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
:
(a) 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