Fast protection method for face-to-face modular multi-level bipolar direct-current transformer

Abstract

The invention discloses a face-to-face modularized multi-level bipolar direct current transformer rapid protection method. The method comprises the steps that S1, a bipolar FTF-MMC-DCT fault circuit equivalent model is constructed; s2, DCT DC voltage and current differential is extracted as a first fault characteristic quantity, and port leakage current is extracted as a second fault characteristic quantity; s3, formulating a first DCT protection scheme based on DC voltage and current differential and a second DCT protection scheme based on leakage current; s4, performing parameter setting on the first DCT protection scheme and the second DCT protection scheme; and S5, comparing the first DCT protection scheme with the second DCT protection scheme from the aspects of protection quickness, transition resistance resistance and noise resistance. According to the invention, the blank of bipolar FTF-MMC-DCT protection is filled up, the fault detection and response capability is improved, the stability and safety are ensured, and the overall stable operation of the multi-stage DC power grid is ensured.

Description

Technical Field

[0001] The present invention relates to the field of flexible direct current transmission system protection, and in particular to a face-to-face modular multi-level bipolar direct current transformer rapid protection method. Background Art

[0002] In recent years, flexible DC transmission technology has gained wide attention due to its advantages of flexible, independent controllability and no commutation failure. With the technological breakthroughs of key equipment such as DC circuit breakers and DC transformers (DCT), large-scale new energy transmission through multi-voltage DC systems will become one of the typical scenarios of flexible DC applications in the future.

[0003] DCT is a hub device that connects DC systems of different voltage levels. It has multiple power conversion levels and a multi-modular topology, 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 an internal or external fault occurs in the DCT, the fault current increases rapidly, which may cause the MMC in the DCT to lock quickly, and even damage the modular multilevel converter (MMC) submodule in severe cases. Therefore, it is of great significance to carry out rapid protection of DCT and realize rapid identification of internal and external faults of DCT for the safe and reliable operation of multi-level DC grids.

[0004] At present, the research on DCT mainly focuses on operation control, topology design, parameter optimization and other aspects. There are few studies on the protection of bipolar face-to-front modular multilevel converter (FTF-MMC-DCT). The research focus is mainly on the bipolar modular multilevel converter high-voltage direct current transmission (MMC-HVDC) system, and no specific protection scheme design has been involved. As the basic unit of FTF-MMC-DCT, the protection research of MMC has achieved certain results, but these methods cannot be directly applied to the protection of FTF-MMC-DCT, because FTF-MMC-DCT has multiple power change levels of input-isolation-output, its topology and control strategy are more complex than MMC, and its fault characteristics are also different. Summary of the invention

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

[0006] The present application discloses a face-to-face modular multi-level bipolar DC transformer fast protection method, comprising the following steps:

[0007] S1. According to the current flow paths under different fault types and the similarity between the DCT equivalent circuit and the MMC equivalent circuit, a bipolar FTF-MMC-DCT fault circuit equivalent model is constructed;

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

[0009] S3, formulating a first DCT protection scheme based on DC voltage and current differential and a second DCT protection scheme based on leakage current according to the first fault characteristic quantity and the second fault characteristic quantity;

[0010] S4, setting 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, transition resistance and noise resistance.

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

[0013] Based on the bipolar FTF-MMC-DCT DC power grid model, the FTF-MMC-DCT internal and external fault equivalent circuit model is obtained;

[0014] The DC fault side of the bipolar FTF-MMC-DCT is equivalent to resistance, inductance and capacitance, and the AC fault side capacitor of the bipolar FTF-MMC-DCT is equivalent to a constant voltage source.

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

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

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

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

[0019]

[0020] Where t is time, U P (0) is the voltage at the initial moment of the P-pole fault, I P is the P-pole fault current, U N is the rated voltage of the transmission line, It indicates the voltage mutation at the measuring point after the P-pole fault. Indicates the sudden change of current at the measuring point after a 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-area line grounding fault, the fault characteristic quantities of the DCT output voltage and current after the fault are as follows:

[0023]

[0024] Among them, U L The rated voltage of the busbar connected to the P pole on the low voltage side of the DCT;

[0025] When the out-of-area line is grounded, the differential current flowing through the measuring point is less than 0;

[0026] For the bipolar short-circuit fault of the DC line outside the area, the fault characteristic quantities of the voltage and current at the DCT outlet after the fault are the same as those of the grounding fault of the DC line outside the area. When the DC line outside the area has a bipolar short-circuit fault, the current differential flowing through the measuring point is less than 0;

[0027] For the AC single-phase grounding fault in the area, the a-phase grounding fault is analyzed, and the fault characteristic quantities of the DCT output voltage and current after the fault are shown as follows:

[0028]

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

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

[0031]

[0032] When two phases of the transformer are grounded, the fault voltage will drop but the drop value is less than (m-1 / 5)U N / dt, the differential current flowing through the measuring point is less than 0.

[0033] Preferably, the second fault characteristic quantity is obtained by analyzing out-of-zone faults and in-zone grounding faults.

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

[0035] The fault characteristics of the P-pole on the low-voltage side of the bipolar FTF-MMC-DCT are analyzed to extract the second fault characteristic quantity;

[0036] For out-of-zone faults, the port leakage current is as follows:

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

[0038] Among them, 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 measurement current of the N pole of the leakage current criterion, I d It is the measured current of the indirect point in the leakage current judgment;

[0039] For an in-zone ground fault, the port leakage current is as follows:

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

[0041] Among them, I f It is the fault current when a ground fault occurs in the zone.

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

[0043] Collect voltage and current differentials, and start protection when the port voltage drops;

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

[0045] If the AC fault voltage drop is greater than the DC fault voltage drop, it is determined to be a DC grounding fault in the area;

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

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

[0048] Collect voltage and current differentials, and start protection when the port voltage drops;

[0049] If the port leakage current |I sum |>0, it is judged as an internal fault;

[0050] If the AC fault voltage drop is greater than the DC fault voltage drop, it is determined to be a DC grounding fault in the area;

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

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

[0053] The starting criterion is set by finding the maximum value of each simulation data through simulation to form a data set, taking the minimum value in the data set as the setting basis, and dividing it by the first reliability coefficient as the starting criterion setting value of the first DCT protection scheme and the second DCT protection scheme;

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

[0055]

[0056] 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, K set1 =(di / dt) set is the set value;

[0057] The second DCT protection scheme has the following in-zone and out-of-zone identification criteria:

[0058] I rel =K rel_3 ×K re ×I max ;

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

[0060] 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;

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

[0062]

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

[0064] Preferably, the comparison result in S5 is: the first DCT protection scheme has better speed than the second DCT protection scheme, and the second DCT protection scheme has better transition resistance resistance and noise resistance than the first DCT protection scheme.

[0065] Beneficial effects of the present invention:

[0066] (1) Accurate fault identification: The two protection schemes based on DC voltage and current differential and leakage current in the present invention 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 the fault and accurately determine the fault type when a system fault occurs. This complementarity makes the protection system more efficient in dealing with various fault situations, ensuring that the fault can be isolated quickly and accurately, minimizing the impact of the fault on the system.

[0068] (3) Enhanced system reliability and safety: The present invention significantly improves the protection capability 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 changeable and complex operating environment. At the same time, this precise protection scheme greatly improves the safety and reliability of the system and avoids possible fault propagation and system crash problems.

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

[0070] (5) Improving the overall performance and efficiency of the system: The present invention provides a more efficient fault response mechanism through two highly complementary protection schemes, ensuring that the bipolar FTF-MMC-DCT system and the multi-level DC power grid including the system can quickly resume normal operation when a fault occurs, thereby minimizing the system downtime and improving the overall operating efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

[0076] FIG6( 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] FIG6( b ) is an equivalent circuit diagram of a P-pole DC bus grounding fault of a bipolar FTF-MMC-DCT ignoring remote feed according to an embodiment of the present invention;

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

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

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

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

[0082] Fig.11 A flow chart of a bipolar FTF-MMC-DCT protection scheme according to an embodiment of the present invention;

[0083] Fig.12 It is a schematic diagram of simulation verification results of the start-up criterion of the protection scheme based on the port DC differential according to an embodiment of the present invention;

[0084] Fig.13 It is a schematic diagram of simulation verification results of the startup criterion of the protection scheme based on leakage current according to an embodiment of the present invention;

[0085] Fig.14 It is a schematic diagram of simulation verification results of positioning criteria of two protection schemes according to an embodiment of the present invention;

[0086] Fig.15 It is a schematic diagram of the simulation verification result of the pole selection criterion of the protection scheme based on the port DC differential according to the embodiment of the present invention;

[0087] Fig.16 It is a schematic diagram of simulation verification results of the pole selection criterion of the second DCT protection scheme according to an embodiment of the present invention. DETAILED DESCRIPTION

[0088] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and examples.

[0089] The present application embodiment discloses a face-to-face modular multi-level bipolar DC transformer fast protection method, the method flow is as follows Figure 1 As shown, the following steps are included:

[0090] S1. According to the current flow path under different fault types, combined with the similarity between the DCT equivalent circuit and the MMC equivalent circuit, a bipolar FTF-MMC-DCT fault circuit equivalent model is constructed. Based on the bipolar FTF-MMC-DCT DC power grid model, the FTF-MMC-DCT internal and external fault equivalent circuit model is obtained; the bipolar FTF-MMC-DCT DC fault side fault is equivalent to resistance, inductance and capacitance, and the bipolar FTF-MMC-DCT AC fault side capacitor is equivalent to a constant voltage source.

[0091] The FTF-MMC-DCT three-phase topology has the characteristics of MMC and can prevent the DC fault current from being transmitted across the side. The two-stage DC grid topology composed of the FTF-MMC-DCT based on the three-phase topology is as follows: Figure 2As shown 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, and H represents the high-voltage side converter; Acx, x = 1 to 4 represents the converter side AC equivalent voltage source 1 to 4. The internal and external faults of FTF-MMC-DCT are divided as follows Figure 3 shown. Figure 3 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 busbar protection area.

[0092] As attached Figure 4 As shown, the bipolar FTF-MMC-DCT three-phase topology adopts a DC-AC-DC connection form. Figure 4 Medium R arm is the bridge arm resistance, L arm is the bridge arm inductance, GND is the grounding point in the system station, U L+ is the positive voltage on the low voltage side of DCT, I L+ is the positive current on the low voltage side of DCT, U H+ is the positive voltage on the high-voltage side of DCT, I H+ is the positive current on the high-voltage side of the DCT, SMx is the MMC submodule (SM), x=1~N are submodules 1~N, I f1 is the main fault current under fault f1. FTF-MMC-DCT converts DC into three-phase AC through MMC, then boosts the voltage through AC transformer, and finally converts the three-phase AC into DC through MMC to achieve DC transformation.

[0093] The faults of bipolar FTF-MMC-DCT are divided into DC side faults and AC side faults. Based on FTF-MMC-DCT, it can prevent the fault current from being transmitted across the sides, and ignore the influence of transformer AC quantity when the DC side fault occurs. When the DC side fault occurs, the MMC of DCT is equivalent to the MMC of conventional converters, and the capacitor discharges quickly when the DC side fault occurs. Figure 4 Under the fault on the middle fault side, the equivalent resistance is R, the inductance is L, and the capacitance is C, and we get Figure 5 . Figure 5 Middle C eq Represents the equivalent capacitance of the single-phase bridge arm SM, R, L, and C represent the equivalent resistance, inductance, and capacitance of the entire MMC respectively. The calculation is shown in the following formula:

[0094]

[0095] Among them, C0 is a single SM capacitor of the bridge arm.

[0096] When a fault occurs on the AC side, the capacitor discharges relatively slowly, and the voltage change of the capacitor is small in the short term after the fault, so the capacitor can be equivalent to a constant voltage source.

[0097] In order to extract the differential of DC voltage and current after a fault, a detailed and accurate equivalent circuit model needs to be constructed. This model should not only comprehensively include all key components in the circuit and their interconnection methods, 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 model under different faults in the multi-level DC established in this embodiment is as follows: Figure 5-Figure 10 shown.

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

[0099] For the DC voltage and current differentials, the FTF-MMC-DCT DC side fault analysis is first performed.

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

[0101] For the ground fault at the DC side outlet of the transformer, Figure 5 In the figure, when a fault occurs at the bipolar FTF-MMC-DCT port, the feed from the remote converter station MMC1-P can be ignored. The DCT has a fault isolation function and ignores the non-faulty high-voltage side MMCH-P feed. The overall equivalent structure is shown in Figure 6(a). In Figure 6(a), f represents the ground fault at the DCT port. The detailed equivalent circuit of the DCT ignoring the remote feed is shown in Figure 6(b). In Figure 6(b), L L , R L , C L MMCL-P equivalent, L Px , R Px , C Px is the equivalent of the converter station MMCx-P, 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 positive direction of the current, IL2, IL4, IL are the currents flowing through lines 2, 4 and the DCT low-voltage side outlet 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 after the fault, respectively. f is the transition resistance of the fault.

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

[0103]

[0104] Among them, i L (0) U L (0) respectively represent the time before the fault occurs I L The current value and U L The voltage value, θ dc is the time constant of the DCT low-voltage P-pole discharge circuit, τ is the initial phase of the DCT low-voltage P-pole discharge circuit, ω dc is the angular frequency of the DCT low voltage P pole discharge circuit, R dis is the resistance of the DCT low-voltage P-pole discharge circuit, 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 Rated voltage of the transmission line.

[0109] I can be obtained L4 ,I L2 As shown below:

[0110]

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

[0112]

[0113] Among them, L Ly =L Py +L T-yL +L yL , represents the equivalent inductance of the discharge circuit, R Ly =R Py +R yL , represents the equivalent resistance of the discharge circuit, y=2, 4 represent lines 2, 4 respectively, according to Kirchhoff's law, we can get I L3 =I L2 +I L4 .

[0114] From the above analysis, it can be seen that when the DC side outlet of the DCT is grounded, the current I flowing into the transformer L Reduced, while MMC2-P, MMC4-P through lines 2, 4, feed current to the fault point. DCT DC side outlet grounding fault is divided into out-of-zone DC bus fault and in-zone DCT DC grounding fault. If it is an out-of-zone DC bus fault, the current flowing through the protection measurement point is I L , within a short time after the fault L Continue to reduce dI P (t) / dt=dI L (t) / dt<0(I P (t) is the voltage at the P-pole protection measurement point). If it is a DCT DC grounding fault in the area, within a short period of time after the fault, L3 Continued increase in dI P (t) / dt=dI L3 (t) / dt>0, dU after fault P (0) / dt=-U N / dt.

[0115] For the grounding fault of the out-of-area line, taking the middle grounding fault of line 4 as an example, the equivalent circuit of the grounding fault of the out-of-area line ignoring the remote feeder is as follows: Figure 7 shown. Figure 7 In, Z 4L =R 4L +L 4Lis the equivalent impedance of line 4, x is the line fault location and 0≤x≤1; is the feed current from line 2 and MMCL-P to the fault point. Other parameters are the same as those in Figure 6(b).

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

[0117]

[0118] Among them, 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, L L <<L L2 It can be seen that dI L2 / dt<<-dI L / dt, and L L5 =L L2 -L L ,,ignore -dI L / dt has dI L5 / dt≈-dI L / dt, while ignoring the resistance R of line 4 4L , and the voltage drop on the resistor at the initial moment of the fault is obtained:

[0119]

[0120] Among them, I L (0) is the moment before the fault occurs. L The current, I P (t) is the voltage of the P-pole protection measurement point. It can be seen from the above formula that when the out-of-zone line is grounded, the differential current flowing through the measurement point is less than 0.

[0121] For bipolar short-circuit faults in DC lines outside the region, Figure 2 The DC grid with bipolar FTF-MMC-DCT shown in the figure has a bipolar short-circuit fault outside the area. Due to its completely symmetrical structure on both sides, it is equivalent to a single-pole R f / 2 The fault characteristics of the P pole are exactly the same as those of the single-pole grounding fault, and the fault characteristics of the N pole are symmetrical with those of the P pole. The fault characteristics of the voltage and current at the DCT outlet after the fault are the same as those of the grounding fault of the out-of-area line. When the out-of-area DC line has a bipolar short circuit fault, the current differential flowing through the measuring point is less than 0.

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

[0123] When an AC fault occurs in the MMCL-P, the MMCL-P cannot be equivalent as a whole because the fault is in the area. During an AC fault, the capacitor discharges relatively slowly and the change in its capacitor voltage is small in a short period of time, so the bridge arm capacitor can be equivalent to a constant voltage source.

[0124] For the AC single-phase grounding fault in the area, the a phase grounding fault is analyzed in this embodiment, and the b and c phases are connected in parallel to be equivalent to R, L, and U, as shown in the attached figure. Figure 8 shown. Figure 8 The b and c phase bridge arm parameters are the same, so the b and c phases can be connected in parallel to be equivalent to R, L, and U, such as Figure 5 shown. Figure 8 In the above equation, R = R arm , L=L arm 、U=U N , I Lbc is the current flowing into the fault point after phase b and phase c are equivalent, I Pa is the upper arm current of phase a, U Pa , U Na are the equivalent power supply voltages of the upper and lower bridge arms of phase a respectively.

[0125] In the few milliseconds before the MMC is blocked, the modulation strategy is considered to have no obvious distortion, and 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 increment of the current 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, get dI Pa / dt<2dI Lbc / dt, when R f =0, a fault occurs at t=0, ignoring the voltage drop on the bridge arm resistor, we get:

[0126]

[0127] Among them, I Pa (0) is the current of the bridge arm of phase a at the moment before the fault occurs, U Pa (0) is the equivalent voltage source voltage of the a-phase bridge arm at the moment after the fault occurs. The above formula is transformed to obtain:

[0128]

[0129] Among them, U Pa (0)≥(1-m)U N, m is the transformer MMC modulation coefficient, and 0<m<1. From the above formula, it can be seen that when a single-phase grounding fault occurs inside the transformer, the fault voltage will drop, but the maximum drop value is greater than (m-1 / 3)U N / dt. Due to the voltage drop at the MMCL-P outlet after the fault, the current dI flowing through the measuring point P P / dt=dI L3 / dt>0.

[0130] For an AC two-phase grounding fault, take the a and b two-phase grounding fault as an example. Similarly, ignoring the AC circuit, the equivalent circuit is as follows: Fig. 9 shown. Fig. 9 Middle I Pa ,I Pb are the upper bridge arm currents of phases a and b respectively, I Lc is the c-phase current, U Px , U Nx is the equivalent power supply voltage of the bridge arm, x=a, b are the two-phase bridge arms a and b respectively, U C is the c-phase equivalent power supply voltage.

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

[0132]

[0133] Transform the above formula to get:

[0134]

[0135] From the above formula, we can know that when there is 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, due to the failure of U P <U N Voltage drop Current dI flowing through the measuring point P P / dt=dI L3 / dt>0.

[0136] The fault characteristics of the MMCL-N side are structurally symmetrical with those of the MMCL-P. The same fault characteristics as those of the MMCL-P can be obtained by inverting the measured values ​​of the MMCL-N.

[0137] Leakage current analysis mainly relies on Kirchhoff's current law (KCL) and Kirchhoff's voltage law (KVL). Unlike a 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 simplifies the analysis process while maintaining the accuracy and practicality of the analysis.

[0138] The equivalent topology of the low-voltage side of the bipolar FTF-MMC-DCT is as follows: Fig.10 shown. Fig.10 Medium Pxy , R Pxy , C Pxy is the equivalent inductance, resistance and capacitance of MMC, Z Pxy is the equivalent impedance of the MMC connection line, x=2, 4 are converter stations 2 and 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 I2 are the currents of MMC4-P and MMC2-P passing through lines 2 and 4, respectively, and f1~f5 are Figure 3 f1 to f5 have the same meaning. f1 represents a bus grounding fault outside the P-pole area on the low voltage side of the DCT, f2 represents a DC grounding fault inside the P-pole area on the low voltage side of the DCT, f3 represents an AC single-phase grounding fault inside the P-pole area on the low voltage side of the DCT, f4 represents an AC two-phase grounding fault inside the P-pole area on the low voltage side of the DCT, and f5 represents a line grounding fault outside the P-pole area on the low voltage side of the DCT. x_xy is the bridge arm current; i X_y is the three-phase AC current of the transformer, subscripts X=P and N are the positive and negative currents, subscripts x=p and n are the upper and lower bridge arms, and subscripts y=a, b, c are the a, b, and c phases respectively; I P_P ,I P_N are the positive upper and lower bridge arm currents, I N_P ,I N_N are the negative upper and lower bridge arm currents and; I d is the grounding point current.

[0139] Since DCT contains active components, traditional transformer differential protection cannot distinguish between internal and external faults. Fig.10 The currents at points 1, 2, and 3 are used for leakage current difference protection, which can accurately distinguish the faults inside and outside the zone and avoid misjudgment of faults caused by noise. The currents flowing through the measurement points 1, 2, and 3 are I P ,I N ,I d , current direction and I d Consistent.

[0140] According to Kirchhoff's current law:

[0141]

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

[0143]

[0144] The alternating current at the N pole is as follows:

[0145]

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

[0147] When an out-of-zone 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 area and the transformer is connected in a triangle, 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 shown as follows:

[0148]

[0149] When an f2 fault occurs in the zone, there is I P =I1+I2,I N_P =-I N_N Since there is no AC fault in the area and the transformer is connected in a triangle, 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 shown as follows:

[0150]

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

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

[0153]

[0154] Among them, I f is the current flowing through the fault point when the f3 or f4 fault occurs. From the leakage current analysis of the above formula, it can be seen that the leakage current of the fault outside the P-pole area is 0, and the leakage current of the ground fault inside the area is greater than 0. The leakage current criterion can clearly distinguish the fault inside and outside the DCT area.

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

[0156] In summary, for out-of-zone faults, the port leakage current is as follows:

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

[0158] Among them, 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 measurement current of the N pole of the leakage current criterion, I d It is the measured current of the indirect point in the leakage current judgment;

[0159] For an in-zone ground fault, the port leakage current is as follows:

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

[0161] Among them, I f It is the fault current when a ground fault occurs in the zone.

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

[0163] According to the above fault analysis, the fault characteristics of the low-voltage side P pole are summarized as shown in Table 1. The characteristics of the N pole fault are opposite to those of the P pole. As shown in Table 1, dI P / dt and I sum Can be used as a criterion for judging whether the area is inside or outside the region, dU P / dt can be used as a criterion for AC and DC positioning. According to the fault characteristics, the DCT protection scheme flow chart based on the port DC differential and leakage current is proposed as follows: Fig.11 As shown. X ,I X Represent the voltage and current of the DCT DC port, respectively. The subscripts X=P and N represent the positive and negative poles of the DCT DC port, respectively. sum =I P +I N +I d Represents leakage current.

[0164] Table 1 Fault judgment characteristics

[0165]

[0166] According to the fault characteristics summarized in Table 1, the first DCT protection scheme based on port DC differential is designed as follows: Fig.11 As shown in (a).

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

[0168] If the P-pole current differential is greater than zero or the N-pole current differential is less than zero, that is, dI P / dt>K set1 or dI N / dt<-K set1 (K set1 When the value is a reference value to avoid false operation), it is determined to be an internal 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 set value), it is determined to be a DC grounding fault in the area;

[0170] Finally, the pole selection is performed. Since the P and N pole faults have little influence on each other, if the It is a P-pole fault, otherwise it is an N-pole fault.

[0171] After determining that it is an internal fault, a protection plan is implemented to achieve fault removal or system isolation through physical devices (such as circuit breakers, relays, etc.) to prevent the fault from expanding or damaging the equipment.

[0172] According to the fault characteristics summarized in Table 1, the second DCT protection scheme based on leakage current is designed as follows Fig.11 As shown in (b).

[0173] The activation criteria are the same as in the first DCT protection scheme;

[0174] The criterion for judging whether the zone is inside or outside is leakage current I sum , if the port leakage current |I sum |>0, it is judged as an internal fault;

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

[0176] The fault 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 determining that it is an internal fault, a protection plan is implemented to achieve fault removal or system isolation through physical devices (such as circuit breakers, relays, etc.) to prevent the fault from expanding or damaging the equipment.

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

[0179] The starting criteria of the first DCT protection scheme and the second DCT protection scheme are both based on the differential value of the DC voltage at the port. The setting value needs to avoid interference and errors and ensure that all faults can be reliably identified. Figure 2 The PSCAD simulation model shown is used for simulation. The maximum value of each simulation data is found out through the current differential under different faults in the simulation area to form a data set. The minimum value in the data set is taken as the setting basis and divided by the first reliability coefficient as the starting criterion setting value of the first DCT protection scheme and the second DCT protection scheme. In this embodiment, the reliability coefficient is 1.2.

[0180] The first DCT protection scheme uses the differential value of the port DC current as the criterion for the zone inside and outside. This embodiment refers to the maximum value scheme for parameter setting, 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 setting basis. This embodiment is set with a transition resistance of 0Ω, and the setting value is calculated as follows:

[0181]

[0182] Among them, K rel_2 is the second reliability coefficient, which is 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, K set1 =(di / dt) set is the set value.

[0183] The second DCT protection scheme uses leakage current as the criterion for in-zone and out-zone judgment. The leakage current protection setting value is calculated as follows:

[0184] I rel =K rel_3 ×K re ×I max ;

[0185] Among them, K rel_3 is the third reliability coefficient, with a value range of 1.05 to 1.1. In this embodiment, it is 1.1. K re is the percentage error of the measuring device, which is taken as 0.1 in this embodiment. max is the maximum overload current of the DC system, I rel is the set value.

[0186] Fault location criterion setting: The first DCT protection scheme and the second DCT protection scheme both use the port DC voltage differential as the fault location criterion. P / dt=-dU N / dt, dU in case of AC fault P / dt=-(m-1 / 5)U N / dt, according to the analysis of AC fault in S3 above, the fault location criterion setting value is: K set2 =(m-1 / 5)U N / dt.

[0187] Fault selection criterion setting: The first DCT protection scheme uses the ratio of the DC current differential of the positive and negative terminals as the fault selection criterion. Due to the characteristics of the bipolar FTF-MMC-DCT structure, the positive and negative faults are not transmitted in a short time. Therefore, the pole selection criterion setting value of the first DCT protection scheme is

[0188] The second DCT protection scheme uses leakage current polarity as the fault selection criterion. Since the leakage current of the positive pole fault is greater than 0, and the leakage current of the 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 transformer low-voltage side fault as an example, under different fault types, the transition resistance is set to 0.01Ω, and the simulation results of the proposed protection scheme within 3ms after the fault are as follows: Figures 12 to 15 As shown. Among them, PG outside the zone and NG outside the zone are respectively the DC bus grounding fault outside the P and N poles; PG inside the zone and NG inside the zone are respectively the DC grounding fault inside the P and N poles; PA-G and NA-G are respectively the AC single-phase grounding fault of the P and N poles; PAB-G and NAB-G are respectively the AC two-phase grounding fault of the P and N poles; I P I P , IN are the DC currents of the P and N pole ports at the low voltage side respectively; U X ,I X are the voltage and current of the DCT DC port, respectively. The subscripts X=P and 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 =1.04kA / ms; K set1 =0.94kA / ms; K set2 =300kV / dt=300(kV / 50us); I N =1.5kA; take twice the margin I max =3kA; K rel Take 1.1 to get I rel =0.33kA.

[0191] For the convenience of observation and comparison, take -dI N / dt, such as Fig.12 As shown in (a) and (b). The differential component of the fault flow in the area (-dI P / dtt, -dI N / dt) minimum value is 1.84kA / ms, which is greater than K set1 , while the fault current differential is negative when the fault occurs outside the zone. Fig.13 It can be seen that the absolute value of the leakage current is 16.9kA at the minimum and 37.1kA at the maximum when the fault occurs inside the zone, while the absolute value of the leakage current is zero and less than 0.3kA when the fault occurs outside the zone. In summary, both the first DCT protection scheme and the second DCT protection scheme can accurately identify faults inside and outside the zone, and the first DCT protection scheme is significantly faster than the second DCT protection scheme.

[0192] according to Fig.14 It can be seen that when R f =0Ω, the maximum value of the differential component of the AC fault voltage in the area is 227kV / 50us, which is less than K set2 , and the DC fault voltage differential in the area is 519kV / 50us, which is greater than K set2 Therefore, the voltage differential can accurately determine the AC and DC faults in the area and perform fault classification.

[0193] according to Fig.15 It can be seen that when R f = 0Ω, taking the P-pole fault as an example, the P-pole current differential after the fault is greater than 1.84kA / ms, while the non-fault N-pole current differential is close to 0. The fault pole can be accurately located by using the ratio of the two-level current differentials. Fig.16 It can be seen that when the P pole fails, the leakage current is greater than 28kA and when the N pole fails, the leakage current is less than -16kA. The leakage current can accurately determine the faulty pole. In summary, the pole selection criteria of the first DCT protection scheme and the second DCT protection scheme can accurately locate the faulty pole.

[0194] After determining that it is an internal fault, a protection plan is implemented to achieve fault removal or system isolation through physical devices (such as circuit breakers, relays, etc.) to prevent the fault from expanding or damaging the equipment.

[0195] S5. The first DCT protection scheme and the second DCT protection scheme are compared in terms of protection speed, resistance to transition resistance and noise resistance, and the comparison results are shown in Table 2. The speed of the first DCT protection scheme is better than that of the second DCT protection scheme, but the resistance to transition resistance and noise resistance of the second DCT protection scheme are better than those of 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. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A face-to-face modular multi-level bipolar DC transformer fast protection method, characterized in that: The following steps are involved: S1. According to the current flow paths under different fault types and the similarity between the DCT equivalent circuit and the MMC equivalent circuit, a bipolar FTF-MMC-DCT fault circuit equivalent model is constructed; S2. Analyze the differences in DCT internal and external fault characteristics of the equivalent circuit model under different faults in multi-level DC, and extract the DCT DC voltage and current differential as the first fault characteristic quantity and the port leakage current as the second fault characteristic quantity; S3, formulating a first DCT protection scheme based on DC voltage and current differential and a second DCT protection scheme based on leakage current according to the first fault characteristic quantity and the second fault characteristic quantity; S4, setting parameters of the first DCT protection scheme and the second DCT protection scheme; S5. Compare the first DCT protection scheme and the second DCT protection scheme in terms of protection speed, transition resistance and noise resistance.

2. The face-to-face modular multi-level bipolar DC transformer fast protection method according to claim 1 is characterized in that: The S1 comprises the following steps: Based on the bipolar FTF-MMC-DCT DC power grid model, the FTF-MMC-DCT internal and external fault equivalent circuit model is obtained; The DC fault side of the bipolar FTF-MMC-DCT is equivalent to resistance, inductance and capacitance, and the AC fault side capacitor of the bipolar FTF-MMC-DCT is equivalent to a constant voltage source.

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

4. The face-to-face modular multi-level bipolar DC transformer fast protection method according to claim 3 is characterized in that: The first fault characteristic quantity is obtained by the following steps: For a ground fault at the DC side outlet of the transformer, ignoring the non-faulty high-voltage side feed, the fault characteristic quantities of the DCT outlet voltage and current after the fault are as follows: Where t is time, U P (0) is the voltage at the initial moment of the P-pole fault, I P is the P-pole fault current, U N is the rated voltage of the transmission line, It indicates the voltage mutation at the measuring point after the P-pole fault. Indicates the sudden change of current at the measuring point after a 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 an out-of-area line grounding fault, the fault characteristic quantities of the DCT output voltage and current after the fault are as follows: Among them, U L It is the rated voltage of the busbar connected to the P pole on the low-voltage side of the DCT. When the line outside the area is grounded, the differential current flowing through the measuring point is less than 0; For the bipolar short-circuit fault of the DC line outside the area, the fault characteristic quantities of the voltage and current at the DCT outlet after the fault are the same as those of the grounding fault of the DC line outside the area. When the DC line outside the area has a bipolar short-circuit fault, the current differential flowing through the measuring point is less than 0; For the AC single-phase grounding fault in the area, the a-phase grounding fault is analyzed, and the fault characteristic quantities of the DCT output voltage and current after the fault are shown as follows: Among them, U Pa (0) is the phase-a voltage during a fault, 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 an AC two-phase grounding fault, the fault characteristic quantities of the DCT output voltage and current after the fault are as follows: When two phases of the transformer are grounded, the fault voltage will drop but the drop value is less than (m-1 / 5)U N / dt, the differential current flowing through the measuring point is less than 0.

5. The face-to-face modular multi-level bipolar DC transformer fast protection method according to claim 4 is characterized in that: The second fault characteristic quantity is obtained by analyzing the out-of-zone fault and the in-zone grounding fault.

6. The face-to-face modular multi-level bipolar DC transformer fast protection method according to claim 5 is characterized in that: The second feature quantity is obtained by the following steps: The fault characteristics of the P-pole on the low-voltage side of the bipolar FTF-MMC-DCT are analyzed to extract the second fault characteristic quantity; For out-of-zone faults, the port leakage current is 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 P pole of the leakage current criterion, I N is the measurement current of the N pole of the leakage current criterion, I d It is the measured current of the indirect point in the leakage current judgment; For an in-zone ground fault, the port leakage current is as follows: I sum =I P +I N +I d =I f >0; Among them, I f It is the fault current when a ground fault occurs in the zone.

7. The face-to-face modular multi-level bipolar DC transformer fast protection method according to claim 6 is characterized in that: The first DCT protection scheme comprises the following steps: Collect voltage and current differentials, and start protection when the port voltage drops; If the P-pole current differential is greater than zero or the N-pole current differential is less than zero, it is determined to be an internal fault; If the AC fault voltage drop is greater than the DC fault voltage drop, it is determined to be a DC grounding fault in the area; like If the fault is P, it is a P-pole fault; otherwise, it is an N-pole fault.

8. The face-to-face modular multi-level bipolar DC transformer fast protection method according to claim 7 is characterized in that: The second DCT protection scheme comprises the following steps: Collect voltage and current differentials, and start protection when the port voltage drops; If the port leakage current |I sum |>0, it is judged as an internal fault; If the AC fault voltage drop is greater than the DC fault voltage drop, it is determined to be a DC grounding fault in the area; If I sum >0, it is a P-pole fault, otherwise it is an N-pole fault.

9. The face-to-face modular multi-level bipolar DC transformer fast protection method according to claim 8 is characterized in that: The parameter setting comprises the following steps: The starting criterion is set by finding the maximum value of each simulation data through simulation to form a data set, taking the minimum value in the data set as the setting basis, and dividing it by the first reliability coefficient as the starting criterion setting value of the first DCT protection scheme and the second DCT protection scheme; The setting value of the in-zone and out-zone identification criterion 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, K set1 =(di / dt) set is the set value; The second DCT protection scheme has the following in-zone and out-of-zone identification criteria: I rel =K rel_3 ×K re ×I max ; Among them, K rel_3 is the third reliability coefficient, K re is the percentage error of the measuring device, 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; Fault selection criterion setting: The fault selection criterion setting value of the first DCT protection scheme is: The fault selection criterion setting value of the second DCT protection scheme is 0.

10. The face-to-face modular multi-level bipolar DC transformer fast protection method according to claim 9 is characterized in that: The comparison result in S5 is that the first DCT protection scheme has a faster performance than the second DCT protection scheme, and the second DCT protection scheme has a better ability to resist transition resistance and noise than the first DCT protection scheme.

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