Small Power Transformers In A Tap; Charging Current Compensation - ABB RELION 650 Series Applications Manual

1MRK 505 363-UEN A
auxiliary transformers are necessary. Instead it is necessary to eliminate the zero-sequence
current by proper setting of the parameter
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.
6.2.2.2

Small power transformers in a tap

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 33.
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.
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Figure 33: Line tap with a small power transformer, the currents of which are not
6.2.2.3

Charging current compensation

There are capacitances between the line phase conductors and between phase conductors and
earth. These capacitances give rise to line charging currents which are seen by the differential
protection as "false" differential currents, as shown in figure 34.
Application manual
Protected zone in instantaneous function
measured, and consequently contribute to a (false) differential current
ZerSeqCurSubtr . If the power transformer is of the
64 kbit/s
Section 6
Differential protection
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