IEEE Power & Energy Magazine - March/April 2022 - 66

dc system continued to
grow and replaced local
dc generation with dcconversion
substations
powered by central acgenerating
stations. It
was expensive but met
demand. AC distribution
to customers was
la rgely confined to
areas of sparse development
insufficient to
amortize the investment
required for dc.
United specialized
in such areas and built a
Automatic
protective
relays were
vital for an
ac network
to achieve
superiority
over dc
distribution.
customer base where electric lights were
a luxury that was affordable by very
few. United perceived those areas as a
huge potential market and moved to establish
a presence there and in the undeveloped
northern area of Manhattan Island.
Utilities then classified customers
as either " lighting, " which meant singlephase
residential and light commercial
load, while " power " customers were
usually industrial, with heavy motor
loads. The feeders were two phase at
60 Hz, with single-phase distribution for
lighting customers, and two-phase lines
to power customers.
The high cost associated with dc
distribution encouraged research to develop
a less-expensive ac distribution
system with equal or greater reliability.
In 1915, Thomas E. Murray, a leading
executive in New York City utilities,
reported that fully 50% of the fixed
plant investment in a dc system was in
the substations and cables. The initial
efforts to distribute ac based on a dc
concept were neither economical nor
efficient. The dc system simply connected
cables with protection (usually
in junction boxes) against overloads.
When tried with ac, problems resulted
from unbalanced transformer loads,
excessive reactive power, and instability.
The available ac motors also lacked
the efficiency of their dc counterparts.
First Steps Elsewhere
Nonetheless, many utilities planned for
the gradual supplementation of dc systems
with ac, some with banked distribution
66
ieee power & energy magazine
transformers in dense areas.
Most of the schemes
used radial distribution
with a specific feeder
or set of feeders and a
transformer bank operated
as a unit to power
the secondary lines to
customer connections.
Some employed loop distribution,
with feeder protection
at each end of the
loop. The next step was the
use of interleaved parallel
feeders with alternate
transformer banks connected
to different primary feeders to
provide power in the event of an outage
on one.
The first system considered to be an
independent standalone network was
installed in 1915 with overhead lines
in 16 city blocks in downtown Peoria,
Illinois. It connected directly to power
station low-tension feeders but had
high-tension feeders to transformers at
the outer ends to address voltage drop.
It operated for 10 years, although it was
subject to outage from any failure and
was as costly as the dc system.
Most of the urban utilities began
to install ac at the fringes of the dense
downtown load areas as an initial step.
By 1922, such companies expanded
ac distribution and limited their dc
territory to the most dense areas and
planned the complete substitution of
ac whenever practical. Soon most of
them were exploring networked transformer
secondaries, and network design
was the prevailing topic of the day in
most trade journals and meetings. The
trend was encouraged by failures of dc
distribution to the extent that a national
conference was held to address that issue.
The New York Edison system was
promoted as the premier example of the
superiority of dc distribution in dense
load areas. That reputation came into
question after a failure in early 1919
blacked out the Midtown Garment District,
an area heavily dependent on electric
power.
As for ac, the primary issue was
that a single feeder to a secondary distribution
network was superior to a radial
system for lighting loads, but power
customers required multiple feeders for
adequate capacity and reliability. Furthermore,
multiple feeders were needed
by customers such as large office
and commercial buildings, depar tment
stores, industrial plants, apartment
houses, theaters, hospitals, and
other concentrated loads. If multiple
feeders to transformers operated at the
transmission voltage there would be
no need for intermediate transformer
substations. The concept could be deployed
initially in areas of light load and
expanded as the need arose.
United Takes the Lead
United distribution relied on manhole and
street vault manual switches; reactive
power compensation was controlled by
manually operated mechanical synchronous
condensers (capacitors) in the
transformer substations. The United
distribution system in Manhattan was
radial, divided into 70 sections with
long feeders from five transformer
substations to the local distribution
transformers. Two-hundred and fifty
manual-switch installations enabled
the transfer of loads to adjacent feeders
in the event of problems, but the time
to reach and operate those switches in
an emergency was excessive. Practicality
necessitated an automatic network
(see Figure 1).
The requirements for such an ac network
were complex. Beyond economics
and reliability superior to dc distribution,
it had to be simple to install and
maintain. It had to be compatible with
existing radial distribution to permit
future substitution without expensive replacement
of components. It had to provide
voltage regulation superior to dc,
and combine light and power loads. It
had to allow expansion to accommodate
new demand and be capable of connecting
to multiple sources. It also had to be
able to connect directly to the power station's
high voltage lines to eliminate the
transformer substations.
Automatic protective relays were
vital for an ac network to achieve
superiority over dc distribution. United
march/april 2022
IEEE Power & Energy Magazine - March/April 2022

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