Cable insulation 2

[eejacky]

Dear all experts,

Is the thickness of the insulation for xlpe cable important to cable design? In other words, whether thick insulation is better than thin insulation?
Thank you!!

 

[DanDel]

The thickness of the cable insulation is, of course, directly proportional to the rated voltage level. 15kV XLP insulation is thicker than 5kV XLP. 133% insulation is thicker than 100% insulation, and will last longer for the same voltage level. All cable must be used for no less than its designed rating, regardless of its thickness. Using a higher-rated cable for a given voltage rating is acceptable, and quite common.

 

[Donale]

Check with the manfacturers it all depends on the conditions of usage. The insulation type may meet the xpl basic requirements at a given voltage but, flexability, extremes in temperature and surrounding atmosphere may drive you toward one type or another. Weight may also be a factor. The same voltage rated cable may have different thickness due to the dielectric constants of the insulation. Work with the cable manufacturer to get the correct cable for your needs.

 

[scottf]

eejacky-

I agree with Donale....judging any insulation system is not as simple as just seeing which one uses more material.

 

[kraigb]

Depending on the operating voltage, there is one other economic factor to consider. Capacitive losses can make a difference at higher voltages and higher enrgy costs. The thinner the insulation wall, the greater the capacitive losses of the cable will be. For transmission voltages, the balance is between the cost of additional insulation material (for a thicker insulation) and the cost of losses over the life of the cable (or whatever your economic cost recovery horizon might be).

 

[scottf]

Kraigb-

I am not a cable guy, so please explain why a thinner insulation increases capacitive losses.

 

[kraigb]

Scottf:
Sorry about the late reponse on this. I haven't been checking the forums as often as I used to...
The relation between capacitance and insulation does seem counterintuitive at first, but I think I can explain it in language that is more or less English.

Capacitance changes with respect to the electric field applied across the insulating dielectric. As the distance between the plates is increased, the region of space where the electric field is stored increases. As the distance increases, the electric field remains constant. This increases the potential difference between the plates, given by V = E*d where d is the distance between the plates. To keep the potential from increasing, charge must be removed from the plates. Hence, the capacitance decreases. Conversely, as you decrease the distance between the plates of the capacitor, the same electric field is distributed across a smaller distance, increasing the charge held on the plates.

You can also look at the following equation used to calculate capacitance for single conductor shielded cables, and note the relationship between capacitance and the thickness of the insulation...

C =(0.00736*K) / (LOG10(D/d))

(C is in units of microfahrads/1000ft in this case)
Where:
K = dielectric constant of the insulation
D = diameter over the insulation in inches
d = diameter of the conductor in inches (or over the conductor shield if present)

This relationship is used by some instrument manufacturing companies to measure insulation thickness if the dielectric constant of the insulation is known.

Dielectric losses for cables can be calculated by multiplying the capacitance by the power factor and the square of the operating voltage (and a conversion constant to get the losses in the units you want).

It's been a while since I had to talk in terms of electric fields, so my terminology may be a little rusty. Hope this helped.
Kraigb

 

[SidiropoulosM]

kraigb, are the dielectric losses important at all voltage levels? I suspect they are not significant except at high voltages, 230 kv and above. Also, how do the dielectric losses compare to conductor losses?

Thanks, Michael Sidiropoulos

 

[Janesh]

Hi guys,

For every voltage level IEC specifies a corresponding nominal insulation thickness. eg:- for 11 kv cables the specified nominal thickness is 3.4 mm. IEC further specifies that the minimum thickness at any point can not be less than 90% of the nominal value by 0.1mm. That means if the insulation thickness at any point if found less than (0.9*3.4 -0.1) mm ie, less than 2.96 mm for an 11 kv rated cable the cable is rejected.

Further the average thickness of insulation (rounded to 0.1 mm) at any cross section of the core shall not be less than the nominal thickness specified above.

If the thickness is on the higher side electricaly there is no problem. Effects of capacitance, dielectric losses etc. are negligible especially at medium voltage level. However higher insulation thickness has the following disadvantages-
1) It requires excess insulation material
2) Higher diameter of the cores makes the diameter of the laid-up cores high, which inturn increases the materials required for fillers,wrapping tapes,inner sheath, armour,outer sheath etc.
3) An excess outer diameter of the cable can make it not suitable for standard sized cable glands etc.

Advantages

1) Increase in outer diameter of the cable slightly improves the current rating of the cable.


Thanks


Janesh

 

[kraigb]

In response to the question from Michael Sidiropoulos:

You are correct that the losses go up as the voltage goes up, but what you consider to be negligable depends more on the economic philosophies of the company owning the lines.

Depending on the ROI (return on investment) horizon a company chooses, the yearly cost of losses ($/watt for every meter of cable installed) can become a factor in the decision. If the company is more interested in the life-cycle cost of the cable, it is quite possible that an increase of $0.03/foot for every year the cable is in service will make a difference. (This, of course, depends also on how long the cable is that you plan to install.)

 

[busbar]

Apologies if this has already been covered, but ANSI-type standards establish a nominal dielectric thickness based on voltage and insulation level, and tolerable variations. AEIC CS8 for medium-voltage cables is one reference.

 

[edjh2]

Remember use surge arresters on underground systems. Normally installed in the pad mont transformer