Alstom MiCOM P545 - 3. WORKED PROTECTION EXAMPLE AND OTHER PROTECTION TIPS; 3.1 Differential protection setting examples; 3.1.1 Differential element

Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-49
AP
3. WORKED PROTECTION EXAMPLE AND OTHER PROTECTION TIPS
3.1 Differential protection setting examples
3.1.1 Differential element
All four settings are user
adjustable. This flexibility in settings allows the relay characteristic
to be tailored to suit particular sensitivity and CT requirements. To simplify the protection
engineer’s task, we strongly recommend three of the settings be fixed to:
s2 = 2.0 pu
k1 = 30%
k2 = 150% (2 terminal applications) or 100% (3 terminal applications)
These settings will give a relay characteristic suitable for most applications. It leaves only
the Is1 setting to be decided by the user. The value of this setting should be in excess of
any mismatch between line ends, if any, and should also account for line charging current,
where necessary.
By considering the circuit shown in Figure 14, the settings for the phase current differential
element can
be established.
33kV
25km
400/1 400/1
33kV
Digital communications link
Steady state charging current = 2.5 A/km - cable
= 0.1 A/km - overhead line
P1010ENa
P1010ENa
Figure 14 Typical plain feeder circuit
The following settings should be set as follows:
s2 = 2.0 pu
k1 = 30%
k2 = 150% (for a two terminal application)
This leaves the setting of
s1 to be established.
In the case that voltage inputs are not in place, no facility to account for line charging current
is available. The setting of s1 must therefore be set above 2.5 times the steady state
charging current value. In this example, assume a cable is used and there are not VT inputs
connected to the relay:
s1 > 2.5 x ch
s1 > 2.5 x (25km x 2.5 A/km)
s1 > 156.25 A
The line CTs are rated at 400 amps primary. The setting of
s1 must therefore exceed
156.25/400 = 0.391 pu.
Therefore select:
s1 = 0.4 pu