EP1149450A1

EP1149450A1 - An electric device

Info

Publication number: EP1149450A1
Application number: EP99965668A
Authority: EP
Inventors: Lars Liljestrand
Original assignee: ABB AB
Current assignee: ABB AB
Priority date: 1998-12-22
Filing date: 1999-12-20
Publication date: 2001-10-31
Also published as: SE9804496D0; AU2135700A; WO2000038292A1; SE9804496L; SE516373C2; JP2002534043A

Abstract

The invention relates to an electric protective device which includes a current limiter (101). The current limiter is arranged to be activated if a predetermined current level above normal current level is reached. According to the invention, detecting means (102) detect whether the increased current level is a fault current or a load transient, and a circuit breaker (103) is arranged to open if there is a fault current, but remain closed otherwise. The invention also relates to a corresponding method, to a circuit provided with the device and to a use of the device.

Description

AN ELECTRIC DEVICE

Technical Field

The present invention relates in a first aspect to an electric protective de- vice of the kind specified in the preamble of claim 1. In a second aspect, the invention relates to an electric circuit as specified in the preamble of claim 9. In a third aspect, the invention relates to a method as specified in the preamble of claim 11. The invention also relates to a use of the electric protective device as specified in the preamble of claim 17. The device and method thus concern protecting components from the effect of a fault current in an electrical power system. Such is the result of a short circuit in the system.

A consequence of a short circuit in an electrical power system is that the current increases from the nominal load current to a value given by the short cir- cuit impedance of the system and the impedance of the fault. The short circuit current can reach levels of more than 100 times the nominal current. The high short circuit current causes thermal and mechanical stress on the equipment upstream the fault. Therefore, short circuit currents have to be interrupted to reduce the stress and the damages. Another consequence of the short circuit is that the voltage will drop in a large part of the power system until the fault is cleared.

After the short circuit is interrupted, a part of the system is disconnected with the result that the power supply is interrupted to that part of the system. It is necessary that the circuit breaker closest to fault interrupts the short circuit current in order to minimize the area with interrupted power supply. It is important that short circuit currents are interrupted with a minimum of delay to avoid damages to the equipment upstream the fault.

However, sometimes an increased load current will occur in the power system during normal operation, which current is not a short circuit current. Examples of normal operations causing temporary high load currents are: • energizing of transformers or reactors

• start of large motors

• cold load pick-up. It is important to avoid interrupting such high load currents.

The need to interrupt a fault current but on the other hand to avoid interrupting a high load current requires distinction between a fault current and a high load current. However, this causes a problem, since the decision to interrupt the current has to be taken within the short time before the current level reaches the dangerous level. This might occur within a fraction of a ms. Within that short time, it is almost impossible to determine whether the increased current is a fault current or a high load.

Against this background, it is the object of the present invention to attain a solution to this problem and provide a device and a method that are able to protect electric equipment in a system from high fault currents, but which do not interrupt the current if an increased current level is the result of temporary high load current.

Description of the invention

This object has been achieved in that a device of the kind specified in the preamble of claim 1 includes the features specified in the characterizing portion of this claim, in that an electric circuit as specified in the preamble of claim 9 includes the features specified in the characterizing portion of this claim, in that a method as specified in the preamble of claim 12 includes the measures specified in the characterizing portion of that claim, and by the use specified in claim 17.

At the occurrence of increased current, the current thus is not immediately interrupted but only limited by the fault current limiter (FCL). Limitation of the current takes place as soon as a tripp condition indicating the increased current level is met and this almost immediately, i.e. within 1 ms. Thereby, the electric equipment is immediately protected, but the current is not yet interrupted. This offers a possibility to postpone the decision whether to interrupt the current or not until it is possible to determine if it is a fault current or a current transient load. Such a determination can be done after about 10 to 20 ms. Within that time, the current has passed through at least one half cycle and it is then easy to distinguish a fault current from other reasons for the increased current level. In the first case, the current is interrupted at that moment, otherwise not. Without limiting the current, a fault current would have destroyed the equipment before it would have been possible to evaluate whether it was a fault current or not.

The main advantages by limiting the amplitude of fault currents at short circuits in power systems are:

• The mechanical and thermal stress of the apparatus and components are reduced.

• Breakers and other equipment with lower rated short circuit currents can be used. • Increased power quality due to a reduced duration of voltage dips.

The basic principle for fault current limiting is to increase the impedance in the circuit between the source and the location of the fault. A large number of fault current limiters (FCL) are discussed in the literature, although there are no products for medium and high voltage systems having multishot function which can be reset for repetitive function. There are a number of possible reasons:

• The relation between cost and achieved function.

• The use of a technique not yet mature.

• The size of the apparatus.

• The desired function cannot be achieved. An important remark is that the cost of a fault current limiter must be compared with the cost benefit of a complete system dimensioned for load current and not merely the cost benefit of a single component.

Very high currents can be achieved when a short circuit occurs in a power system. The amplitude of the current depends on the short circuit impedance of the system at the location of the fault and the type of fault (single line ground fault, 2-phase line-to-line fault or 3-phase fault).

A limited fault current has a constant amplitude. When connecting a transformer or another load, there can be a high transient current with decreasing amplitude. If this current is limited, the time constant will decrease due to the in- creased resistance. It is thus possible to distinguish between a fault current and an inrush current. As a result, there is time enough for a protective circuit to decide whether to trip the circuit breaker of not. The amount of energy dissipated by the FCL will also be much less as the FCL will limit the current during a short period only until stable conditions are reached. A FCL will not only limit the current, but also allow stable conditions to be resumed much faster.

According to a preferred embodiment of the invention, the trip condition is the current level itself. Thereby, a reliable and simple way of activating the FCL is achieved.

In a preferred embodiment, the FCL is activated when the current reaches a level of 2 - 5 times the normal load current, preferably about 3 times.

It is desirable that the opening of the circuit breaker occurs as soon as possible after the current is limited. According to a preferred embodiment, the detection of the reason for the increased current therefore is made within 50 ms and preferably within 20 ms after activation of the FCL.

According to a further preferred embodiment, the FCL includes a variable resistor that can alternate quickly between a state of low and high resistance. Thanks to a resistor with the specified properties being included as an essential component in the FCL, 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 increase of resistance also enables oscillations in the circuit to be quickly dampened. A FCL of this type can also be made extremely small, simple and inexpensive, and without electric arcs.

According to still a further preferred embodiment, 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 PCT/SE 98/00679, the content of which hereby is incorpo- rated in this application by reference. The resistor used in the present application is suitably of the type described in said PCT-application.

In a preferred embodiment of the invention, the device includes means for resetting the FCL after the detection of a increased current level. The FCL is reset irrespective of the detected increased level being due to a fault current or an inrush current. Preferably, the circuit breaker is arranged to be closed again after its opening and after resetting the FCL.

Advantageous embodiments of the invented method include measures corresponding to the mentioned preferred embodiments of the invented device. The above and other advantageous embodiments of the invention are specified in the dependent claims.

Brief Description of the Drawings

Figure 1 is a schematic representation of the principle of the invention, Figure 2 is a graph illustrating fault current and inrush current without current limitation, Figure 3 is a graph similar to that of fig. 2, but with current limitation, Figure 4 is a schematic illustration of a power network illustrating when short circuit occurs, Figure 5 is a graph illustrating the current as a function of time when the invention is applied to the network of fig. 4, Figure 6 is a similar graph, but without current limitation, Figure 7 is a schematic illustration of the power network of fig. 4 illustrating energizing of a transformer, Figure 8 is a graph similar to that of fig. 5 when the invention is applied to the network of fig. 7, Figure 9 is a similar graph, but without current limitation,

Figure 10 shows schematically a resistor in compressed state according to an embodiment of the invention, Figure 1 1 shows schematically the resistor of fig. 10 in expanded state. Description of Preferred Embodiments of the Invention

In Figure 1 , the working principle of the invention is illustrated in a block diagram. Block 104 represents measuring one or more parameters directly or indirectly indicating a high current. This might be the current level itself, the voltage 5 level or some other parameter or combination of parameters. If it is detected in block 104 that the trip condition is met, e.g. that the current level is more than three times the normal current level, the FCL 101 is activated to limit the current. The detection of whether the trip condition is met and the activating of the FCL is performed very rapidly, within less than one ms. Therethrough, the current is lim- l ϋ ited before the increased current causes any damage to the electric equipment downstream the protective device. This gives time to analyse whether the increased current is a fault current or not.

Block 102 represents detecting means to evaluate if the increased current is a fault current or a transient load current. Since the current now is limited, it is

15 no longer urgent to do this very rapidly. Normally, this evaluation can be done within one full cycle, i.e. 20 ms.

Should it be detected that the increased current level is caused by a transient load current, i.e. no fault, the process follows line I, whereby the FCL is reset and the power supply continues as represented by block 105. 0 Should a fault current be detected, the process follows line I I , whereby the circuit breaker (CB) 103 is opened to interrupt the current. The operating time of the CB is about 30 ms. The fault clearing time, the duration from the instant the fault occurs until the fault current is interrupted, thus will be about 50 ms (the sum of the detection time and the fault clearing time). 5 Figure 2 is a graph illustrating the difference between a fault current

(curve F,) and an inrush current (T,) when there is no current limitation.

Figure 3 is a graph correspondingly illustrating the currents when current limitation is applied, where F₂ is a fault current and T₂ is an inrush current. The activation of the current limiter is arranged to occur at about 3kA in the example 0 shown. As is apparent from Figure 3, the fault current can be clearly distinguished from the inrush current already during the first cycle after limiting the current, i.e. within 20 ms By evaluating the information as represented in Figure 3, it can be determined whether the FCL was activated due to fault current or not, and consequently the circuit breaker is opened correspondingly, or maintained closed

The two situations are further illustrated in figs 4 to 9 Figure 4 is a sche- 5 matic diagram of a part of an electric network to which the invention is applied In branch 15 of the network, three electrical components 1 1 , 12, 13 are illustrated, of which 12 is a transformer and 13 a motor With reference 14, a short circuit is represented In the branch 15 upstream of the equipment, a protective device 10 according to the invention is provided

I t⁾ Figure 5 illustrates the situation in Figure 4 At point A, an increase in the current exceeding trip condition is detected Almost immediately (within 1 ms), the current is limited At point B, occurring one cycle later, the increased current is identified as a fault current, and at point C, another 30 ms later, the current is interrupted

15 Figure 6 illustrates what would have happened, if the protective device 10 had not been present, i e when using the technique of directly interrupting the current as soon as a fault current is identified The very high current due to the short circuit would last during the time it takes to identify a fault current and the time required to activate the CB As shown in the figure, this time from A' to C 0 adds up to more than two cycles, maybe even three cycles

Figure 7 illustrates the same network as in fig 5 in a situation when the transformer 12 is to be energized by closing the contact 16 No short circuit is at hand

Figure 8 is a diagram similar to that of fig 5 illustrating the course of ac- 5 tion during the energizing of the transformer As soon as the circuit breaker 16 is closed, the energizing current increases the current level above trip level, which is detected at A" and the current is limited accordingly At point B", one cycle later, it is detected that the current is not a fault current Therefore, the current limited is reset and the power supply continues without interruption 0 Figure 9 illustrates the situation shown in fig 7, but without current limitation As shown, the main difference is that with current limitation (fig 8), the cur- rent is limited for about one half-cycle, which is an advantage, since the transformer reaches steady state faster

Figures 10 and 1 1 illustrate the principle of a variable resistor according to one embodiment of the invention Figure 10 shows the resistor in a position with very low resistance and in Figure 1 1 , the resistance is very high The device can easily be described as a container 1 of non-conducting material, having a wall 2, 3 of conducting material at each end, each of the end walls 2, 3 being connected to a conductor 4, 5 In the container is a powder, e g T1B2, with a particle size of approximately 10 μm The end wall 3 is displaceable laterally in the figure and is in a position in

Figure 10 where it presses strongly against the powder In Figure 1 1 , the end wall is withdrawn so that it does not compress the powder

In the state shown in Figure 10, the powder 6 is firmly compressed so that it acquires electric properties similar to those for equivalent material in solid form When a material like TιB₂, with good conductivity, is used the powder will therefore constitute a good conductor with a resistivity in the vicinity of mΩcm The compression force is a few MPa

In the expanded state shown in Figure 1 1 , the particles 6 have been given space to separate from each other so that they will brush against each other with small contact surfaces Each contact surface gives rise to a contact resistance and the powder as a whole forms a large number of chain-linked such resistances, together producing considerable resistance between the end walls 2 and 3 In this fluffy, loosened state the powder will have a resistivity in the order of MΩcm or higher, i e 10⁹ times higher than in compressed state The powder thus becomes insulating

The displacement of the movable end wall 3 from the position shown in Figure 10 to that shown in Figure 1 1 takes place in approximately 100 μs and the increase in volume is approximately 20% The negative pressure wave caused by the rapid increase in volume together with the inherent spring force in the powder particles caused by the increase in volume results in the powder particles spreading through the available space so that the loosened state ensues The invention, however, is not limited to the use of the described technique for current limiting. Other operation principles for the FCL, such as mechanical switches with arc-chambers, LC-resonance limiters, semiconductor limiters, increasing impedance fuses and superconductors fall within the scope of the present invention.

Claims

1 An electric protective device including a circuit breaker (103) for breaking the current on occurrence of a fault current, characterized in that the device fur-

^ ther includes a current limiter (101 ) and detecting means (102), the current limiter (101 ) being arranged to be activated at a predetermined trip condition representing an increased current level, the detecting means (102) being arranged to detect whether said increased current level is a fault current or a transient load current, and in that the circuit breaker (103) is arranged to open the circuit if a fault 0 current is detected by the detecting means (102), but remain closed if a transient load current is detected by the detecting means (102)

2 An electric protective device according to claim 1 , wherein said trip condition is a predetermined current level above normal current level 5

3 An electric protective device according to claim 2, wherein said predetermined current level is a multiple of the normal current level in the range of 2 - 5, preferably about 3

0 4 An electric protective device according to any of claims 1-3, wherein the detecting means (102) is arranged to perform the detection within 50 ms, preferably within 20 ms

5 An electric protective device according to any of claims 1-4, wherein the ^ current limiter comprises a variable resistor (6) with resistance variable between a very low value corresponding to a conducting state and a very high value corresponding to a substantially insulating state which resistor (6) is arranged to be able to switch between said states continually and in a short time

0 6 An electric protective device according to claim 5, wherein the resistance of the resistor (6) is pressure-dependent

7 An electric protective device according to claim 6, wherein the resistor (6) comprises a powder (6)

8 An electric protective device according to any of claims 1-7, further in- eluding current limiter resetting means (105) and circuit breaker resetting means

9 An electric circuit (108-1 12), characterized in that it is provided with at least one electric protective device (10) as claimed in any of claims 1-8

10 An electric circuit according to claim 9, comprising a plurality of branch conductors (15) connected with a plurality of loads (1 1 12, 13), wherein said at least one electric protective device (10) is arranged in one or more of the branch conductors (15)

1 1 A method for breaking the current in an electric system, characterized in that the current is limited at a predetermined trip condition representing an increased current level, whereafter it is determined whether the increased current level is a fault current or a current transient load, and the current is broken if a fault current is detected but remains uninterrupted if a current transient load is detected

12 A method according to claim 1 1 , in which said trip condition is a predetermined current level above normal current level, preferably a multiple of the normal current load in the range of 2-5 and preferably about 3

13 A method according to any of claims 1 1 -13, in which the detection is performed within 50 ms from activating the current limiter, preferably within 20 ms

14 A method according to any of claims 1 1 -13 in which the current limiter in- eludes a variable resistor which is activated by bringing the resistor to a substantially insulating state with very high resistance from a conducting state with very low resistance

15. A method according to claim 14, in which the variable resistor includes the features specified in any of claims 6-7.

5 16. A method according to any of claims 11-15, in which the current limiter is reset after detection and, in case the current has been cut off, after the cutting of the current.

17. Use of the electric protective device according to any of claims 1-9 to pre- I 0 vent damage caused by fault current and to avoid unnecessary breaking when an increased current level is reached due to other causes.