1 Mw solar DG system interconnection

[bigsolar]

we will be suppling 23kv to the utility through a wye/wye transformer.

Question...Should the secondary(PV)connection xo float or would it be best to connect xoho. My main concern is zero sequence harmonics causing havoc on the transformer and also the ability to clear ground faults.

 

[davidbeach]

X0 and H0 will both need to be grounded. Far better would be a delta on the generation side and a grounded wye on the utility side.

 

[bigsolar]

Here is the entire design...Tell me what you think


2-500Kw Satcon inverters@200vac (no neutral)each supply delta/wye isolation transformer(1:1)go through switchgear and feed utility transformer wye/wye to 23Kv.

we actually had a problem soon after start-up

Contractor hooked up Grounding conductors to XO of the utility transformer and they ended up melting and caused a ground fault that opened B phase at the riser cutout on the pole.

also the ground wire on pole showed signs of arcing.

 

[alehman]

It's hard to say what happened. As David said though, for YY you should have X0 and H0 grounded. If they aren't you won't have a grounded system and hence no way to sense a ground fault. If harmonics are a concern, talk to the inverter mfg. about a filter.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[waross]

Ground or neutral? The terms are often used interchangeably but they are not the same. Construction standards and expectations may differ between grounding conductors and neutral conductors.
A system may be grounded by an electrode or grid. The purpose of a ground is to limit voltages between ground and exposed conducting surfaces. A ground may supply a return path for fault currents.
A neutral supplies a path for normal and fault currents to return to the source. It is most often a dedicated conductor from the source to the load or transformer bank. A neutral is often grounded at one or more points but if the application requires a neutral, it is a grounded neutral not a ground.
Continuous current through a ground grid or electrode may result in accelerated corrosion and/or increasing resistance.
A continuous current to ground may heat the soil surrounding the grounding electrodes and dry it out resulting in much higher than design resistance.
23 kV sounds like a wye distribution circuit. If your distribution or PV phase voltages/currents are not balanced you will have current returning on the neutral/ground?. How is the workman ship of the installation? A poor connection may fail/melt at a relatively low current if it develops arcing in the connection. I have seen failures when feeder conductors were connected by residential electricians who did not understand how to properly tighten the connectors on the feeders. The work looked good until you moved the conductor and discovered that the connection was not properly tightened.

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

 

[bigsolar]

Here is my plan...

Ground wye on Primary side HO (22800)
Float wye on secondary side XO (200VAC)
Float wye on xo of delta/wye iso transformer (delta from inverter side)

All metallic surfaces are bonded back to primary Transformer

Will there be any issue with clearing phase to ground faults on secondary side.

remember, this is a PV system and primary and secondary switch direction.

Is this considered a separately derived system?

 

[alehman]

Yes, you won't have any ground fault current on the secondary unless the system is grounded elsewhere. For the transformer to present a solid ground on either side, *both* neutrals must be grounded.


Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[davidbeach]

That's what I said 6 Jun 09 20:04. All wyes grounded.