ABB Relion REG670, Applications Manual

Page: 760 / 780

24.1.6.3 Breaker failure protection
The CTs must have a rated equivalent limiting secondary e.m.f. E
al
that is larger than
or equal to the required rated equivalent limiting secondary e.m.f. E
alreq
below:
sr R
al alreq op ct L
2
pr
r
I S
E E 5 I R R
I I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION1380 V2 EN
(Equation 305)
where:
I
op
The primary operate value (A)
I
pr
The rated primary CT current (A)
I
sr
The rated secondary CT current (A)
I
r
The rated current of the protection IED (A)
R
ct
The secondary resistance of the CT (
W
)
R
L
The resistance of the secondary cable and additional load ( W ). The loop resistance containing
the phase and neutral wires, must be used for faults in solidly earthed systems. The resistance
of a single secondary wire should be used for faults in high impedance earthed systems.
S
R
The burden of an IED current input channel (VA). S
R
=0.020 VA/channel for I
r
=1 A and S
R
=0.150
VA/channel for I
r
=5 A
24.1.6.4 Restricted earth fault protection (low impedance differential)
The requirements are specified separately for solidly earthed and impedance earthed
transformers. For impedance earthed transformers the requirements for the phase CTs
are depending whether it is three individual CTs connected in parallel or it is a cable
CT enclosing all three phases.
Neutral CTs and phase CTs for solidly earthed transformers
The neutral CT and the phase CTs must have a rated equivalent limiting secondary
e.m.f. E
al
that is larger than or equal to the maximum of the required rated equivalent
limiting secondary e.m.f. E
alreq
below:
2
30
sr R
al alreq rt ct L
pr r
I S
E E I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION2237 V2 EN
(Equation 306)
2
2
sr
R
al alreq etf ct L
pr
r
I S
E E I R R
I I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION2238 V2 EN
(Equation 307)
Section 24
1MRK 502 071-UEN -
Requirements
754 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual