Technology Improvements: Exploring new transformer designs and features

Exploring new transformer designs and features

The growth in the power transformer segment is attributed to the demand for high voltage (HV) and extra high voltage (EHV) transformers as well as growth in exports. Meanwhile, the growth in the distribution transformer segment is led by domestic demand, especially rural electrification programmes. Transformer technologies have evolved considerably to meet the emerging requirements of utilities and adapt to the changing grid. New designs and features have been developed to provide space optimisation, lower costs, increase asset life, improve core materials, ensure safety, add digital hubs, reduce failure rates and cut noise levels, among other things. Advanced computational tools are being used at the design stage to address the increasing complexities of power systems and optimise the material for cost competitiveness. High quality and automated manufacturing processes are essential to make design improvements.

Transformer technologies

HVDC converter transformers

High voltage direct current (HVDC) technology has gained traction in recent years owing to its ability to transmit a large amount of electricity over long distances while reducing losses. An HVDC system can reduce transmission losses by around 50 per cent as compared to a HVAC system. HVDC converter transformers form the core of HVDC projects as they transfer power between an AC system and a DC transmission network. HVDC transformers also provide an appropriate tapping range for DC voltage and for reactive power control, thereby serving as an integral part of the DC transmission network.

While back-to-back HVDC bipole (±500 kV) lines have been in operation in India for a long time, ±800 kV multi-terminal HVDC projects have been commissioned only recently or are under way. The ±800 kV multi-terminal Northeast-Agra UHVDC link (Phase I) was commissioned by Power Grid Corporation of India Limited (Powergrid) in 2015 while the ±800 kV Raigarh-Pugalur, Champa-Kurukshetra lines, and the ±320 kV Pugalur-North Trichur line are still under construction and are expected to be commissioned in 2020-21.

Phase shifting transformers

Phase shifting transformers (PSTs) are special-purpose transformers that are used to control active power flow in the network by regulating the phase of line voltage. These transformers are used in networks where intensive power wheeling takes place due to deregulation. PSTs help ensure the optimum utilisation of transmission lines, thereby enhancing their efficiency. These transformers are site specific and are planned on a case-by-case basis through proper system studies.

BHEL commissioned its first indigenously developed PST at the Kothagudem thermal power station in Telangana in 2014. The transformer is used to control and improve the power flow between 400 kV and 220 kV networks in either direction by shifting the phase as the system requires. According to the Central Electricity Authority, preliminary studies have been undertaken for the deployment of PSTs in the interregional link between the southern grid and the NEW (north eastern western) grid.

Coupling transformers

Coupling transformers find application in flexible AC transmission systems (FACTS). They enhance the control and stability of the transmission system, and increase its power transfer capabilities. These transformers connect the grid with the static synchronous compensator (STATCOM), which is a FACTS device that ensures the supply of dynamic, precise and adjustable amount of reactive power to the AC power system that they are connected to. STATCOM cannot be connected directly to HV grids (220 kV and 400 kV). Therefore, a coupling transformer is needed as the intermediate device to connect the STATCOM system and the HV grid with a bidirectional power flow.

A coupling transformer does not require any tap changer since its voltage is controlled by the STATCOM system. The step-up duty is critical in a STATCOM application and the coupling transformer must be specifically designed to meet these requirements. STATCOM devices installed in the correct location of the system enhance loadability and stability, and reduce network losses through the optimisation process.

Traction transformers

Traction transformers are a critical component in the traction chain, and impact both the train performance and operator services. Indian Railways (IR)’ electrification plans have pumped up the demand for traction transformers. The Mission Electrification initiative of IR aims to electrify an additional 24,000 km of railways. These transformers feed power into essential train functions such as traction, lighting, heating and ventilation, brakes, signalling and communication. They are a critical link in the traction chain, driving train performance and operator services. Many transformer manufacturers are expanding their facilities to cater to the growing demand.

Smart transformers

Smart transformers are an integral part of digital substations, which independently regulate voltage and maintain contact with the smart grid, allowing remote administration and providing 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 smart monitoring and diagnostic features. Further, as smart transformers consume less energy, they help in reducing greenhouse gas emissions. A smart transformer provides the accurate amount of power required and immediately responds to fluctuations in the power grid, acting as a voltage regulator. These features make smart transformers ideal for power systems that are designed for renewable energy integration.

Green transformers

The concept of green transformers is also emerging. Green transformers offer several benefits such as a low carbon footprint, slow ageing, noise reduction, increased safety against fire with the use of ester oil, and lower pollution risks. Such transformers also have lower life cycle costs as they are equipped with hermetically sealed tanks, which require no oil conservator and no associated devices, thereby reducing maintenance requirements. The vacuum tap changers prevent contact erosion and further reduce maintenance needs.

Dry-type transformers

Dry-type transformers do not incorporate liquid to dissipate excess heat and meet temperature classification requirements. The coils within a dry-type transformer are constructed of a gaseous or dry insulation medium. These transformers require minimum maintenance to provide many years of reliable, trouble-free service. A dry-type transformer has no moving parts and is a completely static solid state device. Under normal operating conditions, it insures a long and trouble-free life. Unlike liquid-filled transformers, which are cooled with oil or fire-resistant liquid dielectric, dry-type units utilise only environmentally safe, high temperature insulation systems. Dry-type transformers provide a safe and reliable power source, which does not require fire-proof vaults, catch basins or the venting of toxic gases. These important safety factors allow the installation of dry-type transformers inside buildings close to the load, which improves the overall system regulation and reduces costly secondary line losses.

Dry-type transformers differ from liquid-filled units in many ways, and separate industry standards are applied for their design, construction and maintenance. These transformers are suitable for both indoor and outdoor applications. Cooling ducts in the windings allow heat to be dissipated into the air. Dry-type transformers can operate indoors under almost all ambient conditions such as commercial buildings and light manufacturing facilities. For outdoor operations, a dry-type transformer’s enclosure usually has louvres for ventilation. However, these transformers are affected by a hostile environment (dirt, moisture, corrosive fumes, conductive dust, etc.) because its windings are exposed to the air.

Issues and challenges

One of the key challenges for the transformer industry is the lack of standard transformer specificatins, online monitoring devices and transformer fittings. Further, indigenous material such as copper rods and insulation is still not adequately available. Also, the lack of infrastructure required for transporting heavy equipment such as power transformers poses a big problem. With the enforcement of quality control orders pertaining to cold rolled grain oriented (CRGO) steel used in distribution transformers for mandatory BIS certification, the demand for high quality CRGO steel has increased manyfold. This has increased the pressure on suppliers and manufacturers. Globally, the production, supply and price of CRGO steel is dominated by 14 to 15 mills located in the US, Japan, Germany, South Korea, the UK, Russia, Poland, Brazil and China.

To conclude, technologies available in the transformer space are constantly evolving to meet the power system requirements and address challenges facing utilities. The right technology can help utilities improve power reliability, minimise faults and optimise asset management.