Conventional dc transmission valve side amplitude phase control topology based on parallel current converter

Abstract

The application belongs to the technical field of power system DC transmission, and specifically discloses a conventional DC transmission valve side amplitude and phase regulation and control topology based on a parallel converter, which comprises a DC line, a conventional DC converter valve group, a coupling transformer, an auxiliary flexible DC converter and a receiving end AC bus; the topology is provided with four auxiliary flexible DC converters, one auxiliary flexible DC converter corresponding to one valve in the conventional DC converter valve group; the DC side of the auxiliary flexible DC converter is connected in parallel with the DC side of the corresponding valve; the AC side of the auxiliary flexible DC converter is connected in parallel or in series with the AC side of the corresponding valve; the auxiliary flexible DC converter is used to adopt a network configuration type control strategy to regulate and control the voltage amplitude and phase angle of the AC side of the auxiliary flexible DC converter. Through the application, the risk of commutation failure of the conventional DC converter station is reduced, and economy is taken into account.

Description

Technical Field

[0001] This application belongs to the field of smart grid technology, specifically relating to the field of DC power transmission technology, and more specifically, relating to a conventional DC transmission valve-side amplitude-phase control topology based on a parallel converter. Background Technology

[0002] Large-scale green power infrastructure such as new energy bases may be distributed in reverse to power load centers. In this context, high-voltage direct current (HVDC) transmission technology has significant advantages over traditional high-voltage alternating current (HVAC) transmission technology, including lower transmission losses, smaller line footprint, and no synchronization stability issues. Therefore, it has gradually become one of the important technical solutions for large-scale long-distance power transmission. Line commutated converter (LCC) transmission technology based on thyristor-controlled switching belongs to conventional DC transmission. Compared with IGBT-based controllable switching MMC (Modular Multilevel Converter) flexible DC transmission technology, it has advantages such as lower operating losses, lower construction costs, and more mature technology. Conventional DC converter valve groups (configured on the valve side of conventional DC converter stations) generally adopt a true bipolar high-low voltage valve group construction scheme. The high and low voltage valves of the positive and negative poles of the conventional DC converter valve group can be LCCs. In this case, the conventional DC converter valve group can be called a conventional DC converter valve group LCC. However, when conventional DC converter stations operate as receiving-end inverter stations, they are heavily reliant on the strength of the receiving-end power grid. When the receiving-end grid strength is insufficient or a near-field AC fault occurs, commutation failure may occur. During commutation failure, the receiving-end system loses all active power support from the conventional DC converter station and may induce continuous commutation failures and multiple DC cascading commutation failures, further weakening the receiving-end grid and posing a significant threat to its operational safety. Therefore, researchers are actively exploring LCC topology improvement strategies to reduce the risk of commutation failure and to enhance the support capabilities of conventional DC converter stations for the receiving-end grid.

[0003] Currently, based on the different commutation processes and commutation mechanisms, the improved topologies of conventional DC transmission technology based on grid commutation (LCC) are mainly divided into two major technical routes: Capacitance Commutated Converter (CCC) and Controlled Line Commutated Converter (CLCC).

[0004] The classic approach to capacitor commutation technology involves connecting a capacitor in series at the AC outlet of the LCC to improve the system's equivalent short-circuit ratio and reduce the risk of commutation failure. However, capacitor commutation technology requires a corresponding increase in valve group insulation levels, leading to more severe overvoltages during faults. Therefore, based on the aforementioned capacitor commutation technology approach, researchers have proposed several improved topology schemes: Absorption and Shunt Capacitance Commutated Converter (ASCCC) and Evolutionary Shunt Capacitor Commutated Converter (ESCCC).

[0005] In addition, the classic solution for controllable device commutation technology is the self-adaptive commutation converter (SLCC), whose topology is: a static synchronous compensator (STATCOM) is connected in parallel between the converter transformer and the LCC converter valve, so that the system has dynamic reactive power compensation capability and can stably maintain the AC bus voltage when the receiving end system fails. However, the above solution has problems such as high construction cost.

[0006] How to reduce the risk of commutation failure in conventional DC converter stations while also considering economic efficiency is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] In view of the shortcomings of the prior art, the purpose of this application is to reduce the risk of commutation failure in conventional DC converter stations while taking into account economic efficiency.

[0008] To achieve the above objectives, in a first aspect, this application provides a conventional DC transmission valve-side phase-controlled topology based on a parallel converter, the topology comprising: a DC line, a conventional DC converter valve group, a connecting transformer, an auxiliary flexible DC converter, and a receiving-end AC bus; DC lines are used to transmit DC power to conventional DC converter valve assemblies; A conventional DC converter valve assembly is used to convert the received DC power to AC power, and then feed the converted AC power into the receiving-end AC bus via a connecting transformer. Conventional DC converter valve assemblies are equipped with high-pressure valves and low-pressure valves at their positive and negative terminals. The topology is configured with four auxiliary flexible DC converters, with each auxiliary flexible DC converter corresponding to one valve in a conventional DC converter valve group; The DC side of the auxiliary flexible DC converter is connected in parallel with the DC side of the corresponding valve; The AC side of the auxiliary flexible DC converter is connected in parallel with the AC side of the corresponding valve, or the AC side of the auxiliary flexible DC converter is connected in series with the AC side of the corresponding valve. An auxiliary flexible DC converter is used to regulate the voltage amplitude and phase angle on the AC side of the auxiliary flexible DC converter using a grid-type control strategy.

[0009] Specifically, since the AC side of the auxiliary flexible DC converter is connected in parallel with the AC side of the corresponding valve, or the AC side of the auxiliary flexible DC converter is connected in series with the AC side of the corresponding valve, this application provides two conventional DC transmission valve-side amplitude-phase control topologies based on parallel converters. One topology (referred to as Scheme A) uses DC-side parallel self-powered and AC-side parallel current compensation for the auxiliary flexible DC converter, while the other topology (referred to as Scheme B) uses DC-side parallel self-powered and AC-side series voltage compensation for the auxiliary flexible DC converter.

[0010] Scheme A is an improved LCC topology scheme with DC-side parallel self-powering and AC-side parallel current compensation, which can be abbreviated as: DP / AP-LCC (self-powering by DC-side Parallel and self-adjustment by AC-side Parallel based Line Commutation Converter); Scheme B is an improved LCC topology scheme with DC-side parallel self-powering and AC-side series voltage compensation, which can be abbreviated as: DP / AS-LCC (self-powering by DC-side Parallel and self-adjustment by AC-side Series based Line Commutation Converter).

[0011] For Scheme A, the DC side of the auxiliary flexible DC converter is connected in parallel to the DC side of the positive / negative poles and high / low voltage valves of the conventional DC converter valve group LCC. This enables self-powering on the DC side and eliminates the need for constant DC voltage control by the MMC, freeing up the MMC's control degrees of freedom to flexibly adjust the AC side output voltage-phase angle characteristics. The AC side of the auxiliary flexible DC converter is directly connected in parallel to the AC side of the corresponding valve of the conventional DC converter valve group LCC and the connecting transformer valve side to achieve current compensation, equivalent to an active filter. This reduces the need for passive filters in the system, effectively improving engineering economy.

[0012] For Scheme B, the DC side of the auxiliary flexible DC converter is connected in parallel to the DC side of the positive / negative poles of the conventional DC converter valve group LCC—the high / low voltage valves. This enables the DC side to self-harvest power and eliminates the need for constant DC voltage control by the MMC, freeing up the MMC's control degrees of freedom to flexibly adjust the AC side output voltage-phase angle characteristics. The AC side of the auxiliary flexible DC converter is connected in series with the AC side of the corresponding valve of the conventional DC converter valve group LCC and the valve side of the connecting transformer via a transformer to achieve voltage compensation. This is equivalent to an active phase shifter, unifying the wiring method of the connecting transformers corresponding to the high / low voltage valve sides, reducing the spare types and number of connecting transformers, and effectively improving the project's economic efficiency.

[0013] DC lines are used to transmit DC power generated from long-distance clean energy (green energy) bases to conventional DC converter valve assemblies.

[0014] The auxiliary flexible DC converter adopts a grid-type control strategy. By regulating the voltage amplitude and phase angle on the AC side of the auxiliary flexible DC converter, it can provide rapid reactive power-voltage support when a fault occurs on the AC side of the system, maintain the voltage stability of the AC grid at the receiving end during transients, help improve the equivalent short-circuit ratio of the AC bus at the receiving end, and effectively reduce the risk of commutation failure in conventional DC converter stations.

[0015] The connecting transformer is used to transform the valve-side AC voltage of a conventional DC converter station to the AC bus voltage of the receiving end, suppress the injection of specific characteristic harmonics generated during the operation of the conventional DC converter station into the AC grid of the receiving end, and achieve electrical isolation between the valve-side AC line of the conventional DC converter station and the AC grid of the receiving end.

[0016] The receiving-end AC bus is used to receive and distribute AC power transmitted from the conventional DC transmission system to the receiving-end AC grid; at the same time, the amplitude and phase stability of its AC voltage directly provides the necessary commutation voltage support for the reliable commutation of the conventional DC converter station.

[0017] In one possible implementation, the positive terminal of the positive high-pressure valve of the conventional DC converter valve group is connected to the positive terminal of the DC line; the negative terminal of the positive high-pressure valve of the conventional DC converter valve group is connected to the positive terminal of the positive low-pressure valve of the conventional DC converter valve group; the negative terminal of the positive low-pressure valve of the conventional DC converter valve group is connected to the positive terminal of the negative low-pressure valve of the conventional DC converter valve group; the negative terminal of the negative low-pressure valve of the conventional DC converter valve group is connected to the positive terminal of the negative high-pressure valve of the conventional DC converter valve group; and the negative terminal of the negative high-pressure valve of the conventional DC converter valve group is connected to the negative terminal of the DC line. The AC side of the following valves in a conventional DC converter valve group is connected to the valve-side winding of the corresponding connecting transformer: positive high-voltage valve, positive low-voltage valve, negative low-voltage valve, and negative high-voltage valve. The grid-side winding of the connecting transformer is connected in parallel to the receiving-end AC bus.

[0018] In one possible implementation, the auxiliary flexible DC-DC converter adopts a modular multilevel converter (MMC) structure. Modular multilevel converter (MMC) is a half-bridge MMC based on half-bridge submodule HBSM, or a hybrid MMC based on half-bridge submodule HBSM and full-bridge submodule FBSM.

[0019] In one possible implementation, where the AC side of the auxiliary flexible DC converter is connected in series with the AC side of the corresponding valve, the AC side of the auxiliary flexible DC converter is connected in series via a specific transformer. A specific transformer is used to provide series voltage compensation.

[0020] In one possible implementation, when the AC side of the auxiliary flexible DC converter is connected in parallel with the AC side of the corresponding valve, the connecting transformers corresponding to the positive high-voltage valve and the negative high-voltage valve adopt a Y / Yn connection method, and the connecting transformers corresponding to the positive low-voltage valve and the negative low-voltage valve adopt a Δ / Yn connection method. Among them, the Y / Yn connection method is that the valve side winding of the transformer is star-connected and the neutral point is not grounded, while the grid side winding is star-connected and grounded through the neutral point; The △ / Yn wiring method means that the transformer valve-side winding is connected in a delta configuration, while the grid-side winding is connected in a star configuration and grounded through the neutral point.

[0021] In one possible implementation, when the AC side of the auxiliary flexible DC converter is connected in series with the AC side of the corresponding valve, all connecting transformers adopt the same wiring method, which is either a Y / Yn connection or a Δ / Yn connection. Among them, the Y / Yn connection method is that the valve side winding of the transformer is star-connected and the neutral point is not grounded, while the grid side winding is star-connected and grounded through the neutral point; The △ / Yn wiring method means that the transformer valve-side winding is connected in a delta configuration, while the grid-side winding is connected in a star configuration and grounded through the neutral point.

[0022] Secondly, this application provides a control method for a conventional DC transmission valve-side phase-controlled topology based on a parallel converter, applied to the topology described in the first aspect or any possible implementation of the first aspect, the control method comprising: A grid-type control strategy is adopted to regulate the voltage amplitude and phase angle on the AC side of the auxiliary flexible DC converter.

[0023] In one possible implementation, the grid-based control strategy is specifically an active power synchronization control strategy. The aforementioned grid-based control strategy regulates the voltage amplitude and phase angle on the AC side of the auxiliary flexible DC converter, including: Reference value of the d-axis component of the AC side voltage based on the auxiliary flexible DC converter d-axis component per unit value q-axis component reference value per unit value of q-axis component By using a constant AC voltage outer loop, the reference value of the d-axis component of the AC side current of the auxiliary flexible DC converter is obtained. and q-axis component reference value ; based on , , and The d-axis modulation ratio component is obtained by decoupling the inner loop with vector current and the voltage feedforward loop. and q-axis modulation ratio components ; Active power reference value on the AC side based on the auxiliary flexible DC converter and active power per unit value The phase angle of the AC side of the auxiliary flexible DC converter is obtained through the power synchronization loop. ; based on , and The modulation ratio of phase A is obtained by transforming the dq coordinate system to the stationary ABC coordinate system. B-phase modulation ratio C-phase modulation ratio , , and Used to regulate the voltage amplitude and phase angle on the AC side of the auxiliary flexible DC converter.

[0024] In one possible implementation, the control method further includes: When the shut-off angle is greater than 30°, the conventional DC converter valve group adopts constant DC voltage control. When the shut-off angle is less than or equal to 30°, the conventional DC converter valve group adopts constant shut-off angle control to limit the shut-off angle to 30°.

[0025] In one possible implementation, constant DC voltage control includes: Based on the DC voltage reference value and DC voltage per unit value of a conventional DC converter valve group, a deviation signal is generated; The turn-off angle is determined based on the deviation signal through a PI control circuit.

[0026] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art: (1) The topology of this application (Scheme A and Scheme B) is configured with an auxiliary flexible DC converter. The auxiliary flexible DC converter adopts a grid-type control strategy. By regulating the voltage amplitude and phase angle of the AC side of the auxiliary flexible DC converter, it can provide rapid reactive power-voltage support when a fault occurs on the AC side of the system, maintain the voltage stability of the AC grid at the receiving end during transients, help improve the equivalent short-circuit ratio of the AC bus at the receiving end, and effectively reduce the risk of commutation failure of conventional DC converter stations.

[0027] (2) For Scheme A, the auxiliary flexible DC converter adopts "DC-side parallel self-powering and AC-side parallel current compensation". The auxiliary flexible DC converter can provide current compensation, actively shunt and absorb harmonic currents of the receiving-end AC bus, which is equivalent to an active filter. By utilizing the active filtering function provided by the auxiliary flexible DC converter in Scheme A, the configuration of passive filters in the system can be reduced, effectively improving the engineering economy.

[0028] (3) For Scheme B, the auxiliary flexible DC converter adopts "DC-side parallel self-powering - AC-side series voltage compensation". The auxiliary flexible DC converter can provide voltage compensation, perform harmonic flexible phase shifting, and realize the harmonic voltage overlap and phase reversal elimination, which is equivalent to an active phase shifter. By utilizing the flexible phase shifting function provided by the auxiliary flexible DC converter in Scheme B, some frequency AC-side voltage harmonics can be eliminated, the connection method of the corresponding connection transformers on the high / low voltage valve side can be unified, the spare type and number of connection transformers can be reduced, and the engineering economy can be effectively improved. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of DP / AP-LCC topology scheme A with DC-side parallel self-powered charging and AC-side parallel current compensation provided in the embodiments of this application; Figure 2 This is a schematic diagram of the DP / AS-LCC topology scheme B with DC-side parallel self-powered and AC-side series voltage compensation provided in the embodiments of this application; Figure 3 This is a block diagram of the control strategy for the output voltage-phase construction loop of the auxiliary flexible DC converter provided in the embodiments of this application; Figure 4 This is a block diagram of the conventional DC converter valve group LCC control strategy provided in the embodiments of this application; Figure 5 This is a flowchart illustrating the implementation of multiple functions of the DP / AP-LCC topology scheme A provided in this application under different working conditions and operating scenarios. Figure 6This is a flowchart illustrating the implementation of multiple functions of the DP / AS-LCC topology scheme B provided in this application under different working conditions and operating scenarios.

[0030] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein: S1 is the positive high-voltage valve of the conventional DC-DC converter valve group LCC; S2 is the positive low-voltage valve of the conventional DC-DC converter valve group LCC; S3 is the negative low-voltage valve of the conventional DC-DC converter valve group LCC; S4 is the negative high-voltage valve of the conventional DC-DC converter valve group LCC; S5 is the filter and smoothing reactor; S6 is the grounding electrode of the conventional DC-DC converter valve group LCC; S7 is the auxiliary flexible DC-DC converter corresponding to the positive low-voltage valve; S8 is the auxiliary flexible DC-DC converter corresponding to the negative low-voltage valve; S15 is the auxiliary flexible DC-DC converter corresponding to the positive high-voltage valve; S16 is the auxiliary flexible DC-DC converter corresponding to the negative high-voltage valve; S9 is the connecting transformer corresponding to the AC side of the positive high-voltage valve; S10 is the connecting transformer corresponding to the AC side of the positive low-voltage valve. The connection transformers are as follows: S11 is the connection transformer corresponding to the AC side of the negative low-voltage valve; S12 is the connection transformer corresponding to the AC side of the negative high-voltage valve; S13 is the receiving-end AC bus; S14 is the receiving-end AC grid; S17 is the specific transformer used for series connection of the AC side of the auxiliary flexible DC converter corresponding to the positive high-voltage valve; S18 is the specific transformer used for series connection of the AC side of the auxiliary flexible DC converter corresponding to the negative high-voltage valve; S19 is the specific transformer used for series connection of the AC side of the auxiliary flexible DC converter corresponding to the positive low-voltage valve; S20 is the specific transformer used for series connection of the AC side of the auxiliary flexible DC converter corresponding to the negative low-voltage valve. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0032] In the embodiments of this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or design. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0033] In the description of the embodiments of this application, unless otherwise stated, "multiple" means two or more, for example, multiple processing units means two or more processing units, multiple elements means two or more elements, etc.

[0034] The embodiments of this application are described below with reference to the accompanying drawings.

[0035] like Figure 1 As shown, this application provides a DP / AP-LCC topology scheme A with DC-side parallel self-powered charging and AC-side parallel current compensation. Scheme A belongs to the LCC improved topology of the controllable device commutation technology route, as shown... Figure 1 As shown, the DP / AP-LCC topology scheme A proposed in this application, which features DC-side parallel self-powered charging and AC-side parallel current compensation, mainly includes major equipment and components such as S1-S16.

[0036] S1, S2, S3, and S4 are, in order, the positive high-pressure valve, the positive low-pressure valve, the negative low-pressure valve, and the negative high-pressure valve of a conventional DC converter valve group LCC.

[0037] The positive terminal of the positive high-pressure valve S1 is connected to the positive terminal of the DC line, and the negative terminal of the positive high-pressure valve S1 is connected to the positive terminal of the positive low-pressure valve S2. The negative terminal of the positive low-pressure valve S2 is connected to the positive terminal of the negative low-pressure valve S3. The negative terminal of the negative low-pressure valve S3 is connected to the positive terminal of the negative high-pressure valve S4. The negative terminal of the negative high-pressure valve S4 is connected to the negative terminal of the DC line.

[0038] S5 is equipped with a filter and smoothing reactor at the connection between the conventional DC converter valve group LCC and the DC line. Its main function is to suppress transient overvoltages during system start-up and shutdown and to reduce DC side harmonic voltages during operation.

[0039] S6 is the grounding electrode of the conventional DC converter valve group LCC, providing a zero potential reference point for the system.

[0040] S7, S8, S15, and S16 are auxiliary flexible DC converters. The DC side of the auxiliary flexible DC converters is connected in parallel to the high / low voltage valve for power extraction, and the AC side of the auxiliary flexible DC converters is connected in parallel to the connecting transformer valve side to provide current compensation.

[0041] like Figure 1 As shown, S7 is the auxiliary flexible DC converter corresponding to S2. The DC side of S7 is connected in parallel to the DC side of S2 to draw energy, and the AC side of S7 is connected in parallel to the AC side of S2 (since the AC side of S2 is connected to the valve-side winding of S10, the AC side of S7 is also connected in parallel to the valve-side winding of the connecting transformer S10 corresponding to S2) to provide current compensation.

[0042] like Figure 1As shown, S8 is the auxiliary flexible DC converter corresponding to S3. The DC side of S8 is connected in parallel to the DC side of S3 to draw energy, and the AC side of S8 is connected in parallel to the AC side of S3 (since the AC side of S3 is connected to the valve-side winding of S11, the AC side of S8 is also connected in parallel to the valve-side winding of the connecting transformer S11 corresponding to S3) to provide current compensation.

[0043] like Figure 1 As shown, S15 is the auxiliary flexible DC converter corresponding to S1. The DC side of S15 is connected in parallel to the DC side of S1 to draw energy, and the AC side of S15 is connected in parallel to the AC side of S1 (since the AC side of S1 is connected to the valve-side winding of S9, the AC side of S15 is also connected in parallel to the valve-side winding of the connecting transformer S9 corresponding to S1) to provide current compensation.

[0044] like Figure 1 As shown, S16 is the auxiliary flexible DC converter corresponding to S4. The DC side of S16 is connected in parallel to the DC side of S4 to extract energy, and the AC side of S16 is connected in parallel to the AC side of S4 (since the AC side of S4 is connected to the valve-side winding of S12, the AC side of S16 is also connected in parallel to the valve-side winding of the connecting transformer S12 corresponding to S4) to provide current compensation.

[0045] The auxiliary flexible DC converter adopts a modular multilevel converter (MMC) structure. The MMC contains multiple sub-modules, which can be a full-bridge MMC based on full-bridge sub-modules (FBSM); the MMC can also be a hybrid MMC based on half-bridge sub-modules (HBSM) and full-bridge sub-modules (FBSM) to improve the engineering economy.

[0046] S9 and S12 are the connecting transformers corresponding to the AC sides of the positive and negative high-pressure valves, respectively. They adopt a "Y / Yn" winding connection method, that is, the valve-side winding of the transformer is star-connected and the neutral point is not grounded, while the grid-side winding is star-connected and grounded through the neutral point. S10 and S11 are the connecting transformers corresponding to the AC sides of the positive and negative low-pressure valves, respectively. They adopt a "Δ / Yn" winding grounding method, that is, the valve-side winding of the transformer is delta-connected, while the grid-side winding is star-connected and grounded through the neutral point, in order to provide a 30° voltage phase shift compared to the high-pressure valve, and to eliminate the 6th low-order harmonic voltage by using transformer phase-shifting overlap technology.

[0047] like Figure 1 As shown, the valve-side windings of S9-S12 are connected to the AC side of S1-S4 respectively, and the grid-side windings of S9-S12 are connected in parallel to the receiving-end AC bus S13.

[0048] S13 and S14 are the receiving-end AC bus and the receiving-end AC power grid, respectively.

[0049] It is worth noting that in actual engineering, it is necessary to have backup transformers such as S9-S12. However, since there are two types of transformers with different wiring methods, it is necessary to add at least one backup transformer with different winding wiring methods.

[0050] It is understandable that, such as Figure 1 As shown, a conventional DC converter station includes conventional DC converter valve groups (S1-S4), auxiliary flexible DC converters (S7, S8, S15, S16), and connecting transformers (S9-S12).

[0051] like Figure 1 As shown, the main functions of each device and component are related to the steady-state operating power, specifically including: S1-S4 are used to achieve basic inverter function. The specific principle is as follows: through the orderly and controllable conduction of thyristors and the self-commutation effect of the power grid, DC voltage is inverted into AC voltage output. The specific voltage modulation relationship is as follows: (1); In the formula: Represents the DC side voltage of the valve. These represent the positive and negative terminals of the LCC in a conventional DC converter valve group, respectively. These represent the single-pole high-pressure valve and the single-pole low-pressure valve of a conventional DC converter valve group (LCC), respectively. This represents the AC side voltage amplitude of the corresponding valve; , These represent the commutation arc-extinguishing angle and the shut-off angle of the corresponding valves, respectively.

[0052] Specifically, S15, S7, S8, and S16 are auxiliary flexible DC converters connected in parallel to the DC side of S1-S4. Their AC sides are connected in parallel to the valve side of the connecting transformer to form a parallel current compensation structure. When the current flow to the grid side is defined as positive, the specific AC side current splitting relationship is as follows: (2); In the formula: For the AC side outlet current phasor of low-pressure or high-pressure valves, This refers to the compensating AC current phasor for the auxiliary flexible DC converter corresponding to the low-pressure or high-pressure valve. The total AC current phasor on the valve side of the connecting transformer corresponding to the low-pressure valve or high-pressure valve.

[0053] Specifically, S9-S12 are the connecting transformers corresponding to S1-S4, respectively. However, since they belong to two different types of transformer winding connection forms, taking the positive terminal as an example, the voltage transformation relationship corresponding to its high / low voltage valves is as follows: (3); In the formula: The voltage phasor of the receiving-end AC bus; , The following are the transformer turns ratios corresponding to the positive high-pressure valve and the positive low-pressure valve, respectively. This represents the AC voltage phasor on the valve side of the transformer corresponding to the positive high-voltage valve. This is the AC voltage phasor on the valve side of the transformer corresponding to the positive low-voltage valve.

[0054] Specifically, the positive direction is defined as the flow of DC current and active power into the conventional DC converter valve group LCC. According to Kirchhoff's theorem, the current splitting relationship on the DC side can be known, and the steady-state power flow relationship is derived as follows: (4); In the formula: , , , The DC current flowing through the high-pressure valve of the conventional DC converter valve group LCC, the DC current flowing through the low-pressure valve of the conventional DC converter valve group LCC, the DC current flowing through the auxiliary flexible DC converter corresponding to the high-pressure valve of the conventional DC converter valve group LCC, and the DC current flowing through the auxiliary flexible DC converter corresponding to the low-pressure valve of the conventional DC converter valve group LCC are as follows: , , , The figures, in order, represent the operating active power of the high-pressure valves of a conventional DC-DC converter valve group (LCC), the operating active power of the low-pressure valves of the LCC, the operating active power of the auxiliary flexible DC converter corresponding to the high-pressure valves of the LCC, and the operating active power of the auxiliary flexible DC converter corresponding to the low-pressure valves of the LCC. It should be noted that in actual engineering projects, the high / low-pressure valves often share the total DC voltage between the DC poles of the system; therefore, the active power distribution in the above formula exhibits a similar distribution relationship to the DC current distribution.

[0055] like Figure 2 As shown, this application provides a DP / AS-LCC topology scheme B with DC-side parallel self-powered and AC-side series voltage compensation. Scheme B also belongs to the LCC improved topology of the controllable device commutation technology route, such as... Figure 2 As shown, the DP / AP-LCC topology scheme B proposed in this application, which features DC-side parallel self-powered and AC-side series voltage compensation, mainly includes major equipment and components such as S1-S20.

[0056] S1, S2, S3, and S4 are, in order, the positive high-pressure valve, the positive low-pressure valve, the negative low-pressure valve, and the negative high-pressure valve of the LCC DC transmission system.

[0057] S5 is a filter and smoothing reactor connected at the DC side outlet of the conventional DC converter valve group LCC. Its main function is to suppress transient overvoltages during system start-up and shutdown, and to reduce DC side harmonic voltages during operation.

[0058] S6 is the grounding electrode of the conventional DC converter valve group LCC, providing a zero potential reference point for the system.

[0059] S7, S8, S15, and S16 are auxiliary flexible DC converters. The DC side of the auxiliary flexible DC converter is connected in parallel to the high / low voltage valve for energy extraction, and the AC side of the auxiliary flexible DC converter is connected in series to the valve side of the connecting transformer to provide voltage compensation. This integrates conventional LCC transmission technology with static series voltage compensator technology. Its advantage is that it can reduce the redundancy of conventional DC converter valve group LCC for multiple types of transformers through flexible phase shifting, thereby improving the economic efficiency of the project.

[0060] like Figure 2 As shown, S7 is the auxiliary flexible DC converter corresponding to S2. The DC side of S7 is connected in parallel to the DC side of S2 to extract energy, and the AC side of S7 is connected in series to the AC side of S2 (since the AC side of S2 is connected to the valve-side winding of S10, the AC side of S7 is also connected in series to the valve-side winding of the connecting transformer S10 corresponding to S2) to provide current compensation.

[0061] like Figure 2 As shown, S8 is the auxiliary flexible DC converter corresponding to S3. The DC side of S8 is connected in parallel to the DC side of S3 to extract energy, and the AC side of S8 is connected in series to the AC side of S3 (since the AC side of S3 is connected to the valve-side winding of S11, the AC side of S8 is also connected in series to the valve-side winding of the connecting transformer S11 corresponding to S3) to provide voltage compensation.

[0062] like Figure 2 As shown, S15 is the auxiliary flexible DC converter corresponding to S1. The DC side of S15 is connected in parallel to the DC side of S1 to draw energy, and the AC side of S15 is connected in series to the AC side of S1 (since the AC side of S1 is connected to the valve-side winding of S9, the AC side of S15 is also connected in series to the valve-side winding of the connecting transformer S9 corresponding to S1) to provide voltage compensation.

[0063] like Figure 2 As shown, S16 is the auxiliary flexible DC converter corresponding to S4. The DC side of S16 is connected in parallel to the DC side of S4 to draw energy, and the AC side of S16 is connected in series to the AC side of S4 (since the AC side of S4 is connected to the valve-side winding of S12, the AC side of S16 is also connected in series to the valve-side winding of the connecting transformer S12 corresponding to S4) to provide voltage compensation.

[0064] S9-S12 are the corresponding transformers connected to the AC side of the positive high-voltage valve, positive low-voltage valve, negative low-voltage valve, and negative high-voltage valve, respectively. They all adopt the following... Figure 2 The winding connection method shown for “△ / Yn” (Type-Ⅰ) or all of them adopt the following Figure 2 The winding connection method shown is “Y / Yn” (Type-Ⅱ).

[0065] S13 and S14 are the receiving-end AC bus and the receiving-end equivalent AC system, respectively.

[0066] S17, S18, S19, and S20 are transformers that provide series voltage compensation. It is worth noting that, compared to existing conventional DC converter station construction and the DP / AP-LCC topology scheme A, the topology scheme B proposed in this application, due to its AC-side series voltage compensation structure, can utilize an auxiliary flexible DC converter to provide flexible phase-shifting functionality, thereby unifying the transformer winding connection. The core advantage of this technology is that it allows high / low voltage valves to share a single cold standby transformer, significantly reducing initial investment costs without compromising project reliability.

[0067] like Figure 2 As shown, S17 is a specific transformer corresponding to S15 (a transformer used to provide series voltage compensation), and the AC side of S15 is connected in series with the AC side of S1 through S17. Since the AC side of S1 is connected to the valve-side winding of S9, the AC side of S15 is also connected in series with the valve-side winding of the connecting transformer S9 corresponding to S1 through S17.

[0068] like Figure 2 As shown, S18 is a specific transformer corresponding to S16 (a transformer used to provide series voltage compensation), and the AC side of S16 is connected in series with the AC side of S4 through S18. Since the AC side of S4 is connected to the valve-side winding of S12, the AC side of S16 is also connected in series with the valve-side winding of the connecting transformer S12 corresponding to S4 through S18.

[0069] like Figure 2 As shown, S19 is a specific transformer corresponding to S7 (a transformer used to provide series voltage compensation), and the AC side of S7 is connected in series with the AC side of S2 through S19. Since the AC side of S2 is connected to the valve-side winding of S10, the AC side of S7 is also connected in series with the valve-side winding of the connecting transformer S10 corresponding to S2 through S19.

[0070] like Figure 2As shown, S20 is a specific transformer corresponding to S8 (a transformer used to provide series voltage compensation), and the AC side of S8 is connected in series with the AC side of S3 through S20. Since the AC side of S3 is connected to the valve-side winding of S11, the AC side of S8 is also connected in series with the valve-side winding of the connecting transformer S11 corresponding to S3 through S20.

[0071] It is understandable that, such as Figure 2 As shown, a conventional DC converter station includes conventional DC converter valve groups (S1-S4), auxiliary flexible DC converters (S7, S8, S15, S16), connecting transformers (S9-S12), and specific transformers (S17-S20).

[0072] like Figure 2 As shown, the main functions and steady-state power relationships of each device and component in topology B are basically similar to those in topology scheme A, and will not be repeated below. It is worth noting that the AC-side series voltage compensation provided by the auxiliary flexible DC converter differs from the above schemes, specifically including: S17, S18, S19, and S20 are transformers providing series voltage compensation. S7, S8, S15, and S16 are auxiliary flexible DC converters connected in parallel to the DC side of S2, S3, S1, and S4, respectively. The AC side of the auxiliary flexible DC converters (S7, S8, S15, and S16) is connected in series to the valve side of the connecting transformer via transformers (S17, S18, S19, and S20) to form a series voltage compensation structure. The specific AC side voltage division relationship is as follows: (5); In the formula: The valve-side AC voltage phasor of the connecting transformer corresponding to the low / high voltage valve; The AC voltage phasor at the AC side outlet of the low / high pressure valves of a conventional DC converter valve group LCC; The AC voltage phasor output of the auxiliary flexible DC converter corresponding to the low / high voltage valves of the conventional DC converter valve group LCC; This refers to the turns ratios of transformers S17, S18, S19, and S20. Specifically, when the shut-off angle of the low-pressure valve remains constant, that is... When the output remains unchanged, the output of the auxiliary flexible DC converter can be flexibly adjusted. The amplitude and phase angle of the voltage provide voltage support for the receiving end system.

[0073] Specifically, S9-S12 are the connecting transformers corresponding to S1-S4 respectively. The difference between them and Scheme A is that the connection method of the connecting transformer windings can be unified. Therefore, taking the positive terminal as an example, the voltage transformation relationship corresponding to its high / low voltage valves is as follows: (6); In the formula: The voltage phasor of the receiving-end AC bus; , The following are the transformer turns ratios corresponding to the positive high-pressure valve and the positive low-pressure valve, respectively. This is the AC voltage phasor on the valve side of the transformer corresponding to the positive high-voltage valve. This is the AC voltage phasor on the valve side of the transformer corresponding to the positive low-voltage valve.

[0074] Figure 3 The diagram shows a control strategy block diagram for the output voltage-phase construction loop of the auxiliary flexible DC converter provided in this application embodiment. Taking the active power synchronization grid construction control strategy as an example, it specifically includes the following control links: S31, constant AC voltage outer loop; S32, vector current decoupling inner loop and voltage feedforward loop; S33, active power synchronization and phase angle generation link; S34, coordinate system transformation link from the dq coordinate system to the stationary ABC coordinate system. It is worth noting that this application includes auxiliary flexible DC converters adopting grid-following (e.g., dual closed-loop constant AC side voltage control, etc.) or grid-construction control strategies (e.g., active power synchronization control strategy or virtual synchronous machine control strategy, etc.). This example uses the active power synchronization control strategy as an example for illustrative explanation.

[0075] S31 corresponds to the outer loop of the AC voltage. Specifically, the reference value for the d-axis component of the AC side voltage of the auxiliary flexible DC converter is set to 1, and the reference value for the q-axis component is set to 0. The actual values ​​of the d-axis and q-axis voltages after acquisition, filtering, and per-unit processing are then compared with the reference values ​​to form a deviation signal. Finally, the feedback deviation signal is passed through a proportional-integral circuit with saturation to form the reference values ​​for the d-axis and q-axis current components of the inner loop. The above control block diagram ( Figure 3 The mathematical form of ) is as follows: (7); in, , These represent the reference values ​​for the d-axis and q-axis components of the current, respectively. and These represent the proportions of the PI components corresponding to the d-axis and q-axis channels of the outer ring, respectively. and These represent the integral coefficients of the PI element corresponding to the d-axis and q-axis channels of the outer ring, respectively. , These represent the reference values ​​for the d-axis and q-axis components of the voltage, respectively. , These represent the per-unit values ​​of the voltage d-axis and q-axis components, respectively.

[0076] Specifically, S32 corresponds to the vector current decoupling inner loop and the voltage feedforward loop. The first step involves subtracting the actual d-axis / q-axis current values ​​(or the per-unit values ​​of the d-axis / q-axis current components on the AC side of the auxiliary flexible DC converter) after sampling, filtering, and per-unit processing from the set d-axis / q-axis current reference values, and passing the deviation through a saturated proportional-integral circuit. The second step involves adding a decoupling signal containing the actual q-axis / d-axis current values ​​to the feedback deviation signal to achieve vector current cross-decoupling. Finally, the actual d-axis / q-axis voltage values ​​are fed forward onto the d-axis / q-axis signal path, ultimately generating the d-axis / q-axis modulation ratio components. The above control block diagram (…) Figure 3 The mathematical form of ) is as follows: (8); in, , These represent the modulation ratio components along the d-axis and q-axis, respectively. and These represent the proportion and integral coefficient of the inner loop PI element in the d-axis current, respectively. and These represent the proportion and integral coefficient of the PI element in the inner loop of the q-axis current, respectively. and These represent the per-unit values ​​of the d-axis current component and the q-axis current component on the AC side of the auxiliary flexible DC converter, respectively. and These represent the reference values ​​for the d-axis current and the q-axis current, respectively. The current equivalent decoupling coefficient; , These represent the per-unit values ​​of the voltage d-axis and q-axis components, respectively. Specifically, S34 represents the coordinate system transformation from the rotating dq coordinate system to the stationary ABC coordinate system, corresponding to the inverse Park transformation. The specific mathematical form of the inverse Park transformation is not elaborated here. The process of the inverse Park transformation is based on... , and By transforming the coordinate system from the dq coordinate system to the stationary ABC coordinate system, we can obtain... , and . Indicates the modulation ratio of phase A, Indicates the B-phase modulation ratio, This indicates the C-phase modulation ratio.

[0077] Specifically, S33 represents the active power synchronization and phase angle generation stage. The first step involves subtracting the actual active power value (after sampling, filtering, and per-unit processing) from the active power reference value, and then processing the feedback deviation signal through a first-order filter. The second step involves adding the fundamental frequency to the resulting signal to obtain the actual controller synchronization frequency. Finally, the frequency is integrated to generate the output phase angle, which participates in the coordinate system transformation stage in S34. The above control block diagram (…) Figure 3 The mathematical form of ) is as follows: (9); In the formula: , These are the active power reference value and the actual active power per unit value on the AC side of the auxiliary flexible DC converter, respectively. , , These are the angular frequency deviation, the fundamental angular frequency, and the phase angle on the AC side of the auxiliary flexible DC converter, respectively. D and D represent the equivalent inertial time constant and equivalent damping coefficient corresponding to the first-order low-pass filter, respectively. It is worth noting that, since active power synchronous control is analogous to the rotor operating equation of a synchronous machine, it can provide a certain power damping effect to the system, which is beneficial to improving system stability.

[0078] Figure 4 The diagram shows the block diagram of the conventional DC converter valve group LCC control strategy in two improved topologies provided in this application. Its main body is based on the constant DC voltage control of the conventional DC converter valve group LCC and the constant turn-off angle control during AC fault ride-through. Specifically, it includes the following control links: S41, DC voltage feedback deviation generation link; S42, feedback deviation PI processing link; S43, backup constant turn-off angle control; S44, control mode switching link.

[0079] like Figure 4 As shown, the implementation logic of each step in the control block diagram is as follows: S41 corresponds to the DC voltage feedback deviation generation stage. Specifically, it sets a reference value for the DC voltage of the conventional DC converter valve group LCC, and compares the actual DC voltage value after acquisition, filtering, and per-unit processing with the DC voltage reference value to form a deviation signal. S42 corresponds to the feedback deviation PI processing stage. Specifically, the deviation signal is processed by a saturated PI control stage to form a turn-off angle signal, and the trigger angle signal is obtained by subtracting the turn-off angle signal from 180°. (See the control block diagram above.) Figure 4 The mathematical form of ) is as follows: (10); in: , These correspond to the firing angle and shut-off angle of the LCC in a conventional DC converter valve group, respectively. and These represent the proportional and integral coefficients of the PI parameters of the DC voltage loop, respectively; T is the time constant of the first-order low-pass filter. , These are the DC voltage reference value and the DC voltage per-unit value, respectively.

[0080] S43 corresponds to backup fixed turn-off angle control. Specifically, when an AC side fault occurs in the conventional DC converter valve group (LCC), to prevent commutation failure caused by the turn-off angle being less than the commutation arc-extinguishing angle, the maximum value of the trigger angle is often fixed at 150°. S44 corresponds to control mode switching. When the turn-off angle is greater than 30°, constant DC voltage control is used; otherwise, it switches to backup fixed turn-off angle control to limit the turn-off angle to 30° to avoid commutation failure. It is worth noting that the above-mentioned fixed turn-off angle backup control cannot completely eliminate the risk of commutation failure. Therefore, the two improved topology schemes A and B proposed in this application are needed to further optimize the operating characteristics of the LCC.

[0081] like Figure 5 As shown in the figure, this application provides flowcharts illustrating the implementation of the active filtering mode, dynamic reactive power compensation mode, and wideband oscillation damping function of the improved DP / AP-LCC topology scheme A under different operating conditions and scenarios. Based on the detailed discussion above of the topology, steady-state operating characteristics, and basic control architecture of the proposed DP / AP-LCC topology scheme A, the implementation process of the various flexible control functions integrated in the DP / AP-LCC topology scheme A will be further discussed below.

[0082] In some implementations, such as Figure 5 As shown, the main integrated functions of the DP / AP-LCC improved topology scheme with DC-side parallel self-powered and AC-side parallel current compensation are: active filtering mode, dynamic reactive power compensation mode, and wideband oscillation damping mode.

[0083] Specifically, the implementation process of the active filtering mode is as follows: First, the harmonic content of the current of the AC bus at the receiving end is detected in real time through harmonic monitoring technology, such as the S52 link; when the harmonic content of the current at the AC bus at the receiving end exceeds the standard, the second step is to use the parallel structure of the AC side of the auxiliary flexible DC converter to put into harmonic current suppression control to realize the active filtering function, such as the S55 link. Specifically, the realization of the active filtering function depends on the current splitting relationship shown in equation (2). The specific implementation principle can be described as follows: by putting into the harmonic current compensation control link of the auxiliary flexible DC converter, the auxiliary flexible DC converter actively splits and absorbs the harmonic current of the AC bus at the receiving end. The specific control strategy is similar to that of the existing active filters. It is worth noting that this application innovatively integrates the active filtering function with the traditional conventional DC converter valve group LCC. On the one hand, it can reduce the configuration requirements of the passive filtering equipment of the newly built conventional DC converter valve group LCC, and on the other hand, it can realize the active hot standby of the passive filtering equipment of the existing conventional DC converter valve group LCC.

[0084] Specifically, the implementation process of the dynamic reactive power compensation mode is as follows: First, obtain the AC side fault information of the system through relay protection measures such as voltage detection and communication, and analyze the voltage drop of the AC bus at the receiving end, such as in the S53 link; when an AC fault is detected, the second step is to use the capacity margin of the auxiliary flexible DC converter to provide rapid reactive power-voltage support, maintain the voltage stability of the AC grid at the receiving end during the S14 transient process, and prevent commutation failure, such as in the S56 link. Specifically, the realization of the dynamic reactive power compensation function depends on the control outer loop action shown in equation (7). The specific implementation principle is as follows: when the voltage drops, a transient deviation will occur between the d-axis voltage component and the reference value, thereby causing the adjustment of the inner loop current reference value, and finally the negative feedback acts on the d-axis voltage component to keep it stable. It is worth noting that when the outer loop control target is set to a constant AC voltage amplitude, the auxiliary flexible DC converter acts in the transient transfer process as equivalent to providing reactive power support; however, there may be problems such as the saturation of the inner loop current reference value and the inability to suppress the voltage drop. Therefore, from the perspective of providing reactive power compensation only, the transient state can be switched to constant reactive power control to prevent current limiting problems caused by reactive power exceeding the limit. The specific control architecture will not be elaborated here.

[0085] Specifically, the implementation process of the broadband oscillation damping mode is as follows: First, the receiving-end grid strength is monitored in real time using grid strength analysis technology, and the broadband oscillation risk under certain operating conditions is assessed online / offline, such as in step S54. When the grid strength is detected to be too low or the stability margin is insufficient, the second step is to use the parameter tuning of the auxiliary flexible DC converter to shape the AC side impedance characteristics to improve the system stability margin, such as in step S57. Specifically, the realization of the broadband oscillation damping function depends on the interaction between the above-mentioned system electrical dynamic characteristics and controller dynamic characteristics, which involves complex impedance monitoring and impedance reshaping technologies. This application only adopts the most basic parameter tuning method in active impedance reshaping technology, which avoids the resonant frequency point or increases the damping effect at the resonant frequency point by adaptively adjusting the LCC parameters of the auxiliary flexible DC converter and the conventional DC converter valve group. It is worth noting that the above scheme is currently only analyzed and used in offline mode. Its online real-time broadband oscillation damping function depends on the cooperation and development of multiple fields such as online impedance identification and online oscillation monitoring.

[0086] like Figure 6 As shown, the main integrated functions of the LCC improved topology scheme with DC-side parallel self-powered and AC-side series voltage compensation are: flexible phase-shifting mode, flexible voltage regulation mode, and stabilization enhancement mode.

[0087] Specifically, the implementation process of the flexible phase shift function is as follows: First, the harmonic content of the grid-connected bus / transformer grid-side outlet voltage is detected in real time through harmonic monitoring technology, such as in the S62 link; when the harmonic content of the grid-connected bus outlet voltage exceeds the standard, the second step is to use the AC side series structure of the auxiliary flexible DC converter (such as full-bridge MMC or hybrid MMC) to perform harmonic flexible phase shift, so as to achieve the effect of harmonic voltage overlap and phase reversal elimination, such as in the S65 link. Specifically, the realization of flexible phase shift depends on the voltage division relationship shown in equation (5), and the specific implementation principle can be described as follows: by putting into the harmonic voltage compensation control link of the auxiliary flexible DC converter, the auxiliary flexible DC converter actively provides tangential phase shift compensation harmonic voltage components. The specific control strategy is similar to that of the existing active phase shifter. It is worth noting that this application innovatively integrates flexible phase shifting with the traditional conventional DC converter valve group (LCC). Compared with the DP / AP-LCC scheme of topology A, it can further unify the transformer winding connection method of the conventional DC converter valve group (LCC), thereby reducing the number of cold standby transformers in the conventional DC converter valve group (LCC) and improving the economic efficiency of the project.

[0088] Specifically, the implementation process of the flexible voltage regulation mode is as follows: First, obtain the AC side fault information of the system through voltage detection, communication and other relay protection measures, and analyze the degree of AC side voltage drop, such as in the S63 link; when an AC fault is detected, the second step is to use the capacity margin of the auxiliary flexible DC converter (such as full-bridge MMC or hybrid MMC) to quickly provide reactive power-voltage support, maintain the voltage stability of the receiving end AC grid during the S14 transient process, and prevent commutation failure, such as in the S66 link. Specifically, the realization of the dynamic reactive power compensation function depends on the control outer loop action shown in equation (7). The specific implementation principle is as follows: when the voltage drops, a transient deviation will occur between the d-axis voltage component and the d-axis reference value, thereby causing the inner loop current reference value to be adjusted, and finally the negative feedback acts on the d-axis voltage component to keep it stable. It is worth noting that the traditional conventional DC converter valve group LCC voltage adjustment strategy mainly relies on the transformer tap changer to perform discrete adjustment between different taps. However, this control strategy can extend the characteristics of conventional DC converter valve group LCC voltage support, and realize flexible, continuous and fast voltage regulation and reactive power support capabilities.

[0089] Specifically, the implementation process of the stabilization enhancement mode is as follows: First, the receiving-end grid strength is monitored in real time using grid strength analysis technology, and the risk of broadband oscillations under certain operating conditions is assessed online / offline, such as in the S64 stage. When the grid strength is detected to be too low, the second step is to use the parameter tuning of the auxiliary flexible DC converter to shape the AC side impedance characteristics, and use the auxiliary flexible DC converter grid construction control mode to improve the system's equivalent short-circuit ratio, such as in the S67 stage. Specifically, the active power synchronous grid construction control mode can be analogous to the rotor motion equation of a synchronous machine, increasing the system's equivalent inertial time constant and equivalent damping coefficient, improving the power damping effect similar to that of a power system stabilizer, and reducing the risk of broadband oscillations and commutation failure.

[0090] In summary, both LCC improved topologies proposed in this application (Solution A: DP / AP-LCC; Solution B: DP / AS-LCC) are based on the controllable device-assisted commutation technology and exhibit certain dual characteristics: Solution A is a parallel structure with AC side current compensation, while Solution B is a series structure with AC side voltage compensation. Furthermore, the basic control strategies of the two improved topologies are similar, and additional functional modes can be adopted according to different operating conditions and scenarios. Solution A, based on the highly controllable device-assisted commutation technology, improves the operating characteristics of the receiving-end LCC and supports multiple support modes (active filtering mode, dynamic reactive power compensation mode, and wideband oscillation damping mode), such as... Figure 5 As shown in section S58; Scheme B is based on the highly controllable device-assisted commutation technology route, which improves the operating characteristics of the receiving-end LCC and supports multiple support modes (flexible phase-shifting mode, flexible voltage regulation mode, and stabilization enhancement mode), such as Figure 6As shown in section S68; the core engineering value of this topology lies in reducing the redundancy of transformer reserves within the station by unifying the transformer winding connection method, thus significantly improving the project's economic efficiency. Therefore, this application classifies the above two schemes into a class of conventional DC transmission valve-side amplitude-phase control topologies and control strategies based on parallel auxiliary converters.

[0091] It is understood that the various numerical designations used in the embodiments of this application are merely for the convenience of description and are not intended to limit the scope of the embodiments of this application.

[0092] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A conventional DC transmission valve-side phase-controlled topology based on a parallel converter, characterized in that, include: DC lines, conventional DC converter valve assemblies, connecting transformers, auxiliary flexible DC converters, and receiving-end AC busbars; DC lines are used to transmit DC power to conventional DC converter valve assemblies; A conventional DC converter valve assembly is used to convert the received DC power to AC power, and then feed the converted AC power into the receiving-end AC bus via a connecting transformer. Conventional DC converter valve assemblies are equipped with high-pressure valves and low-pressure valves at their positive and negative terminals. The topology is configured with four auxiliary flexible DC converters, with each auxiliary flexible DC converter corresponding to one valve in a conventional DC converter valve group; The DC side of the auxiliary flexible DC converter is connected in parallel with the DC side of the corresponding valve; The AC side of the auxiliary flexible DC converter is connected in parallel with the AC side of the corresponding valve, or the AC side of the auxiliary flexible DC converter is connected in series with the AC side of the corresponding valve. An auxiliary flexible DC converter is used to regulate the voltage amplitude and phase angle on the AC side of the auxiliary flexible DC converter using a grid-type control strategy. The positive terminal of the high-pressure valve on the positive side of a conventional DC converter valve assembly is connected to the positive terminal of the DC line; the negative terminal of the high-pressure valve on the positive side of a conventional DC converter valve assembly is connected to the positive terminal of the low-pressure valve on the positive side of the conventional DC converter valve assembly; the negative terminal of the low-pressure valve on the positive side of a conventional DC converter valve assembly is connected to the positive terminal of the low-pressure valve on the negative side of the conventional DC converter valve assembly; the negative terminal of the low-pressure valve on the negative side of a conventional DC converter valve assembly is connected to the positive terminal of the high-pressure valve on the negative side of the conventional DC converter valve assembly; and the negative terminal of the high-pressure valve on the negative side of a conventional DC converter valve assembly is connected to the negative terminal of the DC line. The AC side of the following valves in a conventional DC converter valve group is connected to the valve-side winding of the corresponding connecting transformer: positive high-voltage valve, positive low-voltage valve, negative low-voltage valve, and negative high-voltage valve. The grid-side winding of the connecting transformer is connected in parallel to the receiving-end AC bus.

2. The conventional DC transmission valve-side phase-controlled topology based on a parallel converter according to claim 1, characterized in that, The auxiliary flexible DC converter adopts the modular multilevel converter (MMC) structure; Modular multilevel converter (MMC) is a half-bridge MMC based on half-bridge submodule HBSM, or a hybrid MMC based on half-bridge submodule HBSM and full-bridge submodule FBSM.

3. The conventional DC transmission valve-side phase-controlled topology based on a parallel converter according to claim 1, characterized in that, When the AC side of the auxiliary flexible DC converter is connected in series with the AC side of the corresponding valve, the AC side of the auxiliary flexible DC converter is connected in series through a specific transformer; A specific transformer is used to provide series voltage compensation.

4. The conventional DC transmission valve-side phase-controlled topology based on a parallel converter according to claim 1, characterized in that, When the AC side of the auxiliary flexible DC converter is connected in parallel with the AC side of the corresponding valve, the connecting transformers corresponding to the positive high-voltage valve and the negative high-voltage valve adopt the Y / Yn connection method, and the connecting transformers corresponding to the positive low-voltage valve and the negative low-voltage valve adopt the Δ / Yn connection method. Among them, the Y / Yn connection method is that the valve side winding of the transformer is star-connected and the neutral point is not grounded, while the grid side winding is star-connected and grounded through the neutral point; The △ / Yn wiring method means that the transformer valve-side winding is connected in a delta configuration, while the grid-side winding is connected in a star configuration and grounded through the neutral point.

5. The conventional DC transmission valve-side phase-controlled topology based on a parallel converter according to claim 1, characterized in that, When the AC side of the auxiliary flexible DC converter is connected in series with the AC side of the corresponding valve, all connecting transformers adopt the same wiring method, which is either Y / Yn connection or Δ / Yn connection. Among them, the Y / Yn connection method is that the valve side winding of the transformer is star-connected and the neutral point is not grounded, while the grid side winding is star-connected and grounded through the neutral point; The △ / Yn wiring method means that the transformer valve-side winding is connected in a delta configuration, while the grid-side winding is connected in a star configuration and grounded through the neutral point.

6. A control method for a conventional DC transmission valve-side phase-controlled topology based on a parallel converter, characterized in that, The control method, applied to a conventional DC transmission valve-side amplitude-phase control topology based on a parallel converter as described in any one of claims 1-5, includes: A grid-type control strategy is adopted to regulate the voltage amplitude and phase angle on the AC side of the auxiliary flexible DC converter.

7. The control method for a conventional DC transmission valve-side amplitude-phase modulation topology based on a parallel converter according to claim 6, characterized in that, The grid-based control strategy is specifically an active power synchronization control strategy. This strategy involves regulating the voltage amplitude and phase angle on the AC side of the auxiliary flexible DC converter, including: Reference value of the d-axis component of the AC side voltage based on the auxiliary flexible DC converter d-axis component per unit value q-axis component reference value per unit value of q-axis component By using a constant AC voltage outer loop, the reference value of the d-axis component of the AC side current of the auxiliary flexible DC converter is obtained. and q-axis component reference value ; based on , , and The d-axis modulation ratio component is obtained by decoupling the inner loop with vector current and the voltage feedforward loop. and q-axis modulation ratio components ; Active power reference value on the AC side based on the auxiliary flexible DC converter and active power per unit value The phase angle of the AC side of the auxiliary flexible DC converter is obtained through the power synchronization loop. ; based on , and The modulation ratio of phase A is obtained by transforming the dq coordinate system to the stationary ABC coordinate system. B-phase modulation ratio C-phase modulation ratio , , and Used to regulate the voltage amplitude and phase angle on the AC side of the auxiliary flexible DC converter.

8. The control method for a conventional DC transmission valve-side phase-controlled topology based on a parallel converter according to claim 6, characterized in that, Also includes: When the shut-off angle is greater than 30°, the conventional DC converter valve group adopts constant DC voltage control. When the shut-off angle is less than or equal to 30°, the conventional DC converter valve group adopts constant shut-off angle control to limit the shut-off angle to 30°.

9. The control method for a conventional DC transmission valve-side phase-controlled topology based on a parallel converter according to claim 8, characterized in that, Constant DC voltage control includes: Based on the DC voltage reference value and DC voltage per unit value of a conventional DC converter valve group, a deviation signal is generated; The turn-off angle is determined based on the deviation signal through a PI control circuit.

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