ABB REG670 Applications Manual page 761

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
Where:
I The rated primary current of the power transformer (A)
rt
I Maximum primary fundamental frequency phase-to-earth fault current that passes the
etf
CTs and the power transformer neutral (A)
I The rated primary CT current (A)
pr
I The rated secondary CT current (A)
sr
I The rated current of the protection IED (A)
r
R The secondary resistance of the CT (Ω)
ct
R The resistance of the secondary wire and additional load (Ω). The loop resistance
L
containing the phase and neutral wires shall be used.
S The burden of a REx670 current input channel (VA). S
R
A and S = 0.150 VA / channel for IR = 5 A
R
In substations with breaker-and-a-half or double-busbar double-breaker arrangement,
the fault current may pass two main phase CTs for the restricted earth fault protection
without passing the power transformer. In such cases and if both main CTs have equal
ratios and magnetization characteristics the CTs must satisfy Requirement
Requirement 307:
I
³ = ×
sr
E E I
al alreq ef
I
pr
EQUATION2239 V2 EN
Where:
I Maximum primary fundamental frequency phase-to-earth fault current that passes two
ef
main CTs without passing the power transformer neutral (A)
Neutral CTs and phase CTs for impedance earthed transformers
The neutral CT and phase CTs must have a rated equivalent limiting secondary e.m.f.
E that is larger than or equal to the required rated equivalent limiting secondary e.m.f.
al
E below:
alreq
I
³ = × ×
E E 3 I
al alreq etf
I
EQUATION2240 V2 EN
Where:
I Maximum primary fundamental frequency phase-to-earth fault current that passes
etf
the CTs and the power transformer neutral (A)
I The rated primary CT current (A)
pr
I The rated secondary CT current (A)
sr
Table continues on next page
æ ö
S
× + +
R
R R
ç ÷
ct L
2
I
è ø
r
æ ö
S
× + +
sr R
R R
ç ÷
ct L
2
I
è ø
pr
r
Section 24
Requirements
=0.020 VA / channel for IR = 1
R
306 and the
(Equation 308)
(Equation 309)
755