Multi-terminal flexible DC control and protection cooperation method based on fault current limiting

Abstract

The invention provides a multi-terminal flexible DC control and protection cooperation method based on fault current limiting, and the method is characterized in that the method comprises the following steps: adjusting the input proportion of an MMC sub-module through a DC current change rate, so as to achieve the suppression of a DC line fault current, and determining a starting threshold value of a DC current change rate current limiting link; the minimum input proportion of the MMC sub-modules is determined, a current amplitude current-limiting link is constructed, and the current amplitude current-limiting link uses the outlet current of the converter station to form an amplitude current-limiting link to adjust the input proportion of the MMC sub-modules in real time; constructing a fault pole selection criterion by using the ratio of the amplitude integrals of the fault voltage components of the positive and negative pole lines; setting integral time of pilot protection fault criterion calculation, and comparing line mode current fault component amplitude integrals on different outgoing lines of the converter station within the integral time to realize fault direction discrimination; and adjusting the starting time of the grid-side current limiting control and the integral window length of the line mode current fault component amplitude integral on different outgoing lines.

Description

Technical Field

[0001] The present invention relates to the field of relay protection and control of power systems, and in particular to a coordinated fault current limiting strategy and a longitudinal directional protection based on line mode current amplitude integration. Background Art

[0002] The multi-terminal flexible direct current transmission system undertakes the important mission of delivering new energy. It can realize multi-power supply and multi-point power receiving, and has broad research and application prospects. However, the DC grid is a low-inertia network. After a fault occurs, multiple converter stations immediately feed current to the fault point, causing the fault current to rise very quickly, threatening the safe operation of primary equipment such as converter stations. At the same time, the fault isolation technology of the multi-terminal flexible direct current grid is not perfect. The lack of mature and economical fault current interrupting equipment and fast and reliable protection solutions restricts the further development of the multi-terminal flexible direct current grid. Therefore, the fault isolation technology and protection technology applicable to multi-terminal flexible direct current need to be improved urgently.

[0003] Fault current limiting control is an effective means to ensure the safety and stability of power equipment in converter stations and extend the protection action time window, which can solve the problem of fault current interruption difficulties to a certain extent. According to the different implementation locations, fault current limiting control can be divided into source side control and grid side control. Source side current limiting control has fast input speed and good economy, and can realize multi-level current limiting control schemes based on electrical quantities such as fault current and voltage, so as to achieve self-adaptation to the severity of the fault, but it will also increase the power fed into the AC side, which may cause the MMC to lock in advance. Compared with the source side current limiting scheme, the grid side current limiting scheme has a slower input speed, but has a stronger current limiting capability. The current limiting DC circuit breaker has both fault current limiting and breaking functions, is flexible and controllable, and can meet the current limiting requirements under different fault scenarios, but only using DC circuit breakers to limit current will lead to a significant increase in costs. In terms of source-grid coordinated fault current limiting, the current limiting control strategy fails to fully combine the advantages of the rapidity of source side current limiting and the strong current limiting capability of grid side current limiting. It is very necessary to study the source-grid coordinated current limiting timing and parameter coordination strategy that can be applied to multi-terminal flexible direct current grids.

[0004] Due to the lack of active current limiting control in actual projects, multi-terminal flexible direct current grids have extremely high requirements for the speed of protection, which makes flexible direct current grids face the current situation of immature main protection and ineffective backup protection. At present, the research on the main protection scheme of flexible direct current grid is still limited to 3-5ms, and the introduction of current limiting control is expected to break the harsh restrictions on protection action time, so that it can adapt to the application scenarios of long-distance and large-capacity power transmission in the future. However, there are relatively few existing studies on protection under current limiting control. The protection scheme constructed only by using the sudden change direction of line current lacks the coordination of protection and control, and the anti-interference ability is relatively limited. Therefore, in the context of more and more active current limiting strategies being proposed and the converter locking problem being alleviated, it is necessary to combine control strategies, improve protection performance, promote the deep integration of the two, and build a new control and protection coordination scheme.

[0005] Control and protection coordination is one of the important solutions to solve the difficulties faced by flexible DC power grids in breaking fault currents and the stringent requirements on protection speed. However, the research on current limiting control using MMC and DC circuit breakers is relatively isolated, and there are difficulties in parameter setting and fault identification. Based on this, the present invention combines source-side current limiting with grid-side current limiting for multi-terminal flexible DC power grids, and designs a selective, fast and effective collaborative current limiting strategy. On the basis of fault current limiting, a longitudinal directional protection scheme based on line-mode current amplitude integration is proposed. Finally, the coordination of fault current limiting control and protection parameters and timing is improved, and a control and protection coordination scheme is constructed, which expands the applicable scenarios of protection and improves the anti-interference ability of protection. Summary of the invention

[0006] The present invention provides a multi-terminal flexible DC control and protection coordination method based on fault current limiting, which mainly includes a coordinated fault current limiting strategy and a longitudinal direction protection based on line mode current amplitude integration, which effectively reduces the fault current amplitude in the flexible DC power grid, expands the application scenarios of the protection scheme, and improves the anti-interference performance of the protection. The technical solution is as follows:

[0007] A multi-terminal flexible direct current control and protection coordination method based on fault current limiting, characterized by comprising the following steps:

[0008] The DC current change rate is used to adjust the MMC submodule input ratio K to suppress the DC line fault current and determine the starting threshold Δ1 of the DC current change rate current limiting link;

[0009] Determine the minimum investment ratio K of MMC submodules min , construct a current amplitude limiting link, wherein the current amplitude limiting link uses the current at the converter station outlet to form an amplitude limiting link to adjust the MMC sub-module input ratio K in real time;

[0010] The fault pole selection criterion is constructed by using the ratio of the fault voltage component amplitude integral of the positive and negative lines.

[0011] Set the integration time for calculating the fault judgment criterion of the longitudinal protection, compare the integral of the fault component amplitude of the line mode current on different outgoing lines of the converter station within this integration time, and realize the fault direction judgment;

[0012] Based on the output of comprehensive control and protection, the start time of the grid-side current limiting control and the integral window length of the line mode current fault component amplitude integral on different outgoing lines are adjusted.

[0013] Furthermore, the starting threshold Δ1 of the change rate current limiting link needs to avoid the maximum value of the current change rate under the out-of-zone fault and normal operation state. The specific calculation formula is as follows:

[0014]

[0015] Where: Δ1 is the starting threshold of the current change rate limiting link, K set1 is the reliability coefficient, i dc，区外 and i dc，负荷 They are the DC current under out-of-zone fault and normal conditions respectively.

[0016] Furthermore, the minimum investment ratio K of the MMC submodule min The setting method is:

[0017] According to the action time t of the longitudinal protection back Determine the time for which the converter station needs to maintain an unblocked state;

[0018] A bipolar metallic fault is set at the head end of each outgoing line of the converter station. back The current of each bridge arm of the converter station does not exceed the withstand current level within the time period. The proportion of MMC submodules is gradually adjusted, and the proportion is guaranteed to be no less than 50%. The minimum proportion K is determined. min .

[0019] Furthermore, the constructed current amplitude limiting link utilizes the DC current change rate and the converter station outlet current i MMC Together they constitute the start criterion, and the formula is as follows:

[0020]

[0021] Where: i dc is the DC line current, It is the rated value of the converter station outlet current.

[0022] Furthermore, the real-time adjustment of K needs to be based on the maximum input ratio K1 and the minimum input ratio K of the submodule that can be output by the amplitude current limiting link. minThe linear regulation curve is constructed by the current amplitude conditions of the converter station output when the two are output. set1 Times and K m times As K1 and K min The output condition of the constructed linear adjustment curve is as follows:

[0023]

[0024] Furthermore, K set1 is 1.1, K1 is 90%, K m Take it as 1.5.

[0025] Furthermore, the method of constructing the fault pole selection criterion using the ratio of the fault voltage component amplitude integral of the positive and negative lines is as follows:

[0026] After the protection is started, the fault selection criterion is constructed by using the ratio of the fault voltage component amplitude integral of the positive and negative lines. The fault voltage component u of the positive and negative lines is dcp 、u dcn They are as follows:

[0027]

[0028] Where: The voltages at the positive and negative measuring points are U p , U n The rated voltages of the positive and negative electrodes are U np , U nn ;

[0029] The specific fault selection criteria are:

[0030]

[0031] Where: t0 is the protection start time; Δt is the fault voltage component amplitude integration window length; k set is a ratio coefficient greater than 1.

[0032] Furthermore, Δt is taken as 0.5ms; k set Take it as 1.2.

[0033] Further by comparing t dw The fault direction can be determined by integrating the amplitude of the line mode current fault component on different outgoing lines of the converter station within a certain period of time. The method is as follows:

[0034] Compare the amplitude integrals of the line mode current fault components on each outgoing line in pairs. Let i linea and i lineb They represent the line mode current fault components on the outgoing lines a and b of the converter station respectively. If line a satisfies the following criteria relative to line b, it is judged that line a has a fault:

[0035]

[0036] Where: k rel is a margin factor greater than 1, t dw is the integration window length for the integral of the amplitude of the line mode current fault component;

[0037] After the fault direction is determined, the judgment result of the converter station at this end is transmitted to the adjacent converter station by using the head end of the communication, and the judgment results of the converter stations at both ends of the line are integrated to identify the fault area.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. A selective collaborative current limiting control strategy is proposed to effectively reduce the fault current of the DC line after the fault. This current limiting strategy utilizes the DC current change rate and the amplitude characteristics of the converter station outlet current. While improving the starting criterion setting value to ensure the selectivity of the source-side current limiting, it can also achieve real-time adjustment of the sub-module input ratio according to the current amplitude. The method for determining the upper and lower limits of the source-side current limiting output value is clarified, and the grid-side current limiting control is started when the output of the source-side current limiting reaches the limit value, which improves the adaptive ability of the collaborative current limiting fault control strategy to the severity of the fault.

[0040] 2. A longitudinal directional protection method based on line mode current amplitude integration that can be applied to fault current limiting is proposed. This method is not affected by the reflected wave of the opposite converter station, and effectively solves the problem of low sensitivity of single-ended protection under high-resistance faults and difficulty in parameter setting under fault current limiting scenarios. In addition, this method only needs to transmit logical signals, with low communication pressure and calculation amount, low hardware requirements for data sampling and data processing, and strong anti-interference ability.

[0041] 3. A complete control and protection coordination scheme is constructed from the perspective of timing coordination and parameter coordination. The starting time of the grid-side current limiting in fault control is used to limit the integral window length of the longitudinal protection. The action time of the traditional main protection is used as a starting method for grid-side current limiting, making the control and protection coordination scheme more flexible and controllable, and improving the adaptability of control and protection to the severity of faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a topological structure diagram of a four-terminal flexible DC power grid;

[0043] Figure 2 It is the current change rate diagram when fault occurs at different positions of the line;

[0044] Figure 3 It is the topological structure diagram of the current limiting link on the source side;

[0045] Figure 4It is the adjustment curve diagram of the current limiting link on the source side;

[0046] Figure 5 This is the fault equivalent circuit diagram taking into account the current limiting control;

[0047] Figure 6 It is the action sequence diagram of the control and protection coordination scheme;

[0048] Figure 7 This is the line mode fault current amplitude diagram of each converter station outgoing line when the positive pole is grounded at F2;

[0049] Figure 8 This is a flow chart of the control and security collaboration solution. DETAILED DESCRIPTION

[0050] First, the basic technical solution of the present invention is described.

[0051] Step (1) determines the starting threshold Δ1 of the rate-of-change current limiting link. After the protection is started, the DC current change rate is used to adjust the MMC submodule input ratio K to a fixed value K1, which can suppress the DC line fault current. K1 can be taken as 90%. The start-up of the source-side current limiting link using the current change rate measured by the protection measurement points at both ends of the line can improve the selectivity of fault control. Its starting threshold needs to avoid the maximum value of the current change rate under the out-of-zone fault and normal operation. The specific calculation formula is as follows:

[0052]

[0053] Where: Δ1 is the starting threshold of the current change rate limiting link, K set1 is the reliability coefficient, which can be taken as 1.1, i dc，区外 and i dc，负荷 They are the DC current under out-of-zone fault and normal conditions respectively.

[0054] Step (2) Determine the minimum investment ratio K of the submodule min , and construct a current amplitude limiting link to adjust the submodule input ratio in real time. MMC The amplitude current limiting link is used to adjust the MMC submodule input ratio K in real time, and the DC current change rate and the converter station outlet current are used to form the start-up criterion. The details are as follows:

[0055]

[0056] Where: i dc is the DC line current, It is the rated value of the converter station outlet current.

[0057] The real-time adjustment of K needs to be based on the maximum input ratio K1 and the minimum input ratio K of the submodule that can be output by the amplitude current limiting link. min The linear regulation curve is constructed by the current amplitude conditions of the converter station output when the two are output. set1 Times and K m times As K1 and K min The output condition of the constructed linear adjustment curve is as follows:

[0058]

[0059] Step (2) Minimum input ratio K of submodules min Setting method: First determine the time for the converter station to maintain the unblocked state, usually the action time t of the longitudinal protection back is the required time. Secondly, a bipolar metallic fault is set at the head end of each outgoing line of the converter station, with t back The current of each bridge arm of the converter station does not exceed the withstand level within the time. The proportion of submodules put into operation is gradually adjusted, and the proportion of put into operation is not less than 50%. The final determination of K min .

[0060] Step (3) constructs the fault selection criterion by using the ratio of the fault voltage component amplitude integral of the positive and negative lines after the protection is started. The present invention defines the voltage of the positive and negative measurement points as U p , U n The rated voltages of the positive and negative electrodes are U np , U nn , therefore, the fault voltage of the positive and negative poles is u dcp 、u dcn They are as follows:

[0061]

[0062] The present invention uses the ratio of positive and negative transient voltages to construct a pole selection criterion, and the specific expression is as follows:

[0063]

[0064] Among them: t0 is the protection start time; Δt is the integral window length. Considering the requirement of protection speed, Δt should not be too long, so it is selected as 0.5ms; k set is the ratio coefficient. Considering a certain margin, its value is usually slightly greater than 1 and can be taken as 1.2.

[0065] Step (4) is to compare t dw The fault direction is determined by integrating the amplitude of the line mode current fault component on different outgoing lines of the converter station within a certain period of time. When line a fails, the following criteria are met:

[0066]

[0067] Where: k rel is the margin factor, usually 1.2, t dw is the integration window length, i linea and i lineb They represent the line mode current fault components on the outgoing lines a and b of the converter station respectively.

[0068] After the fault direction is determined, the judgment result of the converter station at this end is transmitted to the adjacent converter station using the head end of the communication. The fault area can be accurately identified by integrating the judgment results of the converter stations at both ends of the line.

[0069] Integration window length t dw Determination: Since the fast mechanical switch of the DC circuit breaker takes at least 2ms to disconnect, and the protection criterion proposed in the present invention is only applicable to the DC circuit breaker before current limiting or disconnection operation, the integral window length t dw It can be selected as 2ms.

[0070] Step (5) Comprehensively control and protect the output conditions, and adjust the start time of the grid-side current limiting control and the integral window length of the protection. The present invention uses the amplitude current limiting link and the main protection action time of the traditional flexible DC system to jointly form the start criterion of the grid-side current limiting control to improve the adaptive ability of the fault control strategy to the severity of the fault. Within the traditional main protection action time t1, if the submodule input ratio does not reach K min , indicating that the fault is relatively minor, at this time, the grid-side current limiting control is not put into operation within the time t1 to avoid a significant impact on the protection criteria, etc.; if the input ratio has reached K within the main protection action time min , indicating that the fault is serious and current limiting control should be used as the main method. The grid-side current limiting control should be immediately put into operation and the integration of the line mode current amplitude should be stopped to determine the fault direction. If the traditional main protection fails to complete the fault identification within the specified time, it indicates that the traditional main protection has failed and the grid-side current limiting control should also be put into operation.

[0071] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0072] 1. Use the electrical quantities measured at the protection measurement points to implement the protection start-up criteria and fault pole selection criteria.

[0073] When there is no fault in the line, the DC voltage remains basically constant; after a fault occurs, the DC voltage drops rapidly. Therefore, the improved voltage gradient algorithm can be used as the starting criterion for protection. When a single-pole grounding fault occurs in the line, the transient voltage amplitude of the fault pole is large, and the transient voltage amplitude generated by coupling on the sound pole is small; when a bipolar fault occurs in the line, the transient voltage fault component amplitudes of the positive and negative poles are similar, so the integral ratio of the positive and negative transient voltage amplitudes can be used to form the pole selection criterion.

[0074] 2. Use the DC current change rate and the current amplitude at the converter station outlet to form a coordinated current limiting strategy.

[0075] Multi-terminal flexible DC system topology Figure 1 As shown in Figure 2, metallic single-pole faults are set at different locations on the DC line. The current change rate detected by the protection at the outgoing line of the MMC1 converter station is as follows: Figure 2 As shown. Even if the most serious PN fault occurs outside the zone, the peak value of the current change rate in a short period of time is still significantly different from that of the fault inside the zone. Therefore, setting the start threshold of the source-side current limiting based on the current change rate when avoiding faults outside the zone and loads can fully ensure the selectivity of the fault current limiting and avoid the expansion of the fault impact caused by the source-side current limiting. The specific start-up criteria are as follows:

[0076]

[0077] Since the current change rate shows an oscillating characteristic at the initial stage of the fault and the peak value continues to decrease, the current limiting capacity of the change rate current limiting link will decrease or temporarily fail. Therefore, it is necessary to use the current amplitude current limiting link for further current limiting control.

[0078] Based on the current amplitude at the converter station outlet, a linear curve is constructed to achieve real-time adjustment of the submodule input ratio. While being adaptive to faults of different severity, it can fully ensure that the power electronic devices in the MMC converter station do not over-current, so that the converter station remains in an unblocked state. The current carrying capacity of the converter station is generally at least twice its rated current. Therefore, the converter station outlet current is selected to reach 1.5 times As K min The input time can ensure the safe operation of power electronic devices. min Parameter setting, in K min Under the condition of not less than 50%, the present invention takes the principle of no overcurrent of power electronic devices as the basis, and combines the action time t of backup protection back By means of simulation and experiment, K min The value of is more reasonable. Figure 3 and Figure 4 The topological diagram and adjustment curve diagram of the source side current limiting control are shown respectively. In addition, the output of the amplitude current limiting link reaches K min As a starting method for grid-side current limiting control, it enables the current limiting device to be put into use more quickly in extremely serious fault scenarios, effectively improving the fault control's ability to suppress DC current.

[0079] 3. Use the line mode current amplitude integral to determine the fault direction.

[0080] According to the superposition principle, the network after a fault occurs can be divided into a normal network and a fault component network. Since there is a coupling relationship between the positive and negative poles of the actual overhead line, it is usually necessary to decouple the collected voltage and current to obtain independent line mode and zero mode components. The decoupling matrix used in the present invention is shown as follows:

[0081]

[0082] Since the startup threshold of the collaborative current limiting strategy proposed in the present invention is relatively high and the minimum input ratio of the submodules is reasonably adjusted, the voltage drop caused by the source-side current limiting strategy of the present invention is much smaller than the voltage drop caused by the fault. Taking a single converter station as an example for analysis, it can be concluded that Figure 5 From the fault equivalent circuit diagram shown in the figure, it can be seen from the differential equation that, compared with the differential equation without adding source-side current limiting, the rate-of-change current limiting link and the current amplitude current limiting link are equivalent to increasing the inductance and resistance values ​​of the converter station.

[0083] Based on the reasonable equivalence of the source-side current limiting control, the traveling wave refraction and reflection process under fault current limiting is further analyzed, and the amplitude of the line-mode current fault component on different outgoing lines of the converter station is compared by difference. It can be proved that in the multi-terminal flexible DC power grid, for the converter stations on both sides of the fault line, the amplitude of the fault current traveling wave of the fault line is greater than the amplitude of the fault current traveling wave of its adjacent line; for the remote converter station, the magnitude relationship of the fault current traveling wave amplitude of its different outgoing lines may change, but the converter stations on both sides of the line will not simultaneously determine that the amplitude of the fault current traveling wave of the line is greater than that of other outgoing lines. Therefore, the amplitude of the line-mode current fault component on different outgoing lines can be used to determine the fault direction. In order to further improve the reliability of the protection criterion, the integral method can be used to improve the criterion to improve the anti-interference performance of the protection criterion. The specific protection criterion is as follows:

[0084]

[0085] 4. Integrate fault current limiting control and protection criteria to build a coordinated control and protection solution.

[0086] Figure 6 The action sequence diagram of the control and protection coordination scheme is shown in Figure 1. The fault pole selection and current change rate limiting links are started simultaneously using the protection start criterion. In the scenario where the fault is serious, the current amplitude limiting link may output the minimum submodule input ratio K within the traditional main protection action time t1. min At this time, current limiting should be the main approach, and grid-side current limiting control should be immediately implemented to minimize the DC fault current; if the output value of the current amplitude limiting link does not reach K min, indicating that the fault is relatively minor, and the grid-side current limiting will not be activated within t1. However, if the protection fails to operate within t1, it indicates that the traditional main protection has failed, and the grid-side current limiting control should also be activated at this time. The activation time of the grid-side current limiting also affects the integral window length t of the longitudinal protection. dw The integral endpoint of the protection criterion must be ensured to be before the grid-side current limiting control starts. Figure 7 This is a waveform diagram of the line-mode fault current amplitude on different outgoing lines of each converter station in the multi-terminal flexible DC grid when a metallic single-pole fault occurs at F2. Each converter station integrates and compares the fault current amplitudes of different outgoing lines to obtain the fault direction and transmits the judgment results to the adjacent converter stations, so that the fault line can be determined in the multi-terminal power grid.

[0087] The flowchart of the control and insurance coordination solution proposed in the present invention is as follows: Figure 8 The specific implementation process is as follows:

[0088] Firstly, the starting threshold Δ1 of the rate-of-change current limiting link in the source-side current limiting is determined based on the simulation and experimental results, and the minimum input ratio K of the submodules in different converter stations is selected. min , set the length of the longitudinal protection integral window t dw . Secondly, after the protection is started, the source-side current limiting control is activated and the fault pole selection is performed. The fault pole is determined by the amplitude ratio of the transient voltage fault component of the positive and negative poles. At the same time, if the fault current change rate meets the starting threshold of the source-side current limiting, the current limiting control is started, the sub-module input is reduced, and the current amplitude current limiting link is started as the fault current continues to increase; if the fault occurs at the far end of the line or there is a large transition resistance at the fault point so that the current change rate does not meet the starting threshold, the source-side current limiting link will not be put into operation during the entire fault handling process. Thirdly, the collected current is decoupled and calculated to obtain the line-mode fault component, and the line-mode fault current amplitude on different outgoing lines of the converter station is integrated. The fault direction is obtained by comparison using the protection criterion, and the judgment result is transmitted to the adjacent converter station. After obtaining the judgment result of the adjacent converter station, the fault line can be determined. Finally, during the entire current limiting and protection judgment process, if the sub-module input ratio reaches K min Or if the traditional main protection fails to complete the fault identification within the specified time, the grid-side current limiting should be activated, and after the grid-side current limiting is activated, the longitudinal protection should immediately stop integrating the line mode current amplitude and determine the fault direction.

Claims

1. A multi-terminal flexible DC control and protection coordination method based on fault current limiting, characterized in that: The steps include: The DC current change rate is used to adjust the MMC submodule input ratio K to suppress the DC line fault current and determine the starting threshold Δ1 of the DC current change rate current limiting link; Determine the minimum investment ratio K of MMC submodules min , construct a current amplitude limiting link, wherein the current amplitude limiting link uses the current at the converter station outlet to form an amplitude limiting link to adjust the MMC sub-module input ratio K in real time; The fault pole selection criterion is constructed by using the ratio of the fault voltage component amplitude integral of the positive and negative lines. Set the integration time for calculating the fault judgment criterion of the longitudinal protection, compare the integral of the fault component amplitude of the line mode current on different outgoing lines of the converter station within this integration time, and realize the fault direction judgment; Based on the output of comprehensive control and protection, the start time of the grid-side current limiting control and the integral window length of the line mode current fault component amplitude integral on different outgoing lines are adjusted.

2. The multi-terminal flexible DC control and protection coordination method based on fault current limiting according to claim 1 is characterized in that: The starting threshold Δ1 of the change rate current limiting link needs to avoid the maximum value of the current change rate under the out-of-zone fault and normal operation state. The specific calculation formula is as follows: Where: Δ1 is the starting threshold of the current change rate limiting link, K set1 is the reliability coefficient, i dc，区外 and i dc，负荷 They are the DC current under out-of-zone fault and normal conditions respectively.

3. The multi-terminal flexible DC control and protection coordination method based on fault current limiting according to claim 2 is characterized in that: The minimum input ratio of the MMC submodule is K min The setting method is: According to the action time t of the longitudinal protection back Determine the time for which the converter station needs to maintain an unblocked state; A bipolar metallic fault is set at the head end of each outgoing line of the converter station. back The current of each bridge arm of the converter station does not exceed the withstand current level within the time period. The proportion of MMC submodules is gradually adjusted, and the proportion is guaranteed to be no less than 50%. The minimum proportion K is determined. min .

4. The multi-terminal flexible DC control and protection coordination method based on fault current limiting according to claim 2 is characterized in that: The constructed current amplitude limiting link uses the DC current change rate and the converter station outlet current i MMC Together they constitute the start criterion, and the formula is as follows: Where: i dc is the DC line current, It is the rated value of the converter station outlet current.

5. The multi-terminal flexible DC control and protection coordination method based on fault current limiting according to claim 4 is characterized in that: The real-time adjustment of K needs to be based on the maximum input ratio K1 and the minimum input ratio K of the submodule that can be output by the amplitude current limiting link. min The linear regulation curve is constructed by the current amplitude conditions of the converter station output when the two are output. set1 Times and K m times As K1 and K min The output condition of the constructed linear adjustment curve is as follows:

6. The multi-terminal flexible DC control and protection coordination method based on fault current limiting according to claim 4 is characterized in that: K set1 is 1.1, K1 is 90%, K m Take it as 1.

5.

7. The multi-terminal flexible DC control and protection coordination method based on fault current limiting according to claim 1 is characterized in that: The method of constructing the fault pole selection criterion using the ratio of the fault voltage component amplitude integral of the positive and negative lines is as follows: After the protection is started, the fault selection criterion is constructed by using the ratio of the fault voltage component amplitude integral of the positive and negative lines. The fault voltage component u of the positive and negative lines is dcp 、u dcn They are as follows: Where: The voltages at the positive and negative measuring points are U p , U n The rated voltages of the positive and negative electrodes are U np , U nn ; The specific fault selection criteria are: Where: t0 is the protection start time; Δt is the fault voltage component amplitude integration window length; k set is a ratio coefficient greater than 1.

8. The multi-terminal flexible DC control and protection coordination method based on fault current limiting according to claim 7 is characterized in that: Δt is taken as 0.5ms; k set Take it as 1.

2.

9. The multi-terminal flexible DC control and protection coordination method based on fault current limiting according to claim 1 is characterized in that: By comparing t dw The fault direction can be determined by integrating the amplitude of the line mode current fault component on different outgoing lines of the converter station within a certain period of time. The method is as follows: Compare the amplitude integrals of the line mode current fault components on each outgoing line in pairs. Let i linea and i lineb They represent the line mode current fault components on the outgoing lines a and b of the converter station respectively. If line a satisfies the following criteria relative to line b, it is judged that line a has a fault: Where: k rel is a margin factor greater than 1, t dw is the integration window length for the integral of the amplitude of the line mode current fault component; After the fault direction is determined, the judgment result of the converter station at this end is transmitted to the adjacent converter station by using the head end of the communication, and the judgment results of the converter stations at both ends of the line are integrated to identify the fault area.

Images