Q3Power System - I
Question
Q.3. Explain the flow of zero sequence current in different three-phase transformer connections.
Answer
Zero-sequence current flow through a 3-phase transformer depends critically on winding connection: Y-Y grounded transformers allow zero-sequence current to pass through only if both neutrals are grounded; delta windings block zero-sequence current from flowing externally but allow it to circulate internally within the closed delta loop; and delta-delta transformers block zero-sequence current entirely on both sides.
The flow of zero-sequence current through a 3-phase transformer depends fundamentally on whether each winding is star (Y) or delta (Δ) connected, and whether any star point is grounded, since zero-sequence current in each phase is identical in magnitude and phase (all three flowing simultaneously in the same direction relative to each phase), requiring a genuine neutral or closed-loop return path to flow at all.
Star-Star (Y-Y) Connection
If both star points are solidly grounded, zero-sequence current can flow from the external network into one winding, through the transformer (via mutual coupling), and out through the other winding's neutral into the external network on that side — the transformer behaves, for zero-sequence current, simply as its normal leakage impedance connected between the two sides. If either neutral is ungrounded, that side has no return path for zero-sequence current, so the entire zero-sequence circuit is open on that side (no zero-sequence current can flow through the transformer at all in that condition), unless a tertiary delta winding is present to provide an alternative closed path.
Star-Delta (Y-Δ) Connection
If the star side neutral is grounded, zero-sequence current can flow into the star winding from the external network; due to transformer action, this induces a corresponding zero-sequence current in the delta winding, but since all three zero-sequence EMFs induced in the delta winding are identical (in phase), they simply drive a circulating current around the closed delta loop internally — this circulating current cannot escape to the external network connected to the delta terminals (since the delta offers no path to a neutral/ground point externally). Thus, the delta side is represented, in the zero-sequence network, as directly connected to the reference (ground) bus, while the star side connects through the transformer leakage impedance (plus any star-side neutral grounding impedance) to the reference bus — effectively, the delta winding acts like a short-circuited (to ground) termination as seen from the star side, for zero-sequence purposes, while blocking zero-sequence current from propagating to the delta side's external network.
Delta-Delta (Δ-Δ) Connection
With no star point present on either side, there is no path at all for zero-sequence current to enter or leave the transformer from either external network — the zero-sequence network is completely open-circuited on both sides. Any zero-sequence EMF that might be induced can only drive a circulating current within each delta winding internally (if a zero-sequence voltage is impressed by some other source elsewhere in that same delta-connected part of the network), but no zero-sequence current can flow between the two sides of the transformer or into either external network through this transformer.
This dependence of zero-sequence current flow on transformer winding connection is of major practical importance in fault analysis: it determines which parts of a power system provide a return path for ground-fault current, which strongly influences protective relay coordination (particularly for ground-fault/earth-fault relays) and dictates where zero-sequence current can and cannot be measured or expected to flow throughout an interconnected transmission and distribution network.