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Wednesday, 17 September 2014

Failure of Distribution Transformers in India

Transformer industry in India:

Transformer industry in India has evolved over a period of time and is now a matured industry capable of manufacturing a wide range of power, distribution and special type of transformers for different applications. Recently it has manufactured a UHV 1200 kV class transformer for the test station at Bina in Madhya Pradesh. 

The power transformer market in India is well organized and their customers are large entities like Central Transmission Utility (CTU) and the State Transmission Utilities (STU) whereas the distribution transformer market is dominated by un-organized players.

The number of distribution transformers currently in service is nearly 4.3 million and the number is adding at an annual rate of approximately 10%. Distribution transformers of 11/0.4 kV class usually come with both aluminium and copper winding. 

Failure rate of Distribution Transformers in India:

"The failure rate of distribution transformers in India is as high as 25 to 30% which is the highest in the world."
Reports say that every year distribution transformers worth rupees 200 Crores fail; which is a great financial loss to the nation and which can be avoided. 

Causes of Failure of Distribution Transformers:

Substantial failure rate of distribution transformers in India is mainly due to:

i)   the design criterion, 

ii)  the material used in manufacturing, 
iii) maintenance practice, 
iv) material used in maintenance, and
v)  Un-authorized electrical loads.

The operating conditions, particularly in rural India, like weather conditions, overloading, through or passing faults, inadequate protection, public interference, poor maintenance of LT and 11 kV lines often results in distribution transformer failure.

Distribution transformers installed in rural areas form the bulk of these transformers. They are very much exposed to vulnerable weather conditions particularly lightning. These transformers feed lengthy Low Tension (LT) lines which are more prone to faults because of these atmospheric conditions. 

Majority of the transformers have poor efficiency because of improper or unbalanced loading conditions. It is common practice to connect additional electrical load on these transformers on the basis of maximum demand recorded at some point of time or on the basis of assessed maximum demand without considering the seasonal variations and the actual diversity factor. 

Un-authorized electrical connections also result in overloading. Wide variation in load and ambient temperature make undesired ingress of moisture, particularly in rural areas, which weakens the dielectric strength of transformer oil, forms sludge and deposits on the winding which on passage of time may obstruct the ducts in the winding provided for oil circulation. 

The routine maintenance of LT and 11 kV lines and protective equipments associated with these transformers are also poor. Figure 1 shows a badly maintained LT fuses on one of the distribution transformers.

Fig. 1: Badly maintained LT fuses

Prolonged operation of distribution transformer under abnormal operating condition such as faults, overloading or unbalance load deteriorate the insulating materials; ultimately leading to failure. Figure 2 below shows the damaged High Voltage (HV) winding of  a 11/0.4 kV distribution transformer.

Fig. 2:  Damaged High Voltage winding of  a 11/0.4 kV distribution transformer.

Repair rate of Distribution Transformers:

The repair rate of distribution transformers is also high in India. Each distribution transformer is repaired 2 to 3 times in its whole life span of 25 years (due to fund paucity) reducing the efficiency further with each repair. The direct economic impact of distribution transformer failure is in terms of cost of repair or replacement whereas indirect economic impact comes in the form of revenue loss due to supply interruption and increased losses.     

How to reduce Failure of Distribution Transformers:

This significant failure rate of distribution transformer in the country can be curbed to some extent by employing Complete Self-Protection (CSP) scheme which enables the transformer to protect itself from faults. 

In the CSP scheme, transformers are equipped with primary fuses, secondary side circuit breakers and lightning arresters. The primary side fuse is mounted inside the primary bushing and is in series with the primary winding to isolate the transformer in case of faults inside the transformer or on its LT side. Secondary circuit breakers and lightning arresters are there to protect the transformer from overloads, LT side faults and lightning strokes respectively.