electrialstandards

Depending upon the time of operation the overcurrent relays are categorized as :- 1.        Instantaneous over-current relays 2.        Inverse time-current relays 3.        Inverse definite minimum time (IDMT) Over-current relay 4.        Very inverse relay and 5.        Extremely inverse relay Now let us study the same one by one:- 1.        Instantaneous Over-current relay:- This relay as clear from it’s name is that it has instantaneous tripping without any delay. Time of operation of such relay is about 0.1 sec. This type of characteristic can be achieved by using hinged armature relays. 2.        Inverse time-current relay:- In that type of relays operating time reduces as the actuating quantity ,i.e. current In case of over-current ,increases in magnitude. More pronounced the effect more inverse will be the characteristics. They are normally more inverse near the pick up value of the actuating quantity and become less inverse as it is increased. This type of characteristic

Electrical Formulas Electrical engineering required certain formula which must be known for an electrical engineer and even a non engineer person so to know certain parameters of appliances around everyone. Lets discuss 1 st about basic parameters used in electrical systems:- I   =   Amperes E   =   Volts kW   =   Kilowatts kVA   =   Kilo volt-Amperes HP   =   Horsepower % eff . =   Percent Efficiency pf   =   Power Factor For Single-Phase load here are the formula’s as below:- TO FIND:- §   Amperes when kVA is known –>     I = kVA x 1000 / E §   Amperes when horsepower is known –>    ( HP x 746) / ( E  x  % eff.  x pf ) §   Amperes when kilowatts are known –>    ( kW x 1000 ) / ( E x pf ) §   Kilowatts  –>    ( I x E x pf ) /1000 §   Kilovolt-Amperes  –>    ( I x E ) / 1000 §   Horsepower  –>    ( I x E x % eff. x pf  ) / 746 §   Watts   –>    E x I x pf §   Energy Efficiency   –>    Load Horsepower x 746 / Load Input kVA x 1000 §   Power Factor   @ cos θ –>

Power can be transferred through Overhead cables or Underground cables. In overhead lines inductance is predominant and in case of underground cables capacitance is predominant. There are always advantages of Overhead transmission lines in comparison to underground cables:- 1.      The conductor used in Overhead transmission lines is less expensive as size of cable required in overhead lines is less than underground cables. As overhead lines have better heat dissipation than underground cables. 2.      Insulation cost in case of underground cables is more than overhead lines. Overhead lines use bare conductors which are well supported and provide sufficient spacing between conductors. But in underground cables insulation is provided by various high grade paper tapes. A metal sheath is also provided so that moisture doesn’t enter the cable. Oil and inert gas is inserted so as to fill voids. Also storage vessels are needed to be installed after some intervals so to makeup the voids creat

Skin effect occurs in transmission lines due to unequal distribution of current over the entire cross section of the conductor being used for long distance power transmission. Skin effect usually occur in Alternating current not in Direct current flow. As Direct current is uniformly distributed across the section so skin effect doesn't takes place.But in alternating current current flow is non-uniform, where outer filaments of conductor takes more current than filament closer to the center. This will leads to higher resistance in conductors due to uneven distribution at alternation current then Direct current. The inner filament carrying currents gives rise to flux which links to inner filaments only where as flux due to current carrying outer filaments enclose both the inner as well as the outer filaments. The flux linkages per ampere to inner strands is more as compared to outer strands. Hence the inductance/impedance of the inner strands is greater than those of the outer strand

Corona Effect Corona phenomenon is the ionization of the surrounding air near power conductor. Free electrons are always present in free space because of radioactivity and cosmic rays. As the potential between the conductors is increased, the gradient around the surface of the conductor increase. The free electrons will move with certain velocity depending upon the field strength.These free electrons will collide with molecules of air and in-case they have high velocity they will dislodge the electrons from molecules and which will leads to increase in no. of electrons. This will form a electron avalanche.  In case the ratio of spacing between conductors to the radius of conductor is less than 15, flashover will occur between conductors before ionization. Corona loss occurs with hissing sound which is very clearly listened in HT lines. There sound is even very clearer during rainy seasons. Corona loss is given by formula as below:- Corona loss=  241X10 -5 f+25  (r/d) 1/2