Calculating Thermal Expansion for Power Cables

Try : Power Cable and their Application ( Siemens Book 1 or book 2)

Chinchano

The question is not very specific.What is the need for the calculation.The rating of the conductor carrying current will be specified for the temperature of 90 deg c. Design of the cable by the supplier is based on the short circuit current prevailing and stipulated by the end user.

during thermal cycles there are thermal forces expanding the entire cable system.

if we know what the thermal forces are as well as maximum calculated expansion, we can assure that cable splices (e.g.) would be able to take and absorb it without being damaged.

Have you ever done this kind of reasearch? Could you please refer?

How are you going to bury the cable?.If it is a direct burial the temperature rise of the cable can be absorbed by the backfilling material to certain extent.

The cable is intalled and it doesn't matter whether it's direct burried or pulled through conduits. the cable rate is calculated and heat dissipation has been already taken into consideration. Max. ampacity is being reached and cable is thermally expanded. No matter what kind of installation this is.

in steady state condition 90C on the conductor is staying 90C, and the cable is stretched. This is easy case.

While emergency operation, when there is 105C-110C on the conductor for 100 hrs or more, it's dificult to calculate the consequences of this kind of thermal impact.

and even more dificult calculation would be in a case of short cct fault, when conductor carries enormous current and is overheated within fractions of the second.

EHV Power , try " Power Cable and their Application " by Siemens Book 1 or 2 you will see those cases.

Thank you, Chinchano.

I have just ordered it.

The temperature you have mentioned is very close to the temp at which the xlpe insulation will lost its shape and properties.

well, you are right.

but it's not necessarily applicable for every case. Cables are built to withstand not only steady state conditions but also beyond that.

So all the currents during emergency or fault operations are still ok for the XLPE as long as it's within specified parameters of temperature/time.

if you are able to find the level of thermal expansion then share it.

According with some exercises made it in factory with phisical cable sample:

CU 2000 mm2 XLP 230 kV AL shield welded +PEAD Jacket.

Ambient temp 0 % exp

90 celsius conductor temp core exp +/- 2.17 %

105 celsuis conductor temp ( overload ) core exp +/-2.65%

( core is defined CU condcutor + triple extruded insulation +cushing tapes )

AV

The dimensions of most materials increase when the material is heated; the material expands. Design engineers and architects recognize the expansion of material with temperature and design accordingly. This is the reason that expansion joints are designed into concrete roads, bridges, etc.

For the cables, the problem is more severe.

1. The temperature range is greater - from -40C to 130C compared to -40C to 60C for building materials.
2. Cables are made of materials with different properties. Some expand on heating but recover to approximately original dimensions on cooling, some expand but do not re- cover. Expansion rates differ. These differences can cause rupture or permanent deformation of components.
3. Should voids in stress relief systems or splices form or loose connections occur, corona and hot spot or heat deterioration soon will lead to cable failure.

For a long-lived cable, components must be selected with their thermal expansion properties in mind. The following graph shows how two different insulating materials can vary in expansion characteristics over a range of temperature. XLP and EP are used as examples. (For reference, copper expands 1.1% in volume, aluminum 1.6% from 25C to 250C. The linear expansions are 0.4 and 0.53% for copper and aluminum.)

Note: At normal 90C maximum operating temperature, XLP expands approximately twice as much as EP. At 130C emergency temperature, XLP expands approximately three times as much as EP. Excessive expansion of cable insulation's can initiate problems that can lead to eventual cable failure, for example:

1. Shielding tapes can rupture.
2. Insulation's can flow under tightly fitting prefabricated slip-on accessories.
3. Terminations can be damaged when thermal expansion increases length as well as diameter.
4. Expansion on heating and contraction on cooling could result in voids at the stress relief component and the insulation.

AV ( Sorry I can´t up the chart with values how ever test were made it and it is accurate ).

Consider rigid system into manholes taking care of cable and joint ( don´t let them move )

Also you must consider expansion efect in axial way conductor´s and insulations have dif. expansión values ) if you cross the molecular limit of the polymer the insulation can´t back to normal size, it means there will be shrink back efect , also you will have longitudinal displacement moving internal part of joint and terminals and you will have a short circuit ( tracking efect ).

regards

AV