WO2014056742A2 - Controlling a mocular converter - Google Patents

Controlling a mocular converter Download PDF

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Publication number
WO2014056742A2
WO2014056742A2 PCT/EP2013/070264 EP2013070264W WO2014056742A2 WO 2014056742 A2 WO2014056742 A2 WO 2014056742A2 EP 2013070264 W EP2013070264 W EP 2013070264W WO 2014056742 A2 WO2014056742 A2 WO 2014056742A2
Authority
WO
WIPO (PCT)
Prior art keywords
converter
link
voltage difference
pulse width
modular
Prior art date
Application number
PCT/EP2013/070264
Other languages
French (fr)
Other versions
WO2014056742A3 (en
Inventor
Akos Mester
Drazen Dujic
Original Assignee
Abb Technology Ag
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Abb Technology Ag filed Critical Abb Technology Ag
Publication of WO2014056742A2 publication Critical patent/WO2014056742A2/en
Publication of WO2014056742A3 publication Critical patent/WO2014056742A3/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/36—Means for starting or stopping converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/0074—Plural converter units whose inputs are connected in series
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/483—Converters with outputs that each can have more than two voltages levels
    • H02M7/49—Combination of the output voltage waveforms of a plurality of converters

Definitions

  • the invention relates to the field of modular converters.
  • the invention relates to a method for controlling a modular converter and a control arrangement for controlling a modular converter.
  • a line frequency transformer (LFT) and a rectifier unit may be used for generating the electric power supplied to the electrical motors.
  • LFT line frequency transformer
  • Such a converter arrangement may be replaced by a modular converter comprising a plurality of converter cells that produce from an AC input voltage a DC output voltage that is supplied to the electrical installations on-board, for example the electrical motors.
  • the AC input voltage is supplied from an overhead line.
  • the converter cells of the modular converter may comprise a DC-to-DC converter, in which a DC-to-AC converter on the line side is connected via a transformer with an AC-to- DC converter on the motor side.
  • Both the DC-to-AC converter and the AC-to-DC converter may be active converters with controllable semiconductor switches.
  • At the line side and at the motor side there may be DC links with DC link capacitors.
  • the following problems may arise when operating the modular converter.
  • the first DC link there may be no energy contained in the modular converter, which can lead to inrushing currents.
  • these currents may be limited by means of a charging circuit and inrushing currents may be no problem.
  • the first and second DC link may be linked by an isolated DC-to-DC converter and inrushing currents into the second DC links may be possible as well as a saturation of the transformer due to the unbalanced voltages on the primary and secondary side of the transformer during the start-up.
  • a difference between the first and the second DC link voltage may be small enough so that a standard PWM (pulse width modulation) pattern (for example with 50% duty cycle and/or with fixed switching frequency) may be applied and the modular converter may work properly.
  • a standard PWM (pulse width modulation) pattern for example with 50% duty cycle and/or with fixed switching frequency
  • the difference between the first and second DC link voltage may become too large (for example above a specific threshold, which may be defined by the controller) and an application of a standard PWM pattern may not be possible any more.
  • the number of active converter cells is dynamically changed during operation. This may be due to optimization of losses, reliability or redundancy requirements. If a converter cell has to be activated after it has been disengaged for a longer period of time, one or both DC links of the converter cell may have to be energized. This may require a procedure in which a part of the modular converter is operating, and a different part is charged. The problem may be the same as before: the discharged part of the modular converter may have to be charged safely.
  • An aspect of the invention relates to a method for controlling a modular converter, which may be a method for energizing and/or maintaining the operation of a modular converter.
  • the modular converter comprises a plurality of converter cells interconnected in series on a first side and interconnected in parallel on a second side.
  • Each converter cell comprises a first DC link, a DC-to-DC converter, and a second DC link connected in cascade (or in series) between the first side and the second side.
  • the modular converter may be realized using a number of identical converter cells connected in series/parallel at their respective connection terminals.
  • the modular converter may be a medium voltage converter.
  • the converter may further comprise an AC-to-DC input converter (for AC input into the converter) connected with the first DC link or a DC-to-DC input converter (for DC input into the converter) connected with the first DC link.
  • the DC-to-DC converter may be a resonant DC-to-DC converter and may comprise a transformer.
  • the modular converter may be galvanically isolated by the transformers of the converter cells.
  • the method comprises the steps of: determining whether a voltage difference between the first DC link and the second DC link of a converter cell is above a predefined threshold value; operating the DC-to-DC converter of the converter cell with a modified pulse width modulation pattern having pulses with a duty cycle smaller than 50%, if the voltage difference is above the predefined threshold value.
  • the voltage difference may be balanced by applying the modified pulse width modulation pattern.
  • the method may be performed by a controller that controls the modular inverter and in particular the converter cells of the converter.
  • the duty cycle of a pulse width modulation pattern may be defined by the relationship between the duration of a full cycle and the duration of a pulse (i.e. in which the voltage is either switched to a positive value or to a negative value). Since a full cycle usually has a positive and a negative pulse, a duty cycle of 50% may mean that the voltage is either positive or negative but not 0.
  • the duty cycles during the modified pulse width modulation pattern may have pulse widths below 40%. It is also possible that there are cycles, in which no switching occurs at all. In the latter case, the duty cycle may be 0%.
  • a switching frequency of a standard pulse width modulation pattern (with 50% duty cycle) and of modified pulse width modulation pattern may be defined via the durations of a full cycle. It has to be noted that the standard pulse width modulation pattern and the modified pulse width modulation pattern both may have the same frequency.
  • the load transferred by the DC-to- DC converter may be much smaller and the currents flowing through the DC-to-DC converter. In such a way, eventual excessive inrush currents and a saturation of the transformer may be prevented.
  • the control method may be adaptive in the sense that it is able to maintain correct operation of the modular converter during disturbances on the terminals (i.e. the input or the output) of the modular converter.
  • the control method may be adaptive in the sense that it is able to maintain correct operation of the modular converter during disturbance in some of the converter cells (for example, there may be a loss of coupling between the first and second DC links).
  • the method may be applied equally during charging and/or start-up scenarios with energy source located either on the side of the first DC link or the side of the second DC link.
  • the method may be applied during the increase and/or the decrease of the number of active converter cells used in the modular converter (for example for reconfiguration if the modular converter incorporates redundancy).
  • the method does not require a much higher switching frequency as the nominal operating frequency (i.e. the frequency of the standard pulse width modulation pattern) of the modular converter.
  • the modular converter comprises a central controller and a plurality of local controllers, each local controller associated with one of the converter cells and adapted for controlling the converter cell.
  • the voltage difference for a converter cell may be determined based on determining an offline/online status of the local controller of the converter cell.
  • the voltage difference may be determined indirectly (i.e. not by measuring voltages and/or currents).
  • the modified pulse width modulation pattern may have to be applied to the DC-to-DC converter. This may be determined based on the operation state of the associated local controller and dependency of their state to the values of DC link voltage, which may be implementation specific. If the associated local controller is offline, this may indicate, that the associated converter cell is in a start-up phase.
  • the offline/online status of the local controller may be determined with the central controller.
  • the method further comprises the steps of: if the local controller is not activated, delaying the operation of the DC-to-DC converter for a predefined delay time or until local controller becomes active, and operating the DC- to-DC converter with the modified pulse width modulation pattern after the delay time.
  • the local controller may finish its start-up sequence.
  • a standard pulse with modulation pattern may be applied and/or the converter cell may start to charge.
  • the voltage difference is determined based on measuring a first voltage in the first DC link and a second voltage in the second DC link.
  • the voltage difference is determined based on a first resonant current at the primary side of the transformer and/or a second resonant current at the secondary side of the transformer.
  • the voltage difference may be determined from measured currents.
  • the time derivate of the resonant current may be related with the voltage drop (difference) between the two DC links.
  • the modified pulse width modulation pattern comprises a sequence of pulses having a pulse width that gradually increases towards a 50% duty cycle.
  • the sequence of pulses may start with a pulse that has no (0%) or nearly no duty cycle. The following cycles may become wider and wider until the duty cycle of 50% is reached again.
  • a time duration, for which the modified pulse width modulation pattern is applied is based on the voltage difference. For example, if the voltage difference is small or differs only for a small amount from the predefined threshold value, also the time duration for application of the modified pulse width modulation pattern may be small. Conversely, if the voltage difference is higher, the time duration for the modified pulse width modulation pattern may be longer, too.
  • a pulse width of switching pulses of the modified pulse width modulation pattern is based on the voltage difference. For example, the bigger the voltage difference is, the smaller the pulse widths (or at least the pulse width of a first pulse of a sequence of increasing pulses) may be set.
  • the method further comprises the step of: if the voltage difference is below the predefined threshold value, operating the DC-to-DC converter with a standard pulse width modulation pattern having (only) pulses with a 50% duty cycle.
  • the modular converter or at least the converter cell may be operated with a standard or normal pulse width modulation pattern, which may only comprise pulses that are half as long as a full cycle.
  • the voltage difference is determined and evaluated after regular time intervals.
  • the voltages between the first side and the second side of the DC-to-DC converter may be balanced with the aid of the method.
  • the voltage difference is (only) determined after start-up of a converter cell. It is also possible that the voltage difference is balanced during the start-up phase of the converter. The method may also be applied in the case an additional (redundant) converter cell is activated, while other converter cells are already operating.
  • the DC-to-DC converter comprises a DC- to-AC converter stage, an AC-to-DC converter stage and a transformer interconnecting the two stages.
  • the pulse width modulation pattern may be applied to semiconductor switches of the first and/or second converter stage.
  • only the DC-to-AC converter or only the AC-to-DC converter stage may be an active stage that is actively switched.
  • the DC-to-DC converter may comprise a first DC-to-AC converter for transforming the DC current in the first DC link into a first AC current supplied to the transformer.
  • the second AC current generated by the transformer may be rectified by the AC-to-DC converter and supplied into the second DC link.
  • Either the DC-to-AC converter or the AC- to-DC converter or both converters may be active converters that are controlled by a controller of the modular converter.
  • a further aspect of the invention relates to a control arrangement for a modular converter, wherein the control arrangement is adapted for performing the method as described in the above and in the following.
  • the control arrangement comprises a central controller and a plurality of local controllers communicatively interconnected with the central controller.
  • Each local controller may be associated with one converter cell of the modular converter and/or may be adapted for switching semiconductor switches of the DC-to-DC converter of the respective converter cell.
  • Each local controller may be adapted for switching semiconductor switches of the AC-to-DC input converter or the DC-to-DC input converter of the respective converter cell.
  • the control arrangement may comprise two control hierarchies, a central controller, which controls the local controllers of the converter cells.
  • a further aspect of the invention relates to a modular converter, which comprises a plurality of converter cells interconnected in series on a first side and interconnected in parallel on a second side, wherein each converter cell comprises a first DC link, a DC-to- DC converter, and a second DC link connected in cascade.
  • the modular converter may comprise a control arrangement for controlling the plurality of converter cells as described in the above and in the following.
  • the modular converter may have to be energized and a special modulation sequence may be applied to charge the DC links in a controllable fashion and to prevent saturation of a transformer and possibly large inrush currents.
  • a desired relationship for example the difference
  • a desired relationship for example the difference
  • a special corrective/modified modulation sequence may be applied to particular converter cells in order to bring the modular converter back into normal operation.
  • Fig. 1 schematically shows a modular converter according to an embodiment of the invention.
  • Fig. 2 schematically shows a modular converter according to a further embodiment of the invention.
  • Fig. 3 shows a diagram with voltages in a converter cell of a modular converter according to an embodiment of the invention.
  • Fig. 4 shows a diagram with currents in a converter cell of a modular converter according to an embodiment of the invention.
  • Fig. 5 shows a diagram with currents in a converter cell of a modular converter according to an embodiment of the invention.
  • Fig. 6 shows a diagram with voltages in a converter cell of a modular converter according to an embodiment of the invention.
  • Fig. 7 shows a diagram with currents in a converter cell of a modular converter according to an embodiment of the invention.
  • Fig. 8 shows a diagram with currents in a converter cell of a modular converter according to an embodiment of the invention.
  • Fig. 9 shows a flow diagram for a method for controlling a modular converter according to an embodiment of the invention.
  • Fig. 10 shows a modified pulse width modulation pattern according to an embodiment of the invention.
  • Fig. 1 1 shows a modified pulse width modulation pattern according to an embodiment of the invention.
  • Fig. 12 shows a modified pulse width modulation pattern according to an embodiment of the invention.
  • Fig. 1 shows a single phase modular multilevel converter 10 comprising a plurality of converter cells 12.
  • the modular converter 10 has AC medium voltage terminals on an AC side 14, in particular a first AC terminal 16 and a second AC terminal 18, and low voltage terminals on a DC side 20, in particular a first DC terminal 22 and a second DC terminal 24.
  • An inductor 26 and a charging circuit 28 are connected between first AC terminal 16 and the first converter cell 12.
  • the charging circuit 28 is realized as a combination of an inrush current limiting resistor and a bypass switch.
  • the converter cells 12 are connected in series on the AC side 14 between the first AC terminal 16 and the second AC terminal 18 in order to meet the AC line voltage between the two terminals 16, 18.
  • the converter cells are connected in parallel on the DC side 20 between the first DC terminal 22 and the second DC terminal 24.
  • Each of the converter cells 4 comprises an AC-to-DC converter 30, which is connected to a first DC link 32 at its output that is floating with respect to the system ground.
  • the first DC link 32 is connected with a DC-to-DC converter 34, which on the second side is connected to a second DC link 36.
  • Both DC links 32, 36 comprise at least one capacitor.
  • the DC-to-DC converter 34 comprises a first power stage or DC-to-AC converter 38, one or more transformers 40 and a second power stage or AC-to-DC converter 42.
  • the DC-to-DC converter 34 is connected to the first DC link 32 with its first power stage 38 which drives the one or more transformers 40 and provides through the second power stage 42, a DC power to the second DC link 36.
  • the first side 14 of the converter 10 is galvanically isolated from the second side 20.
  • Each of the converter cells 12 comprises a local controller 44 that is adapted to control the AC-to-DC converter 30, and the DC-to-DC converter 34 and in particular the converter stages 30, 38, 42. In Fig. 1 only one local controller 44 is shown.
  • the modular converter 10 comprises a central or main controller 46 for controlling the local controllers 44.
  • the local controllers 44 and the central controller 46 belong to a control arrangement 48 for controlling the modular converter 10.
  • the overall modular converter 10 may be realized with redundant cells, although this is not shown in Fig. 1 .
  • each or at least some of the converter cells 12 may comprise a bypass switch 50 that is adapted for connecting the converter cell 12 between the terminals 16, 18 or for bypassing the converter cell 12.
  • the modular converter 10 interconnects an AC side 14 with a DC side 20.
  • the side 14 may be a (medium voltage) DC side and the converter 30 may be a DC-to-DC converter.
  • the remaining functionality of the converter 10 may be the same as explained with respect to Fig. 1 .
  • the modular converter 10 may be operated either as medium voltage AC to low voltage DC galvanic isolated converter or as medium voltage DC to low voltage DC converter.
  • Fig. 2 shows a three phase modular multilevel converter 10' that comprises three single phase modular converters 10, which may have the same components as the single phase converter 10 shown in Fig. 1 .
  • the first terminals 16 are connected to the three phases of a three phase medium voltage
  • the second terminals 18 are connected to the ground.
  • the modular converters 10 are connected in parallel.
  • a delta connection may be applied on the medium voltage AC side 14.
  • / may be a number between 1 and N, wherein N is the total number of converter cells
  • the voltage V 2 of the second DC links 36 is identical for all converter cells 12. Therefore, the voltage V 1 . i of the first DC link 32 of each converter cell 12 may have slightly different values.
  • V threshold may be defined for variations of (2) which should not be exceeded (on a converter cell level) in absolute terms, during normal operation of the modular converter 10, i.e.:
  • the first DC links 32 and the second DC links 36 are discharged, and thus no energy content is present in the modular converter 10.
  • inrush current will start flowing through the inductor 26, charging circuit 28 and series connected AC-to-DC converters 30. This current is limited by the resistor present in the charging circuit 28 and thus there may be no severe inrush currents into the first DC links 32.
  • first DC links 32 and the second DC links 36 are empty, differences between two DC links voltages are small and conditions (1 ) are satisfied making equations (3) true. So it could be said that conditions before turning on the modular converter 10 are "normal” at least in the sense of definitions introduced in (1 )-(3). Therefore, a standard PWM pattern may be applied to the first converter stage 38 of the DC-to-DC converter 34.
  • the standard PWM pattern has a 50% duty cycle with a fixed switching frequency. This results that all first DC links 32 and the second DC links 36 are charged simultaneously (in parallel).
  • Fig. 3 the voltage 60 of a first DC link 32 and the voltage 62 of a second DC link 36 (times the turn ratio of the corresponding transformer 40) of a converter cell 12 during parallel charging are shown.
  • rated value of the first DC link 32 is 3600V and 1500V for the second DC link 36, while the turn ratio of the transformer 40 is 2.4. It can be seen that the voltage 60 of the first DC link 32 rises quickly and that the voltage 62 of the second DC link 62 follows with the same slope.
  • the inrush current is limited by the charging resistor of the charging circuit 28. Both DC link voltages 60, 62 reach a certain value (below the rated value) and then at some time instant, the charging resistor is bypassed and the AC-to-DC converters 30 are activated to boost the DC link voltages 60, 62 to its rated values. All the time, the DC-to-DC converters 34 are operated using the standard PWM pattern.
  • Fig. 4 and 5 show the current waveforms during the parallel charging as explained with respect to Fig. 3.
  • Fig. 4 shows the resonant current 64 and the magnetizing current 66 on the first side 14.
  • Fig. 5 shows the resonant current 68 on the second side 20. From Fig. 4 and 5, it can be seen that the primary and secondary currents 64, 66, 68 are limited and controlled. Also saturation of the transformer 40 is avoided.
  • Fig. 3 to 5 show a normal operation of the modular converter. However, during start-up or during normal operation, it may happen that conditions (3) are violated in some of the converter cells 12, due to various disturbances either on the AC side 14 or DC side 20. Violation of (3) means that the voltage difference between the voltage of the first DC link 32 and the voltage of the second DC link 34, has exceeded a specified, predefined threshold value. An application of the standard PWM pattern may lead to saturation of the transformer 40 or to a large inrush current.
  • a corrective or modified PWM pattern is applied in order to bring the voltages of the two DC links 32, 36 towards their rated values.
  • a modified PWM pattern may be applied to one converter cell 12 or to several converter cells 12, depending where violation of (3) has been detected. However, in some cases, a modified PWM pattern may be applied to the whole converter arrangement 10 as will be explained next.
  • Fig. 6 to 8 are figures analogously to Fig. 3 to 5.
  • the DC link voltages 60, 62 are shown during sequential charging.
  • Fig. 6 shows the resonant current 64 and the magnetizing current 66 on the first side 14.
  • Fig. 8 shows the resonant current 68 on the second side.
  • Fig. 6 to 8 illustrate the extreme case of the start-up of the modular converter 10, when the first DC links 32 are charged and the second DC links 36 are completely discharged (or are only partially charged). Such a situation may occur, if the normal charging procedure described in the previous section was interrupted due to an unexpected event.
  • the local controller 44 may happen that the local controller 44 has blocked operation of the DC-to-DC converter 34 and no PWM pattern has been applied to the first stage 38 of the DC-to-DC converter 34 immediately after connecting the modular converter 10 to the AC source. This may be the results of a controller algorithm, or because the local controller hardware cannot operate if the first DC links 32 are not charged. Thus, the first DC link 32 is charged and the second DC link 36 remains discharged, since there is no energy transfer through the DC-to-DC converter 34. Condition (3) is not true and the control arrangement 48 selects modified PWM pattern for the first stage 38 of the DC-to-DC converter 10.
  • the control arrangement 68 applies a modified PWM pattern to the first stage 38 of the DC-to-DC converter 34, transferring the energy to the second DC link 36, whose voltage 62 then starts to rise.
  • the modified PWM pattern may be applied as long as conditions (3) are not true. Once (3) is satisfied, the control arrangement 48 will continue operating the DC-to-DC converter 34 using the standard PWM pattern.
  • both DC link voltages 60, 62 reach a specific value (below a rated value) at some time instant, the charging resistor may be bypassed and the AC-to-DC converter 30 is activated to boost the DC link voltages 60, 62 to its rated values.
  • This illustrative example is the worst possible case when the voltage difference between the first and second DC links 30, 32 is large.
  • Parallel charging is used, when both DC link voltages 60, 62 rise at the same time and a standard PWM pattern is applied after the connection of the modular converter 10 to the AC source. There may be no delay between the connection to the AC source and the application of the standard PWM pattern to the first stage 38 of the DC-to-DC converter 10.
  • Sequential charging is used, when only the first DC link 32 is charged after the connection of the modular converter 10 to the AC source and then a modified PWM pattern is used to transfer the energy to the second DC link 10. There may be a delay between the connection to the AC source and the application of the modified PWM pattern to the first stage 38 of the DC-to-DC converter 34.
  • control algorithms are described, with which the above described charging procedure may be achieved. Furthermore, the control algorithm may be used during the normal operation of the modular converter 10, after the charging.
  • Fig. 7 shows a flow diagram with a control algorithm for charging the power converter 10 and for maintaining the operation of the modular converter 10.
  • a central controller 46 is responsible for an overall control of the modular converter 10. It may be assumed that the central controller 46 is always available (powered on), in particular prior to the connection of the modular converter 10 to an AC source.
  • Each converter cell 12 comprises a local controller 44 responsible for the control of the converter cell 12. The local controller 44 may or may not be available (online) prior to the connection of the modular converter 10 to the AC source.
  • Each of these controllers 44, 46 may perform various further control and protection functions which are not discussed herein.
  • step 80 a start-up of the modular converter 10 is initiated, which may involve specific steps as discussed above.
  • step 82 it is determined, whether the local controllers 44 are online or offline.
  • the central controller verifies in step 84, whether the conditions (3) are satisfied.
  • step 86 a standard PWM pattern is applied to the DC-to-DC converter 34. I.e. the start-up and charging procedure of the modular converter 10 will continue using a standard PWM pattern.
  • step 88 a modified PWM pattern is applied to the DC-to-DC converter 34. i.e. the start-up and charging procedure of the modular converter 10 will continue using a corrective PWM pattern.
  • step 90 the local controller 44 regularly checks whether condition (3) is satisfied for its own converter cell 12. If the condition (3) is satisfied, the application of a standard PWM pattern will be continued by the local controller 44.
  • the local controller 44 switches its mode of operation from application of a standard PWM pattern in step 86 into an application of a modified PWM pattern in step 88. It is also possible that the local controller checks during the application of modified PWM pattern in step 88, whether condition (3) is satisfied and switches to the application of a standard PWM pattern in step 86, when this is the case.
  • step 82 If it is determined in step 82, that the local controllers 44 are offline, the central controller waits for a delay time in step 92. Since the local controllers 44 are offline and the start-up process is initiated, a delay will be introduced as a consequence of the need for the voltage 60 of the first DC links 32 to reach a specific value when the local controllers 44 will change its status from offline to online.
  • control arrangement 48 proceeds with a modified PWM pattern in order to transfer energy to the second DC links 36 in a controlled fashion without saturation of a transformer 40 or excessive inrush currents into the second DC links 36.
  • the local controller 44 regularly checks in step 90 the validity of the condition (3) for its own converter cell 12 and may switch its mode of operation from the modified PWM pattern into the standard PWM pattern (and vice versa).
  • step 84 There may be following differences and similarities between step 84 and step 90:
  • Step 84 may be performed by the central controller 46 and 90 may be performed by a local controller 44.
  • Step 84 and step 90 may both compare the individual DC link voltages of each converter cell 12. However, step 90 may only compare the DC link voltages of one converter cell 12 and this may be done in every converter cell 12 individually. On the other hand, step 84 may compare the DC link voltages for every converter cell 12 individually and this may be done by the central controller 46 only once.
  • the method may operate with only step 84, only step 90 and also using steps 84, 90 both.
  • converter cells 12 may be in standard mode (step 86), and others may be in corrective mode (step 88) at a given point in time.
  • Fig. 10 to 12 show diagrams with a modified PWM pattern 94.
  • time is running to the left and the y-axis indicates the voltage u output by the DC-to-AC converter 38 of the DC-to-DC converter 34.
  • the DC-to-AC converter 38 is adapted for generating a voltage of +DC, 0 and -DC, wherein +DC is the voltage of the DC link 32.
  • pulses 96 Connected regions of the generated voltage, in which the voltage is not 0, may be seen as pulses 96.
  • pulses 96 with positive voltage +DC alternate with pulses 96 with negative voltage -DC.
  • the distance (i.e. time) between the centers (or symmetry axes) of two pulses 96 with positive voltage +DC (or negative voltage -DC) may define the length or width of a full cycle 98.
  • the inverse of a full cycle may be the frequency of the PWM pattern 94 (in the case the length of the full cycle does not change).
  • the positive and negative pulses 96 may have the same width and/or frequency or a different width and/or frequency.
  • the apparent switching frequency may change during the charging (sequential charging). It may be possible that the frequency never changes and that the frequency of a modified PWM pattern 94 is the frequency of the standard PWM pattern.
  • the symmetry axis or the center of the positive and negative pulses 96 may move compared to the positions shown in Fig. 9.
  • the widths 100 before and after a pulse 96 may differ.
  • the distance between two neighboring pulses 96 may define the width of a half cycle 100 of the PWM pattern.
  • the duty cycle of a PWM pattern or of a pulse 96 is the width of a pulse 96 divided by the width of the corresponding full cycle 98 (which may be the full cycle 98 surrounding the pulse 96).
  • a modified PWM pattern 94 may be characterized in that the duty cycle of some or all pulses 96 of the modified PWM pattern 94 is smaller than 50%, i.e. that there are regions in which the voltage u is 0. In other words, in a modified pulse pattern 94 reaching the transformer 40, the voltage may have at least three levels.
  • a standard PWM pattern may be characterized by an operation with a 50% duty cycle with optionally a fixed switching frequency.
  • the width or duty cycle of pulses 96 of a modified PWM pattern 94 and/or the duration of the application of a modified PWM pattern 94 may be determined by one of the controllers 44, 48, for example based on the voltage difference.
  • a modified PWM pattern 94 may have very narrow pulses 96 at a certain switching frequency, for example with a duty cycle below 20% or below 10%.
  • the width of the pulses 96 may change between 0% and 50%.
  • a modified PWM pattern 94 may have very wide pulses 96 at a certain switching frequency, for example with a duty cycle between 40% and 50%.
  • the width of the pulses 96 changes during a sequence of pulses 96 of a modified PWM pattern 94.
  • Fig. 10 to 12 may show parts of a sequence of pulses 96 that gradually increases. Pulses 96 in the beginning of the sequence (Fig. 10) may be small, may become broader in the middle of the sequence and nearly may reach the width of a half cycle at the end of the sequence.
  • a modified PWM pattern 96 may be transitioned into a standard PWM pattern.
  • consecutive pulses 96 may go through the transition (as shown from Fig. 10 to 12). This transition may be over when the modified PWM pattern 94 becomes a standard PWM pattern (with 50% duty cycle and fixed switching frequency).
  • the length or duration of the sequence of pulses 96 and/or the width of the first pulse 96 of the sequence may be determined by one of the controllers 44, 48, for example based on the voltage difference.

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Abstract

1. A method for controlling a modular converter (10), the modular converter (10) comprising a plurality of converter cells (12) interconnected in series on a first side (14) and interconnected in parallel on a second side (20); wherein each converter cell (12) comprises a first DC link (32), a DC-to-DC converter (34), and a second DC link (36) connected in cascade; the method comprising the steps of: determining whether a voltage difference between a first DC link (32) and a second DC link (36) of a converter cell (12) is above a predefined threshold value; if the voltage difference is above the predefined threshold value: operating the DC- to-DC converter (34) of the converter cell (12) with a modified pulse width modulation pattern (94) having pulses (96) with a duty cycle smaller than 50%, characterized in that the modular converter (1 0) comprises a central controller (46) and a plurality of local controllers (44), each local controller (44) associated with one of the converter cells (1 2) and adapted for controlling the one converter cell (1 2), wherein the voltage difference is determined based on determining an offline/online status of a local controller (44), and in the case the local controller (44) is offline: delaying the operation of the DC-to-DC converter (34) for a predefined delay time or until the local controller (44) is online, and operating the DC-to-DC converter (34) with the modified pulse width modulation pattern (94) after the delay time.

Description

Controlling a modular converter
FIELD OF THE INVENTION
The invention relates to the field of modular converters. In particular, the invention relates to a method for controlling a modular converter and a control arrangement for controlling a modular converter. BACKGROUND OF THE INVENTION
In electric trains or trams, a line frequency transformer (LFT) and a rectifier unit may be used for generating the electric power supplied to the electrical motors. Such a converter arrangement may be replaced by a modular converter comprising a plurality of converter cells that produce from an AC input voltage a DC output voltage that is supplied to the electrical installations on-board, for example the electrical motors. Usually, the AC input voltage is supplied from an overhead line.
The converter cells of the modular converter may comprise a DC-to-DC converter, in which a DC-to-AC converter on the line side is connected via a transformer with an AC-to- DC converter on the motor side. Both the DC-to-AC converter and the AC-to-DC converter may be active converters with controllable semiconductor switches. At the line side and at the motor side there may be DC links with DC link capacitors.
DESCRIPTION OF THE INVENTION
During the start-up of the modular converter or during the start-up of one converter cell (in the case additional active converter cells may be added to already operating converter cells), situations may arise in which, due to the unbalanced loaded DC link capacitors, high currents may flow through at least parts of one of the DC-to-DC converters. In this case, the modular converter may not work properly or the transformer may be saturated, which may result in a low efficiency. Similar situations may arise in the case, when the load on the modular converter changes.
In particular, the following problems may arise when operating the modular converter. During the start-up, there may be no energy contained in the modular converter, which can lead to inrushing currents. In case of the first DC link, these currents may be limited by means of a charging circuit and inrushing currents may be no problem. However, the first and second DC link may be linked by an isolated DC-to-DC converter and inrushing currents into the second DC links may be possible as well as a saturation of the transformer due to the unbalanced voltages on the primary and secondary side of the transformer during the start-up.
During normal operation of the modular converter (once it has been charged), a difference between the first and the second DC link voltage may be small enough so that a standard PWM (pulse width modulation) pattern (for example with 50% duty cycle and/or with fixed switching frequency) may be applied and the modular converter may work properly. However, due to various disturbances, the difference between the first and second DC link voltage may become too large (for example above a specific threshold, which may be defined by the controller) and an application of a standard PWM pattern may not be possible any more.
Furthermore, it may be possible that the number of active converter cells is dynamically changed during operation. This may be due to optimization of losses, reliability or redundancy requirements. If a converter cell has to be activated after it has been disengaged for a longer period of time, one or both DC links of the converter cell may have to be energized. This may require a procedure in which a part of the modular converter is operating, and a different part is charged. The problem may be the same as before: the discharged part of the modular converter may have to be charged safely.
These problems might be solved by applying a much higher frequency to the transformer than the nominal operating frequency. However, with high power semiconductors such high frequencies may be not applicable and/or very high losses may be generated.
It is an object of the invention to provide a simple and secure control scheme for a modular converter, which can maintain operation of the modular converter during these disturbances.
This object is achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.
An aspect of the invention relates to a method for controlling a modular converter, which may be a method for energizing and/or maintaining the operation of a modular converter.
According to an embodiment of the invention, the modular converter comprises a plurality of converter cells interconnected in series on a first side and interconnected in parallel on a second side. Each converter cell comprises a first DC link, a DC-to-DC converter, and a second DC link connected in cascade (or in series) between the first side and the second side. The modular converter may be realized using a number of identical converter cells connected in series/parallel at their respective connection terminals. The modular converter may be a medium voltage converter.
The converter may further comprise an AC-to-DC input converter (for AC input into the converter) connected with the first DC link or a DC-to-DC input converter (for DC input into the converter) connected with the first DC link.
The DC-to-DC converter may be a resonant DC-to-DC converter and may comprise a transformer. The modular converter may be galvanically isolated by the transformers of the converter cells.
According to an embodiment of the invention, the method comprises the steps of: determining whether a voltage difference between the first DC link and the second DC link of a converter cell is above a predefined threshold value; operating the DC-to-DC converter of the converter cell with a modified pulse width modulation pattern having pulses with a duty cycle smaller than 50%, if the voltage difference is above the predefined threshold value. In such a way, the voltage difference may be balanced by applying the modified pulse width modulation pattern.
Usually, the method may be performed by a controller that controls the modular inverter and in particular the converter cells of the converter.
The duty cycle of a pulse width modulation pattern may be defined by the relationship between the duration of a full cycle and the duration of a pulse (i.e. in which the voltage is either switched to a positive value or to a negative value). Since a full cycle usually has a positive and a negative pulse, a duty cycle of 50% may mean that the voltage is either positive or negative but not 0.
The duty cycles during the modified pulse width modulation pattern may have pulse widths below 40%. It is also possible that there are cycles, in which no switching occurs at all. In the latter case, the duty cycle may be 0%.
A switching frequency of a standard pulse width modulation pattern (with 50% duty cycle) and of modified pulse width modulation pattern may be defined via the durations of a full cycle.. It has to be noted that the standard pulse width modulation pattern and the modified pulse width modulation pattern both may have the same frequency.
During the modified pulse width modulation pattern, the load transferred by the DC-to- DC converter may be much smaller and the currents flowing through the DC-to-DC converter. In such a way, eventual excessive inrush currents and a saturation of the transformer may be prevented.
The control method may be adaptive in the sense that it is able to maintain correct operation of the modular converter during disturbances on the terminals (i.e. the input or the output) of the modular converter. The control method may be adaptive in the sense that it is able to maintain correct operation of the modular converter during disturbance in some of the converter cells (for example, there may be a loss of coupling between the first and second DC links).
The method may be applied equally during charging and/or start-up scenarios with energy source located either on the side of the first DC link or the side of the second DC link.
The method may be applied during the increase and/or the decrease of the number of active converter cells used in the modular converter (for example for reconfiguration if the modular converter incorporates redundancy).
The method does not require a much higher switching frequency as the nominal operating frequency (i.e. the frequency of the standard pulse width modulation pattern) of the modular converter.
According to an embodiment of the invention, the modular converter comprises a central controller and a plurality of local controllers, each local controller associated with one of the converter cells and adapted for controlling the converter cell. In this case, the voltage difference for a converter cell may be determined based on determining an offline/online status of the local controller of the converter cell.
As an embodiment, the voltage difference may be determined indirectly (i.e. not by measuring voltages and/or currents).
In particular, after the start-up of the modular converter or a start-up of one converter cell, the modified pulse width modulation pattern may have to be applied to the DC-to-DC converter. This may be determined based on the operation state of the associated local controller and dependency of their state to the values of DC link voltage, which may be implementation specific. If the associated local controller is offline, this may indicate, that the associated converter cell is in a start-up phase.
The offline/online status of the local controller may be determined with the central controller.
According to an embodiment of the invention, the method further comprises the steps of: if the local controller is not activated, delaying the operation of the DC-to-DC converter for a predefined delay time or until local controller becomes active, and operating the DC- to-DC converter with the modified pulse width modulation pattern after the delay time. In the predefined delay time, the local controller may finish its start-up sequence.
In an alternative embodiment, if the local controller is activated at start-up, a standard pulse with modulation pattern may be applied and/or the converter cell may start to charge..
According to an embodiment of the invention, the voltage difference is determined based on measuring a first voltage in the first DC link and a second voltage in the second DC link.
According to an embodiment of the invention, the voltage difference is determined based on a first resonant current at the primary side of the transformer and/or a second resonant current at the secondary side of the transformer. As a further embodiment for determining the voltage difference in a more direct way, the voltage difference may be determined from measured currents. For example, the time derivate of the resonant current may be related with the voltage drop (difference) between the two DC links.
According to an embodiment of the invention, the modified pulse width modulation pattern comprises a sequence of pulses having a pulse width that gradually increases towards a 50% duty cycle. For example, the sequence of pulses may start with a pulse that has no (0%) or nearly no duty cycle. The following cycles may become wider and wider until the duty cycle of 50% is reached again.
According to an embodiment of the invention, a time duration, for which the modified pulse width modulation pattern is applied, is based on the voltage difference. For example, if the voltage difference is small or differs only for a small amount from the predefined threshold value, also the time duration for application of the modified pulse width modulation pattern may be small. Conversely, if the voltage difference is higher, the time duration for the modified pulse width modulation pattern may be longer, too.
According to an embodiment of the invention, a pulse width of switching pulses of the modified pulse width modulation pattern is based on the voltage difference. For example, the bigger the voltage difference is, the smaller the pulse widths (or at least the pulse width of a first pulse of a sequence of increasing pulses) may be set.
According to an embodiment of the invention, the method further comprises the step of: if the voltage difference is below the predefined threshold value, operating the DC-to-DC converter with a standard pulse width modulation pattern having (only) pulses with a 50% duty cycle. After (and/or before) the application of the modified pulse width modulation pattern, the modular converter or at least the converter cell may be operated with a standard or normal pulse width modulation pattern, which may only comprise pulses that are half as long as a full cycle.
According to an embodiment of the invention, the voltage difference is determined and evaluated after regular time intervals. In this case, also during normal operation of the modular converter (and not only during the startup phase), the voltages between the first side and the second side of the DC-to-DC converter may be balanced with the aid of the method.
According to an embodiment of the invention, the voltage difference is (only) determined after start-up of a converter cell. It is also possible that the voltage difference is balanced during the start-up phase of the converter. The method may also be applied in the case an additional (redundant) converter cell is activated, while other converter cells are already operating.
According to an embodiment of the invention, the DC-to-DC converter comprises a DC- to-AC converter stage, an AC-to-DC converter stage and a transformer interconnecting the two stages. The pulse width modulation pattern may be applied to semiconductor switches of the first and/or second converter stage. For example, only the DC-to-AC converter or only the AC-to-DC converter stage may be an active stage that is actively switched.
The DC-to-DC converter may comprise a first DC-to-AC converter for transforming the DC current in the first DC link into a first AC current supplied to the transformer. The second AC current generated by the transformer may be rectified by the AC-to-DC converter and supplied into the second DC link. Either the DC-to-AC converter or the AC- to-DC converter or both converters may be active converters that are controlled by a controller of the modular converter.
A further aspect of the invention relates to a control arrangement for a modular converter, wherein the control arrangement is adapted for performing the method as described in the above and in the following.
According to an embodiment of the invention, the control arrangement comprises a central controller and a plurality of local controllers communicatively interconnected with the central controller. Each local controller may be associated with one converter cell of the modular converter and/or may be adapted for switching semiconductor switches of the DC-to-DC converter of the respective converter cell. Each local controller may be adapted for switching semiconductor switches of the AC-to-DC input converter or the DC-to-DC input converter of the respective converter cell.
The control arrangement may comprise two control hierarchies, a central controller, which controls the local controllers of the converter cells.
A further aspect of the invention relates to a modular converter, which comprises a plurality of converter cells interconnected in series on a first side and interconnected in parallel on a second side, wherein each converter cell comprises a first DC link, a DC-to- DC converter, and a second DC link connected in cascade.
According to an embodiment of the invention, the modular converter may comprise a control arrangement for controlling the plurality of converter cells as described in the above and in the following.
It has to be understood that features of the method as described in the above and in the following may be features of the control arrangement and/or the modular converter as described in the above and in the following and vice versa. Summarized, the described method is simple and may prevent a saturation of a medium frequency transformer within a modular converter, both during start-up and normal operation when disturbances may occur.
During the start-up, the modular converter may have to be energized and a special modulation sequence may be applied to charge the DC links in a controllable fashion and to prevent saturation of a transformer and possibly large inrush currents.
During normal operation, a desired relationship (for example the difference) between voltages of the first and second DC links may be invalidated for certain converter cells, due to the various disturbances, in which case a special corrective/modified modulation sequence may be applied to particular converter cells in order to bring the modular converter back into normal operation.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.
Fig. 1 schematically shows a modular converter according to an embodiment of the invention.
Fig. 2 schematically shows a modular converter according to a further embodiment of the invention.
Fig. 3 shows a diagram with voltages in a converter cell of a modular converter according to an embodiment of the invention.
Fig. 4 shows a diagram with currents in a converter cell of a modular converter according to an embodiment of the invention.
Fig. 5 shows a diagram with currents in a converter cell of a modular converter according to an embodiment of the invention.
Fig. 6 shows a diagram with voltages in a converter cell of a modular converter according to an embodiment of the invention.
Fig. 7 shows a diagram with currents in a converter cell of a modular converter according to an embodiment of the invention.
Fig. 8 shows a diagram with currents in a converter cell of a modular converter according to an embodiment of the invention.
Fig. 9 shows a flow diagram for a method for controlling a modular converter according to an embodiment of the invention. Fig. 10 shows a modified pulse width modulation pattern according to an embodiment of the invention.
Fig. 1 1 shows a modified pulse width modulation pattern according to an embodiment of the invention.
Fig. 12 shows a modified pulse width modulation pattern according to an embodiment of the invention.
In principle, identical parts are provided with the same reference numerals in the figures. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
Fig. 1 shows a single phase modular multilevel converter 10 comprising a plurality of converter cells 12.
The modular converter 10 has AC medium voltage terminals on an AC side 14, in particular a first AC terminal 16 and a second AC terminal 18, and low voltage terminals on a DC side 20, in particular a first DC terminal 22 and a second DC terminal 24.
An inductor 26 and a charging circuit 28 are connected between first AC terminal 16 and the first converter cell 12. The charging circuit 28 is realized as a combination of an inrush current limiting resistor and a bypass switch.
The converter cells 12 are connected in series on the AC side 14 between the first AC terminal 16 and the second AC terminal 18 in order to meet the AC line voltage between the two terminals 16, 18. The converter cells are connected in parallel on the DC side 20 between the first DC terminal 22 and the second DC terminal 24.
Each of the converter cells 4 comprises an AC-to-DC converter 30, which is connected to a first DC link 32 at its output that is floating with respect to the system ground. The first DC link 32 is connected with a DC-to-DC converter 34, which on the second side is connected to a second DC link 36. Both DC links 32, 36 comprise at least one capacitor.
The DC-to-DC converter 34 comprises a first power stage or DC-to-AC converter 38, one or more transformers 40 and a second power stage or AC-to-DC converter 42. The DC-to-DC converter 34 is connected to the first DC link 32 with its first power stage 38 which drives the one or more transformers 40 and provides through the second power stage 42, a DC power to the second DC link 36.
Because of the transformers 40, the first side 14 of the converter 10 is galvanically isolated from the second side 20.
Each of the converter cells 12 comprises a local controller 44 that is adapted to control the AC-to-DC converter 30, and the DC-to-DC converter 34 and in particular the converter stages 30, 38, 42. In Fig. 1 only one local controller 44 is shown. The modular converter 10 comprises a central or main controller 46 for controlling the local controllers 44.
The local controllers 44 and the central controller 46 belong to a control arrangement 48 for controlling the modular converter 10.
The overall modular converter 10 may be realized with redundant cells, although this is not shown in Fig. 1 . For connecting and disconnecting converter cells 12 to the AC side 14, each or at least some of the converter cells 12 may comprise a bypass switch 50 that is adapted for connecting the converter cell 12 between the terminals 16, 18 or for bypassing the converter cell 12.
Without any loss of generality, the explanations related to Fig. 1 assume that the modular converter 10 interconnects an AC side 14 with a DC side 20. Alternatively, the side 14 may be a (medium voltage) DC side and the converter 30 may be a DC-to-DC converter. In this case, the remaining functionality of the converter 10 may be the same as explained with respect to Fig. 1 .
The modular converter 10 may be operated either as medium voltage AC to low voltage DC galvanic isolated converter or as medium voltage DC to low voltage DC converter.
Fig. 2 shows a three phase modular multilevel converter 10' that comprises three single phase modular converters 10, which may have the same components as the single phase converter 10 shown in Fig. 1 . In Fig. 2, on the AC side 14, the first terminals 16 are connected to the three phases of a three phase medium voltage, the second terminals 18 are connected to the ground. On the DC side 20, the modular converters 10 are connected in parallel. In an alternative embodiment, a delta connection may be applied on the medium voltage AC side 14.
In the following, it is assumed that an initial charging of a modular converter 10 is done from the AC side 14, since the location of the charging circuit 28 is indicated as such in Fig. 1 . However, a charging may be performed either from the AC side 14 or the DC side 20.
If the voltage of the first DC link 32 of the /th converter cell 12 is l ?.,, the voltage of the second DC link 36 is V2, and the turn ratio of the /th transformer 40 is n„- then in general following relation is true during normal operation of the modular converter 10:
V^ n^ (1 )
/ may be a number between 1 and N, wherein N is the total number of converter cells
12. There may be a small difference between the voltages l ?., of two first DC links 32 on the first side, especially since a plurality of first DC links 32 exists, which float with respect to the system ground potential, and which are connected through a number of transformers 40 to the second side 20.
On the other hand, due to the parallel connection of the DC terminals 22, 24 on the second side 20, the voltage V2 of the second DC links 36 is identical for all converter cells 12. Therefore, the voltage V1.i of the first DC link 32 of each converter cell 12 may have slightly different values.
Small variations in circuit parameters may cause that two first DC links 32 deviate from each other. These deviations may be expressed for each converter cell 12 separately as:
Figure imgf000011_0001
ΔΥΝ = V1-N - nN V2 (2)
A certain threshold value Vthreshold may be defined for variations of (2) which should not be exceeded (on a converter cell level) in absolute terms, during normal operation of the modular converter 10, i.e.:
A L Vl <—V vthreshold
A L V2 <V vthreshold
AV3 <Vthreshold (3)
A °V V N <—V threshold
If conditions (3) are not true, a specific corrective action may be needed to bring each converter cell 12 into normal operating mode.
In the following, a start-up sequence of the modular converter 10 will be explained.
Before the turn-on event, the first DC links 32 and the second DC links 36 are discharged, and thus no energy content is present in the modular converter 10. After connecting the AC terminals 16, 18 to a source, inrush current will start flowing through the inductor 26, charging circuit 28 and series connected AC-to-DC converters 30. This current is limited by the resistor present in the charging circuit 28 and thus there may be no severe inrush currents into the first DC links 32.
Since the first DC links 32 and the second DC links 36 are empty, differences between two DC links voltages are small and conditions (1 ) are satisfied making equations (3) true. So it could be said that conditions before turning on the modular converter 10 are "normal" at least in the sense of definitions introduced in (1 )-(3). Therefore, a standard PWM pattern may be applied to the first converter stage 38 of the DC-to-DC converter 34. The standard PWM pattern has a 50% duty cycle with a fixed switching frequency. This results that all first DC links 32 and the second DC links 36 are charged simultaneously (in parallel).
In Fig. 3, the voltage 60 of a first DC link 32 and the voltage 62 of a second DC link 36 (times the turn ratio of the corresponding transformer 40) of a converter cell 12 during parallel charging are shown.
The time instant t=0s indicates the connection of the modular converter arrangement to the AC source. For the sake of illustration, rated value of the first DC link 32 is 3600V and 1500V for the second DC link 36, while the turn ratio of the transformer 40 is 2.4. It can be seen that the voltage 60 of the first DC link 32 rises quickly and that the voltage 62 of the second DC link 62 follows with the same slope. The inrush current is limited by the charging resistor of the charging circuit 28. Both DC link voltages 60, 62 reach a certain value (below the rated value) and then at some time instant, the charging resistor is bypassed and the AC-to-DC converters 30 are activated to boost the DC link voltages 60, 62 to its rated values. All the time, the DC-to-DC converters 34 are operated using the standard PWM pattern.
Fig. 4 and 5 show the current waveforms during the parallel charging as explained with respect to Fig. 3. Fig. 4 shows the resonant current 64 and the magnetizing current 66 on the first side 14. Fig. 5 shows the resonant current 68 on the second side 20. From Fig. 4 and 5, it can be seen that the primary and secondary currents 64, 66, 68 are limited and controlled. Also saturation of the transformer 40 is avoided.
Fig. 3 to 5 show a normal operation of the modular converter. However, during start-up or during normal operation, it may happen that conditions (3) are violated in some of the converter cells 12, due to various disturbances either on the AC side 14 or DC side 20. Violation of (3) means that the voltage difference between the voltage of the first DC link 32 and the voltage of the second DC link 34, has exceeded a specified, predefined threshold value. An application of the standard PWM pattern may lead to saturation of the transformer 40 or to a large inrush current.
In this case, a corrective or modified PWM pattern is applied in order to bring the voltages of the two DC links 32, 36 towards their rated values.
It is important to note that a modified PWM pattern may be applied to one converter cell 12 or to several converter cells 12, depending where violation of (3) has been detected. However, in some cases, a modified PWM pattern may be applied to the whole converter arrangement 10 as will be explained next. Fig. 6 to 8 are figures analogously to Fig. 3 to 5. In Fig. 6, the DC link voltages 60, 62 are shown during sequential charging. Fig. 6 shows the resonant current 64 and the magnetizing current 66 on the first side 14. Fig. 8 shows the resonant current 68 on the second side. Fig. 6 to 8 illustrate the extreme case of the start-up of the modular converter 10, when the first DC links 32 are charged and the second DC links 36 are completely discharged (or are only partially charged). Such a situation may occur, if the normal charging procedure described in the previous section was interrupted due to an unexpected event.
For example, it may happen that the local controller 44 has blocked operation of the DC-to-DC converter 34 and no PWM pattern has been applied to the first stage 38 of the DC-to-DC converter 34 immediately after connecting the modular converter 10 to the AC source. This may be the results of a controller algorithm, or because the local controller hardware cannot operate if the first DC links 32 are not charged. Thus, the first DC link 32 is charged and the second DC link 36 remains discharged, since there is no energy transfer through the DC-to-DC converter 34. Condition (3) is not true and the control arrangement 48 selects modified PWM pattern for the first stage 38 of the DC-to-DC converter 10.
This is illustrated in Fig. 6 where time instant t=0s indicates the connection of the modular converter 10 to the AC source. It can be seen that the voltage 60 of the first DC link 32 rises quickly and that the voltage 62 of the second DC link 36 remains at the zero value.
Around time instant t=0.006s, 70 the control arrangement 68 applies a modified PWM pattern to the first stage 38 of the DC-to-DC converter 34, transferring the energy to the second DC link 36, whose voltage 62 then starts to rise. The modified PWM pattern may be applied as long as conditions (3) are not true. Once (3) is satisfied, the control arrangement 48 will continue operating the DC-to-DC converter 34 using the standard PWM pattern.
After both DC link voltages 60, 62 reach a specific value (below a rated value) at some time instant, the charging resistor may be bypassed and the AC-to-DC converter 30 is activated to boost the DC link voltages 60, 62 to its rated values.
This illustrative example is the worst possible case when the voltage difference between the first and second DC links 30, 32 is large.
However, it can be understood that during normal operation, disturbances may not be so severe and differences between voltages are not so large. Still, corrective PWM pattern may be activated whenever these differences violate conditions (4). Regarding the start-up procedure and the charging method, both normal and modified PWM patterns may be effectively used, in which case one could distinguish between parallel charging and sequential charging, wherein the decision may be made by the control arrangement.
Parallel charging is used, when both DC link voltages 60, 62 rise at the same time and a standard PWM pattern is applied after the connection of the modular converter 10 to the AC source. There may be no delay between the connection to the AC source and the application of the standard PWM pattern to the first stage 38 of the DC-to-DC converter 10.
Sequential charging is used, when only the first DC link 32 is charged after the connection of the modular converter 10 to the AC source and then a modified PWM pattern is used to transfer the energy to the second DC link 10. There may be a delay between the connection to the AC source and the application of the modified PWM pattern to the first stage 38 of the DC-to-DC converter 34.
In the following, control algorithms are described, with which the above described charging procedure may be achieved. Furthermore, the control algorithm may be used during the normal operation of the modular converter 10, after the charging.
Fig. 7 shows a flow diagram with a control algorithm for charging the power converter 10 and for maintaining the operation of the modular converter 10.
Two layers of control can be recognized. A central controller 46 is responsible for an overall control of the modular converter 10. It may be assumed that the central controller 46 is always available (powered on), in particular prior to the connection of the modular converter 10 to an AC source. Each converter cell 12 comprises a local controller 44 responsible for the control of the converter cell 12. The local controller 44 may or may not be available (online) prior to the connection of the modular converter 10 to the AC source.
Each of these controllers 44, 46 may perform various further control and protection functions which are not discussed herein.
In step 80, a start-up of the modular converter 10 is initiated, which may involve specific steps as discussed above. After the start-up 80, in step 82 it is determined, whether the local controllers 44 are online or offline.
If the local controllers 44 are online, the central controller verifies in step 84, whether the conditions (3) are satisfied.
If the conditions (3) are satisfied, in step 86, a standard PWM pattern is applied to the DC-to-DC converter 34. I.e. the start-up and charging procedure of the modular converter 10 will continue using a standard PWM pattern.
If the conditions (3) are not satisfied in step 84, in step 88, a modified PWM pattern is applied to the DC-to-DC converter 34. i.e. the start-up and charging procedure of the modular converter 10 will continue using a corrective PWM pattern.
During the application of the standard PWM pattern in step 86, for example during parallel charging, in step 90, the local controller 44 regularly checks whether condition (3) is satisfied for its own converter cell 12. If the condition (3) is satisfied, the application of a standard PWM pattern will be continued by the local controller 44.
If the condition (3) is not satisfied in step 90, the local controller 44 switches its mode of operation from application of a standard PWM pattern in step 86 into an application of a modified PWM pattern in step 88. It is also possible that the local controller checks during the application of modified PWM pattern in step 88, whether condition (3) is satisfied and switches to the application of a standard PWM pattern in step 86, when this is the case.
If it is determined in step 82, that the local controllers 44 are offline, the central controller waits for a delay time in step 92. Since the local controllers 44 are offline and the start-up process is initiated, a delay will be introduced as a consequence of the need for the voltage 60 of the first DC links 32 to reach a specific value when the local controllers 44 will change its status from offline to online.
After the delay in step 92, since conditions (3) are not true anyhow, the control arrangement 48 proceeds with a modified PWM pattern in order to transfer energy to the second DC links 36 in a controlled fashion without saturation of a transformer 40 or excessive inrush currents into the second DC links 36.
During the process of sequential charging, the local controller 44 regularly checks in step 90 the validity of the condition (3) for its own converter cell 12 and may switch its mode of operation from the modified PWM pattern into the standard PWM pattern (and vice versa).
There may be following differences and similarities between step 84 and step 90:
Step 84 may be performed by the central controller 46 and 90 may be performed by a local controller 44.
Step 84 and step 90 may both compare the individual DC link voltages of each converter cell 12. However, step 90 may only compare the DC link voltages of one converter cell 12 and this may be done in every converter cell 12 individually. On the other hand, step 84 may compare the DC link voltages for every converter cell 12 individually and this may be done by the central controller 46 only once.
The method may operate with only step 84, only step 90 and also using steps 84, 90 both.
Furthermore, some converter cells 12 may be in standard mode (step 86), and others may be in corrective mode (step 88) at a given point in time.
Fig. 10 to 12 show diagrams with a modified PWM pattern 94. In the diagrams, time is running to the left and the y-axis indicates the voltage u output by the DC-to-AC converter 38 of the DC-to-DC converter 34. The DC-to-AC converter 38 is adapted for generating a voltage of +DC, 0 and -DC, wherein +DC is the voltage of the DC link 32.
Connected regions of the generated voltage, in which the voltage is not 0, may be seen as pulses 96. In the modified PWM pattern 94, pulses 96 with positive voltage +DC alternate with pulses 96 with negative voltage -DC.
The distance (i.e. time) between the centers (or symmetry axes) of two pulses 96 with positive voltage +DC (or negative voltage -DC) may define the length or width of a full cycle 98. The inverse of a full cycle may be the frequency of the PWM pattern 94 (in the case the length of the full cycle does not change).
The positive and negative pulses 96 may have the same width and/or frequency or a different width and/or frequency.
In particular, the apparent switching frequency may change during the charging (sequential charging). It may be possible that the frequency never changes and that the frequency of a modified PWM pattern 94 is the frequency of the standard PWM pattern.
Furthermore, the symmetry axis or the center of the positive and negative pulses 96 may move compared to the positions shown in Fig. 9. For example, the widths 100 before and after a pulse 96 may differ.
The distance between two neighboring pulses 96 (i.e. a negative pulse 96 and a positive pulse 96) may define the width of a half cycle 100 of the PWM pattern.
The duty cycle of a PWM pattern or of a pulse 96 is the width of a pulse 96 divided by the width of the corresponding full cycle 98 (which may be the full cycle 98 surrounding the pulse 96).
A modified PWM pattern 94 may be characterized in that the duty cycle of some or all pulses 96 of the modified PWM pattern 94 is smaller than 50%, i.e. that there are regions in which the voltage u is 0. In other words, in a modified pulse pattern 94 reaching the transformer 40, the voltage may have at least three levels.
Contrary to this, a standard PWM pattern may be characterized by an operation with a 50% duty cycle with optionally a fixed switching frequency.
The width or duty cycle of pulses 96 of a modified PWM pattern 94 and/or the duration of the application of a modified PWM pattern 94 may be determined by one of the controllers 44, 48, for example based on the voltage difference.
It can be seen from Fig. 10 that in an extreme case a modified PWM pattern 94 may have very narrow pulses 96 at a certain switching frequency, for example with a duty cycle below 20% or below 10%.
In general, the width of the pulses 96 may change between 0% and 50%. In Fig. 12 it is shown that a modified PWM pattern 94 may have very wide pulses 96 at a certain switching frequency, for example with a duty cycle between 40% and 50%.
Furthermore, it is possible that the width of the pulses 96 changes during a sequence of pulses 96 of a modified PWM pattern 94.
Fig. 10 to 12 may show parts of a sequence of pulses 96 that gradually increases. Pulses 96 in the beginning of the sequence (Fig. 10) may be small, may become broader in the middle of the sequence and nearly may reach the width of a half cycle at the end of the sequence.
For example, in such a sequence a modified PWM pattern 96 may be transitioned into a standard PWM pattern.
During application of a modified PWM pattern 94, consecutive pulses 96 may go through the transition (as shown from Fig. 10 to 12). This transition may be over when the modified PWM pattern 94 becomes a standard PWM pattern (with 50% duty cycle and fixed switching frequency).
The length or duration of the sequence of pulses 96 and/or the width of the first pulse 96 of the sequence may be determined by one of the controllers 44, 48, for example based on the voltage difference.
While the invention 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; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, 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 article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. LIST OF REFERENCE NUMERALS
10 modular converter
12 converter cell
14 AC side
16 first AC terminal
18 second AC terminal
20 DC side
22 first DC terminal
24 second DC terminal
26 inductor
28 charging circuit
30 AC-to-DC-converter
32 first DC link
34 DC-to-DC converter
36 second DC link
38 DC-to-AC converter
40 transformer
42 AC-to-DC converter
44 local controller
46 central controller
48 control arrangement
50 bypass switch
60 voltage of a first DC link
62 voltage of a second DC link
64 resonant current on first side
66 magnetizing current on first side
68 resonant current on second side
70 time instant for application of modified PWM pattern
80 start-up
82 local controllers online?
84 conditions satisfied?
86 application of standard PWM pattern 88 application of modified PWM pattern
90 condition for converter cell satisfied?
92 application of delay
94 modified PWM pattern
96 pulse
98 full cycle
100 half cycle

Claims

1 . A method for controlling a modular converter (10),
the modular converter (10) comprising a plurality of converter cells (12) interconnected in series on a first side (14) and interconnected in parallel on a second side (20);
wherein each converter cell (12) comprises a first DC link (32), a DC-to-DC converter (34), and a second DC link (36) connected in cascade;
the method comprising the steps of:
determining whether a voltage difference between a first DC link (32) and a second DC link (36) of a converter cell (12) is above a predefined threshold value;
if the voltage difference is above the predefined threshold value: operating the DC- to-DC converter (34) of the converter cell (12) with a modified pulse width modulation pattern (94) having pulses (96) with a duty cycle smaller than 50%, characterized in that the modular converter (1 0) comprises a central controller (46) and a plurality of local controllers (44), each local controller (44) associated with one of the converter cells (1 2) and adapted for controlling the one converter cell (1 2), wherein the voltage difference is determined based on determining an offline/online status of a local controller (44), and in the case the local controller (44) is offline : delaying the operation of the DC-to-DC converter (34) for a predefined delay time or until the local controller (44) is online, and operating the DC-to-DC converter (34) with the modified pulse width modulation pattern (94) after the delay time.
2. The method of one of the preceding claims,
wherein the voltage difference is determined based on measuring a first voltage the first DC link (32) and a second voltage in the second DC link (36).
3. The method of one of the preceding claims,
wherein the voltage difference is determined based on a first resonant current at the primary side of a transformer (40) of the DC-to-DC converter (34) and/or a second resonant current at the secondary side of a transformer (42).
4. The method of one of the preceding claims,
wherein the modified pulse width modulation pattern (94) comprises a sequence of pulses (96) having a pulse width that gradually increases towards a 50% duty cycle. 5. The method of one of the preceding claims,
wherein a time duration, for which the modified pulse width modulation pattern (94) is applied, is based on the voltage difference.
6. The method of one of the preceding claims,
wherein a pulse width of pulses (96) of the modified pulse width modulation pattern
(94) is based on the voltage difference.
7. The method of one of the preceding claims, further comprising the step:
if the voltage difference is below the predefined threshold value: operating the DC- to-DC converter (34) with a standard pulse width modulation pattern having pulses with a 50% duty cycle.
8. The method of one of the preceding claims,
wherein the voltage difference is determined and evaluated after regular time intervals.
9. The method of one of the preceding claims,
wherein the voltage difference is determined after start-up of a converter cell (12). 10. The method of one of the preceding claims,
wherein the DC-to-DC converter (34) comprises a DC-to-AC converter stage (38), an AC-to-DC converter stage (42) and a transformer (40) interconnecting the two stages, wherein the pulse width modulation pattern is applied to semiconductor switches of the first and/or second converter stage (38, 40).
1 1 A control arrangement (48) for a modular converter, the control arrangement (48) adapted for performing the method of one of the claims 1 to 12.
12. The control arrangement (48) of claim 13 further comprising:
a central controller (46);
a plurality of local controllers (44) communicatively interconnected with the central controller (46), each local controller (44) associated with one converter cell (12) of the modular converter (10) and adapted for switching semiconductor switches of the DC-to- DC converter (34) of the converter cell (12).
13. A modular converter (10), comprising:
a plurality of converter cells (10) interconnected in series on a first side (14) and interconnected in parallel on a second side (20), wherein each converter cell (12) comprises a first DC link (32), a DC-to-DC converter (34), and a second DC link (36) connected in series;
a control arrangement (48) according to claim 13 or 14 for controlling the plurality of converter cells (12).
PCT/EP2013/070264 2012-10-10 2013-09-27 Controlling a mocular converter WO2014056742A2 (en)

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EP12188016.5 2012-10-10

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