Improved Dual-Clamped Sub-Module, Control Method and Modular Multilevel Converter

Abstract

The present invention provides an improved dual-clamp sub-module, a control method and a modular multilevel converter. The improved dual-clamp sub-module includes a first half-bridge sub-module, a second half-bridge sub-module, a third half-bridge sub-module, diode D1, diode D2, diode D3 and switch T7. It has a simple structure, requires fewer devices and has a low cost. The control method specifically analyzes the states of each device in the normal mode and the blocked mode of the improved dual-clamp sub-module, and finally gives the output voltage of the improved dual-clamp sub-module in different states, realizing the control of the improved dual-clamp sub-module. The control process is simple and easy to implement. The modular multilevel converter composed of the improved dual-clamp sub-module and the half-bridge sub-module has the ability to clear faults, does not require additional IGBTs, has a low economic cost and low losses.

Description

Technical Field

[0001] The present invention relates to the technical field of DC power transmission, and particularly to an improved double-clamp sub-module, a control method and a modular multilevel converter. Background Art

[0002] The unified power flow controller (UPFC) is a new generation of flexible AC power transmission device with the most powerful functions and superior characteristics. Traditional low-level topologies such as two-level and three-level cannot meet the requirements of high voltage and high power of UPFC. The modular multilevel converter (MMC) has been relatively maturely developed in the high-voltage field. At present, the MMC-UPFC system composed of MMC and UPFC - the 220kV Nanjing West Ring Network UPFC, 220kV Shanghai Yunzaobang UPFC, and 500kV Suzhou UPFC projects have been put into use, and their fault characteristics have also attracted much attention. Since the MMC-UPFC system still relies on the DC bus to transmit power, a short-circuit fault on the DC side of the converter station will not only cause the interruption of the power transmission of the entire system, but also the IGBT may be damaged due to overcurrent. The MMC cascaded by half-bridge sub-modules (HBSM) does not have the ability to clear faults. After the switching tubes are turned off under a fault, its current path is as attached Figure 1 , La represents the arm reactance, Rdc is the resistance. At this time, the MMC operates in the diode uncontrolled rectification mode. If the AC side does not trip, the DC side fault current will always exist and the MMC cannot be restarted; if the AC side trips, the restart of the DC side requires complex timing coordination and the reconstruction of the AC voltage takes a long time, and the MMC will be out of service for a long time. Most of the existing MMCs use cable power transmission. The failure rate on the DC side is lower than that of traditional overhead lines, but the cost of cables is relatively high, and it can only reduce the incidence of DC faults and cannot clear faults; there is also a scheme to deploy a DC circuit breaker on the DC line to clear faults, but at present, the DC circuit breaker has only been relatively well developed in the medium and low voltage power transmission fields and is still insufficient in the high voltage and ultra-high voltage fields. Moreover, the additional deployment of a DC circuit breaker will bring a relatively high economic cost. In view of the fault characteristics on the DC side, the introduction of a clamped sub-module can effectively clear faults, such as a full-bridge sub-module (FBSM), a clamp double sub-module (CDSM), etc. Because this kind of fault clearing method does not require the AC side to trip, and the reclosing and DC voltage reconstruction can be completed in only dozens of milliseconds, but it will introduce additional IGBTs, which will bring additional economic costs and losses. Summary of the Invention

[0003] To overcome the deficiencies in the prior art that the modular multilevel converter does not have fault clearing ability, high economic cost and large losses, the present invention provides an improved dual-clamp sub-module, a control method and a modular multilevel converter. The improved dual-clamp sub-module includes a first half-bridge sub-module, a second half-bridge sub-module, a third half-bridge sub-module, diode D1, diode D2, diode D3 and switch tube T7. It has a simple structure and requires fewer devices. The modular multilevel converter composed of the improved dual-clamp sub-module has fault clearing ability, does not require additional IGBTs, has low economic cost and small losses.

[0004] To achieve the above-mentioned invention purpose, the present invention adopts the following technical solutions:

[0005] On the one hand, the present invention provides an improved dual-clamp sub-module, including a first half-bridge sub-module, a second half-bridge sub-module, a third half-bridge sub-module, diode D1, diode D2, diode D3 and switch tube T7;

[0006] The anodes of diode D1, the anode of diode D2, and the collector of switch tube T7 are all connected to the positive electrode of energy storage capacitor C3 in the third half-bridge sub-module. The cathode of diode D1 is connected to the positive electrode of energy storage capacitor C1 in the first half-bridge sub-module. The cathode of diode D2 is connected to the positive electrode of energy storage capacitor C2 in the second half-bridge sub-module. The emitter of switch tube T7 is simultaneously connected to the negative electrode of energy storage capacitor C2 in the second half-bridge sub-module and the cathode of diode D3. The anode of diode D3 is connected to the negative electrode of energy storage capacitor C3 in the third half-bridge sub-module.

[0007] The first half-bridge sub-module further includes switch tubes T1 and T2;

[0008] Switch tube T1 includes IGBT1 and diode D11 anti-parallel to IGBT1;

[0009] Switch tube T2 includes IGBT2 and diode D12 anti-parallel to IGBT2;

[0010] The collector of IGBT1 is connected to the positive electrode of energy storage capacitor C1, and its emitter is connected to the common point A. The collector of IGBT2 is connected to the common point A, and its emitter is connected to the negative electrode of energy storage capacitor C1.

[0011] The second half-bridge sub-module further includes switch tubes T3 and T4;

[0012] Switch tube T3 includes IGBT3 and diode D21 anti-parallel to IGBT3;

[0013] Switch tube T4 includes IGBT4 and diode D22 anti-parallel to IGBT4;

[0014] The collector of the IGBT3 is connected to the positive electrode of the energy storage capacitor C2, and its emitter is simultaneously connected to the collector of the IGBT4 and the negative electrode of the energy storage capacitor C1. The emitter of the IGBT4 is connected to the negative electrode of the energy storage capacitor C2.

[0015] The third half-bridge sub-module further includes a switching transistor T5 and a switching transistor T6;

[0016] The switching transistor T5 includes an IGBT5 and a diode D31 connected in anti-parallel with the IGBT5;

[0017] The switching transistor T6 includes an IGBT6 and a diode D32 connected in anti-parallel with the IGBT6;

[0018] The collector of the IGBT5 is connected to the positive electrode of the energy storage capacitor C3, and its emitter is connected to the common point B. The collector of the IGBT6 is connected to the common point B, and its emitter is connected to the negative electrode of the energy storage capacitor C3.

[0019] The switching transistor T7 includes an IGBT7 and a diode D7 connected in anti-parallel with the IGBT7.

[0020] On the other hand, the present invention also provides a control method for an improved double-clamping sub-module. When the improved double-clamping sub-module is in the normal mode, the control method includes:

[0021] State 11): IGBT1, IGBT3, IGBT6, and IGBT7 are turned on, and IGBT2, IGBT4, and IGBT5 are turned off. The output voltage of the improved double-clamping sub-module is +3Uc;

[0022] State 12): IGBT1, IGBT3, IGBT5, and IGBT7 are turned on, and IGBT2, IGBT4, and IGBT6 are turned off, or IGBT1, IGBT4, IGBT6, and IGBT7 are turned on, and IGBT2, IGBT3, and IGBT5 are turned off, or IGBT2, IGBT3, IGBT6, and IGBT7 are turned on, and IGBT2, IGBT4, and IGBT5 are turned off. The output voltage of the improved double-clamping sub-module is +2Uc;

[0023] State 13): IGBT1, IGBT4, IGBT5, and IGBT7 are turned on, and IGBT2, IGBT3, and IGBT6 are turned off, or IGBT2, IGBT4, IGBT6, and IGBT7 are turned on, and IGBT1, IGBT3, and IGBT5 are turned off, or IGBT2, IGBT3, IGBT5, and IGBT7 are turned on, and IGBT1, IGBT4, and IGBT6 are turned off. The output voltage of the improved double-clamping sub-module is +Uc;

[0024] Status 14): IGBT2, IGBT4, IGBT5, and IGBT7 are conducting, while IGBT1, IGBT3, and IGBT6 are turned off, and the output voltage of the improved dual-clamping sub-module is 0.

[0025] In the said Status 11), when the current flows from common point A to common point B, the current path is: common point A → diode D11 → energy storage capacitor C1 → diode D21 → energy storage capacitor C2 → diode D7 → energy storage capacitor C3 → diode D32 → common point B;

[0026] When the current direction is from common point B to common point A, the current path is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → energy storage capacitor C2 → IGBT3 → energy storage capacitor C1 → IGBT1 → common point A.

[0027] In the said Status 12), when IGBT1, IGBT3, IGBT5, and IGBT7 are conducting and IGBT2, IGBT4, and IGBT6 are turned off, when the current flows from common point A to common point B, the current path is: common point A → diode D11 → energy storage capacitor C1 → diode D21 → energy storage capacitor C2 → diode D7 → IGBT5 → common point B; when the current direction is from common point B to common point A, the current path is: common point B → diode D31 → IGBT7 → energy storage capacitor C2 → IGBT3 → energy storage capacitor C1 → IGBT1 → common point A;

[0028] When IGBT1, IGBT4, IGBT6, and IGBT7 are conducting and IGBT2, IGBT3, and IGBT5 are turned off, when the current flows from common point A to common point B, the current path is: common point A → diode D11 → energy storage capacitor C1 → IGBT 4 → diode D7 → energy storage capacitor C3 → diode D32 → common point B; when the current direction is from common point B to common point A, the current path is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → diode D22 → energy storage capacitor C1 → IGBT1 → common point A;

[0029] When IGBT2, IGBT3, IGBT6, and IGBT7 are conducting and IGBT2, IGBT4, and IGBT5 are turned off, when the current flows from common point A to common point B, the current path is: common point A → IGBT2 → diode D21 → energy storage capacitor C2 → diode D7 → energy storage capacitor C3 → diode D32 → common point B; when the current direction is from common point B to common point A, the current path is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → energy storage capacitor C2 → IGBT3 → diode D12 → common point A.

[0030] In the state 13), when IGBT1, IGBT4, IGBT5, and IGBT7 are turned on and IGBT2, IGBT3, and IGBT6 are turned off, the current path when the current flows from the common point A to the common point B is: common point A → diode D11 → energy storage capacitor C1 → IGBT4 → diode D7 → IGBT5 → common point B; the current path when the current direction is from the common point B to the common point A is: common point B → diode D31 → IGBT7 → diode D22 → energy storage capacitor C1 → IGBT1 → common point A;

[0031] When IGBT2, IGBT4, IGBT6, and IGBT7 are turned on and IGBT1, IGBT3, and IGBT5 are turned off, the current path when the current flows from the common point A to the common point B is: common point A → IGBT2 → IGBT4 → diode D7 → energy storage capacitor C3 → diode D32 → common point B; the current path when the current direction is from the common point B to the common point A is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → diode D22 → diode D12 → common point A;

[0032] When IGBT2, IGBT3, IGBT5, and IGBT7 are turned on and IGBT1, IGBT4, and IGBT6 are turned off, the current path when the current flows from the common point A to the common point B is: common point A → IGBT2 → diode D21 → energy storage capacitor C2 → diode D7 → IGBT5 → common point B; the current path when the current direction is from the common point B to the common point A is: common point B → diode D31 → IGBT7 → energy storage capacitor C2 → IGBT3 → diode D12 → common point A.

[0033] In the state 14), the current path when the current flows from the common point A to the common point B is: common point A → IGBT2 → IGBT4 → diode D7 → IGBT5 → common point B;

[0034] The current path when the current direction is from the common point B to the common point A is: common point B → diode D31 → IGBT7 → diode D22 → diode D12 → common point A.

[0035] On the other hand, the present invention provides a control method for an improved dual-clamped sub-module. When the improved dual-clamped sub-module is in the locked mode, the control method includes:

[0036] State 21): When IGBT1, IGBT2, IGBT3, IGBT4, IGBT5, IGBT6, and IGBT7 are all turned off and the current flows from the common point A to the common point B, the output voltage of the improved dual-clamped sub-module is +3Uc;

[0037] State 22): When IGBT1, IGBT2, IGBT3, IGBT4, IGBT5, IGBT6, and IGBT7 are all turned off and the current flows from common point B to common point A, the output voltage of the improved dual-clamped sub-module is -Uc.

[0038] In the said state 21), the current path is: common point A → diode D11 → energy storage capacitor C1 → diode D21 → energy storage capacitor C2 → diode D7 → energy storage capacitor C3 → diode D32 → common point B.

[0039] In the said state 22), the current path includes:

[0040] 1) common point B → diode D31 → energy storage capacitor C3 → diode D3 → diode D22 → diode D12 → common point A;

[0041] 2) common point B → diode D31 → diode D1 → energy storage capacitor C1 → diode D12 → common point A;

[0042] 3) common point B → diode D31 → diode D2 → energy storage capacitor C2 → diode D22 → diode D12 → common point A.

[0043] Furthermore, the present invention provides a modular multilevel converter, which includes three phase units. The phase unit includes an upper bridge arm and a lower bridge arm. The upper bridge arm and the lower bridge arm are formed by mixing and connecting in series the improved dual-clamped sub-module and the half-bridge sub-module.

[0044] Compared with the closest prior art, the technical solution provided by the present invention has the following beneficial effects:

[0045] The improved dual-clamped sub-module provided by the present invention includes a first half-bridge sub-module, a second half-bridge sub-module, a third half-bridge sub-module, diode D1, diode D2, diode D3, and switch tube T7. It has a simple structure, requires fewer devices, and has a low cost;

[0046] The control method of the improved dual-clamped sub-module provided by the present invention specifically analyzes the states of each device in the improved dual-clamped sub-module in two modes: normal mode and locked mode, and finally gives the output voltage of the improved dual-clamped sub-module in different states, realizing the control of the improved dual-clamped sub-module. The control process is simple and easy to implement;

[0047] The modular multilevel converter provided by the present invention, which is composed of the improved dual-clamped sub-module and the half-bridge sub-module, has the ability to clear faults, does not require additional IGBTs, has a low economic cost and low losses;

[0048] Compared with the half-bridge sub-module, the improved dual-clamp sub-module provided by the present invention has 1 / 3 more IGBTs and 4 / 3 more diodes on average for each half-bridge sub-module, with a lower economic cost. Moreover, the number of IGBTs in the current path of the improved dual-clamp sub-module provided by the present invention is less at any time, resulting in lower losses and higher efficiency.

[0049] In the improved dual-clamp sub-module provided by the present invention, the three groups of levels of IGBT1 and IGBT2, IGBT3 and IGBT4, and IGBT5 and IGBT6 are complementary respectively, and the pulse distribution method of the half-bridge sub-module can be adopted. IGBT7 is always on and does not require a pulse distribution program, so the control process is relatively simple.

[0050] In the technical solution provided by the present invention, after the improved dual-clamp sub-module is blocked, the energy storage capacitors C1, C2, and C3 are connected in parallel, and the equivalent capacitance is 3C0. The equivalent capacitance is relatively large, and under the same current, the voltage rises slowly. Therefore, the improved dual-clamp sub-module has a lower requirement for the withstand voltage of the sub-module capacitor voltage.

[0051] In the technical solution provided by the present invention, for a DC side short-circuit fault, the blocked improved dual-clamp sub-module isolates the DC fault and supports the AC voltage at the same time. The AC side does not need to trip, the restart time is short, and the power supply stability is relatively high.

[0052] The technical solution provided by the present invention has a wide range of applications and can be specifically applied to devices composed of modular multilevel converters, including but not limited to unified power flow controllers, inter-line power flow controllers, flexible DC transmission, and flexible DC distribution. Description of the Drawings

[0053] Figure 1 It is a schematic diagram of the current path after the switch tube is turned off in the MMC formed by cascading half-bridge sub-modules in the prior art.

[0054] Figure 2 It is a structural diagram of the improved dual-clamp sub-module in Embodiment 1 of the present invention. Detailed Embodiments

[0055] The present invention will be further described in detail below with reference to the drawings.

[0056] Embodiment 1

[0057] Embodiment 1 of the present invention provides an improved dual-clamp sub-module, and the specific structure is as Figure 2 shown. The improved dual-clamp sub-module includes a first half-bridge sub-module, a second half-bridge sub-module, a third half-bridge sub-module, diode D1, diode D2, diode D3, and switch tube T7; switch tube T7 includes IGBT7 and diode D7 anti-parallel to IGBT7.

[0058] Specifically, the anode of diode D1, the anode of diode D2, and the collector of switch T7 are all connected to the positive electrode of energy storage capacitor C3 in the third half-bridge sub-module. The cathode of diode D1 is connected to the positive electrode of energy storage capacitor C1 in the first half-bridge sub-module. The cathode of diode D2 is connected to the positive electrode of energy storage capacitor C2 in the second half-bridge sub-module. The emitter of switch T7 is simultaneously connected to the negative electrode of energy storage capacitor C2 in the second half-bridge sub-module and the cathode of diode D3. The anode of diode D3 is connected to the negative electrode of energy storage capacitor C3 in the third half-bridge sub-module.

[0059] The above-mentioned first half-bridge sub-module further includes switch T1 and switch T2 in addition to energy storage capacitor C1;

[0060] Switch T1 includes IGBT1 and diode D11 anti-parallel to IGBT1;

[0061] Switch T2 includes IGBT2 and diode D12 anti-parallel to IGBT2;

[0062] Specifically, the collector of IGBT1 is connected to the positive electrode of energy storage capacitor C1, and its emitter is connected to common point A;

[0063] The collector of IGBT2 is connected to common point A, and its emitter is connected to the negative electrode of energy storage capacitor C1.

[0064] The above-mentioned second half-bridge sub-module further includes switch T3 and switch T4 in addition to energy storage capacitor C2;

[0065] Switch T3 includes IGBT3 and diode D21 anti-parallel to IGBT3;

[0066] Switch T4 includes IGBT4 and diode D22 anti-parallel to IGBT4;

[0067] Specifically, the collector of IGBT3 is connected to the positive electrode of energy storage capacitor C2, its emitter is simultaneously connected to the collector of IGBT4 and the negative electrode of energy storage capacitor C1, and the emitter of IGBT4 is connected to the negative electrode of energy storage capacitor C2.

[0068] The above-mentioned third half-bridge sub-module further includes switch T5 and switch T6 in addition to energy storage capacitor C3;

[0069] Switch T5 includes IGBT5 and diode D31 anti-parallel to IGBT5;

[0070] Switch T6 includes IGBT6 and diode D32 anti-parallel to IGBT6;

[0071] Specifically, the collector of IGBT5 is connected to the positive electrode of the energy storage capacitor C3, and its emitter is connected to the common point B; the collector of IGBT6 is connected to the common point B, and its emitter is connected to the negative electrode of the energy storage capacitor C3.

[0072] The economic cost and losses of the improved dual-clamped sub-module are introduced separately as follows:

[0073] 1) In terms of economic cost:

[0074] The voltage withstand requirement of diode D1 is 2Uc, and the voltage withstand requirements of other devices are all Uc. Since the cost of diodes is relatively low, adding three diodes will not cause a significant increase in cost; diodes D1 - D3 are always non-conductive under normal operation and no current flows through them. When the improved dual-clamped sub-module is blocked and the current direction is B→A, the fault current is divided into three paths and flows through the three diodes D1 - D3 respectively. Therefore, the current withstand requirements of diodes D1, D2, and D3 are relatively low.

[0075] One improved dual-clamped sub-module can replace three half-bridge sub-modules in terms of output voltage function. Comparing one improved dual-clamped sub-module with three half-bridge sub-modules, one IGBT7 and four diodes, namely D1, D2, D3, and D7, are added. On average, each half-bridge sub-module adds 4 / 3 diodes and 1 / 3 IGBT.

[0076] 2) In terms of losses:

[0077] 2-1) Switching losses:

[0078] During normal operation, IGBT7 is always turned on and does not perform switching, so there are no switching losses. Under the same modulation and voltage equalization strategy, the switching of the other six IGBTs is the same as that of the half-bridge sub-module. Therefore, its switching losses are exactly the same as those of the half-bridge sub-module and there is no additional increase.

[0079] 2-2) Conduction losses:

[0080] No current flows through diodes D1, D2, and D3 under normal operation, so there are no conduction losses; the switching mode of IGBT1 - IGBT6 is the same as that of the half-bridge under normal operation, so these six IGBTs do not increase conduction losses; IGBT7 and diode D7 are always conducting under normal operation, and the increased conduction losses mainly come from IGBT7 and diode D7. According to the calculation method of conduction losses, the on-state loss of the IGBT is the integral of the forward conduction voltage drop and the conduction current over a current cycle. The increased on-state loss is normalized by the number of capacitors, and the conduction loss of each half-bridge sub-module increases by 1 / 3 on average.

[0081] Embodiment 2

[0082] Embodiment 2 of the present invention provides a control method for the improved dual-clamp sub-module in Embodiment 1 above. When the improved dual-clamp sub-module is in the normal mode, the specific process of this control method is as follows:

[0083] State 11): IGBT1, IGBT3, IGBT6, and IGBT7 are turned on and IGBT2, IGBT4, and IGBT5 are turned off. The output voltage of the improved dual-clamp sub-module is +3Uc;

[0084] When the current flows from common point A to common point B, the current path is: common point A → diode D11 → energy storage capacitor C1 → diode D21 → energy storage capacitor C2 → diode D7 → energy storage capacitor C3 → diode D32 → common point B;

[0085] When the current direction is from common point B to common point A, the current path is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → energy storage capacitor C2 → IGBT3 → energy storage capacitor C1 → IGBT1 → common point A.

[0086] State 12): IGBT1, IGBT3, IGBT5, and IGBT7 are turned on and IGBT2, IGBT4, and IGBT6 are turned off, or IGBT1, IGBT4, IGBT6, and IGBT7 are turned on and IGBT2, IGBT3, and IGBT5 are turned off, or IGBT2, IGBT3, IGBT6, and IGBT7 are turned on and IGBT2, IGBT4, and IGBT5 are turned off. The output voltage of the improved dual-clamp sub-module is +2Uc;

[0087] 1) When IGBT1, IGBT3, IGBT5, and IGBT7 are turned on and IGBT2, IGBT4, and IGBT6 are turned off, when the current flows from common point A to common point B, the current path is: common point A → diode D11 → energy storage capacitor C1 → diode D21 → energy storage capacitor C2 → diode D7 → IGBT5 → common point B; when the current direction is from common point B to common point A, the current path is: common point B → diode D31 → IGBT7 → energy storage capacitor C2 → IGBT3 → energy storage capacitor C1 → IGBT1 → common point A;

[0088] 2) When IGBT1, IGBT4, IGBT6, and IGBT7 are turned on and IGBT2, IGBT3, and IGBT5 are turned off, when the current flows from common point A to common point B, the current path is: common point A → diode D11 → energy storage capacitor C1 → IGBT 4 → diode D7 → energy storage capacitor C3 → diode D32 → common point B; when the current direction is from common point B to common point A, the current path is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → diode D22 → energy storage capacitor C1 → IGBT1 → common point A;

[0089] 3) When IGBT2, IGBT3, IGBT6, and IGBT7 are turned on and IGBT2, IGBT4, and IGBT5 are turned off, the current path when the current flows from common point A to common point B is: common point A → IGBT2 → diode D21 → energy storage capacitor C2 → diode D7 → energy storage capacitor C3 → diode D32 → common point B; the current path when the current flows from common point B to common point A is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → energy storage capacitor C2 → IGBT3 → diode D12 → common point A.

[0090] State 13): When IGBT1, IGBT4, IGBT5, and IGBT7 are turned on and IGBT2, IGBT3, and IGBT6 are turned off, or when IGBT2, IGBT4, IGBT6, and IGBT7 are turned on and IGBT1, IGBT3, and IGBT5 are turned off, or when IGBT2, IGBT3, IGBT5, and IGBT7 are turned on and IGBT1, IGBT4, and IGBT6 are turned off, the output voltage of the improved dual-clamped sub-module is +Uc;

[0091] 1) When IGBT1, IGBT4, IGBT5, and IGBT7 are turned on and IGBT2, IGBT3, and IGBT6 are turned off, the current path when the current flows from common point A to common point B is: common point A → diode D11 → energy storage capacitor C1 → IGBT4 → diode D7 → IGBT5 → common point B; the current path when the current flows from common point B to common point A is: common point B → diode D31 → IGBT7 → diode D22 → energy storage capacitor C1 → IGBT1 → common point A;

[0092] 2) When IGBT2, IGBT4, IGBT6, and IGBT7 are turned on and IGBT1, IGBT3, and IGBT5 are turned off, the current path when the current flows from common point A to common point B is: common point A → IGBT2 → IGBT4 → diode D7 → energy storage capacitor C3 → diode D32 → common point B; the current path when the current flows from common point B to common point A is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → diode D22 → diode D12 → common point A;

[0093] 3) When IGBT2, IGBT3, IGBT5, and IGBT7 are conducting and IGBT1, IGBT4, and IGBT6 are off, the current path when the current flows from common point A to common point B is: common point A → IGBT2 → diode D21 → energy storage capacitor C2 → diode D7 → IGBT5 → common point B; the current path when the current direction is from common point B to common point A is: common point B → diode D31 → IGBT7 → energy storage capacitor C2 → IGBT3 → diode D12 → common point A.

[0094] State 14): When IGBT2, IGBT4, IGBT5, and IGBT7 are conducting and IGBT1, IGBT3, and IGBT6 are off, the output voltage of the improved double-clamped sub-module is 0;

[0095] The current path when the current flows from common point A to common point B is: common point A → IGBT2 → IGBT4 → diode D7 → IGBT5 → common point B;

[0096] The current path when the current direction is from common point B to common point A is: common point B → diode D31 → IGBT7 → diode D22 → diode D12 → common point A.

[0097] Embodiment 3

[0098] Embodiment 3 of the present invention provides a control method for the improved double-clamped sub-module in Embodiment 1. When the improved double-clamped sub-module is in the locked mode, the specific process of the control method provided in Embodiment 3 is as follows:

[0099] State 21): When IGBT1, IGBT2, IGBT3, IGBT4, IGBT5, IGBT6, and IGBT7 are all off and the current flows from common point A to common point B, the output voltage of the improved double-clamped sub-module is +3Uc;

[0100] The current path is: common point A → diode D11 → energy storage capacitor C1 → diode D21 → energy storage capacitor C2 → diode D7 → energy storage capacitor C3 → diode D32 → common point B.

[0101] State 22): When IGBT1, IGBT2, IGBT3, IGBT4, IGBT5, IGBT6, and IGBT7 are all off and the current flows from common point B to common point A, the output voltage of the improved double-clamped sub-module is -Uc;

[0102] The current path includes:

[0103] 1) common point B → diode D31 → energy storage capacitor C3 → diode D3 → diode D22 → diode D12 → common point A;

[0104] 2) Common point B → Diode D31 → Diode D1 → Energy storage capacitor C1 → Diode D12 → Common point A;

[0105] 3) Common point B → Diode D31 → Diode D2 → Energy storage capacitor C2 → Diode D22 → Diode D12 → Common point A.

[0106] Embodiment 4

[0107] Embodiment 4 of the present invention provides a modular multilevel converter, which includes three phase units. Each phase unit includes an upper bridge arm and a lower bridge arm. The upper bridge arm and the lower bridge arm are formed by mixing and connecting in series the improved double-clamped sub-module and the half-bridge sub-module provided by Embodiment 1. The modular multilevel converter provided by Embodiment 4 has the ability to clear faults.

[0108] For the convenience of description, each part of the above-mentioned device is described separately as various modules or units according to its function. Of course, when implementing the present application, the functions of each module or unit can be realized in one or more software or hardware.

[0109] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0110] The present application is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or block in the flowchart and / or block diagram can be realized by computer program instructions, and the combination of the processes and / or blocks in the flowchart and / or block diagram can also be realized by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate a device for realizing the functions specified in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0111] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory generate a manufactured product including an instruction device, and the instruction device realizes the functions in the process Figure 1 one process or multiple processes and / or blocksFigure 1 The functions specified in one or more boxes.

[0112] These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable device provide for implementing the steps of the functions specified in one or more processes and / or boxes Figure 1 One process or more processes and / or boxes Figure 1 The steps of the functions specified in one or more boxes.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Those of ordinary skill in the art can still modify or equivalently replace the specific implementation manners of the present invention with reference to the above embodiments. Any such modifications or equivalent replacements that do not depart from the spirit and scope of the present invention are within the scope of the claims of the present invention pending approval.

Claims

1. A control method for an improved dual-clamped sub-module, characterized in that, When the improved double-clamping sub-module is in the normal mode, the control method includes: State 11): IGBT1 in the first half-bridge sub-module, IGBT3 in the second half-bridge sub-module, IGBT6 in the third half-bridge sub-module, and IGBT7 in the switching transistor T7 are turned on, and IGBT2 in the first half-bridge sub-module, IGBT4 in the second half-bridge sub-module, and IGBT5 in the third half-bridge sub-module are turned off. The output voltage of the improved double-clamping sub-module is +3Uc. State 12): IGBT1, IGBT3, IGBT5, and IGBT7 are turned on and IGBT2, IGBT4, and IGBT6 are turned off, or IGBT1, IGBT4, IGBT6, and IGBT7 are turned on and IGBT2, IGBT3, and IGBT5 are turned off, or IGBT2, IGBT3, IGBT6, and IGBT7 are turned on and IGBT2, IGBT4, and IGBT5 are turned off. The output voltage of the improved double-clamping sub-module is +2Uc. State 13): IGBT1, IGBT4, IGBT5, and IGBT7 are turned on and IGBT2, IGBT3, and IGBT6 are turned off, or IGBT2, IGBT4, IGBT6, and IGBT7 are turned on and IGBT1, IGBT3, and IGBT5 are turned off, or IGBT2, IGBT3, IGBT5, and IGBT7 are turned on and IGBT1, IGBT4, and IGBT6 are turned off. The output voltage of the improved double-clamping sub-module is +Uc. State 14): IGBT2, IGBT4, IGBT5, and IGBT7 are turned on and IGBT1, IGBT3, and IGBT6 are turned off. The output voltage of the improved double-clamping sub-module is 0. Wherein, the improved double-clamping sub-module specifically includes a first half-bridge sub-module, a second half-bridge sub-module, a third half-bridge sub-module, a diode D1, a diode D2, a diode D3, and a switching transistor T7. The anodes of the diode D1, the diode D2, and the collector of the switching transistor T7 are all connected to the positive electrode of the energy storage capacitor C3 in the third half-bridge sub-module. The cathode of the diode D1 is connected to the positive electrode of the energy storage capacitor C1 in the first half-bridge sub-module. The cathode of the diode D2 is connected to the positive electrode of the energy storage capacitor C2 in the second half-bridge sub-module. The emitter of the switching transistor T7 is simultaneously connected to the negative electrode of the energy storage capacitor C2 in the second half-bridge sub-module and the cathode of the diode D3. The anode of the diode D3 is connected to the negative electrode of the energy storage capacitor C3 in the third half-bridge sub-module. The first half-bridge sub-module further includes a switching transistor T1 and a switching transistor T2. The switching transistor T1 includes an IGBT1 and a diode D11 anti-parallel to the IGBT1. The switching transistor T2 includes an IGBT2 and a diode D12 anti-parallel to the IGBT2. The collector of the IGBT1 is connected to the positive electrode of the energy storage capacitor C1, and its emitter is connected to the common point A. The collector of the IGBT2 is connected to the common point A, and its emitter is connected to the negative electrode of the energy storage capacitor C1. The second half-bridge sub-module further includes a switching transistor T3 and a switching transistor T4; The switching transistor T3 includes an IGBT3 and a diode D21 anti-parallel connected to the IGBT3; The switching transistor T4 includes an IGBT4 and a diode D22 anti-parallel connected to the IGBT4; The collector of the IGBT3 is connected to the positive electrode of the energy storage capacitor C2, and its emitter is simultaneously connected to the collector of the IGBT4 and the negative electrode of the energy storage capacitor C1. The emitter of the IGBT4 is connected to the negative electrode of the energy storage capacitor C2; The third half-bridge sub-module further includes a switching transistor T5 and a switching transistor T6; The switching transistor T5 includes an IGBT5 and a diode D31 anti-parallel connected to the IGBT5; The switching transistor T6 includes an IGBT6 and a diode D32 anti-parallel connected to the IGBT6; The collector of the IGBT5 is connected to the positive electrode of the energy storage capacitor C3, and its emitter is connected to the common point B. The collector of the IGBT6 is connected to the common point B, and its emitter is connected to the negative electrode of the energy storage capacitor C3; The switching transistor T7 includes an IGBT7 and a diode D7 anti-parallel connected to the IGBT7.

2. The control method of the improved double-clamping sub-module according to claim 1, characterized in that, In the state 11), when the current flows from the common point A connected to the emitter of the IGBT1 to the common point B connected to the collector of the IGBT6, the current path is: common point A → diode D11 in the first half-bridge sub-module → energy storage capacitor C1 → diode D21 in the first half-bridge sub-module → energy storage capacitor C2 → diode D7 in the switching transistor T7 → energy storage capacitor C3 → diode D32 in the third half-bridge sub-module → common point B; When the current direction is from the common point B to the common point A, the current path is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → energy storage capacitor C2 → IGBT3 → energy storage capacitor C1 → IGBT1 → common point A.

3. The control method of the improved double-clamping sub-module according to claim 2, characterized in that, In the state 12), when the IGBT1, IGBT3, IGBT5, and IGBT7 are turned on and the IGBT2, IGBT4, and IGBT6 are turned off, when the current flows from the common point A connected to the emitter of the IGBT1 to the common point B connected to the collector of the IGBT6, the current path is: common point A → diode D11 in the first half-bridge sub-module → energy storage capacitor C1 → diode D21 in the first half-bridge sub-module → energy storage capacitor C2 → diode D7 in the switching transistor T7 → IGBT5 → common point B; when the current direction is from the common point B to the common point A, the current path is: common point B → diode D31 in the third half-bridge sub-module → IGBT7 → energy storage capacitor C2 → IGBT3 → energy storage capacitor C1 → IGBT1 → common point A; When IGBT1, IGBT4, IGBT6, and IGBT7 are conducting and IGBT2, IGBT3, and IGBT5 are turned off, the current path when the current flows from common point A to common point B is: common point A → diode D11 → energy storage capacitor C1 → IGBT 4 → diode D7 → energy storage capacitor C3 → diode D32 → common point B; the current path when the current direction is from common point B to common point A is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → diode D22 → energy storage capacitor C1 → IGBT1 → common point A; When IGBT2, IGBT3, IGBT6, and IGBT7 are conducting and IGBT2, IGBT4, and IGBT5 are turned off, the current path when the current flows from common point A to common point B is: common point A → IGBT2 → diode D21 → energy storage capacitor C2 → diode D7 → energy storage capacitor C3 → diode D32 → common point B; the current path when the current direction is from common point B to common point A is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → energy storage capacitor C2 → IGBT3 → diode D12 → common point A.

4. The control method of the improved double-clamping sub-module according to claim 1, characterized in that, In the state 13), when IGBT1, IGBT4, IGBT5, and IGBT7 are conducting and IGBT2, IGBT3, and IGBT6 are turned off, the current path when the current flows from the common point A connected to the emitter of IGBT1 to the common point B connected to the collector of IGBT6 is: common point A → diode D11 in the first half - bridge sub - module → energy storage capacitor C1 → IGBT4 → diode D7 in the switching transistor T7 → IGBT5 → common point B; the current path when the current direction is from common point B to common point A is: common point B → diode D31 in the third half - bridge sub - module → IGBT7 → diode D22 in the second half - bridge sub - module → energy storage capacitor C1 → IGBT1 → common point A; When IGBT2, IGBT4, IGBT6, and IGBT7 are conducting and IGBT1, IGBT3, and IGBT5 are turned off, the current path when the current flows from common point A to common point B is: common point A → IGBT2 → IGBT4 → diode D7 → energy storage capacitor C3 → diode D32 → common point B; the current path when the current direction is from common point B to common point A is: common point B → IGBT6 → energy storage capacitor C3 → IGBT7 → diode D22 → diode D12 → common point A; When IGBT2, IGBT3, IGBT5, and IGBT7 are conducting and IGBT1, IGBT4, and IGBT6 are turned off, the current path when the current flows from common point A to common point B is: common point A → IGBT2 → diode D21 → energy storage capacitor C2 → diode D7 → IGBT5 → common point B; the current path when the current direction is from common point B to common point A is: common point B → diode D31 → IGBT7 → energy storage capacitor C2 → IGBT3 → diode D12 → common point A.

5. The control method of the improved double-clamping sub-module according to claim 1, characterized in that In the state 14), when the current flows from the common point A connected to the emitter of the IGBT1 to the common point B connected to the collector of the IGBT6, the current path is: common point A → IGBT2 → IGBT4 → diode D7 in the switching transistor T7 → IGBT5 → common point B; When the current direction is from the common point B to the common point A, the current path is: common point B → diode D31 → IGBT7 → diode D22 → diode D12 → common point A.

6. A control method for an improved dual-clamp sub-module, characterized in that When the improved dual-clamped sub-module is in the locked mode, the control method includes: State 21): When the IGBT1 and IGBT2 in the first half-bridge sub-module, the IGBT3 and IGBT4 in the second half-bridge sub-module, the IGBT5 and IGBT6 in the third half-bridge sub-module, and the IGBT7 in the switching transistor T7 are all turned off and the current flows from the common point A connected to the emitter of the IGBT1 to the common point B connected to the collector of the IGBT6, the output voltage of the improved dual-clamped sub-module is +3Uc; State 22): When the IGBT1, IGBT2, IGBT3, IGBT4, IGBT5, IGBT6, and IGBT7 are all turned off and the current flows from the common point B to the common point A, the output voltage of the improved dual-clamped sub-module is -Uc; It includes a first half-bridge sub-module, a second half-bridge sub-module, a third half-bridge sub-module, a diode D1, a diode D2, a diode D3, and a switching transistor T7; The anode of the diode D1, the anode of the diode D2, and the collector of the switching transistor T7 are all connected to the positive electrode of the energy storage capacitor C3 in the third half-bridge sub-module. The cathode of the diode D1 is connected to the positive electrode of the energy storage capacitor C1 in the first half-bridge sub-module. The cathode of the diode D2 is connected to the positive electrode of the energy storage capacitor C2 in the second half-bridge sub-module. The emitter of the switching transistor T7 is simultaneously connected to the negative electrode of the energy storage capacitor C2 in the second half-bridge sub-module and the cathode of the diode D3. The anode of the diode D3 is connected to the negative electrode of the energy storage capacitor C3 in the third half-bridge sub-module; The first half-bridge sub-module further includes a switching transistor T1 and a switching transistor T2; The switching transistor T1 includes an IGBT1 and a diode D11 anti-parallel to the IGBT1; The switching transistor T2 includes an IGBT2 and a diode D12 anti-parallel to the IGBT2; The collector of the IGBT1 is connected to the positive electrode of the energy storage capacitor C1, and its emitter is connected to the common point A. The collector of the IGBT2 is connected to the common point A, and its emitter is connected to the negative electrode of the energy storage capacitor C1; The second half-bridge sub-module further includes a switching transistor T3 and a switching transistor T4; The switching transistor T3 includes an IGBT3 and a diode D21 anti-parallel to the IGBT3; The switching transistor T4 includes an IGBT4 and a diode D22 anti-parallel to the IGBT4; The collector of the IGBT3 is connected to the positive electrode of the energy storage capacitor C2, and its emitter is simultaneously connected to the collector of the IGBT4 and the negative electrode of the energy storage capacitor C1. The emitter of the IGBT4 is connected to the negative electrode of the energy storage capacitor C2; The third half-bridge sub-module further includes a switching transistor T5 and a switching transistor T6; The switching transistor T5 includes an IGBT5 and a diode D31 anti-parallelly connected to the IGBT5; The switching transistor T6 includes an IGBT6 and a diode D32 anti-parallelly connected to the IGBT6; The collector of the IGBT5 is connected to the positive electrode of the energy storage capacitor C3, its emitter is connected to the common point B, the collector of the IGBT6 is connected to the common point B, and its emitter is connected to the negative electrode of the energy storage capacitor C3; The switching transistor T7 includes an IGBT7 and a diode D7 anti-parallelly connected to the IGBT7.

7. The control method of the improved double-clamping sub-module according to claim 6, characterized in that, In the state 21), the current path is: common point A → diode D11 in the first half-bridge sub-module → energy storage capacitor C1 → diode D21 in the second half-bridge sub-module → energy storage capacitor C2 → diode D7 in the switching transistor T7 → energy storage capacitor C3 → diode D32 in the third half-bridge sub-module → common point B.

8. The control method of the improved double-clamping sub-module according to claim 6, characterized in that, In the state 22), the current path includes: 1) common point B → diode D31 in the third half-bridge sub-module → energy storage capacitor C3 → diode D3 → diode D22 in the second half-bridge sub-module → diode D12 in the first half-bridge sub-module → common point A; 2) common point B → diode D31 → diode D1 → energy storage capacitor C1 → diode D12 → common point A; 3) common point B → diode D31 → diode D2 → energy storage capacitor C2 → diode D22 → diode D12 → common point A.

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