Micom P721, Technical Manual

Page: 165 / 334

Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 29/52
4. SETTING EXAMPLES
4.1 Restricted earth fault application
4.1.1 Example 1
FIGURE 27 shows the application of a high impedance REF to protect the LV winding of a
power transformer.
In this example assume that the Input selection in the P721 is set to Voltage. Therefore, the
Stabilizing Resistor N needs to be set. The P721 calculates the relay operating current
using the set Voltage and Stabilizing Resistor N. Note that this current must be within the
setting range of [87N] Threshold I Diff. If the calculated relay operating current is not within
the range (0.01In-1In), then the warning led in the relay is asserted and the message
“Inconsistent V/R ratio” is displayed.
A
B
C
11kV
415V
1500/5A
P1527ENc
Transformer: 1MVA
Data
11kV/415V
Burdens:
Dyn11, X=5%
R = 0.04 W
L
RCT = 0.3 W
RCT
RCT
RL
RL
RL
RL
Restricted
Earth Fault
Protection
FIGURE 27: RESTRICTED EARTH FAULT PROTECTION ON A TRANSFORMER
4.1.1.1 Stability voltage calculation
The transformer full load current, I
FLC
, is:
1391.2A
3 415
10 1
6
FLC
=
×
×
=
I
To calculate the stability voltage the maximum through fault level should be considered. The
maximum through fault level, ignoring the source impedance,
I
F
, is:
27824A
0.05
1391.2A
X
TX
FLC
F
= =
=
I
I
Required relay stability voltage, V
S
, and assuming one CT saturated is:
( )
L CT F S
2R R K V
+ =
I
Assume that the Measurement Filter is set to Fourier. Then the graphic shown in FIGURE
28 should be used to determine the K factor and the operating time.