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Zero sequence current can flow from line to transformer bank if the windings are in
Grounded star/delta
Delta/star
Star/ grounded star
Delta/delta
Grounded star/delta
Zero sequence current (I0тАЛ) can only flow in a transformer winding if there is a path provided by a grounded neutral or a delta connection. In a Grounded Star/Delta connection, the grounded star allows zero sequence currents to flow from the line into the primary winding, which then induces a circulating zero sequence current in the delta winding.
Zero sequence current (I0тАЛ) can only flow in a transformer winding if there is a path provided by a grounded neutral or a delta connection. In a Grounded Star/Delta connection, the grounded star allows zero sequence currents to flow from the line into the primary winding, which then induces a circulating zero sequence current in the delta winding.
I0тАЛ=31тАЛ(IaтАЛ+IbтАЛ+IcтАЛ) тАФ Definition of Zero Sequence Current
Zero sequence currents are in-phase in all three phases and require a return path through the neutral. In a grounded star connection, the neutral provides this path. If the secondary is delta-connected, the zero sequence current circulating in the delta acts to balance the ampere-turns, allowing the flow to exist without producing a magnetic flux in the core (as I0aтАЛ+I0bтАЛ+I0cтАЛ=0 is not required for the excitation of the magnetic circuit in the same way as positive sequence).
Zero sequence currents cannot exist in a 3-wire system without a neutral connection.
A delta connection provides a path for zero sequence currents to circulate internally, but cannot draw them from an external 3-wire line.
For zero sequence currents to flow from the line, the transformer must provide a return path to the system neutral (grounded star).
Delta windings serve to trap zero sequence currents, preventing them from appearing in the lines on the other side.
Provides a reference ground for protection relays.
Allows measurement of zero sequence components for earth fault protection.
Increases fault current magnitude during line-to-ground faults.
Requires careful insulation of the neutral point.
Transformer banks in transmission substations.
Grounding transformers for ungrounded systems.
In a Delta/Delta connection, zero sequence currents cannot flow from the line because there is no neutral connection to ground.
In a Star (ungrounded)/Delta connection, the lack of a neutral return path prevents zero sequence current flow from the source.
A is correct тАФ The Grounded Star/Delta connection provides a physical path through the neutral for zero sequence currents to return to the source.
Remember: If there is no path to ground (neutral), I0тАЛ cannot flow. If the transformer is delta-connected on the side of the fault, it will act as a trap for zero sequence current.