DC fault management in power system

Abstract

The invention relates to DC fault management in a power system. Embodiments herein provide a protection method (700) of protecting a modular multilevel converter (80) for DC fault protection in a symmetric single stage configuration. The MMC for each of the at least one phase comprises a branch (70) comprising a first branch and a second branch, the second branch comprising pilot valves connected in series. Furthermore, the branches of the MMC (80) are connected in series between the DC poles (25, 30). The method (700) comprises measuring DC pole voltages relative to a common ground and determining a DC fault (21) accordingly identifying a DC pole-to-ground fault of one of the DC poles. Upon determining the DC fault, the method (700) includes controlling a pilot valve (52, 54) of a second branch (55) of each branch (70) to bypass the DC poles through the second branch (55) for protecting the DC poles other than the DC poles identified with the DC fault.

Description

Technical Field

[0001] The present disclosure generally relates to a power system having a symmetrical unipolar configuration. More specifically, the present disclosure relates to providing a protection method, a controller, and a computer program product for protecting a modular multilevel converter (MMC) to achieve direct current (DC) fault protection in a symmetrical unipolar configuration. Background Art

[0002] Renewable energy sources (e.g., wind farms, photovoltaic (PV) energy, etc.) play an important role in moving towards a carbon-neutral energy system. Power systems, such as High-Voltage Direct Current (HVDC) systems, provide the flexibility, controllability, and resilience required to integrate renewable energy on a large scale and transmit power from offshore stations to onshore stations. The power system includes voltage source converters arranged in a symmetrical unipolar configuration to transmit power (i.e., direct current DC power) from offshore stations to onshore stations through transmission lines connected between the voltage source converters.

[0003] Figure 1A An example power system 100 according to the prior art is disclosed. For simplicity, Figure 1A An electric power system 100 is shown comprising two voltage source converters (VSC) 10 arranged in a symmetrical unipolar configuration. Each voltage source converter 10 is connected to an alternating current (AC) power source 15 and to a transmission line via DC poles 25 and 30 of the voltage source converter 10. The transmission line comprises DC cables 25a and 30a adapted to transmit electric power / DC power with positive and negative polarities.

[0004] In a symmetrical monopole configuration, a DC fault 21 identifying a DC pole-to-ground fault of one of the DC poles raises the voltage of the other DC pole relative to ground. For example, a DC fault 21 of a DC pole 25 raises the voltage of a DC pole 30 relative to ground, wherein the aforementioned voltage may exceed a predefined maximum voltage for the DC pole 30. Such an overvoltage of the DC pole 30 may cause damage to the power system 100 including the DC cables 25a and 30a, which further causes significant interruptions and economic losses to the power supplier.

[0005] Figure 1B An example protection device according to the prior art is disclosed for protecting DC poles 25 and 30 during a DC fault. Figure 1BAs shown, the protection device includes a DC lightning arrester 20 for each DC pole 25 and 30. When a DC fault 21 of another DC pole is determined, the DC lightning arrester 20 protects the corresponding DC pole. For example, due to the DC fault of the DC pole 20, the voltage of the DC pole 30 may increase. When the increased voltage of the DC pole 30 is about to exceed a predefined threshold, the DC lightning arrester 20 corresponding to the DC pole 30 may be short-circuited to protect the DC pole 30.

[0006] In the prior art, the voltage level of the DC arrester 20 generally needs to be designed to be greater than (e.g., 1.7 times) the steady-state peak voltage. The rated voltage of each DC cable will be determined based on the withstand voltage level of the DC arrester 20. However, this voltage level design of the DC arrester 20 and the DC cable may involve huge costs and technical challenges, because the rated voltage design of the existing type of DC cable (e.g., extruded DC cable) may still be limited. For example, if the voltage of the DC cable exceeds 640 kilovolts (KV), and the rated voltage is increased due to the DC fault 21, the existing type of DC cable may not be able to withstand. In addition, in most cases, even if the voltage of the DC cable exceeds 525kV, when considering the DC fault 21, there may be technical challenges to meet the overvoltage requirements of the DC cable.

[0007] Therefore, there is a need for an improved method and apparatus for DC fault protection in an electrical system having a symmetrical single-pole configuration. Summary of the invention

[0008] Therefore, an object of the present disclosure is to provide a protection method, a controller and a computer program product for direct current (DC) fault protection in a symmetrical single-pole configuration to mitigate, alleviate or eliminate all or at least some of the above-mentioned disadvantages of currently known solutions.

[0009] This object and others are achieved by the protection method, controller and computer program product as defined in the appended claims.In this context, the term "exemplary" should be understood as serving as an example, instance or illustration.

[0010] According to a first aspect of the present disclosure, a protection method for protecting a modular multilevel converter MMC is provided for direct current (DC) fault protection in a symmetrical unipolar configuration. The MMC for each phase of at least one phase includes a branch. The branch includes a first branch having an upper arm and a lower arm connected in series. The upper arm and the lower arm each include a submodule configured to contribute to the waveform of the output voltage of the MMC. The branch also includes a second branch having a first pilot valve and a second pilot valve connected in series. The first branch and the second branch are connected in parallel. The branches of the MMC are connected in series between the DC poles of the MMC. The protection method includes measuring a DC pole voltage relative to a common ground. The protection method includes determining a DC fault of a DC pole-to-ground fault identifying one of the DC poles based on the measured DC pole voltage. After determining the DC fault of the DC pole, the protection method includes controlling the pilot valve of the second branch of each branch to bypass the DC pole through the second branch of the pilot valve for protecting the DC pole other than the DC pole identified with the DC fault.

[0011] According to a second aspect of the present disclosure, a controller for protecting a modular multilevel converter (MMC) is provided for DC fault protection in a symmetrical single-pole configuration. The MMC for each phase of at least one phase includes a branch. The branch includes a first branch having an upper arm and a lower arm connected in series. The upper arm and the lower arm each include a submodule configured to contribute to the waveform of the output voltage of the MMC. The branch also includes a second branch having a first pilot valve and a second pilot valve connected in series. The first branch and the second branch are connected in parallel. The branches of the MMC are connected in series between the DC poles of the MMC. The controller includes a measurement module configured to measure a DC pole voltage relative to a common ground. The controller also includes a determination module configured to determine a DC fault of a DC pole-to-ground fault identifying one of the DC poles based on the measured DC pole voltage. The controller also includes a protection module for controlling the pilot valve of the second branch of each branch after determining the DC fault of the DC pole to bypass the DC pole through the second branch of the pilot valve for protecting the DC pole other than the DC pole identified with the DC fault.

[0012] A third aspect is a device comprising the controller of the second aspect.

[0013] According to a fourth aspect of the present disclosure, a power system is provided. The power system includes a transmission line, which includes a direct current (DC) cable, a modular multilevel converter (MMC), and a controller of the second aspect. The MMC is arranged between a first terminal and a second terminal of the transmission line in a symmetrical unipolar configuration for transmitting power through the transmission line. The MMC for each phase of at least one phase includes a branch. The branch includes a first branch having an upper arm and a lower arm connected in series. The upper arm and the lower arm each include a submodule configured to contribute to the waveform of the output voltage of the MMC. The branch also includes a second branch having a first pilot valve and a second pilot valve connected in series. The first branch and the second branch are connected in parallel. The branches of the MMC are connected in series between the DC poles of the MMC. The controller of the second aspect is configured to be connected to each branch of the MMC.

[0014] According to a fifth aspect of the present disclosure, a computer program product is provided, comprising a non-transitory computer readable medium having a computer program including program instructions thereon. The computer program can be loaded into a data processing unit and is configured to execute a method according to any one of the first to fourth aspects when the data processing unit runs the computer program.

[0015] Advantageously, the proposed invention provides an efficient protection device and method for DC fault protection, reducing the cost and footprint of the converter. Through DC fault protection, the withstand voltage level of a DC cable in a symmetrical single-pole configuration can be reduced at a lower cost.

[0016] Other advantages may be apparent to those skilled in the art. Certain embodiments may have some or all of the advantages mentioned. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The foregoing will become apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings, in which like reference numerals refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead being placed upon illustrating example embodiments.

[0018] Figure 1A An example power system according to the prior art is disclosed;

[0019] Figure 1B Disclosed is an example protection device for protecting a DC pole due to a DC fault according to the prior art;

[0020] Figure 2 A power system according to an embodiment of the present invention is disclosed, comprising a controller for protecting a modular multilevel configuration (MMC) to implement DC fault protection in a symmetrical single-pole configuration;

[0021] Figure 3A ,3B and 3C disclose components of a branch of an MMC connected to a controller according to one embodiment of the present invention;

[0022] Figure 4 The functional components of a controller according to one embodiment of the present invention are disclosed;

[0023] Figure 5A and 5B shows DC fault management in an example power system according to one embodiment of the present invention; and

[0024] Figure 6 Disclosed is a method according to an embodiment of the present invention Figure 4 an exemplary hardware implementation of the illustrated controller; and

[0025] Figure 7 is a flow chart illustrating example method steps according to one embodiment of the present invention. DETAILED DESCRIPTION

[0026] Aspects of the present disclosure will be described more fully below with reference to the accompanying drawings. However, the apparatus and methods disclosed herein may be implemented in many different forms and should not be construed as being limited to the aspects set forth herein. The same numbers in the accompanying drawings always refer to the same elements.

[0027] The terms used herein are only used to describe specific aspects of the present disclosure and are not intended to limit the present invention. It should be emphasized that the term "include / comprise" used in this specification is used to specify the presence of the features, integers, steps, or components, but does not exclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. Unless the context clearly indicates otherwise, the singular forms "one", "an", and "the" used herein also include plural forms.

[0028] Direct current DC pole voltage: As used herein, “DC pole voltage” may refer to the output voltage of a DC pole of a modular multilevel converter MMC.

[0029] DC fault: As used herein, a "DC fault" may refer to a DC pole-to-ground fault.

[0030] Unhealthy DC pole: As used herein, an "unhealthy DC pole" may refer to a DC pole that is determined to have a DC fault.

[0031] Healthy DC pole: As used herein, a “healthy DC pole” may refer to a DC pole other than a DC pole determined to have a DC fault.

[0032] Generally speaking, power systems are needed to integrate renewable energy on a large scale and transmit electricity from offshore stations to onshore stations.

[0033] According to the existing technology, Figure 1A and 1B As shown, the power system includes a converter (e.g., a voltage source converter) arranged in a symmetrical monopole configuration to be connected to an alternating current (AC) power source and connected to a transmission line via a direct current (DC) pole for transmitting power. The transmission line includes a DC cable suitable for transmitting power. In the symmetrical monopole configuration, a DC fault identifying a DC pole-to-ground fault of one of the DC poles may cause the voltage of the other DC fault to rise and exceed a maximum voltage predefined for the other DC fault. Thereby causing damage to the DC pole.

[0034] In the prior art, DC lightning arresters can be used to protect DC poles during DC faults. However, the rated voltage design of DC lightning arresters may involve huge costs and technical challenges.

[0035] Compared to the prior art, embodiments herein provide a power system with an improved protection device for DC fault protection in a symmetrical single-pole configuration.

[0036] Figure 2 A power system 100 is disclosed. The power system 100 described herein can be connected to renewable energy sources, such as large wind farms, photovoltaic PV energy sources, etc., to support high-power onshore and offshore renewable energy transmission. In some examples, the power system 100 can be a high-voltage direct current HVDC system.

[0037] like Figure 2 As shown, the power system 100 includes a transmission line 85. The transmission line 85 includes DC cables 25a and 30a suitable for transmitting power (eg, DC power).

[0038] The power system 100 also includes a modular multilevel converter MMC 80. The MMC 80 is arranged in a symmetrical unipolar configuration. At a first end, the MMC 80 is connected to an alternating current AC power source 15 via a resistor 16. At a second end, the MMC 80 is connected between a first terminal and a second terminal of a transmission line 85 via DC poles 25 and 30. The MMC 80 is configured to perform voltage conversion and transmit power (i.e., DC power) via the transmission line 85.

[0039] According to an embodiment disclosed herein, the MMC 80 includes branches 70 for phases 15a, 15b, and 15c of the AC power source 15. The branches 70 are connected in series between the DC poles 25 and 30 of the MMC 80. In one example, as Figure 2 As shown, the branch is connected in series between DC poles 25 and 30 of MMC 80 via resistors 42 and 44 .

[0040] Figure 3A , 3Band 3C disclose a branch 70 for each phase. Figure 3A , 3B The branch 70 of the MMC 80 disclosed in FIG. 3C includes a first branch 65 and a second branch 55. The first branch 65 and the second branch 55 are connected in parallel.

[0041] The first branch 65 includes an upper arm 62 and a lower arm 64. The upper arm 62 and the lower arm 64 are connected in series. The upper arm 62 and the lower arm 64 each include a submodule configured to contribute to the waveform of the output voltage of the MMC 80. In one example, Figure 3B As shown, the submodule of each of the upper arm 62 and the lower arm 64 may include a full-bridge submodule / full-bridge inverter. The full-bridge submodule may include four switching elements 62a / 64a with unit capacitors 62b / 64b. In another example, as Figure 3C As shown, the submodule of each of the upper arm 62 and the lower arm 64 may include a half-bridge submodule / half-bridge inverter. The half-bridge submodule may include two switching elements 62a / 64a with a unit capacitor 62b / 64b. Examples of the switching elements 62a / 64a described herein may include, but are not limited to, insulated gate bipolar transistors IGBTs, integrated gate commutated thyristors IGCTs, and the like.

[0042] The second branch 55 includes a first pilot valve 52 and a second pilot valve 54 connected in series. Figure 3B and 3C As shown, each of the first pilot valve 52 and the second pilot valve 54 includes a series-connected anti-parallel thyristor pair (52a, 52b) / (54a, 54b). Therefore, the modular multi-level configuration of the proposed MMC 80 can reduce the cost and footprint of the converter and handle ultra-high DC voltages.

[0043] Reference again Figure 2 , the power system 100 also includes a controller 95 connected to each branch 70 of the MMC 80. The controller 95 is configured to protect the MMC 80 against DC faults in a symmetrical monopole configuration. For DC fault protection, the controller 95 measures a DC pole voltage relative to a common ground. The DC pole voltage here refers to the output voltage of the DC poles 25 and 30 of the MMC 80 relative to the common ground. Based on the measured DC pole voltage, the controller 95 determines a DC fault 21, which identifies a DC pole-to-ground fault of one of the DC poles 25 and 30. After determining the DC fault of one of the DC poles 25 and 30, the controller 95 controls the pilot valve of the second branch of each branch 70 in the MMC 80 to bypass the DC pole through the second branch of the pilot valve. Therefore, an effective protection device and method are provided for DC fault protection in a symmetrical monopole configuration.

[0044] Various embodiments of DC fault protection in a symmetrical single-pole configuration are disclosed in conjunction with the accompanying drawings later in this specification.

[0045] Figure 4 The functional components of the controller 95 are disclosed. The controller 95 is configured to be connected to each branch 70 of the MMC to protect the MMC to achieve DC fault protection in a symmetrical single-pole configuration. Figure 4 As shown, the branch 70 includes a second branch 55 having a first pilot valve 52 and a second pilot valve 54 .

[0046] Figure 4 The controller 95 shown includes a measurement module 90 , a determination module 91 , and a protection module 92 .

[0047] The measurement module 90 is configured to measure the DC pole voltage relative to the common ground. The DC pole voltage refers to the output voltage of the DC pole of the MMC relative to the common ground. The measurement module 90 receives a voltage parameter from the voltage measurement device 45. The voltage measurement device 45 can be configured to be connected to the DC pole of the MMC. In some examples, the voltage parameter from the voltage measurement device 45 can identify the output voltage of the DC pole. Based on the received voltage parameter, the measurement module 90 measures the DC pole voltage relative to the common ground.

[0048] The determination module 91 is configured to determine a DC fault that identifies a DC pole-to-ground fault of one of the DC poles. The determination module 91 determines whether the measured DC pole voltage relative to the common ground exceeds a predefined threshold voltage (e.g., 1.3-1.4 pu) with a dynamic margin. When it has been determined that one of the measured DC pole voltages relative to the common ground exceeds the predefined threshold voltage, the determination module 91 determines a DC fault of the corresponding DC pole.

[0049] Upon determining a DC fault in one of the DC poles, the protection module 92 is configured to control the pilot valves 52 and 54 of the second branch 55 of each branch 70 in the MMC to bypass the DC pole through the second branch 55 of the pilot valves 52 , 54 .

[0050] According to some embodiments, in order to control the pilot valves 52 and 54 of the second branch 55, the protection module 92 may determine a time interval for controlling the pilot valves of the second branch 55 of each branch 70 of the MMC. The protection module 92 may send a command sequence for controlling the pilot valves 52 and 54 of the second branch 55 of each branch 70 within the determined time interval to bypass the DC pole through the second branch 55 of the pilot valves 52, 54.

[0051] For example, the control of the bypass DC pole by the second branch 55 of the pilot valves 52 and 54 reduces the voltage of the DC poles other than the DC pole identified with the DC fault. The aforementioned voltage of the DC poles other than the DC pole identified with the DC pole increases due to the DC fault and exceeds the maximum voltage predefined for the DC pole. Therefore, controlling the voltage of the DC pole can protect the aforementioned DC pole from the influence of the determined DC fault of another DC pole.

[0052] Figure 5A and 5B 1 shows DC fault management in an electrical system 100. The electrical system 100 includes an MMC 80 arranged in a symmetrical single-pole configuration between an AC power source 15 and a transmission line 85. The MMC 80 is connected to the AC power source 15 via suitable components such as a transformer 12, a resistor 16, etc. The MMC 80 is connected to the transmission line 85 via DC poles 25 and 30 of the MMC 80. The transmission line 85 includes DC cables 25a and 30a adapted to transmit power / DC power with positive and negative polarities.

[0053] The MMC 80 described herein may be a series modular multi-level converter configured for voltage conversion. Figure 5A and 5B The illustrated MMC 80 includes a branch 70 for three AC phases 15a, 15b, and 15c. The branch 70 is connected in series between the DC poles 25 and 30 (+Udc and -Udc) of the MMC 80. In some examples, the branch 70 may be connected in series between the DC poles 25 and 30 via resistors 42 and 44. In addition, the DC poles 25 and 30 may be associated with capacitors 46 and 48.

[0054] Each phase leg 70 includes a first branch 65 having an upper arm 62 and a lower arm 64 connected in series. Each of the upper arm 62 and the lower arm 64 includes a submodule configured to contribute to the waveform of the output voltage of the MMC 80. In some examples, the submodule may include a full-bridge submodule / full-bridge inverter, such as Figure 5A and 5B As shown. The full-bridge submodule includes four switching elements with unit capacitors. In some examples, the submodule may include a half-bridge submodule / half-bridge inverter. The half-bridge submodule may include two switching elements with unit capacitors. Examples of switching elements described herein may include, but are not limited to, insulated gate bipolar transistors IGBTs, integrated gate commutated thyristors IGCTs, and the like.

[0055] The branch 70 further comprises a second branch 55 arranged to be connected in parallel to the first branch 65. The second branch 55 comprises a first pilot valve 52 and a second pilot valve 54. Each of the first pilot valve 52 and the second pilot valve 54 comprises a series connected anti-parallel thyristor pair (52a, 52b) / (54a, 54b).

[0056] Reference Figure 5A , the power system 100 includes a controller 95 connected to each branch 70 of the MMC 80. The controller 95 is configured to manage DC faults in the power system 100.

[0057] The controller 95 measures the DC pole voltage relative to the common ground (i.e., the output voltage of the DC poles 25 and 30 relative to the common ground). Based on the measured DC pole voltage, the controller 95 determines a DC fault 21 that identifies a DC pole-to-ground fault of one of the DC poles 25 and 30. For example, Figure 5A and 5B As shown, the controller 95 determines a DC fault 21 of a DC pole 25 (ie, the DC pole 25 here is an unhealthy DC pole). The DC fault 21 of the DC pole 25 increases the voltage of another DC pole 30, which may exceed the maximum voltage predefined for the DC pole 30, thereby damaging the healthy DC pole 30.

[0058] In order to protect the DC pole 30, the controller 95 controls the pilot valves 52 and 54 of each second branch 55 of each branch 70. In particular, in order to control the pilot valves 52 and 54 of the second branch 55 of each branch 70, the controller 95 determines a time interval and sends a command sequence for controlling the pilot valves 52 and 54 within the determined time interval. Controlling the pilot valves 52 and 54 may refer to turning on / firing each thyristor (52a, 52b, 54a and 54b) of each pilot valve 52 and 54 within the determined time interval, and controlling the gate voltage of each thyristor (52a, 52a, 54a and 54a) of each pilot valve 52 and 54. In some examples, the command sequence may indicate a voltage limit, and the gate voltage of each thyristor (52a, 52b, 54a and 54b) of each pilot valve 52 and 54 must be limited to the voltage limit. The voltage limit may be determined based on the withstand voltage of the thyristor. Similar to the protective firing of thyristors, the thyristor firing method proposed in this paper is executed very quickly.

[0059] like Figure 5BAs shown, such firing of the thyristors of the pilot valves 52 and 54 bypasses the DC poles 25 and 30 through the second branch 55 of the pilot valves 52, 54. As a result, the DC poles 25 and 30 can be short-circuited, which results in a lower voltage over the entire length of the DC cables 25a and 30a, and there may be no source of traveling waves, which may have a huge impact on the overvoltage in the middle of the DC cables 25a, 30a. The short-circuiting of the DC poles 25 and 30 further controls / reduces the voltage increase of the DC pole 30 caused by the DC fault 21 of the DC pole 25. Therefore, the healthy DC pole 30 can be protected from damage caused by the voltage increase relative to the ground.

[0060] In some examples, after MMC 80 is blocked, second branch 55 through pilot valves 52 and 54 may be activated to bypass DC poles 25 and 30 , thereby creating a more efficient bypassing scheme.

[0061] Understandably, Figure 5A and 5B In the embodiment, the DC fault 21 of the DC pole 25 is considered. However, the DC fault 21 may also occur in another DC pole 30. In this case, the controller 95 may perform the above-mentioned operation to protect the DC pole 25 from the DC fault 21 of the DC pole 30.

[0062] Figure 6 An exemplary implementation of a controller in programmable signal processing hardware is disclosed. Figure 6 The signal processing device 400 shown includes an input / output I / O section 410 for receiving voltage parameters of the DC pole of the MMC and sending a command sequence for controlling the pilot valve of the second branch of each branch in the MMC. The signal processing device 400 also includes a processor 420, a working memory 430, and an instruction storage device 440 storing computer-readable instructions, which, when executed by the processor 420, cause the processor 420 to perform the processing operations described below to protect the MMC for DC fault protection in a symmetrical single-pole configuration. The instruction storage device 440 may include a read-only memory ROM or a similar type of memory, and the computer-readable instructions may be input therein from a computer program product, such as a computer-readable storage medium 450, such as an optical disk, CD-ROM, etc., or a computer-readable signal 460 carrying computer-readable instructions.

[0063] In this embodiment, Figure 6 The combination of hardware components 470 shown, including a processor 420, a working memory 430 and an instruction storage device 440, is configured to implement the functions of the above-mentioned measurement module 90, determination module 91 and protection module 92, and will now be referred to Figure 7 Detailed description.

[0064] Figure 7 is a flow chart showing the method steps of a protection method 700 performed by a controller for protecting an MMC in a symmetrical single-pole configuration to achieve DC fault protection. Figure 2 , 3, 4 and 5a-5b are described in detail, and therefore, the repeated description of MMC is omitted here.

[0065] In step 701, the measurement module 90 measures the DC pole voltage relative to the common ground. In some embodiments, the measurement module 90 may receive voltage parameters from a voltage measurement device connected to the DC pole and measure the DC pole voltage relative to the common ground based on the received voltage parameters.

[0066] At step 702, the determination module 91 determines a DC fault based on the measured DC pole voltages, which identifies a DC pole-to-ground fault of one of the DC poles. In some embodiments, the determination module 91 may determine whether the measured DC pole voltage relative to the common ground exceeds a predefined threshold voltage. When it has been determined that one of the measured DC pole voltages relative to the common ground exceeds the predefined threshold voltage, the determination module may determine a DC fault of the corresponding DC pole.

[0067] After determining the DC fault of one of the DC poles, the protection module 92 controls the pilot valve of the second branch of each branch to bypass the DC pole through the second branch of the pilot valve to protect the DC poles other than the DC pole identified with the DC fault in step 706. The voltage of the DC poles other than the DC pole identified with the DC fault increases due to the DC fault and exceeds the maximum voltage predefined for the DC pole. Therefore, by controlling the pilot valve to bypass the DC pole, the overvoltage of the DC pole caused by the DC fault of another DC pole can be reduced.

[0068] In some embodiments, the protection module 92 may determine the time intervals and send a command sequence for controlling the pilot valve of the second branch of each branch within the determined time intervals. Thus, in a symmetrical unipolar configuration, the protection of the MMC for DC fault protection is improved.

[0069] Any appropriate steps, methods, features, functions, or benefits disclosed herein may be performed by one or more functional units or modules of one or more virtual devices. Each virtual device may include multiple of these functional units. These functional units may be implemented by a processing circuit, which may include one or more microprocessors or microcontrollers, and other digital hardware, which may include a digital signal processor DSP, dedicated digital logic, etc. The processing circuit may be configured to execute program codes stored in a memory, which may include one or more types of memory, such as a read-only memory ROM, a random access memory RAM, a cache, a flash memory device, an optical storage device, etc. The program code stored in the memory includes program instructions for executing one or more telecommunications and / or data communication protocols and instructions for executing one or more technologies described herein. In some implementations, according to one or more embodiments of the present disclosure, a processing circuit may be used to cause a corresponding functional unit to perform a corresponding function.

[0070] The above description of the specific embodiments will fully reveal the general nature of the embodiments herein, so that others can easily modify and / or adapt these specific embodiments to adapt to various applications by applying current knowledge without departing from the general concept, and therefore, these adaptations and modifications should and are intended to be understood within the meaning and scope of the equivalents of the disclosed embodiments. It should be understood that the words or terms used herein are for description rather than limitation. Therefore, although the embodiments herein are described according to preferred embodiments, those skilled in the art will recognize that the embodiments herein can be modified to practice within the scope of the present disclosure.

Claims

1. A protection method (700) for protecting a modular multilevel converter (MMC) (80) for direct current (DC) fault protection in a symmetrical unipolar configuration, wherein: The MMC (80) for each of at least one phase comprises: The branch (70) comprises: a first branch (65) having an upper arm (62) and a lower arm (64) connected in series, wherein the upper arm (62) and the lower arm (64) each include a submodule configured to contribute to a waveform of an output voltage of the MMC (80); and A second branch (55) having a first pilot valve (52) and a second pilot valve (54) connected in series; wherein the first branch (65) and the second branch (55) are connected in parallel; The branch (70) of the MMC (80) is connected in series between the DC poles (25, 30) of the MMC, and the method (700) comprises: measuring (702) a DC pole voltage relative to a common ground; determining (704) a DC fault (21) based on the measured DC pole voltage, the DC fault identifying a DC pole-to-ground fault of one of the DC poles (25, 30); and After a DC fault of a DC pole is determined, the pilot valve (52, 54) of the second branch (55) of each branch (70) is controlled (706) to bypass the DC pole (25, 30) through the pilot valve (52, 54) of the second branch (55) for protecting DC poles other than the DC pole identified with the DC fault.

2. The protection method (700) according to claim 1, wherein: The voltage of the DC poles other than the DC pole identified by the DC fault increases due to the DC fault (21) and exceeds a maximum voltage predefined for the DC pole.

3. The protection method (700) according to any one of the preceding claims, wherein: The step (706) of controlling the pilot valve (52, 54) of the second branch (55) of each branch (70) comprises: determining a time interval for controlling the pilot valve (52, 54) of the second branch (55) of each branch circuit (70); and A command sequence for controlling the pilot valve (52, 54) of the second branch (55) of each branch (70) is sent at determined time intervals.

4. The protection method (700) according to any one of the preceding claims, wherein: The step (704) of determining the DC fault (21) comprises: determining whether the measured DC pole voltage relative to common ground exceeds a predefined threshold voltage; and When it has been determined that one of the measured DC pole voltages relative to common ground exceeds said predefined threshold voltage, said DC fault of the corresponding DC pole is determined.

5. The protection method (700) according to any one of the preceding claims, wherein: The step (702) of measuring the DC pole voltage comprises: receiving a voltage parameter from a measurement device configured to be connected to the DC pole (25, 30); and The DC pole voltage is measured relative to a common ground based on the received voltage parameter.

6. A controller (95) for protecting a modular multilevel converter MMC (80) for DC fault protection in a symmetrical single-stage configuration, wherein: The MMC (80) for each of at least one phase comprises: The branch (70) comprises: a first branch (65) having an upper arm (62) and a lower arm (64) connected in series, wherein the upper arm (62) and the lower arm (64) each include a submodule configured to contribute to a waveform of an output voltage of the MMC (80); and A second branch (55) having a first pilot valve (52) and a second pilot valve (54) connected in series; wherein the first branch (65) and the second branch (55) are connected in parallel; The branch (70) of the MMC (80) is connected in series between the DC poles (25, 30) of the MMC, and the controller (95) comprises: A measurement module (90) configured to measure a DC pole voltage relative to a common ground; a determination module (91) configured to determine a DC fault (21) based on the measured DC pole voltage, the DC fault identifying a DC pole-to-ground fault of one of the DC poles (25, 30); and A protection module (92) is configured to control the pilot valve (52, 54) of the second branch (55) of each branch (70) after determining a DC fault of the DC pole, so as to bypass the DC pole (25, 30) through the pilot valve (52, 54) of the second branch (55) for protecting the DC poles other than the DC pole identified by the DC fault.

7. The controller (95) of claim 6, wherein: The voltage of the DC poles other than the DC pole identified with the DC pole increases due to the DC fault and exceeds a maximum voltage predefined for the DC pole.

8. The controller (95) according to any one of claims 6-7, wherein: The protection module (92) is configured to control the pilot valve (52, 54) of the second branch (55) of each branch (70) in the following manner: determining a time interval for controlling the pilot valve (52, 54) of the second branch (55) of each branch circuit (70); and A command sequence for controlling the pilot valve (52, 54) of the second branch (55) of each branch (70) is sent at determined time intervals.

9. The controller (95) according to any one of claims 6 to 8, wherein: The determination module (91) is configured to determine the DC fault (21) in the following manner: determining whether the measured DC pole voltage relative to common ground exceeds a predefined threshold voltage; and The DC fault of the DC pole is determined when it has been determined that a measured DC pole voltage relative to a common ground corresponding to the pole exceeds the predefined threshold voltage.

10. The controller (95) according to any one of claims 6 to 9, wherein: The measuring module (90) is configured to measure the DC pole voltage by: receiving a voltage parameter from a measurement device configured to be connected to the DC pole (25, 30); and The DC pole voltage is measured relative to a common ground based on the received voltage parameter.

11. An apparatus (400) comprising the controller (95) according to any one of claims 6 to 10.

12. An electric power system (100), comprising: A transmission line (85) comprising a direct current (DC) cable (25a, 30a); A modular multilevel converter MMC (80) arranged in a symmetrical monopolar configuration between a first terminal and a second terminal of the transmission line (85) for transmitting power through the transmission line (85), wherein the MMC (80) for each of at least one phase comprises a branch (70), Wherein, the branch (70) comprises: a first branch (65) having an upper arm (62) and a lower arm (64) connected in series, wherein the upper arm (62) and the lower arm (64) each include a submodule configured to contribute to a waveform of an output voltage of the MMC (80); and A second branch (55) having a first pilot valve (52) and a second pilot valve (54) connected in series; wherein the first branch (65) and the second branch (55) are connected in parallel; wherein the branch (70) of the MMC (80) is connected in series between the DC poles (25, 30) of the MMC; and A controller (95) according to any one of claims 6 to 10, the controller (95) being configured to be connected to each branch (70) of the MMC (80).

13. The power system (100) according to claim 12, wherein: The first pilot valve (52) and the second pilot valve (54) each include a pair of anti-parallel thyristors ((52a, 52b), (54a, 54b)) connected in series.

14. The power system (100) according to any one of claims 12-13, wherein: The power system (100) is a high voltage direct current (HVDC) system.

15. A computer program product comprising a non-transitory computer readable medium having a computer program including program instructions thereon, the computer program being loadable into a data processing unit and configured to execute the method according to any one of claims 1 to 5 when the data processing unit runs the computer program.

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