UNIT 3.pptx

electrical engineering

1. UNIT-3 Protection of Alternator and transformers

2. Differential over current protection

3. Conti… Differential over current protection system is also called the merz -price protection system It is used for protection of alternator and transformer This system works on current balance system

4. Difficulties in differential protection The difficulties encountered in this type of protection are: Difference in resistance due to change in the length of the pilot wires Ratio error in CT with heavy fault current

5. Difference in Resi . Due to pilot wire

6. Ratio error in CT with heavy fault current

7. Protection of Transformer Protection of power transformer is relatively simple compared to the other equipments Merz -price protection, Buchholz relay etc. are included in protection of power transformer.

8. Probable faults in PT The following abnormalities and faults occurring in power transformer Over heating of core Improper contacts on the tap changer Failure of the cooling system Phase to phase fault Earth fault Inter turn fault Over load Over voltage

9. Merz -price protection

10. Conti… Merz price protection system can be used to give protection against the phase to phase fault and phase to earth fault in power transformer The primary winding is connected in delta and CTs connected to that side are connected in star The secondary winding is connected in star and the CTs on that side are connected in delta

11. Conti… Relay is connected and kick fuses are connected across the relay coils No unbalanced current flows through the relay as the secondary winding currents of CTs are balanced The initial inrush of magnetizing current taken by the transformer when it is switched on passes through the kick fuses so relay can not operate The current balance is disturbed when there is internal fault in transformer. The fuse blow off when fault occurs

12. Buchholz relay

13. Conti… Buchholz relay is a gas operated relay This is the only type of relay which gives warning before occurrence of the fault All other relay operates only after the occurrence of fault When there is incipient fault, gas is produced in the oil in small quantity Gas bubbles travel upward to the conservator as these are light in weight

14. Conti… The buchholz relay is connected between the transformer tank and the conservator so the gas bubbles are collected in the top portion of the relay This reduces the oil level in the relay causing the upper float come down This makes the mercury switch to become horizontal and the wires kept in it are shorted by mercury The relay operates and the alarm circuit is activated

15. Conti… Buchholz relay provides protection against the following fault : Failure of insulation over the core bolt Local over heating Leakage of air in the tank Shorting of the limitations Reduction of oil level due to oil leakage Short circuit between the two phases

16. Conti… Advantages: Construction is simple Prediction of some fault can be made Disadvantages: It is used only for oil cooled transformer with conservator Setting of mercury switch can not be done with high sensitivity because otherwise the switch may get operated due to small vibration

17. Protection system for alternator Abnormalities and faults occurring in power system : Abnormalities: Failure of prime mover Failure of exciter Overloading Unbalanced loading Over voltage Over speed (b) Faults: Stator faults: Phase to phase fault Phase to earth fault Inter turn fault Rotor fault: Earth fault Inter turn fault

18. Thank you