INTRODUCTION

The new TRILINK (S.A. Pat. 89/3502) range of gang-operated disconnectors is a complete review of existing problems incorporating new materials by a design team with extensive worldwide experience in medium voltage switchgear design. The design has concentrated on the cost effectiveness of devices for rural electrification, especially for those in the Third World environment. High standards of performance and operator safety are combined with low installed cost and vandal resistance.

This design review was not merely  another design of isolator, but was conceived in such a way as to create a basic module. This module was required to establish a form of geometry, which would extend to the various other devices. It was envisaged that there would be an entire new range of devices capable of allowing improved system design, performance and reliability.

INSULATION

The TRILINK incorporates Cycloaliphatic Epoxy insulators, which have extended inserts (S.A. Pat. 87/4484) to give improved voltage stress control over the end fittings, resulting in extraordinary low RF noise. The low mass of the operating insulator is a key factor in the minimal operating energy required. This results in the high closing speed, which is attainable when using a spring actuator as described later. The vandal resistance of this class of insulator is now well known.

NEW PATENTED DESIGN

The triangular formation of the insulators and the contact blade results in a very strong arrangement with all the operating forces and reactions being contained within the assembly on its own steel cross-arm. As a result the interconnecting phase linkage is in no way dependent upon the pole structure for its continued freedom of movement. The assembly merely requires two bolts to attach it to the pole. The standard arrangement is designed for operation by a linesman's portable insulated operating rod, (link-stick) with a downward movement for both opening and closing. This PULL-TO-OPEN, PULL-TO-CLOSE action eliminates all problems associated with ground level actuators and the attendant costs and risks in earth mats and reliable earth connections. A portable locking device with an insulated insert can be applied to the operating lever to lock in either position - see drawing no. LG0945.

LOW LEVEL MECHANISM

When a padlocking facility is an essential requirement, a low level operating mechanism can be provided as an optional extra. This mechanism incorporates a special feature, which enables it to be used with the TRILINK spring-close mechanism. Refer to drawing no. LG0529.

RATINGS

The standard off-load unit for rural lines is 200Amps with a tested short time current rating of 4kA for 3 seconds, equal to 6kA for 1 second. Alternative ratings of 10 and 16kA are also available for higher fault ratings, also including up to 630Amps. Voltage ratings are based on 24kV highest system voltage, with a BIL of 150kV. A 36kV design is available for isolator / disconnector duty only.

CONTACTS

The revolutionary design of the TRILINK contacts is quite new to this type of equipment, because by using the compressive forces available from the actuator, increased contact pressures can be obtained. These spring loaded wedge-type silver plated contacts are biased to the open position, and the moving contacts are ejected as soon as the opening stroke is commenced. When the contacts are fully open, an isolation distance of 320 mm is provided with a BIL across the gap in excess of 175 kV.

ENHANCED PERFORMANCE

The hand-operated TRILINK described above is a cost effective device, designed for low installed cost as a ganged disconnector for rural applications, but there are circumstances in which the system designer may wish to enhance the level of performance and operational facilities. Standard options, which can be added at the time of specifying the equipment, bringing the device more in line with the specification of a switch- disconnector are as follows: a) Spring assisted operation can be provided for the closing operation making the closing speed quite independent of the operators’ effort. This enables the disconnector to be used for closing on load at selected points in the network. b) Whip-lash type arc control devices can be added to operate during opening operations, giving a true fast load-break separation once the main contacts are open, hence protecting them against any possibility of arc damage. This feature is intended for spur-line connections or other circuits with connected loads of the order of 500/750kVA for 12/24kV application only. For main line switches with load-breaking capacity of 630Amp/I 1 kV and 200Amp/22kV see TRISWITCH.