During laboratory analysis, it has been found that the permanent elongation of Zebra conductor is of the order of 600 mm/km (or more normally 600 microstrains). The coefficient of linear expansion for Zebra is 20× 10−6 per◦C, and this represents a change in length of 20 mm/km per◦C. Hence, to fully compensate for the predicted increase in sag that the creep will produce, it is possible to erect Zebra at such a tension equivalent to a temperature shift, below ambient, of−600/20 = −30◦C. A similar method is used with predictor equations.
The drawback here is for a large overtension remaining for quite some period after erection, which may possibly cause vibration problems and possibly a higher tension in the conductor (and hence tower fittings) than has been allowed for in the original design. For this latter reason, NGT employs a third ruling condition for their basic sag/tension calculations, namely a fixed tension for the temperature shift employed. In the case of Zebra, this is 40 kN at−30◦C on the assumption that stringing is unlikely to be undertaken when the actual ambient temperature is below 0◦C.
7.8.3
Positive temperature shift
Here, instead of reducing the erection tension, the temperature shift is added on to the design temperature and the sag template drawn accordingly. This allows the erection tension to be the same as the design tension at that temperature. The drawback here is the increase in costs due to the taller structure heights that will become necessary.
7.8.4
Prestressing
Since the rate of creep is large early in a conductor’s life, it is possible to partially compensate for this by prestressing the conductor to some arbitrarily high tension (i.e. maximum working tension or up to 70 per cent of UTS) for a fixed period of time (one or two hours), preferably being re-regulated during that time as the tension falls back. The drawback here is the standing time required by the line gang while the conductor is being prestressed.
Once the level of creep compensation has been determined, a conductor erection table can be formulated and given to the erection gang.
7.9
Conductor clashing
Sag calculations are additionally used to calculate the minimum phase spacing between conductors to avoid conductor clash. A number of equations have been successfully used over the years, some more empirical than others. The method out- lined in Engineering Technical Report ENATR 111 [4] gives greater phase spacings than earlier equations and considers the action of ice-loaded conductors as sustained by an upper and lower component of mean wind pressure (termed the gust and lull pressures). Assuming a horizontal phase separation, the gust and lull pressures have been calculated as 1.832 and 0.546 times the mean pressure, respectively. This method has been included in the software associated with ENATS 43-40 (both issues) and ENATS 43-121. This method is retained in the UK NNA BSEN 50341-3-9. Further details are included in ENATR 111 published by Energy Networks Association.
The method outlined in DIN VDE 0210 [5] for phase spacing is used quite extensively around the world. It is of the form:
mid-span spacing= k√(sag40+ I) + SAM (7.31) where I is suspension insulator string length (m) (if used, otherwise 0), sag40is the still air sag at 40◦C for the maximum span (m), SAMis a voltage-related factor (for 11 kV, this is 0.10 m) and k is a conductor orientation and type factor (for horizontal conductors up to 150 mm2alloy, this is 0.7).
7.10
Additional issues to be considered
7.10.1
Uplift
As stated earlier, one of the sag/tension calculations necessary is the bare conduc- tor sag at the cold design temperature (in the UK,−5.6◦C). The sag calculated is plotted at appropriate scales on to the plastic sag template and used to assess the presence of uplift at intermediate structures. Uplift is the action of the conductor, due to contraction, to attempt to lift vertically away from a suspension structure thereby damaging the conductor, cross-arm and/or fittings. The effect can be calculated using equation (7.28), or more simply by applying the template to the line profile and
Weather loads, conductor sags and tensions 107 cold curve A uplift hot curve C B
Figure 7.5 Uplift in a valley
checking visually whether uplift is a problem. In Figure 7.5 the curve is placed so that it touches structures A and C, and any uplift present at the intervening structure, B, will be instantly seen if the cold curve is above the structure.
To overcome this problem, structures A and/or C could be moved closer to B; alternatively, a taller intermediate structure at B could be used or the structure could be changed to a section (i.e. where the conductor is deadended on both sides of the cross-arm).
7.10.2
Earthwires
The calculation of sags/tensions for earthwires essentially follows that for phase wires as described above. However, it is essential that the earthwire sag does not exceed that of the phase wires, as this will increase the earthwire shield angle beyond the normally effective 30◦. In extreme cases, there is also the possibility of introducing an increased risk of conductor/earthwire clash.
For these reasons, it is usual practice to specify that the earthwire design sag be matched to the design sag of the phase conductor at everyday temperature in still air. Hence, for the sag/tension calculation, the limiting criterion would additionally include the appropriate tension at everyday temperature.