Method and device for isolating ground fault on valve side of bipolar direct current system

Abstract

The application provides a bipolar direct current system valve side ground fault isolation method and device, and relates to the technical field of high-voltage direct current transmission fault processing. The bipolar direct current system comprises a first direct current pole, a second direct current pole, a grounding unit and two groups of alternating current circuit breakers. The method comprises the following steps: when a ground fault occurs at the valve side of the bipolar direct current system converter transformer, differential protection is started for the valve side area until the alternating current system is isolated from the fault point; the fault pole converter valve is blocked, the first alternating current circuit breaker of the fault pole is controlled to be disconnected, and after a delay preset time length, the second alternating current circuit breaker of the fault pole is controlled to be disconnected, so as to isolate the alternating current system from the fault point; when the direct current is less than a preset direct current threshold, the first switch and the second switch are controlled to be disconnected, so as to complete the isolation of the direct current system from the fault point. The application can delay the tripping of the two alternating current circuit breakers, does not need to judge the fault phase and whether the alternating current phase current appears a zero point, and can quickly act to complete the isolation of the valve side ground fault.

Description

Technical Field

[0001] This invention relates to the field of high-voltage direct current transmission fault handling technology, and in particular to a method and device for isolating valve-side grounding faults in a bipolar direct current system. Background Technology

[0002] This section is intended to provide background or context for the embodiments of the invention set forth in the claims. The description herein is not an admission that it is prior art simply because it is included in this section.

[0003] DC systems employing symmetrical bipolar connections are widely used in high-voltage, high-capacity DC transmission systems due to their flexible operation and ability to reduce rated operating voltage. In recent years, flexible DC systems based on modular multilevel converters (MMCs) have been widely used in large-scale centralized integration of new energy sources due to their advantages such as flexible power control, low harmonic content in output voltage and current, and high degree of modularity. Meanwhile, uncontrolled rectifiers based on diode converters show promise for large-scale offshore wind power transmission systems due to their low cost, small footprint, and low operating losses.

[0004] When an asymmetrical ground fault (single-phase or two-phase ground fault) occurs on the valve side of the DC pole, which consists of one or more MMC converters and an uncontrolled rectifier bridge connected in series, the unidirectional conduction of the diodes in the converter will cause the AC current to be DC biased. In severe cases, there is no current zero-crossing point in the AC current, which makes it difficult to isolate and clear such faults. Summary of the Invention

[0005] This invention provides a method and apparatus for isolating valve-side grounding faults in a bipolar DC system, which can quickly and reliably isolate grounding faults in the valve-side area of ​​a converter transformer.

[0006] In a first aspect, embodiments of the present invention provide a method for isolating a valve-side grounding fault in a bipolar DC system. The bipolar DC system includes a first DC pole, a second DC pole, a grounding unit, and two sets of AC circuit breakers. Each set of AC circuit breakers includes a first AC circuit breaker and a second AC circuit breaker. One end of the first DC pole is electrically connected to the positive terminal via a first switch. One end of the second DC pole is electrically connected to the negative terminal via a second switch. The other ends of the first DC pole and the second DC pole are respectively electrically connected to the AC system via one or more sets of AC circuit breakers. The first DC pole and the second DC pole are grounded through the grounding unit. The method includes:

[0007] When a ground fault is detected on the valve side of the converter transformer of the bipolar DC system, differential protection is activated in the valve side area until the AC system is isolated from the fault point.

[0008] The faulty pole converter valve is locked, the first AC circuit breaker of the faulty pole is opened, and after a preset delay, the second AC circuit breaker of the faulty pole is opened to achieve isolation between the AC system and the fault point.

[0009] When the DC current is less than the preset DC threshold, the first switch and the second switch are disconnected to isolate the DC system from the fault point.

[0010] Secondly, embodiments of the present invention also provide a valve-side grounding fault isolation device for a bipolar DC system. The bipolar DC system includes a first DC pole, a second DC pole, a grounding unit, and two sets of AC circuit breakers. Each set of AC circuit breakers includes a first AC circuit breaker and a second AC circuit breaker. One end of the first DC pole is electrically connected to the positive pole via a first switch. One end of the second DC pole is electrically connected to the negative pole via a second switch. The other ends of the first DC pole and the second DC pole are respectively electrically connected to the AC system via one or more sets of AC circuit breakers. The first DC pole and the second DC pole are grounded through the grounding unit. The device includes:

[0011] The protection module is used to activate differential protection in the valve side area when a ground fault is detected on the valve side of the converter transformer of the bipolar DC system, until the AC system is isolated from the fault point;

[0012] The AC control module is used to lock the faulty pole converter valve, control the first AC circuit breaker of the faulty pole to open, and after a preset delay, control the second AC circuit breaker of the faulty pole to open, so as to isolate the AC system from the fault point.

[0013] The DC control module is used to control the first switch and the second switch to disconnect when the DC current is less than a preset DC threshold, so as to isolate the DC system from the fault point.

[0014] Thirdly, embodiments of the present invention also provide an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the above-described method for isolating valve-side grounding faults in a bipolar DC system.

[0015] Fourthly, embodiments of the present invention also provide a computer-readable storage medium storing a computer program for performing the above-described method for isolating valve-side grounding faults in a bipolar DC system.

[0016] The embodiments of the present invention bring the following beneficial effects: The embodiments of the present invention provide a method and device for isolating a valve-side grounding fault in a bipolar DC system. The bipolar DC system includes a first DC pole, a second DC pole, a grounding unit, and two sets of AC circuit breakers. Each set of AC circuit breakers includes a first AC circuit breaker and a second AC circuit breaker. One end of the first DC pole is electrically connected to the positive pole through a first switch. One end of the second DC pole is electrically connected to the negative pole through a second switch. The other ends of the first DC pole and the second DC pole are respectively electrically connected to the AC system through one or more sets of AC circuit breakers. The first DC pole and the second DC pole are grounded through the grounding unit. The method includes: when a grounding fault is detected on the valve side of the converter transformer in the bipolar DC system, differential protection is activated in the valve-side area until the AC system is isolated from the fault point; the converter valve of the fault pole is locked, the first AC circuit breaker of the fault pole is controlled to open, and after a preset delay, the second AC circuit breaker of the fault pole is controlled to open to achieve isolation between the AC system and the fault point; when the DC current is less than a preset DC threshold, the first switch and the second switch are controlled to open to complete the isolation between the DC system and the fault point. The embodiments of the present invention do not require determining whether the faulty phase and the AC phase current have reached zero crossing, and can quickly complete the isolation of the valve side grounding fault.

[0017] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention are realized and obtained in accordance with the structures particularly pointed out in the description, claims and drawings.

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0019] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 A flowchart of a method for isolating a valve-side grounding fault in a bipolar DC system provided in an embodiment of the present invention;

[0021] Figure 2 A structural block diagram of a bipolar DC system valve-side grounding fault isolation device provided in an embodiment of the present invention;

[0022] Figure 3 This is a schematic diagram of the composition of a bipolar DC system provided in an embodiment of the present invention;

[0023] Figure 4 A schematic diagram of an MMC converter unit composed of power modules based on a half-bridge topology, provided for an embodiment of the present invention;

[0024] Figure 5 A schematic diagram of a converter unit composed of a diode rectifier bridge provided in an embodiment of the present invention;

[0025] Figure 6 This is a schematic diagram of the current loop during a single-phase fault provided in an embodiment of the present invention;

[0026] Figure 7 A schematic diagram of the system composition structure of an electronic device provided in an embodiment of the present invention;

[0027] Figure 8 This is a schematic diagram of the equivalent circuit of a non-faulty phase provided in an embodiment of the present invention;

[0028] Figure 9 This is a timing diagram for implementing the fault isolation switch control according to an embodiment of the present invention;

[0029] Figure 10 When the converter transformer provided in the embodiment of the present invention adopts YY connection, a single-phase ground fault occurs on its valve side, and the AC fault current and circuit breaker trip signal waveforms on the grid side of the converter transformer are shown.

[0030] Figure 11 A schematic diagram of the fault current waveform of one phase on the AC side when the converter transformer adopts YY connection, provided for an embodiment of the present invention;

[0031] Figure 12 This invention provides a waveform of the fault current of the second AC circuit breaker Q2 interrupting the other two phases of the AC side, as provided in an embodiment of the invention.

[0032] Figure 13 The following is a waveform diagram of the AC fault current interruption process when a single-phase ground fault occurs on the valve side of the converter transformer provided in the embodiment of the present invention using YY connection. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] In the event of a ground fault in the valve-side region of the converter transformer in a bipolar DC system composed of MMC-HVDC or diode rectifier bridge converters, the converter must be shut down immediately, and the fault point must be isolated from the AC system. Due to the unidirectional conduction characteristics of the diodes and diode rectifiers under the MMC half-bridge submodule, the AC side current exhibits a DC bias. In the event of a single-phase ground fault on the converter transformer valve side, the AC side current may not cross zero, making it difficult to isolate the converter unit from the AC system. If the protection system misjudges or misinterprets the situation, it will lead to equipment damage.

[0035] Currently, among the solutions for isolating valve-side grounding faults in bipolar DC systems, the first existing technology relies on detecting the zero-crossing point of the AC phase current and using AC circuit breakers with phase-by-phase tripping capabilities. Detecting the zero-crossing point of the AC phase current places high demands on the measurement accuracy of the AC current measuring device under normal current conditions while simultaneously requiring continuous operation at low currents (near zero). Furthermore, the protection system requires a considerable processing time to determine the faulty phase and the current zero-crossing point, posing a risk of misjudgment and erroneous assessment, thus threatening equipment safety. The second existing technology involves adding a branch circuit breaker auxiliary unit in parallel on the valve side of the DC system. This unit includes impedance element series switches. When the AC circuit breaker current fails to cross zero, closing the branch switch in the auxiliary unit ensures the circuit breaker current crosses zero, thus achieving valve-side fault isolation by artificially creating a symmetrical grounding fault. However, this can expand the fault range and cause unnecessary impact on equipment in the DC system.

[0036] Based on this, the present invention provides a method and apparatus for isolating valve-side grounding faults in a bipolar DC system. This method can effectively isolate the fault point when a grounding short-circuit fault occurs on the valve side of the converter transformer in a bipolar flexible DC system based on a modular multilevel converter (MMC) or an uncontrolled rectifier bridge converter. When a grounding short-circuit fault occurs between the DC system converter transformer and the MMC converter, the faulty pole converter valve is locked after detecting the grounding fault in that area, and then the fault point is isolated by delaying the opening of two AC circuit breakers with long arc-burning capabilities.

[0037] To facilitate understanding of this embodiment, a detailed description of a method for isolating valve-side grounding faults in a bipolar DC system disclosed in this embodiment of the invention will be provided first.

[0038] This invention provides a method for isolating valve-side grounding faults in a bipolar DC system. (See also...) Figure 3The bipolar DC system includes a first DC pole, a second DC pole, a grounding unit, and two AC circuit breakers; each set of AC circuit breakers includes a first AC circuit breaker and a second AC circuit breaker; one end of the first DC pole is electrically connected to the positive pole through a first switch; one end of the second DC pole is electrically connected to the negative pole through a second switch; the other ends of the first DC pole and the other ends of the second DC pole are respectively electrically connected to the AC system through one or more sets of AC circuit breakers; the first DC pole and the second DC pole are grounded through the grounding unit.

[0039] It should be noted that the first DC electrode and the second DC electrode can be connected to the grounding unit via a metal return wire or a grounding wire.

[0040] It should also be noted that each converter unit in the DC pole needs to be connected to the AC system through a set of AC circuit breakers. If there are multiple converter units in the DC pole, multiple sets of AC circuit breakers are required to connect them to the AC system.

[0041] See Figure 1 The flowchart shown illustrates a method for isolating a valve-side grounding fault in a bipolar DC system. This method includes the following steps:

[0042] Step S102: When a ground fault is detected on the valve side of the converter transformer in the bipolar DC system, differential protection is activated in the valve side area until the AC system is isolated from the fault point.

[0043] In an embodiment of the present invention, see Figure 9 The timing diagram shown illustrates the control of the fault isolation switch. When a valve-side ground fault occurs in the bipolar DC system, differential protection is activated in the valve-side area. For example, differential protection or bridge arm overcurrent protection can be implemented in the valve-side area until the AC system is isolated from the fault point.

[0044] It should be noted that each DC pole consists of one or more converter units. The number and connection method of the converter units can be set according to actual needs, and this embodiment of the invention does not impose specific limitations on this. Each converter unit is connected to the AC system through a set of AC circuit breakers.

[0045] Step S104: Lock the faulty pole converter valve, control the first AC circuit breaker of the faulty pole to open, and after a preset delay, control the second AC circuit breaker of the faulty pole to open, so as to isolate the AC system from the fault point.

[0046] In this embodiment of the invention, the fault is most likely to be either the first DC pole or the second DC pole. Both the first DC pole and the second DC pole include a converter valve. The converter valve in the faulty DC pole is locked, and an AC circuit breaker between the faulty DC pole and the AC system is controlled to trip. After a preset delay, another AC circuit breaker between the faulty DC pole and the AC system is controlled to trip, so as to isolate the AC system from the fault point.

[0047] It should be noted that the preset duration can be set according to actual needs. This embodiment of the invention does not impose a specific limitation on this. For example, the delay time is related to the short-circuit impedance characteristics of the AC system and the parameters of the converter transformer, and is generally within 200ms, which can be obtained through simulation calculation.

[0048] Step S106: When the DC current is less than the preset DC threshold, control the first switch and the second switch to disconnect, so as to complete the isolation of the DC system from the fault point.

[0049] In this embodiment of the invention, after the second AC circuit breaker is tripped, a certain time is delayed until the DC current is less than a preset DC threshold. Then, the first and second switches are controlled to open, so as to isolate the DC system from the fault point and thus achieve isolation of the valve side grounding fault of the bipolar DC system.

[0050] It should be noted that the preset DC threshold can be set according to actual needs, and this embodiment of the invention does not impose specific limitations on it. For example, it can be set to 30A.

[0051] It should also be noted that the first switch can be a neutral line switch (NBS) and the second switch can be a pole line fast switch (HSS).

[0052] This invention provides a method for isolating a valve-side grounding fault in a bipolar DC system. The bipolar DC system includes a first DC pole, a second DC pole, a grounding unit, and two sets of AC circuit breakers. Each set of AC circuit breakers includes a first AC circuit breaker and a second AC circuit breaker. One end of the first DC pole is electrically connected to the positive pole via a first switch. One end of the second DC pole is electrically connected to the negative pole via a second switch. The other ends of the first and second DC poles are respectively electrically connected to the AC system via one or more sets of AC circuit breakers. The first and second DC poles are grounded through the grounding unit. The method includes: when a valve-side grounding fault is detected in the bipolar DC system, differential protection is activated in the valve-side area until the AC system is isolated from the fault point; the converter valve of the fault pole is locked, the first AC circuit breaker of the fault pole is controlled to open, and after a preset delay, the second AC circuit breaker of the fault pole is controlled to open to achieve isolation between the AC system and the fault point; when the DC current is less than a preset DC threshold, the first and second switches are controlled to open to complete the isolation between the DC system and the fault point. This invention can delay the tripping of two AC circuit breakers without needing to determine whether the faulty phase or the AC phase current has reached zero-crossing, and can quickly isolate the valve-side grounding fault.

[0053] In one embodiment, the first DC pole includes one or more MMC converter units or diode rectifier bridge converter units; the second DC pole includes one or more MMC converter units or diode rectifier bridge converter units.

[0054] In this embodiment of the invention, the first DC pole can be set as an MMC converter unit or a diode rectifier bridge converter unit according to actual needs, and the MMC converter unit or the diode rectifier bridge converter unit is connected in series; the second DC pole can also be set as an MMC converter unit or a diode rectifier bridge converter unit, and this embodiment of the invention does not specifically limit this.

[0055] In one embodiment, the MMC converter unit includes at least a start-up resistor, a converter transformer, a converter valve with a half-bridge topology, and a converter reactor; the diode rectifier bridge converter unit includes at least a converter transformer, a converter valve, and a smoothing reactor.

[0056] In this embodiment of the invention, the structure of the MMC converter unit can be found in [reference needed]. Figure 4 The diagram shown depicts an MMC converter unit composed of power modules based on a half-bridge topology. The structure of a diode rectifier bridge converter can be found in [reference needed]. Figure 5 The diagram shown is composed of a converter unit based on a diode rectifier bridge.

[0057] In one embodiment, the method further includes: using a differential protection device to determine that a valve-side grounding fault has occurred in the bipolar DC system based on the valve-side current of the converter transformer, the upper arm current of the converter bridge, the lower arm current of the converter bridge, and a preset rated value.

[0058] In one embodiment, a differential protection device is used to determine whether a valve-side ground fault has occurred in the bipolar DC system based on the converter transformer valve-side current, the converter bridge upper arm current, the converter bridge lower arm current, and a preset rated value. This includes determining whether the following formula holds true; if it does, then a valve-side ground fault has occurred in the bipolar DC system: Ivx - Ipx - Inx > k × IvAN; where Iv is the converter transformer valve-side current, Ip is the converter bridge upper arm current, In is the converter bridge lower arm current, k is the protection setting proportional coefficient, and x is the subscript of a certain phase.

[0059] In one embodiment, the neutral point on the grid side of the converter transformer is grounded via a surge arrester or directly grounded.

[0060] In this embodiment of the invention, the neutral point on the grid side of the converter transformer is not directly grounded. In this implementation, the neutral point on the grid side of the converter transformer can be grounded through a surge arrester.

[0061] In one embodiment, the arcing duration of the first AC circuit breaker and the second AC circuit breaker is not less than a preset duration threshold, and the first AC circuit breaker and the second AC circuit breaker are AC circuit breakers capable of interrupting half-wave short-circuit current.

[0062] In this embodiment of the invention, the preset duration threshold can be set according to actual needs, and this embodiment of the invention does not impose a specific limitation on it. For example, it can be set to 300ms. Two AC circuit breakers with long arc-breaking capability and the ability to interrupt large DC fault currents are installed between the AC system and the DC converter unit.

[0063] The implementation of this method will be described below with a specific example.

[0064] A bipolar DC system consists of at least one rectifier station and at least one inverter station. The rectifier station and the inverter station each consist of two DC poles. In the rectifier station, each DC pole consists of at least one modular multilevel converter (MMC) unit or a diode rectifier bridge converter unit. In the inverter station, each DC pole consists of at least one modular multilevel converter (MMC) unit.

[0065] The MMC converter unit includes main components such as starting resistors, converter transformers, converter valves, and converter reactors. The MMC converter valve adopts a half-bridge topology. The diode rectifier bridge converter can be composed of converter transformers, converter valves, and smoothing reactors.

[0066] The bipolar DC system is grounded directly at the bipolar neutral point, or grounded through a small resistor (less than 10 ohms), or grounded at the grounding electrode through a grounding electrode line with an equivalent resistance of less than 10 ohms.

[0067] The bipolar DC system MMC converter unit is connected to the AC system via two AC circuit breakers Q1 and Q2 with long arc-breaking capability. See details... Figure 3 .

[0068] The converter unit is equipped with current measuring devices at the valve-side bushing of the converter transformer, the AC inlet of the converter valve, between the converter valve and the converter reactor, and at the DC pole line. The converter unit is also equipped with valve-side differential protection and valve-side overcurrent protection devices.

[0069] The AC circuit breaker trips using a time-delay trip strategy. Upon detecting a valve-side ground fault via valve-side differential protection or valve-side overcurrent protection, it immediately locks the converter valve (MMC converter unit) and simultaneously trips one circuit breaker. Then, after a certain delay (usually within 240ms), the other circuit breaker trips. The AC circuit breaker is capable of withstanding prolonged arcing (arc duration not less than 300ms). A ground fault in the converter transformer valve-side area may be a single-phase ground fault, a two-phase ground fault, or a three-phase ground fault.

[0070] For the changes in AC current waveform, please refer to Figures 10-13 In the diagram, BRK1 and BRK2 represent circuit breaker status signals (0 represents the circuit breaker is in the closed state, and 1 represents the circuit breaker is in the open state), Is1 represents the three-phase current on the AC side of the transformer, sec represents the time unit second, and KA represents the current unit kiloampere.

[0071] in, Figure 10 The figure shows the AC current on the grid side of the converter transformer and the circuit breaker trip signal waveforms when a single-phase ground fault occurs on the valve side of the YY-connected converter transformer. In this figure, Is1 is the three-phase current on the AC system side of the converter transformer when a single-phase fault occurs on the valve side; BRK1 and BRK2 are the status signals of the two circuit breakers, which are both 0 when superimposed, indicating that both circuit breakers are in the closed state. Figure 11 The waveform of the AC fault current when the first AC circuit breaker Q1 of the YY-connected converter transformer breaks a phase fault on the AC side is shown. In this figure, BRK1 changes from 0 to 1, representing the tripping time of the first circuit breaker Q1, which is between 1.64 seconds and 1.66 seconds. BRK2 remains unchanged and is always in the closed state. Figure 12 The waveform of the fault current in the second AC circuit breaker Q2 of the YY-connected converter transformer when it interrupts the fault current in the other two phases on the AC side is shown in the figure. In this figure, BRK2 changes from 0 to 1, which represents the tripping time of the second circuit breaker Q2, between 1.76 seconds and 1.78 seconds. BRK1 is not shown, and its value is always 1, which means that it is always in the open state. Figure 13The diagram illustrates the entire process of interrupting AC fault current in a YY-connected converter transformer. If the converter transformer uses a YΔ connection, the same process can be used to interrupt the AC fault current.

[0072] Taking a high-voltage direct current bipolar system where one pole consists of one or more MMC converter units connected in series as an example:

[0073] 1. In a DC bipolar system, the positive or negative pole is composed of one or more converter units connected in series. The converter unit may consist of main equipment such as a starting resistor, a converter transformer, a half-bridge topology MMC converter valve, and a converter reactor.

[0074] 2. The DC bipolar system is connected to the AC system via two AC circuit breakers with long arc-breaking capability and the ability to interrupt half-wave short-circuit current (approximately one cycle between zero points). The circuit breakers are located on the grid side of the converter transformer and the starting resistor. The grid-side neutral point of the converter transformer is not directly grounded; in this implementation scheme, the grid-side neutral point of the converter transformer is grounded via a surge arrester.

[0075] 3. In a DC bipolar system, the neutral point of the converter unit is directly grounded, or grounded at the grounding electrode via the neutral point and the grounding electrode line, or the neutral point is grounded via a small resistor of less than 10 ohms. In addition to the positive and negative lines, the sending and receiving ends of a DC bipolar system can be connected by independent metallic return lines.

[0076] 4. The DC bipolar system is equipped with pole fast switch HSS and neutral bus switch NBS at the high voltage pole and neutral line.

[0077] 5. Current measuring devices are installed at the converter transformer valve side bushing, the AC inlet of the converter valve, between the converter valve and the converter reactor, and at the DC pole line of the converter unit.

[0078] 6. The converter unit is equipped with valve-side differential protection and valve-side overcurrent protection devices.

[0079] 7. In this implementation case, see Figure 6 When a single-phase ground fault occurs in the area from the converter transformer to the converter valve, the ground fault is detected by the differential protection criterion: Ivx-Ipx-Inx>k×IvAN, where Iv is the current at the bushing T1 on the converter transformer valve side, Ip and In are the currents T3 of the upper and lower arms of the converter bridge, and their current directions are from the AC side to the converter valve, k is the protection setting proportional coefficient, which can generally be taken as 0.3, and x is the subscript of a certain phase.

[0080] 8. In this implementation case, the non-faulty phase current has a zero-crossing point in the valve-side current, but the fault current waveform has a DC bias. The equivalent circuit for the non-faulty phase is as follows: Figure 8 As shown.

[0081] When the system voltage is greater than zero, the current flowing through the loop increases; when the system voltage is less than zero, the current flowing through the loop decreases. Due to the freewheeling current of the reactor, the current flowing through the diode does not immediately turn off. If the influence of the loop resistance is ignored, the diode will not turn off within one cycle. If the equivalent resistance of the loop is taken into account, the diode experiences current interruption within one cycle. The time the diode is off is related to the magnitude of the equivalent resistance of the loop; the larger the resistance, the longer the duration of the loop current crossing zero. The equivalent resistance of the fault loop mainly consists of the system short-circuit impedance resistance, the equivalent resistance of the converter transformer, the equivalent resistance of the bridge arm reactor, and the equivalent resistance of the fault point. Under more stringent conditions, when the equivalent resistance is small, the duration of the fault current crossing zero in the non-faulty phase is short, requiring Q1 and Q2 to have the ability to interrupt current within one cycle.

[0082] In a bipolar system, the converter valve side is ungrounded, so there is no zero-sequence current path. The fault phase current is the superposition of the non-fault phase currents, and the non-fault phase currents are 60° out of phase. Therefore, it can be concluded that the fault phase current does not have a current zero-crossing point.

[0083] When the converter transformer adopts a YY connection, the grid-side current in the non-faulty phase has a zero-crossing point, and the time interval between these zero-crossing points is close to one cycle time. Circuit breakers Q1 and Q2 have the current breaking capacity under this current waveform. When the converter transformer adopts a Y-delta connection, one non-faulty phase of the grid-side current has a zero-crossing point.

[0084] By opening Q1, the phase with a zero-crossing point in the grid-side current can be disconnected. After the first phase current is interrupted, the fault currents of the other two phases will decay to zero within a certain period of time. The decay time constant is τ = L / R, where L includes the equivalent inductance of the converter transformer, the bridge arm reactor, the connecting conductor, and the equivalent inductance of the ground fault circuit; and R includes the equivalent resistance of the converter transformer, the bridge arm reactor, the connecting conductor, and the equivalent resistance of the ground fault point.

[0085] In this implementation scheme, Q1 is used to disconnect the phase with a zero-crossing point in the grid-side current, and Q2 is turned on after a delay to disconnect the fault current of the remaining two phases, thereby achieving isolation between the AC system and the fault point.

[0086] 9. The fault isolation protection action sequence of this implementation plan is as follows: Figure 9 As shown, at time t0 of the single-phase ground fault on the valve side, the differential protection starts at time t1. At time t2, the protection immediately blocks the converter valve and trips the first AC-side circuit breaker Q1, which can interrupt at least one phase of fault current. After a delay of 160ms, at time t3, the second AC-side circuit breaker Q2 trips. At time t4, the AC circuit breaker Q2 completes the interruption of the other two phases of fault current, realizing the isolation between the AC system and the DC system.

[0087] 10. After the AC system is isolated from the DC system, a freewheeling current will flow through the bridge arm reactor on the neutral bus. The current decay time constant is τ = L / R, where L is the inductance of the fault current-carrying loop (mainly composed of the inductance of the bridge arm reactor and connecting wires), and R is the equivalent resistance of the fault current-carrying loop (mainly composed of the DC resistance of the bridge arm reactor, the resistance of the connecting wires, and the resistance of the grounding loop). See also Figure 9 DC current measuring devices T4 and T5 were detected. Figure 4 and Figure 5 The current is less than 30A, at time t5 ( Figure 9 Open the neutral line switch NBS and the pole line fast switch HSS to isolate the faulty pole.

[0088] This invention provides a method and apparatus for isolating ground faults on the valve side of a bipolar DC system. When a ground fault occurs in the valve side region of the converter transformer in an MMC-HVDC bipolar DC system, the converter of that pole needs to be stopped immediately, and the fault point needs to be isolated from the AC system. Applying this method eliminates the need for the protection system to determine the fault type and fault phase, eliminates concerns about the protection system failing to operate or maloperating, and eliminates the need to determine whether the current of a certain phase has a zero crossing point. It can quickly and reliably complete the isolation of ground faults in the valve side region of the converter transformer.

[0089] This invention also provides a device for isolating valve-side grounding faults in a bipolar DC system, as described in the following embodiments. Since the principle by which this device solves the problem is similar to that of the method for isolating valve-side grounding faults in a bipolar DC system, the implementation of this device can refer to the implementation of the method for isolating valve-side grounding faults in a bipolar DC system; repeated details will not be elaborated further.

[0090] See Figure 2 The diagram shows a structural block diagram of a valve-side grounding fault isolation device for a bipolar DC system. The bipolar DC system includes a first DC pole, a second DC pole, a grounding unit, and two sets of AC circuit breakers. Each set of AC circuit breakers includes a first AC circuit breaker and a second AC circuit breaker. One end of the first DC pole is electrically connected to the positive terminal via a first switch; one end of the second DC pole is electrically connected to the negative terminal via a second switch; the other ends of the first and second DC poles are respectively electrically connected to the AC system via one or more sets of AC circuit breakers; the first and second DC poles are grounded through the grounding unit. The device includes:

[0091] Protection module 21 is used to activate differential protection on the valve side area when a ground fault is detected on the valve side of the bipolar DC system converter transformer until the AC system is isolated from the fault point; AC control module 22 is used to lock the converter valve of the fault pole, control the first AC circuit breaker of the fault pole to open, and control the second AC circuit breaker of the fault pole to open after a preset delay, so as to achieve isolation between the AC system and the fault point; DC control module 23 is used to control the first switch and the second switch to open when the DC current is less than a preset DC threshold, so as to complete the isolation of the DC system from the fault point. The device includes:

[0092] In one embodiment, the first DC pole includes an MMC converter unit or a diode rectifier bridge converter unit; the second DC pole includes an MMC converter unit or a diode rectifier bridge converter unit.

[0093] In one embodiment, the MMC converter unit includes at least a start-up resistor, a converter transformer, a converter valve with a half-bridge topology, and a converter reactor; the diode rectifier bridge converter unit includes at least a converter transformer, a converter valve, and a smoothing reactor.

[0094] In one embodiment, the device further includes a judgment module for determining, using a differential protection device, that a valve-side ground fault has occurred in the bipolar DC system based on the valve-side current of the converter transformer, the upper arm current of the converter bridge, the lower arm current of the converter bridge, and a preset rated value.

[0095] In one embodiment, the judgment module is specifically used to: determine whether the following formula is true; if true, then determine that a valve-side ground fault has occurred in the bipolar DC system: Ivx-Ipx-Inx>k×IvAN; where Iv is the valve-side current of the converter transformer, Ip is the upper arm current of the converter bridge, In is the lower arm current of the converter bridge, k is the protection setting proportional coefficient, and x is the subscript of a certain phase.

[0096] In one embodiment, the neutral point on the grid side of the converter transformer is grounded via a surge arrester or directly grounded.

[0097] In one embodiment, the arcing duration of the first AC circuit breaker and the second AC circuit breaker is not less than a preset duration threshold, and the first AC circuit breaker and the second AC circuit breaker are AC circuit breakers capable of interrupting half-wave short-circuit current.

[0098] Based on the same inventive concept, this invention also provides an embodiment of an electronic device for implementing all or part of the above-described method for isolating valve-side grounding faults in a bipolar DC system. This electronic device specifically includes the following components:

[0099] The device comprises a processor, memory, a communications interface, and a bus; wherein the processor, memory, and communications interface communicate with each other via the bus; the communications interface is used to realize information transmission between related devices; the electronic device can be a desktop computer, tablet computer, or mobile terminal, etc., and this embodiment is not limited to these. In this embodiment, the electronic device can be implemented with reference to the embodiments for implementing the above-mentioned bipolar DC system valve-side ground fault isolation method and the embodiments for implementing the above-mentioned bipolar DC system valve-side ground fault isolation device, the contents of which are incorporated herein, and repeated details will not be described again.

[0100] Figure 7 This is a schematic diagram of the system composition structure of an electronic device provided in an embodiment of the present invention. Figure 7 As shown, the electronic device 70 may include a processor 701 and a memory 702; the memory 702 is coupled to the processor 701. It is worth noting that... Figure 7 This is an example; other types of structures can also be used to supplement or replace this structure to achieve telecommunications functions or other functions.

[0101] In one embodiment, the functionality of the bipolar DC system valve-side ground fault isolation method can be integrated into the processor 701. The processor 701 can be configured to perform the following controls: when a valve-side ground fault is detected in the bipolar DC system, activate differential protection for the valve-side region until the AC system is isolated from the fault point; lock the converter valve of the faulty pole, control the first AC circuit breaker of the faulty pole to open, and after a preset delay, control the second AC circuit breaker of the faulty pole to open, thereby achieving isolation between the AC system and the fault point; when the DC current is less than a preset DC threshold, control the first and second switches to open, thereby completing the isolation between the DC system and the fault point.

[0102] As can be seen from the above, the electronic equipment provided in the embodiments of the present invention can delay trip two AC circuit breakers without needing to determine whether the faulty phase and the AC phase current have reached zero crossing point, and can quickly complete the isolation of the valve side grounding fault.

[0103] In another embodiment, the bipolar DC system valve-side ground fault isolation device can be configured separately from the processor 701. For example, the bipolar DC system valve-side ground fault isolation device can be configured as a chip connected to the processor 701, and the function of the bipolar DC system valve-side ground fault isolation method can be realized through the control of the processor.

[0104] like Figure 7As shown, the electronic device 70 may further include: a communication module 703, an input unit 704, an audio processing unit 705, a display 706, and a power supply 707. It is worth noting that the electronic device 70 does not necessarily need to include these components. Figure 7 All components shown; in addition, the electronic device 70 may also include Figure 7 For components not shown, please refer to existing technologies.

[0105] like Figure 7 As shown, processor 701, sometimes also referred to as controller or operation control, may include a microprocessor or other processor device and / or logic device, which receives input and controls the operation of various components of electronic device 70.

[0106] The memory 702 may be, for example, one or more of a cache, flash memory, hard drive, removable media, volatile memory, non-volatile memory, or other suitable devices. It may store the aforementioned failure-related information, and also store a program for executing that information. The processor 701 may execute the program stored in the memory 702 to perform information storage or processing, etc.

[0107] Input unit 704 provides input to processor 701. Input unit 704 may be, for example, a keypad or touch input device. Power supply 707 provides power to electronic device 70. Display 706 displays images and text. Display may be, for example, an LCD display, but is not limited thereto.

[0108] The memory 702 can be a solid-state memory, such as a read-only memory (ROM), random access memory (RAM), a SIM card, etc. It can also be a memory that retains information even when power is off, can be selectively erased, and contains more data; examples of this type of memory are sometimes referred to as EPROMs. The memory 702 can also be some other type of device. The memory 702 includes a buffer memory 7021 (sometimes called a buffer). The memory 702 may include an application / function storage unit 7022 for storing application programs and function programs or processes for executing the operation of the electronic device 70 via the processor 701.

[0109] The memory 702 may also include a data storage unit 7023 for storing data, such as contacts, digital data, pictures, sounds, and / or any other data used by the electronic device. The driver storage unit 7024 of the memory 702 may include various drivers for the electronic device's communication functions and / or for performing other functions of the electronic device (such as messaging applications, address book applications, etc.).

[0110] The communication module 703 is a transmitter / receiver that transmits and receives signals via the antenna 708. The communication module (transmitter / receiver) 703 is coupled to the processor 701 to provide input signals and receive output signals, which is the same as in a conventional mobile communication terminal.

[0111] Based on different communication technologies, multiple communication modules 703 can be configured in the same electronic device, such as cellular network modules, Bluetooth modules, and / or wireless LAN modules. The communication module (transmitter / receiver) 703 is also coupled to a speaker 709 and a microphone 710 via an audio processing unit 705 to provide audio output via the speaker 709 and receive audio input from the microphone 710, thereby realizing typical telecommunications functions. The audio processing unit 705 may include any suitable buffer, decoder, amplifier, etc. Additionally, the audio processing unit 705 is also coupled to a processor 701, enabling on-device recording via the microphone 710 and on-device playback of stored audio via the speaker 709.

[0112] In embodiments of the present invention, a computer-readable storage medium is also provided for implementing all steps of the bipolar DC system valve-side ground fault isolation method in the above embodiments. The computer-readable storage medium stores a computer program that, when executed by a processor, implements all steps of the bipolar DC system valve-side ground fault isolation method in the above embodiments. For example, when the processor executes the computer program, it implements the following steps:

[0113] When a valve-side grounding fault is detected in the bipolar DC system, differential protection is activated in the valve-side area until the AC system is isolated from the fault point; the fault pole converter valve is locked, the first AC circuit breaker of the fault pole is opened, and after a preset delay, the second AC circuit breaker of the fault pole is opened to achieve isolation between the AC system and the fault point; when the DC current is less than the preset DC threshold, the first switch and the second switch are opened to complete the isolation between the DC system and the fault point.

[0114] As can be seen from the above, the computer-readable storage medium provided in the embodiments of the present invention can delay trip two AC circuit breakers without determining whether the faulty phase and the AC phase current have reached zero crossing point, and can quickly complete the isolation of the valve side grounding fault.

[0115] While this invention provides the method operation steps as described in the embodiments or flowcharts, more or fewer operation steps may be included based on conventional or non-inventive labor. The order of steps listed in the embodiments is merely one possible execution order among many and does not represent the only possible execution order. In actual device or client product execution, the methods shown in the embodiments or drawings can be executed sequentially or in parallel (e.g., in a parallel processor or multi-threaded processing environment).

[0116] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0117] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0118] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0119] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0120] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

[0121] In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, without necessarily requiring or implying any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. The terms "upper," "lower," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as limiting the invention.

[0122] Unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0123] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention is not limited to any single aspect, nor to any single embodiment, nor to any combination and / or substitution of these aspects and / or embodiments. Each aspect and / or embodiment of the present invention can be used alone, or in combination with one or more other aspects and / or other embodiments.

[0124] Finally, it should be noted that the above-described embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the technical scope disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for isolating valve-side grounding faults in a bipolar DC system, characterized in that, The bipolar DC system includes a first DC pole, a second DC pole, a grounding unit, and two sets of AC circuit breakers; each set of AC circuit breakers includes a first AC circuit breaker and a second AC circuit breaker; one end of the first DC pole is electrically connected to the positive pole through a first switch; one end of the second DC pole is electrically connected to the negative pole through a second switch; the other ends of the first DC pole and the other ends of the second DC pole are respectively electrically connected to the AC system through one or more sets of AC circuit breakers; The first DC electrode and the second DC electrode are grounded through the grounding unit; the method includes: When a ground fault is detected on the valve side of the converter transformer of the bipolar DC system, differential protection is activated in the valve side area until the AC system is isolated from the fault point. The faulty pole converter valve is locked, the first AC circuit breaker of the faulty pole is opened, and after a preset delay, the second AC circuit breaker of the faulty pole is opened to achieve isolation between the AC system and the fault point; the preset delay time can be set within 200ms according to actual needs. When the DC current is less than the preset DC threshold, the first switch and the second switch are disconnected to isolate the DC system from the fault point; the preset DC threshold is set to 30A according to actual needs. Using a differential protection device, a valve-side ground fault is determined in the bipolar DC system based on the valve-side current of the converter transformer, the upper arm current of the converter bridge, the lower arm current of the converter bridge, and preset rated values. This includes determining whether the following formula holds true; if it does, a valve-side ground fault is determined in the bipolar DC system: Ivx-Ipx-Inx>k×IvAN Where Iv is the valve-side current of the converter transformer, Ip is the upper arm current of the converter bridge, In is the lower arm current of the converter bridge, k is the protection setting proportional coefficient, and x is the subscript of a certain phase.

2. The method according to claim 1, characterized in that, The first DC pole includes one or more MMC converter units or diode rectifier bridge converter units; the second DC pole includes one or more MMC converter units or diode rectifier bridge converter units.

3. The method according to claim 2, characterized in that, The MMC converter unit includes at least a starting resistor, a converter transformer, a converter valve with a half-bridge topology, and a converter reactor; the diode rectifier bridge converter unit includes at least a converter transformer, a converter valve, and a smoothing reactor.

4. The method according to claim 3, characterized in that, The neutral point on the grid side of the converter transformer is grounded via a surge arrester or directly grounded.

5. The method according to any one of claims 1-3, characterized in that, The arcing duration of the first AC circuit breaker and the second AC circuit breaker is not less than a preset duration threshold, and the first AC circuit breaker and the second AC circuit breaker are AC circuit breakers with the ability to interrupt half-wave short-circuit current.

6. A valve-side ground fault isolation device for performing the method of claim 1 in a bipolar DC system, characterized in that, The bipolar DC system includes a first DC pole, a second DC pole, a grounding unit, and two sets of AC circuit breakers; each set of AC circuit breakers includes a first AC circuit breaker and a second AC circuit breaker; one end of the first DC pole is electrically connected to the positive pole through a first switch; one end of the second DC pole is electrically connected to the negative pole through a second switch; the other ends of the first DC pole and the other ends of the second DC pole are respectively electrically connected to the AC system through one or more sets of AC circuit breakers; The first DC electrode and the second DC electrode are grounded through the grounding unit; the device includes: The protection module is used to activate differential protection in the valve side area when a ground fault is detected on the valve side of the converter transformer of the bipolar DC system, until the AC system is isolated from the fault point; The AC control module is used to lock the converter valve of the faulty pole, control the first AC circuit breaker of the faulty pole to open, and control the second AC circuit breaker of the faulty pole to open after a preset delay, so as to isolate the AC system from the fault point; the preset delay can be set within 200ms according to actual needs. The DC control module is used to control the first switch and the second switch to disconnect when the DC current is less than a preset DC threshold, so as to isolate the DC system from the fault point; the preset DC threshold is set to 30A according to actual needs.

7. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the valve-side grounding fault isolation method for any one of claims 1 to 5 in a bipolar DC system.

8. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by a processor, implements the computer program of the bipolar DC system valve-side ground fault isolation method according to any one of claims 1 to 5.

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