digital protection methods os transformer

1. TRANSFORMER
PROTECTION
B Y
K R I S H N A M O O RT H Y M
E L E C T R I C A L E N G N I E E R

2. CONTENT
• Types of fault in transformer
• Transformer protection methods
• Video links

3. TYPES OF FAULT IN TRANSFORMER
• Internal faults
1.Short circuit faults
2.Incipent faults
• External faults

4. TRANSFORMER PROTECTION METHODS
• Percentage differential protection
• Protection against magnetizing inrush current
• Buchholz relay
• Over heating protection
• Over fluxing protection

5. PERCENTAGE DIFFERENTIAL
PROTECTION
• Rating of transformer should be 5MVA &above
• Current entering and leaving end has been marked as +ve and
–ve respectively.
• Under normal condition operating current flowing opposition to
normal current.
• Under fault condition operating current and normal current are
same.
• It cause the operation of the relay.

7. BUCHHOLZ RELAY
• Whenever there will be a minor internal fault in the transformer
such the insulating transformer oil will be decomposed in
different hydrocarbon gases, CO2 and CO.
• By collecting the accumulated gases from the gas release
pockets on the top of the relay and by analyzing them one can
predict the type of fault in the transformer.
• Fall of oil level means lowering the position of the float and
thereby tilting the mercury switch.

8. • . The contacts of this mercury switch are closed and an alarm
circuit energized.
• More severe types of faults, such as short circuit between
phases or to earth and faults in the tap changing equipment, are
accompanied by a surge of oil which strikes the baffle plate and
causes the mercury switch of the lower element to close.
• This switch energized the trip circuit of the circuit breaker
associated with the transformer and immediately isolate the
faulty transformer.

10. PROTECTION AGAINST MAGNETIZING
INRUSH CURRENT
• What is Inrush current ?
• Second harmonics diverted in to restraining coil.
• When it exceeds 15% of fundamental current leads relay to
operate. The minimum operating time is about 2 cycles.
• To remove dc offset harmonics an instantaneous over current
relay is also incorporated with harmonic restrained scheme.
• Alternative method is harmonic blocking scheme. A separate
blocking relay whose contact is series with biased differential
relay.

11. OVER FLUXING PROTECTION
• Flux is directly proportional to the voltage and inversely
proportional to the frequency.
• E/f ratio should me maintained
• 10% of over fluxing is allowed

12. OVER HEATING PROTECTION
• Load is proportional to temperature
• Permissible temperature is 95 degree Celsius and this is
measured by thermal image technique.
• CT on secondary wdg power the small heater. The temperature
sensing device and heater placed in a small pocket.
• Silicon resistor or silistor also used.
• The voltage across the silistor have a static control over cooling
pumps and fans.

13. VIDEO LINKS
• Buchholz relay - https://youtu.be/qhl5ZEKegXc
• Transformer inrush current basics-
https://youtu.be/kn9QOF5d-Sw