Underlying secrets of procuring power transformers
Ajay K Dhagat, Advisor, Prolec GE India, writes why the reliability of transformers is important for continuous power
Electrical systems design needs extra care, precaution and safety factor to achieve a reliable electrical distribution network. In transmission or distribution, nearly 50 per cent of investment cost goes toward power transformers.
A transformer is 99 per cent efficient, robust equipment which maintain the grid balance. In India, it is believed that a transformer performs well if there is no oil leakage or failure. However, this is a myth. Taking advantage of a non-compliant transformer, working normally without any failure, corrupt manufacturers supply underrated or high-loss transformers without the knowledge of the end users.
The cost of transformer varies depending on weight and grade of used core and copper. This encourages non-complaint manufacturers to use less material to gain cost benefits of 7-15 per cent. Import data from a major Indian port showed that more than one third of transformer core material imported in a year through that port was second quality. The Ministry of Steel recently has released an amendment, banning import of low-grade CRGO steel.
Another way of non-compliance is manipulation of losses during inspection by doctoring the measuring instruments. Many non-compliant manufacturers do not follow standards and make huge errors, sometimes intentionally. There can be a difference of 4 to 5 per cent with such practices which include using shunts in measurement circuit and manipulation of heat run test (HRT), current and potential transformers, and winding resistance measurement.
Unlike other power equipment, transformer efficiency is difficult to check at site. Following are the concerns that remain:
Whether losses committed and offered are same?
Whether temperature rise when full losses occur will be as specified or not?
Whether core material specified to achieve particular losses has been supplied or not?
Whether flux density and current density limitations, if specified, have been observed or not?
Thereby, the perception-a given brand of transformer has not failed and is successfully running-alone can not be the criteria.
Case historyRecently a challenge test was conducted on 5 MVA and 8 MVA transformers supplied by a non-compliant manufacturer to a utility in Andhra Pradesh showed that the losses of transformers exceeded from 30 to 100 per cent over and above the guaranteed loss.
Recommendations to buyersBefore floating an enquiry, contact reliable manufacturers to get recommended loss figures. Select reasonable loss based on the incurred energy costs. Calculate capitalisation values for NLL and LL, considering the average loadInsist for standard GTP where weight of bare copper and finished laminations is to be specified along with stray losses. Verify loss figures during design review, check the quantity and grade of copper, and core lamination at manufacturing stage. Insist on core mill certificate. Use ERDA or knowledgeable third-party inspection agency who can question the test set-up. Ensure to pick one transformer from the lot randomly and test at independent lab.
Authored by:Ajay K Dhagat, Advisor, Prolec GE India, is an electrical engineer with more than 40 years of experience in T&D industry. He has functioned in senior positions with several multinational companies and power utilities across India. He has served as president of IEEMA from 2006 to 07.