SE1300326A1

SE1300326A1 - Förfarande och arrangemang för styrning av överföring av elektrisk kraft i ett överföringssystem för elektrisk kraft

Info

Publication number: SE1300326A1
Application number: SE1300326A
Authority: SE
Inventors: Pierluigi Tenca
Original assignee: Abb Technology Ltd
Priority date: 2013-05-08
Filing date: 2013-05-08
Publication date: 2013-05-15

Abstract

SAMMANDRAG. Förfarande och arrangement för styrning av överföring av elektrisk kraft i ett överföringssystem för elektrisk kraft. Överföringssystemet innefattar ett kraftö-verföringssystem för högspänd likström, HVDC, och åtminstone en omvandlare ansluten till kraftöverföringssystemet för HVDC och anslutbar till ett kraftsystem för växelström. Den åtminstone ena omvandlaren innefattar ett flertal kaskadceller. (101) (cascaded cells), och respektive omvandlare innefattar åtminstone en arm innefattande en eller ett flertal kaskadceller och är anordnad att omvandla växelström till likström för inmatning till kraftöverföringssystemet för HVDC och/eller likström till växelström för inmatning till kraftsystemet för växelström. Åtminstone en kaskadcell är induktivt kopplad till åtminstone en av de andra kaskadcellerna, t.ex. medelst en eller ett flertal transformatorer (106). Arrangemanget är anordnat att tillhandahålla kraft från åtminstone en av kaskadcellerna till åtminstone en av de andra kaskadcellerna åtminstone genom induktiv koppling. Förfarandet innefattar steget att styra överföringen av elektrisk kraft genom att induktivt koppla åtminstone en av kaskadcellerna till åtminstone en av de andra kaskadcellerna och genom att tillhandahålla kraft från åtminstone en av kaskadcellerna till åtminstone en av de andra kaskadcellerna åtminstone genom induktiv koppling.(Fig.1)

Description

15 20 25 30 2 The above-mentioned object of the present invention is attained by provid- ing an arrangement and a method, respectively, as defined in the appended claims.

By means of the method and arrangement according to the present inven- tion the following positive effects are attained: A reduction in size/dimensions of the DC link capacitors of the cascaded cells, which in turn results in a more com- pact converter. The reduction of the DC link capacitors of the cells means less stored energy in them (by the same voltage) with reduced problems for the explo- sion of the switches in case of shoot-through fault. For STATCOM applications, the present invention can extend the range of possible negative sequence currents that the converter can provide. By using one of the arms to supply power to the other arms, some arms can deliver non-zero average power with increased scope of application, hence broader market possibilities, for the converters. The inventive solution can use standard three-phase inverter modules to realize a full-bridge cell, with consequent large market availability of suppliers. An embodiment of the in- vention, based on the availability of fast thyristors (such as the emerging SiC thy- ristors), provides a current-source DC bus concept that does not need current in- terruption. The latter embodiment also allows exchange of non-zero average power via DC power su pplies, instead of AC as in prior art. Additionally, the possi- bility of DC current simplifies the usual prior-art installation problems, originating from varying electromagnetic fields, for the surrounding bodies of the EMI. Simpli- fied pre-charge of the DC-link capacitors is possible, because by means of the present invention there is an additional path to provide energy to them. Even dif- ferent converters can exchange energy through the additional and galvanically insulated path, which can improve the availability and emergency operation of a plant.

Advantageous embodiments of the arrangement and the method, respec- tively according to the present invention and further advantages with the present invention emerge from the detailed description of embodiments.

Detailed Description of Embodiments The present invention will now be described, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings including Figs. 1-5. The present invention relates to an arrangement for controlling the electric power transmission or distribution in an electrical power transmission 10 15 20 25 30 3 or distribution system. The electrical power transmission distribution system com- prises a HVDC power transmission system, and at least one converter connected to the HVDC power transmission system and connectable to an alternating current (AC) power system. The converter may also be called converter station, or HVDC station. The at least one converter may comprise a plurality of cascaded cells (hereinafter also called “ce||s”). Each converter may comprise at least one arm comprising one or a plurality of cascaded cells and is arranged to convert alter- nating current to direct current for input to the HVDC power transmission system and/or direct current to alternating current for input to the alternating current power system. Each cascaded cell may include a cell capacitor and may in general in- clude power semiconductor switches and diodes.

Each cascaded cell may be a cascaded half-bridge cell (also called Cas- caded Two-Level, CTL, cell). Each cascaded cell may be a cascaded full-bridge cell. A cascaded half-bridge cell and a cascaded full-bridge cell per se and their structure are well known to the person skilled in the art and therefore not disclosed or discussed in more detail. As stated above, the converter may comprise a plu- rality of cascaded cells, which may be half-bridge cells, or cascaded full-bridge cells, or a mixture thereof. The converter may thus comprise a mixture of cas- caded half-bridge cells and cascaded full-bridge cells. The plurality of cascaded cells may be interconnected to one another.

The converter may comprise one or a plurality of arms, e.g. one positive arm and one negative arm. Each of the arms may comprise one or a plurality of cascaded cells. Each cascaded cell may comprise one or a plurality of power semiconductor switches. Each cascaded cell may comprise one or a plurality of diodes.

Each cascaded cell may comprise at least one cell capacitor, which may be called DC link capacitor.

The converter may be based on the Modular Multilevel Converter (M2LC) topology. Reference is for example made to US2012/0032512-A1 which discloses examples of M2LC cells. However, other topologies for the converter are possible.

The cascaded cells of the prior-art, or classic, M2LC voltage-source arms exchange energy, or power, between them and with the surrounding environment only through their terminals. This is limiting for the possible operations of the arms, and consequently of the converter, and it makes it necessary to implement modu- 10 15 20 25 30 4 lation strategies aimed first of all at maintaining the capacitor voltages within ac- ceptable bounds. As an additional consequence of this prior-art structure, the en- ergy stored in each cell must be significant in order to maintain the ripple of the state variable (voltage) sufficiently reduced, even when the best possible known moduiations are applied. The large stored energy in turn increases the explosion risks for the desired cheap packages, different from the StakPak, with a conse- quent need to develop expensive countermeasures. The pre-charge of the capac- itors is nowadays performed through a complex procedure that is essentially forced by the fact that energy can enter the cells only through their terminals.

The inventor of the present invention has found that in order to reduce the volume and costs of the voltage-source converters (VSC) employing two-level cells, it is required to reduce the energy stored in the DC link capacitors, also be- cause this can turn in less expensive ways of dealing with the consequences of shoot-through failure. The stored energy can be reduced if one solves the problem of exchanging energy between the cells, especially the cells that belong to differ- ent arms, without excessive circuit complexity and while preserving the galvanic insulation, as well as fault tolerance. ln principal there are several prior-art net- works that could exchange energy between all cells, also without galvanic insula- tion, but the number and type of passive components, the number and type of switches, as well as the fault propagation that they imply, render them unsuitable for practical applications. The scenario of feeding the DC link of each cell with a dedicated AC/DC converter that uses a transformer in the classic voltage-imposed operation (like in the case of cascaded H cells drives) is unfeasible with a high number of cells and high voltages. The set of transformers/windings/buses that should ultimately channel the power towards a common voltage source in a classic voltage-based concept would become complex and delicate.

According to one embodiment of the arrangement according to the present invention, at least one of the cascaded cells is inductively coupled to at least one of the other cascaded cells, for example by means of one or a plurality of trans- formers. A plurality of cascaded cells may be inductively coupled to a plurality of other cascaded cells, or a plurality of cascaded cells may be interconnected. The arrangement is arranged to supply power from at least one of the cascaded cells to at least one of the other cascaded cells at least through inductive coupling, in order to control the electric power transmission or distribution. 10 15 20 25 30 5 The energy supply of thyristor drivers through high-voltage cables carrying high-frequency alternating current (AC) and passing through local toroidal cores (one for each thyristor driver) is a solution previously known to the skilled person.

The above-mentioned US2012/O032512-A1 renovates this concept for M2LC modules. Additionally, a similar concept but aimed at supplying the M2LC cells for inductive power transfer is found in the article “A Modu/ar Multi-level Converter (M2LC) based on /nductive Power Transfer (IPT) technology/I Sustainable Energy Technologies (ICSET), 2012 IEEE Third International Conference, pp. 5459, 24, by B.S. Riar and U. Madawala. However, the solutions mentioned in the two above-mentioned publications need a centralized AC power supply, and the solu- tion of the above-mentioned article needs a resonant circuit or circuits between the cable and the DC-link capacitor of each cell. Conversely, the solution according to the present invention presents the following distinguishing features: 1) The possi- ble absence of a dedicated power supply feeding the cable. According the present invention, all cells contribute in a distributed manner to the energy stored in the cable loop. 2) The possible absence of a centralized controller. 3) A further em- bodiment of the inventive concept may be specifically designed for pure DC cur- rent in the cable, not AC current as in prior art. This feature simplifies the typical problems associated with varying electromagnetic fields in the cable loop and sur- rounding bodies. 4) The fact that the current of the cable can be prominently DC with a superimposed small ripple, when the balance of the energy exchanged be- tween or among all cells is desired to be zero over a sufficiently long time interval. 5) The required insulation level can be provided by the power cable itself, via ex- isting cable technologies.

With reference to Fig 1, according to embodiments of the present inven- tion, an energy bus 102 may be provided to exchange energy among or between the cells 101. The energy bus may be constituted by at least one current (instead of classic voltage), ﬂowing into at least one closed loop cable 104, which is elec- tromagnetically coupled with all participating cells in the fashion of a current transformer 106. The cable 104 may include an inductor. The arrangement may essentially be a current-based transformer (not a voltage-based transformer as in the general imagined realizations) with multiple and independently driven second- aries (secondary windings). One embodiment is schematically illustrated in Fig. 1 which illustrates an embodiment of the concept for one arm and shows the struc- 10 15 20 25 30 6 ture of each single cell. The cable 104 constitutes the transformer primary winding which is in common to a plurality of cells, e.g. all cells, and driven in current. Each cell has its own secondary winding, driven in voltage, and may be built with the typical toroidal geometry, e.g. each secondary may be wound on a toroidal core inside which the cable 104 passes. Fig. 2 schematically illustrates an embodiment of the concept of exchanging energy between N different arms which may be composed of different numbers of cells. As illustrated in the embodiment of Fig. 2, all cells of all energetically coupled arms may contribute in a distributed manner to the establishment and maintenance of the comment current i(t), which constitutes the means (or vehicle) for temporary storing and retrieving energy in the magnetic field created by the closed loop cable. The arms may also belong to different con- verters. The cable 204 passes through all the cells 201 which exchange energy.

An inductance, e.g. an inductor 212, may be present, but may also be excluded.

The magnetic field is the common energy storage here, through which a plurality of cell, e.g. all cells, can exchange energy between them. Each secondary belongs to one specific cell and the voltage at its terminals is imposed by the action of the two switches at the right of the DC-link capacitor (see Fig. 1). The DC-link capaci- tor has been split in two halves to realize a half-bridge single-phase inverter con- figuration in order to reduce components and losses. When the only aim is the en- ergy exchange between the coupled cells, the arrangement does not need a dedi- cated AC power supply. “108” in Figs. 1 and 4 represents a typical unidirectional voltage blocking, bidirectional current carrying switch, like the typical combination of IGBT/IGVT/MOSFET with a diode. The value of i(t) may be sensed locally by each cell. As a consequence each cell can decide autonomously whether it can insert or extract energy in/from the common loop. Each cell can make the decision by weighing both its high/low DC-link voltage needs as well as the vicinity of i(t) to its acceptable extrema defined by the system design. As the common quantity easily measured by each cell is the current i(t), the current control can be totally distributed and fault-tolerant. Still under the specific need of energy exchange between cells, without exchange of energy from the environment surrounding the system, the current i(t) can have a significant DC component to which an unavoid- able ripple (because the loop inductance is always finite) is superimposed. Thus, the current may be controlled by the common and distributions action of each cell. 10 15 20 25 30 7 With reference to Fig. 2, the terminal of the arms may be connected to different points in general.

The current i(t) should never be interrupted and thus no DC breakers are necessary. Sections of the system may be isolated by short circuiting the cells with the switches or with provided by-passes 110 in case of failures, but there is no need to break the current i(t) abruptly. ln case of emergency, the current i(t) can be brought to zero rapidly by inserting a planned resistor in series with the cable loop that remains short-circuited during normal operation.

Fig. 3 schematically illustrates another embodiment applied to a common Delta-connected STATCOM. Owing to the separated path for energy exchange among the cells 301 (different from the cell AC output) the energy can be relo- cated among the cells as needed in order to improve the amount of negative se- quence currents that the STATCOM can generate. An inductance, e.g. an inductor 312, may be present, but may also be excluded. lf a very fast thyristor is available, faster than the existing (and expensive) ones in the Si technology, the embodiment schematically illustrated in Fig. 4 can be provided. This embodiment has the key advantage to use pure DC current in the cable and possibly a DC output converter to supply non-zero average power to the system of all the coupled arms. Controllable switches based on SiC, e.g. SiC thyristors may be used, which provides small high-frequency transformers that have the primary driven in current and the secondary driven in voltage. The insu- lation level could be moved to the high-frequency transformers instead of being provided by the cable as in the first embodiment of Fig. 1. The thyristors in Fig. 4 may be commutated by the voltage created by the half-bridge inverter and, being bidirectional voltage blocking devices, they allow both polarities for the DC voltage in series with the common cable. The thyristors in Fig. 4 may be commutated by the primary-side controlled square voltage. The DC voltage can be positive or negative (because of thyristors), hence, energy can be exchanged in both direc- tions. As it occurs in classic thyristor-based line-commutated inverters, but now much more welcome for the current-source nature of the cable primary, this prop- erty allows bidirectional energy exchange between each cell 401 and the cable 404, hence ultimately between/among all cells of all arms. Also for the embodi- ment of Fig. 4, if the final end is only the energy exchange between the cells, no external power supply is required, but now the cable current i(t) can be pure DC 10 15 20 25 8 with increased system compatibility and reduced Iosses in the cable by less skin effect. lf, on the contrary, it is desired to inject or extract energy into/from the sys- tem of all cells, one can do that by using a dedicated DC power supply.

Fig. 5 schematically illustrates a further embodiment of the arrangement, where the inventive concept has been extended to arms composed of current- source cells.

The present invention also relates to a method for controlling the electric power transmission or distribution in an electrical power transmission or distribu- tion system, e.g. as disclosed above. The method comprises the step of controlling the electric power transmission by inductively coupling at least one of the cas- caded cells, or a plurality of cascaded cells, to at least one of the other cascaded cells, or to a plurality of the other cascaded cells, and by supplying power from at least one of the cascaded cells to at least one of the other cascaded cells at least through inductive coupling. According to an embodiment of the method according to the present invention, where the electrical power transmission system com- prises a plurality of arms, and each arm comprises one or a plurality of cascaded cells, the method comprises the step of controlling the electric power transmission by inductively coupling at least one cascaded cell of a first arm to at least one cas- caded cell of a second arm, and by supplying power from at least one cascaded cell of the first arm to at least one cascaded cell of the second arm at least through inductive coupling.

The features of the different embodiments of the arrangement and the method, respectively, disclosed above may be combined in various possible ways providing further advantageous embodiments.

The invention shall not be considered limited to the embodiments illus- trated, but can be modified and altered in many ways by one skilled in the art, without departing from the scope of the appended claims.

Claims

10 15 20 25 30 CLAIMS

1. An arrangement for controlling the electric power transmission or distribu- tion in an electrical power transmission or distribution system, the electrical power transmission or distribution system comprising a high voltage direct current, HVDC, power transmission system, and at least one converter connected to the HVDC power transmission system and connectable to an alternating current power system, the at least one converter comprising a plurality of cascaded cells, wherein each converter comprises at least one arm comprising one or a plurality of cascaded cells and is arranged to convert alternating current to direct current for input to the HVDC power transmission system and/or direct current to alternating current for input to the alternating current power system, wherein at least one of the cascaded cells is inductively coupled to at least one of the other cascaded cells, for example by means of one or a plurality of transformers, and wherein the arrangement is arranged to supply power from at least one of the cascaded cells to at least one of the other cascaded cells at least through inductive coupling.

2. A method for controlling the electric power transmission or distribution in an electrical power transmission or distribution system, the electrical power trans- mission or distribution system comprising a high voltage direct current, HVDC, power transmission system, an alternating current power system, and a least one converter connected to the HVDC power transmission system and connected to the alternating current power system, the a least one converter converts alternat- ing current to direct current for input to the HVDC power transmission system and/or direct current to alternating current for input to the alternating current power system, wherein the at least one converter comprises a plurality of cascaded cells, and each converter comprises at least one arm comprising one or a plurality of cascaded cells, wherein the method comprises the step of controlling the electric power transmission by inductively coupling at least one of the cascaded cells to at least one of the other cascaded cells, and by supplying power from at least one of the cascaded cells to at least one of the other cascaded cells at least through inductive coupling. 10 10

3. A method according to claim 2, wherein the electrical power transmission system comprises a plurality of arms, each arm comprising one or a plurality of cascaded cells, wherein the method comprises the step of controlling the electric power transmission by inductively coupling at least one cascaded cell of a first arm to at least one cascaded cell of a second arm, and by supplying power from at least one cascaded cell of the first arm to at least one cascaded cell of the second arm at least through inductive coupling.