ABB REC650 Applications Manual page 55

1MRK 511 286-UUS A
Application manual
145 kV REC650
3
Y
1
ANSI11000124 V1 EN
Figure 8: Fault calculation for phase overcurrent protection
A phase-phase-ground short circuit is applied. In this calculation the short circuit power
of the feeder shall be minimized (the source impedance maximized).
1. Set Pickup2 to 140 % of IBase
2205 A primary current.
The longest 22 kV feeder has an impedance of Z = 3 + j10 Ω. The external network
has a maximum source impedance of Zsc = j10 Ω (145 kV level). This impedance is
transformed to the 22 kV level:
2
æ ö
22
= × =
Z j 10
ç ÷
sc ,22
è 145 ø
GUID-BA67F1FC-2DCE-4E04-BBB3-FABB86FDEA3A V1 EN
The transformer impedance, referred to 22 kV level is:
The phase-to-phase fault current can be calculated as follows:
3 22000 / 3
= ×
I
sc ph 2
+
2 j 0.23
GUID-BAA8989B-EF8F-4277-9B2B-EBBCA35ED7C2 V1 EN
This current is smaller than the required minimum setting to avoid unwanted trip at
large load current. This means that the 22 kV phase overcurrent protection cannot
serve as complete back-up protection for the outgoing 22 kV feeders.
2. Set TD4 to 0.15
The feeder short circuit protections has the following setting:
I>: 300 A.
I>>: 6 000 A.
Characteristic: IEC Normal Inverse with k-factor = 0.25
The setting of the k-factor for the 22 kV phase overcurrent protection is derived from
graphical study of the inverse time curves. It is required that the smallest time
22 kV
1
Ph-Ph
1
Y
Y
3
W
j 0.23
=
949 A
+ +
j 0.97 3 j 10
Section 3
REC650 setting examples
51_67
ANSI11000124-1-en.vsd
(Equation 11)
(Equation 12)
49