KNEE POINT TEMPERATURE
When the temperature of a composite conductor is increased, the aluminium (or aluminium alloy) envelope wires usually expand at a higher rate than the core. This expansion is accompanied by a corresponding reduction in its share of the total tensile load on the conductor.
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With increasing temperature, the aluminium will transfer its mechanical load to the core resulting in the core carrying most, if not all, of the mechanical load.
The temperature at which the envelope of the conductor becomes mechanically “unloaded”, the conductor being supported only by its core. This temperature is called Knee Point temperature.
This is the point below which the sag-tension relationship of the conductor is determined by the whole conductor whereas above this point it is governed by the core.
For High-Temperature conductors, since aluminium has a larger Coefficient of Thermal Expansion (CTE) than the core, the thermal elongation properties of the core control the maximum sag of the conductor.
The knee point temperature is different for different types of conventional HT/HTLS conductors. It depends on the installation conditions and for some conductor technologies, the knee-point can be shifted to a lower temperature by pre-tensioning. This is more effective with conductors containing annealed aluminium envelopes.
The knee-point is not a fixed value and depends on many factors like span length, mechanical tension, ruling conditions, and conductor constituent characteristics (e.g. proportion of envelope section over total section, coefficient of thermal expansion, modulus of each component).
It should be further noted that creep in the conductor will also shift the conductor knee point to lower temperatures over time, because conductor creep is primarily driven by the aluminium constituent, and the tensile load in the envelope shifts to the core while it elongates (due to creep).
For an HT conductor to be fully effective, the rated temperature of the line needs to be above the knee-point, to have full benefit from a low-expansion core.
FACTORS AFFECTING THE KNEE POINT
Different conductor materials have varying physical properties and different temperatures of the knee points considering unique thermal conductivities. Considering, alloying as a major manufacturing process for electrical conductors, the knee point can shift based on the impurities present and the composition of the alloy. Knee point also depends on the grain structure of the material and the heat treatment process can effectively alter it.
GRAPH OF KNEE POINT TEMPERATURE VS SAG
The lower the temperature at which the knee point of the conductor is reached, the more will be the provision of temperature rises before the maximum sag of the conductor is reached. Thus, depending on the required sag limit of the line, the sag-temperature relationship of the conductor can be designed very specifically. With the low knee point of the conductor, the thermal performance of the line will be high. However, the installation and construction challenges of lines using such special conductors are equally difficult.
In a sag-temperature curve, the knee point is the point at which the slope of the sag-temperature changes. The lower this slope is changed the more will be the effect of the sag temperature relationship on the steel core. Thus, using a low expansion core in the conductor will result in higher temperature limits and low sag.
CONCLUSION
It may therefore be understood from above that at the knee point temperature the core will be under tension for which it is specifically designed. If the line is operated below the knee point temperature of the conductor, there will be mechanical loading in the envelope of the conductor and it may cause damage to the envelope if usage is prolonged at or below the knee point temperature.
