Underground System Design TADP 547 - Gonzaga University

Underground System Design

TADP 547

Operation & Maintenance

Practice

Presentation 6.3

Instructor: Frank Frentzas

Operation & Maintenance Practice

With proper operation and maintenance procedures under-

ground cable systems will provide long term and trouble free

service when properly maintained.

Without proper maintenance cables can still operate well for

many years. However, when problems or failures occur the

repair costs will likely to be high with repair durations running

from days to months.

The reliability of any underground cable system is a direct

function of the operation and maintenance programs being

performed.

Utilities generally develop maintenance procedures specific to

their system operation and workload.

Operation & Maintenance Practice (cont.)

System operators should have procedures available that cover

operational requirements of underground cable systems.

Operators should be aware of the key factors that can effect

cable operation and cable life.

– Thermal loading/ampacity

– Pressure requirements

– Circuit reclosure schemes

Key Factors

Thermal loading:

System operators should be aware of and follow both normal and

emergency conductor temperatures - based on ampacity rating.

Pressure requirements:

Pressures are critical for cable operation. Cables should be operated

within the specified cable pressure range to avoid break-down/failure

of the insulation. Cables operating outside the normal pressure range

should be de-energized.

Circuit Reclose Scheme:

When underground cable circuits trip on a fault a reclose operation

should be prevented - since you will likely be closing back into the

fault. When overhead lines are a part of the cable circuit, a reclose

should only be allowed when the distance relay indicates the fault

was on the overhead line portion.

Standard Practice Practice

It is vitally important that maintenance procedures incorporate

both safe work practices and environmental requirements and

regulations.

Since no two maintenance programs are alike, we will cover

general maintenance procedures for HPFF, HPGF, LPFF and

XLPE cable systems.

We will start by covering maintenance procedures for Pipe

Type cable systems.

HPFF & HPGF Procedures

A life expectancy of fifty plus years is not unusual

for Pipe Type cable systems. With such longevity

replacement parts from original manufacturers can

be problematic or non-existent. The resulting

difficulties for regular system repairs provide a

powerful incentive for ensuring effective preventive

maintenance programs are in place.

Line Route Patrols

Focused inspections to verify no unauthorized

excavation or construction has taken place along

the cable route.

Prevent third party damage to pipe coating which

may lead to future pipe leaks.

Verify no ground shifting, sinking or wash-outs

have taken place along cable route.

Frequency of patrols is typically weekly.

Manhole Inspections

Underground manhole inspections verify the

condition of the HV cable systems they

contained.

Interior visual inspection of structural integrity

of manhole interior walls, floors, and ceilings.

Verification that pipe coating shows is not

damaged and there is no evidence of coating

disbanding from the pipe surface.

Verification that all connections are properly

coated and no signs of corrosion are present.

Manhole Inspections (cont.)

Verifies there are no dielectric fluid leaks from

the joints, piping, or valves.

Grounding conductors are securely connected.

All cathodic connections properly connected

and insulated, and bond leads are not in contact

with any structures that would compromise the

cathodic protection system.

Frequency of inspections can vary from 2 - 5

years.

Termination Inspections

Verify there are no dielectric fluid leaks from

terminations, piping, or valves.

Grounding conductors are securely connected to

the structure.

All cathodic connections are properly connected.

Check and re-torque base and top plate bolts.

Insulator checked for cracked or damage sheds,

and for contamination.

Frequency of inspections can vary from 1 - 5 years.

Fluid Samples

Fluid samples can provide baseline condition of the

line, and are periodically checked every 1 to 5 years

for changes in gas levels.

Fluid samples drawn from joints and terminations to

examine the condition of the cable insulation.

Dielectric fluid samples are taken from the bottom

of the joint and termination using a glass jar. The

test checks for moisture, dielectric strength, power

factor, and color.

Dissolved Gas Analysis (DGA)

Dissolved Gas Analysis (DGA) used to determine the

paper insulation condition. The technology was

extensively used on power transformers and adopted by

the cable industry.

DGA samples are taken using a glass syringe with a small

end tube to draw fluid from the top of joints and

terminations.

All samples must be sealed and protected from moisture

and heat until they reach the laboratory for analysis.

Samples are analyzed for several gasses using a gas

chromatograph to measure the concentration of each gas.

Dissolved Gas Analysis (DGA)

The concentration of each gas gives an indication of the

insulation condition.

High concentration of Acetylene indicates arcing is taking

place and that the joint or termination should be inspected as

soon as possible.

High levels of Carbon Monoxide and Dioxide indicate

decomposition of cellulose components (impregnated paper),

possibly due to overheating.

IEEE 1406 specifies acceptable and concern levels for gas

and moisture content.

Pressurization Plants

Pressurization plants maintain line pressure and

are essential to cable system reliability.

Pumping plants are typically inspected once a

month, and more frequently for older plants with

weekly chart recorders which require the chart to

be changed.

Unit should be visually inspected for fluid leaks and

damage to any mechanical components.

The annunciator alarm panel should be checked for

proper operation.

Pressurization Plants (cont.)

Pressure gauges, switches, timers, and regulating

valves should be checked and calibrated annually

for proper operation.

If the HPFF circuit is equipped with a forced cooling

unit it should be checked and calibrated once a

year.

Nitrogen cabinets on HPGF system should be

checked annually for proper calibration and proper

alarm operation.

Corrosion Inspection

Most pipe systems fail from the outside, meaning

the pipe fails before the cable system. A corrosion

protection system should therefore be inspected

annually for proper operation.

Pipe coating defects, also refer to as holidays,

should be identified and repaired to avoid corrosion

leaks. Pipe defects are identified by performing a

coating resistance test and an above ground

survey.

The frequency of these tests is anywhere from 3 - 10

years, depending on the pipe history and location.

Low Pressure Fluid Filled Systems

Low Pressure Fluid Filled systems are similar to pipe type

systems in frequency and procedures for line patrols, manhole

inspections, and fluid samples.

Manhole inspections involve an inspection of cable supports

and link boxes used for sheath bonding system, and the fluid

reservoir for proper level and pressure.

Pressure alarms must be inspected and tested every (1 to 3

years) for proper operation

A cable sheath test must be performed every 3 - 5 years to

verify the cable jacket integrity.

In some cases, sheath current readings are also measured

every 3 - 5 years.

XLPE Cable Systems

XLPE systems require less maintenance than other

cable systems since no hydraulic pressure equip-

ment is used.

However, XLPE systems require the same main-

tenance as other cable systems when it comes to

line route patrols, manhole inspections, sheath

tests, and sheath current measurements.

Line inspections can prevent contractors from

digging into the cable or damaging the jacket/

sheath, which can lead to localized hot spots and a

cable failure.

XLPE Cable Systems (cont.)

Manhole inspections are similar to other cable systems in that

they verify structure condition and, most importantly, inspect

the cable system.

Check racking system for any movement, and the grounding

and bonding system.

Manhole inspections typically performed on a 3 - 5 year

frequency.

Sheath tests should be performed every 3 - 5 year to identify

jacket or sheath damage, and the condition of bonding

connections - including link boxes and surge arrestors.

XLPE Cable Systems (cont.)

Sheath currents can be measured on a 3 - 5 year

cycle to verify the bonding system is operating as

designed.

In some cases, real-time optical fibers embedded

temperature measurements can be taken to provide

actual cable operating temperatures.

On-line partial discharge (PD) tests are now being

performed to detect cable and accessories defects.

Most utilities perform a PD test during the initial

installation and just prior to warranty expiration.

Typical Manhole Inspection Form

Actual Sheath Test Form: Note Damage on C Phase

Summary

Providing cable systems are operated per design

and a comprehensive maintenance program is

performed the systems will operate trouble free

for many years.