ABB REG670 - Page 762

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 I
r
= 1 A and S
R
= 0.150 VA / channel for I
r
= 5 A
In case of three individual CTs connected in parallel (Holmgren connection) on the
phase side the following additional requirements must also be fulfilled.
The three individual 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
2
sr R
al alreq tf ct Lsw
pr
r
I S
E E I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION2241 V2 EN (Equation 310)
Where:
I
tf
Maximum primary fundamental frequency three-phase fault current that passes the
CTs and the power transformer (A).
R
Lsw
The resistance of the single secondary wire and additional load (Ω).
In impedance earthed systems the phase-to-earth fault currents often are relatively
small and the requirements might result in small CTs. However, in applications where
the zero sequence current from the phase side of the transformer is a summation of
currents from more than one CT (cable CTs or groups of individual CTs in Holmgren
connection) for example, in substations with breaker-and-a-half or double-busbar
double-breaker arrangement or if the transformer has a T-connection to different
busbars, there is a risk that the CTs can be exposed for higher fault currents than the
considered phase-to-earth fault currents above. Examples of such cases can be cross-
country faults or phase-to-phase faults with high fault currents and unsymmetrical
distribution of the phase currents between the CTs. The zero sequence fault current
level can differ much and is often difficult to calculate or estimate for different cases.
To cover these cases, with summation of zero sequence currents from more than one
CT, the phase side CTs must fulfill the Requirement
311 below:
2
sr
R
al alreq f ct L
pr
r
I
S
E E I R R
I
I
æ ö
³ = × × + +
ç ÷
è ø
EQUATION2242 V2 EN (Equation 311)
Section 24 1MRK 502 071-UEN -
Requirements
756 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual