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Case Study – Generator Step Up Transformer (Gsu) Configuration

Case Study – Generator Step Up Transformer (Gsu) Configuration

Fatimajabeen A, SD Geethalakshmi

Generator step-up transformers

(GSUs) are the essential element of a power plant (thermal, nuclear, hydro and gas based). They are the critical link between the generating station and the transmission network. GSUs step-up the generating voltage level to the required evacuating transmission voltage level.

GSU being one of the expensive and critical electrical equipment in the power plant, selection of rating and configuration of GSUs is of paramount importance. In case of gas turbine based power plants, output of the gas turbine varies with the change in the site ambient temperature. Hence, GSU rating will be selected to fully evacuate the gas turbine output at site ambient temperatures from minimum to maximum.

In Indian power industry, the most widely followed configuration for combined cycle plants is having dedicated generator transformer with two windings for each GTG and STG. This is a proven design and a default configuration adopted in most of the combined cycle power plants. Reliability in this configuration is higher as dedicated transformers are provided for each gas turbines and steam turbine generators. Failure of single transformer will result in outage of only that particular unit.


    • However, this scheme requires transformers equal to the number of generators i.e. for 2GTGs + 1STG plant, three (3) numbers of GSUs are to be provided for transmitting the generated power & three (3) numbers of switchyard bays are required for connecting these three GSUs.
    • TCE, in one of the projects, carried out GSU configuration study and suggested an alternate optimised GSU configuration scheme. With the advance technologies being implemented in the transformer industry, failure rate of transformers has reduced considerably. Considering this fact and also the huge capital cost involved in the GSUs, following alternate scheme was arrived at.
    • One number of three-winding GSU for GTG-1 & STG and one number of two winding GSU for GTG-2 were adopted instead of conventional configuration with two winding transformers.


    • This configuration has following benefits
    • Cost optimisation due to reduced number of GSUs & switchyard bays ( 2 against the 3 as per conventional design) Space optimisation due to reduced number of GSUs & switchyard bays Operation and Maintenance ease as the number GSU and switchyard equipment are reduced
    • Above discussed configuration was suggested after a thorough study critically examining the reliability and availability of GSUs. TCE with its expertise and experience could perform these type of studies and propose the most optimal solution for the GSU configuration for combined cycle power plants.