Calculating Settings For Step 1 - ABB REC650 Applications Manual

Section 3
REC650 setting examples
3.1.4.2
44
For the choice of the time delay characteristic IEC Normal inverse is used in this
network.

Calculating settings for step 1

1. Set Pickup1 to 140% of IBase (334 A primary current)
The first requirement is that the phase overcurrent protection shall never trip for load
current during the extreme high load situations. It is assumed that the transformer
shall be able to be operated up to 130 % of the rated power during limited time. The
protection resetting ratio shall be considered as well. The reset ratio is 0.95. The
minimum setting can be calculated as:
×
1 60 1000
³ × ×
I 1 3 .
pu
×
0 95 .
3 145
GUID-23DBA3B9-4E20-4210-901B-3CB4B8B2BC38 V1 EN
The next requirement is that the protection shall be able to detect all short circuits
within the defined protected zone. In this case it is required, if possible, that the
protection shall detect phase-to-phase short circuit at the most remote point of the
outgoing feeders as shown in figure 6.
145 kV
3
ANSI11000124 V1 EN
Figure 6: Fault calculation for phase overcurrent protection setting
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).
The longest 22 kV feeder has an impedance of Z = 3 + j10 Ω. The external network
has a maximum source impedance of Z
transformed to the 22 kV level:
=
327 A
REC650
1
Ph-Ph
1
Y
Y
3
Y
1
= j10 Ω (145 kV level). This impedance is
sc
1MRK 511 286-UUS A
51_67
22 kV
ANSI11000124-1-en.vsd
Application manual
(Equation 2)