ADR233B
Ref ID : ADR233B/IM/PS
Rev No. : 05
Page No. : 128 of 479
circulates around the delta connected windings (unless an earthing transformer is used). So in
the case of a Y-delta transformer, an external fault will cause zero sequence current to be
measured by the Y-side CTs. However, because this zero sequence current is trapped on the
delta side, it is not measured by the delta-side CTs. This could cause maloperation if not
compensated for. Before the advent of numerical IEDs, this was handled by a configuration
involving interposing CTs.
The ADR233B provides this feature by filtering out the zero sequence components in software
when the Y-connected windings grounded.
In “Equipment Data” settings menu the “W1 Grounding” and “W2 Grounding” setting can
be set as grounded or ungrounded.
Where;
• Grounded = zero sequence filter on
• Ungrounded = zero sequence filter off
The zero-sequence current is calculated by using the amplitude-matched phase currents.
Iamp, Zero = (IA, amp+ IB, amp+ IC, amp)/3
PHASE COMPENSATION
There are two ways of connecting a three-phase transformer winding:
• Star-connected (sometimes known as Y, or Wye)
• Delta-connected (sometimes known as D or D)
In some transformers, the windings are split at the centre point and terminals are brought out so
that they can also be interconnected. These windings can be zig-zag connected (Z-connected).
The more common connection types are:
Y–y D–d
Y–d D–y
Y–z D–z
The transformer vector group number represents the phase shift between the current in a low
voltage winding with respect to its counterpart on the high voltage winding. This corresponds to
the position of the number of a standard clock face. The table1 below shows examples of
connections with the clock vectors Midnight, 1 o’ clock, 6 o’ clock and 11 o’clock, which is
equivalent to a phase shift of 0°, -30°, -180° and +30° respectively. The enough amount of
differential current generated due to the transformer phase difference and then it may lead to
undesirable tripping during normal operating condition.
To Next Page
Loading...
