A bypass method and system for a fault sub-module of an MMC type converter

Abstract

A bypass method and system for a fault sub-module of an MMC converter. The bypass method comprises: after detecting that the self sub-module has a fault, triggering an IGBT conduction command by a local control board of the sub-module, and sending a closing instruction of a bypass switch. The bypass method further comprises: a bypass state detection method for detecting a return state of the bypass switch by the local control board of the fault sub-module; and if an electrical signal of the bypass switch state cannot be detected, the state of the bypass switch is estimated. The present application can realize reliable bypass when a sub-module of the MMC converter has a fault, and realize rapid detection of the bypass state when the bypass state cannot be sent, so as to ensure the continuous operation of the MMC converter.

Description

Technical Field

[0001] This invention belongs to the field of power electronic control technology, and more specifically, relates to a bypass method and system for a fault submodule of an MMC converter. Background Technology

[0002] With the rapid growth of urban areas and the rapid development of information technology, the number of sensitive loads, nonlinear loads, and other important loads in the power grid is increasing, exacerbating problems such as high line losses and tight power supply corridors in AC distribution networks. Voltage fluctuations, increased grid harmonics, instantaneous voltage drops, and worsening three-phase imbalances are also worsening. The existing distribution network structure and methods urgently need to be changed. DC distribution networks offer advantages such as reduced line losses, improved power quality on the user side, increased power supply capacity, isolation of fault areas, and convenient and flexible access for renewable energy and energy storage devices. Therefore, MMC converters are widely used in DC transmission and distribution systems.

[0003] The smallest unit of an MMC converter is the submodule that makes up the converter valve, and the bypass switch is an indispensable key component of the submodule. When a submodule fails, the controller of that submodule issues a command to close the bypass switch, bypassing the submodule and thus isolating the impact of the fault on the system. If the bypass switch cannot accurately and quickly close, the faulty submodule is considered uncontrolled, and the fault may escalate, causing the system to trip and requiring a power outage for maintenance. Therefore, the reliable operation of the submodule bypass switch is the guarantee of the safe operation of the entire flexible DC transmission system. In existing technologies, the isolation of faulty submodules relies entirely on the bypass switch, and the success of the bypass switch's closing depends entirely on its feedback status.

[0004] Existing methods for bypassing MMC power modules involve a bypass switch malfunction or voltage rise to the overvoltage detection circuit board threshold. When the bypass switch fails to operate or the voltage rises to the threshold, the overvoltage detection circuit board sends a request to the power device control and drive board of the adjacent normal power module to trigger a bypass command for the current power module. Upon receiving the command, the power device control and drive board triggers the corresponding switch module of the faulty power module to form a bypass channel. Specifically, when the faulty power module is a full-bridge power module, the corresponding switch modules are T2 and T4 or T1 and T3; when the faulty power module is a half-bridge power module, the corresponding switch module is T2. The drawback is that each power module requires a bypass switch to trigger adjacent modules and a drive circuit for the power devices, significantly increasing the manufacturing cost and size of the power modules. The improvement of this invention compared to the prior art is that it achieves power module bypass through the power devices of the faulty module itself, ensuring that the module does not lose power or experience overvoltage after a failure, and it monitors the bypass status of the module in real time. The advantages of this invention compared to prior art document 1 are that the MMC device has lower cost, smaller size, higher reliability, and requires no additional redundant drive or power supply circuits. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a novel method for bypassing and detecting the bypass status of faulty submodules in MMC converters. This method ensures that the bypass operation of the faulty submodule does not entirely depend on the bypass switch, and the detection of the bypass status does not entirely depend on the open / close position signal of the bypass switch, thereby improving the reliability of the converter and enhancing the continuity of grid operation. Furthermore, a bypass method and system for faulty submodules in MMC converters are proposed.

[0006] The present invention adopts the following technical solution.

[0007] A bypass method for a fault submodule of an MMC type converter, characterized in that the bypass method includes: Step 1: After detecting a fault in its own submodule, the local control board of the submodule triggers the IGBT turn-on command corresponding to the submodule bypass and simultaneously sends the bypass switch closing command.

[0008] Step 2: The submodule detects the feedback status of the bypass switch. If the submodule does not detect that the feedback status of the bypass switch is closed, it triggers the submodule to automatically switch between the locked state and the bypass state according to the module voltage Udcx of the submodule.

[0009] Step 3: Set a fixed value Udc_up as the charging limit of the faulty submodule. When the voltage Udcx of the faulty submodule exceeds Udc_up, the local control board of the submodule controls the faulty submodule to enter the bypass state. Step 4: set a fixed value Udc_dn as the discharge lower limit of the faulty submodule. When the voltage Udcx of the faulty submodule is lower than Udc_dn, the local control board of the submodule controls the faulty submodule to enter the blocked state, and the submodule capacitor is charged through the anti-parallel diode of the IGBT.

[0010] Preferably, the submodules of the MMC-type converter include full-bridge submodules and half-bridge submodules, wherein the full-bridge module comprises a DC bus capacitor (C), a bypass switch (K), 4 sets of IGBTs (T1~T4) and anti-parallel diodes (D1~D4), and the half-bridge module comprises a DC bus capacitor (C), a bypass switch (K), 2 sets of IGBTs (T1~T2) and anti-parallel diodes (D1~D2). The bypass state of the full-bridge submodule is: T1 and T3 are conducted while T2 and T4 are not conducted, or T2 and T4 are conducted while T1 and T3 are not conducted; the bypass state of the half-bridge submodule is: T2 is conducted while T1 is not conducted.

[0011] Preferably, the blocked state means that all IGBTs are not conducted.

[0012] Preferably, in the step 2, the voltage Udcx of the submodule in the blocked state is calculated through the sampling values of modulation wave, bridge arm current, bridge arm inductor and AC line voltage. Preferably, step 2 comprises: Step 2.1: detect the return state of the bypass switch through the local control board of the faulty submodule; Step 2.2: if the electrical signal of the bypass switch state cannot be detected, estimate the state of the bypass switch.

[0013] Preferably, the estimation method in said step 2.2 is: Step 2.2.1: determine whether the voltage Udcx of the submodule in the blocked state exceeds the voltage set value Udc_set. If Udcx>Udc_set, set the uncontrolled charging flag MxChargeFlag of the faulty submodule bypassed by IGBT to 1; if Udcx<Udc_set and the duration for which MxChargeFlag is set to 1 is greater than or equal to 1s, reset MxChargeFlag to 0; Step 2.2.2: when the first falling edge of MxChargeFlag occurs, start timing the black module detection period of 30s, detect the number of falling edges FallEdgeCnt of MxChargeFlag within 30s. If FallEdgeCnt is greater than 3 times within 30s, it is determined that the bypass switch fails to close successfully; otherwise, it is determined that the bypass switch closes successfully; Step 2.2.3: After 30 seconds, reset FallEdgeCnt to 0.

[0014] Preferably, the bypass method can be applied to bypass all faults of the submodules of the MMC converter, including but not limited to: capacitor overvoltage in the submodule, communication failure, overtemperature failure, and undervoltage failure.

[0015] This invention also discloses a bypass system for a fault submodule of an MMC converter, the bypass system operating using the bypass method for a fault submodule of an MMC converter described above, the bypass system comprising: A bypass action module, comprising a mechanical switch and an IGBT, is used to bypass a faulty submodule; The bypass status detection module is used to detect the bypass status and check whether the bypass is successful after the bypass command is issued by the faulty submodule. The submodule local control board is used to generate control commands to control the mechanical switches and IGBTs of the submodule for on / off and interlocking.

[0016] The beneficial effects of the present invention are as follows: Compared with the prior art, the present invention has the following advantages: This invention can be applied to bypassing and bypass status detection of submodules in a chain-structure converter system with a similar topology. Firstly, by applying the bypass method of this invention, when the bypass switch K can bypass normally, the bypass IGBT operates faster because the bypass commands of the bypass IGBT and the bypass switch are triggered simultaneously. This effectively reduces the inrush current when the bypass switch closes, extending the service life of the bypass switch. When the bypass switch K itself fails and closing fails, bypassing using existing IGBTs can quickly disconnect the faulty module. Furthermore, the faulty module can switch between bypass and lockout states based on its own situation without shutting down the MMC converter.

[0017] Secondly, by applying the bypass status detection method of this invention, bypass status detection can still be achieved even when the bypass switch fails to operate or the bypass switch status does not provide normal feedback. Even when the bypass switch operates normally but no bypass switch closing signal is detected, the MMC converter can still determine that the faulty module bypassed successfully, allowing the MMC to continue operating and improving the converter's reliability. Attached Figure Description

[0018] Figure 1 Topology diagrams for MMC type converters and full / half-bridge submodules are provided. Figure 2 This is a schematic diagram of a bypass action module and a bypass status detection module for a fault submodule of an MMC type converter. Figure 3A flowchart of the bypass method for the faulty submodule; Figure 4 This is a flowchart of the bypass status detection method for faulty submodules. Detailed Implementation

[0019] The present application will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, and should not be construed as limiting the scope of protection of the present application.

[0020] The main circuit topology of the MMC converter system includes six bridge arms, each consisting of N power submodules and one bridge arm inductor connected in series. The submodules include full-bridge and half-bridge submodules. A full-bridge module includes four IGBTs, anti-parallel diodes, a DC bus capacitor, and a bypass switch. A half-bridge module includes two IGBTs, anti-parallel diodes, a DC bus capacitor, and a bypass switch. The IGBTs and bypass switches within each submodule are controlled by pulse signals output from a local control board located within the submodule for on / off switching. Figure 1 As shown.

[0021] This disclosure discloses a bypass method and system for the fault submodule of an MMC type converter, such as... Figure 2 As shown, it includes: a bypass action module and a bypass status detection module. The detailed design of these two functional modules is described below: The bypass action module is mainly for implementing the bypass method of the MMC-type fault submodule. The specific bypass method process is as follows: Figure 3 As shown, the bypass action module, based on the traditional bypass switch operation, adds the function of forming a bypass loop by utilizing the IGBT conduction of the MMC power submodule, thereby realizing the bypass disconnection function of the faulty submodule. Specifically, the full-bridge submodule uses the conduction of T1 and T3, or T2 and T4, to form the bypass loop, while the half-bridge submodule uses the conduction of T2. Thus, even when the bypass switch of the faulty submodule fails to operate, the bypass disconnection function of the faulty submodule can still be achieved. Control commands are generated by the submodule local control board to control the IGBT conduction and latch-up of the submodule. Since the control power of the submodule local control board is taken from the DC bus capacitor of the submodule, it is necessary to monitor the capacitor voltage of the submodule after bypassing it using the IGBT. If the capacitor voltage of the submodule is insufficient to support the normal operation of the submodule local control board power supply, the capacitor of the faulty submodule needs to be charged. Therefore, two setpoints, Udc_up and Udc_dn, are set as the upper limit for charging and the lower limit for discharging the faulty submodule. When the voltage Udcx of the faulty submodule is greater than Udc_up, the faulty submodule is bypassed using IGBTs; when the voltage Udcx of the faulty submodule is less than Udc_dn, the faulty submodule blocks all IGBTs and charges through anti-parallel diodes.

[0022] The bypass state detection module mainly implements the following functions: after a bypass command for a faulty sub-module is issued, it detects the bypass state and checks whether the bypass is successful. The specific process is as Figure 4 shown. On the basis of the traditional method that only detects the return state of the bypass switch, the bypass state detection module adds estimation of the bypass switch state. When the local control board of the faulty sub-module cannot detect the electrical signal of the bypass switch state, the state of the bypass switch can be obtained through an estimation method, which avoids direct fault shutdown when the bypass switch has been successfully closed but the bypass switch state is not collected. The specific algorithm steps of the estimation method are as follows: 1) Calculate the voltage Udcx of the sub-module in the blocked state through the sampled values of the modulation wave, bridge arm current, bridge arm inductance and AC line voltage. Taking the blocked sub-module located on the upper bridge arm of phase A or phase B as an example, the calculation formula is as follows:

[0023] In the formula, Uab is the sampled value of AB phase line voltage, Lm is the bridge arm inductance, Iaup is the A-phase upper bridge arm current, Ibup is the B-phase upper bridge arm current, Modaup is the A-phase upper bridge arm modulation wave, and Modbup is the B-phase upper bridge arm modulation wave. Similarly, the voltage of sub-modules in blocked state on other bridge arms can be calculated.

[0024] 2) Determine whether the blocked sub-module voltage Udcx exceeds the voltage set value Udc_set (usually set to 0.5 times the rated capacitance voltage of a single sub-module). If Udcx > Udc_set, set the uncontrolled charging flag MxChargeFlag of the faulty sub-module bypassed by IGBT to 1; if Udcx < Udc_set and the duration for which MxChargeFlag is set to 1 is greater than or equal to 1s, reset MxChargeFlag to 0.

[0025] 3) When the first falling edge of MxChargeFlag occurs, start timing the black module detection period of 30s, and detect the number of falling edges FallEdgeCnt of MxChargeFlag within 30s. If FallEdgeCnt is greater than 3 times within 30s, it is determined that the bypass switch closing has failed; otherwise, it is determined that the bypass switch closing has succeeded.

[0026] 4) After 30s, reset FallEdgeCnt to 0.

[0027] The applicant of this invention has provided a detailed description of the embodiments of the invention in conjunction with the accompanying drawings. However, those skilled in the art should understand that the above embodiments are merely preferred embodiments of the invention. The detailed description is only intended to help readers better understand the spirit of the invention and is not intended to limit the scope of protection of the invention. On the contrary, any improvements or modifications made based on the inventive spirit of the invention should fall within the scope of protection of the invention.

Claims

1. A bypass method for a fault submodule of an MMC type converter, characterized in that, The bypass method comprises: Step 1: after the local control board of the submodule detects that its own submodule fails, trigger the IGBT corresponding to the submodule bypass to conduct, and simultaneously send a bypass switch closing command; Step 2: the submodule detects the feedback state of the bypass switch, if the submodule does not detect that the feedback state of the bypass switch is the closed position, trigger the submodule to automatically switch between the blocked state and the bypass state according to the module voltage Udcx of the submodule; Step 2 comprises the following steps: Step 2.1: detect the feedback state of the bypass switch through the local control board of the faulty submodule; Step 2.2: if the electrical signal of the bypass switch state cannot be detected, estimate the state of the bypass switch; The estimation method in said Step 2.2 is: Step 2.2.1: determine whether the voltage Udcx of the submodule in the blocked state exceeds the voltage set value Udc_set, if Udcx>Udc_set, set the flag MxChargeFlag for uncontrolled charging of the faulty submodule bypassed by IGBT to 1; if Udcx<Udc_set and the duration for which MxChargeFlag is set to 1 is greater than or equal to 1s, reset MxChargeFlag to 0; Step 2.2.2: when the first falling edge of MxChargeFlag occurs, start timing the black module detection period of 30s, detect the number of falling edges FallEdgeCnt of MxChargeFlag within 30s, if FallEdgeCnt is greater than 3 times within 30s, it is determined that the bypass switch has not been closed successfully, otherwise it is determined that the bypass switch has been closed successfully; Step 2.2.3: after 30s, reset FallEdgeCnt to 0.

2. The bypass method for a fault submodule of an MMC converter according to claim 1, characterized in that, The bypass method comprises: Step 3: set a fixed value Udc_up as the charging upper limit of the faulty submodule, when the voltage Udcx of the faulty submodule exceeds Udc_up, the local control board of the submodule controls the faulty submodule to enter the bypass state; Step 4: set a fixed value Udc_dn as the discharging lower limit of the faulty submodule, when the voltage Udcx of the faulty submodule is lower than Udc_dn, the local control board of the submodule controls the faulty submodule to enter the blocked state, and the submodule capacitor is charged through the anti-parallel diode of the IGBT.

3. The bypass method for a faulty submodule of an MMC-type converter according to claim 1, characterized in that, the submodules of the MMC-type converter comprise full-bridge submodules and half-bridge submodules, wherein, said full-bridge submodule comprises a DC bus capacitor (C), a bypass switch (K), four IGBTs (T1~T4) and anti-parallel diodes (D1~D4), said half-bridge submodule comprises a DC bus capacitor (C), a bypass switch (K), two IGBTs (T1~T2) and anti-parallel diodes (D1~D2), the bypass state of said full-bridge submodule is: T1 and T3 conduct while T2 and T4 do not conduct, or T2 and T4 conduct while T1 and T3 do not conduct; The bypass state of the half-bridge submodule is: T2 is on while T1 is off.

4. A bypass method for a fault submodule of an MMC converter according to claim 1, characterized in that, The latching state is that all IGBTs in the submodule are not conducting.

5. A bypass method for a fault submodule of an MMC converter according to claim 1, characterized in that, In step 2, the voltage Udcx of the submodule in the locked state is calculated using the modulation wave, bridge arm current, bridge arm inductance, and AC line voltage sampling value.

6. A bypass method for a fault submodule of an MMC converter according to claim 1, characterized in that, The bypass method can be applied to bypass all faults in the submodules of the MMC converter, including but not limited to: capacitor overvoltage in the submodule, communication failure, overtemperature failure, and undervoltage failure.

7. A bypass system for a fault submodule of an MMC converter, the bypass system operating using the bypass method for a fault submodule of an MMC converter as described in any one of claims 1-6, characterized in that, The bypass system includes: A bypass action module, comprising a mechanical switch and an IGBT, is used to bypass a faulty submodule; The bypass status detection module is used to detect the bypass status and check whether the bypass is successful after the bypass command is issued by the faulty submodule. The submodule local control board is used to generate control commands to control the mechanical switches and IGBTs of the submodule for on / off and interlocking.

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