Electricity: electrical systems and devices – Safety and protection of systems and devices – Impedance insertion
Reexamination Certificate
1999-12-08
2001-09-18
Fleming, Fritz (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Impedance insertion
C338S013000, C338S100000
Reexamination Certificate
active
06292338
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric coupling device, to an electric circuit provided with such an electric coupling device and to methods for connecting, disconnecting and/or limiting the current in an electric circuit.
2. Discussion of the Background
Considerable advantages could be gained if the short-circuiting current in an electric power system could be limited. No expedient current limiter exists today apart from fuses. Increased demands on the quality of electric power accentuates the need for current limiters. Current limiters based on electric arcs, resonance circuits or semiconductors are usually so bulky and expensive that they cannot be considered for other than extremely specialised purposes.
Various types of surge arresters have long been used to limit the voltage in electrical systems and providing a current limiter which, similar, is inexpensive, small and simple would satisfy an urgent need. It is desirable for the voltage over the current limiter to be dependent only on the current strength.
An equivalent need also exists in connection with connecting and disconnecting the load current in an electric circuit and it would be desirable to achieve this without electric arcs and with more efficient attenuation of oscillations in the system.
SUMMARY OF THE INVENTION
Against this background the object of the present invention is to provide an electric coupling device for limiting, connecting or disconnecting the current in an electric circuit, which can also be used for high currents, which is simple, small and inexpensive, and which is free from electric arcs and also enables rapid attenuation. The object is also to provide a method whereby these objects are achieved.
From a first aspect the object of the invention has been achieved by means of an electric coupling device having the features defined in the characterizing part of claim
1
. The invention relates to both direct current and alternating current.
Thanks to a resistor with the specified properties being included as an essential component in the electric coupling device, the current can be greatly limited when the resistor is controlled to its state with very high resistance. Since this occurs very rapidly the current can be limited before it causes damage to the electrical equipment connected. The continual alteration in the resistance also enables oscillations in the circuit to be quickly dampened. An electric coupling device of this type can also be made extremely small, simple and inexpensive, and without electric arcs.
Similar advantages are achieved when using the device as breaker or connector.
In a preferred embodiment the electric coupling device comprises an isolating switch arranged in series with the variable resistor, which facilitates the connection, disconnection and limiting functions of the electric coupling device.
Most suitably the resistor is of a type having pressure-dependent resistance so that the resistance decreases with increasing pressure, and vice versa. With a resistor of this type it is easy to rapidly alternate from the extreme states of the resistor. This therefore constitutes a preferred embodiment of the invention.
In a particularly preferred embodiment the pressure-dependent resistor is of a type comprising a powder where, in compressed state, the powder behaves as a homogenous conductor and in decompressed state has a large number of contact points and consequently high total resistance. A powder-based variable resistor is described in SE 971372-6, from which priority has been claimed for the present patent application. The resistor used in the present application is suitably of the type described in said Swedish patent application.
The resistor should be so constructed that the relation between the resistance in its substantially insulating state and in its conducting state is greater than 10
6
, preferably greater than 10
9
. The resistor should also be arranged for action times of less than 3 ms, preferably less than 200 &mgr;s. The short operating time ensures that no damaging current strength levels are reached during disconnection and current limiting. Furthermore, in the case of a powder-based resistor, a dynamic force is generated in the disintegration of the powder that facilitates a considerable change in resistance.
In another preferred embodiment the electric coupling device is provided with a resistor connected in parallel with the variable resistor. This second resistor may be linear or non-linear.
In yet another preferred embodiment a second contact is arranged in parallel with the variable resistor. The variable resistor can then be made more simple since a slightly larger resistance can be accepted in its low resistance state.
Alternatively a second contact may be arranged in series with the variable resistor and the first isolator.
An important application for the electric coupling device is as current limiter. The alteration in state of the variable resistor can then be arranged to occur depending on sensing of the current in the circuit. This constitutes another preferred embodiment of the invention.
The invention also relates to an electric circuit comprising the electric coupling device according to the invention when used for connecting and disconnecting an electric circuit and for limiting the current. Such an electric circuit exploits the advantages of the electric coupling device according to the invention as described above.
In such an electric circuit it is advantageous to arrange the electric coupling device close to the power consumption points in each branch of the electric circuit.
The invention also relates to the method of connecting, disconnecting and limiting the current in an electric circuit, the method being performed in a manner corresponding to the function of the electric coupling device according to the invention and offering equivalent advantages.
The invention offers advantages from many aspects. Protracted disturbances in the faulty branch downstream of the electric circuit device are reduced in the event of short-circuiting and there is a chance of immediate reparation. Brief disturbances in operation upstream of the electric coupling device are reduced and cause shorter or negligible voltage drop. The same applies to brief operational disturbances in parallel branches.
As regards the loads, higher available short-circuiting effect is possible for heavy starts, for instance, and the transformers can therefore be specified with lower short-circuiting reactance. Conditional connection of local generation is also possible so that extra transformers or other power detours are eliminated. Faster and more efficient protection of generators and machines is achieved and a greater freedom of choice in zero-point earthing for the machines is available. The connecting reactor or resistor is eliminated in capacitor banks and sensitive loads.
Lower requirements can be set for the mechanical dimensioning and strength of the equipment, thus enabling freer constructions and an opportunity of selecting compact and optimal geometries.
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Bernhoff Hans
Breder Henrik
Isberg Jan
Jonsson Lars
Liljestrand Lars
ABB AB
Fleming Fritz
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Roberto Rios
LandOfFree
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