27_Borsi_MV HV Cables GIS Intro and Technology.pdf

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MV and HV Cables, GIS Introduction and Technology

Prof. Dr. – Ing. Habil. H. Borsi

Bali 2013

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1925 1950 1975 2000 1825 1850 1875 1900 Time

Power cable with paper dielectric

1890 10-kV-Massekabel (für Wechselstrom)

1913 33-kV-Massekabel (Radialfeldkabel/Höchstädter)

1924 132 kV low-pressure oil-filled cablel

1931 External gas pressure cables

1931 High-pressure oil cable

1936 220 kV low-pressure oil-filled cable

1937 Internal gas pressure cable



1980 1100-kV low-pressure oil-filled cable (test cable)

Beginning of cable technology

1847 First communication cable with gutta-percha insulation

1880 First power cable with gutta-percha insulation (dc)

1882 First power cables with impregnated fiber fabric insulation and lead sheath

Historical developement

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1980 1990 2000 1940 1950 1960 1970 Time

Power cables with plastic dielectric

Historical developement

1960 20 kV cables with PE insulation



1979 275 kV cables with XLPE insulation

1986 400 kV cables with VPE insulation

1988 500 kV cables with VPE insulation


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The main structural elements of a single core cable and its functions

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1994 1978 1998

TB RE MI FRH MZA

IPH

1998 2000

Reuter Mitte Friedrichshain Marzahn

Vattenfall Europe Transmission Wolmirstedt

Vattenfall Europe Transmission Neuenhagen

Oil cabel 7.6 km

Overhead cable 1.1 km (Berlin territory)

Overhead cable 2.6 km

Oil cabel 8.1 km

XLPE-cabel 6.5 km

XLPE-cabel 5.4 km

Overhead 7.0 km

(Berlin territory)

(1) (1) Tunnel Teufelsseekanal: 0.32 km (2) Tunnel Havelquerung: 0.52 km (3) Tunnel Schlosspark: 1.10 km (4) Tunnel Spree: 0.27 km

380-kV-overhead line 21.4 System-km 380-kV-oil cable 31.4 System-km 380-kV-XLPE- cable 23.8 System-km (laying in the tunnel) 27.4 System-km

(2) (3)

6,3 km Tunnel 5,2 km Tunnel

(4)

Overview

The 380 kV diagonal

High and extra high voltage cables for big city supply

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High voltage transmission technique (380 kV) Development of high voltage cable technology

Oil cable - Proven system of over 30 years of use under service conditions - Disadvantage through increased demands on secondary systems by oil-paper insulation and the use of forced cooling

With the use of appropriate and high quality pure synthetic plastics as well as the further development of the extrusion process (since 1960), there was the possibility for the production of high-voltage cables with solid insulation.

XLPE cable - Very good operating experience Bewag / Vattenfall Europe Berlin since 1998 - Maintenance-free and easy-to-install cable with Increased transmission capacity

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About medium voltage cables

• Despite fission product, free polymerization of highly polar material due to the presence of permanent dipoles. • High loss factor and permittivity by orientation polarization

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Countermeasure: Addition of basic lead compounds in order to accumulate free radicals (obstruction of the zipper effect) and bind the fission products ‘’HCl’’ to electrical neutralization; coloring in order to protect the material from the light. Effects of aging: Early increase of the loss factor in the ionic conduction area embrittlement

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Aging of PVC by UV light or heat

Zipper-like separation of the chain molecule with the formation of double bonds and elimination of hydrochloric acid (schematically)

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Continuous crosslinking system ("continousvulcanisation" CV) for the production of plastic cables with EPR insulation; the free slack of the conductor during the crosslinking and cooling sections are matched, indirect heat transfer in the crosslink pipe by means of superheated steam

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Chain line for the production of EPR cables

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Properties of EPR as Medium Voltage Insulation

Permittivity at 50Hz Loss factor at 50 Hz, 20-90 ºC Specific direct current conductivity at 20-90 ºC Short-term dielectric strength of samples ≤ 1 mm thick, 50 Hz Dielectric strength cable, 5 m long, 3-4 mm thick, 15-20 Hz, Continuous operation temperature (max.) Short circuit temperature Mechanical tensile strength at room temperature Elongation

2.5-3.0 20-60*10-4

10-16 -10-14 S/cm 100-160 kV/mm 50 kV/mm 90 ºC 250 ºC 5-8 N/mm2

250-350%

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Clear superiority of EPR in the high temperature range, excellent thermal reserves in case of overload and short-circuit

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Mechanical tensile strength of EPR compared to cross-linked polyethylene (XLPE)

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Structural formula of polyethylene chain (schematically) By-theoretically-absolutely symmetrical molecular structure and the absence of polar reaction products exceptionally high dielectric material with an extremely small loss factor (≤10-4) and low DZ (~2.2).

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Polyethylen and XLPE cables Polyethylene (PE) = polymerization product of ethene or ethylene C2H4

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The proportion of the less dense amorphous zones with an irregular molecular structure increases with the number of tertiary carbon atoms, and with the temperature. Consequently, LDPE contains larger amorphous content than HDPE. Above the crystalline melting temperature (110-130 ºC) the materials are completely amorphous.

Molecular structure of semi-crystalline materials, as an example polyethylene (schematic)

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Material data of polyethylene at room temperature 1) XLPE specification are valid for dry crosslinking 2) AC voltage values are valid for short-term load

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Properties LDPE HDPE XLPE General Production Melting behavior Specific gravity (g/cm3) Crystallite (vol. -%) Molecular weight Flammability Maximum continuous temperature (Cantigrad) Dielectric properties Dielectric constant at 50 Hz Dissipation factor at 50 Hz (.10-4) Resistivity (Ω.m) Dielectric Strength (kV/mm) Physical limit (partially failed) Model samples 1mm (50 Hz) VDE plates 1 mm (50 Hz) Cable 5.5 mm (50 Hz) Cables (power surge)

Polymerization Thermoplastic

0.918 40-45

70.000-100.000 yes 70

2.2-2.3 1-2

> 1017

800 150

50-100 30-50

about 100

Polymerization Thermoplastic

> 0.95 70-80

150.000 yes 80

2.3-2.4 1-3

> 1017

800 150

50-100 40-60 ca 100

Polymerization Thermoelastic

0.918 40-45

70.000-100.000 yes 90

2.2-2.3 1-3

> 1017

800 150

50-100 50-70

about 100

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Weak interface polarization determines the loss factor reduction from 0 to 80 ºC. Reducing the DZ as a result of increasing crystallite melting over temperature results in decreasing the density.

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Loss factor and permittivity (DZ) of LDPE and XLPE at 2 KV/mm

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Structure plus interface and screen pad to avoid sticking of the shield wires to the plastic jacket and protect the outer conductive before wire screen prints

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Standard XLPE cable

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Longitudinal and radial watertight XLPE cable For high voltage quasi-standard

Polymer-coated Al-foil overlaps along a cross-water protection

Swelling tape, rubber or powder coat for inner longitudinal water seal

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Inner and outer "conductive layers" in the form of overlaps wound Al-coated Höchstädter papers; Paper insulation wound with a gap of several layer paper tapes

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Cables with impregnated paper insulation Basic structure of the paper insulation of a cable

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The need of the gaps between neighboring strips of paper at bending When it comes to narrow gaps at bending it causes compression of the paper with cavity formation

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Advantages Paper Kabel Disadvantages compared to XLPE cables • Low susceptibility of PD by automatic cavity filling with impregnating • "self-healing" local damage by post flow impregnating • high resistance to aging, time-tested performance up to 400 (500) kV. • for oil-filled cables possibility of impregnating process on installed cables • no risk of water treeing • Easy on-site testing capability with DC

• approximately 10 times higher loss factor (20/10-4) and 50% larger DZ: at the same voltage 30 times higher loss power • complex production with drying, degassing, vacuum impregnation • complicated sets (socket terminals + Links), no prefabricated elements used • Risk of the outflow of impregnating material in the mountains • not maintenance free (gas pressure or Oil monitoring) • damage causes environmental hazard because of leaking impregnating oil

Advantage and disadvantage of high voltage cables with impregnated paper insulation compared with XLPE cables

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Nitrogen-filled membrane body (same volume) changes due to thermal cycles of impregnating oil automatically. Distance between neighboring expansion tanks: 0.3 ... 1 km.

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Pressure maintenance in oil cable systems overcompensation vessels

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Impregnating oil with low viscosity and hollow channel in the lead to maintain the impregnating supply.

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Cross section of a low-pressure oil-filled cable Main features:

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Properties of low-pressure oil-filled cables

Loss factor tanδ (50 Hz, 20 ° C) Permittivity ε r (50 Hz, 20 °C) Impulse withstand voltage(1...5/50µ S) Short-term strength (50 Hz) Duration withstand voltage (50 Hz) Operating field strength at the conductor E0 Max. continuous operating temperature Operating pressure (Nitrogen N2)

20-30*10 -4 3.5 100-130 kV/mm 45-50 kV/mm 40-45 kV/mm 13-16 KV/mm 85 ºC ~ 3 bar

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Max. permissible system pressure on the valley side for mechanical reasons about 6.5 bar; minimum pressure required on the mountain side for electrical reasons about 2 bar. Accordingly all 60-70 m height differential locking sleeves must be provided with expansion tanks.

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Distribution of oil-filled cable systems in mountainous region into sections with separate pressure maintenance

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Water as a polar molecule increases the loss factor by orientation polarization and promotes ion conduction (a). In addition, water may evaporate at high concentration under field exposure and form bubbles: reduction of dielectric strength (b). Consequence: effective drying

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Influence of water content on the electrical properties of paper impregnated dielectrics at 50 Hz / 20 ºC

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Components of a paper-cable manufacturing line (without impregnating and coating equipment)

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Based on the respective transmission power only <0.1% loss development, but absolutely considerable energy. Main sources of loss: Current heat in conductor and dielectric losses at 380 kV oil cable? -1600 mm2 conductor cross section respectively -Conductor losses valid for rated current 1.1 kA

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Loss sources in high and extra high voltage cables

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Greatly restricted thermal dissipation very close to the cable due to dehydration of the soil and heating leads to excessive heating of the dielectric. In addition, unfavorable conditions by mutual influence of several neighboring conductors in three-phase system.

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Temperature gradient in the surroundings of a high- power cable

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Direct heat dissipation from the cable surface by cooling water enables improved performance three to four times compared to normal burial.

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Increasing the transmission capacity of cable systems through forced cooling

The application of cooling method in the 400 kV cable network BEWAG, Berlin:

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Methods of forced cooling for high power cables

Indirect water-cooling of the cable surface

Laying of cables and cooling pipes in lean concrete -Advantages: Easy to install, protected route -Disadvantage: Limited cooling capacity

Direct water- cooling of the cable surface

Laying up of all three phases in a common trench or cooling - channel -Advantage: Compact route, effective cooling -Disadvantage: In case of damage difficult repair Laying up in separate cooling tubes -Benefit: Effective cooling -Disadvantage: Large line width

Lead-Direct cooling (water or oil)

Cooling passage in each conductor -Advantage: Very effective cooling directly "at source" -Disadvantages: Expensive cables and links

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Main components are: • Conductor as a hollow tube for enlarging the radius of curving (reduction of surface field strength consequently) • Housing at earth potential as return conductor • Supporting insulators with special design for the central guiding of the conductor • SF6 insulating gas under increased pressure as the main dielectric

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Basic structure of gas pipe cables

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Applications for Gas-Insulated Lines (GIL)

- Very low capacitive and resistive losses reduce operating costs and avoid reactive power compensation.

- Very low electromagnetic fields allow the use in the neighborhood of residential areas and technically sensitive areas.

- Connecting load centers with higher power requirements via the distance of a few kilometers

- With transmission power of 3000 MVA reaches the GIL the transmission capacity of overhead lines and exceeds the transmission capability of solid-insulated cables.

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Advantages and disadvantages of SF6 Gas pipe cables

Advantages Disadvantages

• Large conductor cross-sections and thus high transmission rates achievable • Extremely low internal losses, based on the nominal power <0.1% per system-km • Environmentally friendly, no risk of soil contamination with liquid impregnation • Suitable for vertical installation without any additional measure • Regenerable dielectric, proven long-term electrical strength • Suitable for voltages> 420 kV

• Large cable dimensions, expensive • Only short pieces transportable, complex piping with welding works • Special spring elements for compensation of thermally induced length changes required • Not fully testable in the factory • Non-maintenance free (compared to XLPE) • Changing the direction of paths only by using special spherical segments

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Countermeasure : Limiting pressure to <5 bar (absolute) or heating the most susceptible parts of the system.

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Falling breakdown voltage through gas condensation below about -32 ºC

Condensation decreases the molecular density in remaining gas chamber and consequently reduces the dielectric strength.

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Field concentration in the edge, and tangential stress on the insulating surface (danger of creeping discharges): Targeted field controlling is required!

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Field and potential lines at the offset edge of the outer conducting layer or Höchstädter foil of high voltage cable (schematic)

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Conclusions from the 380 kV development have also led to the development of the sets in the 110 kV voltage level!

sleeve body

control electrodes

Conducting layer XLPE- cabel

Cross-bonding cable

conductor connection sleeve housing

One-piece deferral sleeve Made of EPDM

One-piece deferral sleeve Made of Silicon

Epoxy resin with Stress cones made of silicone rubber

Testing and Quality Assurance Complete set

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Application area: in plastic cables up to 400 kV Advantages: Low dielectric loss, easy assembly on site, complete prefabrication and testing at the factory Disadvantage: design, especially for higher voltages

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a) Geometric-capacitive Conductive deflector control, embedded in elastic insulating compound, reduces the volume capacity on the cable end, extends the field distribution on the electrical load and shifts electrical stress in sustainable solid (Silicon rubber, EPR, EPDM).

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With unchanged geometry and frequency, the increase of the induced voltage is proportional to current and conductor length: It is the reason of the current displacement in the neighboring conductors (proximity effect) and transient currents in double-grounded cable sheath (jacket flow losses)

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Voltage induction due to the magnetic field of the flowing current in conductor

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Consequence: compensation of the induced sheath voltage by symmetric out crossing ("cross bonding") of the cable sheaths or screens With absolutely identical path lengths of the individual sections in the symmetric crossed system, the induced sheath voltages add up to zero at the cable end: No compensation currents - no additional losses

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Shield losses by inducing currents at both earthed sides of jacket or shield, based on total losses of the 3-phase system

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History of the induced sheath voltage in out crossing (arbitrary units)

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Full-crossing of the cable shealds (cross bonding)

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Testing and Quality Assurance

Section length Heating/cooling cycle max. conductor temp.

Test voltage at 50 Hz

Duration of test

120 m 16 h / 32 h 90°C (+ 5 K, - 0 K) 1.7 x U0 = 1.7 x 230 kV = 400 kV *) 1 year

Switching impulse (250/2500 µs)

Lightning impulse

voltage (1.2/50 µs)

*) With continuous partial discharge measurements on all sets

950 kV (5 pos., 5 neg.) 1175 kV (10 pos., 10 neg.)

Experimental setup at CESI in Italy

Test for 400 kV long-term test

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Summary of routine tests acc. to IEC, VDE

Examination Application to ... Short-duration power frequency test

cable with paper and plastic insulation

loss factor measurement cables with paper insulation Partial discharge measurement cable with plastic insulation On site test (Power-on)

all cable systems after installation

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Decoupling of PD signals via coupling capacitor connected in parallel to the cable. Advantage: With cable breakdown, no galvanic infeed of high voltage in quadrupole Disadvantage: Reduced sensitivity (PD signals can be closed to circuit by Z)

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a) Parallel connection:

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Quadrupole coupling in series with the test object To increase the sensitivity an additional "return path capacity“ in parallel

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b) Coupling series:

Advantage: good sensitivity Disadvantage: when cable breaks down, galvanic infeed of the high voltage securing in four pole - effective overvoltage protection required

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Two coupling quadrupoles form a bridge with coupling capacitor and cable. Advantage: sensitivity as b), additionally limited noise suppression by external extinction pulses in the detector input Disadvantage: as b), additionally complicated

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c) Bridge circuit: basic circuits for partial discharge tests on cables

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The PD pulses, originated from the fault location, propagate with constant speed in both directions. At the open end, a pulse is reflected and thus meets later the measuring instrument. The time difference Δt is proportional to the distance difference Δl = 2a. Requirements for the determination of a from Δt: calibration of the total running time over 2L by impulse Injection at the cable beginning (duration until the arrival of the reflected pulse: t = 2L/v).

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Principle of the Location

27_Borsi_MV HV Cables GIS Intro and Technology.pdf

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