3500 kVA Transformer Nameplate Guide: Ratings, Vector Group & Impedance

The nameplate of a 3500 kVA transformer tells you the rated power, voltages, currents, vector group, impedance, cooling, insulation levels and mass. Read these values before you buy, install or connect the unit in parallel with another.

A 3500 kVA (3.5 MVA) transformer is a common size for industrial plants, substations, solar farms and utility feeders. A mistake on the nameplate, or a misreading of it, can mean a failed inspection or a damaged unit.

This guide explains each key item on the plate, with a worked example and a checklist you can use at delivery.

Why the Nameplate Matters

It Is the Transformer's Identity

The nameplate is a permanent record of what the factory built and tested. It is the first thing an inspector, utility engineer or maintenance crew will check.

It Protects Your Project

Protection settings, cable sizes, paralleling and spare part orders all depend on nameplate data. If the plate and your specification disagree, find out why before energizing.

Sample Nameplate: 3500 kVA, 33/11 kV

The table below shows an illustrative nameplate. Your values will differ with voltage level, standard and design.

Item	Example Value
Standard	IEC 60076
Type	Three-phase, oil-immersed, outdoor
Rated power	3500 kVA
Frequency	50 Hz
Rated voltage HV / LV	33 kV / 11 kV
Rated current HV / LV	61.2 A / 183.7 A
Vector group	Dyn11
Impedance voltage	6.5% at rated current
Cooling	ONAN
Tap range	±2 × 2.5% on HV
Insulation level HV / LV	LI 170 kV, AC 70 kV / LI 75 kV, AC 28 kV

Rated Power and Voltages

Rated Power (kVA)

Rated power is the apparent power the transformer can deliver continuously at rated voltage, frequency and temperature limits. It is given in kVA, not kW.

The real power you get depends on the load power factor. At 0.9 power factor, a 3500 kVA unit supplies about 3150 kW.

Rated Voltages

The nameplate lists the HV and LV rated voltages. The LV value is the no-load voltage, as it is on any transformer.

Tap positions change the HV voltage. The plate should show the full tap table, not only the principal tap.

Rated Current Calculation

The Formula

I = S ÷ (√3 × U)

Here S is rated power in kVA, U is line voltage in kV, and I is line current in amperes.

Results for the Example

Side	Voltage	Rated Current
HV	33 kV	61.2 A
LV	11 kV	183.7 A

At 0.4 kV, the same 3500 kVA unit would carry about 5,052 A on the LV side. That is why this rating is more common at medium-voltage levels.

Vector Group Explained

What It Tells You

The vector group shows how the windings are connected and the phase shift between them. A capital letter is the HV winding. A lowercase letter is the LV winding.

Reading Dyn11

D means the HV winding is delta. y means the LV winding is star. n means the LV neutral is brought out.

The number 11 is the clock position of the LV phasor. It means the LV voltage leads the HV voltage by 30°.

Why Dyn11 Is Popular

The delta winding traps third harmonics and gives a stable supply. The star side has an accessible neutral for earthing and unbalanced loads.

Other Common Groups

Vector Group	Phase Shift	Typical Use
Dyn11	LV leads by 30°	Distribution and industrial supply
Dyn1	LV lags by 30°	Some regional grids
YNd11	LV leads by 30°	Step-up and generator transformers
Yyn0	No shift	Small units, special networks

Impedance Voltage

Definition

Impedance, or short-circuit voltage, is the percent of rated voltage needed to drive rated current through the windings when the other side is shorted. For our example, 6.5% of 33 kV is about 2,145 V.

Why It Matters

Impedance controls two things. A higher value limits fault current but increases voltage drop. A lower value gives better regulation but raises fault current.

Fault Current Estimate

With an infinite source, the fault current is the rated current divided by the impedance.

Item	Calculation	Result
LV fault current	183.7 A ÷ 0.065	About 2,826 A
HV fault current	61.2 A ÷ 0.065	About 942 A
Fault level	3.5 MVA ÷ 0.065	About 54 MVA

Real systems have source impedance, so actual fault current is lower. Use these figures only for a first check of breaker ratings.

Voltage Regulation from Impedance

The Approximate Formula

Regulation is roughly R × cos φ + X × sin φ, where R and X are the resistance and reactance in percent. R comes from load loss, and X comes from impedance.

Worked Example

Assume a load loss of 27 kW. R is then about 0.77% (27 ÷ 3500). With 6.5% impedance, X is about 6.45%.

Load Power Factor	Approximate Regulation
1.0	About 0.8%
0.9 lagging	About 3.5%
0.8 lagging	About 4.5%

What It Means

Poor power factor makes the voltage drop much larger. On a plant with low power factor, the impedance value on the plate has a direct effect on voltage at the load.

Choosing Between Lower and Higher Impedance

Feature	Lower Impedance	Higher Impedance
Fault current	Higher	Lower
Voltage drop	Smaller	Larger
Breaker duty	More demanding	Easier
Best fit	Weak grids, motor starting	Strong grids, high fault levels

Choose the value that keeps fault current within breaker ratings and voltage drop within limits. State the target in your specification.

Tap Changer Data

Off-Circuit vs On-Load

An off-circuit tap changer is adjusted only when the transformer is de-energized. An on-load tap changer (OLTC) adjusts voltage under load and is common on utility transformers.

Example Tap Table

Tap	HV Voltage
+5%	34,650 V
+2.5%	33,825 V
Principal	33,000 V
-2.5%	32,175 V
-5%	31,350 V

Using Nameplate Data for Protection Settings

Rated Current Sets the Baseline

Overcurrent relays and fuses are selected from the rated current of each side. For our example, that is 61.2 A on the HV side and 183.7 A on the LV side.

Inrush Current

When a transformer is energized, magnetizing inrush can reach 8 to 12 times rated current for a short time. For this unit, that is about 1,800 A on the 11 kV side.

Relay settings must ride through inrush without tripping the breaker.

Impedance and Fault Duty

The impedance sets the through-fault current the windings must withstand. Check that the short-circuit withstand rating on the test report fits your system fault level.

Cooling Class and Temperature Rise

Cooling Codes

ONAN means oil natural, air natural. The transformer cools by natural convection of oil and air.

ONAF adds fans on the radiators. It usually raises the rating by roughly 20% to 30%.

If the plate shows two ratings, such as ONAN and ONAF, the higher one is available only with fans running.

Temperature Rise Limits

IEC 60076-2 sets limits for oil-immersed units: 60 K top-oil rise and 65 K average winding rise at the standard ambient. Check that the plate states these values.

Insulation Levels

What the Plate Shows

The plate lists the lightning impulse (LI) and power-frequency (AC) withstand voltages for each winding. These must suit the system voltage and the local surge risk.

Example Values

For a 33 kV winding, a typical set is LI 170 kV and AC 70 kV. For 11 kV, it is LI 75 kV and AC 28 kV. Confirm the exact values against IEC 60076-3 and your project specification.

Other Markings on the Plate

Identification Items

Look for the manufacturer name, serial number and year of manufacture. These link the unit to its test report and warranty.

Winding Diagram and Polarity

A connection diagram shows terminal marks and the internal winding layout. Use it when you connect cables or test the unit.

Oil and Accessories

The plate may state the oil type and volume. Accessories such as a Buchholz relay, pressure relief device and winding temperature indicator are usually listed on a separate plate or in the manual.

Site Conditions

Altitude and ambient temperature limits may be marked. A unit built for 1,000 m altitude may need derating at higher sites.

Losses, Efficiency and Mass

Loss Data

Some nameplates show no-load loss and load loss. If not, they appear on the test report. Both values affect running cost over the life of the unit.

Mass and Oil

The plate shows total mass, oil mass and often untanking mass. Use these for crane selection, foundation design and oil containment sizing.

Parallel Operation: What Must Match

Parameter	Requirement
Vector group	Must be identical
Voltage ratio	Same, within a small tolerance
Impedance	Within about 10% of each other
Rated power	Ratio no more than about 3 to 1
Frequency	Same

A mismatch in vector group causes a large circulating current and cannot be corrected by tap changes.

Nameplate Acceptance Checklist

Check at Delivery

Item	What to Verify
Rated power	3500 kVA, with ONAF rating if ordered
Voltages and taps	Match the purchase order and tap table
Vector group	Matches the single-line diagram
Impedance	Within the specified value and tolerance
Insulation levels	Match system voltage and standard
Serial number and year	Match test report and documents
Mass and oil volume	Match shipping and foundation data

Common Mistakes

Mixing Up kVA and kW

A 3500 kVA transformer does not deliver 3500 kW unless the load power factor is 1.0.

Ignoring Impedance Tolerance

Standards allow a tolerance on impedance, commonly ±7.5% to ±10% of the declared value. Include it in fault level and paralleling checks.

Overlooking the Vector Group

Two transformers of the same kVA but different vector groups cannot run in parallel. Check this before ordering a second unit.

Reading Only the Principal Tap

The rated current and voltage on the plate refer to the principal tap. At other taps, currents differ.

Frequently Asked Questions

What is a typical impedance for a 3500 kVA transformer?

Many 3.5 MVA units fall between 6% and 8%, depending on voltage level and the standard. Follow your utility or project specification.

Is Dyn11 or Dyn1 better?

Neither is better. Choose the group that matches your grid or the transformers it must run with.

Does ONAF mean I can load the transformer higher?

Yes, but only while the fans run and within the plate rating. Plan the fan power supply and control.

Can I change the vector group after manufacture?

No. The vector group is fixed by the internal winding connection. A different group needs a new design.

What should I send to a manufacturer?

Send the voltages, kVA, frequency, vector group, impedance, tap range, cooling, standard and site conditions. Add loss limits if you have them.

The nameplate of a 3500 kVA transformer is a short document with a lot of information. Read the ratings, vector group and impedance carefully, and compare them with your specification.

Use the checklist at delivery, and confirm paralleling needs before you order a second unit. Need help preparing a specification for a 3500 kVA project? Send your requirements to our team and we will recommend a suitable design.