6.2.6
Monitored data
PID-6990-MONITOREDDATA v1
Table 62: HZPDIF (87) Monitored data
Name
Type
Values (Range)
Unit
Description
MEASVOLT
REAL
-
kV
Measured RMS voltage on CT secondary
side
6.2.7
Operation principle
IP14242-1 v2
M13075-3 v11
High impedance protection system is a simple technique which requires that all CTs, used in the
protection scheme, have relatively high knee point voltage, similar magnetizing characteristic and
the same ratio. These CTs are installed in all ends of the protected object. In order to make a
scheme all CT secondary circuits belonging to one phase are connected in parallel. From the CT
junction points a measuring branch is connected. The measuring branch is a series connection of
one variable setting resistor (or series resistor) RS with high ohmic value and an over-current
element. Thus, the high impedance differential protection responds to the current flowing
through the measuring branch. However, this current is result of a differential voltage caused by
this parallel CT connection across the measuring branch. Non-linear resistor (that is, metrosil) is
used in order to protect entire scheme from high peak voltages which may appear during internal
faults. Typical high impedance differential scheme is shown in Figure
is shown in this figure.
I> (50)
R
S
V
I
1
2
4
3
5
GUID-C7509DA3-F6AA-4BA0-9D08-EAA376663AFA V1 EN-US
Figure 48: HZPDIF scheme
Where in the Figure:
1.
shows one main CT secondary winding connected in parallel with all other CTs, from the same
phase, connected to this scheme.
2.
shows the scheme earthing point.
Section 6
1MRK 502 066-UUS B
Differential protection
146
Technical manual
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