This document discusses transformer protection methods. It describes various types of faults that can occur in transformers, including internal short circuit faults and incipient faults, as well as external faults. The document then outlines several protection methods for transformers, including percentage differential protection, Buchholz relays, protection against magnetizing inrush current, overheating protection, and overfluxing protection. Videos are provided that explain how Buchholz relays and transformer inrush current work.

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