El estudio del patrimonio cultural en el actual contexto global

Application

Cables intended for the transmission and distribution of electrical energy are mainly used in power plants, in distribution sys- tems and substations of power supply utili- ties, and in industry.

They are preferably used where overhead lines are not suitable, e.g. in densely built- up areas, in cities (pedestrian zones), in- dustrial installations and buildings. For power supply cables there are two main fields of application with different stresses (Fig. 1):

Stresses and requirements

These cables, especially the insulation (electrical strength) of buried cables, must be reliable and have a long service life. In order to fulfil this requirement for Siemens cables, the cable construction as well as the materials and manufacturing processes are permanently improved with a lot of de- velopment work.

The different stresses determined by the function form the basis for the definition of the cable requirements (Fig. 2).

In view of the possible external stresses for power cables, cables are be divided into two standard cable types, i. e. one for laying in the ground (distribution cables) and one for installation in air (installation cables) (Fig. 3).

High-voltage cables are often designed according to the specific stresses of each special case of application.

The Siemens instructions AR 320-220 and AR 320-1-220 contain detailed information on the application of cables, e. g. permis- sible pulling forces, limit temperatures, bending radii, cable fixing, storage and transport, etc.

Voltages ■Rated voltage

– Power cables are classified according to the rated voltages U0/U and Um.

– U0 is the rms value between con-

ductor and ground or grounded metallic covering (concentric conduc- tor, screen, armor, metal sheath). – U is the rms value between phase

conductors.

– Um is the maximum rms value

between phase conductors. In an a. c. system, the rated voltage Ummust be at least equal to the highest

voltage of the system Ub maxfor which

it is intended. U0 = U/ 3

– For application in three-phase and single-phase systems the main stan- dard rated voltages (rounded values) in compliance with IEC 183 are given in Fig. 4.

The maximum continuous operating-volt- age at normal operation for low voltage cables with rated voltage of 0.6/1kV (Um = 1.2 kV) is

– 1.8 kV in d.c. systems – 3.6 kV in a.c. systems

for PVC-insulated cables having a concentric conductor or armor and conductor cross-sectional areas from 240 mm2 _{and above.}

Directly in the ground

Laying in the ground

Outdoors Installation in air In ducts In concrete In water Indoors In channels

Fig. 1: Fields of application

Stresses determined by the function:

Current Voltage Thermal stress Electrical stress Thermal/ mechanical stress Thermal stress Electrical stress Normal operation Short-circuit Ground fault Transient waves Operation under fault con- ditions

5/3 Siemens Power Engineering Guide · Transmission & Distribution

Mechanical Chemicals (permanent influence), oil, acids Moisture (water) Temperature Fire propagation Chemical Climatic Fire behavior

Tensile strength (laying) Impact strength (civil works) Abrasion

Termites, rodents, etc.

Chemicals

(short-term influence) Ozone

Moisture (rain, humidity) UV radiation

Temperature (cold, heat) Fire propagation

Corrosive combustion gases Smoke density

Circuit integrity of cable installation Tensile strength (laying)

Pressure force (cleats) Vibrations

Laying in the ground Installation in air

Power Cables – General

Fig. 3: Stresses determined by the installation method

Fig. 4 Current ratings

For safe project planning of cable installa- tions, the cross-sectional area of conductor shall be determined such that the require- ment

current-carrying capacity Iz≥ loading Ib

is fulfilled for all operating conditions which can occur. A distinction is made between the current-carrying capacity

■for normal operation

■and for short-circuit

(operation under fault conditions) Especially in low-voltage systems, the cross-sectional area of the conductor must be additionally determined in respect of the permitted voltage drop ∆U. In order to avoid thermal overloading of the cable a suitable protective device also has to be selected. Besides that, the relevant instal- lation rules shall be observed.

With regard to these criteria, brief in- structions for project planning are given in Part 2 of the book “Power Cables and their Application”. They are sufficient for most cases when using the values listed in this book. The procedure is shown by examples.

More comprehensive calculation meth- ods with detailed project planning data can be taken from Part 1 of the book “Power Cables and their Application”.

Order-Nr.: Part 1: A19100-L531-F159-X-7600 Part 2: A19100-L531-F506-X-7600. For high-voltage cables, the current-carry- ing capacity is to be examined for each special case of application. It depends on a lot of special laying and installation con- ditions so that it is not possible to give standard values.

Standards

To ensure the operational properties and the high quality of all types of Siemens cables, short-term and long-term tests are carried out. They are based on national and international standards such as VDE and IEC. A perfect quality system accord- ing to ISO 9001 ensures a maximum of reliability of Siemens cables.

0.6 3.6 6 12 18 64 76 87 127 160 290 1.2 7.2 12 24 36 123 145 170 245 300 525 1.4 8.3 14 28 42 142 168 196 284 346 606 0.7 4.1 7 14 21 71 84 98 142 173 303 Uo Um in three-phase systems

Um in single-phase a.c. systems Both phase conductors insulated One phase conductor grounded [kV] [kV] [kV] [kV]

5/4 Siemens Power Engineering Guide · Transmission & Distribution

To protect the insulation of PROTODUR or PROTOTHEN X cables against permanent, intensive ingress of fuels, oils or solvents, a lead sheath can be provided under the PVC sheath.

For high-voltage cables a lead sheath is normally used for low-pressure oil-filled cables, but it is also available for cables with XLPE insulation. For these high-volt- age PROTOTHEN X cables, however, normally a screen of round copper wires with a cross-sectional area of 35 mm2 _or

50 mm2 _{is used together with an alumi-}

num laminated PE sheath.

For all types of insulation used for high- voltage cables, a metal sheath of alumi- num is available as well. The advantage of such a cable design with a corrugated aluminum sheath is the very high mechani- cal protection and, under fault conditions, the very high ground-fault current-carrying capacity.

Outer coverings

For low-voltage PROTODUR cables with PVC insulation and PROTOTHEN X cables with XLPE insulation, a PVC sheath is nor- mally applied.

Medium-voltage XLPE-insulated cables nor- mally have a sheath of polyethylene which is more resistant with respect to the me- chanical properties. PVC sheaths can also be provided, especially for underground mining or indoor installation (flame retar- dance according to IEC 332-2).

As already mentioned, high-voltage cables having a screen of round copper wires are provided with an aluminum laminated PE sheath consisting of an aluminum tape coated with PE copolymer on the outer

Power Cables – General

Constructional elements