Combined Cycle is a term applied to a power plant using both a gas turbine and a steam turbine or diesel engine and a steam turbine to generate power together. It is different from a conventional power plant where power is generated, only in a steam turbine.  

High Efficiency Of Combined Cycle:

In the case of a conventional steam power plant, the maximum efficiency attainable is limited due to the application of high temperature fluid. Whereas in case of an open cycle gas 

turbine power plant though we are using high temperature fluid, the efficiency is lower due to high rejection of heat in the outlet as exhaust.

.Therefore, by combining these two cycles the maximum efficiency can be achieved (i.e. keeping the gas turbine cycle as a topping cycle (high inlet temperature) and the steam turbine cycle as the bottoming cycle (low exhaust temperature)). The below image  gives the efficiency comparison of the various types of power plant.  From this figure, it can be seen that plant efficiency goes as high as 50% whereas the best efficiency even with a reheat steam turbine plant cannot exceed about 40%.