Electricity: electrical systems and devices – Safety and protection of systems and devices – Voltage regulator protective circuits
Reexamination Certificate
1994-10-26
2001-04-17
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Voltage regulator protective circuits
C361S100000
Reexamination Certificate
active
06219214
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus for protecting switching circuits having parallel multiple switching elements with the characteristic of self-firing due to an over-voltage and a method of protecting the switching circuits.
2. Description of the Prior Art
Over-voltages are generated in the field windings of generators and motors according to asynchronous inputs, short circuits, and the like. Over-voltage protection circuits, in which switching elements, for example thyristors, are applied to the protection of these devices from over-voltages.
FIGS. 3 and 4
show prior art over-voltage protection circuits having thyristors as switching elements.
In
FIG. 3
,
41
is a generator,
42
is a field winding of generator
41
,
43
is a thyristor rectifier which supplies direct current (DC) power to the field winding
42
and
44
is an over-voltage protection circuit in which thyristors
45
are used for protecting from over-voltages generated in the field winding
42
.
The thyristors
45
in over-voltage protection circuit
44
are caused to fire by over-voltages generated in the field coil
42
and form a short circuit, thus protecting the generator
41
and the thyristor rectifier
43
. In order to improve the reliability of this over-voltage protection circuit, thyristors
45
are used which have the characteristic of self-firing without a gate signal when a certain level of voltage is impressed (hereafter, these thyristors
45
are referred to as VBO free thyristors).
FIG. 4
is a detailed diagram of a prior art over-voltage protection circuit which uses VBO free thyristors. Although a suitable number of VBO free thyristors may be connected in series or parallel according to the voltage and current used in the over-voltage protection circuit,
FIG. 4
shows the case of
3
parallels. Elements
10
,
20
and
30
are reactors which equalize the current shares between the parallel circuits. Elements
11
,
12
,
21
,
22
,
31
and
32
are VBO free thyristors. Element
55
is a gate circuit which supplies gate pulses to VBO free thyristors
11
,
12
,
21
,
22
,
31
and
32
when an ON command is supplied to the over-voltage protection circuit.
In the circuit in
FIG. 4
, VBO free thyristors
11
,
12
,
21
,
22
,
31
and
32
can be switched ON by detecting faults such as circuit over-voltage or generator short-circuits. However, even when there is no ON command, VBO free thyristors
11
,
12
,
21
,
22
,
31
and
32
can fire by themselves and prevent over-voltage.
In the prior art circuit in
FIG. 4
, when VBO free thyristors
11
,
12
,
21
,
22
,
31
and
32
self-fire due to over-voltage, they do not fire simultaneously due to randomness of firing voltage levels between individual thyristors and temperature variations. If it is assumed that the firing level of VBO free thyristor
11
is the lowest, VBO free thyristor
11
will fire first. The voltage impressed on VBO free thyristor
12
will increase due to the firing of VBO free thyristor
11
. Therefore, VBO free thyristor
12
will also fire almost simultaneously. In this way, the series circuit which includes VBO free thyristors
11
and
12
will fire.
On the other hand, when the self-firing levels of the series circuits which include VBO free thyristors
21
,
22
,
31
and
32
are high, over-voltage is inhibited due to the firing of the series circuit which includes VBO free thyristors
11
and
12
. Therefore, the series circuits which include VBO free thyristors
21
,
22
,
31
and
32
will not fire. For this reason, the circuit current concentrates in one of the three parallel circuits, so that three times the current flows through VBO free thyristors
11
and
12
, and VBO free thyristors
11
and
12
will break down.
As described above, due to randomness between the self-firing voltages of individual VBO free thyristors and temperature variations, the flow of current becomes concentrated in one of the parallel circuits. In order to prevent this, devices with completely identical characteristics were selected from a suitable number of VBO free thyristors and these were connected in parallel. In addition, temperature control for the unit was required in order to prevent the temperatures from becoming uneven. Thus, there was the problem of the unit becoming bulky and also very costly.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an apparatus for protecting switching circuits and an effective method of protecting the switching circuits which prevents current concentration in one circuit of the switching elements which are connected in parallel as described above.
In order to achieve the above object, an apparatus of this invention is comprised of a switching circuit composed by connecting in parallel multiple circuits in which at least one switching element having the characteristic of self-firing according to an over-voltage are connected in series; detect means for detecting the self-firing of at least one switching element in the switching circuit; fire means for applying an firing pulse to all the switching elements in response to an output signal from the detect means. And a method of this invention entails detecting the self-firing of at least one switching element in the switching circuit and applying a firing pulse to all the switching elements in response to the detected self-firing.
In the invention constructed as above, a firing pulse is supplied to the other'switching elements which are connected with a switching element which has fired according to over-voltage. Therefore, current concentration in one of the parallel connected multiple circuits can be prevented.
REFERENCES:
patent: 4697219 (1987-09-01), Mitsuoka
patent: 4796146 (1989-01-01), Takahashi
Foley & Lardner
Huynh Kim
Kabushiki Kaisha Toshiba
Sherry Michael J.
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