A Fast Protection Method for Face-to-Face Modular Multilevel Bipolar DC Transformer

Abstract

The present invention discloses a fast protection method for a face-to-face modular multilevel bipolar DC transformer, which includes: S1, constructing an equivalent model of a bipolar FTF-MMC-DCT fault circuit; S2, extracting the differential of the DC voltage and current of the DCT as the first fault feature quantity, and extracting the port leakage current as the second fault feature quantity; S3, formulating a first DCT protection scheme based on the differential of the DC voltage and current and a second DCT protection scheme based on the leakage current; S4, performing parameter setting for the first DCT protection scheme and the second DCT protection scheme; S5, comparing the first DCT protection scheme and the second DCT protection scheme in terms of protection speed, ability to withstand transition resistance, and anti-noise ability. The present invention fills the blank of the protection of bipolar FTF-MMC-DCT, improves the fault detection and response ability, ensures stability and safety, and guarantees the overall stable operation of the multilevel DC power grid.

Description

Technical Field

[0001] The present invention relates to the field of flexible DC transmission system protection, and particularly to a fast protection method for a face-to-face modular multilevel bipolar DC transformer. Background Art

[0002] In recent years, flexible DC transmission technology has received extensive attention due to its advantages such as flexible independent controllability and no commutation failure. With the technological breakthroughs of key equipment such as DC circuit breakers and direct current transformers (DCTs), the large-scale collection and transmission of new energy through DC systems of multiple voltage levels will become one of the typical scenarios for future flexible DC applications.

[0003] The DCT is a hub device connecting DC systems of different voltage levels, with a multi-power conversion stage and a multi-modular topology structure, integrating functions such as voltage conversion, power transmission, and fault isolation. However, due to the low inertia and weak damping characteristics of the DC grid, when internal or external faults occur in the DCT, the fault current increases rapidly, which may cause the MMC in the DCT to quickly lock, and in severe cases, even damage the sub-modules of the modular multilevel converter (MMC). Therefore, carrying out fast protection of the DCT and realizing fast identification of internal and external faults of the DCT is of great significance for the safe and reliable operation of multi-level DC grids.

[0004] Currently, the research on DCT mainly focuses on operation control, topology design, parameter optimization, etc. There is less research on the protection of the bipolar face-to-face modular multilevel type DCT (Front-to-Front Modular Multilevel Converter, FTF-MMC-DCT). The research focus is mainly on the modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system, and no specific protection scheme design has been involved yet. As the basic unit constituting the FTF-MMC-DCT, certain research results have been achieved in the protection of the MMC, but these methods cannot be directly applied to the protection of the FTF-MMC-DCT because the FTF-MMC-DCT has a multi-power change stage of input-isolation-output, its topology structure and control strategy are more complex than those of the MMC, and the fault characteristics are also different. Summary of the Invention

[0005] In order to solve the problem of the lack of a protection scheme for the bipolar FTF-MMC-DCT, the present invention proposes a fast protection method for a face-to-face modular multilevel bipolar DC transformer to solve the above problems.

[0006] The present application discloses a fast protection method for a face-to-face modular multilevel bipolar DC transformer, including the following steps:

[0007] S1. According to the current flow path under different fault types, combining the similarity between the DCT equivalent circuit and the MMC equivalent circuit, construct an equivalent circuit model of a bipolar FTF-MMC-DCT fault circuit;

[0008] S2. Analyze the differences in the internal and external fault characteristics of the DCT in the equivalent circuit model under different faults in a multi-level DC, and accordingly extract the DC voltage and current differentials of the DCT as the first fault characteristic quantity, and extract the port leakage current as the second fault characteristic quantity;

[0009] S3. According to the first fault characteristic quantity and the second fault characteristic quantity, formulate a first DCT protection scheme based on the DC voltage and current differentials and a second DCT protection scheme based on the leakage current;

[0010] S4. Set the parameters of the first DCT protection scheme and the second DCT protection scheme;

[0011] S5. Compare the first DCT protection scheme and the second DCT protection scheme in terms of protection speed, ability to withstand transition resistance, and anti-noise ability.

[0012] Preferably, the S1 includes the following steps:

[0013] Based on the bipolar FTF-MMC-DCT DC grid model, obtain the equivalent circuit model of the internal and external faults of the FTF-MMC-DCT;

[0014] Equivalent the bipolar FTF-MMC-DCT DC fault side under a fault to a resistor, an inductor, and a capacitor, and equivalent the capacitor on the bipolar FTF-MMC-DCT AC fault side to a constant voltage source.

[0015] Preferably, the first fault characteristic quantity is obtained by analyzing the ground fault at the DC side outlet of the transformer, the ground fault of the external line, the bipolar short circuit fault of the external DC line, the internal AC single-phase ground fault, and the AC two-phase ground fault;

[0016] The bipolar FTF-MMC-DCT adopts a completely symmetric structure for the positive and negative poles and the same structure for the high and low voltage sides. Only one pole on one side needs to be analyzed for faults. Therefore, analyze the fault characteristics of the P pole on the low voltage side of the bipolar FTF-MMC-DCT and extract the first fault characteristic quantity.

[0017] Preferably, the first fault characteristic quantity is obtained through the following steps:

[0018] For the ground fault at the DC side outlet of the transformer, ignoring the non-faulty high voltage side feed current, the fault characteristic quantities of the DCT outlet voltage and current after the fault are shown as follows:

[0019]

[0020] Among them, t is time, and U P (0) is the voltage at the initial moment of P - pole fault, and I P is the P - pole fault current, and U N is the rated voltage of the transmission line; represents the sudden change in the voltage at the measurement point after the P - pole fault, and represents the sudden change in the current at the measurement point after the P - pole fault;

[0021] When a ground fault occurs at the DC - side outlet of the transformer, the current flowing into the transformer decreases, and the voltage drop after the fault is dU P (0) / dt = - U N / dt;

[0022] For an out - of - zone line ground fault, the fault characteristic quantities of the voltage and current at the DCT outlet after the fault are shown as follows:

[0023]

[0024] Among them, U L is the rated voltage of the bus connected to the P - pole of the DCT low - voltage side;

[0025] When an out - of - zone line ground fault occurs, the differential of the current flowing through the measurement point is less than 0;

[0026] For an out - of - zone DC line bipolar short - circuit fault, the fault characteristic quantities of the voltage and current at the DCT outlet after the fault are the same as those of an out - of - zone line ground fault. When an out - of - zone DC line bipolar short - circuit fault occurs, the differential of the current flowing through the measurement point is less than 0;

[0027] For an in - zone AC single - phase ground fault, analyzing the a - phase ground fault, the fault characteristic quantities of the voltage and current at the DCT outlet after the fault are shown as follows:

[0028]

[0029] Among them, U Pa (0) is the voltage of phase a during the fault, and U Pa (0) ≥ (1 - m)U N , where m is the MMC modulation coefficient of the transformer and 0 < m < 1; when a single - phase ground fault occurs inside the transformer, the fault voltage will drop, but the maximum value of the drop is greater than (m - 1 / 3)U N / dt, and the differential of the current flowing through the measurement point is less than 0;

[0030] For an AC two - phase ground fault, the fault characteristic quantities of the voltage and current at the DCT outlet after the fault are shown as follows:

[0031]

[0032] When there is a two-phase grounding fault inside the transformer, the fault voltage will drop, but the drop value is less than (m - 1 / 5)U N / dt, and the differential current flowing through the measurement point is less than 0.

[0033] Preferably, the second fault characteristic quantity is obtained by analyzing external faults and internal grounding faults.

[0034] Preferably, the second characteristic quantity is obtained through the following steps:

[0035] Analyze the fault characteristics of the P pole on the low-voltage side of the bipolar FTF-MMC-DCT, and extract the second fault characteristic quantity;

[0036] For external faults, the port leakage current is as shown in the following formula:

[0037] I sum = I P + I N + I d = 0;

[0038] Where, I sum is the leakage current, I P is the measured current of the P pole of the leakage current criterion, I N is the measured current of the N pole of the leakage current criterion, I d is the measured current of the intermediate grounding point of the leakage current criterion;

[0039] For internal grounding faults, the port leakage current is as shown in the following formula:

[0040] I sum = I P + I N + I d = I f > 0;

[0041] Where, I f is the fault current when an internal grounding fault occurs.

[0042] Preferably, the first DCT protection scheme includes the following steps:

[0043] Collect the differential voltage and current. When the port voltage drops, start the protection;

[0044] If the differential current of the P pole is greater than zero or the differential current of the N pole is less than zero, it is determined as an internal fault;

[0045] If the AC fault voltage drop is greater than the DC fault voltage drop, it is determined as an internal DC grounding fault;

[0046] If It is a P - pole fault, otherwise it is an N - pole fault.

[0047] Preferably, the second DCT protection scheme includes the following steps:

[0048] Collect the voltage and current differential components, and start the protection when the port voltage drops.

[0049] If the port leakage current |I sum | > 0, it is determined as an in - zone fault.

[0050] If the AC fault voltage drop is greater than the DC fault voltage drop, it is determined as an in - zone DC grounding fault.

[0051] If I sum > 0, it is a P - pole fault, otherwise it is an N - pole fault.

[0052] Preferably, the parameter setting includes the following steps:

[0053] Start - criterion setting: Find the maximum value of each simulation data through simulation to form a data set, take the minimum value in the data set as the setting basis, and divide it by the first reliability coefficient as the start - criterion setting value of the first DCT protection scheme and the second DCT protection scheme.

[0054] In - zone and out - of - zone identification - criterion setting: The in - zone and out - of - zone identification - criterion setting value of the first DCT protection scheme is:

[0055]

[0056] where K rel_2 is the second reliability coefficient, {(di / dt) max} min represents the minimum value in the data set formed by the maximum values of di / dt under different working conditions, and K set1 =(di / dt) set is the setting value.

[0057] The in - zone and out - of - zone identification - criterion setting value of the second DCT protection scheme is:

[0058] I rel =K rel_3 ×K re ×I max ;

[0059] where K rel_3 is the third reliability coefficient, K re is the percentage error of the measuring device, and I max is the maximum overload current of the DC system.

[0060] Fault location criterion setting. The fault location criterion setting values for the first DCT protection scheme and the second DCT protection scheme are: K set2 =(m - 1 / 5)U N / dt;

[0061] Fault pole selection criterion setting. The fault pole selection criterion setting value for the first DCT protection scheme is:

[0062]

[0063] The fault pole selection criterion setting value for the second DCT protection scheme is 0.

[0064] Preferably, the comparison result in S5 is: The quick-acting performance of the first DCT protection scheme is better than that of the second DCT protection scheme, and the anti-transition resistance ability and anti-noise ability of the second DCT protection scheme are better than those of the first DCT protection scheme.

[0065] Advantages of the present invention:

[0066] (1) Accurately identify faults: The two protection schemes of the present invention based on the differential of DC voltage and current and on leakage current can highly accurately identify various fault types in the bipolar FTF-MMC-DCT system. Each scheme targets different fault characteristics and has excellent fault detection accuracy, ensuring that the system can respond promptly and accurately when a fault occurs.

[0067] (2) Strong complementarity and rapid fault isolation: The two protection schemes of the present invention have strong complementarity and can quickly isolate faults and accurately judge fault types when a fault occurs in the system. This complementarity makes the protection system more efficient in dealing with various fault situations, ensuring that faults can be quickly and accurately isolated, minimizing the impact of faults on the system.

[0068] (3) Enhance system reliability and security: The present invention significantly improves the protection ability of the bipolar FTF-MMC-DCT system in different operating scenarios through accurate fault identification and rapid isolation measures, ensuring the stability of the system in a changing and complex operating environment. At the same time, this accurate protection scheme greatly improves the security and reliability of the system, avoiding possible fault spread and system collapse problems.

[0069] (4) Ensure the stability of the multi-level DC power grid: The protection scheme of the present invention is not only applicable to the bipolar FTF-MMC-DCT system but also can effectively ensure the stability of the multi-level DC power grid containing the bipolar FTF-MMC-DCT. In a multi-level DC power grid, the system may face more complex fault situations. The present invention effectively prevents fault spread through accurate fault detection and rapid isolation mechanisms, ensuring the stable operation of the entire multi-level DC power grid.

[0070] (5) Improve the overall system performance and efficiency: Through two highly complementary protection schemes, the present invention provides a more efficient fault response mechanism, ensuring that the bipolar FTF-MMC-DCT system and the multi-level DC power grid containing this system can quickly resume normal operation when a fault occurs, minimizing the system outage time, and thus improving the overall operation efficiency. Description of the Drawings

[0071] Figure 1 It is a flowchart of the face-to-face modular multi-level bipolar DC transformer fast protection method according to an embodiment of the present invention;

[0072] Figure 2 It is a topological structure diagram of a four-terminal flexible DC power grid containing a bipolar FTF-MMC-DCT according to an embodiment of the present invention;

[0073] Figure 3 It is a schematic diagram of the internal and external fault division of a four-terminal flexible DC power grid containing a bipolar FTF-MMC-DCT according to an embodiment of the present invention;

[0074] Figure 4 It is a topological structure diagram of the positive pole of a bipolar FTF-MMC-DCT according to an embodiment of the present invention;

[0075] Figure 5 It is a schematic diagram of the equivalent model of the MMC on the fault side of a bipolar FTF-MMC-DCT according to an embodiment of the present invention;

[0076] Figure 6(a) is an equivalent circuit diagram of a DC grounding fault at the P-pole outlet of a bipolar FTF-MMC-DCT according to an embodiment of the present invention;

[0077] Figure 6(b) is an equivalent circuit diagram of a DC bus grounding fault at the P-pole of a bipolar FTF-MMC-DCT ignoring the remote feed current according to an embodiment of the present invention;

[0078] Figure 7 It is an equivalent circuit diagram of a DC line grounding fault outside the P-pole region of an FTF-MMC-DCT ignoring the remote feed current according to an embodiment of the present invention;

[0079] Figure 8 It is an equivalent circuit diagram of an AC single-phase grounding fault at the P-pole of a bipolar FTF-MMC-DCT according to an embodiment of the present invention;

[0080] Figure 9 It is an equivalent circuit diagram of an AC two-phase grounding fault at the P-pole of a bipolar FTF-MMC-DCT according to an embodiment of the present invention;

[0081] Figure 10 It is a low-voltage side equivalent topological structure diagram of a bipolar FTF-MMC-DCT according to an embodiment of the present invention;

[0082] Figure 11 Flow chart of the bipolar FTF-MMC-DCT protection scheme according to an embodiment of the present invention;

[0083] Figure 12 Schematic diagram of the simulation verification results of the starting criterion of the protection scheme based on the differential of the port DC current according to an embodiment of the present invention;

[0084] Figure 13 Schematic diagram of the simulation verification results of the starting criterion of the protection scheme based on the leakage current according to an embodiment of the present invention;

[0085] Figure 14 Schematic diagram of the simulation verification results of the location criterion of two protection schemes according to an embodiment of the present invention;

[0086] Figure 15 Schematic diagram of the simulation verification results of the pole selection criterion of the protection scheme based on the differential of the port DC current according to an embodiment of the present invention;

[0087] Figure 16 Schematic diagram of the simulation verification results of the pole selection criterion of the second DCT protection scheme according to an embodiment of the present invention. Detailed implementation manners

[0088] To make the objectives, technical solutions and advantages of the present application clearer and more understandable, the following examples are given with reference to the accompanying drawings to further elaborate on the present application in detail.

[0089] An embodiment of the present application discloses a fast protection method for a face-to-face modular multilevel bipolar DC transformer. The method flow is as Figure 1 shown and includes the following steps:

[0090] S1. According to the current flow paths under different fault types, combining the similarity between the DCT equivalent circuit and the MMC equivalent circuit, construct an equivalent model of the bipolar FTF-MMC-DCT fault circuit. Based on the bipolar FTF-MMC-DCT DC grid model, obtain the equivalent circuit models of internal and external faults of the FTF-MMC-DCT; when there is a fault on the DC fault side of the bipolar FTF-MMC-DCT, it is equivalent to a resistor, an inductor and a capacitor, and when there is a fault on the AC fault side of the bipolar FTF-MMC-DCT, the capacitor is equivalent to a constant voltage source.

[0091] The three-phase topology of the FTF-MMC-DCT has the characteristics of the MMC and can prevent the cross-side transfer of DC fault current. The bipolar DC grid topology composed of the FTF-MMC-DCT based on the three-phase topology is as Figure 2As shown in the figure. In the figure: MMCx-P and MMCx-N are the positive and negative poles of the bipolar converter respectively, where x = 1 to 4, representing converter stations 1 to 4, L represents the low-voltage side converter, H represents the high-voltage side converter; Acx, x = 1 to 4 represent the AC equivalent voltage sources 1 to 4 on the converter side. The internal and external fault division of FTF-MMC-DCT is as Figure 3 shown. Figure 3 In the figure, the shaded area is the line protection area, the grid area is the FTF-MMC-DCT protection area, and the dotted box area is the bus protection area.

[0092] As shown in the Figure 4 appendix, the three-phase topology of the bipolar FTF-MMC-DCT adopts the DC-AC-DC connection form. Figure 4 In it, R arm is the arm resistance, L arm is the arm inductance, GND is the system internal grounding point, U L+ is the positive voltage on the low-voltage side of the DCT, I L+ is the positive current on the low-voltage side of the DCT, U H+ is the positive voltage on the high-voltage side of the DCT, I H+ is the positive current on the high-voltage side of the DCT, SMx is the MMC sub-module (submodule, SM), x = 1 to N are the sub-modules 1 to N respectively, I f1 is the main fault current under f1 fault. FTF-MMC-DCT converts the DC quantity into a three-phase AC quantity through the MMC, then steps up the voltage through the AC transformer, and finally converts the three-phase AC quantity into a DC quantity through the MMC to achieve DC voltage conversion.

[0093] The faults of the bipolar FTF-MMC-DCT are divided into DC side faults and AC side faults. Based on the fact that FTF-MMC-DCT can prevent the cross-side transfer of fault current, the influence of the AC quantity of the transformer is ignored during DC side faults. When there is a DC side fault, the MMC of the DCT is equivalent to that of the conventional converter, and aiming at the rapid discharge of the capacitor during DC side faults, the Figure 4 is equivalent to a resistor R, an inductor L, and a capacitor C under the fault of the fault side, and Figure 5 is obtained. Figure 5 In it, C eq represents the equivalent capacitance of the single-phase arm SM, and R, L, and C represent the equivalent resistance, inductance, and capacitance of the entire MMC respectively. Its calculation is shown as follows:

[0094]

[0095] Among them, C0 is the capacitance of a single SM in the arm.

[0096] During AC side faults, the capacitor discharges relatively slowly, and the change range of the capacitor voltage in a short time after the fault is small, so the capacitor can be equivalent to a constant voltage source.

[0097] To extract the differentials of DC voltage and current after a fault, it is necessary to construct a detailed and accurate equivalent circuit model. This model should not only comprehensively include all the key components in the circuit and their interconnections, but also accurately reflect the electrical characteristics of the components and the impact of the fault on the circuit, so as to ensure that the subtle characteristics of the voltage and current changes in the circuit after the fault can be captured during the differential operation, providing strong data support for subsequent fault diagnosis. The equivalent circuit models under different faults in the multi-level DC established in this embodiment are as Figures 5 - 10 shown.

[0098] S2. Analyze the differences in the internal and external fault characteristics of the DCT in the equivalent circuit model under different faults in the multi-level DC, and accordingly extract the differentials of the DCT DC voltage and current as the first fault characteristic quantity, and extract the port leakage current as the second fault characteristic quantity.

[0099] For the differentials of DC voltage and current, first conduct a fault analysis on the DC side of the FTF-MMC-DCT.

[0100] The bipolar FTF-MMC-DCT adopts a completely symmetric structure for the positive and negative poles and the same structure for the high and low voltage sides. Only one pole on one side needs to be analyzed for faults. In this embodiment, taking the fault of the P pole on the low voltage side of the bipolar FTF-MMC-DCT as an example, analyze the changes in the DC quantities at the ports under different fault types of the FTF-MMC-DCT, and extract the first fault characteristic quantity. The analysis object is as Figure 5 shown.

[0101] For the grounding fault at the DC side outlet of the transformer, Figure 5 In, when a fault occurs at the port of the bipolar FTF-MMC-DCT, the feeding current of the remote converter station MMC1-P can be ignored. The DCT has a fault isolation function. Ignoring the feeding current of the non-faulty high voltage side MMCH-P, its overall equivalent structure is shown in Figure 6(a). In Figure 6(a), f represents the grounding fault at the DCT port. According to Figure 6(a), the detailed equivalent circuit of the DCT ignoring the remote feeding current is shown in Figure 6(b). In Figure 6(b), L L 、R L 、C L are equivalent to MMCL-P, L Px 、R Px 、C Px are equivalent to the converter station MMCx-P, where x = 2, 4 are the positive poles of converter stations 2 and 4 respectively. L T is the reactance of the current limiting inductor, R xL 、L xLis the line equivalent impedance, x = 2, 4 are the branches where MMC2-P and MMC4-P are located respectively, the red arrow is the specified positive direction of the current, IL2, IL4, and IL are the currents flowing through line 2, line 4, and the outlet of the low-voltage side of the DCT respectively, I L3 is the total current fed into the fault point by the external line, U P2 , U P4 , U L are the voltages on the equivalent capacitors of MMC2-P, MMC4-P, and MMCL-P respectively after the fault, R f is the transition resistance of the fault.

[0102] According to Kirchhoff's law, when R f = 0 and a fault occurs at t = 0, the post-fault current equations are as follows:

[0103]

[0104] Among them, i L (0), U L (0) respectively represent the current value of I L and the voltage value of U L at the moment before the fault occurs, θ dc is the time constant of the discharge circuit of the low-voltage P pole of the DCT, τ is the initial phase of the discharge circuit of the low-voltage P pole of the DCT, ω dc is the angular frequency of the discharge circuit of the low-voltage P pole of the DCT, R dis is the resistance of the discharge circuit of the low-voltage P pole of the DCT, and the parameter value calculation formula is as follows:

[0105]

[0106] Ignoring the voltage drop on the resistor at the initial moment, we get:

[0107]

[0108] Among them, U P is the voltage at the transformer outlet, U N is the rated voltage of the transmission line.

[0109] We can obtain I L4 , I L2 as follows:

[0110]

[0111] Among them, y = 4, x = 1 represents the I L4 current, y = 2, x = 4 represents the I L2 current, i Ly (0) represents the current flowing through the line at the moment before the fault occurs, U Ly(0) represents the equivalent capacitor voltage of the line connecting to the MMC at the moment before the fault occurs. y = 2 and 4 represent Lines 2 and 4 respectively. τ x , θ dcx , ω dcx , R disx where x = 1, 2 represent the time constant, initial phase, angular frequency, and resistance of the discharge circuits of Lines 2 and 4 respectively. The specific calculation formulas are as follows:

[0112]

[0113] Among them, L Ly = L Py + L T-yL + L yL , representing the equivalent inductance of the discharge circuit. R Ly = R Py + R yL , representing the equivalent resistance of the discharge circuit. y = 2, 4 represent Lines 2 and 4 respectively. According to Kirchhoff's law, I L3 = I L2 + I L4 .

[0114] From the above analysis, it can be seen that when a grounding fault occurs at the DC side outlet of the DCT, the current flowing into the transformer I L decreases. At the same time, MMC2-P and MMC4-P feed current to the fault point through Lines 2 and 4. The grounding fault at the DC side outlet of the DCT is divided into an external DC bus fault and an internal DCT DC grounding fault. If it is an external DC bus fault, the current flowing through the protection measurement point is I L . After the fault, I L continuously decreases in a short period of time, and dI P (t) / dt = dI L (t) / dt < 0 (I P (t) is the voltage of the P-pole protection measurement point). If it is an internal DCT DC grounding fault, in a short period of time after the fault, L3 continuously increases, and dI P (t) / dt = dI L3 (t) / dt > 0. After the fault, dU P (0) / dt = -U N / dt.

[0115] For an external line grounding fault, taking the intermediate grounding fault of Line 4 as an example, the equivalent circuit of the external line grounding fault ignoring the far-end feed current is as Figure 7 shown. Figure 7 Among them, Z 4L = R 4L + L 4Lis the equivalent impedance of Line 4, x is the line fault location and 0≤x≤1; is the current fed from Line 2 and MMCL-P to the fault point. Other parameters are the same as those in Fig. 6(b).

[0116] According to Kirchhoff's law, when R f = 0 and a fault occurs at t = 0, the post-fault current equations are as follows:

[0117]

[0118] where, UP is the voltage at point P at the outlet of MMCL-P, L L2 is the total inductance of Line 2, R L2 is the total resistance of Line 2. From L L << L L2 it can be known that dI L2 / dt << -dI L / dt, and there is L L5 = L L2 - L L ,, ignoring -dI L / dt, we have dI L5 / dt ≈ -dI L / dt. At the same time, ignoring the resistance R 4L of Line 4, as well as the voltage drop across the resistance at the initial moment of the fault, we get:

[0119]

[0120] where, I L (0) is the current of I L at the moment before the fault occurs, and I P (t) is the voltage at the measurement point of the P-pole protection. From the above formula, it can be known that when there is a ground fault on the external line, the differential component of the current flowing through the measurement point is less than 0.

[0121] For a bipolar short-circuit fault on the external DC line, for a DC power grid with bipolar FTF-MMC-DCT as shown in Figure 2 , for its external DC bipolar short-circuit fault, due to its completely symmetric structure on both sides, it is equivalent to a ground fault at the R f / 2 of the single-pole on both sides. The fault characteristics of its P-pole are exactly the same as those of the single-pole ground fault, and the fault characteristics of the N-pole are symmetric with those of the P-pole. Then the fault characteristic quantities of the voltage and current at the outlet of the DCT after the fault are the same as those of the ground fault on the external line. When there is a bipolar short-circuit fault on the external DC line, the differential component of the current flowing through the measurement point is less than 0.

[0122] Then, the fault analysis of the AC side of FTF-MMC-DCT is carried out.

[0123] When an AC fault occurs in the MMCL-P, since the fault is within the zone, the MMCL-P cannot be equivalently represented as a whole. During an AC fault, the capacitor discharges relatively slowly, and the change in its capacitor voltage is not significant in a short period of time. The arm capacitors can be equivalently represented as a constant voltage source.

[0124] For an in-zone AC single-phase grounding fault, in this embodiment, the a-phase grounding fault is analyzed. The b and c phases are equivalently represented in parallel as R, L, and U, as shown in the appendix Figure 8 as shown. Figure 8 Since the arm parameters of the b and c phases are the same, the b and c phases can be equivalently represented in parallel as R, L, and U, as shown in Figure 5 as shown. Figure 8 where R = R arm L = L arm U = U N , I Lbc is the current flowing into the fault point after the equivalent of the b and c phases, I Pa is the current of the upper arm of the a phase, U Pa , U Na are the equivalent power supply voltages of the upper and lower arms of the a phase respectively.

[0125] Within a few milliseconds before the MMC is blocked, the modulation strategy is considered to have no obvious distortion. The MMC4-P feeds current to the fault point through lines 2 and 4. Lines 2 and 4 contain two current-limiting reactances, and their reactance values are much larger than L. At the initial moment of the fault, the current increment fed into the fault point by lines 2 and 4 is small, that is, dI L3 / dt < dI Lbc / dt. According to Kirchhoff's law, dI Pa / dt = dI L3 / dt + dI Lbc / dt, and we get dI Pa / dt < 2dI Lbc / dt. When R f = 0 and a fault occurs at t = 0, ignoring the voltage drop across the arm resistance, we get:

[0126]

[0127] where I Pa (0) is the current of the a-phase arm at the moment before the fault occurs, U Pa (0) is the equivalent voltage source voltage of the a-phase arm at the moment after the fault occurs. Transforming the above formula, we get:

[0128]

[0129] where U Pa (0) ≥ (1 - m)U N, where m is the modulation coefficient of the transformer MMC, and 0 < m < 1. It can be seen from the above formula that for a single-phase ground fault inside the transformer, the fault voltage will drop, but the maximum value of the drop is greater than (m - 1 / 3)U N / dt. Since the current dI flowing through the measurement point P due to the voltage drop at the outlet of the MMCL-P after the fault P / dt = dI L3 / dt > 0.

[0130] For an AC two-phase ground fault, taking the a and b phase ground fault as an example, similarly ignoring the AC circuit, the equivalent circuit is as Figure 9 shown. Figure 9 where I Pa and I Pb are the upper-bridge arm currents of phases a and b respectively, I Lc is the current of phase c, U Px and U Nx are the equivalent power supply voltages of the bridge arms, x = a, b are the bridge arms of phases a and b respectively, and U C is the equivalent power supply voltage of phase c.

[0131] At the initial moment of the fault, the fault current mainly comes from the non-faulty phase c. Ignoring dI L3 / dt, assuming U LPa < U LPb , the differential of the port voltage is obtained as follows:

[0132]

[0133] Transforming the above formula gives:

[0134]

[0135] It can be seen from the above formula that for a two-phase ground fault inside the transformer, the fault voltage will drop, but the drop value is greater than (m - 1 / 5)U N / dt. Since the voltage drop of U P < U N after the fault, the current dI flowing through the measurement point P P / dt = dI L3 / dt > 0.

[0136] The fault characteristics on the MMCL-N side are structurally symmetric to those on the MMCL-P side. Taking the inverse of the MMCL-N measurement values can obtain the same fault characteristics as those of the MMCL-P.

[0137] Leakage current analysis mainly relies on Kirchhoff's Current Law (KCL) and Kirchhoff's Voltage Law (KVL). Different from the detailed equivalent model that requires a detailed description of the characteristics of each component in the circuit, leakage current analysis only requires a circuit equivalent model that can reflect the basic connection relationship and electrical characteristics of the circuit. This method not only simplifies the analysis process but also maintains the accuracy and practicality of the analysis.

[0138] The equivalent topology of the low-voltage side of the bipolar FTF-MMC-DCT is as Figure 10 shown. Figure 10 Among them, L Pxy , R Pxy , C Pxy are the equivalent inductance, resistance, and capacitance of the MMC, Z Pxy is the equivalent impedance of the MMC connection line. x = 2, 4 represent Converter Station 2 and Converter Station 4 respectively, y = +, - represent the P pole and N pole respectively, L T is the reactance of the current-limiting inductor. The red arrow is the specified positive direction of the DC current. I1 and I1 are the currents of MMC4-P and MMC2-P passing through lines 2 and 4 respectively. f1~f5 have the same meaning as f1~f5 in Figure 3 . f1 represents the grounding fault of the external busbar in the P-pole area of the low-voltage side of the DCT, f2 represents the DC grounding fault in the P-pole area of the low-voltage side of the DCT, f3 represents the single-phase AC grounding fault in the P-pole area of the low-voltage side of the DCT, f4 represents the two-phase AC grounding fault in the P-pole area of the low-voltage side of the DCT, and f5 represents the grounding fault of the external line in the P-pole area of the low-voltage side of the DCT. i x_xy is the arm current; i X_y is the three-phase AC current of the transformer. The subscript X = P, N represents the positive and negative pole currents, the subscript x = p, n represents the upper and lower arms, and the subscript y = a, b, c represents the a, b, c phases respectively; I P_P , I P_N are the upper and lower arm currents of the positive pole respectively, I N_P , I N_N are the upper and lower arm currents of the negative pole respectively, and I d is the grounding point current.

[0139] Since the DCT contains active components, the traditional transformer differential protection cannot distinguish between internal and external faults. Using the leakage current differential protection with the currents at points 1, 2, and 3 in Figure 10 can not only accurately distinguish between internal and external faults but also avoid false fault judgments caused by noise. The currents flowing through the measurement points 1, 2, and 3 are I P , I N , I d respectively, and the current direction is the same as I d .

[0140] According to Kirchhoff's Current Law, we have:

[0141]

[0142] The AC current of the P pole is as follows:

[0143]

[0144] The AC current of the N pole is as follows:

[0145]

[0146] Take the occurrence of faults f1, f2, f3, f4, and f5 at the P pole as an example.

[0147] When an external f1 or f5 fault occurs, there is I P = I P_P ,I N_P = -I N_N Since there is no AC fault in the zone and the transformer is connected in a delta configuration, then there is i P_a + i P_b + i P_c = 0, i N_a + i N_b + i N_c = 0, I N_P = -I N_N The leakage current is obtained as shown in the following formula:

[0148]

[0149] When an internal f2 fault occurs, there is I P = I1 + I2, I N_P = -I N_N Since there is no AC fault in the zone and the transformer is connected in a delta configuration, then there is i P_a + i P_b + i P_c = 0, i N_a + i N_b + i N_c = 0, I P_P = -I P_N ,I N_P = -I N_N The leakage current is obtained as shown in the following formula:

[0150]

[0151] Among them, I f2 is the current flowing through the fault point when the f2 fault occurs.

[0152] When an internal f3 or f4 fault occurs, there is I P = I1 + I2, I N_N = -I N_P, since the N pole has no fault, there is i N_a +i N_b +i N_c =0, I N_P =-I N_N , and the leakage current is obtained as shown in the following formula:

[0153]

[0154] where I f is the current flowing through the fault point when faults f3 or f4 occur. From the above analysis of the leakage current, it can be seen that the leakage current of the external fault in the P-pole region is 0, and the leakage current of the internal grounding fault is greater than 0. The leakage current criterion can clearly distinguish between internal and external faults in the DCT region.

[0155] Since the N pole and the P pole are completely symmetrical, it can be known that the leakage current of the external fault in the N-pole fault area I P +I N +I d =0, and the leakage current of the internal grounding fault in the N-pole area I P +I N +I d <0.

[0156] In summary, for external faults, the port leakage current is as shown in the following formula:

[0157] I sum =I P +I N +I d =0;

[0158] where I sum is the leakage current, I P is the measured current of the P-pole of the leakage current criterion, I N is the measured current of the N-pole of the leakage current criterion, I d is the measured current of the intermediate grounding point of the leakage current criterion;

[0159] For internal grounding faults, the port leakage current is as shown in the following formula:

[0160] I sum =I P +I N +I d =I f >0;

[0161] where I f is the fault current when an internal grounding fault occurs.

[0162] S3. According to the first fault characteristic quantity and the second fault characteristic quantity, formulate the first DCT protection scheme based on the differential of DC voltage and current and the second DCT protection scheme based on the leakage current.

[0163] According to the above fault analysis, the fault characteristics of the low-voltage side P pole are summarized in Table 1, and the characteristics of the N pole fault are opposite to those of the P pole. As can be seen from Table 1, dI P / dt and I sum can be used as the in-zone and out-of-zone criteria, and dU P / dt can be used as the AC and DC positioning criteria. According to the fault characteristics, the flowcharts of the DCT protection schemes based on the differential of the DC quantity at the port and the leakage current are as shown in Figure 11 . In the figure, U X and I X represent the DC port voltage and current of the DCT respectively, the subscripts X = P, N represent the positive and negative poles of the DCT DC port respectively, and I sum = I P + I N + I d represents the leakage current.

[0164] Table 1 Fault criterion characteristics

[0165]

[0166] According to the fault characteristics summarized in Table 1, the first DCT protection scheme based on the differential of the DC quantity at the port is designed as shown in Figure 11 (a) below.

[0167] First, collect the differential quantities of voltage and current. When it satisfies that the port voltage drops, that is, |dU X / dt| > K set0 , the protection starts, where X = P, N is the P pole or N pole, and K set0 = 0.1|U N / dt| is the reference value to avoid interference;

[0168] If it satisfies that the differential of the P pole current is greater than zero or the differential of the N pole current is less than zero, that is, dI P / dt > K set1 or dI N / dt < -K set1 (K set1 is the reference value to avoid misoperation), then it is determined as an in-zone fault;

[0169] If the AC fault voltage drop is greater than the DC fault voltage drop, that is, dU P / dt < -K set2 or dU N / dt > K set2 (K set2 is the setting value), then it is determined as an in-zone DC grounding fault;

[0170] Finally, pole selection is performed. Since the faults of the P and N poles have little mutual influence, if it satisfies It is a P - pole fault, otherwise it is an N - pole fault.

[0171] After judging as an internal fault, implement the protection scheme, and achieve fault removal or system isolation through physical devices (such as circuit breakers, relays, etc.) to prevent the expansion of the fault or damage to the equipment.

[0172] According to the fault characteristics summarized in Table 1, design the second DCT protection scheme based on the leakage current as Figure 11 shown in (b) below.

[0173] The starting criterion is the same as that in the first DCT protection scheme;

[0174] The in - zone and out - of - zone criterion uses the leakage current I sum , if the port leakage current |I sum | > 0, it is judged as an in - zone fault;

[0175] The fault location criterion is the same as that in the first DCT protection scheme;

[0176] The fault pole selection uses the port leakage current. If I sum > 0, it is a P - pole fault, otherwise it is an N - pole fault.

[0177] After judging as an internal fault, implement the protection scheme, and achieve fault removal or system isolation through physical devices (such as circuit breakers, relays, etc.) to prevent the expansion of the fault or damage to the equipment.

[0178] S4. Tune the parameters of the first DCT protection scheme and the second DCT protection scheme.

[0179] Tune the starting criterion. The starting criteria of both the first DCT protection scheme and the second DCT protection scheme are the micro - component of the port DC voltage. Its setting value needs to avoid interference and errors and ensure that all faults can be reliably identified. For the Figure 2 PSCAD simulation model shown below, conduct simulations. Find the maximum value of the current micro - component under different in - zone faults in each simulation to form a data set. Take the minimum value in the data set as the tuning basis and divide it by the first reliability coefficient as the setting value of the starting criterion for the first DCT protection scheme and the second DCT protection scheme. In this embodiment, the reliability coefficient is taken as 1.2.

[0180] Tune the in - zone and out - of - zone identification criterion. The first DCT protection scheme uses the micro - component of the port DC current as the in - zone and out - of - zone criterion. In this embodiment, refer to the maximum - value type scheme for parameter tuning, that is, find the maximum value of each simulation data to form a data set, and take the minimum value in the set as the tuning basis. This embodiment tunes with a transition resistance of 0Ω, and the setting value calculation is as follows:

[0181]

[0182] Among them, K rel_2 is the second reliability coefficient, which is taken as 1.1 in this embodiment, {(di / dt) max} min represents the minimum value in the data set formed by the maximum value of di / dt under different working conditions, and K set1 =(di / dt) set is the setting value.

[0183] The second DCT protection scheme uses the leakage current as the criterion for internal and external faults. The calculation of the setting value of the leakage current protection is as follows:

[0184] I rel =K rel_3 ×K re ×I max ;

[0185] Among them, K rel_3 is the third reliability coefficient, and its value range is 1.05 - 1.1. It is taken as 1.1 in this embodiment. K re is the percentage error of the measuring device, which is taken as 0.1 in this embodiment. I max is the maximum overload current of the DC system, and I rel is the setting value.

[0186] For the setting of the fault location criterion, both the first DCT protection scheme and the second DCT protection scheme use the micro-component of the DC voltage at the port as the fault location criterion. During a DC fault, dU P / dt = -dU N / dt. During an AC fault, dU P / dt = -(m - 1 / 5)U N / dt. According to the analysis of the AC fault in S3 above, the setting value of the fault location criterion is: K set2 =(m - 1 / 5)U N / dt.

[0187] For the setting of the fault pole selection criterion, the first DCT protection scheme uses the ratio of the micro-components of the DC currents at the positive and negative ports as the fault pole selection criterion. Due to the structural characteristics of the bipolar FTF-MMC-DCT, the faults at the positive and negative poles are not transmitted in a short time. Therefore, the setting value of the pole selection criterion of the first DCT protection scheme is

[0188] The second DCT protection scheme uses the polarity of the leakage current as the fault pole selection criterion. Since the leakage current during a positive pole fault is greater than 0, while the leakage current during a negative pole fault is less than 0, 0 is taken as the setting value of the pole selection criterion.

[0189] In a specific embodiment, taking the fault on the low-voltage side of the transformer as an example, under different fault types, the transition resistance is set to 0.01 Ω. The simulation results of the proposed protection scheme within 3 ms after the fault are asFigures 12 - 15 As shown. Among them, P-G outside the zone and N-G outside the zone are DC bus grounding faults outside the P and N poles respectively; P-G inside the zone and N-G inside the zone are DC grounding faults inside the P and N poles respectively; PA-G and NA-G are single-phase AC grounding faults of the P and N poles respectively; PAB-G and NAB-G are two-phase AC grounding faults of the P and N poles respectively; I P I P , IN are the DC currents at the P and N extreme ports on the low-voltage side respectively; U X , I X are the DC port voltage and current of the DCT respectively, and the subscripts X = P, N are the positive and negative poles of the DCT DC port respectively.

[0190] The parameter is set to K set0 = 41 (kV / 50us). When the transition resistance is 0Ω, {(di / dt) max} min = 1.04 kA / ms; K set1 = 0.94 kA / ms; K set2 = 300 kV / dt = 300 (kV / 50us); I N = 1.5 kA; Taking twice the margin I max = 3 kA; K rel Taking 1.1 gives I rel = 0.33 kA.

[0191] For convenient observation and comparison, take -dI N / dt, as shown in Figure 12 (a) and (b). The minimum value of the differential component of the fault current inside the zone (-dI P / dtt, -dI N / dt) is 1.84 kA / ms, which is greater than K set1 , while the differential component of the fault current during an external fault is negative. According to Figure 13 , it can be known that the minimum value of the absolute value of the leakage current during an internal fault is 16.9 kA, and the maximum value is 37.1 kA, while the absolute value of the leakage current during an external fault is zero, less than 0.3 kA. In summary, both the first DCT protection scheme and the second DCT protection scheme can accurately identify internal and external faults, and the quick-acting performance of the first DCT protection scheme is significantly higher than that of the second DCT protection scheme.

[0192] According to Figure 14 , it can be known that when R f = 0Ω, the maximum value of the differential component of the internal AC fault voltage is 227 kV / 50us, which is less than K set2 , while the differential component of the internal DC fault voltage is 519 kV / 50us, which is greater than K set2 . Therefore, the differential component of the voltage can accurately judge internal AC and DC faults and perform fault grading.

[0193] According to Figure 15 , it can be known that when R f = 0 Ω, taking the P-pole fault as an example, the differential component of the P-pole current after the fault is greater than 1.84 kA / ms, while the differential component of the non-fault N-pole current is close to 0. The fault pole can be accurately located by using the ratio of the two-stage current differential components. According to Figure 16 , it can be known that the leakage current is greater than 28 kA during P-pole fault and less than -16 kA during N-pole fault. The leakage current can accurately judge the fault pole. In summary, the pole selection criteria of the first DCT protection scheme and the second DCT protection scheme can both accurately locate the fault pole.

[0194] After judging as an internal fault, the protection scheme is implemented, and the fault is removed or the system is isolated through physical devices (such as circuit breakers, relays, etc.) to prevent the fault from expanding or damaging the equipment.

[0195] S5. Compare the first DCT protection scheme and the second DCT protection scheme in terms of protection speed, ability to withstand transition resistance, and anti-noise ability. The comparison results are shown in Table 2. The protection speed of the first DCT protection scheme is better than that of the second DCT protection scheme, but the second DCT protection scheme has better anti-transition resistance ability and anti-noise ability than the first DCT protection scheme.

[0196] Table 2 Comparison of protection performance of the same scheme

[0197]

[0198] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A fast protection method for a face-to-face modular multilevel bipolar DC transformer, characterized in that It includes the following steps: S1. According to the current flow paths under different fault types and combining the similarity between the DCT equivalent circuit and the MMC equivalent circuit, construct an equivalent model of the bipolar FTF-MMC-DCT fault circuit; S2. Analyze the differences in the internal and external fault characteristics of the DCT in the equivalent circuit model under different faults in the multi-level DC, and accordingly extract the DC voltage and current differentials of the DCT as the first fault feature quantity, and extract the port leakage current as the second fault feature quantity; S3. According to the first fault feature quantity and the second fault feature quantity, formulate a first DCT protection scheme based on the DC voltage and current differentials and a second DCT protection scheme based on the leakage current; The first DCT protection scheme includes the following steps: Collect the voltage and current differentials. When the port voltage drops, start the protection; If the differential of the P-pole current is greater than zero or the differential of the N-pole current is less than zero, it is determined as an internal fault; If the AC fault voltage drop is greater than the DC fault voltage drop, it is determined as an internal DC grounding fault; If it is a P-pole fault, otherwise it is an N-pole fault; The second DCT protection scheme includes the following steps: Collect the voltage and current differentials. When the port voltage drops, start the protection; If the port leakage current |I sum | > 0, it is determined as an internal fault; If the AC fault voltage drop is greater than the DC fault voltage drop, it is determined as an internal DC grounding fault; If I sum > 0, it is a P-pole fault; otherwise, it is an N-pole fault; S4. Set the parameters for the first DCT protection scheme and the second DCT protection scheme; Set the starting criterion. Through simulation, find the maximum value of each simulation data to form a data set, take the minimum value in the data set as the setting basis, and divide it by the first reliability coefficient as the setting value of the starting criterion for the first DCT protection scheme and the second DCT protection scheme; Set the in-out zone identification criterion. The in-out zone identification criterion setting value of the first DCT protection scheme is: Among them, K rel_2 is the second reliability coefficient, {(di / dt) max} min represents the minimum value in the data set formed by the maximum value of di / dt under different working conditions, and K set1 =(di / dt) set is the setting value; The in-out zone identification criterion setting value of the second DCT protection scheme is: I rel = K rel_3 × K re × I max ; Among them, K rel_3 is the third reliability coefficient, and K re is the percentage error of the measuring device, and I max is the maximum overload current of the DC system; Fault location criterion setting, the fault location criterion setting values of the first DCT protection scheme and the second DCT protection scheme are: K set2 =(m - 1 / 5)U N / dt; Set the fault pole selection criterion. The fault pole selection criterion setting value of the first DCT protection scheme is: The fault pole selection criterion setting value of the second DCT protection scheme is 0; S5. Compare the first DCT protection scheme and the second DCT protection scheme in terms of protection speed, ability to withstand transition resistance, and anti-noise ability.

2. The fast protection method for the face-to-face modular multilevel bipolar DC transformer according to claim 1, wherein The said S1 includes the following steps: Based on the bipolar FTF-MMC-DCT DC grid model, obtain the equivalent circuit models of the internal and external faults of the FTF-MMC-DCT; Equivalent the DC fault side of the bipolar FTF-MMC-DCT under faults to resistors, inductors and capacitors, and equivalent the capacitors on the AC fault side of the bipolar FTF-MMC-DCT to constant voltage sources.

3. The fast protection method for a face-to-face modular multilevel bipolar DC transformer according to claim 2, characterized in that The first fault feature quantity is obtained by analyzing the grounding fault at the DC side outlet of the transformer, the grounding fault of the out-of-zone line, the bipolar short-circuit fault of the out-of-zone DC line, the internal AC single-phase grounding fault, and the AC two-phase grounding fault; The bipolar FTF-MMC-DCT adopts a completely symmetrical structure for the positive and negative poles and the same structure for the high and low voltage sides. Only one pole on one side needs to be analyzed for faults. Therefore, analyze the fault characteristics of the P-pole on the low voltage side of the bipolar FTF-MMC-DCT and extract the first fault feature quantity.

4. The fast protection method for a face-to-face modular multilevel bipolar DC transformer according to claim 3, characterized in that The first fault feature quantity is obtained through the following steps: For the grounding fault at the DC side outlet of the transformer, ignoring the non-fault high-voltage side feed current, the fault characteristic quantities of the DCT outlet voltage and current after the fault are shown as follows: Among them, t is time, U P (0) is the voltage at the initial moment of P-pole fault, I P is the P-pole fault current, U N is the rated voltage of the transmission line, represents the sudden change in the voltage at the measurement point after the P-pole fault, represents the sudden change in the current at the measurement point after the P-pole fault; When a ground fault occurs at the DC side outlet of the transformer, the current flowing into the transformer decreases, and the voltage drop after the fault is dU P (0) / dt = -U N / dt; For the grounding fault of the external line, the fault characteristic quantities of the DCT outlet voltage and current after the fault are shown as follows: Among them, U L is the rated voltage of the busbar connected to the P pole on the low-voltage side of the DCT; During the grounding fault of the external line, the differential component of the current flowing through the measurement point is less than 0; For the bipolar short-circuit fault of the external DC line, the fault characteristic quantities of the DCT outlet voltage and current after the fault are the same as those of the grounding fault of the external line. During the bipolar short-circuit fault of the external DC line, the differential component of the current flowing through the measurement point is less than 0; For the single-phase grounding fault in the internal AC system, analyzing the a-phase grounding fault, the fault characteristic quantities of the DCT outlet voltage and current after the fault are shown as follows: Among them, U Pa (0) is the phase-a voltage during a fault, and U Pa (0) ≥ (1 - m)U N , where m is the modulation coefficient of the transformer MMC and 0 < m < 1; when a single-phase ground fault occurs inside the transformer, the fault voltage will drop, but the maximum value of the drop is greater than (m - 1 / 3)U N / dt, and the differential component of the current flowing through the measurement point is less than 0; For the two-phase grounding fault in the AC system, the fault characteristic quantities of the DCT outlet voltage and current after the fault are shown as follows: When there is a two-phase grounding fault inside the transformer, the fault voltage will drop, but the drop value is less than (m - 1 / 5)U N / dt, and the differential current flowing through the measurement point is less than 0.

5. The fast protection method for a face-to-face modular multilevel bipolar DC transformer according to claim 4, characterized in that, The second fault characteristic quantity is obtained by analyzing the external fault and the internal grounding fault.

6. The fast protection method for a face-to-face modular multilevel bipolar DC transformer according to claim 5, characterized in that, The second fault characteristic quantity is obtained through the following steps: Analyze the fault characteristics of the P pole on the low-voltage side of the bipolar FTF-MMC-DCT to extract the second fault characteristic quantity; For the external fault, the port leakage current is shown as follows: I sum = I P + I N + I d = 0; Among them, I sum is the leakage current, I P is the measured current of the leakage current criterion P pole, I N is the measured current of the leakage current criterion N pole, I d is the measured current of the intermediate ground point of the leakage current criterion; For the internal grounding fault, the port leakage current is shown as follows: I sum = I P + I N + I d = I f > 0; Among them, I f is the fault current during an in-zone ground fault.

7. The fast protection method for the face-to-face modular multilevel bipolar DC transformer according to claim 6, characterized in that, The comparison result in S5 is that the quick-acting performance of the first DCT protection scheme is better than that of the second DCT protection scheme, and the anti-transition resistance ability and anti-noise ability of the second DCT protection scheme are better than those of the first DCT protection scheme.

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