ABB ret650, Руководство по применению, Страница: 376 / 392

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-US
(Equation 68)
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
69 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-US
(Equation 69)
Section 19
1MRK 504 169-UEN A
Requirements
370 Transformer protection RET650 2.2 IEC
Application manual