WO2010040388A1 - Multilevel converter and method for compensating active and reactive power in a high voltage network - Google Patents

Multilevel converter and method for compensating active and reactive power in a high voltage network Download PDF

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Publication number
WO2010040388A1
WO2010040388A1 PCT/EP2008/063403 EP2008063403W WO2010040388A1 WO 2010040388 A1 WO2010040388 A1 WO 2010040388A1 EP 2008063403 W EP2008063403 W EP 2008063403W WO 2010040388 A1 WO2010040388 A1 WO 2010040388A1
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WO
WIPO (PCT)
Prior art keywords
cells
power
reactive power
multilevel converter
active
Prior art date
Application number
PCT/EP2008/063403
Other languages
French (fr)
Inventor
Jan R Svensson
Original Assignee
Abb Technology Ag
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Abb Technology Ag filed Critical Abb Technology Ag
Priority to PCT/EP2008/063403 priority Critical patent/WO2010040388A1/en
Publication of WO2010040388A1 publication Critical patent/WO2010040388A1/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
    • H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
    • H02J3/1842—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
    • H02J3/1857—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters wherein such bridge converter is a multilevel converter
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
    • H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
    • H02J3/1864—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein the stepless control of reactive power is obtained by at least one reactive element connected in series with a semiconductor switch
    • 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
    • H02M7/49—Combination of the output voltage waveforms of a plurality of converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/28—Arrangements for balancing of the load in a network by storage of energy
    • H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/10—Flexible AC transmission systems [FACTS]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/20—Active power filtering [APF]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P80/00—Climate change mitigation technologies for sector-wide applications
    • Y02P80/10—Efficient use of energy, e.g. using compressed air or pressurized fluid as energy carrier

Definitions

  • the invention relates to power compensation, e.g. power compensation of a high voltage transmission line including a conductor for electric power transmission within the range of 6 kV and upwards.
  • the apparatus comprises a multilevel converter including an energy storage.
  • a STATCOM comprises a voltage source converter (VSC) having an AC side connected to the AC network (transmission line) via an inductor in each phase.
  • the DC side is connected to a temporary electric power storage means such as capacitor means.
  • VSC voltage source converter
  • the voltage magnitude output on the AC side is controlled thus resulting in the compensator supplying reactive power or absorbing reactive power from the transmission line. Since the active power transfer is zero, the voltage over the DC capacitors is constant when assuming that the converter losses are negligible.
  • the VSC comprises at least six self-commutated semiconductor switches, each of which is shunted by a reverse or anti-parallel connected diode. Since a STATCOM apparatus has no active power source it can only compensate for reactive power.
  • WO96/14686 illustrates a multilevel converter for each phase of an AC system, wherein the multilevel converter is configured as a chain comprising a plurality of converter cells, often called chain link.
  • Each converter cell comprises, for example, four switching devices, insulated gate bipolar transistors (IGBTs), configured as two parallel phase-legs together with a parallel capacitor for providing energy and thus has a DC voltage (figure 3).
  • the chain link converter includes a control arrangement for switching the valves in the converter cells so that an AC voltage is obtained.
  • batteries can replace the capacitors and thus be used as the power source or sink in each of the chain links.
  • WO2007/102758 illustrates (fig. 1 ) a power compensator 1 connected to a transmission line via a transformer 2.
  • the power compensator 1 comprises a VSC 4, a capacitor 6 and an energy storage device 5.
  • On the DC side of the VSC the energy storage and the capacitor are connected in parallel.
  • the energy storage device comprises a plurality of series connected battery units.
  • the battery units contain a number of battery cells, each providing for instance 1.7 to 3.1 V. For a 100MVA converter that is connected to a 33kV network, a DC side of approximately 66kV is needed that requires many battery cells connected in series to reach the 66kV voltage level.
  • the invention provides a chain link converter having cells for providing reactive power and also at least one cell providing a source of active power. It is a further object of the invention to provide reactive and active power in a cost-efficient manner.
  • the invention provides a multilevel converter having chain link topology of cells, wherein a plurality of the cells are adapted for providing reactive power and comprises a capacitor. Such cells are also denoted reactive power cells.
  • the multilevel converter is characterised in that it also comprises a least one converter cell in the chain per phase comprising a battery unit for providing active power. Such cells are also denoted active power cells.
  • the multilevel converter can be used for reactive and active power compensation of a power supply, for example in transmission lines, and can also be installed at industrial sites compensating the consumption of large industrial loads.
  • the converter is useful for balancing weak grids, such as a part isolated from the main grid by a power failure.
  • weak grids such as a part isolated from the main grid by a power failure.
  • it can be arranged at remote sites in a grid, sites having limited number of backup possibilities during disturbances or in remote areas in association to power production plants, to provide island operation during power variations.
  • the multilevel converter can also be used for compensation during disturbances or blackouts at hospitals or other critical facilities, wherein the active power cells are used to compensate for the active power loss and provide back-up power.
  • figure 5 illustrates a prior art arrangement comprising a voltage source converter 50 (VSC) connected at its AC- side via an inductance L to a transmission line.
  • VSC voltage source converter
  • the DC-side of the VSC is coupled to energy storage arrangements 51 , 52 in parallel with capacitors 53, 54.
  • the energy storage arrangements comprise a large number of battery units in series connection to provide a total voltage at the same level as the voltage level of the DC capacitors.
  • the inventive multilevel converter can provide both reactive power and active power, without the active power source being dimensioned for the full voltage of the DC side of the VSC.
  • the active power source need only be dimensioned for providing a compensating power and a voltage adapted to the DC voltage of the cell in the chain link converter, which is a fraction of the DC voltage for the VSC. This provides for cost-efficient power compensation.
  • less than 50% of the cells in a chain comprise a battery unit, preferably less than 10% of the cells in a chain comprise a battery, thus, being adapted for high reactive power variation and compensation, and relatively low active power compensation.
  • each of the converter cells which use a DC capacitor as energy storage, has an H-bhdge structure.
  • each of the converter cells which use a battery as energy storage, has an H-bhdge structure.
  • An embodiment also provides for compensation of active and reactive power of a three-phase network by means of an arrangement of three single-phase multilevel converters, each being a multilevel converter in accordance with any of the mentioned embodiments, said three single-phase multilevel converters being interconnected in a delta or in Y-connection.
  • Such a power compensating arrangement can be arranged close to an industrial load for balancing mainly a reactive power from the load, at remote parts of a grid, at power plants or in backup power installations for example at hospitals.
  • the arrangement can, thus, be used for improving power quality, provide for uninterrupted power during blackouts and balance a small isolated grid during island operation of power supply to a varying load.
  • This multilevel converter arrangement preferably comprises means for sensing voltage and current in a connection point of a three-phase transmission line, and a control system adapted to track the reference active and reactive power.
  • the three- phase transmission line thus being compensated both in reactive power by switching the reactive power cells, and in active power by switching the active power cells.
  • the reactive power cells can only provide reactive power
  • the active power cells can besides active power also provide reactive power. Both active and reactive power can be controlled to a desired reference level for each active power cell.
  • a preferred embodiment of the control system uses the converter cells, which have DC capacitors, to inject or consume reactive power into the three-phase network in order to track the reference reactive power.
  • the control system uses the converter cells, which have batteries, to inject or consume active power into the three-phase network in order to track the reference active power.
  • the active power reference has a higher priority than the reactive power reference.
  • the invention also provides a method for compensating active and reactive power in a high voltage network, said network being coupled to at least one multilevel converter in accordance with any of the mentioned embodiments, and a control system for controlling the reactive power cells and the at least one active power cell.
  • the method is characterised by the step of tracking the reference active and reactive power to the network, and activating the reactive power cells and the at least one active power cell as a response to the determined active and reactive power.
  • the voltage and current in a connection point of the network can be measured and the determined voltage and current used as a basis for activating the reactive power cells and active power cells, respectively.
  • Figure 1 illustrates a cell with a capacitor unit for providing reactive power compensation.
  • the cell is denoted by "reactive power cell.”
  • Figure 2 illustrates a cell with a battery unit for providing active power compensation.
  • the cell is denoted by "active power cell.”
  • Figure 3 illustrates a Y-connection of three chains, each chain including reactive and active power cells.
  • Figure 4 illustrates chains of reactive and active power cells arranged in delta configuration connected to a three-phase transmission line.
  • FIG. 5 illustrates a prior art arrangement of a voltage source converter provided with an energy storage. Detailed description of the invention
  • FIG. 1 illustrates a converter cell comprising four valves 1 -4 and a capacitor unit 5 connected in an H-bhdge.
  • the H-bhdge is configured as two parallel phase-legs, each comprising two valves in series, and the capacitor unit 5 connected in parallel with the phase-legs.
  • Each valve includes transistor switches, such as IGBTs
  • insulated-gate bipolar transistor In parallel with each IGBT is a freewheeling diode, or anti-parallel diode, for conducting in the opposite direction.
  • a plurality of such cells in series makes up a voltage source with controllable amplitude, frequency and phase.
  • FIG. 2 illustrates a similar H-bhdge cell with four valves 1 -4, but including a battery unit 6 instead of a capacitor unit 5.
  • This battery unit equipped cell provides an active power source but also reactive power as the cell described above.
  • the battery unit 6 in turn comprises a plurality of battery cells, each providing a few volts.
  • a preferred embodiment of the invention comprises a series of such cells in a multilevel converter adapted for power compensation in high power AC transmissions.
  • the series, or chain of cells both reactive power sources and active power sources are provided.
  • Each cell in the chain has the same bridge topology of valves and power source, some has a battery unit as power source, but most of the cells has a capacitor unit as power source.
  • Each cell comprises either a capacitor unit or a battery unit.
  • the voltage over the active power cells of a chain is only a fraction of the voltage over the whole chain. For example, less than 50% of the cells are active power cells, typically less than 10% of the cells are active power cells that each comprise a battery unit 6, and more than 90% are reactive power cells that each comprise a respective capacitor unit 5.
  • Three such multilevel strings are suitably arranged for a three-phase high power system.
  • Figure 3 illustrates three Y-connected multilevel converter strings. For simplicity only four cell levels are illustrated, three cells 10-12, 20-22, 30-32 in each string includes capacitor units 5 to provide a reactive power source, whereas one cell 13, 23, 33 in each string includes a battery unit 6 to provide an active power source.
  • Each string is connected to a respective phase P1 , P2, P3 of the transmission network via a respective inductance L1 , L2, L3.
  • the arrangement in figure 3 also includes a control system 40 for conditioning the transmissions in each of the three phases P1 , P2, P3 of the transmission system.
  • the control system 40 includes means for monitoring 41 , 42, 43 the voltages and currents of each phase P1 , P2, P3 and includes means for switching 44 the valves of the active and reactive power cells 10-13, 20-23, 30-33 in order to track the reference active and reactive power, which are determined in order to effectively compensate and support the transmission line.
  • the control system 40 also monitors the charge level in the batteries and controls the active power to the network in such a way that the battery charge level does not become too high or too low.
  • Each of the three strings of multilevel converters 10-13, 20-23, 30-33 are connected to each phase P1 -P3 of the transmission network via a filtering inductor L1 -L3.
  • Figure 4 illustrates three multilevel converters connected in delta, including strings of reactive power cells 10-12, 20-22, 30-33 and active power cells 13, 23, 33 in serial connection, and also a control system and inductors L1-L3 for connection to a three- phase transmission network, similar to the arrangement of figure 3, but, of course, the topology is different.

Abstract

A multilevel converter (10-13, 20-23, 30-33) having chain link topology of cells in serial connection (10-13, 20-23, 30-33), wherein a plurality of the cells (10-12, 20-22, 30-32) comprises a capacitor unit (5) and is adapted for providing reactive power, the multilevel converter is characterised in that it also comprises a least one converter cell in the chain per phase comprising a battery unit for providing active power. A method for compensating active and reactive power using a multilevel converter and an arrangement including three multilevel converters are also provided.

Description

Multilevel converter and method for compensating active and reactive power in a high voltage network
Technical Field The invention relates to power compensation, e.g. power compensation of a high voltage transmission line including a conductor for electric power transmission within the range of 6 kV and upwards. Especially the invention concerns an apparatus for providing an exchange of power with a high voltage AC electric power system. The apparatus comprises a multilevel converter including an energy storage.
Background to the invention
Within flexible alternating current transmission system (FACTS) a plurality of control apparatus are known. One such FACTS apparatus is the static compensator (STATCOM). A STATCOM comprises a voltage source converter (VSC) having an AC side connected to the AC network (transmission line) via an inductor in each phase. The DC side is connected to a temporary electric power storage means such as capacitor means. In a STATCOM the voltage magnitude output on the AC side is controlled thus resulting in the compensator supplying reactive power or absorbing reactive power from the transmission line. Since the active power transfer is zero, the voltage over the DC capacitors is constant when assuming that the converter losses are negligible. The VSC comprises at least six self-commutated semiconductor switches, each of which is shunted by a reverse or anti-parallel connected diode. Since a STATCOM apparatus has no active power source it can only compensate for reactive power.
WO96/14686 illustrates a multilevel converter for each phase of an AC system, wherein the multilevel converter is configured as a chain comprising a plurality of converter cells, often called chain link. Each converter cell comprises, for example, four switching devices, insulated gate bipolar transistors (IGBTs), configured as two parallel phase-legs together with a parallel capacitor for providing energy and thus has a DC voltage (figure 3). The chain link converter includes a control arrangement for switching the valves in the converter cells so that an AC voltage is obtained. Instead of having capacitors in each converter cell of the converter, batteries can replace the capacitors and thus be used as the power source or sink in each of the chain links.
Note that for a cell using a battery, a small capacitor is still present in order to handle commutations. However, the energy in the capacitor is so small compared with the battery that from now on only the battery will be mentioned.
WO2007/102758 illustrates (fig. 1 ) a power compensator 1 connected to a transmission line via a transformer 2. The power compensator 1 comprises a VSC 4, a capacitor 6 and an energy storage device 5. On the DC side of the VSC the energy storage and the capacitor are connected in parallel. The energy storage device comprises a plurality of series connected battery units. The battery units contain a number of battery cells, each providing for instance 1.7 to 3.1 V. For a 100MVA converter that is connected to a 33kV network, a DC side of approximately 66kV is needed that requires many battery cells connected in series to reach the 66kV voltage level.
Summary of the invention
It is an object of the invention to provide active and reactive power to a supply system of high voltage power to a load, wherein disadvantages of the prior art are overcome or at least alleviated. In particular, the invention provides a chain link converter having cells for providing reactive power and also at least one cell providing a source of active power. It is a further object of the invention to provide reactive and active power in a cost-efficient manner.
For this purpose the invention provides a multilevel converter having chain link topology of cells, wherein a plurality of the cells are adapted for providing reactive power and comprises a capacitor. Such cells are also denoted reactive power cells. The multilevel converter is characterised in that it also comprises a least one converter cell in the chain per phase comprising a battery unit for providing active power. Such cells are also denoted active power cells. The multilevel converter can be used for reactive and active power compensation of a power supply, for example in transmission lines, and can also be installed at industrial sites compensating the consumption of large industrial loads.
Moreover, the converter is useful for balancing weak grids, such as a part isolated from the main grid by a power failure. For this purpose, it can be arranged at remote sites in a grid, sites having limited number of backup possibilities during disturbances or in remote areas in association to power production plants, to provide island operation during power variations.
The multilevel converter can also be used for compensation during disturbances or blackouts at hospitals or other critical facilities, wherein the active power cells are used to compensate for the active power loss and provide back-up power.
For facilitating understanding of the invention, figure 5 illustrates a prior art arrangement comprising a voltage source converter 50 (VSC) connected at its AC- side via an inductance L to a transmission line. The DC-side of the VSC is coupled to energy storage arrangements 51 , 52 in parallel with capacitors 53, 54. The energy storage arrangements comprise a large number of battery units in series connection to provide a total voltage at the same level as the voltage level of the DC capacitors.
Using both active power cells and reactive power cells in serial connection in the multilevel converter according to the invention makes the use of this type of separate energy storage arrangement unnecessary.
The inventive multilevel converter can provide both reactive power and active power, without the active power source being dimensioned for the full voltage of the DC side of the VSC. The active power source need only be dimensioned for providing a compensating power and a voltage adapted to the DC voltage of the cell in the chain link converter, which is a fraction of the DC voltage for the VSC. This provides for cost-efficient power compensation. Moreover, in an embodiment less than 50% of the cells in a chain comprise a battery unit, preferably less than 10% of the cells in a chain comprise a battery, thus, being adapted for high reactive power variation and compensation, and relatively low active power compensation.
In a preferred embodiment, each of the converter cells, which use a DC capacitor as energy storage, has an H-bhdge structure.
In a preferred embodiment, each of the converter cells, which use a battery as energy storage, has an H-bhdge structure.
An embodiment also provides for compensation of active and reactive power of a three-phase network by means of an arrangement of three single-phase multilevel converters, each being a multilevel converter in accordance with any of the mentioned embodiments, said three single-phase multilevel converters being interconnected in a delta or in Y-connection.
Such a power compensating arrangement can be arranged close to an industrial load for balancing mainly a reactive power from the load, at remote parts of a grid, at power plants or in backup power installations for example at hospitals. The arrangement can, thus, be used for improving power quality, provide for uninterrupted power during blackouts and balance a small isolated grid during island operation of power supply to a varying load.
This multilevel converter arrangement preferably comprises means for sensing voltage and current in a connection point of a three-phase transmission line, and a control system adapted to track the reference active and reactive power. The three- phase transmission line thus being compensated both in reactive power by switching the reactive power cells, and in active power by switching the active power cells. While the reactive power cells can only provide reactive power, the active power cells can besides active power also provide reactive power. Both active and reactive power can be controlled to a desired reference level for each active power cell. Since the active power cells will provide both active and reactive power, a preferred embodiment of the control system uses the converter cells, which have DC capacitors, to inject or consume reactive power into the three-phase network in order to track the reference reactive power. Moreover, the control system uses the converter cells, which have batteries, to inject or consume active power into the three-phase network in order to track the reference active power. To utilize the converter cells that have batteries optimally, the active power reference has a higher priority than the reactive power reference.
The invention also provides a method for compensating active and reactive power in a high voltage network, said network being coupled to at least one multilevel converter in accordance with any of the mentioned embodiments, and a control system for controlling the reactive power cells and the at least one active power cell. The method is characterised by the step of tracking the reference active and reactive power to the network, and activating the reactive power cells and the at least one active power cell as a response to the determined active and reactive power.
The voltage and current in a connection point of the network can be measured and the determined voltage and current used as a basis for activating the reactive power cells and active power cells, respectively.
Brief description of the drawings
Figure 1 illustrates a cell with a capacitor unit for providing reactive power compensation. The cell is denoted by "reactive power cell." Figure 2 illustrates a cell with a battery unit for providing active power compensation.
The cell is denoted by "active power cell."
Figure 3 illustrates a Y-connection of three chains, each chain including reactive and active power cells.
Figure 4 illustrates chains of reactive and active power cells arranged in delta configuration connected to a three-phase transmission line.
Figure 5 illustrates a prior art arrangement of a voltage source converter provided with an energy storage. Detailed description of the invention
Figure 1 illustrates a converter cell comprising four valves 1 -4 and a capacitor unit 5 connected in an H-bhdge. The H-bhdge is configured as two parallel phase-legs, each comprising two valves in series, and the capacitor unit 5 connected in parallel with the phase-legs. Each valve includes transistor switches, such as IGBTs
(insulated-gate bipolar transistor). In parallel with each IGBT is a freewheeling diode, or anti-parallel diode, for conducting in the opposite direction. A plurality of such cells in series makes up a voltage source with controllable amplitude, frequency and phase. When connecting the series-connected cells through an inductor to a network, reactive power can be controlled without affecting the cell energy and thus the voltage across the capacitors in the converter cells. Note that the converter losses are thus neglected.
Figure 2 illustrates a similar H-bhdge cell with four valves 1 -4, but including a battery unit 6 instead of a capacitor unit 5. This battery unit equipped cell provides an active power source but also reactive power as the cell described above. The battery unit 6 in turn comprises a plurality of battery cells, each providing a few volts.
A preferred embodiment of the invention comprises a series of such cells in a multilevel converter adapted for power compensation in high power AC transmissions. In the series, or chain, of cells both reactive power sources and active power sources are provided. Each cell in the chain has the same bridge topology of valves and power source, some has a battery unit as power source, but most of the cells has a capacitor unit as power source. Each cell comprises either a capacitor unit or a battery unit.
The voltage over the active power cells of a chain is only a fraction of the voltage over the whole chain. For example, less than 50% of the cells are active power cells, typically less than 10% of the cells are active power cells that each comprise a battery unit 6, and more than 90% are reactive power cells that each comprise a respective capacitor unit 5. Three such multilevel strings are suitably arranged for a three-phase high power system. Figure 3 illustrates three Y-connected multilevel converter strings. For simplicity only four cell levels are illustrated, three cells 10-12, 20-22, 30-32 in each string includes capacitor units 5 to provide a reactive power source, whereas one cell 13, 23, 33 in each string includes a battery unit 6 to provide an active power source. Each string is connected to a respective phase P1 , P2, P3 of the transmission network via a respective inductance L1 , L2, L3.
The arrangement in figure 3 also includes a control system 40 for conditioning the transmissions in each of the three phases P1 , P2, P3 of the transmission system. The control system 40 includes means for monitoring 41 , 42, 43 the voltages and currents of each phase P1 , P2, P3 and includes means for switching 44 the valves of the active and reactive power cells 10-13, 20-23, 30-33 in order to track the reference active and reactive power, which are determined in order to effectively compensate and support the transmission line. The control system 40 also monitors the charge level in the batteries and controls the active power to the network in such a way that the battery charge level does not become too high or too low.
Each of the three strings of multilevel converters 10-13, 20-23, 30-33 are connected to each phase P1 -P3 of the transmission network via a filtering inductor L1 -L3.
Figure 4 illustrates three multilevel converters connected in delta, including strings of reactive power cells 10-12, 20-22, 30-33 and active power cells 13, 23, 33 in serial connection, and also a control system and inductors L1-L3 for connection to a three- phase transmission network, similar to the arrangement of figure 3, but, of course, the topology is different.

Claims

Claims
1. Multilevel converter (10-13, 20-23, 30-33) having chain link topology of cells (10-13, 20-23, 30-33), wherein a plurality of the cells (10-12, 20-22, 30-32) comprises a capacitor unit (5) and is adapted for providing reactive power, the multilevel converter is c h a r a c t e r i s e d in that at least one of the cells (13, 23, 33) comprises a battery unit (6) for providing active power.
2. Multilevel converter according to claim 1 , wherein each of the plurality of the cells (10-12, 20-22, 30-32) comprising a capacitor unit has an H-bhdge structure with two phase-legs.
3. Multilevel converter according to claim 1 or 2, wherein the at least one cell (13, 23, 33) comprising a battery unit has an H-bhdge structure.
4. Multilevel converter according to any of claims 2 to 3, wherein each of the capacitors in the plurality of the cells (10-12, 20-22, 30-32) is connected in parallel with the two phase-legs of the H-bhdge structure.
5. Multilevel converter (10-13, 20-23, 30-33) according to claim 4, wherein each of the cells (10-13, 20-23, 30-33) comprises either a capacitor unit (5) or a battery unit (6).
6. Multilevel converter according to any of claims 1 to 5, wherein less than 50% of the cells in the chain comprise a battery.
7. Multilevel converter according to claim 6, wherein less than 10% of the cells in the chain comprise a battery.
8. Arrangement of three multilevel converters for power compensation of active and reactive power in a three-phase network, each being a multilevel converter in accordance with any of claims 1 -7, said multilevel converters being interconnected in a delta or in Y-connection.
9. Multilevel converter arrangement according to claim 8, comprising means for sensing voltages and currents (41 -43) in a connection of the three-phase network, and a control system (40) adapted for controlling (44) the power from each of the multilevel converters as a response to sensed voltages and currents.
10. Method for compensating active and reactive power in a high voltage network, said network being coupled to at least one multilevel converter in accordance with any of claims 1 -8, and a control system for activating the reactive power cells and the at least one active power cell, c h a r a c t e r i s e d by the step of determining the voltage and current, respectively, in the network, and activating the reactive power cells and the at least one active power cell as a response to the determined active and reactive power.
PCT/EP2008/063403 2008-10-07 2008-10-07 Multilevel converter and method for compensating active and reactive power in a high voltage network WO2010040388A1 (en)

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