WO2012093942A1

WO2012093942A1 - Energy conversion system

Info

Publication number: WO2012093942A1
Application number: PCT/NO2011/000351
Authority: WO
Inventors: Paal Keim OLSEN
Original assignee: Smartmotor As
Priority date: 2011-01-07
Filing date: 2011-12-21
Publication date: 2012-07-12
Also published as: NO20110018A1; EP2661807A4; EP2661807A1; NO332201B1

Abstract

An energy conversion system is disclosed, which comprises an electrical machine (32) or a group of electrical machines (32), where each separate winding group (50) inside one electrical machine (32) or individual electrical machine (32) in the group of electrical machines (32) is galvanically connected to an associated power converter (rectifier or inverter) unit (33). Energy storage circuits (DC links) (34) of the converters (33) are connected to one another in series forming a common high DC voltage (45). Alternatively AC/AC converter elements (33) are connected in series forming a high AC voltage (121). Furthermore, each separate winding group (50) inside one electrical machine (32) or individual electrical machine (32) in the group of electrical machines (32) is surrounded by an electrical screen (60, 60A-E) of conductive or semi-conductive material for redistribution of electric potentials inside the system. The potential redistribution allows slot insulation of electric machines (32) to be designed for considerably lower voltage than the common high AC voltage (121) or high DC voltage (45). This means thinner slot insulation, better heat transfer in the machine, better fault detection and the machine size considerably smaller than of the corresponding machine with high-voltage insulation.

Description

Energy conversion system

The invention relates to an energy conversion system according to the preamble of claim 1. Especially the invention relates to an energy conversion system which includes at least one electrical machine comprising winding groups, where each winding group consists of at least one winding, and at least two power converters having machine side terminals and non-machine side terminals, where the power converters are connected to one or more winding groups on the machine side terminals and the power converters are connected in series through the non- machine side terminals, where

a) at least one winding group in the electrical machine is surrounded by at least one electrical screen, and where at least one of the electrical screens is connected to an AC or DC voltage source, or

b) at least one of the power converters or parts of the power converters are surrounded by at least one electrical screen, and where at least one of the electrical screens are connected to an AC or DC voltage source,

or a combination of a) and b).

Background

There are many energy conversion and energy transfer applications where the use of high- voltage connection between energy conversion part and the grid would be beneficial in terms of the total system cost and efficiency. Application examples are, first of all, power production from renewable sources such as wind, waves, tidal and on-land hydro streams (where the systems work in generating mode), but also marine systems (both motor and generator modes) and subsea systems for oil & gas industry (motor mode).

The systems producing energy from renewable sources are often located at remote areas, far away from where the consumption of energy is located. Usually the generation units are connected in parks with points of common coupling (hubs). From the hubs the power is transferred to the shore over long distances. For the long transmission line, DC voltage is preferred as the losses are lower and it requires less complex transmission cables. Transformer stations and AC/DC converters are used to achieve HVDC (High-Voltage Direct Current) transmission systems. Typical electric drive train of the modern wind- or tidal energy converter includes generator, power electronics converter (usually AC/DC converters connected back-to-back by their DC-links), step-up transformer and protection devices.

Examples of electrical system structure for a small wind park are schematically shown in Figures 1 and 2. The part of the drive train which is located in the wind turbine tower and/or nacelle may vary depending on generator type. Figure 1 presents the system with doubly-fed induction generators, while Figure 2 shows the system with permanent magnet synchronous generator.

It can be seen that energy conversion from AC to DC and back takes place four times in this system, between the generator and the grid. In addition there are three voltage step-ups in the transformers. Each such energy conversion or voltage step-up incurs losses of approximately 1 % of the total generated power.

In addition to power losses, numerous components reduce reliability of the total system and increase its cost. The transformers are the most problematic components as they are heavy, especially compared to electronics components of the same power range, require maintenance and may cause environmental problems (oil transformers). High weight of the transformers directly influences the wind turbine design, especially floating ones, and the offshore hub (platform) design, bringing high costs to the offshore projects.

There have been efforts in both academia and industry to find system configurations without the step-up transformers. For example, wind turbines can be connected in series to produce a high voltage directly. The idea of such a system is presented, for example, in dissertation of Stefan Lundberg (2006) (Wind Farm Configuration and Energy Efficiency Studies - Series DC versus AC Layouts. Goteborg: Chalmers University of Technology. Diss. ISBN/ISSN: 978-91-7291-884-9). See also Figure 3. The content of the wind turbines is not specified but some means of an AC/DC rectifier is included in the nacelle to produce a DC voltage. It should be noted that the idea of connecting the converters in series by their DC-links is not new and is widely used in for example HVDC systems for power transfer over long distances.

The system from Lundberg (2006) combined with the grid coupling elements from Figures 1 and 2 can be presented on a more detailed level, showing the generators and AC/DC inverters in the turbines, as illustrated in Figure 4. Here AC voltage from each turbine generator is converted by an AC/DC rectifier to a DC voltage. The AC/DC rectifiers are connected in series creating a high DC voltage which is connected to a DC cable for energy transmission to a main grid.

Similar ideas on systems without transformers have been proposed in some patents. In US 2009212568 (Al) (Maibach, 2009), for example, it is described a converter system using multiple AC/DC converters where the DC-links of the converters are connected in series. The AC-side of each converter is connected to one or more windings of a generator, thus each winding or winding group delivers power to only one converter. The system is presented in Figure 5. The generator comprises stator windings where three winding elements are connected to the AC voltage side of the converter associated with the respective stator winding. The converters are connected in series on the DC voltage side, creating a high total DC voltage which in turn can be connected to a high voltage DC cable for transmission of electrical power to an inverter connected to a main electricity grid.

In US 4,780,659 (Bansal, 1988) a similar converter system is described where multiple generators are connected to diode bridges which then are connected in series.

Another high voltage transmission concept is described in WO 2010/058028 (A2) where a frequency converter is connected to the generator windings. The frequency converter is constituted by a plurality of elements arranged in columns and coupled in cascading order to add inverted voltage.

The ideas in US 200921258 (Al), US 4,780,659 and WO 2010/058028 (A2) have considerable unsolved challenges that increase the weight and size of both the generator and the converters used. The AC/DC converters or AC/AC converter elements would each float on a voltage potential level equal to the voltage potential on their non-machine side which means that the converter components must be insulated from their surroundings. The converters used must also be custom made to be able to withstand the high voltages.

The stator windings would also have an offset equal to the non-machine side voltage of the corresponding converter and thick slot wall insulation is required to avoid spark over, voltage breakdown or corona activity. Thick insulation will result in worse heat removal from the hot spots in the winding and will consequently require larger machine size. Object

Tne main object of the invention is to provide an electric energy conversion system with lower total system cost, and higher efficiency and reliability than prior art electric energy systems.

Another object of the invention is to provide a solution for the problems of prior art related to the thick high-voltage insulation in the electric machines and high electric potentials of the series- connected converters in the high-voltage DC-link.

The invention

An energy conversion system according to the invention is described in claim 1. Preferable features and advantages of the energy conversion system are described in claims 2-17.

In the present invention an energy conversion system is disclosed, which includes at least one electrical machine comprising winding groups, where each winding group consists of at least one winding, and at least two power converters having machine side terminals and non-machine side terminals, where the power converters are connected to one or more winding groups on the machine side terminals and the power converters are connected in series through the non- machine side terminals, where

a) at least one winding group in the electrical machine is surrounded by at least one electrical screen, and at least one of the electrical screens is connected to an AC or DC voltage source, or

b) at least one of the power converters or parts of the power converters are surrounded by at least one electrical screen, and at least one of the electrical screens are connected to an AC or DC voltage source,

or a combination of a) and b).

The electrical machine can be a rotary machine, a linear machine, an actuator, a transformer or another inductive apparatus, while the power converters can be rectifiers (AC/DC), inverters (DC/AC), frequency converters (AC/AC or AC/DC/AC), DC-to-DC converters (DC/DC), transformers or a combination of those.

The electrical screens are according to the invention connected to an AC or DC voltage source to minimize the voltage difference between the electrical screen and the components surrounded by the screen. If the system includes converters having one or more DC-links, the screen can be connected to the corresponding DC-link. The AC or DC voltage source can also be the non-machine side terminals of the corresponding converter or a connection point within the corresponding converter. If the electrical screen surrounds winding groups, the voltage source can be the neutral point of the winding group, one of the ends of one of the windings or the middle of one of the windings. Any other AC or DC voltage source can also be used, either from an existing energy conversion system or from an external source.

The electrical screens are made of a conductive or semi-conductive material and redistributes electrical potentials inside the energy conversion system. The electrical screens in the machines can be the active iron carrying magnetic flux in the machine, the housing of the machine or a semi- conductive or conductive material forming a shell, mesh, layer or coating. In converters the electrical screen can be the housing or a shell, mesh, layer or coating of a semi-conductive or conductive material. The electrical screens redistribute voltage potentials in the energy conversion system. In electrical machines this allows slot insulation to be designed for considerably lower voltages than the common high DC voltage or high AC voltage. This means thin slot insulation, better heat transfer in the machine and a machine size considerably smaller than prior art machines with high-voltage insulation. For converters, the redistribution of voltage potentials allows more compact converters where the components can stand closer together without risking spark overs or breakdowns. The present invention solves the problems associated with the use of thick high-voltage insulation that occur when using the solution from US 200921258 (Al) by introducing electrical screens and redistributing electric potential in the electrical machines and the converters. By connecting each electrical screen to an electric potential near the offset of the corresponding windings, the insulation needed between the windings and the stator slot is reduced to a minimum only having to withstand the alternation of the voltage in the winding. Similarly the converter would be provided an electrical screen having the same voltage potential as on a DC-link of the converter.

The problems associated with thick high-voltage insulations that occurs in WO 2010/058028 (A2) can also be solved by the present invention. Each electrical screen surrounding a winding group can according to the invention be connected to one of the non-machine side AC terminals of the corresponding converter element. Similarly the converter element can have an electrical screen connected to the non-machine side terminals to minimize the voltage difference between the electrical components inside the converter element and the surround environment.

The weight and cost of the electrical machine would also be drastically reduced compared to a machine in the system presented in US 200921258 (Al) or WO 2010/058028 (A2). For wind turbines, the tower does not need to be able to carry as much weight and the nacelle would be reduced in both weight and size.

The present invention enables a higher efficiency by eliminating the transformers, using one- stage conversion (AC/DC) instead of two-stage (AC/DC/AC), using high voltage transmission already on a single energy conversion unit level, and using DC voltage transmission or distribution instead of AC voltage. By removing transformers in tower and on the hub, the DC/ AC stage of the converter in each turbine and the high-voltage AC/DC rectified in the hub the total system efficiency could be increased by 3-5 %.

A higher reliability is obtained when using the present invention as it uses fewer components and has a simpler design than present energy conversion systems. The reliability is further increased by being able to disconnect malfunctioning windings while keeping the other windings and converters in operation.

Better fault detection is obtained by monitoring the voltage of the electrical screens and the current flowing to and from the electrical screens. By this, winding faults can be detected at an earlier stage and partial discharges can be detected with a higher degree of accuracy.

The cost of the total energy conversion system will also be considerably lower than of a conventional system. The elimination of the transformers, using off the shelf low voltage converters with no modifications other than an extra casing, thinner cables to transport less current and lighter generators all contribute to reducing the costs of the energy production system. Furthermore, by eliminating the transformers, the offshore hub design may be changed and the platform may be replaced with a less expensive submerged or floating solution.

These and further advantages and features of the present disclosure will become evident from the following detailed description of exemplary embodiments of the disclosure, in conjunction with the drawings.

Example

The invention will below be described in detail with references to the attached drawings, where:

Figure 1 shows an example of a standard wind turbine park according to prior art using doubly- fed machines,

Figure 2 shows an example of a wind turbine park according to prior art using permanent magnet machines,

Figure 3 shows another prior art wind turbine park system,

Figure 4 shows an example of a wind turbine park configuration using the concept shown in Figure 3 and the grid connection shown in Figures 1 and 2,

Figure 5 shows a prior art energy conversion system with series connected converters, Figure 6 shows a schematic example of an energy conversion system of a first embodiment according to the present invention,

Figure 7 shows a schematic example of an energy conversion system of a second embodiment according to the present invention,

Figure 8 shows a schematic example of a third embodiment according to the invention, Figure 9a-c shows a schematic example of a fourth embodiment according to the invention, Figure 10 shows a schematic example of a high voltage energy conversion system according to the invention to be used in for instance tidal turbine parks,

Figure 11 shows an extension of Figure 10,

Figure 12 shows an energy conversion system according to the invention comprising medium or high voltage electrical machines,

Figure 13 shows an extension of Figure 12,

Figure 14 shows a radial cross section view of a radial-flux electrical machine according to the invention,

Figure 15 shows an axial cross section view of a radial flux electrical machine according to the invention, seen from the side,

Figure 16 shows another example of a cross section view of a radial flux machine according to the invention, seen in an axial direction, Figure 17 shows an example of an axial flux, ironless machine according to the invention, Figure 18a and 18b show schematic examples of arrangements of electrical screens according to the invention,

Figures 19a and b show further schematic examples of arrangements of the electrical screens according to the invention, and

Figure 20 shows a schematic example of an energy conversion system according to the invention utilizing a high AC voltage.

An energy conversion system according to the invention includes at least one electrical machine and at least two power converters, where the converters are connected in series on a non- machine side to provide a high voltage. According to the invention the electrical machine comprises winding groups and is provided with electrical screens surrounding one or more of the winding groups. The electrical screens are connected to points with certain electrical potential to minimize the potential difference between the electrical screens and the windings within them. For systems where the power converter is an AC/DC converter having DC-links and the electrical machine is connected to one or more AC/DC converters, the electrical screens may be connected to the positive, negative or neutral point of the corresponding DC-link. For systems where the power converter is an AC/AC or AC/DC/AC frequency converter and the electrical machine is connected to one or more frequency converters, the electrical screens may be connected to one of the terminals on the frequency converter not connected to the machine, i.e. non-machine side terminals.

The electrical screen may alternatively be connected to the neutral point of the windings it surrounds. If the electrical screen only surrounds one phase of the windings, it can be connected to one of the ends of the windings or to the middle of the winding. Alternatively, the electric screen can be connected to some other voltage source. The voltage source can be a DC voltage source coming from, for example, an external capacitor bank or energy storage circuit connected to a corresponding AC/DC converter or the source may be some other external AC or DC voltage source.

An electrical screen according to the invention can be made of a material that is either conductive or semi-conductive. In machines including an iron stator, for example, of laminated steel, the iron may act as the electrical screen. For electrical machines including stators created of composite material or other non-magnetic materials the screen can be a shell, sheet, mesh, coating or some other means of layer of a conductive or semi-conductive material surrounding one or more windings in the machine. Alternatively the housing of the machine can be the electrical screen. The purpose of the electrical screens is to split the electrical potential difference between the windings and earth into two separate areas in the machine so that the windings and the surrounding stator slots has a minimum potential difference, thus reducing the slot wall insulation to a minimum. For electrical machines including iron an extra insulation must be arranged between the stator iron and the stator housing so that the stator iron can act as the electrical screen and float on a voltage potential different from ground potential. For electrical machines having a stator made of a non-magnetic material, referred to as an ironless stator, the stator material, for instance a composite material, acts as insulation. The electrical screen has an important role also in machines with ironless stators. By having an electrical screen around the windings, floating at an electric potential near the potential of the windings, one can ensure that there only is an AC voltage between the windings and the screen, while the voltage between the screen and the machine housing is only DC. Splitting the voltage into an AC and a DC part enables the possibility of only using high quality composite material between the windings and the screen while the insulation outside the screen can be of lower quality. The electrical machine can according to the invention be a rotary machine, linear machine, actuator, transformer or another inductive apparatus.

According to the invention the power converter can be provided with an electrical screen surrounding power parts of the converter or the whole converter. For an AC/DC converter having a DC-link, the screen can be connected to the positive, negative or neutral point of the DC-link corresponding to the converter, to a point in an energy storage circuit connected to the converter or to some other external DC voltage source. For an AC/AC or AC/DC/AC frequency converter, the screen can be connected to a non-machine side of the converter or to some other external AC voltage source.

One proposed use of the invention is to use "off the shelf" low- or medium voltage converters and use the housing of the converter as an electrical screen. Insulation should then be added around the converter housing to avoid any spark over or voltage breakdowns between the converter housing and its surroundings.

Electrical screens according to the invention, used for electrical machines, converters or other electric equipment, give advantages in applications where high voltages are needed and it is possible to connect low or medium voltage equipment in series to create the desired high voltage. A further advantage is that the electrical screen provides a quicker and more accurate way of detecting faults. By measuring the voltage changes in each screen or monitoring currents going to or from each screen, a fault can be detected in the windings before it has fatal consequences. The electrical screen is preferably as close to the windings as possible. If for instance a turn fault occurs, the winding would heat up and melt the copper and insulation around the copper. This would lead to a short circuit between the winding and the corresponding screen which may be detected by some measuring means.

Reference is now made to Figure 1 which shows an example of a standard wind turbine park connection for far distributive systems. Each wind turbine 31 is connected to an electrical machine 32. The electrical machine 32 is doubly fed, meaning that it is connected to a back-to-back frequency converter 33, 34, 35 and a transformer 36. The transformer 36 converters the low or medium AC voltage from the generator to a medium AC voltage for instance 36 kV. The transformer 36 can be located in a nacelle 30 of the wind turbine 31, in a tower of the wind turbine or outside the tower.

The medium AC voltage is connected to a high voltage transformer 37 together with the other wind turbines 31 in the wind turbine park. The high AC voltage is then transformed into a high DC voltage by an AC/DC rectifier 33 which is connected to a DC voltage transmission cable 38. The DC cable 38 is further connected to a DC/AC inverter 35 which is connected to a main grid 39 by means of the transformer 37.

Reference is now made to Figure 2 which shows an example of a wind turbine park according to prior art using permanent magnet machines 32. Each wind turbine generator 32 is connected to a frequency converter 33, 34, 35, more specifically a back-to-back configuration of an AC/DC rectifier 33 and a DC/AC inverter 35. The frequency converter is then connected to a transformer 36 to convert the low or medium AC voltage to a medium AC voltage of higher magnitude. The transformer 36 may be located in the nacelle 30 of the wind turbine, in the tower of the wind turbine or outside the tower.

The medium AC voltage is then connected to a high voltage transformer 37 following the same configuration as in Figure 1 with a DC transmission line 38 before connecting to a main grid 39. Reference is now made to Figure 3 which shows a prior art wind turbine park system. To obtain a high voltage system, wind turbines 31 are connected in series to produce a high DC voltage 45. The content of the wind turbines 31 is not specified but some means of an AC/DC rectifier 33 is included in the nacelle 30 to produce a DC voltage.

Reference is now made to Figure 4 which shows an example of a wind turbine park

configuration using the concept shown in Figure 3 and the grid connection shown in Figures 1 and 2. The AC voltage from each turbine generator 32 is converted by an AC/DC rectifier 33 to a DC voltage. The AC/DC rectifiers 33 are connected in series creating a high DC voltage 45 which is connected to a DC cable 38 for energy transmission before connected to a main grid 39.

Reference is made to Figure 5 which shows prior art (US 2009212568 (Al), Maibach, 2009) with converters having series connected DC-links 34. The energy conversion system 46 comprises a wind power turbine 31, for example in the case of a wind energy system, or a water power turbine 31, for example in the case of a tidal power energy system, which is connected to a generator 32. The generator comprises stator windings where three windings 51A, 51B, 51C form a winding group 50. Each winding group 50 is connected to the AC voltage side of a converter 33 associated with the respective stator winding group 50. The converters 33 have a corresponding DC-link 34 that are connected in series, creating a common high DC voltage 45 which in turn can be connected to a high voltage DC cable 38 for transmission of electrical power to an inverter 35 connected to a main electricity grid 39.

Reference is now made to Figure 6 which shows a schematic example of an energy conversion system 46 according to a first embodiment of the present invention that can utilize a high DC voltage 45. The system illustrated includes windings 51A-C in electrical machines 32, preferably three phase windings 51A-C, and the electrical machines 32 can run in either motor mode or generator mode and can be synchronous machines or asynchronous machines. The electrical machines 32 can further be radial flux machines or axial flux machines and stators in the electrical machines 32 can be made of iron or be ironless. The windings 51A-C illustrated are wye-connected but they can also be delta-connected or connected in some other configuration. The windings 51A- C can be distributed in different electrical machines 32 or all can be in the same machine 32. A stator of a machine 32 including all the windings 51A-C may be segmented, where the windings 51A-C in each segment is referred to as a winding group 50. For the case where the windings 51A- C are distributed in different machines 32, the windings 51A-C in each machine 32 are referred to as a winding set 50. The winding groups or sets 50 are surrounded by electrical screens 60 with different electrical potentials. In electrical machines 32 with iron stators, the electrical screens 60 are preferred to be the stator iron. In electrical machines 32 with ironless stators, the electrical screens 60 can comprise of shells, mesh, layer or coatings of some kind of semi-conducting or conducting materials surrounding the winding groups 50.

In the example illustrated in Figure 6, each three phase winding group 50 is connected to a converter, particularly an AC-DC converter 33, by means of a three phase connection 63. The non- machine side terminals of the converter, the DC voltage side in Figure 6, may be connected to a DC-link 34. The DC-links 34 and converters 33 are connected in series with each other, creating a common high DC voltage 45. The system illustrated comprises a connection 61 between the electrical screen 60 surrounding a winding group 50 and a midpoint of the corresponding DC-link 34. The connection 61 may include a fault detection device 62 to monitor for any short circuit between windings 51A-C and electrical screen 60 or partial discharges. A further example of the invention is shown in Figure 6 where an electrical screen 60 is arranged around each converter 33. The electrical screen 60 is connected to an energy storage circuit/DC-link 34 of the corresponding converter 33 to minimize the voltage difference between components inside the converter 33 and the electrical screen 60 surrounding them. The electrical screen 60 may alternatively be connected to some other DC voltage source.

The total number of converters, N, in Figure 6 is even, as is the number of winding groups 50. Ground potential 64 in Figure 6 is connected between converter number N/2 and N/2 + 1 to minimize the voltage between converter number 1 and ground, and converter number N and ground.

Reference is now made to Figure 7 which shows a schematic example of an energy conversion system 46 according to a second embodiment according to the invention, where the total number of converters, N, is odd. The grounded point 64 is therefore connected to a midpoint of DC-link 34 corresponding to converter number (N+l)/2 to minimize the voltage between converter number 1 and ground and converter number N and ground.

Reference is now made to Figure 8 which shows a schematic example of a third embodiment according to the invention, where the electrical screen 60 is connected to a neutral point 65 of the corresponding winding group 50. The electrical screen potential is then equal to the voltage offset of the winding potential and the insulation between each winding and the corresponding stator slot is minimized.

The connection 61 between the screen 60 and the neutral point 65 may include a fault detection device 62 to detect winding faults.

Reference is now made to Figures 9a-c which shows a schematic example of a fourth embodiment according to the invention, where each electrical screen 60 only surrounds one phase/winding 51. The illustrations show cases where the electrical screen 60 is connected to some point 70/71 of the corresponding winding 51. Figure 9a shows an example where the electrical screen 60 is connected to a midpoint 70 of one of the corresponding windings 51. Figures 9b and 9c shows examples where the electrical screen 60 is connected to one of the ends 71 of the corresponding winding 51.

In Figures 9a-c each electrical screen 60 only surrounds one winding 51 in the same phase, but the electrical screen 60 according to the invention may surround more than one winding 51 of the same phase. The windings 51 illustrated in Figures 9a-c may be connected to other windings 51 by wye-connection, delta-connection or by some other means of connection. The windings may alternatively be connected directly to a single-phase converter, preferably an AC-DC converter 33. The windings illustrated in both Figure 8 and Figures 9a-c may be connected to converters 33 illustrated in Figure 6 and 7, or may be connected to other windings 51 to create a high AC voltage.

Reference is now made to Figure 10 which shows a schematic example of a high voltage energy conversion system 46 according to the invention to be used in for instance tidal power. In Figure 10, each electrical machine 32 is connected to a turbine 31, thus working as a generator. Each power turbine 31 including an electrical machine 32 is connected to an AC/DC-converter 33. The DC-links 34 of the AC/DC-converters 33 from different turbines 31 are connected in series creating a common high DC voltage 45. The high DC voltage 45 is connected to a DC cable 38 which in turn is connected to a DC/AC inverter 35 before connecting to a main grid 39 through a transformer 37. In the example illustrated in Figure 10, the electrical screen 60 according to the invention is connected to the neutral point 76 of the corresponding converter 33. Alternatively the electrical screen 60 could be connected to the positive or negative side of the DC-link 34 of the

corresponding converter 33, the neutral point of the winding group inside the electrical screen or to some other DC voltage.

Figure 10 shows an energy conversion system 46 according to the invention including a fault detection device 62 on the electrical screen connection 61 to monitor for any short circuits or partial discharges within electrical screens 60. Furthermore a short circuit device 54 is connected to each of the converter DC-links 34. In normal operation all of the short circuit devices 54 are open. If one of the converters 33 fails or a voltage break down occurs within one of the screens 60, the corresponding short circuit device 54 can be triggered to short circuit the DC-link 34 and disconnecting the damaged winding group 50 and converters 33.

Reference is now made to Figure 11 which shows an energy conversion system 46 according to the invention. The figure is an extension of Figure 10 where more than one energy conversion systems 46 are connected in parallel 80 to use the same DC cable 38 for energy transmission. The parallel connection 80 may be arranged on a separate platform, in a subsea hub or a floating unit. Reference is now made to Figure 12 which shows an energy conversion system 46 according to the invention comprising medium or high voltage electrical machines 32. Each electrical machine 32 includes more than one winding group 50 where each winding group 50 is surrounded by an electrical screen 60. In Figure 12 each winding group 50 is made of three windings 51A-C and the winding groups 50 are connected to separate AC/DC converters 33.

The AC/DC converters 33 are connected in series on the DC link 34 side according to the invention and create a medium or high voltage output. The voltage output is further connected in series with other voltage outputs to create a common high DC voltage 45 which can be connected to a high voltage DC cable 38 before connected to a main grid 39.

Reference is now made to Figure 13 which shows a high voltage energy conversion system 46 according to the invention. The figure is an extension of Figure 12 where more than one high voltage energy conversion systems 46 are connected in parallel to use the same DC cable 38 for energy transmission.

Reference is now made to Figure 14 which shows an example of an electrical machine 32 according to the invention to be used in an energy conversion system 46. The electrical machine 32 is a radial flux machine with a stator including stator laminations 92. The stator lamination 92 acts as an electrical screen surrounding all of the stator windings 51 and is provided with insulation 90 around the stator to avoid short circuits. The electric machine 32 in Figure 14 includes a rotor 94 with a shaft 95 inside. The rotor 94 can have the same voltage potential as the stator laminations 92 or it can be connected to ground. In the latter case the air filled or liquid (oil) filled gap 93 between the stator 92 and rotor 94 acts as insulation. Axial channels 91 are further added in the stator 92 for cooling the machine 32. The axial channels 91 can have any cross section shape and can be used to transport fluids or gases for cooling the stator laminations 92.

Reference is now made to Figure 15 which shows a cross section view of a radial flux machine 32 according to the invention, seen from the side. The machine 32 includes stator lamination 92 that acts as an electrical screen 60. The machine is further surrounded by a housing 102 and some means of non-conducting fluid 101, for example oil. The oil 101 circulates inside the machine flowing through a gap 93 between stator 92 and rotor 94, around the end of the stator 92 and between the stator laminations 92 and the housing 102. The oil 101 acts as insulation between the stator laminations 92 and the rotor 94, and between the stator laminations 92 and the housing 102. The oil 101 may also act as a cooling fluid. The stator laminations 92 are connected to the machine housing 102 by some means of non-conductive carrying elements 100.

Reference is now made to Figure 16 which shows another example of a cross section view of a radial flux machine 32 according to the invention, seen in an axial direction. The electrical machine 32 illustrated in Figure 16 is similar to Figure 14, but has an additional machine housing 102 around the insulation 90 of the machine 32. The machine housing 102 includes axial channels 91 which can transport fluids or air for cooling of the machine 32. The flow of fluid goes one way 101A on one side of the insulation 90 and the other 101B on the other side. The fluid flow 101A-B is used for cooling the machine in the way that it transports heat generated inside the machine 32 around the insulation to the machine housing 102.

Reference is now made to Figure 17 which shows an example of an axial flux, ironless machine 32 according to the invention that can be used in an energy conversion system 46 illustrated in Figure 12. The ironless machine 32 shown in the figure has one rotor 94 on each side of an ironless stator 112 and each rotor is provided with magnets 110 arranged on an inner side of the rotor 94. The ironless machine 32 further includes windings 51 which are surrounded by electrical screens 60. Each electrical screen 60 has an electrical potential that minimizes the voltage between the electrical screen 60 and the windings 51 it surrounds. The air gap 93 between the ironless stator 112 and the rotor 94 is used to insulate between the screen 60 and ground potential.

Reference is now made to Figures 18a and 18b which show examples of arrangements of the electrical screens 60A-E schematically according to the invention to minimize the potential difference between the neighbouring screens and thickness of the insulation needed between the electrical screens 60A-E when the electrical screens 60A-E are connected to a DC voltage source, i.e. the DC-link 34 of the corresponding converter, the neutral point 65 (shown in Figure 8) of the corresponding winding group 50 or to some other external DC voltage source. Figure 18A shows an arrangement of electrical screens 60A-E where the number of electrical screens 60A-E with the same electric potential is odd. The electrical screens 60A-E are arranged after electric potential and every second group 60B,D of potential is arranged with increasing voltage potential in one direction around the machine 32, and the other groups 60A,C,E of electric potentials are arranged with increasing potential in the other direction around the machine 32. The number of electrical screens 60A-E having the same electrical potential is as shown in Figure 18a, but may be different from one. Each electrical screen 60A-E includes three windings 51A-C that are wye-connected to each other. The number of windings 51 may be different from three and they may be connected in some other way or not be connected to each other at all.

The maximum voltage between two neighbouring electrical screens 60A-E arranged according to the invention would be two times the voltage on the DC-link 34 of the corresponding converter 33.

Figure 18b shows an example of a schematic arrangement of electrical screens 60A-E where the number of electrical screens 60A-E with the same electric potential is even. Half of the electrical screens 60A-E, including at least one of each electrical potential, are arranged in increasing order of potential in one direction around the electrical machine 32. The other electrical screens 60A-E are arranged in increasing order of potential in the other direction around the electrical machine 32. The number of electrical screens 60A-E having the same electric potential is two in Figure 18b, but it may be different from two. In Figure 18b, each electrical screen 60A-E includes three windings 51A-C that are wye-connected to each other. The number of windings 51A-C may be different from three and they may be connected in some other way or not be connected to each other at all.

The maximum voltage between two neighbouring electrical screens 60A-E arranged as described above, would be the voltage on the DC-link 34 of the corresponding converter 33. Reference is now made to Figures 19a and b which show examples of arrangements of the electrical screens 60A-E schematically according to the invention to minimize the thickness of the insulation needed between the electrical screens 60A-E when the electrical screens 60A-E are connected to alternating voltage sources, i.e. to the midpoint 70 (shown in Figures 9a) of the corresponding winding 51A, to one of the ends 71 (shown in Figures 9b-c) of the corresponding winding 51A or to some other means creating an alternating voltage. Figure 19a shows an arrangement of the electrical screens 60A-E when there is only one phase/winding 51A in the electrical machine 32. The electrical screens 60A-E are arranged after potentials and every second screen 60B, 60D is arranged with increasing potential in one direction around the electrical machine 32, and the other electrical screens 60A, 60C, 60E are arranged with increasing voltage potentials in the other direction around the electrical machine 32. The number of windings 51A in one phase is five in Figure 19a, but a machine according to the invention may have a number of phase windings 51A-C different from five. Further, there is only one winding 51A inside each electrical screen 60A-E, but a machine 32 according to the invention may have more than one winding 51A inside each screen 60A-E.

Figure 19b shows an arrangement with electrical screens 60A-E where there is more than one phase in the electrical machine 32. The electrical screens 60A-E corresponding to each of the phases are arranged in increasing order of electric potential magnitude. The neighbouring screen 60A-E of two different phases are then the screen 60E with highest electric potential in one phase and the screen 60A with lowest electric potential in the other phase.

In Figure 19b the number of electrical screens 60A-E in each phase is five. The number of electrical screens 60A-E in each phase may be different from five in a machine 32 according to the invention. Further, the figure shows a machine 32 having three phases, but the number of phases inside a machine 32 according to the invention may be different from three.

Reference is now made to Figure 20 which shows a high AC voltage energy conversion system. Each winding group 50 is connected to a frequency converter element 33 that converts voltages between 3 phase and single phase. The non-machine side terminals, single phase terminals in Figure 20, are connected in series to utilize a high AC voltage 121. Three such arrangements are used to create a high voltage 3 phase system 122. Electrical screens 60 are connected to the non- machine side terminals of the frequency element 33 and have the same alternating voltage as the corresponding high voltage phase 121.

Another benefit with the electrical screens, in addition to what is described above, is that they give environmental protection. The radiation emission from the electrical components inside the electrical screen can be reduced.

Further, the electrical screens can also protect against external radiation emissions so that the components inside the screens are not disturbed. Modifications

The examples given describe different options to what the electrical screen can be connected to. Another alternative is to connect the electrical screen to more than one voltage source, for instance to both the DC-link of the AC/DC converter and to the neutral point of the corresponding winding groups.

In Figures 6 and 7, the three phase winding group is connected to the corresponding AC/DC converter by means of a three phase connection. An alternative is to have a number of phases different from three.

In the description of the invention the number of winding groups is equal to the number of AC/DC converters. An alternative is to have more converters than winding groups, where each winding group is connected to more than one converter. Another alternative is to have more winding groups than converters, where each converter is connected to more than one winding group.

In Figures 9a-c the electrical screen is connected to either the middle or to one of the ends of the corresponding winding. The electrical screen may be connected to some other part of the corresponding winding.

The windings of the electrical machine may be superconductive windings.

The description of renewable energy production systems according to the invention describes wind and tidal power plants, but the invention can also be used for wave energy or other energy conversion systems. The converter units used in wave energy are often low power where each generator is connected to individual AC/DC rectifiers. The rectifiers are low voltage (100-1000 V) and by connecting them in series a medium DC voltage output (1000 - 10000 V) may be achieved. This ensures lower losses in cables at power transfer to the point of common coupling or the grid.

The energy conversion systems mentioned in the invention mainly describe production systems for renewable energy but may just as well be used for other energy conversion systems working in generating as well as motoring mode, for example marine systems or subsea pump systems.

An alternative application according to the invention is induction furnaces, where each furnace comprises at least one winding and can be called a winding group.

A further alternative embodiment, which is applicable for offshore wind turbines, is that the top structure of the turbine, including the nacelle and parts of the tower, all are a part of the electrical screen. A part of the tower must then be coated with an insulation layer or be made of an insulating material so that the parts being a part of the electrical screens are not in contact with the ocean water surrounding it. An insulation means can for instance be a tower made of composite materials.

Claims

1. An energy conversion system having at least one electrical machine (32) comprising winding groups, where each winding group (50) consists of at least one winding (51, 51A-C), and at least two power converters (33) having machine side terminals and non-machine side terminals, where the power converters (33) are connected to one or more winding groups (50) on the machine side terminals and the power converters (33) are connected in series through the non-machine side terminals, characterized in that

a) at least one winding group (50) in the electrical machine (32) is surrounded by at least one electrical screen (60, 60A-E), and that at least one of the electrical screens (60, 60A-E) are connected to an AC or DC voltage source; or

b) at least one power converter (33) or parts of a power converter is surrounded by at least one electrical screen (60, 60A-E), and that at least one of the electrical screens (60, 60A-E) are connected to an AC or DC voltage source;

or a combination of a) and b).

2. An energy conversion system according to claim 1, characterized in that the electrical machine (32) is a rotary machine, linear machine, actuator, transformer or another inductive apparatus.

3. An energy conversion system according to claim 2, characterized in that the electrical machine (32) is either an ironless machine or a machine containing iron cores.

4. An energy conversion system according to claim 1, characterized in that the electrical screen (60, 60A-E) surrounding the winding group (50) is connected to:

- a neutral point (65) of the corresponding winding group (50),

- the middle of a single winding (51, 51A-C),

- one end (71) of a single winding (51, 51A-C),

- a point within the corresponding power converter (33),

- the non-machine side terminals of the corresponding power converter, or

- a point on a DC-link of the corresponding power converter.

5. An energy conversion system according to claims 1-2, characterized in that the electrical screen (60, 60A-E) is the iron core carrying magnetic flux in the electric machine (32), a machine housing (90) or a semi-conductive or conductive material forming a shell, mesh, layer or coating.

6. An energy conversion system according to claim 1, characterized in that the electrical machine (32) includes at least two winding groups (50), each surrounded by an electrical screen (60, 60A-E).

7. An energy conversion system according to claim 1, characterized in that the number of electrical machines (32) is at least two and that each electrical machine (32) includes a winding group (50) surrounded by an electrical screen (60, 60A-E).

8. An energy conversion system according claim 6 or 7, characterized in that the number of winding groups (50) and electrical screens (60A-E), which either corresponds to individual electrical machines (32) in a group of electrical machines (32) or to parts of one electrical machine (32), is even and where ground potential (64) is set between two middle winding groups (50).

9. An energy conversion system according to claim 6 or 7, characterized in that the number of winding groups (50) and electrical screens (60, 60A-E), which either corresponds to individual electrical machines (32) in a group of electrical machines (32) or to parts of one electrical machine (32), is odd and where ground potential (64) is at a winding group (50) in the middle.

10. An energy conversion system according to claim 6, characterized in that the electrical screens (60, 60A-E) inside an electrical machine (32) are arranged so that the electrical screens (60, 60A-E) with lower electric potential differences are arranged close to each other and electrical screens (60, 60A-E) with higher electric potential differences are arranged further apart from each other.

11. An energy conversion system according to claim 1, characterized in that the electrical machines (32) are provided with a housing (102) and an opening between the housing (102) and active parts of stator, where the opening is partly occupied by non-conductive carrying elements (100) and partly by some cooling and insulating fluid (101).

12. An energy conversion system according to claim 2, characterized in that the electrical machine

(32) includes superconductive windings (51, 51A-C).

13. An energy conversion system according to claim 1, characterized in that the power converters

(33) are rectifiers (AC/DC), inverters (DC/AC), frequency converters (AC/AC or AC/DC/AC), DC-to- DC converters (DC/DC), transformers (AC/AC) or a combination of these.

14. An energy conversion system according to claim 13, characterized in that the electrical screen (60, 60A-E) surrounding the power converter (33) or parts of the power converter (33) is connected to a point within the power converter (33), to the non-machine side terminals of the power converter (33) or to a point on a DC-link of the power converter (33).

15. An energy conversion system according to claim 13, characterized in that the power converters (33) have s short circuit means (54) connected to the non-machine side terminals.

16. An energy conversion system according to any one of the claims 1-15, characterized in that the energy conversion system (46) comprises more than one electrical screen (60, 60A-E) and that at least one electrical screen (60, 60A-E) surrounds one or more other electrical screens.

17. An energy conversion system according to any one of the claims 1-16, characterized in that a fault detection device (62) is connected between the electrical screen (60, 60A-E) and the AC or DC voltage source.