AC Extra High Voltage (EHV) power transformers are an essential component of the power system enabling change in AC voltage and thus allowing operators to interconnect AC networks of different voltage levels to each other. Power transformers must be built to withstand severe electrical stress from fault currents and transients. Their availability and longevity have a major impact on grid reliability and profitability.
Key functions of Power transformers with tap changers are:
- Voltage step-up and -down: Since increasing voltage will reduce the currents required to distribute the same electrical power, step-up transformers are used to minimize transmission line losses. Step-down power transformers are used to bring down transmission voltages to usable voltage level for end-customer connections.
- Slow dynamic regulation to adjust to changing network conditions supporting voltage stability of the AC-grid.
Integrating a tap changer with the transformer allows for the regulation of the output voltage by adjusting the number of transformer windings (the transformation ratio). Having in mind that the effects on the network depend on the network itself, this however enables more flexibility to the operator compared to a fixed voltage step up or down ratio.
The main technologies of power transformers highlighting some specificities are reminded below:
Components & enablers
The exact components depend on the specific transformer. Typical components of an AC power transformers are:
- Laminated core
- Insulating materials
- Transformer oil
- Tap changer
- Oil conservator
- Cooling units
Advantages & field of application
The transformers constitute an integrated cornerstone in the power systems. The acceptance of these devices from a public acceptance and environmental impact is assessed by the audible footprint and the loss levels. The actual physical footprint is essential since the ‘not in my backyard’ discussions are expected to increase in intensity and frequency.
Technology Readiness Level
It is a mature technology - TRL 9 – System ready for full scale deployment (for standard components). However lower TRL prevail for some components development which could lead to a change in the TRL once these components are ready to be commercialised (e.g. development aiming at better performances of the transformers in terms of losses and environmental impacts: cooling system, mechanical design, core and magnetics, insulation system, acoustic/noise, use of ester oil).
Research & Development
The coming generations of transformers will be characterized by a shift on environmental focus. The responsibility of reaching a lower impact on the environment will be shared between producers and users. It includes all steps from production to operation over the lifetime of the equipment and decommissioning/recycling. This shift will impact the following areas:
Design and manufacturing: the design of the transformers is key to optimize their impact in terms of weights and losses results, more specifically the use of materials (steel, copper) will be carefully considered at that stage. The type of oil will be also part of the discussion, ester oil having lower risk impact for the surrounding areas. Manufacturers will be required to produce equipment with a minimum impact on the environment during their production.
A key design feature for installation of large power transformers are the transportability constraints due to their dimensions and heavy weight that could require specialized railroad freight infrastructure with high costs.
Operation: Life-time assessment will become more important:
- Importance of losses: optimize losses for the operation.
- An increased utilization, which improves return on investment for the users (higher focus on the actual hot spot). Incorrect or sub-optimized design can lead to temperature rise by 6-8 degrees on the actual hot spot, which significantly decreases the lifetime of the equipment.
Best practice performance
Large Power transformers (LPT) refer to units with power rating higher than 200 MVA or with voltage ratings higher than 275 kV for the three-phase units .
The United States Department of Energy defines the estimated magnitude of large power transformers . Typical characteristics for one-phase transmission transformers: 765-345 kV transformers with 500 MVA capability rating and a weight of 235 tons. The three-phase transmission transformers encompass several classes:
- 230-115 kV transformers with 300 MVA capability rating and a weight of 170 tons
- 345-138 kV transformers with 500 MVA capability rating and a weight of 335 tons
- 765-138 kV transformers with 750 MVA capability rating and a weight of 410 tons.
On high and extra high voltage, commercial catalogue of manufacturers detail LTP equipment with similar performances: three-phase units up to 1100 MVA and single-phase units up to 500 MVA, or Extra High Voltage Transformers of 1000 MVA, 500/275/63 kV 3 phase ; transformers well above 1300 MVA , up to 1,200 MVA, and voltages up to 765 kV .
Best practice application
420 kV power transformer (rated power of 400 MVA) for a substation for TransnetBW (TSO state of Baden-Württemberg) to link the 380 kV voltage level with the 110 kV grid.
power transformer with ester oil as insulation. All permissible (over)temperatures have been rated according to IEC 60076-2.
420 kV extra-high voltage level using natural ester. Due to the lower flammability, the transformer also has a higher fire protection class (K instead of O), so that the equipment can be used in densely populated areas.
Siberian Generating Company has commissioned a new 125 MVA transformer at Tom-Usinsk Power Plant as part of the project to improve the reliability of the largest power plant in Kuzbass.
Siemens, 125-MVA autotransformer.
The extreme temperatures in Siberia place special demands on transformers. The autotransformer fulfils these requirements, and losses are below the value stipulated in the Russian Governmental standard.
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