Have you looked at the EGSA Performance Standards? Take a look here,
Take a look at EGSA 107T-1999 Performance Standard for Generator Test Methods, it may be helpful
Also, have you looked at ISO 8528_5?
Scope
This part of IS0 8528 defines terms and specifies design criteria arising out of the combination of a reciprocating
internal combustion (RIC) engine and an alternating current (a.c.) generator when operating as a unit.
It applies to a.c. generating sets driven by RIC engines for land and marine use, excluding generating sets used
on aircraft or to propel land vehicles and locomotives.
For some specific applications (for example, essential hospital supplies, high-rise buildings, etc.) supplementary
requirements may be necessary. The provisions of this part of IS0 8528 should be regarded as a basis.
For generating sets driven by other reciprocating-type prime movers (e.g. sewage gas engines, steam engines),
the provisions of this part of IS0 8528 should be used as a basis.
Variations in how a plant like that should operate depends on a large number of factors, types of loads, critical nature of the site operation, is life safety a concern, types of power sources, etc. Finding a one size fits all solution is pretty hard. Are you only looking at the electrical performance of the system, and trying to see if available utility service is adequate to power your site, or if you can parallel to the local utility to improve stability and reliability? Or are you trying to do some kind of a performance analysis to see if a utility service is a more cost effective solution for your power needs than your existing plant?
MikeL