Common Tests Performed on Transformers

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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 :

Transformer Name plate – What information is required

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Transformers are usually provided with a rating plate of weatherproof material situated in a visible position. All entries on the plate are usually indelibly marked. According to the IEC 60076-1 standard for transformers, the following are the minimum information required for a transformer name plate. If you are involved in specifying transformers, purchasing transformers or work with transformers, here is a checklist for typical transformer name plate information:

How Transformers are Cooled

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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:

Instrument Transformers – Basic Operating Principles

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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 Transformer

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

What are Autotransformers?

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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:

NEMA Insulation Classes for Transformers

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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:

The class number  = the maximum °C of the transformer insulation

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

Basics of Transformer Ratings

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

(a) KVA rating

(b) Impedance rating

KVA 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.

How a Voltage Transformer Works

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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: