Control method and system for improving transient supporting capacity of flexible loop closing device

Abstract

The invention discloses a control method and system for improving the transient support capability of a flexible loop closing device, and the method comprises the steps: enabling an active output P < e > of a self-synchronizing voltage source to output a phase theta through active / frequency control, enabling a reactive power Q < e > outputted by the self-synchronizing voltage source to output a reference voltage E < m > through reactive / voltage control, the reference voltage E < m > and the phase theta are combined to obtain an alternating-current voltage reference signal e < abc >, the voltage difference formed by the alternating-current voltage reference signal e < abc > and the actual output voltage v < abc > acts on virtual impedance to obtain an inductive current reference signal i < Ldq > * through Park change, the inductive current reference signal i < Ldq > * and an actual inductive current signal i < Ldq > are subjected to PI control to generate a modulation signal v < dq > *, and a driving signal of a switching tube is generated through SVPWM; the method has the advantages that the control strategy of the flexible loop closing device does not need to be switched in different operation modes, and transient stability of the system is facilitated.

Description

Technical Field

[0001] This invention relates to the field of grid-connected inverter control technology, specifically to a control method and system for improving the transient support capability of flexible loop-connected devices. Background Technology

[0002] With a high proportion of renewable energy being integrated into the distribution network, the pressure on the grid to absorb renewable energy is increasing. Flexible loop-connecting devices can connect different distribution network lines to perform power flow regulation, thereby improving the distribution network's ability to absorb renewable energy. However, the increased proportion of renewable energy also leads to a decrease in the strength of the distribution network, specifically manifested in transient fluctuations in the amplitude and frequency of distribution network voltage, and a decline in power quality under fault-based power transfer modes.

[0003] The control method of flexible loop-connected devices is the key to improving operational performance. The paper "Zhu Enze, Zhang Guoju, Ge Xuefeng, et al. Network operation and fault overcurrent suppression strategy of energy storage flexible interconnection equipment [J]. Electric Power Engineering Technology, 2025, 44(4): 33-41." points out that the shortcomings of grid-connected control are that it lacks frequency support capability and cannot provide support for interconnection feeders, which is the characteristic of flexible loop-connected devices. The grid-connected control converter can be equivalent to a voltage source and impedance in series, and the output control is completed by controlling the port voltage. Common grid-connected control methods include droop control, virtual oscillator and matching control methods. Nonlinear virtual oscillator and matching control are still in the research stage and the technology is not yet mature. Droop control has been demonstrated in some projects. It adopts constant DC voltage and constant active power control. The control strategy of flexible loop-connected devices needs to be switched in different operating modes, which is not conducive to the transient stability of the system. Summary of the Invention

[0004] The technical problem to be solved by the present invention is that the control method of the existing flexible loop closing device requires switching the control strategy of the flexible loop closing device under different operating modes, which is not conducive to the transient stability of the system.

[0005] This invention solves the above-mentioned technical problems through the following technical means: a control method for improving the transient support capability of a flexible loop closing device, wherein two flexible loop closing devices are arranged symmetrically back to back. When both ports of the two flexible loop closing devices are in grid-connected state, the system operation mode is normal mode. When one of the two flexible loop closing devices is in off-grid state, the system operation mode is divided into fault transfer mode. In normal mode and fault transfer mode, the system drives the switching transistors in the flexible loop closing device through the following strategy, the strategy including: Active power output of self-synchronizing voltage source P e Output phase after active power / frequency control θ Reactive power output from self-synchronous voltage source Q e The reference voltage is output after reactive power / voltage control.E m Reference voltage E m and phase θ Combined to obtain AC voltage reference signal e abc AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and the Parker transformation yields the inductor current reference signal. i Ldq * With respect to the actual inductor current signal i Ldq The modulation signal is generated after PI control. v dq * The drive signal for the switching transistor is generated through SVPWM.

[0006] This invention utilizes the active power output of a self-synchronizing voltage source. P e and output reactive power Q e Through a series of control operations, a modulation signal vdq* is generated, which is then used to generate drive signals for the inverter switching transistors via SVPWM. This allows for real-time drive control of the switching transistors based on the acquired signals. This enables power transfer to power-deficient loads without switching control strategies during fault transfer, and also provides transient inertia support for the distribution network in normal mode. This significantly improves the transient support capability of the flexible loop-closing device and the power supply reliability of the distribution substation. The overall solution does not require switching the control strategy of the flexible loop-closing device under different operating modes, which is beneficial to the transient stability of the system.

[0007] Furthermore, the expression for the active power / frequency control is:

[0008] in, Indicates the reference active power. This indicates the active power output of the self-synchronizing voltage source. This represents the active power / frequency droop factor. Indicates the power frequency angular frequency. This indicates the output angular frequency of the self-synchronizing voltage source. Represents the virtual moment of inertia. Indicates the current moment. This represents the phase angle output by the virtual synchronizer.

[0009] Furthermore, the expression for the reactive power / voltage control is:

[0010] in, This represents the voltage regulation integral coefficient. This represents the internal potential of the self-synchronizing voltage source. Indicates reference reactive power. This indicates the reactive power output of the self-synchronizing voltage source. This represents the reactive power / voltage droop factor. Indicates the rated voltage. This indicates the output voltage of the self-synchronizing voltage source.

[0011] Furthermore, the AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and its expression is:

[0012] in, Indicates the AC voltage reference signal. Indicates the actual output voltage. Indicates virtual resistance. Indicates inductor current. This represents a virtual inductance.

[0013] Furthermore, the expression for generating the modulated signal is:

[0014] in, This indicates the modulated signal component on the d-axis. This represents the proportionality coefficient of the current loop. Indicates the integral coefficient of the current loop. Represents a complex variable. This represents the component of the reference inductor current along the d-axis. This represents the component of the inductor current along the d-axis. This represents the d-axis component of the output voltage. Indicates the filter inductance. This represents the q-axis component of the inductor current. This indicates the q-axis component of the modulated signal. This represents the q-axis component of the reference inductor current. This represents the q-axis component of the inductor current. This represents the q-axis component of the output voltage. This represents the d-axis component of the inductor current.

[0015] The present invention also provides a control system for improving the transient support capability of a flexible loop closing device, including a topology building module and a drive control module; The topology building module is used for two flexible loop-connecting devices to be set up symmetrically back to back. When both ports of the two flexible loop-connecting devices are in grid-connected state, the system operation mode is normal mode. When one of the two flexible loop-connecting devices is in off-grid state, the system operation mode is divided into fault transfer mode. The drive control module is used to drive the switching transistors in the flexible loop-closing device in normal mode and fault transfer mode using the following strategies: active power output from a self-synchronizing voltage source. P e Output phase after active power / frequency control θ Reactive power output from self-synchronous voltage source Q e The reference voltage is output after reactive power / voltage control. E m Reference voltage E m and phase θ Combined to obtain AC voltage reference signal e abc AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and the Parker transformation yields the inductor current reference signal. i Ldq * With respect to the actual inductor current signal i Ldq The modulation signal is generated after PI control. v dq * The drive signal for the switching transistor is generated through SVPWM.

[0016] Furthermore, the expression for the active power / frequency control is:

[0017] in, Indicates the reference active power. This indicates the active power output of the self-synchronizing voltage source. This represents the active power / frequency droop factor. Indicates the power frequency angular frequency. This indicates the output angular frequency of the self-synchronizing voltage source. Represents the virtual moment of inertia. Indicates the current moment. This represents the phase angle output by the virtual synchronizer.

[0018] Furthermore, the expression for the reactive power / voltage control is:

[0019] in, This represents the voltage regulation integral coefficient. This represents the internal potential of the self-synchronizing voltage source. Indicates reference reactive power. This indicates the reactive power output of the self-synchronizing voltage source. This represents the reactive power / voltage droop factor. Indicates the rated voltage. This indicates the output voltage of the self-synchronizing voltage source.

[0020] Furthermore, the AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and its expression is:

[0021] in, Indicates the AC voltage reference signal. Indicates the actual output voltage. Indicates virtual resistance. Indicates inductor current. This represents a virtual inductance.

[0022] Furthermore, the expression for generating the modulated signal is:

[0023] in, This indicates the modulated signal component on the d-axis. This represents the proportionality coefficient of the current loop. Indicates the integral coefficient of the current loop. Represents a complex variable. This represents the component of the reference inductor current along the d-axis. This represents the component of the inductor current along the d-axis. This represents the d-axis component of the output voltage. Indicates the filter inductance. This represents the q-axis component of the inductor current. This indicates the q-axis component of the modulated signal. This represents the q-axis component of the reference inductor current. This represents the q-axis component of the inductor current. This represents the q-axis component of the output voltage. This represents the d-axis component of the inductor current.

[0024] The advantages of this invention are: (1) The present invention utilizes the active power output of a self-synchronizing voltage source Pe and output reactive power Q e Through a series of control operations, a modulation signal vdq* is generated, which is then used to generate drive signals for the inverter switching transistors via SVPWM. This allows for real-time drive control of the switching transistors based on the acquired signals. This enables power transfer to power-deficient loads without switching control strategies during fault transfer, and also provides transient inertia support for the distribution network in normal mode. This significantly improves the transient support capability of the flexible loop-closing device and the power supply reliability of the distribution substation. The overall solution does not require switching the control strategy of the flexible loop-closing device under different operating modes, which is beneficial to the transient stability of the system.

[0025] (2) The existing technology (Chinese Patent Publication No. CN107994620A) operates in the same way under normal and fault conditions of the power grid. However, the present invention operates in normal mode when the power grid is stable; and in fault mode when the power grid fails, the device operates in fault transfer mode, with one mode operating in grid-connected mode and the other in off-grid mode. Moreover, the present invention provides driving strategies for both modes. The present invention provides better frequency and voltage support for the power grid under fault conditions, which is more conducive to the transient stability of the system. Attached Figure Description

[0026] Figure 1 The topology of the flexible loop closing device is shown in the control method for improving the transient support capability of the flexible loop closing device disclosed in the embodiments of the present invention. Figure 2 This invention discloses a self-synchronization control structure for a flexible loop-closing device in a control method for improving the transient support capability of a flexible loop-closing device according to an embodiment of the present invention. Figure 3 The DC voltage waveform of the flexible loop closing device during the fault transfer transient process in a control method for improving the transient support capability of a flexible loop closing device disclosed in an embodiment of the present invention is shown below. Figure 4 The output power curve of VSC2 during the fault-to-supply transient process in a control method for improving the transient support capability of a flexible loop closing device disclosed in an embodiment of the present invention; Figure 5 The output voltage and current waveforms of VSC2 during the fault transfer transient process in a control method for improving the transient support capability of a flexible loop closing device disclosed in an embodiment of the present invention; Figure 6 The output voltage amplitude and frequency curves of VSC2 during the fault transfer transient process in a control method for improving the transient support capability of a flexible loop closing device disclosed in an embodiment of the present invention; Figure 7This is a schematic diagram of the output frequency and active power of the flexible loop closing device when responding to frequency regulation of the power grid system in a control method for improving the transient support capability of a flexible loop closing device disclosed in an embodiment of the present invention. Figure 8 The frequency curves of the flexible loop device in response to power grid frequency regulation under different rotational inertia coefficients are shown in the control method for improving the transient support capability of the flexible loop device disclosed in the embodiments of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. 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.

[0028] Example 1 like Figure 1 and Figure 2 As shown, Embodiment 1 of the present invention provides a control method for improving the transient support capability of a flexible loop-closing device. The flexible loop-closing device replaces the conventional tie switch and flexibly interconnects the regional distribution network. It is composed of two or more VSCs connected on the DC side. The topology of the flexible loop-closing device based on the back-to-back structure is shown in the attached figure. Figure 1 As shown. Based on the grid-connected and off-grid states of the flexible loop-closing device's ports, its operating modes are divided into normal operation mode (power flow balancing) and fault transfer mode. During steady-state operation, the flexible loop-closing device can balance the load rate of the distribution area, achieving efficient utilization of the distribution transformer capacity through power transfer and optimized power flow layout.

[0029] The overall scheme obtains the voltage and current signals in the output abc coordinate system through voltage and current sampling, and then performs Parker transformation to obtain the components in the dq coordinate system. The dq components are used for instantaneous power calculation on the one hand, and provide input for subsequent control links on the other hand.

[0030] Active power output of self-synchronizing voltage source calculated from instantaneous power. P e and output reactive power Q e The output phase is controlled by active power / frequency. θ The reference voltage is output after reactive power / voltage control. E m The amplitude and phase are combined to obtain the AC voltage reference signal. e abc Reference voltage and actual output voltage v abcThe resulting voltage difference, acting on the virtual impedance, can be transformed by Parker transformation to obtain the inductor current reference signal. i Ldq * With respect to the actual inductor current signal i Ldq The modulation signal is generated after PI control. v dq * The inverter switching transistors are driven by SVPWM. This allows for power transfer to power-deficient loads without switching control strategies during fault conditions, and also provides transient inertia support for the distribution network in normal mode, significantly improving the transient support capability of the flexible loop-closed device and the power supply reliability of the distribution substation.

[0031] The active power / frequency control incorporates the inertia characteristics of a synchronous generator based on droop control, as expressed below:

[0032] in, Indicates the reference active power. This indicates the active power output of the self-synchronizing voltage source. This represents the active power / frequency droop factor. Indicates the power frequency angular frequency. This indicates the output angular frequency of the self-synchronizing voltage source. Represents the virtual moment of inertia. Indicates the current moment. This represents the phase angle output by the virtual synchronizer.

[0033] The reactive power / voltage control references the primary voltage regulation control of the synchronous machine, and the expression is as follows:

[0034] in, This represents the voltage regulation integral coefficient. This represents the internal potential of the self-synchronizing voltage source. Indicates reference reactive power. This indicates the reactive power output of the self-synchronizing voltage source. This represents the reactive power / voltage droop factor. Indicates the rated voltage. This indicates the output voltage of the self-synchronizing voltage source.

[0035] The virtual impedance control references the stator electrical characteristics of the synchronous generator, and its expression is as follows:

[0036] in, Indicates the AC voltage reference signal. Indicates the actual output voltage. Indicates virtual resistance. Indicates inductor current. This represents a virtual inductance.

[0037] The generated modulation signal v dq * This is mainly achieved through current loop control. The current loop control applies PI control to the difference between the inductor current reference value and the actual value, and adds the dq-axis components of the output voltage. It also considers a decoupling control method for the dq components. The expression is as follows:

[0038] in, This indicates the modulated signal component on the d-axis. This represents the proportionality coefficient of the current loop. Indicates the integral coefficient of the current loop. Represents a complex variable. This represents the component of the reference inductor current along the d-axis. This represents the component of the inductor current along the d-axis. This represents the d-axis component of the output voltage. Indicates the filter inductance. This represents the q-axis component of the inductor current. This indicates the q-axis component of the modulated signal. This represents the q-axis component of the reference inductor current. This represents the q-axis component of the inductor current. This represents the q-axis component of the output voltage. This represents the d-axis component of the inductor current.

[0039] The main circuit setup in this embodiment is shown in the attached diagram. Figure 1 As shown, attached Figure 2 Table 1 shows the block diagram of the self-synchronization control of the flexible loop closing device and the main parameters of the simulation model of the flexible loop closing device.

[0040] Table 1 Main parameters of the flexible loop closing device

[0041] The following simulation verification process demonstrates the transient support capability under the fault-to-supply operation mode: The first AC load is S L1 =200+j100KVA, the second AC load is S L2 =400 + j200KVA. Let... tAt 4 seconds, transformer T2 on the VSC2 side experiences a fault and exits operation, with VSC1 taking over DC voltage control. Considering the characteristics of transient response, the power command decision issuance and execution require time, during which the Flexible Interconnect Device (FID) continues to operate according to the previous command. Considering a delay of 1 second, a comparison of the FID transient response under different control methods is shown in the appendix. Figures 3-6 As shown.

[0042] From the appendix Figure 3 It can be seen that the DC voltage rises when there is no energy demand on the AC side, corresponding to the attached... Figure 5 The phenomenon where the current waveform is zero for a 10ms time period. t During the 4-5 second period, there is a deviation between the power dispatch command and the load demand. The active / frequency droop control of droop control and self-synchronizing voltage source control will cause frequency deviation. After the load transfer command arrives, it will return to the rated value of 50Hz. Depending on the load power demand, the power output of the traditional control method drops significantly, the voltage waveform becomes distorted, and the current is zero during the 10ms strategy switching process. (See attached...) Figure 6 As shown, under traditional control methods, voltage amplitude drops are severe, with both amplitude and frequency drops exceeding the response under self-synchronizing voltage source control. The control method employed in this invention exhibits a relatively fast transient response during mode switching, effectively improving the power supply reliability of the distribution network.

[0043] The simulation process for participating in power grid system frequency regulation under normal operating conditions is as follows: Using a synchronous generator as the synchronous grid, frequency changes are simulated by load switching, and the frequency response of traditional control and self-synchronizing voltage source control under the same operating conditions is compared. The FID (Functional Identification Device) using self-synchronizing voltage source control has the capability to participate in primary grid regulation. The simulation condition is designed as follows: the AC grid at both ends consists of a synchronous generator, and the second AC load is located on the side... t =3s to put in a 300kW active load and in t Exit at 4s to simulate frequency fluctuations.

[0044] From the appendix Figure 7 It can be seen that, under the same operating conditions causing frequency fluctuations, the self-synchronizing voltage source controlled FID reduces the depth of instantaneous frequency fluctuations and the degree of deviation from the rated value in steady state by increasing active power generation, thus enabling it to participate in frequency regulation of the distribution network interconnection system and providing transient support. (See attached...) Figure 8 As shown, by appropriately adjusting the virtual moment of inertia parameters, the rated frequency deviation and frequency change rate can be reduced, thus enhancing the effect of the moment of inertia in frequency modulation. The adjustable virtual moment of inertia participating in frequency modulation during dynamic processes is a manifestation of the inertia of flexible interconnected devices. The sources of inertia power for flexible loop-connected devices include the power supply connected to the DC side and the AC systems at both ends.

[0045] Through the above technical solutions, when a power outage occurs in the feeder connected to the flexible loop device, the power supply can be immediately and automatically switched to the non-power outage line. The flexible loop device automatically provides voltage and frequency support for the load of the faulty line. In normal operation mode, it can also participate in the frequency regulation of the distribution network system, thereby improving the stability and reliability of the simulated power supply system.

[0046] Example 2 Based on Embodiment 1, Embodiment 2 of the present invention also provides a control system for improving the transient support capability of a flexible loop closing device, including a topology building module box drive control module; The topology building module is used for two flexible loop-connecting devices to be set up symmetrically back to back. When both ports of the two flexible loop-connecting devices are in grid-connected state, the system operation mode is normal mode. When one of the two flexible loop-connecting devices is in off-grid state, the system operation mode is divided into fault transfer mode. The drive control module is used to drive the switching transistors in the flexible loop-closing device in normal mode and fault transfer mode using the following strategies: active power output from a self-synchronizing voltage source. P e Output phase after active power / frequency control θ Reactive power output from self-synchronous voltage source Q e The reference voltage is output after reactive power / voltage control. E m Reference voltage E m and phase θ Combined to obtain AC voltage reference signal e abc AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and the Parker transformation yields the inductor current reference signal. i Ldq * With respect to the actual inductor current signal i Ldq The modulation signal is generated after PI control. v dq * The drive signal for the switching transistor is generated through SVPWM.

[0047] Specifically, the expression for the active power / frequency control is:

[0048] in, Indicates the reference active power. This indicates the active power output of the self-synchronizing voltage source. This represents the active power / frequency droop factor. Indicates the power frequency angular frequency. This indicates the output angular frequency of the self-synchronizing voltage source. Represents the virtual moment of inertia. Indicates the current moment. This represents the phase angle output by the virtual synchronizer.

[0049] Specifically, the expression for the reactive power / voltage control is:

[0050] in, This represents the voltage regulation integral coefficient. This represents the internal potential of the self-synchronizing voltage source. Indicates reference reactive power. This indicates the reactive power output of the self-synchronizing voltage source. This represents the reactive power / voltage droop factor. Indicates the rated voltage. This indicates the output voltage of the self-synchronizing voltage source.

[0051] Specifically, the AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and its expression is:

[0052] in, Indicates the AC voltage reference signal. Indicates the actual output voltage. Indicates virtual resistance. Indicates inductor current. This represents a virtual inductance.

[0053] Specifically, the expression for generating the modulated signal is:

[0054] in, This indicates the modulated signal component on the d-axis. This represents the proportionality coefficient of the current loop. Indicates the integral coefficient of the current loop. Represents a complex variable. This represents the component of the reference inductor current along the d-axis. This represents the component of the inductor current along the d-axis. This represents the d-axis component of the output voltage. Indicates the filter inductance. This represents the q-axis component of the inductor current. This indicates the q-axis component of the modulated signal. This represents the q-axis component of the reference inductor current. This represents the q-axis component of the inductor current. This represents the q-axis component of the output voltage. This represents the d-axis component of the inductor current.

[0055] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications 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.

Claims

1. A control method for improving the transient support capability of a flexible loop-closing device, characterized in that, Two flexible loop closing devices are arranged symmetrically back-to-back. When both ports of the flexible loop closing devices are in grid-connected state, the system operates in normal mode. When one of the two flexible loop closing devices is in off-grid state, the system operates in fault transfer mode. In normal mode and fault transfer mode, the system drives the switching transistors in the flexible loop closing devices through the following strategies, which include: Active power output of self-synchronizing voltage source P e Output phase after active power / frequency control θ Reactive power output from self-synchronous voltage source Q e The reference voltage is output after reactive power / voltage control. E m Reference voltage E m and phase θ Combined to obtain AC voltage reference signal e abc AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and the Parker transformation yields the inductor current reference signal. i Ldq * With respect to the actual inductor current signal i Ldq The modulation signal is generated after PI control. v dq * The drive signal for the switching transistor is generated through SVPWM.

2. The control method for improving the transient support capability of a flexible loop-closing device according to claim 1, characterized in that, The expression for the active power / frequency control is: in, Indicates the reference active power. This indicates the active power output of the self-synchronizing voltage source. This represents the active power / frequency droop factor. Indicates the power frequency angular frequency. This indicates the output angular frequency of the self-synchronizing voltage source. Represents the virtual moment of inertia. Indicates the current moment. This represents the phase angle output by the virtual synchronizer.

3. The control method for improving the transient support capability of a flexible loop-closing device according to claim 1, characterized in that, The expression for the reactive power / voltage control is: in, This represents the voltage regulation integral coefficient. This represents the internal potential of the self-synchronizing voltage source. Indicates reference reactive power. This indicates the reactive power output of the self-synchronizing voltage source. This represents the reactive power / voltage droop factor. Indicates the rated voltage. This indicates the output voltage of the self-synchronizing voltage source.

4. The control method for improving the transient support capability of a flexible loop-closing device according to claim 1, characterized in that, The AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and its expression is: in, Indicates the AC voltage reference signal. Indicates the actual output voltage. Indicates virtual resistance. Indicates inductor current. This represents a virtual inductance.

5. The control method for improving the transient support capability of a flexible loop-closing device according to claim 1, characterized in that, The expression for generating the modulated signal is: in, This indicates the modulated signal component on the d-axis. This represents the proportionality coefficient of the current loop. Indicates the integral coefficient of the current loop. Represents a complex variable. This represents the component of the reference inductor current along the d-axis. This represents the component of the inductor current along the d-axis. This represents the d-axis component of the output voltage. Indicates the filter inductance. This represents the q-axis component of the inductor current. This indicates the q-axis component of the modulated signal. This represents the q-axis component of the reference inductor current. This represents the q-axis component of the inductor current. This represents the q-axis component of the output voltage. This represents the d-axis component of the inductor current.

6. A control system for improving the transient support capability of a flexible loop-closing device, characterized in that, Includes topology building module and driver control module; The topology building module is used for two flexible loop-connecting devices to be set up symmetrically back to back. When both ports of the two flexible loop-connecting devices are in grid-connected state, the system operation mode is normal mode. When one of the two flexible loop-connecting devices is in off-grid state, the system operation mode is divided into fault transfer mode. The drive control module is used to drive the switching transistors in the flexible loop-closing device in normal mode and fault transfer mode using the following strategies: active power output from a self-synchronizing voltage source. P e Output phase after active power / frequency control θ Reactive power output from self-synchronous voltage source Q e The reference voltage is output after reactive power / voltage control. E m Reference voltage E m and phase θ Combined to obtain AC voltage reference signal e abc AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and the Parker transformation yields the inductor current reference signal. i Ldq * With respect to the actual inductor current signal i Ldq The modulation signal is generated after PI control. v dq * The drive signal for the switching transistor is generated through SVPWM.

7. The control system for improving the transient support capability of a flexible loop-closing device according to claim 6, characterized in that, The expression for the active power / frequency control is: in, Indicates the reference active power. This indicates the active power output of the self-synchronizing voltage source. This represents the active power / frequency droop factor. Indicates the power frequency angular frequency. This indicates the output angular frequency of the self-synchronizing voltage source. Represents the virtual moment of inertia. Indicates the current moment. This represents the phase angle output by the virtual synchronizer.

8. The control system for improving the transient support capability of a flexible loop-closing device according to claim 6, characterized in that, The expression for the reactive power / voltage control is: in, This represents the voltage regulation integral coefficient. This represents the internal potential of the self-synchronizing voltage source. Indicates reference reactive power. This indicates the reactive power output of the self-synchronizing voltage source. This represents the reactive power / voltage droop factor. Indicates the rated voltage. This indicates the output voltage of the self-synchronizing voltage source.

9. A control system for improving the transient support capability of a flexible loop-closing device according to claim 6, characterized in that, The AC voltage reference signal e abc With actual output voltage v abc The resulting voltage difference acts on the virtual impedance, and its expression is: in, Indicates the AC voltage reference signal. Indicates the actual output voltage. Indicates virtual resistance. Indicates inductor current. This represents a virtual inductance.

10. A control system for improving the transient support capability of a flexible loop-closing device according to claim 6, characterized in that, The expression for generating the modulated signal is: in, This indicates the modulated signal component on the d-axis. This represents the proportionality coefficient of the current loop. Indicates the integral coefficient of the current loop. Represents a complex variable. This represents the component of the reference inductor current along the d-axis. This represents the component of the inductor current along the d-axis. This represents the d-axis component of the output voltage. Indicates the filter inductance. This represents the q-axis component of the inductor current. This indicates the q-axis component of the modulated signal. This represents the q-axis component of the reference inductor current. This represents the q-axis component of the inductor current. This represents the q-axis component of the output voltage. This represents the d-axis component of the inductor current.

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