HV Cable Current Ratings For A Challenging Installation

Published 23 May 2022

HV Cables in Multiple Backfills

HV Cable Current Ratings

FEM Current Rating Solution

The customer required HV cable current ratings for an installation involving multiple (6) zones of backfilled soil with Thermal Resistivity (TR) varying between 1.2 – 5.2 K.m/W plus the resistivity of the native soil needed to be considered.

The IEC Standard methods do not cover such a complex installation. The most IEC 60287 can deal with is a single backfill. The native soil thermal resistivity was 1.5 K.m/W which is lower than the backfill areas – the IEC Standard equations are invalid for this condition alone.

The Finite Element Method (FEM) was needed to accurately solving the heat flux distributions for this installation.

How does the FEM model work?

In FEM the space is subdivided into smaller parts known as finite elements using a technique called meshing. These elements take the shape of a tetrahedron or a triangle.

The domain of interest is space discretized and this discretization is implemented by constructing a mesh of the object. The formulation of finite element method results in a system of algebraic equations.

These equations that model the finite elements are then assembled into a larger system of equations that models the entire problem. The solution to the problem is reached by minimizing the mean squared error error function.

For the analysis of HV cables, it is assumed to be in a 2D space. The heat generated by the cable conductors is considered along with the ambient temperature. The FEM model creates the temperature array and a large set of equations are solved.

Another important consideration of the HV cable system which is modelled using FEM is the ground surface and ambient air temperature interface which involves heat convection – this aspect is neglected using the IEC method.

How much difference does FEM make?

The calculated current ratings of HV cables using FEM cannot even be compared with the (approximate) IEC Standard method because the standard method is not valid for this installation.

The current rating for the 11kV single core, 300sqmm copper conductor cables surrounded with 6 zones of soil with different resistivity as well as the native soil resistivity, soil ambient temperature of 25 deg.C. and soil/air interface was as follows:

FEM method = 283 A.
IEC method (uniform soil resistivity 5.2 K.m/W) = 201 A (-29 % underestimate).
IEC method (uniform soil resistivity 1.5 K.m/W) = 366 A (+29 % overestimate)

In conclusion, only using FEM will give an accurate calculation of HV cable current ratings for this complicated installation.

Jayson Patrick – Technical Director, Electrotechnik

Jayson is the Technical Director at Electrotechnik, a company which develops leading electrical power system design and analysis software and has extensive experience working on large-scale power systems projects in high voltage design, testing/commissioning, and power system analysis roles along with two decades of experience in professional software development.

Jayson has a master’s degree in electrical power engineering and is a professional software developer. He leads a multi-disciplinary team of developers, cloud architects, UI designers, electrical power engineers and PhD researchers. Jayson’s role at Electrotechnik involves developing new software and complex algorithms for electrical power systems design, where he is using a combination of technologies, multiple programming languages and Agile development practices.

Jayson has a passion for UI and UX design and believes in building powerful software which is accurate while being intuitive and easy to use.

Earthing Conference

Jayson will be delivering a talk at Earthing Conference UK in Birmingham this month.

1:30pm – Day One

Analysis of Earthing Grids in Multilayer Soils at High Frequencies

Jayson Patrick – Technical Director, Electrotechnik

Modelling earthing system behaviour during both power frequency fault conditions and at high frequencies, especially related to lightning phenomena, is of interest to earthing system designers. The behaviour of earthing systems at varying frequencies is quite different and this paper will present details of the algorithm developed and results in the form of a parametric analysis for multiple grids in different soil conditions for high frequencies.

THORNE & DERRICK

T&D are Specialist Distributors to UK Distribution Network Operators (DNO’s), NERS Registered Service Providers, ICP’s and HV Jointing Contractors of an extensive range of LV, MV & HV Jointing, Earthing, Substation & Electrical Eqpt – this includes 11kV/33kV/66kV cable joints, terminations and connectors for both DNO and private network applications.

Contact our UK Power Team for competitive quotations, fast delivery from stock and technical support or training on all LV-HV products.