Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H1/00—Details of emergency protective circuit arrangements
  • H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
  • H02H1/003—Fault detection by injection of an auxiliary voltage
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
  • H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current

Definitions

• the present invention relates to a device for current limitation and protection against faults in a current fed to an electrical load, in the form of an electrical apparatus or installation.
• the device is designated a fault current limiter and comprises an electrically conductive current limiting element.
• a device of the above mentioned kind is for example a fuse comprising an element which upon overload or a large over-current, a fault, will trip, i.e. melt, when the faulty current has caused a sufficiently large energy development in the current limiting element to bring the fuse- element to a temperature above its melting point.
• This device exhibit an energy controlled trip with little sensitivity as to the nature of the fault.
• European patent document EP 0 487 920 discloses a device for current limitation and protec- tion against overload and shortcircuit currents which comprises a current-limiting element based on a so called PTC-element.
• the PTC-element disclosed in this document comprises a polymer-based electrically conductive body that exhibits a resistivity with positive temperature coefficient and at least two electrodes, where the polymer-based body is in free contact with at least one of the electrodes or an other electrically conductive body.
• the expression, in free contact, means in this application that the body is not bonded to an electrode by welding, soldering, adhesives, fusion or any other chemical or physical bond but that the electrode and body is held together and electrical contact is maintained over such contact surface by a com- pressive force applied by pressure-applying means to hold the electrode against this contact surface of the body.
• This will give rise to a contact pressure between body and electrode in such a contact surface. Since current transition over such surface only occur in a number of contact points, a current constriction arises at the surface, which result in a voltage drop. This voltage drop will be located substantially in a layer of the body which exhibits the highest resistivity, i.e. the polymer based body.
• a low contact resistance is maintained at the contact surface in that the pressure-applying means applies a sufficient pressure to ensure current transition in a sufficient large number of contact points.
• the temperature will increase at the contact points resulting in a local melting and/or gasification of the polymeric material at some of these contact points.
• a layer of gasified polymer/carbon will develop in the vicinity of this contact surface, whereby the resistance is increased sharply and markedly across this layer and the current through the PTC-element is limited, the PTC- element trips. This increase in resistance will limit the current passing through the PTC- element.
• the sharp change in resistance when the PTC-element switches from low-resistivity to high-resistivity state and vice- versa at gasification and restoration respectively is for a material such as polyethylene with a carbon black filler in the order of from a tenths of a m ⁇ or less to hundreds of m ⁇ or more.
• the international application WO-96/25783 discloses a current limiter for which it is possible to control at which electrode tripping will occur and thereby offer a faster and more reliable current limitation at short-circuit currents or other fault currents.
• This current limiter comprises a polymer based electrically conductive body with two contact surfaces, where one of the contact surfaces is contacted, i.e. the voltage drop at this contact surface and its corresponding electrode is minimised while the other contact surface of the body is in free contact with its electrode. This is achieved by arranging the body with a surface layer, which exhibits a lower resitivity than the bulk of the body, at the contacted contact surface.
• a contacted surface of this kind comprising a surface layer with lower resistivity than the bulk of the body is in this application designated, a contacted contact surface.
• the current limiters described are all energy controlled, i.e. they will trip after the fault current have caused a sufficient energy development in the current limiting element mainly concen- trated to the surface(s). This suits applications operating at a low rated current, i.e.
• a device for current limitation and protection against faults in a current fed to an electrical load e.g. a load in the form of an electrical apparatus or installation to be protected, is in this application called a fault current limiter.
• a fault current limiter according to the present invention comprises an energy controlled current limiting element in the form of an electrically conductive body such as a PTC-element or a fuse, electrically connected in series with the load.
• a circuit for current injection is electrically connected with the current limiter.
• the current supplied to the load through the fault current limiter is in this application called the load current and its current path the main circuit.
• the circuit for current injection comprises according to the present invention a capacitor and a switch connected in series.
• the switch which can be either a mechanical switch or a switch based on a power semicon- ductor such as a thyristor, is controlled and operated by a control unit based on changes in the properties of the load current.
• the charge of the capacitor is sufficient to upon release cause the current limiter to trip on its own or in combination with the fault current cause an energy development in the current lim- iting element exceeding trip energy.
• the control unit which is arranged to control and operate the switch comprises;
• a processor that analyses the properties of the monitored load current and on detection of certain deviations in these properties activates means which operates the switch.
• These means operating the switch can be electromechanical means or an out signal generated in the processor which when received by the switch activates the switch or thyristor
• Such a current limiting element comprises;
• the trip zone of the polymer based body is predetermined to a thin layer adjacent to the elec- trode in free contact, the tripping side, while the electrode and the body on the contacted side, the non-tripping side, as well as the bulk of the body remains essentially unaffected by the trip, i.e. the body exhibits a tripping side and a non-tripping side.
• a substantial part of the energy developed in the body is developed in the trip zone, and essentially all localised heating and gasification in other areas of the body is avoided due to the low energy development at the contacted surface of the body and the similarly low energy development in the bulk of the body.
• the gas evolution in the trip zone will give an even current limitation effect over the whole contact surface on the tripping side.
• the gas produced by the polymer and the filler due to energy development in the trip zone will be sufficient to separate the whole electrode on the trip side from the body.
• the current limitingelement comprises a divided electrode on one side of the polymer-based body. At least one part of the divided electrode is connected to the main circuit and at least one part of the divided electrode is connected to the current injection circuit.
• the use of a divided electrode allows for a reduction in the current injection energy while still achieving the same current limiting effect.
• the gas generated in a tripping zone in the part of the body comprising a contact surface in contact with the current injection part-electrode will spread over the whole contact surface on the tripping side, separating the whole electrode on the tripping side from the body.
• the divided electrode can be either contacted to its contact surfaces on the polymer body or held in free contact with these surfaces.
• the use of a divided electrode on one side of the body results in that the current injection energy, i.e. the energy released when the capacitor is discharged upon detection of a fault in the load current can be reduced so can also the size of the capacitor and the switch.
• the divided electrode is contacted with its contact surfaces on the non-tripping side and one large electrode is held in free contact with the contact surface on the other side, the tripping side. But provided that the divided electrode is supported by a mechanically stiff supporting plate the divided electrode can be arranged on the tripping side. The supported and divided electrode will then be held in free contact with its contact surfaces while the one large electrode on the other side can be either contacted or held in free contact.
• the supporting plate need to be electrically insulating so that the parts of the divided electrode is electrically isolated from each other.
• the current limiting element in addition to the divided electrode also exhibits a division of the electrically conductive body arranged so that one first body is connected to the circuit for current injection, that one second body is connected to the main circuit.
• the electrically conducting bodies are on one side of the fault current limiter connected to two separate electrodes, one for the main circuit and one for the current injection circuit, while on the other side a common electrode is used.
• the common electrode is preferably held in free contact with its contact surfaces on both bodies. But provided that a divided electrode is supported by a mechanically stiff and electrically insulating supporting plate also the supported and divided electrode can be held in free contact with its contact surfaces on the bodies.
• an divided electrode be used on both sides.
• the first body which is included in the current injection circuit, is preferably a polymer-based body as described in the forgoing while a body exhibiting an equal or lower resistivity is favourably chosen for as the second body, i.e. the body in the main circuit.
• the possibility to use a body with a lower resistivity for the main circuit offers a possibility for reduction in losses at normal operation conditions at or below rated current. Such an improvement will not be offset by any delay in response to a detected fault as the capacitor still will be discharged into the current injection circuit comprising the current limiting element.
• the gas evolved from the trip-zone in the polymer based body has proven sufficient also to separate a common electrode or a supported divided electrode on the trip side from the body in the main circuit.
• Suitable material for such a body included in the main circuit is graphite, polymer-based composites with equal or a higher carbon content or a metal, metal composite, ceramic or ceramic com- posite and thus an equal or lower resistivity than the first body.
• electromechanical means is arranged electrically in series with a current injection circuit. These means are adopted to upon discharge of the capacitor through them generate a mechanical force. The generated mechanical force will coun- teract a contact pressure applied over electrodes and a polymer based body by pressure applying means.
• the fault current limiter used for this embodiment comprises; - electrodes;
• - pressure-applying mechanical means such as springs or other resilient element to apply an elastic force that holds the electrodes and polymer based body together and applies a contact pressure in the contact surfaces.
• the electromechanical means will on occurrence of a fault in the load current the capacitor is discharged through them and a mechanical force that counteracts and reduces the pressure applied on contact surfaces is generated by the electromechanical means.
• the contact pressure thus is reduced on discharge of the capacitor in the current injection circuit the contact resistance in a surface where the polymer based body is in free contact with its electrode is increased and tripping is initiated at such a side.
• one side is made the tripping side by arranging one side in free contact with its electrode and the other side contacted but both sides can be held in free contact with respective electrode.
• Suitable electromechanical means are flat coils with opposite winding directions, connected in se- ries with each other and the current injection circuit, arranged within pressure-applying means, preferably within a rigid mechanical framework included in these means to ensure that the applied mechanical force is evenly distributed over the whole contact surface.
• One coil can be replaced with a conducting surface acting like a short-circuit winding.
• the electromechanical means as described in the foregoing and the principle of current injection can be combined in different way to increase the sensitivity of the fault current limiter.
• the mechanical means are connected in series with the current injection circuit which is electrically connected to the current limiting element.
• both the electromechanical means and the polymer based current limiting element are electrically connected to the current injection circuit.
• a plurality of current injection circuits be arranged and connected so that at least one is connected to the current limiting element and one or more to the electromechanical means.
• fault current limiters According to the present invention which is caused to trip based on a fault in the main load current by a control unit using a current injection circuit comprising a capacitor and a switch operated by the same control unit there is no problem to assure simultaneous tripping of a plurality of fault current limiters which are included in the same main circuit and electrically connected,
• each group comprising a plurality of current limiting element electrically connected in parallel, electrically in series to simultaneously increase both the voltage and current capabilities, as the current injection in all fault current limiters will be triggered by the same fault. It will even be possible and suitable to use the same control unit to trig and operate all switches thus connected.
• Figure 1 is a schematic sketch showing the main circuit and the current injection circuit ac- cording to one embodiment of the invented fault current limiter.
• Figure 2 is a schematic sketch of an embodiment with a divided electrode on one side of the polymer based body.
• Figure 3 is a schematic sketch of an embodiment with a divided electrode on one side and a divided body comprising the polymer based body in the current injection circuit and second electrically conductive body in the main circuit.
• Figure 4 is a schematic sketch of an embodiment with a divided electrode on both sides and a divided body comprising of the polymer based body in the current injection circuit and second electrically conductive body in the main circuit. DESCRIPTION OF THE PREFERRED EMBODLEMENTS
• the fault current limiter according to embodiment illustrated in figure one is installed in the main power supply line 10 feeding a load current to a load 14 in the form of one or a multiple of electrical apparatuses or installation.
• the load 14 can be any kind of electrical installation from a single electrical device to a network.
• the faulty current can be a short-circuit current, a lasting overload current but also a disturbance in the time derivative of the current or any other disturbance of the properties deemed to be a fault.
• the electrode 1 la on one side, e.g.
• the side facing away from the load as shown in the figure is preferably contacted, i.e. its surface has been prepared such that the resistance over the electrode and contact surface is reduced compared to an untreated contact surface, while the conductive polymer based body in the current limiter shown in figure 1 is on the load side held in free contact with its electrode 1 lb.
• the side of the current limiting element facing the load 14 constitute the tripping side and the side facing away from the load 14 will constitute the non-tripping side. It is, however not necessary that the load side is the tripping side but the contacted electrode and the electrode held in free contact can be switched around or both electrodes can be held in free contact with its contact surface.
• a circuit for current injection 20 is electrically connected with the current limiting element in the main circuit 10.
• This circuit for current injection comprises a capacitor 21 with a charge of a size sufficient to cause the current limiter to trip upon discharge, and a switch, preferably a thyristor 22 connected in series with the capacitor.
• the current limiter is also equipped with a control unit 30 operating the switch 22 so that on detection of a disturbance in the current deemed to be a fault the circuit 20 for current injection is closed and the capacitor 21 is discharged.
• the energy released upon discharge of the capacitor 21 results in a current being injected through the body 12 that is sufficient to cause the current limiter to trip and only a limited current will pass through the resistor 41 provided in a second circuit 40 in parallel with the current limiting element.
• the discharge can be sufficient to cause the limiter to trip on its own or in combination with the faulty current.
• the control unit have a sensor 31 for monitoring the load current in the main circuit. Such a sensor might be a current transformer linked to suitable instruments for analysing the current as to magnitude, time derivative and other critical properties. Suitable algorithms for use in the control unit is known. On detection of certain deviations in these properties control means 32 in the control unit 30 will operate the switch as described in the foregoing to discharge the capacitor 21.
• the control means 32 can be an out-signal from the control unit 30 to the switch 22 or a motor operating a mechanical switch or the thyristor.
• a fault current limiter according to the embodiment as shown in figure 2 exhibit a further reduction of the required energy for trip. This further reduction have been accomplished by dividing the electrode on at least one side of the body 12.
• the current limiting element shown in figure 2 has the divided electrode arranged on the side facing away from the load but a divided electrode can be arranged on either side or on both sides of the body provided that the divided electrode when held in free contact with the polymer body on a tripping side is supported by a mechanically stiff supporting plate, not shown.
• a supporting plate for a divided electrode need to be in an electrically insulating material such that the two part electrodes 110,111 are electrically insulated from each other.
• a fault current limiter according to the embodiment shown in figure 3 has the losses in the current limiting element at normal operating conditions at or below rated current been substantially reduced. This has been accomplished by also dividing the body in two.
• the two part- bodies are connected at one side by a common electrode 112 and on the other side by the di- vided electrode, exhibiting one part electrode 111 for the current injection circuit and one part electrode for the main circuit 110.
• the conductive body 120 used for the main circuit 10 have a resistivity equal to or lower than the resistivity exhibited by the polymer based current limiting element 121 in its low-resistivity state.
• Suitable materials for the second body to be used in the main circuit is a polymer-based composite with a similar carbon content or a higher carbon content, preferably the same type of carbon is used. Also other conductive or semi- conductive materials, e.g. metals, ceramic based materials or a graphite based disc or felt can be used.
• the fault current limiter shown in figure 3 has a third electrically insulating body 122 or a layer sandwiched between the two bodies 121 and 120 respectively.
• the part electrodes 110,111 comprised in the divided electrode is in one suggested embodiment arranged contacted while the common electrode 112 on the other side is held in free contact with its con- tact surfaces on the two bodies 120,121 but also the divided electrode can be held in free contact.
• a divided electrode can be held in free contact with the polymer body provided that it is supported by a mechanically stiff supporting plate, not shown.
• a supporting plate for a divided electrode need to be in an electrically insulating material such that the two part electrodes 110, 111 are electrically insulated from each other.
• a divide electrode is arranged on both side of the body, which is arranged compris- ing a divided electrically conductive body 120,121 as already described for the embodiment shown in figure 3.
• the part electrodes 110, 111, 115, 116 of either of the divided electrodes is preferably contacted to its contact surfaces while the other divided electrode is supported with a supporting plate, not shown, and arranged such that its part electrodes 110, 111, 115, 116 are held in free contacts with its contact surfaces.
• both the divided electrodes are pro- vided with a supporting plate and arranged such that its part electrodes 110, 111 , 115, 116 are held in free contact with the contact surfaces of the bodies 120,121.

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• 1997
  • 1997-06-18 SE SE9702335A patent/SE9702335D0/xx unknown
• 1998
  • 1998-06-12 WO PCT/SE1998/001135 patent/WO1998058435A1/en not_active Application Discontinuation
  • 1998-06-12 EP EP98928787A patent/EP0990294A1/de not_active Withdrawn

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