MMC power module overvoltage protection circuit and multi-stage bypass method

Abstract

This invention discloses an overvoltage protection circuit and multi-stage bypass method for an MMC power module. The circuit includes a bypass switch connected in parallel across the MMC power module. When the capacitor voltage of the MMC power module exceeds a set safety value, the bypass switch closes, bypassing the MMC power module. A thyristor is also connected in parallel across the MMC power module. When the thyristor breaks down due to overvoltage, a reliable short-circuit path is formed, bypassing the MMC power module and stopping the capacitor charging. The breakdown voltage of the thyristor is greater than the set safety value but less than the rated voltage of the IGBT in the MMC power module. This invention, through the double protection of the bypass switch and thyristor, ensures that the IGBT of the power module is not uncontrolled. When the power module is continuously charged, it is reliably bypassed, preventing the IGBT and capacitor from exploding, thus not affecting other power modules and the continued operation of the MMC converter valve flexible DC system, thereby improving system reliability.

Description

Technical Field

[0001] This invention relates to the field of power technology, specifically to an overvoltage protection circuit for an MMC power module and a multi-stage bypass method. Background Technology

[0002] With the development of high-power flexible DC transmission technology, modular multilevel converters (MMCs) are increasingly being used in engineering projects due to their advantages such as low switching frequency, low loss, and flexible control. Each arm of this type of converter has hundreds of power modules connected in series. Each power module can adopt a full-bridge or half-bridge structure. When a power module becomes uncontrollable due to a failure in the power supply or main control board, the operating capacitor will be continuously charged. This continuous charging current will cause the capacitor voltage to exceed its tolerance range, eventually leading to insulation breakdown, internal vaporization causing capacitor expansion, and even potential explosion, posing a significant hazard to surrounding equipment. Summary of the Invention

[0003] To address the issue of capacitors in existing MMC power modules potentially being continuously charged during operation, this invention provides an overvoltage protection circuit and a multi-stage bypass method for MMC power modules to prevent the phenomenon of capacitors being continuously charged.

[0004] To achieve the above objectives, the technical solution of the present invention is as follows:

[0005] In a first aspect, embodiments of the present invention provide an overvoltage protection circuit for an MMC power module, comprising:

[0006] A bypass switch is connected in parallel across the two ends of the MMC power module; when the capacitor voltage of the MMC power module exceeds the set safety value, the bypass switch closes, causing the MMC power module to bypass.

[0007] A thyristor is connected in parallel across the two ends of the MMC power module. When the thyristor breaks down due to overvoltage, a reliable short-circuit path is formed, causing the MMC power module to bypass and the capacitor of the MMC power module to stop charging. The breakdown voltage of the thyristor is greater than a set safety value but less than the rated voltage of the IGBT of the MMC power module.

[0008] Secondly, this invention provides an example of a multi-stage overvoltage bypass method for an MMC power module, comprising:

[0009] A bypass switch and a thyristor are connected in parallel at both ends of the MMC power module;

[0010] First bypass: The voltage value across the capacitor of the MMC power module detected by the voltage sampling circuit is transmitted to the main control board. The main control board determines whether the voltage value across the capacitor has reached the first overvoltage setting value V1. If it has, the main control board sends a signal to trigger the bypass switch to bypass the MMC power module.

[0011] Second bypass: After the first overvoltage bypass fails due to a main control board malfunction, the voltage across the capacitor is directly detected by two hardware overvoltage detection circuits and compared with the second overvoltage setting value V2. Only when the voltage values ​​detected by the two hardware overvoltage detection circuits are greater than the second overvoltage setting value V2 will a signal be sent to trigger the bypass switch, so that the MMC power module is bypassed.

[0012] Third bypass: After the first and second overvoltage bypass methods fail, when the capacitor voltage exceeds the breakdown voltage value V3 of the thyristor, the thyristor is broken down, and the power module is bypassed.

[0013] Among them, V1 <V2<V3。

[0014] Compared with the prior art, the advantages of this invention are as follows:

[0015] This invention provides dual protection through a bypass switch and a thyristor, ensuring that the IGBT of the power module is uncontrolled. When the power module is continuously charged, it is reliably bypassed, preventing the IGBT and capacitor from exploding and affecting other power modules. This also ensures the continued operation of the MMC converter valve flexible DC system, thereby improving system reliability. Attached Figure Description

[0016] Figure 1 A schematic diagram of an MMC power module overvoltage protection circuit applied to a full-bridge power module;

[0017] Figure 2 A schematic diagram of an MMC power module overvoltage protection circuit applied to a half-bridge power module;

[0018] Figure 3 A flowchart of the multi-stage overvoltage bypass method for MMC power modules;

[0019] Figure 4 This is a control principle diagram for the multi-stage overvoltage bypass method of the MMC power module. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0021] Example 1:

[0022] The overvoltage protection circuit for the MMC power module provided in this example mainly includes a bypass switch K and a thyristor T. The bypass switch K is connected in parallel across the two ends of the MMC power module. When the voltage across the capacitor C of the MMC power module exceeds a set safe value, the bypass switch closes, causing the MMC power module to bypass and stopping the charging of capacitor C. This effectively prevents the continuous charging of capacitor C and ensures safety.

[0023] When the bypass switch K fails to close in time, since the thyristor T is also connected in parallel across the two ends of the MMC power module, as the capacitor C continues to charge, its voltage continuously increases. Once the voltage reaches the breakdown voltage of the thyristor T, the thyristor breaks down due to overvoltage, forming a reliable short circuit path, thus bypassing the MMC power module and stopping the capacitor of the MMC power module from charging. Of course, the breakdown voltage of the thyristor is greater than the set safety value but less than the rated voltage of the IGBT of the MMC power module to prevent the IGBT from breaking down before the thyristor.

[0024] As a preferred embodiment of this example, the thyristor T mentioned above is a gate-short-circuited thyristor. Since it is directly gate-short-circuited, no trigger circuit is required. It automatically breaks down when the voltage reaches the breakdown value, serving as a backup bypass with higher reliability.

[0025] like Figure 1 As shown, when the MMC power module is a full-bridge power module, the thyristor uses two bidirectional thyristors connected in anti-parallel; for example... Figure 2 Therefore, when the MMC power module is a full-bridge power module and is used on the rectifier side, a bidirectional bypass thyristor is selected, while when it is used on the inverter side, a unidirectional bypass thyristor is used to further improve economy and safety.

[0026] Example 2:

[0027] The multi-stage overvoltage bypass method for MMC power modules provided in this embodiment first connects a bypass switch K and a thyristor T in parallel across the two ends of the MMC power module. Then, a three-stage bypass method is adopted. The first-stage bypass is triggered by the software in the main control board through calculation. The second-stage bypass is triggered by the output of the hardware comparison circuit to the bypass switch K. The third-stage backup bypass uses a hard-breakdown type thyristor connected in parallel across the two ends of the power module, with the gates of the thyristor short-circuited. The hardware configuration of the three-stage bypass method is described in [link to hardware configuration]. Figure 1 Or as shown in Figure 2. When the active board fails and the bypass switch fails to operate, the capacitor voltage rises to the backup protection value. The parallel bypass thyristor breaks down due to overvoltage, forming a reliable short-circuit path, bypassing the power module. The remaining power modules are unaffected, and the system operates reliably. For example... Figure 3 As shown, the power module is equipped with three overvoltage bypasses as follows:

[0028] First bypass: When the main control board detects that the capacitor voltage reaches the first overvoltage set value V1 through the voltage sampling circuit, it determines through the software written in the FPGA and sends a signal to trigger the bypass switch to bypass the power module.

[0029] Second bypass: After the first overvoltage bypass fails due to a main control board failure, when the capacitor voltage reaches the second overvoltage set value V2, the voltage across the capacitor is directly compared with the set value through two hardware overvoltage detection circuits. The bypass switch can be triggered only after the two hardware voltage detection circuits send trigger signals simultaneously to bypass the power module.

[0030] Third bypass: After the first and second overvoltage bypass methods both fail, when the capacitor voltage reaches the third backup protection set value V3, the thyristors in parallel with the power module are directly broken down without an additional control circuit to achieve reliable bypass.

[0031] As Figure 4 shown, it is a specific bypass control schematic diagram. The hardware overvoltage detection circuit is set to two paths. To prevent mis-triggering of one path of overvoltage detection and improve bypass reliability, the two hardware overvoltage detection circuits determine overvoltage simultaneously and then send a bypass trigger signal through the AND gate output.

[0032] The bypass switch trigger circuit is two paths. To improve trigger reliability and prevent the failure of one trigger circuit, any one trigger circuit can trigger the bypass switch when it receives a trigger command.

[0033] After the first or second overvoltage bypass switch triggers and acts, the auxiliary contacts also act. The action signal is sent back to the control board through the detection circuit, and the control board uploads it to the valve control device through the reporting optical fiber.

[0034] The relationship between the voltage set values of the three bypasses is V1 < V2 < V3. Only when the bypass switch refuses to operate, the third overvoltage bypass thyristor will be directly broken down to bypass the power module.

[0035] For the third bypass method, the gate of the bypass thyristor is short-circuited without an additional trigger circuit to prevent the reduction of reliability caused by the failure of the trigger circuit. When the voltage exceeds the breakdown value of the thyristor, the thyristor is broken down to form a reliable path.

[0036] For the third bypass method of the bypass thyristor, the breakdown value should be greater than the second bypass set value but also less than the rated voltage value of the IGBT to prevent the IGBT from being broken down before the thyristor.

[0037] For the third bypass method of the bypass thyristor, a bidirectional thyristor with two thyristors in anti-parallel is used for the full-bridge power module. When the half-bridge power module is used on the rectifier side, a bidirectional bypass thyristor is selected, and a unidirectional bypass thyristor can be used on the inverter side.

[0038] The third bypass method bypasses the thyristor without requiring a control circuit and an energy extraction circuit, resulting in high reliability. As the final bypass for the power module, it can reliably bypass the power module.

[0039] The bypass thyristor described in the third bypass method has long-term reliable current-carrying characteristics, and can be maintained until the next maintenance without stopping the system, thus improving the availability and reliability of the flexible DC system.

[0040] Therefore, the MMC power module overvoltage multi-stage bypass method provided in this embodiment has the following technical advantages compared with the prior art:

[0041] 1. By setting up three bypasses, the reliability of the power module bypass is greatly improved, avoiding the shutdown of the flexible DC system due to unreliable bypass of the power module.

[0042] 2. By setting up two voltage hardware detection circuits and using AND gate outputs, the bypass switch is prevented from closing due to malfunction of a single hardware detection circuit.

[0043] 3. By setting up two bypass switch trigger circuits, the trigger signals are all connected to the bypass switches. If either trigger circuit receives a trigger signal, it can trigger the bypass switch, thus improving the reliability of the bypass operation.

[0044] 4. The third bypass thyristor is directly short-circuited at the gate, requiring no trigger circuit. It automatically breaks down when the voltage reaches the breakdown value, serving as a backup bypass with higher reliability.

[0045] Through the above measures, the IGBTs of the power module are kept uncontrolled. When the power module is continuously charged, it is reliably bypassed, and the IGBTs and capacitors do not explode, thus not affecting other power modules and the continued operation of the MMC converter valve flexible DC system, thereby improving system reliability.

[0046] The above embodiments are merely illustrative of the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made based on the essence of the content of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for overvoltage multistage bypass of MMC power modules, characterized in that, The application relates to an overvoltage protection method for an MMC power module. A bypass switch and a thyristor are connected in parallel at two ends of the MMC power module; First bypass: the voltage value between the two ends of the capacitor of the MMC power module detected by a voltage sampling loop is transmitted to a main control board, and whether the voltage value between the two ends of the capacitor reaches a first overvoltage setting value V1 is judged by the main control board; if yes, the main control board sends a signal to trigger the bypass switch, so that the MMC power module is bypassed; Second bypass: after the first overvoltage bypass fails due to main control board failure, the voltage between the two ends of the capacitor is directly detected by two overvoltage detection loops, and the detected voltage is compared with a second overvoltage setting value V2; when the voltage detected by the two overvoltage detection loops is greater than the second overvoltage setting value V2, a signal is sent to trigger the bypass switch, so that the MMC power module is bypassed; Third bypass: after the first and second overvoltage bypass methods fail, when the voltage of the capacitor exceeds the breakdown voltage value V3 of the thyristor, the thyristor is broken down, and the power module is bypassed; Wherein, V1 < V2 < V3 < the rated voltage value of the IGBT of the MMC power module; When the MMC power module is a full-bridge power module, the thyristor is a bidirectional thyristor with two thyristors in anti-parallel connection; When the MMC power module is a full-bridge power module and is used in the rectification side, a bidirectional bypass thyristor is selected, and when the MMC power module is used in the inverter side, a unidirectional bypass thyristor is selected.

2. The MMC power module overvoltage multi-stage bypass method of claim 1, wherein, The thyristor is a gate short thyristor.

Images