Transformer Technologies: New solutions aimed at meeting power system requirements

New solutions aimed at meeting power system requirements

Transformer technologies have evolved considerably to meet the emerging requirements of utilities and enhance power system reliability. Over the years, a number of technological improvements have been made to transformers, which perform the important function of adapting voltage levels, stepping up to ensure efficient long distance high voltage transmission and stepping down for distribution of electricity to consumers.

Snapshot of the transformer industry

In India, there are over a 100 transformer companies catering to both power and distribution transformers. Currently, the annual turnover of the transformer industry is over Rs 137 billion, of which more than 48 per cent comes from the power transformer segment and the remaining from the distribution transformer segment.

The industry, on an average, has witnessed a compound annual growth rate of 5-6 per cent in the past five years. During this period, while the power transformer segment grew at the rate of 8 per cent, the distribution transformer segment grew at only about 3 per cent. Power transformers are expected to see higher growth with increasing renewable energy penetration into the grid, the government’s focus on electrification schemes and significant investments planned for the power transmission segment. Further, with emphasis on railway electrification, the demand for traction transformers is expected to pick up. Another potential growth area for the industry will be the renovation and refurbishment business.

In terms of manufacturing capacity, the production of power transformers has increased from 152,190 MVA in 2014-15 to 242,263 MVA in 2017-18. However, the industry is currently operating at only 60 per cent of the installed capacity.

New and emerging transformers

Product-wise, the latest transformer technologies include 1,100 kV high-voltage direct current (HVDC) converter transformers, 800 kV HVDC converter transformers as well as phase shifting transformers. Controlled shunt reactors that allow control of reactive power thereby maximising line efficiency are also gaining traction. Transformers for renewable energy applications too are gaining a presence. Coupling transformers for STATCOM applications have become important from the grid stability point of view. Other new technologies such as traction transformers for the dedicated freight corridor and higher capacity generator transformers (for 800/1,000 MW) are still nascent in India.

There is an increasing focus of utilities towards smart transformers, which are an integral part of digital substations. Smart transformers independently regulate voltage and maintain contact with smart grids in order to allow remote administration and real-time feedback on power supply parameters. The use of these transformers is gaining ground at the distribution and transmission levels. These transformers are equipped with intelligent electronic devices, and intelligent monitoring and diagnostics features. Besides, they provide web, and supervisory control and data acquisition (SCADA) interfaces.

The dependence on digital technologies is increasing with the growing use of online monitoring devices, which use artificial intelligence in analysis. Further, there is the emergence of low noise transformers for ultra low applications and green transformers, which are used with ester oil.

Technology trends in materials used

In terms of basic core materials, used in transformer manufacturing, there has been a gradual shift from CRGO M4 steel sheets to amorphous steel to laser scribed core material grade ZDKH. Further, the materials used in transformer windings have changed from paper insulated regulator copper conductors to epoxy coated continuously transposed conductors. This has resulted in compact size and better space utilisation.

For transformer insulation paper, high density kraft paper and thermally upgraded paper are being used currently. Also, the use of ester oil is growing as compared to conventional transformer oil due to environmental concerns. Ester oil offers several advantages, including biodegradability, higher flashpoint, higher thermal conductivity, lower calorific value, higher temperature stability, and better benefit-to-cost ratio than mineral oil.

New design and construction practices

Transformer manufacturing practices have also changed evidently over the years. There is now greater availability of various simulation tools to carry out electrostatic and electromagnetic analysis. This has optimised the usage of transformers in recent years.

Further, new trends in core construction are  minimising losses and expediting production. Step lap construction of transformer cores is now being adopted instead of the conventional non-step lap (NSL) type to reduce the building factor in transformer cores (by 5-8 per cent), bring down the no load current and decrease the noise level relative to conventionally stacked NSL cores.

For transformer processing, a drying technique that is gaining importance is vapour phase drying, which uniformly removes moisture. Vapour phase drying, like hot oil spray drying, uses a hydrocarbon to add heat concurrent with vacuum drying. The hydrocarbon selected has a boiling range that is narrow and relatively low, so it can be removed from the drying process prior to the introduction of transformer oil. The vapour phase process provides the most efficient combination of heat addition and vacuum application. This results in the reduction of processing time by approximately 25 per cent over hot oil spray processing, and approximately 40 per cent over hot air processing.

The majority of transformer failures used to be due to bushing failures but this has changed in recent times. The overall reliability of a power transformer depends to a great extent on the sound operation of bushing. Oil impregnated paper (OIP) bushings are conventionally used in power transformers. However, now a new technology,  Resin impregnated paper (RIP) bushing, has replaced OIP bushing. RIP bushings have superior thermal and electrical performance. In RIP bushings, the major insulation consists of a core wound from paper and subsequently impregnated with epoxy resin.

Also, tap changers, which were earlier oil filled, are now either vacuum type or no top changers at all. Temperature monitoring has also seen improvement with fibre optic hot spot measurement devices for greater accuracy now being used as opposed to oil temperature indicators (OTIs), which measure the top oil temperature. Besides, there is widespread use of online monitoring devices for condition monitoring of transformers.

Issues and challenges

The key challenges for the transformer industry are the lack of standardisation of transformer specifications, and insufficient online monitoring devices and transformer fittings. Moreover, the unavailability of indigenous materials, for example, copper rods, insulation, CRGO and online devices still prevails. Further, with increasing focus on short circuit tests, the monopoly of labs poses concerns. Better infrastructure is also needed for the transportation of heavy equipment.

To conclude, transformer technologies are constantly evolving to meet power system requirements and address the challenges faced by utilities. To this end, the right technology selection can help utilities improve power reliability, minimise faults and optimise asset management.

Twinkle Agarwal