Note: Descriptions are shown in the official language in which they were submitted.
~ ~53~g l7GE--2573
- STATIC EXCITATION SYSTEM
Background of the Invention
~_ . . _
The present invention relates to excitation
systems for large fluid-cooled dynamoelectric machines
and in particular to static excitation systems.
Excitation systems for large dynamoelectric
machines, such as the turbine-generator combinations
employed by electric utilities, have grown in power
rating along with the power ratings of the generators
themselves. Early excitation systems included rotat-
ing power sources such as a separate dc generator
driven by the turbine-generator shaft which supplied
excitation curren-t to the rotating field windings
through slip rings and brushes. Another approach has
employed an ac exciter driven by the turbine-generator
with rectification and control of the excitation volt-
age in external stationary rectifier banks. Still
another approach has employed the use of diode rec-
tifiers on the rotor. In these arrangements, the whole
rectification power source rotates, with control effect-
ed through electromagnetic flux linkages with the rotat-
ing components. An example of an excitation system
,
having rotating rectification means is found in U.S.
Patent No. 3,768,502 issued October 23, 1973 to Drexler
et al.
A separate category of excitation systems
is referred to as "static" because the excitation power
source does not rotate, but is stationary or static.
A compound excitation system of the static type is
described, for example, in U.S. Patent No. 3,702,965
issued November 14, 1072 to Drexler et al. The patent
"
~' ~
1 16~38~ 17GE-2573
to Drexler et al. describes an excitation system
receiving energy from both potential windings and
current windings. Hence, the term compound source
is applied. The current windings provide electrical
energy to the excitation system in response to the
output current from the generator armature winding,
that is, from the main machine output winding. The
potential windings supply electrical energy to the
excitation system in response to the voltage across
the armature windings. Such compound source excita-
tion systems exhibit response ratios of approximately
3.5 or better. The potential windings for such
machines are typically found within the stator slots,
lying over, hut insulated from, the main armature
winding. On the other hand, the current windings for
the excitation system generally surround each phase
of the three typical phases of the armature output
winding leads.
However, simpler excitation systems are
possible, particularly where a high response ratio
is not required. A simple excitation system, for
example, appears to be disclosed in U.S. Patent No.
3,132,296 issued May 5, 1964 to Nippes. However, the
excitation system in this patent is solely directed
toward utilizing the third and higher harmonics of
the fundamental frequency of the rotor magnetic flux.
Such systems as disclosed therein are not practical
for large dynamoelectric machines. (As used herein,
the term large dynamoelectric machine refers to one
which has a power rating in excess of approximately
50 megawatts.) The third harmonic or other higher
harmonic is not capable of providing sufficient power
~ 3~ 17GE-2573
to the excitation systern to produce such high levels of
generator output. Unlike the cooling requirements of
large machines, the cooling requirements for the exci-
tation system components of a machine employing only
third or hiyher harmonics of the rotor magnetic flux,
are minimal. However, certain excitation system com-
ponents of large machines require cooling to operate
effectively and reliably over extended periods of time.
Summary of the Invention
In accordance with a preferred embodiment
of the present invention a static excitation system
for a large, fluid-cooled dynamoelectric machine having
a rotating field winding and a stator core with a set
of main windings disposed in slots in said stator core
comprises an excitation winding disposed in the same
stator slots so that alternating current is induced
therein by the action of the rotating field winding,
the excitation winding being connected so as to be
responsive to the first harmonic frequency of the rotor
magnetic flux. The excitation system further comprises
a transformer means coupling the excitation winding to
a rectification means which provides direct current
to the rotating field winding, the transformer means
preferably being disposed within the cooling fluid path
of the machine.
` The excitation system of the present invention
- is simple and reliable. Additionally, the excitation
system of the present invention does not require cur-
rent windings and accordingly is less expensive than
compound source excitation systems.
Accordingly, it is an object of the present
invention to provide a simple, inexpensive and reliable
~ 17GE-2573
excitation system for a large dynamoelectric machine.
It is a further object of the present inven-
tion to provide an excitation system having a high
initial responseO
Description of the Drawings
. _ .
FIGURE l is a schematic, partially electrical
and partially mechanical, drawing illustrating the
excitation system of the present invention.
FIGURE 2 is a schematic diagram particularly
illustrating the excitation transformer.
Detailed Description of the Invention
- - -
Figure l illustrates the excitation system of
the present invention employed in a large dynamoelectric
generator such as that which would be employed by an
electric utility to provide power. The generator com-
prises an outer, pressurizable shell or enclosure 15
throughout which a cooling fluid, such as hydrogen gas,
is circulated to cool the machine. Within pressurizable
shell 15 there is disposed a stator ll of conventional
construction such as that disclosed in the above-mentioned
3,702,965 patent. The stator construction typicaLly
comprises a large plurality of sectorially shaped metal
punchings stacked so as to form a hollow cylindrical
structure, the inner periphery of which possesses a
plurality of longitudinal slots in which main windings
12 are disposed. It is from main windings 12 that the
output electrical power of the generator is provided
through high voltage bushings 22. Disposed within the
hollow cylindrical portion of the stator assembly ll,
there is disposed rotor lO which is typically coupled
to a steam turbine or other motive source. To maintain
cooling fluid 28 within the generator, seals 27 are
J ~ ~3~9 17GE-2573
provided. Rotor 10 comprises a large cylindrical
metal forging into which longitudinal slots have
been cut. Within these slots there is typically
provided two or more field windings 13. It is these
field windings 13 which are energized through the
excitation system of the present invention. Field
windings 13 produce a radially directed field of
magnetic flux which cuts across the main windings
12 during rotation, thereby producing the desired
electrical power output.
Because of the large amounts of electrical
power generated by such dynamoelectric machines as
illustrated in Figure 1, even the slightest ineffic-
iencies in machine operation can be very costly and
can produce large amounts of thermal energy within
the machine which must be removed for the long-term
reliable operation demanded by the intended use. It
is for such reasons that the main windings 12 are
typically cooled with a li~uid coolant such as water.
This is relatively easily arranged because of the
stationary nature of the main windings. However, it
is also necessary to cool rotor 10. This is typically
accomplished by circulating therethrough a cooling
fluid 28, such as hydrogen gas, which is preferred
because it not only exhibits the capability of absorb-
ing and transporting large amounts of thermal energy,
but also because its density reduces windage losses
in the machine. By means of rotor-mounted fans (not
shown) and other conventional fluid circwlating means,
the coolant fluid 28 is circulated past coolers 19
disposed in domes 16 and 17 atop the generator. It
is these coolers 19 which remove heat from the cooling
17GE-2573
fluid before it is cycled back to the interior of
the main generator housing and in particular before
it is recirculated back to the gap between the rotor
10 and the stator 11.
Next is considered the excitation system
; itself and its relation to the other components of
the generator. The essential feature of the excita-
tion system of the present invention is the winding
set 14a, 14b and 14c shown in Figure 1. The windings
may comprise one or more conductive bars disposed
; in the stator slots. These windings are the potential
source windings of the excitation system and are more
generally referred to as "P-bars". These windings
are placed in the stator slots along with main winding
12. However, the P-bar windings are required to carry
substantially less current than the main stator windings
themselves. Typically, the P-bar windings carry cur-
rents of less than approximately 2,000 amperes. These
P-bar windings (generally designated by reference
number 14) are placed in the slots in a manner such
as that shown in the above-mentioned 3,702,965 patent.
Typically they constitute single bars of copper located
at 120 intervals about the inner circumference of the
generator stator. Preferably, one of these three P-bars
is located at the topmost slot in a horizontal generator
stator. This leaves the bottom 120 of the stator
assembly open and more amenable to the insertion of
the rotor forging during generator assembly. At one
end, all of the P-bar windings are referenced to a
neutral ground so as to be arranged in an electrical
wye configuration. This function is preferably perform-
; ed by leading the P-bar winding connections out from
17GE-2573
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dome 16 through bushing 23 to circuit 25. Circuit
25 performs conventional fusing and circuit breaker
functions for the potential windings 14, both ind-
ividually and collectively. At the other end of the
generator, the collector end, the P-bar windings 14
are coupled directly to excitation transformer 18.
This transformer is discussed in more detail below.
The output leads of the transformer leave the press-
urized generator housing from dome 17 through bushing
24 so as to couple the excitation transformer with
rectifier bridge 20. The rectifier bridge provides
direct-current output to fixed brushes and thence to
slip rings 21 which rotate with the rotor l0. The
brushes conventionally comprise carbon. Thus, elec-
trical energy induced in the P-bar windings in altern-
ating-current form is provided as an input to excita-
tion transformer 18 after which it is rectified and
supplied to the field winding as a source for the
rotating magnetic flux. As is of course known, since
residual magnetism in the rotor l0 is generally not
sufficient for the purposes of generator startup,
other electrical circuits (not shown) are generally
present to provide electrical current to the slip rings
21 during initial generator startup. Following this,
however, the generator is self-exciting.
Figure 2 more particularly illustrates the
detailed construction of the excitation transformer 18.
In particular, the primary of the transformer 29 is
preferably configured in a wye circuit and the second-
ary of the excitation transformer is configured in a
delta circuit. One of the significant economies
brought about by the excitation system of the present
17G~ 2573
invention is the elimination of the need for certain
large linear reactive circuits. Because the excita-
tion system voltages are no longer directly dependent
upon the output current, these current-limiting reac-
tances in the P-bar circuit are no longer required.
Instead, suitable reactance may be provided by leak-
age flux within the excitation transformer itself.
This leakage reactance is lndicated by inductors 26a,
26b and 26c shown within the dotted line of Figure 2.
Accordingly, that which is inside the dotted line is
a simplified equivalent circuit and the inductors
26a, 26b and 26c are not required to be separately
provided.
As indicated above, it is desirable that P-bar
windings 14 not be required to carry large amounts of
current, that is to say, currents in excess of approxi-
mately 2,000 amperes. This is highly desirable because
separate cooling facilities for the P-bars need not be
required. Since low current through the P-bar is desir-
able in large dynamoelectric machines, transformer 18
is required so that the excitation system be capable
of supplying a sufficient amount of electrical power
to the field winding 13. However, a significant ad-
vantage of the present transformer, as compared with
those that have been employed in compound excitation
systems, is that now the large current winding through
the transformer is no longer required, thereby signif-
icantly reducing the cost and complexity of its construc-
tion, not to mention its size. It is this size reduc-
tion which facilitates placement of the transformer
within the domes for the cooling system presently pro-
vided on large generators.
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~ ~53~
Another alternative to generator system design
in which there is a low level of current in the P-bars
involves increasing the number o~ P-bars per generator
phase. This does not present significant design problems
since adequate space in the stator slots is usually
available in machines with high power ratings.
A significant advantage of the present inven-
tion a~ises because a smaller excitation transformer 18
-~; is now needed because of the lack of need for the current
winding. This aids placing excitation transformer 18
in cooling dome 17, directly in the path of flow for
the generator coolant 28. This further simplifies
design of this transformer.
The rectifier bridge is a conventional three-
phase, full-wave rectifier bridge with thyristors, as
needed~ for voltage control. Also, the transformer may
also comprise a set of three two-phase transformers
rather than the single three-phase transformer as shown.
In one desirable embodiment there is a relatively weak
magnetic link between the primary and the secondary so
as to provide sufficient leakage reactance to limit the
~; current under fault conditions in the output of the
excitation system.
From the above it may be appreciated that the
simplified excitation system for large dynamoelectric
machines as provided by the present invention offers
significant advantages not to be found in other excita-
tion systems. The system of the present invention is
simpler and therefore more reliable. It eliminates
the need for separately provided current limiting react-
ances in the excitation system circuitry. These react-
ances are large, relatively expensive and must generally
~ g 17GE-2573
be separately cooled. Fur~hermore, the excitation
system described herein requires a much simpler
transformer having only primary and secondary wind-
ings without the need of providing separate current
windings, which are necessarily difficult, cumbersome
: and expensive to provide since use of the highly
energized main winding output conductors is involved~
Nonetheless, the system provides a high initial response
and permits direct cooling of the required excitation
transformer.
While the invention has been described in
detail herein in accord with certain preferred em-
bodiments thereof, many modifications and changes
therein may be effec*ed by those skilled in the art.
Accordingly, it is intended by the appended claims
to cover all such modifications and changes as fall
within the true spirit and scope of the invention.
