Synchronizing a diesel or inverter-based generator with the utility grid without a ground reference 1

[spark706]

I am attempting to understanding some finer technical aspects for synchronizing a rotational-based or inverter-based generator with the utility system without the generator and the utility system in a delta-delta or a floating wye-floating wye configuration. For now please disregard the concept of an ungrounded generator as this is a hypothetical question based on a conversation I had with another engineer today.

My understanding is that a common ground reference is necessary with the utility system which is easily accomplished with a generator that is grounded or via the use of a shunt grounding bank. Such a configuration will provide necessary ground reference with the utility to allow a closed transition of a microgrid back from islanding or connect a rotational generator to the grid for parallel operation. I believe that for the two systems to align the Phase Sequence, Voltage Magnitude, Frequency, and Phase Angle must align, but I was under the impression that to ensure that would happen, the ground reference is crucial. Essentially, when trying to align four separate criteria without a known quantity that the the generator and the utility system share would be extremely difficult or impossible to achieve.

So, with the above said, is it possible to synchronize an ungrounded rotational-based generator, for instance, through a delta/wye-floating ground transformer to a utility system that is or is not grounded? If so, would you please point me to some specific literature that discusses the actual need for the ground component during synchronization.

Many Thanks for any help or guidance.

 

[badservo]

"Ground" is not the same thing as "Neutral".
Delta,open delta, wye, floating neutral, resistance grounded neutral, solidly or grounded neutral are chosen based on load requirements and how you want it to fail when something goes wrong.
You need every metal surface grounded to protect humans from touch and step potential. You don't need a neutral (or ground) to synchronize.

 

[JaPaBo]

I agree with badservo. I work with inverters and see systems synchronizing to the power grid through isolation transformers where the inverter-facing windings are floating / not ground referenced and there is no neutral connection to the inverter.

You won't find "specific literature that discusses the actual need for the ground component during synchronization" as it's not generally required (other than as applicable by local electrical codes).

 

[waross]

Been there, done that.
A ground is not required to synchronize two generators.
A neutral is not required to synchronize two generators. (Unless one is a single phase, two wire generator and one of the wires is considered a neutral.)
The synchronization scheme at a small plant that I supervised:
Voltage; Voltmeters, phase to phase, A to B.
Rotational angle; Synchroscope, phase to phase, A to B.
The phase sequence is verified at the time commisioning and is hard wired.
The synchroscope responds to the frequency and the phase angle.
The 'scope actually responds to the frequency difference between the two sources.
The frequency is indicated by the speed of the needle.
One rotation equals one cycle.
The switch is normally closed when the incoming set is slightly faster than the working set.
That avoids reverse power trips.
Yes, I know, the trip settings must be such that synchronization does not cause a reverse power trip.
Despite that, stuff happens.
Stuff is much less likely to happen when the incoming set is a little fast.
None of our gear required a neutral connection nor a ground connection.
A couple of our sets had disconnect switches in the grounding jumper.
I found that an operator had left one of the switches open and one generator was ungrounded.
It made no difference at all to synchronizing, before or after i closed the switch.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

We recently commissioned a marine application where all the sources of power on the vessel (including the shore connnection) were ungrounded (IT System to IEC 60364), no issues at all with synchronising.

Where the ground reference can become more important, particularly from the utility side, is the ability to prevent feeding into a ground fault somewhere in the network. This isn't a synchronising issue for islanded microgrids or similar, its more a requirement of being able to disconnect all sources of supply to a faulted network.



EDMS Australia

 

[spark706]

After seeing badservo's response, I would like to clarify that we I say grounded I am referencing the neutral as in a floating-wye versus grounded-wye. Apologies for that.

I found this PowerPoint slide mentioning using a grounding bank to provide the ground reference for synchronizing an inverter-based generator.

 

[waross]

found this PowerPoint slide

Sales technobabble.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[spark706]

Thanks for the help!

 

[DM61850]

I don't a ground reference is needed. I don't see why you couldn't sync phase-to-phase voltages. I could see issues if one side was faulted to ground or floating far from ground when you sync in. You probably have other protection to detect this faulted condition anyways. I suppose that wouldn't matter if your generator was connected in delta.