Delta-delta secondary L-G measurements

In a delta system, there is no "real" reference of the transformer windings & phases to ground other than how good the insulation is in the system. Therefore, the 3 phases are only approximately the phase V divided by √3 to ground. If the insulation is fairly good in all 3 phases, you will see the voltages about equal when measured to ground. If one phase has poorer insulation (= a high-resistance connection to ground) you see a lower voltage than the other 2, and the others are relatively higher due to the vector relationship of the phases.

That's one reason that delta is used with a system that is predominantly motor loads - you can have an actual ground fault of one phase, and since the phase relationship doesn't depend on ground for reference, you can keep running. There needs to be ground fault monitoring, so that you can diagnose & repair the ground fault, but it doesn't actually shut you down until you have a second fault on another phase.

In the case mentioned would you agree that a problem in the insulation maybe developing and may need megger testing of cables and motors to ascertain their condition?

It would not be a bad idea, just to determine IF a problem is developing. However, it may just be due to variations caused by other factors including different types of insulation or environmental issues (humidity, dirt/contamination, etc.). It may not be something "failing," only that it needs some maintenance or cleaning.

Thanks very much.