Technical data
24
1MRS751108-MUM
Protection Relay
Technical Reference Manual, General
REX 521
5.1.6. Analog channels
The protection relay measures the analog signals needed for protection, measuring,
etc. via galvanically separated matching transformers. In addition, current sensors
(Rogowski coil) and voltage dividers developed by ABB can be used with REX 521.
The different versions of REX 521 are provided with the following matching
transformers and sensor inputs:
A letter in the order number specifies whether the protection relay is equipped with
basic, medium, high or sensor measuring input modules. (Refer to section “Ordering
information” on page 61).
5.1.6.1. Scaling the rated values of the protected unit for analog
channels
A separate scaling factor can be set for each analog channel. The factors enable
differences between the ratings of the protected unit and those of the measuring
device (CTs, VTs etc.) The setting value 1.00 means that the rated value of the
protected unit is exactly the same as that of the measuring device.
The scaling factor is calculated channel by channel as follows:
Scaling factor = I
nmd
/ I
np,
where
Example:
Version Matching transformers Sensor inputs
Basic CT1, CT2, CT3, CT4 -
Medium CT1, CT2, CT3, CT4, CT5, VT1 -
High CT1, CT2, CT3, CT4, CT5, VT1, VT2, VT3, VT4 -
Sensor CT4, CT5, VT1 RS1, RS2, RS3, VD1, VD2, VD3
When scaling factors are used, it should be noted that they affect the
operation accuracy of the relay. The accuracies stated in the
description of each function block (in the CD-ROM Technical
Descriptions of Functions) only apply with the default values of the
scaling factors. For example, a high factor affects the operation of
sensitive protection functions such as the directional earth fault
protection. To ensure the proper operation of the function blocks, it
must be checked that the analog scales (pu scales) of the phase
currents I
L1
, I
L2
, and I
L3
, and correspondingly, the analog scales of
the phase-to-phase voltages U
12
, U
23
, and U
31
or phase-to-earth
voltages U
1
,
U
2
,
and U
3
are identical.
I
nmd
Rated primary current [A] of the measuring device
I
np
Rated primary current [A] of the protected unit connected to the
channel
Rated primary current of current trafo = 500 A: I
nmd
= 500 A
Rated current of the protected unit = 250 A: I
np
= 250 A
Scaling factor for current channels: 500 A / 250 A = 2.00