Note: Descriptions are shown in the official language in which they were submitted.
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Device and method relating to protection of an object against over-currents
comprising over-current reduction and current limitation.
Applicant: ASEA BROWN BOVERI AB
Device and method relating to pra~tection of an object against
over-currents comprising over-current reduction and current
limitation.
FILED OF THE INVENTION AND PRIOR ART
This invention is related to a device in an electric power
plant for protection of an object: connected to an electric
power network or another equipment in the electric power
plant from fault-related over-currents, the device com
prising a switching device in a line between the object
and the network/equipment. In addition, the invention
includes a method for protecting the object from over
currents.
The electric object in question is preferably formed by a
rotating electric machine having' a magnetic circuit, for
instance a generator, motor (both synchronous and asyn-
chronous motors are included) o:r synchronous compensator
requiring protection against fault-related over-currents,
i.e. in practice short-circuii: currents. As will be
discussed in more detail hereunder, the structure of the
rotating electric machine may bE: based upon conventional
as well as non-conventional technique.
The present invention is intended to be applied in connec-
tion with medium or high voltagE~. According to IEC norm,
medium voltage refers to 1-72,5 kV whereas high voltage is
>72,5 kV. Thus, transmission, sub-transmission and distri-
bution levels are included.
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In prior power plants of this nature one has resorted to,
for protection of the object in question, a conventional
circuit-breaker (switching device) of such a design that
it provides galvanic separation on breaking. Since this
circuit breaker must be designed to be able to break very
high currents and voltages, it will obtain a comparatively
bulky design with large inertia, which reflects itself in
a comparatively long break-time. It is pointed out that
the over-current primarily intended is the short-circuit
current occurring in connection with the protected object,
for instance as a consequence of faults in the electric
insulation system of the protected object. Such faults
means that the fault current (short-circuit current) of
the external network/equipment will tend to flow through
the arc created in the object. The result may be a very
large breakdown. It may be mentioned that for the Swedish
power network, the dimensioning short-circuit
current/fault-current is 63 kA. In reality, the short-
circuit current may amount to 40-50 kA.
A problem with said circuit-breaker is the long-break time
thereof. The dimensioning break-time (IEC-norm) for com-
pletely accomplished breaking is 150 milliseconds (ms). It
is associated to difficulties to reduce this break-time to
less than 50-130 ms depending upon the actual case. The
consequence thereof is that when there is a fault in the
protected object, a very high current will flow through
the same during the entire time required for actuating the
circuit-breaker to break. During this time the full fault
current of the external power network involves a consider-
able load on the protected object. In order to avoid dam-
age and complete breakdown with respect to the protected
object, one has, according to the prior art, constructed
the object so that it manages, without appreciable damage,
to be subjected to the short-circuit current/fault current
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during the break-time of the circuit breaker. It is
pointed out that a short-circuit current (fault current)
in the protected object may be composed of the own
contribution of the object to lthe fault current and the
current addition emanating from the network/eguipment. The
own contribution of the object to the fault current is not
influenced by the functioning of the circuit-breaker but
the contribution to the fault: current from the net-
work/equipment depends upon the operation of the circuit
breaker. The requirement for constructing the protected
object so that it may withstand a high short-circuit cur-
rent/fault current during a considerable time period means
substantial disadvantages in tree form of more expensive
design and reduced performance.
The rotating electric machine:a intended here comprise
synchronous machines mainly used. as generators for connec-
tion to distribution and transmission networks collec-
tively denoted power networks hereunder. The synchronous
machines are also used as motors and for phase compensa-
tion and voltage regulation, then as mechanically idling
machines. The technical field also comprises double-fed
machines, asynchronous converter cascades, external pole
machines and synchronous flux machines.
The magnetic circuit referred to in this context may be
air-wound but may also comprise: a magnetic core of lami-
nated, normal or oriented, sheet or other, for example
amorphous or powder based, material, or any other action
for the purpose of allowing an alternating flux, a wind-
ing, a cooling system etc., and may be disposed in the
stator or the rotor of the machine, or in both.
It is, according to the invention, primarily the intention
to protect a non-conventional rotating electric machine
for direct connection to all kinds of high voltage power
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networks. Such a machine has its magnetic circuit designed
with a threaded conductor, which is insulated with a solid
insulation and in which earth has been incorporated.
In order to be able to explain and describe the non-con-
ventional machine, a brief description of a rotating elec-
tric machine will first be given, exemplified on the basis
of a synchronous machine. The first part of the
description substantially relates to the magnetic circuit
of such a machine and how it is constructed according to
classical technique. Since the magnetic circuit referred
to in most cases is located in the stator, the magnetic
circuit below will normally be described as a stator with
a laminated sheet metal core, the winding of which will be
referred to as a stator winding and slots arranged for the
winding in the laminated core will be referred to as
stator slots or simply slots.
Most synchronous machines have a field winding in the
rotor, where the main flux is generated by direct current,
and an AC winding in the stator. The synchronous machines
are normally of three-phase design and the invention
mainly relates to such machines. Sometimes the synchronous
machines are designed with salient poles. However, cylin-
drical rotors are used for two- or four-pole turbo genera-
tors and for double-fed machines. The latter have an AC
winding in the rotor and this may be designed for the
voltage levels of the power network.
The stator body for large synchronous machines are often
made of sheet steel with a welded construction. The lami-
nated core is normally made from varnished 0.35 or 0.5 mm
electric sheet. For radial ventilation and cooling, the
laminated core is, at least for medium size and large
machines divided into packages with radial or axial ven-
tilation channels. For larger machines, the sheet is
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punched into segments, which arE: attached to the stator
body by means of wedges/dovetails. The laminated core is
retained by pressure fingers and pressure plates. The
stator winding is located in slots in the laminated core
5 and the slots have, as a rule, a cross section as a rec-
tangle or as a trapetzoid.
Polyphase AC windings are designed either as single layer
or two-layer windings. In the case of single-layer wind-
ings, there is only one coil side per slot, and in the
case of two-layer windings there: are two coil sides per
slot. By coil side is meant one or more conductors brought
together in height and/or width and provided with a common
coil insulation, i.a. an insulation intended to withstand
the rated voltage of the machinE= relative to earth. Two
layer windings are usually designed as diamond windings,
whereas the single-layer windings, which are relevant in
this connection may be designed as diamond windings or as
a flat winding. In the case of a diamond winding, only one
coil span (or possibly two coil spans) occurs, whereas
flat windings are designed as concentric windings, i.e.
with a greatly varying coil span. By coil span is meant
the distance in circular measure between two coil sides
belonging to the same coil, either in relation to the
relevant pole pitch or in the number of intermediate slot
pitches. Usually different variants of chording are used,
for example fractional pitch, 1to give the winding the
desired properties.
The type of winding substantially describes how the coils
in the slots, that is the coil sides, are connected to-
gether outside the stator, that is at the coil ends. A
typical coil side is formed by :>o called Roebel bars, in
which certain of the bars have been made hollow for a
coolant. A Roebel bar comprises a plurality of rectangu-
lar, parallel connected copper' conductors, which are
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transposed 360 degrees along the slot. Ringland bars with
transpositions of 540 degrees and other transpositions
also occur. The transposition is necessary to avoid circu-
lating currents. Between each strand there is a thin insu-
lation, e.g. epoxy/glass fibre. The main insulation be-
tween the slot and the conductors is made, e.g., of ep-
oxy/glass fibre/mica and has externally a thin semicon-
ducting earth potential layer used for equalizing the
electrical field. Externally of the sheet stack, one does
not have any outer semiconducting earth potential layer,
but an electric field control in the form of so called
corona protection varnish intended to convert a radial
field into an axial field, which means that the insulation
on the coil ends occurs at a high potential relative to
ground. The field control is a problem which sometimes
gives rise to corona in the coil-end region, which may be
destructive.
Normally all large machines are designed with a two-layer
winding and equally large coils. Each coil is placed with
one side in one of the layers and the other side in the
other layer. This means that also coils cross each other
in the coil-end. If more than two layers are used, these
crossings render the winding work difficult and deterio
rate the coil-end.
What has been stated above may be said to belong to clas-
sical technique when it comes to the rotating electrical
machines in view.
During the last decades, there have been increasing re-
quirements for rotating electric machines for higher volt-
ages than what has previously been possible to design and
produce. The maximum voltage level which, according to the
state of the art, has been possible to achieve for syn-
chronous machines with a good yield in the coil production
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is around 25-30 kV. It is also generally known that con-
nection of a synchronous machi.ne/generator to a power
network must be made via a 0/Y-connected so called step-up
transformer, since the voltage of the power network nor-
orally lies at a higher level than the voltage of the ro-
tating electric machine. Together with a synchronous ma-
chine, this transformer thus constitutes integrated parts
of a plant. The transformer constitutes an extra cost and
also entails the disadvantage that the total efficiency of
the system is lowered. If it were possible to manufacture
machines for considerably higher voltages, the step-up
transformer could thus be omitted.
Certain attempts to a new approach as regards the design
of synchronous machines are described, inter alia, in an
article entitled "Water-and-oil-cooled Turbogenerator TVM-
300" in J. Elektrotechnika, No. 1, 1970, pp 6-8, in US 4
429 244 "Stator of generator" and in the Russian patent
document CCCP patent 955369.
The water- and oil-cooled synchronous machine described in
J. Elektroteknika is intended for voltages up to 20 kV.
The article describes a new in~;ulation system consisting
of oil/paper insulation, which makes it possible to emerse
the stator completely in oil. The oil can then be used as
a coolant while at the same time using it as insulation.
To prevent oil in the stator from leaking out towards the
rotor, a dielectric oil-separating ring is provided at the
internal surface of the core. The stator winding is made
from conductors with an oval hallow shape provided with
oil and paper insulation. The coil sides with their insu-
lation are secured in the slots made with rectangular
cross section by means of wedges. As coolant oil is used
both in the hollow conductors and in holes in the stator
walls. Such cooling systems, however, entail a large num-
ber of connections of both oi.l and electricity at the
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coil-ends. The thick insulation also entails an increased
radius of curvature of the conductors, which in turn re-
sults in an increased size of the winding overhang.
The above mentioned US patent relates to the stator part
of a synchronous machine which comprises a magnetic core
of laminated sheet with trapetsoidal slots for the stator
winding. The slots are tapered since the need of insula-
tion of the stator winding is smaller towards the interior
of the rotor where that part of the winding which is lo-
cated nearest the neutral point is located. In addition,
the stator part comprises a dielectric oil-separating
cylinder nearest the inner surface of the core. This part
may increase the magnetization requirement relative to a
machine without this ring. The stator winding is made of
oil-immersed cables with the same diameter for each coil
layer. The layers are separated from each other by means
of spacers in the slots and secured by wedges. What is
special for the winding is that it comprises two so called
half-windings connected in series. One of the two half-
windings is located, centered, inside an insulating
sleeve. The conductors of the stator winding are cooled by
surrounding oil. A disadvantage with such a large quantity
of oil in the system is that the risk of leakage and the
considerable amount of cleaning work which may result from
a fault condition. Those parts of the insulating sleeve
which are located outside the slots have a cylindrical
part and a conical termination, the duty of which is to
control the electric field strength in the region where
the cable leaves the laminated core.
From CCCP 955369 it is clear, in another attempt to raise
the rated voltage of the synchronous machine, that the
oil-cooled stator winding comprises a conventional high-
voltage cable with the same dimension for all the layers.
The cable is placed in stator slots formed as circular,
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radially located openings corresponding to the cross sec-
tion area of the cable and the necessary space for fixing
and for coolant. The different radially located layers of
the winding are surrounded by and fixed in insulated
tubes. Insulating spacers fix 'the tubes in the stator
slot. Because of the oil-cooling, an internal dielectric
ring is also here needed for sealing the oil-coolant
against the internal air gap. The: structure shown does not
have any reduction of the insulation or of the stator
slots. The structure comprises a very thin radial waist
between different stator slots, which means a large slot
leakage flux which significantly influences the magnetiza-
tion requirement of the machine.
Machine designs according to t:he pieces of literature
accounted for mean that the electromagnetic material in
the stator is not used to an optimum. The stator teeth
should adjoin as closely to the casing of the coil sides
as possible from a magnetical point of view. It is highly
desirable to have a stator tooth having, at each radial
level, a maximum width since the width of the tooth af-
fects considerably the losses of the machine and, accord-
ingly, the need for magnetization. This is particularly
important for machines with higher voltage since the num-
ber of conductors per slot becomes large therein.
OBJECT OF THE INVENTION
The primary object of the present invention is to devise
ways to design the device and the method so as to achieve
better protection for the object and, accordingly, a
reduced load on the same, a fact which means that the
object itself does not have to be designed to withstand a
maximum of short-circuit currents/fault currents during
relatively long time periods.
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A secondary object with the invention is to design the
protection device and method such that an adequate protec-
tion is achieved for rotating electric machines, the de-
sign of which is based upon non-conventional design prin-
5 ciples, which may mean that the design does not have the
same resistance to fault-related over-currents, internal
as well as external, as the conventional machines of to-
day.
10 ~L1MMARY OF THE INVENTION
According to the invention, the object indicated above
is achieved in that the line between the object and the
switching device is connected to an overcurrent reducing
arrangement, which is actuatable for overcurrent reduc-
tion with assistance of an overcurrent conditions de-
tecting arrangement within a time period substantially
less than the break time of the switching device, and
that between the connection of the overcurrent reducing
arrangement to the line and the object, there is pro-
vided a current limiter.
Thus, the invention is based upon the principle not to
rely for breaking purposes only upon a switching device
which finally establishes galvanic separation, but in-
stead use a rapidly operating overcurrent reducing ar-
rangement, which, without effecting any real breaking of
the overcurrent, nevertheless reduces the same to such
an extent that the object under protection will be sub-
jected to substantially reduced strains and, accord-
ingly, a smaller amount of damage. The reduced overcur-
rent/fault current means, accordingly, that when the
switching device establishes galvanic separation, the
total energy injection into the protected object will
have been much smaller than in absence of the overcur-
rent reducing arrangement. Besides, there will a further
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reduction of the fault current flowing to (or from) the
object by means of the current limiter. Also the current
limiter is of such a nature that: it is rapidly operating
for current reduction to such an extent that the strains
imposed on the object will be dramatically reduced with-
out the current limiter having to effect any total
breaking of the overcurrent/fau7_t current.
According to a preferred embodiment of the invention,
the overcurrent reducing arrangEament is designed as com-
prising an overcurrent diverter for diversion of over-
currents to earth or otherwise another unit having a
lower potential than the network/equipment.
The current limiter according t:o the invention is suit-
ably based on current limitation by means of a constant
or variable inductance and/or resistance or other imped-
ance.
As is more closely defined in the claims, the invention is
applicable on rotating electric machines having magnetic
circuits designed by means of cable technology. These
machines may under certain cond_Ltions become sensitive to
electrical faults. Such a design may for instance be given
a lower impedance than what is considered conventional
today within the power field. 'This means a lower resis-
tance against fault-related over-currents than that pre-
sented by conventional machines of today. If the machines,
besides, have been designed from the start to operate with
a higher electrical voltage thar.~ the conventional machines
of today, the strain on the electrical insulation system
of the machine, caused by the resulting higher electrical
field, becomes, of course, greater. This means that the
machine may be more efficient, more economical, mechani-
cally lighter, more reliable, less expensive to use and
generally more economical than conventional machines, and
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the machine may manage without the usual connection to
other electromagnetic machines, but such a machine places
great demands on the electrical protection to eliminate,
or at least reduce, the consequences of a breakdown in the
machine in question. A combination of the protection de-
vice according to the invention and a rotating electric
machine designed in this way means, accordingly, an opti-
mization of the plant in its entirety.
The electric machine primarily intended with the invention
operates with such a high voltage that the ~/Y-connected
step-up transformer mentioned above may be omitted, i.e.
machines with a considerably higher voltage than machines
according to the state of the art is intended in order to
be able to perform direct connection to power networks at
all types of high voltage. This means considerably lower
investment costs for systems with a rotating electric
machine and the total efficiency of the system can be
increased.
A rotating electric machine according to the invention
entails a considerably reduced thermal stress on the sta-
tor. Temporary overloads of the machine does become less
critical and it will be possible to drive a machine at
overload for a longer period of time without running the
risk of damage arising. This means considerable advantages
for owners of power generating plants, who are forced
today, in case of operational disturbances, to rapidly
switch to other equipment in order to ensure the delivery
requirements laid down by law. With a rotating electric
machine of such a design here contemplated, the mainte-
nance costs can be significantly reduced because a trans-
former does not have to be included in the system for
connecting the machine to the power network.
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The invention also includes a synchronous compensator
directly connected to the power network.
To increase the power of a rotating electric machine, it
is known to attempt to increase the current in the AC
coils. This has been achieved b:y optimizing the quantity
of conducting material, that is by close-packing of rec-
tangular conductors in the rectangular rotor slots. The
aim has been to handle the increase in temperature result-
ing from this by increasing the quantity of insulating
material and using more temperature resistant and hence
more expensive insulating materials. The high temperature
and field load on the insulation have also caused problems
with the life of the insulation. In the relatively thick-
walled insulating layers which are used for high voltage
equipment, for example impregnated layers of mica tape,
partial discharges, pd, constitute a serious problem. When
manufacturing these insulating layers, cavities, pores and
the like, will easily arise, in which internal corona
discharges arise when the insulation is subjected to high
electric field strengths. These corona discharges gradu-
ally degrade the material and may lead to electric break-
down through the insulation.
In order to be able to increase the power of a rotating
electric machine in a technically and economically j usti-
fiable way, this must be achieved by ensuring that the
insulation is not broken down by the phenomena described
above. This can be achieved by means of an insulation
system produced so that the risk. for cavities and pores is
minimal. The insulation system about said at least one
current-carrying conductor included in the winding in
question comprises an electrically insulating layer of a
solid insulating material, about which there is arranged
an outer layer of a semiconducting material. An inner
layer of semiconducting material is arranged inwardly of
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the insulating layer. Said at least one conductor is ar-
ranged inwardly of the inner layer.
In order to obtain a good thermal resistance, it is pre-
y ferred that at least one of the inner and outer layers
have substantially equal coefficients of thermal expansion
as the insulating material. In practice, both layers and
the insulating material have substantially equal thermal
coefficients of expansion. This in combination with the
fact that the inner and outer layers are bonded relative
to the insulating material along substantially the entire
interface means that the insulating material as well as
inner and outer layers will form a monolithic part such
that defects due to different temperature expansion do not
occur. The electrical load on the insulation increases as
a consequence of the f act that the semiconducting layers
about the insulating material will form equipotential
surfaces meaning that the electrical field in the insulat-
ing material will be distributed evenly over the same. The
outer semiconducting layer is suitably connected to earth
potential or otherwise a low potential. This means that
for such a cable the outer layer about the insulating
material may be kept at earth potential for the whole
length of the cable.
The outer semiconducting layer may also be cut off at
suitable locations along the length of the conductor and
each cut-off partial length may be directly connected to
earth potential. Around the outer semiconducting layer
there may also be arranged other layers, casings and the
like, such as a metal shield and a protective mantle. A
further improvement of the invention is achieved by making
the coils and the slots, in which the coils are placed,
round instead of rectangular. By making the cross section
of the coils round, these will be surrounded by a constant
magnetic field without concentrations where magnetic sepa-
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ration may arise. Also the electric field in the coil will
be distributed evenly over the cross section and local
loads on the insulation are considerably reduced. In
addition, it is easier to place circular coils in slots in
5 such a way that the number of coil sides per coil group
may increase and an increase of i~he voltage may take place
without the current in the conductors having to be
increased.
10 Additional improvements may also be achieved by composing
the conductor from smaller part:, so called strands. The
strands may be insulated from each other and only a small
number of strands may be left uninsulated and in contact
with the inner semiconducting layer to ensure that this is
15 at the same potential as the conductor.
The outer semiconducting layer should present such elec-
trical properties that a potential equalization along the
conductor is ensured. However, the outer layer may not
present such conduction properties that a current will be
carried along the surface, which could give cause to
losses, which in turn could cau;~e undesired thermal load.
The inner semiconducting layer must have a sufficient
electrical conductivity to ensure potential equalization
and, accordingly, equalization of the electric field out
side the layer but this requires, on the other hand, that
the resistivity may not be too small. It is preferred that
the resistivity for the inner and outer layers is in the
range 10-6 S2cm - 100 kS2 cm, s>uitably 10-3 - 1000 SZcm,
preferably 1-500 ~cm.
The use of a cable of a flexible type for forming the
winding means that the winding work may occur by means of
a threading operation where the cable is threaded into the
openings of the slots in the magnetic cores.
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Since the outer semiconducting layer is connected to earth
potential or otherwise a relatively low potential, it will
essentially operate for enclosing the electrical field
inwardly of the layer. The use of an insulation system
comprising a solid insulation surrounded by inner and
outer semiconducting layers for enclosing the electrical
field in the insulation means a substantial improvement
compared with the prior art and eliminates entirely the
need for resorting to liquid or gaseous insulation
materials.
In order to master the problems occurring with direct
connection of rotating electric machines to all kinds of
high voltage power networks, a machine according to the
invention has a number of features, which substantially
distinguishes it from the prior art with respect to clas-
sical machine technology and the machine technology which
has been published during the last years:
- as has been mentioned, the winding is made of a cable
having one or more solidly insulated conductors with a
conducting layer around the insulation. A few typical
conductors of this kind is XLPE-cable (Cross linked poly-
ethylene) or a cable with EP-rubber insulation (EP - eth-
ylene-propylene); however, the cable must be further de-
veloped both as far as the strands of the conductor and as
far as the semiconducting layers are concerned
- cables are preferably used with a circular cross sec-
tion. However, in order to obtain a better packing den-
sity, cables with another cross section may be used
- use of such a cable allows the magnetic core to be de-
signed in a new and optimal manner according to the inven-
tion both with respect to slots and teeth
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- the winding is carried out with a trapped insulation for
the best possible use of the magnetic core
- the design of the slots is adapted to the cross section
of the cable of the winding in :such a way that the slots
are formed as a number of axially and radially outwardly
of each other extending cylindrical openings with an open
waist running between the layers of the stator winding
- the design of the grooves is adjusted to the cable cross
section in view
- the design of the slots is adapted to the trapped insu-
lation of the slots
- the development with respect -to the strands means that
the conductor of the winding consists of a number of lay-
ers combined with each other, ~~.e. not necessarily ade-
quately transposed with respect -to each other, of strands,
including both uninsulated and insulated strands
- the development with respect to the outer casing means
that the outer casing is cut off at suitable locations
along the length of the conductor and each cut-off partial
length is directly connected to Earth potential
- the winding is preferably carried out as a multi-layer
concentrical cable winding to decrease the number of coil-
end crossings.
These features involve a number of advantages relative to
machines according to the prior art:
- the trapped insulation means> that a nearly constant
tooth width may be used independently of the radial propa
gation
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18
- the use of such a cable means that the outer semicon-
ducting layer of the winding may be kept at earth poten-
tial along the whole length thereof
- an important advantage is that the electrical field is
near zero in the coil-end region outside the outer semi-
conducting layer and that the electrical field does not
have to be controlled when the layer is at earth poten-
tial. This means that one cannot get any field concentra-
tions, neither in the core, in coil-end regions nor in the
transition therebetween
- the mixture of insulated as well as uninsulated combined
strands and transposed strands alternatively involve low
additional costs.
To summarize, a rotating electric machine according to the
invention means a considerable number of important advan-
tages in relation to corresponding prior art machines.
First of all, the machine can be connected directly to a
power network at all types of high voltage. Another impor-
tant advantage is that earth potential has been consis-
tently conducted along the whole winding, which means that
the coil-end region can be made compact and that support
means in the coil-end region can be applied at practically
earth potential. Still another important advantage is that
oil-based insulation and cooling systems disappear. This
means that no sealing problems may arise and that the
dielectric ring previously mentioned is not needed. One
advantage is also that all forced cooling can be made at
ground potential. A considerable space and weight saving
from the installation point of view is obtained with a ro-
tating electric machine according to the invention, since
it replaces a previous insulation design with both a ma-
chine and a step-up transformer.
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19
Further advantages and features of the invention, particu-
larly with respect to the method according to the inven-
tion, appears from the following description and claims.
HRIEF DESCRIPTION OF THE DRAWING:i
With reference to the enclosed drawings, a more specific
description of an embodiment example of the invention
follows hereinafter.
In the drawings:
Fig 1 is a purely diagrammat:ical view illustrating the
basic aspects behind 'the solution according to
the invention,
Figs 2a-
2d are diagrams illustrating in a diagrammatical
form and in a comparative way fault current de
velopments and the energy development with and
without the protection device according to the
invention;
Fig 3 is a diagrammatical view illustrating a conceiv-
able design of a device according to the inven-
tion;
Figs 4-9 are views partly corresponding to Fig 3 of dif-
ferent alternative embodiment of the invention
with regard to the current limiter denoted 6;
Fig 10 is a diagrammatical view illustrating a possible
design of the overcurrent reducing arrangement;
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Fig 11 is a diagrammatical view illustrating the device
according to the invention applied in connec
tion with a power plant comprising a generator,
a transformer and a power network coupled
5 thereto;
Fig 12 illustrates parts contained in a cable intended
to form the winding for a magnetic circuit of a
rotating electric machine of a kind particularly
10 well suited to be protected by the protection
device according to the invention; and
Fig 13 illustrates in an axial end view an embodiment
of a sector/pole pitch of a magnetic circuit in
15 a rotating electric machine, for which the pro-
tection device according to the invention is
particularly well suited.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
An electric power plant comprising a protected object 1 is
shown in Fig 1. As is described hereunder, this object
could for instance consist of a generator. This object is
connected, via a line 2, to an external distribution net-
work 3. Instead of such a network, the unit denoted 3
could be formed by some other equipment contained in the
power plant. The power plant involved is conceived to be
of such a nature that it is the object 1 itself which
primarily is intended to be protected against fault cur-
rents from the network/equipment 3 when there occurs a
fault in the object 1 giving rise to a fault current from
the network/equipment 3 towards the object 1 so that the
fault current will flow through the object. Said fault may
consist in a short-circuit having been formed in the ob-
ject 1. A short-circuit is a conduction path, which is not
intended, between two or more points. The short-circuit
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21
may far instance consist of an arc. This short-circuit and
the resulting violent current flow may involve consider-
able damages and even a total break-down of the object 1.
It is already pointed out that with at least some types of
protected electrical objects 1, short-circuit cur-
rents/fault currents harmful to the object in question may
flow from the protected ob;,ject towards the net-
work/equipment 3. Within the scoF~e of the invention, it is
intended to be used for protect~.on purposes not only for
protection of the object from e:Kternally emanating fault
currents flowing towards the object but also from internal
fault currents in the objects flowing in the opposite
direction. This will be discussed in more detail in the
following.
In the following, the designation 3 will, to simplify the
description, always be mentioned. as consisting of an ex-
ternal power network. However, it should be kept in mind
that some other equipment may be. involved instead of such
a network, as long as said equipment causes violent cur-
rent flows through the object 1 when there is a fault.
A conventional circuit breaker 4 is arranged in the line 2
between the object 1 and the network 3. This circuit
breaker comprises at least one: own sensor for sensing
circumstances indicative of the fact that there is an
overcurrent flowing in the line 2. Such circumstances may
be currents/voltages but also other indicating that a
fault is at hand. For instance, the sensor may be an arc
sensor or a sensor recording short circuit sound etc. When
the sensor indicates that the overcurrent is above a cer-
tain level, the circuit breaker 4 is activated for break-
ing of the connection between the object 1 and the network
3. The circuit breaker 4 must, however, break the total
short circuit current/fault current. Thus, the circuit
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22
breaker must be designed to fulfil highly placed require-
ments, which in practice means that it will operate rela-
tively slowly. In Fig 2a it is illustrated in a cur-
rent/time-diagram that when a fault, for instance a short
circuit in the object 1, occurs at the time tfault~ the
fault current in the line denoted 2 in Fig 1 rapidly as-
sumes the magnitude il. This fault current il is broken by
means of the circuit breaker 4 at tl, which is at least
within 150 ms after tfault- Fig 2b illustrates the diagram
i2~t and, accordingly, the energy developed in the pro
tected object 1 as a consequence of the short circuit
therein. The energy injection into the object occurring as
a consequence of the short-circuit current is, accord
ingly, represented by the total area of the outer rectan
gle in Fig 2d.
It is in this connection pointed out that the fault cur-
rent in Figs 2a-c and the currents in Fig 2d represent the
envelope of the extreme value. Only one polarity has been
drawn out in the diagram for the sake of simplicity.
The circuit breaker 4 is of such a design that it estab-
lishes galvanic separation by separation of metallic con-
tacts. Accordingly, the circuit breaker 4 comprises, as a
rule, required auxiliary equipment for arc extinguishing.
According to the invention the line 2 between the object 1
and the switching device 4 is connected to an arrangement
reducing overcurrents towards the apparatus 1 and
generally denoted 5. The arrangement is actuatable for
overcurrent reduction with the assistance of an
overcurrent conditions detecting arrangement within a time
period substantially less than the break time of the
circuit breaker 4. This arrangement 5 is, accordingly,
designed such that it does not have to establish any
galvanic separation. Therefore, conditions are created to
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23
very rapidly establish a current reduction without having
to accomplish any total elimination of the current flowing
from the network 3 towards the protected object 1. Fig 2b
illustrates in contrast to the ease according to Fig 2a
that the overcurrent reducing arrangement 5 according to
the invention is activated upon occurrence of a short
circuit current at the time tfault for overcurrent
reduction to the level i2 at the time t2. The time
interval tfault-t2 represents, accordingly, the reaction
time of the overcurrent reducing arrangement 5. The task
of the arrangement 5 not to break but only reduce the
fault current, the arrangement may be caused to react
extremely rapidly, which will be discussed more closely
hereunder. As an example, it may be mentioned that current
reduction from the level il to th.e level i2 is intended to
be accomplished within one or a few ms after unacceptable
overcurrent conditions having been detected. It is then
aimed at to accomplish the current reduction in a shorter
time than 1 ms, and preferably more rapidly than 1
microsecond.
As appears from Fig 1, the device comprises a current
limiter generally denoted 6 and. arranged in the line 2
between the connection of the arrangement 5 to the line 2
and the object 1. This current limiter is adapted to oper-
ate for current limitation primarily in a direction to-
wards the object 1 but in certain fault cases also in a
direction away from the object. The current limiter 6 may
be arranged to be brought into operation for current limi-
tation as rapidly as or even more rapidly than the over-
current reducing arrangement 5. According to a further
alternative involving less strain on the current limiter
6, the current limiter could be designed to be activated
for current limitation not until the over-current from the
network 3 towards the object 1 has been reduced by means
of the over-current reducing arrangement 5, but of course
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24
the current limiter 6 should be brought to activity for
current limitation substantially more early than the time
when the circuit breaker 4 breaks. From that stated it
appears that it is suitable that the current limiter 6 is
coupled to the line 2 in such a way that it is the current
reduced by means of the over-current reducing arrangement
which in an even more reduced extent will flow through the
current limiter 6.
Fig 2b illustrates the action of the current limiter 6. In
said Figure it has been chosen to indicate that the cur-
rent limiter 6 enters into operation for current limita-
tion at the time t3, which in the example would mean that
the duration of the current i2 reduced by means of the
over-current reducing arrangement 5 has been substantially
limited, namely to the time span t2-t3. It is again
pointed out that the representations in Fig 2 are to be
considered as purely diagrammatical. The time t3, when the
current limiter 6 is activated, may be much earlier and
even earlier than the time for activation of the over-
current reducing arrangement 5 at the time t2. It appears
from Fig 2b that the fault current after the time t3 is
reduced to the level i3. This remaining fault current i3
is finally broken by means of the circuit breaker 4 at a
time tl. However, the fault current i3 is so comparatively
small as a consequence of adequate dimensioning of the
current limiter 6 that the fault current in question may
be endured by the object in question and also other parts
of the power plant. The consequence of the reduction and
limitation respectively of the fault current, which the
energy injection from the network 3 caused by said fault
current will give rise to, in the protected object 1 is
represented by the surfaces marked in Fig 2d with oblique
lines. It appears that a drastic reduction of the energy
injection is achieved. In this connection it is pointed
out that since, according to a specific model, the energy
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increases with the square of the: current, a reduction by
one half of the current reduces the energy injection to a
fourth. It is illustrated in Fig 2c that the fault current
will tend to flow through the device 5. That part i3 of
5 the total fault current il, which will continue to flow
through the current limiter 6 after the time t3 is also
marked in Fig 2c.
In reality, the dimensioning of the arrangement 5 and the
10 current limiter 6 is conceived to be carried out such that
the arrangement 5 reduces the fault current and the volt-
age to be restricted by means of the current limiter 6 to
substantially lower levels. A realistic activation time as
far as the current limiter 6 i;s concerned is 1 ms, the
15 dimensioning possibly being po:a ible to carry out such
that the current limiter 6 is caused to delimit the cur-
rent not until after the arrangement 5 has reduced the
current flowing through the limiter 6 to at least a sub-
stantial degree. As pointed out, this is not a requirement
20 but the opposite case would also be possible.
It is illustrated in more detai:L in Fig 3 how the device
may be realized. It is pointed out that the invention is
applicable in direct current (also HVDC - High Voltage
25 Direct Current) and alternating current connections. In a
multi phase arrangement with alternating current, the line
denoted 2 may be considered as farming one of the phases
in a multi phase alternating current system. However, it
should be noted that the device according to the invention
may be realized so that either all phases are subjected to
the protecting function according to the invention in case
of a detected error, or that only that or those phases
where a fault current is obtained is subjected to current
limitation.
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26
It appears from Fig 3 that the overcurrent reducing ar-
rangement generally denoted 5 comprises an overcurrent
diverter 7 for diverting overcurrents to earth 8 or other-
wise another unit having a lower potential than the net-
s work 3. Thus, the overcurrent diverter may be considered
as forming a current divider which rapidly establishes a
short circuit to earth or otherwise a low potential 8 for
the purpose of diverting at least a substantial part of
the current flowing in the line 2 so that said current
does not reach the object 1 to be protected. If there is a
serious fault in the object 1, for instance a short cir-
cuit, which is of the same magnitude as the short circuit
that the overcurrent diverter 7 is capable of establish-
ing, it may be said that generally speaking a reduction
to one half of the current flowing to the object 1 from
the network 3 is achieved as a consequence of the overcur-
rent diverter 7 in case the fault is close to the latter.
In comparison with Fig 2b, it appears, accordingly, that
the current level i2 illustrated therein and being indi-
sated to amount to approximatively half of il may be said
to represent the worst occurring case. Under normal condi-
tions, the purpose is that the overcurrent diverter 7
should be able to establish a short circuit having a bet-
ter conductivity than the one corresponding to the short
circuit fault in the object 1 to be protected so that
accordingly a main part of the fault current is diverted
to earth or otherwise a lower potential via the overcur-
rent diverter 7. It appears from this that, accordingly,
in a normal fault case, the energy injection into the
object 1 in case of a fault becomes substantially smaller
than that which is indicated in Fig 2d as a consequence of
lower current level i2 as well as shorter time span t2-t3.
The overcurrent diverter 7 comprises switch means coupled
between earth 8 or said lower potential and the line 2
between the object 1 and the network 3. This switch means
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27
comprises a control member 9 and a switch member 10. This
switch member may for instance be formed by at least one
semiconductor component, for instance a thyristor, which
is open in a normal state, i.e. isolating in relation to
earth, but via the control member 9 may be brought into an
active, conducting state in a very short time in order to
establish current reduction by diversion to earth.
Fig 3 also illustrates that an overcurrent conditions
detecting arrangement may compriae at least one and pref-
erably several sensors 11-13 suitable for detecting such
overcurrent situations requiring activation of the protec-
tion function. As also appears i:rom Fig 3, these sensors
may include the sensor denoted 13 located in the object 1
or in its vicinity. Furthermore, the detector arrangement
comprises a sensor 11 adapted to sense overcurrent condi-
tions in the line 2 upstreams of the connection of the
overcurrent reducing arrangement 5 and the line 2. As is
also explained in the following, it is suitable that a
further sensor 12 is provided to sense the current flowing
in the line 2 towards the object 1 to be protected, i . e.
the current which has been reduced by means of the over-
current reducing arrangement 5. '.Cn addition, it is pointed
out that the sensor 12, as well as possibly the sensor 13,
is capable of sensing the current: flowing in the line 2 in
a direction awav from the object 1, for instance in cases
where energy magnetically stored in the object 1 gives
rise to a current directed away i:rom the object 1.
It is pointed out that the sensors 11-13 do not necessar-
ily have to be constituted by only current and/or voltage
sensing sensors. Within the scope of the invention, the
sensors may be of such nature that they generally speaking
may sense any conditions indicative of the occurrence of a
fault of the nature requiring initiation of a protection
function.
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28
In cases where such a fault occurs that the fault current
will flow in a direction away from the object 1, the de-
vice is designed such that the control unit 14 thereof
will control the further breaker 6 to closing, in case it
would have been open, and, in addition, the overcurrent
reducing arrangement 5 is activated such that the short
circuit current may be diverted by means of the same.
When, for example, the object 1 is conceived to consist of
a transformer, the function on occurrence of a short cir-
cuit therein could be such that the short circuit first
gives rise to a violent flow of current into the transfor-
mator, which is detected and gives rise to activation of
the arrangement 5 for the purpose of current diversion.
When the current flowing towards the transformer 1 has
been reduced in a required degree, the current limiter 6
is caused to reduce the current, but, controlled by means
of the control unit 14, possibly not earlier than leaving
time for the energy, in occurring cases, magnetically
stored in the generator 1 to flow away from the generator
1 and be diverted via the arrangement 5.
Furthermore, the device comprises a control unit generally
denoted 14. This is connected to the sensors 11-13, to the
overcurrent reducing arrangement 5 and to the current
limiter 6. The operation is such that when the control
unit 14 via one or more of the sensors 11-13 receives
signals indicating occurrence of unacceptable fault cur-
rents towards the object 1, the overcurrent reducing ar-
rangement 5 is immediately controlled to rapidly provide
the required current reduction. The control unit 14 may be
arranged such that when the sensor 12 has sensed that the
current or voltage has been reduced to a sufficient de-
gree, it controls the current limiter 6 to obtain opera-
tion thereof for breaking when the overcurrent is below a
predetermined level. Such a design ensures that the cur-
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29
rent limiter 6 is not caused to limit the current until
the current really has been reduced to such a degree that
the current limiter 6 is not given the task to break such
a high current that it is not adequately dimensioned for
that purpose. However, the embodiment may alternatively
also be such that the current limiter 6 is controlled to
limit the current a certain predetermined time after the
overcurrent reducing arrangement having been controlled to
carry out current reduction.
The circuit breaker 4 may comprise a detector arrangement
of its own for detection of overcurrent situations or
otherwise the circuit breaker may be controlled via the
control unit 14 based upon information from the same sen-
sors 11-13 also controlling the operation of the overcur-
rent reducing arrangement.
In the embodiment illustrated in Fig 3 the current limiter
6 is formed by an inductance 27 provided in the line 2.
Such an inductance achieved by means of a coil has the
result that at a certain increase of the current, a back
electromotive force arises, which counteracts increase of
current. An advantage with this embodiment is that it is
extremely simple and furthermore, it gives rise to, when a
fault occurs, a rapid limitation of the current flow to-
wards the object 1 without need for active control.
As the device has been described until now, it operates in
the following way: In absence of a fault, the circuit
breaker is closed whereas the switch means 10 of the over-
current reducing arrangement 5 is open, i.e. in a non-
conductive state. In this situaltion the switch means 10
must, of course, have an adequate electrical strength so
that it is not unintentionally brought into a conducting
state. Over-voltage conditions appearing in the line 2 as
a consequence of athmospheric (lightning) circumstances or
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coupling measures may, thus, not cause the voltage
strength of the closing means 10 in its non-conducting
state to be exceeded. For this purpose, it is suitable to
couple at least one surge arrester 22 in parallel over the
5 switch means 10. In the example, such surge arresters are
illustrated on either side of the switch means 10. The
surge arresters have, accordingly, the purpose to divert
such over-voltages which otherwise could risk to cause
inadvertent breakthrough in the switch means 10.
When an over-current state has been registered by any of
the sensors 11-13 or the own sensor of the circuit breaker
4 (it is of course understood that information from the
own sensor of the circuit breaker 4 can be used as a basis
for control of the over-current reducing arrangement 5
according to the invention) and this over-current state is
of such magnitude that a serious fault of the object 1 can
be expected to be present, the breaking function is initi-
ated as far as the circuit breaker 4 is concerned. In
addition, the control unit 14 controls the over-current
reducing arrangement 5 to effect such reduction, and this
more closely by causing the switch means 10 into an elec-
trically conducting state via control member 9. As de-
scribed before, this may occur very rapidly, i.e. in a
fraction of the time required for breaking by the circuit
breaker 4, for what reason the object to be protected
immediately is relieved from the full short-circuit cur-
rent from the network 3 by the switch means 10 diverting
at least an important part and in practice the main part
of the current to earth or otherwise a lower potential.
The current limiter 6 may, as well, enter into a rapid
function to limit the current flowing into the line 2
towards (or possibly from) the object 1.
When these incidents have occurred, breaking is carried
out as the last measure by means of the circuit breaker 4.
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31
It is important to note that 'the over-current reducing
arrangement 5 as well as the current limiter 6 according
to a first embodiment are designed to be able to function
repeatedly. Thus, when it has bean established by means of
the sensors 11-13 that the circuit breaker 4 has closed
the switch means 10 is reset in-t:o a non-conducting state,
and the current limiter 6 is ready, so that the next time
the circuit breaker 4 closes, the protective device is in
a completely operational state. According to another em-
bodiment, the arrangement 5 may :require exchange of one or
more parts in order to operate again.
Fig 4 illustrates an alternative embodiment of the current
limiter 6a. This embodiment comprises an inductance 28 and
a capacitor 29, which form, in unison, a resonance cir-
cuit, which at resonance gives a very high impedance. The
inductance and the capacitor are: coupled parallel to each
other. A switch 30 and the capacitor 29 are coupled in
parallel over the inductance 28 placed in the line 2.
Accordingly, the switch 30 and the condensator 29 are
coupled inparallel over the inductance 28 placed in the
line 2. Accordingly, the switch 30 and the condensator 29
are placed in series with each other. The coupler 30 has
one or more contacts, which by means of a suitable operat-
ing member 31 may be controlled for closing or opening
respectively via the control unlit 14.
The current limiter 6a illustrated in Fig 4 operates in
the following way: during normal operational conditions,
the switch 30 is open. The impedance of the current lim-
iter 6a is given by the inductance and the resistance of
the inductor. In case of a fault current of a sufficient
magnitude, the control unit 14 will control the switch
means 10 for closing for the purpose of overcurrent di-
version and furthermore, the control unit 14 will con-
trol the switch 30 to closing such that the capacitor 29
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32
is coupled in and a parallel resonance circuit, which
should be adjusted to the power frequency, is formed.
The impedance of the current limiter 6a will be very
high at resonance. As is also apparent from a compara-
tive study of Fig 2b, a considerable current reduction
down to the drawn current level i3 is obtained.
In Fig 5 an alternative embodiment of the current lim-
iter 6b is shown, this embodiment being based upon a se-
ries resonance circuit comprising an inductance 32 and a
capacitor 33 in series with each other and a switch 34
coupled in parallel over the capacitor 33. An operating
member 35 for operating the contact or contacts of the
switch 34 is under control from the control unit 14.
During normal operation, the switch 34 over the capaci-
tor 33 is open. The coil 32 in series with the capacitor
33 in series resonance (at for example 50 Hz) has a very
small impedance. Transient fault currents are blocked by
the coil 32. In case of a fault, the voltage over the
capacitor as well as the inductance is increased. By
closing the switch 34 over the capacitor, the same is
shortcircuited. This involves a drastic increase of the
total impedance, for what reason the current is limited.
As is indicated in Fig 5, the inductance 32 may be made
variable, for instance by short-circuiting parts of the
winding or a winding located on the same core. In this
way it becomes possible to continuously adjust the cur-
rent limiter 6b to minimize the voltage drop over the
current limiter during normal load. Another modification
not shown in Fig 5 is to use a self-triggered spark gap
instead of the switch 34 over the capacitor 33. In this
way, a self-triggered function is achieved, i.e. the em-
bodiment becomes passive in the sense that no particular
control from any control unit is required.
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In the variant illustrated in F.ig 6, the current limiter
6c comprises a switch 36 arranged in the line 2 and in
parallel over this switch a capacitor 37 and a resistor
38, the capacitor and resistor :being coupled in parallel
relative to each other. The switch 36 has in reality the
character of a vacuum circuit breaker provided with
transversely directed coils 39 'to increase the arc volt-
age and achieve current commutation into the limiting
resistor 38. The control unit 7_4 is arranged to control
the switch 36 via an operating member 40.
Fig 7 illustrates a current li:miter 6d formed by a me-
chanical switch 41 having a cornmutation element 42 con-
sisting of a large number of series-connected arc cham-
hers. The arc chambers are made of a resistive material.
When the switch 41 opens, the arc short-circuits the re-
sistive arc chamber. When the arc moves into the arc
chamber, the arc is divided into many subarcs. In this
way the arcs are increasing then length of the resistive
path between the contacts and an increasing resistance
is achieved.
As before, the control unit 1~4 is arranged to control
the operation of the switch 41 via an operating member
43.
Fig 8 illustrates a further embodiment of a current lim-
iter 6e. This limiter comprisEas, in the embodiment, a
fast semiconductor switch 44 anal a parallel current-lim-
iting impedance 45 and a voltage-limiting element 46,
for instance a varistor. The semiconductor switch 44 may
be formed by means of gate turn-off thyristors (GTO
thyristors). A resistor is usEad as a current limiting
impedance. The varistor 46 limits the over-voltage when
the current is restricted. Under normal load conditions,
the current flows through the semiconductors 44. When a
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34
fault is detected, the semiconductor switch 44 is opened
under control via the control unit 14, preferably via a
suitable operating member 47, and the current is commu-
tated to the resistor 45.
Finally, a current limiter 6f is illustrated in Fig 9,
this limiter comprising a coil 48 connected in the line 2.
The coil 48 is included in a reactor having an iron core
49. Between the iron core 49 of the reactor and the coil
48 there is provided a superconducting tubular screen 50.
Under normal operation, the superconducting screen 50
screens-off the iron core from the coil, the inductance
thus being relatively low. When the current exceeds a
certain level, the superconduction ceases and the induc-
tance increases drastically. Thus, a strong current limi-
tation is obtained.
In the embodiment according to Fig 9, the screening of the
iron core from the coil occurs due to the Meissner-effect.
An advantage with the embodiment according to Fig 9 is, as
far as current limiter 6f is concerned, that a small in
ductance is at hand in normal operation. A disadvantage is
that in order to achieve superconduction, cooling to very
low temperatures, for instance by liquid nitrogen, is
required.
In all embodiments Figs 4-9 dust described, only the dif-
ferences with respect to the current limiter relative to
the design according to Fig 3 have been described more
closely. With respect to other constituents, the descrip-
tion relating to Fig 3 is referred to.
Fig 10 illustrates an alternative embodiment of the over-
current reducing arrangement 5. Instead of relying on a
semiconductor switch means as in Fig 3, the embodiment
according to Fig 10 is intended to involve causing of a
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medium present in a gap 24 between electrodes 23 to assume
electrical conductivity by means of a control member 9a.
This control member is arranged to control the operation
of members 25 for causing or at least initiating the me-
5 dium or a part thereof in the ~gap 24 into a conducting
state. Said member 25 is in the example arranged to cause
the medium in the gap 24 to assume electrical conductivity
by causing or at least assisting in causing the medium to
ionization/plasma. It is preferred that the members 25
10 comprise at least one laser, which by energy supply to the
medium in the gap 24 provides for the ionization. As ap-
pears from Fig 10, a mirror 26 rnay be used for necessary
diverting of the laser beam bundle. It is in this connec-
tion pointed out that the embodiment according to Fig 10
15 may be such that the means 25 do not alone give rise to
ionization/plasma in the entire electrode gap. Thus, the
intention may be that an electrical field imposed over the
gap should contribute in ionization/plasma formation, only
a part of the medium in the gap being ionized by means of
20 the members 25 so that thereafter the electrical field in
the gap gives rise to establishment of plasma in the en-
tire gap. It is in this connection pointed out that there
may be in the electrode gap not only a medium consisting
of various gases or gas mixture: but also vacuum. In the
25 case of vacuum, initiation by means of laser occurs at at
least one of the electrodes, which, accordingly, will
function as an electrone and ion transmitter for estab-
lishment of an ionized environmE:nt/a plasma in the elec-
trode gap.
Fig 11 illustrates a conventional embodiment in the sense
that a generator lb via a transformer la is coupled to a
power network 3a. The objects to be protected are, accord-
ingly, represented by the transformer la and the generator
1b. The over-current reducing arrangement 5a and the cur-
rent limiter 6g and the ordinary circuit breaker 4a are,
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36
as can be seen, arranged similar to what appears from Fig
1 for the case' that the object 1 shown therein is con-
ceived to form the object la according to Fig 11. Accord-
ingly, reference is in this regard made to the descrip-
tions delivered with respect to Fig 1. The same is due for
the protection function of the over-current reducing ar-
rangement 5c and the current limiter 6i with respect to
the generator lb. In this case, the generator lb could,
accordingly, be considered equivalent with the object 1 in
Fig 1 whereas the transformer la could be considered
equivalent to the equipment 3 in Fig 1. Thus, the over-
current reducing arrangement 5c and the current limiter 6i
will, in combination with the conventional circuit breaker
4b, be able to protect the generator lb against violent
flow of current in a direction away from the transformer
1a.
As an additional aspect in Fig 11, the additional over-
current reducing arrangement 5b with associated current
limiter 6h are present. As can be seen, there will be
over-current reducing arrangements 5a and 5b on either
side of the transformer la. It is then pointed out that
the current limiters 6g and 6i respectively are arranged
in the connections between said over-current reducing
arrangements 5a and 5b and the transformer la. The further
over-current reducing arrangement 5b is intended to pro-
tect the transformer la from current flows towards the
transformer from the generator lb. As can be seen, the
circuit breaker 4b will be able to break independently of
in which direction between the objects la and lb a protec-
tion function is desired.
With the assistance of Figs 12 and 13, an embodiment will
now be described which is "non-conventional" in contrast
to the one in Fig 11 in the sense that a rotating electric
machine with a magnetic circuit or high voltage is in-
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37
tended to be connectable directly to the high voltage
power network 3, 3a without any intermediate step-up
transformer.
An important condition for being able to manufacture a non-
conventional magnetic circuit is to use for the winding a
conductor cable with a solid electrical insulation with a
semiconducting layer both at the conductor and casing. Such
cables are available as standard cables for other power
engineering fields of use. As mentioned before, a further
developed embodiment of such a standard cable is used as a
stator winding. To be able to describe an embodiment,
initially a short description of a standard cable will be
made. The inner current-carrying conductor comprises a
number of non-insulated strands. Around the strands there
is a semiconducting inner casing. Around this semiconduct-
ing inner casing, there is an insulating layer of solid
insulation. An example of such :solid insulation is cross-
linked polyethylene (XLPE), alternatively ethylene-propyl-
ene (EP)-rubber. This insulating layer is surrounded by an
outer semiconducting layer which in turn is surrounded by a
metal shield and a mantle Such a cable will be referred to
hereunder as a power cable.
A preferred embodiment of the further developed cable
appears from Fig 12. The cable: 51 is described in the
figure as comprising a current-carrying conductor 52 which
comprises transposed both non-insulated and insulated
strands. Electromechanically transposed, solidly insulated
strands are also possible. Around the conductor there is an
inner semiconducting layer or caging 53 which, in turn, is
surrounded by a layer 54 of a solid insulation material.
The cable used as a winding in the preferred embodiment
does not have metal shield and external sheath. To avoid
induced currents and losses as:~ociated therewith in the
outer semiconducting layer, this is cut off, preferably in
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38
the coil end, that is, in the transitions from the sheet
stack to the end windings. Each cut-off part is then
connected to ground, whereby the outer semiconducting layer
55 will be maintained at, or near, ground potential in the
whole cable length. This means that, around the solid in-
sulated winding at the coil ends, the contactable surfaces,
and the surfaces which are dirty after some time of use,
only have negligible potentials to ground, and they also
cause negligible electric fields.
To optimize a rotating electric machine, the design of the
magnetic circuit as regards the slots and the teeth, re-
spectively, is of decisive importance. As mentioned above,
the slots should connect as closely as possible to the
casing of the coil sides. It is also desirable that the
teeth at each radial level are as wide as possible. This
is important to minimize the losses, the magnetization
requirement, etc., of the machine.
With access to a conductor for the winding such as for
example, the cable described above, there are great possi-
bilities of being able to optimize the laminated magnetic
core from several points of view. In the following, a
magnetic circuit in the stator of the rotating electric
machine is referred to. Figure 13 shows an embodiment of
an axial end view of a sector/pole pitch 56 of a machine
according to the invention. The rotor with the rotor pole
is designated 57 In conventional manner, the stator is
composed of a laminated core of electric sheets
successively composed of sector-shaped sheets. From a back
portion 58 of the core, located at the radially outermost
end, a number of teeth 59 extend radially inwards towards
the rotor. Between the teeth there is a corresponding
number of slots 60. The use of cables 51 according to the
above among other things permits the depth of the slots
for high-voltage machines to be made larger than what is
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39
possible according to the state of the art. The slots have
a cross section which is reduced towards the rotor since
the need of cable insulation becomes lower for each
winding layer towards the rotor. As is clear from the
figure, the slot substantially consists of a circular
cross section 62 around each layer of the winding with
narrower waist portions 63 between the layers. With some
justification, such a slot cross section may be referred
to as a "cycle chain slot". S:ince a relatively large
numbers of layers will be required in such a high voltage
machine and the availability of actual cable dimensions as
far as insulation and outer semiconductor are concerned is
restrictive, it may, in practice, be difficult to achieve
a desirable continuous reduction of the cable insulation
and the stator slot respectively. In the embodiment shown
in Figure 13, cables with three different dimensions of
the cable insulation are used, arranged in three
correspondingly dimensioned sections 64, 65 and 66, that
is, in practice a modified cycle chain slot will be
obtained. The figure also shows that the stator tooth can
be shaped with a practically constant radial width along
the depth of the whole slot.
In an alternative embodiment, the cable which is used as a
winding may be a conventional power cable as the one de-
scribed above. The grounding of the outer semiconducting
shield then takes place by stripping the metal shield and
the sheath of the cable at suitab:Le locations.
The scope of the invention accommodates a large number of
alternative embodiments, depending on the available cable
dimensions as far as insulation Bind the outer semiconduc-
tor layer etc. are concerned, of a so-called cycle chain
slot.
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As mentioned above, the magnetic circuit may be located in
the stator and/or the rotor of the rotating electric ma-
chine. However, the design of the magnetic circuit will
largely correspond to the above description independently
5 of whether the magnetic circuit is located in the stator
and/or the rotor.
As winding, a winding is preferably used which may be de-
scribed as a multilayer, concentric cable winding. Such a
10 winding means that the number of crossings at the coil
ends has been minimized by placing all the coils within
the same group radially outside one another. This also
permits a simpler method for the manufacture and the
threading of the stator winding in the different slots.
It should be noted that the description presented herein-
above only should be considered as exemplifying for the
inventive idea, on which the invention is built. Thus, it
is obvious for the man skilled in the art that detailed
modifications may be made without leaving the scope of the
invention. As an example, it may be mentioned that it
would be possible to use as a switch means 10 a mechanical
switch.
