Reverse resistance-half-bridge hybrid submodule, DC / DC converter and control method thereof

Abstract

The present invention relates to a reverse resistance half-bridge hybrid submodule, a DC / DC converter and a control method thereof. The reverse resistance half-bridge hybrid submodule comprises a half-bridge submodule, an IGBT module VT6, a reverse resistance half-bridge submodule and a diode D5. The present invention adopts a hybrid form of a reverse resistance half-bridge submodule and a traditional half-bridge submodule, utilizes the forward blocking capability of the reverse resistance half-bridge submodule to block the forward current, utilizes the unidirectional conductive characteristic of the diode to block the reverse current, avoids the increase of the current flowing through each switching device during a bipolar short circuit fault, and causes damage to the switching device, thereby achieving the goal of a high-voltage and large-capacity DC / DC converter using the advantages of the submodule topology to clear the fault current.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converters, and in particular to a reverse resistance-half bridge hybrid submodule, a DC / DC converter and a control method thereof. Background Art

[0002] Offshore wind power has the advantages of abundant wind resources, no land occupation, and proximity to load centers. Offshore DC wind farms with DC collection and DC transmission use high-power density power electronic converters, eliminating the bulky AC step-up transformers and reducing the size and weight of offshore converter stations. At the same time, DC cables are used instead of AC cables inside offshore wind farms to reduce the probability of excessive reactive voltage. Therefore, it has broad development prospects. Among them, high-voltage and large-capacity DC / DC converters are key equipment for offshore DC wind farms. In addition to completing large-capacity and high-voltage ratio energy transmission under steady-state conditions, they must also solve the problem of cutting off fault current when a short-circuit fault occurs in the DC line.

[0003] Since the inverter side (i.e., the low-voltage side) of the high-voltage and large-capacity DC / DC converter is connected to the collection network, and the rectifier side (i.e., the high-voltage side) is connected to the high-voltage transmission line, and the energy of the entire wind farm needs to be transmitted at the same time, when a bipolar short circuit fault occurs on either side of the converter, the capacitor of the MMC (Modular Multilevel Converter) sub-module and the short-circuit point form a discharge loop, the short-circuit point current rises rapidly, and the current flowing through each switching device also increases greatly, causing serious damage to the switching devices. Summary of the invention

[0004] In view of this, the present invention provides a reverse resistance-half bridge hybrid submodule, a DC / DC converter and a control method thereof to avoid an increase in current flowing through each switching device during a bipolar short circuit fault, thereby preventing the switching device from being damaged.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A reverse resistance-half bridge hybrid submodule, the hybrid submodule comprising: a half bridge submodule, an IGBT module VT6, a reverse resistance half bridge submodule and a diode D5;

[0007] The half-bridge sub-module, the IGBT module VT6 and the reverse resistance half-bridge sub-module are sequentially connected in series to form a first series branch;

[0008] The diode D5 is connected in anti-parallel with the first series branch.

[0009] Optionally, the reverse resistance half-bridge submodule includes: an IGBT module VT3, a RB-IGBT module VT4, a RB-IGBT module VT5 and a capacitor C2;

[0010] The IGBT module VT3 and the RB-IGBT module VT4 are connected in series to form a second series branch;

[0011] The RB-IGBT module VT5 is connected in reverse parallel to the RB-IGBT module VT4;

[0012] The capacitor C2 is connected in parallel with the second series branch.

[0013] Optionally, the half-bridge submodule includes: an IGBT module VT1, an IGBT module VT2 and a capacitor C1;

[0014] The IGBT module VT1 and the IGBT module VT2 are connected in series to form a third series branch;

[0015] The capacitor C1 is connected in parallel with the third series branch.

[0016] Optionally, the IGBT module VT1, the IGBT module VT2, the IGBT module VT3 and the IGBT module VT6 are respectively connected in reverse parallel with a diode D1, a diode D2, a diode D3 and a diode D4.

[0017] Optionally, when the hybrid submodule is in the first input state, the IGBT module VT1, the IGBT module VT3, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT2 and the RB-IGBT module VT4 are in the off state;

[0018] When the hybrid submodule is in the second input state, the IGBT module VT2, the RB-IGBT module VT4, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT1 and the IGBT module VT3 are in the off state;

[0019] When the hybrid submodule is in the third input state, the IGBT module VT1, the RB-IGBT module VT4, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT2 and the IGBT module VT3 are in the off state;

[0020] When the hybrid submodule is in the cut-off state, the IGBT module VT2, the IGBT module VT3, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT1 and the RB-IGBT module VT4 are in the off state;

[0021] When the hybrid submodule is in a locked state, the IGBT module VT1, the IGBT module VT2, the IGBT module VT3, the RB-IGBT module VT4, the RB-IGBT module VT5 and the IGBT module VT6 are all in an open state.

[0022] A DC / DC converter based on MMC, wherein the submodule in each phase MMC of the DC / DC converter is the above-mentioned hybrid submodule.

[0023] A control method for a DC / DC converter based on MMC, the control method is applied to the above-mentioned DC / DC converter based on MMC, and the control method comprises the following steps:

[0024] Determine the target input state of the hybrid submodule according to the target voltage that the hybrid submodule in each phase MMC of the DC / DC converter needs to output; the target input state is the first input state, the second input state or the third input state;

[0025] When no bipolar short-circuit fault occurs on the inverter side and the rectifier side of the DC / DC converter, the modulation strategy of the half-bridge submodule is adopted to control the switching between the target input state and the cut-off state of the hybrid submodule in each phase MMC of the DC / DC converter;

[0026] When a bipolar short circuit fault occurs in at least one of the inverter side and the rectifier side, each hybrid submodule in the at least one side is switched to a locked state.

[0027] Optionally, determining the target input state of the hybrid submodule according to the target voltage that the hybrid submodule in each phase MMC of the DC / DC converter needs to output specifically includes:

[0028] When the target voltage that the hybrid submodule needs to output is U C1 , determining that the target input state of the hybrid submodule is a first input state;

[0029] When the target voltage that the hybrid submodule needs to output is U C2 , determining that the target input state of the hybrid submodule is a second input state;

[0030] When the target voltage that the hybrid submodule needs to output is U C1 +U C2, determining that the target input state of the hybrid submodule is a third input state;

[0031] Among them, U C1 is the voltage across capacitor C1, U C2 is the voltage across capacitor C2.

[0032] A control system for a DC / DC converter based on MMC, the control system is applied to the above-mentioned DC / DC converter based on MMC, and the control system comprises:

[0033] A target input state determination module is used to determine the target input state of the hybrid submodule according to the target voltage that the hybrid submodule in each phase MMC of the DC / DC converter needs to output; the target input state is the first input state, the second input state or the third input state;

[0034] A modulation module, used for switching the hybrid submodules in each phase MMC of the DC / DC converter between a target input state and a cut-off state by using a modulation strategy of a half-bridge submodule when no bipolar short circuit fault occurs on both the inverter side and the rectifier side of the DC / DC converter;

[0035] The protection module is used to switch each hybrid submodule in at least one side to a locked state when a bipolar short circuit fault occurs in at least one side of the inverter side and the rectifier side.

[0036] Optionally, the target input state determination module specifically includes:

[0037] The first target input state determination submodule is used when the target voltage to be output by the hybrid submodule is U C1 , determining that the target input state of the hybrid submodule is a first input state;

[0038] The second target input state determination submodule is used when the target voltage to be output by the hybrid submodule is U C2 , determining that the target input state of the hybrid submodule is a second input state;

[0039] The third target input state determination submodule, when the target voltage that the hybrid submodule needs to output is U C1 +U C2 , determining that the target input state of the hybrid submodule is a third input state;

[0040] Among them, U C1 is the voltage across capacitor C1, U C2 is the voltage across capacitor C2.

[0041] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0042] The present invention discloses a reverse resistance-half bridge hybrid submodule, a DC / DC converter and a control method thereof. The reverse resistance-half bridge hybrid submodule comprises a half bridge submodule, an IGBT module VT6, a reverse resistance half bridge submodule and a diode D5. The present invention adopts a reverse resistance half bridge submodule and a traditional half bridge submodule in a hybrid form, utilizes the forward blocking capability of the reverse resistance half bridge submodule to block the forward current, utilizes the unidirectional conductive characteristic of the diode to block the reverse current, and when applied to a DC / DC converter, can avoid damage to the switch device due to the increase of the current flowing through each switch device during a bipolar short circuit fault, thereby achieving the goal of clearing the fault current by utilizing the advantages of the submodule topology in a high voltage and large capacity DC / DC converter.

[0043] The reverse-resistance-half-bridge hybrid submodule proposed in the present invention adopts a hybrid mode of a reverse-resistance half-bridge submodule and a traditional half-bridge submodule. Therefore, the modulation strategy of the MMC composed of half-bridge submodules (such as the double phase-shift modulation strategy) is also applicable to the MMC composed of reverse-resistance-half-bridge hybrid submodules. At present, the control research on the MMC composed of half-bridge submodules is relatively mature, so the control difficulty of the MMC composed of reverse-resistance-half-bridge hybrid submodules is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technical personnel in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0045] Figure 1 A schematic structural diagram of a reverse resistance-half-bridge hybrid submodule provided in Embodiment 1 of the present invention;

[0046] Figure 2 A schematic diagram of the structure of a DC / DC converter provided in Embodiment 1 of the present invention;

[0047] Figure 3 A schematic diagram of the structure of a half-bridge submodule provided in Embodiment 1 of the present invention;

[0048] Figure 4 A schematic diagram of the structure of a full-bridge submodule provided in Example 1 of the present invention;

[0049] Figure 5 A schematic diagram of the structure of a clamped twin module provided in Embodiment 1 of the present invention;

[0050] Figure 6 A schematic diagram of the working state of the half-bridge submodule provided in Embodiment 1 of the present invention;

[0051] Figure 7 A schematic diagram of the working state of the full-bridge submodule provided in Example 1 of the present invention;

[0052] Figure 8 A schematic diagram of the working state of the clamping twin module provided in Embodiment 1 of the present invention;

[0053] Fig. 9 A schematic diagram of the structure of a reverse resistance half-bridge submodule provided in Embodiment 1 of the present invention;

[0054] Fig.10 A schematic diagram of the current flow path of the reverse resistance-half-bridge hybrid submodule in a locked state provided in Embodiment 1 of the present invention;

[0055] Fig.11 An equivalent circuit diagram of the reverse resistance-half-bridge hybrid submodule provided in Embodiment 1 of the present invention when the current is positive in the locked state;

[0056] Fig.12 An equivalent circuit diagram of the reverse resistance-half-bridge hybrid submodule provided in Embodiment 1 of the present invention when the current is negative in the locked state;

[0057] Fig.13 A simulation diagram of the upper bridge arm current of the A phase of the DC / DC converter based on the MMC composed of half-bridge submodules provided in Example 2 of the present invention;

[0058] Fig.14 This is a simulation diagram of the A-phase upper bridge arm current of the DC / DC converter based on the MMC composed of hybrid sub-modules provided in Example 2 of the present invention. DETAILED DESCRIPTION

[0059] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technical personnel in this field without creative work are within the scope of protection of the present invention.

[0060] The object of the present invention is to provide a reverse resistance-half bridge hybrid submodule, a DC / DC converter and a control method thereof, so as to avoid the increase of current flowing through each switching device during a bipolar short circuit fault, thereby preventing the switching device from being damaged.

[0061] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0062] Example 1

[0063] Embodiment 1 of the present invention provides a reverse resistance-half bridge hybrid submodule, such as Figure 1 As shown, the hybrid sub-module includes: a half-bridge sub-module, an IGBT module VT6, a reverse-resistance half-bridge sub-module and a diode D5; the half-bridge sub-module, the IGBT module VT6 and the reverse-resistance half-bridge sub-module are connected in series in sequence to form a first series branch; the diode D5 is connected in reverse parallel to the first series branch.

[0064] The reverse resistance half-bridge submodule includes: an IGBT module VT3, a RB-IGBT module VT4, a RB-IGBT module VT5 and a capacitor C2; the IGBT module VT3 and the RB-IGBT module VT4 are connected in series to form a second series branch; the RB-IGBT module VT5 is connected in reverse parallel to the RB-IGBT module VT4; and the capacitor C2 is connected in parallel to the second series branch.

[0065] The half-bridge submodule includes: an IGBT module VT1, an IGBT module VT2 and a capacitor C1; the IGBT module VT1 and the IGBT module VT2 are connected in series to form a third series branch; the capacitor C1 is connected in parallel to the third series branch.

[0066] The IGBT module VT1 , the IGBT module VT2 , the IGBT module VT3 and the IGBT module VT6 are respectively connected in reverse parallel with a diode D1 , a diode D2 , a diode D3 and a diode D4 .

[0067] 1. DC / DC converter based on MMC

[0068] As the key equipment of offshore DC wind farms, high-voltage and large-capacity DC / DC converters need to realize large-capacity and high-voltage ratio energy transmission. Therefore, MMC-based DC / DC converter structures are adopted. Figure 2 As shown, each bridge arm on the inverter side and the rectifier side adopts MMC, and each MMC consists of multiple sub-modules SM ( Figure 2 The exemplary embodiment is a half-bridge type sub-module) connected in series.

[0069] On the inverter side, U MV is the DC input voltage on the inverter side, m is the number of submodules in each bridge arm of the single-phase MMC on the inverter side, L p is the bridge arm inductance, u ua 、u ub 、u la 、u lb is the bridge arm voltage, i ua 、i ub 、i la 、i lbis the bridge arm current, subscript u represents the upper bridge arm, subscript l represents the upper and lower bridge arms, and subscripts a and b represent phases a and b respectively; on the rectifier side, U HV is the DC output voltage on the rectifier side, M is the number of submodules in each bridge arm of the single-phase MMC on the rectifier side, and L s is the bridge arm inductance, u uA 、u uB 、u lA 、u lB is the bridge arm voltage, i uA 、i uB 、i lA 、i lB is the bridge arm current, the subscripts u and l are consistent with the corresponding parameters on the inverter side, and the subscripts A and B represent phases A and B respectively. The ratio of the medium-frequency transformer connecting the MMC on both sides of the inverter and rectifier is 1:K T ,i p 、i s are the currents of the primary and secondary sides of the transformer, U p , U s are the primary and secondary voltages of the transformer respectively.

[0070] At present, the commonly used sub-modules in MMC are mainly half-bridge type, full-bridge type and clamped twin sub-module type. Figure 3-5 As shown, Figure 2 The submodule shown in FIG. 1 is a half-bridge type.

[0071] The half-bridge submodule consists of two fully-controlled devices VT1 and VT2 connected in series. Each fully-controlled device is anti-parallel connected with a diode to form a half-bridge, and a capacitor is connected in parallel to form a half-bridge submodule.

[0072] According to the on-off status of the fully controlled devices VT1 and VT2, the half-bridge submodule can work in three working states: blocking, switching on, and cutting off. At the same time, due to the different current flow directions, each working state is further divided into two modes. Therefore, there are a total of 6 working states of the half-bridge submodule, such as Figure 6 As shown, let the capacitor voltage be U C .

[0073] Figure 6 0 means off, 1 means on. In the locked working state, all the control devices VT1 and VT2 are off. When the current flows from point A to point B, the current starts from point A, passes through the anti-parallel diode of VT1, flows through the capacitor and returns to point B. At this time, the port voltage is equal to the capacitor voltage U C ; When the current flows in the reverse direction, the current starts from point B and returns to point A through the anti-parallel diode of VT2. At this time, the port voltage is equivalent to a short circuit and the output voltage is 0.

[0074] In the working state, the fully controlled device VT1 is turned on and VT2 is turned off. When the current flows from point A to point B, the flow path is the same as the flow path of the current from point A to point B in the locked working state. The external current charges the capacitor, and the port output voltage is the capacitor voltage U C When the current flows in the reverse direction, it starts from point B, flows through the capacitor, and returns to point A through VT1. The port voltage is still the capacitor voltage U C , at this time the capacitor discharges to the external circuit. Therefore, when the half-bridge submodule is working in the on state, regardless of the direction of the current flow, the port voltage is the capacitor voltage U C , but the direction of current flow will affect the charging and discharging state of the capacitor.

[0075] In the cut-off working state, the fully controlled device VT1 is turned off and VT2 is cut off. When the current flows from point A to point B, the current flows from point A to point B through VT2. Ignoring the resistance on the device, the port is equivalent to being short-circuited, and the port voltage is 0; when the current flows from point B to point A, the current directly returns to point A from point B through the anti-parallel diode of VT2, and the port voltage is also 0. Therefore, when the half-bridge submodule works in the cut-off state, regardless of the direction of the current flow, the port voltage is 0, the capacitor works in the bypass mode, and is not in the current flow path, so the capacitor charging and discharging phenomenon will not occur.

[0076] The structure of the full-bridge submodule is as follows Figure 4 As shown in the figure, due to the increase in the number of devices, its working principle is more complicated than that of the half-bridge submodule. The biggest difference is that the port voltage of the full-bridge submodule can output a negative level. Therefore, referring to the classification of the working state of the half-bridge submodule, the working state of the full-bridge submodule will have an additional negative input working state. According to the opening of the full-control device, the full-bridge submodule can be divided into four working states, namely, locking, positive input, negative input, and bypass. At the same time, according to the different current flow directions, each working state is further divided into two, so there are a total of eight working states, such as Figure 7 shown.

[0077] 1) Locking working state: All the fully controlled devices VT1~VT4 are turned off, and the current flows in different directions. Figure 7 As shown in the first column, when the current flows from point A to point B, the port voltage is +U C , otherwise, the port voltage is -U C , but no matter how the current flows, the external current charges the capacitor.

[0078] 2) Positive working state: Fully controlled devices VT1 and VT4 are turned on, and the current flows in different directions. Figure 7 As shown in the second column, no matter how the current direction changes, the port voltage is +U C, the direction of current flow only affects the charging and discharging state of the capacitor.

[0079] 3) Negative input working state: Fully controlled devices VT2 and VT3 are turned on, and the current flows in different directions. Figure 7 As shown in the third column, no matter how the current direction changes, the port voltage is -U C , the direction of current flow only affects the charging and discharging state of the capacitor.

[0080] 4) Bypass working state: Fully controlled devices VT1, VT3 or VT2, VT4 are turned on, Figure 7 The current flow path of the last column of bypass working state takes VT1 and VT3 as an example. In this working state, the capacitor does not participate in the operation, the port of the submodule is equivalent to a short circuit, and the output voltage is 0.

[0081] The structure of the clamped twin module is as follows Figure 5 As shown in the figure, compared with the full-bridge submodule, the clamped twin submodule adds a pair of IGBTs and their anti-parallel diodes, a capacitor and two separate diodes. Under normal working conditions, the fully controlled device VT5 is always in the on state. By controlling VT1-VT4, the clamped twin module can output 0, U C , 2U C Three levels; when locked, current flows from port A to port B, clamping the twin module output 2U C , clamp the twin module output when the current is reversed -U C The output level and current flow path of the clamping twin module under different IGBT switching states and different current directions are as follows: Figure 7 shown.

[0082] When a bipolar short circuit fault occurs, the system gives a signal to the MMC submodule after a delay of 2-3ms. Figure 6 It can be seen from the figure that after the MMC composed of half-bridge sub-modules is locked, when the current flows into point B of the half-bridge sub-module, it will directly flow out from point A through the reverse-parallel diode of VT2. Therefore, when the current is reversed, the half-bridge sub-module does not have the fault blocking capability; Figure 4 After the MMC composed of the full-bridge submodules shown is locked, no matter whether the current flows in from A or B, the current charges the capacitor, and the capacitor provides a reverse voltage to the diode. The unidirectional conductive characteristic of the diode can be used to clear the fault current. However, the number of switching devices in the full-bridge submodule is doubled compared to the half-bridge submodule; Figure 5The MMC composed of the clamped twin modules shown in the figure is locked. When the current flows in from point A, it will flow out from point B through two series capacitors, but when the current flows in from point B, it will flow out from point A through two parallel capacitors. Although the capacitors are in a charged state under different current directions, the MMC can use the reverse voltage provided by the capacitors of the clamped twin modules and the unidirectional conductive characteristics of the diodes to cut off the fault current, but the coupling relationship of the capacitors inside the clamped twin modules is different under different current directions, which makes it more complicated to control. In summary, it is necessary to study a new submodule that can also have fault blocking capability under current reverse conditions, with a small number of components and a simple capacitor coupling relationship.

[0083] 2. Reverse resistance - half-bridge new hybrid sub-module

[0084] when Figure 2 When a bipolar short circuit fault occurs on either side of the DC / DC converter shown in the figure, before all submodules are locked, each switch tube is still in a normal triggering mechanism. At this time, the half-bridge submodule capacitors in the upper and lower bridge arms of each phase and the short circuit point form a discharge loop, and the capacitors of the half-bridge submodules discharge to the short circuit point through VT1, causing the DC current at the fault point to increase rapidly. After the submodules (half-bridge submodules) of the MMC on the fault side (inverter side or rectifier side) are locked, the normally working non-fault side MMC will continue to input current to the DC fault point through the isolation transformer and the locked fault side MMC. The flow path of this continuous current is mainly through the AC current of the isolation transformer through the anti-parallel diode of the half-bridge submodule VT2 to the fault position. Therefore, even if the submodules of the fault side MMC are locked, there is still a stable current that continues to flow to the fault point, and the fault side MMC cannot block the fault current through its own topology and control.

[0085] The reverse resistance half-bridge submodule is Figure 3 In the traditional half-bridge submodule, VT2 (traditional IGBT) and its anti-parallel diode are replaced with reverse blocking (RB) IGBT. RB-IGBT is a new type of power semiconductor device with reverse blocking capability proposed by Fuji Electric Corporation of Japan. The anti-parallel diode of the traditional IGBT is eliminated inside it. It can withstand the same voltage in both forward and reverse directions and has the characteristics of low conduction loss. The structure of the reverse blocking half-bridge submodule is as follows: Fig. 9 As shown, the reverse resistance half-bridge sub-module includes an IGBT module VT1, a diode D1, a RB-IGBT module VT2, a RB-IGBT module VT3 and a capacitor C.

[0086] When the reverse-resistance half-bridge sub-module is working normally, it is necessary to ensure that the B-IGBT module VT3 is always in the on state. The output level of the sub-module can be controlled by controlling the switching state of the IGBT module VT1 and the RB-IGBT module VT2. Therefore, the working mode of the reverse-resistance half-bridge sub-module is consistent with the traditional half-bridge sub-module.

[0087] The present invention connects the reverse resistance type half-bridge submodule and the traditional half-bridge submodule in series to form a Figure 1 The reverse resistance-half-bridge hybrid sub-module shown has reverse blocking capability, and at the same time, when the bridge arm current reverses, the voltage borne by the RB-IGBT module is clamped at the voltage level of the capacitor, preventing the RB-IGBT module in the hybrid sub-module that is locked in advance from bearing the AC side voltage alone when the sub-modules are not locked at the same time, thereby avoiding the phenomenon that the voltage borne by the RB-IGBT module is too large, which may cause the switching tube to be broken down.

[0088] Figure 1 When the hybrid submodule shown is working normally, it is necessary to ensure that VT5 and VT6 are always in the open state. By triggering VT1-VT4, the hybrid submodule can output 0, U C1 , U C2 , U C1 +U C2 .

[0089] When the hybrid submodule is in the first input state, the IGBT module VT1, the IGBT module VT3, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT2 and the RB-IGBT module VT4 are in the off state.

[0090] When the hybrid submodule is in the second input state, the IGBT module VT2, the RB-IGBT module VT4, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT1 and the IGBT module VT3 are in the off state.

[0091] When the hybrid submodule is in the third input state, the IGBT module VT1, the RB-IGBT module VT4, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT2 and the IGBT module VT3 are in the off state.

[0092] When the hybrid submodule is in the cut-off state, the IGBT module VT2, the IGBT module VT3, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT1 and the RB-IGBT module VT4 are in the off state.

[0093] When the hybrid submodule is in a locked state, the IGBT module VT1, the IGBT module VT2, the IGBT module VT3, the RB-IGBT module VT4, the RB-IGBT module VT5 and the IGBT module VT6 are all in an open state.

[0094] The triggering states of the switch tubes of the reverse resistance-half bridge hybrid submodule in the normal working state and the locked state and the corresponding output levels of the submodule are shown in Table 1. arm >0 means the current flows in from A and out from B, and the input current is recorded as positive. Conversely, the current flows in from B and out from A, and the input current is recorded as negative. arm <0.

[0095] Table 1 Working status of hybrid submodule

[0096]

[0097] In the latching working state, when the input current direction is different, the current flow path is as follows Fig.10 When the current is positive, the current flows in from A, passes through two diodes, two capacitors in series, and two anti-parallel RB-IGBTs, and flows out from B. The equivalent circuit is as follows Fig.11 As shown, at this time, the current charges the capacitor, the cathode of the diode is connected to the positive voltage end of the capacitor, and the anode is connected to the negative voltage end of the capacitor. Therefore, the diode is subjected to reverse voltage and is in an off state, so that the hybrid sub-module plays a role in clearing the forward fault current. In addition, the two RB-IGBT modules have forward and reverse blocking capabilities, which can forcefully block the fault current, thereby enhancing the hybrid sub-module's blocking capability for the forward fault current.

[0098] When the input current is negative, the current passes through D3, D5, and D2 in sequence to charge the series capacitor inside the hybrid submodule. The equivalent circuit is as follows: Fig.12As shown, the capacitor voltage provides a reverse voltage for the diode, and the unidirectional conductive characteristics of the diode can be used to limit the inflow of current, thereby blocking the transmission of current from the AC side to the fault point. In summary, regardless of whether the current input to the hybrid module is positive or negative, the reverse resistance-half-bridge hybrid module has fault blocking capability, and at the same time, the two capacitors inside it are always in series mode when the hybrid module is locked, and the coupling state of the two capacitors inside the clamping twin module will not change with the change of current direction when the clamping twin module is locked.

[0099] When a bipolar short circuit fault occurs on the DC side of the DC / DC converter, the short circuit point current mainly consists of two parts: one part is the capacitor discharge current in the discharge loop formed by the submodule capacitors of the upper and lower bridge arms of each phase and the short circuit fault point, and the other part is the stable current fed to the short circuit fault point by the non-fault side MMC through the AC transformer. During the period from the occurrence of the short circuit fault to the locking of all submodules of the fault side MMC, the capacitor discharge current accounts for the main component of the short circuit point current and the bridge arm current, so the input current of each submodule is negative. When the system gives the fault side MMC a locking signal, the capacitor cannot be put into the discharge loop, so the capacitor stops discharging, but due to the effect of the bridge arm inductance, the bridge arm current cannot change suddenly, and the magnetic field energy stored in the bridge arm inductance before locking is released. Therefore, in the early stage of the fault side MMC locking, the bridge arm current direction is still consistent with the capacitor discharge current direction, and the submodule input current is still negative.

[0100] From the above analysis, it can be seen that when the MMC on the rectifier side and the inverter side of the DC / DC converter is composed of a reverse resistance-half-bridge hybrid submodule, in the early stage of the reverse resistance-half-bridge hybrid submodule locking after a bipolar short circuit fault occurs, the input current is negative. Due to the inconsistency between the system locking command and the action time of each reverse resistance-half-bridge hybrid submodule, the locking time of each reverse resistance-half-bridge hybrid submodule is slightly different. The voltage borne by the RB-IGBT module in the reverse resistance-half-bridge hybrid submodule that is locked in advance is clamped to the voltage of capacitor C2 (see the current flow path). Fig.10 ), the RB-IGBT module will not need to bear the entire AC voltage of the AC transformer near the fault side, thus avoiding the phenomenon that the RB-IGBT module is broken down due to excessive voltage. Fig. 9The reverse resistance type half-bridge submodule shown constitutes the MMC. After the fault side MMC is locked, the submodule locked in advance will bear the entire voltage on the AC side, which will cause the RB-IGBT module to be broken down, which is not conducive to the recovery of the DC / DC converter after the fault is cleared. If a clamped twin submodule is used to form the MMC, after the submodule is locked, since the input current is negative, the two capacitors inside it are in parallel. Compared with the reverse resistance-half-bridge hybrid submodule, the state of the two capacitors in series after locking, the fault blocking capability of the clamped twin submodule is reduced by half. Although the full-bridge submodule also has good fault blocking capability, the number of IGBT modules and diodes used when outputting unit levels under normal working conditions is the largest among the submodules mentioned in the present invention, and the submodule has a weaker advantage in the number of switching devices. Table 2 specifically lists the number of devices used by the full-bridge submodule, the clamped twin submodule and the hybrid submodule on a single bridge arm when the submodule input current is negative and has the same blocking capability.

[0101] Table 2 Number of different types of submodule switch devices used on a single bridge arm

[0102]

[0103] In Table 2, N is

[0104]

[0105] Where U dc The formula (1) represents the theoretical value of the number of submodules used in a single bridge arm to maintain the stability of the DC side voltage when the conventional half-bridge submodules are used to form the MMC.

[0106] Comparing the numbers of IGBT modules, RB-IGBT modules, diodes and capacitors used by different submodule types in Table 2 when the input current is negative and the MMC composed of them has the same reverse blocking capability, it can be found that the clamped twin modules are also NU in order to ensure that their reverse blocking capability is C The number of capacitors is twice that of the other two sub-modules, but in fact, a single clamping twin module has two capacitors inside, and can output 2U under normal circumstances. C Therefore, in order to ensure the principle of maintaining the DC side voltage stability under normal operation as expressed in formula (1), only N / 2 clamping twin modules are required on a single bridge arm of the MMC. At this time, the number of capacitors is also N. However, when the submodule is locked and the input current is negative, as shown in Figure 8 As shown, the submodule outputs -U CIn order to make the MMC composed of clamped twin submodules have the same reverse blocking fault current capability as the full-bridge MMC, the number of submodules needs to be doubled, and the number of switch devices and capacitors also increases accordingly, which not only increases the size of the DC / DC converter, but also increases the investment cost. However, after the reverse resistance-half-bridge hybrid submodule is locked, when the input current is negative, its two internal capacitors are connected in series to provide reverse voltage for the diode. The connection relationship of the capacitors in the locked state is the same as that of the submodule output 2U in the normal state. C (i.e. U C1 +U C2 , U C1 and U C2 Equal to U C ) level, the coupling state of the two capacitors is consistent, so the number of reverse resistance-half-bridge hybrid sub-modules required by the DC / DC converter in normal operation and blocking conditions is N / 2. There is no need to add extra reverse resistance-half-bridge hybrid sub-modules to make the hybrid sub-module type MMC have the same reverse blocking capability as the full-bridge sub-module type MMC. At the same time, the number of switches required in the bridge arm of the former is less than that of the latter, which can reduce investment costs and improve the economy of DC / DC converters based on MMC.

[0107] In summary, the MMC composed of reverse resistance-half-bridge hybrid sub-modules has the advantages of strong reverse blocking capability and a small number of switching devices. At the same time, since the reverse resistance-half-bridge hybrid sub-module uses RB-IGBT modules, the overall conduction loss is reduced. Therefore, the use of MMC composed of reverse resistance-half-bridge hybrid sub-modules in DC / DC converters has a broad development prospect.

[0108] Example 2

[0109] Embodiment 2 of the present invention provides a DC / DC converter based on MMC, wherein the sub-modules in each phase MMC of the DC / DC converter are hybrid sub-modules in Embodiment 1.

[0110] Figure 2 The submodule of MMC in the DC / DC converter shown is used Figure 1 The reverse resistance-half bridge hybrid submodule shown in the figure is replaced, and the model is built using MATLAB / Simulink software. The s / m modulation based on double phase shift is adopted, and the input inverter voltage is set to 6kV, the operating frequency of the medium frequency transformer is 1kHz, and the transformation ratio is K T is 2.5, the bridge arm inductance is 1mH, and the transformer leakage inductance L calculated to the primary side T The submodule capacitance (including capacitor C1 and capacitor C2) is 4700 μF , the rated voltage is 1.5kV, the number of sub-modules of the inverter side (i.e. the low-voltage side bridge arm is 4, and the number of sub-modules of the rectifier side (i.e. the high-voltage side) bridge arm is 40.

[0111] At 2.4s, a bipolar short-circuit fault was set on the rectifier side (i.e., high-voltage side) of the DC / DC converter. In order to verify the fault clearing capability of the hybrid submodule, a DC / DC converter based on MMC composed of half-bridge submodules was used for comparative verification in the simulation. In the DC / DC converter composed of half-bridge submodules, a permanent fault occurred on the high-voltage DC side at 2.4s. Fig.13 The current waveform of the upper bridge arm of phase A in the rectifier MMC is shown. Fig.13 It can be seen from the figure that when the fault current stabilizes, there is a stable AC component, which is consistent with the analysis of the fault current composition in Example 1. After the capacitor discharge is completed, there is still a stable fault current transmitted from the non-fault side MMC to the fault point through the AC transformer. Therefore, the bridge arm current first increases rapidly due to the capacitor discharge and then remains stable. For the DC / DC converter based on the MMC composed of hybrid submodules, a bipolar short circuit fault is set at 2.4s. After 2ms, all submodules of the fault side MMC are locked. Fig.14 It can be seen from the bridge arm current of phase A of the rectifier MMC that after the submodule is locked, the bridge arm current gradually decreases to 0, thereby verifying that the hybrid submodule has the ability to block fault current.

[0112] Through the above simulation analysis, it can be concluded that when a short circuit fault occurs on the DC side, the use of a submodule with fault blocking capability is an effective way to clear the fault current. The present invention proposes a reverse resistance-half-bridge hybrid submodule. By analyzing the flow path of the current in the locked state, the reason why the hybrid submodule has the ability to cut off the fault current is explained in principle, and the submodule is compared with the traditional half-bridge, full-bridge and clamped twin submodules, and the conclusion is obtained that the hybrid submodule has the advantages of strong blocking capability, a small number of switching devices and low conduction loss. Finally, the model is built on the MATLAB / Simulink simulation software to verify that the hybrid submodule has the ability to clear the fault current.

[0113] Example 3

[0114] Embodiment 3 of the present invention provides a control method for a DC / DC converter based on MMC. The control method is applied to the above-mentioned DC / DC converter based on MMC. The control method comprises the following steps:

[0115] The target input state of the hybrid submodule is determined according to the target voltage that the hybrid submodule in each phase MMC of the DC / DC converter needs to output; the target input state is the first input state, the second input state or the third input state.

[0116] When no bipolar short circuit fault occurs on both the inverter side and the rectifier side of the DC / DC converter, the modulation strategy of the half-bridge sub-module is adopted to control the switching between the target input state and the cut-off state of the hybrid sub-module in each phase MMC of the DC / DC converter.

[0117] When a bipolar short circuit fault occurs in at least one of the inverter side and the rectifier side, each hybrid submodule in the at least one side is switched to a locked state.

[0118] The step of determining the target input state of the hybrid submodule according to the target voltage to be output by the hybrid submodule in each phase MMC of the DC / DC converter specifically includes:

[0119] When the target voltage that the hybrid submodule needs to output is U C1 , determining that the target input state of the hybrid submodule is a first input state;

[0120] When the target voltage that the hybrid submodule needs to output is U C2 , determining that the target input state of the hybrid submodule is a second input state;

[0121] When the target voltage that the hybrid submodule needs to output is U C1 +U C2 , determining that the target input state of the hybrid submodule is a third input state;

[0122] Among them, U C1 is the voltage across capacitor C1, U C2 is the voltage across capacitor C2.

[0123] Based on the above embodiments, the advantages of the present invention are as follows:

[0124] Since the inverter side of the DC / DC converter based on MMC is connected to the collection network, the rectifier side is connected to the high-voltage transmission line, and the energy of the entire wind farm needs to be transmitted at the same time, when a bipolar short circuit fault occurs on either side of the converter, the submodule capacitor of the MMC and the short circuit point form a discharge loop, the current at the short circuit point rises rapidly, and the current flowing through each switch device also increases greatly, causing serious damage to the switch device. For this reason, the present invention proposes a reverse resistance-half-bridge hybrid submodule, which can effectively clear the fault current by using the topological structure of the submodule itself. When the current is input into the submodule in the forward direction, the unidirectional conductive characteristics of the diode and the RB-IGBT module have the same forward and reverse fault blocking capabilities to cut off the fault current; when the current is reversed, the capacitor provides a reverse voltage for the diode. At the same time, since the input current is negative at the initial stage of the fault lockout, even if the submodule lockout time is inconsistent under this current direction, the capacitor can clamp the voltage borne by the RB-IGBT module at the capacitor voltage level to prevent the RB-IGBT in the submodule that is locked in advance from bearing all the AC voltage.

[0125] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0126] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core ideas of the present invention. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A reverse resistance-half bridge hybrid submodule, characterized in that: The hybrid submodule includes: a half-bridge submodule, an IGBT module VT6, a reverse resistance half-bridge submodule and a diode D5; The half-bridge sub-module, the IGBT module VT6 and the reverse resistance half-bridge sub-module are sequentially connected in series to form a first series branch; The diode D5 is connected in reverse parallel with the first series branch; The reverse resistance half-bridge submodule includes: an IGBT module VT3, a RB-IGBT module VT4, a RB-IGBT module VT5 and a capacitor C2; The IGBT module VT3 and the RB-IGBT module VT4 are connected in series to form a second series branch; The RB-IGBT module VT5 is connected in reverse parallel to the RB-IGBT module VT4; The capacitor C2 is connected in parallel with the second series branch; The half-bridge submodule includes: IGBT module VT1, IGBT module VT2 and capacitor C1; The IGBT module VT1 and the IGBT module VT2 are connected in series to form a third series branch; The capacitor C1 is connected in parallel with the third series branch; The IGBT module VT1, the IGBT module VT2, the IGBT module VT3 and the IGBT module VT6 are respectively connected in reverse parallel with a diode D1, a diode D2, a diode D3 and a diode D4; When the hybrid submodule is in the first input state, the IGBT module VT1, the IGBT module VT3, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT2 and the RB-IGBT module VT4 are in the off state; When the hybrid submodule is in the second input state, the IGBT module VT2, the RB-IGBT module VT4, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT1 and the IGBT module VT3 are in the off state; When the hybrid submodule is in the third input state, the IGBT module VT1, the RB-IGBT module VT4, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT2 and the IGBT module VT3 are in the off state; When the hybrid submodule is in the cut-off state, the IGBT module VT2, the IGBT module VT3, the RB-IGBT module VT5 and the IGBT module VT6 are in the on state, and the IGBT module VT1 and the RB-IGBT module VT4 are in the off state; When the hybrid submodule is in a locked state, the IGBT module VT1, the IGBT module VT2, the IGBT module VT3, the RB-IGBT module VT4, the RB-IGBT module VT5 and the IGBT module VT6 are all in an open state.

2. A DC / DC converter based on MMC, characterized in that: The submodule in each phase MMC of the DC / DC converter is the hybrid submodule as claimed in claim 1.

3. A control method for a DC / DC converter based on MMC, characterized in that: The control method is applied to the MMC-based DC / DC converter according to claim 2, and the control method comprises the following steps: Determine the target input state of the hybrid submodule according to the target voltage that the hybrid submodule in each phase MMC of the DC / DC converter needs to output; the target input state is the first input state, the second input state or the third input state; When no bipolar short-circuit fault occurs on the inverter side and the rectifier side of the DC / DC converter, the modulation strategy of the half-bridge submodule is adopted to control the switching between the target input state and the cut-off state of the hybrid submodule in each phase MMC of the DC / DC converter; When a bipolar short circuit fault occurs in at least one of the inverter side and the rectifier side, each hybrid submodule in the at least one side is switched to a locked state.

4. The control method of the DC / DC converter based on MMC according to claim 3, characterized in that: The step of determining the target input state of the hybrid submodule according to the target voltage to be output by the hybrid submodule in each phase MMC of the DC / DC converter specifically includes: When the target voltage that the hybrid submodule needs to output is U C1 , determining that the target input state of the hybrid submodule is a first input state; When the target voltage that the hybrid submodule needs to output is U C2 , determining that the target input state of the hybrid submodule is a second input state; When the target voltage that the hybrid submodule needs to output is U C1 +U C2 , determining that the target input state of the hybrid submodule is a third input state; Among them, U C1 is the voltage across capacitor C1, U C2 is the voltage across capacitor C2.

5. A control system of a DC / DC converter based on MMC, characterized in that: The control system is applied to the MMC-based DC / DC converter according to claim 2, and the control system includes: A target input state determination module is used to determine the target input state of the hybrid submodule according to the target voltage that the hybrid submodule in each phase MMC of the DC / DC converter needs to output; the target input state is the first input state, the second input state or the third input state; A modulation module, used for switching the hybrid submodules in each phase MMC of the DC / DC converter between a target input state and a cut-off state by using a modulation strategy of a half-bridge submodule when no bipolar short circuit fault occurs on both the inverter side and the rectifier side of the DC / DC converter; The protection module is used to switch each hybrid submodule in at least one side to a locked state when a bipolar short circuit fault occurs in at least one side of the inverter side and the rectifier side.

6. The control system of the DC / DC converter based on MMC according to claim 5, characterized in that: The target input state determination module specifically includes: The first target input state determination submodule is used when the target voltage to be output by the hybrid submodule is U C1 , determining that the target input state of the hybrid submodule is a first input state; The second target input state determination submodule is used when the target voltage to be output by the hybrid submodule is U C2 , determining that the target input state of the hybrid submodule is a second input state; The third target input state determination submodule, when the target voltage that the hybrid submodule needs to output is U C1 +U C2 , determining that the target input state of the hybrid submodule is a third input state; Among them, U C1 is the voltage across capacitor C1, U C2 is the voltage across capacitor C2.

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