Cable Ampacity Calculation


CYMCAP is designed to simulate the thermal behavior of power cable installations. The original version of this package was developed jointly by Ontario Hydro (Hydro One), McMaster University and CYME International, under the auspices of the Canadian Electricity Association.

Program Features

CYMCAP is dedicated to performing ampacity and temperature rise calculations for power cable installations.

Determining the maximum current power cables can sustain without deterioration of any of their electrical properties is important to the design of electrical installations.

The accuracy of CYMCAP provides increased confidence when upgrading existing power cable installations and designing new ones, thus maximizing the benefits from the considerable capital investment associated with them.

CYMCAP software

Analytical Capabilities

* Iterative techniques based on Neher-McGrath and IEC-60287 methods.
* Full compliance with North American practice and support all pertinent IEC standards IEC 60287, IEC 60228, IEC 601042, IEC 60853, etc.
* A detailed graphical representation of virtually any type of power cable. This facility can be used to modify existing cables and enrich the cable library with new ones. This includes single-core, three-core, belted, pipe-type, submarine, sheathed, and armored cables.
* Different cable installation conditions such as directly buried, thermal backfill, underground ducts or duct banks.
* Cables in pipes with the pipe directly buried or in a thermal backfill.
* Independent libraries and data bases for cables, duct-banks, load curves, heat sources and installations.
* Simulation of cables in air on riser poles, groups of cables in air, moisture migration, nearby Heat sources and Heat sinks, etc.
* Different cable types within one installation.
* Non-isothermal earth surface modeling.
* Cyclic loading patterns as per IEC-60853.
* Multiple cables per phase with proper modeling of the sheath mutual inductances which greatly influence circulating current losses and thus de-rating.
* All bonding arrangements for flat and triangular formations are supported with explicit modeling of minor section lengths, unequal cable spacing, etc

CYMCAP software

Transient Analysis

The program supports a Transient Thermal Analysis Option which includes the following:

* Ampacity given time and temperature.
* Temperature analysis given time and ampacity.
* Time to reach a given temperature, given the ampacity.
* Ampacity and temperature analysis as a function of time.
* User-defined load profiles per circuit.
* Multiple cables per installation.
* Circuits can be loaded simultaneously or one at a time.

CYMCAP software

CYMCAP/MDB, Multiple Duct Banks

The Multiple Duct Banks module (MDB) is the extension to CYMCAP designed to determine the steady state ampacity of cables installed in several neighboring duct banks and/or backfills with different thermal resistivity. The module presents a unique solution combining standard and non-standard calculation methods. The module computes the values of T4 (the external to the cable thermal resistance) using finite elements and then the ampacity (or operating temperature) of the cable system is obtained using the IEC standardized solution method.

CYMCAP/MDB features many modeling facilities. The following capabilities can be highlighted:
* Modeling up to eleven rectangular areas with different thermal resistivity.
* Modeling up to three duct banks in a single installation.
* Modeling one heat source or sink in the installation.
* Computation of the steady state ampacity or temperature.

CYMCAP software

CYMCAP/OPT, Duct Bank Optimizer Module

The duct bank optimizer is an add-on module to CYMCAP that allows the user to determine the optimal placement of several circuits within a duct bank. More specifically, the module can recommend the various circuit disposition within the duct bank in order that:

* The duct bank overall ampacity, i.e. the sum of the ampacities for all circuits, is maximized.
* The duct bank overall ampacity, i.e. the sum of the ampacities for all circuits, is minimized.
* The ampacity of any given circuit is maximized.
* The ampacity of any given circuit is minimized.

CYMCAP software

For a 3 by 4 duct bank with three trefoils and one three-phase circuit (one phase per conduit), there are over 110,000 possible combinations. CYMCAP elaborated mathematical algorithm prevents the repetitive calculation of equivalent cases, therefore the solution is obtained very efficiently. The condition illustrated on the right hand side, below, shows the cable locations for maximum ampacity.

CYMCAP/SCR, Short Circuit Cable Rating

The Short Circuit Cable Rating (SCR) add-on module to CYMCAP is dedicated to the rating of cables for short circuit currents. The implemented method is described in the IEC Standard 949 (1988) "Calculation of thermally permissible short-circuit currents, taking into account non-adiabatic heating effects". CYMCAP computes both adiabatic and non-adiabatic ratings. CYMCAP /SCR offers two possibilities according to the known input data:

* Compute the maximum short-circuit current that a cable component can carry given the short circuit time together with the initial and final temperatures.
* Compute the final temperature that a given cable component will reach for a specified short circuit current and initial temperature.

The short circuit rating can be computed for up to the five metallic layers in the CYMCAP model:

* Conductor
* Sheath
* Sheath Reinforcement
* Concentric Neutral / Skid Wires
* Armour

CYMCAP software

Cables in Tunnels

The optional Cables in Tunnels Module allows the user to determine the temperature, steady state, cyclic and transient ampacity of cables installed in unventilated tunnels. Note that only equally loaded cables having the same type and loading are considered. This add-on module supports a large variety of cable arrangements for single core (flat formations or trefoils) and three-core cables. The cables can be laid down on a floor, hanging from supports clamped on a wall, installed in ladder racks or in cable trays. Major features are:

* Modeling of a large variety of installation methods: laying on a floor; hanging from a wall; in ladder-type racks; or in cable trays.
* Cables and groups of cables can be single-core or three-core. Single-core cables can be arranged in flat formations (vertically or horizontally) or in trefoil.
* Computation of the steady state ampacity or temperature. Cyclic loading using daily, weekly and yearly load factors. Computation of emergency ratings.

CYMCAP software

Magnetic Fields

The Magnetic Fields Module (EMF) is an optional add-on that can be connected to CYMCAP. Although this module is not directly related to cable thermal rating, it offers convenience to the CYMCAP users. After an ampacity or a temperature steady state simulation the module computes the magnetic flux density at any point on or above the ground of an underground cable installation. The output is a plot (or a table) of magnetic flux density versus position. Modeling features include:

* Infinite-length thin-wire two-dimensional approach.
* Consideration of time-varying currents producing an elliptically polarized rotating magnetic vector.
* The currents in a three-phase circuit can be unbalanced (in magnitude and phase).
* All media is assumed homogenous, isotropic and linear.
* The induced currents are neglected.

CYMCAP software

Cables in Troughs

The Cables in Troughs Module (CYMCAP / TRO) is an optional add-on to CYMCAP that allows the user to determine the thermal rating of cables installed in filled or in unfilled troughs.

A trough (or a trench) is understood in this module as a long shallow rectangular-shaped excavation, where the walls, bottom and cover are made of concrete. The cables can be installed on the floor, hanging from supports on the walls or racks. The trough can be filled with a thermally good material or left unfilled (air filled). The heat transfer mechanism is different for each and therefore they are treated independently.

Unfilled Troughs

The rating of the cables is calculated as for cables in free air, but the temperature inside the trough is computed according to the IEC Standard 60287-2-1. The following capabilities can be highlighted:

* Computes the equally loaded ampacity or temperature when the cables are equally loaded.
* Computes the average temperature of the air inside the trough.

Note that load factors are not permitted and that all unfilled troughs are assumed to be flush with the surface.

CYMCAP software

Filled Troughs

Filled troughs are treated in CYMCAP as multiple backfills. Cables in filled troughs are rated in CYMCAP using:

* Finite elements method to compute the external-to-the-cable thermal resistance T4.
* IEC Standards procedures to perform efficiently ampacity calculations.

Besides, CYMCAP / TRO offers many possibilities according to the known input data:

* Computes the temperature and steady state unequally loaded ampacity as customary.
* Facilities to move the troughs down and model asymmetrical troughs.
* Considers cyclic loading through the use of load factors.

CYMCAP software

Multiple Casings

The Multiple Casings module (MCAS) is an optional add-on to CYMCAP that allows the user to determine the steady state unequally loaded ampacity and/or temperature rating of cables installed in one or more non-magnetic casings. A casing is understood in CYMCAP as a large non-magnetic conduit filled with air, inside which cables in ducts and cables not in ducts can be installed. Casings can be immersed in water, placed on the sea bed or buried underground. No other filling material than air is considered in the casing(s) or in the duct(s).

CYMCAP/ MCAS features many modeling facilities among which the following capabilities can be highlighted:

* Different burial environments are allowed: water or underground.
* Modeling of any number of casings in parallel in the same installation.
* Modeling of any number of ducts inside one or more casings at the same time.
* Capable of modeling any number of circuits inside a casing and a duct.
* Circuits in ducts and in casings can be multiple cables per phase.
* Several materials are available to model ducts and casings, including non-magnetic metallic materials (PVC, Polyethylene, Earthenware, non-magnetic metal, etc.)
* Sizes of ducts and casings are not limited.

CYMCAP software

Cable Impedance Calculation

The Cables Impedance calculation module (ZMat) is the optional add-on to CYMCAP that determines the electrical parameters for cables necessary for performing network studies at the power frequency (50/60 Hz). The estimation of parameters is performed after an ampacity or temperature steady state simulation has been successfully completed. The final results of ZMat are the positive and zero sequence impedances and admittances for all the cables present in an installation.

All impedance and admittance matrices are displayed in the report: starting from the primitive matrices per section per metallic component, the transposed matrixes (if they exist), then the reduced to phase conductor matrices and finally the resulting symmetrical components matrices.

* Computation of the sequence impedances for all the cables present in an installation.
* Model of the sequence admittances for all cables present in an installation.
* Multiple cables per phase are supported.
* One or more neutrals can be represented and are taken into account in the calculations.

CYMCAP software

Cable Crossing

The Cable Crossing (Xing) module is the optional add-on to CYMCAP that allows the user to determine the steady state ampacity of circuits crossing each other.

When two circuits cross each other, each behaves as a heat source for the other one. The amount of generated heat, the vertical distance between the crossing circuits and the crossing angle are the important parameters that influence the crossing rating. In the absence of crossing calculations, the general practice is to use the conservative result where the circuits are assumed to be parallel. When the circuits are parallel, the thermal interaction is maximum. It goes to a minimum when they cross each other at a right angle. The conservative approach unnecessarily derates both circuits. By using the Cables Crossing module, one can achieve ratings up to 20% higher than the conservative ampacities that are obtained based on the parallel installation scenario.

* Capable of modeling two circuits crossings each other in the same installation.
* Cable crossing is supported in directly buried underground, buried ducts and buried pipes underground.
* Rating approach follows the IEC standard 60287-3-3.

CYMCAP software