Corner grounded Delta - Photovoltaic system

[KENELEC]

Hello,

New member here but use the site frequently. I have a question regarding the integration of a PV system to an existing facility with a corner grounded Delta. I have attached the SLD below fro reference. I don't have any experience with corner grounded Delta systems and am concerned that there is a risk of circulating currents of voltage imbalances. the below are snips from the drawings for reference. Any insight will be gratefully appreciated.

 

[ThatShouldHaveWorked]

I'm probably missing something. I haven't done a big PV system. That said I don't see the point of the up / down transformer coming from the PV.

Are they using 277? How you have the indoor switchboard labeled would lead me to think no. The same would be true with the indoor transformers.

Just because the city gives you a neutral doesn't mean you have to use it. I've seen systems where the neutral comes into the main disconnect and doesn't come out.

When in doubt ask the AHJ how they want it done.

 

[KENELEC]

the reason for the step-up/step down transformers is due to a 1000' run. Voltage drop was a main concern.

 

[waross]

I don't have any experience with corner grounded Delta systems and am concerned that there is a risk of circulating currents of voltage imbalances.

With corner grounding, there will be leakage currents through the insulation resistance and capacitive currents from the two ungrounded phases to ground.
These are insignificant and may generally be ignored.

If your inverter feeds are ungrounded or have no ground reference you should have no problems.

But be aware that unbalanced phase loading will not cause circulating currents but will cause a division of current.
Consider a delta bank.
Refer to the transformers as "A:, "B" and "C".
For simplicity we will consider unity PF loading.
If a single phase load of 10 Amps is applied to transformer "A", the current will split almost 50/5 and 5 Amps will flow in transformer "A"
The remaining 5 Amps will flow through transformer "B" at 50% PF and through transformer "C" at 50% PF. One phase will be leading and one phase will be lagging.
The higher the X/R ratio, the closer to 50/50 will be the current split.


Another possible issue is the 277/480:12.47 kV transformer.
Two possible issues.
Any uneven phase to phase loading severe enough to cause uneven voltages on the 277/480 Volt side will cause circulating current in the delta.
Should you lose one phase on the 277 Volt side, the delta will circulate enough current to backfeed that phase.
If you pull a fuse or open one line of the 277 Volts, there will be 277 Volts backfed with an Available Short Circuit Current of 1/3 of the transformer bank.
When a wye delta transformer is used in a distribution circuit, and one supply phase is lost, the wye delta bank will backfeed into the missing phase enough power to pick up other loads on the missing phase.
This will probably not be an issue in normal operation but it is something to be aware of during troubleshooting.
Opening only one phase on the wye/delta primary will not "safe" that phase.
There will be full voltage and a lot of Amps at both sides of the fuse holder or open circuit.

 

[ThatShouldHaveWorked]

You need a bonding locknut or bushing on one end of the metal raceways between the PVs and the fused disconnect.

Otherwise bonding bushings on EMT conduit fittings are only require if they are in concentric / eccentric knockouts. With 3.5" / 4" I would expect the holes to be punched.

If you do use bonding bushings they need to be connected to the EGC not the enclosure.

On the RGC two locknuts one inside and one outside is permitted. I'd probably use a bonding bushing to hedge my bet.

Since you'll also have a bunch of pull boxes I want to mention you only need the jumper in a junction box.

I would probably use a binding bushing in the box between the PVC and metal race ways

Over use of bonding bushings is a pet peeve of mine.