Solar PV park transformer connection

[Power0020]

Reviewing a design for a solar PV park with inverter step up transformer (to 22 kV) connected in Dyn11, with the delta facing 22 kV and wye facing 600 V (inverter side).

Array power is evacuated to a 22 kV switchgear through radial feeder with ring main units.

The receiving substation (some 2-3 km away) has a step-up 22/220 kV ddYN three winding transformer and grounding (zig-zag) transformer is used to detect SLG faults. feeders are arranged on normally opened bus sections on 22 kV side.

I wonder why inverter step up transformer is connected in Dyn11 with wye facing power flow direction rather than YNd11? Note that with YNd11 connection, the additional zig-zag transformer will not be needed.

Any clues?

 

[waross]

The step up transformer is probably a 22kV:0.6 kV distribution transformer being reverse fed.
Delta/Wye is common for distribution transformers.
A 400 Volt delta feeding a 22000 Volt wye will probably be a special order.
When you back feed the delta:wye it becomes a wye delta.
It works as long as everything is perfect. When something goes wrong, a more interesting things may happen.
If you use a four wire connection (wye point connected to the supply neutral) ant unbalance of voltages or phase angles will cause circulating currents.
With the loss of a phase you will have a back feed to the lost phase.

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

 

[Marmite]

The Dy transformer arrangement is typical for wind turbines, solar farms, small distributed generation etc.
The LV system needs an earth reference, and you need to be able to detect and clear faults. The source of fault current is the transmission/distribution system, not the PV. (Especially at night!)
Standard Delta/Star distribution transformers are also cheap and widely available.
Regards
Marmite

 

[davidbeach]

...cheap and widely available... and misapplied. When the PV park is feeding a line-ground fault on the 22kV system after the rest of the 22kV system has cleared then you get significant overvoltages on the unfaulted phases. A much better transformer selection would be a wye-delta-wye; the delta could be buried or it could be brought out.

 

[prc]

Marmite, to my knowledge these are not standard transformers but multi winding inverter transformers. LV side neutral is never grounded. Please see thread238-424040