Laminations in Transformer Core and Motor Stator

Electric machines, especially AC machines such as transformers and alternators are exposed to alternating magnetic fields during operation. 

This alternating magnetic field causes the induction of eddy currents in the core of transformers and the stator of motors.  The eddy current creates a loss of energy in the form of heat loss and hysteresis loss.

In order to avoid this, the core of transformers and the stator of motors and generators are made of a set of laminated steel sheets. Silicon Steel is used.  This steel is cold rolled and has special grain orientation.  Each steel sheet is around .3 mm thick. 

The sheets are insulated on both sides and laid of top of one another.  This arrangement ensures that the eddy current is reduced as it cannot flow over a wide area of cross section.  The laminated surfaces need to be very clean.  Presence of foreign particles can cause laminar faults which lead to core damage. 

Ferranti Effect

Ferranti Effect refers to a phenomenon in long transmission wherein the receiving end voltage is higher than the voltage at the sending end.    The line capacitance in long transmission lines cause the higher voltage at the receiving end.  The Ferranti Effect is more pronounced when the transmission line is lightly loaded eg. at night. 

A transmission line draws a charging current from the current source.  This charging current causes a voltage drop due to line inductance which is in series.  This voltage drop is in phase opposition to the voltage at the receiving end.  Thus, it causes the voltage at the sending end to drop lower than the receiving end.

The Ferranti effect can also be observed in cables based transmission systems.  The effect is more in cables even for shorter lengths as cables have higher capacitances.

What is Electrical Steel

Electrical steel is a special type of steel used in the construction the cores of transformers and the stator of motors and generators.  This steel is also known as silicon steel.  It is an alloy of Iron with silicon. The silicon content can be upto 15%  The silicon increases the resistivity of steel minimizing eddy currents and the resulting heat loss.

Electrical steel also has a small hysteresis curve, reducing iron losses.  Heat treatment is done which increases the grain size of the steel and reduces the hysterisis loss.

In grain-oriented silicon steel, the orientation of the grain structure is made in a specific direction in order to increase the flux density and reduce the magnetic saturation.  This type of steel is used in the construction of transformer core where the direction of the magnetic field is predictable and in a specific direction.  The orientation of the grain structure ensures that all the molecules have poles are in the same direction.  This reduces the hysteresis loss.  This is usually more expensive.

In motors and generators though, the magnetic circuit is more complex, hence non-oriented silicon steel may be used.

In electric machines such as transformers, motors, etc., the steel is made in the form of thin sheets which are laminated on both sides. This is done in order to prevent eddy currents from circulating in the core. 

Electrical steel should be handled carefully.  Wrong bending or rough handling can adversely affect the magnetic properties of the steel.

Water in Transformer Oil

Water in Transformer oil is a serious issue which can reduce the insulating properties of the transformer oil.  Water also causes degradation of the paper based insulation.  This seriously and permanently damages the insulation of the winding.

Water enters the transformer through three ways

Through the Silica gel breather

If the silica gel is not monitored adequately, dry silica gel is always blue.  The silica gel should be replaced when it changes color.  If the silica gel is saturated, it may no longer filter the air and water can enter the transformer in the form of moisture.

Through improper sealing of accessories
Leaking gaskets, loose fitting components, cracked bushings can cause rain water to enter the transformer.

Water in Transformer oil exists in two forms as free water and as water in solution with the oil.  Pure oil and water do not mix.  However, over a period of time due to contamination and degradation the oil absorbs some water.

Disintegration of Cellulose
The insulation of the transformer winding is made of cellulose based paper.  At high temperatures, cellulose disintegrates and one of the byproducts is water.

Sampling the oil for Water content
The oil should be sampled for water content periodically.  It should be ensured that the sample collected is representative of the entire oil.  For instance, free water usually is at the bottom of the transformer tank.  Thus oil collected from the bottom valve may indicate a higher value of water content.  Thus samples should be collected from the upper and the lower valves of the transformer.

If the water content is found to be excessive.  The oil needs to be drained and dehydrated. 

Hot washing of insulators

Hot washing of insulators refers to cleaning the insulators in transmission lines when the lines are live. Transmission lines can afford very little downtime. Cleaning the hundreds of insulators has to be carried out when the lines are live with voltage.

Insulators get dirty due to dust, moisture, bird droppings and chemicals from smoke. These deposits will form a layer over the surface and can contribute to a flashover between the conductor and the grounded frame of the transmission tower. Thus, periodic cleaning of the insulators is essential.

Hot washing involves cleaning the insulator surfaces with de-mineralized water. De-mineralized has high resistivity (greater than 50000 ohm cm). The water is pressurized and sprayed in jets from special cleaning machines. These cleaning machines are stationed on the ground or in some cases fixed on helicopters which hover near the lines and clean the insulators.

The Hot washing is usually carried out from the bottom of the insulator. The whole insulator is not made wet at any given point of time. The bottom of the insulator is washed and then the washing proceeds to the middle sections and the then to the top of the insulator.

The water spray from cleaning one insulator should not fall on another insulator. This may cause a flashover. The wind direction should also be taken into account.

Paper Insulation

Paper being an insulating and dielectric medium finds wide application as an insulation for cables. Paper is made up of cellulose a very good dielectric.

Some of the common types of insulation paper are the kraft paper and the crepe paper.  kraft paper is obtained from the pulp of soft wood.  When this kraft paper is impregnated with liquids to enhance BDV and maintain pH, crepe paper is obtained.

Crepe paper is flexible and tough which enables it to be wound over sharp turns in the windings.  Semi-conducting insulation paper is used in some transformer winding to ground the leakage winding.  

Transformerboard is a kind of paper insulation which provides has a rigid structure.  It is widely used for transformer insulation.

Fish Paper is another kind of insulating paper which is vulcanized.  Fishpaper can even be machined to form a specific shape.  It has excellent insulating and mechanical properties