WO2011060812A1 - High voltage dcdc converter - Google Patents

High voltage dcdc converter Download PDF

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Publication number
WO2011060812A1
WO2011060812A1 PCT/EP2009/065327 EP2009065327W WO2011060812A1 WO 2011060812 A1 WO2011060812 A1 WO 2011060812A1 EP 2009065327 W EP2009065327 W EP 2009065327W WO 2011060812 A1 WO2011060812 A1 WO 2011060812A1
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WO
WIPO (PCT)
Prior art keywords
terminal
voltage source
chain
inductor
source converter
Prior art date
Application number
PCT/EP2009/065327
Other languages
French (fr)
Inventor
Colin Davidson
David Trainer
Ruchira Withanage
Original Assignee
Areva T&D Uk Limited
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 Areva T&D Uk Limited filed Critical Areva T&D Uk Limited
Priority to PCT/EP2009/065327 priority Critical patent/WO2011060812A1/en
Publication of WO2011060812A1 publication Critical patent/WO2011060812A1/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
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/02—Conversion of dc power input into dc power output without intermediate conversion into ac
    • H02M3/04—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters
    • H02M3/10—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M3/145—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M3/155—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/156—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators
    • H02M3/158—Conversion of dc power input into dc power output without intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only with automatic control of output voltage or current, e.g. switching regulators including plural semiconductor devices as final control devices for a single load
    • 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
    • H02M1/0095—Hybrid converter topologies, e.g. NPC mixed with flying capacitor, thyristor converter mixed with MMC or charge pump mixed with buck
    • 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
    • H02M3/33507—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 with automatic control of the output voltage or current, e.g. flyback 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
    • 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
    • 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
    • H02M7/4837—Flying capacitor 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
    • 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
    • H02M7/487—Neutral point clamped inverters

Abstract

A DC voltage source converter for high voltage DC power transmission comprises at least one inductor (50) and at least one chain-link converter (48) connected between first and second DC terminals (52, 54), the or each chain-link converter (48) including a chain of modules (56) in series, each module including one or more semiconductor switches (58) and one or more energy storage devices (60), the semiconductor switches (58) being controllable in use to provide a continuously variable voltage source wherein the or each chain-link converter (48) is operable when DC networks are connected in use to the first and second DC terminals (52, 54) to control switching of the modules (56) to selectively enable one DC network to charge the or each inductor (50), or enable the or each inductor (50) to discharge into the other DC network.

Description

HIGH VOLTAGE DCDC CONVERTER
This invention relates to a DC voltage source converter for high voltage direct current (HVDC) power transmission.
In HVDC power transmission networks, alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables. This conversion reduces the cost per kilometre of the lines and/or cables, and thus becomes cost-effective when power needs to be transmitted over a long distance. DC transmission and distribution networks are needed to support the emergence of HVDC power transmission. However, interconnecting the DC transmission and distribution networks to form a DC power grid is difficult because different DC networks may operate at different voltage levels. In order to interconnect two DC networks operating at different voltage levels, a DC-to-DC voltage source converter is required.
DC-to-DC voltage conversion can be carried out using a buck-boost converter 30 or a flyback converter 42. Both converters 30, 42 rely on the use of a semiconductor switch 34, a diode 38 and a storage inductor to transfer energy from an input capacitor 32 to an output capacitor 40. The DC input capacitor 32 may be replaced by a "stiff" input voltage if only unidirectional power transfer from the input capacitor 32 to the output capacitor 40 is required.
Figure la shows the configuration of the buck-boost converter 30 which includes a series arrangement of an input DC capacitor 32 and a semiconductor switch 34 connected in parallel with a storage inductor 36 and connected in parallel with a diode 38 and an output DC capacitor 40.
Figure lb shows the configuration of the flyback converter 42. Instead of using the storage inductor, the flyback converter 42 uses a mutually coupled pair of inductors 44,46 to store energy. The flyback converter 42 includes a series arrangement of the input capacitor 32, the semiconductor switch 34 and a first mutually coupled inductor 44; and a series arrangement of the output capacitor 40, the diode 38, a second mutually coupled inductor 46 whereby the first and second mutually coupled inductors 44,46 are coupled to each other.
Both converters are capable of step-down or step-up operation depending on the duty ratio of the switch 34. The duty ratio is determined by Vout /Vin = D/ (1-D) where Vout is the output voltage across the output capacitor 40, Vin is the input voltage across the input capacitor 32 and D is the duty ratio of the switch 34. A feature of both buck-boost and flyback converters 30,42 is that the polarity of the output voltage is opposite to that of the input voltage. The operation of each converter 30,42 involves two distinct phases in each cycle, the phases being the charging phase and the discharging phase . During the charging phase, the semiconductor switch 34 is closed. This allows current to flow through the semiconductor switch 34 and through the storage inductor 36 or first mutually coupled inductor 44. In the buck-boost converter 30, there is no current flow from the input DC capacitor 32 to the output capacitor 40 because the diode 38 is reverse-biased at this stage. In the flyback converter 42, the changing magnetic field in the first mutually coupled inductor 44 induces a voltage drop across the second mutually coupled inductor 46 but the second mutually coupled inductor 46 does not conduct because the diode 38 is reverse biased. The flow of current through the storage inductor 36 or first mutually coupled inductor 44 allows the storage of energy in the respective inductor due to the formation of a magnetic field.
During the discharging phase, the semiconductor switch 34 is switched to an open state. Since the semiconductor switch 34 is open, the current flowing through the semiconductor switch 34 and the input capacitor 32 drops to zero. At this stage the magnetic field resists the change in current by reversing the polarity of the voltage drop across the inductor 36 or the first mutually coupled inductor 44. In the buck- boost converter 30, the change in voltage drop polarity maintains the inductor current which flows through the now forward-biased diode 38 and the output capacitor 40. In the flyback converter 42, the change in magnetic field induces a voltage drop in the second mutually coupled inductor which forward biases the diode 38 and therefore allows current flow through the second mutually coupled inductor 46 and the output DC capacitor 40. As a result, electrical power is therefore transferred from the input DC capacitor 32 to the output DC capacitor 40.
The conventional buck-boost and flyback converters are however not suitable for transferring 10' s and 100' s of MW of electrical power due to the low voltage ratings of available semiconductor switches and diodes. In order to transfer electrical power at high voltage, multiple insulated gate bipolar transistor (IGBT) devices can be connected in series to form a high power electronic switch to replace the semiconductor switch in the conventional DC-to-DC converter. Similarly a large number of diodes can be used to form a diode stock to replace the single diode in the conventional DC-to-DC converter. However, the series string of IGBT devices requires complex active gate drives and large passive components to adequately control voltage sharing between the IGBT devices. This results in complex, large and expensive converter hardware.
According to an aspect of the invention, there is provided a DC voltage source converter for high voltage DC power transmission comprising at least one inductor and at least one chain-link converter connected between first and second DC terminals, the or each chain-link converter including a chain of modules in series; each module including one or more semiconductor switches connected to one or more energy storage devices; wherein the or each chain-link converter is operable when DC networks are connected in use to the first and second DC terminals to control switching of the modules to selectively enable one DC network to charge the or each inductor, or enable the or each inductor to discharge into the other DC network.
The provision of a chain-link converter allows DC-to-DC conversion to be carried out at high voltage levels because the structure of the chain-link converter allows the build-up of a combined voltage via the insertion of multiple voltage-providing modules into the chain-link converter. The combined voltage can be used to offset the high voltage levels of the DC networks to enable the converter to switch between the inductor charging and discharging phases.
In order to provide a unidirectional chain-link converter, a pair of semiconductor switches may be connected in parallel with the respective energy storage device in a half-bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct current in both directions . In order to provide a bidirectional chain-link converter, two pairs of semiconductor switches may be connected in parallel with the respective energy storage device in a full-bridge arrangement to define a 4-quadrant bipolar module that can provide positive or negative voltage and can conduct current in both directions.
In such embodiments, each semiconductor switch is connected in parallel with a free-wheel diode. The inclusion of the free-wheel diode in each half- bridge and full-bridge module means that the chain-link converter can be operated as a diode stack.
In order to provide a unidirectional chain-link converter, the semiconductor switches may be connected with the respective energy storage device in a half- bridge multilevel converter arrangement to define a 2- quadrant unipolar module that can provide zero or positive voltage and can conduct current in both directions.
In order to provide a bidirectional chain-link converter, the semiconductor switches may be connected with the respective energy storage device in a full- bridge multilevel converter arrangement to define a 4- quadrant bipolar module that can provide positive or negative voltage and can conduct current in both directions . In embodiments employing the use of a multilevel converter arrangement, the multilevel converter may be a flying capacitor converter or a neutral point diode clamped converter.
The provision of a multilevel converter arrangement increases the number of possible combinations of open and closed semiconductor switches of each chain-link module and thereby increases the number of possible operating modes of the chain-link converter. In embodiments employing the use of bipolar modules, the or each chain-link converter may be connected in series to the first or second DC terminals and may be operable to generate a voltage to oppose the flow of current created by a fault in the DC network connected to the respective DC terminal.
Preferably the semiconductor switches are insulated gate bipolar transistors, field effect transistors, gate turn-off thyristors, gate commutated thyristors (GCT) , integrated gate commutated thyristors (IGCT) and/or injection enhanced gate turn-off thyristors (IEGT) .
The use of semiconductor switches is advantageous because such devices are small in size and weight, and have relatively low power dissipation, which minimises the need for cooling equipment. It therefore leads to significant reductions in power converter cost, size and weight. Each energy storage device is preferably a capacitor, battery, fuel cell, photovoltaic cell, auxiliary AC generator with associated rectifier or another energy source or sink.
Each energy storage device may be any device that is capable of storing and releasing electrical energy to provide a voltage. This design flexibility is useful in designing converter stations in different locations, such as off-shore wind farms, where the availability of equipment may vary due to locality or transport difficulties .
In embodiments of the invention each energy storage device may be linked to an auxiliary coupling bus.
The provision of an auxiliary coupling bus ensures that the voltage levels of the modules are balanced through the exchange of energy between the energy storage devices via the coupling bus.
Preferably the voltage source converter includes at least one capacitor connected in parallel with the first DC terminal and/or second DC terminal.
The provision of a capacitor connected in parallel with an input DC terminal limits the undesirable effects of energy flowing back into the input DC terminal. In addition, a capacitor connected to an output DC terminal can be charged during the inductor discharging phase so that it can provide a voltage to the DC network connected to the output DC terminal during the inductor charging phase.
In other embodiments the voltage source converter may include at least one diode connected in series with the second DC terminal, the diode being reverse-biased during charging of the or each inductor to prevent current flow into the second DC terminal and being forward biased during discharging of the or each inductor to allow current flow into the second DC terminal .
The diode blocks current flow into the second DC terminal during the inductor charging phase so that the current from the first DC terminal flows through the inductor only.
In further embodiments the semiconductor switches may be controllable in use so that the chain-link converter permits current flow in one direction only.
The chain-link converter can be configured to behave like a diode and therefore be used to block current flow into the second DC terminal during the inductor charging phase.
To provide a unidirectional DC voltage source converter, a series arrangement of the chain-link converter and the first DC terminal may be connected in parallel with the inductor and connected in parallel with a series arrangement of the diode and the second DC terminal .
The provision of a chain-link converter, which enables the voltage source converter to switch between the inductor charging and discharging phases, means that it is not necessary to rely on series strings of semiconductor switches which require complex active gate drives and large passive components.
To provide a bidirectional DC voltage source converter, a series arrangement of a first chain-link converter and the first DC terminal may be connected in parallel with the or each inductor and connected in parallel with a series arrangement of a second chain-link converter and the second DC terminal .
The provision of chain-link converters connected to each DC terminal allows bidirectional power transfer between the two DC terminals since the tasks of switching between inductor charging and discharging phases, and blocking current flow into a DC terminal during the inductor charging phase are interchangeable between the first and second chain-link converters.
In embodiments of the invention the inductor may be an autotransformer . Such an autotransformer may be iron- cored or air-cored. In embodiments employing the use of an autotransformer, a series arrangement of a first chain-link converter and the first DC terminal may be connected in series with a first terminal and a common terminal of the autotransformer, and a series arrangement of a diode and the second DC terminal may be connected in series with a second terminal and the common terminal of the autotransformer .
In other such embodiments employing the use of an autotransformer, a series arrangement of a first chain- link converter and the first DC terminal may be connected in series with a first terminal and a common terminal of the autotransformer ; and a series arrangement of a second chain-link converter and the second DC terminal may be connected in series with a second terminal and the common terminal of the autotransformer .
The autotransformer can achieve large step-down or step-up ratios in an effective manner.
In embodiments employing the use of an autotransformer the common terminal may be connected to ground.
In embodiments of the invention, the voltage source converter may include first and second inductors mutually coupled to each other.
In such embodiments the voltage source converter may include a series arrangement of the chain-link converter, the first DC terminal and the first mutually coupled inductor; and a series arrangement of the second DC terminal, the diode and the second mutually coupled inductor.
In other such embodiments the voltage source converter may include a series arrangement of a first chain-link converter, the first DC terminal and the first mutually coupled inductor; and a series arrangement of the second DC terminal, a second chain-link converter and the second mutually coupled inductor.
As well as providing the same advantages as the autotransformer, the mutually coupled inductors also allows the interconnection between the first DC terminal and the first mutually coupled inductor to be connected to ground; and the interconnection between the second DC terminal and the second mutually coupled inductor to be connected to ground.
Grounding the isolated DC terminals allows the polarities of the voltages at the first and second DC terminals to be the same.
In embodiments of the invention a series arrangement of the first DC terminal connected between first and third chain-link converters may be connected in series with the first mutually coupled inductor and a series arrangement of the second DC terminal connected between second and fourth chain-link converters may be connected in series with the second mutually coupled inductor. The symmetrical arrangement of the chain-link converters provides a voltage source converter which is suitable for bipolar DC networks. In other embodiments a first pole of each DC terminal may be connected in use to a positive DC voltage and a second pole of each DC terminal may be connected in use to a negative DC voltage such that the polarities of the voltages across the first and second DC terminals are the same.
Bipolar DC networks may be connected in use to the DC voltage source converters with magnetic isolation between the first and second DC terminals.
According to a second aspect of the invention, there is provided a bipole DC voltage source converter comprising two DC voltage source converters wherein the DC voltage source converters are interconnected and connected in use to ground so that the two DC voltage source converters share a common pole at neutral potential .
In embodiments of the invention the respective interconnections between the common terminal of the autotransformer and second poles of the first and second DC terminals of the DC voltage source converters may be connected via superposition to define a common pole which is connected in use to ground; and a first pole of each DC terminal is connected in use to a positive or negative potential. In other embodiments the respective interconnections between the first mutually coupled inductor and a second pole of the first DC terminal of the DC voltage source converters may be connected via superposition to define a first common pole which is connected in use to ground; the respective interconnections between the second mutually coupled inductor and a second pole of the second DC terminal of the DC voltage source converters may be connected via superposition to define a second common pole which is connected in use to ground; and a first pole of each DC terminal may be connected in use to a positive or negative potential. The provision of a bipole DC voltage source converter allows the poles of the input and output terminals to function independently of each other so the DC voltage source converter can still operate normally in the event of failure of one of the poles.
According to a third aspect of the invention, there is provided an arrangement of a plurality of DC voltage source converters wherein the plurality of DC voltage source converters are connected in series in a cascade arrangement to define a two-terminal voltage source converter .
Connecting an even number of DC voltage source converters in a cascade arrangement allows the polarities of the voltages at the input and output terminals of the cascade arrangement to be the same. Preferred embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which:
Figure la and lb shows, in schematic form, prior art DC-to-DC voltage source converters;
Figure 2a shows a unipolar half-bridge module;
Figure 2b shows a bipolar full-bridge module;
Figure 3 shows the different operating modes for a full-bridge module;
Figures 4a and 4b show a neutral-point diode clamped converter arrangement for half-bridge and full- bridge modules ;
Figures 5a and 5b show a flying capacitor converter arrangement for half-bridge and full-bridge modules ;
Figure 6 shows a DC voltage source converter according to a first embodiment of the invention;
Figure 7 shows the inductor charging and discharging phases for the voltage source converter of Figure 6;
Figure 8 shows a circuit analysis of the voltage source converter of Figure 6 during the inductor discharging phase;
Figure 9 shows a DC voltage source converter according to a second embodiment of the invention;
Figure 10 shows a power transfer process including the inductor charging and discharging phases for the voltage source converter of Figure 9; Figure 11 shows another power transfer process including the inductor charging and discharging phases for the voltage source converter of Figure 9;
Figure 12 shows a DC voltage source converter according to a third embodiment of the invention;
Figure 13 shows a DC voltage source converter according to a fourth embodiment of the invention;
Figure 14 shows a DC voltage source converter according to a fifth embodiment of the invention;
Figure 15 shows a DC voltage source converter according to a sixth embodiment of the invention;
Figure 16 shows a DC voltage source converter according to a seventh embodiment of the invention;
Figure 17 shows a DC voltage source converter according to an eighth embodiment of the invention;
Figure 18 shows a DC voltage source converter according to a ninth embodiment of the invention;
Figure 19 shows a DC voltage source converter according to a tenth embodiment of the invention;
Figure 20 shows a DC voltage source converter according to an eleventh embodiment of the invention; and
Figure 21 shows a DC voltage source converter according to a twelfth embodiment of the invention.
Referring to Figure 6, a DC voltage source converter 47 for high voltage DC power transmission according to a first aspect of the invention includes at least one inductor 50 and at least one chain-link converter 48 connected between first and second DC terminals 52,54. The or each chain-link converter 48 includes a chain of modules 56 in series, each module including one or more insulated gate bipolar transistors (IGBT) 58 and one or more capacitors 60. The or each chain-link converter 48 is operable when DC networks are connected in use to the first and second DC terminals 52,54 to control switching of the modules 56 to selectively enable one DC network to charge the or each inductor 50, or enable the or each inductor 50 to discharge into the other DC network .
The DC voltage source converter can be configured to allow the unidirectional transfer of electrical power from one DC terminal to the other, or the bidirectional transfer of electrical power between both DC terminals 52,54.
During the charging of each inductor 50, electrical current flows through the inductor 50 so that a magnetic field is formed. The magnetic field resists the increase in current by inducing a voltage drop across the inductor 50 of opposite polarity to the change in current. More energy can be stored in the magnetic field by further increasing the inductor current 86 (Figure 7) . The inductor 50 is therefore capable of storing energy in the form of a magnetic field created by the flow of electrical current through the inductor 50.
During the discharging of each inductor 50, the supply of electrical current to the inductor 50 is reduced. The decrease in inductor current 86 causes the magnetic field to decrease in strength. The magnetic field resists the change in field strength by inducing a voltage drop of opposite polarity to the change in inductor current 86 in order to maintain the inductor current 86. Eventually the inductor current 86 drops to zero and the magnetic field collapses, thus fully discharging the inductor 50.
The chain-link converter 48 is used to facilitate switching between the inductor charging and discharging phases because the structure of the chain-link converter 48 allows the build-up of a combined voltage via the insertion of multiple voltage-providing modules 56 into the chain-link converter 48. The combined voltage can be used to offset the high voltage levels of the DC networks to enable the converter to switch between the inductor charging and discharging phases.
In embodiments of the invention it is envisaged that the capacitor 60 of each of the modules 56 may be replaced by a battery, fuel cell, photovoltaic cell, auxiliary AC generator with associated rectifier or another energy source or sink. In order to provide a unidirectional chain-link converter 48, the insulated gate bipolar transistors 58 may be connected in parallel with the respective capacitor 60 in a half-bridge arrangement to define a 2-quadrant unipolar module 62, such as that shown in Figure 2a, which can provide zero or positive voltage and can conduct current in both directions. In order to provide a bidirectional chain-link converter 48, the insulated gate bipolar transistors 58 may be connected in parallel with the respective capacitor 60 in a full-bridge arrangement to define a 4-quadrant bipolar module 64, such as that shown in Figure 2b, which can provide positive or negative voltage and can conduct current in both directions. Each half-bridge and full-bridge module 62,64 may include a free-wheel diode 66 connected in parallel with each IGBT 58.
In use, the IGBTs 58 are controllable to operate the half-bridge module 62 in three half-bridge modes; blocked, bypassed and output mode. In half-bridge blocked mode, both the IGBTs 58 are opened. For one polarity current flows through one free-wheel diode 66 and bypasses the capacitor 60. For the other, opposite, polarity current flow through the other free-wheel diode 66 and the capacitor 60 charges up until its voltage reaches that of the external source. In half- bridge bypassed mode, one of the IGBTs 58 is closed to form a short circuit such that current flows through the closed IGBT 58 and bypasses the capacitor 60. In half-bridge output mode, one of the IGBTs 58 is closed such that current flows through the closed IGBT 58 and the capacitor 60 which provides a voltage phase to the voltage source converter. In use, the IGBTs 58 are controllable to operate the full-bridge module 64 in full-bridge blocked, output, bypassed and diode modes, as shown in Figure 3. In full-bridge blocked mode 68, all the IGBTs 58 are opened. In full-bridge output mode 70a, 70b, the IGBTs 58 are configured so that current flows through the closed IGBTs 58 and the capacitor 60 which presents a positive or negative voltage to the voltage source converter. In full-bridge bypass mode 72, the IGBTs 58 are configured to form a short circuit so that current flows through the closed IGBTs 58 and bypasses the capacitor 60. In full-bridge diode mode
74a, 74b, 74c, 74d, one IGBT 58 device is closed to form a short circuit while the rest are open so that current flows through a free-wheel diode 66 and the short circuit .
Since there are four IGBTs 58 in each full-bridge module 64, there are four possible configurations for full-bridge diode mode 74a, 74b, 74c, 74d in which two configurations results in a diode equivalent which permits current flow in one direction only, while the other two configurations results in a diode equivalent which permits current flow in the other direction only.
In other embodiments the insulated gate bipolar transistors 58 may be connected with the respective capacitor 60 in a half-bridge multilevel converter arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct current in both directions. In further embodiments the insulated gate bipolar transistors 58 may also be connected with the respective capacitor 60 in a full- bridge multilevel converter arrangement to define a 4- quadrant bipolar module that can provide positive or negative voltage and can conduct current in both directions .
The multilevel converter arrangement may be a neutral point diode clamped converter arrangement 76,78, as shown in Figures 4a and 4b, or a flying capacitor converter arrangement 80,82, as shown in Figures 5a and 5b. It is envisaged that other operating modes are possible for the multilevel converter half-bridge and full-bridge modules 76,78,80,82 using various combinations of open and/or closed IGBTs 58.
In other embodiments of the invention it is envisaged that the insulated gate bipolar transistors 58 may be replaced by field effect transistors, gate turn-off thyristors, gate commutated thyristors (GCT) , integrated gate commutated thyristors (IGCT) and/or injection enhanced gate turn-off thyristors (IEGT).
The voltage source converter 47 shown in Figure 6 includes a series arrangement of the chain-link converter 48 and the first DC terminal 52 connected in parallel with the inductor 50 and connected in parallel with a series arrangement of a diode 38 and the second DC terminal 54. The first DC terminal 52 is connected in parallel with an input capacitor 32 while the second DC terminal 54 is connected in parallel with an output capacitor 40. The chain-link converter 48 consists of a chain of half-bridge modules 62 connected in series. The direction of the diode 38 is such that it is reverse-biased during charging of the inductor 50 to prevent current flow into the second DC terminal 54 and is forward biased during discharging of the inductor 50 to allow current flow into the second DC terminal 54.
In use, this configuration permits unidirectional power transfer from a first DC network connected to the first DC terminal 52 to a second DC network connected to the second DC terminal 54.
In order to charge the inductor 50, the half-bridge modules 62 are operated in half-bridge bypassed mode. A voltage across the first DC terminal 52 results in an increase in current through the first DC terminal 52, chain-link converter 48 and the inductor 50 while bypassing the capacitors 60 in the chain-link converter 48, as shown in Figure 7. At this stage, the current 84,86 through the chain-link converter 48 and the inductor 50 increases at the same rate. The increasing current 86 through the inductor 50 causes the inductor 50 to store energy in the form of a magnetic field. The diode 38 is reverse-biased which means that the established current 84,86 in the chain-link converter 48 and the inductor 50 respectively cannot flow through the diode 38 and the second DC terminal 54. It is envisaged that some of the half-bridge modules 62 may be operated in half-bridge output mode during the charging phase to provide fine control of the current. In order to discharge the inductor 50, the half-bridge modules 62 may be operated in half-bridge blocked or output modes.
When the half-bridge modules 62 are operated in half- bridge output mode, the chain-link converter 48 provides a voltage 90, which is opposite in direction to the input voltage at the first DC terminal 52 and is at least equal to the sum of the voltages at the first and second DC terminals 52,54. Application of the chain-link converter voltage 90 in the reverse direction causes the current 84 through the chain link converter 48 to decrease. This results in a decrease in inductor current 86 and therefore a decrease in magnetic field strength which causes the magnetic field to induce a voltage drop of opposite polarity to the change in inductor current 86 in order to maintain the inductor current 86. Since the application of the chain-link converter voltage 90 in the reverse direction causes the diode 38 to be forward-biased, the established current 86 in the inductor 50 can now flow through the output capacitor 40 and the forward-based diode 38, and thus resulting in an increasing diode current 88 which charges the output capacitor 40. The inductor current 86 will continue to decrease until it reaches zero, or the voltage source converter reverts to the inductor charging phase. If the voltage of the chain-link converter 48 is equal to the sum of the voltages at the first and second DC terminals 52,54, the current through the inductor 50 falls at an approximately steady rate given by Equation 1. Since the chain-link converter 48 needs to provide a combined voltage 90, the minimum number of chain-link modules 56 required in the chain-link converter 48 is determined by the sum of the input and output voltages, and the voltage range of the individual chain-link module 56.
Figure imgf000025_0001
Where dIL/dt is the change of inductor current 86 with time, Vout is the voltage across the second DC terminal 54 and L is the inductance value of the inductor 50.
If the voltage of the chain-link converter 48 is more than the sum of the voltages at the first and second DC terminals 52,54, the current through the inductor 50 falls at an approximately steady rate given by Equation 2, as shown in Figure 7.
Figure imgf000025_0002
Where V±n is the voltage across the first DC terminal 52 and Vi is the voltage provided by the chain-link converter 48.
Stray inductance in the circuit is a factor in determining the speed of switching from the inductor charging phase to the inductor discharging phase. In order to determine the effect of stray inductance, the DC voltage source converter is analysed by dividing the problem into three separate circuits and considering inductor 50 to be two parallel inductors each having an inductance value of 2L. The change of current with time for each circuit can be determined separately and added .
In a first of the circuits, a first circuit current 92 (Figure 8) flows through the first DC terminal 52, chain-link converter 48, first parallel inductor 50 and the stray inductance in the first circuit. The rate of change in current in the first circuit is calculated using Equation 3.
Figure imgf000026_0002
Where dli/dt is the rate of change in current in the first circuit and Ls i is the stray inductance in the first circuit.
In a second of the circuits, a second circuit current 94 (Figure 8) flows through the second DC terminal 54, second parallel inductor 50 and the stray inductance in the second circuit. The rate of change in current in the second circuit is calculated using Equation 4.
Figure imgf000026_0001
Where dl∑/dt is the rate of change in current in the second circuit and LS2 is the stray inductance in the second circuit.
In a third of the circuits, a third circuit current 96
(Figure 8) flows through the first and second DC terminals 52,54, chain-link converter 48 and both the stray inductances. The rate of change in current in the third circuit is calculated using Equation 5.
Figure imgf000027_0001
Where dls/dt is the rate of change in current in the third circuit.
Figure 7 shows the rate of change in current in each of the chain-link converter 48, diode 38 and inductor 50 during the discharging phase which is calculated using Equations 6, 7 and 8.
Figure imgf000027_0002
Where Iv is the current in the chain-link converter 48, ID is the current in the diode 38 and IL is the current in the inductor 50. When the half-bridge modules 62 are operated in half- bridge blocked mode, the chain-link converter 48 acts as a reverse-biased diode stack which prevents current flowing through the chain-link converter 48, inductor 50 and the first DC terminal 52. The decrease in inductor current 86 causes the magnetic field to decrease in strength. In response to the change in field strength, the magnetic field induces a voltage drop of opposite polarity to the change in inductor current 86 in order to maintain the inductor current 86. As a result, the diode 38 is forward-biased and the established current 86 in the inductor 50 can now flow through the output capacitor 40 and the forward-based diode 38. This results in an increasing diode current 88 which charges the output capacitor 40. The inductor current 86 will continue to decrease until it reaches zero, or the voltage source converter reverts to the inductor charging phase.
After the DC voltage source converter reverts to the inductor charging phase, the inductor current 86 starts increasing to charge the inductor 50 while the output capacitor 40 releases its stored energy to the DC network connected to the second DC terminal 54. As a result, electrical power is transferred from the first DC terminal 52 to the second DC terminal 54 via the charging and discharging of the inductor 50. Each capacitor 60 may be linked to an auxiliary coupling bus to allow the exchange of energy between the different capacitors 60. Alternatively, each half- bridge module 62 may include an energy source or sink, such as a battery, fuel cell or photovoltaic cell, which is capable of injecting power into or removing power from the capacitor 60. The provision of the auxiliary coupling bus and the energy source or sink ensures that the net real power into the chain-link converter 48 is zero in order to balance the voltage levels of the capacitors 60.
For half-bridge modules 62, which are capable of providing a unipolar voltage phase, it is necessary for the current 84 through the chain-link converter to change direction, as shown in Figure 7, for part of the voltage conversion process in order to achieve capacitor voltage balance. The input capacitor 32 connected in parallel with the first DC terminal 52 limits the undesirable effects of energy flowing back into the DC terminal 52.
However, in half-bridge blocked mode, the current 84 through the chain-link converter 48 never changes direction which means that the capacitors 60 can only ever charge, not discharge. This means that half-bridge blocked mode can only be used in the DC voltage source converter if an auxiliary coupling bus or an energy sink is connected to each capacitor 60. A voltage source converter 49 according to a second embodiment of the invention is shown in Figure 9.
The voltage source converter has the same configuration as the embodiment in Figure 6 except that the half- bridge modules 62 are replaced by full-bridge modules 64.
In use, this configuration permits unidirectional power transfer from a first DC network connected to the first DC terminal 52 to a second DC network connected to the second DC terminal 54.
During the inductor charging phase, the full-bridge modules 64 are initially operated in full-bridge bypassed mode 72 (Figure 3) . A voltage across the first DC terminal 52 results in the flow of current through the first DC terminal 52, the chain-link converter 48 and the inductor 50 while bypassing the capacitors 60 in the chain-link converter 48. At this stage, the current 84,86 through the chain-link converter 48 and the inductor 50 increases at the same rate. The increasing current 86 through the inductor 50 causes the inductor 50 to store energy in the form of a magnetic field. The diode 38 is reverse-biased which means that the established current 84,86 in the chain- link converter 48 and the inductor 50 cannot flow through the diode 38 and the second DC terminal 54. For part of the inductor charging phase, the full- bridge modules 64 are operated in full-bridge output mode to partially discharge and provide a voltage phase which adds to the voltage across the first DC terminal 52. The voltage phase is minimised to avoid increasing the voltage rating required for the diode 38.
In order to discharge the inductor 50, there are two ways of operating the DC voltage source converter.
One way is to operate the full-bridge modules 64 in full-bridge output mode 70b (Figure 3) , which results in the discharging of the inductor 50 similar to the inductor discharging phase for the embodiment in Figure 6 and shown in Figure 10. Another way is to operate the full-bridge modules 64 in full-bridge blocked mode 68. The bipolar characteristic of the full-bridge module 64 means that in full-bridge blocked mode 68, the voltage 90 provided by the chain- link converter 48 can be reversed for part of the cycle. This provides an extra degree of freedom since it is possible to control the amplitude and duration of the output voltages in both directions. The combination of operating in full-bridge output mode 70b during part of the inductor charging phase and full-bridge blocked mode 68 during the inductor discharging phase allows capacitor voltage balancing to be achieved during the voltage conversion process without the need for an auxiliary coupling bus or a real power source. When the full-bridge modules 64 are operated in full- bridge blocked mode 68 as shown in Figure 11, the chain-link converter 48 provides a voltage 90 in the opposite direction to the voltage across the first DC terminal 52 and the partially discharged capacitors 60 of the chain-link modules 56 begin to recharge. At this point the chain-link converter current 84 and the inductor current 86 begin to decrease. The decrease in inductor current 86, and therefore a decrease in magnetic field strength, causes the magnetic field to induce a voltage drop of opposite polarity to the change in inductor current 86 in order to maintain the inductor current 86. Since the application of the chain-link converter voltage 90 in the reverse direction also causes the diode 38 to be forward- biased, the established current 86 in the inductor 50 can now flow through the output capacitor 40 and the forward-biased diode 38. This results in an increasing diode current 88 which charges the output capacitor 40.
When the chain-link converter current 84 reaches zero, the chain-link converter 48 in full-bridge blocked mode 68 effectively becomes an open switch which prevents current flow through the chain-link converter 48. This means that operating in full-bridge blocked mode 68 also has the advantage of not requiring precise control of the switching instants between the inductor discharging and charging phases. The inductor current 86 will continue to decrease until it reaches zero, or the voltage source converter reverts to the inductor charging phase. The embodiment in Figure 9 also has the advantage of being able to limit damage caused by faults in the DC network connected to the first DC terminal 52. When there is a fault in the DC network connected to the first terminal 52, each full-bridge module 64 is operated in full-bridge output mode 70 to provide a voltage to oppose the flow of current created by a fault in the DC network connected to the first DC terminal 52 until the fault current in the DC network is extinguished.
A DC voltage source converter 51 according to a third embodiment of the invention is shown in Figure 12.
The DC voltage source converter 51 includes a series arrangement of a first chain-link converter 48a and the first DC terminal 52 which is connected in parallel with at least one inductor 50 and connected in parallel with a series arrangement of a second chain-link converter 48b and the second DC terminal 54. The first DC terminal 52 is connected in parallel with an input capacitor 32 while the second DC terminal 54 is connected in parallel with an output capacitor 40. The first and second chain-link converters 48a, 48b include a chain of half-bridge modules 62 in series.
In use, this configuration permits bidirectional power transfer between the DC network connected to the first and second DC terminals 52,54. During the inductor charging phase, the DC voltage source converter is operated in the same manner as the first embodiment shown in Figure 6, except that the half-bridge modules 62 in the second chain-link converter 48b are operated in half-bridge blocked mode to form a diode stack which permits current flow in one direction only. At this stage the diode stack is reverse-biased which means that there is no current flow through the diode stack and the second DC terminal 54.
During the inductor discharging phase, the DC voltage source converter is operated in the same manner as the first embodiment shown in Figure 6, except that the diode stack formed by the second chain-link converter 48b, instead of the diode 38, becomes forward biased to allow current flow through the diode stack and the second DC terminal 54. Power transfer in the opposite direction is performed by switching the roles of the first and second chain- link converters 48a, 48b so that the first chain-link converter 48a is operated in half-bridge blocked mode to form a diode stack and the second chain-link converter 48b is operated to switch between the inductor charging and discharging phases.
A DC voltage source converter 53 according to a fourth embodiment of the invention is shown in Figure 13. The DC voltage source converter 53 includes a series arrangement of a first chain-link converter 48 and the first DC terminal 52 which is connected in parallel with at least one inductor 50 and connected in parallel with a series arrangement of a second chain-link converter 48 and the second DC terminal 54. The first DC terminal 52 is connected in parallel with an input capacitor 32 while the second DC terminal 54 is connected in parallel with an output capacitor 40. The first and second chain-link converters 48 include a chain of full-bridge modules 64 in series.
In use, this configuration permits bidirectional power transfer between the DC network connected to the first and second DC terminals 52,54.
During the inductor charging phase, the DC voltage source converter is operated in the same manner as the first embodiment shown in Figure 8, except that the half-bridge modules 62 in the second chain-link converter 48 is operated in full-bridge diode mode 74b, 74d (Figure 3) to form a diode stack which permits current flow in one direction only. At this stage the diode stack is reverse-biased which means that there is no current flow through the diode stack and the second DC terminal 54.
During the inductor discharging phase, the DC voltage source converter is operated in the same manner as the first embodiment shown in Figure 8, except that the diode stack formed by the second chain-link converter 48, instead of the diode 38, becomes forward biased to allow current flow through the diode stack and the second DC terminal 54. Power transfer in the opposite direction is performed by switching the roles of the first and second chain- link converters 48 so that the first chain-link converter 48 is operated in full-bridge diode mode 74a, 74c (Figure 3) to form a diode stack and the second chain-link converter 48 is operated to switch between the inductor charging and discharging phases.
The embodiment in Figure 13 also has the advantage of being able to limit damage caused by faults in either of the DC networks connected to the first and second DC terminals 52,54.
When there is a fault in either of the DC networks, each full-bridge module 64 is operated in full-bridge output mode 70 to provide a voltage to oppose the flow of current created by a fault in either DC network connected to the DC terminals 52,54 until the fault current in either of the DC networks is extinguished. For each of the embodiments shown in Figures 6, 8, 12 and 13, the voltages across the first and second DC terminals 52,54 have opposite polarities to each other.
In other embodiments, the inductor 50 shown in Figures 6, 9, 12 and 13 may be replaced by an autotransformer 98, which may be a tapped air-cored reactor. In further embodiments, the inductor 50 shown in Figures 6, 9, 12 and 13 may be replaced by mutually coupled inductors 100,102 to provide magnetic isolation between the first and second DC terminals 52,54. The magnetic isolation between the two DC terminals 52,54 means that it is possible to earth either DC terminal in any location to ensure that the polarities of the voltages across the first and second DC terminals 52,54 are the same.
In such an embodiment as shown in Figure 14, the DC voltage source converter includes a series arrangement of a first chain-link converter 48 and a first DC terminal 52 is connected in series with a first terminal 106 and a common terminal 110 of the autotransformer and a series arrangement of a second chain-link converter 48 and the second DC terminal 54 is connected in series with a second terminal 108 and the common terminal 110 of the autotransformer 98. The first DC terminal 52 is connected in parallel with an input capacitor 32 while the second DC terminal 54 is connected in parallel with an output capacitor 40. It is envisaged that the second chain-link converter 48 may be replaced by a diode 38 which is reverse-biased during charging of the inductor 50 to prevent current flow into the second DC terminal 54 and is forward biased during discharging of the inductor 50 to allow current flow into the second DC terminal 54. The autotransformer 98 comprises a single conductive winding with three terminals; first, second and common terminals 106,108,110. The common terminal 110 is connected to a first end of the autotransformer 98 while the first and second terminals 106,108 may be connected to any turn of the conductive winding. The ratio of the voltage across the first and common terminals 106,110 to the voltage across the second and common terminals 108,110 is dependent on the ratio of the number of winding turns between the first and common terminals 106,110 to the number of winding turns between the second and common terminals 108,110.
The autotransformer-based DC voltage source converter operates in the same manner as the inductor-based converter except that the autotransformer 98 permits improved voltage step-up and step-down capabilities.
The common terminal 110 in the embodiment shown in Figure 14 may be connected to ground 104 to form a DC voltage source converter, as shown in Figure 15. A first pole of each DC terminal 52, 54 is at a positive DC voltage and a second pole of each DC terminal 52,54 is at neutral potential.
In another such embodiment as shown in Figure 16, the DC voltage source converter includes a series arrangement of a first chain-link converter 48a, the first DC terminal 52 and the first mutually coupled inductor 100; and a series arrangement of the second DC terminal 54, a second chain-link converter 48b and the second mutually coupled inductor 102. The first DC terminal 52 is connected in parallel with an input capacitor 32 while the second DC terminal 54 is connected in parallel with an output capacitor 40.
It is envisaged that the second chain-link converter 48b may be replaced by a diode 38 which is reverse- biased during charging of the first mutually coupled inductor 100 to prevent current flow into the second DC terminal 54 and is forward biased during discharging of the second mutually coupled inductor 102 to allow current flow into the second DC terminal 54.
The DC voltage source converter operates in the same manner as the inductor-based converters except for the charging and discharging of the mutually coupled inductors 100,102. During the charging phase, the first mutually coupled inductor 100 stores energy in the form of a magnetic field. Although the changing magnetic field in the first mutually coupled inductor 100 induces a voltage drop across the second mutually coupled inductor 102, the second mutually coupled inductor 102 does not conduct because the second chain- link converter 48b in the form of a diode stack is reverse biased. During the discharging phase, the change in magnetic field strength in the first mutually coupled inductor 100 induces a voltage drop in the second mutually coupled inductor 102 which is opposite in direction and thereby forward biases the diode stack, permitting current flow through the second mutually coupled inductor 102 and the output capacitor 40.
Electrical power is therefore transferred from the first DC terminal 52 to the second DC terminal 54 via the charging and discharging of the mutually coupled inductors 100,102. Power transfer in the opposite direction is possible by switching the roles of the first and second chain-link converters 48a, 48b.
The first and second DC terminals 52,54 may be connected to ground 104 to form a DC voltage source converter, as shown in Figure 17, in which the voltages across the first and second DC terminals 52,54 have the same polarity. A first end of each DC terminal 52,54 is at a positive DC voltage and a second end of each DC terminal 52,54 is at neutral potential.
In conventional DC voltage source converters, voltage step-up or step-down is achieved by adjusting the duty ratio of the semiconductor switch. However, in embodiments employing the use of an autotransformer 98 or mutually coupled inductors 100,102, it is possible to configure the autotransformer 98 or mutually coupled inductors 100,102 to achieve large voltage step-up or step-down ratios at duty ratios of approximately 50%. This allows the voltage converter to operate at high efficiency while performing voltage step-down or step- up between the voltages across the first and second DC terminals 52,54. In situations where the same polarity is required for the voltages across the first and second DC terminals 52, 54, it can be achieved by connecting part of the voltage source converter circuit to ground 104 as described earlier, or by forming bipolar or bipole DC voltage source converters.
In order to form a bipolar DC voltage source converter, a first pole of each DC terminal 52,54 is connected in use to a positive DC voltage and a second pole of each DC terminal 52, 54 is connected in use to a negative DC voltage such that the polarities of the voltages across the first and second DC terminals 52, 54 are the same. The embodiment in Figure 16 can be configured in this manner to form a bipolar DC voltage source converter.
Figure 18 shows another example of a bipolar DC voltage source converter in which a series arrangement of the first DC terminal 52 connected in series between first and third chain-link converters 48a, 48c is connected in series with the first mutually coupled inductor 100 and a series arrangement of the second DC terminal 54 connected between second and fourth chain-link converters 48b, 48d is connected in series with the second mutually coupled inductor 102.
To provide a bipole converter, two DC voltage source converters are interconnected to define a two terminal bipole DC voltage source converter. Each terminal has three poles respectively connected in use to a positive, negative and neutral potential. Other than allowing the polarity of the input and output voltages to be the same, bipole converters also have the advantage of positive and negative poles which function independently of each other. In the event that one of the poles fails, the bipole converter can still operate using the remaining functional pole.
In one such embodiment as shown in Figure 19, the interconnection between the common terminal 110 of the autotransformer 98 and second poles of the first and second DC terminals 52,54 of the DC voltage source converters are connected to define a common pole which is connected in use to ground 104; and a first pole of each DC terminal 52,54 is connected in use to a positive or negative potential. The positive and negative poles of one terminal of the bipole converter may be crossed over so that the polarity at the terminals of the bipole converter is the same. In another such embodiment as shown in Figure 20, the respective interconnections between the first mutually coupled inductor 100 and a second pole of the first DC terminal 52 of the DC voltage source converters are connected to define a first common pole which is connected in use to ground 104; the respective interconnections between the second mutually coupled inductor 102 and a second pole of the second DC terminal 54 of the DC voltage source converters are connected to define a second common pole which is connected in use to ground 104; and a first pole of each DC terminal 52,54 is connected in use to a positive or negative potential. In this configuration, a crossover of positive and negative poles is not required to achieve the same polarity at the terminals of the bipole converter
Another way of achieving the same polarity for the voltages across the first and second DC terminals 52,54 may be carried out by arranging an even number of DC voltage source converters in series in a cascade arrangement to define a two-terminal voltage source converter. As shown in Figure 21, in the cascade arrangement, the second DC terminal 54 of a first DC voltage source converter is connected to the first DC terminal 52 of a second DC voltage source converter. As a result, a two-terminal voltage source converter is formed in which the input voltage across its first DC terminal 52 has the same polarity as the output voltage across its second DC terminal 54.

Claims

1. A DC voltage source converter (47) for high voltage DC power transmission comprising at least one inductor (50) and at least one chain-link converter (48) connected between first and second DC terminals (52, 54), the or each chain-link converter (48) including a chain of modules (56, 62, 64) in series; each module including one or more semiconductor switches (58) connected to one or more energy storage devices (60); wherein the or each chain- link converter (48) is operable when DC networks are connected in use to the first and second DC terminals (52, 54) to control switching of the modules (56, 62, 64) to selectively enable the DC network connected to one DC terminal to charge the or each inductor (50), or enable the or each inductor (50) to discharge into the DC network connected to the other DC terminal .
2. A DC voltage source converter according to Claim 1 wherein each module (62) includes a pair of semiconductor switches (58) connected in parallel with the respective energy storage device (60) in a half- bridge arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct current in both directions.
3. A DC voltage source converter according to Claim 1 wherein each module (64) includes two pairs of semiconductor switches (58) connected in parallel with the respective energy storage (60) device in a full- bridge arrangement to define a 4-quadrant bipolar module that can provide positive or negative voltage and can conduct current in both directions.
4. A DC voltage source converter according to Claim 2 or Claim 3 wherein each semiconductor switch (58) is connected in parallel with a free-wheel diode (66) .
5. A DC voltage source converter according to Claim 1 wherein each module includes semiconductor switches
(58) connected with the respective energy storage devices (60) in a half-bridge multilevel converter arrangement to define a 2-quadrant unipolar module that can provide zero or positive voltage and can conduct current in both directions.
6. A DC voltage source converter according to Claim 1 wherein each module includes semiconductor switches (58) connected with the respective energy storage devices (60) in a full-bridge multilevel converter arrangement to define a 4-quadrant bipolar module that can provide positive or negative voltage and can conduct current in both directions.
7. A DC voltage source converter according to Claim 5 or Claim 6 wherein the multilevel converter arrangement is a flying capacitor converter arrangement (80, 82) or a neutral point diode clamped converter arrangement (76, 78) .
8. A DC voltage source converter according to Claim 4 or Claim 7 wherein the or each chain-link converter (48) is connected in series with the first or second DC terminals (52, 54) and is operable to generate a voltage to oppose the flow of current created by a fault in the DC network connected to the respective DC terminal .
9. A DC voltage source converter according to any of the preceding claims wherein the semiconductor switches
(58) are insulated gate bipolar transistors, field effect transistors, gate turn-off thyristors, gate commutated thyristors, integrated gate commutated thyristors and/or injection enhanced gate turn-off thyristors.
10. A DC voltage source converter according to any of the preceding claims wherein each energy storage device (60) is a capacitor, battery, fuel cell, photovoltaic cell, auxiliary AC generator with associated rectifier or another energy source or sink.
11. A DC voltage source converter according to any of the preceding claims wherein each energy storage device (60) is linked to an auxiliary coupling bus.
12. A DC voltage source converter according to any of the preceding claims wherein the voltage source converter includes at least one capacitor (32, 40) connected in parallel with the first DC terminal (52) and/or second DC terminal (54) .
13. A DC voltage source converter according to any of the preceding claims further including at least one diode (38) connected in series with the second DC terminal (54), the diode being reverse-biased during charging of the or each inductor to prevent current flow into the second DC terminal and being forward biased during discharging of the or each inductor to allow current flow into the second DC terminal.
14. A DC voltage source converter according to any of Claims 1 to 12 wherein the semiconductor switches (58) are controllable in use so that the or each chain-link (48) converter permits current flow in one direction only.
15. A DC voltage source converter according to Claim 13 wherein a series arrangement of the chain-link converter (48) and the first DC terminal (52) is connected in parallel with at least one inductor (50) and connected in parallel with a series arrangement of the diode (38) and the second DC terminal (54) .
16. A DC voltage source converter according to Claim 14 wherein a series arrangement of a first chain-link converter (48a) and the first DC terminal (52) is connected in parallel with at least one inductor (50) and connected in parallel with a series arrangement of a second chain-link converter (48b) and the second DC terminal (54) .
17. A DC voltage source converter according to any of Claims 1 to 14 wherein the inductor is an autotransformer (98).
18. A DC voltage source converter according to Claim 17 wherein the autotransformer (98) is a tapped air- cored reactor.
19. A DC voltage source converter according to Claim 17 or Claim 18, when each claim is dependent from Claim 13 wherein a series arrangement of a chain-link converter (48) and the first DC terminal (52) is connected in series with a first terminal (106) and a common terminal (110) of the autotransformer and a series arrangement of a diode (38) and the second DC terminal (54) is connected in series with a second terminal (108) and the common terminal (110) of the autotransformer .
20. A DC voltage source converter according to Claim 17 and Claim 18, when each claim is dependent from any of Claims 1 to 12 or Claim 14, wherein a series arrangement of a first chain-link converter (48a) and the first DC terminal (52) is connected in series with a first terminal (106) and a common terminal (110) of the autotransformer and a series arrangement of a second chain-link converter (48b) and the second DC terminal (54) is connected in series with a second terminal (108) and the common terminal (110) of the autotransformer .
21. A DC voltage source converter according to Claim 19 or Claim 20 wherein the common terminal (110) is connected to ground.
22. A DC voltage source converter according to any of Claims 1 to 14 wherein the voltage source converter includes first and second inductors (100, 102) mutually coupled to each other.
23. A DC voltage source converter according to Claim 22 when dependent from Claim 13 wherein the voltage source converter includes a series arrangement of the chain-link converter (48), the first DC terminal (52) and the first mutually coupled inductor (100); and a series arrangement of the second DC terminal (54), the diode (38) and the second mutually coupled inductor (102) .
24. A DC voltage source converter according to Claim 22 when dependent from any of Claims 1 to 12 or Claim
14 wherein the voltage source converter includes a series arrangement of a first chain-link converter (48a), the first DC terminal (52) and the first mutually coupled inductor (100); and a series arrangement of the second DC terminal (54), a second chain-link converter (48b) and the second mutually coupled inductor (102).
25. A DC voltage source converter according to Claim 23 or Claim 24 wherein the interconnection between the first DC terminal (52) and the first mutually coupled inductor (100) is connected to ground; and the interconnection between the second DC terminal (54) and the second mutually coupled inductor (102) is connected to ground.
26. A DC voltage source converter according to Claim 22 when each claim is dependent from any of Claims 1 to 12 or Claim 14 wherein a series arrangement of the first DC terminal (52) connected in series between first and third chain-link converters (48a, 48c) is connected in series with the first mutually coupled inductor (100) and a series arrangement of the second DC terminal (54) connected between second and fourth chain-link converters (48b, 48d) is connected in series with the second mutually coupled inductor (102) .
27. A DC voltage source converter according to Claim 24 or Claim 26 wherein a first pole of each DC terminal (52, 54) is connected in use to a positive DC voltage and a second pole of each DC terminal is connected in use to a negative DC voltage such that the polarities of the voltages across the first and second DC terminals are the same.
28. A bipole DC voltage source converter comprising two DC voltage source converters according to any of Claims 19, 20, 23 or 24 wherein the DC voltage source converters are interconnected to define a two terminal voltage source converter, each terminal having three poles respectively connected in use to a positive, negative and neutral potential.
29. A bipole DC voltage source converter according to Claim 28 when dependent from Claim 19 or Claim 21 wherein the respective interconnections between the common terminal (110) of the autotransformer (98) and second poles of the first and second DC terminals (52, 54) of the DC voltage source converters are connected via superposition to define a common pole which is connected in use to ground; and a first pole of each DC terminal (52, 54) is connected in use to a positive or negative potential.
30. A bipole DC voltage source converter according to Claim 28 when dependent from Claim 23 or Claim 24 wherein the respective interconnections between the first mutually coupled inductor (100) and a second pole of the first DC terminal (52) of the DC voltage source converters are connected via superposition to define a first common pole which is connected in use to ground; the respective interconnections between the second mutually coupled inductor (102) and a second pole of the second DC terminal (54) of the DC voltage source converters are connected via superposition to define a second common pole which is connected in use to ground; and a first pole of each DC terminal is connected in use to a positive or negative potential.
31. An arrangement of a plurality of DC voltage source converters according to any of the preceding claims wherein the plurality of DC voltage source converters are connected in series in a cascade arrangement to define a two-terminal voltage source converter.
PCT/EP2009/065327 2009-11-17 2009-11-17 High voltage dcdc converter WO2011060812A1 (en)

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