Earthing transformer creates an artificial neutral point for ungrounded 6 kV, 10 kV and 35 kV distribution networks. It works in coordination with arc‑suppression coils or neutral‑earthing resistors to mitigate earth‑fault capacitive current, suppress arc‑ground over‑voltage and ferro‑resonance, protect equipment insulation and improve power‑supply reliability.
With expanding cable‑based distribution networks, the system capacitive earth current may exceed 10 A. Single‑phase earth faults may generate intermittent arcs, which trigger destructive over‑voltages, phase‑to‑phase short‑circuits, VT burnout and surge‑arrester failure. Earthing transformers become essential equipment for safe network operation.
Three‑phase earthing transformers are mostly built with ZNyn (zig‑zag / Z‑connection). The integrated secondary winding can serve as station‑service transformer to save substation space and investment. Single‑phase earthing transformers are commonly matched with neutral‑earthing resistor cabinets for generators or main power transformers.
| Connection Group | Technical Features | Limitations |
|---|---|---|
| YNyn | Neutral point accessible on HV side. On 3‑limb core, zero‑sequence flux closes through tank and clamping structure and causes local overheating. | Permitted arc‑suppression‑coil capacity ≤ 20 % of transformer rating; poor capacity utilization. Rarely used as dedicated earthing transformer in China. |
| YNd | Delta secondary provides zero‑sequence circulating path with reduced zero‑sequence reactance. | When secondary is fully loaded, arc‑suppression‑coil capacity limited to 50 % transformer rating; delta winding is inconvenient for mixed lighting‑power loads. |
| YN‑open delta | Resistors/reactors can be connected across open‑delta terminals to adjust zero‑sequence impedance and suppress ferro‑resonance. | Cannot supply station‑service power; complex construction and higher cost; limited application. |
| ZNyn (Zig‑zag, Preferred Worldwide) | Zero‑sequence magnetomotive forces of two opposite‑direction windings on each core limb counteract each other. Zero‑sequence impedance reaches only 6‑10 Ω. It can carry arc‑suppression‑coil load up to 90‑100 % of transformer rating. The yn‑connected LV winding can supply substation auxiliary loads. | Z‑connection requires approx. 1.15 times more turns than wye‑connection; transformer rated capacity should be sized 1.05‑1.15 times of arc‑suppression‑coil capacity. |
ZNyn is the dominant connection for earthing transformers. Typical auxiliary‑winding rating ranges from 80 kVA to 200 kVA.
During single‑phase‑to‑earth fault, fault current flows via earth to the neutral point of earthing transformer and splits equally into three‑phase windings.
The zero‑sequence magnetomotive forces from oppositely‑arranged winding sections on each core limb largely cancel each other. Zero‑sequence leakage flux is minimized, yielding very low zero‑sequence impedance. The neutral point allows smooth flow of compensation current from arc‑suppression coil, while three‑phase line‑to‑line voltages remain symmetrical. The system is allowed to operate with earth‑fault condition for 2 hours.
Under normal balanced operation, positive‑ and negative‑sequence magnetic flux circulates inside core limbs, presenting high positive‑/negative‑sequence impedance and low no‑load losses.
Manufacturing tolerances prevent perfect flux cancellation, resulting in practical zero‑sequence impedance of 6‑10 Ω, far lower than ~600 Ω of ordinary wye‑connected transformers.
ZNyn zig‑zag earthing transformers are widely deployed for 6‑35 kV distribution networks in China to fulfill both neutral‑grounding compensation and substation auxiliary‑power supply. Zig‑zag earthing transformers are also popular overseas for distribution‑network earth‑fault protection. Direct neutral‑grounding for this equipment remains under research and has not been adopted in practical projects in China.
