Zero Sequence Confirmation 3

[Energyfit24381]

Looking for confirmation on zero sequence philosophy as it pertains to Distributed Energy Resources. I am converting a site with multiple meters to a singular main primary PCC/primary service point. I am creating the protection scheme which is pretty much done but was told im still lacking zero sequence continuity to the customer secondary. We have a 12.47/480 V Y-Y transformer that will feed mutliple transfer switches and gens.
Specifically, the concern that was bought up to me is that the protective devices that i have so far on the secondary are not able to detect and control for phase to ground faults on utility primary equipment without zero sequence continuity. Can someone shed light on zero sequence philosophy and how to implement?

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[wcaseyharman]

I’m not sure what is meant by zero sequence continuity, I will be interested when someone familiar with that term chimes in. In my utility we have seen that some DERs may not supply zero sequence current, which means they will not supply fault current to ground faults without a Ygnd-D transformer. However, Ygnd-D transformers have their own issues on distribution systems as well.
One of the problems without having a zero sequence source is if during a ground fault the substation breaker opens, the phase-ground voltage will rise to 173% (line to line) voltage if the DER continues to operate. Often utilities design their systems to be effectively grounded, with limits the rise to less than 140%. Customers, equipment and surge arrestors which are all designed for effectively grounded system will see a significant overvoltage causing damage.
It’s an interesting topic.

 

[cranky108]

When isolated from the utility, the lack of a zero-sequence path means a ground fault is very difficult to detect. What some IBR based sites do is to install a zig-zag transformer to establish a zero-sequence path. Another method is to use a wye-delta transformer with no secondary load, to establish a zero sequence path.
Note a Y-Y transformer is not a very good zero-sequence path. It is a path, but without a delta winding, it is not a very good one. BTW, is that grounded or ungrounded?

 

[Kiribanda]

If you draw the zero seq. network for a YGND/ YGND transformer, you will find that due to the high transformer mag. reactance the zero seq
components on the primary side for a primary side single phase to ground fault is almost zero. In other words, a YGND/ YGND transformer
is effectively a DELTA/ wye GND transformer so that for high side faults no zero seq currents exists on the primary.
Therefore, you should have either a Y GND/ DELTA or incorporate a third winding as DELTA to catch the zero seq. currents.
Otherwise, a single line to ground fault on the 12.47kV will be undetected and not cleared creating a safety hazard.
The logic is when the DER is connected to the 12.47kV system you have to ENSURE that the 12.47kV system is continued to be an EFFECTIVELY grounded system.

 

[Energyfit24381]

Thanks @wcasey, we will have means for the utility to trip the system offline in the case of a fault but of course that does not wave the requirement for secondary protection. This is great feedback.

@cranky yes this will be a grounded Y-Y transformer. We will be adding differential protections on the secondary side to satisfy the utility. This is great info!

@kiribanda this is great info!!

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[prc]

I am confused about the discussion.
1) Normally what I have seen in solar DER in India is Dy transformers (ie HV-delts LV- ungrounded star as required by inverter supplier) with a zig-zag grounding transformer on y side for a group of transformers.
2) With YN/yn transformers (i.e. both sides star connected with neutral effectively grounded), why is a primary side L-G fault to be detected by secondary protection? Will it not be detected by primary protection? In India, almost all MV transformers have a Ynyn connection without any stabilizing tertiary winding.

 

[mb3928]

Energyfit2438, you should be able to detect a utility side ground fault from the DG side as long as both neutrals share the same ground reference. One way to do what (I think) your AHJ is asking for us to connect the utility side and DG side neutrals together. In that case you must not bond the DG side neutral to ground, and it is not a separately derived system. This may or may not result in issues with voltage drop/rise due to fault currents, you will need to evaluate.

You didn’t specify the DG type but note that most inverter DG these days is not a ground zero sequence current source. Depending on the network constants you may need to add a grounding transformer to maintain effective grounding. This could also be what your AHJ is referring to. EPRI has a good paper on this topic.

 

[Energyfit24381]

Thanks everyone, as always i appreciate the support and expertise. This is all awesome. The DER i am referencing above is purely generators, there is no inverter in the system. We are going from primary through a 12.47/.48 Y-Y grounded XFMR to Switchgear to generators and back so to speak. And by “back” i mean export to utility when suitable.

After discussing with the utility provider we added a relay to tie the secondary equipment in so faults upstream in the primary service will shut down the system as before we only showed primary protections.

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-