China Oil Immersed Grounding Transformer Manufacturer 15kV-35kV

The IEC cooling code naming rule uses the first letter to identify the internal cooling medium. ONAN and ONAF start with “O”, which means mineral oil is the primary internal coolant and are the standard options for oil‑immersed units.

AN means air natural cooling and AF means air forced cooling, and these two codes are originally defined for dry‑type transformers without liquid insulation.

For oil‑immersed earthing transformers, AN and AF are not the primary design codes; they are only listed as alternative references for special compact cabinet‑integrated layouts where air convection dominates the thermal dissipation.

When placing an order, ONAN or ONAF should be selected as the formal cooling classification to avoid confusion in tender documents and grid approval files.

Earthing transformer follows short‑time fault‑duty design philosophy, fundamentally different from conventional power transformer rated for continuous load. Its core design basis is short‑time neutral fault current together with fault clearing duration, rather than continuous power‑transfer capacity.

If users specify capacity according to continuous kVA of regular power transformer, the result will be either over‑sized with unnecessary high purchase cost and larger footprint, or wrong thermal design which cannot withstand required I²t fault energy during real single‑phase earth fault.

Proper specification work should start from system capacitive earth‑fault current, then derive required short‑time rating, instead of copying power‑transformer selection logic.

In general, parallel‑operation of multiple zig‑zag earthing transformers on one MV busbar is not recommended unless specially engineered. Each unit has its own manufacturing tolerance of zero‑sequence impedance. When paralleled, unbalanced zero‑sequence circulating currents will appear between transformers even without actual earth fault.

These circulating currents produce extra heating loss and may trigger mis‑operation of zero‑sequence protection relays. If project conditions really require parallel deployment, every unit’s zero‑sequence impedance tolerance, winding polarity and phase‑angle characteristics must be strictly matched, and detailed calculation on circulating‑current magnitude needs to be completed at design phase.

Most standard projects adopt one single earthing transformer for each isolated bus‑section as preferred solution.

Even though the optional auxiliary winding can deliver station‑service power under normal operating condition, its priority ranks below the main neutral‑earthing protection function.

During single‑phase earth‑fault event, large zero‑sequence currents flow through primary zig‑zag windings, which will induce considerable over‑voltage and over‑current on secondary side. Under fault condition, it is not allowed to keep critical station‑service loads running continuously via this auxiliary winding.

Project design should avoid relying entirely on the earthing‑transformer secondary winding for essential uninterruptible auxiliary power. Independent station‑service transformer or UPS power source shall be configured for important loads that cannot lose supply during earth‑fault incidents.

Its loss characteristics are highly related to operating characteristics: ① No-load loss refers to the iron loss (core hysteresis and eddy current loss) during normal operation. Due to long-term no-load/light load status, it is the main operating loss; ② Load loss refers to the copper loss (winding resistance loss) during faults, which is only generated during short-time faults and accounts for a very low proportion. The impact on operating costs needs to be divided by scenarios: For low-voltage and medium-voltage distribution networks (earthing transformers operate for a long time), the impact of no-load loss is obvious, and models with low no-load loss (such as national standard level 1 energy efficiency) should be preferred during selection; for high-voltage/extra-high voltage systems (earthing transformers work for a short time during faults), the proportion of no-load loss is small, and fault tolerance performance can be prioritized. Overall, its annual loss is much lower than that of ordinary power transformers, and the operating cost is relatively low