My system is powered by 60 V battery. First converter is 60 V / 12 V isolated, second converter is 60 V / 12 V non-isolated. Two converters power own system component. When I measure the voltage between converters' grounds, the voltage is -6.90 V. Why is the voltage like this? And How can I find the problem?
editing: I measure the voltage between converters seconds(outputs) grounds.
If one converter is isolated, there's no reason there should be any specific voltage between its output ground and any other point in the system.
You should be able to connect the two grounds together, which will solve your 'problem'.
I think the question you’re asking is, how does an isolated power supply develop a measurable voltage difference between the primary (grounded) and secondary (floating) side?
The answer is, the isolation isn’t perfect. There’s some leakage across the galvanic isolation (transformer in this case). Its measureable using a high impedance meter from floating secondary to primary ground.
You can measure leakage current too - this should be quite low, less than a mA probably for this kind of supply.
Is there a specific reason one part of your load is isolated? A medical device or special instrumentation for example? Then you may need to find or design a supply that has less leakage than the one you’re using.
If I correctly understand your problem, your converters are connected in parallel for powering their own load and you are measuring the differential potential between the ground of your non isolated converter and the primary ground of the isolated converter ?
How do you do your measurement ? With an oscilloscope or with a multimeter ? I think you are trying to measure the average voltage across the capacitor between the primary and the secondary of your transformer of your isolated converter.
--------------------------E D I T --------------------------
As you can see you are trying to measure the voltage across your transformer isolation, ie the potential difference between the primary ground and the secondary ground.
As @hacktastical said, the isolation is not perfect. Nevertheless I think your measure is bad as you are paralleling a high impedance (the one of your multimeter) and the high isolation of your transformer. Your measure modify the system. It is an "intrusive" measurement. And the voltage value given by the multimeter is not correct as you need to measure an impedance negligeable with respect to the internal impedance of your multimeter for having correct measurement. So it is complicated to say (at least to me) what is the meaning of your voltage measurement.
