I am designing electrical system for a power plant. I need to know the BIL of 5 MVA, 13.8 KV synchronous generators for the insulation level coordination studies. But the generators do not have BIL ratings, to do the calculation I am making following assumption.

I know the rotating machine winding can withstand a voltage wave with the crest value of 1.25 times the hi-pot test voltage in 10 microseconds. There is a formula in IEEE for the value hi-pot test voltage for stator coils during manufacture, which is test voltage = 2*(2U+1).

Using this criteria the value of crest of voltage wave would become 2*(2*13.8+1) *1.41*1.25=100.815 KV

Can I consider 100 KV as a BIL of this these generators in my insulation level coordination studies?

What is your experience on this? How do you deal with synchronous generators BIL?
I need your expert judgment on this, together with reference to relevant standards
Thank you

The required BIL is worked out based on the level of lightening impulse which a Generator is expected to successfully withstand. Rating 5 MVA is small enough to enable one to assume that the generated power shall be used only in-house through a distribution network that has no exposure to atmosphere.
However, if this is not the case, no response can be given unless full information is available about the system of Over Head Transmission System to which the particular Generator shall be connected. Obviously such transmission system shall not be at 13.8 kV but rather at 110 kV or 132 kV ... etc. The BIL of such transmission system is required to be known and details of Generator Transformer are also required.
In case, the 5 MVA Generator feeds an internal distribution system comprising other Generators and there is a common interconnecting transformer for connecting this system to the grid system, full details of such a system are required to work out BIL requirement of the Generator.
However, manufacturers produce generators of standard ratings such as 5 MVA is BIL values already Type Tested by them. It is always advisable to chose such standard BIL value from manufacturer's standard product list and to engineer the rest of Generator's electrical network to suit it. This is done by then selecting the right type of Lightning Arresters for installation at Generator Terminals.

I guess you do not need to include the generator into your insulation coordination.

You can make some overvoltages simulations considering two posible scenarios: lightning and switching operations.

Switching simulations could be done by means of statistical tools to have a better results.

Usually at that voltage level lightning is the main parameter and switching is not necessary, but better to check both conditions.

Once you have done such simulations you can check the resultant overvoltages against the machine winding impulse capability. If results exceded your machine capability you can use surge arresters, capacitor or a combination of both.

You can review at Impulse voltage strength of AC rotating machines, IEEE transactions, PAS-100, No. 8, 1981.

Some basic literature can be found in IEEE 141.

Good luck