Transformer Differential Protection - ABB REG670 Applications Manual

1MRK 502 071-UEN -
24.1.6.2
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual

Transformer differential protection

The current transformers must have a rated equivalent limiting secondary e.m.f. E
that is larger than the maximum of the required rated equivalent limiting secondary
e.m.f. E below:
alreq
I
³ = × ×
E E 30 I
al alreq rt
I
EQUATION1412 V2 EN
I
³ = × ×
sr
E E 2 I
al alreq tf
I
pr
EQUATION1413 V2 EN
where:
I The rated primary current of the power transformer (A)
rt
I Maximum primary fundamental frequency current that passes two main CTs and
tf
the power transformer (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 (W)
ct
R The resistance of the secondary wire and additional load (W). The loop resistance
L
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 The burden of an IED current input channel (VA). S
R
and S
In substations with breaker-and-a-half or double-busbar double-breaker arrangement,
the fault current may pass two main CTs for the transformer differential protection
without passing the power transformer. In such cases and if both main CTs have equal
ratios and magnetization characteristics the CTs must satisfy equation
equation 304.
æ
I
³ = × ×
sr
E E I
ç
al alreq f
I
è
pr
EQUATION1414 V2 EN
where:
I Maximum primary fundamental frequency current that passes two main CTs without passing
f
the power transformer (A)
æ ö
S
× + +
sr R
R R
ç ÷
ct L
2
I
è ø
pr r
æ ö
S
× + +
R
R R
ç ÷
ct L
2
I
è ø
r
=0.150 VA/channel for I =5 A
R r
ö
S
+ +
R
R R
÷
ct L
2
I
ø
r
Section 24
Requirements
al
(Equation 302)
(Equation 303)
=0.020 VA/channel for I =1 A
R r
302 and
(Equation 304)
753