connectionofearthingcircuitsinhvsystems

8/6/2019 ConnectionofEarthingCircuitsinHVSystems

1/51


2/51

0. Introduction 3

1. General screen connection principles 5

2. Earthing circuit conditions and tests 13

3. Standardized screen connection diagrams 15

4. Elements making up the screen circuit 18

5. Multiple circuits 43

6. Tests 44

7. Earthing of lightning arresters 49

1

CONTENTS


3/51

This Engineering Recommendation deals with the earthing of cable screens which constitute

an insulated three phase line, installed either in a service gallery, in an indoor installation

or directly buried. To reduce the losses on the line and optimize the transport capacity,

connection systems tend to be adopted which reduce the intensity of the currents induced

on the screens. These systems involve specific connections of the screens to each other

and to earth and give rise to permanent current and current surges on the screen circuits

which must be considered.

0.1 SCOPE

This Recommendation describes the systems and components used in the connections

of the screen circuits of three phase power lines for voltages equal to or greater than 66

kV, made up of single-core cables with metallic screen and insulating sheath. Specific

means may be required to withstand the permanent current and current surges which can

occur between different parts of the screen circuit, and between the same and the earth,

and the testing and checking of the continuity of this circuit should be allowed.

0.2 DEFINITIONS

This Recommendation uses the definitions of the International Electrotechnical Vocabulary

(publication CEI 50), and in Appendix A of "The Design of Specially Bonded Cable Systems,

(part 1)" of Electra no. 28, of May 1973.

The following definitions are moreover added:

Screen. Continuous concentric conductor which surrounds the main conductor and

insulation, aimed at confining the electric field and at conducting the possible short-

circuit currents. It can be made up of a continuous metallic covering or a corona of

wires possibly complemented with metallic tapes. It should be connected to earth

directly or indirectly.

Sheath Voltage Limiters (SVL ). Devices with two terminals of strongly non-linear

voltage-current characteristic, aimed at limiting the differences of transitory potential

which, on the occasion of impulse, atmospheric or manoeuvring surges, can appear

between elements of the screen circuit with limited dielectric strength.

Connection box. Blockable box(1), built to house the connections of the screens, of

the earthing cables and the associated SVL when they exist. It should be connected

to earth if it is metallic.

0. INTRODUCTION

(1) Which requires a tool or a key to open it

3


4/51

Indirect earthing Earthing of the screens of the cables, or of a section of them, so that

the screen circulation currents that would otherwise occur are reduced or eliminated.

Other sections of screen from the same or another phase may be involved in series in

this connection between a screen and an earth electrode. Indirect earthing causes small

permanent current at some points of the screens or at the terminal.

4


5/51


6/51

1.1 RIGID EARTHING SYSTEMS

In these systems the screens of the three phases are connected together directly and to

earth so that, at all points of the line, the voltages of the screens between each other and

in relation to earth are maintained close to zero. No provision is adopted to prevent thecirculation of currents along the screens under a permanent regime. These currents, induced

by the main conductors, will give rise to an additional production of heat, with the consequent

reduction of the lines transport capacity.

The screens should be joined together and to earth at both ends of the lines. If necessary,

with the aim of limiting the screen voltages which could appear in the event of a defect on

the line itself, the screens will be joined together at other points, which can also be earthed. (2)

Generally speaking, single-core cables which constitute a three phase line are arranged

in a triangle as close as possible, to reduce the currents on the screens, which increase

on increasing the separation between phases.

With this arrangement, the transpositions of phases along the line are only useful to reduce

their inductive influence on adjacent cables. When the arrangement is flat, or in some other

way asymmetric, the transposition of conductors can moreover achieve the equality ofcurrents on the screens, and thus obtain the minimum value of losses in this type of

connection. Under no circumstances can the transposition of conductors be used to cancel

the currents on the screens.

Joints

Fig. 1

Rigid earthing system

6

(2) It is prudent to connect the screens together and to earth every 2 to 3 km, but this is only required

if the voltages generated between phases or between phase and earth due to voltage drop because

of the circulation of fault current exceeds the perforation voltage of the sheath.


7/51

1.2 SPECIAL EARTHING SYSTEMS

The permanent circulation of current along the screens during the normal operation of the

line leads to a rise in the losses and to the need to oversize the section of the main

conductors in order to obtain a certain transporting capacity for the line. With the aim of

preventing these inconveniences, the screens are earthed following diagrams which exclude

the formation of closed circuits between them, or they manage to cancel the electromotive

forces induced in the possible closed circuits, without them ceasing to fulfil the safety

conditions already set forth.

Having attained this objective, a further two advantages appear, in addition to those sought:

The separation between phases can be increased in order to obtain a better dissipation

of the heat to the surroundings. As the necessary section of the main conductor is smaller, the capacitive current

absorbed by the line is somewhat lower.

Certain inconveniences should be taken into account:

The screen system (cables and accessories) should withstand permanent currents (in

the order of some tens of volts) and current surges (in the order of some kilovolts), and

consequently it should be completely insulated in relation to earth, except at the

connection points foreseen.

In certain cases, joints have to be used which allow the electric discontinuity on the

screens.

Limitation elements should be installed for the current surges of short duration at the

discontinuity points of the screens and, in certain cases, at the ends.

In cases of earthing at a single point, an earth bonding conductor should be installed.

The losses in the screens cannot be eliminated completely on cancelling the complete

intensity which passes along them. The proximity effect generates stray currents in the

closed paths within the frame of the screen (Foucaults losses) which also cause losses.

7


8/51

1.2.1 Cross bonded system

This system consists of dividing the total length of the line into sections by means of

discontinuities in the screens, making a crossed screen connection at each change of

section. The natural points to establish the discontinuity are the joints. The typical diagram

is that formed by one or more major sections, each containing three minor sections.

We thus achieve that in each major section the total EMF induced is practically nil. A

sufficient, although not total, reduction of this EMF can be achieved with unequal sections

or with a number which is not a multiple of three. The screens are earthed at the two ends

of the line and, possibly, at the ends of the major sections. If the arrangement of the cables

is not symmetrical (flat arrangement, for example) and, for this reason, transpositions of

the same are carried out, the crossing of the connections of the screens should be carried

out in the opposite direction to that of the transposition, so that the screens in series remain

in the same spatial position all along the run.

8

Fig. 3

Crossing of screens in the event of transposition of cables

Fig. 2

Cross bonded system(1 major section, 3 minor sections)

Minor section Minor section Minor section

Major section

Cross-bondinglink boxes


9/51

Two variants are distinguished for the purposes of the connection of the screens to each

other and to earth: Sectionalised cross bonding: The screens are joined to each other

and to earth at the ends of the circuit and at the ends of all the major sections.

9

Continuous cross bonding: The screens are joined to each other and to earth only at the

ends of the circuit.

Fig. 4

Sectionalised cross bonding system

(2 major sections, 6 minor sections)(Intermediate earthing)

Major section Major section


Fig. 5

Continuous cross bonding system

(2 major sections, 6 minor sections)

Cross bonding link boxeswith SVL

Cross bonding link boxeswith SVL


10/51

1.2.2 Single-point earthing system

With short stretches or those which do not require more than one section of cable, the

cross bonding system may be uneconomical. In these cases we resort to a diagram in

which each screen has a single point of connection to the rest and to earth, Normally, thepoint at which the screens are joined and are connected to earth is one of the ends of the

line but, if the permanent screen voltages which appear in this case are excessive, the

earth connection point can be transferred to an intermediate point. The screen circuit of

a line can be divided into isolated sections, with each section having a single earthing

point. Both in the previous case and in the latter, the system is called single-point earthing.

With the single-point system, each section of the screen circuit is joined to earth by

connecting it to the earth continuity conductor (described in 1.2.3), either at its ends or at

intermediate points.

Fig. 6, Fig. 7 and Fig. 8 show typical examples of single-point earthing for lines of one or

two sections.

Fig. 6

Single point bonding system(Earthing at one end)

10

SVLlink box

Fig. 7

Single point bonding system(Earthing at the middle point)

Earth continuity conductor


SVLlink box

SVLlink box


11/51

1.2.3 Earth continuity conductor

If there is no low impedance joint between the earthing electrodes of each end of the line,

in the event of circulation along the line of short-circuit currents with a strong homopolar

component there will be an earth return. In this case very high voltages may be induced

in the screen circuit and in conductors close to and parallel to the line. In order to provide

a low impedance path for the homopolar current, a conductor link is arranged between the

earth electrodes of both ends of the line, formed by one (or several) conductor(s) with a

sufficient section to admit the expected short-circuit current. This conductor, installed

adjacent to the line, is transposed at intermediate points to balance the EMF induced and

thus prevent the permanent circulation of current during the normal operation of the line.

The earth continuity conductor should be insulated all along its run. It is tested at a test

voltage of 10 kV, 50 Hz, 1 min.

1.2.4 Surges in the screen circuit

With the special connection systems, the circulation of fault currents external or internal

to the line induces voltages of industrial frequency on the screens, above those of permanent

operation. These voltages should not exceed the admissible values of the external cable

sheath and accessories or of the discontinuity insulation of screens on the joints and

terminals. Their value depends on the length of the smaller sections, on the electric and

geometric parameters of the line, on the intensity of short-circuit and on the screen circuit

diagram. The cables and accessories are designed to resist these surges.

The rapid transitory phenomena of atmospheric or manoeuvring origin cause abrupt front

surges which propagate around the screen circuit and can reach very high values at the

points of interruption of the screens and at the terminals. To limit these surges it is necessary

to install voltage limiting devices at certain points, as detailed in section 1.2.5. These limiters

must not, under any circumstances, initiate the conduction with the industrial frequency

voltage, caused by the fault currents, which can occur at their terminals.

11

Fig. 8

Single point bonding system(Earthing at both ends with interruption of

screen at the middle point)

SVLlink boxes



12/51

1.2.5 Sheath voltage limiters (SVL)

The limiting devices indicated in the previous section, also called surges dischargers, are

strongly non-linear zinc oxide conductor elements. They present a very high resistance to

the reduced voltages that appear under normal operating conditions, and they do not

therefore modify the screen circuit diagram. They weakly conduct the industrial frequency

voltages originated during mains failures, without having any limiting effect on these voltages.

On the other hand, they conduct intensely for the short disturbances of atmospheric or

manoeuvring origin which would cause very high voltages at the ends and at the discontinuity

points, limiting these voltages to admissible values. This conduction is accompanied by

production of heat in the discharger, for which the latter has a reduced storage capacity.

For this reason, they are only adequate to limit surges of an order of duration lower than

1 ms, being able to be destroyed if an industrial frequency voltage, even of short duration,

exceeds their free conduction threshold.

It is necessary to limit the voltages which appear between screens and the local earth and

which subject the insulating sheath of the cable and the supporting insulators of the

terminals to dielectric stress, and the voltages that are presented between the two ends

of the screen which concur at the same joint with screen discontinuity, which must be

withstood by a very small thickness of insulating material inside the joint.

1.3 SCREEN CONNECTIONS

To allow periodic verifications of insulation and continuity of the screen circuit, the connections

between screens, to earth and to the SVL are carried out using detachable elements (3). The

connection of the SVL must also be detachable.

These connections and the SVL must be placed out of reach of unauthorized people, either

in adequate boxes, with insertion of physical barriers, or next to the external terminal base

if they are not accessible from the ground.

12

(3) The detachable connections are connected by means of blades or screws and can easily be

removed and replaced, without subjecting the elements of the circuit to stress or bending.


13/51


14/51

If there is no earth continuity conductor, and in the location there is a local earth system

intended for other uses, to carry out the connection to this system it will be necessary

to justify that it can accept the impulses of intensity which would originate in the event

of atmospheric or manoeuvring discharge and the industrial frequency intensities that

would appear in the event of action of one of the SVL ensuring the safety of people and

equipment.

If none of the possibilities indicated in the above hypotheses is available, the centre of

the star of the SVL will not be earthed. In these cases, the connection of the SVL in a

triangle is preferred.

14


15/51

3.1 RIGID EARTHING SYSTEMS

With this system the screens are directly joined to each other and to earth at both ends

and, possibly, at some intermediate point, without there being any interruption in their

continuity. Fig.1 illustrates the general case, in which both ends of the line and possibly

the joint points are directly earthed.

3. STANDARDIZED SCREEN CONNECTION DIAGRAMS

Joints

Fig. 1

Rigid earthing system

3.2 CROSS BONDED CONNECTION SYSTEMS

Fig. 2 describes the diagram to be used in the event of a major single section and Fig. 5

in the event of two major sections. With this system surge limiting devices are only necessary

at the intermediate points of discontinuity of the screens.

Fig. 2

Cross bonting system

(1 Major section, 3 minor sections)

Minor sectionMinor section Minor section

Major section

15

Cross bondinglink boxeswith SVL


16/51

The SVL can be connected in the following manners (see Fig 9):

In a triangle, each of them being parallel with the continuity interruption space of each

screen.

In a star, with its centre connected to a local earth system or to the bonding conductor

(or to one of the conductors), if it exists (see section 2.3).

In a star, with its centre insulated.


Fig. 3

Continuous cross bonding system(2 major sections, 6 minor sections)

16

Cross bonding link boxes with SVL

Fig. 9

Detail of SVL box and screen crossing

Cross bondinglink boxwith SVL

SVL instar

SVL intriangle


17/51

3.3 SINGLE POINT EARTHING SYSTEMS

These systems, which can be made up of one or several sections, are characterized by the

discontinuity of the screens in some (or in all) the changes of section and by the joining

of the screens to each other and to earth in a single point within each section. Surge limitingdevices assembled in a star with centre earthed must be installed at each end of section

which does not coincide with the point of its earthing. There must be an earth continuity

conductor, and the earth connections, both of the screens and of the surge limiting devices,

must be carried out directly to this conductor (Figs. 6, 7 and 8).

17

Fig. 6

Single point bonded system

(Earthing at one end)

Fig. 7


(Earthing at the middle point)


SVLlink boxes


Fig. 8

Single point bonded system(Earthing at both ends with interruption

of screen at the middle point)

SVLlink box

SVLlink box

Earth continuity conductor SVLlink box


18/51

4. ELEMENTS MAKING UP THE SCREEN CIRCUIT

(4) The connection boxes are of a watertight horizontal type, and are used in chests or galleries.

The cupboards are of vertical type protected against the elements and used fastened to walls ofsubstations or metallic structures (pylons or towers).(5) An internal short-circuit generates a very considerable excess pressure. If the box breaks, thisexcess pressure will be transmitted to the chest (of small volume) and may be sufficient to pull offthe cover and cause an accident.

18

The elements necessary to establish the connections between the elements of the screen

circuit that each system requires, and for the subsequent verification of the screens, are

described below.

4.1 CONNECTION BOXES OR CUPBOARDS (4)

The ends of the junction cables and of the SVL (except the SVL connected directly between

sectioning insulation on terminals of equipment with metallic casing or on external terminals

see section 4.5.1), will be enclosed in connection boxes capable of containing the effects

of thermal or electric failure of any of the elements housed without damage being causedto neighbouring external elements. In the cases of boxes installed in chests or enclosures

of small volume it is recommended that they should be capable of containing the effects

of an internal short-circuit.(5)

In the case of voltages of 245 kV upwards, with their corresponding high fault levels, it is

accepted that, in certain situations, the insulation of the SVL and of the connections may

not be capable of withstanding the conditions to which they are subjected in the event of

an internal fault of the line itself. When this situation is foreseen, additional measures should

be adopted to confine the consequences of this type of fault.

Metallic connection boxes should always be earthed, by means of a connection independent

from the earthing of the elements contained inside them (connections of the screens or of

the SVL) with a sufficient level of insulation. The internal insulation of the connection boxes

should fulfil the conditions of section 6.3.

The boxes or cupboards should be equipped with some means to prevent incorrect

connection positions and shall be equipped with a label which shows the normal connectionarrangement.

The connections of the terminals, junction cables and SVL must be designed so that they

can achieve and easily maintain the contact resistances of section 6.6 (d). The terminals

and junction cables must also fulfil the short-circuit resistance conditions given in the Table

of section 4.2.


19/51

4.1.1 Watertight connection boxes

They must fulfil protection grade IP68. Those which contain SVL and which are assembled

at ground level must be of a horizontal type, with covers equipped with adequate bolts for

installation in not very deep chests under the surface of the ground. The highest point ofthe box assembled in a chest (without considering the lifting eyes) should not be more than

1 m below the level of the ground, to aid handling.

The boxes foreseen for external installation must be weatherproof and adequate for assembly

on watertight terminal support structures.

The metallic boxes must be earthed by means of connection to adjacent main earth

electrodes or to the earth bonding conductor. They can also be connected just to a local

earth system, independent from the main system, if the internal insulation of the boxguarantees an adequate safety level.

Each box should contain the elements of just one circuit.

4.1.2 Protected connection cupboards or boxes

All these boxes should fulfil protection grade IP54. When they are in vertical position they

are called cupboards. The cupboards should fulfil the following conditions:

(1) The back part of the cupboard should be a fixed panel.

(2) The door(s) should be closed by means of a security lock or an alternative method. When

the cupboard has two doors, only one of them needs to be blockable; the other door can

be secured by means of internal espagnolette bolts on the upper and lower part.

(3) Each circuit should have its own cables for connection to screens and to earth, SVL

and earth cables. These elements cannot be shared by two circuits.

When the cupboard is situated within the area of an important earth network (for example

of a substation), the internal earth bar should be connected to this network. If the cupboard

is metallic, it should also be connected to this earth network, it being possible to carry out

this connection to the earth bar inside it.

When the box is not situated thus (for example at joint points all along the cable run), the

earth bar to which the connecting cables are connected (by SVL, in the appropriate cases)

in normal operation should be connected to the earthing system of the location of the joint.

The box, if it is metallic, should be connected to its own local earth. The insulation between

the internal earth bar and the box should be adequate to resist the test level between

junction cable and earth specified under Point 6.3. The insulation between the casing of

the SVL and the box should also fulfil this condition.

When the box is metallic, its earth connection terminal shall be independent from the

elements housed in it. The set of connection devices may be covered by a transparent

methyl methacrylate insulating screen, or by another suitable material.

19


20/51

If higher values are foreseen in a specific installation, suitable larger sections should be adopted.

The connecting cables to be used should be:

In cable systems with rigid connection: All the connecting cables will be single-core.

In cable systems with special connection: Except in the cases which are indicated below,

all the connecting cables shall be concentric. In a joint with screen sectioning, the conductors

on both sides of the sectioning shall be the inside and the outside of the connecting cable.

In the terminals, the junction cables connected to the screens of the cables should be the

interior conductors of the concentric cables; the exterior conductors of the aforementioned

cables shall be connected in all cases to the metallic parts of the terminal support structures.

If metallic connection boxes are used, the exterior conductors of the concentric cables of

the terminals shall be connected in all cases to the metallic box.

4.2 JUNCTION CABLES BETWEEN SCREENS AND CONNECTION BOXES

AND/OR SVL HOUSING

These cables must be made up of stranded copper conductors insulated with XLPE, and

should be one of the following types:

(i ) Single-core construction, in accordance with Table 1A.

(ii) Concentric construction, in accordance with Table 1B.

Tables 1A and 1B show the minimum section of connecting conductor associated with the

nominal voltages and with the systems short-circuit current levels. The connecting cables

must comply with the conditions of UNE-HD-603 in everything which is applicable to them,

except as regards the testing voltages, which shall be those indicated under Point 6.4 of

this recommendation.

The section of the connecting cables, both for the interior and exterior conductors of the

concentric cables and for the single-core cables in any type of connection, shall be determined

by the maximum value of the short-circuit current foreseen for the installation. Depending

on the highest voltage for the material, the minimum levels of this current to be taken into

account in the design of the connecting conductors are established in Tables 1A and 1B,

as follows:

20

U 72,5 kV 16 kA for 1 second

72,5 kV < U 145 kV 25 kA for 1 second

145 kV < U 245 kV 40 kA for 1 second

245 kV < U 63 kA for 1 second


21/51

The SVL and their detachable connections can be situated directly on the pylon or metallic

support structure.

The connections should be designed to minimize the length of the cables. Whenever

possible, no concentric type connecting cable should be more than 10 m long.

Joints will not be accepted in the connecting conductors in new installations, but they can

be accepted in subsequent interventions, for example in maintenance or repair work.

The exceptional cases in which single-core connecting cables can be used in systems with

special screen connection are:

Junction cables used for the earthing of the screens of the cables at the common earthing

point of two sections with single-point connection, as shown in Fig. 7.

21

Fig. 7


(Earthing at the middle point)


Junction cables used in terminals normally earthed.

Junction cables used for earthing or other connections in the terminals in equipment

with metallic casings, as in Figs. 20, 21 and 22.

SVLlink box

SVLlink box


22/51

Fig. 10

Screen connection diagram in terminals with metallic casing(Direct earthing)

22

Connections close to metallic casings

Blockable

connection box

assembledon frame

Main earth of the system orearth continuity conductor

The bridges must be connected directly to both sides of the

separation insulation and the metallic casings must be

connected to the main earth of the system.

NOTE:

Connection function:

Element:

Applicable to:

Direct earthing

Terminal of the cable

For direct earthing in systems with class SF61insulated screen.


23/5123

Fig. 11

Screen connection diagram in terminals with metallic casing(Direct earthing)

Connections far from metallic casings

Blockable

connection box

on frame

Connecting cable

to pass through the

CT (if appropriate)

Main earth of the system orearth continuity conductor

The SVL must be connected directly to both sides of the



NOTE:


Element:

Applicable to:

Direct earthing


Direct earthing in systems with class SF62insulated screen.


24/51

Fig. 12

Screen connection diagram in terminals with metallic casingNot earthing)

24

Connections of metallic casings

Connecting cable

(Can pass outside

the CT if appropriate)

Main earth of the system or

earth continuity conductor

The SVL are connected directly to both sides of the

separation insulation and the surrounding boxes are


NOTE:


Element:

Applicable to:

SVL connection

Terminal of the cable without earthing

Connections in systems with class SF63

insulated screen.

The cable and

the earthing box

can be replaced by

a portable earthing

when it acts on the

SVL

Blockable connection

box without bridge

connected

in normal service


25/51

4.3 LINE END TERMINALS

4.3.1 Terminals for installation in open air.

All the connecting cables between screens or metallic parts of the terminal base, the connection

boxes and the earthing points should be of one of the standardized insulated types.

The insulation level between screens (or metallic parts connected to them) and earth

should allow the application of the continuous high voltage rigidity tests specified for the

cable sheath. They must also withstand the surges indicated in column 3 of Table 2A.

The insulation in relation to earth of the metallic bases of the exterior terminals will be

carried out by means of pedestal insulators.

4.3.2 Terminals for direction connection to equipment with metallic casing.

The terminals for screened installations (GIS or transformers) shall incorporate screen

separation insulation.

When the cables end in equipment with metallic casing (with SF6 or oil insulation), special

arrangements may be necessary for the connection, the earthing and the use of SVL. The

main circuits can be divided into two classes, OIL and SF6.

4.3.2.1 OIL circuits

This class includes terminals with metallic shielding in a transformer (generally

submerged in oil). The connection to the winding of the transformer is carried out

directly or through an isolator (not manoeuvrable in service). For this application

it is permitted to use the same connection and earthing arrangements as the air

terminals, that is Figs. 10 to 12 inclusive. The connection and earthing arrangements

for class SF6 circuits can also be used if they are more economic or more

appropriate for the conditions of the site.

25


26/51

Fig. 13

Screen connection diagrams in rigid connection terminals(Direct earthing)

26

Blockable

connection box

Connecting cable

to pass through an CT

(if appropriate)

Main ea r th

of the system

On the screens of the cables there will be

circulation, or capacitive, currents.

This diagram is used preferably in terminals

assembled on conversion towers.

NOTE:(a)

(b)

Connection function:Element:

Applicable to:

Direct earthingTerminal

Except for class GIS terminations in equipment with

metallic shielding (see Figs. 10 and 11).


27/5127

Fig. 14

Screen connection diagrams in terminals(Direct earthing)

Blockable

connection box

assembled on

frame

Connecting cable

to pass through

an CT (if appropriate)

Main earth

of the system


Element:

Applicable to:

Direct earthing

Terminal

Systems of all kinds with connection of their ends to

weatherproof type terminals and to terminals of

equipment with OIL class metallic casing.

NOTE: (a)

(b)

On the screens there will be circulation, or capacitive,

currents.This diagram is used preferably in terminals assembled

on individual towers.


28/51

Direct earthing

Terminal

Systems of all kinds whose ends are equipped with

weatherproof type terminals or terminals in equipment

with OIL class metallic casing.

Fig. 15

Screen connection diagrams in terminals(Direct earthing)

28

Blockable

connection box

Concentric

cables

Main earth of the system


Element:

Applicable to:

Only the internal

connecting cable

should pass

through the

CT (if appropriate)

This diagram will give rise to

circulation or capacitive currents in

the screens of the cables.

NOTE:


29/51

4.3.2.2SF6 circuits

This class includes terminals with metallic shielding when there is no transformer

between them and an automatic switch or manoeuvrable isolator. In general these

terminals are submerged in SF6.

For this type of terminal it is essential that the sectioning insulation of the screens

is protected against current surges during the manoeuvring operations, by means

of short-circuit bars or SVL, as appropriate, connected directly between both sides

of this insulation. Normally, to obtain an effective suppression of the impulse surges,

two connections of this kind must be assembled (whenever possible), separated

some 180 around the circumference of the terminal(6). Each connection, including

those of the SVL, if appropriate, must be as short as possible and under nocircumstances must exceed 0.5 m. They should be fastened by screws to the

adjacent metallic parts in order to be able to remove them for the screen insulation

tests (see 6.6).

(a) Class SF61 refers to terminals earthed in which the earth connection of the

shielding is close to the terminals of the cables. In this case, the sectioning

insulation should be short-circuited using bridges connected directly to both

sides of the insulation, as Fig. 10 shows. If the current transformers are not

housed inside the assembly with metallic shielding of the substation, the SF62

connection should be used.

(b) Class SF62 refers to terminals earthed in which the earth connection of the

shielding is far from the terminals of the cables. In this case, the sectioning

insulation should be protected by means of SVL, as Fig. 11 shows.

(c) Class SF63 refers to terminals not earthed. The sectioning insulation of the

terminals should be protected by means of SVL, as Fig. 12 shows.

(6)

Under these conditions there is no impedance between the switch and the cable and dischargescan occur with very marked wave fronts. The impulse frequency is very high and the wavelengthshort. To maintain the protection level of the cable it is necessary to reduce as far as possible thedistance between the SVL and the screen and divide the length of the screen sectioning insulationplacing two SVL or connection bars parallel.

29


30/51

Fig. 10

Screen connection diagram in terminals with metallic casing

(Direct earthing)

30

Connections close to metallic casings

Blockable

connection box

assembled

on frame



The bridges must be connected directly to both sides of the



NOTE:


Element:

Applicable to:

Direct earthing


For direct earthing in systems with class SF61insulated screen.


31/5131

Fig. 11

Screen connection diagram in terminals with metallic casing

(Direct earthing)

Connections far from metallic casings

Blockable

connection box

on frame



The SVL must be connected directly to both sides of the



NOTE:


Element:

Applicable to:

Direct earthing


Direct earthing in systems with class SF62insulated screen.

Connecting cable

to pass through the

CT (if appropriate)


32/51

Fig. 12

Screen connection diagrams in terminals with metallic casing

(Not earthed)

32


Blockable connection

box without bridge

connected

in normal service

The cable and

the earthing box

can be replaced by

a portable earthing

when it acts on the

SVL

Connecting cable

(Can pass outside

the CT if appropriate)



The SVL are connected directly to both sides of the

separation insulation and the surrounding boxes are


NOTE:


Element:

Applicable to:

SVL connection


Connections in systems with class SF63insulated screen.


33/51

4.3.3 Connection diagrams with intensity transformers

When intensity transformers are installed adjacent to the terminals, the connection diagrams

will normally be those described in the corresponding figure. When the connecting cable

is concentric, as in Fig 12, this entails part of the cable where the two conductors are

separated, and the connection diagram experiences a reduction in its efficiency in the

limitation of the transient voltage.(7)

33

(7) On separating the two conductors this increases the inductance of the circuit and the voltage drop at this point.

Direct earthing

Terminal

All kinds of system whose ends are equipped with

weatherproof type terminals or with terminals in

equipment with OIL class metallic casing.

Fig. 15

Screen connection diagrams in terminations

(Direct earthing)

Blockable

connection box

Concentric

cables

Main earth of the system


Element:

Applicable to:

Only the internal

connecting cable

should pass

through the

CT (if appropriate)

This diagram will give rise to

circulation or capacitive currents on

the screens of the cables.

NOTE:


34/51

4.3.4 Metallic shielding insulation of the terminals

Any oil or gas piping which penetrates the terminal must be connected through insulation

couplings or straps, so that it can be earthed permanently while the screens of the cables

are left insulated in relation to earth.

When the permanent screen voltage on a terminal exceeds 10 V, the metallic parts of the

shielded terminal base must be screened against accidental contacts by means of adequate

insulating protection.

4.4 JOINTS

The following conditions should be observed:

4.4.1 External insulation

The joint boxes and the connecting cables should be insulated externally in such a way

that allows the performance on the ground of the direct voltage rigidity tests of the cable

sheath.

4.4.2 Joints without screen sectioning

Any joint installed where screen sectioning is not required shall have direct metallic continuity

between the screens of two cables which access the joint.

The joints with screen sectioning (see 4.4.3) can be adapted to be used as joints without

sectioning if short connectors are added which directly and permanently bypass the

sectioning insulation. These connectors will be integrated in the general external insulation

of the joint box.

4.4.3 Joints with screen sectioning

Any joint installed where screen sectioning is required must incorporate an insulating part

and be arranged in a way that provides the insulation levels necessary both to earth, and

between screens, of the cables which access the joint.

All the joints with screen sectioning must be arranged in a way which accommodates the

concentric connecting cables (see 4.2). The connection elements shall be situated as close

as possible to the screen sectioning insulation.

When a cable system with insulated screen has to be connected to a non-insulated screen

cable system, the joint must be of the type with screen sectioning (see Fig. 16).

34


35/51

When the total length of a cable system with rigid connection of insulated screen is big,

it may be convenient, to aid the location and repair of possible defects on the cable sheath,

to subdivide the system into two or more sections. This subdivision can be achieved with

the inclusion of one or more groups of joints with screen sectioning.

35

Blockable connection box

with sheath voltage limiters

Concentric

cables

Earth of the substation or


Joints with

screen discontinuity

Fig. 16

Joint between a system with earth connection

and a system with screen insulated at this point


Element:

Applicable to:

Rigid connection and SVL connection

Joint between different sections, one of whichis not earthed.

Systems which incorporate sections withcrossed connection and sections with single-point connection.


36/5136

4.4.4 Open-air joints

The external insulation of any joint which is going to be installed in the open air must be

identical to that of the buried joint. The insulation cannot be achieved on the basis of

distances in the air.

4.5 SURGE LIMITING DEVICES.

4.5.1 Application

To minimize the current surges in the screens of the cables, sheath voltage limiters (SVL)

must be installed:

On the screen cross joints and on the non-earthed terminals of sections with single-point

connection.

On non-earthed terminals, connected directly to equipment with metallic shielding of the

type defined in 4.3.2.2. The use of SVL should be avoided as far as possible at these

points, by means of the adoption of an alternative connection and earthing device.

At the non-earthed terminals of all the other types.

Under certain circumstances it may be necessary to install SVL in earthed terminals

when they are installed in equipment with metallic shielding (see 4.3.2.2).

Figs. 15, 17, 18, 22 and 23 show these applications.


37/51

Fig. 17

Screen connection diagram in terminations

(Not earthed)

37


Element:

Applicable to:

Connection of SVL

Terminations not earthed.

Systems of all kinds whose ends are equipped with

weatherproof type terminations or terminations in

equipment with OIL class metallic enclosure.

Only the internalconnecting cable

should pass

through the CT

(if appropriate)

Concentric

cables

Blockable link box

with S.V.L.

Substation earth or



38/51

Fig. 18

Screen connection diagram in joints

in cross bonded systems

38


with sheath voltage limiter

Concentric

cables

Earth system of the joint location


Element:

Applicable to:

Crossed connection protected with SVL

Joints at sectioning points of minor sections.

Cross bonded systems.

Joints with



39/51

Fig. 19

Screen connection diagram in screen interruption joints

without earthing between sections with single-point bonding

39

Concentric

cables

Main earth

of the system


Element:

Applicable to:

SVL connection

Screen not earthed.

Systems with single-point bonding.

Joints with


Main earth

of the system

Double link box with

SVL. Can be replaced with

two single boxes

Depending on the system design, the schematic diagram of a joint

between minor sections single point bonded can be this one or that

of Fig. 16 and 19.

NOTE:


40/5140

Fig. 12

Screen connection diagrams in terminals with metallic casing

(Not earthed)


Blockable

connection box

on frame without

bridge connected

in normal service

Connecting cable

(Can pass outside

the CT if convenient)



The SVL are connected directly to both sides of the separation

insulation and the surrounding boxes are connected to the main

earth of the system.

NOTE:


Element:

Applicable to:

Connection of SVL


Connections in systems with class SF63

insulated screen.


41/5141


with sheath voltage limiters

Concentric

cables

Earth of the substation or



Element:

Applicable to:

Rigid connection and SVL connection

Joint between different sections, one of which is

not earthed.

Systems which incorporate sections with crossed

connection and sections with single-point connection.

Joints with


Fig. 16

Joint between a system with earth connection

and a system with screen insulated at this point


42/5142

4.5.2 Operating conditions

The units must be capable of withstanding the situations detailed below, already described

in section 1.2.5 of this Recommendation.

(1) The units must be capable of withstanding continuously the permanent screen voltage

induced by the nominal charging current.

(2) The units must be capable of withstanding for a period of 1 second each, two

applications in quick succession of their nominal voltage (at industrial frequency),

which must not be lower than the screen voltage induced under DC conditions external

to the line.

(3) The units must be capable of withstanding the transient voltages and the corresponding

currents to which they are subjected under abnormal conditions of the system; that isto say, in routine manoeuvres, in nominal atmospheric discharges, in nominal manoeuvre

surges, or during a short-circuit current of nominal value. However, it is accepted that

occasionally the units are not capable of withstanding the conditions to which they are

subjected in the event of internal defect in the cable system (see 4.1).

(4) The units must be capable of limiting the transient voltages to which they are subjected

(residual peak voltage), to no more than the impulse voltage admissible in the connection

enclosure between links and earth (see 6.3). If the residual peak voltage exceeds the

value of 20 kV, it may be necessary to review the insulation levels of the cable sheathand of the joint protection.

The units which are used to protect the sectioning insulation of the earthed terminals in

equipment with metallic shielding should fulfil only conditions (3) and (4).

4.5.3 Earthing

The earth connection of the SVL devices shall be carried out taking into account that

indicated in section 2.3 of this Recommendation.

4.5.4 Installation of SVL

When they are installed in enclosures with metallic casing or in external insulators, directly

in the air, they shall be encapsulated and weather resistant. The SVL must be installed in

such as way that they can be easily disconnected.


43/5143

5. MULTIPLE CIRCUITS

An installation can contain two or more triads of single-core cables. The following conditions

must be fulfilled for the whole to be considered as a single line (several conductors per

phase) for the purposes of this recommendation:

The run shall be identical for all the triads, as shall their relative distances.

All the triads shall have the same screen connection diagram.

All the triads must be earthed at the same points of their run and at the same points of

the earth network.

All the triads must be connected rigidly in parallel in HV at both ends, without the insertion

of switchgear.

It must be impossible to leave some of the triads out of service and to maintain the rest

in service.

Under these conditions, the following may be common to all the triads: the bonding

conductor, the connection boxes or enclosures and the surge limiting device housing boxes

or enclosures.

When any of the conditions indicated is not fulfilled and, in particular, when it is a question

of circuits projected as independent, there should not be common elements in the screen

circuits of the different triads.


44/5144

6. TESTS

6.1 EXTERNAL INSULATION OF THE JOINTS.

The joint protection box should withstand mechanical resistance tests, immersion in 1m

of water column and 20 heating and cooling cycles, followed by direct voltage tests of 20

kV during 1 minute and impulse tests between screen ends and between each screen and

earth as indicated in the following table. In accordance with IEC 60840 Appendix H or IEC

62067 Appendix D

Table 2A

Impulse tests with nominal wave of 1/50 ms to IEC 60230

Voltage ofJoint boxes

the system Between connectors Between connectors and earth

kV kVp kVp

66 60 30

132 75 37,5

275 95 47,5

400 125 62,5

6.2 WATERTIGHTNESS OF THE BURIED CONNECTION BOXES

Buried connection boxes for installation under ground level should be subjected to a

watertightness type test.

Connection boxes with small lengths of junction cable connected should be submerged

in water up to a depth of no less than 1 m in relation to the highest point of the box or must

be subjected to an external excess pressure of 0.1 bar for a period of 7 days.

The connection box must be built in such a way that it withstands this test without damage

or leak. At the end of the test, the appearance of small drops of water inside the box is

accepted, which overall are not considered to be harmful for the life or for the operation

of the connection box.

6.3 INTERNAL INSULATION OF THE CONNECTION ENCLOSURES

The connection enclosures shall be capable of withstanding type tests of:

(a) Impulse voltage (nominal wave form of 1/50 microseconds in accordance with IEC

60230) three positives and three negatives of 35 kVp between phases and of 17.5 kVp

between phases and earth.

(b) Direct voltage of 25 kV for 5 minutes between phases and between phases and earth.


45/5145

Table 2B

6.4 CONNECTING CABLES

The connecting cables must be subjected to routine tests in accordance with HD 603,

except as regards the test voltages, which should be as follows:

Insulation of single-core cables 20 25 DC 1 minute

Internal insulation of concentric cables of120 mm2, 240 mm2 and 300 mm2

25 15 AC 5 minutes

Internal insulation of concentric cables of500 mm2

30 20 AC 5 minutes

External insulation of concentric cables 20 25 DC 1 minute

Description

Test voltage

Dry rigidity testspark test

kV AC

Voltage teston finished cables

kV


46/5146

6.5 SHEATH VOLTAGE LIMITERS

The routine and maintenance tests on finished assemblies shall be as follows:

Zinc oxide units

The voltage test at AC 50Hz must be adapted to give a peaktest intensity of1.6 mA and the voltage measured must be within the range given below.

In the finished installation tests the visual examination must show that the junction cables

and the external surface of the unit or of the container are free from important protuberances

or from other signs of deterioration.

(a) Internal insulation resistances for SVL enclosures with earth cables.

The resistance between the earth cables and any metallic case should not be less than

10 M measured with a 1000 V megaohmmeter.

Type of limiter Peak voltage Peak voltage/ 2

(kV) (kV)

CPA-03 5,05 5,56 3,57 3,93

CPA-06 10,10 11,12 7,14 7,86


47/51

6.6 COMPLETE CIRCUITS ONCE INSTALLED

(a ) After installing a complete circuit tests shall be performed to ensure that the

screen and the sectioning insulation of the screen at all points are capable of

withstanding a rigidity test at direct voltage of 10 kV for 1 minute. The SVL should

be disconnected on performing these tests.

The connections and the earthing of the screens should also be checked.

(b ) It is recommended, at regular intervals, to test the integrity of the screen-earth

insulation at a direct voltage of 5 kV for 1 minute. The SVL should be checked

at the same time, in accordance with Point 6.5.

(c ) If there is an internal defect in a cable system with special connection, the tests

referred to in section (b) above should be carried out and the continuity of the

screen-earth metallic path should be checked before putting the cable back inservice.

(d ) After the installation a measurement should be taken of the resistance of all the

contacts of the junction cables of the screens in each connection box or cupboard,

using a digital microhmmeter. The contact resistance should not be greater than

20 .

The contact resistance of the SVL connections in the boxes or cupboards should

be measured in a similar manner and should not be greater than 50 .

When possible the contact resistance of the external earth cable should bemeasured in a similar manner, and should not be greater than 50 .

Note

If a fault current has circulated through a cable system with special connection,

it is recommended at the first opportunity available to check the integrity of the

screen-earth insulation, the integrity of the connection cables and the characteristics

of the SVL.

47


48/5148

Table 1A

CONSTRUCTION OF SCREEN CONNECTION SINGLE-CORE CABLES

*Also for connections of 132kV and 275kV systems not subjected to the short-circuit current, for example connections to SVL.

Table 1B

CONSTRUCTION OF SCREEN CONNECTION CONCENTRIC CABLES

12,8 13,3 17,8 18,7 20,2 21,0 26,4 28,0

1,2+2,0 19,2 20,7 1,7+1,0 23,4 25,3 1,8+1,0 25,8 27,9 2,2+3,0 36,8 39,7

24,4 26,4 33,6 36,3 36,0 38,9 47,4 51,2

1,8 28,4 30,7 1,8 37,6 40,6 2,1 40,6 43,8 2,4 52,6 56,8

Dimensions(mm)

Nominal sectionof the conductor

Voltage of the as-sociated system

Central conductor

XLPE Insul. +PVC internal sheath

30 up to 66 kV 132 kV 275 kV 400 kV

120* mm2 240 mm2 300 mm2 500 mm2

Concentricconductor

PE external sheath

Thickness Diameter

Min Max

Thickness Diameter

Min Max

Thickness Diameter

Min Max

Thickness Diameter

Min Max

12,8 13,3 17,8 18,7 20,2 21,0 26,4 28,0

1,2+1,5 18,2 19,7 1,7+1,7 24,6 26,6 1,8+1,8 27,4 29,6 2,2+2,0 35,3 38,1

Thickness Diameter

Min

Dimensions

(mm)

Nominal section

of the conductor

Voltage of the as-

sociated system

Conductor

XLPE Insul. +

PVC Sheath

Max

30 up to 66 kV 132 kV 275 kV 400 kV

120 mm2 240 mm2 300 mm2 500 mm2

Thickness Diameter

Min Max

Thickness Diameter

Min Max

Thickness Diameter

Min Max


49/5149

7. EARTHING OF LIGHTNING ARRESTERS

7.1. INTRODUCTION.

The object of this chapter is to determine how lightning arresters should be connected to

earth in high voltage circuits.

The earthing is standardized by the substation regulation MIE RAT 13 Earthing installations

of the Regulation on Technical Conditions and Safety Guarantees in Electric Power Plants,

Transformer Stations and Substations. To summarize, this regulation says:

- In an installation there cannot be a live accessible point which can damage people,

whether under conditions of normal operation or in short-circuit. Consequently, any

cable which conducts current should be insulated.

- The earthing circuit cables which connect the electrodes should be bare, resistant

to corrosion and preferably visible.

- The dischargers have to be connected to the earth of the accessory that they

protect.

- The earth circuits should be straight without forced bends and as short as possible.

The earthing of lightning arresters (insulated cable or bare conductor) is not determined.

There is no reason to use one or another type of conductor.

Taking the case of a lightning arrester at the top of a support. The earth connection can

be carried out with an insulated cable or with a bare conductor, and in actual fact is carried

out in both ways depending on what the customer decides. It should be taken into account

that:

- It is preferable to use an insulated cable, which is on the safety side.

- In any case the customer can make any decision without it affecting the safety of

the circuit.

- We can opt for a bare cable, linked electrically and mechanically to the support

in short sections, which will not have problems of perforation of insulation in the event

of an important impulse.


50/5150

7.2. CONNECTION TO THE EARTH CIRCUIT.

The connection of the different earthings indicated (direct earthing, earthing of the

discharger, earthing of the lightning arresters) will be carried out as follows:

The lightning arrester earthing cable can be the same for the three lightning arresters of

a circuit.

The same cable cannot be used for lightning arresters of different circuits.

The earth connection will be carried out to the earth cable indicated by the customer.

The connection elements must be suitable for the section of the conductor connected.

The project completion documentation must indicate where the connection of each circuitis carried out.


51/51

HEADQUARTERS

Casanova, 150 - 08036 BARCELONATel.: +34 93 227 97 00 - Fax: +34 93 227 97 [email protected]

BRANCHES IBERIA

ANDALUSIAAverroes, 6, Edificio Eurosevilla, 4, 741020 SEVILLATel.: +34 95 499 95 18 - Fax: +34 95 451 10 [email protected]. Mvil: +34 626 014 918 - Fax: +34 95 225 99 [email protected]

CENTREvila, Badajoz, Cceres, Ciudad Real,Guadalajara, Madrid, Segovia y ToledoAvda. Ciudad de Barcelona, 81 A, 4 A - 28007 MADRIDTel.: +34 91 309 66 20 - Fax: +34 91 309 66 30

[email protected], Len, Palencia, Salamanca, Valladolid y ZamoraTel. Mvil: +34 609 154 594 - Fax: +34 983 24 96 [email protected]

EAST COASTAlbacete, Comunidad Valenciana, Cuenca y MurciaCirilo Amors, 27 - 6 C - 46004 VALENCIATel.: +34 96 350 92 58 - Fax: +34 96 352 95 [email protected]

NORTHEASTAndorra, Aragn, Baleares y CataluaAragn, 177-179, 2 planta - 08011 BARCELONATel.: +34 93 467 85 78 - Fax: +34 93 467 46 [email protected]

NORTHlava, Asturias, Cantabria y VizcayaJuan de Ajuriaguerra, 26 - 48009 BILBAOTel.: +34 94 424 51 76 - Fax: +34 94 423 06 [email protected], La Rioja, Navarra, SoriaTel.: +34 629 34 85 22 - Fax +34 948 23 46 [email protected] GALICIABESIGA COMERCIAL, S.L.Av. Tierno Galvn, 11215178 MAIANCA - OLEIROS (La Corua)Tel.: +34 981 61 71 94 - Fax: +34 981 61 74 [email protected]

CANARIES Branchngel Guerra, 23 - 1

35003 LAS PALMAS DE GRAN CANARIATel.: +34 928 36 11 57 - Fax: +34 928 36 44 [email protected]

PORTOR. Gonalo Cristovo, 312 - 4 B e C4000-266 PORTOTel.: +351 223 392 350 - Fax: +351 223 323 878

EXPORT

Casanova, 150 - 08036 BARCELONA (Spain)Tel.: + 34 93 227 97 24 - Fax: + 34 93 227 97 [email protected]

FACTORIES

ABRERA (Spain)Carrer del Metall, 4 (Polgon Can Sucarrats) - 08630 ABRERA (Barcelona)Tel.: +34 93 773 48 00 - Fax: +34 93 773 48 48

MANLLEU (Spain)Ctra. Rusiol, 63 - 08560 MANLLEU (Barcelona)Tel.: +34 93 852 02 00 - Fax: +34 93 852 02 22

MONTCADA I REIXAC (Spain)Ctra. de Ribas, Km. 13,250 - 08110 MONTCADA I REIXAC (Barcelona)Tel.: +34 93 227 95 00 - Fax: +34 93 227 95 22

VITORIA (Spain)Portal de Bergara, 36 - 01013 VITORIA-GAZTEIZTel.: +34 945 261 100 - Fax: +34 945 267 146 - [email protected] - www.ecn.es

MONTEREAU (France)SILEC CABLE - Rue de Varennes Prolonge - 77876 MONTEREAU CEDEX (France)Tel.: +33 (0) 1 60 57 30 00 - Fax: +33 (0) 1 60 57 30 [email protected] - www.sileccable.com

MORELENA (Portugal)Av. Marqus de Pombal, 36-38 Morelena - 2715-055 PRO PINHEIRO (Portugal)Tel.: +351 219 678 500 - Fax: +351 219 271 942

NORDENHAM (Germany)NSW - Kabelstrae 9-11 - D-26954 NORDENHAM (Deutschland)Tel.: +49 4731 82 0 - Fax: +49 4731 82 1301 - [email protected] - www.nsw.com

BISKRA (Algeria)ENICAB - Zone Industrielle - B.P. 131 07000 RP BISKRA (Algrie)Tel.: +213 033 75 43 21/22 - Fax: +213 033 74 15 19 - [email protected]

LUANDA (Angola)CONDEL - Fbrica de Condutores Elctricos de Angola, SARL5 Av N 9, Zona Industrial do Cazenga, Caixa Postal n 3043 LUANDA (Angola)Tel.: +244 2 380076/7/8/9/17 - Fax +244 2 33 78 12 - [email protected]

INTERNATIONAL

ABU DHABIP.O. Box No. 112478 - Next Showroom Building - Nazda Street, ABU DHABI (UAE)Tel.: +971 -2- 6338991 - Fax: +971 -2- 6338993 - [email protected]

ALGERIAENICABCentre Commercial de LEgtc local n A21 - 170 Rue, Hassiba Ben Bouali El Hamma016000 ALGER - Telf: +213 021 67 61 73 - Fax: +213 021 67 61 75 - [email protected]

NORWAYRandemveien 17 - 1540 VESTBY (Norway)

Tel.: +47 64955900 - Fax: +47 64955910 - [email protected]

UNITED KINGDOMRegus House, Herons Way, Chester Business Park,CH4 9QR CHESTER (United Kingdom)Tel.: +44 1244 893 245 - Fax: +44 1244 893 101 - [email protected]

RUSSIAAzovskaya str., 13 - (Russia) MOSCOWTel.: +7 495 617 0005 - Fax: +7 495 617 0006 - [email protected]

AGENCIES

ARGENTINA

connectionofearthingcircuitsinhvsystems

Documents