There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why?
The motor has max load current compare to that of transformer because the motor consumes real power.. and the transformer is only producing the working flux and its not consuming.. hence the load current in the transformer is because of core loss so it is minimum.
what is buchholz relay and the significance of it in to the transformer?
Buchholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay. whenever any internal fault occurs in a transformer, the buchholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself other wise it trips the circuit by its own tripping mechanism.
What will happen if DC supply is given on the primary of a transformer?
Mainly transformer has high inductance and low resistance.In case of DC supply there is no inductance ,only resistance will act in theelectrical circuit. So high electrical current will flow through primary side of the transformer.So for this reason coil and insulation will burn out.
Why transformer rating in KVA instead of KW?
Any device is rated depending on losses of the machine. In a transformer the losses are iron losses which depends on voltage and copper losses which depends on current so there are no losses which depend on power factor hence transformer is rated in KVA. There are 2 losses in transformer. One is copper loss which depends on current and the other is iron loss which depends on voltage. These two factors are not affected by the power factor. This is why transformers are rated in KVA and not KW. Single phase KVA = Amps x Volts/1000. Single phase KW = Amps x Volts x pf/1000. 3 phase KVA = Amps x Volts x 1.73/1000. 3 phase KW = Amps x Volts x 1.73 x pf/1000 yes there are two type of losses in transformer is that ohmic losses and core losses . ohmic losses due to transformer current and core losses due to transformer voltage. these losses are due to transformer voltage and current. and these losses not depends on load ie KW that’s the way its rating in KVA.
Explain the principle of Power transformer
- A transformer consists of a closed magnetic core and two or more windings
- When one of the windings( 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 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.
What is an auto transformer ? What are its advantages and its limitations?
- 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.
What are the essential requirements for parallel operation of two or more power transformers ?
- 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
Why should the voltage ratio of the transformers for parallel operation be same?
- If voltage ratios are not identical, 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
Is it essential to have same rated KVA/MVA for two transformers for parallel operation? OR Why impedance of parallel transformers be the same?
- No
- But their % 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
What is the function of Conservator ?
- 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
What is the function of Breather ?
- 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
- 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
State the functions of tertiary
- 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
What is polarity of transformer winding ?
- Polarity of winding refers to positive and negative terminals with reference to induced voltage
What are the main causes of insulation deterioration ?
- Moisture contamination
- Dirt
- Excessive heat in contact with oxygen
State the characteristics of transformer oil
- 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)
What is the thermal time constant of a transformer ?
- 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
What is the impedance voltage ? What is its significance ?
- The impedance of a transformer is expressed in terms of impedance voltage
- It is defined as the voltage required to be applied to the line terminals of a winding to cause rated current to flow through these terminals, with the terminals of other winding being short circuited
- It is also called short circuit voltage (Vsc)
- The value is related to reference temperature
- It is determined by conducting short circuit test
- Expressed as % of rated voltage
- % impedance in the range of 4.5% to 15%
Significance :
- Determines the regulation and efficiency of a transformer
- Its high value means higher impedance, higher voltage regulation, higher losses and lesser efficiency
Explain transformer losses
(1)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
(2) 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
Why core of transformer is laminated ? OR How eddy current loss is minimized ?
- 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
How the hysterisis loss is minimized ?
- By using laminated steel sheets of high grade grain oriented steel sheets
- Laminated surface is subjected to in organic insulation treatment
- The insulating varnish ensures insulation between the surface
State the significance of no load circuit losses( iron losses)
- The defects in magnetic circuit of the transformer, cause an increase in the no load current and losses
- Hence, efficiency reduces and may lead to excessive over heating and reduction in the life span
What is the condition of loading for maximum efficiency of a power transformer and its significance ?
- 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
State various losses occurring in a power transformer
- 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
Explain over fluxing protection
- Applied with GT or on both the sides of the power transformers with voltage class 400/220KV
- Measures ratio of V/f , alarm at ratio 2.4 and trip on further increasing in the value
- Over flux relay operates during run-up or run down of the generator
- Load shedding in system may cause excessive voltage rise operating the over flux relay
- Flux developed in the core of a transformer is directly proportional to voltage and inversely proportional to frequency
- Thus the flux level will go up in case the voltage increases or frequency decreases
- Increase in the flux level above than the design level, will tend to saturate the core which will force the flux to cover those areas which normally don’t carry any flux( non laminated parts)
What is the significance of vector grouping in Power Transformers?
Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference betweent the current and voltage. EG. DYN11 means Delta primary, Wye Secondry and the current is at 11 o clock reffered to the voltage.
Function of OTI and WTI?
OTI (Oil Temperature Indicator) measure Oil temperature in oil filled transformer. It’ use for protection of transformer. Separate OTI meter placed in transformer.
WTI (Winding Temperature Indicator) measure HV and LV winding temperature. It’s use for protection of transformer. In WTI meter, R,Y,B phase winding temperature indicates.