Making sense of a transformer nameplate data

The nameplate of a power transformer contains the following details as per standard, then additional information could be provided varying from manufacturer to manufacturer.

Name Of Manufacturer

Serial Number

Year Of Manufacture

Connection Symbol – This tells you the HV winding and LV winding configuration of a transformer and difference in phase angle between them. Example DYN11 where the first letter represents the HV winding and indicates that it is connected in delta, the second letter represents the LV winding and indicates that it is connected in Star(wye)  and the third letter N indicates that the LV winding has a Neutral while 11 denotes a 30 degree lead in phase angle. This article explains more on the Vector group of a transformer and why it is important.  (Transformers connected in parallel must have the same vector group i.e. same phase angle shift to avoid circulating currents. This is a situation where one source will become load to the other source.)

Maximum Allowable Temperature Rise – this is the average temperature rise above the ambient temperature that would occur in the winding when the transformer is loaded at its nameplate rating. A transformer with a 55°C temp rise would stabilize its temperature at 55°C above ambient temperature.

Rating in kVA – This is the amount of power the transformer can deliver a without exceeding the temperature rise indicated on the nameplate rating i.e. if you have a 3 phase, 440V transformer rated 250kVA with 55°C temperature rise.  The temperature of the transformer would not rise above 55°C of the ambient temperature as long as the total current pulled from the transformer doesn’t exceed 328A.

Frequency – Every transformer is designed to operate at a particular frequency; 50Hz or 60Hz. If the transformer operated at a lower frequency losses will increase causing the transformer to overheat under load or/and run hot on no load.

Number of phases – The transformer could either be 3 phase or single phase.

Cooling type – Losses in the transformer appear as heat and it may burn the transformer if allowed to build up. The nameplate would carry a piece of information like ONAN meaning Oil Natural Air Natural.

Let me explain the four letters represent the medium and mechanism with which the transformer is cooled  

First Letter Cooling Medium Internal (O – for Oil)

Second Letter Cooling mechanism Internal (N for Natural)

Third Letter Cooling Medium External (A – for Air)

Fourth Letter Cooling Mechanism External (N - for Natural)

There is a table to guide you

First Letter	Letter	Description
	O	Liquid usually oil with a flash point less than or equal to 300°C
	K	Liquid with a flash point greater than 300°C
	L	Liquid with no measurable flash point
Second Letter	Letter	Description
	N	Cooled by Natural convection. Hot oil naturally flows to the upper portion of the transformer tank and the vacant place is occupied by cold oil
	F	Forced circulation - The oil in the tank is flowing around with the help of a pump used to circulate oil within the closed loop of transformer tank.
	D	Forced circulation directed through windings and radiator.
Third Letter	Letter	Description
	A	Air
	W	Water
Fourth Letter	Letter	Description
	N	Natural
	F	Forced circulation( fan cooled)

Insulation Class – The capacity of a transformer is limited by the temperature that the insulation can tolerate. The insulation class corresponds to a particular temperature rating, and for longevity, the transformer should not be operated above its temperature rating continuously.

The useful operating life of the transformer is cut in half for every 10°C increase above its rated temperature. The insulation class of a transformer would tell us the maximum allowable operating temperature

Insulation System Class	Standard Temperature rise	Maximum allowable operating temp.
Class 220 (H)	150 degree Celsius rise	220°C
Class 180 (F)	115 degree Celsius rise	180°C
Class 150 (B)	80 degree Celsius rise	150°C
Class 105 (A)	50 degree Celsius rise	105°C

Percentage Impedance – If the secondary side of a transformer is shorted, the voltage required on the primary side to circulate the rated full-load current on the shorted secondary side is known as the Impedance Voltage. The impedance voltage is used to calculate the % impedance of the transformer using the formula below:

%Z= (Impedance Voltage/Rated Secondary Voltage)*100

This information can be used to determine the voltage drop that would occur in the transformer at full load loading. It can also be used to calculate maximum short circuit current the transformer would deliver if there is a short circuit in the secondary circuit.

Approximate Weights of the transformer – The nameplate of many transformers normally show the approximate weight of the transformer Coil, Core, Oil, and tank.

Voltage Rating of Primary and secondary. (Tap Voltages) -This shows the no load values of the HV and LV side of the transformer. You can see something like this 11000 -3300Y/1905 this would mean HV = 11,000.  LV = 3300KV Phase to Phase and 1905V phase to neutral.

You could come across can see a nameplate that lists it with more clarity like HV = 11000V, LV – 415Y/240. For a transformer with taps, the details of taps would also be given along with the Full Load Amps.

Let the table below guide you 

Voltage Marking	Meaning
-	Indicates voltages from different windings eg 6600 – 440 is 6.6KV in the primary side and 440 on the secondary side.
/	Indicates voltages from same winding example LV: 415/240 is 415V in the LV side and 240V to neutral or center tap
x	Indicates voltages obtained by series or parallel connection of two-part windings example 240x120 Two part winding that can be connected in series for 240 V and parallel for 120 V
Y	Indicates a Star (wye) connected winding.

Conducting material - The type of material used to make the windings of the transformer would be listed in the nameplate too, an example would read HV/LV Conductor = AL/AL used for each winding.

Basic Insulation Level (BIL) – BIL rating is also known as impulse withstand voltage, it specifies the magnitude of the voltage surge that a transformer can tolerate without any damage to the windings and insulation of the transformer or the maximum voltage stress the transformer can handle.