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 7.3.2.1 Constant-Current Transformers. Constant-current transformers are commonly
used to supply current, usually 6.6 or 20 A to the many lamps connected in series. The
moving-coil constant-current transformer is a single-phase device with inherently high leakage
reactance. Movement of the coil varies this reactance automatically to maintain constant-current
output. The transformer is capable of regulating current within 1 percent for all loads within its
rating, at any supply voltage within 5 percent of normal, and with any ordinary variation in
frequency and temperature. Full-load power factor is approximately 75 percent and three-fourths
load power factor is about 56 percent. Input kVA is substantially constant over the entire range
of loads. In certain designs, capacitors improve the low power factor to a value near unity for
normal loads. The indoor or station type of constant-current transformer is supplied from the
substation high-tension bus. Indoor ratings vary from 5 to 70 kW, with 30 kW and 60 kW being
the most popular. The secondary winding of larger ratings is usually designed with two coils,
and leads from each coil are cross-connected to supply two series circuits. The pole- and
submersible-type constant-current transformers are built in sizes from 2 to 30 kW in oil-filled
tanks, usually with capacitors. They are mounted in the immediate vicinity of the lamps they
supply, thus realizing economy in circuit construction. Distribution primaries supply the
constant-current transformers with single-phase power.
7.3.2.2 Series Circuits. Current in a series circuit passes directly, without transformation,
through the lamps and circuit. Most circuits operate at 6.6 A. Circuits serving large lamps in the
heart of a city often operate at 20 A. Series circuits are usually carried on the same crossarms
with primary wires in overhead distribution and in separate ducts or pipes in underground
distribution. In parkways, series-circuit cables are often buried. Individual lamps are provided
with series cutouts which automatically close the circuit when lamps are removed. A film cutout
designed to puncture at about 1,000 V is connected across terminals of each lamp; if the lamp
filament breaks or burns out, the film cutout is punctured, the lamp is short-circuited, and the
circuit remains closed.
(a) Series circuits may be laid out on the parallel- or the open-loop plan. In the
parallel-loop, outgoing and incoming wires of the circuit are carried along the same route; in the
open-loop plan, the circuit goes out along one street and returns by another. The parallel-loop
requires greater mileage of wire to supply lamps in a given area than the open-loop method.
(b) Three disadvantages of the series circuit are:
o
The necessity for insulating lamps, leads, fixtures, and circuits for full operating
voltage to ground.
o The large exposure of circuit wire in which a break causes interruption to all
lamps.
o The economic necessity of installing streetlighting wires on the same crossarm
with primaries, causing wire congestion and exposing other distribution to
possible faulting if a streetlighting wire breaks.
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