2. Comparison of Vertical and Catenary CV Lines
Cross-linked polyethylene (XLPE) insulating compound is one of the most
reliable materials for manufacture of high voltage and extra voltage power
cables.
Cross-linked Polyethylene (XLPE) is the end product of a chemical reaction of
high molecular polyethylene with organic peroxide cross-linking additives, under
a specific temperature and pressure.
In cross-linking, the processing technique and the degree of purity of the raw
material used are of paramount importance
MD1
3. Comparison of Vertical and Catenary CV Lines
Seamless interfaces are required between the insulation and the semi-conductive
inner and outer shields, constituting three layers referred to as the “Cable core”
Such an interface is achieved by:
The simultaneous extrusion of the inner semi-conductive, insulation and the
outer semi-conductive layers
Use of specially developed high performance cross heads
Use of super smooth, extra clean raw materials
Use of temperature controlled curing in CDCC (Completely Dry Curing and
Cooling) line
This one step triple extrusion process guarantees the following characteristics:
• Homogeneous insulation free from micro-voids.
• Very smooth surfaces ensuring good contact between the insulation and semi-
conductive layers.
• High impulse and ac breakdown strength.
• Long life and service reliability.
4. Comparison of Vertical and Catenary CV Lines
The dimensions of the core elements need to be maintained with close tolerances
for thickness and eccentricity.
The compounds must be free of moisture and contaminants.
Typical processes used for “core manufacture “are:
-Triple extrusion: Conductor screen (inner semicon), XLPE insulation and
insulation screen (outer semicon) are formed in one pass
-Cross-linking
-Degassing to remove cross-linking by-product gases
5. Comparison of Vertical and Catenary CV Lines
This one step continuous process guaranties homogeneous insulation, free from
micro voids, and a very smooth contact between the insulation and semi
conductive layers.
This ensures high impulse and AC breakdown strength, long life, and service
reliability which are highly critical factors at high kV levels.
Catenary Tube:
A typical catenary tube is approximately 140m in length
44 m length is utilized for heating and 96 m length used for cooling.
Dry Curing (Cross-linking):
After extrusion, the conductor passes through the curing (cross linking) zone
wherein controlled heat treatment takes place in a pressurized atmosphere of
inert gas (nitrogen).
There are 6 heating zones each 6 mt (20 ft) long in catenary tube where nitrogen
is circulated.
6. Comparison of Vertical and Catenary CV Lines
Vulcanization of the extruded core is achieved by circulating nitrogen at a
pressure of 10 bars having a descending temperature profile from 400
degrees C to 300 degrees C.
Degree of cross-linking depends on the time under heating and pressure.
During the cross-linking process, peroxide in the cross-linking agent present
in polyethylene decomposes into two components which react with PE,
resulting in cross-linking, thus obtaining excellent mechanical and electrical
properties in the insulation.
The major residual of cross-linking process is “gas” that gradually diffuses
out of the insulation.
All manufacturing parameters, operating speed, and thermic parameters are
computer controlled.
Dry Cooling:
The hot cross-linked core then passes into the cooling zone of the catenary
line; cooled once again in an atmosphere of pressurized nitrogen.
The core then leaves the CV tube through the exit seal, thus completing the
dry curing and cooling processes.
7. Comparison of Vertical and Catenary CV Lines
A brief history of the processes:
Historically, the VCV and MDCV processes were developed for the curing of natural
rubber, involving the passage of a cable through a long tube filled with steam at high
pressure.
Process wise, the vertical tube was perceived to have an advantage as there was no
undue sag of thick-walled soft extrudate as it left the die.
However, as very long vertical tubes of around of 150 meters (500 ft.) became
necessary for EHV production, the VCV became an expensive process.
Until the ‘70s, VCV was the only available technology for production of EHV cables with
insulation thickness greater than 25 mm (1 inch) and larger sized conductors above
1600 mm2.
Subsequent improvements to the CCV process and its relative lower cost has made it
the process of choice for cable manufacturers.
8. Comparison of Vertical and Catenary CV Lines
There are three different types of extrusion lines used for manufacturing of HV and EHV XLPE
cables:
CCV line (catenary line for continuous vulcanization)
The curing tube is shaped like a catenary designed to replicate the flow path of the
core as it exits the extruder.
To avoid sag, the conductor is rotated with suitable tension as it traverses in the tube.
Nitrogen gas is used to maintain pressure and cooling
VCV line (vertical line for continuous vulcanization)
The curing tube is vertical
Cable axis is controlled to be in the center by applying tension.
Nitrogen gas is used to maintain pressure and cooling
MDCV line (Mitsubishi-Dainichi horizontal line continuous vulcanization-Long land die)
The curing tube is horizontal
The die size is the same as the final size and the core fills the die
No nitrogen is used in this process.
Does not have the “conductor sag” issues but it becomes critical to maintain the
conductor axis due to weight.
9. Comparison of Vertical and Catenary CV Lines
With the development of new generation of compound materials and equipment the
constraints of CCV lines have successfully been overcome.
Various tension retaining devices are employed to maintain the core central axis
of the core.
Reinforced caterpillar systems carry higher loads imposed by heavier cores.
For EHV cables with large size of conductors, material suppliers have developed
special ultra clean XLPE materials with a special melt flow index.
XLPE has been used for over 35 years as a cable insulation compound, commencing
with MV and HV cables.
Since the early 1990s usage of XLPE for 400kV networks has been proven (e.g., in
utility of Berlin “Bewag”).
11. Comparison of Vertical and Catenary CV Lines
The “cable core” is the most critical element of
high and extra high voltage power cables.
Such an interface is achieved by the
simultaneous extrusion of the inner semi-
conductive, insulation, and the outer semi-
conductive layers.
This is achieved by specially developed high
performance cross heads and clean raw
materials with temperature controlled curing in
CDCC (Completely Dry Curing and Cooling) line.
Triple head extruder
12. Comparison of Vertical and Catenary CV Lines
1 x 2000 mm2 400 kV XLPE cable produced on a catenary –CCV line.
27 mm insulation thickness (nominal).
15. Comparison of Vertical and Catenary CV Lines
Over the past 25 years, our facility in Turkey has manufactured and shipped cables up to
400 kV, using CCV lines with an overall supply record of over 28 million feet of XLPE HV,
EHV cables.
All of the manufactured/shipped cables have been performing successfully.
8639 KM ( 5638 miles) is 2.5 times the distance from east coast to west coast of the US
16. Comparison of Vertical and Catenary CV Lines
The installed base for CV lines worldwide consists of over 400 catenary CV (CCV)
lines and about 70 vertical and MD CV lines.
CCV and VCV lines both use nitrogen gas to maintain pressure and ensure uniform
cooling. This ensures a superior level of degassing of the core.
Based on available statistics, 90% of the world output of HV and EHV cables is
produced on CCV lines.
Conclusion:
Manufacturing of HV and EHV cables utilizing any of the above lines will meet with
the current US and IEC standards for eccentricity, routine, and special tests.
Factory test results for eccentricity and insulation thickness demonstrate the
superior quality of cables processed on CCV lines.
