Backup protection for a Delta–Wye (Δ–Yg) transformer looks straightforward — put overcurrent relays on both sides and be done — but the transformer's own winding connection changes what the relays see. A fault on the Wye side does not appear the same way on the Delta side: phase faults come through as a 2:1:1 current split, and earth faults do not come through at all. This post explains what the backup relays must cover and why two special earth-fault relays are needed.

Figure 6(c): Backup protection of a Delta–Wye (Δ–Yg) transformer. Phase faults appear as a 2:1:1 distribution on the Delta side; earth faults need the 51G and 51N relays because zero-sequence current cannot cross the delta.

Phase faults: the 2:1:1 distribution

A phase-to-phase fault on the Wye side produces a 2:1:1 current distribution in the three Delta supply lines: one line carries twice the current of each of the other two. The fault current is therefore not shared equally, and a relay that only watched one phase could miss it. That is why three phase IDMT overcurrent relays (51R, 51Y, 51B) are provided on both the Delta primary side and the Wye secondary side, time-graded so that they act as backup if the main protection fails to clear an external phase fault.

Earth faults: why zero sequence cannot cross the delta

A line-to-ground fault on the grounded Wye side sends a zero-sequence current 3I₀ back through the Wye neutral to earth. But inside the Delta winding, the balancing zero-sequence current simply circulates inside the closed delta loop — it cannot exit into the Delta supply lines at all. So a relay on the Delta side looking at the line currents will never see a Wye-side earth fault, and a relay on the Wye side looking at the phase currents will see it only weakly. Two dedicated relays close this gap:

• (i) Neutral Standby Earth Fault Relay (51G) — connected to the CT on the Yg neutral, it measures the 3I₀ returning through the neutral directly. It gives sensitive backup protection for Wye-side ground faults, which is exactly the current the phase relays cannot see.
• (ii) Residual Zero-Sequence Relay (51N) — connected across the residual sum of the three Delta-side CTs, it responds to the zero-sequence current of the Delta system itself. It operates exclusively for upstream Delta-system ground faults, because zero-sequence current from the Wye side can never reach the Delta-side CTs.

The two relays are complementary: 51G watches the grounded Wye side's earth faults through the neutral, and 51N watches the Delta side's earth faults through the residual sum. Together with the six phase overcurrent relays, they give the transformer complete backup cover — phase and earth, on both sides, with nothing left unseen.

Key points to remember

• Phase-to-phase fault on the Wye side → 2:1:1 current distribution in the three Delta supply lines.
• Three phase IDMT overcurrent relays (51R, 51Y, 51B) on both the Δ primary and Y secondary sides, time-graded as backup.
• L–G fault on the grounded Wye side: 3I₀ returns via the neutral; inside the delta, balancing zero-sequence current circulates in the closed loop and never reaches the supply lines.
• 51G (neutral standby E/F relay on the Yg neutral CT) gives sensitive backup for Wye-side ground faults.
• 51N (residual zero-sequence relay across the three Delta-side CTs) responds only to upstream Delta-system ground faults.

Based on Power System Protection & Switchgear, Paper II, Q6(c), Exam 11 June 2026.