WO2001052379A2

WO2001052379A2 - Electric power system based on renewable energy sources

Info

Publication number: WO2001052379A2
Application number: PCT/SE2000/002616
Authority: WO
Inventors: Gunnar Asplund; Harry Frank; Lars Gertmar; Bengt Rothman; Inger Segerqvist
Original assignee: Abb Ab
Priority date: 1999-12-23
Filing date: 2000-12-21
Publication date: 2001-07-19
Also published as: AU2420401A; SE518121C2; SE9904740D0; SE9904740L; WO2001052379A3

Abstract

An electric power system has at least two electric power plants, each one with an ac machine (3a..3n) driven by a renewable energy source (2a..2n). Each one of the ac machines is connected to its own converter (4a..4n), which operates as a rectifier. Further, the electric power system has an HVDC transmission line (7) for transmission of the power produced by the electric power stations. In addition thereto, the electric power system comprises a converter (8), operating as an inverter, for further transmission via a power transformer (9) of the power, transmitted by means of the HVDC transmission line, to a distribution or transmission network. A series link is formed, consisting of a series connection of the output sides of the rectifiers and the input side of the inverter via the HVDC transmission line.

Description

Electric power system based on renewable energy sources

TECHNICAL FIELD

The present invention relates to an electric power system based on renewable energy sources . The electric power system comprises at least two electric power plants, each one including an ac machine driven by the energy source and connected to a converter operating as a rectifier, the electric power system further comprising a dc transmission line and a converter operating as an inverter for transmitting a power, transmitted through the transmission line, further to a distribution or transmission network.

In relation to the background art described below with regard to series-connected energy sources and their connection to transmission or distribution networks, the invention comprises a new method of designing the connection to the electric power network and to the consumers .

The electric power system will substantially be described below with reference to a wind power plant . The electric power plants thus comprise wind power stations where the renewable energy source is wind power. The voltage of the wind power ac generators, driven by the wind turbines, is rectified and series-connected and connected to an HVDC transmission line, preferably a cable intended to be submerged into water, for transmission of the dc power produced by the wind power stations to a, preferably land- based, transmission or distribution network.

In addition to wind power stations, the electric power plants may comprise so-called minihydroelectric power stations, geothermal power stations and power stations based on solar cells, etc.

Although the advantages of the invention in the following will substantially be dealt with in connection with the wind turbines, the generators and the converters being placed at sea or in lakes, the invention may also entail advantages where some of or all of the ac machines and the generators and converters of the wind turbines, respectively, are placed on land and where the connection, which then does not necessarily have to consist of a cable but instead may be realized in the form of overhead lines or cables, interconnects several such electric power plants to the distribution or transmission network.

BACKGROUND ART, THE PROBLEMS

When locating the above-mentioned parts of a wind power station at sea, it is required, for obtaining economy in the project, that large groups of wind power plants be located within a limited area. Sea-based wind power stations require relatively large plants, that is, in this connection, normally 3 MW and thereabove, and a total plant power of 50-100 MW is considered suitable. So far, the planning for such wind power plants has substantially assumed that the electric power transmission takes place through traditional ac transmission in a three-phase ac voltage submarine cable. The generator is then normally a three-phase asynchronous generator. There are also examples of synchronous generators having been used directly connected to the network. However, this has often resulted in the necessity of installing complicated mechanical resilience between the generator and the engine housing to damp power oscillations which arise because of the varying character of the wind load. The reason for this is that the rotor dynamics of a synchronous generator behaves mechanically like a spring against a stiff ac voltage network whereas an asynchronous generator behaves like a damper.

In an ac voltage-based wind power station, a conventional asynchronous generator of 3 MW would probably be designed for 3 -6 kV and be connected to a transformer for stepping up, say, 24 kV in a first stage. In a wind power plant with 30-40 wind power stations, the generators/transformers are then connected in parallel and then, via a central trans- former, the voltage is stepped up further to, say, 130 kV.

The advantage of such a system is that it is inexpensive and does not require any complicated sub-systems . One disadvantage of such a system is the difficulty in trans- mitting power over long distances along a high-voltage ac voltage cable. This is due to the cable producing capaci- tive reactive power which increases proportionally to the length of the cable and to the voltage squared. The current through the conductor and in the cable screen then increases to such an extent that the cable cannot be realized for long distances. Another disadvantage resides in the fact that the varying wind load gives rise to voltage variations on the transmission line, which may affect the electricity consumers which are connected in the vicinity of the plant. This applies especially if the network is weak, that is, has a low short-circuit power. Owing to the above-mentioned technical problems with long cable transmission distances, it is sometimes necessary to connect the wind power plant to a weak network. According to certain regulations, the amplitude of the voltage variations on the network must not be more than 4 % . Different countries have different regulations in this respect. The regulations are often less severe at a lower voltage level on the transmission line. There are also different regulations regarding rapid voltage variations.

As is well known, these may give rise to so-called flicker, that is, light variations in light bulbs.

One solution to the above-mentioned problems with long cable distances is to transmit the power by means of high- voltage direct current, a so-called HVDC transmission. This makes it possible to extend the cable all the way to a strong network. Another advantage is that the dc transmission involves lower losses than ac transmissions. The cable distance may then, in principle, be of any length, from a technical point of view.

An HVDC link consists of a rectifier station, a transmission line in the form of a cable or an overhead line, a station for inversion and one or more filters for eli i- nating harmonics generated during the inversion. In an older variant of an HVDC link, thyristors are used both for rectification and inversion. Thyristors are capable of being fired but not capable of being extinguished. The commutation takes place near the zero crossing of the voltage, which zero crossing is determined by the ac voltage and the inverters . These converters are therefore referred to as line-co mutated. A disadvantage of this technique is that inversion consumes reactive power and leads to harmonics which are sent out onto the network. In the following, converters which are used for inversion which be referred to as inverters .

In a more modern dc voltage solution, so-called IGBTs are used instead of thyristors in the inverters. Such an installation is described, inter alia, in an article entitled "HVDC Light-DC transmission based on voltage sourced converters" published in ABB Review 1/1998, pp. 4- 9. An IGBT (Insulated Gate Bipolar Transistor) is capable of being both fired and extinguished and, in addition, has a high switching frequency. This enables the inverters to be made according to a completely different principle, such as so-called self-commutated inverters . In summary, the advantages of self-commutated converters are that they may deliver and consume reactive power. This makes possible active compensation of the voltage level on the network side if the connection has been made to a weak network. Consequently, this makes this type of converter superior to that of the older technique. The high-frequency switching technique also results in a reduction of the problems concerning harmonics compared with the older generation of HVDC installations. However, IGBT inverters entail higher losses in the inverter station. The cost of the IGBT inverter station itself is also higher than for such a station with thyristors . A self-commutated converter is characterised by the voltage being built up by a rapid pulse pattern, which is generated by the converter. The voltage difference between the pulse pattern and the sinusoidal line voltage will lie over the inductance on the network side. There are two types of self-commutated inverters with somewhat different properties. One is a so-called voltage source inverter, VSI, with at least one capacitor on the dc side, and the other is a so-called current source inverter, CSI .

Self-commutated converters have been utilized for feeding and dc transmission of active power for wind power at Nas on Gotland (Sweden) . They have also been utilized for compensation of the ac voltage for wind power at Re sby Hede on Jutland (Denmark) .

For various reasons, most wind power stations have a gear between the wind turbine and the generator of the wind power station. When problems arise on such designs, the gear is often the cause. Therefore, it is desirable to have a direct-driven generator. Starting from a speed of rotation which is relevant to the wind turbine, it is preferable for a gearless system to be designed with a multi-pole generator. As far as multi-pole machines are concerned, the synchronous machine is preferable to an asynchronous machine. To obtain good economy in a synchronous machine, it is required to operate at a relatively low frequency, say 5-10 Hz for MW installations. An electric power network operates with a higher frequency, that is, 50 or 60 Hz, which implies that there has to be a frequency converter between the generator and the electric power network. The frequency converter may, as usual, be designed as a cyclo- converter, a matrix converter, or with a dc intermediate link. As regards sea-based wind-driven plants, as described above, because of long transmission lines, power transmission via dc cables is preferable. The frequency converter is then replaced by an ac/dc converter. A plurality of different principles are documented for transmitting direct current from the wind power stations to the inverter station which shall subsequently convert the transmitted dc power into power for the electric power network. US 5,083,039, "Variable speed Wind turbine" describes how each generator in a wind power plant may have its own internal dc voltage link. After inversion of the transmitted power of each busbar, they are connected (combined) via a common transformer to the electric power network. WO 97/45908, "Windenergiepark", which will be described in more detail below, shows how a plurality of wind power generators, after respective ac/dc converters, are connected in parallel to a common dc transmission line.

One problem which is associated with wind power stations is that the varying wind speed will influence the frequency of the generators. This problem has been dealt with in, inter alia, the above-mentioned US 5,083,039. Another way to be able to adapt the frequency of the electric generator of a wind power station, at varying wind speed, to the frequency of a connected power network occurs with a brushless system, OPTI-SLIP®, produced by Vestas Danish Wind Techno1. A/S, Denmark, described in an article entitled "Semi- variable speed operation - a compromise?", presented at the Proceedings of 17 Annual Conference, British Wind Energy

Association, 19-21 July 1995, Warwick, UK. The principle of the control is based on the well-known method involving loss control with the aid of varying external rotor resistors connected by means of slip rings to the rotor winding of the electric generator. As opposed to the well- known slip-ring method, the OPTI-SLIP® installation is arranged with a rotating ac-dc-dc converter and rotating fixed rotor resistors, connected directly to the rotor winding. The ac-dc conversion is carried out with a diode rectifier which, in turn, is short-circuited by a dc-dc converter. The speed control of the wind power station is performed via an internal rotor-current control. The loss power associated with the control is thus developed in the rotating rotor resistor and is then discharged into the surrounding air. From the conference paper it is clear that the speed may be up to 4 % above the synchronous speed, resulting in a loss of power of the same percentage as losses in the rotor circuit.

The problem with a varying wind speed and the associated speed variations of the generator of the wind power station has also been studied and tested experimentally at the testing facility of the Chalmers University of Technology at Hδnδ. The tests are described in, inter alia, Technical Report No. 17βL: "Asynkrongenerator och frekvensomriktare for drift av vindkraftverk med variabelt varvtal" ("Asynchronous generator and frequency converter for operation of wind power plants with a variable speed") and in Technical Report No. 185: "Analys av synkrongeneratorer med frekvensomriktare for elgenerering vid variabelt varvtal", ("Analysis of synchronous generators with frequency converters for electricity generation at variable speed") , both from the Electrotechnical Section of the Chalmers University of Technology in Gδteborg. In the embodiments .described, the ac generator voltage of the wind power plant is disconnected from the frequency of the network via a dc link at low voltage, typically at the 400- volt or 600-volt level. Having a variable speed of the turbine gives an energy profit and lower stresses in the mechanical system while at the same time it normally results in a possibility of utilizing the speed variations for eliminating, from the electric power network, the rapid power pulsations of the tower shadow, which give rise to the above-mentioned flicker. Expressed in popular terms, the moment of inertia of the turbine will function as an intermediate layer for kinetic energy. On the other hand, of course, the slow power variations, which are inherent in the wind power, cannot be eliminated. If it is desired to have a direct-driven generator and thus to eliminate the need of a gearbox between the turbine and the generator, the generator has to be synchronous because it will have a large number of poles. In other words, a direct-driven generator requires a frequency converter. If a controlled rectifier is used, it is furthermore possible to control the moment actively by changing the control angle. In most concepts involving a variable speed, in addition, an external active speed control is provided by means of so-called pitch control, which means that the blade angle of the turbine is changed. One disadvantage of variable speed according to the concepts described is the cost of the necessary power electronics, and another disadvantage is that maintenance of such power electronics at sea is difficult and costly.

In the above-mentioned WO 97/45908, "Windenergiepark", a variable speed system with an HVDC link, of an older model according to the above, is described. According to this technical solution, see Figure 3/8 of this patent application, a rectifier with an inductor is to be connected to each of the sea-based generators . The voltages from each generator, thus rectified, are connected in parallel with the common dc transmission link. On the network side there is a central inverter with an associated inductor which is connected to a mains transformer in a conventional way. The system seems primarily to be intended for mains-commutated, or in any case current-source, rectifiers and inverters, since inductors in the dc circuit make it a current-source system. The system has one advantage since the dc voltage after the rectifier may be varied within a large range. This is necessary during operation with a variable speed since the generator in the wind power plant at low speed can only provide a low output voltage. One disadvantage of a current-source inverter, however, is that it cannot control the reactive power to the network as efficiently as a voltage-source inverter. In this embodiment, thus, it is that direct current which flows out of the sea-based generators, and which is conveyed by the dc transmission link, that goes into the inverter on land, as opposed to the situation disclosed in the reference below. Otherwise, it is clear from the patent document that the voltage is assumed to lie at a level of 6-10 kV, which is typical of conventional generators. This, in turn, implies that the dc voltage is at a maximum of 12 kV, which is an unrealisti- cally low voltage for transmitting a total power of 50-100

MW along a lengthy distance, since the current to be transmitted then entails heavy losses in the cable. If it is desired to keep the cable losses at a low level, a cable with a large conductor area can be used, which, however, is an uneconomical solution because of high cable costs .

For a wind power plant of a magnitude of 50-100 MW, it would instead be desirable to transmit the power with a dc voltage level of about 100 kV. This could be possible by connecting a transformer for stepping up to about 100 kV for each generator. After rectification, according to the above-mentioned patent document, the wind power stations would be connected in parallel for connection to the dc transmission link.

One way to be able to transmit the power from a wind park with a high dc voltage is described in PCT/SE99/ 00943 , "A wind power plant". Briefly, this takes place by connecting, to the parallel-connected sea-based wind power generators/rectifiers, a dc/dc converter with its low- voltage side towards the rectifiers and with its high- voltage side via a dc transmission link to a land-based inverter. The dc/dc converter is arranged as a "dc/dc transformer", that is, it is to step up the dc voltage of the rectifiers to the desired dc transmission voltage level. This, in turn, implies that the dc/dc converter steps down the transmitted current in the same proportion (see the above-mentioned reference) . Sum current from the rectifiers is thus no longer equal to the current in the dc transmission link and the current of the inverter. In one embodiment of the sea-based part of the wind power plant, the rectifiers consist of passive diode rectifiers in series with a local step-up dc voltage converter comprising an inductor, series-connected IGBT valves and series- connected diodes. The dc/dc converter may also have the same basic design. As far as the land-based part is concerned, it is preferred that the inverter is in the form of a voltage-source self-commutated system. According to the above-mentioned PCT/SE application, such a system may comprise a capacitor connected in parallel across the inverter on the dc side with inductors connected in series with each phase on the network side.

Since the publication of WO 97/45919, "Rotating electric machines with magnetic circuit for high voltage and method for manufacturing the same", it is known that, in rotating electric machines, the stator winding may be designed for high voltage. The technique for achieving this is based on the stator winding being designed from an electric conductor surrounded by an insulation system comprising an inner semiconducting layer, an extruded PEX/XLPE insulation, that is, crosslinked polyethylene or ethylene propylene, and an •outer semiconducting layer. Such an insulated conductor is generally referred to, and will be referred to in this application, as a "cable". With this possibility, it would seem obvious, in connection with wind power, to manufacture the wind power generators for a higher insulation voltage than in existing conventional generators at 10-12 kV. In principle, it is therefore possible to replace the electromagnetic step-up transformation to, for example, 100 kV, as described above, and also the dc/dc converter which is described in the PCT/SE application, by high-voltage generators. However, this implies that the costs for the con- verters become higher than for the design with relatively low voltage. According to the prior art, it is assumed that the wind power stations shall be connected in parallel to the dc transmission link.

example in. a brpchure "Submarine Power Cables", ABB SEHVC

M-012E from ABB High Voltage Cables.

There have been discussions to replace these dc cables by cables which have the same insulation system as ac transmission and distribution cables with the inner and outer semiconductive layers described above and with an intermediate layer of solid extruded XLPE. It has been found that this is a difficult technical problem since the solid extruded insulation because of the dc stress must have other insulation properties than those which are needed in an ac cable. However, in an article written by P.

Carsteiisen, K. Johannesson and A. Gustafsson: "Extruded DC power cables and accessories for use in HVDC transmission systems", published at the ICC Fall Meeting 1999, pp 1-7, a newly developed cable for this purpose is described.

The development of the same cable is also described by M. Byggeth, K. Johannesson, C. Liljegren, U. Axelsson et al in an article "The development of an extruded HVDC cable system and its first application in the Gotland HVDC Light project", Fifth International Conference on Insulated Power Cables 20-24 June, 1999, Versailles, France. The article also describes the use of the cable in connection with a so-called HVDC Light installation. The rated power transmitted by means of the cable is 50 MW at 80 kV and the transmission is made with a bipolar 72 km extruded HVDC power cable .

Transmission of electric power via the high-voltage connection of an alternating current may also be an attractive method under certain special conditions . An installation for such transmission comprises, in addition to the high-voltage connection, a device for generating an alternating current with a frequency which is considerably lower than the frequency of an alternating current fed to connected end consumers . Such a plant is described in the patent application PCT/SE99/00944, "A wind power plant and a method for control". When choosing the relevant frequency of the high-voltage connection, the length of the connection, the maximum transmissible power, the level of the plant voltage at which the high-voltage connection is designed to lie as well as the inductance and the capacitance of the high-voltage connection are taken into consideration. To sum up, the transmission frequency is chosen dependent on the electrical conditions of the plant.

As is clear from the above, the described examples of the prior art have shown how to connect a plurality of generators in parallel with connected converters, possibly after having increased the dc voltage in various ways in connection with the connection to a common dc transmission line.

However, there is one technique with series-connected dc machines which was especially used during the first decades of the 20^th century. Chapter 23 of a book by Arnold la Cour: "Die Gleichstrom aschine", Zweiter Band, Springer, Berlin 1927, describes "Serienkraftubertragungssysterne mit veranderlichem und konstantem Strom". Reference will be made here to two somewhat different systems called "Oerlikons Kraftubertragungssystem" and "Thurys Kraft- ubertragungssystem". It is stated here that 20 to 30 dc machines with data 5000 V and 120 A were series-connected for transmission at 100-150 kV. In such systems the machines must be insulated up from ground potential and this has been made by mechanical insulation.

From the above it can be read that the dc machines included substantially had working points with equally large powers. The system was equipped with exceedingly simple actuators which, among other things, is clear from a description of the "Thury" system on page 3 in "Direct Current Trans- mission", Vol 1, by E Wilson Kimbark. It is stated here that "The system operated at constant current. The voltage of each machine in the HV series was regulated by shifting the brushes".

Dc power transmission during the first decades of the 20^th century is also described by E Uhlmann in "Power Transmission by Direct Current", Springer 1975. On page 1 thereof, E Uhlmann states that: "Of the various direct current systems that existed around that period, the Thury system has become the best known. The largest plant, which Thury built, was still in operation in France up to the thirties. By means of a cascade of series connected d.c. generators, mounted on insulators, a direct voltage of 57 kV was produced, a power of 4 MW was transmitted over a distance of about 180 km and at the far end, by means of a corresponding cascade of motor generators, the transmission voltage was reduced to the required consumer voltage level".

US 4,057,736, "Electrical power generation and distribution system" from the middle of the 1970 's, describes a system which seems to be a new invention of the described Thury system. The Abstract of the patent document states that "... a plurality of relatively low voltage generating stations are connected in series to cumulatively produce the high voltage needed for long-distance transmission line delivery. Power-generating devices of successive stations are supported on insulative structures of progressively greater height and are driven or supplied with fuel through insulative means . The generating devices may take various forms including, for example, AC or DC generators driven through insulative drive shafts or fuel cells or magnetohydrodynamic devices supplied with fuel through insulative pipes, and the system is adaptable to large-scale power production from scattered energy sources

for solving the_ problems associated with this are described, nor is there any fundamental description of how to solve these problems . As far as the ac series connection is concerned, and especially from the points of view of insulation and grounding, the description gives the impression that the systems are only adapted to single- phase designs, although the word "multiphase" is mentioned. Single-phase systems imply that the reactances of the transmissions and the distribution lines, respectively, become higher, by a power of ten, than the corresponding reactances of three-phase systems with otherwise the same restrictions in the transmission capacity.

Referring to the US patent and the series-connected dc systems with the converter mentioned above, it is stated that "Rectifiers ... may utilize diodes, tyristors or the like in known circuit configurations ... ". It is not clear from the description if there are actuators for control/adaptation of the voltage of the thyristors. However, there seems to be some form of actuators, primarily concerning any disconnection of parts of parallel-connected energy sources to be able to maintain the operation with reduced energy supply.

The text in column 9 of the US patent, lines 5-9, further states that "in some instances, the voltage step-up provided by the generators (18a) of the several stations multiplied by the number of such stations may not equal the high voltage which is desired to apply to the transmission line." Thus, it seems as if the system with series connection is based on all generating sources having the same voltage and powers equal to the common current.

Finally, to supplement what has been stated previously regarding HVDC transmissions, it can be briefly commented that in these installations/systems, a series connection is made of a number, depending on the actual dc voltage level, of both individual power semiconductors and converter modules, which are supplied with ac voltage from so-called converter transformers. These are transformers which are different from conventional, normally oil- insulated, transformers in such a way that the windings, from the point of view of insulation, must be able to withstand the high dc potential to which they are subjected. An example of this is described, inter alia, in WO97/45907, "Rotating electrical machine plants".

In certain applications, so-called dc-dc converters have been provided with an intermediate high-frequency ac link in the form of a transformer arrangement to achieve a galvanic insulation between the two dc sides. One such con- verter is described, inter alia, in an article entitled

"Coaxially Wound Transformers for High-Power High-Frequency

Application", presented in IEEE Trans on Power Electronics, Vol. 7. No. 1, January 1992. A corresponding dc-dc converter is described in WO 91/07807, "DC/DC power Transformer". It is not clear from these documents how a high insulation level between the two dc sides should be overcome, nor how the high dc potential relative to ground should be overcome. A dc-dc converter with an intermediate ac link for the application which is intended in this connection re- lates to a transformer with windings of the above- mentioned, extruded HVDC power cable, or possibly of the above-mentioned MIND cable.

SUMMARY OF THE INVENTION, ADVANTAGES

The object of the invention is to provide electric power systems, based on the development of power-electronic power converters in the last decade, and based on supply of a number of adjacently positioned, relatively small, electric generating plants arranged at a not-insignificant distance from the electric-power consumer. •

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A ε 03

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•ti Φ υ β (ti ε A

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power plants are utilized to limit the fault currents in case of ground faults .

BRIEF DESCRIPTION OF THE DRAWINGS

Figures la-le describe the converter symbols which will be used in connection with the description of the embodiments of the invention.

Figure 2 shows in the form of a block diagram the prin

ciple of a basic design of the invention, that is, a high- voltage, series-connected dc collection, transmission and conversion system for electrically utilizing wind energy and for transmitting and feeding the energy into an ac power network. Generally, for Figure 2 and the following figures, the following applies: the invention comprises a number of wind power plants, each having one ac machine and one ac-dc converter, by successive series connection on the dc side, a dc loop is formed which

- via a dc cable is connected to a dc-ac converter, the ac side of which is connected to an ac power network.

Figure 3 shows, in principle, how the ac-dc converters,

connected to the rotating ac machines, may be designed with an internal dc-dc converter for adaptation to optimum current or rated power of the wind power plants .

Figure 6 shows how the internal dc-dc converter according to Figure 5 may be designed with an internal ac intermediate link of relatively high frequency.

Figure 7 shows how a wind power plant according to the invention may consist of wind power stations of various designs .

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As mentioned by way of introduction, and as is clear from the appended claims, the invention relates to an electric power system which comprises at least two electric power plants, each one comprising an ac machine driven by a re- newable energy source. The embodiments which will be described below show the invention as applied to electric power systems in the form of wind power plants/wind power stations. The embodiments shown may, however, represent the invention, generally seen, also as far as renewable energy sources are concerned since the only thing that is needed is to replace the driving power of the ac machine by some other renewable energy source.

Especially with regard to wind power stations, the driven ac machines in the preferred embodiments are designed as relatively slowly rotating multi-pole synchronous machines with frequencies up to about ten Hz. The ac machines may be direct-driven by the wind turbine, or a simple mechanical gear may be used inside the wind power stations between the wind turbine and the ac machine.

The individual wind power stations in a wind power plant may have different data as far as rated power, rated speed, rated voltage, etc., are concerned. They can operate at different working points as far as wind speed is concerned. For a wind power plant, at one and the same time, the wind power stations may be driven at different speeds due to the fact that the wind speed may vary within different parts of the plant ., his_ means that the various ac machines may generate voltage of different frequencies . Although the ac machines and the connected ac-dc converter, respectively, may have different rated power, all of them must, however, be dimensioned for the rated current common to the wind power plant. Depending on the supply of wind power, of course, the actual current may vary.

As will have been clear both from the "Background Art" and the "Summary of the Invention", converters constitute an important part of the invention. It may, therefore, be appropriate, during the introductory part of the description of the various embodiments of the invention, to give a brief account of the various converter connections and their main fields of application.

The simplest converter connection that may be used is a diode converter. Since it is not controllable, however, other controllable units are required when using such a converter.

Converter connections for large powers are currently normally designed with silicon-based thyristors, so-called SCR. These connections often require large reactive powers because the currents ^' of"^*the ac network will have a large phase shift relative to its voltages. This implies that the dimensioning of converters and of machines for converter- based systems, in addition to the purely active power flows, must take into consideration reactive power flows, that is, reactive losses in the reactances of machines and of the ac network, as well as the need of phase compensation.

Converter connections for medium-magnitude powers are at present, to an increasingly greater extent, designed with silicon-based transistors . One advantage of such converter connections is, as also described under the background art, that the problems with reactive powers and harmonics may be avoided by utilizing so-called self-commutated connections. This type of converter connections is thus exceedingly well suited for the power ranges which are accorrimodated within the scope of the electric power systems comprised within the invention. Currently, they are primarily designed with power semiconductors such as GTO, IGBT and IGCT. The connections which are utilized may generally be designed with all semiconductors capable of being extinguished.

As will be clear from the brief description of the drawings, Figures la-le show the converter symbols which are used in connection with the description of the embodiments of the invention. In this connection,

Figure la relates to conversion from alternating current to direct current, that is, ac-dc conversion,

Figure lb relates to conversion from direct current to alternating current, that is, dc-ac conversion,

Figure lc relates to conversion from alternating current to alternating current, that is, ac-ac conversion,

Figure Id relates to conversion from direct current to direct current, a direct conversion of one dc voltage to another dc voltage, that is, dc-dc conversion,

Figure le relates to conversion from direct current to direct current via alternating current, an indirect conversion of one dc voltage to another dc voltage with an ac voltage intermediate link, that is, dc-dc conversion.

A preferred fundamental embodiment of the invention is shown in Figure 2. The figure shows an electric power system comprising a number of electric power plants/wind power stations. In the exemplified embodiment, the electric power system comprises a number of^* wind power stations 1, .. , L, .. , N, each one comprising a wind turbine 2a ... 2n, en ac machine 3a ... 3n and an ac-dc converter 4a ... 4n. The dashed part "L" in the figure indicates that there may be a number of wind power stations between the wind power stations 1 and N. In accordance with the invention, the figure shows how the dc sides of the converters are series-connected. The power generated in the wind power station is transmitted via a dc transmission line 7, preferably consisting of two extruded HVDC power cables, to a dc-ac converter 8 operating as an inverter for further transmission of the generated power via a power transformer 9 to a distribution or transmission network.

For an embodiment according to Figure 2 to function, it is assumed that the ac machines are provided with excitation windings for regulation of the generated ac voltage.

There is a great freedom of choice as regards the choice of ac-dc converters 3a .. 3n. They may be designed as diode converters, thyristor converters or with silicon- based transistors. On the basis of what has been described above, converter connection with silicon-based transistors constitute a preferred embodiment. As will be clear from the following, the individual wind power stations in the wind power plant may be designed with different embodi- ments, distinguished from each other, of converter connections for the ac-dc converters 4a .. 4n.

As regards the inverter 8, it may also be based on thyristor converters or converters with silicon-based transistors. Also in this case, the embodiment with silicon-based transistors is a preferred embodiment.

The problems relating to the nominal voltage of the ac machines and the requirements for the high-voltage insula- tion level of the ac machines relative to ground may, according to the invention, be overcome by winding the machine with the previously mentioned extruded HVDC power cable. Another alternative within the scope of the inven- 03 1 03 Φ β

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in Figure 4- Wind power station "M" shows a variant of the wind power stations according to Figure 1, where the wind power station has been provided with a mechanical gear 12m between the wind turbine and the ac machine, and a trans- former 13m has been introduced between the ac machine and the converter 4m. The wind power station "M" also indicates how the ac machine may be grounded via a high-ohmic resistor 14m at the neutral point of the ac machine. The transmission line between the wind power stations and the in- verter station 8 may consist of two extruded HVDC power cables 7' and an overhead line 7''. As shown, the system may also be grounded in the power transformer 9 via a high-ohmic resistor 15m. In addition, Figure 7 indicates how the produced wind power energy may be transported further to a remote distribution or transmission network. The wind power station/stations "M" may very well be a wind power station/wind power stations which was/were previously connected to a 3 -phase ac network.

Still other similar embodiments, for example by other combinations of embodiments according to the figures and by internal series and/or parallel connection of converters and/or power semiconductors, are within the scope of the invention.

Claims

1. An electric power system with at least two electric power stations, each one with an ac machine (3a..3n) driven by a renewable energy source (2a..2n), whereby each one of the ac machines is connected to its own converter (4a..4n), operating as a rectifier, and an HVDC transmission line (7) for transmission of the power produced by the electric power plants, characterized in that the electric power system, in addition thereto, comprises a converter (8) operating as an inverter for further transmission via a power transformer (9) of the power, transmitted by means of the HVDC transmission line, to a distribution or transmission network, and a series link consisting of a series connection of the output sides of the rectifiers and the input side of the inverter via the HVDC transmission line.

2. An electric power system according to claim 1, characterized in that the renewable energy sources

(2a..2n) have different rated powers and working points.

3. An electric power system according to claim 1, characterized in that the ac machines (3a..3n) and the connected converters (4a..4n) have different rated powers, rated voltages, and rated speeds.

4. An electric power system according to claim 1, characterized in that all the ac machines (3a..3n) and the connected converters (4a..4n) are designed for a rated current common to the electric power system.

5. An electric power system according to claim 1, characterized in that the renewable energy source is any of wind power, minihydroelectric power, geothermal power and solar power.

6. An electric power system according to claim 1, characterized in that the converter (4a..4n) which is connected to the ac machine (3a..3n) is a high-voltage ac/dc power electronic power converter designed with IGBT,

IGCT, GTO or SCR.

7. An electric power system according to claim 1, characterized in that the dc transmission line (7) is an HVDC transmission line.

8. An electric power system according to claim 1, characterized in that the converter operating as an inverter (8) is a high-voltage dc/ac power electronic power converter.

9. An electric power system according to claim 8, characterized in that said high-voltage dc/ac power- electronic power converter is designed with self- commutated semiconductor valves .

10. An electric power system according to claim 8, characterized in that said high-voltage dc/ac power- electronic power converter (8) is pulsewidth-modulated.

11. An electric power system according to claim 8, characterized in that said high-voltage dc/ac power electronic power converter (8) is designed with line- commutated semiconductor valves .

12. An electric power system according to claim 6, characterized in that said ac/dc power-electronic power converter is designed with an internal dc/dc converter

(5a..5n) for dc adaptation to the optimum current level of the dc series connection at different rated powers or different working points of the renewable energy sources.

13. An electric power system according to claim 12, characterized in that the internal dc/dc converter (5a..5n) is pulsewidth-modulated.

14. An electric power system according to claim 6, characterized in that said ac/dc power-electronic power converter is designed with an internal dc/dc converter

(6a..6n) with an insulating ac intermediate link, for dc adaptation to the optimum current level of the dc series connection at different rated powers or different working points of the renewable energy sources and for achieving a high-voltage insulation level.

15. An electric power system according to claim 14, characterized in that the internal dc/dc converter (6a..6n) in the ac/dc power-electronic power converter is pulsewidth-modulated.

16. An electric power system according to claim 8, characterized in that said high-voltage dc/ac power- electronic power converter (8) is designed with a dc/dc converter (10) for voltage matching to the high-voltage dc/ac power-electronic power converter.

17. An electric power system according to claim 16, characterized in that said dc/dc converter (10) for voltage matching to the high-voltage dc/ac power- electronic power converter (8) is pulsewidth-modulated.

18. An electric power system according to claim 8, characterized in that said high-voltage dc/ac power- electronic power converter (8) is designed with a dc/dc converter (11) with an insulating ac intermediate link, for voltage matching to the high-voltage dc/ac power- electronic power converter.

19. An electric power system according to claim 18, characterized in that said dc/dc converter (11) for voltage matching to the high-voltage dc/ac power- electronic power converter (8) is pulsewidth-modulated.

20. An electric power system according to claim 1, characterized in that the ac machines (3a..3n) are designed for a nominal phase voltage of 1-10 kV.

21. An electric power system according to claim 1, characterized in that the ac machines (3a..3n) are insulated for at least 40 kV towards ground.

22. An electric power system according to claim 8, characterized^* in that said dc/ac power-electronic power converter (8) is designed for a nominal dc voltage of at least +/- 20 kV and preferably for +/- 50 kV.

23. An electric power system according to claim 1, characterized in that the series link is high-ohmic- grounded at one point .

24. An electric power system according to claim 7, characterized in that the power transformer (9) is high- ohmic-grounded at its neutral point.

25. An electric power system according to claim 1, characterized in that the ac machines (3a..3n) are wound with an extruded HVDC power cable .

26. An electric power system according to claim 1, characterized in that the ac machines (3a..3n) are wound with a MIND power cable.

27. An electric power system according to any of claims 13 and 15, characterized in that the insulated ac intermediate links comprise transformers.

28. An electric power system according to claim 23, characterized in that the transformers are wound with an extruded HVDC power cable.

29. An electric_^ power system according to claim 23, characterized in that the transformers are wound with a extruded MIND cable.

30. An electric power system according to claim 23, • characterized in that the nominal operating frequency of the insulated ac intermediate links is greater than 60 Hz.

31. Use of an extruded HVDC power cable as ac winding in an ac machine/transformer and as HVDC transmission line in an electric power system according to claim 1.

32. Use of a MIND cable as ac winding in an ac machine/transformer and as HVDC transmission line in an electric power system according to claim 1.

33. Use of an extruded HVDC power cable at a high dc potential relative to ground in ac-fed windings of electric machines/transformers , the magnetic flux carriers of which are at ground potential.

34. Use of a MIND cable at a high dc potential relative to ground in ac-fed windings of electric machines/transformers, the magnetic flux carriers of which are at ground potential.

35. A method for winding an ac winding in an electric machine/transformer, the magnetic flux carrier of which is intended, during operation, to be at ground potential and which winding is intended, during operation, to be at a high dc potential relative to ground, characterized in that the winding is wound with an HVDC power cable.

36. A method according to claim 35, characterized in that the HVDC cable is provided with an insulation system consisting of an inner and an outer semiconductive layer and with an intermediate layer of solid extruded PE material .

37. A method according to claim 36, characterized in that said intermediate layer of solid extruded PE material is formed on the basis of a polymeric material.

38. A method according to claim 37, characterized in that said intermediate layer of solid extruded PE material is formed on the basis of a polymeric, crosslinked material.

39. A method according to claim 35, characterized in that the HNDC cable is provided with an oil-based insulation system.

40. A method according to claim 39, characterized in that the HVDC cable is provided with a paper insulation impregnated with viscous oil.