Use of HVDC-insulated conductor in magnetic flux carriers TECHNICAL FIELD The present invention relates to the use of insulated electric conductors at a high dc potential relative to ground in ac-fed windings of electric machines/- electromagnetic circuits, the magnetic flux carriers of which lie at ground potential. In this context, the insulation of the conductor is stressed by -an ac voltage generated or induced in the electric machine/the electromagnetic circuit, and by -an HVDC potential, relative to ground potential, which is caused by the connection of the electric machine/the electromagnetic circuit to a surrounding electric power network.

In this context, the HVDC stress is at least of the same order of magnitude as that ac stress which is associated with the generated or induced ac voltage.

The HVDC-insulated conductor may be designed with a solid insulant with an inner and an outer screen, or be a so- called MIND cable, that is, a cable which has a paper insu- lation or the like, impregnated with viscous oil. Both the high-voltage solid insulated conductor and the MIND cable will be referred to below as an HVDC cable.

In known electric power applications with currently used electric machines/electromagnetic circuits and solid insu- lants, the ac stress has so far essentially dimensioned the insulation of the high-voltage insulated conductor.

BACKGROUND ART, THE PROBLEMS In this context, ac electric machines/electromagnetic circuits comprise

-rotating electric machines such as motors and genera- tors, and -static electric machines such as (power) transformers and (power) inductors.

Both the rotating and the static electric machines comprise two parts, essentially different in character, in the form of -flux-carrying parts such as a stator/rotor core and a transformer/inductor core, respectively, which have a low magnetic resistance, reluctance, to magnetic flux, and -current-carrying conductors which have a low electric resistance to electric current.

The current-carrying conductors which carry alternating current have hitherto been designed essentially for high voltage with an oil-based insulation systems or with an insulation in solid form, which is normally stressed by a pure ac voltage relative to the flux-carrying parts which are at ground potential.

In certain contexts, however, the current-carrying conduc- tors of the above-mentioned rotating and static electric machines may be stressed both by a high ac voltage and a high dc voltage by means of various connections with con- verters. The problem which the invention is intended to solve is to create a reliable and environmentally friendly solution, which requires little maintenance, for electric machines in this connection.

Rotating electric machines, such as motors and generators, comprise a stator and a rotor. The stator is a stationary part which essentially consists of a magnetic core composed of laminated sheet, and of an ac winding arranged in slots in the core. The rotating rotor is arranged in a cavity shaped as a cylinder and consists essentially of a magnetic core, composed of laminated sheet or solid iron, and of an ac or dc winding arranged in slots in the core. In the ac

winding of the stator an ac electromotive force, emf, is generated, which, when being connected to the surrounding electric power network of the rotating electric machine, stresses the ac winding by means of an ac voltage, the magnitude of which is essentially determined by the mentioned emf.

Static electric machines, such as (power) transformers and (power) inductors, as far as their electrical function is concerned, are integrated into a stationary piece of equip- ment which essentially consists of a magnetic core, compo- sed of laminated sheet, and -for (power) transformers, of at least two ac windings arranged in so-called windows in the core. In one of the ac windings, when the static electric machine is connected to a surrounding electric power network, an ac voltage is induced which, by magnetic connection to the other ac winding, induces an additional ac voltage in this winding, and -for (power) inductors, of at least one winding arranged in a so-called window in the core. If this winding is an ac winding, then, when connecting the electric power network surrounding the static electric machine, an ac voltage is induced, and an alternating current flows in the winding. If this winding is a dc winding, the direct current in question is driven stationarily by a very small dc voltage across the winding/the (power) inductor, whereas a large ac voltage component may be superimposed on the dc winding.

For static electric machines, the ac voltages used are of magnitudes which are essentially determined by the ac vol- tage of the above-mentioned electric power network.

Ac electric machines/electromagnetic circuits of the above- mentioned kind are used in power-electronic systems, that is, systems which comprise converters built up with power semiconductors as thyristors, diodes, power transistors etc. Series connection of converters is often utilized

-to reduce the negative effect of the converters as far as reactive power or harmonics in the electric power network are concerned, or -to create the high-voltage levels which are required for transmitting or distributing electric power over major or minor distances, or -to interconnect power networks with equal or different frequencies by means of more or less controllable connection links.

Power plants and large motor drives for variable speed are designed with so-called full-power 12-pulse converters to obtain low harmonics in the ac power supply network. 12- pulse-connected converters are designed either with -a series connection of two 6-pulse converters, whereby the ac windings of the stator are stressed by an ac voltage and by a dc component, or with -a parallel connection of two 6-pulse converters, whereby the ac windings of the stator are stressed by a pure ac voltage.

High-voltage direct current transmission by means of con- verters, HVDC transmission, is utilized for transmission of electric power, with direct current and dc voltage, between various points in the same, synchronously or asynchron- ously, operating electric power network. Static electric machines, such as (power) transformers and (power) inductors, are utilized for feeding ac power to the ac connections of the converters and/or for arranging filters, which reduce the negative effect of the converters on electric power supply networks and/or on adjoining telephony equipment. Examples of such connections with so- called converter transformers are disclosed, inter alia, in WO 97/45907"Rotating Electrical Machines".

In this connection the following applies: -the mentioned (power) transformers connect ac power to the ac connections of the converters which are at HVDC

potential relative to ground by connecting the dc connections of the converters in series, and -the mentioned (power) inductors may be connected as series and/or shunt elements into the dc connection with a high HVDC potential relative to ground.

Great pains have been taken, in oil-based insulation sys- tems, to deal with the HVDC potential which thus arises in the dielectric insulation system which is formed by the ac windings, oil and grounded casings. To take up the high- voltage HVDC potential, to which the so-called converter transformers are subjected, these are normally designed as oil-insulated (power) transformers.

The cables which have been used for HVDC transmission, both on land and as cables disposed on the seabed, are of two types. One is a so-called MIND cable which is a cable in which the conductor has a paper insulation impregnated with viscous oil (Mass Impregnated Non Drained). The other cable is a paper-insulated oil-filled cable. This cable is useful for both ac transmission and HVDC transmission. These cables are described in a number of publications, for example in a brochure Submarine Power Cables, ABB SEHVC M-012E from ABB High Voltage Cables, and in the cable patents and other publications mentioned below.

For several decades there have been ac transmission and distribution cables which have been insulated with a layer of solid extruded XLPE, provided with an inner and an outer semiconductive layer. Insulated conductors of this design have been used in rotating ac machines, for example according to WO 97/45919,"Rotating electric machines with magnetic circuit for high voltage". This document describes high-voltage rotating electric machines for transformerless connection to an ac power network. Such insulated conductors have also been used in connection with transformers, for example in WO 97/45847,

"Transformer, Reactor", which describes a transformer with an oil-free ac insulation.

There have been discussions to replace the above-mentioned HVDC transmission cables by cables which have the same insulation system as cables with the inner and outer semiconductive layers described above and with an inter- mediate 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.

Recently, a number of important improvements in the insulation field, and then especially in systems with extruded insulation, have been made. The result of these has been that it is nowadays possible to manufacture cables with a capacity to withstand a high dc potential.

The improvements have been documented in various publi- cations such as, for example,"Development of materials for extruded DC power cables"by Nilsson, et. al., published at ICC Fall Meeting 1999, and in an article by Boström, et. al."Cross-linked polyethylene materials for dc power cables", published at Fifth International Con- ference on Insulated Power Cables, JICABLE'99, Paper C2.12.

Cables for high-voltage dc transmission are described in, inter alia,"Extruded DC power cables and accessories for use in HVDC transmission systems'by Carstensen, Johannesson and Gustafsson, published at ICC Fall Meeting 1999, and in"The development of an extruded HVDC cable system and its first application in the Gotland HVDC Light project"by Bygget, Johannesson, Liljegren, Palmqvist, Axelsson, Jonsson and Tornkvist, published at the Fifth

International Conference on Insulated Power Cables, JICABLE'99, Paper B7.5.

Cable accessories for jointing and terminating, that is, transition between different cables, and from PEX/XLPE cable to, for example, an overhead line, are very impor- tant parts in a high-voltage insulation system. These accessories are also described in the literature mentioned in the preceding paragraph.

One of the first installations with transistor-equipped converters is disclosed, inter alia, in"Small scale trans- mission to AC networks by HVDC Light"by Eriksson, Jonsson and Tollerz, published at the 12th Cepsi Conference in Pattaya, Thailand, Nov. 1999.

An HVDC transmission for transmission of the power from a wind part with a high dc voltage is described in PCT/SE99/00943,"Wind power plant". 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 an HVDC 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.

Dc-dc conversion may be performed with an intermediate high-frequency ac link in the form of a coaxial trans- former arrangement, for example according to the patent documents DE19802760, EP0932168, or according to DE19717554, EP0874377. These patent documents relate to dc-dc converters which are used for track-bound traction.

A dc-dc converter is also described in"Coaxially wound transformers for high-power high-frequency applications" by Kheraluwala, Novotny and Divan, IEEE Trans. Power

Electronics, Vol. 7, No. 1, Jan. 1992, pp. 54-62.

Throughout, the voltages and the powers are much lower in these publications than the levels which applications of dc-dc converters in accordance with the invention are intended to cover.

WO 91/97807,"DC/DC Power Transformer", discloses a dc-dc transformation of electric dc power in the megawatt region.

The patent describes neither the choice of frequency nor dc stresses on the insulation system for its internal ac transformers or dc inductors. It is therefore assumed that the relevant ac transformers and dc inductors are oil- insulated with advanced voltage distribution in the dielec- tric.

To sum up, it can be stated that, for ac machines, no solution has been presented for dc-insulating their high- voltage ac windings, normally located in the stator. The problem has been solved by parallel-connected 12-pulse connections. As far as (power) transformers and (power) inductors are concerned, oil-based dc insulation has been used during the last few decades, with the ensuing risk of environmental influence caused by the (transformer) oil.

The research and development around extruded ac cables has been intensified during the last few years and has pro- gressed so far that the first HVDC transmissions have been taken into service with extruded HVDC cable. Further, as mentioned by way of introduction as the problem intended to be solved by the invention, there is a clear need, in connection with converter-based solutions for variable speed and/or high-voltage HVDC transmission, to be able to have ac windings in ac machines or in (power) transformes with a high dc potential relative to ground. Also in connection with ac and dc inductors, filter circuits are often utilized for converter connections where the conduc- tor in the winding of the inductor may be subjected to a high dc potential relative to ground.

SUMMARY OF THE INVENTION, ADVANTAGES One object of the invention is to provide electric machines/electromagnetic circuits, comprising magnetic flux carriers at ground potential and windings with insulated conductors at a high-voltage dc potential relative to ground, with such a high insulation level that there is a minimum risk of flashover between conductors and ground. By high-voltage dc potential in this context is meant that the cable is subjected to a dc stress which is at least of the same order of magnitude as the ac stress which is associa- ted with the generated or induced ac voltage.

According to the invention, this object is achieved by using such HVDC-insulated cables and conductors, respec- tively, as have been described above, in the windings of the electric machines/the electromagnetic circuits. The mentioned cables, that is, the MIND cables, the paper- insulated oil-filled cables, and the recently developed conductors with extruded HVDC insulation with inner and outer semiconductive layers which are included in the above-mentioned extruded cables for HVDC transmission, shall now, according to the invention, be used in the form of windings in flux carriers at ground potential where the insulated conductor may be at a high-voltage dc potential relative to ground.

By using this technique, having a capacity to withstand a high dc potential, in windings in magnetic flux carriers, advantages may be obtained both in rotating and stationary electric machines. The technique may be used in systems which have previously required costly and unwieldy designs. An example of this is that oil-insulated conver- ter transformers may be replaced by so-called dry trans- formers. The technique also implies that new system solu- tions may be used, for example the possibility of series- connecting, instead of parallel-connecting, wind power stations and/or other alternative energy sources for HVDC

transmission to consumers far away from the energy sources. In US 4,057,736,"Electrical Power Generation and Distribution System", it has been made sure that each one of the series-connected electricity-generating machines are"being supported on insulative structure".

The great advantage of using the above-mentioned HVDC- insulated conductor with solid extruded insulation and an inner and an outer semiconductive screen and the above- mentioned oil-insulated cables, respectively, is that there is no electric field outside the outer semicon- ductor. The electric field achieved by the electric conductor only occurs in the solid insulation and in the oil insulation, respectively. These are the same advan- tages as those which are afforded by the solid extruded cable which is used in the above-mentioned WO 97/45919, "Rotating electric machines with magnetic circuit for high voltage"and in WO 97/45847,"Transformer, Reactor". Use of the HVDC-insulated conductors/cables referred to thus implies that both the windings and the core of magnetic flux carriers, as transformers and inductors, which may be exposed to a high dc potential may be designed without taking into account any electric field distribution, provided that the outer semiconductive screen is grounded in a reliable manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The accompanying drawing shows an example of a possible use, according to the invention, of HVDC-insulated cables and conductors, respectively, in magnetic flux carriers.

The figure relates to a wind power plant in which the invention may advantageously be used.

A wind power plant may comprise a plurality of wind power stations, in the figure exemplified as wind power stations 1,.., K,.., L,.., M,.., N etc. Otherwise, the

figure shows how the different wind power stations in a wind power plant may be designed. Why they are equipped in different ways and how these different pieces of equipment operate will not be described in this document. A more detailed explanation of this is given, inter alia, in Swedish patent application No. 9904740-9,"Electric power system based on renewable energy sources", filed con- currently herewith.

All the wind power stations according to the figure are equipped with a wind turbine 2a.. 2n and an ac machine 3a .. 3n. Converters 4a.. 4n are connected to the ac machines, either directly or via a gear. One or more of the wind power stations (K) may be provided with a dc-dc converter 5k."L"indicates that a plurality of wind power stations of different designs may be connected into the series link. One or more of the wind power stations (N) may also be provided with a dc-dc converter 6n to be able galvanically to separate the inputs and outputs of the converter. Such dc-dc converters normally comprise a transformer arrangement. One or more of the wind power stations M may be provided with a transformer 13m between the ac machine 3m and the converter 4m. The figure other- wise shows how the accumulated series-connected electri- city production of the wind power stations may be trans- mitted via HVDC cables 7'and/or overhead lines 7''to a converter station. This comprises converters 8'and 8'' for inversion of the transmitted HVDC power and power transformers 9'and 9''for feeding a 3-phase distribution and transmission network. In addition, the converter station may comprise inductors 16'and 16''for smoothing as well as filters 17'and 17''of various kinds.

It is obvious that, in a wind power plant according to the figure, that is, with series-connected wind power stations, windings in electric machines and electromagne- tic circuits, such as inductors and filters, will be sub- jected to a high dc potential relative to ground. To

protect the plant against insulation flashover, measures must thus be taken which prevent a breakdown of the insu- lation in machines and other components included. Other parts of such a plant, such as converters, breakers, protection devices and so on, may, of course, also be subjected to a high dc potential. The design of insulation protection for these parts is outside the scope of this invention.

Now, for manufacture of windings in those of the flux- carrying parts of the plant which are subjected to a high dc potential, that is, windings in the ac machines 3a..

3n, windings in those dc-dc converters which permit gal- vanically separated inputs and outputs, windings in inductors and the filter arrangements which are integrated in the converter station as well as windings in the power transformers 9'and 9'', the invention comprises using some of the above-mentioned HVDC-insulated cables/conductors. In the accompanying figure, the short arrows show the electric machines/electromagnetic circuits with magnetically flux-carrier-based components and conductors which are subjected to a high dc potential.

Embodiments of electric machines/electromagnetic circuits in which HVDC-insulated cables/conductors are used as windings in the flux-carrying parts will preferably be used, according to the invention, in connection with power-electronic systems, that is, where converters with thyristors, diodes, power transistors, etc., are included.