Nitrogen in Transformers

Oil filled Transformers usually have a conservator which maintains the level of oil.  The conservator has a layer of air on top of the oil.  This air communicates with the atmosphere through a breather containing a desiccant,usually, silica gel. 

In some transformers, the tank of the transformer is filled with a layer of nitrogen above the oil in which the windings and the core are immersed.  Nitrogen, being an inert gas, reduces the risk of a fire hazard.  It also protects the transformer oil and the transformer insulation from reacting with oxygen in the air and deteriorating. 

In transformers containing nitrogen, the nitrogen is led into the space above the conservator through a pressure reducing valve.  When the oil level in the transformer rises and the transformer breathes out, the nitrogen inside is released into the atmosphere.  When the oil levels fall due to a reduction in ambient temperature or the load, fresh nitrogen is released into the conservator through the cylinder and the valves. 

Vacuum Circuit Breakers

Vacuum Circuit breakers are a very popular type of circuit breaker used in the industry.  Vacuum Circuit Breakers or VCBs as they are popularly known use vacuum as the quenching medium.

Vacuum Circuit breakers work by opening the contacts in an evacuated chamber.  The vacuum in the chamber ensures that there are very few ionisable molecules which can sustain the arc.  The arc is, thus, unable to sustain itself and is extinguished.

Vacuum circuit breakers have been used up to 36 kV and can interrupt up to 4000A.  When the arc is initiated during contact separation, metallic vapour is produced from the contacts.  If the contacts are of soft metal, very little vapour is produced.  While this ensure quick arc extinction.  The current waveform can get chopped and lead to high voltage transients.  If too soft metals are used in the contacts, the arc would cause erosion of the material.  Hence, the contacts are made of materials which are neither too soft or hard.  Common materials used are alloys Copper-bismuth or copper-chrome.

C Rating and battery capacity.

The capacity of a battery is indicated in Ampere-hours (Ah).  For instance, 200Ah, 80 Ah, etc.  The rate at which the battery can be discharged is indicated by the C rating.  

Batteries are usually rated as C/10, c/8, etc.  a battery with a 200 Ah capacity and a C/10 rating indicates that the battery can supply 20 A for 10 hours. 

The capacity of a battery varies inversely with the discharge rate. Batteries usually have different discharge rates for a fixed capacity. 

Combined Instrument Transformers

Combined Instrument Transformers are special measuring transformers which combine the functions of a current and a voltage transformers.  They are generally used where space is a constraint.  Typical scenarios are when measuring transformers need to be added in a functioning substation.  

Combined instrument Transformers usually have multiple secondaries for both current and voltage measurement.   These specialized transformers have been used upto 300 kV.

What are Primary and Secondary Cells ?

Primary cells are cells in which the chemical reaction which produces the power is not reversible. That is, the cell cannot be charged.  A common example is the dry cell.  Primary cells are cheaper and find application in radios, flash lights, torches, etc.    

Secondary cells are those cells in which the chemical reaction can be reversed by passing an electric current in the reverse direction.  The active material of the battery is reformed and the battery is recharged.  

Rod Earthing

Rod Earthing is an extremely popular form of Earthing as they are easy to install and retrofit.  Rod earthing involves driving metal rods made of copper, copper clad steel or stainless steel.  

The Rods used for earthing are usually about a metre long with a diameter of around 12 mm.  The rods should be driven to a depth of around 2.5 metres.  Usually the rods have threaded sections on one end so that the rods can be driven one on top of another to achieve the required resistance.

The Rods can be hammered into the ground.  This would be possible in sandy soil.  If the soil is hard, a Earthing rod driver which is a tool, usually pneumatic, which drives the earth rods into the soil is used. 

Capacitance Grading

Capacitance Grading is a method of distributing the electrostatic stress across the insulation of cables by using materials with different dielectric strength. When a cable is live, the insulation around the conductor is in a state of electrostatic stress. 

This stress is high near the centre and reduces towards the periphery. This uneven electrostatic stress can cause the failure of insulation. To prevent this kind of catastrophic failure, the insulation is graded. The permittivity of the insulating material is made to vary inversely as the distance from the centre. 

It is not possible to have a single material which has a permittivity which varies depending on the distance. Hence, the insulation is made of a number of layers of insulation made from different materials. 

Each of these materials has a different value of permittivity.   This ensures that the insulation is approximately uniform throughout the cross section of the insulation.