How to Read a Transformer Nameplate: A Complete Guide

How to Read a Transformer Nameplate: A Complete Guide

A transformer nameplate is not just a label. It's the unit's ID, its technical summary, and its installation guide - all in one place. If you buy, install, or maintain transformers, knowing how to read a nameplate is a basic survival skill.

The model number is the first thing you see on a nameplate. It's usually a string of letters and numbers, like SZ-8000-33-11 or SFS-25000KVA. This isn't random - it encodes the transformer type, capacity, and voltage class.

Letters (based on Chinese GB standard):

S: Three-phase. For single-phase, it's usually D.

Z: On-load tap changing. If it's off-circuit tap changing, this position usually has no letter.

F: Three-winding. For two-winding, this position usually has no F.

S (second position): Indicates the windings are three-phase.

Other common letters:

O: Autotransformer.

G: Dry-type transformer.

C: Cast resin.

P: Forced oil circulation.

D: Single-phase.

X: Special purpose (e.g., rectifier transformer, furnace transformer).

Numbers:

First group: Usually rated capacity. 8000 means 8000 kVA.

Second group: Usually voltage class. 33-11 means 33kV on the HV side, 11kV on the LV side.

Third group: Sometimes a design code or special identifier.

Why the model matters:

Quick identification: Experienced engineers can tell the basic specs from the model alone.

Selection reference: When choosing a transformer, the model helps you quickly check if it fits the project.

Spare parts and service: Give the model to the manufacturer, and they can pull the drawings and parts for you.

Examples:

SZ-8000-33-11: Three-phase, on-load tap changing, 8000 kVA, 33kV/11kV.

SFS-25000KVA: Three-phase, three-winding, 25000 kVA.

SCB10-1000/10: Three-phase, cast resin dry-type, 1000 kVA, 10kV.

Measured in kVA or MVA. A 10000 kVA unit means it can handle 10,000 kilovolt-amperes. This is the most important number for load planning. If you exceed it, the transformer overheats and its life shortens.

Usually written as high voltage / low voltage, like 33±2×2.5%/11 kV. The ±2×2.5% part tells you the tap range. This means the high-voltage side can be adjusted by ±2 steps of 2.5% each to match grid voltage.

The connection symbol tells you two things: how the windings are connected, and the phase difference between high and low voltage.

Letters:

Uppercase letters refer to the high-voltage winding.

Lowercase letters refer to the low-voltage winding.

D / d: Delta connection.

Y / y: Star connection.

N / n: Neutral brought out.

Z / z: Zigzag connection, often used to reduce harmonics or handle unbalanced loads.

Numbers:

The number is the "clock position." It shows how much the low-voltage side lags the high-voltage side. Each step is 30°.

0: In phase, 0° difference (e.g., Yyn0).

11: LV lags HV by 330° (e.g., Dyn11).

1: LV lags HV by 30° (e.g., Yd1).

5: LV lags HV by 150° (e.g., Yd5).

Common examples:

Dyn11: HV delta, LV star with neutral, LV lags by 330°. The most common distribution transformer connection.

YNyn0: Both HV and LV star with neutral, in phase. Often used where the HV side needs grounding.

YNd11: HV star with neutral, LV delta, LV lags by 330°. Common in HV and EHV power transformers.

YNyn0d5: Three-winding transformer. HV star with neutral, MV star with neutral, LV delta, LV lags by 150°.

Why it matters:

Two transformers can only be paralleled if their connection symbols match. If one is Dyn11 and the other is Yyn0, paralleling them directly will cause a short circuit.

The current the transformer can carry continuously on each side. Written in amps. This helps you size cables, breakers, and protection settings.

Impedance voltage (also called short-circuit impedance) is usually expressed as a percentage, like 5.82% or 8%. It's the voltage applied to one side of the transformer to produce rated current on the other side when the second side is short-circuited, expressed as a percentage of rated voltage.

What it affects:

Short-circuit current: Lower impedance means higher fault current. A transformer with 5% impedance will have a short-circuit current about 20 times its rated current; at 8%, it's about 12.5 times. This directly determines breaker and protection settings.

Voltage drop: Lower impedance means better voltage regulation under load changes. But too low, and the fault current becomes very high, stressing equipment and cables.

Parallel operation: Transformers running in parallel must have similar impedance. If one is 5% and another is 8%, load sharing will be uneven - the lower-impedance unit will overload.

System stability: High impedance means poor voltage regulation; low impedance means high fault current and higher equipment cost. Selection is a balance between the two.

Typical ranges:

Distribution transformers (10kV class): usually 4% to 6%.

Medium and large power transformers (35kV to 110kV): usually 6% to 10%.

HV and EHV transformers (220kV and above): usually 10% to 20%.

The cooling method determines how long the transformer can run and how much load it can carry. Here are the common types:

AN (Air Natural): Natural air cooling. The most common method for dry-type transformers. No fans, just natural convection. Simple structure, low maintenance.

AF (Air Forced): Forced air cooling. Fans are added to dry-type transformers, increasing capacity by about 40% to 50%. If the fans stop, the transformer automatically drops back to its AN rating.

ONAN (Oil Natural, Air Natural): Natural oil circulation, natural air cooling. Oil circulates by convection, heat dissipates through radiators. No fans or pumps. The most basic oil-immersed cooling method, usually for small to medium capacity.

ONAF (Oil Natural, Air Forced): Natural oil circulation with fans. Fans blow air across the radiators, increasing capacity by about 20% to 30%. With fans running, the transformer can run above its ONAN rating, but continuous operation should still stay within design limits.

OFAF (Oil Forced, Air Forced): Forced oil circulation with forced air cooling. Pumps circulate the oil and fans blow air across the coolers. Higher cooling efficiency, used for large capacity transformers.

ODAF (Oil Directed, Air Forced): Directed oil flow with forced air cooling. Pumps direct oil into the windings. The best cooling performance, used for very large or special-duty transformers.

If the nameplate says ONAN/ONAF, it means the transformer has two ratings: one for ONAN operation (fans off) and one for ONAF operation (fans on). When selecting protection settings and planning load, check both ratings - don't just look at the higher one.

Something like LI/AC 75/35. LI means lightning impulse withstand voltage, AC means power-frequency withstand. These numbers matter for insulation coordination and surge protection.

Total weight, core and coil weight, oil weight - these matter for transport, foundation design, and crane capacity. A 50-ton transformer needs a different logistics plan than a 5-ton one.

Look for IEC 60076, GB 1094, or IEEE C57.12.00. These tell you what standard the unit was built to. The serial number is your reference for warranty, spare parts, and service history.

Quick Tips

Always photograph the nameplate before installation.

Check that the nameplate matches the actual application - voltage, capacity, connection.

If the nameplate is unreadable, ask the manufacturer for a duplicate.

Keep a digital copy for every unit in your fleet.

Reading a transformer nameplate takes five minutes. Not reading it can cost you a failed unit, a burned cable, or a project delay. If you need help identifying a transformer or sourcing a replacement, contact us.

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