ABB REG670 - Monitored Data

6.2.6 Monitored data
Table 52: HZPDIF Monitored data
Name
Type Values (Range) Unit Description
MEASVOLT REAL - kV Measured RMS voltage
on CT secondary side
6.2.7 Operation principle
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
45. Note that only one phase is shown in this figure.
I> (50)
R
S
U
I
1
2
4
3
5
GUID-5CEAF088-D92B-45E5-B98F-3083894A694C V1 EN
Figure 45: 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.
1MRK502052-UEN B Section 6
Differential protection
143
Technical manual