switching between network following and network construction modes under asymmetric fault conditions.

[0050] Figure 3 A detailed control block diagram for seamless switching between network following and network construction modes under asymmetric fault conditions.

[0051] Figure 4 The simulation waveform diagram shows the mode switching of the converter using the control method of the present invention under asymmetrical fault conditions.

[0052] Figure 5 The simulation waveform diagram is for the traditional direct switching method that does not employ the control method of this invention.

[0053] Figure 6 This is a schematic diagram of the electronic device terminal structure of the present invention. Detailed Implementation

[0054] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. Technical features in the various embodiments of the present invention can be combined accordingly without mutual conflict.

[0055] Figure 1 This is a diagram showing the main circuit topology and overall control architecture of the converter grid-connected system under asymmetrical faults in an embodiment of the present invention.

[0056] like Figure 1 As shown in (a), the main circuit hardware topology consists primarily of an energy storage converter, an LC filter, a Δ / Y step-up transformer, and an AC power grid. The location of the asymmetric fault is clearly marked in the figure, and PCC stands for Point of Common Coupling in the power grid.

[0057] like Figure 1 As shown in (b), under asymmetrical fault conditions, a dual-loop independent control architecture with positive and negative sequences is adopted, where PWM stands for Pulse Width Modulation. The converter control system generates a trigger drive signal g by sampling the AC voltage U and AC current I at the converter outlet in real time. vsc This controls the switching of the power semiconductor transistors inside the converter, enabling bidirectional AC-DC energy conversion. Specifically:

[0058] First, by combining the positive-sequence reference phase and the negative-sequence reference phase, the system transforms the sampled converter output voltage and current from the abc coordinate to the dq coordinate, thereby obtaining the d-axis voltage and current signals containing positive-sequence and negative-sequence coupling, as well as the d-axis voltage and current signals containing negative-sequence and positive-sequence coupling.

[0059] Furthermore, by filtering out the coupling in their respective dq signals using a notch filter, positive and negative sequence separation is achieved, and the positive and negative sequence d-axis and q-axis components of voltage and current under asymmetric transient conditions (i.e., positive sequence d-axis and q-axis voltages) are extracted. , With negative sequence d-axis and q-axis voltages , and positive sequence d-axis and q-axis currents , With negative sequence d-axis and q-axis currents , ).

[0060] Subsequently, the separated positive and negative sequence components enter the core control loop, and different control paths are executed according to the current operating mode of the converter, as follows:

[0061] Under network control, the positive and negative sequence voltage reference values ​​(i.e., the positive sequence d-axis and q-axis voltage reference values) are... , Reference values ​​of negative sequence d-axis and q-axis voltages , Subtract from the virtual impedance (where the positive and negative sequence virtual impedances include the virtual resistance as the real part) , and virtual reactance as the imaginary part , The specific value is dynamically calculated based on the deviation between the measured current amplitude and the converter current limit. The resulting virtual voltage drop is used as a new voltage reference value and fed into the voltage loop. It is then used for closed-loop adjustment with the measured positive and negative sequence dq axis components of the voltage, and positive and negative sequence current reference commands are output. , and , ;

[0062] In grid-connected mode, the outer voltage loop is not activated, and the positive and negative sequence current reference commands are used. , and , It is given directly by the superior command.

[0063] In both grid-following and grid-building control modes, the aforementioned positive and negative sequence current reference commands are input to the current loop for closed-loop tracking and error adjustment, thereby generating positive and negative sequence d-axis and q-axis voltage modulation signals. These signals are then recombined with the positive and negative sequence reference phases, transforming the dq coordinate system into the abc coordinate system to obtain the positive and negative sequence voltage modulation signals. Finally, the positive and negative sequence voltage modulation signals are summed and then modulated by PWM to generate the underlying drive signal g. vsc .

[0064] In the coordinate transformation and grid synchronization stage, this architecture abandons the redundant design of the positive and negative sequence dual phase-locked loops and adopts a single positive sequence reference phase θ; while the coordinate transformation of the negative sequence component directly selects -θ, the inverted version of the positive sequence reference phase, for orientation. This method not only eliminates the complex parameter tuning process of the traditional positive and negative sequence dual phase-locked loops, but also fundamentally avoids the risk of phase angle jumps and system instability caused by the dynamic response delay of the negative sequence phase-locked loop under asymmetrical faults.

[0065] Figure 2 This is a flowchart illustrating the seamless switching control method between network following and network construction modes under asymmetric faults in an embodiment of the present invention. Figure 3 This is a detailed control block diagram. Combined with... Figure 2 process and Figure 3 The internal control logic of the present invention, and the specific implementation steps of the method are as follows:

[0066] Step 1: Set the basic control architecture of the converter in grid-connected mode and grid-connected mode.

[0067] During the entire process of full-power operation and mode switching of the converter, the control system uses a notch filter to separate the positive and negative sequences of the sampled AC voltage U and AC current I, accurately extracting the fundamental positive and negative sequence components (i.e., the positive and negative sequence d-axis voltages). , and , With positive and negative sequence d-axis currents , and , ).

[0068] Based on this, the system executes the following different control architectures in network following mode and network construction mode respectively:

[0069] (1) Network control architecture

[0070] In the grid synchronization stage, the system locks the true phase of the grid voltage through a positive-sequence phase-locked loop, and its control equation is:

[0071]

[0072] In the formula, This is the angular frequency of the positive-sequence phase-locked loop output; K is the rated angular frequency of the power grid. p_pll and K i_pll These are the proportional and integral coefficients of the positive-sequence phase-locked loop PI controller, respectively. The actual component of the positive-sequence q-axis voltage; θ pll t represents the grid synchronization reference phase output by the phase-locked loop, and t is the sampling time.

[0073] At this point, θ is selected. pll It serves as the reference angle for the positive-sequence Park coordinate transformation; at the same time, the positive-sequence phase is directly inverted and used as the reference angle for the negative-sequence Park coordinate transformation.

[0074] In the control loop section, the voltage loop is not engaged in grid-connected mode, and the positive and negative sequence current reference commands are used. , and , It is given directly by the superior command.

[0075] The current reference command is fed into the current loop. For the positive sequence channel, the current loop sends the deviation between the positive sequence current reference value and the actual sampled positive sequence current to the proportional-integral (PI) controller to calculate the positive sequence d- and q-axis modulated wave reference command. At the same time, the negative sequence channel uses the exact same method to calculate the negative sequence d- and q-axis modulated wave reference command.

[0076] Because the positive-sequence and negative-sequence components move in different directions of rotation in physical space—the positive-sequence component rotates in the positive direction of the grid fundamental wave, while the negative-sequence component rotates in the opposite direction—they must each undergo coordinate transformation using their respective corresponding reference angles. For the positive-sequence component, the control system uses θ. pllAs a reference angle, the positive-sequence d- and q-axis modulation wave commands are transformed to the abc coordinate system using standard Park inverse transform and Clark inverse transform to obtain the positive-sequence three-phase voltage modulation signal; simultaneously, the negative-sequence channel uses -θ pll Using the same reference angle, the negative sequence three-phase voltage modulation signal is obtained.

[0077] The positive and negative sequence three-phase voltage modulation signals are directly added together, and after PWM modulation, the converter switching transistor drive signal is obtained, thereby realizing the control of the converter in grid-connected mode.

[0078] (2) Network-based control architecture

[0079] In the grid synchronization stage, the system adopts a virtual synchronous generator control strategy. Its active power-frequency control loop simulates the swing equation of the synchronous generator rotor, autonomously integrates to generate a virtual angular velocity, and further generates the synchronization phase in the phase integrator.

[0080]

[0081] In the formula, J is the virtual inertia of the virtual synchronous generator; ω vsg Virtual angular velocities generated by autonomous integration; P0 and P e These are the reference command value and the actual measured value of active power, respectively; D p θ is the active damping coefficient; vsg This is the generated positive-sequence virtual synchronization phase.

[0082] At this point, θ is selected. vsg As the reference angle for positive-order coordinate transformation, it is also directly inverted and used as the reference angle for negative-order coordinate transformation.

[0083] In the control loop section, firstly, a positive-sequence d-axis voltage reference value is generated through the reactive power-voltage droop control loop. (Setting the orientation to the d-axis, i.e., the positive sequence q-axis voltage reference value) (Set to 0), and to suppress negative sequence imbalance under asymmetric transients, the negative sequence dq axis voltage reference value is set to 0. , Set to 0:

[0084]

[0085] In the formula, U0 is the rated voltage amplitude; k q The reactive power droop factor; Q0 and Q e These are the reference value for reactive power and the actual measured value calculated based on the measured voltage U and measured current I, respectively.

[0086] Secondly, in order to achieve fault-limited current, a virtual impedance is introduced to generate a virtual voltage drop, which in turn generates the final voltage reference value input to the positive and negative sequence voltage loops. , and , As a voltage command:

[0087]

[0088] Subsequently, with the voltage loop engaged in network mode, the corrected voltage command is sent into the voltage loop to output the current reference command value in network mode. , and , Taking the positive and negative sequence d-axis channels as an example, its adjustment equation is:

[0089]

[0090] In the formula, This is the d-axis positive sequence current reference command for the network configuration mode output by the voltage loop. The d-axis negative sequence current reference command for the grid configuration mode output by the voltage loop; , These are the proportional and integral coefficients of the positive-sequence voltage loop PI controller, respectively. , These are the proportional and integral coefficients of the negative sequence voltage loop PI controller, respectively. and These are the integral terms of the voltage loop PI controller in the positive-sequence control loop and the negative-sequence control loop, respectively.

[0091] Finally, the current reference command generated by the voltage loop is sent into the current loop for closed-loop tracking and regulation, generating the final switching signal to drive the converter.

[0092] Step 2: Monitor the system's operating status and mode command flag changes in real time.

[0093] This invention defines a mode command flag bit (flag) within the control system to indicate the current operating status of the converter, and reads this status in real time. When the system receives flag = 0, it indicates that the converter is currently operating in grid-connected mode; conversely, when it receives flag = 1, it indicates that the converter is operating in grid-connected mode.

[0094] To accurately capture the transient moment of mode switching (denoted as t0), the control system detects and captures the rising edge of the command flag bit (flag) within each discrete control cycle, and performs corresponding judgments based on the rising edge detection value Δflag. The rising edge detection value Δflag is obtained by subtracting the flag value at the current sampling moment from the flag value at the previous sampling moment.

[0095] If Δflag = 0: it means that the command has not changed, the system is in a steady state, and the converter continues to maintain the currently active grid-following or grid-connecting control mode;

[0096] If Δflag = -1 (falling edge detection): This indicates that the system has received a command to switch the converter from grid-connected mode to grid-following mode;

[0097] If Δflag = 1 (rising edge detection): This indicates that the system has received a command to switch the converter from grid-following mode to grid-connecting mode.

[0098] Step 3: Execute the corresponding seamless switching control strategy.

[0099] Based on the switching direction of the mode command flag detected in step two, the control system executes the corresponding seamless switching control strategy on the converter at the moment of switching triggering, so as to eliminate the system transient impact caused by the sudden change in control structure and the virtual impedance input.

[0100] (1) When the system determines that the switching direction is from network construction mode to network following mode (i.e. Δflag = -1), the initial phase angle value is synchronized and the current command is pre-aligned.

[0101] At the instant of switching from network construction mode to network following mode, the control system faces a switch in the orientation phase angle of the rotating coordinate system (due to the virtual phase θ). vsg Switching to phase-locked loop physical phase θ pll This includes switching the current loop reference command source. To achieve a smooth transition, the following operations are performed in sequence:

[0102] 1) Phase angle initial value synchronization:

[0103] The control system at the sampling time before officially switching to the network mode (denoted as ), extracting the virtual phase of the active-frequency control loop output in the network configuration mode. This value is then written as the initial value into the phase integrator of the positive-sequence phase-locked loop:

[0104]

[0105] This operation ensures that the output of the positive-sequence PLL at the instant the tracking mode is activated is strictly equal to the virtual phase at the moment before the switch. Simultaneously, the reference phase for the negative-sequence coordinate transformation seamlessly switches to the inverted -θ phase. pll This eliminates the phase angle jump of the coordinate system at its source.

[0106] 2) Current command preset alignment:

[0107] In network-connected mode, the reference command for the current loop is calculated and generated by the voltage loop; while in network-following mode, this command is directly given by the external scheduling system. If the externally given value is used directly at the moment of switching, the deviation between it and the actual output value of the outer voltage loop at the moment before switching will cause a transient current surge.

[0108] The control system also intercepts Measurement values ​​of the actual positive and negative sequence current output of the converter at any time , and , At time t0, reset the d-axis and q-axis current reference commands of the current loop in grid mode to the above physical measurement values:

[0109]

[0110] By aligning the instruction reference value to the actual physical value in both directions, seamless connection of current loop instructions before and after switching is ensured.

[0111] (2) When the system determines that the switching direction is from the following mode to the network construction mode (i.e., Δflag = 1), the initial phase angle synchronization, virtual impedance matching and voltage loop dual integrator reset are executed in sequence.

[0112] When the converter switches to grid-connected mode to handle severe grid faults such as asymmetric voltage drops, the control structure undergoes abrupt changes and introduces positive and negative sequence virtual impedances. To mitigate the resulting severe impact, the following steps are performed:

[0113] 1) Phase angle initial value synchronization:

[0114] To ensure that the rotating coordinate system does not undergo a phase jump during the switching process, a truncated section is taken. The true physical phase output by the positive sequence phase-locked loop at any given time This initial value is written into the phase integral register of the active-frequency control loop in the network configuration mode:

[0115]

[0116] Phase reset ensures that the positive and negative sequence virtual phases generated by the grid configuration mode after the switch start smoothly from the current grid reference phase point, eliminating power oscillations caused by phase angle jumps from the source.

[0117] 2) Virtual impedance matching:

[0118] Under asymmetrical fault conditions, if a virtual impedance with an initial value of zero is directly applied during mode switching, the huge deviation between the actual voltage and the reference voltage will cause the voltage loop output to jump.

[0119] This invention uses the voltage and current physical quantities at the moment of switching to inversely calculate and assign a precise initial virtual impedance.

[0120] Taking the ascending sequence control channel as an example:

[0121] Control system extraction Positive sequence voltage reference value at time 1 and Measured value of positive sequence voltage and , and the measured current value and .

[0122] To ensure the input proportional term deviation of the outer voltage loop is zero, the compensation voltage drop caused by the virtual impedance must completely offset the difference between the reference voltage and the actual voltage. Taking the positive sequence as an example, in the rotating dq coordinate system, according to Ohm's law, the voltage drops along the d-axis and q-axis caused by the virtual impedance satisfy the following equality relationship:

[0123]

[0124] By simultaneously solving the above system of equations, the positive-sequence virtual resistance at the instant of switching can be calculated. With positive sequence virtual reactance The initial value is:

[0125]

[0126] Similarly, for the negative sequence control channel, the negative sequence reference voltage is set to 0 (i.e., Combining the measured values ​​of negative sequence voltage and current, the initial value of the negative sequence virtual impedance is calculated by substituting these values ​​into the calculation. and .

[0127] After the mode switch is completed, in the network configuration mode, the positive and negative sequence virtual impedances are dynamically adjusted from their initial values ​​according to the current limiting requirements.

[0128] This invention, through the reverse matching based on Ohm's law, enables the input virtual impedance voltage drop to perfectly offset the command error, achieving a disturbance-free connection of the control loop. This avoids the huge deviation between the voltage outer loop reference command and the actual physical quantity caused by the traditional switching method defaulting to zero initial value of virtual impedance at the moment of switching.

[0129] 3) Voltage loop dual integrator coordinated reset:

[0130] Under the aforementioned virtual impedance matching mechanism, the deviation between the reference voltage and the measured voltage has been eliminated by the virtual voltage drop. For the PI controller in network mode, the input deviation of its proportional term is zero, and the output of the proportional element is strictly zero.

[0131] Therefore, to ensure that the current reference command output by the voltage loop at time t0 is exactly equal to the actual physical current flowing just before the switch, it is only necessary to directly set the initial values ​​of the internal state registers of the integral terms of the positive and negative sequence PI controllers (the initial values ​​of the integral terms of the positive sequence d and q axis PI controllers). and Initial values ​​of the integral terms of the negative-sequence d-axis and q-axis PI controllers and Reset to the actual current measurement value at time t0.

[0132] Taking the ascending sequence channel as an example, its reset logic is as follows:

[0133]

[0134] Similarly, the integral terms of the negative-order channels are also symmetrically reset, i.e. , .

[0135] The above strategy completely eliminates the reference command deviation that occurs when switching from network following mode to network construction mode under asymmetric faults.

[0136] Step 4: After the integrator initial value reset and current command preset alignment operations are completed, the control system seamlessly switches to the corresponding target control mode, completely eliminating the transient impact caused by the sudden change in control structure, and realizing the smooth mode switching of the system under asymmetric faults.

[0137] To verify the practicality of the seamless switching method for converters and grid connection under asymmetrical faults proposed in this invention, a simulation model of the converter grid-connected system was built in this embodiment. Simulations were then performed comparing converters using the strategy of this invention with those using the traditional direct hard switching strategy. The simulations were conducted using the following parameter settings: DC bus voltage 1.6kV, AC voltage base value 0.69kV, AC grid fundamental frequency 50Hz, rated capacity 5MVA, and switching frequency 10kHz.

[0138] The simulation timeline is set as follows: the converter initially operates in grid-connected mode; at 1.0s, an asymmetrical dip fault occurs in the grid; at 2.0s, the converter receives a command to switch from grid-connected mode to grid-following mode; at 3.0s, the converter receives another command to switch back from grid-following mode to grid-connected mode.

[0139] Figure 4 The system simulation waveforms of the converter using the control method of this invention are shown during the entire process of asymmetrical fault and bidirectional mode switching. From top to bottom, the waveforms are the positive-sequence d-axis current, q-axis current, negative-sequence d-axis current, q-axis current, three-phase current at the converter port, and three-phase voltage at the port as a function of time. pu is an abbreviation for per unit, i.e., per unit value. Figure 4 As shown, after an asymmetrical fault occurs at 1.0s, the system responds quickly and operates stably. During the two mode switching moments at 2.0s and 3.0s, thanks to the phase angle initial value synchronization, bidirectional alignment of current commands, and virtual impedance reverse calculation and integrator reset mechanisms proposed in this invention, both positive-sequence and negative-sequence dq-axis currents achieve a smooth and seamless transition. The magnified views of the port current and port voltage clearly show that at the moment of switching triggering, the three-phase port current waveforms maintain extremely high continuity and good sinusoidal properties, without any transient spikes; simultaneously, the port voltage also remains stable, achieving seamless switching between converter grid connection and grid configuration modes under severe asymmetrical operating conditions.

[0140] In contrast. Figure 5 The simulation waveforms of a converter system without the control method of this invention (i.e., using the traditional hard switching method) are shown. PU stands for per unit. First, at 2.0s, when switching from grid-connected mode to grid-following mode, the sudden change in the reference command due to the lack of current command pre-alignment causes significant jumps in both the positive and negative sequence dq-axis currents, resulting in transient distortion of the port currents. More seriously, at 3.0s, when switching from grid-following mode to grid-connected mode, the lack of phase angle synchronization and the sudden addition of virtual impedance cause a severe deviation between the voltage outer loop integrator state and the actual physical quantities, resulting in violent transient oscillations in both the positive and negative sequence dq-axis currents; consequently, the three-phase currents at the converter ports exhibit extremely large overcurrent spikes. This severe overcurrent impact, accompanied by severe distortion of the port voltage, can easily trigger the overcurrent protection of the converter's underlying hardware in practical engineering, leading to equipment damage or tripping and disconnection from the grid.

[0141] contrast Figure 4 and Figure 5 Simulation results fully demonstrate that traditional switching methods, due to their failure to handle phase delays and integrator initial value errors under asymmetrical operating conditions, induce severe transient overcurrents and distortions at the moment of switching. In contrast, this invention, thanks to the proposed phase angle initial value synchronization and virtual impedance precise matching mechanism, effectively avoids transient impacts during switching, and achieves smooth and seamless transitions between positive and negative sequence dq axis currents. This fully verifies that the control strategy of this invention not only possesses extremely high safety under severe asymmetrical faults but also has excellent flexible support potential.

[0142] It should also be noted that the seamless switching control method for converter-grid connection and grid construction modes under asymmetric faults in the above embodiments can essentially be executed by a computer program. Therefore, similarly, based on the same inventive concept, another preferred embodiment of the present invention also provides a computer electronic device corresponding to the method provided in the above embodiments, which includes a memory and a processor;

[0143] The memory is used to store computer programs;

[0144] The processor is configured to implement the seamless switching control method for converter-grid connection and grid construction modes under asymmetric faults in the above embodiments when executing the computer program.

[0145] From a hardware perspective, such as Figure 6 The diagram shown is a hardware structure diagram provided in this embodiment. In addition to the processor, memory, network interface and non-volatile memory shown in the diagram, any device with data processing capabilities in the embodiment may also include other hardware depending on the actual function of the device with data processing capabilities, which will not be described in detail here.

[0146] When the logical instructions in the aforementioned memory can be implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product, which is stored in a storage medium.

[0147] Therefore, based on the same inventive concept, another preferred embodiment of the present invention also provides a computer-readable storage medium corresponding to the method provided in the above embodiments. The storage medium stores a computer program, which, when executed by a processor, can realize the seamless switching control method for converter grid connection and grid construction modes under asymmetric faults in the above embodiments.

[0148] It is understood that the computer-readable storage medium can be an internal storage unit of any data processing device described in any of the foregoing embodiments, such as a hard disk or memory. The computer-readable storage medium can also be an external storage device of any data processing device, such as a plug-in hard disk, smart media card (SMC), SD card, flash card, etc., equipped on the device. Furthermore, the computer-readable storage medium can include both internal storage units and external storage devices of any data processing device. The computer-readable storage medium is used to store the computer program and other programs and data required by the data processing device, and can also be used to temporarily store data that has been output or will be output.

[0149] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained through equivalent substitution or transformation fall within the protection scope of the present invention.

Claims

1. A control method for seamless switching between converter grid connection and grid construction modes under asymmetrical faults, characterized in that, The method includes: S1, determine whether the converter's mode has changed. If it has changed from grid-connecting mode to grid-following mode, proceed to S2; if it has changed from grid-following mode to grid-connecting mode, proceed to S3. S2, when switching to grid-connected mode, firstly, the positive-sequence virtual synchronous phase generated by the virtual synchronous generator before switching is captured and written as the initial value into the phase integrator of the positive-sequence phase-locked loop; secondly, the current reference values ​​of the positive-sequence control loop and the negative-sequence control loop in grid-connected mode are reset to the actual measured current values ​​of the corresponding loops in grid-connected mode before switching. After the switching is completed, the Park transform reference phase of the positive sequence control loop uses the positive sequence phase-locked loop output, and the Park transform reference phase of the negative sequence control loop is obtained by inverting the positive sequence phase. S3, when switching to the grid-connected mode, firstly, the positive-sequence phase output of the positive-sequence phase-locked loop before switching is captured and written into the phase integrator of the virtual synchronous generator as the initial value; secondly, based on the measured voltage value, measured current value and voltage reference value before switching, the positive and negative sequence virtual impedances are obtained and used as the initial values ​​of the virtual impedance in the grid-connected mode; finally, the initial values ​​of the integral terms of the voltage loop PI controllers of the positive-sequence control loop and the negative-sequence control loop are reset to the measured positive and negative sequence current values ​​before switching. After the switch is completed, the Park transformation reference phase of the positive sequence control loop uses the positive sequence virtual synchronous phase output by the virtual synchronous generator, and the Park transformation reference phase of the negative sequence control loop is obtained by inverting the positive sequence virtual synchronous phase.

2. The seamless switching control method for converter grid connection and grid construction modes under asymmetrical faults according to claim 1, characterized in that, In S1, the determination of whether the mode of the converter has switched is achieved by the edge detection value.

3. The seamless switching control method for converter grid connection and grid construction modes under asymmetrical faults according to claim 1, characterized in that, In step S2, the process of extracting the positive-sequence virtual synchronization phase generated by the virtual synchronous generator before the switchover is as follows: Obtain the AC voltage and AC current at the converter outlet at the sampling moment before the mode switch occurs, and obtain the measured value of active power; In the active-frequency control loop of the virtual synchronous generator, the virtual angular velocity is generated autonomously by integrating the swing equation of the synchronous generator rotor based on the active power reference value and the measured active power value. In the phase integrator of the virtual synchronous generator, the virtual angular velocity is integrated to obtain the positive-sequence virtual synchronous phase.

4. The seamless switching control method for converter grid connection and grid construction modes under asymmetrical faults according to claim 1, characterized in that, In S2, the process of obtaining the positive-sequence phase output of the positive-sequence phase-locked loop is as follows: Obtain the AC voltage at the converter outlet after switching and input it into the positive sequence control loop; In the positive sequence control loop, the AC voltage is transformed by Park coordinates to obtain the measured value of the positive sequence voltage. The measured value of the positive sequence q-axis voltage is then sent to the PI controller of the positive sequence phase-locked loop, and the adjustment value is obtained after proportional-integral adjustment. The adjustment amount is superimposed with the rated angular frequency of the power grid to obtain the angular frequency of the positive sequence phase-locked loop; In the phase integrator of the positive-sequence phase-locked loop, the angular frequency is integrated to obtain the positive-sequence phase.

5. The seamless switching control method for converter grid connection and grid construction modes under asymmetrical faults according to claim 1, characterized in that, In step S3, the process of extracting the positive-sequence phase output of the positive-sequence phase-locked loop before switching is as follows: Obtain the AC voltage at the converter outlet at the sampling moment before the mode switch occurs, and input it into the positive sequence control loop; In the positive sequence control loop, the AC voltage is transformed by Park coordinates to obtain the measured value of the positive sequence voltage. The measured value of the positive sequence q-axis voltage is then sent to the PI controller of the positive sequence phase-locked loop, and the adjustment value is obtained after proportional-integral adjustment. The adjustment amount is superimposed with the rated angular frequency of the power grid to obtain the angular frequency of the positive sequence phase-locked loop; In the phase integrator of the positive-sequence phase-locked loop, the angular frequency is integrated to obtain the positive-sequence phase.

6. The seamless switching control method for converter grid connection and grid construction modes under asymmetrical faults according to claim 1, characterized in that, In step S3, the calculation process for the positive and negative sequence virtual impedances, based on the measured voltage, measured current, and voltage reference value before switching, is as follows: Based on the measured voltage, measured current and voltage reference value, and according to Ohm's law, establish the equivalent relationship between the product of virtual impedance and measured current and the difference between voltage reference value and measured voltage value; In the positive sequence control loop, the measured voltage, measured current and voltage reference values ​​of the positive sequence control loop before switching are substituted into the above-mentioned equal relationship to obtain the positive sequence virtual impedance; In the negative sequence control loop, the voltage reference value is set to 0. The measured voltage and current values ​​of the negative sequence control loop before switching are substituted into the above-mentioned equal relationship to obtain the negative sequence virtual impedance.

7. The seamless switching control method for converter grid connection and grid construction modes under asymmetrical faults according to claim 6, characterized in that, The expression for the equality relationship is: ； Where the superscript x represents the positive or negative order component. and These are the real and imaginary parts of the virtual impedance, respectively. and These are the d-axis and q-axis voltage reference values, respectively. and These are the measured voltage values ​​for the d-axis and q-axis, respectively. To switch to the previous sampling time, For the moment of switching, and These are the measured values ​​of the d-axis and q-axis currents, respectively.

8. The seamless switching control method for converter grid connection and grid construction modes under asymmetrical faults according to claim 6, characterized in that, In the positive sequence control loop, the voltage reference values ​​include a positive sequence d-axis voltage reference value and a positive sequence q-axis voltage reference value. The positive sequence q-axis voltage reference value is 0, and the positive sequence d-axis voltage reference value is obtained from a virtual synchronous generator, specifically: In the reactive power-voltage droop control loop of the virtual synchronous generator, the measured reactive power value is obtained based on the AC voltage and AC current at the converter outlet; combined with the reactive power reference value and the rated voltage amplitude, the positive sequence d-axis voltage reference value is obtained.

9. The seamless switching control method for converter grid connection and grid construction modes under asymmetrical faults according to claim 1, characterized in that, In step S3, the process of obtaining the positive-sequence virtual synchronization phase output by the virtual synchronous generator is as follows: Obtain the AC voltage and AC current at the converter output after switching, and get the measured value of active power; In the active-frequency control loop of the virtual synchronous generator, the virtual angular velocity is generated autonomously by integrating the swing equation of the synchronous generator rotor based on the active power reference value and the measured active power value. In the phase integrator of the virtual synchronous generator, the virtual angular velocity is integrated to obtain the positive-sequence virtual synchronous phase.

10. A computer electronic device, characterized in that, Including memory and processor; The memory is used to store computer programs; The processor is configured to, when executing the computer program, implement the converter grid connection and grid construction mode seamless switching control method under asymmetric faults as described in any one of claims 1 to 9.

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