Advertisement
The chosen Metrosil 'C' value must;
1) Ensure negligible current flows through the Metrosil at relay operate voltage (Vs), and,
2) Limit over-voltages for operational and safety reasons i.e. 1.09C (I
A 'C' value of 450 is generally acceptable where the relay operate voltage is less than 100V, a 'C' value
of 1000 is recommended for settings above 100V.
Metrosil short time power rating must;
Be sufficient to dissipate the heat created by the flow of maximum secondary internal fault current. The
Metrosil is chosen so that it can withstand I
breaker condition the back up protection clearance time must be considered – typically a one second
rating is sufficient.
4
I
P
1SEC
F
2.1.4 3 Pole High Impedance Differential Protection
Three pole differential protection (87/50) can be applied to various primary plant connection arrangements, Figure
2-2 shows differential protection applied to a busbar.
Figure 2-2
The zone of protection is defined by the position of the CTs. The protection provides a low operate current (fault
setting) for in zone earth faults and stability during external faults.
To achieve stability of the high impedance elements and limit overvoltages external series stabilising resistors and
Metrosils are wired into the scheme.
The operating voltage of the relay/stabilising resistor combination is calculated taking into account the r.m.s. value
of the symmetrical component of the maximum external (through fault) current.
The relay current setting is calculated taking into account: the required operate level for in-zone phase and earth
faults (fault setting).
©2013 Siemens Protection Devices Limited
T
V
K
AC Connections - 3 Phase Differential Protection
Chapter 7) 7SR23 DAD Applications Guide
Fint
for the maximum fault clearance time. For a failed circuit
Fint
) < 3kV
Chapter 7 Page 10 of 42
Advertisement
Chapters
