Application Guide - NP900 Series
320 (504
)
A996A
Figure 1.1.1.4-154 Yd1 transformer internal connection in principle.
In modern relays these standard vector groups (wye, delta, lead or lag) are defined by a
setting selection and there is no need for interposing transformers. If the transformer
vector group is not standard it should still be settable within the relay (in case of zigzag
transformers).
In this case in NP900 differential relay the differential function applies following translation
to delta side currents (note that the correction is not only to the angles but also to the
amplitudes since the per unitized delta side has
√
3
relation amplitude difference to star
connected side)
IL1DS
��������
LV
=
(IL1
LV
�������
-IL2
LV
�������
)
√
3
IL2DS
��������
LV
=
(IL2
LV
�������
-IL3
LV
�������
)
√
3
IL3DS
��������
LV
=
(IL3
LV
�������
-IL1
LV
�������
)
√
3
This is the so called vector group matching for the per-unitized currents of the transformer.
This matching is necessary when either of the transformer side is connected to delta but
another is connected to star.
In non numeric relays this matching was done with
interposing CTs which were connected in the power transformer star side to delta and
delta side to star to get the HV and LV side vectors matching each other. When these got
currents were summed in the relay inputs HV and LV side currents negate each other
when there is no difference thus not causing trip. If the currents would have difference
