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
  • 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
• • 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
  • H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
• • 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
  • H02M1/0003—Details of control, feedback or regulation circuits
  • H02M1/0006—Arrangements for supplying an adequate voltage to the control circuit of converters

Definitions

• the invention relates to converter control for a multilevel voltage source converter for high voltage DC power transmission and reactive power compensation .
• each module In multilevel voltage source converters, the current and voltage rating of each module is finite and largely determined by the capabilities of the semiconductor switching devices employed. It is common practice to achieve high power equipment by connecting a large number of modules in series, which effectively increases the total operating voltage of the converter and hence the MVA rating.
• the series arrangement of the individual modules in the multilevel voltage source converter means that the failure of one of the modules leads to the failure of the entire multilevel voltage source converter because there is no flow path for a converter current .
• a possible solution is the inclusion of additional, suitably packaged, high current power electronic devices such as a pair of reverse connected parallel thyristors.
• the power electronic devices attached across the module reverts to a robust and safe bidirectional short circuit which maintains the flow of the converter current in the multilevel voltage source converter .
• this solution maintains the operation and availability of the multilevel voltage source converter, it is however unsuitable for use in low cost power conversion equipment.
• the inclusion of additional devices to each module can, depending on the number of modules, substantially increase the overall cost of the multilevel voltage source converter.
• a mechanical bypass switch is provided for each module in the multilevel voltage source converter to maintain the operation and availability of the high power multilevel voltage source converter, and at a lower cost.
• the mechanical bypass switch is connected in parallel to each module and, in the event of module failure, the mechanical bypass switch is activated to cause a short circuit so that the converter current flows through the mechanical bypass switch in both directions
• the operation of the mechanical bypass switch is however normally irreversible due to reasons such as contact welding and loss of local power supplies. As a result, the mechanical bypass switch may need to be replaced after the mechanical bypass switch is activated to remove the short circuit.
• a multilevel voltage source converter for high voltage DC power transmission and reactive power compensation comprising at least one power electronic module including at least one switch and an electronic bypass system, wherein the electronic bypass system is controllable to activate the or each switch in the module to cause a short circuit through the activated switch and thereby maintain a converter current through the module and the multilevel voltage source converter.
• Utilization of the existing switches in the module is advantageous because it removes the need for additional devices, which could otherwise increase equipment costs.
• the electronic bypass can be reversed simply by controlling the electronic bypass system to deactivate the or each switch and open the or each short circuit.
• a mechanical bypass may be permanently latched due to contact welding and may thereby require maintenance in order to reverse the bypass .
• the electronic bypass system may be controllable locally or remotely, and the short circuit is preferably sustained.
• the electronic bypass system is controllable to activate the or each switch in the module to cause a short circuit during abnormal operating conditions of the module.
• the purpose of the electronic bypass system is to implement the electronic bypass as soon as abnormal operating conditions such as control failure and equipment failure are detected.
• the electronic bypass system may be controllable in other embodiments to activate the or each switch in the module to control the voltage of the module .
• the module may include a local control system to receive control signals from and send alarm and status data back signals to a global control system of the multilevel voltage source converter.
• the local control system receives control signals from the global control system, which contains instructions to operate the module, and sends alarm and status data back signals to the global control system, which include information on the status of the individual module.
• the global control system may monitor the status of the electronic bypass system of the module via information provided in the alarm and status data back signals.
• the alarm and status data back signals enable the global control system to be updated with the status of the electronic bypass system such as whether the electronic bypass system is currently activated or removed, which is advantageous when it comes to maintenance of the module.
• the electronic bypass system may activate the or each switch to cause the short circuit in the event of loss of communication of control signals or data back signals between the local and global control systems
• Losing the control signal means that the module cannot be switched in and out in the correct sequence in relation to the other modules in the converter.
• the loss of data back signals means that there is a possibility of the module operating outside its normal operating conditions, which could lead to equipment failure such as damage from over-voltage or loss of local power supplies if the voltage falls too low.
• the electronic bypass system therefore activates at least one switch in the module to cause the short circuit to bypass the module to maintain the converter current and thus prevents excessive charging of the module DC capacitor.
• the or each switch may be a semiconductor switch.
• the or each semiconductor switch may be an insulated-gate bipolar transistor.
• a semiconductor switch is advantageous because a semiconductor switch is operable faster than a mechanical switch in performing a switching action.
• the module may include an external or local power supply to supply power to operate the or each switch when it is activated by the electronic bypass system in order to maintain the short circuit.
• an external power supply in the module allows the switches to be activated indefinitely and thereby allows the operation and availability of the multilevel voltage source converter to be maintained at all times.
• the local power supply may be derived from the multilevel converter module capacitor .
• the capacitor supplies power by discharging its stored energy when the electronic bypass is in operation.
• the use of a capacitor means that the electronic bypass can only be maintained temporarily until the stored energy has been used.
• the module includes a resistor connected in parallel with the capacitor.
• the electronic bypass system may be controllable to temporarily switch off the or each activated switch so that the converter current is directed through the capacitor in order to recharge the capacitor and therefore the local power supply, which enables the electronic bypass to continue.
• the capacitor discharges over time and will eventually run out of stored energy.
• the activated switches are briefly switched off to recharge the capacitor and switched back on to resume the short circuit.
• the temporary transition of the electronic bypass system from short circuit mode to recharge mode allows the multilevel voltage source converter to be run indefinitely in the event of module failure as opposed to arrangements in which the capacitor is left to discharge over time without recharging.
• the temporary transition of the electronic bypass system from short circuit mode to recharge mode means that an extra voltage step is introduced into the output waveform during the period of capacitor recharge. This has minimal effect on the AC side power quality and harmonic distortion because in practice there are a very large number of voltage steps produced by the multilevel converter. The addition of one more step has a negligible effect and the transition to recharge mode will occur infrequently.
• the local power supply may include a local current transformer that derives energy from the flow of the converter current.
• a local current transformer as a power supply means that the electronic bypass will not be interrupted because the transformer continually extracts energy from the converter current, even during the electronic bypass.
• the transformer can therefore supply power to the activated switches so as to maintain the short circuit indefinitely.
• the local power supply may include a combination of one or more capacitors and/or one or more local current transformers .
• the electronic bypass may be removed when normal operating conditions of the module are restored.
• the reversibility of the electronic bypass system is advantageous because it means that the electronic bypass system is reusable and does not require any additional maintenance such as repair or replacement .
• the multilevel voltage source converter may include a mechanical bypass switch connected in parallel with the module such that activation of the mechanical bypass switch causes current flow to pass through the mechanical bypass switch instead of the module.
• the inclusion of the mechanical bypass switch provides a backup bypass system in the event of failure of the electronic bypass system.
• the module may include a pair of switching devices and a capacitor connected in parallel in a half-bridge arrangement to define a 2-quadrant unipolar module, and each switching device may include a diode connected in anti-parallel with a switch.
• the 2-quadrant unipolar module is unidirectional, i.e. it produces voltage steps in one polarity only, and can develop zero or positive voltage .
• the module may includes two pairs of switching devices and a capacitor connected in parallel in a full-bridge arrangement to define a 4-quadrant bipolar module, and each switching device may include a diode connected in anti-parallel with a switch.
• the 4-quadrant bipolar module is bidirectional, i.e. it produces voltage steps in both positive and negative polarities, and can develop a zero, positive or negative voltage.
• the multilevel voltage source converter may include a plurality of modules arranged in series.
• the series arrangement of the individual modules forms a single phase multilevel converter.
• the single phase multilevel voltage source converter can have unidirectional or bidirectional characteristics depending on the type of switching arrangement in the modules.
• Figure 1 shows a module which forms one element of a multilevel converter
• Figure 2a shows a module operating in electronic bypass mode
• Figure 2b shows a module operating in capacitor recharge mode
• Figure 3a shows a multilevel voltage source converter according to a first embodiment of the invention.
• Figure 3b shows a multilevel voltage source converter according to a second embodiment of the invention .
• a multilevel voltage source converter 11 according to a first embodiment of the invention is shown in Figure 1.
• the multilevel voltage source converter 11 includes a module 40 including at least one switch 12 and an electronic bypass system, wherein the electronic bypass system is controllable to activate the or each lower switch 12 in the module 40 to cause a short circuit through the activated lower switch 12 and thereby maintain a converter current through the module 40 and the multilevel voltage source converter 13.
• the electronic bypass system is controllable to activate the or each lower switch 12 in the module 40 to cause a short circuit during abnormal operating conditions of the module 40.
• Abnormal operating conditions may include failure of certain components in the module or control failure.
• the module 40 includes a local control system 14 which receives control signals 16 from and sends alarm and status data back signals 18 to a global control system 20 of the multilevel voltage source converter 13, which include information on the status of the individual module.
• the global control system 20 monitors the status of the electronic bypass system of the module 40 via the information provided in the alarm and status data back signals 18.
• the alarm and status data back signals 18 enable the global control system 20 to be updated with the status of the electronic bypass system, such as whether the electronic bypass system is currently activated or removed.
• the electronic bypass system activates the or each switch 12 to cause the short circuit in the event that the communication of control signals 16 or data back signals 18 between the local control system 14 and global control systems 20 is lost.
• control signals 16 or data back signals 18 signifies control failure, and therefore the electronic bypass system activates the or each switch 12 to short circuit the module.
• the switch 12 is provided in the form of an insulated-gate bipolar transistor .
• the module 40 includes a local power supply 22 which supplies power to the or each switch 12 that is activated by the electronic bypass system in order to maintain the short circuit.
• the local power supply draws energy from a module DC capacitor 24.
• electronic bypass as shown in
• the capacitor 24 is recharged by temporarily switching off the or each activated switch 12 so that the converter current 36 is directed through the capacitor 24, as shown in Figure 2b.
• the electronic bypass system When the module capacitor has sufficiently recharged, the electronic bypass system reactivates the switches 12 to resume the short circuit.
• the temporary transition of the electronic bypass system from short circuit mode to recharge mode allows the capacitor 24 to recharge itself and to continue to feed the local power supply for the activated switches 12.
• the module 40 includes a resistor 26 connected in parallel with the capacitor 24.
• the local power supply 22 may include a local current transformer which is continuously charged by the converter current, even during the short circuiting of the module 40.
• the local power supply may include a combination of any one of the local power supply 22, the capacitor 24 or the local current transformer to utilise their respective advantages.
• the local power supply may be replaced by an external power supply.
• the electronic bypass is removed when normal operating conditions of the module 40 are restored.
• the module 40 includes a mechanical bypass switch 28 connected in parallel with the module, and the activation of the mechanical bypass switch 28 causes the current flow to pass through the mechanical bypass switch 28 instead of the module 40.
• the mechanical bypass switch 28 serves as a backup to the electronic bypass system in the event that the electronic bypass system fails.
• the module 40 may be provided in the form of a 2-quadrant unipolar module 42, such as that shown in Figure 3a.
• Figure 3a includes a pair of switching devices 10 and a capacitor 24 connected in parallel in a half-bridge arrangement.
• Each switching device includes a diode 30 connected in anti-parallel with a switch 12.
• the module 42 can develop zero or positive voltage and allows the voltage source converter 11 to produce voltage steps in one polarity only.
• the module 40 may be provided in the form of a 4-quadrant bipolar module 44, such as that shown in Figure 3b.
• the 4-quadrant bipolar module 44 shown in Figure 3b includes two pairs of switching devices 10 and a capacitor 24 connected in parallel in a full- bridge arrangement.
• the or each switching device includes a diode 30 connected in anti-parallel with a switch 10.
• the module 44 can develop positive or negative voltage and allows the voltage source converter 11 to produce voltage steps in both positive and negative polarities.
• the multilevel voltage source converter 13 may include a plurality of modules 40 arranged in series.
• the series arrangement of the individual modules 40 forms a single phase multilevel voltage source converter 13.
• the single phase multilevel converter can have unidirectional or bidirectional characteristics depending on whether the modules 40 are 2-quadrant unipolar modules 42 or 4- quadrant bipolar modules 44.

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• Inverter Devices (AREA)

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• 2009
  • 2009-06-15 EP EP09779756A patent/EP2443733A1/de not_active Withdrawn
  • 2009-06-15 WO PCT/EP2009/057381 patent/WO2010145688A1/en not_active Ceased

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