Current limiting control method and device for meshed flexible direct current power transmission system and electronic equipment

Abstract

The application provides a current-limiting control method and device for a grid-forming flexible direct current transmission system and electronic equipment, and relates to the technical field of flexible direct current transmission. The method measures the actual voltage at the grid connection point of a converter, dynamically calculates the maximum value of a virtual internal potential and the maximum values of active power and reactive power allowed to be output by the grid-forming control of the converter according to the internal impedance characteristics of the converter, limits the virtual internal potential output of the grid-forming controller, and thus realizes the effect that the alternating current output does not exceed the set value under any operating condition.

Description

Technical Field

[0001] This invention relates to the field of flexible DC transmission technology, and in particular to a current limiting control method, device, and electronic equipment for grid-type flexible DC transmission systems. Background Technology

[0002] Grid-type flexible DC transmission systems exhibit voltage source characteristics at the AC grid connection point, and their internal control strategy lacks an inner-loop current control mechanism. When a fault occurs in the AC grid or the voltage at the converter's grid connection point drops, the converter's modulation wave maintains its internal electromotive force stability, resulting in an extremely large short-circuit current. This excessive short-circuit current exceeds the converter's own tolerance capacity, jeopardizing the safety of DC engineering equipment. Furthermore, it increases the short-circuit current level of the surrounding power grid, potentially causing short-circuit currents in the nearby DC power grid to exceed limits.

[0003] To limit the short-circuit current of grid-connected flexible DC transmission during AC grid faults, existing technical solutions mainly fall into four categories: 1. By decoupling the grid connection point voltage and grid connection current of the converter, a current inner loop is constructed, thereby limiting the AC output current in the current inner loop controller; 2. Utilizing the inherent current-limiting characteristics of conventional grid-connected control, grid faults are identified by monitoring the voltage and current at the grid connection point, and the control mode is switched to grid-connected control when a grid fault occurs, thereby achieving fault current limiting; 3. By monitoring and identifying grid faults, the converter power or voltage reference value is recalculated based on the residual voltage at the grid connection point during the grid fault period, so that the output current during the fault period is maintained within the acceptable overload capacity range of the converter. 4. Introduce a virtual impedance between the equivalent internal potential of the converter and the grid connection point voltage, and indirectly limit the output current of the converter by adjusting the magnitude of the virtual impedance.

[0004] The first type of technical approach in the existing schemes is only applicable to two-level and three-level converter topologies with centralized filter capacitors at the grid connection point. For the modular multilevel converter topologies that are the mainstream in the field of high-voltage direct current transmission, since there are no centralized filter capacitors on the AC side, it is impossible to use the capacitor current link to achieve decoupling control of the AC voltage and AC current of the converter. Therefore, the first type of technical approach cannot be used to achieve fault current limiting. The second type of technical approach relies on the identification of grid faults and only switches to the current limiting state when the AC side voltage drops or the current surges to a certain extent. There is a certain risk of misjudgment and missed judgment. Moreover, after entering the current limiting state, it returns to the grid-following mode. At this time, the grid voltage itself is in a fault state, which is not conducive to the stable phase-locked operation of the converter. The third and fourth types of technical approaches also rely on the identification of grid faults, which has a certain risk of misjudgment and missed judgment. Furthermore, these two schemes indirectly achieve current limiting by limiting virtual internal potential and virtual impedance, respectively, and cannot accurately and intuitively set the allowable overcurrent level in the control strategy. In addition to affecting the short-circuit current level, the virtual impedance introduced in the fourth type of technical route is also closely related to the active / reactive decoupling control performance of the converter, the system damping strength, and the power angle operating range. Its large-scale adjustment and change may also affect the operating stability of the converter system.

[0005] In summary, existing current limiting control schemes still have many problems, such as being applicable only to specific converter topologies, relying on grid fault identification, and causing sudden state changes due to control mode switching. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a current limiting control method, device and electronic equipment for a grid-type flexible DC transmission system. The method measures the actual voltage at the grid connection point of the converter and dynamically calculates the maximum virtual internal potential and the maximum active and reactive power values ​​allowed for the grid control output of the converter based on the internal impedance characteristics of the converter. This limits the virtual internal potential output of the grid-type controller, thereby achieving the effect that the AC output current does not exceed the set value under any operating condition.

[0007] In a first aspect, embodiments of the present invention provide a current limiting control method for a grid-type flexible DC transmission system, the method comprising: Based on the connection transformer impedance, bridge arm reactance, and virtual impedance used in grid control of the converter in the flexible DC transmission system, calculate the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system. Using the current output current and maximum allowable overcurrent setting value of the converter actually output to the grid connection point, calculate the allowable output current range of the converter according to the current limiting priority mode option command; The virtual internal potential limit of the converter's allowable output is calculated using the current output voltage at the grid connection point, the total circuit impedance, and the current range. Calculate the allowable output power range of the converter using the current output voltage and current range of the grid connection point; The modulation voltage of the converter is calculated based on the virtual internal potential limit and power range, and the modulation of the converter is driven by the voltage waveform corresponding to the modulation voltage, so as to realize the dynamic current limiting of the flexible DC transmission system.

[0008] Optionally, the step of calculating the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system, based on the connecting transformer impedance, arm reactance, and virtual impedance used in the grid control of the converter in the flexible DC transmission system, includes: Obtain the connection transformer impedance of the converter in a flexible DC transmission system ; Obtain the bridge arm reactance of the upper and lower bridge arms in the phase unit of the converter. ; The virtual impedance of the converter is obtained based on the virtual circuit used in the grid control process. ;in, For virtual resistance components, For virtual reactance components, The imaginary unit; The total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system is calculated using the connection transformer impedance, bridge arm reactance, and virtual impedance. The total loop impedance is obtained through the following formula: ; The resistive component of the total impedance of the circuit; This represents the reactance component of the total circuit impedance.

[0009] Optionally, the step of calculating the allowable output current range of the converter using the current output current actually output to the grid connection point and the maximum allowable overcurrent setting value, according to the current limiting priority mode option instruction, includes: Obtain the current AC current actually output from the converter to the grid connection point. And obtain the d-axis current projection component in the rotating Cartesian coordinate system after the current AC current undergoes Park transformation. and q-axis current projection components ; Obtain the maximum allowable overcurrent setting value of the converter And obtain the rate limiting priority mode option command; When the converter's current-limiting priority mode option command is set to active power priority mode, the range of active current allowed to be output by the converter is specified. and reactive current range The result is obtained through the following formula: , ;in, This is the lower limit of the d-axis current. This is the upper limit value of the d-axis current; This is the lower limit of the q-axis current. This is the upper limit of the q-axis current. When the converter's current-limiting priority mode option command is set to reactive power priority mode, the range of active current that the converter is allowed to output is... and reactive current range The result is obtained through the following formula: , ; The allowable output current range of the converter is determined based on the active current range and the reactive current range.

[0010] Optionally, the virtual internal potential limit of the converter's allowable output can be calculated using the current output voltage at the grid connection point, the total loop impedance, and the current range, including: Obtain the current AC voltage at the grid connection point in the flexible DC transmission system. and obtain the current AC voltage. The d-axis projection component of the grid-connected point voltage in the rotating Cartesian coordinate system obtained after Park transformation. and the q-axis grid-connected point voltage projection component ; Using the d-axis grid connection point voltage projection component q-axis grid-connected point voltage projection components Total circuit impedance The range of active current allowed to be output by the converter and the range of reactive current allowed to be output by the converter. Calculate the virtual internal potential limit allowed for the converter's output; where the upper limit of the virtual internal potential is... and the lower limit of virtual internal potential The result is obtained through the following formula: ; ; In the formula, This is the lower limit of the d-axis current. This is the upper limit value of the d-axis current; This is the lower limit of the q-axis current. This is the upper limit of the q-axis current. The resistive component of the total impedance of the circuit; This is the reactance component of the total circuit impedance; It is the imaginary unit.

[0011] Optionally, obtain the current AC voltage. The d-axis projection component of the grid-connected point voltage in the rotating Cartesian coordinate system obtained after Park transformation. and the q-axis grid-connected point voltage projection component ,include: Obtain the three-phase instantaneous voltage of the current AC voltage. , and ;in, The instantaneous voltage of phase A. This is the instantaneous voltage of phase B. This is the instantaneous voltage of phase C; Real-time phase angle of virtual internal potential output by network control As an angular reference value, after performing a Parker transformation on the three-phase instantaneous voltage, the d-axis and q-axis projection components of the grid-connected voltage corresponding to the current AC voltage in the rotating rectangular coordinate system are obtained; among them, the d-axis grid-connected voltage projection component... and the q-axis grid-connected point voltage projection component The result is obtained through the following formula: ; in, The real-time phase angle of the virtual internal potential output by the network controller; t is time; This represents the rotational angular velocity corresponding to the virtual internal potential of the converter.

[0012] Optionally, the allowable output power range of the converter can be calculated using the current output voltage and current range at the grid connection point, including: Obtain the d-axis grid connection point voltage projection component based on the current output voltage of the grid connection point. and the q-axis grid-connected point voltage projection component ; The active current range that the converter can output is obtained based on the current range. and reactive current range ;in, This is the lower limit of the d-axis current. This is the upper limit value of the d-axis current; This is the lower limit of the q-axis current. This is the upper limit of the q-axis current. Using the d-axis grid connection point voltage projection component q-axis grid-connected point voltage projection components d-axis current upper limit and the upper limit of q-axis current Calculate the upper limit of the active power output allowed by the converter. and upper limit of reactive power Among them, the upper limit of active power and upper limit of reactive power The result is obtained through the following formula: ; Using the d-axis grid connection point voltage projection component q-axis grid-connected point voltage projection components d-axis current lower limit and the lower limit of q-axis current Calculate the lower limit of the active power that the converter can output. and the lower limit of reactive power Among them, the lower limit of active power and the lower limit of reactive power The result is obtained through the following formula: ; Based on the upper limit of active power and the lower limit of active power Determine the active power range of the converter And based on the upper limit of reactive power and the lower limit of reactive power Determine the reactive power range of the converter .

[0013] Optionally, the modulation voltage of the converter is calculated based on the virtual internal potential limit and power range, including: Obtain the current AC current output from the converter to the grid connection point in the flexible DC transmission system. And obtain the d-axis current projection components of the three-phase components of the current alternating current in the rotating rectangular coordinate system. and q-axis current projection components ; The virtual impedance of the converter is obtained based on the virtual circuit used in the grid control process. ;in, For virtual resistance components, For virtual reactance components, The imaginary unit; The internal potential limit of the converter during grid construction control is determined by using the virtual internal potential limit value, and the power limit of the converter during grid construction control is determined by using the power range. Determining the virtual internal potential of the converter based on internal potential limiting and power limiting And utilize virtual internal potential and d-axis current projection components. q-axis current projection components Virtual impedance calculation of the d-axis modulation voltage projection component of the converter in a rotating rectangular coordinate system and q-axis modulated voltage projection components ; The modulation voltage of the converter is calculated using the d-axis modulation voltage projection component and the q-axis modulation voltage projection component.

[0014] Optionally, the d-axis modulated voltage projection component is calculated using the following formula: ; The q-axis modulated voltage projection component is calculated using the following formula: .

[0015] In a second aspect, the present invention provides a current limiting control device for a grid-type flexible DC transmission system, the device comprising: Loop Impedance Calculation Module: This module is used to calculate the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system based on the connection transformer impedance, arm reactance, and virtual impedance used in the grid control of the converter in the flexible DC transmission system. Current range calculation module: Used to calculate the current range that the converter can output, based on the current output current and the maximum allowable overcurrent setting value of the converter actually output to the grid connection point, according to the current limiting priority mode option command; The first current limiting calculation module is used to calculate the virtual internal potential limit of the converter's allowable output using the current output voltage of the grid connection point, the total circuit impedance, and the current range. The second current limiting calculation module is used to calculate the power range that the converter can output using the current output voltage and current range of the grid connection point. Current limiting execution control module: It is used to calculate the modulation voltage of the converter based on the virtual internal potential limit and power range, and drive the modulation of the converter using the voltage waveform corresponding to the modulation voltage, so as to realize the dynamic current limiting of the flexible DC transmission system.

[0016] Thirdly, embodiments of the present invention also provide an electronic device, which includes a processor and a memory. The memory stores computer-executable instructions that can be executed by the processor. The processor executes the computer-executable instructions to implement the steps of the current limiting control method for a grid-type flexible DC transmission system provided in the first aspect.

[0017] This invention provides a current-limiting control method, device, and electronic equipment for a grid-type flexible DC transmission system. In the process of current-limiting control of the flexible DC transmission system, the method first calculates the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system based on the connecting transformer impedance, bridge arm reactance, and the virtual impedance used for grid control of the converter in the flexible DC transmission system. Then, using the current output current of the converter actually output to the grid connection point and the maximum allowable overcurrent setting value, the allowable current range of the converter is calculated according to the current-limiting priority mode option instruction. Subsequently, the allowable virtual internal potential limit of the converter is calculated using the current output voltage of the grid connection point, the total loop impedance, and the current range, and the allowable power range of the converter is calculated using the current output voltage and current range of the grid connection point. Finally, the modulation voltage of the converter is calculated based on the virtual internal potential limit and the power range, and the modulation of the converter is driven by the voltage waveform corresponding to the modulation voltage to achieve dynamic current limiting of the flexible DC transmission system. This method measures the actual voltage at the converter's grid connection point and dynamically calculates the maximum virtual internal potential and the maximum active and reactive power values ​​allowed for the converter's grid-connected control output based on the converter's internal impedance characteristics. This limits the virtual internal potential output of the grid-connected controller, thereby achieving the effect that the AC output current does not exceed the set value under any operating condition.

[0018] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention are realized and obtained through the structures particularly pointed out in the description and the drawings.

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 A flowchart of a current limiting control method for a grid-type flexible DC transmission system provided in an embodiment of the present invention; Figure 2 This is a structural diagram illustrating the relationship between relevant controllers in a current-limiting control method for a grid-type flexible DC transmission system provided in an embodiment of the present invention. Figure 3A converter primary circuit topology diagram provided in an embodiment of the present invention for a current limiting control method for a grid-type flexible DC transmission system; Figure 4 A general control logic block diagram of a current limiting control method for a grid-type flexible DC transmission system provided in an embodiment of the present invention; Figure 5 This invention provides a current limiting control method for a grid-type flexible DC transmission system, including a control principle diagram of the current upper and lower limit calculation stage. Figure 6 The control principle diagram of the first current limiting link in a current limiting control method for a grid-type flexible DC transmission system provided in an embodiment of the present invention; Figure 7 This is a schematic diagram of the structure of a current limiting control system for a grid-type flexible DC transmission system provided in an embodiment of the present invention; Figure 8 This is a schematic diagram of the structure of an electronic device provided in an embodiment of the present invention.

[0022] icon: 100 - Loop impedance calculation module; 200 - Current range calculation module; 300 - First current limiting calculation module; 400 - Second current limiting calculation module; 500 - Current limiting execution control module; 101 - Processor; 102 - Memory; 103 - Bus; 104 - Communication interface. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0024] To facilitate understanding of this embodiment, a current-limiting control method for a grid-type flexible DC transmission system disclosed in this embodiment of the invention will first be introduced. The method is as follows: Figure 1 As shown, it includes: Step S101: Based on the connection transformer impedance, bridge arm reactance, and virtual impedance used in the grid control of the flexible DC transmission system, calculate the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system.

[0025] Based on the connection transformer impedance and arm reactance parameters of the primary circuit of the converter in the flexible DC transmission system, as well as the virtual impedance parameters used in the grid-type control strategy, the total circuit impedance between the AC grid connection point of the converter and the virtual internal potential of the grid control is calculated and determined.

[0026] In the modular multilevel converter topology, the upper and lower arm reactances of each phase unit exhibit virtual parallel characteristics in the impedance relationship between the modulation voltage and the grid connection point voltage. Therefore, the parallel equivalent value of the arm reactance is taken in the calculation of the total loop impedance. The connection transformer impedance, the parallel equivalent arm reactance, and the virtual internal impedance of the grid control are connected in series to obtain the total loop impedance values ​​under the d-axis and q-axis, respectively, which serve as the impedance basis parameters for subsequent current limiting calculations.

[0027] Step S102: Using the current output current and maximum allowable overcurrent setting value of the converter actually output to the grid connection point, calculate the allowable output current range of the converter according to the current limiting priority mode option instruction.

[0028] This step mainly involves collecting the actual output current from the converter to the AC grid connection point, combining it with the pre-set maximum allowable overcurrent setting value, and calculating and determining the allowable output current range of the converter in real time according to the active power priority or reactive power priority calculation strategy mode selected by the relevant current limiting priority mode option command.

[0029] In specific scenarios, the three-phase AC current output by the converter can be converted into d-axis and q-axis current projection components in a rotating dq coordinate system using Park transformation. Based on the externally given priority mode options, if the active power priority mode is adopted, the maximum allowable overcurrent setting is used as the constraint boundary to prioritize the output capability of the d-axis active current, thereby calculating the upper and lower limits of the converter's allowable output d-axis and q-axis currents. If the reactive power priority mode is adopted, the output capability of the q-axis reactive current is prioritized, thereby calculating the corresponding upper and lower limits of the d-axis and q-axis currents. Finally, the allowable output current range of the converter is determined using the upper and lower limits of the d-axis and q-axis currents.

[0030] Step S103: Calculate the virtual internal potential limit of the converter's allowable output using the current output voltage of the grid connection point, the total circuit impedance, and the current range.

[0031] In the specific implementation process, the three-phase grid voltage at the grid connection point can be converted into d-axis voltage components and q-axis voltage components through Park transformation. Based on the actual d-axis and q-axis voltages at the grid connection point, the voltage drop generated by the allowed output d-axis and q-axis currents on the total impedance of the loop is superimposed, and the upper and lower limits of the virtual internal potential amplitude allowed to maintain the converter output current in the current range are calculated to form the virtual internal potential limit.

[0032] Step S104: Calculate the allowable power range of the converter output using the current output voltage and current range of the grid connection point.

[0033] In practice, the actual d-axis and q-axis voltages at the grid connection point can be multiplied by the upper and lower limits of the allowable output d-axis and q-axis currents, respectively, to calculate the upper and lower limits of the active power or reactive power that the converter can output under the current grid voltage conditions, thereby obtaining the corresponding power range.

[0034] Step S105: Calculate the modulation voltage of the converter based on the virtual internal potential limit and power range, and use the voltage waveform corresponding to the modulation voltage to drive the modulation of the converter, so as to realize the dynamic current limiting of the flexible DC transmission system.

[0035] The virtual internal potential limit value calculated in step S103 and the power range calculated in step S104 are respectively applied to the corresponding control loops of the grid-type controller to limit the virtual internal potential and power reference value output by the grid-type controller. Based on the limited virtual internal potential and the current actual operating current of the converter, the modulation voltage of the converter is calculated, and the converter is driven to operate using the modulation voltage to realize dynamic current limiting control of the flexible DC transmission system under all operating conditions.

[0036] Specifically, the upper and lower limits of the virtual internal potential are used as the output limits of the reactive or voltage control links of the grid controller to limit the virtual internal potential. The upper and lower limits of active power are used to construct the active power range of the synchronous control link of the grid controller, and the upper and lower limits of reactive power are used to construct the reactive power range of the reactive or voltage control link of the grid controller to limit the output power of the grid controller and avoid secondary overcurrent during fault recovery. Based on this, the voltage drop across the virtual impedance is calculated according to the limited virtual internal potential and the current actual d-axis and q-axis currents of the converter, thus obtaining the d-axis and q-axis components of the final modulated voltage. After Parker inverse transformation, a three-phase instantaneous modulated voltage waveform is generated to drive the power module of the converter for modulation, thereby ensuring that the AC output current of the converter does not exceed the preset maximum allowable overcurrent value under any operating condition.

[0037] Specifically, the aforementioned current-limiting control method applies to the current-limiting controller and the grid-building controller in the flexible DC transmission system. The relationship structure diagram of these controllers is as follows: Figure 2 As shown. Figure 2 middle, , , These are the externally given current limiting controller tuning parameters, representing the virtual impedance value, the maximum allowable overcurrent value, and the option to use active power priority mode or reactive power priority mode when limiting current. , These are the converter operating variables that the current limiting controller needs to measure, representing the instantaneous grid voltage at the converter's grid connection point and the instantaneous AC current output by the converter, respectively. , , , ,as well as , These are the upper and lower limits of key variables output from the current limiting controller to the converter grid control controller. They represent the upper and lower limits of active power (corresponding to active power range), reactive power (corresponding to reactive power range), and the upper and lower limits of virtual internal potential amplitude allowed during grid control, respectively.

[0038] It should be noted that, Figure 2 This section explains the connection relationship between the current limiting controller and the original grid controller of the converter, mainly focusing on the functional positioning and working mode of the current limiting controller in the overall control architecture of the converter.

[0039] Figure 2 The current limiting controller is the main implementing mechanism of the technical solution of this invention, and its internal control block diagram is as follows: Figure 4 As shown. The grid controller is the original control mechanism of the converter, and its functions are described below: The grid-connected controller employs a grid-based control strategy to synchronize the converter with the AC grid and deliver a specified power output. Its inputs include the converter's active power reference value P. ref Or DC voltage reference value U dcref Reactive power reference value Q ref Or AC voltage reference value U acref These reference values ​​are then sent to the grid controller as control commands input by the operators. Secondly, the grid controller's input also includes the actual active power P of the converter. ac Actual DC voltage U dc Actual reactive power Q ac Actual AC voltage U ac These variables are obtained by measuring the real-time state of the converter's primary circuit. The grid controller performs grid control functions based on these inputs, outputting the virtual internal potential amplitude E of the converter. v And rotational angular velocity ω. In this embodiment, the network controller also employs P output from the current limiting controller. max / P min Q max / Q mi n and E max / E min As upper and lower limits, this restricts E v The output of ω and ω thus achieves the effect of current limiting.

[0040] In terms of overall framework, the above-mentioned current limiting control method is a supplement to the original basic network control strategy. Figure 2 The grid controller in the converter performs the original control functions and runs a basic grid-based control strategy. The current limiting controller collects the voltage and current status at the AC port of the converter and calculates the upper and lower limits of key control variables such as active power, reactive power, and virtual internal potential in real time according to the set parameters, and outputs them to the grid controller. The grid controller uses the received upper and lower limits of active power, reactive power, and virtual internal potential to limit its internal control functions, thereby ensuring that the AC output current of the converter always operates within the given maximum allowable overcurrent range.

[0041] Optionally, step S101, which calculates the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system based on the connecting transformer impedance, arm reactance, and virtual impedance used in the grid control of the converter in the flexible DC transmission system, includes the following steps: Step S201: Obtain the impedance of the connecting transformer of the converter in the flexible DC transmission system. .

[0042] Obtain the impedance of the transformer connected in the primary circuit of the converter in a flexible DC transmission system. This impedance is the actual impedance parameter between the AC side of the converter and the grid connection point.

[0043] Step S202: Obtain the arm reactance of the upper and lower arms in the phase unit of the converter. .

[0044] Simultaneously, based on the topology of the converter phase unit, the upper arm reactance and lower arm reactance in each phase unit are... After performing parallel equivalent processing, the parallel equivalent value of the bridge arm reactance can be obtained. Based on the working principle of the modular multilevel converter, the upper and lower arms of each phase unit exhibit virtual parallel characteristics in the impedance relationship between the modulation voltage and the AC grid connection point voltage. Therefore, the arm reactance is taken as the equivalent value after the upper and lower arm reactances are connected in parallel.

[0045] Step S203: Obtain the virtual impedance of the converter based on the virtual circuit used in the grid control process. ;in, For virtual resistance components, For virtual reactance components, It is the imaginary unit.

[0046] Based on the virtual circuit model used in the converter grid-type control strategy, the virtual impedance parameters of the grid-type control are obtained. ;in, For virtual resistance components, For virtual reactance components, The virtual impedance is an imaginary unit. It is a setting parameter for grid-connected control and can be selected based on the grid strength at the converter's grid connection point, as well as the virtual damping and grid power supply strength required by the system. It is used to simulate the stator impedance characteristics of a synchronous generator and improve the system's damping and power angle operating range.

[0047] Step S204: Calculate the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system using the connection transformer impedance, bridge arm reactance, and virtual impedance; wherein, the total loop impedance is calculated using the following formula: ; The resistive component of the total impedance of the circuit; This represents the reactance component of the total circuit impedance.

[0048] The connection transformer impedance obtained in step S201, the parallel equivalent bridge arm reactance, and the virtual impedance obtained in step S202 are superimposed in series to calculate and determine the total loop impedance between the AC grid connection point of the converter and the virtual internal potential of the grid control.

[0049] Specifically, the total circuit impedance consists of a resistive component R and a reactive component X. The calculation relationship is as follows: the total circuit impedance equals the sum of the connecting transformer impedance, 0.5 times the bridge arm reactance (i.e., the equivalent value of the upper and lower bridge arms in parallel), and the virtual impedance. The 0.5 coefficient before the arm reactance reflects the parallel equivalent relationship between the upper and lower arm reactances of the modular multilevel converter. The total loop impedance serves as the basic impedance parameter for subsequent current limiting calculations, used to establish a quantitative relationship between the virtual internal potential, grid connection point voltage, and output current.

[0050] The calculation of the total circuit impedance can refer to... Figure 3 Primary circuit topology diagram of the medium-voltage converter. Figure 3 middle, This indicates the impedance of the connecting transformer in a modular multilevel converter. This represents the arm reactance of a modular multilevel converter; The AC modulation voltage of the converter is generated by the modulation wave output by the grid controller, which drives the converter in the upper and lower bridge arms.

[0051] From the perspective of the AC power grid, the impedance between the modulation voltage of each phase unit of the converter and the AC grid connection point voltage is composed of the impedance of the aforementioned connecting transformer and the arm reactances of the upper and lower bridge arms. The arm reactances of the upper and lower bridge arms exhibit a virtual parallel relationship. Therefore... Figure 3 The bridge arm reactance consists of two values. The reactances are connected in parallel.

[0052] Figure 3 From The initial internal circuitry is a virtual circuit used in the converter grid control process, and therefore is drawn with dashed lines. , This refers to the virtual resistance component in network control. This refers to the virtual reactance component in network control. , The values ​​can be selected based on the grid conditions at the converter's grid connection point, according to the required virtual damping and grid power intensity. These values ​​are generally far from sufficient to limit the short-circuit current on the AC grid side of the converter to an acceptable range.

[0053] Figure 3 In , That is Figure 2 The current limiting controller needs to measure the instantaneous grid voltage at the converter's grid connection point and the instantaneous AC current output of the converter.

[0054] Optionally, step S102, which calculates the allowable output current range of the converter using the current output current actually output to the grid connection point and the maximum allowable overcurrent setting value according to the current limiting priority mode option instruction, includes the following steps: Step S301: Obtain the current AC current actually output from the converter to the grid connection point. And obtain the d-axis current projection component in the rotating Cartesian coordinate system after the current AC current undergoes Park transformation. and q-axis current projection components .

[0055] Collect the current AC current output from the converter to the AC grid connection point. The three-phase instantaneous values ​​are: (Instantaneous current of phase A) (Instantaneous current of phase B) (Instantaneous current of phase C); using the real-time phase angle of the virtual internal potential output by the grid-type control as an angular reference, based on the current AC current. The collected three-phase alternating current is subjected to a Parker transformation to convert the projected components of the alternating current in the stationary three-phase coordinate system to the rotating rectangular coordinate system, thereby obtaining the corresponding d-axis current projected components. and q-axis current projection components .

[0056] Specifically, the calculation is as follows: ;in, The real-time phase angle of the virtual internal potential output for network control.

[0057] Step S302: Obtain the maximum allowable overcurrent setting value of the converter. And obtain the rate limiting priority mode option command.

[0058] Obtain the pre-set maximum allowable overcurrent setting value of the converter. This setting value represents the maximum allowable current amplitude on the AC side of the converter. At the same time, the current limiting priority mode option instruction is obtained, and the priority mode strategy adopted by the current limiting control is determined by the current limiting priority mode option instruction. That is, the active power priority mode option instruction or the reactive power priority mode option instruction is selected according to the system operation requirements, which serves as the mode basis for subsequent current range calculation.

[0059] Step S303: When the converter's current limiting priority mode option command is set to active power priority mode, the range of active current allowed to be output by the converter is... and reactive current range The result is obtained through the following formula: , ;in, This is the lower limit of the d-axis current. This is the upper limit value of the d-axis current; This is the lower limit of the q-axis current. This is the upper limit of the q-axis current.

[0060] Based on the current limiting priority mode option instruction determined in step S302, calculate the range of active current allowed to be output by the converter. and reactive current range .

[0061] When the converter's current limiting priority mode option is set to active power priority mode, the maximum permissible overcurrent setting value is used. To constrain the amplitude, priority is given to ensuring the output range of the active current along the d-axis, and the active current range is calculated accordingly. and reactive current range Among them, the active current range corresponds to the upper and lower limits of the d-axis current, both of which are equal to the maximum allowable overcurrent setting value. The reactive current range corresponds to the upper and lower limits of the q-axis current, which are determined by the square root of the difference between the square of the maximum allowable overcurrent setting and the square of the current d-axis current projection component.

[0062] Step S304: When the converter's current limiting priority mode option command is set to reactive power priority mode, the range of active current allowed to be output by the converter is as follows: and reactive current range The result is obtained through the following formula: , .

[0063] When the converter's current limiting priority mode option is set to reactive power priority mode, the maximum allowable overcurrent setting value is used. To constrain the amplitude, priority is given to ensuring the output range of the q-axis reactive current, and the active current range is calculated accordingly. and reactive current range Among them, the reactive current range corresponds to the upper and lower limits of the q-axis current, both of which are equal to the maximum allowable overcurrent setting value. The active current range corresponds to the upper and lower limits of the d-axis current, which are determined by the square root of the difference between the square of the maximum allowable overcurrent setting and the square of the current q-axis current projection component.

[0064] Step S305: Determine the allowable output current range of the converter based on the active current range and the reactive current range.

[0065] The active current range calculated based on steps S303 and S304 and reactive current range Together, these constitute the current range that the converter can output, namely the current operating range of the active current range and the reactive current range defined by the upper and lower limits of the d-axis current and the upper and lower limits of the q-axis current, which serve as the current constraint basis for subsequent virtual internal potential limit calculation and power range calculation.

[0066] Optionally, the virtual internal potential limit of the converter's allowable output can be calculated using the current output voltage at the grid connection point, the total loop impedance, and the current range, including the following steps: Step S401: Obtain the current AC voltage at the grid connection point in the flexible DC transmission system. and obtain the current AC voltage. The d-axis projection component of the grid-connected point voltage in the rotating Cartesian coordinate system obtained after Park transformation. and the q-axis grid-connected point voltage projection component .

[0067] Collect the current AC voltage at the grid connection point of the converter in the flexible DC transmission system. The three-phase instantaneous values ​​are obtained; using the real-time phase angle of the virtual internal potential output by the grid-type control as an angular reference, a Parker transformation is performed on the acquired three-phase AC voltage to convert the voltage projection components in the stationary three-phase coordinate system to the rotating rectangular coordinate system, thus obtaining the corresponding d-axis grid-connected point voltage projection components. and the q-axis grid-connected point voltage projection component This serves as the basic voltage parameter for subsequent current limiting calculations.

[0068] Step S402: Utilize the d-axis grid connection point voltage projection component q-axis grid-connected point voltage projection components Total circuit impedance The range of active current allowed to be output by the converter and the range of reactive current allowed to be output by the converter. Calculate the virtual internal potential limit allowed for the converter's output; where the upper limit of the virtual internal potential is... and the lower limit of virtual internal potential The result is obtained through the following formula: ; ; In the formula, This is the lower limit of the d-axis current. This is the upper limit value of the d-axis current; This is the lower limit of the q-axis current. This is the upper limit of the q-axis current. The resistive component of the total impedance of the circuit; This is the reactance component of the total circuit impedance; It is the imaginary unit.

[0069] The specific calculation process for the upper limit of the virtual internal potential allowed for the converter output is as follows: First, calculate the upper limit of the d-axis component of the virtual internal potential. and the upper limit of the q-axis component The upper limit of the d-axis component Equal to the d-axis grid-connected point voltage projection component With the upper limit of the d-axis current In resistor Pressure drop and the upper limit of q-axis current In reactance Pressure drop The sum; upper limit of the q-axis component Equal to the q-axis grid-connected point voltage projection component With the upper limit of q-axis current In resistor Pressure drop Subtract the upper limit of the d-axis current In reactance Pressure drop The difference. Then, the upper limit of the d-axis component. and the upper limit of the q-axis component By synthesizing the sum of squares, the square root of the sum of squares is obtained to obtain the upper limit of the amplitude of the virtual internal potential. .

[0070] The specific calculation process for the lower limit of the virtual internal potential allowed for converter output is as follows: First, calculate the lower limit of the d-axis component of the virtual internal potential. and the lower limit of the q-axis component The lower limit of the d-axis component Equal to the d-axis grid-connected point voltage projection component With d-axis current lower limit In resistor Pressure drop and the lower limit of q-axis current In reactance Pressure drop The sum; lower limit of q-axis components Equal to the q-axis grid-connected point voltage projection component With q-axis current lower limit In resistor Pressure drop Subtract the lower limit of the d-axis current In reactance Pressure drop The difference. Then, the lower limit of the d-axis component. and the lower limit of the q-axis component By synthesizing the sum of squares, the square root of the sum of squares is taken to obtain the lower limit of the amplitude of the virtual internal potential. .

[0071] The aforementioned virtual internal potential upper limit and lower limit value Together, they constitute the virtual internal potential limit range, which is applied to the reactive power or voltage control link of the grid controller to limit the output of the virtual internal potential, thereby ensuring that the AC output current of the converter does not exceed the set maximum allowable value.

[0072] Optionally, obtain the current AC voltage. The d-axis projection component of the grid-connected point voltage in the rotating Cartesian coordinate system obtained after Park transformation. and the q-axis grid-connected point voltage projection component This includes the following steps: Step S501: Obtain the three-phase instantaneous voltage of the current AC voltage. , and ;in, The instantaneous voltage of phase A. This is the instantaneous voltage of phase B. This is the instantaneous voltage of phase C.

[0073] The three-phase instantaneous voltage values ​​of the current AC voltage at the grid connection point of the converter in the flexible DC transmission system are collected, and the instantaneous voltage of phase A is obtained respectively. Instantaneous voltage of phase B and the instantaneous voltage of phase C The three-phase instantaneous voltage is a time-domain sampled value in a stationary three-phase coordinate system, reflecting the real-time state of the grid voltage at the grid connection point.

[0074] Step S502: Real-time phase angle of the virtual internal potential output by the network control. As an angular reference value, after performing a Parker transformation on the three-phase instantaneous voltage, the d-axis and q-axis projection components of the grid-connected voltage corresponding to the current AC voltage in the rotating rectangular coordinate system are obtained; among them, the d-axis grid-connected voltage projection component... and the q-axis grid-connected point voltage projection component The result is obtained through the following formula: ; in, The real-time phase angle of the virtual internal potential output by the network control; t is time; This represents the rotational angular velocity corresponding to the virtual internal potential of the converter.

[0075] Using the real-time phase angle of the virtual internal potential output by the network control as the angular reference value, a Parker transformation is performed on the three-phase component voltages acquired in step S501 to transform the voltage projection components in the stationary three-phase abc coordinate system to the rotating rectangular dq coordinate system, thus obtaining the d-axis component voltage corresponding to the current AC voltage. and q-axis component voltage .

[0076] Specifically, the Parker transformation uses a 2 / 3 transformation matrix. The transformation relationships are as follows: the d-axis component equals the sum of the three-phase voltages multiplied by the cosine of their corresponding phase angles, then multiplied by a coefficient of two-thirds; the q-axis component equals the sum of the three-phase voltages multiplied by the sine of their corresponding phase angles, then multiplied by a coefficient of two-thirds. The angles in the transformation matrix represent the real-time phase angles of the virtual internal potential for grid control. , Let be the synchronous angular frequency of the three-phase component voltages. The Parker transformation described above achieves the conversion from a three-phase stationary coordinate system to a two-phase rotating coordinate system, facilitating subsequent current-limiting control calculations in the synchronous rotating coordinate system.

[0077] Optionally, the allowable output power range of the converter can be calculated using the current output voltage and current range at the grid connection point, including the following steps: Step S601: Obtain the d-axis grid-connected point voltage projection component based on the current output voltage of the grid connection point. and the q-axis grid-connected point voltage projection component .

[0078] Based on the current output voltage of the grid connection point, obtain the d-axis projection component of the grid connection point voltage in the rotated Cartesian coordinate system after Parker transformation. and the q-axis grid-connected point voltage projection component The projected voltage components of the grid connection point on the d and q axes reflect the amplitude and phase information of the grid voltage at the grid connection point in the synchronous rotating coordinate system, serving as the basic voltage parameters for subsequent power range calculations.

[0079] Step S602: Obtain the allowable active current range of the converter output based on the current range. and reactive current range ;in, This is the lower limit of the d-axis current. This is the upper limit value of the d-axis current; This is the lower limit of the q-axis current. This is the upper limit of the q-axis current.

[0080] The upper limit of the d-axis current allowed to be output by the converter is obtained from the aforementioned current range calculation step. d-axis current lower limit q-axis current upper limit and the lower limit of q-axis current The aforementioned current ranges collectively define the allowable operating range of the converter's AC output current, serving as the current constraints for calculating the power range.

[0081] Step S603: Utilize the d-axis grid connection point voltage projection component q-axis grid-connected point voltage projection components d-axis current upper limit and the upper limit of q-axis current Calculate the upper limit of the active power output allowed by the converter. and upper limit of reactive power Among them, the upper limit of active power and upper limit of reactive power The result is obtained through the following formula: .

[0082] Using the d-axis projection component of the grid connection point voltage q-axis grid-connected point voltage projection components and the upper limit of the d-axis current. and the upper limit of q-axis current Calculate the maximum active power and maximum reactive power that the converter is allowed to output under the current current limiting priority mode option command.

[0083] Specifically, the upper limit of active power equal to d-axis voltage With the upper limit of the d-axis current The product plus the q-axis voltage With the upper limit of q-axis current The product of these values ​​reflects the maximum active power that the converter is allowed to output under the current grid voltage conditions; the upper limit of reactive power... equal to q-axis voltage With the upper limit of the d-axis current The product minus the d-axis voltage With q-axis current upper limit The product of these values ​​reflects the maximum reactive power that the converter is allowed to output under the current current-limiting priority mode option command.

[0084] Step S604: Utilize the d-axis grid connection point voltage projection component q-axis grid-connected point voltage projection components d-axis current lower limit and the lower limit of q-axis current Calculate the lower limit of the active power that the converter can output. and the lower limit of reactive power Among them, the lower limit of active power and the lower limit of reactive power The result is obtained through the following formula: .

[0085] Using the d-axis projection component of the grid connection point voltage q-axis grid-connected point voltage projection components and the lower limit of the d-axis current. and q-axis current lower limit Calculate the minimum active power and minimum reactive power that the converter is allowed to output under the current current limiting priority mode option command.

[0086] Specifically, the lower limit of active power equal to d-axis voltage With d-axis current lower limit The product plus the q-axis voltage With q-axis current lower limit The product of these values ​​reflects the minimum active power that the converter is allowed to output under the current grid voltage conditions; the lower limit of reactive power. equal to q-axis voltage With d-axis current lower limit The product minus the d-axis voltage With q-axis current lower limit The product of these values ​​reflects the minimum reactive power that the converter is allowed to output under the current current-limiting priority mode option command.

[0087] Step S605: Based on the upper limit of active power and the lower limit of active power Determine the active power range of the converter And based on the upper limit of reactive power and the lower limit of reactive power Determine the reactive power range of the converter .

[0088] The upper limit of active power calculated based on step S603 The lower limit of active power calculated in step S604 Together they constitute the active power range of the converter. The upper limit of reactive power calculated based on step S603. The lower limit of reactive power calculated in step S604 Together they constitute the reactive power range of the converter. .

[0089] The calculated active power range is applied to the synchronous control link of the grid controller, and the reactive power range is applied to the reactive or voltage control link of the grid controller to limit the output power of the grid controller, thereby avoiding secondary overcurrent during the fault recovery phase and ensuring that the output current of the converter does not exceed the set maximum allowable overcurrent value under all operating conditions.

[0090] Optionally, the modulation voltage of the converter is calculated based on the virtual internal potential limit and power range, including the following steps: Step S701: Obtain the current AC current output from the converter to the grid connection point in the flexible DC transmission system. And obtain the d-axis current projection components of the three-phase components of the current alternating current in the rotating rectangular coordinate system. and q-axis current projection components .

[0091] Collect the current AC current output from the converter to the grid connection point in the flexible DC transmission system. The three-phase instantaneous values ​​are obtained; using the real-time phase angle of the virtual internal potential output by the grid-type control as an angular reference, the Parker transformation is performed on the collected three-phase AC current to transform the current projection components in the stationary three-phase coordinate system to the rotating rectangular coordinate system, thus obtaining the corresponding d-axis current projection components. and q-axis current projection components This serves as the current feedback parameter for subsequent modulation voltage calculations.

[0092] Step S702: Obtain the virtual impedance of the converter based on the virtual circuit used in the grid control process. ;in, For virtual resistance components, For virtual reactance components, It is the imaginary unit.

[0093] Based on the virtual circuit model used in the converter grid-type control strategy, the virtual impedance parameters of the grid-type control are obtained. ;in, For virtual resistance components, For virtual reactance components, The virtual impedance is an imaginary unit. It is an inherent tuning parameter of the network-type control and is used to establish a quantitative relationship between the virtual internal potential and the modulation voltage.

[0094] Step S703: Determine the internal potential limit of the converter during the grid construction control process using the virtual internal potential limit value, and determine the power limit of the converter during the grid construction control process using the power range.

[0095] Using the virtual internal potential limit value calculated above, the internal potential limiting range of the converter during grid construction control is determined, i.e., upper and lower limits are imposed on the amplitude of the virtual internal potential output by the grid construction controller. Simultaneously, using the power range calculated above, the power limiting range of the converter during grid construction control is determined, i.e., upper and lower limits are imposed on the active and reactive power reference values ​​output by the grid construction controller. Through this dual limiting mechanism, the output current of the converter is ensured to remain within the allowable range at all times.

[0096] Step S704: Determine the virtual internal potential of the converter based on internal potential limiting and power limiting. And utilize virtual internal potential and d-axis current projection components. q-axis current projection components Virtual impedance calculation of the d-axis modulation voltage projection component of the converter in a rotating rectangular coordinate system and q-axis modulated voltage projection components .

[0097] Based on the internal potential and power limiting determined in step S703, the output of the grid controller is limited to obtain the virtual internal potential of the actual output. Then, using this virtual internal potential and the d-axis current projection component obtained in step S701... and q-axis current projection components And the virtual impedance obtained in step S702, calculate the d-axis projection component and q-axis projection component of the converter's modulation voltage in the rotating rectangular coordinate system.

[0098] Specifically, the d-axis component of the modulation voltage Equal to virtual internal potential Subtract d-axis current In virtual resistance Pressure drop Subtract the q-axis current. In virtual reactance Pressure drop ;Right now q-axis component of the modulation voltage Equal to q-axis current In virtual resistance Pressure drop Add d-axis current In virtual reactance Pressure drop ;Right now The above calculations enabled the conversion from virtual internal potential to actual modulation voltage.

[0099] Step S705: Calculate the modulation voltage of the converter using the d-axis modulation voltage projection component and the q-axis modulation voltage projection component.

[0100] The d-axis and q-axis voltage projection components calculated in step S704 are subjected to an inverse Parker transformation, transforming the rotating rectangular dq coordinate system back to the stationary three-phase abc coordinate system, thus obtaining the three-phase instantaneous modulated voltage waveform of the converter. This modulated voltage waveform is used to drive the power modules of the converter to perform pulse width modulation, thereby controlling the actual output of the converter and achieving dynamic current limiting under all operating conditions.

[0101] Optionally, the d-axis modulated voltage projection component is calculated using the following formula: The q-axis modulated voltage projection component is calculated using the following formula: Based on this, step S705, which calculates the modulation voltage of the converter using the d-axis voltage projection component and the q-axis voltage projection component, includes the following steps: Step S801: After performing Parker inverse transformation on the d-axis voltage projection component and the q-axis voltage projection component, the three-phase component voltage of the modulation voltage is obtained.

[0102] Projection component of the modulated voltage along the d-axis in a rotating rectangular coordinate system and q-axis modulated voltage projection components Performing an inverse Parker transformation, the projected components of the modulation voltage in the dq rotating coordinate system are transformed back to the stationary three-phase abc coordinate system, yielding the three-phase components of the modulation voltage, namely the A-phase modulation voltage. B-phase modulation voltage and C-phase modulation voltage .

[0103] Specifically, the Parker inverse transform uses a 3 / 2 transform matrix form, and its transformation relationship is as follows: each phase component of the three-phase modulated voltage is composed of the sum of the d-axis voltage projection component multiplied by the cosine of the corresponding phase angle and the q-axis voltage projection component multiplied by the sine of the corresponding phase angle. The angles in the transform matrix represent the real-time phase angles of the virtual internal potential for grid control. , ω is the angular frequency of the three-phase component voltage.

[0104] Step S802: Determine the modulation voltage of the converter based on the three-phase component voltages; wherein, the modulation voltage is calculated using the following formula: ; In the above formula, Modulation voltage; , , These are the three-phase component voltages on phases A, B, and C of the modulation voltage, respectively; The projected voltage component is the d-axis projection component. , For virtual internal potential, and For virtual impedance, The d-axis current projection component. This refers to the q-axis current projection component; The q-axis projected voltage component. ; ω is the angular frequency of the three-phase component voltage.

[0105] The three-phase instantaneous modulated voltage waveform obtained by the above-mentioned d-axis and q-axis modulated voltage projection components after Parker inverse transformation is directly used to drive the power module of the converter to perform pulse width modulation, thereby realizing precise control of the converter output voltage and current, and ensuring that the AC output current of the converter does not exceed the preset maximum allowable overcurrent value under any operating condition.

[0106] The overall control logic block diagram of the current limiting control method for grid-type flexible DC transmission systems described above is as follows: Figure 4 As shown, it mainly includes voltage and current acquisition, current upper and lower limit calculation, converter circuit total impedance calculation, and first and second current limiting.

[0107] The voltage and current acquisition stage measures the actual grid voltage at the AC grid connection point and the actual AC current output from the converter to the grid connection point. Using the synchronous voltage phase angle adopted for real-time internal potential control of the grid as an angular reference value, the d-axis component of the actual grid voltage phasor at the grid connection point is calculated. and q-axis components And the d-axis component of the actual output current phasor of the converter. and q-axis components .

[0108] The current upper and lower limit calculation process is based on the maximum allowable overcurrent setting value. and the actual output current of the converter , According to the prescribed active power priority mode or reactive power priority mode, the upper and lower limits of the allowable output d-axis current of the converter are calculated in real time. , and q-axis current upper and lower limits , .

[0109] The total circuit impedance calculation is based on the actual primary circuit connection transformer impedance of the converter. Bridge arm reactor and the virtual impedance used in network control Calculate and determine the total loop impedance between the converter grid connection point and the virtual internal potential. ;in, For virtual resistance components, For virtual reactance components, The imaginary unit; The resistive component of the total circuit impedance; This is the reactance component of the total circuit impedance.

[0110] The first current limiting stage is based on the current actual voltage. , Allowable upper and lower limits of output current , , , Calculate the upper and lower limits of the virtual internal potential amplitude allowed in the grid control of the converter. , It then outputs its data to the network controller.

[0111] The second current-limiting stage is based on the current actual voltage. , and the upper and lower limits of the allowable output current. , , , Calculate the upper and lower limits of active power output allowed at the grid connection point. , upper and lower limits of reactive power , It then outputs its data to the network controller.

[0112] The control principle diagram of the first current limiting stage in the above process is as follows: Figure 5 As shown, according to the tuning options The value of represents the upper and lower limits of the AC current allowed to be output by the converter. , , , The calculations are performed according to either the active power priority mode or the reactive power priority mode.

[0113] The control principle diagram of the first current limiting stage in the above process is as follows: Figure 6 As shown, this control loop uses the current actual voltage. , Based on this, consider the converter operating at the maximum allowable output current. , or minimum current , At this time, these currents are in the total impedance of the converter circuit. The voltage drop across the voltage drop is used to calculate and determine the upper and lower limits of the virtual internal potential allowed for the grid control output. , .

[0114] The aforementioned current-limiting control method for grid-connected flexible DC transmission systems measures the actual voltage at the converter's grid connection point. Based on the converter's internal impedance characteristics and the manually set maximum allowable output current, it dynamically calculates the upper and lower limits of the virtual internal potential, as well as the upper and lower limits of active and reactive power, that allow the converter's grid-connected control output. These upper and lower limits are then applied to a basic grid-connected controller to limit the modulation wave of the converter by the grid-connected controller. This ensures that the AC output current does not exceed the set value under any operating condition, thereby achieving the goal of limiting the short-circuit current level of the converter during AC-side grid faults.

[0115] Compared with existing technical solutions, the beneficial effects of this technical solution include: (1) There is no explicit current inner loop, so there is no requirement for a concentrated filter capacitor at the AC grid connection point. It can also be used for modular multilevel converters.

[0116] (2) No need to detect AC grid faults, no need to switch control modes when there is a grid fault, no risk of misjudging or missing faults, and the fault current limiting process is smoother.

[0117] (3) The current limiting link, as part of the network control under normal operating conditions, always participates in the controller regulation. The current limiting function continues to play a role under any operating condition, and can realize dynamic current limiting under all operating conditions. The current limiting characteristics are more universal and universal.

[0118] (4) The current limiting function does not depend on the adjustment of the virtual impedance, nor does it require the virtual impedance value to meet specific conditions. The grid controller can set the virtual impedance to any constant or variable according to the virtual power intensity requirements of the converter, the oscillation damping of the grid, impedance matching and other requirements, which improves the flexibility of system configuration.

[0119] Corresponding to the above embodiments of the current limiting control method for grid-type flexible DC transmission systems, this invention also provides a current limiting control device for grid-type flexible DC transmission systems, such as... Figure 7 As shown, the device includes: Loop impedance calculation module 100: used to calculate the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system based on the connection transformer impedance, bridge arm reactance, and virtual impedance used in the grid control of the converter in the flexible DC transmission system. Current range calculation module 200: Used to calculate the current range that the converter can output according to the current output current and the maximum allowable overcurrent setting value actually output to the grid connection point of the converter, according to the current limiting priority mode option instruction. First current limiting calculation module 300: used to calculate the virtual internal potential limit value of the converter's allowable output using the current output voltage of the grid connection point, the total circuit impedance, and the current range; Second current limiting calculation module 400: used to calculate the power range that the converter can output using the current output voltage and current range of the grid connection point; The current limiting execution control module 500 is used to calculate the modulation voltage of the converter based on the virtual internal potential limit and power range, and to drive the modulation of the converter using the voltage waveform corresponding to the modulation voltage, so as to realize the dynamic current limiting of the flexible DC transmission system.

[0120] As can be seen from the current limiting control device for grid-type flexible DC transmission systems described above, this device measures the actual voltage at the converter grid connection point and dynamically calculates the maximum virtual internal potential and the maximum active and reactive power values ​​allowed for the grid control output of the converter based on the internal impedance characteristics of the converter. This limits the virtual internal potential output of the grid-type controller, thereby achieving the effect that the AC output current does not exceed the set value under any operating condition.

[0121] The current limiting control device for grid-type flexible DC transmission systems provided in this embodiment of the invention has the same implementation principle and technical effects as the aforementioned current limiting control method embodiment for grid-type flexible DC transmission systems. For the sake of brevity, any parts not mentioned in the device embodiment can be referred to the corresponding content in the aforementioned current limiting control method embodiment for grid-type flexible DC transmission systems.

[0122] This embodiment also provides an electronic device, the structural schematic diagram of which is shown below. Figure 8 As shown, the device includes a processor 101 and a memory 102; wherein, the memory 102 is used to store one or more computer instructions, which are executed by the processor to implement the steps of the current limiting control method for the grid-type flexible DC transmission system described above.

[0123] Figure 8 The electronic device shown also includes a bus 103 and a communication interface 104, with the processor 101, communication interface 104 and memory 102 connected via the bus 103.

[0124] The memory 102 may include high-speed random access memory (RAM) and may also include non-volatile memory, such as at least one disk storage device. The bus 103 may be an ISA bus, PCI bus, or EISA bus, etc. The bus can be divided into address bus, data bus, control bus, etc. For ease of representation, Figure 8 The symbol is represented by a single double-headed arrow, but this does not mean that there is only one bus or one type of bus.

[0125] The communication interface 104 is used to connect to at least one personnel terminal and other network units through a network interface, and to send encapsulated IPv4 packets or IPv4 packets to the personnel terminal through the network interface.

[0126] Processor 101 may be an integrated circuit chip with signal processing capabilities. In implementation, each step of the above method can be completed by the integrated logic circuitry in the hardware of processor 101 or by instructions in software form. The processor 101 can be a general-purpose processor, including a Central Processing Unit (CPU), a Network Processor (NP), etc.; it can also be a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field-Programmable Gate Array (FPGA), or other programmable logic devices, discrete gate or transistor logic devices, or discrete hardware components. It can implement or execute the methods, steps, and logic block diagrams disclosed in the embodiments of this disclosure. The general-purpose processor can be a microprocessor or any conventional processor. The steps of the methods disclosed in the embodiments of this disclosure can be directly manifested as execution by a hardware decoding processor, or execution by a combination of hardware and software modules in the decoding processor. The software module can reside in a mature storage medium in the art, such as random access memory, flash memory, read-only memory, programmable read-only memory, electrically erasable programmable memory, or registers. This storage medium is located in memory 102. The processor 101 reads the information in memory 102 and, in conjunction with its hardware, completes the steps of the method described in the foregoing embodiments.

[0127] This invention also provides a storage medium storing a computer program, which, when executed by a processor, performs the steps of the current limiting control method for a grid-type flexible DC transmission system described in the foregoing embodiments.

[0128] In the several embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, devices, and methods can be implemented in other ways. The system embodiments described above are merely illustrative. For example, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. Furthermore, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Additionally, the coupling or direct coupling or communication connection shown or discussed may be through some communication interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0129] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0130] In addition, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit.

[0131] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a processor-executable, non-volatile, computer-readable storage medium. Based on this understanding, the technical solution of this invention, essentially, or the part that contributes to the prior art, or a portion of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, electronic device, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0132] Finally, it should be noted that the above-described embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the technical scope disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A current-limiting control method for a grid-type flexible DC transmission system, characterized in that, The method includes: Based on the connection transformer impedance, arm reactance, and virtual impedance used in the grid control of the flexible DC transmission system, calculate the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system. Using the current output current and maximum allowable overcurrent setting value of the converter actually output to the grid connection point, the allowable output current range of the converter is calculated according to the current limiting priority mode option instruction; The virtual internal potential limit of the converter's allowable output is calculated using the current output voltage of the grid connection point, the total impedance of the circuit, and the current range. The allowable output power range of the converter is calculated using the current output voltage of the grid connection point and the current range. The modulation voltage of the converter is calculated based on the virtual internal potential limit and the power range, and the modulation of the converter is driven by the voltage waveform corresponding to the modulation voltage, so as to realize the dynamic current limiting of the flexible DC transmission system.

2. The current limiting control method for a grid-type flexible DC transmission system according to claim 1, characterized in that, The steps for calculating the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system, based on the connecting transformer impedance, arm reactance, and virtual impedance used in grid control of the converter in the flexible DC transmission system, include: Obtain the connection transformer impedance of the converter in a flexible DC transmission system ; Obtain the bridge arm reactance of the upper and lower bridge arms in the phase unit of the converter. ; The virtual impedance of the converter is obtained based on the virtual circuit used in the grid control process. ;in, For virtual resistance components, For virtual reactance components, The imaginary unit; The total loop impedance between the grid connection point and the virtual internal potential of the converter is calculated using the impedance of the connecting transformer, the arm reactance, and the virtual impedance; wherein the total loop impedance is calculated using the following formula: ; The resistive component of the total impedance of the circuit; This is the reactance component of the total impedance of the circuit.

3. The current limiting control method for a grid-type flexible DC transmission system according to claim 1, characterized in that, The step of calculating the allowable output current range of the converter using the current output current actually output to the grid connection point and the maximum allowable overcurrent setting value, according to the current limiting priority mode option instruction, includes: Obtain the current AC current actually output by the converter to the grid connection point. And obtain the d-axis current projection component in the rotating Cartesian coordinate system obtained after the current AC current undergoes Park transformation. and q-axis current projection components ; Obtain the maximum permissible overcurrent setting value of the converter. And obtain the rate limiting priority mode option command; When the current-limiting priority mode option command for the converter is set to the active power priority mode option, the range of active current that the converter is allowed to output is... and reactive current range The result is obtained through the following formula: , ;in, This is the lower limit of the d-axis current. This is the upper limit value of the d-axis current; This is the lower limit of the q-axis current. This is the upper limit of the q-axis current. When the current limiting priority mode option command for the converter is set to the reactive power priority mode option, the range of active current that the converter is allowed to output is... and reactive current range The result is obtained through the following formula: , ; The allowable output current range of the converter is determined based on the active current range and the reactive current range.

4. The current limiting control method for a grid-type flexible DC transmission system according to claim 1, characterized in that, The virtual internal potential limit of the converter's allowable output is calculated using the current output voltage of the grid connection point, the total circuit impedance, and the current range, including: Obtain the current AC voltage at the grid connection point in the flexible DC transmission system. and obtain the current AC voltage. The d-axis projection component of the grid-connected point voltage in the rotating Cartesian coordinate system obtained after Park transformation. and the q-axis grid-connected point voltage projection component ; Using the d-axis grid-connected point voltage projection component The q-axis grid-connected point voltage projection component The total impedance of the circuit The range of active current allowed to be output by the converter. and the range of reactive current allowed to be output by the converter. Calculate the virtual internal potential limit allowed for the converter's output; wherein, the upper limit of the virtual internal potential is... and the lower limit of virtual internal potential The result is obtained through the following formula: ； ； In the formula, This is the lower limit of the d-axis current. This is the upper limit value of the d-axis current; This is the lower limit of the q-axis current. This is the upper limit of the q-axis current. The resistive component of the total impedance of the circuit; The reactance component of the total impedance of the circuit; It is the imaginary unit.

5. The current limiting control method for a grid-type flexible DC transmission system according to claim 4, characterized in that, Obtain the current AC voltage The d-axis projection component of the grid-connected point voltage in the rotating Cartesian coordinate system obtained after Park transformation. and the q-axis grid-connected point voltage projection component ,include: Obtain the three-phase instantaneous voltage of the current AC voltage. , and ;in, The instantaneous voltage of phase A. This is the instantaneous voltage of phase B. This is the instantaneous voltage of phase C; Real-time phase angle of virtual internal potential output by network control As an angular reference value, after performing a Parker transformation on the three-phase instantaneous voltage, the d-axis grid-connected point voltage projection component and the q-axis grid-connected point voltage projection component corresponding to the current AC voltage in a rotating rectangular coordinate system are obtained; wherein, the d-axis grid-connected point voltage projection component and the q-axis grid-connected point voltage projection component The result is obtained through the following formula: ； in, The virtual internal potential output by the network controller is the real-time phase angle; t is time. The rotational angular velocity of the virtual internal potential corresponding to the converter.

6. The current limiting control method for a grid-type flexible DC transmission system according to claim 1, characterized in that, The allowable output power range of the converter is calculated using the current output voltage of the grid connection point and the current range, including: Obtain the d-axis grid connection point voltage projection component based on the current output voltage of the grid connection point. and the q-axis grid-connected point voltage projection component ; The active current range that the converter can output is obtained based on the current range. and reactive current range ;in, This is the lower limit of the d-axis current. This is the upper limit value of the d-axis current; This is the lower limit of the q-axis current. This is the upper limit of the q-axis current. Using the d-axis grid-connected point voltage projection component The q-axis grid-connected point voltage projection component The upper limit of the d-axis current and the upper limit value of the q-axis current Calculate the upper limit of the active power that the converter is allowed to output. and upper limit of reactive power The upper limit of the active power is mentioned above. and the upper limit of reactive power The result is obtained through the following formula: ； Using the d-axis grid-connected point voltage projection component The q-axis grid-connected point voltage projection component The lower limit value of the d-axis current and the lower limit value of the q-axis current Calculate the lower limit of the active power that the converter can output. and the lower limit of reactive power Wherein, the lower limit of active power and the lower limit of reactive power The result is obtained through the following formula: ； Based on the aforementioned upper limit of active power and the lower limit of active power Determine the active power range of the converter. And based on the upper limit of reactive power. and the lower limit of reactive power Determine the reactive power range of the converter. .

7. The current limiting control method for a grid-type flexible DC transmission system according to claim 1, characterized in that, The modulation voltage of the converter is calculated based on the virtual internal potential limit and the power range, including: Obtain the current AC current output from the converter to the grid connection point in the flexible DC transmission system. And obtain the d-axis current projection component of the three-phase components of the current alternating current in the rotating rectangular coordinate system. and q-axis current projection components ; The virtual impedance of the converter is obtained based on the virtual circuit used in the grid control process. ;in, For virtual resistance components, For virtual reactance components, The imaginary unit; The internal potential limit of the converter during grid construction control is determined using the virtual internal potential limit value, and the power limit of the converter during grid construction control is determined using the power range. The virtual internal potential of the converter is determined based on the internal potential limit and the power limit. And using the virtual internal potential and the d-axis current projection component The q-axis current projection component The virtual impedance calculation calculates the d-axis modulation voltage projection component of the converter in the rotating rectangular coordinate system. and q-axis modulated voltage projection components ; The modulation voltage of the converter is calculated using the d-axis modulation voltage projection component and the q-axis modulation voltage projection component.

8. The current limiting control method for a grid-type flexible DC transmission system according to claim 7, characterized in that, The d-axis modulated voltage projection component is calculated using the following formula: ; The q-axis modulated voltage projection component is calculated using the following formula: .

9. A current-limiting control device for a grid-type flexible DC transmission system, characterized in that, The device includes: Loop impedance calculation module: used to calculate the total loop impedance between the grid connection point and the virtual internal potential of the flexible DC transmission system based on the connection transformer impedance, bridge arm reactance, and virtual impedance used in the grid control of the converter in the flexible DC transmission system. Current range calculation module: used to calculate the current range that the converter can output according to the current output current actually output to the grid connection point and the maximum allowable overcurrent setting value, according to the current limiting priority mode option instruction; First current limiting calculation module: used to calculate the virtual internal potential limit value allowed to be output by the converter using the current output voltage of the grid connection point, the total impedance of the circuit and the current range; The second current limiting calculation module is used to calculate the power range that the converter can output using the current output voltage of the grid connection point and the current range. Current limiting execution control module: used to calculate the modulation voltage of the converter based on the virtual internal potential limit and the power range, and use the voltage waveform corresponding to the modulation voltage to drive the modulation of the converter, so as to realize the dynamic current limiting of the flexible DC transmission system.

10. An electronic device, characterized in that, The electronic device includes a processor and a memory, the memory storing computer-executable instructions that can be executed by the processor, the processor executing the computer-executable instructions to implement the steps of the current limiting control method for a grid-type flexible DC transmission system according to any one of claims 1 to 8.

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