DC power transmission network construction type control method and system

Abstract

The invention relates to a direct-current power transmission network construction type control method and system, and belongs to the technical field of direct-current power transmission. The outer ring d axis of the VSC is an AC tie line voltage amplitude control link, and the outer ring q axis is an AC tie line active power control link; the voltage control instruction corresponding to the VSC inner ring q-axis is obtained according to the control quantity output by the VSC outer ring q-axis, and the voltage control instruction corresponding to the inner ring d-axis is obtained according to the amplitude of the vector difference between the voltage control instruction corresponding to the VSC inner ring q-axis and the control quantity output by the VSC outer ring d-axis; performing coordinate rotation transformation on the voltage control instruction corresponding to the inner ring q axis of the VSC and the voltage control instruction corresponding to the inner ring d axis to obtain a voltage control instruction of the network construction type VSC; in the direct current power transmission system, if the connection structure of a sending end converter station is a two-port network structure, the reference angle of coordinate rotation transformation is obtained according to a virtual rotor motion equation; and if the connection structure of the sending end converter station is a three-port network structure, the reference angle is obtained according to a phase-locked loop.

Description

Technical Field

[0001] The present invention relates to a grid-forming control method and system for DC power transmission, belonging to the technical field of DC power transmission. Background Art

[0002] China will vigorously build a new power system with new energy as the main body to facilitate the transformation of China's energy structure. Due to the reverse distribution of new energy consumption and supply in China, UHVDC power transmission technology remains the most economical way for cross-regional energy transmission and consumption in the future power system.

[0003] Although the vision of building a new power system is promising, it poses higher requirements for HVDC power transmission. For example, the randomness and volatility of wind power and photovoltaic power generation result in the uncertainty of power output on the power source side, which is likely to cause the operation of tap changers and the switching of filter circuit breakers, etc., affecting the safe and reliable operation of converter station equipment; the system inertia provided by wind turbines is much smaller than that of thermal power units. In the new power system, once there is an active power disturbance, the fluctuation range of frequency will expand and the speed will increase, and the stability of AC / DC systems will deteriorate; new energy is difficult to provide reactive power support to the system, and new energy is mainly connected to low-voltage power grids. After large-scale connection, the system voltage regulation ability decreases significantly, further affecting the operation performance of the DC power transmission system.

[0004] Chinese Patent Application Publication No. CN118589472A discloses a control method and system for a grid-forming SVG. This method realizes the control of the grid-forming SVG by using the amplitude-phase control principle, making its control characteristics match those of the LCC converter, reducing the harmonic oscillation generated between the SVG and the LCC converter, and ensuring the stability of the island operation of the DC power transmission system. Among them, this control method uses a phase-locked loop for phase synchronization. However, in a weak grid operation scenario such as a two-port network (the network structure corresponding to the sending converter station being only connected to a new energy power station or only connected to the AC power grid), using this phase-locked loop synchronous control method is prone to phase-locked loop instability, which may lead to the instability of grid voltage and current, and further affect the stability and security of the power system. Summary of the Invention

[0005] The purpose of the present invention is to provide a grid-forming control method and system for DC power transmission to solve the problem that in a weak grid operation scenario such as a two-port network, the existing control method for grid-forming SVG using a phase-locked loop for synchronous control is prone to phase-locked loop instability, which may lead to the instability of grid voltage and current, and further affect the stability and security of the DC power transmission system.

[0006] The present invention provides a DC power transmission grid-forming control method to solve the above technical problems. This method includes an outer-loop control and an inner-loop control; the outer-loop d-axis of the VSC is an AC tie-line voltage amplitude control link, and the outer-loop q-axis is an AC tie-line active power control link; the voltage control command corresponding to the inner-loop q-axis of the VSC is obtained according to the control quantity output by the outer-loop q-axis of the VSC, and the voltage control command corresponding to the inner-loop d-axis is obtained according to the amplitude of the vector difference between the voltage control command corresponding to the inner-loop q-axis of the VSC and the control quantity output by the outer-loop d-axis of the VSC; the voltage control commands corresponding to the inner-loop q-axis of the VSC and the inner-loop d-axis are subjected to a coordinate rotation transformation to obtain the voltage control command of the grid-forming VSC.

[0007] In a DC power transmission system, if the connection structure of the sending-end converter station is a two-port network structure, the reference angle of the coordinate rotation transformation is obtained according to the virtual rotor motion equation; if the connection structure of the sending-end converter station is a three-port network structure, the reference angle of the coordinate rotation transformation is obtained according to a phase-locked loop.

[0008] Further, the two-port network structure is the network structure corresponding to the sending-end converter station being only connected to a new energy power station or only connected to an AC power grid; the three-port network structure is the network structure corresponding to the sending-end converter station being connected to a new energy power station and also connected to an AC power grid at the same time.

[0009] Further, the method for obtaining the voltage control command corresponding to the inner-loop q-axis of the VSC according to the control quantity output by the outer-loop q-axis of the VSC includes: adding the control quantity output by the outer-loop q-axis of the VSC to the control quantity corresponding to the output of the VSC reactive power output balance control to obtain the voltage control command corresponding to the inner-loop q-axis of the VSC.

[0010] In a DC power transmission system, if the connection structure of the sending-end converter station is a two-port network structure, the control method of the AC tie-line active power control link includes: setting the output of the AC tie-line active power closed-loop control link to zero so that the control quantity output by the outer-loop q-axis of the VSC is zero.

[0011] If the connection structure of the sending-end converter station is a three-port network structure, the control method of the AC tie-line active power control link includes: performing a closed-loop PI control through the difference between the actual value of the AC tie-line active power and the set active power reference value to control the actual value of the AC tie-line active power to be near the set active power reference value.

[0012] Further, the method for obtaining the reference angle of the coordinate rotation transformation according to the virtual rotor motion equation includes: obtaining the reference angle of the coordinate rotation transformation according to the virtual rotor motion equation determined by the reference angle, the difference between the AC tie-line active power reference value and the actual value of the AC tie-line active power, the inertia time constant, and the damping coefficient.

[0013] Further, in the DC power transmission system, if the two-port network structure is such that the sending-end converter station is only connected to the new energy power station, the reference value of the active power of the AC tie line is set to be equal to the actual value of the active power of the AC tie line.

[0014] Further, the methods for setting the output of the closed-loop control link of the active power of the AC tie line to zero include: disabling the active power control link of the outer-loop q-axis of the AC tie line so that the output of the active power control link of the AC tie line is zero.

[0015] Further, the control method for obtaining the voltage control command corresponding to the inner-loop d-axis according to the magnitude of the vector difference between the voltage control command corresponding to the inner-loop q-axis of the VSC and the control quantity output by the outer-loop d-axis of the VSC includes:

[0016] Taking the positive square root of the difference between the square of the voltage control command corresponding to the inner-loop q-axis of the VSC and the square of the control quantity output by the outer-loop d-axis of the VSC to obtain the magnitude of the vector difference between the voltage control command corresponding to the inner-loop q-axis of the VSC and the control quantity output by the outer-loop d-axis of the VSC, and adding the control quantity corresponding to the output of the sub-module voltage balance control between VSCs to this magnitude to obtain the voltage control command corresponding to the inner-loop d-axis.

[0017] Further, the control method of the AC tie line voltage amplitude control link includes:

[0018] Performing closed-loop PI control according to the difference between the reference value of the AC tie line voltage amplitude control and the actual value of the AC tie line voltage amplitude to control the actual value of the AC tie line voltage amplitude near this reference value; this reference value is obtained according to the difference between the per-unit value of the AC tie line voltage amplitude and the control quantity corresponding to the output of the AC tie line reactive power control.

[0019] Further, the control method of the sub-module voltage balance control between VSCs includes:

[0020] Taking the actual value of the average voltage of the VSC sub-modules on the Y side of the DC power transmission system as the reference value of the sub-module voltage balance control between VSCs, taking the actual value of the average voltage of the VSC sub-modules on the D side of the DC power transmission system as the feedback value of the sub-module voltage balance control between VSCs, and performing closed-loop control according to the difference between the reference value and the feedback value of the sub-module voltage balance control between VSCs to obtain the control quantity corresponding to the output of the sub-module voltage balance control between VSCs.

[0021] Further, the control method of the VSC reactive power output balance control includes:

[0022] Taking the actual value of the VSC reactive power on the Y side of the DC transmission system as the reference value for the VSC reactive power output balance control, and taking the actual value of the VSC reactive power on the D side of the DC transmission system as the feedback value for the VSC reactive power output balance control, and performing closed-loop control according to the difference between the reference value and the feedback value of the VSC reactive power output balance control to obtain the control quantity corresponding to the output of the VSC reactive power output balance control.

[0023] Beneficial effects: The present invention provides a new grid-forming control method for DC transmission. Considering the operation scenario of a weak power grid such as a two-port network, it is easy for the phase-locked loop to become unstable when using this phase-locked loop synchronization control method. Therefore, in a DC transmission system, if the connection structure of the sending converter station is a two-port network structure, the reference angle (the reference angle is also called the reference phase) of the coordinate rotation transformation performed on the voltage control command corresponding to the q-axis of the VSC inner loop and the voltage control command corresponding to the d-axis of the VSC inner loop is obtained according to the virtual rotor motion equation. Using the virtual rotor motion equation for phase synchronization introduces inertia support, damping and other synchronous generator control characteristics into the DC transmission system, avoiding the situation that the phase-locked loop is prone to instability under extreme conditions in the operation scenario of a weak power grid such as a two-port network, and improving the safety and stability of the system; when the connection structure of the sending converter station is a three-port network structure, compared with the phase synchronization method using the virtual rotor motion equation, the parameters are easier to tune when using the phase synchronization method of the phase-locked loop. In addition, compared with the two-port network structure, the operation scenario under the three-port network belongs to a strong power grid operation scenario, and the phase synchronization method of the phase-locked loop is not prone to phase-locked loop instability when the three-port network is used; therefore, in a DC transmission system, if the connection structure of the sending converter station is a three-port network structure, the reference angle of the coordinate rotation transformation performed on the voltage control command corresponding to the q-axis of the VSC inner loop and the voltage control command corresponding to the d-axis of the VSC inner loop is obtained according to the phase-locked loop. This method solves the problem that in the operation scenario of a weak power grid such as a two-port network, the existing grid-forming VSC control method using this phase-locked loop for synchronization control is prone to phase-locked loop instability, which may lead to the instability of the grid voltage and current, and further affect the stability and safety of the DC transmission system, and improves the stability and safety of the DC transmission system.

[0024] The present invention also provides a grid-forming control system for DC transmission, including a processor, and the processor is used to execute computer program instructions to implement the steps of the above-mentioned grid-forming control method for DC transmission.

[0025] This grid-forming control system for DC transmission can achieve the same beneficial effects as the above-mentioned grid-forming control method for DC transmission. Description of the Drawings

[0026] Figure 1It is the structural schematic diagram of the DC power transmission system in the embodiment of the DC power transmission grid-forming control method of the present invention;

[0027] Figure 2 It is the topological structure diagram of the VSC converter in the embodiment of the DC power transmission grid-forming control method of the present invention;

[0028] Figure 3 It is the example diagram of the two-port network structure corresponding to the connection of the sending-end converter station only to the new energy power station in the embodiment of the DC power transmission grid-forming control method of the present invention;

[0029] Figure 4 It is the example diagram of the two-port network structure corresponding to the connection of the sending-end converter station only to the AC power grid in the embodiment of the DC power transmission grid-forming control method of the present invention;

[0030] Figure 5 It is the example diagram of the three-port network structure in the embodiment of the DC power transmission grid-forming control method of the present invention;

[0031] Figure 6 It is the control function schematic diagram corresponding to the connection structure of the sending-end converter station being a two-port network structure in the embodiment of the DC power transmission grid-forming control method of the present invention;

[0032] Figure 7 It is the control function schematic diagram corresponding to the connection structure of the sending-end converter station being a three-port network structure in the embodiment of the DC power transmission grid-forming control method of the present invention;

[0033] Figure 8 It is the control principle block diagram of the sub-module voltage balance control between VSCs in the embodiment of the DC power transmission grid-forming control method of the present invention;

[0034] Figure 9 It is the control principle block diagram of the VSC reactive power output balance control in the embodiment of the DC power transmission grid-forming control method of the present invention. Specific Embodiments

[0035] The following further describes the specific embodiments of the present invention with reference to the accompanying drawings.

[0036] Embodiment of the DC Power Transmission Grid-Forming Control Method

[0037] This embodiment provides a technical solution for a DC power transmission network-forming control method. In the DC power transmission system, if the connection structure of the sending-end converter station is a two-port network structure (when the operating scenario under the two-port network belongs to a weak grid operating scenario), the reference angle for the coordinate rotation transformation of the voltage control command corresponding to the q-axis of the VSC inner loop and the voltage control command corresponding to the d-axis of the inner loop is obtained according to the virtual rotor motion equation; if the connection structure of the sending-end converter station is a three-port network structure (when the operating scenario under the three-port network belongs to a strong grid operating scenario), the reference angle for the coordinate rotation transformation of the voltage control command corresponding to the q-axis of the VSC inner loop and the voltage control command corresponding to the d-axis of the inner loop is obtained according to the phase-locked loop. This method respectively considers the characteristics of the operating scenarios under the two-port network and the three-port network, and sets different synchronization control methods for the two operating scenarios to obtain the reference angle for the coordinate rotation transformation, solving the problem that in a weak grid operating scenario such as a two-port network, the existing control method of the network-forming VSC using this phase-locked loop for synchronization control is prone to phase-locked loop instability, which may lead to instability of the grid voltage and current, and further affect the stability and security of the DC power transmission system, and improving the stability and security of the DC power transmission system.

[0038] This method specifically includes outer loop control and inner loop control; the d-axis of the VSC outer loop is an AC tie line voltage amplitude control link, and the q-axis of the outer loop is an AC tie line active power control link; the voltage control command corresponding to the q-axis of the VSC inner loop is obtained according to the control quantity output by the q-axis of the VSC outer loop, and the voltage control command corresponding to the d-axis of the inner loop is obtained according to the amplitude of the vector difference between the voltage control command corresponding to the q-axis of the VSC inner loop and the control quantity output by the d-axis of the VSC outer loop; the voltage control command corresponding to the q-axis of the VSC inner loop and the voltage control command corresponding to the d-axis of the inner loop are subjected to coordinate rotation transformation to obtain the voltage control command of the network-forming VSC.

[0039] In the DC power transmission system, if the connection structure of the sending-end converter station is a two-port network structure, the reference angle for the coordinate rotation transformation is obtained according to the virtual rotor motion equation; if the connection structure of the sending-end converter station is a three-port network structure, the reference angle for the coordinate rotation transformation is obtained according to the phase-locked loop.

[0040] This method solves the problem that in a weak grid operating scenario such as a two-port network, the existing control method of the network-forming VSC using this phase-locked loop for synchronization control is prone to phase-locked loop instability, which may lead to instability of the grid voltage and current, and further affect the stability and security of the DC power transmission system, and improves the stability and security of the DC power transmission system.

[0041] The structural schematic diagram of the DC power transmission system in this embodiment is as Figure 1As shown in the figure, the HVDC transmission system consists of a sending-end converter substation (sending-end converter station), a receiving-end converter substation (receiving-end converter station), and a DC overhead line connecting the two stations. The sending-end converter substation consists of a full-capacity LCC converter and a partial-capacity VSC converter. The two converters are connected in parallel on the AC side. The VSC converter can be connected in parallel on the secondary side or the primary side of the LCC converter transformer; the reactive power capacity of the VSC converter can be 40% - 60% of the capacity of the LCC converter, and it can adopt a star-connected structure or a delta-connected structure composed of a cascaded H-bridge chain topology; the receiving-end converter substation consists of a conventional LCC converter; the topology structure diagram of the VSC converter is as shown in Figure 2 As shown, the VSC converter can adopt a cascaded H-bridge to form a Y-type topology or a D-type topology. Among them, the connection structure of the sending-end converter station is divided into a two-port network structure and a three-port network structure. The two-port network structure is the network structure corresponding to the sending-end converter station only connected to the new energy power station or only connected to the AC power grid. An example diagram of the two-port network structure corresponding to the sending-end converter station only connected to the new energy power station is as shown in Figure 3 As shown, the sending-end converter station includes an LCC converter and a VSC converter, and the ports of the LCC converter and the VSC converter are connected to the new energy power station;

[0042] An example diagram of the two-port network structure corresponding to the sending-end converter station only connected to the AC power grid is as shown in Figure 4 As shown; the ports of the LCC converter and the VSC converter are directly connected to the AC power grid;

[0043] The three-port network structure is the network structure corresponding to the sending-end converter station connected to the new energy power station and the AC power grid at the same time. An example diagram of the three-port network structure is as shown in Figure 5 As shown, energy transfer occurs among the sending-end converter station, the new energy power station, and the AC power grid. The three-port network structure has two degrees of freedom for power flow control, that is, for a three-port (one of the sending-end converter station, the new energy power station, and the AC power grid is used as the input end, and the other two are the output ends; or one is the output end, and the other two are the input ends) network, it is possible to control the input or output of two of the ports to adjust the output of the third port.

[0044] Figure 3 、 Figure 4 And Figure 5 The P in g refers to the power transmitted from the AC power grid side, and P w refers to the power transmitted from the new energy power station side, and P lcc refers to the power transmitted from the LCC converter side, and P vsc refers to the power transmitted from the VSC converter side.

[0045] Taking this HVDC transmission system as an example, the grid-forming control method of HVDC transmission will be specifically described below:

[0046] In this embodiment, in a DC power transmission system, when the connection structure of the sending converter station is a two-port network structure, the corresponding control function schematic diagram is as Figure 6 shown. When the connection structure of the sending converter station is a three-port network structure, the corresponding control function schematic diagram is as Figure 7 shown. This grid-forming control method for DC power transmission includes outer-loop control and inner-loop control; the d-axis of the VSC outer loop is the AC tie-line voltage amplitude control link, which is implemented in the d / q space, and the q-axis of the outer loop is the AC tie-line active power control link;

[0047] In this embodiment, the control method of the AC tie-line voltage amplitude control link is specifically:

[0048] According to the difference between the reference value of the AC tie-line voltage amplitude control and the actual value of the AC tie-line voltage amplitude, closed-loop PI control is performed to control the actual value of the AC tie-line voltage amplitude to be near the reference value; this reference value is obtained from the difference between the per-unit value of the AC tie-line voltage amplitude and the control quantity corresponding to the output of the AC tie-line reactive power control. In this embodiment, the reference value of the AC tie-line voltage amplitude control is taken as 1.0 pu.

[0049] Among them, the control method of the AC tie-line reactive power control is a prior art, and this control method is specifically: through the difference between the actual value Q of the AC tie-line reactive power act and the set reactive power reference value (in this embodiment, the input of the set reactive power reference value is 0), closed-loop PI control is performed to obtain the control quantity corresponding to the output of the AC tie-line reactive power control.

[0050] The voltage control command corresponding to the q-axis of the VSC inner loop is obtained from the control quantity output by the q-axis of the VSC outer loop, and the voltage control command corresponding to the d-axis of the inner loop is obtained from the magnitude of the vector difference between the voltage control command corresponding to the q-axis of the VSC inner loop and the control quantity output by the d-axis of the VSC outer loop; the control method of obtaining the voltage control command corresponding to the d-axis of the inner loop from the magnitude of the vector difference between the voltage control command corresponding to the q-axis of the VSC inner loop and the control quantity output by the d-axis of the VSC outer loop is specifically:

[0051] Take the positive square root of the difference between the square of the voltage control command corresponding to the q-axis of the VSC inner loop and the square of the control quantity output by the d-axis of the VSC outer loop to obtain the magnitude of the vector difference between the voltage control command corresponding to the q-axis of the VSC inner loop and the control quantity output by the d-axis of the VSC outer loop, and add this magnitude to the control quantity U sm balYD corresponding to the output of the sub-module voltage balance control between VSCs to obtain the voltage control command v gd * corresponding to the d-axis of the inner loop. Among them, the control principle block diagram of the sub-module voltage balance control between VSCs is asFigure 8 As shown Figure 8 The overall balance control of the VSC sub-module voltage in is an existing technology and will not be elaborated here. The inter-VSC sub-module voltage balance control is composed of a proportional-integral controller. The specific control method of the inter-VSC sub-module voltage balance control is as follows:

[0052] Taking 1 / 3*(U smag_AY +U smag_BY +U smag_CY ) of the actual value of the average voltage of the VSC sub-module on the Y side of the DC transmission system as the reference value for the inter-VSC sub-module voltage balance control, and taking 1 / 3*(U smag_AD +U smag_BD +U smag_CD ) of the actual value of the average voltage of the VSC sub-module on the D side of the DC transmission system as the feedback value for the inter-VSC sub-module voltage balance control. According to the difference between the reference value and the feedback value of the inter-VSC sub-module voltage balance control, closed-loop control is performed to obtain the control quantity U sm_balYD .

[0053] Perform coordinate rotation transformation (d / q transformation) on the voltage control command corresponding to the q-axis of the VSC inner loop and the voltage control command corresponding to the d-axis of the inner loop to obtain the voltage control command of the grid-forming VSC; the transformation matrix used for the d / q transformation is:

[0054]

[0055] In the formula, U a is the phase-A voltage, U b is the phase-B voltage, U c is the phase-C voltage, θ is the reference angle of the coordinate rotation transformation, U d is the d-axis voltage, and U q is the q-axis voltage.

[0056] Referring to Figure 6 , in the DC transmission system, if the connection structure of the sending converter station is a two-port network structure, then the reference angle of the coordinate rotation transformation (the reference angle θ is determined by Figure 6obtained from the phase synchronization control link in []. It is obtained according to the virtual rotor motion equation; the method for obtaining the reference angle θ of the coordinate rotation transformation for the voltage control command corresponding to the q-axis of the VSC inner loop and the voltage control command corresponding to the d-axis of the inner loop according to the virtual rotor motion equation includes: obtaining the reference angle of the coordinate rotation transformation for the voltage control command corresponding to the q-axis of the VSC inner loop and the voltage control command corresponding to the d-axis of the inner loop according to the virtual rotor motion equation determined by the reference angle, the difference between the active power reference value of the AC tie line and the actual active power value of the AC tie line, the inertia time constant, and the damping coefficient; specifically, the virtual rotor motion equation determined by the difference between the active power reference value of the AC tie line and the actual active power value of the AC tie line, the inertia time constant, and the damping coefficient is:

[0057]

[0058] where T j is the inertia time constant, ω is the angular frequency, P ref is the active power reference value of the AC tie line, P e is the actual active power value of the AC tie line, D is the damping coefficient, t is the time; θ is the reference angle;

[0059] In this embodiment, in the DC power transmission system, if the two-port network structure is such that the sending converter station is only connected to the new energy power station, then the value of the active power reference value P ref of the AC tie line is set to be equal to the actual active power value P e of the AC tie line.

[0060] Referring to Figure 7 , if the connection structure of the sending converter station is a three-port network structure, then the reference angle of the coordinate rotation transformation (the reference angle is θ obtained from the phase synchronization control link in Figure 7 ) is obtained according to the phase-locked loop.

[0061] In this embodiment, referring to Figure 6 or Figure 7 , the method for obtaining the voltage control command corresponding to the q-axis of the VSC inner loop according to the control quantity output by the q-axis of the VSC outer loop includes: adding the control quantity output by the q-axis of the VSC outer loop to the control quantity Q act_balYD corresponding to the output of the VSC reactive power output balance control to obtain the voltage control command V gq * corresponding to the q-axis of the VSC inner loop; where the control principle block diagram of the VSC reactive power output balance control is as shown in Figure 9 , and the control method of the VSC reactive power output balance control (the VSC reactive power output balance control corresponds to the VSC reactive power balance control between VSCs in Figure 9 ) is specifically:

[0062] Taking the actual reactive power Q of the VSC on the Y side of the HVDC system act_Y as the reference value for the VSC reactive power balance control, and taking the actual reactive power Q of the VSC on the D side of the HVDC system act_D as the feedback value for the VSC reactive power balance control. According to the difference between the reference value and the feedback value of the VSC reactive power balance control, closed-loop control is performed to obtain the control quantity Q corresponding to the output of the VSC reactive power balance control. ref_balYD .

[0063] Referring to Figure 6 , in the HVDC system, if the connection structure of the sending-end converter station is a two-port network structure, the control method of the AC tie-line active power control link includes: setting the output of the AC tie-line active power closed-loop control link to zero, so that the control quantity output by the outer-loop q-axis of the VSC is zero; in this embodiment, the specific method of setting the output of the AC tie-line active power closed-loop control link to zero is: disabling the AC tie-line active power control link of the outer-loop q-axis, so that the output of the AC tie-line active power control link is zero. Since the two-port network has only one input terminal and one output terminal, it is impossible to perform AC tie-line active power control. Therefore, here the AC tie-line active power control link of the outer-loop q-axis is disabled, so that the output of the AC tie-line active power control link is zero.

[0064] Referring to Figure 7 , if the connection structure of the sending-end converter station is a three-port network structure, the control method of the AC tie-line active power control link includes: through the difference between the actual AC tie-line active power P act and the set active power reference value P ref , perform closed-loop PI control to control the actual AC tie-line active power P act to be near the set active power reference value P ref . Since the three-port network structure has two degrees of freedom for power flow control and can control the output of the third port by adjusting the input or output of two of the ports, it is more conducive to the suppression and control of the power flow during the large-scale new energy consumption process. Therefore, the three-port network structure can enable the AC tie-line active power control link.

[0065] Embodiment of the HVDC grid-forming control system

[0066] This embodiment provides a technical solution for an HVDC grid-forming control system, which includes a processor for executing a computer program to implement the steps of the HVDC grid-forming control method.

[0067] Since the specific implementation process and principle of the HVDC grid-forming control system in this embodiment have been described in detail in the embodiment of the HVDC grid-forming control method, no further elaboration will be provided here.

[0068] It should be understood that the above specific embodiments of the present invention are only used for illustrative explanation or interpretation of the principle of the present invention, and do not constitute a limitation on the present invention.

Claims

1. A method for controlling a DC transmission grid configuration, characterized in that: It includes outer loop control and inner loop control; the outer loop d axis of the VSC is the voltage amplitude control link of the AC tie line, and the outer loop q axis is the active power control link of the AC tie line; the voltage control command corresponding to the inner loop q axis of the VSC is obtained according to the control quantity output by the outer loop q axis of the VSC, and the voltage control command corresponding to the inner loop d axis is obtained according to the amplitude of the vector difference between the voltage control command corresponding to the inner loop q axis of the VSC and the control quantity output by the outer loop d axis of the VSC; the voltage control command corresponding to the inner loop q axis of the VSC and the voltage control command corresponding to the inner loop d axis of the VSC are subjected to coordinate rotation transformation to obtain the voltage control command of the meshed VSC; In a DC transmission system, if the connection structure of the sending-end converter station is a two-port network structure, the reference angle of the coordinate rotation transformation is obtained according to the virtual rotor motion equation; if the connection structure of the sending-end converter station is a three-port network structure, the reference angle of the coordinate rotation transformation is obtained according to the phase-locked loop.

2. The method for controlling a DC transmission grid configuration according to claim 1, characterized in that: The two-port network structure corresponds to a network structure in which the sending-end converter station is only connected to a new energy station, or a network structure in which the sending-end converter station is only connected to an AC power grid; the three-port network structure corresponds to a network structure in which the sending-end converter station is connected to a new energy station and also to an AC power grid.

3. The method for controlling a DC transmission grid configuration according to claim 1 or 2, characterized in that: The method of obtaining the voltage control command corresponding to the VSC inner loop q axis according to the control amount output by the VSC outer loop q axis includes: superimposing the control amount output by the VSC outer loop q axis on the control amount corresponding to the output of the VSC reactive power balance control to obtain the voltage control command corresponding to the VSC inner loop q axis; In a DC power transmission system, if the connection structure of the sending-end converter station is a two-port network structure, the control method of the AC tie line active power control link includes: setting the output of the AC tie line active power closed-loop control link to zero, so that the control amount of the VSC outer ring q-axis output is zero; If the connection structure of the sending-end converter station is a three-port network structure, the control method of the active power control link of the AC interconnection line includes: performing closed-loop PI control based on the difference between the actual value of the active power of the AC interconnection line and the set active power reference value, so as to control the actual value of the active power of the AC interconnection line to near the set active power reference value.

4. The method for controlling a DC transmission grid configuration according to claim 2, characterized in that: The method of obtaining the reference angle of the coordinate rotation transformation according to the virtual rotor motion equation includes: obtaining the reference angle of the coordinate rotation transformation according to the virtual rotor motion equation determined by the reference angle, the difference between the reference value of the active power of the AC interconnection line and the actual value of the active power of the AC interconnection line, the inertia time constant and the damping coefficient.

5. The method for controlling a DC transmission grid configuration according to claim 4, characterized in that: In a DC power transmission system, if the two-port network structure is that the sending-end converter station is only connected to the new energy station, the value of the AC interconnection line active power reference value is set to be equal to the actual value of the AC interconnection line active power.

6. The method for controlling the DC transmission grid configuration according to claim 3, characterized in that: The method of setting the output of the AC tie line active power closed-loop control link to zero includes: disabling the AC tie line active power control link of the outer ring q axis to make the output of the AC tie line active power control link zero.

7. The method for controlling a DC transmission grid configuration according to claim 1 or 2, characterized in that: The control method of obtaining the voltage control command corresponding to the inner ring d axis according to the amplitude of the vector difference between the voltage control command corresponding to the VSC inner ring q axis and the control quantity output by the VSC outer ring d axis includes: Take the positive square root of the difference between the square of the voltage control command corresponding to the VSC inner loop q-axis and the square of the control quantity output by the VSC outer loop d-axis to obtain the amplitude of the vector difference between the voltage control command corresponding to the VSC inner loop q-axis and the control quantity output by the VSC outer loop d-axis. Superimpose the amplitude on the control quantity corresponding to the output of the voltage balancing control between the VSC sub-modules to obtain the voltage control command corresponding to the inner loop d-axis.

8. The method for controlling a DC transmission grid configuration according to claim 1 or 2, characterized in that: The control method of the AC tie line voltage amplitude control link includes: According to the difference between the reference value of the AC tie line voltage amplitude control and the actual value of the AC tie line voltage amplitude, closed-loop PI control is performed to control the actual value of the AC tie line voltage amplitude to be near the reference value; the reference value is obtained according to the difference between the per-unit value of the AC tie line voltage amplitude and the control quantity corresponding to the output of the AC tie line reactive power control.

9. The method for controlling a DC transmission grid configuration according to claim 7, characterized in that: The control method of the voltage balancing control between the VSC sub-modules includes: The actual value of the average voltage of the VSC submodules on the Y side of the DC transmission system is used as the reference value of the voltage balancing control between the VSC submodules, and the actual value of the average voltage of the VSC submodules on the D side of the DC transmission system is used as the feedback value of the voltage balancing control between the VSC submodules. According to the difference between the reference value and the feedback value of the voltage balancing control between the VSC submodules, closed-loop control is performed to obtain the control quantity corresponding to the output of the voltage balancing control between the VSC submodules.

10. The method for controlling a DC transmission grid configuration according to claim 3, characterized in that: The control method of the VSC reactive power output balance control includes: The actual reactive power value of the VSC on the Y side of the DC transmission system is taken as the reference value of the VSC reactive output balance control, and the actual reactive power value of the VSC on the D side of the DC transmission system is taken as the feedback value of the VSC reactive output balance control. According to the difference between the reference value and the feedback value of the VSC reactive output balance control, closed-loop control is performed to obtain the control quantity corresponding to the output of the VSC reactive output balance control.

11. A DC transmission grid control system, comprising a processor, characterized in that: The processor is used to execute computer program instructions to implement the steps of the direct current transmission grid type control method as described in any one of claims 1-10.

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