Isolated dc-dc converter and fault protection method

Abstract

The application provides an isolated DC transformer and a fault protection method. The isolated DC transformer comprises three single-phase converters connected in series on a high-voltage side, an intermediate isolation transformer, and a three-phase converter on a low-voltage side, characterized in that the intermediate isolation transformer comprises a first type winding on the high-voltage side and a second type winding on the low-voltage side, comprising: the DC ports of the three single-phase converters are connected in series to form a high-voltage DC port of the isolated DC transformer, and the AC ports of the three single-phase converters are connected to the three first type windings of the intermediate isolation transformer, respectively; the second type winding is connected in star connection, and the neutral point is grounded through a resistor; or the second type winding is connected in delta connection; the three-phase terminals of the second type winding are connected to the AC ports of the three-phase converter; and the low-voltage DC port of the three-phase converter is connected to a low-voltage DC power grid.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronic applications, in particular to an isolated DC transformer and a fault protection method. BACKGROUND

[0002] In recent years, new energy represented by photovoltaic and wind power has been widely used. Large-scale intermittent energy sources such as photovoltaic and wind power are integrated into traditional alternating current power grids, which will face challenges such as access current balance coordination control complexity, poor power supply stability, and relatively low power conversion efficiency. If the new energy direct current collection and direct current sending mode is adopted, it is beneficial to reduce the energy loss in the process of alternating current / direct current (DC / AC or AC / DC) conversion, avoid the frequency stability and complex harmonic problems of alternating current system, realize the stable and efficient long-distance transmission of new energy, and promote the large-scale development of large-scale base photovoltaic and offshore wind power cluster sending.

[0003] The high-voltage direct current transformer is a key device for interconnecting between direct current grid lines of different voltage levels, and is an important part of building future multi-voltage level and multi-terminal direct current grid. The operation mode of the high-voltage direct current transformer in the multi-voltage level direct current grid is often working in the voltage source operation mode to undertake the task of stabilizing the voltage of the low-voltage direct current grid. The low-voltage direct current grid is composed of a low-voltage direct current bus and multiple branch feeders. In actual operation, the single-pole grounding fault of the low-voltage direct current grid caused by lightning strike on the direct current overhead line and mechanical damage to the direct current cable due to external force is one of the most common fault types of the direct current system. Especially when a single branch feeder fails, the system should quickly locate the fault and cut off the faulty feeder through protection action to ensure the uninterrupted operation of the non-fault feeder interval, thereby improving the power supply reliability.

[0004] The fault current characteristics relied on for fault location when the system single-pole grounding fault occurs are closely related to the wiring group and grounding mode of the direct current transformer. In addition, the wiring group and grounding mode of the isolated direct current transformer should also adapt to the device converter topology form, and be able to provide a path for zero-sequence current and station power supply under steady-state operating conditions.

[0005] The above information disclosed in the background section is only used to strengthen the understanding of the background of the present application, and therefore it can include information that does not constitute prior art known to those of ordinary skill in the art. SUMMARY

[0006] To solve the above problems, the present application provides an isolated DC transformer and a fault protection method.

[0007] According to a first aspect of the present application, an isolated DC transformer is provided, which comprises three single-phase converters connected in series on a high-voltage side, an intermediate isolation transformer, and a three-phase converter on a low-voltage side, the intermediate isolation transformer comprising a first type of winding on the high-voltage side and a second type of winding on the low-voltage side, comprising:

[0008] The DC ports of the three single-phase converters are connected in series to form a high-voltage DC port of the isolated DC transformer, and the AC ports of the three single-phase converters are respectively connected to the three first type of windings of the intermediate isolation transformer.

[0009] The second type of winding is star-connected, and the neutral point is grounded through a resistor; or is delta-connected.

[0010] The three-phase terminals of the second type of winding are connected to the AC ports of the three-phase converter.

[0011] The low-voltage DC port of the three-phase converter is connected to a low-voltage DC power grid.

[0012] According to some embodiments, the intermediate isolation transformer further comprises a third type of winding:

[0013] The third type of winding is delta-connected, and is used to provide a zero-sequence current path for the three single-phase converters and the three-phase converter, and the three-phase terminals of the third type of winding are led out as a station transformer winding to supply power to a load.

[0014] According to some embodiments, the three first type of windings are discrete windings without an electrical connection point with each other.

[0015] According to some embodiments, the high-voltage side of the isolated DC transformer is pseudo-bipolar connected, and the total voltage of the high-voltage DC port is U dch , the high-voltage DC port comprises a positive DC pole line and a negative DC pole line, the voltage of the positive DC pole line is 1 / 2U dch , and the voltage of the negative DC pole line is -1 / 2U dch ,

[0016] In the first type of winding, the winding close to the positive DC pole line bears a positive DC bias voltage of 1 / 3U dch to ground, the middle winding bears a zero DC bias voltage to ground, and the winding close to the negative DC pole line bears a negative DC bias voltage of -1 / 3U dch to ground.

[0017] According to some embodiments, the high-voltage side of the isolated DC transformer is true bipolar connected, and the total voltage of the high-voltage DC port is U dch, the high voltage direct current port includes a ground pole line and a direct current pole line, the voltage of the ground pole line is 0V, and the voltage of the direct current pole line is U dch ,

[0018] In the first type of winding, the winding close to the ground pole line bears 1 / 6U dch DC bias voltage to ground, the middle winding bears 1 / 2U dch DC bias voltage to ground, and the winding close to the direct current pole line bears 5 / 6U dch DC bias voltage to ground.

[0019] According to some embodiments, the intermediate isolation transformer is a three-phase integrated transformer, and the three-phase windings of the first type of winding, the second type of winding and the third type of winding share a core magnetic circuit.

[0020] According to some embodiments, the intermediate isolation transformer is a three-phase separated transformer, and each phase winding of the first type of winding, the second type of winding and the third type of winding is configured with a separate core, and the three-phase windings do not share a core magnetic circuit.

[0021] According to some embodiments, the resistor includes a high resistance, a low resistance or a combination of switchable resistors, and the grounding loop further includes a current Hall detection device and a lightning arrester overvoltage protection device.

[0022] According to some embodiments, the single-phase converter and the three-phase converter include a full-bridge type converter, a half-bridge type converter, a three-level type converter, a modular multi-level type converter and / or a combination type converter.

[0023] The single-phase converter and the three-phase converter include internal capacitors and converter reactor elements.

[0024] According to some embodiments, the operating frequency of the intermediate isolation transformer includes low frequency, power frequency, medium frequency and / or high frequency.

[0025] According to some embodiments, the voltage between the terminals of each first type of winding is determined by the output voltage of the alternating current port of the single-phase converter connected thereto, wherein:

[0026]

[0027] M is the AC / DC modulation ratio, U dc is the direct current port voltage of the single-phase converter, U ac is the output voltage of the alternating current port of the single-phase converter.

[0028] The AC / DC modulation ratio is determined by the number of modules configured internally in the single-phase converter.

[0029] According to a second aspect of the present application, a fault protection method for the isolated DC transformer of any one of the first aspect is provided, comprising:

[0030] determining whether a single-pole earth fault occurs in the low-voltage DC power grid;

[0031] in response to the single-pole earth fault of the low-voltage DC power grid, the second type of winding injects a short-circuit current to the earth fault point;

[0032] performing fault location according to the short-circuit current, and determining whether a positive-negative pole differential current at the earth fault point reaches a line protection action threshold;

[0033] in a case where the positive-negative pole differential current does not reach the line protection action threshold, a first resistor in a resistor combination is put into operation;

[0034] in a case where the positive-negative pole differential current at the earth fault point reaches the line protection action threshold, a faulty feeder is cut off;

[0035] in a case where the positive-negative pole differential current does not reach the line protection action threshold after the first resistor is put into operation, a second resistor in the resistor combination is put into operation;

[0036] in a case where the positive-negative pole differential current still does not reach the line protection action threshold after the resistor combination with the minimum resistance value is put into operation, an isolated DC transformer zero-sequence current protection is implemented.

[0037] The present application provides an isolated DC transformer and a fault protection method. Three first type discrete windings on a high-voltage side of the intermediate isolated transformer are connected with three single-phase converters in series on the high-voltage side, which adapts to the series topology of the three single-phase converters on the high-voltage side. Three-phase second type windings on a low-voltage side are connected with three-phase converters on the low-voltage side. The three-phase second type windings on the low-voltage side of the isolated transformer are connected in star, and the neutral point is grounded through a resistor. This grounding mode can inject a certain short-circuit current to the fault point when a single-pole earth fault occurs in the low-voltage DC power grid, and realize fault point location. Meanwhile, a third winding is configured to be connected in delta, which not only provides a zero-sequence current path for steady-state operation of the DC transformer, but also provides power supply for station power supply.

[0038] It should be understood that the foregoing general description and the following detailed description are only examples, and cannot limit the present application. BRIEF DESCRIPTION OF DRAWINGS

[0039] The above and other objects, features and advantages of the present application will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which example embodiments of the present application are shown. The following drawings described below are only some embodiments of the present application and are not a limitation of the present application.

[0040] Figure 1 A schematic diagram of a three-phase integrated isolated DC transformer of an example embodiment is shown;

[0041] Figure 2 A schematic diagram of a three-phase integrated isolated DC transformer of an example embodiment is shown;

[0042] Figure 3 A schematic diagram of a three-phase integrated isolated DC transformer of an example embodiment is shown;

[0043] Figure 4 A schematic diagram of a high voltage side modular single-phase full-bridge converter of an example embodiment is shown;

[0044] Figure 5 A schematic diagram of a low voltage side modular three-phase half-bridge converter of an example embodiment is shown;

[0045] Figure 6 A low voltage DC grid branch feeder single-pole ground fault short circuit current loop diagram of an example embodiment is shown;

[0046] Figure 7 A fault protection method flow diagram of an isolated DC transformer of an example embodiment is shown;

[0047] Figure 8 A fault protection method flow diagram of an isolated DC transformer of an example embodiment is shown. DETAILED DESCRIPTION

[0048] Example embodiments now will be described more fully hereinafter with reference to the accompanying drawings. Example embodiments, however, can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the example embodiments to those skilled in the art. Like reference numerals refer to like elements throughout the several views.

[0049] The described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of embodiments of the disclosure. One skilled in the relevant art will recognize, however, that the techniques described can be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In these instances, well-known structures, methods, devices, implementations, materials, and so forth can not be described in detail.

[0050] The flowcharts shown in the drawings are only illustrative, and are not necessarily required to include all the contents and operations / steps, nor are they necessarily executed in the order described. For example, some operations / steps can be further divided, and some operations / steps can be combined or partially combined, so the actual execution order can be changed according to actual conditions.

[0051] The terms "first", "second", and the like in the specification and claims of the present application and the above drawings are used to distinguish different objects, and are not used to describe a specific order. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but can optionally further include steps or units not listed, or can optionally further include other steps or units inherent to the process, method, product, or device.

[0052] Those skilled in the art can understand that the drawings are only schematic diagrams of example embodiments, and the modules or flows in the drawings are not necessarily required to implement the present application, and therefore cannot be used to limit the protection scope of the present application.

[0053] Figure 1 A schematic diagram of a three-phase integrated isolation type DC transformer is shown.

[0054] The wiring group and grounding mode of the isolation type DC transformer are shown in Figure 1 The isolation type DC transformer includes three series-connected single-phase converters 1a, 1b, 1c on the high-voltage side, an intermediate isolation transformer 2, and a three-phase converter 3 on the low-voltage side. The intermediate isolation transformer 2 includes at least three types of windings 21, 22, 23. The DC ports of the three single-phase converters 1a, 1b, 1c on the high-voltage side are connected in series to form the high-voltage DC ports DCH+ and DCH- of the isolation type DC transformer. The three single-phase converters are connected in series at points M and N. The AC ports AX, BY, CZ of the three single-phase converters are connected to the three first type windings 21 on the high-voltage side of the intermediate isolation transformer 2. The three-phase second type windings 22 on the low-voltage side of the intermediate isolation transformer 2 are connected in star configuration, with the neutral point grounded through a resistor R. The three-phase terminals a, b, c of the second type windings 22 are connected to the AC ports of the three-phase converter 3 on the low-voltage side. The low-voltage DC ports DCL+ and DCL- of the three-phase converter 3 on the low-voltage side are connected to the low-voltage DC grid.

[0055] According to an example embodiment, the intermediate isolation transformer 2 further includes a third type winding 23. The third type winding 23 is connected in delta configuration and provides a zero-sequence current path for the three single-phase converters and the three-phase converter. The three-phase terminals u, v, w of the third type winding 23 are connected to the load as a station transformer winding.

[0056] According to some embodiments, the three first type windings 21 on the high voltage side of the intermediate isolation transformer 2 are discrete windings without electrical connection points to each other, and the three discrete windings bear different DC bias voltages to ground. The high voltage side of the isolation type DC transformer adopts pseudo bipolar connection, and the total voltage between the high voltage DC ports DCH+ and DCH- is U dch , and the winding close to the positive DC pole line bears 1 / 3U dch , the positive DC bias voltage to ground, the intermediate winding bears zero DC bias voltage to ground, and the winding close to the negative DC pole line bears -1 / 3U dch , the negative DC bias voltage to ground.

[0057] According to some embodiments, taking the positive and negative DC pole line voltages as +200kV and -200kV respectively as an example, the total voltage between the high voltage DC ports DCH+ and DCH- is 400kV, then the winding AX close to the positive DC pole line bears +133.3kV positive DC bias voltage to ground, the intermediate winding BY bears zero DC bias voltage to ground, and the winding CZ close to the negative DC pole line bears -133.3kV negative DC bias voltage to ground. The present application takes 400kV as an example, but the present application is not limited thereto.

[0058] According to some embodiments, the three first type windings 21 on the high voltage side of the intermediate isolation transformer 2 are discrete windings without electrical connection points to each other, and the three discrete windings bear different DC bias voltages to ground. The high voltage side of the isolation type DC transformer adopts pseudo bipolar connection, and the total voltage between the high voltage DC ports DCH+ and DCH- is U dch , and the winding close to the positive DC pole line bears 1 / 3U dch , the positive DC bias voltage to ground, the intermediate winding bears zero DC bias voltage to ground, and the winding close to the negative DC pole line bears -1 / 3U dch , the negative DC bias voltage to ground. dch , the negative DC bias voltage to ground. dch , the negative DC bias voltage to ground. dch , the negative DC bias voltage to ground. dch , the negative DC bias voltage to ground.

[0059] According to some embodiments, taking the positive and negative DC pole line voltages as +200kV and -200kV respectively as an example, the total voltage between the high voltage DC ports DCH+ and DCH- is 400kV, then the winding AX close to the positive DC pole line bears +133.3kV positive DC bias voltage to ground, the intermediate winding BY bears zero DC bias voltage to ground, and the winding CZ close to the negative DC pole line bears -133.3kV negative DC bias voltage to ground. The present application takes 400kV as an example, but the present application is not limited thereto.

[0060] According to some embodiments, the grounding resistance R can be high resistance, low resistance or a combination of switchable resistances, the grounding loop is configured with a current Hall detection device for detecting the neutral point-to-ground zero sequence current, and an overvoltage protection device such as an arrester is selected and configured.

[0061] According to some embodiments, the intermediate isolation transformer has a working frequency of a common typical low frequency of 20 Hz, a power frequency of 50 Hz, a medium frequency of 100-400 Hz or a high frequency of 1 kHz.

[0062] According to some embodiments, the three first type windings 21 of the high voltage side of the intermediate isolation transformer 2 have an end-to-end voltage of each winding output by the single-phase converter AC port voltage U ac Determination: AC / DC modulation ratio U dc is the single-phase converter DC port voltage.

[0063] According to some embodiments, the DC port voltage U dc Under certain conditions, by reducing the number of modules configured inside the single-phase converter, reducing the AC / DC modulation ratio M, and further reducing the end-to-end voltage U ac of the winding, the winding current is improved.

[0064] According to some embodiments, the intermediate isolation transformer 2 is a three-phase integrated transformer in physical structure, and the three types of three-phase windings are wound on the same core and share the core magnetic circuit, as shown in Figure 1 .

[0065] According to some embodiments, the three series-connected single-phase converters 1a, 1b and 1c on the high voltage side of the isolation type DC transformer and the three-phase converter assembly 3 on the low voltage side are full-bridge converters, half-bridge converters, three-level converters, modular multi-level converters and combination converters composed of the above topologies, which contain capacitors and converter reactors inside, Figure 4 is a typical high voltage side modular single-phase full-bridge converter schematic diagram, Figure 5 is a typical low voltage side modular three-phase half-bridge converter schematic diagram.

[0066] According to the example embodiments, the application proposes an isolated DC transformer and a fault protection method. Three first-type discrete windings on the high-voltage side of the intermediate isolation transformer are connected with the AC ports of three series-connected single-phase converters on the high-voltage side, which adapts to the series topology of the three single-phase converters on the high-voltage side; three-phase second-type windings on the low-voltage side are connected with the AC ports of three-phase converters on the low-voltage side, the three-phase second-type windings on the low-voltage side of the isolation transformer are star-connected, and the neutral point is grounded through a resistor. This grounding mode can inject a certain short-circuit current to the fault point when a single-pole grounding fault occurs in the low-voltage DC power grid, so as to realize fault point positioning; meanwhile, the third winding is configured to be delta-connected, which not only provides a zero-sequence current path for the steady-state operation of the DC transformer, but also provides power supply for station power supply.

[0067] Figure 2 Another embodiment of an example three-phase integrated isolated DC transformer schematic diagram is shown.

[0068] As shown in Figure 2 , the circuit shown in Figure 2 is basically the same as the circuit shown in Figure 1 , the difference is that the second-type winding 22 of the intermediate isolation transformer 2 is delta-connected.

[0069] Figure 3 Another embodiment of an example isolated DC transformer schematic diagram is shown.

[0070] As shown in Figure 3 , the circuit shown in Figure 3 is basically the same as the circuit shown in Figure 1 , the difference is that the intermediate isolation transformer 2 is a three-phase separated transformer in physical structure, each winding is configured with a separate core, and the three-phase windings do not share the core magnetic circuit: i.e. the three-type windings AX, a, and u of phase A are wound on one core; the three-type windings BY, b, and v of phase B are wound on one core; and the three-type windings CZ, c, and w of phase C are wound on one core.

[0071] According to some embodiments, the second-type winding 22 on the low-voltage side of the intermediate isolation transformer 2 is star-connected YN, and the neutral point is grounded through a resistor R.

[0072] Figure 7 A flowchart of a fault protection method of an example isolated DC transformer is shown.

[0073] S701, determine whether a single-pole grounding fault occurs in the low-voltage DC power grid.

[0074] According to the example embodiments, if a single-pole grounding fault occurs in the low-voltage DC power grid connected with the low-voltage DC port of the isolated DC transformer, go to S702.

[0075] S702, the second type winding injects a short circuit current to the grounding fault point.

[0076] According to the example embodiment, the isolation type DC transformer continues to keep the unlocked operation, and a certain short circuit current is injected to the grounding fault point through a loop formed by the grounding point of the neutral point of the second type winding of the intermediate isolation transformer low voltage side of the isolation type DC transformer and the branch feeder 5 grounding fault point, and the short circuit current loop is as shown in the figure Figure 6 In the present application, the branch feeder 5 grounding fault is taken as an example, but the present application is not limited thereto.

[0077] S703, it is judged whether the positive and negative electrode difference flow reaches the line protection action threshold.

[0078] According to the example embodiment, the line protection configured by the branch feeder interval performs fault positioning according to the short circuit current, and it is judged whether the positive and negative electrode difference flow of the grounding fault point reaches the line protection action threshold.

[0079] S704, a small resistance is put in.

[0080] According to the example embodiment, when the positive and negative electrode difference flow cannot reach the line protection action threshold, the first resistance in the grounding resistance combination is put in; by reducing the loop resistance, the positive and negative electrode fault difference flow value reaches the line protection action threshold.

[0081] According to the example embodiment, after the first resistance is put in, it is judged again whether the positive and negative electrode difference flow reaches the line protection action threshold: if the line protection action threshold is not reached, the second resistance is continued to be put in; if the line protection action threshold is reached, it is turned to S705.

[0082] According to some embodiments, the present application reduces the resistance value of the resistance combination by increasing the parallel resistance, but the present application is not limited thereto.

[0083] The small resistance put in also includes the protection method after the smallest resistance value resistance is put in, and the flow chart is as shown in the figure Figure 8 .

[0084] S801, it is judged whether the smallest resistance value resistance combination is put in.

[0085] According to the example embodiment, if the smallest resistance value resistance combination is put in, it is turned to S802.

[0086] S802, it is judged whether the positive and negative electrode difference flow reaches the line protection action threshold.

[0087] According to the example embodiment, when the smallest resistance value resistance combination is put in, if the branch feeder interval line protection fault difference flow value reaches the protection action threshold, it is turned to S705; if the protection action threshold is still not reached, it is turned to S803.

[0088] S803, the isolated DC transformer zero sequence current protection.

[0089] According to the example embodiment, when the minimum resistance combination is put in, the branch feeder interval line protection fault differential current value still cannot reach the protection action threshold, and it is determined that it is a DC bus single-pole grounding fault. The isolated DC transformer zero sequence current protection is used as a backup protection action, and the low-voltage DC power grid whole section fault bus is cut off.

[0090] S705, cutting off the fault feeder.

[0091] According to the example embodiment, by reducing the loop resistance, the positive and negative pole fault differential current value reaches the line protection action threshold, and then the fault feeder is cut off by the primary breaking device configured in the feeder interval, and the low-voltage DC power grid healthy part continues to run.

[0092] It should be clearly understood that the present application describes how to form and use specific examples, but the present application is not limited to any details of these examples. Instead, based on the teachings of the disclosure of the present application, these principles can be applied to many other embodiments.

[0093] In addition, it should be noted that the above-described figures are only schematic illustrations of the processes included in the method according to the example embodiments of the present application, and are not for limiting purposes. It is easy to understand that the processes shown in the above-described figures do not indicate or limit the time sequence of these processes. In addition, it is also easy to understand that these processes can be executed synchronously or asynchronously, for example, in multiple modules.

[0094] The example embodiments of the present application are specifically shown and described above. It should be understood that the present application is not limited to the detailed structure, arrangement or implementation method described herein; on the contrary, the present application is intended to cover various modifications and equivalent arrangements included in the spirit and scope of the appended claims.

Claims

1. An isolated DC voltage transformer comprising three single-phase converters connected in series on the high-voltage side, an intermediate isolation transformer and three-phase converters on the low-voltage side, characterized in that, The intermediate isolation transformer comprises a first type winding at a high voltage side and a second type winding at a low voltage side, comprising: The DC ports of the three single-phase converters are connected in series to form the high voltage DC port of the isolation type DC transformer, the AC ports of the three single-phase converters are connected to the three first type windings of the intermediate isolation transformer respectively, and the three first type windings are discrete windings without electrical connection points with each other; The second type winding adopts star connection, and the neutral point is grounded through a resistor; or adopts delta connection; The three-phase terminals of the second type winding are connected to the AC ports of the three-phase converter; The low voltage DC port of the three-phase converter is connected to a low voltage DC power grid; The high-voltage side of the isolation type direct current transformer adopts pseudo-bipolar wiring, the total voltage of the high-voltage direct current port is U dch , the high-voltage direct current port includes a positive direct current pole line and a negative direct current pole line, the voltage of the positive direct current pole line is 1 / 2U dch , the voltage of the negative direct current pole line is -1 / 2U dch , in the first type winding, the winding close to the positive direct current pole line bears 1 / 3U dch positive direct current bias voltage to ground, the middle winding bears zero direct current bias voltage to ground, and the winding close to the negative direct current pole line bears -1 / 3U dch negative direct current bias voltage to ground; or The high-voltage side of the isolation type direct current transformer adopts true bipolar wiring, and the total voltage of the high-voltage direct current port is U dch The high-voltage direct current port includes a ground pole line and a direct current pole line, the voltage of the ground pole line is 0V, and the voltage of the direct current pole line is U dch In the first type of winding, the winding close to the ground pole line bears 1 / 6U dch The direct current bias voltage to ground, the middle winding bears 1 / 2U dch The direct current bias voltage to ground, and the winding close to the direct current pole line bears 5 / 6U dch The direct current bias voltage to ground.

2. The isolating DC voltage transformer according to claim 1, wherein The intermediate isolation transformer further comprises a third type winding: The third type winding adopts delta connection, and is used to provide a zero sequence current path for the three single-phase converters and the three-phase converter, and the three-phase terminals of the third type winding are led out as a station transformer winding to supply power to a load.

3. The isolating DC voltage transformer according to claim 2, wherein The intermediate isolation transformer is a three-phase integrated transformer, and the three-phase windings of the first type winding, the second type winding and the third type winding share a core magnetic circuit.

4. The isolating DC voltage transformer according to claim 2, wherein The intermediate isolation transformer is a three-phase separated transformer, and each phase winding of the first type winding, the second type winding and the third type winding is configured with a separate core, and the three-phase windings do not share a core magnetic circuit.

5. The isolating DC -DC converter according to claim 1, wherein The resistor comprises a high resistance, a low resistance or a combination of switchable resistors, and the grounding loop further comprises a current Hall detection device and a lightning arrester overvoltage protection device.

6. The isolating DC -DC converter according to claim 1, wherein The single-phase converter and the three-phase converter comprise a full-bridge type converter, a half-bridge type converter, a three-level type converter, a modular multi-level type converter and / or a combination type converter. The single-phase converter and the three-phase converter comprise internal capacitor and converter reactor elements.

7. The isolating DC -DC converter according to claim 1, wherein The working frequency of the intermediate isolation transformer comprises low frequency, power frequency, medium frequency and / or high frequency.

8. The isolating DC -DC converter according to claim 1, wherein The terminal voltage of each first type winding is determined by the output voltage of the AC port of the single-phase converter connected thereto, wherein: M= M is the AC / DC modulation ratio, U dc U is the DC port voltage of the single-phase converter ac U is the AC port output voltage of the single-phase converter The AC / DC modulation ratio is determined by the number of modules configured inside the single-phase converter.

9. A fault protection method for an isolating DC voltage transformer as claimed in any one of the claims 1 - 8, characterized in that, Comprising: Judging whether a single-pole grounding fault occurs in the low voltage DC power grid; In response to the single-pole grounding fault of the low voltage DC power grid, the second type winding injects a short-circuit current to the grounding fault point; According to the short-circuit current, judging whether a difference current between the positive and negative poles at which the grounding fault point is located reaches a line protection action threshold; In the case that the difference current between the positive and negative poles at which the grounding fault point is located does not reach the line protection action threshold, a first resistor in the resistor combination is put into operation; In the case that the difference current between the positive and negative poles at which the grounding fault point is located reaches the line protection action threshold, a faulty feeder is cut off; In the case that the difference current between the positive and negative poles does not reach the line protection action threshold after the first resistor is put into operation, a second resistor in the resistor combination is put into operation; In the case that the difference current between the positive and negative poles still does not reach the line protection action threshold after the resistor combination with the smallest resistance value is put into operation, isolation type DC transformer zero sequence current protection is implemented.

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