As stated earlier, many of the current-limiting fuselinks which are produced are in the back-up or partial-range category. Their constructions and behaviour are described
in the following sections and then fuselinks in the full-range category are described in Section 5.2.3.
5.2.1 Constructions of back-up or partial-range fuselinks
These are of the cartridge type, constructed basically as described in Section 4.1. The special features associated with HV fuselinks are outlined below.
Fuselinks with low rated currents have silver-wire elements. Those with higher ratings have elements made from silver strip, with restricted sections at regular inter-vals along their lengths. These are produced by punching out appropriately shaped notches or holes. All elements have low-melting-point metals applied to them to enable the M-effect to be obtained.
It was explained in Chapter 3 that the speed of arc extinction depends on the voltage dropped across the arcs. It was also explained that the sum of the voltages across the arcs tends to limit the current rise initially and thereafter to cause the current to reduce. For this process to be effective in HV circuits, the total voltage across the arcs in a fuselink must be high, and to achieve this requires either a very long arc or many short ones, the latter being somewhat more effective because of the anode- and cathode-fall voltages associated with each arc.
In consequence and because the total length of break, after operation, must be sufficient to withstand the system voltage and provide adequate isolation, the ele-ments in high-voltage fuselinks are long and have many restrictions. These lengths exceed the lengths of the fuse bodies needed to prevent external flashovers between the end caps at voltages of about 3·6 kV and above and therefore to make compact designs at these voltage levels the elements are accommodated in a helical form by winding them on formers. These are usually made of a refractory material and have a star-shaped cross-section. Such a former is in contact with the elements wound on it only at a number of localised points, and maximum contact between the quartz filling material and the elements is thus achieved. The inclusion of for-mers assists in obtaining consistent performance because they ensure that elements are correctly spaced from the bodies and in multiple-element fuselinks the individual elements can be equally spaced from each other. The former, together with one or more parallel-connected elements, is mounted in the fuse body which is cylindrical and usually made of ceramic material which is sometimes reinforced by adding a glass-fibre covering. The complete arrangement of a typical fuselink is shown in Figure 5.5.
It is common practice in the UK for HV fuselinks to be immersed in oil in various pieces of equipment, such as ring-main fuse-switch units. Several advantages arise because of the cooling effect of the oil, which allows a given current rating to be obtained with a smaller cross-section of element than is necessary in a fuselink which is to be used in air. The smaller element operates more rapidly at very high current levels and the fuselink can be made physically smaller and cheaper.
Fuselinks which are to be immersed in oil must be sealed and tested to ensure that they will prevent the ingress of oil. They are normally fitted with cylindrical cap contacts of 63·5 mm diameter and are produced in two basic overall lengths of 254 and
end cap
Figure 5.5 Construction of typical high-voltage fuselink
Figure 5.6 Fuselinks suitable for oil immersion
359 mm. The caps which are of copper or brass are tin or nickel plated. Examples of these fuselinks are shown in Figure 5.6.
These fuselinks, which have references F01 and F02, are available for use in three-phase circuits, principally at 11 or 13·8 kV, and in current ratings up to about 100 A. Voltage ratings from 3·6 up to 24 kV are also available. They are capable of breaking duties in excess of 250 MVA.
In recent years, over-zealous safety regulations and more restrictive maintenance budgets are resulting in lower usage of oil-immersed equipment. In such cases air type current-limiting HV fuses may be included in the T-off circuit of an SF6ring
Figure 5.7 Fused end box
main unit. Alternatively full range fuses (see Section 5.2.3) may be fitted as sole protection, e.g. in a fused end box attached to the side of the transformer tank or in a simple non-striker tripped switch unit. An example of a fused end box is shown in Figure 5.7 which illustrates two types of insulation methods for the fuselinks.
High-voltage fuselinks, given the reference TA3, are produced for use in air and these generally have brass or copper spade terminations with holes in them to allow bolted connections to be made. These fuselinks are available for use in three-phase systems principally at 11 kV but ranging in voltage rating from 3·6 to 15·5 kV and with current ratings up to 100 A or more.
Fuselinks with cylindrical cap contacts of either 50 or 76 mm diameter and having references FA1–FA5 are produced in lengths of 359, 565 and 914 mm, these dimen-sions being necessary because of their high levels of rated voltage, up to 72·5 kV.
They are for use in air. Typical examples of FA1–FA5 and TA3 fuselinks are shown in Figure 5.8.
Fuselinks used for the protection of three-phase motors generally have elements which are corrugated in the manner shown in Figure 5.9. This enables them to with-stand the cyclic mechanical stresses which can arise during periods when the motors which they protect are being started and stopped repeatedly. These fuselinks gener-ally contain a greater number of wider elements than type TA3 fuses, a combination which enables a high operating current at about 10 s to be obtained.
Designs for use at voltages up to 7·2 kV do not usually contain a former on which the elements are wound and supported. This increases the space available for elements in particular body sizes and thus makes it possible to include a greater number of
Figure 5.8 Fuselinks for use in air
Figure 5.9 Corrugated elements
Figure 5.10 Group of two fuselinks for motor-circuit protection
elements. Motor-protection fuselinks are sometimes mounted in groups of two or three in parallel to achieve the high current ratings needed and bolted connections are usual. Typical voltage ratings are 3·6 to 7·2 kV with current ratings up to about 500 A. A group of two fuselinks is shown in Figure 5.10.
Many fuselinks for use in air outdoors are fitted with weatherproof seals.
Most of the above fuselinks can be provided with strikers to give indication of operation or, more important, to trip associated switchgear. As stated earlier, this latter feature is often essential in three-phase circuits as continued operation after a fuse has opened to clear a single-phase fault might be very harmful. The energy to drive the strikers may be derived from springs or small pyrotechnic devices, the latter being favoured by British manufacturers. In this type, which can be seen in Figure 5.5, a cylinder in the end of the fuselink houses the striker pin and a small charge, of gunpowder, into which an ignition wire is centrally placed. This wire and a series-connected high-resistance fuse wire are connected in parallel with the main fuse elements. After the main elements have melted, the voltage across the fuselink drives sufficient current through the ignition wire to heat it and ignite the gunpowder.
The resulting explosion drives the pointed end of the striker through the fuselink end cap with sufficient force to operate fuse-switch trip mechanisms directly. The penetration of the striker through the end cap is clearly an indication of operation, and it may be used solely for this purpose.
The use of fuses to protect electromagnetic voltage transformers, which supply instruments and relays, is discussed in Section 7.6. The fuses are included in series with the high-voltage windings which normally carry very low currents by power-system standards. Whilst it would be desirable to use fuses with very low rated and minimum-fusing currents to give protection in the event of interturn faults in either set
of windings, such fuselinks would have delicate elements that might break because of vibration or for other non-electrical reasons. This could de-energise relaying equip-ment and jeopardise major plant on the network. The risk cannot be taken and it is therefore accepted that the fuses role is for circuit isolation rather than transformer protection, the common rating available in the UK is 3·15A. The construction of these fuselinks is similar to that of the conventional current-limiting fuselinks described earlier in this chapter. Because of their low-rated current they have only one element or two parallel-connected elements of silver wound on ceramic formers. They are pro-duced for use in three-phase circuits with line voltages ranging from 3·3 to 33 kV and have cylindrical cap contacts or screwed end studs for connection purposes, exam-ples being shown in Figure 5.11. They are not provided with strikers or operation indicators. Both sealed types for use in oil or outdoors in air, and unsealed designs for use indoors in air, are available.
The common practice of mounting low-voltage fuselinks in fuse carriers and bases is not adopted for high-voltage fuselinks because of their relatively large dimensions.
The arrangements vary with the different applications and special mountings are often produced to enable complete fuse units to be incorporated within pieces of equipment.
Figure 5.11 Voltage-transformer fuselinks
Figure 5.12 Fuses mounted in ring-main unit
As an example, the fuselinks in some ring-main units are clipped into a withdrawable three-phase carrier which is mounted in the same oil-filled chamber as the switch mechanism. An illustration of this arrangement is shown in Figure 5.12.
5.2.2 Current-interrupting abilities and categories of partial-range fuselinks
When a high-voltage fuselink with one or more conventional parallel-connected ele-ments is clearing a fault current, short breaks are formed at the element restrictions and short arcs result. The process continues until the burn backs have a total gap length which can withstand the recovery voltage present after the arcs have extin-guished at a current zero. The total gap lengths needed in HV applications are clearly considerable, and the times required to melt and vaporise sufficient element material are correspondingly long. Consequently, only at currents above a certain level is the rate of lengthening of the arcs in any particular fuselink sufficient to so limit the arcing time that the temperature of the filling material is prevented from reaching a level at which its arc-extinguishing properties are so reduced that the fuselink would fail to clear. There is thus a range of currents above the minimum fusing level within which satisfactory clearance will not be effected. This situation, which does not arise with low-voltage fuselinks, can be accepted for some applications as stated later, but it is nevertheless undesirable and manufacturers sought to lower the minimum
safe clearance currents. Experiments demonstrated that improvements were obtained by using large numbers of parallel-connected elements of small cross-sectional area rather than a smaller number of thicker elements. The improvement occurred because all the short arcs set up at each of the restrictions when a relatively low fault current is being cleared do not continue to burn in each of the parallel-connected elements.
In practice they tend to commutate around the elements, the gaps in those elements, which are arcing, extending rapidly because of the high current densities in them. The voltages across these gaps rise to levels where the now shorter gaps in other elements ionise and cause the previously burning arcs to extinguish. This process shortens the time taken for current interruption to be effected. Performance was also improved by employing elements with long restricted sections of small cross-sectional area, which again increased the current densities.
It was nevertheless difficult to produce fuselinks which could safely clear all currents above their minimum fusing currents up to their rated breaking capacities, and therefore three internationally recognised categories of HV fuselinks were introduced, namely:
Back-up Fuses in this category must be able to interrupt all currents between a minimum value specified by the manufacturer and the full rated breaking capacity.
General purpose Fuses in this category must be able to interrupt currents from the rated breaking capacity down to the level at which the operating time is 1 h.
Full range Fuses in this category must be able to interrupt all currents from the rated breaking current down to the smallest current which causes the fuse elements to melt.
It is the practice of the UK Electricity Supply Industry and of supply authorities in numerous overseas territories influenced by UK practice, e.g. Australia, South Africa, India and the Middle and Far East, to use fuse-switch ring-main units.
In the UK, such fuse-switch ring-main units of 250 MVA rating at 11 kV are covered by Electricity Supply Industry standard ESI41-12, which stipulates that the fuselinks must be provided with strikers which trip the switch instantaneously when one or more fuselinks operate.
This not only prevents single-phasing of any motors fed from the transformers but, equally important, eliminates the possibility of trouble if the equipment is subjected to a fault current less than the minimum breaking current of the fuse.
Typically, the total time to trip the switch from inception of arcing in the fuselink may be only 30–50 ms whereas it will generally take at least ten times as long as this for any persistent low-level arc within the fuse to cause any trouble.
It is, of course, only for fault currents below the stated minimum-breaking current for the fuse that such external aid is necessary. At higher fault currents the more usual series-multiple-arcing mode of fuselink operation takes over and ensures easy circuit interruption.
The UK fuse-switch gear is thus fully self-protecting at all possible fault levels and consequently the fuselinks may be of the back-up variety. Such fuselinks having minimum breaking currents (MBC) in the region of 2·5 to 5·0 times their rated current are suitable. Satisfactory co-ordination may be achieved by making the switches capable of breaking at least seven times the rated current of the largest fuselink used.
The statistical chances of faults occurring below the minimum safe breaking cur-rent of the fuse are, in UK practice, quite small in any case, since unearthed neutral points (which could result in small capacitive earth fault currents flowing) are not used and low-voltage secondary-feeder fuses ensure that low-voltage (LV) faults or overloads do not have to be cleared by the high-voltage fuses. Both back-up and general-purpose fuselinks are suitable for such circuits and the latter type has, in the past, been preferred in some applications where instantaneous striker-tripping facil-ities were not provided because the ratios of their minimum safe breaking currents to minimum fusing currents are lower than those of back-up fuselinks. The usage of general-purpose fuselinks is now reducing, however, because of recent developments which have led to the introduction of high-voltage current-limiting fuselinks capa-ble of clearing all currents between their minimum-fusing and rated-breaking levels (i.e. full-range fuses).
As early as 1965, Mikulecky published a paper entitled ‘Current limiting fuse with full-range clearing ability’ [35]. The term full-range has recently been included in fuse specification IEC 60282-1 and it is now widely used by manufacturers and users to describe any HV current-limiting fuse which can, unaided, safely interrupt currents from the rated breaking capacity down to the smallest current which will cause melting of the elements, even under conditions of restricted air circulation.
Clearly such fuselinks may be used as the sole protection in circuits operating over a wide range of normal and abnormal conditions.
5.2.3 Full-range fuselinks
Full-range fuselinks of different designs which are now produced by manufacturers, are described below:
(i) Fuselinks with elements operating at high current densities
It was stated earlier that the minimum safe breaking current of a fuselink reduces as the current densities in its individual elements at rated current are raised. In the past, there were constructional difficulties associated with producing fuselinks with large numbers of elements of very small cross-sectional area. More recently, new assembly methods have emerged which have largely overcome such problems.
(ii) Fuselinks in which electronegative gases are produced
The heat produced when arcing takes place in these fuselinks causes large quantities of electronegative gases to be liberated from solid materials within the bodies. The turbulence, cooling and de-ionising effects of the gases cause the arcs to extinguish after many cycles of burnback. In some designs the solid gas-evolving material forms
Figure 5.13 Full-range fuse with two series elements
part of the supporting former for the elements and in others it is in the form of plates or beads attached to the elements at points along their lengths.
(iii) Fuselinks with two series elements
Expulsion fuselinks, which are not current-limiting, have been described earlier in Section 5.1.1.
In the past, such fuselinks were comparatively large, but the sizes of the com-ponents have now been reduced sufficiently to enable them to be mounted in fuse barrels of standard dimensions in series with current-limiting elements to form integral full-range fuselinks. A typical fuselink of this type is shown in Figure 5.13.
Fault currents above the minimum fusing level up to about five times the rated value are cleared by the expulsion process, the higher currents up to the rated breaking capacity being cleared by the current-limiting elements. The resulting dual-element time–current characteristics closely match the withstand capabilities of protected equipment such as transformers and the co-ordination with other protection devices used on systems is superior to that which can be achieved with many other types of fuses.
(iv) Other types of fuselinks
In recent years several other full-range fuselinks have been marketed or proposed.
In one design, minute explosive charges are placed at points along elements to ensure the formation of breaks simultaneously when the element temperatures exceed the melting-point level.
Full-range fuses of various types are now being produced and installed in many parts of the world. The international standard IEC 60282-1 has been amended to include special tests for these fuses and some designs have already been tested and approved.
The range of types and ratings available is increasing rapidly, and their costs are
The range of types and ratings available is increasing rapidly, and their costs are