transformer protection

The basic over view of protecting devices for transformers and there working principal.

1. Transformer protection Prepared by Jason Electrical and electronic engineering

2. Contents Introduction. Transformer Protection Overview. Transformer Failures. Transformer Protection Review. Types of Protection. Conclusion. References.

3. Introduction Transformers are a critical and expensive component of the power system. Due to the long lead time for repair of and replacement of transformers, a major goal of transformer protection is limiting the damage to a faulted transformer . Some protection functions, such as overexcitation protection and temperature-based protection may aid this goal by identifying operating conditions that may cause transformer failure. The comprehensive transformer protection provided by multiple function protective relays is appropriate for critical transformers of all applications.

4. Induction Law The transformer is based on two principles: 1. An electric current can produce a  magnetic field .  2. A changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction).

5. Transformer Protection Overview The type of protection for the transformers varies depending on the application and the importance of the transformer. Transformers are protected primarily against faults and overloads. The type of protection used should minimize the time of disconnection for faults within the transformer and to reduce the risk of catastrophic failure to simplify eventual repair. Any extended operation of the transformer under abnormal condition such as faults or overloads compromises the life of the transformer, which means adequate protection should be provided for quicker isolation of the transformer under such conditions

6. Transformer Failures winding failures due to short circuits (turn-turn faults, phase-phase faults, phase-ground, open winding ). core faults (core insulation failure, shorted laminations ). terminal failures (open leads, loose connections, short circuits ). on-load tap changer failures (mechanical, electrical, short circuit, overheating ). abnormal operating conditions ( overfluxing , overloading, overvoltage ). external faults.

7. Conditions Internal. Winding Phase-Phase, Phase-Ground faults. Winding inter-turn faults. Core insulation failure, shorted laminations. Tank faults. Over fluxing. External Overloads . Overvoltage . Over fluxing . External system short circuits.

8. Transformer Protection Review Internal Short Circuit - Phase Faults - Ground Faults System Short Circuits (Back up Protection) - Buses and Lines * Phase Faults * Ground Faults Abnormal Condition Open Circuit Over Excitation Abnormal Frequency Abnormal Voltage Breaker Failure Overload Geo-magnetically induced current (GIC)

9. Types of protection Accumulation Gases Arcing by- production (EX: Buchholz Relay) Pressure Relays Arcing causing pressure wave in oil or gas space (Sudden Pressure Relay) Thermal Caused by overload, overexcitation , harmonics and Geo- magnetiocally indued current Hot spot temperature Top Oil LTC Overheating

10. BUCHHOLZRELAY It is a gas actuated relay. When a fault develops slowly,it produces heat, thereby decomposing solid or liquid insulating material in the transformer . The decomposition of the insulating material produces inflammable gases. The Buchholz relay gives an alarm when a specified amount of gas is formed. The analysis of the gas collected in the relay chamber indicates the type of the incipient fault.

11. Working When an incipient fault such as a winding-to-core fault or an inter-turn fault occurs on the transformer winding, there is severe heating of the oil. This causes gases to be liberated from the oil around 350 °C. There is a build-up of oil pressure causing oil to rush into the conservator . A vane is placed in the path of surge of oil between the transformer and the conservator. A set of contacts, operated by this vane, is used as trip contacts of the Buchholz relay This output of Buchholz relay may be used to trip the transformer .

12. PROTECTION AGAINST FIRE Power transformers are subject to fires from many sources. They often occur because of deterioration of insulation in the transformer . This produces arcing which in turn overheats the insulating oil and causes the tanks to rupture; further arcing then will start a fire. Fires are also initiated by lightning and occasionally by dirty insulators on the outside of the tanks.

13. PROTECTION AGAINST LIGHTNING Lightning overvoltage surges originate from atmospheric discharges and they can reach their peak within a few microseconds and subsequently decay very rapidly. The surge voltage can reach up to 10 times the rated transformer voltage and they pose the greatest threat to transformers on the distribution networks. The charge from the surge produces both short duration high current impulse and long duration continuing current impulse which affects the transformer insulation system.

14. Conclusion. The protective equipment discussed is engineered to limit the damage and system disturbance caused by faults which can occur in a transformer . The choice of protective equipment depends on the size and the connection of a transformer, voltage level, power system grounding and the protective relays of the power network. The power companies also have different opinions about the extent and choice of pr:otective equipment for a transformer . No general recommendations can therefore be made.

15. References http:// sites.ieee.org/fw-pes/files/2013/01/transfguide.pdf https:// en.wikipedia.org/wiki/Power-system_protection https:// en.wikipedia.org/wiki/Transformer_protection