Westinghouse KRD-4, Installation, Operation, & Maintenance Instruction

Page: 5 / 114

-TYP E K RD-4 D I RECTIONAL O V E RCURRENT
GROUND RELAY
1 .
Directional Unit opening times for current,
voltage, or dual polarized.
2.
Directional unit closing times for current,
voltage or dual polarized.
3 .
Directional unit closing time for
5
volts
voltage polarized.
Fig,
6
shows the instantaneous overcurrent unit
closing time
The voltage polarized curve (curve B in Fig.
5)
begins to deviate from curve A at about
1 0
volts
polarization,
Both the directional unit and the overcurrent unit
must operate before the trip circuit can be completed.
Hence·, the unit which takes the longer time to oper­
ate determines when the break er will be tripped. The
overcurrent unit contacts cannot operate until the
back contacts of the directional unit open; therefore,
the total time for the overcurrent unit to operate is
its closing tim e given in Fig.
6
plus the directional
unit's opening time given in Fig.
5.
The total closing
time for the directional unit is given in Fig.
5.
The
two examples below will serv e to illu strate the use
of the curves.
(Example One) definition of symbols ar e shown
on Fig.
5.
let: lpol
=
1 . 5
amp.
lop
=
3
amp
tap value (T)
=
0 . 5
amp.
1:1
=
oo
for a current polarized relay:
* MPP
-
loplpol cos
0
0.25
*
( 3 ) ( 1 . 5)
MPP
=
=
1 8
0 . 2 5
Entering the curves i n Fig.
5
at multiples o f product
pickup of
18
the directional unit opening time is
4
m s , and the closing time for this unit is
3 3
m s .
For the overcurrent unit :
1
multiples of pickup
=
3
-
-
- 6
- 0 . 5 -
Entering the curve in Fig.
6
at multiples of pickup
equal to
6
the closing tim e for the ov ercurrent is
1 4
ms. However, the total operating tim e for the overcurrent
unit is
14
plus
4
m s , which is the opening time of back
contacts of the directio nal unit, or
18
ms total operating
time for the overcurrent unit. The total operating time
for the directional unit is
33
ms; and since this i s
the longest tim e,
3 3
m s is the total operating time o f
the relay .
(Example Two)
let: lpol
=
1 5
amp
lop
=
25
amp
T (tap)
=
1
amp .
0=0
MPP
0 . 2 5
MPP
=
1 500
referring to Fig.
5
the directional unit closing time
is
8
ms, and the op ening time of its back contacts is
3
m s. The total operating time for the directional
unit is
8
m s.
For the overcurrent unit:
lop
multiples of pick up
=-­
T
=
25
referring to Fig. 6 the ov ercurrent unit contact clos­
ing time is
10
ms. Therefore, the total operating time
for this unit is
10 + 3
ms or
1 3
ms. In this case the
total operating time of the relay is
13
ms.
Trip Circui t
The m ain contacts will safely close
30
amperes
at
250
volts d-e and the seal-in contacts of the indi­
cating contactor switch will safely carry this current
long enough to trip a circuit bre aker.
The indicating contactor switch has a pickup of
approximately
1
ampere. Its d-e resistance is
0. 1
ohms.
Cyl i nder Unit Contacts
The moving contact ass embly has been factory
adj usted for low contact bounce performance and
should not be changed.
The set screw in each stationary contact has
been shop adjusted for optimum fo llow and this ad­
justm ent should not be disturbed.
5
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