Electricity: electrical systems and devices – Safety and protection of systems and devices – Motor protective condition responsive circuits
Reexamination Certificate
1999-12-13
2001-09-04
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Motor protective condition responsive circuits
C361S023000, C361S087000
Reexamination Certificate
active
06285533
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of and apparatus for controlling the operation of a variable speed gearing capable of preventing fluctuations in the power on a transmission line, when a fault, such as a one-line ground or a three-line ground, has occurred in the transmission line, and thereby improving the reliability of power generation facilities.
FIG. 8
schematically shows an example of an operation controller for a variable speed gearing, such as an induction generator-motor.
In
FIG. 8
, the primary circuit of an induction generator-motor
5
is connected to a transmission line
1
and a transmission line
2
via breakers CB
11
, CB
12
and breakers CB
21
, CB
22
, a breaker CB
3
, a main transformer
3
, disconnectors DS
1
, DS
2
, and a parallel breaker
4
in that order.
An exciting current (secondary exciting current) to the secondary circuit
10
of the induction generator-motor
5
is produced in such a manner that the alternating current on the transmission lines
1
and
2
inputted via an exciting power supply breaker
6
and an exciting transformer
7
is converted into a direct current by a power rectifier
8
in the presence of a power rectifier control signal
13
from a variable speed controller
11
and then the direct current is converted into an alternating current of variable frequency by a power inverter
9
in the presence of a power inverter control signal
14
from the variable speed controller
11
.
The application of the secondary exciting current enables the induction generator-motor
5
to be operated at variable speed in synchronization with the alternating current on the transmission lines
1
and
2
.
On the other hand, when a one-line ground or a three-line ground has occurred at a fault point
19
on the transmission line
2
, the induction generator-motor
5
supplies a fault current to the transmission-line fault point
19
. As a result, the fault current flows in the primary circuit of the induction generator-motor
5
. The fault current then produces electromotive force in the secondary winding of the induction generator-motor
5
, resulting in excess current flowing in the secondary circuit of the induction generator-motor
5
.
FIG. 9
is a waveform diagram to help explain an example of excess current in the secondary circuit
10
of the induction generator-motor
5
when an fault has occurred in the transmission line.
In
FIG. 9
, the current waveform
21
of the secondary circuit
10
, which is generally of a three-phase balanced current, varies with time in the direction of the arrow shown by t.
On the other hand, when a fault has occurred in the transmission line
2
, the current in the secondary circuit
10
of the induction generator-motor
5
increases, because the balance of the three-phase current is lost at the transmission line fault point
22
due to the induced voltage in the secondary circuit
10
caused by the fault current supplied from the induction generator-motor
5
.
The current in the secondary circuit
10
of the induction generator-motor
5
is sensed by a secondary current sensor
15
provided for each of the three phases of the secondary circuit
10
. Excess current is sensed, provided that an excess current sensor
16
senses that the current has exceeded a preset value. The excess current sensor
16
starts to operate at an excess current sensing point
23
.
Then, when the excess current sensor
16
has started to operate, the variable speed controller
11
judges that it is impossible to continue the operation of the induction generator-motor
5
and outputs a generator trip signal
20
to a generator controller
12
, thereby tripping the parallel breaker
4
and exciting power supply breaker
6
, which not only disconnects the induction generator-motor
5
from the transmission lines
1
and
2
but also stops the operation of the induction generator-motor
5
.
In
FIG. 9
, the disconnected state of the induction generator-motor
5
is shown at the generator trip point
24
.
On the other hand, when a transmission line fault has occurred, a transmission protective device
17
and a transmission protective device
18
provided in the transmission line
2
operate, closing the breakers CB
21
, CB
22
in the transmission line
2
again, restoring the transmission line
2
to normal conditions.
At this time, when the ground fault at the transmission line fault point
19
is transient and lasts for a short time, the re-closing of the circuit is successful, restoring the transmission line
2
to the state before the fault occurred. When the ground fault at the transmission line fault point
19
has lasted for a long time, the re-closed circuit is broken finally, tripping the breakers CB
21
, CB
22
at both ends of the transmission line fault point
19
to disconnect the fault point
19
from the transmission line
2
, which keeps the transmission line
2
in good condition.
With such a method, when excess current in the secondary circuit
10
of the induction generator-motor
5
has been sensed, it cannot be judged whether the fault has occurred in the transmission line
2
or in the secondary circuit
10
of the induction generator-motor
5
. For this reason, the operation of the induction generator-motor
5
is stopped in the presence of excess current in the secondary circuit
10
, although the fault in the transmission line
2
might be remedied.
As described above, with a conventional variable speed gearing operation controller, when a fault, such as a one-line ground or a three-line ground, has occurred at the fault point
19
in the transmission line
2
, the transmission line protective devices
17
and
18
provided on the transmission line
2
side operate, restoring the transmission line
2
to normal conditions by the re-closing of the circuit or by the final cutoff of the re-closed circuit, which stops the operation of the induction generator-motor
5
in the presence of excess current in the secondary circuit
10
, although the fault in the transmission line might be remedied.
Namely, with such a method, although the induction generator-motor
5
itself has not gone wrong, the operation of the induction generator-motor
5
is stopped. As a result, the induction generator-motor
5
, which may be operated without being disconnected from the transmission line, is disconnected, preventing the supply of power. Furthermore, the power cannot be supplied to the power system which is short of power due to the fault in the transmission line
2
, causing fluctuations in the power on the transmission line
2
.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide a method of and apparatus for controlling the operation of a variable speed gearing which, when a fault, such as a one-line ground or a three-line ground, has occurred in a transmission line, not only maintains the operating state of the induction generator-motor and causes the induction generator-motor to operate so as to compensate for the power shortage at the time of transmission line fault, thereby preventing the power on the transmission line from fluctuating, but also improves the reliability of the power generation facilities.
According to a first aspect of the present invention, there is provided a method of controlling the operation of a variable speed gearing, comprising: the step of causing a power rectifier to convert an alternating current of a transmission line inputted via an exciting transformer into a direct current in the presence of a power rectifier control signal from a variable speed controller; the step of causing a power inverter to convert the direct current into an alternating current of variable frequency in the presence of a power inverter control signal from the variable speed controller to produce a secondary exciting current; the step of supplying the secondary exciting current to the secondary circuit of an induction generator-motor whose primary circuit is connected to the transmission line via at least a main transformer and thereby operating the induction gene
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sherry Michael J.
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