The system depicted involves bonding and earthing of cable screens at both ends to form part of a closed loop which is electromagnetically linked with the loop formed by the conductors. In such a system, ciruclating currents are set up in the cable screen, resulting in heat loss and consequent derating of current carrying capacity. Such losses are minimized when cables are laid in trefoil "close-touching" formation, but increase with the spacing between cables.

2. Special Bonding Systems

Special bonding systems have been developed to keep circulating current losses to the minimum. Advantages which follow are :

a) Economical conductor size for optimum current ratings

b) Laying of cables with spacing, for example, in flat formation, to reduce the mutual heating effect due to proximity.

Special bonding is generally advantageous for high-amperage cable circuits. In such a system, standing voltages will appear in the cable screen which needs to be adequately insulated. Further, from the safety angle, a limiting value of such voltages becomes important. For the purpose, specially designed Sheath Voltage Limiters (SVL) which serve to restrict the voltage rise, particularly under transient conditions, are to be employed.

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Two types of special bonding systems are in use:

a) Single Point Bonding

In such a system the screens are connected and earthed at one end of the route. At all other points, the screen being insulated from earth will have a standing voltage which will be proportional to the circuit length, conductor current and cable spacing, and be maximum at the furthest point from the earth bond. Since there is no closed circuit, screen circulating current is eliminated. Single point bonding is normally used for limited route lengths to keep the standing voltage to the minimum and render the cable installation safe against "touch-voltage".

b) Cross Bonding

Cross bonding essentially consists of sectionalising the cable screen into elementary sections called minor sections and cross connecting them so as to neutralise the total induced voltage in three consecutive sections. Three minor sections together make a major section. In cross bonding system, the route is split up into groups of three drum lengths with the screens bounded and earthed together at both ends of a major section, but interrupted and connected in series at all other points. The purpose is to allow a standing voltage between screen and earth in each major section but eliminate circulating currents. With such an arrangement, the current carrying capacity can be considerably enhanced particularly for large conductor sizes and further, application is possible for longer route lengths.

When cable are lain in flat formation, transposition is resorted to, so that each phase cable is arranged to occupy over equal lengths of the route, each of the three geometrical positions in the laying formation. This results in balancing of induced voltages in the screen because of equal relative proximity of each single core cable with respect to the other.

Laying Methods

As dicussed in the preceding paragraphs, cables can be laid in trefoil or flat formation, depending upon design requirements and end use suitability. Installation conditions play a major role in optimising the performance of an EHV cable circuit, as also rendering it safe for long term reliability and usage. For the purpose, utmost care is to be exercised in handling an EHV cable installation so that it can serve its useful life to the fullest extent.

Standard methods of laying employed for power cable circuits apply to EHV cables as well, guidelines for which are available from Codes of Practice formulated by the BIS. Attention in particular needs to be paid to the consolidation of the surrounding soil environment in direct burial conditions for improvement of heat dissipation properties.

Bending Radius

While installing TROPOTHEN-S cables, the following minimum bending radius should be observed for single core cables, so that the cable and especially the insulation are not unduly stressed.

20 x D where D is the overall diameter of the cable.

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