Question

Explain the potential hazards if liquid-cooled transformers are used.

What are the losses in transformers? Mention the various factors which affect these losses.

Liquid cooled transformer are not used on ship board installation due to the following reasons.

In case transformers is oil cooled

If the oil due to any reason leaks into the winding, it will cause fire, apart from shorting the winding.

Though the oil used for cooling may not ignite easily but once ignited will result in uncontrolled fire, forcing a threat to bigger fire and safety of personals.

Due to sea environment and continuous agitation, the oil is subjected to deterioration at ea, the oil may get contaminated and lose its properties, leading to overheated transformer.

Additional maintenance involved, as the oil would need to be changed at a specified interval. Transformer would then be needed to isolate which may not be possible most times.

May result in spillage resulting in slippage and undesired work.

There are 2 losses in transformer

Iron core loss

Core losses are caused by two factors:  hysteresis and eddy current losses.  

Hysteresis loss is that energy lost by reversing the magnetic field in the core as the magnetizing AC rises and falls and reverses direction. Eddy current loss is a result of induced currents circulating in the core.

Cu losses.

Copper  loss  is  power  lost  in  the  primary  and secondary windings of a transformer due to the ohmic resistance of the windings.   

Copper loss, in watts, can be found using Equation

Copper  Loss = I²_P  R_P + I²_S R_S

Where

I_P    =    primary current

I_S    =    secondary current

R_P  =    primary winding resistance

R_S  =    secondary winding resistance

Transformer losses are divided into losses in the windings, termed copper loss, and those in the magnetic circuit, termed iron loss.

Losses in the transformer arise from:

Winding resistance

Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.

Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.

Eddy currents

Ferromagnetic materials are also good conductors, and a solid core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness.

Magnetostriction

Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers, and in turn causes losses due to frictional heating in susceptible cores.

Mechanical losses

In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise, and consuming a small amount of power.

Stray losses

Leakage inductance is by itself lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat.