Note: Descriptions are shown in the official language in which they were submitted.
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WO 99/17432 PCT/SE98/01741
A ROTARY ELECTRIC MACHINE
Technical field
The present invention relates to a rotary electric machine of alternating
current
type designed to be connected directly to a distribution or transmission
network
and comprising at least one electric winding. The invention also relates to an
electric power plant comprising such an electric machine, and also to a method
of
exciting a rotary electric machine.
Background art
The rotary electric machine according to the invention may be a synchronous ma-
chine, dual-fed machine, external pole machine or synchronous flow machine.
To connect machines of this type to distribution or transmission networks, in
the
following referred to as power networks, transformers have hitherto been used
to
step up the voltage to network level, i.e. to the range of 130-400 kV.
Generators having a rated voltage of up to 36 kV are described by Paul R.
Siedler
"36 kV Generators Arise from Insulation Research", Electrical World, 15
October
1932, pages 524-527. These generators comprise windings of high-voltage cable
in which the insulation is divided into different layers with different
dielectric con-
stants. The insulating material used consists of various combinations of the
three
components mica-foil mica, varnish and paper.
It has now been found that, by manufacturing the above-mentioned winding of
the
electric machine from an insulated electric high-voltage conductor with a
solid in-
sulation of a type similar to that used in cables for power transmission, the
ma-
chine voltage can be increased to such levels that the machine can be
connected
directly to any power network without the use of intermediate transformers. A
typical operating range for these machines is 30 to 800 kV.
Nowadays static exciters or brushless exciters with rotating diode rectifier
bridges
are used in rotary electric machines. The excitation equipment is frequently
re-
quired to be able to produce a peak voltage and peak current of 1.5 to 3 times
greater than equivalent magnitudes in the case of rated load excitation for
the
machine in question, for a duration of 10-30 seconds. The excitation equipment
shall also be able to produce a field current equivalent to the rated load
excitation
current for 25% voltage on the stator terminal of the machine. The excitation
system shall preferably be "maintenance free", i.e. an excitation system
without
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WO 99/17432 2 PCTlSE98/01741
slip rings. The response and transient times at network disturbances shall
also be
rapid, i.e. the excitation equipment shall ~be able to generate both positive
and
negative field voltage. In the case of synchronous compensators, the
excitation
system shall generally be able to produce both positive and negative field
current
and demands for peak voltage factors greater than 3 times the rated load
excita- .
tion voltage may occur.
Brushless exciters eliminate the problems of dirt from carbon dust from
brushes
and slip rings. However, brushless exciters in accordance with known
technology
exhibit poorer control performance than static exciters.
The object of the present invention is thus to provide a rotary electric
machine that
can be connected directly to a power network and that is provided with a
"maintenance free" excitation system with improved control performance, and an
electric power plant comprising such an electric machine, as well as to
propose a
method for excitation of a rotary electric machine.
Description of the invention
This object is achieved with a rotary electric machine of the type described
in the
introduction, having the characterizing features of claim 1, an electric power
plant
in accordance with claim 17 and a method in accordance with claim 18.
The insulating conductor or high-voltage cable used in the present invention
is
flexible and is of the type described in more detail in WO 97/45919 and
WO 97/45847. The insulated conductor or cable is described further in
WO 97/45918, WO 97/45930 and WO 97/45931.
Thus, in the device in accordance with the invention the windings are
preferably of
a type corresponding to cables having solid, extruded insulation, like those
cur-
rently used for power distribution, such as XLPE-cables or cables with EPR-
insulation. Such a cable comprises an inner conductor composed of one or more
strands, an inner semi-conducting layer surrounding the conductor, a solid
insulat-
ing layer surrounding this semiconducting layer and an outer semiconducting
layer
surrounding the insulating layer. Such cables are flexible, which is an
important
property in this context since the technology for the device according to the
inven-
tion is based primarily on winding systems in which the winding is formed from
cables which are bent during assembly. The flexibility of a XLPE-cable
normally
corresponds to a radius of curvature of approximately 20 cm for a cable 30 mm
in
diameter, and a radius of curvature of approximately 65 cm for a cable 80 mm
in
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WO 99/17432 3 PCT/SE98/01741
diameter. In the present application the term "flexible" is used to indicate
that the
winding is flexible down to a radius of curvature of the order of four times
the ca-
ble diameter, preferably eight to twelve times the cable diameter.
The winding should be constructed to retain its properties even when it is
bent
and when it is subjected to thermal or mechanical stress during operation. It
is
vital that the layers retain their adhesion to each other in this context. The
mate-
rial properties of the layers are decisive here, particularly their elasticity
and rela-
tive coefficients of thermal expansion. In a XLPE-cable, for instance, the
insulat-
ing layer consists of cross-linked, low-density polyethylene, and the
semiconduct-
ing layers consist of polyethylene with soot and metal particles mixed in.
Changes
in volume as a result of temperature fluctuations are completely absorbed as
changes in the radius of the cable and, thanks to the comparatively slight
differ-
ence between the coefficients of thermal expansion in the layers in relation
to the
elasticity of these materials, the radial expansion can take place without the
ad-
hesion between the layers being lost.
The material combinations stated above should be considered only as examples.
Other combinations fulfilling the conditions specified and also the condition
of be-
ing semiconducting, i.e. having a resistivity within the range of 10-1-106 ohm-
cm,
e.g. 1-500 ohm-cm, or 10-200 ohm-cm, naturally also fall within the scope of
the
invention.
The insulating layer may consist, for example, of a solid thermoplastic
material
such as low-density polyethylene (LDPE), high-density polyethylene (HDPE),
polypropylene (PP), polybutylene (PB), polymethyl pentane (PMP), cross-linked
materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene
propylene rubber (EPR) or silicon rubber.
The inner and outer semiconducting layers may be of the same basic material
but
with particles of conducting material such as soot or metal powder mixed in.
The mechanical properties of these materials, particularly their coefficients
of
thermal expansion, are affected relatively little by whether soot or metal
powder is
mixed in or not - at least in the proportions required to achieve the
conductivity
necessary according to the invention. The insulating layer and the semiconduct-
ing layers thus have substantially the same coefficients of thermal expansion.
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WO 99/17432 4 PCT/SE98/01741
Ethylene-vinyl-acetate copolymers/nitrile rubber, butylymp polyethylene,
ethylene-
acrylate-copolymers and ethylene-ethyl-acrylate copolymers may also constitute
suitable polymers for the semiconducting layers.
Even when different types of material are used as base in the various layers,
it is
desirable that their coefficients of thermal expansion are of the same order
of
magnitude. This is the case with the combination of the materials listed
above.
The materials listed above have relatively good elasticity, with an E-modulus
of
E<500 MPa, preferably <200 MPa. The elasticity is sufficient for any minor
differ-
ences between the coefficients of thermal expansion for the materials in the
lay-
ers to be absorbed in the radial direction of the elasticity so that no cracks
or other
damage appear and so that the layers are not released from each other. The ma-
terial in the layers is elastic, and the adhesion between the layers is at
least of the
same magnitude as the weakest of the materials.
The conductivity of the two semiconducting layers is sufficient to
substantially
equalize the potential along each layer. The conductivity of the outer semicon-
ducting layer is sufficiently large to contain the electrical field in the
cable, but suf
ficiently small not to give rise to significant tosses due to currents induced
in the
longitudinal direction of the layer.
Thus, each of the two semiconducting layers essentially constitutes one equipo-
tential surface, and the winding with these layers will substantially enclose
the
electrical field within it.
There is, of course, nothing to prevent one or more additional semiconducting
layers being arranged in the insulating layer.
By providing the electric machine in question with a brushless excitation
system
switchable between positive and negative excitation, a "maintenance free"
system
is obtained having rapid response and transient times at network disturbances,
for
instance, since the excitation system is able to generate both positive and
nega-
tive field voltage and thus positive and negative field current.
According to an advantageous embodiment of the machine in accordance with the
invention, the excitation system comprises two controllable antiparallel-
connected
current converter devices for feeding the field winding of the alternating
current
machine, a two-way field over-voltage protection means or discharge circuit
con-
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WO 99/17432 S PCT/SE98/O1?41
nected across the field winding, and also control equipment for controlling
the cur-
rent converters and over-voltage protection means or discharge circuit. This
is a
simple construction requiring no galvanically separated supply sources and cur-
rent-limiting reactances and no separate short-circuiting devices for
extinguishing
conducting thyristors. The excitation system is also well suited for
synchronous
machines such as synchronous compensators. The present invention thus ex-
ploits the ability offered by semiconductor technology to temporarily change
the
polarity in a simple manner, which facilitates rapid commutation of the field
current
from static current converter bridge to short-circuiting circuit and vice
versa when
a change of current direction is required in the field circuit of the machine.
Brief description of the drawings
To explain the invention more clearly embodiments of the machine in accordance
with the invention, selected by way of example, will now be described in more
detail with reference to the accompanying drawings, in which
Figure 1 shows the insulated cable used in the machine in accordance with
the invention,
Figure 2 shows a circuit diagram of the excitation system in the machine in
accordance with the invention, and
Figures 3a-f show the voltage and current variation upon bridge switching in
the excitation system shown in Figure 2.
Description of a preferred embodiment
Figure 1 shows a cross section through an insulated conductor 11 intended for
use in the windings of the machine in accordance with the present invention.
The insulated conductor 11 thus comprises a number of strands 35 having
circular
cross section and consisting of copper (Cu), for instance. These strands 35
are
arranged in the middle of the insulated conductor 11. A first semiconducting
layer
13 is an-anged around the strands 35. An insulating layer 37, e.g. XLPE insula-
tion, is arranged around the first semiconducting layer 13. A second
semiconduct-
ing layer 15 is arranged around the insulating layer 37. The insulated
conductor is
flexible and retains this property throughout its service life. Said three
layers are
constructed so that they adhere to each other even when the insulated
conductor
is bent. The insulated conductor has a diameter within the interval 20-250 mm
and a conducting area within the interval 80-3000 mm2.
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WO 99/17432 6 PCT/SE98/01741
Figure 2 shows a circuit diagram for the excitation system in the machine in
ac-
cordance with the invention. The field winding 4 of the machine, which may be
stationary or rotating, is connected to two antiparallel-connected current
converter
bridges 1, 2. A two-way over-voltage protection means comprising two antiparal-
lel-connected thyristors 8, 10 with associated ignition circuits 12, 14, is
also pro-
vided over the field winding 4.
The current converter bridges 1, 2 are supplied from a source 16 and
controlled
from a switching logic 18 via control pulse amplifiers 20, 22. A control pulse
gen
erator 28 for the current converter bridges 1, 2 in the form of thyristor
bridges is
also arranged to emit control pulses to the pulse amplifiers 20, 22. Measuring
in-
struments 24, 26 are also arranged to measure the currents IFB1 and IFB2, re-
spectively, from the current converter bridges 1, 2, and transmit the measured
re-
sults to the switching logic 18 for control purposes. Connection of the
thyristors 8,
10 of the over-voltage protection means is also controlled from the switching
logic
18 via the ignition circuits 12, 14. The over-voltage protection means is con-
nected to a current-limiting resistor R. In the system with field breakers
this resis-
for R serves as discharge resistor.
The procedure for switching from bridge 1 to bridge 2 is as follows: Initially
bridge
1 is assumed to be conducting, which means that the current direction IF
through
the field winding 4 is positive, see Figures 3a and 3b. The control signal
Ust, see
Figure 2, to the control pulse generator 28 and the switching logic 18 will be
nega-
tive, resulting in bias reduction and thus a change of polarity of the bridge
1, see
Figure 3a. The time interval for bias change, t2-t1 according to Figure 3b,
from
maximum positive peak voltage to maximum negative peak voltage is approxi-
mately 8.3 ms at a frequency of 50 Hz and 6-pulse two-way bridge.
At the time t3, when the current IFg1 is still greater than 0, an ignition
pulse is
transmitted to the discharge thyristor 10 and a blocking signal to the bridge
1. As
a result of the free-wheel effect at negative bias, a momentary transmission
of
excitation current IFg1 to the over-voltage protection circuit is obtained,
and the
bridge 1 becomes currentless. A signal from the measuring instrument 24 that
the
bridge 1 is currentless initiates unblocking of bridge 2 and blocking of the
ignition
circuit 14 for the thyristor 10. The time interval t4-t3 according to Figure
3, i.e. the
period from the blocking of bridge 1 until the bridge 2 is connected is
approxi-
mately 5 ms, see Figure 3. It is apparent from Figure 3d that the current IF
in the
field circuit 4 during this switching interval is maintained as a result of
the induc-
tance of the field winding 4. As apparent from Figures 3d and 3e, the biased
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WO 99/17432 ~ PCT/SE98/01741
bridge 2 now forces a current IR, see Figure 3f, through the thyristor 10 and
the
current-limiting resistor R, and also a current IF through the field winding 4
of the
synchronous machine. At the time t5 the field current IF has changed polarity
and
the discharge thyristor 10 is extinguished through temporary biasing reduction
of
the bridge 2, i.e. a temporary change in polarity to force a current in the
reverse
direction of the short-circuiting circuit or the over-voltage protection
means.
A suitable choice of current levels for generating blocking and detecting
signals
ensures that the time interval is brief for connecting the two-way field over-
voltage
protection means 8, 10, 12, 14 serving as auxiliary circuit or the two-way
thyristor
discharge circuit.
Switching from negative current direction to positive current direction at a
positive
control signal occurs in corresponding manner by temporary connection of the
thyristor 8 in the over-voltage protection means.
An embodiment of the rotary electric machine in accordance with the invention
is
described above by way of example. However, several modifications are of
course feasible within the scope of the invention. The principle described can
thus be used for both stationary and rotating thyristor bridges for exciting
syn-
chronous machines or for supplying motors for drive systems. Temporary or
pulsed biasing reduction may also be used to reset an activated over-voltage
protection means. In a first phase, an over-voltage signal then gives a signal
for
alarm and resetting the protection means. A continuous error signal after a
num-
ber of resetting attempts will generate a tripping signal.
The introduction and use of extinguishable semiconductor elements can also
shorten the time interval for switching between positive and negative
excitation or
vice versa. The introduction of extinguishable semiconductor elements in the
two-
way over-voltage protection makes temporary reversal of the field voltage
unnec-
essary in order to extinguish an activated and conducting semiconductor
element.
