Underground Cable Ampacity Research Articles

Analysis of Underground Transmission Cable Ampacity for On-site Applications

Analysis of Underground Transmission Cable Ampacity for On-site Applications

Fast calculation method of directly buried cables ampacity

Accurate calculation of the ampacity of underground cables is an important prerequisite for ensuring the safe and economical operation of cables, the realization of this goal mainly depends on the accuracy of the calculation of the external thermal resistance of the cable. The accuracy of numerical method is high but slow calculation speed, the analytical algorithm (IEC standard supply) is fast, but the calculation error is large when dealing with the non-uniform soil problems with poor heat conduction performance backfill soil. In this paper, the external thermal resistance of cable is calculated by finite volume method and equivalent thermal circuit method, substitutes the calculation result into the analytical calculation formula, which can quickly and accurately calculate the ampacity of the cable group. This method is faster than the pure numerical method, and which is more accurate than the analytical algorithm. The superiority of this method is proved by comparing with analytical algorithm, coordinate combination method and field-circuit combination method in uniform and non-uniform soils.

The Heat and Buried Cable Conundrum: A Method to Help Determine Underground Cable Ampacity

One of the most important factors affecting underground cable ampacity is the thermal resistivity of the soil. A soil's thermal resistivity varies with moisture content, and the heat generated by cables can cause the soil to dry, increasing its resistivity. The ability of the soil to maintain its thermal resistivity in the presence of a heat source is known as thermal stability. Since no soil has perfect stability, accounting for the fluctuating thermal resistivity of soil makes the cable ampacity calculations difficult. This article will examine the information available from standard soil tests and the information these tests may provide relating to the migration of moisture in the soil and the resulting changes in soil resistivity. Furthermore, we suggest a method for including this information in underground cable ampacity calculations.

Cable Ampacity Calculations: A Comparison of Methods

When designing electrical power systems, it is often necessary to determine underground cable ampacity. Various methods are in use today, including computer simulation, ampacity tables, and a method that has recently been suggested that includes the effects of moisture migration through soil. Each of these methods can yield substantially different ampacity results for the same installation. Regardless of the method, using the correct value of soil thermal resistivity is critical and using the wrong value can result in cables that are incorrectly sized. This paper examines several commonly used methods and their underlying assumptions. Examples are provided to illustrate the differences in the results obtained from various methods and the consequences of using incorrect assumptions. It is hoped that these examples will provide guidance on the implementation of each method.

Analysis of underground cable ampacity considering non-uniform soil temperature distributions

An analytical model for the assessment of cable ampacity due to non-uniform underground temperature distribution is presented. The uneven underground temperature is usually caused by a non-uniform surface heating (e.g., a street or parking crossing, etc.), which is extended for certain length along the cable installation and in depth. The underground temperature distribution is obtained by solving the heat equation with the respective boundary conditions. The cable ampacity reduction is calculated as a function of the surface temperature distribution and the cable depth of burial. The model is validated and several numerical results are obtained for different installation conditions.

Refinements to the Neher-McGrath model for calculating the ampacity of underground cables

The Neher-McGrath method has been widely accepted as an accurate and relatively simple way to calculate the ampacity of underground cables. It is based on a number of assumptions that simplify the mathematics, but at the same time limit the accuracy and scope of the model. Furthermore, the model relies upon correlations that are now dated, because more up-to-date and accurate heat transfer correlations are now available. This paper examines improvements to the Neher-McGrath model in three different areas: a more accurate expression for the mutual heating parameter that accounts for unequal heating among the cables; improved correlations for the thermal resistance of a fluid layer that exists in pipe-type cables and cables installed in ducts; and a more accurate model for the thermal resistance of concrete duct banks with nonsquare cross-sections. Example installations are then considered to illustrate how the incorporation of these changes will affect the ampacity of the installation. The refinements suggested result in a more complex mathematical algorithm and require more computational time, but the changes can result in significantly different ampacity values than the ones provided by the Neher-McGrath model.

Ampacity of Electric Power Cables in Vertical Protective Risers

A method is presented to compute the ampacity of an electric power cable located in both vented and nonvented circular protective risers. While techniques for calculating the ampacity of underground cables and overhead lines are well established, techniques for rating the riser portion of a cable circuit have not been as thoroughly studied. Usually cable ampacities are based on the assumption that the underground portion of the circuit will be cooler than the portion located in the riser. Therefore, approximate derating factors are used to reduce the underground cable circuit ampacity when risers are present.

Ampacities of Underground Electric Power Cables in the Presence of External Forced Cooling

This paper presents a method of computing the ampacity of underground cables that are cooled by external pipes carrying a coolant. The method utilizes conduction shape factors and an effective thermal resistivity which represents the presence of both the surrounding soil and the cooling pipes. When used in place of the native soil resistivity, the effective resistivity value will produce the same cable temperatures as would exist when a cable is buried near external cooling pipes. The effective resistivity value can therefore be used in ampacity tables or existing ampacity programs, thereby extending their application to cases that involve forced cooling. An experimental horizontal three-cable/two-pipe system was used to verify the results obtained from the mathematical analysis. The predicted effective thermal resistivities for this system were used in an ampacity program and the resulting conductor temperatures were within a maximum of 3.4?? C of the experimentally measured temperatures for conductor temperatures up to 105?? C.

Ampacities of Underground Electric Power Cables in the Presence of External Forced Cooling

This paper presents a method of computing the ampacity of underground cables that are cooled by external pipes carrying a coolant. The method utilizes conduction shape factors and an effective thermal resistivity which represents the presence of both the surrounding soil and the cooling pipes. When used in place of the native soil resistivity, the effective resistivity value will produce the same cable temperatures as would exist when a cable is buried near external cooling pipes. The effective resistivity value can therefore be used in ampacity tables or existing ampacity programs, thereby extending their application to cases that involve forced cooling. An experimental horizontal three-cable/two-pipe system was used to verify the results obtained from the mathematical analysis. The predicted effective thermal resistivities for this system were used in an ampacity program and the resulting conductor temperatures were within a maximum of 3.4?? C of the experimentally measured temperatures for conductor temperatures up to 105?? C.

A Numerical Procedure for the Calculation of the Temperature Rise and Ampacity of Underground Cables

A numerical procedure employing the finite element technique is developed for the calculation of the heat dissipation from a distribution duct bank. The procedure is used for calculating the conductor temperature and the ampacity of cables with and without forced cooling. The accuracy of the procedure is demonstrated by the comparison of the results with experimental findings. The accuracy, versatility and ease-of-use of the numerical procedure over presently employed analytical procedures show clearly the advantages of the numerical procedure.

A Numerical Procedure for the Calculation of the Temperature Rise and Ampacity of Underground Cables

A Numerical Procedure for the Calculation of the Temperature Rise and Ampacity of Underground Cables