23-01-2013, 04:22 PM
TRANSFORMER PROTECTION:-
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INTRODUCTION
This presentation is to refresh the Transformer related knowledge and understand the intricacies involved in Transformer protection. Some basic things about transformers General things about Power Transformers. Reasons of Transformer Failures/Protection overview Discussions on different topics including Differential Protection , Through Fault , Phasor Diagrams etc. Objective & Contents Objective Contents
What is effect of Frequency on the transformer ? Why the core of the Transformer is Laminated ? What is the Transformer dot notations in PTs & CTs? What is inrush current of the transformer ? Why transformers explode ? What are Dy11 or Dy1connections ? Some common Questions
TYPICAL TRANSFORMER:
TYPICAL TRANSFORMER Transformer is the highly efficient electric Machine which changes the voltage level of the power system and transfers the electrical energy from one voltage level to another voltage level . The ratio of change in voltage level is directly proportional to the ratio of primary number of turns and secondary number of turns Transformer Action
Some Basic Things about Transformer:
Some Basic Things about Transformer Transformer changes the voltage level and the power received by us from Generators pass through several transformers. Two interesting questions 1 Why voltage level at which the power is consumed is different in the developed and developing countries ? 2 Which parameter is same through out the power system ? Vector Diagram No load current . Magnetizing current Im , Iw ,secondary current , effect of secondary current on primary side etc
Power Transformer :
Power Transformer Cut-away view of three-phase oil-cooled transformer. The oil reservoir is visible at the top. Radiative fins aid the dissipation of heat. Transformer inside
Basic Functions of the Power Transformer:
Basic Functions of the Power Transformer To increase the voltage level to enable transfer of power at high voltage there by reducing the transmission losses. To reduce the voltage level to enable consume power on the safe value To keep the power losses to minimum level. To ensure safe operation on normal and overload conditions and also when abnormal through faults occur.
Why Faults occur in the Transformers ?:
Why Faults occur in the Transformers ? Load on power transformers There has been constant increase in the load and the rate at which the load js increasing and the rate at which transformers are upgraded/replaced donot match in the developing countries.As a result transformers get overstressed The Moisture The increase in moisture in power transformers can be caused by floods, leaky pipes, leaking roofs and water entering the tanks through leaking bushings or fittings. Deterioration of Insulation Insulation deterioration is the cause of failure over the past several years. The reasons why insulation deteriorates are ,overload , heavy short circuit outside transformers and, moisture .
Transformer Failures :
Transformer Failures Failures in transformers can be classified into 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) Tap changer failures (mechanical, electrical, short circuit, overheating) Abnormal operating conditions (overfluxing, overloading, overvoltage) External faults
Faults:
Faults Transformer Faults can be classified into 3 types 1 Faults in the auxiliary equipment which is part of the transformer. Transformer oil Gas Cushion Oil pumps and forced air fans Core and winding insulation 2 Faults in the Transformer winding and connections Faults between adjacent turns Faults between winding and ground Faults between primary and secondary 3 Overloads and external short circuits
What is the impact of Transformer Failure?:
What is the impact of Transformer Failure? The direct economic impact of repairing or replacing the Transformer is Huge cost involved. The indirect economic impact due to production loss The repair time or complete replacement time are the factors which will decide the losses . The availability of back up power feed or replacement of the transformer and the cost of each option are other concerns.
Causes of Transformer Failures:
Causes of Transformer Failures Flood – 2 out of 94 The flood category includes failures caused by inundation of the transformer due to man-made or natural caused floods. It also includes mudslides. Loose Connections – 6 out of 94 This category includes workmanship and maintenance in making electrical connections. One problem is the improper mating of dissimilar metals, although this has decreased somewhat in recent years. Another problem is improper torquing of bolted connections. Loose connections could be included in the maintenance category, but we customarily report it separately. Lightning - 3 out of 94 Lightning surges are considerably fewer in number than previous studies we have published. Unless there is confirmation of a lightning strike, a surge type failure is categorized as “Line Surge”. Moisture 1 out of 94 The moisture category includes failures caused by leaky pipes, leaking roofs, water entering the tanks through leaking bushings or fittings, and confirmed presence of moisture in the insulating oil. Moisture could be included in the inadequate maintenance or the insulation failure category above, but we customarily report it separately.