Page 84 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
82 Chapter Two

load which is unbalanced between phases. It is not normally required
and is mentioned here because ISO 8528 includes a reference to it.
■ Voltage Restrained Overcurrent Relays. The opening paragraphs of
this section explain the behavior of a generator under fault condi-
tions. When running from a standby generator the low value of short
circuit current can lead to problems with fault clearance. The voltage
restrained relay has two time/current characteristics and may help
to resolve the problems. Under conditions of normal voltage the relay
has a long time characteristic which is intended to ensure that the
generator remains on line for as long as possible. If conditions of low
voltage arise during fault clearance, the relay operates to a standard
inverse characteristic and, hence, trips in a predictably shorter time.

A Note on Current Transformers and Relays
Current transformers used for protection purposes must be of the cor-
rect class for the duty. It is important that their cores do not saturate
at the maximum currents expected, it is also important, for differential
and earth fault systems, that the ratio and secondary phase angle are
sufficiently accurate to ensure proper operation.
Until the 1980s most protection relays were electromechanical and
based on the induction disc or cup, the principle having remained in
use for many years. Current adjustment was by a “plug setting” and a
tapped autotransformer, and time adjustment was by a “time multi-
plier” which determined the angle through which the disc had to rotate
to close its contacts. The parts required precision manufacture and
skilled assembly and, over a long period of time, as electronic tech-
niques improved, the electromechanical relays became obsolescent and
were superseded by electronic versions. For this reason all relays are
now electronic in operation and have a very different appearance.
Similar comments have already been made in this book in connection
with voltage regulators and speed governors.
The modern relays perform the same functions as their predeces-
sors and the same terminology is used, adjustments are made in a dif-
ferent way to achieve the same result. Electronic relays are more
accurate than the electromechanical devices, overshoot is less
because there are no moving parts, and the current transformer bur-
dens are less. The relays are more versatile and provide, within a sin-
gle unit, a choice of characteristics such as standard inverse, very
inverse, extremely inverse, or definite time. The final output circuits
may use electromagnetic relays with metallic contacts, which are better
suited to the inductive loads likely to be encountered than are elec-
tronic components.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.