Basler BE1-CDS220, Instruction Manual

Page: 19 / 476

BE1-CDS220 General Information 1-7
1 2 3 4
1
2
3
O
P
E
R
A
T
I
N
G
C
U
R
R
E
N
T
(
I
N
M
U
L
T
I
P
L
E
S
O
F
T
A
P
)
MAXIMUM RESTRAINT CURRENT (IN MULTIPLES OF TAP)
D2837-11.DWG
01-05-99
SL
OP
E
=
MINIMUM PICKUP
= 0.01 to 1.00
TIMES TAP
5
I
I
RE
ST
R
OP
*
10
0
=
(1
5
TO
6
0%
)
Figure 1-1. Restrained Differential Characteristic
The BE1-CDS relay has two settable restraint current calculation methods: average and maximum.
Average restraint emulates the operating characteristics of common electromechanical relays. Maximum
restraint is recommended because it uses the current from the best performing CT to determine the
restraint to use during a fault condition. In addition, digital technology provides a transient monitor function
that enables the BE1-CDS relay to detect the onset of CT saturation to ride through the condition to
further enhance security from misoperation caused by poorly performing CTs.
Finally, Basler Electric addresses the source of false differential current at its roots. Active CT technology
used on the current inputs provides low burden to extend the linear range of power system CTs and wide
dynamic range to reduce measurement errors at high current levels. Sixteen bit ADC performance and
digital anti-aliasing filters also contribute to minimizing magnitude and angle measurement errors.
Problem 2: Measured Current Magnitude Mismatch
General
The currents measured by each set of current inputs can be transformed from their primary values by
different CT ratios. This is illustrated in Figure 1-2. When the zone of protection includes a transformer,
there is another source of magnitude mismatch that must be accounted for. The primary currents that the
differential relay must monitor will be on different voltage bases in most cases. That is—due to the trans-
former action, the current on each side of the transformer is transformed by the inverse of the voltage
transformation ratio. This is illustrated in Figure 1-3.