Multi-terminal flexible HVDC grid double-ended protection method based on simplified voltage ratio

Abstract

This invention relates to a dual-terminal quantity protection method for multi-terminal flexible DC power grids based on simplified voltage ratios, comprising: Step 1: determining fault identification criteria for faults inside and outside the protection zone; Step 2: determining fault initiation criteria; Step 3: determining fault pole selection criteria; Step 4: determining the protection scheme. The technical solution provided by this invention can quickly and reliably identify faults inside and outside the protection zone of a DC line, reliably protect the entire line length, has strong resistance to transition resistance and noise, does not require real-time synchronization of data at both ends of the line, and has a faster operating speed than traditional current differential protection.

Description

Technical Field

[0001] This invention belongs to the field of power system protection and control technology, specifically relating to a dual-terminal quantity protection method for multi-terminal flexible DC power grids based on simplified voltage ratios. Background Technology

[0002] Compared to traditional high-voltage direct current (HVDC) converters, modular multilevel converters (MMCs) offer advantages such as independent active and reactive power control, lower losses, and no commutation failures, demonstrating significant advantages in renewable energy grid integration. With the development of the renewable energy sector, MMC-based flexible DC transmission technology is increasingly widely applied in practical engineering projects, becoming a hot research topic. Flexible DC transmission systems are weakly damped systems; after a line fault, the fault current rises rapidly. The electronic components in MMCs have low overcurrent tolerance, thus requiring isolation of the faulty line within milliseconds. To cope with the rapid rise in fault current after a DC line fault, DC reactors are typically installed at both ends of the line, often used as line boundary elements.

[0003] Compared to flexible DC transmission at both ends, the emergence of multi-terminal DC (MTDC) grids makes fault characteristics more complex and places higher demands on DC line protection.

[0004] DC line protection schemes are divided into single-ended quantity protection and double-ended quantity protection: DC line single-ended quantity protection only uses the electrical quantity on this side to determine the location of the fault, does not rely on double-ended communication, and the protection action speed is faster, which is of great significance for the protection of DC lines in multi-terminal flexible DC grids; double-ended quantity protection uses the electrical quantity information at both ends of the line to form the protection criteria, requires protection communication at both ends of the line, but the protection reliability is higher and can protect the entire length of the line.

[0005] Currently, there are few protection schemes for multi-terminal flexible DC transmission lines, and most schemes treat the MMC converter station as an inductor, resulting in inaccurate research models that are also affected by the MMC model and control system. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a dual-terminal quantity protection method for multi-terminal flexible DC power grids based on simplified voltage ratios. This method can quickly and reliably identify faults inside and outside the DC line area, reliably protect the entire line length, has strong resistance to transition resistance and noise, does not require real-time synchronization of data at both ends of the line, and operates quickly.

[0007] The technical problem solved by this invention is achieved through the following technical solution:

[0008] A method for dual-terminal quantity protection of multi-terminal flexible DC power grids based on simplified voltage ratios, characterized in that: the steps of the method are as follows:

[0009] S1. Determine the fault identification criteria inside and outside the zone.

[0010] The criteria for fault identification inside and outside the zone are constructed as follows:

[0011] The fault point is located at protection P ij In the positive direction, the simplified bus voltage u of the DC bus BUSi in With protection P ij Voltage reverse traveling wave u ijf equal;

[0012] If the fault point is located at protection P ij In the opposite direction, protect P. ij Voltage reverse traveling wave u ijf =0, simplified bus voltage u in Not zero; utilizes the simplified bus voltage u during faults in both forward and reverse directions. in The reverse traveling wave of the voltage at the protection installation point u ijf The protection criterion is constructed by the ratio difference between the two values. The amplitudes of the two values ​​within the data window are summed and the ratio is calculated. This ratio is defined as the simplified voltage ratio K. ij Its expression is shown in the following formula:

[0013]

[0014] Where: N is the number of sampling points in the data window;

[0015] When a positive direction fault occurs, u in =u ijf Therefore, K ij =1;

[0016] When a reverse fault occurs, u ijf Approximately equal to 0, therefore K ij >>1; Therefore, there is no need to rely on simulation values ​​to select action setting values. The fault identification criterion is constructed as follows:

[0017]

[0018] When both ends of the line are identified as positive faults by the protection, it is determined to be an internal fault and the protection will operate; otherwise, it is determined to be an external fault.

[0019] S2. Establish fault start criteria

[0020] During normal system operation, the simplified bus voltage u in and protection installation location anti-traveling wave u ijf Approximately 0; after the fault, the u values ​​at both ends of the faulty line are... in Following the anti-traveling wave uijf Rapid descent, utilizing u in and u ijf Together, we determine whether a fault has occurred in the DC system and calculate u. in and u ijf The voltage gradient expression is shown in the following equation:

[0021]

[0022] The protection start criterion is set as shown in the following formula: when at least one voltage gradient is greater than the set value, the protection starts, U set1 The selection of the appropriate voltage rating needs to avoid voltage fluctuations during normal operation and ensure reliable startup even under high-impedance faults. Based on simulation results, U is selected. set1 =10kV,

[0023] (Δu in (k)>U set1 )∪(Δu ijf (k)>U set1 ) = 1

[0024] This criterion utilizes two-point voltage; when the voltage at either point meets the starting criterion, the protection can be reliably started.

[0025] S3. Establish fault polarity selection criteria

[0026] The ground mode component exists only in unipolar faults, and the polarity of the ground mode component is different under positive and negative faults. It can be used to construct a pole selection criterion and to simplify the ground mode component u of the bus voltage within the time window T. in0 The average value is calculated using the following formula:

[0027]

[0028] Where: f s Where N is the sampling frequency, and N is the number of sampling points in the data window;

[0029] U set0 The selection of U needs to avoid the maximum unbalanced voltage during a bipolar fault while retaining a certain margin. Based on simulation results, this paper selects U. set0 =10kV, the pole selection criterion is constructed as follows:

[0030]

[0031] S4. Determine the protection plan

[0032] Current, voltage, and bus voltage are continuously collected from both sides of the line, and the reverse traveling wave of the simplified bus voltage and the voltage at the protection installation point is calculated. If the voltage at the DC line protection installation point at a certain end meets the fault start criterion, the protection start element at that end is activated, and the simplified voltage ratio is calculated. When the simplified voltage ratio at both ends of the line is less than the threshold value, it is judged as an in-zone fault. When the simplified voltage ratio at either end is greater than the threshold value, it is judged as an out-of-zone fault. If it is judged as an in-zone fault, the fault pole is selected by using the ground mode component of the simplified bus voltage.

[0033] Furthermore, the identification criteria are based on the voltage, current, and bus voltage on both sides of the line within a very short time after a fault is detected. The data window duration should be less than 2T, where T = 1 line / v,l line 2T refers to the length of the DC transmission line, and v refers to the propagation speed of the fault traveling wave, which is approximately equal to the speed of light. In a multi-terminal flexible DC grid, the distance between DC lines is often around 200km. Therefore, 2T is generally greater than 1.4ms. Considering the redundancy of the protection action time, the protection can act within 2.5ms after the fault occurs.

[0034] When there is a fault in the forward direction, the simplified voltage ratio is close to 1; when there is a fault in the reverse direction, the simplified voltage ratio is theoretically infinite, much greater than 1; therefore, the threshold value in the fault identification criterion between the inside and outside of the zone can be set to 2 with good reliability.

[0035] The advantages and beneficial effects of this invention are as follows:

[0036] 1. This invention is a dual-terminal quantity protection method for multi-terminal flexible DC power grids based on simplified voltage ratios. It has low computational load and can reliably and quickly identify faults inside and outside the DC line area, protecting the entire length of the line.

[0037] 2. The present invention is a dual-terminal quantity protection method for multi-terminal flexible DC power grid based on simplified voltage ratio. It has strong resistance to transition resistance and noise resistance, and its operating speed is faster than that of traditional current differential protection.

[0038] 3. The present invention is a dual-terminal quantity protection method for multi-terminal flexible DC power grid based on simplified voltage ratio. The protection at both ends only needs to transmit logic signals and does not require real-time synchronization of data at both ends of the line, thus having lower requirements for communication channels. Attached Figure Description

[0039] Figure 1 This is a diagram of the four-terminal flexible DC power grid topology of the present invention;

[0040] Figure 2 This is the equivalent diagram of the system fault of the present invention;

[0041] Figure 3 This is a schematic diagram of the time-varying current source of the present invention.

[0042] Figure 4 This is a simplified Peterson equivalent circuit for the present invention;

[0043] Figure 5 This is a flowchart of the protection method of the present invention;

[0044] Figure 6 This is a simulation diagram of the fault within the region according to the present invention;

[0045] Figure 7 This is a simulation diagram of an external fault in this invention. Detailed Implementation

[0046] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0047] This invention provides a dual-terminal quantity protection method for multi-terminal flexible DC power grids based on simplified voltage ratios, comprising:

[0048] Figure 1 The diagram shows two typical four-terminal flexible DC grid structures, where all four converters employ a symmetrical bipolar MMC structure; ij It is a DC transmission line; P ij Protection devices are installed at the beginning and end of each line; BUSi is the DC bus (where i, j = 1, 2, 3, 4); DC reactors L are installed on the line to limit the rate of rise of fault current.

[0049] For DC transmission lines, there is coupling between the positive and negative poles. The fault components at the positive and negative poles can be decoupled into line-mode and ground-mode components using the following formula, where F0 and F1 are the ground-mode component and the line-mode component, respectively. p With F n These represent the positive and negative fault components, respectively. Since the line-mode wave velocity and line-mode impedance amplitude change very little with frequency, while the ground-mode wave velocity and ground-mode impedance amplitude change much more, and the line-mode wave velocity is faster, this paper uses the line-mode component in fault analysis unless otherwise specified.

[0050]

[0051] With line l 23 up f a Taking a fault as an example, protect P 23 Analysis revealed that after the fault occurred, a traveling wave propagated from the fault point to both ends of the line. Upon reaching the DC bus, the bus voltage dropped, and MMC2 began to discharge. For protection P... 23 In this regard, the fault current of MMC2 can be obtained through measurement. According to the substitution theorem, the fault model of MMC2 can be equivalent to a time-varying current source i. m2The current source magnitude is equal to the fault current magnitude of MMC2; simultaneously, a fault voltage source u is introduced at the fault point. f The system's fault component diagram is as follows: Figure 2 As shown. The voltage u2 at DC bus BUS2 can be considered as the superposition of the combined effects of the time-varying current source and the fault voltage source, and the DC bus voltage components under the individual effects of the two are respectively denoted as u. 2m and u 2n The fault components when either factor acts alone are analyzed below.

[0052] After the fault traveling wave first arrives at BUS2, the capacitor of the MMC2 submodule discharges rapidly, and the fault current i m2 Rapidly rises and simultaneously enters line l 23 and l 12 To analyze current i m2 The transmission situation to the two lines needs to be studied in the complex frequency domain using the line impedance. The complex frequency domain model of current transmission is as follows: Figure 3 As shown, the equivalent time-varying current source of MMC2 is denoted as I. m2 (s), bus voltage is U 2m (s), protect P 23 The voltage and current at point U are respectively 23m (s), I 23m (s). Since the voltage levels of all lines in a multi-terminal flexible DC transmission system are the same, different transmission lines often use the same DC reactor L and line parameters. Since the line surge impedance is only related to the line parameters, the line-mode surge impedance Z of different outgoing lines from the DC bus varies. C1 Same. Obtain the bus voltage U. 2m The expression for (s) is:

[0053]

[0054] After inverse Laplace transform, the time-domain expression for the DC bus voltage under the action of a time-varying current source alone is obtained as follows:

[0055]

[0056] according to Figure 3 The protection P under the sole action of the time-varying current source can be obtained. 23 Time-domain solution of voltage and current fault components u 23m i 23m As shown in the following formula, substituting into the above formula yields the protection P under the sole action of the time-varying current source. 23 The voltage reverse ripple at that point is 0.

[0057]

[0058] Due to the protection P acting alone under the time-varying current source 23The voltage reverse ripple at the location is 0, therefore the protection P is activated when the fault voltage source acts alone. 23 The reverse voltage traveling wave at a given point is equal to the total reverse voltage traveling wave, denoted as u. 23f The following text will no longer emphasize that this voltage reverse traveling wave is generated by the fault voltage source acting alone. The formula for calculating the extracted voltage reverse traveling wave is shown below, where u 23 i 23 Protect P respectively 23 Voltage and current fault components.

[0059]

[0060] When the fault voltage source acts alone, the time-varying current source can be considered as an open circuit, and the traveling wave of the fault line-mode voltage propagates to the protection P. 23 The Peterson equivalent circuit at that time is as follows Figure 4 As shown in the figure. U 23f (s) is from line l 23 Directional protection P 23 Linear mode reverse traveling wave; U 2n (s) is the DC bus voltage that removes the influence of the local MMC. It is defined as the simplified bus voltage. The protection principle will be constructed using this voltage later. The time-domain solution calculation process is shown in the following formula, which is the total fault component minus the fault component under the action of the time-varying current source alone.

[0061]

[0062] according to Figure 4 It can be known that the simplified voltage U of the bus is 2n (s) and the anti-traveling wave U 23f The relationship between (s) is shown in the following equation.

[0063] U 2n (s)=U 23f (s)

[0064] Line l 23 up f a After a fault occurs at point BUS2, a fault traveling wave propagates from the fault point to both ends of the line. The time when the fault traveling wave reaches BUS2 is defined as t1. The fault traveling wave travels along line l... 12 The wave continues to propagate and reaches BUS1, generating a reflected wave. The time when this reflected wave reaches BUS2 is defined as t2. During the time interval t1 to t2, the protection P... 23 reverse wave u 23f Equal to the simplified bus voltage u 2n Protect P 21 reverse wave u 21f =0; after t2, by line l 12 The transmitted traveling wave caused the protection P 21 reverse wave u21f Not zero.

[0065] During the time interval t1 to t2 after the fault occurs, the fault point is located at protection P. ij In the positive direction, the simplified bus voltage u of the DC bus BUSi in With protection P ij Voltage reverse traveling wave u ijf Equal; if the fault point is located at protection P ij In the opposite direction, protect P. ij Voltage reverse traveling wave u ijf =0, simplified bus voltage u in Not zero. The bus voltage u can be simplified during faults in both forward and reverse directions. in The reverse traveling wave of the voltage at the protection installation point u ijf The ratio difference is used to construct a protection criterion. To increase protection reliability, the amplitudes of the two values ​​within the data window are summed and the ratio is calculated again. This ratio is defined as the simplified voltage ratio K. ij Its expression is shown in the following formula, where N is the number of sampling points in the data window.

[0066]

[0067] When a positive direction fault occurs, u in =u ijf Therefore, K ij =1; when a reverse fault occurs, u ijf Approximately equal to 0, therefore K ij >>1. Therefore, there is no need to rely on simulation values ​​to select action setting values. The fault identification criterion is constructed as follows:

[0068]

[0069] When both ends of the line are identified as having a positive-direction fault, the fault is determined to be within the zone, and the protection operates; otherwise, it is determined to be an external fault. To meet the requirement of u within the sampling time... in =u ijf The time window T should be selected within the time interval t1 to t2 after the fault. Assuming the length of the adjacent line at the protection installation point is l, and the traveling wave transmission speed of the line mode is v, T should satisfy the inequality condition shown in the following equation.

[0070]

[0071] Considering both protection speed and reliability, the selection principle of T in this paper is as shown in the following formula, which can ensure that T does not exceed 0.5ms.

[0072]

[0073] The protection scheme was determined as follows:

[0074] 1. Fault identification criteria inside and outside the zone:

[0075] Within the time interval t1 to t2 after the fault occurs, if the fault point is located at protection P ij In the positive direction, the simplified bus voltage u of the DC bus BUSi in With protection P ij Voltage reverse traveling wave u ijf Equal; if the fault point is located at protection P ij In the opposite direction, protect P. ij Voltage reverse traveling wave u ijf =0, simplified bus voltage u in Not zero.

[0076] The bus voltage u can be simplified during faults in both forward and reverse directions. in The reverse traveling wave of the voltage at the protection installation point u ijf The ratio difference is used to construct a protection criterion. To increase protection reliability, the amplitudes of the two values ​​within the data window are summed and the ratio is calculated again. This ratio is defined as the simplified voltage ratio K. ij Its expression is shown in the following formula, where N is the number of sampling points in the data window.

[0077]

[0078] When a positive direction fault occurs, u in =u ijf Therefore, K ij =1; when a reverse fault occurs, u ijf Approximately equal to 0, therefore K ij >>1. Therefore, there is no need to rely on simulation values ​​to select action setting values, and the fault identification criterion can be constructed as follows:

[0079]

[0080] When both ends of the line are identified as having a positive-direction fault, the fault is determined to be within the zone, and the protection operates; otherwise, it is determined to be an external fault. To meet the requirement of u within the sampling time... in =u ijf The time window T should be selected within the time interval t1 to t2 after the fault. Assuming the length of the adjacent line at the protection installation point is l, and the traveling wave speed of the line mode is v, T should satisfy the following inequality:

[0081]

[0082] Considering both protection speed and reliability, the selection principle of T in this paper is as shown in the following formula, which can ensure that T does not exceed 0.5ms.

[0083]

[0084] 2. Establish fault start criteria:

[0085] During normal system operation, the simplified bus voltage u in and protection installation location anti-traveling wave u ijf Approximately 0; after the fault, the u values ​​at both ends of the faulty line are... in Following the anti-traveling wave u ijf It drops rapidly, so u can be used in and u ijf Together, determine whether a fault has occurred in the DC system. Calculate u. in and u ijf The voltage gradient expression is shown in the following equation:

[0086]

[0087] The protection activation criterion is set as shown in the following formula: the protection is activated when at least one voltage gradient is greater than the set value. set1 The selection of the appropriate voltage rating needs to avoid voltage fluctuations during normal operation and ensure reliable startup even under high-impedance faults. Based on simulation results, this paper selects U... set1 =10kV.

[0088] (Δu in (k)>U set1 )∪(Δu ijf (k)>U set1 ) = 1

[0089] This criterion utilizes two-point voltage. When the voltage at either point meets the start-up criterion, the protection can be reliably started, increasing the reliability of the protection and reducing the risk of failure to operate due to sampling element errors.

[0090] 3. Establish fault polarity selection criteria:

[0091] The ground mode component exists only in unipolar faults, and the polarity of the ground mode component differs under positive and negative pole faults, which can be used to construct pole selection criteria. The ground mode component u of the simplified bus voltage within the time window T is... in0 The average value is calculated using the following formula:

[0092]

[0093] The pole selection criterion is constructed as follows, where f s U is the sampling frequency, and N is the number of sampling points in the data window. set0 The selection of U needs to avoid the maximum unbalanced voltage during a bipolar fault while retaining a certain margin. Based on simulation results, this paper selects U. set0 =10kV.

[0094]

[0095] 4. Protection Logic:

[0096] Current, voltage, and bus voltage are continuously collected from both sides of the line. The simplified bus voltage and the reverse traveling wave of the voltage at the protection installation point are calculated. If the voltage at the protection installation point of a DC line at a certain end meets the fault initiation criterion, the protection initiation element at that end is activated, and the simplified voltage ratio is calculated. When the simplified voltage ratio at both ends of the line is less than the threshold value, it is judged as an in-zone fault; when the simplified voltage ratio at either end is greater than the threshold value, it is judged as an out-of-zone fault. If it is determined to be an in-zone fault, the fault pole is selected using the ground mode component of the simplified bus voltage. The protection flowchart is as follows: Figure 5 As shown.

[0097] Simulation verification

[0098] Build the following in the PSCAD / EMTDC simulation platform respectively Figure 1 Simulation verification was performed on two four-terminal flexible DC grid topology models. MMC1 employs constant reactive power control and constant DC voltage control, with a rated DC voltage of ±500kV. MMC2-MMC4 employ constant active power control and constant reactive power control. The overhead lines between converter stations use a frequency-dependent line model with positive and negative poles mounted on the same tower, each 180km long; the line-mode wave impedance is 222Ω, and the line-mode traveling wave speed is approximately 3×10⁻⁶. 8 m / s; sampling frequency is 20kHz.

[0099] Simulation results for faults within the zone:

[0100] Set line l at time 0 23 A positive metallic ground fault occurs at the midpoint f2, and protection P... 23 P 21 Simulation results of the reverse traveling wave of the line mode and the simplified voltage of the BUS2 bus are as follows: Figure 6 As shown in (a), protect P 32 P 34 Simulation results of the reverse traveling wave and simplified voltage of bus BUS3 are as follows: Figure 6 As shown in (b).

[0101] Depend on Figure 6 (a) It can be seen that 0.3 ms after the fault occurs, the traveling wave of the fault line mode propagates to the line boundary, and the protection P... 23 Linear mode reverse wave u 23f Simplified voltage u with BUS2 bus 2n Rapidly decreasing and approximately equal, protecting P 21 Linear mode reverse wave u 21f Approximately 0; at 0.9ms, the protection P 23 For the first time, I experienced the power of line l 23 The following wave, u 23f It rose rapidly again, u 2nDespite the changes, the two remain approximately equal; the fault traveling wave passes through protection P. 21 Then, along line l 12 Spreading and protecting P 12 The reflection occurs at point P, and the reflected wave arrives at protection P at 1.5ms. 21 Protect P 21 A reverse traveling wave was detected. (To be continued) Figure 6 The analysis in (b) is similar.

[0102] The protection start criterion at 0.35ms yields Δu. 2n =Δu 23f =250kV>10kV, protection P 23 P 21 Start; calculate K 23 =1.004<2, K 21 =193>2, therefore protect P 23 The fault is determined to be in the positive direction; protection P is activated. 21 The fault is determined to be in the opposite direction; Δu is calculated. 2n0 =-345kV <-10kV, judged as a positive pole fault. Similarly, protection P 32 The fault is determined to be in the positive direction; protection P is activated. 34 The fault was determined to be in the reverse direction, and the longitudinal protection identified it as line l. 23 A positive electrode fault has occurred.

[0103] Simulation results for out-of-area faults:

[0104] When a metallic positive ground fault occurs on DC bus BUS2, protection P 23 P 21 Simulation results of the reverse traveling wave of the line mode and the simplified voltage of the BUS2 bus are as follows: Figure 7 As shown in (a), protect P 32 P 34 Simulation results of the reverse traveling wave and simplified voltage of bus BUS3 are as follows: Figure 7 As shown in (b).

[0105] Depend on Figure 7 (a) It can be seen that after a metallic fault occurs in BUS2, the simplified voltage u of the BUS2 busbar... 2n The value rapidly drops to negative, and the fault traveling wave is directed towards line l. 23 l 12 Dissemination and protection of P 23 P 21 Linear mode reverse wave u 23f u 21f The value is 0; at 1.2ms, the traveling wave reflects back to the protection installation point at the end of the line, and both protection devices detect the reverse traveling wave. After a fault occurs on the DC bus, u 2n <0, consistent with the previous analysis. (Regarding...)Figure 7 The analysis process for (b) is the same as that for line faults.

[0106] K was calculated 23 =345>2, K 21 =261>2, therefore protect P 23 P 21 Both were judged as reverse faults, and the longitudinal protection identified it as l. 23 Faults outside the line area and l 12 The fault occurs outside the line zone. Therefore, the protection described in this paper can avoid false tripping when a fault occurs on the DC bus.

[0107] This protection scheme replaces the MMC with a time-varying current source, thus it is unaffected by MMC parameters and the control system. It exhibits good applicability across different topologies, requires only logic signal transmission without data synchronization, and has low requirements for communication equipment. Simulation verification shows that this protection scheme has strong noise and transition resistance immunity and can operate normally at lower sampling frequencies.

[0108] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A dual-terminal quantity protection method for multi-terminal flexible DC power grids based on simplified voltage ratios, characterized in that: The steps of the method are as follows: S1. Determine the fault identification criteria inside and outside the zone. The criteria for fault identification inside and outside the zone are constructed as follows: The fault point is located at protection P ij In the positive direction, the simplified bus voltage u of the DC bus BUSi in With protection P ij Voltage reverse traveling wave u ijf equal; If the fault point is located at protection P ij In the opposite direction, protect P. ij Voltage reverse traveling wave u ijf =0, simplified bus voltage u in Not zero; utilizes the simplified bus voltage u during faults in both forward and reverse directions. in The reverse traveling wave of the voltage at the protection installation point u ijf The protection criterion is constructed by the ratio difference between the two values. The amplitudes of the two values ​​within the data window are summed and the ratio is calculated. This ratio is defined as the simplified voltage ratio K. ij Its expression is shown in the following formula: Where: N is the number of sampling points in the data window; When a positive direction fault occurs, u in =u ijf Therefore, K ij =1; When a reverse fault occurs, u ijf Approximately equal to 0, therefore K ij >>1; Therefore, there is no need to rely on simulation values ​​to select action setting values. The fault identification criterion is constructed as follows: When both ends of the line are identified as positive faults by the protection, it is determined to be an internal fault and the protection will operate; otherwise, it is determined to be an external fault. S2. Establish fault start criteria During normal system operation, the simplified bus voltage u in and protection installation location anti-traveling wave u ijf Approximately 0; after the fault, the u values ​​at both ends of the faulty line are... in Following the anti-traveling wave u ijf Rapid descent, utilizing u in and u ijf Together, we determine whether a fault has occurred in the DC system and calculate u. in and u ijf The voltage gradient expression is shown in the following equation: The protection start criterion is set as shown in the following formula: when at least one voltage gradient is greater than the set value, the protection starts, U set1 The selection of the appropriate voltage rating needs to avoid voltage fluctuations during normal operation and ensure reliable startup even under high-impedance faults. Based on simulation results, U is selected. set1 =10kV, (Δu in (k)>U set1 )∪(Δu ijf (k)>U set1 )=1 This criterion utilizes two-point voltage; when the voltage at either point meets the starting criterion, the protection can be reliably started. S3. Establish fault polarity selection criteria The ground mode component exists only in unipolar faults, and the polarity of the ground mode component is different under positive and negative faults. It can be used to construct a pole selection criterion and to simplify the ground mode component u of the bus voltage within the time window T. in0 The average value is calculated using the following formula: Where: f s Where N is the sampling frequency, and N is the number of sampling points in the data window; U set0 The selection of U needs to avoid the maximum unbalanced voltage during a bipolar fault while retaining a certain margin. Based on simulation results, this paper selects U. set0 =10kV, the pole selection criterion is constructed as follows: S4. Determine the protection plan Current, voltage, and bus voltage are continuously collected from both sides of the line, and the reverse traveling wave of the simplified bus voltage and the voltage at the protection installation point is calculated. If the voltage at the DC line protection installation point at a certain end meets the fault start criterion, the protection start element at that end is activated, and the simplified voltage ratio is calculated. When the simplified voltage ratio at both ends of the line is less than the threshold value, it is judged as an in-zone fault. When the simplified voltage ratio at either end is greater than the threshold value, it is judged as an out-of-zone fault. If it is judged as an in-zone fault, the fault pole is selected by using the ground mode component of the simplified bus voltage.

2. The method for dual-terminal quantity protection of multi-terminal flexible DC power grid based on simplified voltage ratio as described in claim 1, characterized in that: The identification criteria are based on the voltage, current, and bus voltage on both sides of the line within a very short time after a fault is detected. The data window duration should be less than 2T, where T = 1. line / v,l line 2T refers to the length of the DC transmission line, and v refers to the propagation speed of the fault traveling wave, which is approximately equal to the speed of light. In a multi-terminal flexible DC grid, the distance between DC lines is often around 200km. Therefore, 2T is generally greater than 1.4ms. Considering the redundancy of the protection action time, the protection can act within 2.5ms after the fault occurs. When there is a fault in the forward direction, the simplified voltage ratio is close to 1; when there is a fault in the reverse direction, the simplified voltage ratio is theoretically infinite, much greater than 1; therefore, the threshold value in the fault identification criterion between the inside and outside of the zone can be set to 2 with good reliability.

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