Table of Contents
Advertisement
7UT51 v3
Measured currents are multiplied by 1/k
Result: If the primary current of the transformer equals to the nominal current,
k_CT_n must be in a range 1/4 to 4. Otherwise, interposing transformers are required.
CT Mismatch Correction
Figure 4.3
4.1.2
Handling Non
Behavior
Since actual applications do not use ideal
components, the differential protection must take into
account the non-ideal behavior of the protected
transformer, the CTs monitoring it, and the rest of the
power distribution system. In particular:
•
A small, constant differential current may exist
(due, for example, to a magnetizing current). To
prevent this current from causing a pickup, the
differential protection has a minimum 87T pickup
level (see Section 4.4.1 on page 56).
•
The effects of variation in the behavior of different
CT's increase as the total current flowing through
the transformer increases (either a normal load or
a high through-current to an external fault).
Hence, the differential current required for a
pickup should be larger for larger transformer
currents (see Section 4.4.3 on page 57 and
Section 4.4.4 on page 58).
•
On the other hand, if the differential current
becomes so large that it cannot be due to an
external fault, the differential protection should
pickup (87HS protection, see Section 4.4.2 on
page 56).
PRIM-2330C
7UT512
1
k
CT
_
1
the relay will also measure nominal current = 5A.
Ideal
-
Transformer Differential Protection (87T and 87HS)
k
1
k
CT
_
2
_CT_n.
•
During an external fault with a large
through-current, an internal fault may also occur. If
this happens, the relay should pickup
(see Section 4.5 on page 58).
•
When the protected transformer is energized, the
inrush current may result in a transient differential
current. As is standard practice, the relay can use
the second- harmonic component of the
differential current to restrain the differential
protection during inrush (see Section 4.6 on
page 59).
•
Asymmetrical currents in the transformer windings
and/or overexcitation of the transformer can also
cause a differential current. The relay can restrain
tripping by measuring the higher-harmonic
components of the differential current, while still
handling the possibility that the CTs themselves
might produce higher-harmonic components in the
differential current during an actual fault (see
Section 4.7 on page 60).
•
Finally, the relay must rapidly respond to a
differential current so large that it can only be due
to an internal fault, regardless of any restraint
addresses. To do so, the relay has an 87HS
function whose algorithm can detect an evolving
fault in less than one cycle (see Section 4.4.2 on
page 56).
I
I
n
_
CT
_
Wn
n
_
Re
=
=
CT
_
n
I
I
n
_
Tr
_
Wn
n
_
Tr
_
Wn
k_CT_n:
Matching Factor for Winding n
In_CT_Wn:
Nominal CT Current of Winding n
In_Tr_Wn:
Nominal Transformer Current
for Winding n.
In_Relay:
Nominal Current of the Relay=5A
In_Tr_Wn_sec
Nominal Current of the Transformer
on the CT secondary
4
lay
_
sec
41
Hide quick links:
Advertisement
Table of Contents
