The installed capacity of variable renewable energy (VRE) such as solar and wind power in India is estimated to rise to 500 GW by 2030. Correspondingly, the exponential capacity growth will necessitate the development of transmission and distribution (T&D) infrastructure such as reactors, transformers, substations, etc., to a similar degree. Transformers are critical components in solar energy production and distribution. Historically, transformers have “stepped-up” or “stepped-down” energy from non-renewable sources. However, in the case of solar and wind energy, they will also need to have specialised inverters to convert DC voltages to AC, switching transients, improved harmonics systems, etc.
For instance, large load swings in a relatively short periods of time are inevitable with solar and wind turbine transformers. At night, there is virtually no load. As the sun rises, early in the day, solar cells begin to conduct and the wind begins to blow. This results in very large load swings as well as pressure changes within the tanks of liquid-filled transformers. Hence, renewable energy such as solar- and wind-based power requires specialised transformers.
Smart transformers for renewable energy
Conventional transformers intrinsically face difficulties at handling and transmitting substantial volumes of electricity, given that VRE installations are volatile and generate electricity intermittently with huge and rapid peaks and troughs in electricity generation. Additionally, in the upcoming years, the rise in installation of rooftop will create more prosumers willing to sell electricity to the grid during the day and withdraw electricity during off-peak hours. Upgrading to smart transformers will ensure bidirectional flow of energy from the grid to buildings as well as from consumers to the grid. Essentially, it will ensure that the electricity is extended to consumers who need it during peak hours at competitive prices, while simultaneously dispensing with the need to store electricity, which is a costly proposition.
Smart transformers will also facilitate energy grids to become more resilient to volatility and grid instability of various kinds. The internet of things (IoT) technology will also help in gathering data on the grid’s performance and report any possible issues to power companies in real time. These insights will allow energy suppliers to fix any inefficiencies in the network, identify the core issue and immediately resolve them. They could then address these problems as fast as possible, avoiding outages and downtime.
IoT, in combination with artificial intelligence/machine learning, will also be able to give utilities advanced warning about any potential issues by modelling the performance and intensity of the usage of components. This, in turn, will help utilities in diagnosing and scheduling repair of possible problems in advance and reduce their operations and maintenance (O&M) costs. Therefore, smart transformers will increase the efficiency and reduce O&M costs of T&D utilities.
New and emerging technologies
Electric power from solar energy is generated by converting solar energy to DC by using photovoltaic (PV) cells. The DC power generated by PV cells is converted to AC by inverters and the AC power is connected to the power grid by a step-up transformer. Additionally, they synchronise the output AC power with the phase frequency and voltage of the available grid in order to feed PV power into the grid. PV inverters are very efficient, generally 96-98 per cent, and inject very minimal DC current, harmonics or reactive power into the grid, which are usually within the allowable range of the grid code.
Solar power systems also have special design issues because the largest solar inverter size is about 500 kVA, which is half of the general solar power output. Therefore, designers are building 1,000 kVA solar transformers by placing two inverter connected windings in one box. The transformer must have separate windings to accept completely separate inputs. Design issues also stem from running cables for long distances to convert DC to AC. Restrictions on inverter size also limit the size of PV systems. Increasing the size by adding more solar inverters into one transformer box is extremely difficult. With the required box size and need to run cabling to convert DC to AC, things get complex.
Hence, inverter technology has been slow to advance, and it remains to be seen whether this comparative disadvantage will be a challenge in the advancement of solar technology to the same level as wind farms. Meanwhile, isolation transformers are typically used to protect inverters from grid-side surges as well as avoid any DC injection from the inverter into the grid. Many inverter models also have inbuilt isolation transformers. However, isolation transformers increase the cost and decrease inverters’ efficiency, hence, many purchase inverters without isolation transformers. Isolation transformers are not required if the PV system utilises another transformer such as a step-up transformer to step up the voltage to 11 kV.
CEA standards for solar park pooling stations
The Central Electricity Authority (CEA) issued guidelines and standards for solar transformers in May 2021, in order to standardise transformers in solar park pooling stations. Generally, electrical energy generated from the solar block is pooled at the 33 kV level and stepped up to 132 kV/220 kV/400 kV at the pooling station of solar power park developers and then connected to the interstate/ intra-state transmission system. As of now, there are different practices among solar park developers across the country regarding ratings, number of low voltage windings, losses, percentage of impedance, provision of on-load tap-changers and tertiary winding, etc., of power transformers. This notification aims to standardise these transformers. Standardisation of transformer equipment will also enable physical interchangeability of transformers of different makes. In case of failure of any transformer, the outage time to replace a failed unit with a spare unit/new unit of different make would be minimised as it can be accommodated in the same space without/with minor modification to the existing structure.
In the coming years, rising renewable capacity will necessitate simultaneous investments in the T&D sector, so as to upgrade it and incorporate smart technologies in order to facilitate volatile, intermittent and bidirectional flow of electricity.