Method for starting up plurality of series-connected subunits of converter

Abstract

The present disclosure relates to a method for starting up a plurality of serially connected subunits (102) of a converter, each of the plurality of subunits connected in series includes a first power electronic switch (113) and a second power electronic switch (114) in a half-bridge arrangement, a chargeable element (115), a first gate drive unit (117) connected with the first power electronic switch, and a second gate drive unit (118) connected with the second power electronic switch. The method comprises: receiving a main voltage from an external voltage source at a plurality of subunits connected in series, the main voltage being thereby distributed between the subunits and charging chargeable elements of the subunits; electrifying the first gate unit in each subunit; temporarily switching the first power electronic switch in at least one, but less than all of the subunits, thereby bypassing the subunits; when the subunit voltage in one subunit in the plurality of subunits reaches the full working voltage level, electrifying the second grid unit in the subunit; and changing those subcells having a first electronic switch temporarily switched to bypass the subcells until the second gate cells of all subcells have been energized.

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to converters. In particular, the present disclosure relates to a method for starting a plurality of series connected sub-units of a converter. BACKGROUND

[0002] Converters, such as modular multilevel converters (MMC), typically comprise one or more converter arms, each converter arm having a number of series connected sub-units. Each sub-unit comprises a number of semiconductor devices, such as, for example, IGBT switches and / or other switching devices, a chargeable element, such as a sub-unit capacitor, and control devices, such as, for example, a control board for controlling the sub-unit and communicating with a central control unit, a gate drive unit (GDU) controlling the switches, etc. In different converter configurations or applications, there can be different initial sub-unit voltages, however, in any case, the sub-unit voltage needs to exceed a lower limit for starting the sub-unit. In some cases, the main voltage applied to the converter and distributed among the sub-units is too low to reach the lower limit of the sub-unit voltage, which makes it impossible to start operation of the converter. SUMMARY

[0003] The present disclosure seeks to remedy at least in part the problems discussed above. To achieve this, a method for starting a plurality of series connected sub-units of a converter and an arrangement of a plurality of series connected sub-units of a converter are provided, as defined in the independent claims. Further embodiments are provided in the dependent claims.

[0004] According to an aspect of the present disclosure, a method for starting a plurality of series connected sub-units of a converter is provided, wherein each sub-unit of the plurality of series connected sub-units comprises a first and a second power electronic switch in a half-bridge arrangement, a chargeable element, a first gate drive unit connected with the first power electronic switch, and a second gate drive unit connected with the second power electronic switch. The method comprises: receiving a main voltage at the plurality of series connected sub-units from an external voltage source, which main voltage is thereby distributed among the sub-units and charges the chargeable elements of the sub-units; energizing the first gate unit in each sub-unit; temporarily switching the first power electronic switch in at least one, but less than all, of the sub-units, thereby bypassing the sub-unit; energizing the second gate unit in a sub-unit of the plurality of sub-units when the sub-unit voltage in that sub-unit reaches a full operating voltage level; and changing those sub-units in which the first electronic switch is temporarily switched until the second gate unit in all sub-units has been energized.

[0005] By bypassing some of the sub-units, the sub-unit voltage of the other sub-units will increase, which in turn results in an increased voltage of the chargeable element. Thereby, a sufficiently high voltage can be reached to start those sub-units. By changing which sub-units are bypassed, eventually all sub-units will be started and running, and the converter will be fully operational.

[0006] According to an embodiment of the method, the operation of energizing the first gate unit is performed when the sub-unit voltage reaches an under- operating voltage level which is lower than the full operating voltage level. Thereby, it is ensured that the switching of the first electronic switch is achieved, even if the main voltage is too low to start all sub-units jointly.

[0007] According to an embodiment of the method, when each sub-unit comprises two main terminals and an external bypass switch arranged between the main terminals, the method further comprises detecting a short circuit in the second power electronic switch of the sub-unit, and closing the external bypass switch.

[0008] According to an embodiment of the method, the operation of closing the external bypass switch comprises energizing a bypass flip-flop by means of the first gate unit, wherein the bypass flip-flop switches the external bypass switch to the closed position.

[0009] According to another aspect of the disclosure, there is provided a converter comprising a valve control unit and a number of valves, each valve comprising a number of sub-units connected in series. Each sub-unit of the number of sub-units connected in series comprises a first power electronic switch and a second power electronic switch in a half-bridge arrangement, a chargeable element, a first gate drive unit connected to the first power electronic switch, a second gate drive unit connected to the second power electronic switch, and a control circuit. The converter is configured to perform a start-up procedure during which: the control circuit of each sub-unit of the number of sub-units connected in series is configured to initially energize only the first gate unit, temporarily switch the first power electronic switch in at least one of the sub-units, but less than all of the sub-units, thereby bypassing the sub-unit, energize the second gate unit in each of the sub-units where the sub-unit voltage reaches the full operating voltage level, change which sub-units the first electronic switch is temporarily switched in, until the second gate unit in all of the sub-units has been energized.

[0010] The described arrangement provides similar advantages as the above-mentioned method. The arrangement can be comprised in different types of converters, such as modular multilevel converters, non-modular converters, etc., where the structure of the above-defined sub-unit arrangement is provided.

[0011] According to an embodiment of the arrangement, each subunit includes an external bypass switch. For example, the external bypass switch may be a sacrificial mechanical switch or a semiconductor switch, or a reusable vacuum switch.

[0012] According to one embodiment of the arrangement, the control circuit is configured to energize the first gate cell when the sub-cell voltage reaches an underoperating voltage level lower than the full operating voltage level.

[0013] According to the embodiment of the arrangement, multiple sub-units constitute one arm of the converter. Attached Figure Description

[0014] Exemplary embodiments of the present disclosure will now be described with reference to the accompanying drawings, in which: Figure 1 This is a block diagram of an embodiment of a modular multilevel converter according to the present disclosure; Figure 2 It is a block diagram of the sub-unit structure included in the modular multilevel converter; and Figure 3 This is a flowchart of an embodiment of the method according to the present disclosure. Detailed Implementation

[0015] The present disclosure will be described below with reference to the accompanying drawings, in which exemplary embodiments are illustrated. However, the present disclosure should not be construed as limiting itself to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this specification will convey the scope of the disclosure to those skilled in the art.

[0016] like Figure 1 As shown, an exemplary structure of the converter, and more specifically an exemplary structure of the modular multilevel converter (MMC) 100, includes converter valves 101, such as three converter valves 101, one for each AC phase. Each converter valve 101 has multiple interconnected sub-units 102, also referred to as sub-modules. The MMC 100 is typically connected to an AC power source 110 (such as an AC grid or some other feasible AC power source) on one side via an AC circuit breaker 111 and a transformer 112, and to a DC system 120 (such as an HVDC system or any other feasible DC system) on the other side. The MMC 100 further includes a valve control unit (VCU) 103 connected to the converter valves 101. The VCU 103 can then be connected to a higher-level controller external to the MMC 100. The higher-level controller can be responsible for controlling the overall operation of the MMC 100 and other MMCs. Each converter valve 101 includes two valve arms 104, which are respectively connected between an AC phase terminal 105 and a corresponding DC terminal 106 (e.g., the positive and negative terminals of a DC system 120). Each arm 104 includes multiple sub-units 102 connected in series. This can be used with...Figure 1 The valve is defined in different ways as illustrated. For example, the valve can constitute one arm 104 or a part of an arm, in particular when the arm comprises a large number of sub-units. Thus, the term “valve” is generally defined herein as a plurality of sub-units 102 connected in series.

[0017] The sub-units 102 are half-bridge HB sub-units. Further reference is made to Figure 2 Each sub-unit 102 comprises a first sub-unit terminal 108 and a second sub-unit terminal 109, a first power electronic switch 113 and a second power electronic switch 114 in a half-bridge arrangement, a chargeable element 115, such as a capacitor, a control circuit, here embodied as a separate control device 116, and a first gate drive unit (GU) 117 and a second gate drive unit 118. The first power electronic switch 113 is connected between the first sub-unit terminal 108 and the second sub-unit terminal 109. The second power electronic switch 114 is connected to the first sub-unit terminal 108 and to a first connection of the chargeable element 115. The other connection of the chargeable element 115 is connected with the second sub-unit terminal 109. As understood by the skilled person, the chargeable element can be some other type of suitable chargeable element, such as a certain type of capacitor known as a super capacitor or a battery. Further, it should be noted that the control circuit, here shown as a separate control device 116, can alternatively be integrated with the first gate drive unit 117 and the second gate drive unit 118, and even all three parts can be embodied as a single unit, as indicated by the dashed box in Figure 2 The control device 116 is connected to the second sub-unit terminal 109 and to the first connection of the chargeable element 115, either directly or via a resistor or a fuse 121 as shown in Figure 2 The first GU 117 is connected to the first power electronic switch 113 and to the control device 116. The second GU 118 is connected to the second power electronic switch 114 and to the control device 116. The first and second GUs 117, 118 are configured to control switching of the respective power electronic switches 113, 114. The power electronic switches 113, 114 can be, for example, a combination of a bi-mode insulated gate transistor (BIGT), an insulated gate bipolar transistor (IGBT) and a freewheeling diode, an integrated gate-commutated thyristor (IGCT), a MOSFET, etc. The control device 116 is configured to communicate with a higher level control unit outside the sub-unit, e.g. a valve control unit 103. Further, each sub-unit 102 can comprise an external bypass switch 119 connected between the first sub-unit terminal 108 and the second sub-unit terminal 109. It should be noted that in Figure 1 and Figure 2In the middle, the structure of the MMC and the sub-units is shown in a most schematic way and is only an example, as these structures are known per se to the person skilled in the art. Other structures can also be used, as long as they can be configured to operate as will be described hereinafter.

[0018] According to an embodiment, the modular multilevel converter 100 is configured to perform a start-up procedure during which: the control device 116 of each sub-unit 102 is configured to initially energize only the first gate unit 117; the valve control unit 103 is configured to command the control device 116 of at least one sub-unit 102, but less than all sub-units 102, in each valve 101 to temporarily close the first power electronic switch 113, thereby bypassing the sub-unit 102; the control device 116 of each sub-unit 102 is configured to energize the second gate unit 118 when a sub-unit voltage in the sub-unit 102 reaches a full operating voltage level; and the valve control unit 103 is configured to change in each valve 101 those sub-units 102 that have the first electronic switch 113 temporarily closed until the second gate unit 118 of all sub-units 102 of the valve 101 have been energized.

[0019] More specifically, when a main voltage is applied to the MMC 100, due to the freewheeling diode function of the power electronic switches 113, 114 (which is a combined function of the power electronic switches, or a separate diode, depending on the type of power electronic switch), the chargeable elements 115 of each subcell 102 are powered and start charging. The control device 116 of each subcell 102 of the MMC 100 is configured to initially only power the first gate cell 117. This is because powering both the first gate cell 117 and the second gate cell 118 requires a higher subcell voltage (herein referred to as full operating voltage level) compared to powering only the first gate cell 117. In case the main voltage is too low to support all subcells 102 to be charged to the full operating voltage level, it can still be sufficient to support charging all subcells 102 to a voltage level that is at least equal to an under operating voltage level, herein referred to as under operating voltage level. The factor that actually influences the voltage level is the difference in characteristics of the components of the subcell. For example, there is a production deviation in the capacitance value of the capacitor used as chargeable element 115. As a practical example, and just for illustration, the under operating voltage level can be about 10% lower, or even lower, than the full operating voltage level. Then, as described above, the valve control unit 103 controls that some of the subcells 102 are bypassed by commanding the control device 116 of the subcell 102 to close the first power electronic switch 113. Advantageously, the valve control unit 103 will first bypass those subcells that have the highest voltage. Thereby, the main voltage is distributed over fewer subcells 102, enabling these subcells to charge the chargeable elements 115 to a higher voltage that is at or above the full operating voltage level. In other words, these subcells achieve bootstrap. In turn, this enables the control device 116 of those remaining subcells 102 that are not bypassed to power the second gate cell 118 as well, and thereby the start-up of these subcells is completed and they are in full operating state. Then, by switching the first power electronic switch 113 of the bypassed subcells 102 back to the open state, and bypassing other subcells 102 (e.g. those that are now in full operating state) in one or more sequences, eventually all subcells 102 have been bootstrapped and thus started up.

[0020] As Figure 3As shown, an embodiment of the method 300 comprises a number of operations, starting with receiving, in block 301, a main voltage Um from an external voltage source at a number of series connected sub-units 102. Thereby, the main voltage Um is distributed between the sub-units 102, and charging of the chargeable elements 115 of the sub-units 102 is started. The method further comprises energizing, in block 302, the control device 116 and the first gate unit 117 in each sub-unit 102. Energizing of the first gate unit 117 can be conditioned on that the sub-unit voltage Uc, which also is approximately the voltage across the chargeable element 115, reaches an under- operating voltage level. Thereby, the first power electronic switch 113 of each sub-unit 102 can be switched. The next operation is block 303, temporarily switching the first power electronic switch in at least one of the sub-units 102, but less than all of the sub-units 102, thereby bypassing the sub-unit. Block 304, by bypassing a suitable or large enough number of sub-units 102, the non-bypassed sub-units 102 are thereby boosted to a higher voltage, so that the second gate unit 118 can be energized when the sub-unit voltage Uc reaches a full operating voltage level. The method further comprises block 305, changing those of the sub-units 102 for which the first electronic switch 113 is temporarily switched to bypass the sub-unit 102, until the second gate unit 118 in all of the sub-units has been energized. Then, the modular multilevel converter 100 can start normal operation, which involves switching of both the first power electronic switch 117 and the second power electronic switch 118, depending on whether the sub-units 102 are controlled to, for example, charge, discharge or bypass.

[0021] If a fault condition occurs, such as if the sub-unit voltage Uc is sufficiently high but the second gate unit 118 does not activate, the reason can be that the second power electronic switch 114 is short-circuited. This condition can be detected, for example, by means of measuring the sub-unit voltage Uc and the voltage across the first power electronic switch 113 to determine the voltage across the second power electronic switch 114. If it is determined that the second power electronic switch 114 is short-circuited, the external bypass switch 119 can be closed by powering a bypass flip-flop (not shown per se) from the first gate unit 117. The bypass flip-flop in turn closes the external bypass switch 119.

[0022] While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered illustrative or exemplary only and not restrictive.

[0023] Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements.

[0024] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed application, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. A single processor or other unit can fulfil the functions of several items recited in the claims. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A method for activating a plurality of series-connected sub-units (102) of a converter (100), wherein, Each of the plurality of series-connected sub-units includes a first power electronic switch (113) and a second power electronic switch (114) arranged in a half-bridge configuration, a rechargeable element (115), a first gate driving unit (117) connected to the first power electronic switch, and a second gate driving unit (118) connected to the second power electronic switch. The method includes: A main voltage is received from an external voltage source at the plurality of series-connected sub-units, the main voltage being distributed among the sub-units and charging the rechargeable elements of the sub-units; The first gate unit is energized in each sub-unit; In at least one, but less than all, of the sub-units, the first power electronic switch is temporarily switched to bypass the sub-unit; When the voltage of a sub-unit in one of the plurality of sub-units reaches the full operating voltage level, the second gate unit in that sub-unit is energized; and The change causes the first electronic switch to temporarily switch those sub-units to bypass the sub-unit until the second gate unit in all sub-units is energized.

2. The method according to claim 1, wherein, When the voltage of the sub-unit reaches an underoperating voltage level that is lower than the full operating voltage level, the operation of energizing the first gate unit is performed.

3. The method according to claim 1 or 2, wherein, Each subunit includes two main terminals and an external bypass switch disposed between the main terminals, the method further comprising: A short circuit in the second power electronic switch of the detection subunit; and Close the external bypass switch.

4. The method according to claim 3, wherein, The closing operation of the external bypass switch includes powering a bypass trigger by means of the first gate unit, thereby switching the external bypass switch to the closed position.

5. A converter (100) includes a valve control unit (103) and a plurality of valves (101), each valve including a plurality of subunits (102) connected in series, wherein, Each of the plurality of series-connected sub-units includes a first power electronic switch (113) and a second power electronic switch (114) arranged in a half-bridge configuration, a rechargeable element (115), a first gate drive unit (117) connected to the first power electronic switch, a second gate drive unit (118) connected to the second power electronic switch, and control circuitry, wherein the control circuitry of each sub-unit is configured to communicate with the valve control unit, and wherein the converter is configured to execute a startup procedure during which: The control circuitry of each sub-unit is configured to initially power only the first gate unit. The valve control unit is configured to command the control circuitry of at least one, but fewer than all, of the sub-units in each valve to temporarily close the first power electronic switch, thereby bypassing the sub-unit. The control circuitry of each sub-unit is configured to energize the second gate unit when the sub-unit voltage in the sub-unit reaches the full operating voltage level, and The valve control unit is configured to change the sub-units in each valve that temporarily close the first electronic switch until the second gate unit in all sub-units of the valve is energized.

6. The converter according to claim 5, wherein, Each subunit includes an external bypass switch.

7. The converter according to claim 5 or 6, wherein, The control circuit is configured to energize the first gate unit when the sub-unit voltage reaches an underoperating voltage level lower than the full operating voltage level.

8. The converter according to any one of claims 5 to 7, wherein, Each valve constitutes one arm of the converter.

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