Assumptions and Limitations

In order to run cable calculations, it is necessary that you have a certain level of knowledge about cables and cable calculations, so that you can make qualified assessments of input parameters. The tool has some known limitations that are important to be aware of.

Assumptions and General Information

Steady State Calculations

Calculations for load variation and load curves in underground power cables are conducted using continuous current over an extended period, aiming to achieve a “steady state” solution. This method assumes a constant load, providing a conservative estimate compared to scenarios that include load variation.

Time to Reach Steady State

The time required for underground power cables to reach a steady state can vary significantly, depending on the thermal properties of the cable and the surrounding environment. Steady state is achieved when transient thermal effects have dissipated, and the cable’s temperature stabilizes. According to various studies and industry standards, this process can take anywhere from hours to days, and in some cases, even years for deeply buried installations. For example:

IEC 60287 Standards:

The IEC 60287 standards indicate that the thermal time constant for reaching a steady state in underground power cables can range from several hours to a few days, depending on the cable’s size, insulation, and soil conditions.

Research by George Anders (1997):

In “Rating of Electric Power Cables in Unfavorable Thermal Environment,” Anders suggests that thermal equilibrium for cables buried in typical soil conditions generally takes 24 to 48 hours. Factors such as soil moisture content and thermal conductivity can significantly impact this duration.

Field Studies by REN and SINTEF:

Field studies conducted by REN in collaboration with SINTEF, Norway’s premier research institution for power systems, have demonstrated that underground cables in Norwegian soil conditions usually reach thermal steady state within 24 to 36 hours. These studies align well with the IEC standards and provide practical verification of theoretical models.

Thermal Resistivity

Thermal resistivity in masses is one of the most important criteria for cable calculations. Choosing the right thermal resistivity therefore becomes crucial for the calculation result.

Phase Configuration

In the case of multiple cables per phase, phase order is of great importance (Especially in flat layout). A different phase order will give a different current carrying capacity. If the user is unsure if the installation of the cable phases will follow the phase configurations in the calculations exactly, the user should find the worst phase configuration as the basis for the project. This is to take account of the worst-case scenario.

Milliken Cable

For Milliken cables (segmented conductors), our solution is to combine IEC’s empirical formulas for Milliken cables with a simplified 2D model in COMSOL. The proximity effect and the skin effect is calculated according to IEC 60287. To ensure accurate results, the distance from a Milliken conductor to any other conductor should be minimum 300 mm (center to center).

Limitations

Screen Currents with Varying Screen Configuration

The program only calculates screen currents and screen voltages in the individual trench section and does not take into account that the layout may vary in the length of the cable. For cable trenches that vary along the length of the cable, the user must take this into account separately. For example, a short section in trefoil formation in a long cable that is mainly laid in a flat formation will show too low shield current in the part that is laid in a triangular layout. This is because the program does not take into account the contribution to screen current from the other trench sections.

Pipe-in-Pipe

For pipe-in-pipe calculations, the program gives a conservative value. The heat distribution in the outer pipe layer is calculated without convection.

Load Variation/Load Curves

The calculations are made with continuous current over a long period of time, which gives a “steady state” solution. This means that a calculation where the maximum load is used will be conservative compared to a load with load variation.

Running water/Groundwater

Running water under ground or groundwater will be able to provide good cooling for cables and result in completely different temperatures than what is calculated.

Cables in Water Filled Pipes

The IEC formula assumes the cable is centered in the pipe. However, shifting the cable to the bottom has minimal impact on the outcome, as conduction remains the primary method of heat transfer.

The formula and constants used are empirically determined and may not fully align with the actual conditions of water, such as water flow characteristics.

Crossing Infrastructure

It is currently not possible to enter crossing infrastructure into the tool. It is possible to add the infrastructure in parallel, which will give a conservative calculation.

Steel Structures. (Magnetic constructions)

The current version of the tool does not take into account the induction and heating of metallic structures near the cables. But there is a specific feature available for cables in metal pipes.

Cables in Metal Pipes

No circulating current losses are considered for magnetic pipes, as the pipe is buried in the ground, where the potential of 0V is assumed.

Shallow Cable Trenches

As standard, the tool calculates with a fixed temperature on the ground’s surface (isothermal surface). Cables with less coverage than 400 mm should be calculated with convection against air and air temperature to take into account that the ground may be heated.

Deep Cable Trenches

For deep cable trenches, the calculations will be conservative as the method assumes that heat must be led away to the surface. Here, one must assume an ambient temperature in the ground. This will vary less over the year at deep locations and will often be close to the annual mean temperature at the site at around 10 m depth.

Solar Radiation

The model does not support solar heating of the ground.

Air Convection

Air convection are not included in custom objects and cable troughs, and simulation results for those items are likely conservative. However, air convection for cables in pipes are calculated using empirical formulas.

Armored Cables

The effects of the armor are not taken into account in the model.