Underground System Design

TADP 547

Basic Cable

Design II

Presentation 4.2

Instructor: Frank Frentzas

Cable Ampacity

Ampacity is the current a cable conductor can

carry without exceeding allowable insulation

temperature under various installation conditions.

Key factors that impact circuit ampacity include:

– conductor material and size

– depth and soil conditions

– phase configuration

– grounding and bonding schemes

– fluid circulation and cooling on pipe type

cables

Cable Ampacity Calculations

Ampacity determined by calculating the maximum

current for an allowable temperature rise based

upon cable construction and installation conditions.

The cable ampacity can be solved using the thermal

equivalent of Ohm’s Law, shown below:

Electrical Circuit

Cable Ampacity Calculations

The circuits can be compared as follows:

– Current I is equivalent to Heat W

– Resistance Rel is equivalent to Thermal Resistance Rth

– Voltage Drop DV is equivalent to Temperature Drop DT

– Most ampacity calculations are based on the J.H. Neher

and M.H. McGrath paper from 1957.

– The International Electrotechnical Commission (IEC) has

developed Standards (IEC-60287 and IEC-60853) to provide

instructions for calculating ampacity based on cable

construction.

Steady State Calculations

A steady state ampacity calculation involves the

transfer of I2R and dielectric losses (in conductor

and cable sheath) to the cable surrounds.

Steady state ampacity is the cable rating under

normal loading conditions over a long time period.

Ampacity calculations are performed using a unit

cable length of 1 meter.

A thermal ohm-meter (C0-m/W) is thermal resistance

that cause a 1C0 rise in conductor temperature when

1 W/meter heat is generated in the cable conductor.

A Thermal Ohm-Meter Diagram

1 meter

1 Watt

DT = 1C0Conductor

Cable Insulation

Ampacity Calculation - Procedure

First choose a conductor size you think should

meet the requirement.

Using industry standards (or manufacturers

recommendations) establish maximum allowable

conductor temperature.

Start by calculating dielectric losses.

Calculate resistance of cable components carrying

current.

Calculate thermal resistance of each component.

Ampacity Calculation - Procedure

Calculate temperature rise from dielectric losses flowing

through thermal resistance of each component and

subtract from total temperature rise.

Solve Ohm’s Law equivalent to determine ampacity for

allowable temperature rise.

Depending on ampacity calculated, you will need to make

adjustments to conductor size and/or installation

conditions, i.e. spacing, thermal soil, etc.

Repeat calculations after adjustments to achieve the

required ampacity.

Thermal Equivalent of Single Conductor Cable

Wd

Tamb

(Earth)

Tj (Jacket)Tc (Conductor)

Tamb

Rearth

Td (Duct)

R ins R j Rjack-duct Rd Renv Rmutual

WC Ws

Ampacity Improvements

Factors which can improve ampacity:

– depth is critical - since heat from cable has to dissipate

through earth and escape to surface

– summer ambient soil temperature and surface covering

with high solar absorption must be considered when

establishing depth, i.e. asphalt tends to increase soil

temperature by as much as 50C

– soil conditions - thermal resistivity is important but the use

of thermal backfills can improve those conditions.

Ampacity Improvements

– thermal crossings, other power cables, steam pipes or

other external heat sources

– phase spacing and configuration - phase spacing may be

increased to compensate for increased depths

– sheath bonding schemes - multiple point bonding can

increase current flow on sheath compared to single point

and cross-bonding

– Sheath circulating currents can cause a substantial

reduction to overall cable ampacity

– depending on sheath losses - 15 to 35% reduction in cable

ampacity can be observed

Ampacity Increase

To increase ampacity on High Pressure Fluid

Filled (HPFF) pipe type systems, circulating

pumps are installed to circulated the fluid via a

return pipe and air coils (radiators) to eliminate

hot spots along the route.

If additional capacity required, mechanical

refrigeration units can be installed to cool the

dielectric fluid further.

Several computer programs exist to calculate

ampacity on new and existing installations.

Electric and Magnetic Fields

Current flow in a conductor creates a Magnetic Field around

the cable.

Electromagnetic Fields (EMF) have been a hot topic when

installing power lines the past 15 years.

There is a concern that EMF’s from power lines may cause

health problems to nearby residents.

Most of the evidence or research of potential health effects

from power lines are inconclusive.

Although EMF’s have been a topic on overhead lines,

underground installations are also starting to get public

attention.

Electromagnetic Fields

EMF tends to be more of an issue in European

countries than it is in North America.

Some states have requirements for magnetic fields

or require “prudent avoidance” for new lines.

Lower requirements are typically imposed as part

of a new line certification process.

New York requires 200 milliGauss (mG)

California requires “Prudent Avoidance”

Florida requires as high as 250 mG, depending on

system voltage.

EMF Issues (cont.)

Tennessee requires 4 mG beyond right-of-way

Washington requires “Prudent Avoidance”

Italy requires 30 mG at ground level and certain

cities require 2 to 4 mG.

Sweden requires 300 mG at ground level in areas

with limited access and 2 mG for sensitive areas.

These requirements are measured at edge of the

right-of-way or at ground level.

Utilities are forced to reduce the EMF levels on new

lines by optimizing cable/phase placement.

Cable Shielding for EMF

A common practices to reduce EMF levels is to

optimize cable location or phase configuration.

In some cases magnetic shielding is used to obtain

desired levels.

Another technique is to place the cables in a steel

pipe, although it will reduce circuit ampacity.

Installing cables deeper can also reduce ground level

EMF.

Also, installing a passive loop with ground continuity

conductors.

Phase Configuration

Reversing cable phase configuration (shown below)

can reduce EMF’s by a factor of 10.

A

B

C

A

B

C

A

B

C

C

B

C

Duct Bank

Concrete Encased

Duct Bank

Concrete Encased

Double Circuit Reduction

For optimum reduction the phase configuration for

a double circuit is shown below:

A

C

C

A

B

B

Duct Bank

Concrete Encased

Shielding Methods

Other methods used to shield cables:

B

A

C

C

A

B

Metal Plate

B

A

C

C

A

B

Side Shields

Other Shielding Methods

Other shielding methods involve steel pipes or

metal troughs to enclose cables. However, these

methods can increase installation costs and, most

importantly, trap heat may result in de-rating the

circuit ampacity.

This explains why HPFF pipe type systems have a

lower EMF value than single conductor cables such

as XLPE or LPFF.

Passive loop shielding use continuity conductors

placed on top of the duct bank and bonded together

at the splice manholes.

Cathodic Protection

Once a cable or pipe has been buried it will start to

corrode or form rust on the surface. To avoid rust

or pitting a cathodic system is used.

The area of steel on which the rust forms is called

an anode.

The area of steel that discharge electrons into the

soil is called a cathode.

There must be a potential difference between the

anode and cathode in order for corrosion to occur.

Stray Current

Another cause of corrosion is Stray Currents,

typically direct current (DC).

DC is usually from trains or DC rectifiers in the

area.

The DC passes through ground to the cable

system which carries it back to the DC source.

The point at which stray current is picked up by

the pipe is the cathode and where it leaves is the

anode, and this is where corrosion occurs.

Cathodic Protection Systems

To prevent the pipe from corroding a cathodic

protection system must be designed to make the

pipe a cathode.

One system is to connect the pipe to a metal that

is higher on the galvanic series. This creates a

potential difference between the two metals so

that current will be discharged from the anodic

metal and be picked up by the pipe. The system is

called the Galvanic Cathodic Protection System

and typically uses magnesium anodes.

Cathodic Protection Systems (cont.)

Another system involves connecting the pipe to

another metal and applying an external potential

between the two metals. The source is a rectifier in

which the positive(+) is connected to the anode and

the negative(-) to the pipe, making that the cathode.

The method is called an impressed current system

and is typically used to protect HPFF cable systems.

In order to protect the pipe, ground must float and

during a fault it needs to go to ground. To

accomplish this Isolated Surge Protector (ISP) are

used.

Protective Coating

Pipes used for cable systems have a protective

coating, in addition to cathodic protection.

Early pipe coatings systems include asphalt, mastic,

and coal-tar enamel. More recently, polyethylene and

polypropylene type coatings have been employed.

If the pipe coating could remain perfect (no damage)

no corrosion system would be required. However,

due to constructions dig-ins, surge or fault currents,

etc. protection is required.

Cathodic Protection - Testing

As the coating is critical to protecting the pipe, a

number of tests are performed to insure coating

integrity.

One test, performed when the pipe is first installed,

is called the “jeep” test. The test applies high

voltage DC across the coating and pipe, and if any

defects (holidays) are detected they are repaired

before the pipe is back filled.

After the back filled operation a coating resistance

test is performed to check for coating defects by

applying current to each pipe section.

Cathodic Protection - Maintenance

To insure the system is properly protected, pipe

coating surveys and rectifier reading are part of

the maintenance program.

A maintenance cycle is three years, but it will also

depend upon local system conditions such as DC

trains in area or heavy construction. If necessary,

the frequency can be changed to annually.

Protecting the pipe coating and avoiding corrosion

can prevent dielectric leaks which can be an

environmental hazard.