Note: Descriptions are shown in the official language in which they were submitted.
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HVnT''lT!T~T!C~T~TC PLANT DRI~FT TUBE WATER LEVEL CONTROL
SPECIFICATION
The invention generally relates to a method for
sensing and thereby controlling draft tube water level in a
5 hydroelectric generator while it is being used as a
synchronous ~.nrlrl~n~r.
When a hydroelectric generator is used as a
synchronous cnntll~nc~r~ the high pressure water used to drive
the unit a5 a generator is shut of f, but the circuit breaker
10 connecting it to the utility's electrical grid remains closed.
The turbine now becomes the r ~h:~ni~l load that the
generator, (now being used as a motor), must turn. In order
to keep the power CV~I~LI. ~tion low it is nc>~ cc:~ry to insure
that the turbine i8 cr;nnjn~ in the injected air above the
15 water level of the draft tube. The turbine elevation often is
below the river or tailrace level. Figure 1 is a simple cross
section of a hydroelectric generator.
All methods currently in use to maintain control
over the draft tube water level use bursts of compressed air
20 injected into the turbine head cover to force the water level
down away from the turbine. This volume of air between the
headcover and the draft tube water level is called the bubble.
The headcover air pressure will gradually dissipate because of
packing leakage. After this bubl:lle is established, it has to
25 be periodically replenished with more air.
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Sensing the correct water level in the draft tube is
critical to det-~n;n;ng when to inject more air. As the water
level rises, due to decreases in headcover air pressure and
5 begins to hit the turbine, all air under the headcover will be
churned into the water. The air is then discharged into the
tailrace of the power plant and the electrical load will
increase to a maximum. This process is known as, "losing the
bubble". If the water level in the draft tube is forced too
10 low by the compressed air in the headcover, air bubbles will
begin to escape into the tailrace and the pressure under the
headcover will rapidly drop. The end result will again be the
"1055 of bubble". Methods in use to maintain the draft tube
water level at a useful level are as follows:
1. Conductivity probes are mounted in the draft
tube at two different elevations. These
probes change resistance when they are
immersed in water. They are connected to a
control circuit which inj ects compressed air
2 o into the headcover when the water level rises
to the upper probe and shuts the compressed
air off when the water level is forced below
the lower probe.
2. Float switches are mounted in the draft tube.
These work on much the same principle as the
conductivity probes.
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3. Pressure switches can be used to sense the
headcover air pressure and operate an air
valve whenever nf~rps~s~ry to maintain a
predet~r~n;n~d pressure inside the turbine
area.
More sophisticated methods 5~ employ level
sensors to monitor the tailrace level itself and use this
information in conjunction with one of the above methods to
determine when air is to be in~ ected .
All these methods have one thing in common. The
process is controlled by devices deriving their logic from the
draft tube or turbine. They sense the water level or pressure
directly at the draft tube or turbine. A T' 5h;~n;~-Al means is
always used to initiate the control process. For this reason
they require continuous maintenance or adjusting. Most
importantly, they do not work reliably. They are illustrated
in Figure 2.
~he unreliability and high maintenance of existing
methods can be overcome through the use of the PROCESS which
2o is the subject invention. I'his invention is based on the
process of using increases in generator power or current
~nnl l ion to determine when to inject ~ 6ed air into
the turbine for the purpose of establishing or maintaining
correct draft tube water level during synchronous condenser
25 operation of that generator. The process will work regardless~
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of generator size (Megawatt rating), draft tube dimensions, or
tailrace level5.
The invention can be implemented by providing in a
vertical shaft hydro powered turbine-generator having a
generator directly coupled to a turbine, which is ~ os~d in
a turbine cavity, a draft tube connected directly below said
turbine cavity, wicket gates disposed around said turbine
cavity, and an air supply valve for the injection of
compressed air to depress the water level in the draft tube
when the turbine-generator is operated as a synchronous
condenser, a control system for synchronous cnn~lt~nc:t~r
operation comprising:
(a) means operatively connected to said gcnerator for
sensing the electrical power consumed by the
gene~ator when operating as a synchronous condenser
and for producing a signal indicative thereof;
(b) reference means providing a signal representative
of the electrical power consumed by the generator
at the instant that rising draft tube water begins
to impact said turbine during synchronous condenser
operation; and
(c) a detector, operatively connected to said generator
electrical power sensing means, and connected in a
controlling relation to said air supply valve for
controlled injection of compressed air in response
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to an increase in said generator electrical power
consumption above a threshold level represented by
the reference mean6 to depress the draft tube water
level below the level of the turbine.
The reference means may alternately provide a signal
representative of the electrical current drawn by the
generator at the instant that draft tube water begins to
impact the turbine during synchronous contl~nc-~r operation. In
such case, the controlled injection of compressed air occurs
in response to an increase in said generator current
consumption above a threshold level represented by the
reference means.
The control system may further include a timer
operatively connected between the detector and the air supply
valve for controlling the period of air injection after
initiation by said detector.
It may also operate on the basis that the
predet~orm;nF~d interval of the timer det~rminPc the distance by
which the water level is depressed below the level of the
2 0 turbine .
These and further features of the invention will be
better understood by reference to the Figures and description
set out next below.
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Figure l is a cross section view of a hydroelectric
turbine generator . This f igure illustrates the parts of the
c~qll; t described earlier.
Figure 2 is an illustration depicting the prior art
5 discussed earlier. All methods presently used for draft tube
water level control use sensors of some type to actually
measure activity inside the turbine area.
Figure 3 is a block diagram of the PROCESS used to
sense draft tube water level and inject air into the
10 headcover. The devices shown may vary in complexity and name,
but their function in the process remains the same. The area
of this drawing inside the dotted line represents the part
which i8 the invention.
When a hydroelectric generator is being motored, the
15 invention resides in the process of sensing the increase in
either the power or the current ~.o~ ,,tion of the GENERATOR
when its turbine begins to hit the draft tube water, then
using this quantity (either directly, or after processing by
a suitable tr~nC~ r), to inject l_ re:ssed air into the
20 headcover/turbine/draft tube area thereby controlling the
water level below the turbine. The area inside the dotted
line in Figure 3 is a representation of the location where the
process which is the basis the invention is effected. This is
a purely electrical process without need for any r-^h~niFm for
25 physically det~rm;n;n~ the draft tube level or pressure.
.
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Many variations are possible on this basic process,
but the process itself remain6 the same. For example, one
method of using this process would be to use the output of a
watt transducer to drive a detector circuit. The detector
5 senses the increase and turns on the compreased air for a
preset time necessary to force the water below the turbine
wheel once again. This process is repeated for as long as
the generator is "motoring/condensing". Variations to this
process would be to use an ampere trAn~ c~r instead of the
10 watt trAn~ r. A computer could be used instead of a simple
electronic detector. Any timing device ~rom a simple relay to
a computer, could be used to control the time the air valve is
open to force the water below the turbine. This same scheme
without any additional ~ can be used to establish the
15 bubble as well as maintain it.
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