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The efficiency of all types of heavy current busbars depends upon careful design, the most important factors being:

To meet these requirements there are many different arrangements of copper busbars using laminations, as well as copper extrusions of various cross-sections.

To obtain the best and most efficient rating for rectangular strip copper conductors they should be mounted whenever possible with their major cross-sectional axes vertical so giving maximum cooling surfaces.

Laminations of 6 or 6.3 mm thickness, of varying widths and with 6 or 6.3 mm spacings are probably the most common and are satisfactory in most a.c. low current cases and for all d.c. systems.

It is not possible to give any generally applicable factors for calculating the d.c. rating of laminated bars, since this depends upon the size and proportions of the laminations and on their arrangement. A guide to the expected relative ratings are given in Table 8 below for a 50 Hz system. The ratings for single bars can be estimated using the methods given in Section 3 and Section 4.

Table 13 (Appendix 2) gives a.c. ratings for various configurations of laminated bars based on test measurements.

For all normal light and medium current purposes an arrangement such as that in Figure 9a is entirely satisfactory, but for a.c. currents in excess of 3000 A where large numbers of laminations would be required it is necessary to rearrange the laminations to give better utilisation of the copper bars.

The effect of using a large number of laminations mounted side by side is shown in Figure 10 for a.c. currents. The current distribution is independent of the total current magnitude.

This curve shows that due to skin effect there is a considerable variation in the current carried by each lamination, the outer laminations carrying approximately four times the current in those at the centre. The two centre laminations together carry only about one-tenth of the total current.

The currents in the different laminations may also vary appreciably in phase, with the result that their numerical sum may be greater than their vectorial sum, which is equal to the line current. These circulating currents give rise to additional losses and lower efficiency of the system. It should also be noted that the curve is non-symmetrical due to the proximity effect of an adjacent phase.

For these reasons it is recommended that alternate arrangements, such as those discussed in the following sections, are used for heavy current a.c. svstems.

Where long low-voltage a.c. bars are carrying heavy currents, particularly at a low power factor, inductive volt drop may become a serious problem with laminated bars arranged as in Figure 9a. The voltage drop for any given size of conductor is proportional to the current and the length of the bars, and increases as the separation between conductors of different phases increases. In the case of laminated bars the inductive volt drop can be reduced by splitting up the bars into an equivalent number of smaller circuits in parallel, with the conductors of different phases interleaved as shown in Figure 9b. This reduces the average spacing between conductors of different phases and so reduces the inductive volt drop.

The unbalanced current distribution in a laminated bar carrying a.c. current due to skin and proximity effects may be counteracted by transposing laminations or groups of laminations at intervals. Tappings and other connections make transposition difficult, but it can be worthwhile where long sections of bars are free from tappings. The arrangement is as shown in Figure 9e.

To obtain a maximum efficiency from the point of view of skin effect, as much as possible of the copper should be equidistant from the magnetic centre of a bar, as in the case of a tubular conductor. This can reduce the skin effect to little greater than unity whereas values of 2 or more are possible with other arrangements having the same cross-sectional area.

With flat copper bars the nearest approach to a unity skin effect ratio is achieved using a hollow square formation as shown in Figure 9c, though the current arrangement is still not as good as in a tubular conductor. The heat dissipation is also not as good as the same number of bars arranged side by side as in Figure 9b, due to the horizontally mounted bars at the top and bottom.

This arrangement (Figure 9d) does not have as good a value of skin effect ratio as the hollow square arrangement, but it does have the advantage that the heat dissipation is much improved. This arrangement can have a current-carrying capacity of up to twice that for bars mounted side by side, or alternatively the total cross-sectional area can be reduced for similar current-carrying capacities.

A tubular copper conductor is the most efficient possible as regards skin effect, as the maximum amount of material is located at a uniform distance from the magnetic centre of the conductor. The skin effect reduces as the diameter increases for a constant wall thickness, with values close to unity possible when the ratio of outside diameter to wall thickness exceeds about 20.

The natural cooling is not as good as that for a laminated copper bar system of the same cross-sectional area, but when the proximity effects are taken into account the one-piece tube ensures that the whole tube attains an even temperature - a condition rarely obtained with laminated bar systems.

Tubular copper conductors also lend themselves to alternative methods of cooling by, for example, forced air or liquid cooling where heat can be removed from the internal surface of the tubes. Current ratings of several times the natural air cooled value are possible using forced cooling with the largest increases when liquid cooling is employed.

A tubular bar also occupies less space than the more usual copper laminated bar and has a further advantage that its strength and rigidity are greater and uniform in all deflection planes. These advantages are, however, somewhat reduced by the difficulty of making joints and connections which are more difficult than those for laminated bars. These problems have now been reduced by the introduction of copper welding and exothermic copper forming methods. Copper tubes are particularly suitable for high current applications, such as arc furnaces, where forced liquid cooling can be used to great advantage. The tube can also be used in isolated phase busbar systems due to the ease with which it can be supported by insulators.

This arrangement is not widely used due to difficulties of support but has the advantage of the optimum combination of low reactance and eddy current losses and is well suited to furnace and weld set applications. It should be noted that the isolated phase busbar systems are of this type with the current in the external enclosure being almost equal to that in the conductor when the continuously bonded three-phase enclosure system is used.

Alternative arrangements to flat or tubular copper bars are the channel and angle bars which can have advantages. The most important of these shapes are shown in the diagrams below.

These are easily supported and give great rigidity and strength while the making of joints and connections presents no serious difficulty.

The permissible alternating current density in free air for a given temperature rise is usually greater in the case of two angle-shaped conductors (diagram (a)) than in any other arrangement of conductor material.

For low voltage heavy current single-phase bars with narrow phase centres, single copper channels with the webs of the 'go' and 'return' conductors towards one another give an efficient arrangement. The channel sizes can be chosen to reduce the skin and proximity effects to a minimum, give maximum dissipation of heat and have considerable mechanical strength and rigidity. Where high voltage busbars are concerned the phase spacing has to be much larger to give adequate electrical clearances between adjacent phases with best arrangement being with the channel webs furthest apart. For high-capacity generators which are connected to transformers and allied equipment by segregated or non-segregated copper busbars, the double angle arrangement gives the best combination with the copper bar sizes still being readily manufactured. The current ratings of these arrangements are given in Table 15 (Appendix 2). The ratings given are the maximum current ratings which do not take the cost of losses into account and hence are not optimised.

The extent to which the a.c. current rating for a given temperature rise of a conductor containing a given cross-sectional area of copper depends on the cross-section shape. The approximate relative a.c. ratings for a typical cross-sectional area of 10 000 mm2 are shown in Figure 11. For cross-sectional areas greater than 10 000 mm2 the factors are greater than those shown, and are smaller for smaller cross-sections. In the case of double-channel busbars, the ratio of web-to-flange lengths and also the web thickness have a considerable effect on the current carrying capacity.

Figure 11 Comparative a.c. ratings of various conductor arrangements each having a cross sectional area of 10,000 mm2 of HC copper

In many cases busbars are surrounded by enclosures, normally metallic, which reduce the busbar heat dissipation due to reduction in cooling air flow and radiation losses and therefore give current ratings which may be considerably less than those for free air exposure. Ventilated enclosures, however, provide mechanical protection and some cooling air flow with the least reduction in current rating.

The reduction in rating for a given temperature rise will vary considerably with the type and size of bar and enclosure. The greatest decrease in current rating occurs with bars which depend mainly on free air circulation and less on uniform current distribution such as the modified hollow square arrangement (Figure 9d). In these cases the rating may be reduced to between 60 and 65% when the conductors are enclosed in non-magnetic metal enclosures. In the case of tubular conductors or those of closely grouped flat laminations, which are normally not so well cooled by air circulation, the ratings may be reduced to about 75% of free air ratings for normal temperature rises.

Where the busbar system is enclosed in thick magnetic enclosures, such as in metal-clad switchgear, the reduction is approximately a further 15%. The effect of thin sheet-steel enclosures is somewhat less. These additional reductions are due to the heat generated by the alternating magnetic fields through hysteresis and eddy current losses. Besides the derating caused by enclosure conditions, other limitations on maximum working temperature are often present, such as when the outside of enclosures should not exceed a given safety value. These deratings are affected by the electrical clearances involved and the degree of ventilation in the enclosure. The above figures and the curves shown in Figure 12 should only be taken as a rough guide to the required derating; an accurate figure can only be obtained by testing.

All parts such as conductor and switch fittings, enclosures and interphase barriers may be subject to appreciable temperature rise due to circulating and eddy current losses when close to the heavy current bars and connections. These losses can be reduced to a minimum by making these parts from high conductivity non-magnetic material such as copper or copper alloy.

Figure 12 Comparison of approximate current ratings for busbars in different enclosures

The current rating of copper immersed in oil or compound depend upon a number of factors which may vary widely with design, and can normally only be confirmed by carrying out temperature rise tests on the complete assembly.

The ratings of enclosed bars are nearly always much lower than the free air ratings. The temperature rise is dependent on the rate at which heat is conducted through the insulating media and dissipated from the outside casing by radiation and convection. There is nearly always a closer phase spacing between conductors giving high proximity effects and higher heat losses in the magnetic outer casings and so giving rise to higher temperature rises.

Proximity effect is often more important for insulated bars than those in air. Laminated bars have fewer advantages when immersed in oil or compound and circular copper conductors either solid or hollow though are often preferred particularly for high-voltage gear and high current generators, transformers, etc., where more effective cooling such as water cooling can be employed to improve conductor material utilisation and hence reduce the overall size of plant.

There is a widening use of plastic continuous insulation as the primary insulation for low current and voltage busbars. This insulation is usually of the shrink-on P.V.C. type though wrap-on tape is sometimes used. This method is used for voltages up to about 15 kV, though much higher levels can be attained when specialised insulation systems such as epoxy resin or similar based tapes and powders are employed. These systems are particularly useful where high atomic radiation levels, or high temperatures (up to 130°C) are encountered, although account must be taken of the possibility of halogen gassing from P.V.C. insulations at temperatures around 100°C. Modified P.V.C. materials with improved high-temperature performance are available.

solated phase busbars consist of a metallic enclosed conductor where each individual phase or pole is surrounded by a separately earthed sheath which is connected at its ends by a full short-circuit current rated bar. The sheath is intended primarily to prevent interphase short-circuit currents developing. They have the further advantage that the high magnetic fields created by the conductor current are almost completely cancelled by an equal and opposite current induced in the enclosure or sheath with reductions of 95% or better in the external magnetic field being possible. An important result is that the likelihood of steelwork overheating when adjacent to the busbar system is considerably reduced except where the sheath short-circuit bars are located. This current flowing in the enclosure makes the method of estimating the performance of the busbar system much more complicated and can only be resolved by obtaining a heat balance between conductor and enclosure using an interactive calculation method.

These busbars are used normally for operating voltages of between 11 kV and 36 kV though equipment using much lower voltages and higher voltages are increasingly changing to this system. Examples of such equipment are exciter connections, switchgear interconnections, generator to transformer connections, high voltage switchgear using SF6 (sulphur hexafluoride) gas insulation (this gas having an insulation level many times better than air). The current flowing in the conductor ranges from as little as 1000 A to in excess of 40 kA. To obtain the higher currents forced cooling is used, the most commonly used cooling media being air and water though other cooling gases or liquids can be used. The use of these cooling systems usually creates much increased heat losses and so their use must be justified by benefits in other areas, e.g., reduced civil costs, reduced physical size where space is at a premium or reduction in size to enable normal manufacturing methods be used both for the basic busbar material and also the complete busbar system.

Another factor which influences the method chosen for forced cooling is the naturally cooled rating of the busbar system and also its ability to sustain overload conditions. The busbars are usually manufactured in single-phase units of transportable length and consist of a central conductor usually tubular of round, square or channel cross-section, supported by porcelain or epoxy resin insulators. The insulators are located by the external metallic sheath through which they are normally removed for servicing.