If you are involved in transformer (oil immersed) maintenance in your company, here is a list of common tests required to be done on oil immersed transformers according to IEC 60076-1 :

Increasing the cooling rate of a transformer increases its capacity to handle more power requirements. Various methods exist for cooling oil immersed industrial transformers to guarantee efficient operation. Cooling methods must not only maintain a sufficiently low average temperature, but must prevent an excessive temperature rise in any portion of the transformer. It must prevent hot spots within the transformer. For this reason, working parts of large transformers are usually submerged in high-grade insulating oil. This oil must be kept as free as possible from moisture and oxygen, dissolved combustible gases, and particulates

Transformer Cooling Methods
The efficiency of the cooling is a crucial factor determining the operational safety and the life span of a transformer. Below are the four common cooling methods for industrial transformers. Each method is illustrated with a diagram:

(a) Natural air cooling with radiators (ONAN)
(b) Radiators additionally cooled by fans (ONAF)
(c) Cooling by separate radiator banks
(d) Water as cooling medium

Instrument transformers are used for measuring and control purposes. They provide currents and voltages proportional to the primary, but there is less danger to instruments and personnel.

There are two distinct classes of instrument transformers: the potential transformer and the current transformer. 

Potential transformers (PTs) are used to step down high voltage while current transformers (CTs) used to step current down. The function of a PT is to accurately measure voltage on the primary, while a CT is used to measure current on the primary.

Potential (voltage) transformers have primary and secondary windings on a common core:

Standard potential transformers are single-phase and are usually designed so that the secondary voltage maintains a fixed relationship with the primary voltage. Potential transformers are used with voltmeters, wattmeters, watt-hour meters, power-factor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, undervoltage and overvoltage trip coils of circuit breakers.

Generally, a potential transformer is designed to be connected in parallel with the lines to transform and step down the line voltage to 115 or 120 volts for metering or relay operation. They are typically rated 50 to 200VA (volt-amperes) at 120 secondary volts. The secondary terminals should never be short circuited because a heavy current will result, which can damage the windings.

A current transformer transforms line current into values suitable for standard protective relays and instruments.  The primary of a current transformer has a  few turns, while the secondary may have a great many turns which results in the stepping down of current as shown in the schematic below:

Current transformers are used with ammeters, wattmeters, power factor meters, watt-hour meters, compensators, protective and regulating relays, and trip coils of circuit breakers. The secondary of current transformers are typically rated 5Amps.

Most times, current transformers have several taps on the secondary winding to adjust the range of current possible to measure on the primary.

Note if the secondary of a current transformer is opened, an extremely high voltage is induced in the secondary which is dangerous to personnel and can destroy the current transformer. For this reason, the secondary of a current transformer should always be shorted before removing a relay from its case or removing any other device that the current transformer operates. This protects the current transformer from overvoltage.

Transformers having only one winding are called autotransformers. This is the most efficient type of transformer since a portion of the one winding carries the difference between the primary and secondary currents.

An autotransformer has the usual magnetic core like a typical transformer but only one winding, which is common to both the primary and secondary circuits. An autotransformer schematic is shown below:

The primary is always the portion of the winding connected to the AC power source. This transformer may be used to step voltage up or down. If the primary is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding (as shown in schematic (a) above), the secondary voltage is �stepped-down.�  If only a portion of the winding is the primary and is connected to the supply voltage and the secondary includes all the winding(as shown above in schematic (b) above) then the voltage will be �stepped-up� in proportion to the ratio of the total turns to the number of connected turns in the primary winding.

The main disadvantage of the autotransformer is that the primary and secondary

circuits are electrically connected and, therefore, could not safely be used for stepping down from high voltage to a voltage suitable for plant loads.

Autotransformers find wide application in the following situations:

(a) Where the system supplied contains an identified-grounded conductor that is solidly connected to a similar identified grounded conductor of the system supplying the autotransformer.

(b) Where an induction motor is to be started or controlled

(c) Where a dimming action is required, as in the theatre lighting

(d) Where the autotransformer is to be a part of a ballast for supplying lighting units

The capacity or rating of a transformer is limited by the temperature that the insulation can tolerate.  The life of a transformer can be extended by making sure it is not operated over and above the temperature rating of the insulation system on a continuous basis. A guiding rule of thumb would be that the useful operating life of the transformer halves for every 10�C  rise above its rated temperature. 

The insulation system of a transformer is rated in degrees Celsius at its maximum temperature rating:

NEMA (National Manufacturer�s Association) has the following thermal or insulation classification as regards transformers (dry type):

Note  that a transformer with a Class 220 insulation system can be designed for a maximum temperature rise that is lower than the standard 150�C . It can be designed for either 125 or 80�C rise. Also, a Class 180 insulation transformer can be designed with 80�C rise. Class 155 and 105 transformers are not typically designed for other than their standard temperature rise

The maximum operating temperature is determined by by adding the rated ambient temperature of the device which is normally 40�C, the maximum temperature rise, and a 10�C hot-spot allowance:

Maximum operating Temperature 

Transformers are rated using several methods. Two common ratings of transformers include the:

(a) KVA rating

(b) Impedance rating

KVA ratings of transformers are obtained by simply multiplying the current times the voltage. The result is a rating in VA or volt amperes. Small transformers are rated in VA. As size increases, the ratings are adjusted accordingly to KVA (kilovolt amperes) or MVA (megavolt amperes)

1KVA = 1,000VA or volt amperes

1MVA = 1,000,000VA

Power transformers are defined as transformers rated 500 kVA and larger. Transformers smaller than 500 kVA are generally called distribution transformers.

The impedance of a transformer is the total opposition offered to an alternating current. When referring to impedance of a transformer, it is the equivalent impedance that is meant. To determine the equivalent impedance of a transformer, one of the windings, usually the low voltage winding is short circuited while just enough voltage is applied to the other winding to create full load current in the short circuited winding. This voltage is known as the impedance voltage.

Impedance rating is a characteristic of large power and distribution transformers. The transformer impedance value is often given in percent on the nameplate

It means that the voltage drop due to the impedance is expressed as a percent of rated voltage.

For example, suppose the impedance rating of a 2400V/240V transformer with full load current of 90 amps is given as 3%. 

Voltage drop due to impedance with low voltage winding short circuited 

= 0.03 X 2400 = 72volts

This means there would be a 72-volt drop in the high-voltage winding at full load due to losses in the windings and core. Only 1 or 2% of the losses are due to the core; about 98% are due to the winding impedance.

We can also calculate the actual impedance value on the high voltage side in ohms:

Where:

Z = impedance in ohms

V = impedance voltage drop

I =  Rated full load current in the primary of transformer

Now V = 72V, I = 90Amp

Therefore Z = 72/90 = 0.8?

Transformer function is based on the principle that electrical energy is transferred efficiently by magnetic induction from one circuit to another. Basically, a transformer consists of two or more windings placed on the same magnetic path. The winding being fed electrical energy is called the primary winding while the winding where the load is connected is called the secondary winding. A typical two winding transformer action is shown below:

When the primary winding of a transformer is energized from an alternating current (AC) source, an alternating magnetic field is established in the transformer core. Alternating magnetic lines of force, called �flux,� circulate through the core. With a second (secondary) winding around the same core, a voltage is induced by the alternating flux lines. A load, connected to the terminals of the secondary winding, results in current flow.

A transformer is made up of two basic non-moving parts:

(a) A laminated iron core

(b) Windings (primary & secondary)

The iron core of a transformer is made up of sheets of rolled iron. This iron is treated so that it has a high magnetic conducting quality (high permeability) throughout the length of the core. Permeability is the term used to express the case with which a material will conduct magnetic lines of force. 

The iron also has a high ohmic resistance across the plates (through the thickness of the core). It is necessary to laminate the iron sheets to reduce heating of the core. There are two common types of transformer cores:

(a) Core type

(b) Shell type

In the core-type (core form) transformer, the windings surround the core. In a shell-type (shell form) transformer, the steel magnetic circuit (core) forms a shell surrounding the windings. In a core form, the windings are on the outside; in a shell form, the windings are on the inside.

A transformer has two windings; the primary winding and the secondary winding. 

The primary winding is the coil which receives the energy.  It is formed, wound and fitted over the iron core. The secondary winding is the coil, which discharges the energy at a transformed or changed voltage.

Transformers are classified according to different criteria. However here is a list of the more common generic types of transformers:

(a) Single phase transformers

(b) Three phase transformers

(c) Potential or voltage transformers

(d) Autotransformers

(e) Current transformers

(f) Power transformers

The voltage on the windings of a transformer is directly proportional to the number of turns on the coils of the windings. This relationship is expressed by the formula:

Where:

Vp = voltage on primary coils, V

Vs = voltage on secondary coils, V

Np = number of turns on primary coils

Ns = number of turns on secondary coils

The ratio Vp / Vs is called the voltage ratio (VR). The ratio Np / Ns is called the turns ratio (TR). 

A voltage ratio of 1:4 (read as 1 to 4) means that for each volt on the transformer primary, there is 4V on the secondary. When the secondary voltage is greater than the primary voltage, the transformer is called a step-up transformer.

A voltage ratio of 4:1 means that for each 4V on the primary, there is only 1V on the secondary. When the secondary voltage is less than the primary voltage, the transformer is called a step-down transformer.

The current in the coils of a transformer is inversely proportional to the voltage in the coils. This relationship is expressed by the equation:

Where:

Ip = current in primary coil, A

Is = current in secondary coil, A

In the equation above we may substitute Np / Ns for Vp / Vs so we have:

The efficiency of a transformer is equal to the ratio of the power output of the secondary winding to the power input to the primary winding.

An ideal transformer is 100 percent efficient because it delivers all the energy it receives.  

However, because of core and copper losses, the efficiency of even the best practical transformer is less than 100 percent. Expressed as an equation:

Where: 

Eff = efficiency

Ps  = power output from secondary = Input power �core losses � Copper losses

Pp  = power input to primary

Efficiencies of well-designed transformers are very high, averaging over 98 percent (%) for power transformers. The only losses, in a transformer, are due to core losses which go to maintaining the alternating magnetic field, resistance losses in the coils, and power used for cooling for large transformers requiring cooling. 

The main reason for high efficiencies of transformers, compared to other equipment, is the absence of moving parts. Transformers are called static AC machines.