Conference PaperPDF Available

Replacement of Pipe Type Cables with Cross-Linked Polyethylene (XLPE) Cables by Utilizing the Existing Steel Pipe

Authors:
Replacement of Pipe Type Cables with Cross-linked
Polyethylene (XLPE) Cables by
Utilizing the Existing Steel Pipe
Vincent Curci
Teruo Nishioka
James Grimstad
Power Delivery
HDR Inc.
Omaha, Nebraska, USA
vincent.curci@hdrinc.com
teruo.nichioka@hdrinc.com
AbstractHigh-pressure fluid-filled (HPFF) pipe type cables
make up the bulk of the underground transmission lines in North
America. These cable systems started to enter commercial
operation in the early 1930s and many are now beyond their
design life. Although HPFF cables have provided excellent
reliability, some now suffer from corrosion, leaks, and failures.
HPFF cables also present system design complexity and require
highly skilled personnel for operation, maintenance, and repair.
Utilities in North America now face the technical and economic
challenges of replacing this infrastructure to maintain reliability
of supply. This paper addresses replacement using XLPE cables
by re-utilizing the steel pipe and manhole system in order to
reduce cost and minimize impact to traffic and stakeholders by
eliminating the need to construct new lines. The paper presents
constraints on pipe inside diameters, limitations on XLPE cable
diameters, insulation thicknesses, cable operating stresses, and
ampacity and bonding options.
Index TermsCable, HPFF, Reconductoring, Transmission,
XLPE
I. INTRODUCTION
Pipe type cables have been used since the 1930s for bulk
power transmission especially in urban areas. They have
provided a reliable system for 69-kV through 345-kV
transmission with a proven track record. In North America, pipe
type cables gained rapid acceptance and became the cable
system of choice. The first installation dates back to 1932 at 66-
kV, which was followed by other significant installations up to
345-kV beginning in the 1960s [1]. Available literature indicates
that there are approximately 4,500 circuit miles of 69-kV
through 345-kV underground transmission cables in service in
North America and more than 75 percent, or approximately
3,400 miles, of these systems are pipe type cables, which
translates to more than 53 million feet of single conductor cable
currently in service [1]. Pipe type cables have also been utilized
worldwide and there are installations in the Middle East, Japan
[2], and Europe [3] [4].
Many of these installations are beyond their original design
life and although many continue to operate reliably, some
failures [5], Fig. 1, have occurred and some systems are
experiencing operational issues such as corrosion and leaks.
Figure 1. Pipe type cable failure.
II. TYPES OF PIPE TYPE CABLES IN USE
Pipe type cables consist of three cable phases that have paper
insulation consisting of either Kraft paper or laminated paper
polypropylene (PPLP) and installed in a steel pipe. High-
pressure fluid-filled (HPFF) cables with oil-filling are
referred to as a high-pressure oil-filled or HPOF, Fig. 2.
Figure 2. 230-kV HPOF pipe type cable.
HPFF cables with gas filling are referred to as high-pressure
gas-filled or HPGF. There are external fluid compression pipe
cables as shown in Fig. 3.
Figure 3. Gas compression pipe type cable.
In this case, the three cables within the pipe have an external
lead sheath or jacket and the gas or oil pressure is applied
external to the cable and the filling fluid does not contact the
cable insulation. These installations are uncommon in North
America but prevalent in Europe.
III. REPLACEMENT WITH XLPE CABLES AND
BENEFITS
Cross-Linked Polyethylene (XLPE) insulated cables
present a viable option for replacement of the paper insulated
pipe type cable in the existing steel pipe. XLPE cables have
been commonly used throughout the United States and
worldwide since the 1970s and have surpassed the installation
of oil-filled cables based on information provided in CIGRE
Technical Brochure 379 [6]. Replacing with XLPE cables
presents several advantages, principally eliminating the need to
install new circuits and the associated excavations, which cause
adverse impacts to residents, traffic, and communities. There
are significant economic advantages as replacement costs are
typically half of the new circuit installation cost. Based on
technical study and analysis done by the authors, not every pipe
type cable can be converted to XLPE cable while retaining the
same conductor size, which is critical for ampacity
considerations. If 50 percent of the 3,400 miles of pipe type
cables could be converted, the estimated economic savings
would approach $4 to $10 billion U.S. dollars. The added
advantages of XLPE cables as compared to HPFF cables are
mainly less insulation losses, less capacitance and less charging
current, and the elimination of ancillary equipment such as
pressurizing systems, pressure alarms, and communication
systems. In addition, XLPE insulated cables contain nil filling
oil or gases and thus pose no environmental threat due to the
possibility of leaks and spills.
Another critical consideration for replacing with the XLPE
cables is with respect to availability of supply. Currently there
is one, possibly two manufacturers left worldwide that can
supply high-pressure fluid-filled, or HPFF paper insulated
cables while another two companies can supply accessories. On
the other hand, numerous suppliers worldwide can supply
XLPE cable and accessories.
IV. XLPE CABLE AND ACCESSORIES DESIGN FOR
REPLACEMENT
There are various extruded XLPE cable constructions but
for installation in steel pipes, but the aluminum laminate,
copper laminate tape, or smooth welded sheath construction,
Fig. 4, offer advantages with respect to overall diameter and
lightweight construction as compared to lead sheath or
corrugated smooth aluminum (CSA) designs.
Figure 4. XLPE cables with AL laminate and smooth aluminum sheath.
Triplex cable construction presents an alternate option to
single core cables. Triplex construction at transmission voltages
have been used primarily in Japan in both tunnels and ducts. For
retrofitting pipe type paper insulated cables, the triplex cable
design offers advantages with respect to thermo-mechanical
properties such as reduced cable expansion, which would
eliminate the need to enlarge manholes due to the smaller cable
offsets required if joints are mounted external to the pipe. The
main disadvantage of triplex cables is the limitation on shipping
lengths due the size and weight of reels and the associated
logistics of shipping and transport and handling for field
installations.
With respect to joints, there are several compact designs
available that would be well suited for installation in existing
manholes, which include heat shrink, cold shrink shown in Fig.
5, three-piece pre-molded, and one-piece pre-expanded joints.
The selection of the joint design will ultimately depend on the
voltage class and on the joint placement and mounting for the
installation. Joint placement in splice casings would require
more compactness as compared to joints externally mounted to
the pipe.
.
Figure 5. Cold shrink joint.
Similar to joint designs, there are termination, Fig. 6,
designs such as heat shrink, cold shrink, dry pre-molded slip-
on, pre-molded oil filled and prefabricated types, which are all
suitable for replacement purposes. The applicable design would
depend on the voltage but terminations are available to 500-kV.
Terminations are also available for cable entrance in oil-filled
or SF6 filled enclosures.
Figure 6. XLPE Cable terminations.
V. PIPE DIAMETER LIMITATIONS AND MAXIMUM
CABLE DIAMETERS
In North America nominal pipe sizes of 4, 5, 6, 8, 10 and 12
inches have been used for pipe type cables. Replacement cables
have to be capable of being installed in existing pipes and
assuming same conductor size. Smaller conductor installation
may result in substantial ampacity reduction for the cable
circuit. The conductor sizing may become a utility decision.
The National Electric Code (NEC) does not govern utility
practices and as a result, pipe clearances are not based on
percent fill. Generally, the pipe clearance for three cable pulls
for pipe type cables has been taken at 0.5 inches. The following
formula (1) can be used to calculate the cable clearance for a
three-cable pull:
=
21.366()+
21
(] (1)
where C is the cable clearance to top of pipe, D is inside
diameter of the pipe, d is the cable outer diameter (for extruded
cables) and k is a safety factor to account for pipe or cable
ovality and normally taken at 1.05.
Equation 1 can be used to calculate the maximum cable
diameter for different pipe sizes. Table I shows the maximum
cable diameters calculated from (1) for clearances of 0.375, and
0.5 inch. The advantage of having cable diameters which
provide for 0.375 or 0.5 pipe clearances is with respect to the
phenomenon of jamming of the cable within the pipe. The jam
ratio is given as shown in Equation 2.
=1.05
(2)
where D and d are the inside diameter of the steel pipe and
the outer diameter of the cable, respectively. Pipe clearances in
the range of 0.375 to 0.5 produce jam ratios of approximately
2.5 or less for which the probability of jamming is low during
cable pulling.
TABLE I. MAXIMUM ALLOWABLE CABLE DIAMETERS
Pipe
Max. Cable OD,
mm
Nominal Size,
in
Inside Diameter,
mm
C=0.375
C=0.5
4
102.3
42.1
41.0
5
128.2
53.6
52.6
6
155.6
65.8
54.8
8
206.4
88.3
87.2
10
260.4
112.1
111.1
12
311.2
134.6
133.6
VI. REDUCED INSULATION THICKNESSES AND
OPERATING AT HIGH STRESS VALUES
Replacement XLPE cable will require reduced insulation
wall thicknesses in order to fit within the pipes for the same
conductor size as the HPFF cables. This requires operating the
cables at internal and external stresses that could be above the
limiting values of stresses recommended by AEIC CS 9 and
provided in Table II below.
TABLE II. AEIC CS 9 STRESS LEVELS
Rated
Voltage Conductor
Sizes
Internal
AC Stress
Nominal
External
AC Stress
Limit
Generic
Insulation
Thickness
(kV) (mm2) (kV/mm) (kV/mm) (mm)
69 240-2000 6 3 12
115 240-2000 8 4 15
138 400-2000 8 4 18
161 400-2000 9 4 20
230 400-2000 11 5 23
345 400-2000 14 6 26
However, in the last 40 years, improvements made both in
material properties such as superclean XLPE insulation
compounds and supersmooth semiconductive shields, and
manufacturing techniques such as dry curing, have led to
increases in stress levels, which resulted in cables with smaller
insulation wall thickness. For example, the internal electrical
stress at the conductor semiconductive shield and the external
stress at the insulation semiconductive shield have increased
from approximately 7 kV/mm and 3 kV/mm, respectively, to
15 kV/mm and 7 kV/mm, respectively, and contributed to the
development and use of 500-kV cables [6] [7].
The need to operate above AEIC CS 9 stress levels is clearly
shown in Table III for 69-kV cables for 5-inch and 6-inch pipes.
The cables would require very thin walls and operation at
external stress levels equivalent to 230-kV through 500-kV
levels. The table also shows that replacement may not be
practically feasible for some pipe type cables, especially for the
smaller pipe sizes. The layers’ thickness would include the
insulation extruded shield 40 mil; bedding tape 10 mil;
shielding wires 63 mil; equalizing tape 5 mil; foil laminate tape
5 mil; and jacket 100 mil. The total thickness of the layers is
233 mil.
TABLE III. REPLACEMENT FEASIBILITY FOR 69-KV CABLES
Pipe ID, in
5.047
6.125
6.125
8.125
8.125
Pipe ID, mm
128.2
155.6
155.6
206.4
206.4
Conductor Size, kcmil
1000
1500
2000
2500
3000
Cond. Diameter, mm
26.9
35.1
40.9
46.2
50.5
Dia Cond. Shield, mm
29.0
37.1
42.9
48.3
52.6
Layers' Thickness, mm
11.8
11.8
11.8
11.8
11.8
Max Cable OD, mm
52.6
64.8
64.8
87.2
87.2
Insulation Thickness, mm
5.9
7.9
5.0
13.6
11.4
Dia over Insulation, mm
40.7
52.9
52.9
75.4
75.4
Internal Stress, kV/mm
8.1
6.0
8.9
3.7
4.2
External Stress, kV/mm
5.7
4.2
7.2
2.4
2.9
As the pipe size get larger, replacement with XLPE cable
becomes more feasible as shown in Table IV for 230-kV cables.
The thickness of the layers would include the insulation
extruded shield 40 mil; bedding tape 10 mil; shielding wires 63
mil; equalizing tape 5 mil; foil laminate tape 5 mil; and jacket
120 mil. The total thickness of the layers is 253 mil. As Table
IV shows, the internal and external stresses are within or close
to the AEIC CS 9 levels. Replacement for these cases seems
feasible as insulation thicknesses would be in the range of 18 to
25 mm depending on the conductor size and pipe size.
TABLE IV. REPLACEMENT FEASIBILITY FOR 230-KV CABLES
Pipe ID, in
8.125
8.125
10.25
10.25
10.25
Pipe ID, mm
206.4
206.4
260.4
260.4
260.4
Conductor Size, kcmil
1000
1500
2000
2500
3000
Conductor Size, mm2
506.7
760.1
1013.4
1266.8
1520.
1
Cond. Diameter, mm
26.9
35.1
40.9
46.2
50.5
Dia Cond. Shield, mm
29.0
37.1
42.9
48.3
52.6
Layers' Thickness, mm
12.9
12.9
12.9
12.9
12.9
Max Cable OD, mm
87.2
87.2
111.1
111.1
111.1
Insulation Thickness, mm
22.7
18.6
27.7
25.0
22.8
Dia over Insulation, mm
74.4
74.4
98.3
98.3
98.3
Internal Stress, kV/mm
9.7
10.3
7.5
7.7
8.1
External Stress, kV/mm
3.8
5.1
3.3
3.8
4.3
VII. AMPACITY CONSIDERATIONS
Ampacity is a critical consideration in the planning process
for replacement of pipe cables with XLPE cables. With respect
to ampacity, HPOF and HPGF cables are in contact with the
pipe and do not offer other bonding options. XLPE cables,
however, offer options for multi-point bonding, single point
bonding, and cross bonding. Single point bonding and cross
bonding will produce higher ampacity as compared to multi-
point bonding. For the case where the joints are installed in
sealed pipe casings, the bonding leads will need to be brought
out of the casing for single point and cross bonding schemes.
Table V shows the ampacity for 230-kV HPFF cables with
oil filling and replacement with XLPE cables for various
conductor sizes. The table shows that the multi-point bonded
scheme will cause a loss in current capacity for the XLPE cables
due to the circulating current in the cable shielding. For the
single point bonded case, however, an increase in current
capacity would result for the XLPE cables with circuit capacity
increases from 10 to 20 MVA.
TABLE V. AMPACITY COMPARISON FOR 230-KV HPOF
AND XLPE REPLACEMENT CABLE
Conductor Size, kcmil
1000
1500
2000
2500
3000
Conductor Size, mm2 506.7 760.1 1013.4 1266.8 1520.1
AMPACITY, A
XLPE- Multi Point
Bond
688
798
879
913
937
XLPE - Single Point
Bond
801
1007
1120
1190
1273
HPFF - Oil Filled
738
939
1091
1159
1219
Calculations for 69-kV and 138-kV systems replaced with
XLPE cables show no increase in current capacity with the
single point bonded option. Circuit capacity loss occurs for both
voltage levels and can range up to 10 MVA for the larger
conductor sizes when using single point bonding.
An option to increase ampacity presents itself with respect
to HPFF cables that have aluminum conductors. In this case,
XLPE cables with copper conductors can be used to replace the
HPFF cables where feasible. Fig. 7 shows ampacity for XLPE
copper cables replacing HPFF aluminum cables for 69-kV.
Figure 7. Ampacity for HPOF aluminum cable and XLPE
copper replacement cables for 69-kV.
The reader is referred to EPRI Report 1001818, “Increased
Power Flow GuidebookUnderground Cables”, which
0
200
400
600
800
1000
1200
1400
0 500 1000 1500 2000 2500 3000
Ampacity, A
Conductor Size, kcmil
HPOF AL Cable
XLPE CU Cable - Multi-Point Bond
XLPE CU Cable - Single Point Bond
discusses methods to increase ampacity for underground cables
including cables in pipes [9].
VIII. PIPE CLEANING AND INTEGRITY INSPECTION
Removal of the existing cables will leave residual fluid in
the pipe such as dielectric filling oils and impregnating oil for
the paper insulation. Pipe cleaning can be done by swabbing,
pressure washing, steam cleaning, and chemical PIGs (Pipe
Inspection Gauge) such as gel trains [8]. Depending on the
method, residual oil may be left in the pipe. This residual oil
will contact cable and accessories outer covering and the short
term and long term impacts and effects on the coverings will
need to be addressed [10]. It will also be necessary to inspect the
pipe wall for integrity. The pipe can be inspected for ovality by
means of conventional diameter PIGs or by smart PIGs, which
use magnetic flux linkage (MFL). Inspection of the pipe for
corrosion or damage can be done also by smart MFL PIGs or
robots that can be assembled in chains or stages to check the
pipe for different parameters concurrently, including the debris
load in the pipe.
IX. OTHER DESIGN AND INSTALLATION
CONSIDERATIONS
Other design considerations are cable pulling tensions and
jamming calculations, thermo-mechanical [11] [12] analysis, pipe
sealing and pressurizing options, joint mounting considerations,
and commissioning testing.
X. QUALIFICATION TESTING AND PROJECTS
Some manufacturers [13] [14] have completed long term
qualification testing for high stress design XLPE cables through
230-kV for test periods of one to three years. However, the tests
were conducted on triplex design cables that are enclosed in flat
strap type armoring and intended for replacement of external
compression pipe-type cables. These cables have been
successfully used for several reconductoring projects in Europe,
principally to replace gas compression pipe type cables,
although a 220-kV project in Russia used XLPE cables to
replace a conventional HPOF pipe cable.
ACKNOWLEDGMENT
The authors gratefully acknowledge the contributions of
Deborah Knudtzon in the preparation of this technical paper
and to the electric power industry.
REFERENCES
[1] Zimnoch, J, “Pipe type cables in the USA an historical
perspective”, Presented at the Spring 2011 Insulated
Conductors Committee Meeting [presentation]
[2] Mashio, S., “Retrofitting existing pipe type cable
economical challenges”, presented at the EPRI
Workshop on Reducing Costs of Underground
Transmission, September 2015 [presentation]
[3] “Nexans in modernisation of high-voltage section in
London first phase of project successfully completed”
[press release]
[4] Bosse, A., Waschk, V., Wollschläger, T., “First 220 kV
city cable for retrofitting of steel pipe cables”, Version
2”, JICABLE 2011, 8th International Conference on
Insulated Power Cables [conference paper]
[5] Curci, V, et al. “Repair of the Scattergood Olympic Line
2”, presented at the Western Underground Committee
[presentation]
[6] CIGRE, Technical Brochure 379, Update of service
experience of HV underground and submarine cable
system, Working Group B1.10, April 2009 [technical brochure]
[7] EPRI Report No. 1001846, “Cable system technology
review of XLPE EHV cables, 220 kV to 500 kV”, 2002
[report]
[8] EPRI Report 1001379, “Methods for cleaning and
evaluating pipe-type cable pipes for retrofit with
extruded dielectric cable systems”, December 2001 [report]
[9] EPRI Report 101818, “Increased power flow guidebook
underground cables”, December 2003 [report]
[10] “Guide for selecting and testing jackets for power,
instrumentation and control cables” IEEE Insulated
Conductors Committee Meeting, 2007 [technical guide]
[11] EPRI Report 1001849 “Mechanical effects on extruded
dielectric cables and joints installed in underground
transmission systems installed in North America” [report]
[12] EPRI 1001848 Update “Technical update - mechanical
effects on extruded dielectric cables and joints installed
in underground transmission systems installed in North
America[report]
[13] Waschk, V., “City cable an innovative cable system
design for retrofitting of pipe cable systems”, presented
at the Insulated Conductors Committee, Fall 2004
[presentation]
[14] Kirchner, M., Modern pipe-type cable system new high
voltage XLPE cable and associated accessories including
transition joints, March 2010 [presentation]
Chapter
The first part of this chapter presents information about the evolution of power cables and their insulation materials as papers, oils, natural and synthetic rubbers, polyethylene, polyvinyl chloride, polypropylene, polytetrafluoroethylene, etc. The second part analyzes comparatively the values of typical properties of these materials, which are important for the operation of insulation, and their changes under the electrical, thermal, mechanical, and environmental stresses. The variations of conductivity and electrical permittivity, loss factors, and dielectric strength under the action of electric field and temperature are described, as well as the accumulation of space charge and tracking, partial discharges, electrical, and water treeing resistance of DC and AC cable insulation. Among the thermal properties, thermal expansion, thermal conductivity, thermal stability, melting effect, and working temperature are analyzed, and among the mechanical ones are the elasticity, elongation at traction, tensile strength, strength, compressibility, and hardness. The chapter continues with the presentation of chemical, microorganisms, termites, rodents, and resistance to radiation and the estimated lifetime of cable insulation. Finally, the conclusions are shown that XLPE has higher values for most of the properties and are recommended in the manufacture of DC and AC power cables.
Retrofitting existing pipe type cable economical challenges
  • mashio
Mashio, S., "Retrofitting existing pipe type cable economical challenges", presented at the EPRI Workshop on Reducing Costs of Underground Transmission, September 2015 [presentation]
First 220 kV city cable for retrofitting of steel pipe cables
  • A Bosse
  • V Waschk
  • T Wollschläger
Bosse, A., Waschk, V., Wollschläger, T., "First 220 kV city cable for retrofitting of steel pipe cables", Version 2", JICABLE 2011, 8th International Conference on Insulated Power Cables [conference paper]
Repair of the Scattergood Olympic Line 2
  • curci
Curci, V, et al. "Repair of the Scattergood Olympic Line 2", presented at the Western Underground Committee [presentation]
Pipe type cables in the USA an historical perspective
  • zimnoch
Zimnoch, J, "Pipe type cables in the USA an historical perspective", Presented at the Spring 2011 Insulated Conductors Committee Meeting [presentation]
City cable an innovative cable system design for retrofitting of pipe cable systems
  • waschk
Waschk, V., "City cable an innovative cable system design for retrofitting of pipe cable systems", presented at the Insulated Conductors Committee, Fall 2004 [presentation]
Modern pipe-type cable system new high voltage XLPE cable and associated accessories including transition joints
  • M Kirchner
Kirchner, M., Modern pipe-type cable system new high voltage XLPE cable and associated accessories including transition joints, March 2010 [presentation]
Guide for selecting and testing jackets for power, instrumentation and control cables
"Guide for selecting and testing jackets for power, instrumentation and control cables" IEEE Insulated Conductors Committee Meeting, 2007 [technical guide]
First 220 kV city cable for retrofitting of steel pipe cables
  • bosse