It should be a good indicator of insulation integrity, and overall condition of your equipment. I'm assuming you've never done it before on this piece of equipment, so it would just be a baseline # for you to reference future tests. I think at that voltage, some special equipment is involved, so it may be pricey.
The diagnostic value of the doble test is improved if you
#1 - perform a tipup test by comparing tests at approx 600v and 2400v for 4kv machine.
#2 - Split the winding neutral and perform phase-to-phase tests in addition to phase-to-ground tests.
It is a better test for 4kv machines than for higher voltage machines because the stress control coatings on higher voltage machines give a nonlinear current vs voltage which masks other problems in the tipup test.
In my opinion, it's probably the next logical step in testing above an insulation resistance/polarization index to get a good feel for insulation condition.
Other more sophisticated/cumbersome options would be opening the machine for inspection. Some of our repair shops can perform off-line partial discharge tests (with machine energized at normal line to ground voltage from ac hipot test set). Also with rotor removed you can do a corona probe test.
I guess that in my attempt to list all of the available tests I left out some very important options.
dc step voltage test hi-pot is a very useful test with fairly inexpensive equipment. Potentially destructive but risks can be minimized if performed correctly. Careful review of the shape of the curve of current vs voltage MAY give you warning of impending failure or help you GUESStimate the margin in the insulation of the machine (how much further could I increase voltage before the curve goes vertical roughly tells us how high is actual breakdwon voltage.... don't count on it but that's a rough way to look at it).
Lots of info on dc testing and other techniques at
For specific info on Doble testing, contact Doble or whoever supplied your test equipment. Ask for the Doble Test Guide for Rotating Machinery. Also relevant section of M4000 user guide and several articles in the proceedings.
