12.1 Introduction

There had been a long-felt need for a compact Busbar System for medium and high voltage power inter-connection in outdoor switchyards and switchgear rooms. Medium voltage Busducts have unwieldy dimensions for a rating of 24 kV and above. When installed indoors, it occupies a disproportionately large volume of the room. When installed outdoors, it requires an expensive supporting arrangement. The typical cross-section dimension of a 36 kV, 2500 A, air-insulated, segregated phase bus is 2400 mm (108”) in width and 800 mm (32”) height. (Air Insulated Circuit Breaker Panels are cost-effective for voltage rating up to 36 kV. To make the panel compact, the Busbars are either sleeved or cast in resin to share the insulation stress. Interfacing a Busduct with the panel is challenging.

Gas Insulated Bus is expensive for medium voltage applications unless it forms part of the gas-insulated switchgear. The arm of the gas-insulated switchgear can be extended for connection to the transformer or neighbouring switchgear. Sulphur Hexafluoride gas has been identified to contribute to the greenhouse gas effect.

In contrast, solid insulated busbars are compact and need less installation space and time. Solid insulated bus are manufactured up to a voltage rating of 170 kV.

Single-core XLPE cable, one cable per phase, has been included in this category as it can very well be installed for medium, high and extra-high voltage interconnections right up to 500 kV. After establishing their viability for a project, they can be considered as an alternative to the Busducts up to 36 kV and GIB beyond 36 kV. (XLPE Cables are manufactured in continuous lengths and wound on drums. It may not be easy to procure small cut lengths to meet the requirements of short Connections).

Organic insulation materials age with time. The cross-linking bonds become weak and break. This happens even under normal operating conditions. When they are subjected to thermal & dielectric stress higher than the rated value, the ageing process accelerates. It is believed that

• An increase in the operating temperature by 8 to 10°C reduces the service life by half.
• An increase in the operating voltage by 8 to 10 % reduces the service life by half.

Insulation subjected to thermal surge (due to overload & short circuit) and repetitive voltage surge (due to lightning & switching surges) can seriously impact the life of insulation.

12.2 Description

There are several categories of single-phase, solid insulated Busbar System that can be used for a wide range of system voltage. These are classified based on the insulating material of the conductor