Comparison of vertical and catenary lines-LN

Comparison of Vertical and Catenary CV Lines

Mo Dawud, General Manager, Mass Int’l Corp


Cross-linked polyethylene (XLPE) insulating compound is one of the mostreliable materials for manufacture of high voltage and extra voltage powercables.

Cross-linked Polyethylene (XLPE) is the end product of a chemical reaction ofhigh molecular polyethylene with organic peroxide cross-linking additives, undera specific temperature and pressure.

In cross-linking, the processing technique and the degree of purity of the rawmaterial used are of paramount importance

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Seamless interfaces are required between the insulation and the semi-conductiveinner 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 theouter 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 andCooling) 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.


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


This one step continuous process guaranties homogeneous insulation, free frommicro voids, and a very smooth contact between the insulation and semiconductive layers.

This ensures high impulse and AC breakdown strength, long life, and servicereliability 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 ofinert gas (nitrogen).

There are 6 heating zones each 6 mt (20 ft) long in catenary tube where nitrogenis circulated.


Vulcanization of the extruded core is achieved by circulating nitrogen at apressure of 10 bars having a descending temperature profile from 400degrees 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 presentin polyethylene decomposes into two components which react with PE,resulting in cross-linking, thus obtaining excellent mechanical and electricalproperties in the insulation.

The major residual of cross-linking process is “gas” that gradually diffusesout of the insulation.

All manufacturing parameters, operating speed, and thermic parameters arecomputer controlled.

Dry Cooling:

The hot cross-linked core then passes into the cooling zone of the catenaryline; cooled once again in an atmosphere of pressurized nitrogen.

The core then leaves the CV tube through the exit seal, thus completing thedry curing and cooling processes.


A brief history of the processes:

Historically, the VCV and MDCV processes were developed for the curing of naturalrubber, involving the passage of a cable through a long tube filled with steam at highpressure.

Process wise, the vertical tube was perceived to have an advantage as there was noundue sag of thick-walled soft extrudate as it left the die.

However, as very long vertical tubes of around of 150 meters (500 ft.) becamenecessary for EHV production, the VCV became an expensive process.

Until the ‘70s, VCV was the only available technology for production of EHV cables withinsulation thickness greater than 25 mm (1 inch) and larger sized conductors above1600 mm2.

Subsequent improvements to the CCV process and its relative lower cost has made itthe process of choice for cable manufacturers.


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.


With the development of new generation of compound materials and equipment theconstraints of CCV lines have successfully been overcome.

Various tension retaining devices are employed to maintain the core central axisof the core.

Reinforced caterpillar systems carry higher loads imposed by heavier cores.

For EHV cables with large size of conductors, material suppliers have developedspecial ultra clean XLPE materials with a special melt flow index.

XLPE has been used for over 35 years as a cable insulation compound, commencingwith MV and HV cables.

Since the early 1990s usage of XLPE for 400kV networks has been proven (e.g., inutility of Berlin “Bewag”).



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


1 x 2000 mm2 400 kV XLPE cable produced on a catenary –CCV line.27 mm insulation thickness (nominal).


View of multiple Catenary lines


Another view of the catenary


Over the past 25 years, our facility in Turkey has manufactured and shipped cables up to400 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


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 uniformcooling. This ensures a superior level of degassing of the core.

Based on available statistics, 90% of the world output of HV and EHV cables isproduced on CCV lines.

Conclusion:

Manufacturing of HV and EHV cables utilizing any of the above lines will meet withthe current US and IEC standards for eccentricity, routine, and special tests.

Factory test results for eccentricity and insulation thickness demonstrate thesuperior quality of cables processed on CCV lines.

Documents

Comparison of vertical and catenary lines-LN