This is about practising Electrical and Electronics Engineering. Though it is not my passion , it is my profession !!! In this section one can see posts with rare questions and surprising answers in practising Eletrical and Electronics Engineering. The author assumes that the readers and followers of this section have basic knowledge of Electrical Engineering and its equipment in the field.
Tinned Copper fuse wire is one of the common and simple fuse element used in semi enclosed fuses for maintaining the safety of electric equipment. The material used is Tin plated Copper. Copper is a good conductor, has good tensile strength and corrosion resistance. Hence bare copper wire is not a bad choice to use as a fuse. But, Tin plated copper fuse wires are much popular and widely used. Why ?
When Tin plating is applied over bare copper wire, it improves the strength and longevity of the copper beneath it. The corrosion resistive power of bare copper wire to humid and wet atmospheres gradually decreases when temperatures exceed especially above 100 degrees Celsius when carrying currents. In such circumstances, Tin plating on the bare Copper wire protects Copper beneath it and improves longevity and strength of the fuse wire. Tin coating does not allow oxidation easily. Tin coating on the fuse wire also makes it convenient for soldering. Even though Tin plated Copper wire is expensive than bare Copper wire, the expenditure will be prudent in the long run usage. Hence, TC fuse wires are widely used in electrical maintenance.
Advantages of TC fuse wire
It is cheapest.
It requires no maintenance.
It requires minimum time to replace.
Disadvantages of TC fuse wire
Due to prolonged exposure to atmosphere, oxidation makes the wire thin and fails.
Operation is unreliable and hence cannot be used for large fault currents.
There is a chance of misuse by using inappropriate rated fuse wire.
Loose connections causes local heating.
Specification & Ratings
The common specification of TC fuse wire is Standard Wire Gauge (SWG) in the market based on the diameter of the wire which is the main parameter to decide the current carrying capacity of the fuse wire. The current carrying capacity of a TC fuse wire is usually mentioned as ‘Rated Current‘ of the wire which indicates the maximum current that the fuse can continuously conduct without interrupting the circuit and ‘Fusing Current‘ which indicates the current at which the fuse blows isolating the equipment from the circuit.
The ratings of TC fuse wires as per IEE regulations are mentioned below for reference and practise of using correct rated TC fuse wires for the electrical equipment under protection.
TINNED COPPER FUSE WIRES
I.E.E Regulation 14th edition Table A1
Approximated size of Fuse Elements Composed to Tinned Copper wire for use in semi-enclosed Fuses.
Magnetic Oil Gauge (MOG) is fixed to the conservator tank of the Transformer to indicate oil level in the conservator tank. The oil levels are generally indicated on the MOG dial as ‘Empty’, 1/4, 1/2, 3/4 and ‘Full’. But in between these levels usually 300C temperature indication is also shown. This is because oil level in the conservator tank changes according to the ambient temperature. The hotter the temperature, the more will be the oil level in the conservator tank due to expansion and vice versa. So, as the oil level depends on the ambient temperature, we must have an indication to what level the oil is to be filled at a certain temperature. This is why the level is indicated in temperature degrees so that the when the ambient temperature is same as the temperature indicated on that MOG dial, the oil level should reach that mark to ensure that the Transformer has sufficient oil level in the conservator. Whenever oil is to be filled in the conservator tank, the level is to be checked properly and to be set according to the ambient temperature prevailing at the time of filling the oil.
DVDF (Double voltage and double frequency) test is popular test conducted on the Transformer before it is commissioned. In this test double the rated voltage and double the rated frequency is applied to the Transformer under test and observed for its withstand capability usually for 1 min. Double voltage is applied to test the Transformer withstand capability for higher voltages which are sometimes occur in the form of lightning surges and faults on the transformer. But, practically, no transformer is subjected to double the rated frequency as the frequency variations are not allowed beyond 0.5% by the power supply grid controlling authority. In such a case, is it essential to test the transformer at 200% i.e. double the rated frequency conditions?
When alternating electrical source is applied to the primary winding of the transformer, it draws magnetizing current which produces alternating flux in the core of the transformer. This flux links both primary and secondary windings and due its alternating nature EMF is induced across both windings and Erms can be deduced by the equation.
Erms= 4.44ᴓmfN volts
Where Erms is RMS voltage induced
ᴓm is maximum flux linked
f is the operating frequency
N is the number of turns in the winding.
As per the above equation, when double the rated voltage is applied while testing the transformer, without doubling the frequency, the maximum amount of flux linked will also be doubled as the number of turns is always constant for a particular design. This causes the abnormal heating of core of the Transformer under test and the magnetizing properties of the core are disturbed permanently. Hence, to avoid this abnormal heating of the Transformer due to increase in flux, applied frequency will also be doubled along with applied voltage to test the high voltage with stand capability of the Transformer. Thus the test is named as Double Voltage Double Frequency test.
It is often overlooked by us that whether sealed Transformer oil drums are stored in horizontal or in vertical positions. But, there is difference, especially when stored outside because of a phenomenon called ‘Breathing of Barrel’. Every sealed Transformer oil drum breathes in and breathes out due to variations in the ambient temperature conditions. Even though oil drum is completely sealed, during hot days the oil drum gets heated and the oil in the drum expands and pushes the air accumulated over the oil to its surroundings. When temperatures are cooled, the oil in the drum contracts and creates partial vacuum in the drum and sucks outside moisture mixed air into the drum. This breathing in and breathing out of oil drums happens even they are not opened once.
When oil barrels are stored vertically outside during rainy season, rain water settles on the top of the barrel with its bung submerged. During breathing in of the oil barrels, the water is sucked into the drum. Once, the water enters the drum, it gradually settles at the bottom and raises the oil level in the drum. As this process continues, more and more water settles at the bottom and deteriorates oil and sometimes deforms the shape of the drum also.
To avoid this, the following advices may be noted while storing transformer oil barrels.
As far as possible Transformer oil drums should not be stored outside.
Transformer oil barrel is to be stored in horizontal position so that; there is no accumulation of water on top of the drum during rainy season.
Transformer oil barrel is to be stored in horizontal position with its bungs in 3 O’clock and 9 O’clock positions, so that positive pressure is maintained at bungs as the oil level in the drum will be above the bungs in this position. This prevents breathing of drum there by entering of moisture and dirt from surroundings into the drum can also be avoided.
Oil drums are to be stored horizontally on good platform and covered.
First in and First out method should be followed.
As far as possible Transformer oil drums should be stored in clean, cool and dry places.
Oil surge relay (OSR) is a single float Buchholtz relay fixed to OLTC and its conservator tank. The purpose of OSR is similar to the Buchholtz relay and acts whenever sudden oil rush towards OLTC conservator tank happens due to flashover in OLTC tank. Generally, Buchholtz relay connected to the main tank contains two floats i.e. upper and lower, electrically connected to alarm and trip circuits. Apart from low oil level condition, upper float acts when gasses accumulate due to minor internal faults in the Transformer and lower float acts when sudden rush in oil towards to conservator tank is sensed due to major internal faults in the Transformer. In the OSR connected to OLTC, only single float is provided which acts due to oil rush caused from OLTC internal fault. If the upper float, which acts due to accumulation of gasses, is also provided in the OSR, many times gasses that are normally generated in the frequent operation of OLTC trips the Transformer without any internal fault. Hence only single float is provided in the OSR connected to OLTC to avoid unwanted interruption of supply to the Transformer and ensures safety tripping when any internal fault occurs or low oil level condition in the OLTC tank.
The silica gel breather cup of a Transformer is partially filled with oil to avoid entering of moisture into the Transformer. Every Transformer breathes in and out because they are filled with oil and tightly sealed with gaskets. As the oil expands due to rise in temperature, the pressure builds up , consequently, the vapour and dry air accumulated in the conservator are breathed out. During the fall of temperature, oil in the Transformer contracts and creates partial vacuum in the tank which makes the Transformer to breathe in air from the outside. In the process of breathing in outside air, moisture and dust particles in the outside air are also sucked in. But, the oil in the breather cup obstructs dust particles and settles them at its bottom. The moisture entered is absorbed by the silica gel in the breather.
The oil in the breather cup is only partially filled up to the mark given on the cup. If oil in the cup is over filled, while brething in, the oil gets into contact and absorbed by the silica gel resulting in bottom portion of silica gel is permanently wasted as the oil absorbed silica gel cannot be reactivated again.
If you observe any substation, the ground of the substation yard is filled with crushed gravel stones. This is because of the following reasons pertain to safety from shock.
1) Although substation grounding is chosen to provide low resistance path, the entire ground is filled with crushed stones to provide a high resistance layer so that it can act as an insulation between our foot and the ground.
2) The stone layer on the substation ground provide high resistance so that the fault currents flow into the ground but not along the ground.
3) To minimise step potential and touch potential voltages.
4) It avoids pool of inflammable oil etc. on the substation ground in case of any spilling of insulation oil from the equipment. This also avoids spreading of fire from one equipment to the other in the substation.
5) The rocky outer layer slows down the evaporation of moisture in the earth’s upper layers.
6) It restricts entering of snakes and other reptiles as the surface would be inconvenient to crawl.
7) It avoids growth of plants and weeds in the substation yard to some extent.
8) Generally 20 to 25mm baby gravel stones are used instead big size stones to facilitate movement of persons and equipment in the substation yard.