EIA-Technology for Volt & Current: EHV/HV Cable Sheath Earthing

Cable SheathEarthing

Introduction:

In urban areas, high voltage underground cables are commonly used for the transmission and distribution of electricity. Such high voltage cables have metallic sheaths or screens surrounding the conductors, and/or armour and metallic wires surrounding the cables. During earth faults applied to directly earthed systems, these metallic paths are expected to carry a substantial proportion of the total fault current, which would otherwise flow through the general mass of earth, while returning to system neutrals. These alternative return paths must be considered when determining the extent of the grid potential rise at an electrical plant due to earth faults.
For safety and reliable operation, the shields and metallic sheaths of power cables must be grounded. Without grounding, shields would operate at a potential considerably above ground. Thus, they would be hazardous to touch and would cause rapid degradation of the jacket or other material intervening between shield and ground. This is caused by the capacitive charging current of the cable insulation that is on the order of 1 mA/ft of conductor length.
This current normally flows, at power frequency, between the conductor and the earth electrode of the cable, normally the shield. In addition, the shield or metallic sheath provides a fault return path in the event of insulation failure, permitting rapid operation of the protection devices.
In order to reduce Circulating current and electric potential difference between the sheathings of single core three-phase cables, the sheathing is grounded and bonded at one or both ends of the cables. If the cable is long, double bonding has to be carried out which leads to circulating currents and increased total power loss. Raising the sheath’s resistance, by decreasing its cross section and increasing its resistivity, can reduce this almost to the level of the core losses.
However, in case of an earth fault, a considerable portion of the fault current flows through the increased sheath resistance, creating much higher power in the sheaths than in the faulty core. A simple solution, a conductor rod buried into the soil above or under the cable can divert this power from the sheaths.

Cable Screen:

(1) Purpose of cable screen:

Cable screen controls the electric field stress in the cable insulation.
Cable Screen Provides return path for Cable neutral and fault current.
If the screen is earthed at two ends than it provides Shielding for electromagnetic radiation.
Enclosing dangerous high voltage with earth potential for safety.

(2) Purpose of bonding cable screens at both ends:

The electric power losses in a cable circuit are dependent on the currents flowing in the metallic sheaths of the cables so by reducing the current flows in metallic sheath by different methods of bonding we can increases the load current carrying capacity (ampacity) of the cable.
It provides low impedance fault current return path and provides neutral point for the circuit.
It provides shielding of electromagnetic field.

(3) Induced voltage & circulating circulating current in cable screen:

Electromagnetic coupling between the core and screen Electromagnetic screen.
If the cable screen is single point bonded, no electrical continuity and mmf generates a voltage.
If the cable screen is bonded at both ends, the mmf will cause circulating current to flow if there is electrical continuity.
The circulating current produces an opposing magnetic field.
Suitable bonding method should be employed to meet the standing voltage limit and keep Circulating current to an acceptable level.

Laying Method of Cable:

The three Single core cables in a 3-phase circuit can be placed in different formations. Typical formations include trefoil (triangular) and flat formations.

(1) Trefoil Formation:

To minimize the electromechanical forces between the cables under short-circuit conditions, and to avoid eddy-current heating in nearby steelwork due to magnetic fields set up by load currents, the three single-core cables comprising the three phases of a 3-phase circuit are always run clamped in ‘Trefoil’ formation.
Advantage:

This type of Formation minimizes the sheath circulating currents induced by the magnetic flux linking the cable conductors and metallic sheath or copper wire screens.
This configuration is generally used for cables of lower voltages (33 to 132kV) and of smaller conductor sizes.

Disadvantages:

The trefoil formation is not appropriate for heat dissipation because there is an appreciable mutual heating effect of the three cables.
The cumulated heat in cables and cable trench has the effect of reducing the cable rating and accelerating the cable ageing.

(2) Flat Formation:

This is a most common method for Laying LT Cable.
This formation is appropriate for heat dissipation and to increase cable rating.
The Formation choice is totally deepened on several factors like screen bonding method, conductor area and available space for installation.

Type of Core and Induced Voltage:

(1) Three Core Cable:

For LT application, typically for below 11 kV.
Well balanced magnetic field from Three Phase.
Induced voltages from three phases sum to zero along the entire length of the cable.
Cable screen should be earthed at both ends
Virtually zero induced voltage or circulating current under steady state operation.

(2) Single Core Cable:

For HV application, typically for 11 kV and above.
Single–core cables neglects the use of ferromagnetic material for screen, sheath and armoring.
Induced voltage is mainly contributed by the core currents in its own phase and other two phases.If cables are laid in a compact and symmetrical formation, induced in the screen can be minimized.
A suitable screen bonding method should be used for single–core cables to prevent Excessive circulating current, high induced standing voltage.igh voltage.

Accessories for HT Cable Sheath Bonding:

(1) Function of Link Box?

Link Box is electrically and mechanically one of the integral accessories of HV underground above ground cable bonding system, associated with HV XLPE power cable systems.
Link boxes are used with cable joints and terminations to provide easy access to shield breaks for test purposes and to limit voltage build-up on the sheath

Lightning, fault currents and switching operations can cause over voltages on the cable sheath. The link box optimizes loss management in the cable shield on cables grounded both sides.
In HT Cable the bonding system is so designed that the cable sheaths are bonded and earthed or with SVL in such way as to eliminate or reduce the circulating sheath currents.
Link Boxes are used with cable joints and terminations to provide easy access to shield breaks for test purposes and to limit voltage build-up on the sheath. The link box is part of bonding system, which is essential of improving current carrying capacity and human protection.

(2) Sheath Voltage Limiters (SVL) (Surge Arrestors):

SVL is protective device to limit induce voltages appearing on the bonded cable system due to short circuit.
It is necessary to fit SVL’s between the metallic screen and ground inside the link box. The screen separation of power cable joint (insulated joint) will be protected against possible damages as a result of induced voltages caused by short circuit/break down.

Type of Sheath Bonding for HT Cable:

There is normally Three Type of Bonding for LT/HT Cable Screen.

(1) Single Point Bonded.

One Side Single Point Bonded System.
Split Single Point Bonded System.

(2) Both End Bonded System

(3) Cross Bonded System

(1) Single point bonded system:

(A) One Side Single Bonded System:

A system is single point bonded if the arrangements are such that the cable sheaths provide no path for the flow of circulating currents or external fault currents.
This is the simplest form of special bonding. The sheaths of the three cable sections are connected and grounded at one point only along their length. At all other points there will be a voltage between sheath and ground and between screens of adjacent phases of the cable circuit that will be at its maximum at the farthest point from the ground bond.
This induced voltage is proportional to the cable length and current. Single-point bonding can only be used for limited route lengths, but in general the accepted screen voltage potential limits the length

The sheaths must therefore be adequately insulated from ground. Since there is no closed sheath circuit, except through the sheath voltage limiter, current does not normally flow longitudinally along the sheaths and no sheath circulation current loss occurs.
Open circuit in cable screen, no circulating current.
Zero volt at the earthed end, standing voltage at the unearthed end.
Optional PVC insulated earth continuity conductor required to provide path for fault current, if returning from earth is undesirable, such as in a coal mine.
SVL installed at the unearthed end to protect the cable insulation during fault conditions.
Induced voltage proportional to the length of the cable and the current carried in the cable .
Zero volt with respect to the earth grid voltage at the earthed end, standing voltage at the unearthed end.
Circulating current in the earth–continuity conductor is not significant, as magnetic fields from phases are partially balanced.
The magnitude of the standing voltage is depended on the magnitude of the current flows in the core, much higher if there is an earth fault.
High voltage appears on the unearthed end can cause arcing and damage outer PVC sheath.
The voltage on the screen during a fault also depends on the earthing condition.

Standing voltage at the unearthed end during earth fault condition.

During a ground fault on the power system the zero sequence current carried by the cable conductors could return by whatever external paths are available. A ground fault in the immediate vicinity of the cable can cause a large difference in ground potential rise between the two ends of the cable system, posing hazards to personnel and equipment.
For this reason, single-point bonded cable installations need a parallel ground conductor, grounded at both ends of the cable route and installed very close to the cable conductors, to carry the fault current during ground faults and to limit the voltage rise of the sheath during ground faults to an acceptable level.
The parallel ground continuity conductor is usually insulated to avoid corrosion and transposed, if the cables are not transposed, to avoid circulating currents and losses during normal operating conditions.
Voltage at the unearthed end during an earth fault consists of two voltage components. Induced voltage due to fault current in the core.

Advantage:

No circulating current.
No heating in the cable screen.
Economical.

Disadvantage:

Standing voltage at the un–earthed end.
Requires SVL if standing voltage during fault is excessive.
Requires additional earth continuity conductor for fault current if earth returned current is undesirable. Higher magnetic fields around the cable compared to solidly bonded system.
Standing voltage on the cable screen is proportional to the length of the cable and the magnitude of current in the core.
Typically suitable for cable sections less than 500 m, or one drum length.

(B) Split Single Point-bonded System:

It is also known as double length single point bonding System.
Cable screen continuity is interrupted at the midpoint and SVLs need to be fitted at each side of the isolation joint.
Other requirements are identical to single–point–bonding system like SVL, Earth continuity Conductor, Transposition of earth continuity conductor.
Effectively two sections of single–point–bonding.
No circulating current and Zero volt at the earthed ends, standing voltage at the sectionalizing joint.

Advantages:

No circulating current in the screen.
No heating effect in the cable screen.
Suitable for longer cable section compared to single–point–bonding system and solidly bonded single-core system.
Economical.

Disadvantages:

Standing voltage exists at the screen and sectionalizing insulation joint.
Requires SVL to protect the un–earthed end.
Requires separate earth continuity conductor for zero sequence current.
Not suitable for cable sections over 1000 m.
Suitable for 300~1000 m long cable sections, double the length of single–point–bonding system.

(2) Both End Solidly Bonded (Single-core cable) systems.

Most Simple and Common method.
Cable screen is bonded to earth grids at both ends (via link box).
To eliminate the induced voltages in Cable Screen is to bond (Earth) the sheath at both ends of the cable circuit.
This eliminates the need for the parallel continuity conductor used in single bonding systems. It also eliminates the need to provide SVL, such as that used at the free end of single-point bonding cable circuits
Significant circulating current in the screen Proportional to the core current and cable length and de rates cable.
Could lay cable in compact trefoil formation if permissible.
Suitable for route length of more than 500 Meter.
Very small standing voltage in the order of several volts.