Generator overvoltage protection

Hi,

I'm Looking for information about MV generator overvoltage protection and surge arresters. Did some research (google) and found only one good source from abb's pages.

http://library.abb.com/global/scot/scot245.nsf/veritydisplay/bb5718362206c19ac125708500333aed/$File/Application%20Guide%20Medium%20voltage%20%20networks.pdf

I'm trying to find different calculation methods for overvoltage protection sizing between generator phases and earth and also verify how different fault conditions affect this. Like how overvoltages vary when it is caused by lightning or operation of the generator cb.

thanks in advance :)