Underground System Design TADP 547 Basic Cable Design III

Underground System Design

TADP 547

Basic Cable

Design III

Presentation 4.3

Instructor: Frank Frentzas

Short Circuit Design

Metallic sheaths are used on cables to protect it

from moisture ingress and to provide enough

capacity to handle short circuit fault currents.

In designing for fault current you must consider

fault duration times which are set in relation to

breaker clearance time - typically 3 to 5 cycles.

Typically, a short circuit is specified at a certain kA

value for a duration of 15 cycles (60 cycles = 1sec)

Some utilities require 40 kA for 15 cycles, and as

much as 63 kA for 15 cycles for larger systems

(N.Y, Chicago, etc.)

Short Circuit Calculations

To adequately design the metallic sheath to the required

fault current level the following formula (ICEA P45-482

standard) is used:

Short Circuit Calculations (cont.)

To simplify the previous equation, Table 1 from ICEA P45-

482 can be used for different types of sheaths:

Sheath Design

For some high resistance sheath designs (lead-alloy,

copper, aluminum foils, as well as stainless steel

sheaths) that require additional fault current capacity a

layer of copper wires is typically added to the cable

design.

Copper

Foil

Copper

wires

Cable Expansion

When the cable system is in operation and carrying load

the conductor’s temperature will increase with load, which

causes the cable to expend. Thermal expansion can be

calculated by the following:

DL=(a)(L)(DT)

DL- change in length (total expansion)

a-coefficient of expansion

DT- change in temperature (T2-T

1)

Coefficient of

expansion

(a)

Aluminum 0.0000236

Carbon Steel 0.0000110

Copper 0.0000168

Lead 0.0000293

Stainless Steel (304) 0.0000170

Cable Expansion (cont.)

Field values of thermal expansion are not the same

as calculated value.

Field experience indicate the expansion is less than

calculated values.

Thermal expansion must be addressed when

designing a cable system, and consideration of the

following variables to determine the best and most

cost effective design solution.

Key Variables – Cable Expansion

Key variables that influence the design include:

– Conduit size

– Manhole size

– Cable dimension

– Design and size of cable splice

– Cable supports

– Cable clamp design

– Route geometry

Conduit Size

Conduit size determined by cable diameter.

Common practice is to size duct diameter (id)

1.5x cable diameter. However, smaller ratios

have been used successfully.

For example, a 4” diameter cable will require a

6” duct.

Cable Expansion

Manhole size determined by splice design and the

allowable cable bending radius.

A specified cable length is required to construct a

splice.

Manhole must have enough room to train cable

with-out violating allowable bending radius,

especially in flexible and semi-flexible

installations.

Handling Thermal Expansion

Three common methods of handling thermal

expansion include:

1) Fixed or rigid system

2) Flexible system

3) Semi-Flexible

● A fixed or rigid system is when cable is clamped in

manhole in the straight through position forcing

expansion back into duct. Expansion in the duct is

called snaking.

Cable Clamps

In the fixed position cable clamps and supports

must be designed to withstand expected thermal-

mechanical forces on cable.

Cable must be fixed and not allowed to move.

In flexible and semi-flexible designs, cable clamps

must allow design movement without exceeding

cable bending radius.

Rigid Clamp System

Rigid clamp on 345 kV XLPE cable installation. Cables are

not allowed to move in the manhole.

Flexible Clamp System

A flexible clamp system accommodates cable

expansion and contraction without violating

the allowable cable bending radius.

Flexible systems typically used at 138 kV and

below for systems with small conductor size.

Flexible Clamp on 138 kV LPFF System

Note: joint is allowed to move on the support.

Semi-Flexible Clamp System

In a semi-flexible clamp system, a cable can

partially move to accommodate some cable

expansion with remainder being taken up in

duct without violating allowable cable

bending radius while the joint is locked down

and can only move as a unit.

Semi-flexible systems are typically used in

smaller manholes where expansion of the

cable is limited.

Semi-flexible Clamp on Double Circuit 138 kV XLPE Cable

Note: joint is fixed on support but allowed to move as a unit.

Cable Route

Route geometry or elevation is very important,

especially if on an incline slope.

An incline slope requires special restrains to

prevent cable from moving in direction of slope.

An “S” bend prior to entering manhole can

alleviate the forces from having to be controlled by

manhole clamping alone.

HPFF System Expansion

HPFF pipe type systems have three paper

insulated cables installed in a steel pipe. The

pipe is filled with dielectric fluid and

pressurized.

Thermal expansion takes place within the

pipe but the joints need to be restrained to

avoid excessive thermal mechanical bending,

which can lead to a failure.

Design Calculations

Design Parameters

Another design perimeter when designing cable-to-

pipe ratios is the Jam Ratio.

Jam Ratio is clearance required to prevent cables

getting jammed during pulling.

If ratio of pipe inside diameter to cable diameter is

in range of 2.95 - 3.1, there is a chance cables will

jam during pulling.

You must, therefore, design the cable and pipe to

fall outside that range.

Jam Ratio

Other Joint Systems

Normal pipe type system joints typically restrained

by spiders (about four per joint, depending on

design).

Another type is the Anchor joint. They are used to

restrain cables from moving and are typically

employed on both sides of river crossings, in tunnel

shafts, or where the circuit is built on an incline.

In some applications, typically in tunnels, skid joints

are used to allow the joint to move during cable

expansion and contraction.

Normal Joint for 138 kV HPFF Cable System

Note: the joint is supported by the spiders.

Typical Drawing of a Normal HPFF Joint

Support Spiders

Normal Joint on 138 kV HPFF Cable Showing Center Spiders

Typical Normal HPFF Joint

Anchor Joint for 138 kV HPFF Cable System

Note: anchor side of the joint

Typical Anchor Joint