Cable Losses

[pilts]

I wish to carry out a study, comparing various different types of H.V. cables (>33kV) to compare the associated losses of each type. I need to know where to start! Any ideas??

 

[kraigb]

There is an earlier thread entitled "help with transmission line losses" (if I remembered the title correctly)... You should be able to find it with the advanced search of this site if you look for the phrase "dielectric losses." A number of other people posted to that thread, so it might be worth taking a quick look.

The choice of insulation material can make some difference
due to the dielectric constant of the insulation material. The diameter affects the average voltage stress on the insulation, so increasing that is another way to control dielectric losses, but that has a trade-off with the cost of insulation materials.

The equation for dielectric loss from the Neher McGrath paper on
calculation of temperature rise of cables (IEEE Transactions Oct. 1957 -
there have been some updates to this, too) is:

Wd = 0.00276(E^2)(SIC)(insulation PF) / log[(2T+Dc)/Dc]

Where:
Wd is dielectric loss in watts per conductor foot,
E is phase to neutral voltage in kV,
SIC is relative dielectric constant of the insulation material,
Insulation PF is the power factor of the insulation material (also called
tangent delta),
T is the insulation thickness,
Dc is the diameter of the conductor (over the semicon shield).

This equation only calculates dielectric losses for a single conductor cable, so if you had in mind the comparison of other factors like inductive losses of three phase cables, it gets a little more involved, but this should give you a start.

 

[electricpete]

As kgraigb menntioned, it's pretty broad subject. The dielectric losses mentioned above are a function of voltage only and form a very small portion of the total losses. Majority of losses are associated with the current. Obviously starting with conductor I^2*R. The larger the conductor and higher the frequency, the higher the effective resistance (due to the conductor skin effect). Cable shielding, adjacent conductors, and adjacent ground path can also effect distribution of current within the main conductor and therefore it's effective resistance(conductor proximity effect).

Shields, conduits, and possibly other paths (the earth under transmission lines) may have circulating currents or eddy currents which will generate I^2R losses in those components. Those other losses will of course "count" toward total losses if you're looking at it from an efficiency standpoint. If you're only considering conductor insulation temperature, then these will have lesser or no effect.

 

[jbartos]

Suggestions:
1. The cable construction is supposed to be carefully scrutinized and appropriate equation for the cable capacitance applied.
2. Alternately, the cable capacitances may be obtained by measurements.
3. Search the Forum for any Reference to Simons paper, Wadhwa book, Mies formula, Atkinson formula, Russell's formulae.