Service tax on services to power projects

We are engaged in construction of power projects.  I understand that customs and central excise duty is exempted for material and equipment purchased for the purposed of construction and installation in power project. 

But similar exemption is not availabed to services rendered to power projects.  I came to know that this service tax exemption is not availabe to all projectgs.

Kindly let me know whether any service tax exemption is available to services rendered to Power Projects.