Transformer technology has advanced significantly to accommodate the changing needs of utilities and improve power system resilience. These transformers perform the important function of adapting voltage levels, that is, stepping up to ensure efficient long distance high voltage transmission and stepping down for distribution of electricity to consumers.
Over the years, new transformer technologies have emerged as a result of rapid advancements in the power industry. Dry-type transformers and advanced smart transformers are replacing traditional oil-filled transformers. Further, transformers for renewable energy applications are gaining traction. Coupling transformers for static synchronous compensator applications have also become important from the grid stability point of view. Digital technologies are increasingly being used to improve operational management and prevent faults. Smart transformers are an important element of digital substations that autonomously control voltage and maintain communication with smart grids to allow remote administration and real-time feedback on power supply characteristics. These new-generation transformers use enhanced core materials, and have higher levels of safety (such as fire resistance), lower space requirement, cheaper prices, lower failure rates, lower noise levels and increased asset life. Apart from this, they are environmentally friendly and adaptable to smart grids.
Power Line presents a round-up of the key advanced transformer technologies and their deployment by Indian utilities…
HVDC converter transformers
High voltage direct current (HVDC) transmission is an economical process for the transmission of bulk power over long distances. Since all generating plants produce alternating current (AC), HVDC converter transformers are required to convert it into direct current (DC). These transformers also convert the DC back to AC for power consumption. They have AC windings connected to the AC system and valve windings connected to converters, which are further connected in a series circuit to build up the necessary level of DC voltage. The advantages of these transformers are high electricity transmission and low transmission losses.
Powergrid has implemented key HVDC projects in the country in recent years. In June 2021, Powergrid commissioned Monopole 1 of the ± 320 kV, 2000 MW Pugalur (Tamil Nadu)-Thrissur (Kerala) voltage source convertor (VSC)-based HVDC system. Monopole 2 of the project was commissioned earlier in February 2021 and with the commissioning of Monopole 1, the project has attained its full capacity. The Pugalur-Thrissur HVDC is a part of the larger 6,000 MW Raigarh-Pugalur-Thrissur HVDC system. Bharat Heavy Electricals Limited along with Hitachi ABB Power Grids had won the order for the 800 kV Raigarh-Pugalur link in 2016. The consortium supplied major equipment including 800 kV converter transformers, converter valves, cooling systems and control and protection technology for the project.
Conventional transformers use mineral oils as the insulating fluid. In ester-filled transformers, natural esters are used for liquid insulation of the transformer. These transformers have the significant advantage of being fire resistant as natural esters have about twice the fire point of mineral oil at 360 ºC, as well as a flashpoint of 320 ºC. In case of leakage, ester-filled transformers fare better than their conventional counterparts because natural esters are biodegradable, and thus risks of soil contamination are minimised.
Recently, in April 2021, Tata Power commissioned India’s largest natural ester-filled 110/33/22 kV, 125 MVA power transformer at the BKC receiving station, as part of its Mumbai transmission network. With this, the company aims to augment its transformation capacity and reduce its environmental footprint.
Dry-type transformers are an emerging technology, wherein the windings with the core are kept within an air-filled, pressurised and sealed tank. They are motionless, solid-state devices without any moving or rotating components. They do not need to be stored in fireproof vaults and do not produce poisonous fumes. The two types of dry-type transformers are cast resin transformers and vacuum pressure impersonated transformers. They can reduce the risk of fire hazards and are thus ideal for areas such as residential buildings, offices, schools, hospitals and metro stations. BSES Yamuna Power Limited has deployed a dry-type transformer at East Delhi and BRPL has deployed a 1,500 kVA dry-type transformer at the Triveni Shopping Complex in Delhi. Dry-type transformers have also been deployed at the Kochi metro station.
Green transformers with low noise for ultra-low applications, along with increased safety against fire with the use of ester oil, are emerging. Green transformers offer several benefits such as a lower carbon footprint and slower ageing compared to traditional transformers. 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 in turn reduce maintenance needs.
Smart transformers are an integral component of digital substations. Smart transformers can independently regulate voltage while allowing remote operation by retaining contact with the smart grid. They change the voltage ratio using applied semiconductor technology and can achieve high power density. They also operate at a high frequency, thus reducing both cost and size, making them economical.
In addition, smart transformers minimise energy consumption and greenhouse gas emissions. They can provide stable, optimal power supply, and protect electrical equipment from power fluctuations, thereby elongating equipment life. Abu Dhabi Transmission and Despatch Company (Transco) and Siemens have installed a connected smart power transformer, the Sensformer, at the Al Foah 220 kV substation in the UAE. The project uses the latest technology to utilise data to optimise the performance of substations.
A mobile transformer is a portable device used in mobile substations. It is used in emergency circumstances, when an interim grid connection or temporary power supply is required, such as load peaks, substation replacement and equipment failure. It is a trailer-mounted, self-contained system that is pre-configured and ready to use, with short installation and commissioning times.
Power utilities can use energy efficient transformers (EETs) to enhance the efficiency of the transmission and distribution (T&D) system, and as a result reduce T&D losses. These transformers are specially engineered to have a high performing design. They use low resistance copper wires, suffer minimal heat losses, require less coolant and have longer running time. EETs can reduce their losses by up to 60 per cent.
Advances in materials
Transformer materials are being improved as well. In terms of the basic core materials utilised in transformer manufacturing, the transition from CRGO M4 steel sheets to amorphous steel to laser scribed core material grade ZDKH has been gradual. In addition, the conductor materials used in transformer winding have shifted from paper-insulated regulator copper conductors to epoxy-coated continuously transposed conductors. This has resulted in space optimisation.
Further, new trends in core construction are helping in loss reduction and expediting the production process. Step-lap construction of transformer cores is being adopted instead of the conventional non-step lap (NSL) type to reduce the building factor in transformer cores by 5-8 per cent, as well as the no-load current and the noise level. Also, for transformer processing, a drying technique is gaining traction. It is vapour phase drying that uniformly removes moisture. Vapour phase drying, like hot oil spray drying, uses hydrocarbons to add heat concurrent with vacuum drying. The vapour phase process provides the most efficient combination of heat addition and vacuum application. This reduces the processing time by approximately 25 per cent over hot oil spray processing, and by 40 per cent over hot air processing.
The grid is experiencing significant changes. To keep pace with the changes, transformers, which are a critical component of the transmission grid, are increasingly adapting and embracing modern technology to make the future grid more resilient and adaptable.