Fluid-filled Underground Transmission Cable Condition Assessment

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					         Fluid-filled Underground Transmission Cable Condition

      Harry ORTON,                  Lisa OGAWA, PhD                     David ARNOLD,
Orton Consulting Engineers      BC Hydro, Vancouver, Canada          EPCOR, Edmonton, Alberta
 International, Vancouver,       (Formerly with Powertech)                   Canada

    1.0 Abstract: Many underground power             satisfaction that the correct decision has
    cable users are faced with aging fluid-filled    been made.
    transmission cables that have exceeded their     3.0 On-site Inspections: On-site inspections
    design life of 30-40 years. Major and            begin with routine visual observations by the
    possibly costly decisions have to be made        same personnel overtime so that any
    whether to replace or to continue to use the     differences become obvious. Good records
    existing cable system [1].                       including photographs of situations that need
    This paper presents a condition assessment       attention or further investigation are kept.
    strategy focused on fluid-filled cables that     Cables, terminations, splices, accessories,
    involves a combination of visual                 fluid lines, tanks and link boxes are
    observations through on-site inspections,        examined closely. Details of any
    laboratory diagnostics, on-site diagnostics,     discoloration, fluid leaks, (see Figure 1),
    operation reports and data trending.             corroded components, damaged, loose or
    2.0 Introduction: The objective of               distorted components are recorded.
    condition assessment is to prevent
    unacceptable consequences such as reduced
    performance, to insure high system
    reliability and long service cable life and to
    prevent customer outages. The ideal
    situation is to use suitable diagnostics to
    locate potential problems before failure and
    hence reduce replacement costs and target
    replacement money. Effective condition
    assessment must be able to identify the
    stress factors, whether environmental,
    electrical, mechanical or chemical that may
    cause degradation. Furthermore it is
    important to identify the degradation
    mechanism and the consequences and so                  Figure 1. 275 kV SCFF Cable SF6
    take timely corrective action.                           Termination with Fluid Leak.
    Once a thorough condition assessment has
    been completed, replacement or continued         Visual inspections will look for choking at
    utilization decisions can be made with the       cable cleats and supports, unusual snaking of
                                                     cables, pungent smells or odors, surface
cracking, ionization or corona discharge,       complete understanding of the cable
surface tracking, carbonization or soot,        condition. Laboratory diagnostics include
shrink back, blanching or crazing of the        chemical, electrical, mechanical and
outer sheathing.                                metallurgical evaluations. These diagnostics
In addition, an on-site inspection will check   are mostly destructive, as small samples
fluid pressures, alarm power supply plus        must be removed from the cable being
ensure that SVL’s are MOV’s, not carbon         assessed. Chemical diagnostics include
block air-gap type. Surge diverter counters     Degree of Polymerization (DP), DGA and
should be in place to record the number of      moisture content. Of these DGA is not
events that actually occur at a specific        destruction and can be carried out while the
location.                                       cable is still in service. See Table 2 for Key
Regular patrols along cable routes should be    Gas Indicators.
carried out to prevent “dig-ins” and an
advertising campaign “call before you dig”
should be implemented.

                                                 Figure 3. Electrical Test Electrodes for
                                                      Permittivity Measurements.
 Figure 2. 275 kV SCFF Cable Right-of-
                     way                        Electrical tests include ACBD dielectric
Visit creek and river crossings after heavy     strength of paper samples, loss angle, radial
rains and major storms. Examine rights-of-      dissipation factor, capacitance, permittivity
ways for large tree growth that may reduce      (Figure 3) and thermal stability. Aging of
soil moisture content and increase thermal      short cable lengths of approximately 9-10
resistivity (Figure 2). All spares should be    metres can provide insight into the present
located and include a spare cable length plus   condition of a cable. See Figure 4 and Table
adequate fluid and at least two repair joints   1. Mechanical tests such as elongation-at-
and one termination. Perishables should be      break, tensile strength and burst test carried
replaced and all tools, crimps, dies, pumps     out on paper samples removed from a cable
etc should be located. If the spares are not    repair or diversion can establish the
available then cable replacement may be         mechanical deterioration of the papers in a
necessary. Arrangements must be made for        fluid cable. Furthermore, the metallic sheath
cable jointers to do repair work at short       condition can be determined by using
notice.                                         metallurgical diagnostics such as Eddy
4.0 Laboratory Diagnostics:          A          Current and a dye penetrant technique. X-
multidiscipline approach can provide a          ray diffraction can be used to assess
                                                corrosion byproducts. See Table 3.
 Figure 4. Short Length 9 m HPFF Cable
5.0 On-site Diagnostics: The first and                     Figure 5. Removing Paper samples for
possibly the cheapest approach is Infrared                         Laboratory Evaluation
Thermography on cable accessories, and the               As well documented in the literature, there
second is Acoustic Emission. But with both               are two major types of insulation
diagnostics it is necessary to have physical             degradation: discrete or incremental, at
access to the joints and terminations. So                voids or cavities resulting in electrical trees
only the terminations of direct buried cable             and PD (Partial Discharge) and average or
systems can examined without additional                  overall degradation or material aging
effort. Sheath current measurements can                  “Global” without PD.
provide details on out-of-balance ac                         Table 2. DGA and Key Gas Indicators.[2]
currents, ineffective cross-bonding and
corrosion currents, both ac and dc.                         Key Gas Indicator        Underlying Cause

Table 1. Absolute Dissipation Factor of 9 m Cable               Hydrogen              Partial Discharge
     Sample at 150 psi, Before and After AC
      Withstand Test at 58 kV for 6 hours.
                RED          BLACK          WHITE        Carbon Monoxide/Dioxide     Paper Degradation
              PHASE          PHASE          PHASE
                D.F.           D.F.           D.F.
            Before/After   Before/After   Before/After          Methane             Overheating Problem
               (E-03)         (E-03)         (E-03)

   5         2.24/2.26      2.42/2.42      2.36/2.36
                                                                Acetylene                  Arcing
   10        2.22/2.24      2.42/2.42      2.35/2.36

   15        2.21/2.22      2.42/2.41      2.34/2.33        Ethane & Ethylene      Overheating Involving a
   20        2.20/2.21      2.42/2.41      2.33/2.32                                       Metal

   25        2.20/2.20      2.41/2.40      2.32/2.30
                                                           Hydrogen and Other       Rusting or Hydrolysis
   30        2.19/2.20      2.42/2.40      2.31/2.29             Gases
   35        2.19/2.19      2.41/2.39      2.30/2.27

   40        2.18/2.18      2.41/2.38      2.29/2.26
                                                         Partial discharges occur in voids or cavities
                                                         in insulation or at interfaces in cables or
                                                         accessories. Mini sparks occur in voids that
emit broadband radiation 50 kHz to >500           frequency plus overvoltages are possible, if
MHz and pulse rise times of 1.0 ns.               required.
Magnitudes can change with time, voltage,         On the downside, the cable condition will
temperature, load and humidity (moisture in       change, such as voltage, temperature and
cable), but the location stays the same.          load. Off-line diagnostics include: 60 Hz ac
Attenuation along a cable, particularly at        test, resonant test sets, VLF test (0.1 Hz or
higher frequencies, background noise,             lower for longer cable lengths). combined ac
multiple discharge sites, cable branches as       and VLF test or Complex Discharge
well as different insulation materials make       Analysis (CDA), Oscillating Wave, VLF 0.1
interpretation difficult. And to date, time-to-   Hz dissipation factor and PD, Isothermal
failure predictions using PD magnitude            Relaxation and Return Voltage. These
alone are not possible.          This makes       diagnostics are summarized in Table 3.
replacement criteria difficult to justify. In     On-line diagnostics look at what is there,
addition, electrical tree growth can be fast      and have the advantage that no switching is
for small PD and slow for high PD or the          required once the diagnostic is in place.
reverse. In addition there is a need to           Trending is possible to relate insulation
distinguish between harmless and harmful          condition and PD to operating conditions of
PD, plus electrical tree erosion rates of         voltage, load, temperature and humidity.
materials are different. Overall or global        However, it is not possible to control the test
deterioration can occur by chemical and           voltage. DGA or dissipation factor can be
mechanical means as there is no water             used to determine the overall or global
treeing in fluid filled cables.                   condition of the cable [3]. See Figure 6 and
                                                  Table 2.

     Figure 6. Fluid Analysis Report.

There are two approaches to on-site               Figure 7. Acoustic Measurements on 110
diagnostics; off-line and on-line. Both                    kV SCFF Terminations
techniques must be non-destructive so as not
to reduce cable life. Off-line diagnostics are    For discrete or localized deterioration;
performed with the cable de-energized so          Ultrasound and PD diagnostics are
switching and grounding of the cable under        employed (Figure 7). A summary of the
test is required. There will be system            available methods is given in Table 3.
contingency concerns, space charge and
grounding issues to consider. But it is           6.0 Records and Trending: Access   to
possible to control the test voltage and          system operation records that include
switching     information,     major    storm                with improved performance. Furthermore
occurrences such as lightning or nearby                      both laboratory and in-situ diagnostics are
system faults is important to assess the                     beneficial, but improved data interpretation
potential of cable damage. Pin holes in the                  is necessary. And finally condition
insulation jacket can lead to major corrosion                assessment must become part of any Asset
events. Keeping good base line data and                      Management program.
follow-up diagnostic data will aid in
Trending or how a specific parameter or                      8.0 References:
condition is changing over time. Is partial                  [1] CEATI Underground Power Cable
discharge at given spot increasing or                        Workshop, Vancouver, Canada, June 2008.
decreasing? Is DGA content increasing over                   [2] IEEE Guidelines for Levels of Gases by
time and at what rate? [3, 4, 5]                             DGA
The laboratory results presented in Table 1                  [3] CIGRE 296, “Recent Developments in
combined with the analysis of Table 2 and                    DGA Interpretation”, Joint Task Force
the DGA gas content profile of Figure 6 can                  D1.01/A2.11, June 2006.
provide an insight into the cable condition.                 [4] CIGRE 228, “Service Aged Insulation
Repeating DGA over time will provide a                       Guidelines on Managing the Aging
trend for condition assessment.                              Process”, Working Group D1-11, June 2003
                                                             [5] CIGRE 279, “Maintenance for HV
7.0 Conclusions:      Cable       condition                  Cables and Accessories”, Working Group
assessment provides improved asset                           B1.04, August 2005.
reliability and increases cable service life

                           Table 3. Summary Table of Diagnostic Techniques

                                                             Non-destructive                 Non-destructive
                                                                  Off-line                       On-line

Dissection                       Electrical       Discrete           Integral             Discrete and Integral

Visual Examination,
                                                                                          Dissipation Factor and
Contaminants, Voids,             ACBD, Step       PD at 50/60        Dissipation Factor
                                                                                          PD Inductive and
Electrical Trees, Paper          Tests,           Hz                 and PD at 0.1 Hz
                                                                                          Capacitive Coupled
Layer Registration

                                                                     DC Hipot/
                                 Permittivity &
Chemical, Moisture, Degree                                           Complex              Acoustic Noise
                                 Capacitance on   PD 0.1 Hz
of Polymerization                                                    Discharge            Thermography
                                                                     Analysis (CDA)

Mechanical, Tensile and                           PD Location                             Eddy Current and Dye
                                 Dissipation                         Return Voltage
Elongation, Burst Test                            (2Uo ac res.)                           Penetrant

Short length tests, 9 m.                                             Isothermal Relax.
                                                  Oscillating                             DGA