ABB RED650, Applications Manual

Page: 85 / 454

eliminates the need for auxiliary interposing current transformers. Necessary
parameters, such as transformer rated voltages and phase shift, must be entered via
the Parameter Setting tool or the LHMI.
Another concern with differential protection on power transformers is that a
differential protection may operate unwanted due to external earth faults in cases
where the zero-sequence current can flow only on one side of the power
transformer but not on the other side, as in the case of Yd or Dy phase shift/vector
groups. This is the situation when the zero-sequence current cannot be transformed
from one side to the other side of the transformer. To make the differential
protection insensitive to external earth faults in these situations, the zero-sequence
current must be eliminated from the terminal currents so that it does not appear as a
differential current. This was previously achieved by means of intermediate current
transformers. The elimination of zero-sequence current is done numerically and no
auxiliary transformers are necessary. Instead it is necessary to eliminate the zero-
sequence current by proper setting of the parameter
ZerSeqCurSubtr . If the power
transformer is of the type Dyn, where yn-windings currents are measured, then the
zero-sequence component will be subtracted when these currents are transformed
to the HV-side.
6.1.2.2 Small power transformers in a tap
M12022-54 v5
If there is a line tap with a comparatively small power transformer (say
1-20MVA) , line differential protection can be applied without the need of current
measurement from the tap. It works such that line differential protection function
will be time delayed for small differential currents below a set limit, making
coordination with downstream short circuit protection in the tap possible. For
differential currents above that limit, the operation will be instantaneous in the
normal way. Under the condition that the load current in the tap will be negligible,
normal line faults, with a fault current higher than the fault current on the LV side
of the transformer, will be cleared instantaneously.
For faults on the LV side of the transformer the function will be time delayed, with
the delay characteristic selected, thus providing selectivity to the downstream
functions, see figure
31 . The scheme will solve the problem with back-up
protection for faults on the transformer LV side where many expensive solutions
have been applied such as intertripping or a local HV breaker. In many such
applications the back-up protection has been lacking due to the complexity in cost
implications to arrange it. Refer also to the setting example below.
1MRK 505 393-UEN B
Section 6
Differential protection
Line differential protection RED650 2.2 IEC 79
Application manual