ABB Relion REG670, Applications Manual

Page: 761 / 780

Where:
I
rt
The rated primary current of the power transformer (A)
I
etf
Maximum primary fundamental frequency phase-to-earth fault current that passes the
CTs and the power transformer neutral (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 (Ω)
R
L
The resistance of the secondary wire and additional load (Ω). The loop resistance
containing the phase and neutral wires shall be used.
S
R
The burden of a REx670 current input channel (VA). S
R
=0.020 VA / channel for IR = 1
A and S
R
= 0.150 VA / channel for IR = 5 A
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
306 and the
Requirement 307 :
2
sr R
al alreq ef ct L
pr r
I S
E E I R R
I
I
æ ö
³ = × × + +
ç ÷
è ø
EQUATION2239 V2 EN
(Equation 308)
Where:
I
ef
Maximum primary fundamental frequency phase-to-earth fault current that passes two
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
al
that is larger than or equal to the required rated equivalent limiting secondary e.m.f.
E
alreq
below:
2
3
sr R
al alreq etf ct L
pr
r
I S
E E I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION2240 V2 EN
(Equation 309)
Where:
I
etf
Maximum primary fundamental frequency phase-to-earth fault current that passes
the CTs and the power transformer neutral (A)
I
pr
The rated primary CT current (A)
I
sr
The rated secondary CT current (A)
Table continues on next page
1MRK 502 071-UEN -
Section 24
Requirements
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 755
Application manual