IEEE Power & Energy Magazine - July/August 2021 - 91

Transactions paper that his concern
about the " rapidly growing central
station systems " where " neither manufacturer
nor operator has placed any
absolute safety limit on just how large
a system the present switch is entirely
capable of protecting. " He points to
the introduction of the steam turbine
as one of the causes of concern. These
high-speed turbines required alternators
with two or four poles to produce
the 25 or 60 Hz that the majority
of power systems used. The reduced
number of poles meant decreased selfinductance.
Therefore, when a short
circuit occurred, there was less impedance
in the circuit than there would
have been with reciprocating steam
engines or hydraulic turbines.
Regardless of the type of prime mover,
alternators were becoming much
larger in terms of power output than
they were at the turn of the 20th century,
increasing from 5,000 to 20,000 kW.
Also, it was becoming common practice
in large urban centers to interconnect
several generating stations in parallel.
This had the effect of producing
one very large source of power to feed
into a short circuit.
In 1911, Schuchardt and Schweitzer
said in an AIEE Transactions paper that
to maintain high reliability, one of the
principal problems is to limit the amount
of energy that can flow into a fault. They
give an account of their intensive study
of the subject. Reactors or choke coils
had been used to limit current in dc circuits
and a part of lightning arrestors,
but now they were being proposed as a
means of limiting short circuit current in
ac systems.
The next year, several AIEE Transactions
papers dealt with power-limiting
reactors. In 1914, they were in use in
many of the larger systems, and, by the
early 1920s, they had become standard
practice, especially for systems of large
power supplied by turbo alternators.
Circuit Breaker Design
While fuses had been in use for some
time, they were typically open links of
metal inserted between two terminals.
As such, they were unsuitable for injuly/august
2021
terrupting heavy short circuits. When a
fault of any large magnitude occurred,
these fuses would virtually explode and
spray molten globs of metal, sometimes
causing more damage to the surrounding
electrical equipment than they prevented.
Circuit breakers had been developed in an
attempt to solve this problem.
The early breakers were nothing
more than a knife switch in which
a latch held the blades closed against
a spring. When an overcurrent condition
occurred, the current flowing in
an electromagnet tripped the latch at a
predetermined rating. Figure 4 shows a
typical dc breaker. Because its greatest
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IEEE Power & Energy Magazine - July/August 2021

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