Table of Contents
Advertisement
TYPE CA-5 PERCENTAGE DIFFERENTIAL RELAY
FOR TRANSFORMER AND GENERATOR PROTECTION
C AUTION
Before putting relays into service, remove
all blocking which may have been inserted for the
purpose of securing the parts during shipment, make
sure that all moving parts operate freely, inspect the
contacts to see that they are clean and close properly.
and operate the relay to check the settings and elec
trical connections.
A PPLICA TION
Type CA-5 relays are used for differential protec
tion of either a generator or a two-winding power
transformer.
The
transformer
differ only in the control spring. The generator uses
a weaker spring for greater sensitivity. The following
paragraphs
describe
the
Transformer Relay
Restraint coil current should not
symetrical r.m.s. amperes during external faults. The
combined a-c saturation error of the main and auxiliary
ratio-matching current transformers on either side of
the bank should not exceed
external fault.
Generator Relay
Current transformer burden in ohms should not
exceed (N
V
)/133; further the burden factor, BF,
p
CL
should not differ by more than a 2 to 1 ratio between
the two sets of CT's. The above terms are defined as:
N
= proportion of total number of CT turns in use
p
V
= current
transformer
CL
voltage
1000 R B
BF =
N
V
p
cL
R
= resistance
of
B
winding resistance.
For example, if the 400/5 tap of a 600/5 multi-ratio
CT is used, N
= 400/600 = 0.67. If this CT has a
p
10L200 rating, V
= 200, and the burden should not
CL
exceed:
N
V
0.67 x 200
P
CL
133
133
SUPERSEDES I.L. 41-339.1A
COMPLETE REVISION
INSTALLATION
INSTRUCTIONS
and
generator
relays
application
requirements.
exceed
100
10% for the maximum
10L
accuracy
class
the
burden,
excluding CT
=
1.0 ohm
Westinghouse
•
OPERATION
Assuming a resistance burden of R
the burden factor, BF, is:
NpV
1000 R B
BF =
=
CL
The other set of CT's may then have a burden
factor as high as 2 x 3.8= 7.6, or as low as 1/2 x 3.8 =
1.9. If the other set of CT's also has a burden of 0.5
ohms, a 10L 100, 10L200, or 10L400 rating would be
satisfactory, since the burden factors are 7.6, 3.8 and
1.9, respectivelY.
In calculating the burden, use the one way lead
burden.
CON STRUCTION
The type CA-5 relay consists of a percentage
differential unit, an indicating contactor switch unit,
and an optional indicating instantaneous trip unit.
The construction and operation of these units are as
follows:
Percentage Differential Unit
This unit has one restraining element, with two
windings, and one operating element which is energized
through an external auxiliary current transformer in
accordance with the current fiowing in the differential
connection
of the main current transformers. Taps
controlling the sensitivity of the relay are incorporated
in the external transformer.
The unit operates on the induction-disc principle,
The front electromagnet is the restraining, and the
with both electromagnets operating on the same disc.
rear electromagnet is the operating element. These
are connected as shown in Figs. 1 and 2.
lower bearing for the shaft is a steel ball riding be
The disc is mounted on a vertical shaft. The
tween concave sapphire jewel surfaces. A pin bearing
is used on the upper end of the shaft.
The moving contact assembly is attached to a
Micarta bushing on the disc s haft. When the moving
contact strikes the stationary contact,
contact spring deflects to provide a wiping action. The
I.L. 41-339.18
•
MAINTENANCE
= 0.5 ohms,
B
1000 x 0. 5
= 3 8
.
0.67 x 200
the moving
1964
EFFECTIVE APRIL
Advertisement
Table of Contents
