Method and system for suppressing multi-infeed subsequent commutation failure considering dc inter-area interaction

Abstract

The application discloses a kind of considering the interaction of direct current between multi-infeed subsequent commutation failure suppression method and system, it is applied to the multi-infeed direct current system of high voltage direct current transmission LCC-HVDC based on grid commutation converter, by constructing equivalent model and inverter side control model, the coupling relationship between each direct current loop is characterized;Based on interaction coupling effect, build multi-infeed interaction factor, and establish direct current command value correction model, to suppress the commutation bus voltage drop, increase the off angle as target, calculate the optimized direct current command value;Simultaneously, construct control strategy switching criterion, switch and restore control mode within the preset time after disturbance;When ground fault occurs and meets the criterion, the optimized instruction value is input into the control model, instead of the original instruction value, to cooperatively suppress the subsequent commutation failure of fault loop and non-fault loop;The application can simultaneously suppress the subsequent commutation failure of fault loop and non-fault loop, and improve the system fault resistance ability.

Description

Technical Field

[0001] This invention belongs to the field of high voltage direct current transmission technology, and particularly relates to a method and system for suppressing subsequent commutation failures of multiple feeders that considers DC-DC interaction. Background Technology

[0002] High-voltage direct current (LCC-HVDC) transmission based on grid-commutated converters has been widely used in long-distance, high-capacity power transmission scenarios in my country due to its advantages such as large transmission capacity, mature technology, and low loss. With the continuous expansion of the power grid, multiple LCC-HVDC transmission projects are densely fed into the same receiving-end AC grid, forming typical multi-infeed DC systems. Load centers in East and South my country have already exhibited a distinct multi-infeed DC transmission pattern. While these systems enhance the long-distance power transmission capacity, they also make the structure of the receiving-end grid increasingly complex, placing higher demands on the safe and stable operation of the system.

[0003] To address the commutation failure problem in high-voltage direct current (HVDC) transmission systems, existing technologies primarily focus on single-circuit LCC-HVDC systems, proposing various suppression measures. For example, some studies use detection of commutation voltage amplitude or commutation area to pre-trigger the low-voltage current limiting controller (VDCOL) and adjust the DC current command to reduce commutation demand; others improve the inverter station turn-off angle control strategy, dynamically increasing the turn-off angle margin after a fault occurs to reduce the risk of commutation failure. Furthermore, some studies employ additional control methods such as virtual resistors and synchronous compensators to enhance the inverter's AC bus voltage support capability. These methods, to some extent, improve the commutation failure resilience of single-circuit DC systems and provide a foundation for the design of control strategies in subsequent complex scenarios.

[0004] However, the aforementioned existing technologies are mainly designed for single-circuit DC systems and do not fully consider the complex coupling and interaction between circuits in multi-infeed DC systems. In actual multi-infeed scenarios, when a ground fault occurs in the receiving-end AC system, not only is commutation failure likely to occur on the faulty circuit, but its voltage fluctuations and power changes can also couple to the non-faulty circuits through the AC network, causing abnormal fluctuations in the electrical quantities of the non-faulty circuits, which in turn can lead to subsequent commutation failures or even blocking on the non-faulty circuits. Because existing control strategies cannot effectively account for the interactive effects between DC systems, it is difficult to distinguish the dynamic response differences between faulty and non-faulty circuits. Therefore, it is impossible to simultaneously suppress subsequent commutation failures in two or more DC systems, resulting in insufficient overall system fault resilience, which may lead to chain reactions or even system collapse in severe cases. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a method and system for suppressing subsequent commutation failures in multi-infeed DC systems that takes into account the interaction between DC and DC, so as to solve the problem that the prior art does not fully consider the coupling interaction between DC and DC, resulting in the inability to simultaneously suppress subsequent commutation failures of faulty return and non-faulty return, and improve the overall fault resistance capability of multi-infeed DC systems.

[0006] Technical Solution: The multi-infeed subsequent commutation failure suppression method considering DC-DC interaction described in this invention is applied to a multi-infeed DC system composed of a grid-commutated DC transmission LCC-HVDC, and includes the following steps:

[0007] S1. Construct an equivalent model and an inverter-side control model for a multi-infeed DC system. The equivalent model is used to characterize the coupling relationship between each DC transmission circuit. The inverter-side control model includes constant current control and constant turn-off angle control, and determines the coupling impedance, equivalent impedance, inverter-side converter bus voltage, inverter-side DC voltage, inverter-side DC current, and control parameters of each DC transmission circuit.

[0008] S2. Based on the equivalent model established in step S1, characterize the interactive coupling effect between DC currents in the multi-infeed DC system, construct the multi-infeed interaction factor, and establish a DC current command value correction model that considers the interaction factor.

[0009] S3. Based on the DC current command value correction model, with the goal of suppressing the excessive reduction of the inverter side converter bus voltage and increasing the turn-off angle, the optimized DC current command value is calculated.

[0010] S4. Construct a control strategy switching criterion to determine whether the inverter side control mode has switched from constant current control to constant turn-off angle control, and restore the original control strategy within a preset time after system disturbance.

[0011] S5. When a ground fault occurs in any DC transmission circuit in the multi-feed DC system and the control strategy switching criterion is met, the optimized DC current command value is input to the inverter-side control model to replace the original DC current command value, which is used to collaboratively suppress subsequent commutation failures of fault-to-DC and non-fault-to-DC.

[0012] This invention, through the equivalent model of the multi-infeed DC system and the inverter-side control model constructed in step S1, systematically quantifies the coupling impedance and electrical parameters between each DC loop for the first time, laying a theoretical foundation for revealing the interaction mechanism between DC loops. Step S2 uses this model to establish a multi-infeed interaction factor and correct the DC current command value, effectively solving the problem of lack of coordinated control caused by neglecting the coupling between DC loops in the prior art. Step S3 optimizes the current command value with the goal of suppressing the voltage drop of the converter bus and increasing the turn-off angle. Step S4 designs a control strategy switching criterion to realize the flexible conversion between constant current and constant turn-off angle modes, ensuring rapid recovery of the original strategy after disturbance. Step S5 inputs the optimized command value into the inverter-side control model of both the faulty and non-faulty circuits simultaneously when a fault occurs, thereby enabling coordinated suppression of subsequent commutation failures of multiple DC circuits and significantly improving the overall fault resistance capability of the multi-infeed DC system under complex fault scenarios.

[0013] Preferably, the multi-infeed DC system described in step S1 includes a dual-infeed DC system, and in the equivalent model of the dual-infeed DC system, the reactive power on the inverter side of any DC transmission circuit HVDCp satisfies the following balance relationship:

[0014]

[0015] In the formula, The reactive power provided to the HVDCp receiving-end AC system The reactive power provided to the HVDCp reactive power compensation device The reactive power consumed by the HVDCp inverter The reactive power exchanged between HVDCp and HVDCq in steady state =0.

[0016] This preferred scheme establishes a precise balance relationship of reactive power on the inverter side for dual-infeed DC systems. It incorporates reactive power from reactive compensation devices, receiving-end AC systems, inverter consumption, and inter-loop exchange into a unified equation, and in particular, introduces an exchange term reflecting reactive power interaction between DC systems. Under the steady-state assumption ( =0) accurately describes the system's baseline operating point, providing a key reactive power constraint basis for quantifying DC-DC coupling effects and correcting current command values ​​in subsequent dynamic processes, thereby enhancing the model's ability to characterize multi-feed interaction.

[0017] Preferably, the inverter consumes reactive power. It is determined by the following relationship:

[0018]

[0019]

[0020] In the formula, Where HVDCp is the ideal no-load voltage, and N is the number of six-pulse converters. The transformer turns ratio on the inverter side. This refers to the inverter-side converter bus voltage. This is the DC current on the inverter side. This is the DC voltage on the inverter side.

[0021] This preferred scheme establishes an analytical expression for the reactive power consumption of the inverter by introducing the correlation between the ideal no-load voltage of the inverter and the converter bus voltage, transformer ratio, and number of converters, and combining DC current and DC voltage parameters. This expression can dynamically reflect the direct impact of factors such as converter bus voltage drop and DC current changes on reactive power consumption, providing a key calculation basis for the closed-loop solution of the reactive power balance equation on the inverter side in step S1, thereby improving the accuracy of the equivalent model of the multi-infeed DC system in representing the reactive power interaction characteristics during transient processes.

[0022] Preferably, after a commutation failure occurs, the circuit... reactive power imbalance Defined as the difference between reactive power supply and demand on the inverter side, and the per-unit change in the converter bus voltage on the inverter side. Expressed as the difference in per-unit voltage values ​​of the converter bus before and after the disturbance, the reactive power imbalance and the change in per-unit voltage values ​​satisfy the following relationship:

[0023]

[0024] In the formula, This refers to the reactive power imbalance after a commutation failure. This represents the per-unit change in converter bus voltage. For loop The corresponding short-circuit capacity of the receiving-end AC system.

[0025] This preferred scheme establishes a quantitative relationship between the reactive power imbalance after a commutation failure and the per-unit change in the inverter-side converter bus voltage, incorporating the short-circuit capacity of the receiving-end AC system as a key proportionality coefficient into the model. This relationship can directly quantify the voltage drop from the perspective of reactive power supply and demand imbalance, providing a clear physical link for the construction of the interaction factor and the correction of the DC current command value in step S2. This allows subsequent optimization calculations to accurately characterize the coupling effect of a DC fault on the voltage of the non-faulty converter bus based on the propagation effect of reactive power imbalance.

[0026] Preferably, the turn-off angle in the inverter-side control model described in step S1 satisfies the following expression:

[0027]

[0028] In the formula, For the shut-off angle, It is the inverter-side commutation reactance. For the inverter-side converter transformer turns ratio, It is direct current. It is the inverter-side converter bus voltage. It is the advanced trigger angle.

[0029] This preferred scheme provides an explicit analytical relationship between the turn-off angle and DC current, commutator bus voltage, commutation reactance, transformer turns ratio, and lead trigger angle, revealing the coupling influence mechanism of various electrical parameters on the turn-off angle during commutation. This expression provides a directly calculable mathematical basis for optimizing the turn-off angle in step S3, enabling the optimized DC current command value to quantitatively reflect changes in commutation margin, thereby improving the prediction and control accuracy of subsequent commutation failure suppression.

[0030] Preferably, the formula for calculating the multi-feed interaction factor in step S2 is as follows:

[0031]

[0032] In the formula, For multi-feed interaction factors, This indicates different high-voltage direct current transmission circuits in a multi-feed DC system. express Equivalent mutual impedance between DC and DC currents express The equivalent self-impedance of DC.

[0033] This preferred scheme defines a multi-feed interaction factor based on the ratio of equivalent mutual impedance to equivalent self-impedance, providing a simple and clear metric for quantifying the coupling strength between different DC transmission circuits. This factor reflects the degree of interaction between voltage changes caused by a DC fault in one circuit and the commutation process of another, laying a quantitative foundation for establishing a DC current command value correction model in step S2. This allows the subsequent collaborative suppression strategy to adjust the current command of the faulty and non-faulty circuits in a targeted manner according to the magnitude of the interaction factor, thereby effectively addressing the impact of inter-DC coupling on the propagation of commutation failure.

[0034] Preferably, considering the interaction between DC and DC currents, the per-unit value of the inverter-side converter bus voltage satisfies:

[0035]

[0036] In the formula, for Return DC converter bus voltage per unit value.

[0037] This preferred scheme establishes an iterative expression for the per-unit value of the inverter-side converter bus voltage considering the DC-DC coupling effect by introducing a multi-feed interaction factor. This relationship can quantitatively describe the degree of coupling influence of the change in the voltage of one DC converter bus on the voltage of another DC circuit, thus explicitly incorporating the DC-DC interaction into the voltage calculation process. This provides a mathematical basis for dynamic voltage updates in the DC current command value correction model in step S2, enabling the subsequently optimized current command value to more accurately reflect the voltage coupling characteristics under actual operating conditions.

[0038] Preferably, the formula for calculating the optimized DC current command value in step S3 is as follows:

[0039]

[0040] In the formula, The optimized DC current command value is HVDCq, where HVDCq is the qth high-voltage DC transmission line in a multi-infeed DC system. For multi-feed interaction factors, This represents the per-unit value of the inverter-side converter bus voltage. The inverter-side AC system short-circuit capacity of the HVDCp system is given by [reference to a specific line in the HVDCp system]. HVDCp is the p-th high-voltage direct current transmission line in a multi-infeed DC system. This indicates the rated reactive power of the reactive power compensation device on the inverter side of the HVDCp return DC transmission circuit. This indicates the rated reactive power of the AC system at the inverter side of the HVDCp return DC transmission circuit. This represents the reactive power exchanged between HVDCp and HVDCq, which is 0 in steady state. Indicates the number of six-pulse converters. The transformer turns ratio on the inverter side. This indicates the rated voltage of the inverter-side converter bus in the HVDCp return DC transmission circuit. It is the DC voltage on the inverter side.

[0041] This preferred scheme provides an optimized calculation formula for the DC current command value, taking into account multiple parameters such as multi-feed interaction factors, per-unit value of converter bus voltage, AC system short-circuit capacity, rated reactive power of reactive power compensation device, rated reactive power of receiving-end AC system, inter-circuit exchange reactive power, number of converters, transformer ratio, rated voltage, and DC voltage. This expression aims to suppress excessive reduction in converter bus voltage and increase the turn-off angle. It explicitly embeds the inter-DC interaction effect into the quantitative calculation of the current command, enabling the optimized current command value to adaptively adjust according to the coupling strength between faulty and non-faulty circuits. This provides precise execution parameters for the control strategy switching and coordinated suppression in steps S4 and S5, alleviating the problem that existing technologies cannot simultaneously suppress subsequent commutation failures of multiple DC circuits.

[0042] Preferably, step S5, which involves inputting the optimized DC current command value into the inverter-side control model, includes:

[0043] The optimized DC current command value is connected to the current deviation control (CEC) link of the inverter-side control model. The CEC link outputs a control signal to the inverter-side constant current control module based on the deviation between the received optimized DC current command value and the actual DC current value, so as to adjust the lead trigger angle of the inverter side and thus change the size of the turn-off angle.

[0044] This preferred scheme clarifies that the optimized DC current command value functions by being integrated into the current deviation control loop of the inverter-side control model. Utilizing the loop's response characteristics to the deviation between the command and actual values, a control signal is output to drive the constant current control module to adjust the lead firing angle, thereby changing the turn-off angle. This mechanism enables the optimized current command value to be transformed into a closed-loop direct intervention in the commutation process, ensuring the effective execution of the theoretical optimization results in the actual controller. It provides a reliable control implementation path for the coordinated suppression of subsequent commutation failures by both fault-free and non-fault-free DC current.

[0045] The multi-infeed DC system based on grid-commutated phase converters and LCC-HVDC transmission, as described in this invention, includes:

[0046] The equivalent modeling and parameter extraction module is used to construct the equivalent model and inverter-side control model of the multi-infeed DC system. The equivalent model is used to characterize the coupling relationship between each DC transmission circuit. The inverter-side control model includes constant current control and constant turn-off angle control, and determines the coupling impedance, equivalent impedance, inverter-side converter bus voltage, inverter-side DC voltage, inverter-side DC current and control parameters of each DC transmission circuit.

[0047] The interaction characterization and current command correction module is used to characterize the interaction coupling between DC systems in a multi-infeed DC system based on the equivalent model, construct a multi-infeed interaction factor, and establish a DC current command value correction model that considers the interaction factor.

[0048] The DC current command optimization calculation module is used to calculate the optimized DC current command value based on the DC current command value correction model, with the goal of suppressing the excessive reduction of the inverter side converter bus voltage and increasing the turn-off angle.

[0049] The control strategy switching judgment module is used to construct control strategy switching criteria to determine whether the inverter side control mode has switched from constant current control to constant turn-off angle control, and restore the original control strategy within a preset time after system disturbance.

[0050] The subsequent commutation failure collaborative suppression execution module is used to input the optimized DC current command value to the inverter side control model to replace the original DC current command value when a ground fault occurs in any DC transmission circuit in the multi-infeed DC system and the control strategy switching criterion is met. This is used to collaboratively suppress subsequent commutation failures in both fault-to-DC and non-fault-to-DC transmission.

[0051] Beneficial Effects: Compared with the prior art, the present invention has the following significant advantages: 1. By constructing an equivalent model and inverter-side control model of a multi-infeed DC system, the present invention characterizes the coupling relationship between each DC transmission circuit, solving the problem that the prior art cannot simultaneously suppress subsequent commutation failures of faulty and non-faulty circuits, and significantly improving the overall fault resistance capability of the multi-infeed DC system; 2. By using the multi-infeed interaction factor to establish a DC current command value correction model, with the goal of suppressing excessive reduction of converter bus voltage and increasing the turn-off angle, the optimized DC current command value is calculated, fundamentally reducing the risk of subsequent commutation failures of faulty and non-faulty circuits; 3. By constructing a control strategy switching criterion, the constant current control is switched to constant turn-off angle control only when the conditions are met after system disturbance, and the original control strategy is restored within a preset time, realizing the precise switching of optimized commands and reducing the adverse effects on the steady-state operation of the system. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of the method flow of the present invention;

[0053] Figure 2 This is a distribution map of high-voltage direct current transmission lines in a certain area of ​​the power grid according to the present invention;

[0054] Figure 3 The equivalent model and inverter-side control block diagram of the multi-infeed DC system of the present invention are shown in (a) and (b) respectively.

[0055] Figure 4 This is a power characteristic diagram of the dual-feed DC system of the present invention;

[0056] Figure 5 This is a block diagram of the improved DC control system of the present invention;

[0057] Figure 6 The following is a simulation result diagram of Example 1 of the present invention (single-phase grounding, fault duration 0.2s);

[0058] Figure 7 The simulation results are shown in Figure 2 of the present invention (single-phase grounding, fault duration 0.4s).

[0059] Figure 8 The figure shows the simulation results of Example 3 of this invention (three-phase grounding, fault duration 0.4s). Detailed Implementation

[0060] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0061] This invention provides a method for suppressing subsequent commutation failure in multi-infeed DC systems considering inter-DC interaction. This method is applied to multi-infeed DC systems composed of LCC-HVDC transmission lines based on grid-commutated converters. Figure 1 As shown, it includes the following steps:

[0062] S1: Establish the equivalent model and inverter-side control block diagram of the multi-infeed DC system, including determining the coupling impedance, equivalent impedance, converter bus voltage, DC voltage, and DC current of each DC transmission circuit in the system, as well as determining the control parameters for constant current control and constant turn-off angle control on the inverter side.

[0063] S2: Based on the interaction coupling between DC currents in a multi-infeed DC system, an optimized control strategy is implemented using DC current command values ​​that take into account the interaction between DC currents. This includes defining a multi-infeed interaction factor. The specific calculation formula is as follows:

[0064]

[0065] In the formula, This indicates different high-voltage direct current transmission circuits in a multi-feed DC system. express Equivalent mutual impedance between DC and DC currents express The equivalent self-impedance of DC return;

[0066] S3: To avoid excessive reduction in inverter-side converter bus voltage and increase the turn-off angle. To achieve this, the multi-feed interaction factors are combined. The optimized formula for calculating the DC current command value, taking into account the interaction between DC and DC currents, is derived as follows:

[0067]

[0068] In the formula, The optimized DC current command value for HVDCq. This represents the per-unit value of the inverter-side converter bus voltage. For the short-circuit capacity of the inverter-side AC system of the HVDCp system, This indicates the rated reactive power of the reactive power compensation device on the inverter side of the HVDCp return DC transmission circuit. This indicates the rated reactive power of the AC system at the inverter side of the HVDCp return DC transmission circuit. This represents the reactive power exchanged between HVDCp and HVDCq, which is 0 in steady state. Indicates the number of six-pulse converters. The transformer turns ratio on the inverter side. This indicates the rated voltage of the inverter-side converter bus in the HVDCp return DC transmission circuit. It is the inverter-side DC voltage of the HVDCp return DC transmission circuit in a multi-infeed DC system;

[0069] S4: Set the control strategy switching criterion of the suppression method as follows: the inverter side of the multi-infeed DC system switches from constant current control to constant turn-off angle control, and 100ms after the system disturbance occurs, the control strategy is switched back to the original inverter side control strategy.

[0070] S5: When a ground fault occurs in any DC transmission circuit in the multi-infeed DC system, if the switching criterion is met, the optimized DC current command value in step S5 is input to the system inverter-side controller to replace the original current command value, thereby suppressing the subsequent commutation failure of the faulty circuit and the non-faulty circuit in the multi-infeed DC system.

[0071] When the switching criteria are not met, the system will switch the optimized current command value back to the original current command value to reduce the impact of the proposed control strategy on the system. That is, the proposed control strategy only works when the risk of subsequent commutation failure is high. If the fault does not cause the inverter to have the risk of subsequent commutation failure, the proposed control strategy will not be triggered, which is consistent with the actual engineering logic.

[0072] Furthermore, the inverter-side control model of the multi-infeed DC system consists of constant current control and constant turn-off angle control. The expression for the inverter turn-off angle is:

[0073]

[0074] In the formula, It is the inverter-side commutation reactance. For the inverter-side converter transformer turns ratio, It is direct current. It is the inverter-side converter bus voltage. It is the advanced trigger angle.

[0075] Furthermore, in the equivalent model of a dual-infeed DC system, the reactive power relationship at the inverter-side converter bus of the HVDCp is expressed as:

[0076]

[0077] In the formula, The reactive power provided to the HVDCp receiving-end AC system The reactive power provided to the HVDCp reactive power compensation device The reactive power consumed by the HVDCp inverter The reactive power exchanged between HVDCp and HVDCq in steady state =0.

[0078] Furthermore, the reactive power consumed by the HVDCp inverter Through the ideal no-load voltage of HVDCp DC voltage on the inverter side and inverter-side DC current It is derived that the ideal no-load voltage The number of six-pulse converters Inverter-side transformer turns ratio and inverter-side converter bus voltage Decide.

[0079] Furthermore, the reactive power imbalance of HVDCp after commutation failure per-unit value of the change in inverter-side converter bus voltage satisfy: and Negative correlation with the short-circuit capacity of the inverter-side AC system in the HVDCp system. Positive correlation.

[0080] Furthermore, multi-feed interaction factors The per-unit expression for the inverter-side converter bus voltage is derived from the coupling impedance between each DC input in the multi-infeed DC system, the equivalent impedance of the AC system on each DC inverter side, the inverter-side converter bus voltage of each DC input, and the DC electrical parameters, as follows:

[0081]

[0082] Furthermore, the optimized DC current command value in step S3 is obtained by modifying the original DC current command value in combination with the multi-feed interaction factor MOVIFpq, thereby achieving coordinated control of the DC current of the fault return and non-fault return circuits.

[0083] Furthermore, the inverter-side control loop in step S5 consists of constant current control and constant turn-off angle control. The optimized DC current command value is connected to the current deviation control (CEC) loop to regulate the inverter-side lead trigger angle, thereby changing the turn-off angle.

[0084] Furthermore, this method is applicable to both dual-infeed DC systems and multi-infeed DC systems, and can effectively suppress subsequent commutation failures under different fault types such as single-phase grounding and three-phase grounding, as well as fault scenarios with different fault durations.

[0085] Figure 2 The diagram shows the distribution of high-voltage direct current (HVDC) transmission lines in a certain regional power grid. The equivalent model and control block diagram of the corresponding multi-infeed DC system are shown below. Figure 3 As shown. The multi-infeed DC system of the actual regional power grid is equivalent to HVDC1 and HVDCp, HVDC, which have abstract representations. q Simultaneously, the equivalent self-impedance of the AC system on the DC inverter side is extracted for each cycle. (etc.), equivalent coupling impedance in each DC-DC system Based on this, Figure 2 Complex AC and DC coupled networks are abstracted into lumped-parameter impedance networks, achieving engineering simplification of complex networks. This is specifically aimed at... Figure 3 This invention further breaks down the two core functions that play a decisive role in commutation failure control: constant current control and constant turn-off angle control. These are presented as two independent control branches in the control block diagram, consistent with the functional logic of the actual controller. Key design / operational parameters for constant current and constant turn-off angle control are extracted, including the minimum turn-off angle. DC current command value issued by the main control Inverter-side DC current command values, etc., are used to assign actual engineering parameters to the control block diagram. In addition, combined with... Figure 3 The control logic and control model of high voltage direct current transmission clarify the key influencing factors of the change in the turn-off angle and subsequent commutation failure, providing a theoretical basis for the proposal of control strategies.

[0086] Figure 3 In (a), These are the coupling impedances between HVDC1 and HVDCp, HVDC1 and HVDCq, and HVDCp and HVDCq, respectively. This is the equivalent impedance of the AC system on the inverter side corresponding to DC. This corresponds to the inverter-side converter bus voltage for DC. This corresponds to the DC voltage on the inverter side. This refers to the DC current on the inverter side corresponding to DC. Figure 3 In (b), This is the DC current command value from the main control unit; It is the shut-off angle; This is the minimum shut-off angle; This is the DC current command value; CEC is for current deviation control; VDCOL is for low-voltage current limiting control. From Figure 3 It can be seen that the inverter-side control mainly consists of constant current control and constant turn-off angle control. and These are the leading trigger angles of the inverter side constant current control and constant turn-off angle control outputs, respectively.

[0087] When a ground fault occurs in the receiving-end AC system of an LCC-HVDC inverter, the inverter is prone to commutation failure. During the commutation failure recovery process, Figure 3 (b) Improper controller interaction can easily lead to subsequent commutation failures in the faulty DC circuit. Due to the interactive coupling between multiple feeder systems, a faulty DC circuit will also cause electrical fluctuations in the non-faulty DC circuits, which may lead to commutation failures and subsequent commutation failures in the non-faulty DC circuits. In severe cases, this may result in blockage or even system collapse.

[0088] While considerable research has been conducted on commutation failure and subsequent commutation failures in single LCC-HVDC transmission lines, research on commutation failure and subsequent commutation failures in multi-infeed DC systems formed by multiple DC transmission lines remains lacking. This is because the interactive coupling between the multiple infeed DC lines exacerbates the analytical complexity and makes it difficult to formulate suppression strategies. Therefore, this invention, starting from the mechanism of subsequent commutation failures and considering the interactive coupling between the multiple infeed DC lines, proposes a method and system for suppressing subsequent commutation failures in multi-infeed DC systems that considers the interaction between DC lines.

[0089] based on Figure 3 Based on the provided equivalent model, the calculation expression for the shut-off angle is deduced as follows:

[0090]

[0091] in, It is the inverter-side commutation reactance. For the inverter-side converter transformer turns ratio, It is direct current. It is the inverter-side converter bus voltage. It is the advanced trigger angle.

[0092] In the later stages of the commutation failure recovery process, the DC current... The inverter-side converter bus voltage is showing an upward trend. The trend is downward, and the premature trigger angle is increasing. It shows a trend of first decreasing and then increasing. (Regarding electrical quantities) , and control quantity Under the influence of γ, combined with Equation (1), it can be seen that the turn-off angle γ shows a decreasing trend. As the commutation failure is recovered, the turn-off angle gradually decreases, and the risk of subsequent commutation failure increases. Among them, the excessive reduction of the inverter-side converter bus voltage is one of the important factors that cause subsequent commutation failure.

[0093] Taking a doubly-fed DC system as an example, its power characteristics are as follows: Figure 4 As shown.

[0094] Figure 4 middle, and The reactive power consumed by the inverters of HVDCp and HVDCq; and The reactive power provided to the reactive power compensation devices of HVDCp and HVDCq; and The reactive power provided to the receiving-end AC systems of HVDCp and HVDCq; This represents the reactive power exchanged between HVDCp and HVDCq, which is 0 in steady state.

[0095] Taking HVDCp as an example, the power relationship at the inverter-side converter bus can be expressed as:

[0096] (2)

[0097] in, Reactive power provided by the reactive power compensation device for HVDCp Represented as:

[0098] (3)

[0099] In formula (2) It can also be expressed as:

[0100] (4)

[0101] In the formula, The ideal open-circuit voltage of HVDCp is expressed as:

[0102] (5)

[0103] In the formula, N represents the number of six-pulse converters. This refers to the transformer turns ratio on the inverter side.

[0104] Substituting equation (5) into equation (4), we get as follows:

[0105] (6)

[0106] After a commutation failure, the power balance of the multi-infeed DC system is disrupted, resulting in reactive power imbalance. Represented as:

[0107] (7)

[0108] Substituting equation (3) into equation (7), the reactive power imbalance quantity Further transformation can be expressed as:

[0109] (8)

[0110] reactive power imbalance This will affect the inverter-side converter bus voltage. per-unit value of the change in inverter-side converter bus voltage The relationship can be represented as:

[0111] (9)

[0112] In the formula, This refers to the short-circuit capacity of the AC system on the inverter side of the HVDCp system.

[0113] Substituting equation (8) into equation (9), the inverter-side converter bus voltage It can be represented as:

[0114] (10)

[0115] Substituting equation (10) into equation (6), the DC current on the inverter side... It can be represented as:

[0116] (11)

[0117] Since the voltage drop of the inverter-side commutator bus is one of the important factors causing subsequent commutation failure, avoiding excessive reduction of the inverter-side commutator bus voltage can effectively reduce the probability of subsequent commutation failure. Combining equation (11), it can be seen that in the denominator... When it is the steady-state value, Decrease, and combining with equation (6), we can see that, Decrease The recovery speed is faster, and the degree of fall is reduced. According to equation (1), The reduction and An increase in electrical quantity will increase the turn-off angle. and The combined effect of these factors reduces the risk of subsequent commutation failure.

[0118] Based on the above analysis, this invention proposes an optimized control strategy for DC current command value considering the interaction between DC and DC currents. The optimized current command value calculation formula is as follows:

[0119] (12)

[0120] The multi-feed interaction factor MOVIFpq can be expressed as:

[0121] (13)

[0122] Equation (13) uses impedance to characterize the interaction between multiple feeders. The effect of fault return DC on non-fault return DC can be represented by impedance. Therefore, the voltage fluctuation of non-fault return can be characterized by the voltage fluctuation of fault return. The formulation of subsequent commutation failure suppression strategy for non-fault return can take into account the effect of fault return and improve the suppression effect.

[0123] Further simplification of equation (13) yields:

[0124] (14)

[0125] Substituting equation (14) into equation (12), the formula for calculating the optimized current command value after considering the interaction between multiple DC lines is as follows:

[0126]

[0127] The improved DC control block diagram is shown in Figure 4.

[0128] Since the turn-off angle gradually decreases during commutation failure recovery, it is relatively large in the early stage of recovery, resulting in a lower risk of subsequent commutation failure and no need for adjustment. However, in the later stage of recovery, even if the inverter side switches from constant current control to constant turn-off angle control, the turn-off angle has already decreased significantly, increasing the risk of subsequent commutation failure. Therefore, the switching criterion for the proposed control strategy is the switch from constant current control to constant turn-off angle control on the inverter side. After a disturbance occurs, the system typically experiences a 100ms power fluctuation. To avoid adverse effects on the system from the proposed control strategy, the system switches back to the original control strategy after 100ms.

[0129] Based on the CIGRE HVDC standard test model, a two-infeed DC system model was built, and the simulation verified the effect of the proposed strategy on suppressing subsequent commutation failure. The basic parameters of the model are shown in Table 1.

[0130] Table 1. Basic parameters of the model

[0131]

[0132] To verify the effectiveness of the proposed strategy under different fault severity types and different fault durations, a ground fault was set up on the p-back DC inverter side. The simulation example settings are shown in Table 2.

[0133] Table 2 Simulation Case Settings

[0134]

[0135] The simulation results for the three examples in Table 2 are as follows: Figures 6-8 As shown, the red line represents the original control strategy of CIGRE HVDC, and the blue line represents the proposed control strategy. Figures 6-8 In the diagram, (a), (b), and (c) represent the turn-off angle of the p-type DC circuit, the inverter-side converter bus voltage, and the DC current; (d), (e), and (f) represent the turn-off angle of the q-type DC circuit, the inverter-side converter bus voltage, and the DC current.

[0136] from Figure 6 and Figure 8 As can be seen, under the original control strategy, subsequent commutation failure occurred in the p-back DC. Due to the interactive coupling between the q-back DC and the p-back DC, the electrical quantity fluctuations caused by the p-back DC led to fluctuations in the electrical quantity of the q-back DC, which in turn caused subsequent commutation failure in the q-back DC. After applying the proposed control strategy, the voltage drop of the inverter-side converter bus of the p-back DC and q-back DC was reduced, the rise of DC current was reduced, and the subsequent commutation failure of the p-back DC and q-back DC was effectively suppressed.

[0137] from Figure 7 It can be seen that under the original control strategy, the p-return DC experienced three commutation failures and the q-return DC experienced two commutation failures, which seriously endangered the safe and stable operation of the power system. After applying the proposed control strategy, the subsequent commutation failures of both the p-return DC and the q-return DC were effectively suppressed, enhancing the system's fault resistance capability. Figures 6-8 The effectiveness of the proposed strategy was verified under different fault severity types and different fault durations.

Claims

1. A method for suppressing subsequent commutation failure in multi-infeed DC systems considering inter-DC interaction, applied to multi-infeed DC systems composed of LCC-HVDC transmission based on grid-commutated converters, characterized in that, Includes the following steps: S1. Construct an equivalent model and an inverter-side control model for a multi-infeed DC system. The equivalent model is used to characterize the coupling relationship between each DC transmission circuit. The inverter-side control model includes constant current control and constant turn-off angle control, and determines the coupling impedance, equivalent impedance, inverter-side converter bus voltage, inverter-side DC voltage, inverter-side DC current, and control parameters of each DC transmission circuit. S2. Based on the equivalent model established in step S1, characterize the interactive coupling effect between DC currents in the multi-infeed DC system, construct the multi-infeed interaction factor, and establish a DC current command value correction model that considers the interaction factor. S3. Based on the DC current command value correction model, with the goal of suppressing the excessive reduction of the inverter side converter bus voltage and increasing the turn-off angle, the optimized DC current command value is calculated. S4. Construct a control strategy switching criterion to determine whether the inverter side control mode has switched from constant current control to constant turn-off angle control, and restore the original control strategy within a preset time after system disturbance. S5. When a ground fault occurs in any DC transmission circuit in the multi-feed DC system and the control strategy switching criterion is met, the optimized DC current command value is input to the inverter-side control model to replace the original DC current command value, which is used to collaboratively suppress subsequent commutation failures of fault-to-DC and non-fault-to-DC.

2. The method according to claim 1, characterized in that, The multi-infeed DC system mentioned in step S1 includes a dual-infeed DC system, and in the equivalent model of the dual-infeed DC system, the reactive power on the inverter side of any DC transmission circuit HVDCp satisfies the following balance relationship: In the formula, The reactive power provided to the HVDCp receiving-end AC system The reactive power provided to the HVDCp reactive power compensation device The reactive power consumed by the HVDCp inverter The reactive power exchanged between HVDCp and HVDCq in steady state =0.

3. The method according to claim 2, characterized in that, The reactive power consumed by the inverter It is determined by the following relationship: ; In the formula, Where HVDCp is the ideal no-load voltage, and N is the number of six-pulse converters. The transformer turns ratio on the inverter side. This refers to the inverter-side converter bus voltage. This is the DC current on the inverter side. This is the DC voltage on the inverter side.

4. The method according to claim 2, characterized in that, After commutation failure occurs, the circuit reactive power imbalance Defined as the difference between reactive power supply and demand on the inverter side, and the per-unit change in the converter bus voltage on the inverter side. Expressed as the difference in per-unit voltage values ​​of the converter bus before and after the disturbance, the reactive power imbalance and the change in per-unit voltage values ​​satisfy the following relationship: In the formula, This refers to the reactive power imbalance after a commutation failure. This represents the per-unit change in converter bus voltage. For loop The corresponding short-circuit capacity of the receiving-end AC system.

5. The method according to claim 1, characterized in that, The turn-off angle in the inverter-side control model described in step S1 satisfies the following expression: In the formula, For the shut-off angle, It is the inverter-side commutation reactance. For the inverter-side converter transformer turns ratio, It is direct current. It is the inverter-side converter bus voltage. It is the advanced trigger angle.

6. The method according to claim 1, characterized in that, The formula for calculating the multi-feed interaction factor mentioned in step S2 is as follows: In the formula, For multi-feed interaction factors, This indicates different high-voltage direct current transmission circuits in a multi-feed DC system. express Equivalent mutual impedance between DC and DC currents express The equivalent self-impedance of DC.

7. The method according to claim 6, characterized in that, When considering DC-DC interaction, the per-unit value of the inverter-side converter bus voltage satisfies: In the formula, for Return DC converter bus voltage per unit value.

8. The method according to claim 1, characterized in that, The formula for calculating the optimized DC current command value mentioned in step S3 is as follows: In the formula, The optimized DC current command value is HVDCq, where HVDCq is the qth high-voltage DC transmission line in a multi-infeed DC system. For multi-feed interaction factors, This represents the per-unit value of the inverter-side converter bus voltage. The inverter-side AC system short-circuit capacity of the HVDCp system is given by [reference to a specific line in the HVDCp system]. HVDCp is the p-th high-voltage direct current transmission line in a multi-infeed DC system. This indicates the rated reactive power of the reactive power compensation device on the inverter side of the HVDCp return DC transmission circuit. This indicates the rated reactive power of the AC system at the inverter side of the HVDCp return DC transmission circuit. This represents the reactive power exchanged between HVDCp and HVDCq, which is 0 in steady state. Indicates the number of six-pulse converters. The transformer turns ratio on the inverter side. This indicates the rated voltage of the inverter-side converter bus in the HVDCp return DC transmission circuit. It is the DC voltage on the inverter side.

9. The method according to claim 1, characterized in that, Step S5, which involves inputting the optimized DC current command value into the inverter-side control model, includes: The optimized DC current command value is connected to the current deviation control (CEC) link of the inverter-side control model. The CEC link outputs a control signal to the inverter-side constant current control module based on the deviation between the received optimized DC current command value and the actual DC current value, so as to adjust the lead trigger angle of the inverter side and thus change the size of the turn-off angle.

10. A multi-infeed DC system based on a grid-commutated phase converter and composed of LCC-HVDC transmission, comprising: The equivalent modeling and parameter extraction module is used to construct the equivalent model and inverter-side control model of the multi-infeed DC system. The equivalent model is used to characterize the coupling relationship between each DC transmission circuit. The inverter-side control model includes constant current control and constant turn-off angle control, and determines the coupling impedance, equivalent impedance, inverter-side converter bus voltage, inverter-side DC voltage, inverter-side DC current and control parameters of each DC transmission circuit. The interaction characterization and current command correction module is used to characterize the interaction coupling between DC systems in a multi-infeed DC system based on the equivalent model, construct a multi-infeed interaction factor, and establish a DC current command value correction model that considers the interaction factor. The DC current command optimization calculation module is used to calculate the optimized DC current command value based on the DC current command value correction model, with the goal of suppressing the excessive reduction of the inverter side converter bus voltage and increasing the turn-off angle. The control strategy switching judgment module is used to construct control strategy switching criteria to determine whether the inverter side control mode has switched from constant current control to constant turn-off angle control, and restore the original control strategy within a preset time after system disturbance. The subsequent commutation failure collaborative suppression execution module is used to input the optimized DC current command value to the inverter side control model to replace the original DC current command value when a ground fault occurs in any DC transmission circuit in the multi-infeed DC system and the control strategy switching criterion is met. This is used to collaboratively suppress subsequent commutation failures in both fault-to-DC and non-fault-to-DC transmission.

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