SWITCHGEAR
During the war Swiss investigations, extensive but on a laboratory
scale, had revived the possibility of rationalizing circuit-breaker
design. Though a considerable amount of testing was done subsequently
on power systems in America, the data obtained proved disappointing
when applied to British designs. At the works every opportunity was
taken of testing at weekends with generators going through dynamo
test, and new oscillographs and other equipment were installed with
the cooperation of G. A. Juhlin, chief engineer of the plant department;
to him were due several improvements in switchgear design, culminating
in the crossjet explosion pot mentioned later. It was, however,
only possible to conduct circuit-breaker tests at rare intervals.
 Indoor
switchgear was normally housed in moulded stone cubicles until 1918,
but the combination of compactness and greater safety rapidly gained
support for metalclad gear. In 1922 W. A. Coates was set the task
of producing designs for a complete range. The circuit breakers
themselves were applicable to either cubicle or drawout metalclad
structures.
The
first of the new breakers were built in the following year. They
were cubiclemounted rated at 1500 MVA 33 kV, and are still in
service in Salford with the original condenser bushings. Shortly
afterwards the first order was received for an 11-kV full metalclad
switchboard for Liverpool, and in 1924 the Company's first 33-kV
drawout metalclad unit was shown at Wembley. From this flying start
horizontal drawout metalclad gear established itself, and it almost
eliminated cubicle construction except for subsidiary services where
cost was all-important.
A
further landmark in this class of switchgear was set up in 1928
when Juhlin patented the cross-jet pot, which by restricting the
arc path and controlling its length transferred circuit-breaker
design from an empirical to a theoretical basis and showed the way
to the apparatus of today.
In
1928 the first metalclad gear for 1500 MVA at 6-6 kV was installed
at Edinburgh. As the currents to be handled made drawout construction
impracticable fixed circuit-breakers and oil-immersed isolators
were used with compoundfilled copper busbars of hollow square
section to solve the problem of heat dissipation. Removable bushings
gave direct access to the cables for test purposes.
Outdoor
switchgear was developed by R. W. Todd for the highest voltages
in use, starting in 1922 with 110-kV plain-break gear for the Mangahao
system in New Zealand. This was quickly followed by the 132-kV gear
for the Victoria Falls Company, using breakers with snap-breaks
and insulated explosion pots. The next stage was the introduction
of oil-blast pots with separate pressure-generating arcs. Several
1500-MVA 132-kV breakers were sold to Russia, and in 1927 similar
gear was ordered for half the switching stations on the Scottish
grid. In all some hundreds of such equipments were sold to the
Central Electricity Board.
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