Table of Contents
Advertisement
,
Type
relays are ground directional
current relays used primarily for the protection of
feeders and transmission lines. They are available
with either inverse, very inverse or extremely in-
verse time characteristics.
They consist of two units, a time overcurrent unit
(top) of
induction disk type, and an instantaneous
power
unit
type. The directional unit is either potential or cur-
rent polarized or both and, by means of its closing
contacts, directionally control,: the operation of the
time overcurrent unit.
Type
relays are used for
of a single line. They
coil which may be rated
peres, although the
available. Under normal conditions,
in either the
nor is there any voltage
coils.
Fig. 9 shows the external connections when the
relay
relays polarized from wye-wye
The polarizing voltage for the ground relay is
obtained by means of an auxiliary wye-broken-delta
potent&l transformer.
10 shows the external connections for the
Fig.
Type
ground relay when current
from a local source of ground current.
On some applications, system conditions may
at
such that potential polarization is
desirable, and at other times be such that current
polarization would be preferred. The Type
relay, with its dual polarization feature, is well suit-
ed for such applications. The curves
the
its performance when either potential or current
polarized alone. The simultaneous use of both sets
of polarizing coils is advantageous on applications
where current and potent&l polarizing sources are
available and there is a possibility that one or the
other source may
The differences between the various models
covered by this instruction book are shown
Inverse time relays should be used on systems
where the fault current flowing through a given relay
Courtesy of NationalSwitchgear.com
INTRODUCTION
of the induction cup
(bottom)
low-range
or
ampere rating is also
or current polarizing coils;
used in conjunction with phase
temporarily lost.
is
acity at the time of the fault. Very inverse time and
extremely inverse time relays should be used in
cases where the fault current magnitude is depend-
ent mainly upon the location of the fault in relation to
the relay, and only slightly or not at all upon tbe
system generating setup. The reason for this is that
relays must
fault current flowing. For fault currents below this
value, the operating time becomes greater as the
current is decreased.
generating capacity, together
circuit current with fault position, the operating time
with minimum fault current may
with very inverse time relays and even longer with
extremely inverse time relays. For such cases, the
inverse time relay is more applicable.
am-
The choice between very inverse and extremely
inverse time relays is more limited than between
them and the inverse time relay as they are more
nearly
curves.
For grading with fuses the extremely in-
verse time relay should
rent curves more nearly match the fuse curve.
Another advantage of the extremely inverse relay
it is
is that
very inverse relays for picking up cold load. For
any
cold load pick-up capability, the resulting
settings will provide faster protection at high fault
currents with the extremely inverse relay than with
the less inverse relays.
2
Very Inverse
Very Inverse
PICKUP
Table 1.
When potential polarized, the directional unit
will operate at 3.6 volt-amperes at the
system generating cap-
by the
set to
selective with maximum
be
be
If there is a wide range in
be
in their time-current characteristic
chosen as the time-cur-
be
suited than both the inverse and
better
TABLE I
Time
Circuit
teristic
contacts
Inverse
One
Inverse
Inverse
I n v e r s e T w o
in
exceedingly long
Fig. 4
Fig. 5
Fig. 6
Advertisement
Table of Contents
