Electricalreads

With the secondary shorted: The primary current of a C.T. is independent of its secondary loading   With the secondary shorted (directly or through the connected burden), the counter flux produced by the secondary keeps the core flux below the saturation level With the secondary open : Secondary  of  CT should never be open circuited when current is flowing in the primary winding      If the secondary gets open circuited with the primary carrying current, the entire primary m.m.f. (ampere turns) is spent in magnetizing the core, producing saturation   This results in increased secondary E.M.F. with the voltage shooting up to very high value depending upon the primary current level and the working/saturation flux levels Effects of open circuiting This secondary voltage may be sufficiently high to puncture the insulation The primary and the core also get over heated It presents the hazard of a dangerous high voltage shock to anyon...

It is an automatic switching off device It offers quick and reliable protection to the power system under fault conditions Different types of circuit breakers : SF6 circuit breakers Air blast circuit breakers Vacuum circuit breakers Bulk oil circuit breakers Minimum oil circuit breakers Advantages of SF6 circuit breaker SF6 breakers gradually replacing the conventional oil & air blast circuit breakers especially at voltage rating 132KV and above due to its following merits (a) The closed circuit gas cycle coupled with low gas velocity gives noise less operation (b) Outstanding arc extinguishing properties result in very short arcing times and hence less contact erosion (c) Drastically reduction in electrical clearances due to its excellent insulating properties (d) As totally enclosed and sealed from atmosphere, practically suitable for use in coal mines or in any such environments where explosion fire hazards exist Vacuum circuit breakers Vacuum is used as qu...

A non sinusoidal waveform can be considered to be composed of several sinusoidal waveforms of fundamental frequency and other harmonic frequencies The frequency of nth harmonic is nth times fundamental frequency Even harmonics, such as 2nd, 4th, 6th, etc. Odd harmonics , such as 3rd , 5th, 7th, etc. The r.m.s. value of nth harmonic is 1/nth of r.m.s. value of fundamental Causes of harmonics generation Caused by the use of high flux densities in the core of the transformers If the core is of insufficient cross section, the core gets saturated Secondary waveform may be non sinusoidal because of magnetic core Hence the primary requirement of transformer is to provide a sinusoidal secondary voltage waveform which is achieved by (a)    providing sufficient cross section of core and (b)    Achieving operating point in the linear region of B/H curve. Disadvantages of harmonics Harmonic currents cause additional  I2R loss , core loss, magn...

Efficiency is maximum at a load factor at which iron losses are equal to copper losses Efficiency changes with load factor The average load factor of a typical power transformer used in distribution system is about 0.5 and those used in generating station and major sub station is 0.8 For maximum efficiency with 50% load factor, copper losses should be 4 times the iron losses Even on higher loads, the efficiency would be above 96 % The efficiency reduces rapidly at lower loads than the load for maximum efficiency While designing a transformer, the weight of iron and copper are selected such that copper loss is equal to iron loss for average transformer load factor. This ensures high ή during the operation

Core laminated to reduce iron losses Eddy current circulating in the core is proportional to square of the thickness of lamination If the thickness of lamination is reduced, eddy current losses and hence heating of core is reduced Thin cold rolled grain oriented silicon iron sheets are used for core lamination

Core loss : resulting from the alternating core magnetization Dielectric loss : resulting in insulating material, oil & solid I²R loss: resulting in the windings Load ( stray ) loss : largely the result of leakage magnetic field including the eddy currents in the tank walls and conductors Iron losses: Due to eddy currents and hysterisis Iron losses can be reduced by using superior quality of cold rolled silicon iron sheets No load/core loss/open circuit loss/iron loss is specified at rated voltage Iron loss can be obtained from open circuit tests Copper loss : I2R loss resulting in the windings Can be reduced by using sufficient cross sections of copper for construction of winding/s Load loss/copper loss/short circuit loss is specified at rated current Copper losses can be obtained from short circuit test without operating the transformer on load How the hysterisis loss is minimized By using laminated steel sheets of high grade grain oriented steel sheets La...

The time required for a transformer loaded at rated KVA and with constant temperature of cooling medium to reach steady temperature is called time constant of  the transformer. It is expressed in hours For transformer capacity above 10 KVA For 11KV voltage class, it is 2.5 hours For 33KV voltage class, it is 2.75 hours    For 66 KV voltage class, it is 3.0 hours For 132KV and above, it is 3.5 hours

Free of suspended particles, water soluble acids and bases, active sulfur and colloidal carbon   Free of moisture as it lowers the dielectric strength   Low viscosity has more fluidity and give better cooling, shorter opening time   High flash point (145ºC)

Moisture contamination   Dirt   Excessive heat in contact with oxygen

To suppress the third harmonic voltages which would other wise appear on the system.   Provide a path for third harmonic currents to avoid interference with telephone circuits.   Tertiary often used to provide station power requirements or tie with synchronous condensers.

Fitted between the air space of conservator and outside vent for power transformer The breathing of transformer during load cycles takes place through breather. Filled with silica gel. The silica gel absorbs moisture. When the load increases, the load expands and the air from the conservator is expelled. During reduced load, the oil level in conservator falls down and the atmospheric air is breathed in side. This incoming air is taken through silica gel breather or activated alumina breather, Silica gel absorbs moisture. Dry silica gel is blue and turns pale pink as it absorbs moisture. Wet silica gel can be reactivated /regenerated by drying.

The conservator is connected to the transformer tank by means of  a pipe. During alternating load cycles, the oil of the transformer expands and contracts. The conservator volume is used as a cushion for accommodating expansion of oil during high loads.

Transformer's % impedance should be nearly equal. If  %  impedance are equal , the loading of these transformers would be proportional to their KVA rating. If  % impedance not same, the load sharing will not be proportional to KVA ratings. As % impedance cannot be exactly equal, it is preferable to operate transformers of same rating in parallel. As a general rule, ratio of KVA rating of transformer operating in parallel should be within 3:1.

If voltage ratios are not identical in transformers, for the same primary voltage, the secondary voltage would be different. It would result in circulating current within the secondary circuits and heating of transformer even on no load.

The polarity should be the same. The voltage ratio should be the same. The percentage impedance should be the same. The phase sequence (rotation) should be the same. The vector diagram and the phase displacement should be the same.

In auto transformer, a single winding is used for primary and secondary circuits. The volt amperes  of primary winding is equal to the volt amperes  of secondary winding. Advantages: Since the same winding is used for primary and secondary, it requires less material for the same output than that required by two winding transformer. Used in 400KV class  power transformers ( capacity 315 MVA/500MVA) and some time also for 220KV class power transformers. Used as a Variac in different electrical testing. Limitations: Low impedance much lesser than that of two winding transformer. Hence, high short circuit currents for short circuits on secondary side. In auto transformer, if section of winding common to primary and secondary is opened, full primary voltage will come across secondary, resulting in higher voltage on secondary causing chances of accidents.

A transformer consists of  a closed magnetic core and two or more windings When one of the wingdings( primary) is connected to AC supply, AC magnetic flux is produced in the core The flux linkage of other winding on the same core changes due to alternating nature of the flux in the core Thereby  e.m.f. is induced in the other winding called secondary winding The e.m.f. is induced in the transformer by electromagnetic induction effect of alternating magnetic flux.

Type test Intended  to check the design characteristics one unit of equipment i.e. performed to Confirm its performance as per design & ratings Are performed on a single unit of equipment Of one type, manufactured by a firm to a given Specifications. Routine Test Performed on each & every unit of one type of equipment to confirm/ensure with the type tests since identical in design Are made on every unit before it leaves factory to ensure that it is in accordance with the specifications.

Presently it is used in all EHV sub station of Power system. Three buses are Main-I, Main-II & Transfer Bus in Power system. Low initial & ultimate cost. Any breaker can be taken out of service,  one at a time, for maintenance or in case of fault, keeping the feeder in service. Requires one extra bay of Transfer bus with separate breaker for clearing the faults on transferred load in sub-station.

Schematic drawing: Generally utilized in EHV sub stations (400KV and above) Needs three breakers for two circuits The number of breakers per circuit is 1½. Hence, the name is given as one and a half breaker scheme. ½ breaker is more per circuit Such arrangement is particularly suitable for switchyard   in large generating stations where very high power block is to be handled by individual circuit ( say 500MW) In GEB, one and a half breaker arrangement scheme is at Wanakbori TPS on 400KV circuits. In PGCIL, it is at Dehgam S/S. The higher cost is justified because of higher security and bypass facility. Either of the two buses (Main bus I or Main bus II) can be taken out of service for maintenance Any circuit can be switched off for the purpose of maintenance without the provision of bypass Bus failure does not remove any circuit from service. 

Insulation Resistance (IR) IR of an electrical equipment refers to the resistance measured between conducting part and earth  or between two circuits separated by insulation It is expressed in MW It is of the order of several tens/ thousand MW or infinity for healthy insulation It reveals quality of insulation of the equipment and the degree of dryness/moisture The subsequent high voltage test can be avoided if  ‘IR’ value found is low Polarizing  Index (PI) The subsequent high voltage test can be avoided if  ‘IR’ value found is low                 IR (10 minutes )    i.e.PI=  ---------------------                 IR (1 minute )  It being a ratio, it is unit less  Its value depends on class of insulation of equipment. For class A insulation, PI should be ≥1.5 and for B clas...

Insulation resistance Resistance measured between conducting part and earth or between two circuits separated by insulation. Measured by means of megger/ megaohm meter. It is of the order of tens/thousands megaohms or infinity for healthy insulation. It reveals quality of insulation of the equipment and degree of dryness/moisture. Earth resistance Measured  for an earthed metallic body or point. Measured by means of earth resistance tester. For good effective earthing, the earth resistance should be of the order of 0.5 Ω to 2.0 Ω. It gives effectiveness of earthing system.

Insulation resistance (IR) of an electrical equipment refers to the resistance between conducting part and earth. It is expressed in MW It can also be measured between two circuits separated by insulation It is measured by Insulation measurement kit(Megger). (500/1000/2500/5000V) IR reveals quality of insulation of the equipment and the degree of dryness/ moisture. The subsequent high voltage test can be avoided if IR is low. How to measure Insulation resistance   Megger consists of a built in mega ohm meter and a D.C. generator. The generator is driven by hand or by a motor. One terminal of megger is connected to conducting part and the other terminal is connected to earth frame. The rotor of the generator is driven by hand or motor. The reading  ( Vdc/ Idc)  read by the megger  gives IR value in MW. The scale of megger is graduated from 0 to µ in MW. When D.C. voltage is applied to the insulation, initially the insulation draws capacitive charging cur...

It is the shortest distance between two conducting parts along the surface of the insulating material

The rate of change of electric intensity at a point along the direction of the field is called potential gradient at that point. Its unit is volts/meter

Distance between ground and the highest earthed point on equipment is called ground clearance.

BIL : Basic Insulation Level It indicates the ability of the insulation of the equipment to withstand the electrical stresses It is the combination of rated voltage, the corresponding impulse withstand voltage and corresponding power frequency withstand voltage which together categorize the insulation quality of the equipment For the sake of convenience the rated insulation of a equipment is designated by rated voltage and impulse withstand voltage.