Method for energizing a modular multilevel converter - Patent application

Abstract

The present disclosure relates to a method for energizing a modular multilevel converter (100) comprising converter valves (101), each having a plurality of interconnected cells (102), each cell comprising a plurality of power electronic switches (105, 106, 107, 108) in a full-bridge configuration, a rechargeable element (109), and a plurality of gate drive units (118), one gate drive unit for each power electronic switch. The method includes supplying power to a modular multilevel converter from an external power source (S201); supplying power to a cell, thereby charging a chargeable element of the cell (S202); generating a cell sub-ready signal for the cell when a cell voltage resulting from the charging exceeds a first threshold voltage level (S203); generating a sub-ready_for_operation signal when at least a predetermined minimum number of cells have generated the sub-ready signal (S204); and performing a deblocking sequence (S205) including closing first power electronics switches (105) in all the cells to electrically configure the cells to be continuously charged as half-bridge cells and keeping the first power electronics switches closed until the cell voltage reaches a converter operable voltage that is higher than a second threshold voltage.

Description

[Technical Field]

[0001] Technical Field The present disclosure relates generally to modular multilevel converters, and more particularly to energizing such modular multilevel converters prior to commencing operation. [Background technology]

[0002] background A modular multilevel converter (MMC) typically includes a converter valve, each having several interconnected cells. Each cell includes several different semiconductor devices, such as IGBT switches and / or other switching devices, cell capacitors, and diodes, as well as a controller, such as a control board for controlling the cells and communicating with a central control unit, and a gate drive unit (GDU) for controlling the switches. Energy is typically harvested from the cell capacitors to power the controller. As a result, the cell capacitors must be charged to a sufficient charge level before the electronic controls can assume control of the semiconductor devices and the MMC, and before the MMC can begin operating as intended when connected to the power grid.

[0003] Control boards can only function properly if they have a sufficient power supply, which can be achieved, for example, through specific function boards and a DC-DC converter that converts high cell voltages, which can be in the range of several kilovolts, to low voltages, which can be in the range of a tenth of a volt to several hundred volts.

[0004] For cost-effective manufacturing, cell designs typically do not change for different applications. However, different converter configurations or applications may have different initial cell voltages. This presents a challenge for power supply design. For a given power supply design, the initial cell voltage may be high enough to allow the power supply to function properly in some converter configurations, but not high enough in other converter configurations to initiate MMC operation.

[0005] The cell capacitors can be energized by supplying power from the power source itself, such as the power grid to which the MMC is connected or will be connected, or from an auxiliary power source, such as a generator, if the power grid is not readily available for charging.

[0006] During the initial charging of the cell capacitors, the MMC normally operates in blocking mode, with all switches in the converter cells open so that current flows only through the associated freewheeling diodes to the cell capacitors. Once the cell voltage reaches a first level, the cell's local control board can begin very basic functions, such as monitoring the cell status and cell voltage. Once the cell voltage reaches a second level, the gate drive unit can have sufficient power to enable proper switching control functions. Only after the cell voltage is higher than the second level is the cell confirmed to be ready for operation, and a "Cell OK" signal can be sent to the central valve control unit (VCU). If more than a defined minimum number of cells indicate "Cell OK" in the VCU, a "Ready For Operation" (RFO) can be indicated to the control and protection system. The MMC is then deblocked and begins operation.

[0007] However, for the reasons mentioned above, it may not be possible to properly power up the MMC to the second level, resulting in the MMC not being deblocked and not starting to operate. Summary of the Invention [Means for solving the problem]

[0008] overview The present disclosure seeks to at least partially remedy the above-mentioned problems. To this end, a method for energizing a modular multilevel converter and a modular multilevel converter are provided as defined by the independent claims. Further embodiments are provided in the dependent claims.

[0009] According to one aspect of the present disclosure, there is provided a method for energizing a modular multilevel converter including converter valves, each converter valve having a plurality of interconnected cells, each cell including a plurality of power electronic switches in a full-bridge configuration, a chargeable element, and a plurality of gate drive units, one gate drive unit for each power electronic switch, the method comprising: providing power to the modular multilevel converter from an external power source; providing power to the cell, thereby charging a chargeable element of the cell; generating a cell sub-ready signal for the cell when the cell voltage resulting from the charging exceeds a first threshold voltage level; generating a sub-ready_for_operation signal when at least a predetermined minimum number of cells have generated a sub-ready signal; and executing a deblocking sequence that includes closing first power electronics switches in all of the cells to electrically configure the cells to be continuously charged as half-bridge cells and keeping the first power electronics switches closed until the cell voltage reaches a converter operable voltage that is higher than a second threshold voltage.

[0010] By energizing the MMC in this manner, the dependency on converter configuration is significantly reduced.

[0011] According to one embodiment of the method, supplying power to the cells may include supplying power to only a first gate drive unit of the plurality of gate drive units of each cell.

[0012] According to one embodiment of the method, the generating of the cell sub-ready signal can be followed by sending the cell sub-ready signal to a valve control unit that controls a valve, the valve control unit being included in the modular multi-level converter.

[0013] According to another aspect of the present disclosure, a modular multilevel converter including converter valves is provided, each having a plurality of interconnected cells, each cell including a plurality of power electronic switches in a full-bridge configuration, a chargeable element, and a plurality of gate drive units, one gate drive unit for each power electronic switch. The MMC is configured, upon initial power-on, to receive power from an external power source and supply power to the cells, thereby charging the chargeable elements of the cells. Each cell is configured to generate a cell sub-ready signal when a cell voltage resulting from the charging exceeds a first threshold voltage level. The MMC is configured to execute a deblocking sequence when at least a predetermined minimum number of cells generate the sub-ready signal, closing first power electronic switches in all cells to electrically configure the cells to be continuously charged as half-bridge cells, and maintaining the first power electronic switches closed until the cell voltage reaches a converter operable voltage higher than a second threshold voltage.

[0014] According to one embodiment of the MMC, a valve control unit may be included that is configured to control the converter valve and receive a sub-ready signal from the cell.

[0015] According to one embodiment of the MMC, each cell may include control logic configured to only provide power to a first gate drive unit of the plurality of gate drive units of each cell at the beginning of initial power-up.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS Exemplary embodiments of the present disclosure are described below with reference to the accompanying drawings. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a block diagram of one embodiment of a modular multilevel converter according to the present disclosure. [Figure 2] FIG. 1 is a circuit schematic diagram of a cell of a modular multilevel converter showing different current paths. [Figure 3] FIG. 1 is a circuit schematic diagram of a cell of a modular multilevel converter showing different current paths. [Figure 4] 1 is a flow chart of one embodiment of a method for energizing a modular multilevel converter according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0018] Detailed Description The present disclosure will now be described with reference to the accompanying drawings, which show exemplary embodiments. However, the present disclosure should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided as examples so that this description will convey the scope of the present disclosure to those skilled in the art.

[0019] An exemplary embodiment of the modular multi-level converter (MMC) 100 includes three converter valves 101, such as one converter valve per phase. Each converter valve 101 has multiple interconnected cells 102, also called sub-modules. The MMC 100 is connectable on one side to an electrical phase line of an AC power source 110, such as an AC power grid or other possible AC power source, typically via an AC circuit breaker 111 and a transformer 112, and on the other side to a DC system 120, such as an HVDC system or other possible DC system. Each converter valve 101 includes two valve arms connected between an AC phase terminal and first and second DC terminals, such as positive and negative poles, of the DC system 120, respectively. The MMC 100 further includes a valve control unit (VCU) 103 connected to the converter valves 101. The VCU 103 may then be connected to a higher-level controller 104 external to the MMC 100. An upper level controller 104 may be responsible for controlling the overall operation of MMC 100 and additional MMCs.

[0020] Each cell 102 comprises a number of power electronic switches 105, 106, 107, 108 and a chargeable element 109 such as a cell capacitor. Furthermore, each of the power electronic switches 105-108 is provided with a freewheeling diode 113-116. Furthermore, the cell 102 comprises a control processing circuit 117 including a control board for controlling the cell and communicating with a central control unit, and gate drive units (GDUs), one GDU for each of the power electronic switches 105-108, which control their switching. IGBT In the case of a switch, the GDU can be thought of as turning the power electronics switch on and off, corresponding to the opening and closing of the switch, respectively.

[0021] Figure 2 shows the cell with the power electronics switches 105-108 on a different side of the converter capacitor 109 than in Figures 1 and 3. This is optional and is merely to show the current path through the cell for clarity purposes; the cell processing circuitry has the same structure in all figures.

[0022] The cell 102 is a full bridge (FB) cell, i.e., a plurality of power electronic switches 105-108 are provided in an FB configuration, and the power electronic switches 105-108, together with their respective freewheeling diodes 113-116, are connected to a cell capacitor 109 in an H-bridge configuration.

[0023] According to one embodiment of the method for energizing an MMC, shown at 200 in the flowchart of FIG. 4, the MMC 100 is first powered from the AC power source 110 by closing the AC circuit breaker 111 (S201). This supplies power to the cell 102 (S202), and the cell capacitor 109 begins charging by supplying current through the freewheeling diodes 113-116. The current path within the cell 102 is shown in FIG. 2 as a solid line for one polarity of supply current and a dashed line for the other polarity of supply current. When the cell voltage within the cell 102 increases and exceeds a first threshold level, a cell sub-ready signal for that cell 102 is generated by the control processing circuit 117 (S203). More specifically, for the cell sub-ready signal to be generated, the cell voltage must reach a level at which the cell 102 is ready for switching of the first power electronics switch 105 (also shown as S1 in the diagram). The cell sub-ready signal is sent to VCU 103. Compared to the prior art method in which the first voltage level causes the control logic present in control processing circuit 117 to activate some basic functions such as monitoring, the first threshold level is a higher voltage, while the first threshold level is lower than the second level of the prior art described above.

[0024] If more than a predetermined minimum number of cells have sent a sub-ready signal to the VCU 103, a sub-ready_for_operation signal is generated by the VCU 103 and sent to the upper-level processing circuit 104 (S204). Deblocking of the MMC 100 is then permitted, and the deblocking sequence begins (S205). The deblocking sequence begins by switching the first power electronics switches 105 in all cells 102 to the closed position, i.e., turning them on. When only the first power electronics switches 105 are closed, the cells 102 function as half-bridge cells, and charging current flows through the freewheeling diodes 115 of the second power electronics switches 108 (also designated S4) of the cells 102, as shown by the dashed lines in FIG. 3. The cell capacitors 109 are not discharged and are charged by a unidirectional current. Now, all cells 102 are charged with the same current, and the voltage across the valve arm can reach twice the peak AC supply voltage. This ensures a cell voltage that is higher than the second voltage level mentioned above in the prior art method and high enough to enable proper functioning of the MMC 100 for all converter configurations. This charging in the half-bridge configuration continues until the cell voltage reaches a converter operable voltage that is higher than the second threshold voltage and high enough to ensure proper functioning of the MMC 100. Then, the first power electronics switch 105 is turned off. This prepares the MMC 100 to be set into operation mode, i.e., to begin operation. Thus, the MMC 100 is finally fully deblocked, thereby completing the deblocking sequence.

[0025] According to one embodiment of the present disclosure, the control processing circuit 117 may include control logic configured to supply power only to the first GDU 118 of the plurality of GDUs 118 of each cell 102, i.e., the GDU of the first power electronics switch 105, at the beginning of initial power-up. The GDUs 118 of all other power electronics switches 106-108 are powered off until the cell voltage reaches a second threshold voltage. Because the total amount of load powered by the cell capacitors is reduced, the GDU of the first power electronics switch 105 may be ready for operation when the cell voltage reaches a third threshold voltage, which is higher than the first level but lower than the second level of the prior art. Once the cell voltage exceeds the third threshold voltage, a cell sub-ready signal for that cell 102 is generated by the control processing circuit 117. The same operation then follows as described for the above embodiment. Note that initially activating only a single power electronics switch may expedite the charging of the cell capacitors to ensure that the remaining GDUs and switches have sufficient power to be operational.

[0026] While the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive.

[0027] 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 the other features and elements.

[0028] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the words "comprising" or "comprising" do not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. A single processor or other unit may fulfill 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 advantageously used. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A method for energizing a modular multilevel converter (100), the modular multilevel converter (100) comprising converter valves (101), each converter valve having a plurality of interconnected cells (102), each cell comprising a plurality of power electronic switches (105, 106, 107, 108) in a full-bridge configuration, a rechargeable element (109), and a plurality of gate drive units (118), one gate drive unit for each power electronic switch; Supplying power to the modular multilevel converter from an external power source (S201); Supplying power to the cell, thereby charging the chargeable element of the cell (S202); When the cell voltage resulting from the charging exceeds a first threshold voltage level, generating a cell sub-ready signal for the cell (S203); generating a sub-ready_for_operation signal when at least a predetermined minimum number of cells have generated the cell sub-ready signal (S204); performing a deblocking sequence (S205) including closing first power electronic switches (105) in all cells to electrically configure the cells to be continuously charged as half-bridge cells and keeping the first power electronic switches closed until the cell voltage reaches a converter operable voltage higher than a second threshold voltage, wherein supplying power to the cells includes initially supplying power to only a first gate drive unit of the plurality of gate drive units (118) of each cell (102), the first gate drive unit being connected to the first power electronic switch; and providing power to the remaining gate drive units in each cell when the second threshold voltage is reached.

2. 2. The method of claim 1, wherein generating the cell sub-ready signal is followed by sending the cell sub-ready signal to a valve control unit that controls the converter valve, the valve control unit being included in the modular multilevel converter.

3. 3. The method of claim 1, wherein the deblocking sequence includes opening the first power electronics switch and setting the modular multilevel converter to an operating mode.

4. A modular multilevel converter comprising: a converter valve (101), each converter valve having a plurality of interconnected cells (102), each cell comprising a plurality of power electronic switches (105, 106, 107, 108) in a full-bridge configuration, a chargeable element (109), and a plurality of gate drive units (118), one gate drive unit for each power electronic switch; configured, upon initial power-up, to receive power from an external power source and provide power to the cells, thereby charging the chargeable elements of the cells; each cell configured to generate a cell sub-ready signal when a cell voltage resulting from said charging exceeds a first threshold voltage level; the modular multilevel converter is configured to execute a deblocking sequence, when at least a predetermined minimum number of cells generate the cell sub-ready signal, closing first power electronic switches (105) in all of the cells to electrically configure the cells to be continuously charged as half-bridge cells, and keeping the first power electronic switches closed until the cell voltage reaches a converter operable voltage that is higher than a second threshold voltage; and each cell (102) comprises control logic (117) configured to supply power to only a first gate drive unit (118) of the plurality of gate drive units of each cell at the beginning of the initial power-up, the first gate drive unit being connected to the first power electronic switch and configured to supply power to the remaining gate drive units in each cell when the second threshold voltage is reached.

5. 5. The modular multilevel converter of claim 4, further comprising a valve control unit (103) configured to control the converter valves (101) and to receive the cell sub-ready signal from the cell (102).

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