500 kVA Oil-Immersed Power Transformer: S11-M Fully Sealed Design

A 500 kVA oil-immersed power transformer is the workhorse rating of a low-voltage distribution network. In the fully sealed S11-M and S13-M families the tank is a permanently closed corrugated steel tank: the corrugated wall flexes to absorb the thermal expansion of the oil, so no conservator or oil storage tank is needed, the oil never meets ambient air, and the unit operates through its service life with only routine external inspection.

What a Fully Sealed Oil-Immersed Transformer Is

In a conventional oil-immersed transformer the oil surface breathes under a conservator, taking in moisture and oxygen as the load and the ambient temperature change. A fully sealed, hermetically closed design removes that interface. The corrugated tank wall expands and contracts elastically as the oil volume changes, the cover is bolted and welded to the tank rim, and the oil is isolated from air. The practical consequences for a distribution network are:

Slower oil ageing and slower degradation of the paper insulation, because oxidation and moisture ingress are largely eliminated.

No oil-level gauges to monitor and no oil treatment between maintenance cycles.

Lower overall height and a smaller footprint, since the conservator and its support structure are omitted.

Stable operation outdoors, in humid basements and in locations with wide daily temperature swings.

Construction of the S11-M and S13-M Series

Both series share the same sealed-tank concept and differ mainly in loss level and efficiency grade. The core is built from grain-oriented silicon steel laminations with mitered, step-lap joints to keep no-load loss and sound level low. Windings use electrolytic-grade copper conductor with insulation matched to the voltage class, and the complete active part is dried and vacuum oil-impregnated before the tank is closed. Heat is transferred by natural oil convection to the corrugated wall and, on larger ratings, by forced oil circulation. The design characteristics generally quoted for the series are low no-load loss, low load loss, low sound level and a long service life under continuous loading.

Technical Parameters Around 500 kVA

The reference values below apply to the S11-M series at 35 kV class, three-phase, 50 Hz. They are typical declared values for orientation during selection and must always be confirmed against the project specification and the ordered connection group.

Model	Rated capacity (kVA)	Connection group	No-load loss (W)	Load loss (W)
S11-M-50/35	50	Yyn0 or Dyn11	160	1200 / 1140
S11-M-100/35	100	Yyn0 or Dyn11	230	2010 / 1910
S11-M-200/35	200	Yyn0 or Dyn11	340	3320 / 3160
S11-M-315/35	315	Yyn0 or Dyn11	480	4750 / 4530
S11-M-500/35	500	Yyn0 or Dyn11	680	6910 / 6580
S11-M-630/35	630	Yyn0 or Dyn11	830	7860
S11-M-1000/35	1000	Yyn0 or Dyn11	1150	11500

Where two load-loss figures appear, the series is declared for two connection groups and the applicable figure depends on the group ordered. Load loss also depends on the reference temperature at which it is declared, so figures from different sources may only be compared at one stated temperature; the reference temperature must be agreed in the purchase specification. No-load loss and no-load current are temperature-independent and remain the primary indicator of core quality.

Connection Groups, Losses and Efficiency

Dyn11 is the usual choice for distribution duty: it provides a low zero-sequence impedance path, limits harmonic circulation and is widely specified for commercial and industrial networks.

Yyn0 is used where the low-voltage winding supplies a three-phase four-wire network with balanced single-phase loading.

Tap range is normally ±5% or ±2×2.5% on the high-voltage winding, selected off-circuit to suit the expected supply voltage band.

Impedance is specified to keep short-circuit currents within the rating of downstream switchgear while limiting voltage drop under motor starting.

No-load loss is determined by core material, lamination thickness, joint design and flux density; specifying a lower-loss core raises first cost and reduces the energy cost of continuous operation.

Selection, Installation and Maintenance Notes

Confirm rated capacity against the calculated maximum demand, allowing for future load growth rather than a short-term peak.

State the high-voltage and low-voltage ratings, vector group and tap range together, since they determine the winding design.

State the altitude and the maximum and minimum ambient temperature; both affect insulation clearances and permissible temperature rise.

Select the transformer oil grade according to the lowest expected ambient temperature, for example a low-pour-point grade for cold regions.

On ratings of 630 kVA and above, accessories such as a gas relay and a temperature controller are normally required; specify them explicitly for the rating ordered.

Allow ventilation clearance around the corrugated wall, keep the tank and neutral earthing intact, and support the unit on a level foundation that will not settle.

Routine servicing is limited to checking seals, paint condition, bushings, cable terminations and protective devices.

Frequently Asked Questions

Q: What is the difference between a fully sealed transformer and a conservator type?
A fully sealed unit has no conservator; the corrugated tank wall absorbs the change in oil volume, so the oil is not exposed to air and requires no oil-level monitoring.

Q: Why is the corrugated tank wall important?
The corrugations give the tank elastic compliance for oil expansion and at the same time increase the cooling surface, replacing the radiator bank of a conventional tank.

Q: What losses should be expected at 500 kVA?
For the S11-M series at 35 kV class, typical declared values are about 680 W no-load loss and about 6910 W or 6580 W load loss, depending on the connection group and the stated reference temperature.

Q: Which connection group suits distribution duty?
Dyn11 is the common choice for commercial and industrial distribution because it limits zero-sequence and harmonic circulation; Yyn0 is selected for balanced four-wire low-voltage networks.

Q: What oil grade is normally used?
The grade is chosen from the lowest expected ambient temperature, with a lower pour point specified for cold regions. The exact grade belongs in the purchase specification.

Q: How often does a sealed unit need maintenance?
There is no routine oil treatment or oil-level work. Servicing is limited to external inspection of seals, paint, bushings, terminations and protective accessories.