ABB 615 Series ANSI, Техническое руководство

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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 329
Technical Manual
Equation 32
Example 1:
The rated burden S
n
of the current transformer 5P20 is 10 VA, the secondary rated current
5A, the internal resistance R
in
= 0.07
Ω
and the rated accuracy limit factor F
n
corresponding to the rated burden is 20 (5P20). Thus the internal burden of the current
transformer is S
in
= (5A)
2
×
0.07
Ω
= 1.75 VA. The input impedance of the IED at a rated
current of 5A is < 20 m
Ω
. If the measurement conductors have a resistance of 0.113
Ω
, the
actual burden of the current transformer is S
a
=(5A)
2
×
(0.113 + 0.020)
Ω
= 3.33 VA. Thus
the accuracy limit factor F
a
corresponding to the actual burden will be about 46.
The CT burden can grow considerably at the rated current 5A. The actual burden of the
current transformer decreases at the rated current of 1A while the repeatability
simultaneously improves.
At faults occurring in the protected area, the fault currents may be very high compared to
the rated currents of the current transformers. Due to the instantaneous stage of the
differential function block, it is sufficient that the current transformers are capable of
repeating the current required for instantaneous tripping during the first cycle.
Thus the current transformers usually are able to reproduce the asymmetric fault current
without saturating within the next 8.33ms after the occurrence of the fault to secure that
the trip times of the IED comply with the retardation time.
The accuracy limit factors corresponding to the actual burden of the phase current
transformer to be used in differential protection shall fulfill the following requirement:
Equation 33
Ik
max
The maximum through-going fault current (in I
R
) at which the protection is not allowed to
operate
T
dc
The primary DC time constant related to Ik
max
ω The angular frequency, that is, 2 x π x f
n
T
m
The time to saturate, that is, the duration of the saturation free transformation
K
r
The remanence factor 1/(1-r), where r is the maximum remanence flux in p.u. from saturation
flux.
The parameter r is the maximum remanence flux density in the CT core in p.u. from
saturation flux density. The value of the parameter r depends on the magnetic material used
and on the construction of the CT. For instance, if the value r = 0.4, the remanence flux
density can be 40 % of the saturation flux density. The manufacturer of the CT has to be
contacted when an accurate value for the parameter r is needed. The value r = 0.4 is
recommended to be used when an accurate value is not available.
The required minimum time-to-saturate T
m
in MPDIF is half fundamental cycle period (8.33
ms when f
n
= 60Hz).
Two typical cases are considered for the determination of the sufficient actual accuracy limit
factor, F
a
:
a in
n in
n a
S S
S S
F F


 
) 1 ) 1 ( (
max
      

dc
m
T
T
dc r a
e T Ik K F
