NGR and SLG faults

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I'm looking to add a NGR at a main transformer to reduce SLG faults. The configurations for the inverter step up transformer are delta (MV)- wye (LV) and wye (MV) - wye (LV). The issue I am facing is that the wye-wye config produces harmonics and the delta wye config leaves the MV feeders ungrounded when the substation breaker opens. What is the best solution when adding a NGR? Is it needed?
 
This isn't really a PV question so you will probably get more answers by posting it in another forum, maybe the Bonding and Grounding forum.
 
The wye wye by definition passes Zero sequence currents. And the MV step up is delta so just use the wye wye transformer to produce the neutral as well as apply the resistor there. Zero sequence currents are blocked in the wye-delta so you can apply an NGR on the source easily.

The trick is detecting the direction with the wye-wye. BYZ CTs placed on the phase conductors will provide that signal. If you truly want an NGR on the delta you can connect it via a zig zag transformer to recreate the neutral and connect the NGR there. Make sure the H0 is not grounded at the wye at the other end but even then I’m pretty sure wye wye still passes zero sequence currents so I don’t know if you can provide isolation.
 
You would be putting the NGR on the secondary side of the transformer to reduce SLG faults on that side. So either a Dyg or a YgYg winding will work. Putting an NGR on the Y neutral on the primary side of a Ygyg is not going to help your SLG fault current on the primary side if the majority of the current is coming from the utility upstream of the transformer. Also, NGRs are not usually used in PV installations so I'm not really sure what your goal is here. You seem to be concerned about providing effective grounding on the primary side in the event of a loss of the utility power in an export PV system?.
 
pv_n00b said:
You would be putting the NGR on the secondary side of the transformer to reduce SLG faults on that side. So either a Dyg or a YgYg winding will work. Putting an NGR on the Y neutral on the primary side of a Ygyg is not going to help your SLG fault current on the primary side if the majority of the current is coming from the utility upstream of the transformer. Also, NGRs are not usually used in PV installations so I'm not really sure what your goal is here. You seem to be concerned about providing effective grounding on the primary side in the event of a loss of the utility power in an export PV system?.
Click to expand...

OP has a step up transformer and wants to protect the transmission line between them.
 
paulengr said:
OP has a step up transformer and wants to protect the transmission line between them.
Click to expand...

Between what?

The subject of this thread is over my head, but I think I know enough to say the configuration hasn't been sufficiently described. I take the OP as considering delta-wye vs. wye-wye for connecting the PV inverter output to another system. I'd assume the LV side is the inverter, in which case I'd call the transformer a step down; terminology should be clarified. Also it isn't clear if the MV system is a service or another system, and whether and how it's grounded.
 
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