Saturday, July 14, 2018

Transformer faults and Protection

Transformer Protection and Transformer Fault 

There are various types of transformers, for example, two winding or three winding electrical power transformers, auto transformer, controlling transformers, earthing transformers, rectifier transformers and so forth. Diverse transformers request distinctive plans of transformer insurance relying on their significance, winding associations, earthing strategies and method of task and so forth.

Usually practice to give Buchholz transfer insurance to each of the 0.5 MVA or more transformers. While for all little size dispersion transformers, just high voltage wires are utilized as primary defensive gadget. For all bigger appraised and vital dispersion transformers, over current security alongside confined earth blame insurance is connected.

Differential assurance ought to be given in the transformers evaluated over 5 MVA.

Contingent on the ordinary administration condition, nature of transformer deficiencies, level of maintained over load, plan of tap changing, and numerous different variables, the appropriate transformer assurance plans are picked.

Nature of Transformer Faults

Despite the fact that an electrical power transformer is a static gadget, however inside anxieties emerging from unusual framework conditions, must be thought about.

A transformer for the most part experiences following kinds of transformer blame

Over current because of over-burdens and outer shortcircuits,

Terminal flaws,

Winding flaws,

Beginning shortcomings.

All the previously mentioned transformer deficiencies cause mechanical and warm worries inside the transformer winding and its interfacing terminals. Warm burdens prompt overheating which at last influence the protection arrangement of transformer. Weakening of protection prompts winding shortcomings. Some time disappointment of transformer cooling framework, prompts overheating of transformer. So the transformer security plans are particularly required.

The short out current of an electrical transformer is typically restricted by its reactance and for low reactance, the estimation of short out current might be too much high. The length of outside shortcircuits which a transformer can support without harm as given in BSS 171:1936.

Transformer % reactance Permitted duration of fault in Seconds

4 %                                                                     2

5 %                                                                      3

6 %                                                                     4

7 % and over                                                      5

The general twisting deficiencies in transformer are either earth blames or between turns shortcomings. Stage to stage twisting shortcomings in a transformer is uncommon. The stage blames in an electrical transformer might be happened because of bushing streak over and blames in tap changer gear. Whatever might be the issues, the transformer must be confined in a split second amid blame generally significant breakdown may happen in the electrical power framework.

Nascent flaws are inside issues which constitute no quick peril. Be that as it may, it these flaws are over looked and not dealt with, these may prompt real blames. The shortcomings in this gathering are for the most part between overlay cut off to protection disappointment between center cover, bringing down the oil level because of oil spillage, blockage of oil stream ways. Every one of these deficiencies prompt overheating. So transformer assurance plot is required for early transformer blames too. The earth blame, closer to unbiased purpose of transformer star winding may likewise be considered as a nascent blame.

Impact of winding associations and earthing on earth blame current greatness.

There are basically two conditions for earth blame current to stream amid twisting to earth flaws,

A present exists for the current to stream into and out of the winding.

Ampere-turns adjust is kept up between the windings.

The benefit of winding earth blame current relies on position of the blame on the twisting, strategy for winding association and technique for earthing. The star purpose of the windings might be earthed either positively or by means of a resistor. On delta side of the transformer the framework is earthed through an earthing transformer. Establishing or earthing transformer gives low impedance way to the zero succession present and high impedance to the positive and negative arrangement streams.

Star Winding with Neutral Resistance Earthed

For this situation the unbiased purpose of the transformer is earthed by means of a resistor and the estimation of impedance of it, is considerably higher than that of twisting impedance of the transformer. That implies the estimation of transformer winding impedance is immaterial contrasted with impedance of earthing resistor. The estimation of earth current is, in this manner, corresponding to the situation of the blame in the winding. As the blame current in the essential twisting of the transformer is corresponding to the proportion of the shortcircuited optional swings to the aggregate turns on the essential winding, the essential blame current will be relative to the square of the level of twisting shortcircuited. The variety of blame current both in the essential and optional winding is demonstrated as follows.

Star Winding with Neutral Solidly Earthed

For this situation the earth blame current extent is constrained exclusively by the winding impedance and the blame is not any more relative to the situation of the blame. The purpose behind this non linearity is unequal transition linkage.