A circuit topology to prevent capacitor shoot-through discharge in the event of a power module failure.

Abstract

This invention discloses a circuit topology to prevent capacitor direct discharge during power module failure. A series component is connected in series in the circuit consisting of capacitor C, bypass switch K1, and the upper conduit. The component includes a mechanical switch K2 and two parallel branches with identical structures but opposite conduction directions. Each branch includes a fully controlled device and a diode connected in series with the fully controlled device. This invention solves the problem of high current caused by closing the bypass switch at the lower conduit when the upper conduit fails to short-circuit; it also solves the problem of how to keep the system running when the bypass switch fails to operate. The overall solution has low current stress requirements on components and requires no heat dissipation.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC transmission technology, specifically relating to a circuit topology that avoids direct discharge of capacitors when power module fails. Background Technology

[0002] Flexible DC transmission technology currently mainly adopts a modular multilevel cascaded topology. To improve reliability, a certain number of redundant sub-modules are generally configured in the bridge arm. When a sub-module fails, a bypass switch is used to bypass the faulty sub-module to ensure that the system continues to operate.

[0003] With the development of flexible DC converter valves, the requirements for the certainty of the state after a module failure are becoming increasingly stringent, and the handling of failures is becoming more and more refined, such as... Figure 1 As shown, the existing technology's protection measure for submodule failures involves connecting a bypass switch in parallel at the lower tube. When a fault occurs in the submodule and a short circuit occurs at the upper tube, directly closing the bypass switch at the lower tube will cause the capacitor to discharge directly through the upper tube and the bypass switch, with a current exceeding hundreds of kiloamperes. If this discharge circuit is not interrupted, it will cause serious damage to components and busbars within the short-circuit path, and may even result in a broken current circuit and system malfunction. Furthermore, if the bypass switch fails to operate for some reason, the submodule cannot be taken out of service. Figure 3 As shown, the DC capacitor of the submodule will be continuously charged until it exceeds the device's withstand stress, which will also cause short-circuit discharge through the capacitor. In severe cases, the system will not be able to work properly.

[0004] Chinese Patent Publication No. CN109149974B, entitled "An MMC Half-Bridge Submodule with Short-Circuit Protection Function for Upper Switch Transistor," describes a protection method, such as... Figure 2 As shown, by connecting switching devices in the main circuit of the submodule, a large current is avoided when closing the bypass switch at the lower tube when the upper tube is short-circuited. However, the switching devices also participate in the operation of the system when the submodule is not faulty. The power requirements of the devices are the same as those of the main power devices, and a large amount of conduction loss and switching loss will be generated. At the same time, the device group has high requirements for the operating current and requires heat dissipation, which brings additional economic burden. Furthermore, no protection measures are considered after the bypass switch fails to operate. Summary of the Invention

[0005] The purpose of this invention is to provide a circuit topology that avoids direct discharge of capacitors when power module fails, in order to solve the problem of high losses of switching devices caused by connecting switching devices in the main circuit of the submodule when the pipeline on the submodule fails, requiring heat dissipation and failing to solve the protection problem when the bypass switch fails to operate.

[0006] To solve the above problems, the present invention adopts the following technical solution;

[0007] A circuit topology for preventing direct discharge of capacitors in the event of a power module failure includes an upper circuit, a lower circuit, a bypass switch K1, a capacitor C, and a group of components. The bypass switch K1 is connected in parallel with the lower circuit, and the group of components is connected in series in the loop formed by the capacitor C, the bypass switch K1, and the upper circuit. The group of components includes a mechanical switch K2 and two branches with the same structure but opposite conduction directions connected in parallel with the mechanical switch K2. The two branches are connected in parallel, and each branch includes a fully controlled device and a diode connected in series with the fully controlled device.

[0008] Furthermore, the upper pipeline includes a fully controlled device and a diode connected in parallel with the fully controlled device.

[0009] Furthermore, the lower conduit includes a fully controlled device and a diode connected in parallel with the fully controlled device.

[0010] Furthermore, a thyristor T1 is connected in parallel on the lower pipeline.

[0011] Furthermore, the component is connected in series between the upper pipeline and the positive terminal of capacitor C.

[0012] Furthermore, the component is connected in series between the lower conduit and the negative terminal of capacitor C.

[0013] Furthermore, the components are connected in series in a circuit consisting of the lower pipeline and the bypass switch K1.

[0014] Furthermore, the fully controlled component adopts IGBT.

[0015] Compared with the prior art, the present invention has the following beneficial technical effects:

[0016] This invention relates to a circuit topology to prevent capacitor direct discharge in the event of a power module failure. By connecting components in series in a loop consisting of capacitor C, bypass switch K1, and upper conduit, capacitor C can be isolated from the main power device after a short circuit occurs in the upper conduit, preventing capacitor C from discharging through other components. The mechanical switch K2 has a resistance of only tens of microohms and only needs to be turned on for a few milliseconds, so K2 will not generate significant heat loss when the submodule is working normally. At the same time, the components in the circuit topology of this invention include two parallel branches with the same structure and opposite conduction directions. The branches include fully controlled devices and series-connected diodes. The two parallel branches ensure the continuity of current in the circuit when the mechanical switch K2 is turned off. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the topology of an existing flexible DC transmission module.

[0018] Figure 2A schematic diagram of a circuit structure with short-circuit protection function for the upper switching transistor provided by the prior art.

[0019] Figure 3 This is a schematic diagram of the circuit for connecting the modules in Embodiment 1 of the present invention.

[0020] Figure 4 This is a schematic diagram of the circuit for connecting the modules in Embodiment 2 of the present invention.

[0021] Figure 5 This is a schematic diagram of the circuit for connecting the modules in Embodiment 3 of the present invention.

[0022] Figure 6 This is a schematic diagram of the circuit for connecting the modules in Embodiment 4 of the present invention. Detailed Implementation

[0023] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0024] This invention provides a submodule circuit topology that avoids capacitor discharge during module failure. It includes an upper circuit, a lower circuit, and a capacitor C. The capacitor C is connected in series with the upper and lower circuits to form a loop. A bypass switch K1 is connected in parallel on the lower circuit. The assembly includes a mechanical switch K2, which has two parallel branches with identical circuit elements but opposite conduction directions. Each branch includes a fully controlled device and a diode connected in series with the fully controlled device. The upper circuit includes the fully controlled device and a diode connected in parallel with it. The lower circuit has the same structure as the upper circuit, also including a fully controlled device and a diode connected in parallel with it. Furthermore, a thyristor T1 is connected in parallel on the lower circuit. Specifically, the fully controlled device can be an IGBT, IEGT, IGCT, etc., or a reverse-resistive device; this invention uses an IGBT.

[0025] For the existing topology of flexible DC transmission modules:

[0026] If a fault occurs in the lower pipeline of the submodule, close the bypass switch K1. In the circuit formed by capacitor C and the upper pipeline connected in series, the controller will control the shutdown of all control devices in the upper pipeline to cut off the large current in the circuit.

[0027] If a fault occurs in the upper pipeline of the submodule, the bypass switch K1 is closed. In the circuit formed by capacitor C and the upper pipeline connected in series, due to the short circuit caused by the fault in the upper pipeline, a huge short circuit current will be generated in the circuit, which will burn out the circuit components.

[0028] If a fault occurs in the pipeline of the submodule, and the bypass switch K1 fails to operate due to other factors, the circuit will continue to charge the capacitor C until it exceeds the withstand capacity of the capacitor C, causing a short-circuit current discharge of several hundred kiloamperes through the capacitor. The technical solution with patent publication number CN109149974B does not take this situation into account. Similarly, its designed circuit cannot solve the problem of the bypass switch K1 failing to operate.

[0029] Example 1

[0030] like Figure 3 As shown, the assembly is connected in series between the upper pipeline and the positive terminal of capacitor C.

[0031] When a fault occurs in the conduit of the submodule, an open command is first sent to mechanical switch K2, and an on signal is sent to S3. The arc voltage when mechanical switch K2 opens is used to conduct S3 and D3 to avoid current interruption. After mechanical switch K2 opens, S3 and D3 are turned off. The opening of mechanical switch K2 only takes a few milliseconds, and the current tolerance of S3 and D3 is also at the millisecond level, so no heat dissipation is required. The addition of D3 is mainly to avoid the formation of reverse voltage across S3, which could damage S3. When the current flowing through K2 is reversed, an on signal is sent to S4 when an open command is sent to mechanical switch K2. The other working process is similar to the process of turning on S3. In actual operation, there is no need to determine the current direction. When K2 needs to be opened, both S3 and S4 are simultaneously turned on. Whether S3 or S4 is activated depends on the direction of the current flowing through K2. The opening of K2 cuts off the short circuit path formed by capacitor C through the upper conduit and K1, avoiding the generation of a large discharge current.

[0032] If a fault occurs in the conduit of the submodule, but the bypass switch K1 does not fail to operate, capacitor C discharges. When mechanical switch K2 opens, an S4 turn-on signal is given, and the arc voltage when mechanical switch K2 opens turns on S4 and D4 to prevent intermittent current. Similarly, the current withstand capability requirements for S4 and D4 are not high, so heat dissipation is not required. The purpose of adding S3 and D3 is the same as that of S4 and D4. Whether S4 and D4 or S3 and D3 work depends on the direction of the current flowing through K2. In actual operation, there is no need to determine the current direction. When K2 needs to be opened, S3 and S4 are simultaneously turned on. Whether S3 or S4 works depends on the direction of the current flowing through K2. Opening K2 cuts off the short-circuit path formed by capacitor C through the conduit and K1, preventing the generation of a large discharge current.

[0033] If a fault occurs in the piping of the submodule and the bypass switch K1 fails to operate, when the mechanical switch K2 opens, an on signal is sent to S3. The arc voltage when the mechanical switch K2 opens is used to turn on S3, preventing current interruption. After the mechanical switch K2 opens, S3 is turned off. The opening of the mechanical switch K2 only takes a few milliseconds, and the current withstand capability of S3 and D3 is also at the millisecond level, so no heat dissipation is required. The addition of D3 is mainly to prevent reverse voltage from forming across S3, which could damage S3. When the current flowing through K2 is reversed, when an on command is sent to the mechanical switch K2, an on signal is sent to S4. The other working process is similar to the on-process of S3. In actual operation, there is no need to determine the current direction. When K2 needs to be opened, both S3 and S4 are simultaneously turned on. Whether S3 or S4 is activated depends on the direction of the current flowing through K2.

[0034] Using this invention, when the submodule detects that K1 fails to operate, it opens K2. Since S2, D2, and T1 are in the closed state at this time, the bridge arm current Iarm forms an induced voltage across its ends under the action of the high impedance of the lower pipeline, forcing S2 or T1 to reach the protection setting value and enter the short-circuit failure mode. Subsequently, as the impedance of the short-circuit device reaches the same level as the conduction state, the system continues to operate. Now, the S2 or T1 selected in the flexible DC system (generally, when S2 does not have long-term short-circuit capability, T1 is configured) has the capability of long-term short-circuit current carrying.

[0035] Example 2

[0036] like Figure 4 As shown, the only difference from Example 1 is that the component is connected in series between the lower pipeline and the negative terminal of capacitor C. The working process and logic of the component are the same as in Example 1.

[0037] Example 3

[0038] like Figure 5 As shown, the only difference from Embodiment 1 is that the component group is connected between terminals A and A”, that is, the component group is connected in series in the loop formed by the emitter of the IGBT and the anode of the diode in the lower pipeline and the anode of the thyristor T1. When a fault short circuit occurs in the upper pipeline, the operation of the component group is the same as in Embodiment 1. In particular, when the bypass switch fails to operate, this scheme can not only isolate the capacitor discharge, but also isolate the lower pipeline part S2 to prevent it from reaching the set value and being broken down, thus maximizing the protection of as few devices as possible in the system. After K2 is opened, the path of the bridge arm current Iarm is cut off. The current will not pass through the location of lower S2 and D2, but will flow through the location of T1. Because T1 is in the closed state at this time, the impedance is high, and the bridge arm current forms a voltage across its two ends. After reaching the protection set value, a short circuit occurs. The system continues to operate.

[0039] Example 4

[0040] like Figure 6 As shown, the difference from Embodiment 3 is that the component group is connected between terminal B and B”, that is, the component group is connected in series in the loop formed by the collector of the IGBT in the lower pipeline, the cathode of the diode, and the cathode of the thyristor T1. When a fault short circuit occurs in the upper pipeline, the operation of the component group is the same as in Embodiment 1. When the bypass switch K1 fails to operate, the operation is as in Scheme 3. The bridge arm current Iarm flows through the position of T1, forming a voltage across its two ends. After reaching the protection setting value, a short circuit occurs. The system continues to operate.

[0041] This invention solves the problem of high current and severe damage caused by closing the bypass switch at the lower transistor when the upper transistor of the power module is short-circuited, without significantly increasing system energy loss. It also provides a solution for continued system operation when the bypass switch fails to operate. The overall solution has low requirements for current stress on components and requires no heat dissipation.

[0042] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A circuit topology for preventing capacitor direct discharge in the event of a power module failure, characterized in that, It includes an upper conduit, a lower conduit, a bypass switch K1, a capacitor C, and a group of components. The bypass switch K1 is connected in parallel with the lower conduit. The group of components is connected in series in the circuit formed by the capacitor C, the bypass switch K1, and the upper conduit. The group of components includes a mechanical switch K2 and two branches with the same structure but opposite conduction directions connected in parallel with the mechanical switch K2. The two branches are connected in parallel. Each branch includes a fully controlled device and a diode connected in series with the fully controlled device.

2. The circuit topology for avoiding direct discharge of capacitors when a power module fails, as described in claim 1, is characterized in that... The upper pipeline includes a fully controlled device and a diode connected in parallel with the fully controlled device.

3. The circuit topology for avoiding direct discharge of capacitors when a power module fails, as described in claim 1, is characterized in that... The lower conduit includes a fully controlled device and a diode connected in parallel with the fully controlled device.

4. The circuit topology for avoiding direct discharge of capacitors when a power module fails, as described in claim 3, is characterized in that... A thyristor T1 is connected in parallel on the lower pipeline.

5. The circuit topology for avoiding direct discharge of capacitors when a power module fails, as described in claim 1, is characterized in that... The components are connected in series between the upper pipeline and the positive terminal of capacitor C.

6. The circuit topology for avoiding capacitor direct discharge during power module failure as described in claim 1, characterized in that, The components are connected in series between the lower conduit and the negative terminal of capacitor C.

7. The circuit topology for avoiding capacitor direct discharge during power module failure as described in claim 1, characterized in that, The components are connected in series in a circuit consisting of the lower pipeline and the bypass switch K1.

8. The circuit topology for avoiding capacitor direct discharge during power module failure as described in claim 1, characterized in that, The fully controlled device uses IGBTs.

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